31P NMR Chemical Shifts of Phosphorus Probes ... - ACS Publications

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31

P NMR Chemical Shifts of Phosphorus Probes as Reliable and Practical Acidity Scales for Solid and Liquid Catalysts

Anmin Zheng,*,† Shang-Bin Liu,*,‡ and Feng Deng*,† †

State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, National Center for Magnetic Resonance in Wuhan, Key Laboratory of Magnetic Resonance in Biological Systems, Wuhan Institute of Physics and Mathematics, the Chinese Academy of Sciences, Wuhan 430071, China ‡ Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan ABSTRACT: Acid−base catalytic reaction, either in heterogeneous or homogeneous systems, is one of the most important chemical reactions that has provoked a wide variety of industrial catalytic processes for production of chemicals and petrochemicals over the past few decades. In view of the fact that the catalytic performances (e.g., activity, selectivity, and reaction mechanism) of acid-catalyzed reactions over acidic catalysts are mostly dictated by detailed acidic features, viz. type (Brønsted vs Lewis acidity), amount (concentration), strength, and local environments (location) of acid sites, information on and manipulation of their structure−activity correlation are crucial for optimization of catalytic performances as well as innovative design of novel effective catalysts. This review aims to summarize recent developments on acidity characterization of solid and liquid catalysts by means of experimental 31P nuclear magnetic resonance (NMR) spectroscopy using phosphorus probe molecules such as trialkylphosphine (TMP) and trialkylphosphine oxides (R3PO). In particular, correlations between the observed 31P chemical shifts (δ31P) of phosphorus (P)containing probes and acidic strengths have been established in conjuction with density functional theory (DFT) calculations, rendering practical and reliable acidity scales for Brønsted and Lewis acidities at the atomic level. As illustrated for a variety of different solid and liquid acid systems, such as microporous zeolites, mesoporous molecular sieves, and metal oxides, the 31P NMR probe approaches were shown to provide important acid features of various catalysts, surpassing most conventional methods such as titration, pH measurement, Hammett acidity function, and some other commonly used physicochemical techniques, such as calorimetry, temperature-programmed desorption of ammonia (NH3-TPD), Fourier transformed infrared (FT-IR), and 1H NMR spectroscopies. 4. Applications of 31P NMR for Acidity Characterization of Various Catalysts 4.1. Metal Oxides and Modified Metal Oxides 4.1.1. Mixed Metal Oxides 4.1.2. Sulfated Zirconia 4.1.3. Titanium Oxides 4.1.4. Zinc Oxides 4.1.5. Other Assorted Metal Oxides 4.2. Porous Catalyst Materials 4.2.1. Microporous Zeolites and Mesoporous Molecular Sieves 4.2.2. Aluminosilicate Zeolites 4.2.3. Borosilicate Zeolites 4.2.4. Quantitative Study and Discernment of Internal vs External Acid Sites 4.2.5. Pore Confinement Effects 4.2.6. Nano-Sized and Hierarchically Structured Zeolites 4.2.7. Characterization of Lewis Acidity

CONTENTS 1. Introduction 2. Acidity Characterization by Solid-State 31P NMR Using Phosphorus Probe Molecules 2.1. 31P-Trimethylphosphine (TMP) NMR Approach 2.2. 31P-Trialkylphosphine Oxides (R3PO) NMR Approach 3. Experimental Considerations for 31P NMR of Phosphorus Probes and Related Methods 3.1. Standard Operation Procedures for 31P NMR of Phosphorus Probes Studies 3.2. Related 31P NMR Methods for Acid Characterization using Phosphorus Probes 3.2.1. Double Resonance and Population Transfer 3.2.2. Two-Dimensional Heteronuclear Correlation 3.2.3. Spectral Editing by Homonuclear Decoupling and Selective Excitation 3.2.4. Two-Dimensional Homonuclear DoubleQuantum Correlation © 2017 American Chemical Society

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12485 Received: May 21, 2017 Published: September 27, 2017

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DOI: 10.1021/acs.chemrev.7b00289 Chem. Rev. 2017, 117, 12475−12531

Chemical Reviews 4.3. Functionalized Porous Organic Catalysts 4.4. Heteropolyacid-Based Catalysts 4.5. Homogeneous Acid Catalysts 4.5.1. Aqueous Acid Solutions 4.5.2. Determination of Gutmann Acceptor Number 4.5.3. Ionic Liquid-Heteropolyacid Hybrids 5. Summary and Outlook Author Information Corresponding Authors ORCID Notes Biographies Acknowledgments Symbols and Abbreviations References

Review

strated that the extra-framework SnO2 species in Sn-containing zeolites, which serve as the Lewis acid centers during catalytic reactions, gave rise to a sharp 119Sn NMR resonance peak at ca. −604 ppm under magic-angle spinning (MAS) condition, while tetrahedral framework Sn sites exhibit resonance peaks within the chemical shift (δ119Sn) range from −420 to −450 ppm in the dehydrated sample.11 The latter tend to shift toward upfield direction (ca. −650 to −730 ppm) upon a sample hydration treatment. To this regard, 27Al MAS NMR has been extensively used for probing the framework structure of aluminosilicate materials such as microporous zeolites and mesoporous molecular sieves. Typically, tetrahedral-coordinated framework Al (FAL) species, which are associated with Brønsted acid sites (i.e., bridging hydroxyls; Si−OH−Al) show NMR resonance with δ27Al in the range of 50−65 ppm, whereas Lewis acidic extra-framework Al (EFAL; mostly arising from five- and sixcoordinated) species have δ27Al < 50 ppm.12 Along the same line, the presence of Brønsted acidity in zeolitic catalysts may be identified directly (through acidic OH groups) by means of Fourier-transform infrared (FT-IR)7,13−15 or 1H NMR spectroscopies.16−19 The Hammett acidity function (H0), which was originally designed for direct measurement of acidity of a concentrated solution of strong acids, invokes the use of a colorimetric indicator (BH+; a relatively weak acid) for probing a specific range of acidic strength.20,21 The deprotonation of the probe molecule BH+, which is the conjugate acid of a weak base B, can be expressed by

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1. INTRODUCTION Acid catalysis plays a key role in most industrial processes for the production of chemicals. Owing to their strong acidic strengths and uniform distributions, mineral liquid acid catalysts such as H2SO4, H3PO4, and HF, have been extensively exploited for homogeneous catalysis over the past few decades. However, serious issues provoked by these conventional liquid acids such as toxicity, equipment corrosion, waste treatment, formidable regeneration and recycling, and so on, have drawn considerable concerns in view of eco-friendly processing. In replacement of these hazardous liquid acids, solid acid catalysts such as zeolites, neat and/or modified metal oxides, functionalized silica/carbon materials, and heteropolyacids (HPAs) etc. have been developed and extensively applied as heterogeneous catalysts in various industrial chemical processes.1−5 In general, two types of acidity may be categorized for solid acid catalysts, namely Brønsted and Lewis acidities, which readily affect the reaction pathway of a heterogeneous reaction, while other acid features such as concentration, distribution, and strength of acid sites dictate the performances (i.e., activity and selectivity) of the catalytic reaction. In this context, a comprehensive understanding of detailed acid features of solid acid catalysts, viz. acid types, concentrations, distributions, and strengths of acid sites, is inevitable for practical designs and applications of industrial processes. By definition, Brønsted acid (BA) sites are proton donors normally associated with acidic hydroxyls (−OH groups), whereas electron-deficient Lewis acid (LA) sites serve as electron pair acceptors.6−8 The latter are commonly associated with active metal centers residing on the surfaces of solid acid catalysts. Thus far, a variety of different analytical and spectroscopic techniques have been developed for acidity characterization of solid acids, as summarized in Table 1. Among them, diffuse reflectance ultraviolet−visible (UV−vis) spectroscopy has been applied for qualitative assessment of metal centers (M) in solid catalysts. For example, the presence of an adsorption band at ca. 200−220 nm in Sn- and Ti-containing zeolites is ascribed to charge transfer from O2− to the tetrahedral M4+ framework sites, whereas those which appear in the range of 240−300 nm are attributed to octahedral structure of bulk metal oxide or oligomeric species.9,10 Alternatively, direct detection of active metal center may also be accomplished by solid-state nuclear magnetic resonance (SSNMR) spectroscopy, which has been shown to be a reliable approach for probing local structures of framework and extra-framework species. It has been demon-

BH+ ⇄ B + H+

(1)

The equilibrium constant, Ka, may therefore be expressed in terms of activities (a): K a = aBa H+ /a BH+

(2)

The activities may further be defined as aB = [B]γB and aBH+ = [BH +]γBH+, where [B], [BH+], γB , and γ BH+ represents concentration and thermodynamic activity coefficient of B and BH+, respectively. By definition of the Hammett acidity function: ⎧ [B] ⎫ ⎬ H0 = pK BH+ + log⎨ ⎩ [BH+] ⎭

(3)

where pKBH+ = −log(Ka) for the dissociation of [BH ], eq 2 may be rewritten by taking the decimal logarithm: +

⎛γ +⎞ H0 = −log(a H+) + log⎜⎜ BH ⎟⎟ ⎝ γB ⎠

(4)

In case of dilute aqueous solution (pH = 0−14) in which H3O+ is the predominant acid species and the activity coefficients are close to unity, the H0 value is approximately equal to the pH. However, the definition of pH value is no longer valid in concentrated liquid acids in which their effective hydrogen-ion activities (hence, the nature of acid species) changes more rapidly than the concentration. By convention, the acidic strength of neat sulfuric acid (18.4 M H2SO4; H0 = −12) is defined as the threshold of superacidity; a more negative H0 value therefore represents a stronger acidity. Later, the Hammett acidity value, H0, was further exploited for determining the acidic strength of solid acids.22−24 In this context, unlike aqueous systems in which the value of H0 becomes more negative with increasing concentration, its value refers to acidic strength of an individual acid site in solid acid systems. As such, information on the 12476


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Table 1. Comparisons of Available Techniques for Acidity Characterization of Solid and Liquid Acid Catalystsa type method

BA

LA

location

concentration

strength

titration/Hammett methods TPD (probe method) ammonia; NH3 IR (direct detection) hydroxyl; OH IR (probe method) pyridine; C5H5N 2,6-ditert-butylpyridine; (CH3C)2C5H3N carbon monoxide; CO NMR (direct detection) metal centers; 27Al, 11B, 119Sn, etc. 1 H (hydroxyl OH) NMR (probe method) 1 H (pyridine-d5; C5D5N) 1 H (acetonitrile-d3; CD3-CN) 13 C (2-13C-acetone; (CH3)213CO) 13 C (mesityl oxide; RCOCH = CR2) 15 N (15N-pyridine; C5H515N) 31 P (trimethylphosphine; (CH3)3P) 31 P (trialkylphosphine oxides; R3PO)

+

−

−

−

+

+

±

−

+

±

+

−

−

−

+

+ + −

+ + +

− ± −

± − −

− ± +

− +

+ −

− −

− +

− −

+ + + + + + +

− − + − + + +

− − − − − − +

± − − − + +(B); +(L) +(B); +(L)

+ + + + + −(B); +(L) +(B); −(L)

a

BA, Brønsted acidity; LA, Lewis acidity; TPD, temperature-programmed desorption; IR, infrared spectroscopy; and NMR, nuclear magnetic resonance spectroscopy. Location: acid sites locating at internal or external surfaces of solid catalysts. Concentration: the amount of acid sites. Strength: the ability to dispel a proton from a Brønsted acid site or to accept electrons by a Lewis acid site. R, CnH(2n+1), (n = 1, 2, 4, and 8); +, superior; ±, satisfactory; and −, inferior.

distribution of acidic strength in solid acid catalysts cannot be specified by the H0 value alone. Moreover, being exclusively defined based on Brønsted acid−base chemistry, H0 can neither be used for determining the strength of Lewis acid sites.22 Alternatively, indirect detection of Brønsted acidity may be accomplished by using the probe method, which normally invokes the adsorption of a desirable base probe molecule on acidic proton (H+). In this context, conventional methods such as temperature-programmed desorption (TPD) of ammonia or alkylamines25−28 and FT-IR spectroscopy of pyridine and carbon monoxide29−31 have been extensively used. Several excellent reviews on relevant areas can be found.32−35 Nonetheless, these indirect techniques for acidity characterization invoke assessment of heat of adsorption/desorption associated with the basic probe molecule and acid site and are therefore capable of determining overall strength of acid sites only through variabletemperature measurements. Among them, NH3-TPD is probably the most popular method by which two or more broad desorption peaks are normally observed.25−28 The low-temperature desorption peaks (typically at T < 523 K) are known due to desorption of NH3 from weak acid sites’ temperatures, while the high-temperature peaks (at T > 573 K) are arising from strong acid sites. As such, relative concentrations of overall strong versus weak acid sites may be estimated from the peak areas of the corresponding desorption curves. Moreover, the position of the peak (i.e., the peak temperature), which is relevant to the enthalpy of adsorption, may be used to discern the average acidic (binding) strength of the surface acid sites. However, the NH3TPD method has a drawback because it is incapable of differentiating Brønsted and Lewis acid sites properly.27 Not to mention that the reliability of the method is also affected by the diffusivity of the adsorbate probe molecule mediated by the pore structure of the catalyst adsorbent and possible formation of adsorption complex such as NH4+·nNH3.25

Compared with the conventional titration, Hammett acidity function and NH3-TPD methods, acidity characterization by spectroscopic techniques has been recognized as a reliable approach for obtaining detailed qualitative and/or quantitative information on acid sites. Among them, FT-IR spectroscopy has been widely applied over the past few decades for direct detection of structural hydroxyl groups (−OH) on solid catalysts. The surface hydroxyl groups typically exhibit characteristic bands spanning over a fundamental stretch region of 3600− 3750 cm−1. The stretching vibrational bands at ca. 3740 and 3600−3650 cm−1 are attributed to weakly acidic terminal silanol (Si−OH) and strongly acidic bridging hydroxyls (Si−OH−Al) groups, respectively. Aside from direct detection of the surface hydroxyls, acidity characterization by FT-IR by probe molecule method is also commonly employed. Again, such probe methods invoke adsorption of a base probe molecule such as pyridine (C5H5N) or 2,6-ditert-butylpyridine. In particular, the pyridineIR (py-IR) technique has been extensively used for probing the surface acidity of solid acids.31 Compared with the other conventional acidity characterization methods (Table 1), the unique advantage of the py-IR technique is capable of identifying Brønsted and Lewis acid sites simultaneously. Typically, IR absorption bands at ca. 1545 and 1450 cm−1 are attributed to pyridine adsorbed on Brønsted and Lewis acid (BA and LA) sites, respectively, while both types of acid sites also contribute to an additional band at ca. 1490 cm−1. It is due to this latter overlapping band that makes determinations of the extinction coefficient of pyridine adsorbed on BA and LA sites impossible.17,30 This, in turn, makes quantitative measurements of acid concentration in solid catalysts unfeasible. In addition to the aforementioned techniques, SSNMR spectroscopy has been shown to be a powerful tool for characterizing acid properties of solid catalysts.18,19,36−39 For example, 1H MAS NMR has been extensively used to probe the 12477


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probes for characterizing acid properties of solid catalysts over the past few decades.19,50−64 Consequently, detailed acid features such as type (BA vs LA), distribution, concentration, and strength of acid sites in solid acids may be attained. Moreover, quantitative correlations between the observed δ31P (of the adsorbed probe molecule) and acid strength have been implemented for both 31P-TMP and 31P-R3PO NMR approaches through complementary theoretical studies.52,60−64 More recently, the latter approach has also been successfully exploited for characterizing acid properties of homogeneous catalysts, for example, liquid acids69−71 and functionalized ionic liquids (ILs).72,73 The objectives of this review aim to summarized research work related to acidity characterization of acid catalysts by means of 31 P-probe molecule NMR approach, particularly focusing on the following key issues: (i) introduction of various 31P NMR approaches invoking different P-containing probe molecules and relevant sample preparation procedures, (ii) correlations between the observed δ31P of different P-containing probes and acidic strengths and feasibility of various approaches for acidity scaling, (iii) comparisons of experimental results with theoretical calculations and/or those obtained by other available analytical and spectroscopic methods for acidity characterization, and (iv) illustrations of some practical applications of various 31P NMR approaches for acidity characterization of solid and liquid acid catalyst systems.

structures of various hydroxyl groups in solid acids. For example, acidic bridging hydroxyls (Al−OH−Si, namely the BA sites) and weakly acidic terminal hydroxyls (Al−OH and Si−OH) are commonly found in aluminosilicates such as zeolites, SAPOs, and AlPOs.16,17,40,41 One of the advantages of NMR spectroscopy is the feasibility for not only qualitative but also quantitative measurements, hence, surpassing most conventional techniques for acidity characterization (see Table 1). Consequently, detailed acid features of solid acids such as type, location, concentration, and strength of acid sites may be investigated by SSNMR by choosing a suitable probe molecule containing the desirable NMR-sensitive nucleus such as 1H, 13C, 15N, and 31P. Among them, extensive investigations have been made by using solidstate 1H and 15N NMR of pyridine (C5H5N),42−44 1H NMR of deuterated acetonitrile (CD3CN), 13C NMR of acetone (CH 3 COCH 3 ) and mesityl oxide (CH 3 COCHC(CH3)2),45−49 and 31P NMR of trimethylphosphine ((CH3)3P; TMP) 50−52 and trimethylphosphine oxide ((CH 3 ) 3 PO; TMPO),53−64 and so on. Moreover, 1H SSNMR of deuterated pyridine (C5D5N; py-d5) is also a common approach for probing surface acidic hydroxyls. Due to the formation of hydrogen bonding between the probe molecule and the surface hydroxyl groups, in general, 1H resonances arising from adsorption of pyridine-d5 on nonacidic or weakly acidic terminal hydroxyls (e.g., Si−OH and Al−OH) give rise to signals with δ1H ranging from ca. 2 to 10 ppm, whereas py-d5 adsorbed on BA sites [i.e., formation of pyridinium ion (C5D5NH+)], normally lead to signals with δ1H in the range of 12−20 ppm.65 Alternatively, 15N NMR of pyridine-d5 may also be used to probe the guest−host interactions, particularly those involving BA and LA sites.66 In addition, acidity characterization by 13C SSNMR of isotopelabeled 2-13C-acetone (CH313COCH3) is also a well-established technique, particularly for probing the relative acidic strength of BA sites in solid acids.49 The technique relies on measurement of δ13C, arising from the interaction between the acidic proton and the carbonyl oxygen. As a result, the hydrogen bond interaction results in a shift of 13C resonance toward downfield; the stronger the Brønsted acidity, the higher the observed δ13C. It is noteworthy that, though capable of determining the relative acidic strength of solid acid catalysts, the 1H-pyridine and 13 C-acetone NMR approaches are limited by some of the key NMR parameters. The proton nucleus (1H; spin quantum number I = 1/2) has a very high natural abundance (99.989%) but a rather narrow observable chemical shift range (Δδ1H; ca. 20 ppm), leading to an inferior spectral resolution. On the other hand, while the 13C nucleus (I = 1/2) has a more desirable resolution (Δδ13C; ca. 300 ppm), its drawback is modest detection sensitivity due to very low natural abundance (1.07%). Thus, most 13C SSNMR experiments rely on the use of 13Cenriched reagents, which is less favorable in terms of experimental cost.67,68

2.1. 31P-Trimethylphosphine (TMP) NMR Approach

The 31P-trimethylphosphine (TMP) NMR approach was pioneered by Lunsford and co-workers.50 Utilizing TMP as the probe molecule, the authors characterized Brønsted and Lewis acidities in a series of calcined H−Y zeolites. The 31P NMR spectrum of TMP adsorbed on H−Y calcined at 400 °C exhibited a single peak at ca. −3 ppm, which was assigned due to formation of [(CH3)3PH]+ complexes between pairs of TMP and Brønsted acidic proton (H+), while chemisorbed TMP on H−Y calcined at elevated temperatures (≥500 °C) showed additional signals in the upfield region (i.e., toward a more negative δ31P value) of −32 to −58 ppm. The latter resonances were attributed to chemisorbed TMP at Lewis acid sites arising from EFAL species in harshly calcined H−Y. Therefore, the 31P-TMP NMR approach has been recognized as a sensitive method for identifying Brønsted (BA) and Lewis acid (LA) sites in solid acid catalysts. Accordingly, it has been adopted for acidity characterization of various solid acid systems such as amorphous silica−alumina,74 dealuminated microporous zeolites,75−77 mesoporous zeolite,78−80 mixed metal oxides,81 sulfated metal oxides,82−84 and heteropolyacids (HPAs).47 In general, TMP bounds at BA sites lead to 31P resonance with δ31P in a rather narrow range of ca. −2 ∼ −5 ppm, while signals responsible for TMP adsorbed on LA sites span over a considerable wider δ31P range of ca. −20 ∼ −60 ppm. As such, the 31P-TMP NMR approach is a more sensitive tool for determining the strength of LA sites and less practical for comparing strength of BA sites. In any case, 31P resonances of adsorbed TMP with a larger δ31P value represents a stronger Brønsted or Lewis acidity regardless of the type of acid site. To afford quantitative scaling of Brønsted acidic strength, the observed δ31P of TMP may be correlated with the deprotonation energy (DPE), a measure of energy difference between protonated and deprotonated forms of a BA site.52 The DPE value, which represents intrinsic strength of BA site, may readily be obtained by density functional theory (DFT) calculations.85,86

2. ACIDITY CHARACTERIZATION BY SOLID-STATE 31P NMR USING PHOSPHORUS PROBE MOLECULES Compared to 1H and 13C, the 31P (I = 1/2) is a more preferable NMR-sensitive nucleus in terms of sensitivity and resolution of detected spectra of phosphorus (P)-containing reagents. This is owing to the 100% natural abundance and considerably wider detection range of chemical shift (Δδ31P > 650 ppm) available for the 31P nucleus. Therefore, P-containing molecules, such as trimethylphosphine (TMP) and trialkylphosphine oxides (R3PO), have been developed and widely utilized as NMR 12478


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Figure 1. Correlations of calculated 31P chemical shift of adsorbed TMP with (a) deprotonation energy of Brønsted acid site and (b) binding energy of Lewis acid sites with B, Al, and Ti metal centers in solid acids. Adapted from ref 52. Copyright 2011 American Chemical Society.

Figure 2. Optimized equilibrium configurations of TMPO adsorbed on the 8T model with Si−H bond lengths of (a) 1.30, (b) 2.00, and (c) 2.50 Å. Selected interatomic distances (in Å) are indicated. Adapted from ref 60. Copyright 2008 American Chemical Society.

TiClnF4−n (n = 0−4) solid acid systems. The variations of predicted δ31P with BE derived from various Lewis acid model systems are shown in Figure 1b. It is indicative that a linear correlation between the theoretical δ31P and BE may be respectively inferred for each Lewis acid system:52

Thus, the smaller the DPE value, the easier the acidic proton may be detached (i.e., deprotonated) from a BA site, and thus the stronger the Brønsted acidic strength. As such, DPE is also commonly inferred as the proton affinity (PA). Indeed, based on the optimized structure of TMP adsorbed on the BA site, an increasing extent of proton transfer (from a BA site to TMP molecule) represents a decrease in DPE (i.e., an increase in acidic strength) and, thus, corresponds to an increase in the observed δ31P, as shown in Figure 1a. It is noteworthy that the wellaccepted threshold DPE value for superacidity is ca. 250 kcal/ mol,87,88 and common solid acid catalysts such as zeolites, mostly possess Brønsted acid sites with moderate acidic strengths, corresponding to a typical DPE value of ca. 270−310 kcal mol−1. In view of the fact that only marginal variations in δ31P (−2 to −5 ppm) are available for the TMPH+ adducts corresponding to DPE values ranging from ca. 250 (super acidic) to 320 (weakly acidic) kcal mol−1,52 it is conclusive that the 31P-TMP NMR approach is inferior for differentiating Brønsted acidic strengths in solid acid catalysts. Similar theoretical approach may be used to infer Lewis acidity in solid acids based on the 31P-TMP NMR approach. A correlation between the observed δ31P and Lewis acidic strength is attainable by deriving the binding energy (BE) of TMP adsorbed on LA sites, which are normally associated with metallic centers. In view of the complexity in possible structures of LA sites, Chu et al. performed a theoretical study using model metal halides incorporated with different metallic centers such as boron (B), aluminum (Al), and titanium (Ti).52 The authors examined a series of BClnF3−n (n = 0−3), AlClnF3−n (n = 0−3), and

BCl nF3 − n (n = 0 − 3): δ 31P = 1.41(± 0.01) − 60.50( ±0.33) × BE;

R2 = 1.00

(5)

AlCl nF3 − n (n = 0 − 3): δ 31P = 2.15( ±0.20) − 129.64( ±7.46) × BE;

R2 = 0.99

(6)

TiCl nF4 − n (n = 0 − 4): δ 31P = 3.37(± 0.86) − 93.28( ±16.00) × BE;

R2 = 0.91

(7)

These results clearly indicate that the predicted δ P spanned over a wide range (−10 to −60 ppm), corresponding to a range of BE covering from 15 to 40 kcal mol−1. Thus, the 31P-TMP NMR approach is indeed a sensitive technique for characterizing Lewis acidity in solid acids; the observed δ31P of adsorbed TMP may readily be used as a quantitative scale for Lewis acidic strength. By comparison, acidity characterization utilizing the 13 C-acetone NMR approach,49,64 by which the observed δ13C not only span over a narrow range (220−250 ppm) but also drawback by severe overlap of NMR resonances arising from both BA and LA sites, is an inferior technique for discernment of acid types. Unfortunately, the availability of a wide variety of metallic centers in solid Lewis acids are normally accompanied by 31

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complex interactions with the TMP probe molecule, which in turn makes a universal correlation between δ31P and Lewis acidic strength impossible for most Lewis acid systems. 2.2. 31P-Trialkylphosphine Oxides (R3PO) NMR Approach

Since the distribution of acid sites, which dictates the catalytic performance of solid acids, depends largely on local proximity and overall structural properties of the catalyst, an inhomogeneous distribution of acidic strengths is anticipated. As discussed above, while the 31P-TMP NMR approach is an effective technique for identifying acid types (i.e., Brønsted and/or Lewis acidity) and for determining subtle differences in Lewis acidic strength in solid acids, it is less sensitive for differentiating Brønsted acidic strength.73−76,78−83 In contrast, the 31P-R3PO NMR approach utilizing the oxidized counterparts of TMP, namely trialkylphosphine oxide (R3PO; R = CnH2n+1, n = 1, 2, 4, and 8) molecules, is more suitable for determining Brønsted over Lewis acidic strengths.53−64 The homologues of R3PO includes trimethylphosphine oxide (TMPO), triethylphosphine oxide (TEPO), tributylphosphine oxide (TBPO), and trioctylphosphine oxide (TOPO). Among them, TMPO is probably the most popular probe molecule with the smallest kinetic diameter (KD) of ca. 0.55 nm, which is comparable to the aperture of typical tenmembered ring (10-MR) zeolites (average pore size ca. 0.6 nm). The 31P NMR resonances of adsorbed TMPO probe molecule typically span over a wide δ31P range of ca. 50−98 ppm, depending on the distribution of acidic strengths in various solid acids, in agreement with data obtained from theoretical DFT calculations.60 In addition, a series of comprehensive theoretical studies have been made to predict the δ31P of R3PO on various model acids by DFT calculations.60−64 For example, Zheng et al. adopted a cluster model consisting of eight T atoms (i.e., 8T model; see Figure 2) to mimic the structure of MFI zeolite,60,61 by which the acid strengths of the model zeolite were tuned by varying the terminal Si−H bond distance therein. Consequently, the structures of R3PO adsorbed on BA sites of the 8T model were optimized to derive the intrinsic Brønsted acidic strengths. For illustration, Figure 2 shows the typical optimized equilibrium configurations of TMPO adsorbed on 8T cluster model with varied Si−H bond lengths. Theoretically, the intrinsic acidic strength may be inferred by probing the extent of proton transfer from the BA site to the oxygen atom of the R3PO (denoted as O(P)). Accordingly, a correlation between the O(P)−H distance and theoretical δ31P of R3PO may be obtained, as illustrated for the case of TMPO adsorbate in Figure 3.89 As such, the O(P)-H distance may be used to classify the acidic strength into three categories, namely weak, strong, and very-strong acid sites with corresponding threshold δ31P values of ca. 66, 76, and 86 ppm, respectively. Furthermore, linear correlations between the theoretical δ31P of the respective R3PO probes and the intrinsic Brønsted acidic strength, as represented by proton affinity (PA; also known as DPE) value, may be obtained, as shown in Figure 4.61 Taking the case of TMPO adsorbate as an example, the linear correlation may be expressed as

Figure 3. Correlation between the O(P)-H distance with calculated 31P chemical shift (δ31P) of TMPO adsorbed on model MFI zeolite. The intercepts (red circles) between experimental (vertical dashed lines) and theoretical δ31P were used to obtained the extrapolated O(P)−H distances. The O(P)−H distance was used to classify the acidic strength into three categories, namely weak, strong, and very-strong acid sites with corresponding threshold δ31P values of ca. 66, 76, and 86 ppm, respectively. Adapted from ref 89. Copyright 2016 American Chemical Society.

Figure 4. (a) Optimized equilibrium configurations of free R3PO (viz. TMPO, TEPO, TBPO, and TOPO) probe molecules. Selected interatomic distances (in Å) are indicated. (b) Correlations of calculated 31 P chemical shift of adsorbed R3POH+ complexes and proton affinity (PA) predicted based on the 8T zeolite cluster model. Adapted from ref 61. Copyright 2008 American Chemical Society.

with one another. Moreover, they also exhibit the same slope with that observed for the adsorbed TMPO (n = 1), except for the consistent δ31P offset of ca. 8 ± 2 ppm (Figure 4b). On the basis of theoretical predictions87,88 and experimental results, while a threshold δ31P value of ca. 86 ppm was attained for superacidity based on the 31P-TMPO NMR approach, a corresponding threshold δ31P value of ca. 92−94 ppm may be inferred for TEPO, TBPO, and TOPO.60,61 Alternatively,

TMPO: δ 31P = 182.87(± 5.31) − 0.390(± 0.02) × PA; R2 = 0.9913

(8)

In the case of other homologues of R3PO (R = CnH2n+1, n = 1, 2, 4, and 8) with larger molecular sizes (Figure 4a), such as TEPO (KD ∼ 0.60 nm; n = 2), TBPO (ca. 0.82 nm; n = 4), and TOPO (ca. 1.10 nm; n = 8), their linear correlation curves nearly overlap 12480


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correlation between the observed δ31P and acidic strength may also be inferred by monitoring variations of δ31P with the enthalpy of the guest/host adsorption system, as demonstrated by Osegovic and Drago.54,55 By comparing experimental results observed for various solid acids based on two different techniques, namely the 31P-TEPO NMR approach and the pyridine-calorimetry study, the authors concluded that both methods are eligible for scaling of acidic strength. This was justified by the linear correlation between the observed δ31P of adsorbed TEPO and enthalpy (−ΔH) of protonation of pyridine, as shown in Figure 5.

probing acid sites residing in both intracrystalline pore channels as well as extracrystalline surfaces of H-ZSM-5. On the other hand, bulkier TBPO and TOPO are too large to enter the pore channels of most zeolites and hence can only probe acid sites located on the external surfaces of the zeolite crystallites. Thus, it is conclusive that the 31P-R3PO NMR approaches not only facilitate a practical scaling for acidic strength (via δ31P) but also their distribution (through spectral analysis), location (e.g., internal vs external surfaces), and concentration of BA sites in solid acids.

3. EXPERIMENTAL CONSIDERATIONS FOR 31P NMR OF PHOSPHORUS PROBES AND RELATED METHODS 3.1. Standard Operation Procedures for 31P NMR of Phosphorus Probes Studies

While acidity characterization by 31P SSNMR of TMP and R3PO has been recognized as a practical technique, a thoughtful sample preparation prior to the spectroscopic (and/or analytical) study is crucial for reliable NMR (and/or relevant) measurements. As an illustration, a typical standard operation procedure for preparing probe molecule loaded samples based on the 31PR3PO NMR approach is discussed herein. The sample preparation procedure invoked for these approaches is illustrated in Figure 6 for adsorption of TMPO and/or TBPO on typical solid acids such as H-form ZSM-5 zeolites.62 Prior to the adsorption of the probe molecule, a desirable amount of solid catalyst was first placed in a sample container (glass tube) equipped with a vacuum stopcock. The sample tube containing the catalyst is then connected to a vacuum manifold for catalyst pretreatment. For typical zeolites, a thorough dehydration treatment at ca. 400−450 °C under vacuum (below 10−3 Pa) for overnight would be desirable. However, to avoid undesirable dealumination, dehydration of zeolite is normally carried out by slowly ramping the heating temperature (typically with a rate of 1−2 °C min−1) until reaching ca. 120 °C, and then proceeding with a higher ramp rate until reaching the targeted temperature, followed by maintaining at the temperature for at least 12 h. The sample tube containing the dehydrated catalyst is sealed and placed in an inert environment (e.g., glovebox or N2 bag). Then, a desirable and known amount of guest solution, namely TMPO (or TBPO) in CH2Cl2, is syringed into the sample vessel containing the dehydrated solid acid. The sample tube containing guest adsorbate and host adsorbent is then sealed, removed from the glovebox (or N2 bag), and reconnected to the vacuum manifold, followed by removal of the CH2Cl2 solvent by evacuation at ca. 50 °C. Subsequently, the guest/host sample system is further subjected to a baking treatment at ca. 145−165 °C for at least 1 h. The above thermal treatment is crucial in ensuring a uniform adsorption of TMPO/TBPO guest molecule on BA sites of the catalyst. Moreover, it is noteworthy that an

Figure 5. Scaling of acidic strength by the change in31P MAS NMR chemical shift of TEPO from the shift of physisorbed TEPO and protonation enthalpy of adsorbed pyridine. Assorted sample includes: sol−gel; silica gel; H-ZSM-5 zeolites (with two different sites; one and two); silica-supported antimony pentachloride (four different sites; Sb-1 ∼ Sb-4); SO3−SG, sulfuric acid washed silica gel; SgWO3SO3, silica supported sulfated tungsten oxide; HPW, 12-tungstophosphoric acid; and AlCl3, silica supported aluminum trichloride. Adapted from ref 54. Copyright 2000 American Chemical Society.

Note that unless there is a constraint for the size of probe molecule, the 31P-R3PO NMR approach represents a practical tool for determining distribution of BA sites with different acidic strengths regardless of the chosen probe molecule.60−64 Furthermore, taking advantage of differences in molecular sizes of R3PO, the 31P-R3PO approach may also be exploited for identifying the location of (internal vs external) acid sites in solid catalysts,56,90 especially those which possess an hierarchical pore structure and/or regular pore channels (vide infra). For example, H-ZSM-5 zeolite is a common microporous acid catalyst possessing strait and zigzag 10-MR pore channels, thus capable of accommodating probe molecules with KD less than ca. 0.6 nm. In this context, TMPO and TEPO molecules are suitable for

Figure 6. Standard operation procedure invoked for the 31P-R3PO NMR approach. 12481


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Figure 7. Typical 31P SSNMR pulse sequences invoking phosphorus probe molecules: (a) cross-polarization (CP), (b) 31P/27A1 TRAPDOR, (c) 1 H−31P HETCOR, and (d) 31P−31P DQ correlation spectroscopy.

Figure 8. 31 P/27A1 TRAPDOR spectra of TMP adsorbed on dehydroxylated H−Y zeolite at −150 °C: (a) without and (b) with 27Al on-resonance irradiation during τ (τ = 725 μs; spinning speed = 5.5 kHz). The difference spectrum is shown in (c). The isotropic resonance at −46 ppm (marked) shows the largest TRAPDOR effect. (d) 31P MAS NMR spectrum of TMP adsorbed in dehydroxylated H−Y and (e) corresponding 2 7Al → 31P INEPT spectrum collected with an evolution time, τ = 0.625 ms, spinning speed = 4.8 kHz, and approximately 70000 transients. Adapted with permission from refs 91. Copyright 1996 Elsevier B.V. Adapted from ref 94. Copyright 1997 American Chemical Society.

lattice relaxation time (T1) and the numbers of accessible BA sites (or Al content in zeolite) present and so on). In comparison, a slightly different procedure is invoked for the 31 P-TMP NMR approach. In particular, the dehydration treatment of the catalyst sample and subsequent adsorption of the guest adsorbate are both performed on a vacuum manifold. It is noteworthy that TMP is an extremely toxic reagent with disgusting odor and tends to oxidize easily in the presence of oxygen to form TMPO, thus the adsorption of this volatile guest molecule must be handled with care. Typically, after sample dehydration, a desirable amount of gaseous TMP may be transferred directly into the solid adsorbent (placed under a liquid N2 cooling bath) through the vacuum line at ambient temperature. Upon uptake of the TMP probe molecule, the guest/host sample system is then equilibrated for at least 0.5 h

excessive loading of adsorbate molecules is desirable in ensuring a complete coverage of acid sites during the sample baking treatment. Finally, the adsorbate-loaded sample (in a sealed vessel) is transferred into a zirconia (ZrO2) MAS rotor and sealed with a gastight Kel-F cap under inert environment (in a N2 bag or glovebox). The well-pretreated sample packed in the rotor is inserted into a MAS probehead for subsequent 31P MAS SSNMR experiments. Typically, the 31P NMR spectra are recorded using a single-pulse sequence with a recycle delay of ca. 10 s and a MAS rate of ca. 12 kHz for a desirable number of accumulations in order to obtain signal with adequate sensitivity (i.e., signal-to-noise ratio). However, the aforementioned experimental parameters largely depend on the characteristics of the guest/host sample system under study (e.g., the spin− 12482


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been utilized to determine various interactions between the TMPO guest adsorbate and BA sites in superacidic heteropolyacid host adsorbents such as 12-tungstophosphoric acid (H 3 PW 12 O 40 ; HPW) 92 and 12-molybdophosphoric acid (H3PMo12O40; HPMo).95 As illustrated in Figure 9, with an

before it is subjected to degassing (by evacuation) at RT for another 0.5 h to remove physisorbed TMP. Finally, the sample tube containing TMP-loaded catalyst is flame-sealed while cooling under a liquid N2 bath. Note that, since the 31P SSNMR measurement is normally performed with a MAS probehead, the sample tube with a prepulled capsule size comparable to the typical MAS rotor (typically, 4 or 7 mm o.d.) is most desirable. The sealed sample capsule is then tight-fitted into a ZrO2 MAS rotor for the subsequent 31P MAS NMR experiments in which the 31P chemical shift was referenced to 1 M aqueous H3PO4 solution. 3.2. Related 31P NMR Methods for Acid Characterization using Phosphorus Probes

Although the 31P SSNMR experiments are normally carried out by means of a single nucleus, single-pulse sequence, which is capable of determining the relative acid concentrations in the solid catalyst by spectral analysis. However, to afford detailed distance information between acid sites, it is important to probe the proximity of the adsorption site of the P-containing guest on acidic protons (or metal ions) to confirm the presences of BA (or LA) sites in the catalyst. In this context, it is inevitable to invoke heteronuclear and homogeneous correlation NMR methods. For example, 31P/27A1 transfer of populations in double resonance (TRAPDOR),91 1H−31P heteronuclear correlation (HETCOR)92 and 31P−31P double-quantum (DQ) correlation spectroscopy93 were commonly used for acid characterization of solid acids; the corresponding pulse sequences are displayed in Figure 7. 3.2.1. Double Resonance and Population Transfer. 31 27 P/ Al TRAPDOR NMR has been widely used to clarify the coupling between the bound P atom of phosphorus guest probe molecule with Al atom of the host adsorbent. For example, to identify the presence of BA and LA sites in dehydroxylated H−Y zeolite, Kao and Grey91 performed 31P/27Al TRAPDOR NMR experiments on the TMP/H−Y guest/host system at −150 °C. For 31P spectrum obtained in the absence of 27Al irradiation, the authors observed three binding sites with isotropic δ31P of −32, −46, and −60 ppm (Figure 8a). The resonances at −32 and −60 ppm are due to TMP strongly bound to LA sites and physisorbed TMP, respectively. Upon 27A1 on-resonance irradiation, the signal at −46 ppm decreased considerably while only negligible effects were observed for the other two resonances (Figure 8b). On the basis of difference spectrum shown in Figure 8c, the large TRAPDOR effects at −46 ppm indicate that the TMP molecules are directly bound to LA sites associated with EFAL species. Furthermore, by comparing with regular 31P MAS NMR (Figure 8d), which showed broad resonances featuring at −32, −46, and −60 ppm, the spectrum obtained from 27Al → 31P insensitive nuclei enhanced by polarization transfer (INEPT) sequence showed only an enhanced signal at −46 ppm (Figure 8e), confirming the coupling between the adsorbed TMP and EFAL species in dehydroxylated H−Y zeolite.94 3.2.2. Two-Dimensional Heteronuclear Correlation. Two-dimensional (2D) 31P Lee−Goldburg cross-polarization (LG-CP) HETCOR NMR is a useful method for probing correlations between the local environments of Brønsted acidic proton (1H) and 31P nuclei (of the phosphorus probe molecule). The combined LG-CP decoupling technique is known to achieve polarization transfer with efficient suppression of signal arising from homonuclear dipole−dipole couplings (e.g., 1H−1H spin diffusion). On the basis of the 31P-TMPO approach, such multinuclei 2D 1H−31P LG-CP HETCOR NMR technique has

Figure 9. 1H−31P LG-CP HETCOR NMR spectrum of TMPO adsorbed on HPMo with a loading of 2.5 TMPO/KU. Prior to the NMR experiment, the TMPO-loaded sample was subjected to baking treatment at 200 °C for 8 h. Adapted from ref 95. Copyright 2010 American Chemical Society.

average loading of 2.5 TMPO molecule per Keggin unit (KU) on the HPMo adsorbent, two main signals in the F1 dimension (1H resonance) with a δ1H range of ca. 1.5−3.5 and 6−13 ppm were observed, which may be assigned due to methyl protons of TMPO and TMPO-bound Brønsted acid protons of HPMo, respectively, whereas for the 31P resonance (F2 dimension), both cross-peak signals at δ31P of ca. 80−90 and 60−65 ppm were correlated with 1H resonances at ca. 1.5−3.5 ppm, which may be attributed to the intramolecular 1H−31P interactions in the TMPO molecule itself. While, the presences of cross-peaks which correlate 31P resonances at ca. 80−90 ppm and 1H resonances at ca. 6−9 (Figure 9a) and 9−13 ppm (Figure 9b) may be assigned due to TMPOH+ ions protonated by BA sites with different acidic strengths, whereas the cross-peaks that correlate the 31P at ca. 60−65 ppm and 1H resonances at ca. 15.5−16.5 ppm should be originated from (TMPO)2 H+ complexes on the external surface of HPMo (Figure 9c). The latter imply adsorption of more than one TMPO on a BA site, hence, indicating that the adsorbed TMPO molecules are in close proximities with one and other. It is noted that the above δ31P assignments were readily confirmed by accurate theoretical calculations by DFT. Accordingly, various adsorption states of TMPO, and thus detailed correlations between the probe molecule and acid sites may be nicely resolved. The acid and transport properties of the anhydrous Keggintype 12-tungstophosphoric acid (H3PW12O40; HPW) were also investigated by the 31P-TMPO NMR approach.92 The 31P MAS NMR spectrum obtained from TMPO adsorbed on HPW (with a loading of 2.5 TMPO/KU) subjected to a thermal pretreatment at 100 (denoted as HPW-2.5−100) and 150 °C (denoted as HPW-2.5−150) are depicted in Figure 10 (panels a and b, respectively). For the HPW-2.5−100 sample, which was subjected to pretreatment at mild temperature at 100 °C for 6 h, multiple resonances with δ31P in the range of 50−95 ppm were observed. For convenience, these resonances were divided into three regions, namely region I (85−95 ppm), region II (75−85 ppm), and region III (53−75 ppm). In comparison, only 31P 12483


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Figure 10. (Top) 31P MAS and (bottom) 2D 31P LG-CP HETCOR NMR spectra of two different TMPO-loaded HPW (loading 2.5 TMPO/KU) samples thermally pretreated at (a and c) 100 and (b and d) 150 °C for 6 h, which are denoted as HPW-2.5−100 and HPW-2.5−150, respectively. Adapted with permission from ref 92. Copyright 2011 Wiley.

cross peak between 31P resonance in region II and 1H resonance was observed for the HPW-2.5−100 sample (Figures 10c), indicating a very weak coupling between TMPO and acidic protons. Such kinds of adducts were most likely formed through relatively weak hydrogen-bonding interactions associated with local steric constraints of the HPW secondary structures, as a result, leading to an incompetent polarization transfer between H+ and the corresponding 31P atoms. Additional studies by 31 P− 31 P DQ correlation NMR spectroscopy and DFT calculations further confirmed92 that the correlations between 31 P resonances in region III and 1H resonances at 14−17 ppm observed for the HPW-2.5−100 (Figure 10c) may be attributed to the presence of (TMPO)2H+ adducts, for which two TMPO molecules on one Brønsted acidic H+ site was due to inadequate sample pretreatment. In addition, by adopting a modified 2D 1H−31P HETCOR NMR in conjunction with homonuclear decoupling, Alonso et al. demonstrated97 that the 31P-TMPO NMR approach is also feasible for differentiating the chemical environments of TMPO bound to BA and LA sites in a silica−alumina catalyst. Upon adsorbing TMPO onto the precipitated silica−alumina catalyst, two 1H signals with δ1H at ca. 1.5 and 7.0 ppm were observed, which may be attributed to methyl groups in TMPO and TMPO adsorbed on Brønsted acidic H+, respectively, as shown in Figure

resonances located in region I with distinct peaks at ca. 92.1, 89.4, and 87.7 ppm were observed for the well-pretreated HPW-2.5− 150 sample (Figure 10b). To afford assignments of these resonances, further experiments by 1H-31P LG-CP HETCOR were also carried out for the HPW-2.5−100 (Figure 10c) and HPW-2.5−150 (Figure 10d) samples. The 2D 1H−31P LG-CP HETCOR spectrum obtained from the HPW-2.5−100 revealed off-diagonal cross-peaks which may also be divided into two major categories: (i) correlations between 31P resonances in region I (85−95 ppm) and 1H resonances within δ1H of 6−8 ppm (due to BA sites) and (ii) correlations between 31P resonances in region III (53−75 ppm) and 1H resonances within δ1H of 14−17 ppm. Surprisingly, no detectable correlation between the weak 31P resonances in region II (75−85 ppm) and 1H resonance was found. On the other hand, for the well-pretreated HPW-2.5−150 sample, only correlations between 31P resonances in region I (85−95 ppm) and 1H resonances at 6−10 ppm were observed (Figure 10d). Thus, the 31P resonances in region I may be undoubtedly attributed to three distinct TMPOH+ adducts associated with the three available Brønsted acidic protons. That these protonated adducts all exhibited δ31P exceeding the threshold for superacidity (86 ppm)60−64 verifying that the three Brønsted acidic H+ sites available for H3PW12O40 are indeed superacidic, whereas no 12484


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Figure 11. (a1) 1H MAS and (a2) 31P CP MAS NMR spectra of TMPO adsorbed on silica−alumina. Variations of 31P signal intensities with contact time τc for crystalline TMPO (○) and for TMPO adsorbed on BA site 1 (■) using the 31P (b1) CP MAS and (b2) CP-LG MAS pulse sequences. Lines are purely indicative. 2D Contour plots obtained from 31P HETCOR experiments by means of the pulse sequence depicted in (c4) with (c1) τc = 0.5 ms, τm = 0 ms, 384 scans, (c2) τc = 12 ms, τm = 0 ms, 384 scans, and (c3) τc = 12 ms, τm = 50 ms, 2048 scans. Vertical 1H spectra correspond to TMPO interacting with (A) acid site 1, and (B) crystalline TMPO obtained from a multistep fitting procedure of all cross-peaks originated from correlations between 1H (vertical axis) and 31P (horizontal axis) resonances. Adapted with permission from ref 97. Copyright 2002 Royal Society of Chemistry.

TMPO interacting with acid sites (referred as “A”; Figure 11c2) also emerged. Moreover, by setting τc = 12 ms with a mixing time τm = 50 ms to allow for proton (1H) spin diffusion, a new correlation peak associated with 31P resonance of TMPO (δ31P = 68 ppm) and bridging hydroxyl protons (Si−OH−Al; at δ1H ca. 7 ppm) was also observed (Figure 11c3). Nonetheless, no correlation between resonances of the 31P at 58 ppm and the 1H at 7 ppm was found, revealing that the peak with δ31P = 58 ppm was indeed due to TMPO bound to the Lewis acid sites. Accordingly, various adsorption states and interactions of the TMPO probe molecule and acid sites may be readily resolved using multinuclear NMR techniques.62−64 3.2.3. Spectral Editing by Homonuclear Decoupling and Selective Excitation. In addition to the 2D 31P HETCOR NMR method discussed above for discernment of BA and LA sites, a direct 1D NMR spectral editing technique has also been reported, as demonstrated by Huang et al. using the TMPO/H− Y guest/host system.98 The authors incorporated a selective 1H excitation pulse on nonacidic protons while observing the 31P resonance in conjunction with the LG-CP sequence to suppress homonuclear dipole−dipole couplings. Compared to the 2D LG-

11a1. On the other hand, the corresponding 31P MAS NMR spectrum exhibited three major 31P resonances with δ31P at 42, 58, and 64 ppm. The sharp 31P resonance with δ31P centering at 42 ppm may be assigned due to crystalline TMPO, whereas the latter two signals were attributed to TMPO adsorbed on LA (site 2; 58 ppm) and BA (site 1; 64 ppm) sites, respectively. To afford understanding of the adsorption state of TMPO on the surfaces of the silica−alumina catalyst, 31P CP MAS NMR experiments without and with LG-CP technique were performed, as shown in Figure 11 (panels b1 and b2). By varying the contact time (τc; see Figure 11b1), the 31P resonances arose from crystalline TMPO predominately appeared at shorter contact time (τc ≤ 1 ms), while that originated from protonated TMPOH+ complexes (i.e., TMPO adsorbed on BA sites 1; Figure 11a2) prevails at longer τc. Likewise, 2D spectra obtained from the 1H−31P HETCOR sequence (Figure 11c4) revealed that, for τc ≤ 0.5 ms with null mixing time (τ m = 0), only a single correlation peak corresponding to correlation between 31P signal of the crystalline TMPO (referred as “B”) and 1H signal of methyl protons was observed (Figure 11c1). Upon increasing τc to 12 ms while keeping τm = 0, an additional correlation peak associated with 12485


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CP HETCOR NMR approach, such 1D 31P selecive LG-CP spectral editing method is particularly suitable for investigating sample systems with only limited amount of acid sites. As shown in Figure 12a, conventional experiment by 31P MAS NMR gave

Figure 13. 1H spectrum of TMPO adsorbed on H−Y zeolite. The upper trace was obtained from regular 1H MAS, whereas the lower trace was the result from selective excitation to suppress signal arising from nonacidic silanols at 2.1 ppm while leaving that of bridging hydroxyl (BA sites) at 7.2 ppm unperturbed. Adapted with permission from ref 98. Copyright 2006 Elsevier.

and 55 ppm observed in the difference spectrum are associated with TMPO adsorbed on LA sites. As such, it is conclusive that peak III (55 ppm) should be mainly due to TMPO adsorbed mostly on LA sites and partially with BA sites, and vice versa for peak IV at 65 ppm.100 3.2.4. Two-Dimensional Homonuclear Double-Quantum Correlation. As demonstrated above, isolated BA and LA sites in solid acid catalysts may readily be identified by means of multinuclear NMR techniques, such as TRAPDOR and HETCOR. To this regard, the proximities of acid centers also play an important role during catalytic reactions especially those involving multiple acid sites. Grey and co-workers demonstrated93 that 2D DQ correlation spectroscopy may be utilized to probe the spatial proximity of two like-spin nuclei, thus, capable of probing distance and concentration of acid sites by varying the chain length and concentration of the phosphorus probe molecule. By employing the 2D 31P−31P DQ NMR using diphenyldiphosphine (i.e., Ph2P(CH2)nPPh2; n = 1−6) as probes,93 the authors observed two types of cross-peaks in the double quantum (DQ) versus single-quantum (SQ) correlation plot. As shown in Figure 14, for two proximal sites with chemical shift of ωj and ωk, a pair of off-diagonal cross-peaks at (ωj, ωj + ωk) and (ωk, ωj + ωk) were observed, while for two identical spins (i.e., ω = ωj = ωk), only a diagonal single peak at (ω, 2ω) was present. The authors concluded that the cross-peaks at (14, −14) and (−28, −14) ppm arised from the 31P resonance at −28 ppm (the nonprotonated end of a singly protonated diphosphine) and 14 ppm (the protonated end of the same diphosphine) adjacent to each other. In addition, the diagonal peaks arising from the doubly protonated diphosphine at (8, 16) and (−1, −2) ppm reveal the distance between two proximal BA sites was ca. 0.30 nm in H−Y zeolite. Accordingly, the authors also reported the P−P distances for Ph2P(CH2)nPPh2 with n = 1, 3, and 6 should be 0.30, 0.56, and 0.94 nm, respectively. It should be noted that as demonstrated by Deng and co-workers,12,48,64 the spatial proximities among different acid centers in zeolites can be also obtained without using probe molecules by 2D 1 H−1H and 27Al−27Al DQ NMR spectroscopy.

Figure 12. (a) Top: 31P MAS NMR spectrum of TMPO adsorbed on H−Y zeolite (Si/Al = 2.6). Bottom: deconvoluted spectrum via Gaussian simulations. (b) 31P LG-CP spectra obtained without (top) and with (middle) selective LG-CP excitation. Bottom: the resultant difference spectrum. Adapted with permission from ref 98. Copyright 2006 Elsevier.

rise to five distinct 31P resonances at 41, 45, 55, 65, and 75 ppm for TMPO adsorbed on H−Y zeolite, which were temporarily assigned as peaks I to V, respectively. Among them, peaks V (75 ppm) and IV (65 ppm) were ascribed due to TMPO adsorbed on BA sites, whereas peak I (41 ppm) due to physisorbed TMPO.99 In this regard, the assignments of peaks II (45 ppm) and III (55 ppm) remain uncertain. The assignments of the latter two peaks may be resolved by the 31P selective LG-CP sequence, in which a Gaussian-shaped pulse was used to selectively suppress 1H signal arising from nonacidic Si−OH groups at 2.1 ppm while leaving the 1H resonances from bridging hydroxyls (i.e., BA sites) at 7.2 ppm unperturbed, as shown in Figure 13.98 Accordingly, while regular 31P LG-CP NMR resulted in a spectrum (Figure 12b; top trace) that is in close resemblance to that obtained by conventional 31P MAS NMR (cf. Figure 12a), spectrum obtained from 31P selective LG-CP sequence showed notable suppression of the resonance peak at ca. 65 ppm, as shown in Figure 12b (middle trace). Since the spectrum obtained from the selective LG-CP method is anticipated to show a much lower signal sensitivity, it was scaled-up intentionally so that the most intense signal at 75 ppm (peak V; due to TMPO adsorbed on BA sites) may be mostly eliminated in the difference spectrum (Figure 12b; bottom trace). As such, the presence of the 31P signal at 45 ppm in the difference spectrum indicates that peak II (see Figure 12a) may be assigned due to TMPO adsorbed on BA sites with weaker acidic strength, while the other two 31P resonances at 65 12486


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− 2 ∼ −5 ppm. As such, while the 31P-TMP NMR approach is advantageous for distinguishing the presences of Brønsted and/ or Lewis acid types, it is also a more sensitive approach to determine strengths of Lewis acid sites. It is for this reason that the 31P-TMP NMR approach is commonly used for characterizing acidity of metal oxide catalysts. 4.1.1. Mixed Metal Oxides. The 31P-TMP NMR approach has been employed to characterize acid properties of various metal oxides and mixed metal oxides.74,76,81,84,101 For example, as the TMP probe molecule is adsorbed on the surfaces of anatase titania (TiO2), a single 31P NMR resonance peak centering at −35 ppm (Figure 15a) may be unambiguously assigned due to TMP adsorbed on LA sites.84 Note that no single arising from BA sites was observed. If it is present, additional 31P resonances near ca. −2 ∼ −5 ppm would be anticipated (cf. section 2.1). Upon further sample treatment by sulfonation, a notable shift of the 31P resonance to −27 ppm (Figure 15b) is evident, indicating an enhancement in acid strength of LA site. Meanwhile, a new 31P signal at −4 ppm was found, revealing the formation of BA sites in the sulfated SO42−/TiO2 catalyst. Similar results were also observed for the bulk alumina (Al2O3) and its sulfated counterpart (SO42−/Al2O3). Upon sulfation, the 31P signal of the adsorbed TMP also shifted toward downfield from −51 ppm (Figure 15c) to − 49 ppm (Figure 15d) together with the presence of a new signal at −3 ppm due to protonated TMPH+. Thus, it is indicative that sulfation treatment of metal oxide catalysts not only provokes enhancement of Lewis acidic strength but also formation of additional BA sites, which took place at the sole expense of the weaker Lewis acidity originally presented in the pristine metal oxide. As a result, the sulfonation treatment is a practical procedure for promoting catalytic performances of metal oxides. The 31P-TMPO NMR approach has also been exploited for characterizing acid sites in γ-Al2O3, boehmite γ-AlO(OH), and TiO2 catalysts.101,102 Over the γ-Al2O3 catalyst, two 31P NMR signals observed at δ31P of 65 and 48 ppm at a modest guest molecule loading may be attributed to TMPO interacting with BA and LA sites, respectively. Upon increasing TMPO loading, a weak shoulder peak at ca. 40 ppm emerged, which may be ascribed due to the presence of bulk crystalline TMPO.56,101 In the case of boehmite γ-AlO(OH), Takagaki et al. showed that a calcination treatment at elevated temperature tends to lower the Lewis acidity of the catalyst, as evidenced by results obtained from the 31P-TMPO NMR approach.102 The authors reported that at a modest calcination treatment at 453 K, the TMPOloaded boehmite sample exhibited a singlet 31P peak at 54 ppm

Figure 14. 2D 31P−31P DQ NMR spectrum of Ph2P(CH2)PPh2 adsorbed on H−Y zeolite (loading 8 molecules/unit cell). A post-C7 DQ sequence was incorporated following the 1H−31P CP to prepare and then reconvert DQ coherences. Optimized experimental variables: DQ excitation time = 2.02 ms; spinning rate = 7.937 ± 0.005 kHz. The asterisks represent spinning sidebands. Adapted from ref 93. Copyright 2004 American Chemical Society.

4. APPLICATIONS OF 31P NMR FOR ACIDITY CHARACTERIZATION OF VARIOUS CATALYSTS 4.1. Metal Oxides and Modified Metal Oxides

Metal oxides are commonly used as catalysts or catalyst supports in refinery and chemical industries. Thus, a fundamental understanding of their acid properties is indispensable for the design, implementation, and applications of such important class of solid acid catalysts. Both coordinatively saturated and unsaturated metal atoms in metal oxides may act as the Lewis acid centers during the catalytic process, while the presences of bridging and/or terminal hydroxyl groups are most associated with Brønsted acid sites. Normally, acidity of metal oxides may further be enhanced by a sulfation pretreatment to promote their catalytic performances. As mentioned in section 2.1, acidity characterization by 31P-TMP NMR is a more practical approach for probing Lewis acidity rather than Brønsted acidity. This is owing to the fact that as TMP molecules are bound to LA sites, they give rise to 31P resonances over a considerable chemical shift range (Δδ31P) of ca. −20 ∼ −60 ppm, while TMP is less sensitive for probing BA sites and the protonated TMPH+ ions normally lead to 31P resonances with a relatively narrow Δδ31P of only ca.

Figure 15. 31P MAS NMR spectra of TMP adsorbed on (a) TiO2, (b) SO42−/TiO2, (c) Al2O3, and (d) SO42−/Al2O3. Asterisks denote spinning sidebands. The weak signal at 34 ppm in (c) is arising from trimethylphosphine oxide (TMPO) due to partial oxidation of TMP. Adapted with permission from ref 83. Copyright 2003 Royal Society of Chemistry. Adapted from ref 84. Copyright 2008 American Chemical Society. 12487


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sulfation of the TiO2, additional resonance peaks were observed for the SO42−/TiO2 at 65, 69, and 72 ppm in addition to the two original peaks at ca. 48 and 58.5 ppm (Figure 16c). The former three new peaks may be attributed to TMPO adsorbed on BA sites with different acidic strengths. Apparently, information on acid sites in pristine and sulfated TiO2 obtained from the 31PTMP NMR approach coincide with those from the 31P-TMPO approach; nonetheless, the latter method clearly showed advantage in providing additional information on distribution and acidic strengths of BA sites. 4.1.2. Sulfated Zirconia. Sulfated zirconia (SO42−/ZrO2; SZ) is an environmentally friendly catalyst with ultrahigh acidity and has been extensively utilized for various catalytic reactions such as isomerization, cracking, and alkylation.103 The 31P-TMP NMR approach has also been used to identify acidity properties of zirconia (ZrO2), sulfated zirconia (SZ), and metal-promoted SZ.82,104−106 Although pristine ZrO2 possesses only Lewis acidity, both BA and LA sites are found in SZ. The sulfation treatment normally leads to enhanced Lewis acidity compared to its pristine counterpart.106 Nonetheless, SZ is also known to be a strongly acidic catalyst vulnerable to deactivation, a drawback which is normally circumvented by incorporating a suitable amount of metal promoter.103,107,108 By means of the 31P-TMPO NMR approach in conjunction with elemental analysis, Chen et al. compared acid properties of pure ZrO2 and various SZ catalysts with varied sulfur contents and metal promoters (M = Al, Ga, and Fe).106 As shown in Figure 17a, the 31P NMR spectrum of TMPO adsorbed on the dehydrated pristine ZrO2 revealed four distinct resonances with δ31P of 62, 53, 41, and 34 ppm. While the latter two peaks may be attributed to physisorbed TMPO, the former two peaks should be associated with TMPO adsorbed on LA sites. These assignments were further confirmed

due to TMPO adsorbed on LA sites. However, as the sample was subjected to calcination at higher temperatures (573 and 773 K), a notable shift of the 31P resonance toward upfield direction with δ31P of ca. 48 ppm was observed.102 On the other hand, for TMPO adsorbed on anatase TiO2, a predominant 31P resonance accountable for physisorbed TMPO was found at 48 ppm along with a weak shouldering peak due to Lewis acidity (Figure 16a).84 The assignment of the latter resonance peak with δ31P of

Figure 16. 31P (a) MAS, and (b) CP MAS NMR spectra of TMPO adsorbed on pristine TiO2 and (c) CP MAS NMR spectrum of sulfated SO42−/TiO2. Asterisks denote spinning sidebands. Adapted from ref 84. Copyright 2008 American Chemical Society.

59 ppm was further confirmed by an additional 31P CP MAS NMR experiment by which the signal was notably enhanced by eliminating undesirable artifacts (Figure 16b). Upon further

Figure 17. 31P MAS NMR spectra of TMPO adsorbed on (a) dehydrated pristine ZrO2 before (top) and after hydration treatments for 1.5 h (middle) and 3.0 h (bottom), and (b) dehydrated SZ-xN (x = 0.5, 1.0, and 2.0) prepared with varied sulfur contents. The dashed curves represent spectral simulation by Gaussian deconvolution, and the asterisks denote spinning sidebands (spinning rate 12 kHz). Adapted with permission from ref 106. Copyright 2006 Elsevier. 12488


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For comparison, simulation results obtained for various samples representing TMPO adsorbed on Brønsted and/or Lewis acid sites with practically the same chemical shift within ca. ± 2 ppm (i.e., similar acid strengths) are aligned in the same column. The δ31P values in brackets represent Lewis acid sites. bFor SZ-xN (x = 0.5, 1.0, and 2.0) and M/SZ (M = Al, Ga, and Fe; x = 1.0) samples, data denote relative concentration of acid sites (%), whereas lower data in parentheses represent acid concentration (±0.002 mmol/g cat) of the corresponding acid site, as derived from elemental analyses by ICP-MS. c Denote the presence of physisorbed TMPO. dResonance peaks with chemical shift lower than 50 ppm, which arise from physisorbed TMPO, were excluded during derivations of acid amount. a

− 1.39 0.89 0.73 0.47 1.89 0.49

B/L (%) total

− 0.043 0.164 0.076 0.151 0.130 0.293 − 0.018 0.087 0.044 0.103 0.045 0.197

LA BA

− 0.025 0.077 0.032 0.048 0.085 0.096 X − − − − − X 49.7% − − − 3.7% (0.006) 2.8% (0.004) 35.6% (0.105) 50.3% 33.4% (0.015) 45.7% (0.075) 51.7% (0.039) 55.8% (0.084) 20.0% (0.026) 27.0% (0.079) − − − − 15.3% (0.023) 41.9% (0.054) 13.7% (0.040) − 53.3% (0.023) 43.2% (0.071) 37.2% (0.028) 9.7% (0.015) 14.4% (0.019) 12.9% (0.038) − − − − 3.1% (0.005) 7.8% (0.010) − − − − − 2.8% (0.004) 3.5% (0.005) − − 7.9% (0.003) 7.4% (0.012) 6.0% (0.005) 5.3% (0.008) 4.1% (0.005) − − − − − − − 4.5% (0.013) ZrO2 SZ-0.5N SZ-1.0N SZ-2.0N Al/SZ Ga/SZ Fe/SZ

− 5.4% (0.002) 3.7% (0.006) 5.1% (0.004) 4.3% (0.006) 5.5% (0.007) 6.3% (0.018)

Pc [53] [62] 65 68 [73] 76 87 [90] [99] sample

chemical shift (ppm)a,b

Table 2. 31P MAS NMR Chemical Shift Assignments and Distribution of Acid Sites for the Pristine ZrO2 and Various SZ and M/SZ Catalysts Loaded with TMPO Probe Molecule106

by comparing results with partially hydrated samples. It is wellknown that, in the presence of water, Lewis acid sites tend to hydrate easily to convert into Brønsted acid sites. Thus, 31P resonances associated with Lewis acidity are anticipated to be diminished upon partial hydration of the catalyst sample. On the other hand, strongly hydrogen-bonded TMPOH+ complexes associated with BA sites are unlikely to be dissociated in the presence of water. As shown in Figure 17a, notable decreases in peak intensities of the resonances at 62 and 53 ppm with increasing degree of hydration are evident, indicating that they indeed arose from LA sites with different acidic strengths. Meanwhile, a significant broadening of the resonances below 50 ppm was also observed, revealing progressive formation of new hydroxyl species with increasing level of exposure to humidity. Similarly, the acid features of sulfated zirconia (SZ-xN, x = 0.5, 1.0, and 2.0, where x represents the concentration of sulfuric acid added during the sulfation treatment) catalysts with varied sulfur contents have also been characterized by means of the 31PTMPO NMR approach.106 Upon sulfation, all SZ-xN samples exhibited distinct 31P resonances at 90, 87, 68, and 63 ppm, as shown in Figure 17b. Again, the assignments of these resonances may also be accomplished by partial hydration of the samples. The authors concluded that the peaks at 90 and 63 ppm, which diminished upon sample hydration may be unambiguously attributed to TMPO adsorbed on LA sites, whereas the peaks at 87 and 68 ppm, which remained intact after hydration were associated with BA sites. Together with the results obtained from elemental analyses, detailed properties of acid sites including types, distribution, strengths (i.e., 31P NMR peak assignment), and concentrations of assorted ZrO2 and SZ catalyst samples may be summarized, as depicted in Table 2. By comparing the results obtained for BA and LA sites, it is clear that the acid properties of SZ may readily be mediated by varying the sulfur content. In terms of overall acidity, the Brønsted/Lewis acidity ratios in SZ catalysts were found to follow the trend: SZ-0.5N > SZ-1.0N > SZ-2.0N. The influences of incorporated metals (M = Al, Ga, and Fe) on acid properties of the metal promoted SZ (M/SZ) catalysts were also investigated.106 By incorporating ca. 1.0 wt % of different metal promoter onto the SZ-1.0N catalyst, notable changes in 31 P NMR spectra of adsorbed TMPO were observable compared to that obtained from their metal-free SZ counterpart with x = 1.0, as shown in Figure 18a. In addition to the 31P resonances at 90, 87, 68, and 63 ppm observed for the SZ-1.0N, four additional peaks with δ31P of 76, 73, 65, and 53 ppm were observed for both TMPO-loaded Al/SZ and Ga/SZ catalysts. Again, by comparing variations of 31P resonances between dehydrated and hydrated samples, the extra resonances responsible for BA (76 and 65 ppm) and LA (73 and 53 ppm) sites in M/SZ may readily be assigned (Figure 18b). In particular, an ultrastrong LA site were observed for Fe/SZ at δ31P of 99 ppm (Table 2). It is conclusive that, incorporation of metal tends to promote additional formation of both BA and LA sites, which spanned over a wide range of acidic strengths (i.e., observed δ31P) mostly at the disposal of the strongest LA sites (at 90 ppm) and medium BA sites (at 68 ppm) observed in the parent SZ-1.0N. These new acid features are responsible for the improved catalytic activity and selectivity observed for the M/SZ catalysts compared to metal-free SZ. 4.1.3. Titanium Oxides. The morphology of metal oxide nanoparticles (NPs), especially those with unique facet characteristics, are known to strongly affect their acid properties and hence catalytic activities.109 In this context, various tactics for

acid amount (mmol/g cat.)d

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Figure 18. 31P MAS NMR spectra of TMPO adsorbed on dehydrated M/SZ (M = Al, Ga, and Fe) (a) before and (b) after hydration treatment (for 1 h). The dashed curves represent spectral simulation by Gaussian deconvolution, and the asterisks denote spinning sidebands (spinning rate 12 kHz). Adapted with permission from ref 106. Copyright 2006 Elsevier.

preparing anatase TiO2 with specific, highly exposed, and active facets having desirable photocatalytic properties have been reported.110−116 For example, Hu et al. investigated the facetdependent acidic and catalytic properties of sulfated titania (ST) with different exposed (001) and/or (101) facets (see Figure 19)

Figure 20. 31P MAS NMR spectra of TMP adsorbed on sulfated TiO2 with different exposed facets, namely ST001, ST001/101, and ST101. The dashed curves indicate results obtained from spectral deconvolution. Adapted with permission from ref 115. Copyright 2015 Royal Society of Chemistry. Figure 19. (a) Proposed bonding modes of sulfated TiO2 and (b) schematic atomic structure of (001) and (101) facets of anatase TiO2. Adapted with permission from ref 115. Copyright 2015 Royal Society of Chemistry.

66 ppm may be attributed to TMPO bound to acid sites due to partial oxidation of the TMP probe molecule on the surfaces of the sulfated TiO2. For ST catalyst prepared with both exposed (001) and (101) facets (i.e., ST001/101), the corresponding 31P NMR spectrum of TMP revealed LA sites with enhanced acidic strengths with δ31P appearing at −26.5 and −34.1 ppm, respectively. Moreover, for catalyst possesses only active (001) facets, the corresponding 31P resonances were found to shift further downfield with respective δ31P value of −25.2 and −33.3 ppm. Apparently, in addition to the BA sites, the ST001 catalyst possesses LA sites with acidic strengths surpassing that of ST001/101 and ST101, mainly owing to the sole presence of (001)

by means of the 31P-TMP NMR approach.115 For catalyst with only exposed (101) facets (i.e., ST101), the observed 31P spectrum revealed resonances not only arising from TMPH+ associated with BA sites (with δ31P at −4.0 ∼ − 4.5 ppm) but also two distinct peaks at −31.6 and −49.9 ppm corresponding to TMP adsorbed on LA sites with different acidic strengths (Figure 20). An additional broad resonance centering at δ31P of ca. 61− 12490


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Figure 21. (a) XRD profiles and (b) 31P MAS NMR spectra of TMP-loaded ZnO with plate, rod, and powder morphologies. (c) Deconvoluted spectra of (b) and corresponding chemical shift assignments and concentrations of various sites. *Amount of adsorbed TMP molecules in μmol g−1. The total coverage of surface OH groups was derived from sites III and IV. (d) Schematic illustrations of optimized structures and corresponding adsorption energies for TMP adsorbed on specific surface sites, namely (100)Zn3C, Zn-(002)Zn3C, Zn-(002)Zn−OH, O-(002)Zn−OH, and (100)Zn−OH. Adapted from ref 117. Copyright 2016 American Chemical Society.

facets that entirely consisted of unsaturated five-coordinated Ti species beneficial for the formation of superacids.116 As a results, the ST001 catalyst was found to exhibit superior catalytic activity during Pechmann condensation of 5,7-dihydroxy-4-methyl coumarin.115 4.1.4. Zinc Oxides. A facet-dependent acid property and photocatalytic activity were also observed for the zinc oxide (ZnO) NPs.117 Owing to their unique properties and superior performances in solar cells and photocatalysis, fabrication and characterization of ZnO NPs with tailorable facets and surface properties have drawn considerable R&D interest recently.118−120 It has been shown that both BA and LA sites are present on the surfaces of ZnO nanocrystals. Among them, LA sites are mostly associated with coordinatively unsaturated Zn2+

cations residing on corners and/or edges of the crystalline surfaces. Structural defects such as oxygen vacancies (VO) are also known to generate Lewis acidity.114 Utilizing the 31P-TMP NMR approach, Peng et al. recently reported some interesting work on surface fingerprinting and facet-specific metal oxide NP photocatalys.117,121 The authors demonstrated that qualitative as well as quantitative information on VO on specific facets of metal oxides may be attained by the probe-molecule-assisted NMR technique.117 Figure 21b displays the 31P SSNMR spectra of TMP adsorbed on ZnO NPs with platelike, rodlike, and powder morphologies. The absence of 31P resonance near ca. −2 ∼ −5 ppm indicates that no Brønsted acidity was present on the surfaces of all ZnO NP photocatalysts. On the other hand, broad 31 P resonance peaks spanning over a δ31P range of ca. −35 ∼ −65 12491


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4.1.5. Other Assorted Metal Oxides. Water-tolerant metal oxides, which are favorable for facile product separation, have been widely utilized as environmentally benign Lewis acid catalysts for hydrolysis reactions.124−126 For example, CeO2 has been exploited as solid acid catalyst for the synthesis of 1,3butanediol through hydrolysis of 4-methyl-1,3-dioxane with high yield (95%) or propylene and formaldehyde via Prins condensation reaction and hydrolysis reaction (yield ca. 60%).126 Such organic reactions, which normally take place under mild conditions invoking heterogeneous acid−base catalysts, may also be monitored by means of the 31P-TMP NMR approach. Through comparing the 31P NMR resonances of TMP adsorbed on fresh, Na+-exchanged, and water-treated (at 180 °C) CeO2, Xu and co-workers were able to monitor variations in Brønsted-Lewis acidities, which dictate the reaction pathway during synthesis of 1,3-dials.126 For the fresh and Na+exchanged CeO2 catalysts, the authors attributed the broad symmetric 31P resonance centering at −19.3 ppm to TMP adsorbed on unsaturated Ce Lewis acid centers on the catalyst’s surface. The spectra obtained from both samples showed 31P peak with nearly identical δ31P value and peak intensity, indicating that sample treatment by Na+ ion-exchange led to no obvious change in overall acidic strength of Lewis acidity nor its concentration. As quantified by pyridine-IR, the concentrations of Lewis acidity in these two samples were both found to be 0.054 mmol g−1. On the other hand, for sample treated at 180 °C in water (in an autoclave), while no obvious change in δ31P was observed, a notable increase in acid concentration may be inferred, as also verified by py-IR measurement which resulted in a value of 0.065 mmol g−1 for acid amount. In this context, since no signal responsible for TMPH+ complexes (i.e., peak with δ31P at ca. 0 ppm) was found, it is indicative that either no Brønsted acidity or only very weak hydroxyl groups were presented in the aforementioned CeO2 catalyst. By further correlating experimental results obtained by 31P NMR with TPD, Raman, and in situ FT-IR, the authors were able to verify that ceria contains surface oxygen vacancy (VO) as LA sites, which may be manipulated by sample preparation method. Accordingly, a relationship between surface VOs (hence Lewis acidity) and catalytic activity may be established to clarify the stability and reaction pathway of water-tolerant CeO2 catalyst during hydrolysis reactions. By using the 1H- and 31P-TMP NMR spectroscopy, Xu and coworkers investigated the effect of adsorbed water on acidic properties of various solid acid catalysts such as H-ZSM-5, γAl2O3, SiO2−Al2O3, Nb2O5·nH2O and Ta2O5·mH2O, as shown in Figure 22.127 The proton-coupled 31P MAS NMR spectrum observed for the dehydrated H-ZSM-5 exhibited signals arising from TMP adsorbed on BA sites (i.e., protonated TMPH+ complexes; δ31P at −4.8 ppm) and weakly physisorbed TMP (δ31P at −62.5 ppm), denoted as B(TMP) and W(TMP), respectively, as shown in Figure 22A (sample a). On the other hand, for the dehydrated γ-Al2O3 (sample b), only a single 31P peak at ca. −51 ppm due to TMP adsorbed on Lewis acid sites was observed, which was denoted as L(TMP), whereas 31P resonances arising from both B(TMP) and L(TMP) were found for the dehydrated silica−alumina (SiO2−Al2O3; sample c), niobic acid (Nb2O5·nH2O; sample d), and tantalum oxyhydrate (Ta2O5·mH2O; sample e) catalysts. In addition, complementary information may be obtained from 1H MAS NMR of various samples. As shown in Figure 22B, the 1H spectra obtained from TMP adsorbed on various catalysts typically exhibited four different

ppm due to TMPO adsorbed on Lewis acidity were observed for various ZnO NP samples. Further spectral analyses revealed that, within an affordable resolution error (ca. ± 2 ppm) anticipated for SSNMR studies, these overlapping resonance features may be divided into four different TMP adsorption sites on surface Lewis acid centers, namely site I (−43 ppm), site II (−48 ppm), site III (−55 ppm), and site IV (−61 ppm). It was found that predominant distribution of adsorption sites at (II, III), (I, III), and (III, IV) may be inferred for the plate, rod, and powder ZnO samples, respectively (see Figure 21c). Again, the above results demonstrated that the probe-molecule-assisted 31P SSNMR approach is a practical and reliable technique for providing qualitative and quantitative information on types, distributions, strengths, and concentrations of acid sites in solid acid catalysts, surpassing that obtained from conventional methods such as photoluminescence (PL) and electron paramagnetic resonance (EPR).121 To gain further insights into the four possible adsorption structures of TMP on the surfaces of ZnO, the most probable adsorption configurations, corresponding adsorption energies, and theoretical δ31P were assessed by DFT calculations. The results are summarized in Figure 21d. Since formation of a TMPmetal complex should invoke the coordination between a P atom and a Lewis acid center, thus a larger positive charge of the metal center should result in a stronger binding energy (BE; i.e., stronger acidic strength) between the probe molecule and LA site. As anticipated, the predicted δ31P values obtained from various adsorption configurations showed strong correlations with calculated BE values. Moreover, the theoretical δ31P values were also in close resemblance with the experimental results. For example, the calculated δ31P value obtained for TMP adsorbed on preferentially exposed (100) facets [i.e., (100)Zn3C; −41.97 ppm] agrees well with the experimental value (−42.9 ppm; site I) observed for the rod-like sample, whereas the experimental δ31P value observed for site II (at ca. − 48 ppm) may be attributed to TMP adsorbed on Zn-(002)Zn3C. Likewise, site III may be assigned to the adsorption of TMP on Zn-(002)Zn−OH and/or O-(002)Zn−OH, which showed comparable adsorption energies, and site IV due to relatively weaker adsorption of TMP on nonpolar (100)Zn−OH on the surface of ZnO NPs. Brønsted acidity present in metal oxides is mostly associated with terminal and/or bridging hydroxyl groups. In particular, when in the presence of water, LA sites on the surfaces of metal oxides will be transformed to BA sites.122 Recently, Liang et al. investigated the surface acid properties of nanosized ZnO catalyst during conversion of biomass by means of the 31P-TMP NMR approach.123 Since no 31P signals responsible for Brønsted acidity with δ31P at ca. −2 ∼ −5 ppm was observed for the ZnO catalyst regardless of the temperature of calcination pretreatment, the authors concluded that the surface hydroxyl groups on the catalyst may be too weak to protonate the TMP probe molecule. On the other hand, for samples calcined at modest temperatures (viz. 250 and 450 °C), a 31P resonance at δ31P of −45 ppm was observed, indicating the presence of Lewis acidity on the surfaces of nano-ZnO catalyst. However, this peak arising from the TMP-Lewis center complexes diminished as the nanoZnO was calcined at an elevated temperature (850 °C), which exhibited an inferior catalytic performance compared to samples calcined at 250 and 450 °C. Again, the above results demonstrated that the 31P-TMP NMR approach is capable of probing the acid property on the surface of nano-ZnO, whose catalytic activity is closely related to the temperature of calcination pretreatment. 12492


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as B(TMPO), also emerged at ca. 53 ppm. Again, the presence of TMPO may be attributed to partial oxidation of TMP during sample hydration treatment. Taking the γ-Al2O3 (sample b) as an example, it is clear that the formation of W(TMPO) was solely at the expense of L(TMP) upon hydration, indicating a complete suppression of LA sites in the presence of water. On the other hand, in the case of hydrated Nb2O5·nH2O (sample d) and Ta2O5·mH2O (sample e), the complete elimination of the L(TMP) 31P resonance led to the simultaneous enhancement of the corresponding B(TMP) signal. In this context, the hydration treatment of the SiO2−Al2O3 (sample c) resulted in formation of B(TMP) with a broad distribution of acidic strength.128 Similar conclusions may also be drawn from the 1H MAS NMR spectra of TMP adsorbed on various hydrated catalyst samples (Figure 22D). As such, it is conclusive that the 31P-TMP NMR approach is also a practical technique for probing variations of acid features (i.e., types, strengths, and concentrations) of acid sites in dehydrated as well as hydrated solid acids, hence, capable of monitoring their catalytic performances when in the presence of water. Niobium phosphate (NbOPO4) exhibits unique acidic properties such as coexistence of Brønsted and Lewis acidities with tunable concentrations, high thermal stability, and watertolerance capability, and so on.129−131 For example, metalincorporated bifunctional catalyst such as Pd/NbOPO4 was found to exhibit excellent catalytic activity for direct hydrodeoxygenation of triglycerides to biodiesel under mild conditions.132 To afford understanding of correlation between surface acidity and catalytic activity, the 31P-TMP NMR approach has been exploited for probing the acid properties of Pd/NbOPO4. Figure 23a displays the 31P NMR spectra of reduced Pd/NbOPO4 with and without the presence of the TMP probe molecule. The spectrum observed for the bulk Pd/ NbOPO4 revealed a broad resonance with δ31P centering at

Figure 22. (A and C) 1H-decoupled 31P and (B and D) 1H MAS NMR spectra of TMP adsorbed on (a) H-ZSM-5, (b) γ-Al2O3, (c) SiO2− Al2O3, (d) Nb2O5·nH2O, and (e) Ta2O5·mH2O measured under (A and B) dehydrated and (C and D) hydrated conditions. Resonance peaks labeled with B, L, and W represent 31P signals of TMP (or TMPO) associated with Brønsted acid, Lewis acid, and weakly physisorbed sites, respectively, whereas H+ and CH3 in the parentheses denote 1H signal arising from protons in TMPH+ (or TMPOH+) and methyl protons in the TMP (or TMPO) molecule, respectively. Adapted from ref 127. Copyright 2016 Boqing Xu.

types of 1H signals, which may be differentiated by their observed δ1H. The 1H resonance spanning over δ1H of ca. 5−9 ppm may be unambiguously attributed to the proton of the TMPH+ complex (i.e., TMP adsorbed on BA sites), denoted as B(TMPH+: H+), while the 1H resonances with δ1H at 1.9−2.0 and 1.6−1.7 ppm were assigned to methyl protons of adsorbed TMP on BA and LA sites, which were denoted as B(TMP: CH3) and L(TMP: CH3), respectively. In addition, the 1H signals arising from methyl protons of weakly adsorbed TMP and TMPO, denoted as W(TMP: CH3) and W(TMPO: CH3), respectively, normally resulted in resonances with δ1H less than 1.6 ppm. The emergence of the latter W(TMPO: CH3) signal was due to partial oxidation of TMP that occurred during sample handling.127 Upon hydration treatment of various samples, while 31P resonances corresponding to B(TMP) and W(TMP) remained visible (with some variations in peak intensities), the signals associated with L(TMP) vanished, as expected (Figure 22C), while a weak peak due to TMPO adsorbed on BA sites, denoted

Figure 23. 31P MAS NMR spectra of (a) reduced Pd/NbOPO4 with and without the presence of TMP probe molecule, (b) the deconvoluted spectrum for the TMP-loaded sample. Adapted with permission from ref 132. Copyright 2016 Royal Society of Chemistry. 12493


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Figure 24. (a) Structural models of the anatase TiO2 nanosheets with viewing directions of [010] and [100], respectively. (b1) 1H and (b2) 31P MAS NMR spectra of calcined TiO2 nanosheets and bulk TiO2, (b2) 31P MAS NMR spectrum of TMP adsorbed on calcined TiO2 nanosheets, and conversion of (c1) benzyl chloride and (c2) benzyl alcohol plotted as a function of reaction time for TiO2 nanosheets, nanopowder, and bulk TiO2. Adapted with permission from ref 135. Copyright 2016 Elsevier B.V.

−15.9 ppm due to PO43− tetrahedra. Upon loading TMP, a notable upfield shift of the 31P resonance to −19.5 ppm with an asymmetric line shape was observed. Further spectral analysis justified that the deconvoluted spectrum obtained from the TMP-loaded catalyst consisted of three overlapping resonances with δ31P of −26.1, −16.6, and −3.9 ppm (Figure 23b). The peak at −16.6 ppm was most likely due to distorted PO43− tetrahedra, whereas peaks at −26.1 and −3.9 ppm may be attributed to TMP adsorbed on surface LA and BA sites, respectively. It is noteworthy that the NMR chemical shift observed for TMP bound to coordinately unsaturated metal Nb centers of reduced Pd/NbOPO4 (δ31P at −26.1 ppm) is comparable to those observed for sulfated metal oxides such as SO42−/ZrO282,104−106 and SO42−/TiO2,84 indicating the presence of ultrastrong Lewis acidity in the Pd/NbOPO4 catalyst. In addition to the facet-dependent acidic and catalytic properties of metal oxides and modified metal oxides,110−113,115,117,118 their catalytic activities also largely depend on crystalline size and morphology.119,120,133−135 For example, Ryoo and co-workers135 showed that anatase TiO2 with bilayer nanosheets (Figure 24a) exhibit unique acidic properties compared to bulk TiO2 and TiO2 nanopowder, leading to extraordinary catalytic activity during Friedel−Crafts alkylation reactions (Figure 24, panels c1 and c2). Likewise, the authors adopted solid-state 1H and 31P MAS NMR spectroscopy to probe the surface acidic properties of TiO2 catalysts with different morphologies. The bulk anatase TiO2 exhibited a broad 1 H resonance centering at 2.5 ppm, which may be ascribed due to terminal Ti−OH (Figure 24b1).136 As for the anatase TiO2

nanosheet, two additional 1H peaks at 6.1 and 7.7 ppm were observed, indicating the presence of acidic protons. Separate measurements using the 31P-TMP NMR approach afford a more precise identification of acid sites. As a result, two distinct 31P resonances with δ31P at −2.9 and −24.1 ppm were observed for the TMP-loaded TiO2 nanosheets (Figure 24b2), which may be assigned to TMP adsorbed on BA and LA sites, respectively. The low-field peak should arise from the TMPH+ complexes, whereas the high-field peak due to TMP bound with coordinatively unsaturated metallic Ti Lewis centers.137 Moreover, as verified by catalytic measurements, it was concluded that the BrønstedLewis acid synergy present in the TiO2 nanosheets was responsible for the superior catalytic activity observed during Friedel−Crafts alkylation of aromatics.135 Niobium oxides represent a new class of water-tolerant heterogeneous catalysts with desirable acidities suitable for catalytic conversions in the presence of water, hence they have drawn considerable R&D attention recently.129,138−144 It has been demonstrated that the concentration and acidic strength of BA and LA sites in niobium oxides depend strongly on their surface structure and morphology.143,144 In general, BA sites on niobium oxides are associated with strongly acidic bridging (Nb−OH−Nb) or terminal (Nb−OH) hydroxyls, which act as the proton donors. On the other hand, LA sites on niobium oxides are involved with coordinatively unsaturated metallic Nb Lewis centers, which act as the electron pair acceptors.141 Nonetheless, there lacks information on detailed structureacidity correlation, hence, the corresponding effects on catalytic activity of the niobium oxide catalysts.142,145 Recently, Kreissl et 12494


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Figure 25. Top: 31P MAS NMR spectra of TMP adsorbed on various niobium oxides: (a) bulk Nb2O5, (b) protonated HNb3O8, (c) monolayer niobium oxide (hy-Nb-TEOA), (d) multilayer niobium oxide (hy-Nb), and (e) mesoporous Nb2O5. Bottom: Distributions of BA and LA sites vs δ31P for various samples, which were color-coded with corresponding 31P NMR spectrum above. Adapted with permission from ref 144. Copyright 2016 Elsevier Inc.

al.144 employed the 31P-TMP NMR approach to compare the acid properties of various niobium oxides with different structural morphologies, including commercial bulk Nb2O5, mesoporous Nb2O5, protonated HNb3O8, and mono- (hy-Nb-TEOA) and multilayered (hy-Nb) niobium oxides. The monolayer niobium oxide was prepared, invoking a triethanolamine (TEOA) surfactant as structure directing agent. To correlate the acid properties of various niobium oxides with their textural and chemical properties, analytical/spectroscopic measurements by XRD, TEM, BET, Hammett acidity measurement, and Raman spectroscopy were also performed. As indicated by the Hammett acidity function, niobium oxides typically possess acidic strength in the range of −5.6 < H0 < −3.0. Nonetheless, results obtained from such color indicator method are unable to provide a reliable quantitative measurement of overall acidity nor for distinguishing the subtle differences in acidic strengths of various samples, mostly due to the availability of indicator reagents with suitable pKa values. Needless to say, conventional acidity characterization methods such as titration and Hammett acidity measurements also failed to provide information on acid types (BA vs LA) of the solid acid catalysts.146,147 In this context, acidity characterization by means of the 31P-TMP NMR approach represents a reliable and straightforward method in determining detailed information on acid sites, including acid types, concentrations/distributions, and strengths.60−64 It was found that the structural morphology of niobium oxides is closely related to their acid properties. As shown in Figure 25, bulk Nb2O5, which possessed an extremely low surface area, exhibited a null 31P resonance, while its H+-exchanged counterpart (HNb3O8) showed only a weak resonance with δ31P of ca. −2.8 ppm, indicating the sole presence of Brønsted acidity. On the other hand, both BA and LA sites were observed in layered and mesoporous Nb2O5; 31P resonances arising from TMP adsorbed on BA and LA sites gave rise to a signal with δ31P in the range of ca. −0.3 ∼ −5.5 and −24 ∼ −42 ppm, respectively. Moreover, catalysts with higher surface areas exhibited 31P resonances with more intense peak intensities (i.e., acid

concentrations). As verified by XRD and TEM measurements, although the mesoporous Nb2O5 was highly amorphous in nature, it possessed interconnected wormlike pores with large BET surface area to render accessibility of surface BA and LA sites (Figures 25e). Likewise, the same conclusion may be drawn for the hy-Nb sample, which exhibited a layered structure consisting of stacked nanosheets with desirable voids to favor molecular transport and exposure of surface acid sites (Figures 25d). These results were further correlated with data obtained from Raman spectroscopy in the range of 850−1000 cm−1, corresponding to stretching of Nb = O terminal strength of the octahedral coordinated structure.142,148 Accordingly, the authors were able to correlate the Raman peak intensity, which represents the degree of distortion of the octahedral structure, with Lewis acidic strength based on the observed δ31P of the adsorbed TMP. Since no peak was visible for mesoporous Nb2O5 in the absorption range of 850−1000 cm−1, the authors attributed the observed Lewis acidity (δ31P at −25.37 ppm; see Figure 25e) to structural disorder.144 On the other hand, the Brønsted acidity observed in the δ31P range from ca. 0 to −3 ppm were ascribed due to strong BA sites associated with Nb−O−Nb bridging bonds located at shared edges or faces, while those with δ31P at ca. −4 to −6 ppm due to weaker BA sites corresponding to a weak Nb···O bond connected to the center of the cagelike octahedral structure. In contrast to the weak Brønsted acidity observed for the mesoporous Nb2O5, BA sites with higher acidic strengths were found in HNb3O8, hy-Nb, and hy-Nb-TEOA materials. Thus, it is indicative that the presence of weak BA sites in mesoporous Nb2O5 was due to lacking long-range structural ordering, most likely arising from terminal hydroxyls of disordered corner-sharing octahedra, whereas the higher Brønsted acidity observed in HNb3O8, hy-Nb-TEOA, and hyNb due to strong BA sites located at shared edges or faces. As mentioned earlier, the design and fabrication of solid acid catalysts with mesoporous or hierarchical pore structure are desirable strategies for perspective developments and practical applications in acid-catalyzed reactions such as Friedel−Crafts 12495


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alkylation, hydrolysis, and esterification reactions.149,150 In general, the acid property and catalytic performance of mixed metal oxides depend strongly on the types and relative concentrations of the incorporated metals. By using the 31PTMPO NMR approach, Domen and co-workers59 compared the acidic strength of mesoporous NbxW10−x oxides prepared with differnt Nb and W concentrations, as shown in Figure 26. In the

ultrastrong Brønsted acidity in the mesoporous Nb3W7 catalyst. This indicates that the Nb3W7 catalyst possess BA sites with acidic strength stronger than the ion-exchange Amberlyst-15 (δ31P ca. 81 ppm) and comparable to strongly acidic H-ZSM-5 and H-mordenite zeolites (ca. 86 ppm).51,56 As confirmed by the NH3-TPD results, the acidic strengths of various mesoporous NbxW10−x oxides follow the anticipated trend with a maximum NH3 desorption temperature (and corresponding heat of adsorption; in kJ mol−1) of 535 (135), 555 (140), and 570 (145) K for Nb7W3, Nb5W5, and Nb3W7, respectively. The acid properties of H+-exchanged layered niobium molybdate (HNbMoO6), which was found to exhibit remarkable catalytic performance for Friedel−Crafts alkylation reaction, has also been investigated by the 31P-TMPO NMR approach.151−153 As shown in Figure 27A, both niobic acid and Nb2O5−MoO3 catalysts displayed sharp peaks with δ31P of ca. 40−42 ppm as well as broad peaks in the region of ca. 60−70 ppm, which may be ascribed due to crystalline TMPO and protonated TMPOH+ complexes associated with BA sites, respectively. At a constant TMPO loading of 0.8 mmol per gram catalyst, signals arising from crystalline TMPO (at ca. 40 ppm) were observed in both niobic acid and Nb2O5−MoO3 but not in nanosheet and layered HNbMoO6 catalysts, indicating the less available number of acid sites for the guest molecule in the former two samples than the latter. Unlike the nanosheet HNbMoO6, which showed resonance peaks at ca. 60 and 72 ppm due to BA sites with medium acidic strengths, layered HNbMoO6 exhibited broad, overlapping resonances with main features at further downfield of 86 and 81 ppm. Again, the presence of 31P resonance of adsorbed TMPO with δ31P ≥ 86 ppm in layered HNbMoO6 revealed the existence of BA sites with superacidity surpassing that of H−Y (65 ppm)53,99 and H-Beta (78 ppm)51,154 and ionexchange resin (e.g., Amberlyst-15; 81 ppm) and being comparable to some of the strongly acidic zeolites such as for H-ZSM-556 and H-MOR51 (86 ppm) in which their acidic strengths were enhanced by additional pore-confinement effect.64 Furthermore, the effect of guest molecule (TMPO) loading over the layered HNbMoO 6 catalyst was also investigated. As shown in Figure 27B, additional resonance peaks at 45 and 42 ppm due to crystalline or physisorbed TMPO were observed only for sample loaded with excessive amounts of guest molecule (3.2 mmol g−1; Figure 27B-b3), but not for the sample exposed to 1.6 mmol g−1 TMPO (Figure 27B-b2),

Figure 26. Left: Room-temperature 31P MAS NMR spectra of adsorbed TMPO (loading: 0.8 mmol g−1) on mesoporous (a) Nb, (b) Nb7W3, (c) Nb5W5, and (d) Nb3W7 oxides. All spectra were acquired with a MAS spinning rate of 10 kHz. Right: corresponding expanded spectra in the Brønsted acidic region. Adapted with permission from ref 59. Copyright 2010 Wiley-VCH.

absence of W, the pure Nb oxide exhibited two distinct 31P resonances: the peak at δ31P of 39 ppm should be arising from physisorbed TMPO, whereas the broad peak centering at 65 ppm is due to TMPO adsorbed on BA sites. It is noteworthy that Nb oxides clearly possess Brønsted acidity comparable to that of H− Y zeolite (H0 < −6.6), which typically gave rise to resonances with δ31P at ca. 55−65 ppm.53,99 Upon increasing the W loading, while the peak at 39 ppm remained practically unchanged, a progressive increase in δ31P value was evident (Figure 26, panels b−d). In particular, overlapping peaks with δ31P at 86, 75, and 63 ppm were observed for the Nb3W7 sample (Figure 26d). The weak peak at 86 ppm, whose δ31P is comparable to the threshold value for superacidity (i.e., H0 ≤ −12), revealed the presence of

Figure 27. Comparisons of 31P MAS NMR spectra of (A) TMPO adsorbed on (a1) Nb2O5·nH2O, (a2) Nb2O5−MoO3, (a3) HNbMoO6 nanosheets, and (a4) layered HNbMoO6 with a constant loading of 0.8 mmol g−1 and (B) layered HNbMoO6 with varied TMPO loading of (b1) 0.8, (b2) 1.6, and (b3) 3.2 mmol g−1. Adapted with permission from ref 151. Copyright 2009 Elsevier. 12496


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shown in Figure 28A-a1, three primary resonances with δ31P at 65, 55, and 41 ppm together with a broad shouldering peak at 40

indicating that the total amount of acid sites in layered HNbMoO6 should be higher than 1.6 mmol g−1 but less than 3.2 mmol g−1. Moreover, by comparing to the spectra obtained with a TMPO loading of 0.8 mmol g−1 (Figure 27B-b1), additional broad resonances centering at ca. 69 ppm were also observed along with the weaker resonances at around 80−87 ppm. This indicates the presence of not only superacidic BA sites in the layered HNbMoO6 but also abundant with BA sites medium acidic strengths comparable to typical zeolitic catalysts. The above results also certify the importance of sample preparation procedure (see the section 3.1) for acidity characterization by the 31P-R3PO approaches. In this regard, it is clear that an adequate probe molecule (i.e., TMPO in this case) loading is requisite to ensure a complete coverage, hence, a comprehensive observation of all accessible acid sites. 4.2. Porous Catalyst Materials

4.2.1. Microporous Zeolites and Mesoporous Molecular Sieves. Zeolites are aluminonsilicates with three-dimensional structures, which are consisting of pore channels and/or cages of molecular dimensions. Theoretically, siliceous zeolites should possess no acidity requisite for catalytic reactions. Partial isomorphous substitutions of framework silicon atoms by heteroatoms (such as Al and B) normally develop a negatively charged framework, which is neutralized by protons bonded to the bridging oxygens. Typically, the structural T (Si or Al) atoms in the framework of aluminosilicate zeolites are tetrahedrally coordinated, whose negatively charged framework is usually compensated by the bridging hydroxyl protons to form acid sites. In accordance with IUPAC classification of pore size, pores with sizes less than 2 nm are called microporous,155 those with sizes exceeding about 50 nm are called macropores, whereas those with intermediate size between 2−50 nm are called mesopores. Zeolites, which mostly possess pore or channels widths less than 2 nm, are termed as microporous materials. Most common acidic proton-form zeolites include H-ZSM-5, H−Y, H-mordenite, Hbeta, and MCM-22, and they have been extensively used as heterogeneous acid catalysts in the petrochemical industries. Likewise, aluminated mesoporous molecular sieves such as AlMCM-41 and Al-SBA-15 are being used for catalytic conversions of large molecules. In general, the catalytic activities and reaction rates of porous acid catalysts depend strongly on concentrations and strengths of acid sites, while the reaction mechanism or pathway mainly relies on acid types (i.e., Brønsted vs Lewis acidities), and the locations of acid sites (internal vs external acidities) strongly dictate reaction product shape-selectivity. As such, a comprehensive understanding of acidic features, namely types, concentrations, locations, and strengths of acid sites, in porous acid catalysts is inevitable for gaining fundamental information such as reactivity, selectivity, and reaction mechanism and/or pathway of heterogeneous catalysis systems.64 4.2.2. Aluminosilicate Zeolites. X- and Y-types zeolite are well-known acid catalysts in hydrocarbon conversions and oil refinery industries.156 To identify various acid sites in zeolite H− Y accessible to the TMPO probe molecule, Mueller and coworkers99 employed perdeuterated TMPO (TMPO-d9) in conjunction with NMR techniques such as transfer of population in double resonance (TRAPDOR)157 and rotational echo double resonance (REDOR)158 techniques. By using TMPO-d9 as a probe, disturbing proton resonances arising from methyl protons may be eliminated, leaving surface acidic protons as the exclusive source for 1H−31P couplings during NMR measurements. As

Figure 28. (A) The 31P MAS NMR spectra of zeolite H−Y loaded with 3.0 mmol g−1 TMPO-d9 recorded (a1) without and (a2) with 1H/31P cross-polarization. (B) 31P/27Al TRAPDOR NMR difference spectra of TMPO-d9 (2.5 mmol g−1) adsorbed on H−Y (b1) without and (b2) with 1H/31P/27Al CP. (C) Comparison of the full echo S0 (solid curve) and reduced echo Sf (dashed curve) signals obtained from 31P/1H REDOR NMR of H−Y loaded with 2.5 mmol g−1. Spinning sidebands are labeled with asterisks. Adapted from ref 99. Copyright 2002 American Chemical Society.

ppm were observed in the spectrum obtained from 31P MAS NMR of H−Y loaded with 3 mmol g−1 of TMPO-d9. The authors attributed the broad signal at 40 ppm due to TMPO-d9 physisorbed on terminal silanols in zeolite H−Y. The spectrum revealed all possible 31P resonances with different Brønsted acidic strengths in the TMPO-d9 loaded sample. On the hand, three resonances at 65, 55 and 40 ppm were observed for the same sample when the NMR spectrum was acquired by using 1 H−31P cross-polarization (CP) MAS technique (Figure 28Aa2). Clearly, a notable enhancement in signal intensity was found for the spectrum acquired with CP and revealed only resonances arising from 31P nuclei in the close proximity of 1H nuclei. Further experiment by rotor-synchronized 31P/27Al TRAPDOR NMR rendered information on connectivities between 31P (nucleus with I = 1/2) of the probe molecule and 27Al (quadrupolar nucleus with I = 7/2) on the zeolite framework to be attained. As a result, the 31P/27Al TRAPDOR difference 12497


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spectrum of TMPO-d9 loaded H−Y revealed only two resonances at 65 and 55 ppm (Figure 28B-b1), which may be attributed to 31P nuclei close to 27Al nuclei. A similar spectrum was observed on the same sample using 1H/31P/27Al CPTRAPDOR NMR (i.e., with implementation of 1H−31P CP), as shown in Figure 28B-b2. Thus, the results obtained from the above TRAPDOR NMR experiments clearly indicated that the resonances were indeed arising from coupling of TMPO-d9 with bridging hydroxyl proton (Al−OH−Si; i.e., Brønsted acidity), as evidenced by the fact that the 31P nuclei (on the probe molecule) are indeed in close proximities to both 27Al and 1H nuclei (on zeolite framework). By comparing the spectrum respectively obtained from 1H/31P CP-MAS and 1H/31P/27Al CP-TRAPDOR NMR, no signals at 41 ppm were observed in the latter. Thus, the resonance at 41 ppm in Figure 28A-a2 is likely due to crystalline TMPO-d9 residing on the surfaces of the zeolite far away from the framework Al species. Additional experiments by 31 1 P/ H REDOR NMR afford determination of internuclear distance between 1H and 31P nuclei on the (TMPO-d9)H+ adsorption complexes. As shown in Figure 28C, while resonance peaks at 65, 55, and 40 ppm showed substantial decrease due to dephasing effect, no obvious change in signal intensity of the crystalline peak at 41 ppm was observed, in good agreement with the results from the above CP and TRAPDOR experiments. On the basis of high-resolution neutron powder diffraction, the most probable Brønsted acidic proton positions in H−Y zeolites should be associated with two of the framework oxygen sites, namely O(1) and O(3),158 in the supercages and sodalite cages, respectively. To unravel these proposed locations of BA sites in supercages and sodalite cages, the authors adopted the 31 P-TMPO NMR approaches for homologous series of partially proton-exchanged Na−Y zeolite samples, denoted as nH,Na−Y (n represents the fraction of exchange level), which were prepared by ion-exchange of ammonium and deammoniation. For example, the 0.8H,Na−Y (n = 0.8) sample would represent an 80% exchange of Na+ ion by H+ in the original Na-form Y zeolite. Since the size of NH4+ ion is larger than the Na+ ion, Na+ in the supercages are preferentially replaced by NH4+ at relatively lower exchange levels (n < 0.3), while Na+ in the smaller sodalite cages may be replaced only at higher exchange levels.14,159 In other words, the bridging Si−OH−Al (i.e., BA) sites can be found in the sodalite cages only in samples with higher H+exchange levels (say, n ≥ 0.3). As shown in Figure 29 (panels a and b), for TMPO adsorbed on 0.1H,Na−Y, a weak signal with δ31P of ca. 65−66 ppm was observed in addition to the two stronger peaks at 42 and 48 ppm due to bulk crystalline TMPO and physisorbed TMPO, respectively. Upon increasing the exchange level beyond 30%, an additional peak at ca. 56 ppm emerged seemingly at the expanse of the resonance responsible for physisorbed TMPO (at ca. 48 ppm; see Figure 29c). As the exchange level further increased to 80%, the 31P spectrum observed for the 0.8H,Na−Y sample showed considerable enhancement in peak intensity of the 56 ppm resonance while that peak at 48 ppm diminished. Again, the peaks with δ31P of ca. 65−66 and 56 ppm may be ascribed due to TMPO adsorbed on BA sites residing in the supercages and sodalite cages of the H−Y zeolites.99,159,160 4.2.3. Borosilicate Zeolites. A practical way to manipulate the acidity of zeolites is by incorporating heteroatoms such as boron (B) onto the framework T-sites either by a hydrothermal or postsynthesis modification procedure.161,162 For example, by substituting framework T atoms with boron, H-[B]-ZSM-5 borosilicate was found to exhibit excellent catalytic activity and

Figure 29. 31P MAS NMR spectra of TMPO adsorbed on dehydrated partially H+ ion-exchanged Na−Y zeolites (a) 0.1H,Na−Y, (b) 0.2H,Na−Y, (c) 0.3H,Na−Y, and (d) 0.8H,Na−Y. Asterisks denote spinning sidebands. Adapted from ref 160. Copyright 2015 American Chemical Society.

shape selectivity during conversion of ethylbenzene and isomerization of xylene and greatly enhance the catalytic activity of Beckmann rearrangement reactions. Detailed acid features, viz. types, concentrations, distributions, and strengths of acid sites in the novel H-[B]-ZSM-5 catalyst, were also investigated by using 31P-TMPO and TBPO NMR approaches.162 As shown in Figure 30a, multiple 31P resonances with δ31P spanning from 88 to 31 ppm were observed for TMPO adsorbed on the dehydrated H-[B]-ZSM-5 zeolite. The peaks at 43 and 31 ppm may be assigned due to crystalline and physisorbed TMPO, respectively,56 and likewise for the peaks observed at 52 and 42 ppm in the case of using TBPO probe molecule (Figure 30b). As discussed earlier (see section 2.2), a linear dependence between the observed δ31P with PA or DPE value (i.e., acidic strength) may be inferred for systems using R3PO as the probe molecules (cf. Figure 4). However, a consistent δ31P offset of 8 ± 2 ppm was also found between systems using TMPO and TBPO as the probe molecule, as observed here. To render assignments of acid types (i.e., Brønsted vs Lewis acidity) for various resonances in the spectra obtained from TMPO/TBPO-loaded H-[B]-ZSM-5 samples, spectra obtained from respective hydrated samples were also examined (Figure 30, panels c and d). As mentioned earlier, the weaker couplings of TMPO-LA complexes will be dissociated upon exposure to moisture to form Brønsted acidity. As such, upon partial hydration of the sample, the 31P resonances associated with LA sites should be diminished or decreased notably to convert to BA sites, while resonances originally pertaining to the BA sites are either unaffected or enhanced. In 12498


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Figure 30. 1H-decoupled 31P MAS NMR spectra of (a) TMPO and (b) TBPO adsorbed on dehydrated H-[B]-ZSM-5 and their corresponding spectra (c) and (d) obtained after exposure to humidity for 2.5 h. * denotes a spinning sideband and # denotes noise. (e) Schematic sketches of possible interactions between TMPO and Brønsted/Lewis acid sites in H-[B]-ZSM-5 zeolite: (1) protonated TMPOH+ arising from bridging hydroxyls, (2) hydrogen-bonded TMPO with terminal B−OH group, (3) TMPO adsorbed on Lewis acid sites, and (4) physisorbed TMPO complexes or unreacted TMPO species over Si−OH groups. Adapted with permission from ref 162. Copyright 2014 Elsevier.

determined to be ca. 280, 300, and 335 kcal mol−1. Giving the δ31P of 88 and 56 ppm responsible for Lewis acidity observed for H-[B]-ZSM-5 using TMPO as the probe indicated the presence of LA sites with very strong and medium acidic strengths. In terms of Lewis acidic strength, its strongest LA site with acidic strength corresponding to δ31P of 88 ppm is comparable to sulfated and metal-promoted ZrO2 (90 ppm; see Table 2),106 whereas the other LA site with modest acidic strength (δ31P = 56 ppm) is comparable to zirconia (ZrO2; 62 ppm),106 silica− alumina, γ-alumina, and zeolite H−Y (ca. 53−55 ppm).48 4.2.4. Quantitative Study and Discernment of Internal vs External Acid Sites. As shown in Figure 31, by using the 31PTMPO NMR approach, qualitative information such as relative distributions, concentrations, and strengths of acid sites in various porous solid acid catalysts may readily be obtained through deconvolution of the observed 31P spectrum.163 In addition, by combining the results obtain from spectral analysis with elemental analysis (e.g., by ICP-MS method), further quantitative information on acid sites in solid acid catalysts may also be attained, affording comprehensive understanding of acid features, viz. type, strength, amount, and distribution of acid sites. As shown in Figure 31 and Table 3, up to seven 31P resonances

the case of adsorbed TMPO, resonances at 88 and 56 ppm diminished upon sample hydration, while the intensity of the peak at 62 ppm decreased notably (Figure 30c). Thus, these peaks may be assigned to LA sites. On the other hand, resonances at 74, 66, and 52 ppm should be associated with BA sites with different acidic strengths. As for the case of adsorbed TBPO, since the size of the probe molecule is too big to enter the pore channels of H-[B]-ZSM-5 zeolite, the resonances observed at 70, 57, 52, and 42 ppm should arise exclusively from acid sites on the external surfaces (Figure 30b).56 Aside from the peaks at 57 and 52 ppm due to crystalline and physisorbed TBPO, the assignments of the resonances at 70 and 57 ppm observed in dehydrated H-[B]-ZSM-5 zeolite remain to be clarified. Again, this was achieved by comparing with the spectrum obtained from hydrated H-[B]-ZSM-5 loaded with TBPO (Figure 30d). Since no significant changes in the main resonances was observed compared to that of dehydrated spectrum (Figure 30b), both resonances at 70 and 57 ppm should both associated with BA sites. Moreover, based on the correlation between the δ31P of the observed TMPO with the Brønsted acid strength (see eq 8), the PA (or DPE) values of BA sites corresponding to the resonances at 74, 66, and 52 ppm in H-[B]-ZSM-5 may be quantitatively 12499


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observed for the H-ZSM-5 (Si/Al = 15) at 86 (0.5%), 75 (22.4%), 67 (37.5%), 63 (36.6%), 59 (11.3%), and 53 (3.0%) ppm are equivalent to absolute acid amount of 0.005, 0.165, 0.275, 0.269, 0.090, and 0.021 mmol g−1, as depicted in Table 3. The above results showed that the H-ZSM-5 zeolite possesses mostly BA sites with medium to strong acid. However, unlike HMCM-22, H-USY, H-beta, and H-AlMCM-41, which possess both BA and LA sites, no trace of Lewis acidity was found in HZSM-5 and H-mordenite. As demonstrated above, detailed acid features such as type, distribution, concentration, and strength of acid sites may be obtained by combining the 31P-TMPO NMR approach with elemental analysis. Nonetheless, information on locations of acid sites remains unresolved. The locations of acid sites, whether in intra- or extra-crystalline surfaces of the catalyst, are crucial for the catalytic performances (i.e., activity and selectivity) of solid acids.90,96,164−167 Available techniques for studying location of acid sites in solid acid catalysts include FT-IR168 or TPD165 of adsorbed probe molecules. By using TPD of ammonia and collidine, Du and Olson165 demonstrated that the locations of internal and external acid sites in H-MCM-22 may be determined. Guisnet and co-workers166 characterize internal and external acidity of H-MCM-22 zeolite using FT-IR of the adsorbed 2,6- and 2,4-dimethylquinoline (DMQ) as probes. The authors concluded that 2,4-DMQ is a suitable probe molecule for probing the external acidity of H-MCM-22, and the external acid sites were found to locate in one of the external pocket sites which were responsible for catalyzing m-xylene transformation at 350 °C.166 Liu and co-workers56 proposed a simple method to afford concurrent qualitative and quantitative determination of internal and external acid sites in 10-MR zeolite catalysts by combining the 31P-TMPO/TBPO NMR approaches with elemental analysis. Since the kinetic diameter (KD) of TMPO (ca. 0.55 nm) is comparable to the pore aperture of typical 10-MR zeolites (ca. 0.60 nm), the TMPO probe molecule can diffuse easily in the intracrystalline channels or pores of the zeolite catalyst, rendering concurrent detection of both internal and external acid sites. On the other hand, TBPO (KD ca. 0.82 nm) is too bulky to penetrate into pore channels of the 10-MR zeolites, hence, it can only detect acid sites located on the external surfaces

Figure 31. 31P MAS spectra of TMPO adsorbed on various dehydrated porous solid acids. All catalysts have similar Si/Al ratio. The dashed curves are results obtained from spectral analysis by Gaussian deconvolution, and the shaded peaks (in pink color) represent the presence of Lewis acidity.163

with δ31P at 86, 75, 67, 63, 59, 53, 43, and 30 ppm were observed for TMPO adsorbed on H-ZSM-5 zeolite with a Si/Al ratio of 15. With the exception of the signal at 43 and 30 ppm, which were attributed to crystalline and physisorbed TMPO that either attached in the intercrystalline voids or weakly adsorbed near the channel pore mouths of the zeolite,56 the other five downfield peaks were assigned due to TMPO adsorbed on BA sites with different acid strengths. Apparently, the 31P-TMPO NMR approach is superior compared to conventional NH3-TPD in terms of providing detailed distribution of acid sites in solid acid catalysts.56,61−64 Obviously, there is a distribution of acid sites with different acid strengths in the H-ZSM-5 zeolite, ranging from weak (δ31P < 60 ppm), medium (60−70 ppm), strong (70− 85 ppm), to superacidic (≥86 ppm). Further analysis by ICP-MS confirmed that the 31P resonances (and corresponding relative concentrations) with descending order of acidic strengths

Table 3. 31P NMR Chemical Shift Assignments and Distribution of Acid Sites for Various Solid Acid Catalysts Loaded with TMPO163

a The upper value in each column represents the observed chemical shift (δ31P); the first data in parentheses (in bold) denotes relative concentration of acid sites (%), whereas the latter data (in italic) represents the acid amount in (±0.005) mmole per gram catalyst. bThe shaded columns indicate contributions from either Lewis acid sites (L) or physisorbed (P) TMPO.

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H-ZSM-5 zeolite, the intensities of the downfield peak with highest δ31P (86 ppm) tend to increase at the expanses of other characteristic peaks at upfield (i.e., peaks with lower δ31P). For example, the resonance peak at 53 ppm disappeared for TMPO adsorbed on H-ZSM-5/26 and H-ZSM-5/75 samples. In addition, the peak at 67 ppm was not observed in the H-ZSM5/75. All 31P resonance features at 86, 75, 67, 63, and 53 ppm remained practically unchanged, even after exposing the TMPOloaded H-ZSM-5/15 to humidity for 1.5 h, indicating that these characteristic peaks may be assigned a priori to TMPO absorbed on BA sites. Should LA sites be presented in the sample, it should immediately react with H2O to form BA sites, hence it should be accompanied by diminishing of specific 31P resonances (which was not observed). Further experiments by the 31P-TBPO NMR approach afford differentiation of internal and external acid sites. Unlike TMPO, which is capable of probing both internal and external acidities, TBPO can only probe acid sites located on the extracrystalline surfaces of 10-MR zeolites such as H-ZSM-5. The 31P NMR spectra of TBPO adsorbed on various H-ZSM-5 catalysts with different Si/Al ratios are displayed in Figure 32b. Regardless of the resonances at 54 and 47 ppm due to crystalline and physisorbed TBPO, respectively, only three distinct peaks with δ31P of 92, 75, and 71 ppm were observed. On the basis of the spectrum obtained from a partially hydrated sample (H-ZSM-5/ 26; see Figure 32b), three resonances above are ascribed due to TBPO adsorbed on BA sites since practically no change in resonance features was identified for dehydrated and hydrated TBPO-loaded catalysts. On the basis of a simple analysis of chemical shift difference (Δδ) between the observed δ31P of the characteristic resonance associated with BA sites relative to that of crystalline bulk TBPO (47 ppm),56 internal and external acid sites with comparable acidic strength may be correlated. By comparing the results obtained from the 31P-TMPO and TBPO NMR approaches (see Table 4), it is indicative that the three resonances at 92, 75, and 71 ppm observed for the TBPO-loaded H-ZSM-5 should have comparable acidic strengths observed by the 31P-TMPO approach at 86, 67, and 63 ppm, respectively. This indicates that resonances at 75 and 53 ppm obtained by using TMPO as probe should be exclusively associated with internal BA sites. Note that a consistent offset of ca. 8 ± 2 ppm between the 31P-TMPO and TBPO approaches was observed, in excellent agreement with the result predicted by theoretical DFT

of zeolite crystallites. Similar techniques have also been applied for studying nanosize164,169 and surface-modified90,164 zeolitic catalysts as well as activity-acidity correlations during de-NOx process170,171 over bifunctional metal-supported zeolites. As an illustration, Figure 32a displays the 31P MAS NMR spectra of TMPO adsorbed on various H-ZSM-5 zeolites with

Figure 32. 31P MAS NMR spectra of (a) TMPO and (b) TBPO adsorbed on various H-ZSM-5 zeolites with different Si/Al ratios of 15, 26, and 75. The lower spectra were obtained from the H-ZSM-5/15 and H-ZSM-5/26 samples exposed to humidity for 1.5 h. The dashed curves indicate results of spectral analyses by Gaussian deconvolution. All spectra were recorded with a sample spinning rate of 10 kHz, and * denotes spinning sidebands. Adapted from ref 56. Copyright 2002 American Chemical Society.

varied Si/Al ratios.56 For example, up to seven resonance peaks at δ31P of 86, 75, 67, 63, 53, 43, and 30 ppm were observed for the H-ZSM-5 with Si/Al = 15 (denoted as H-ZSM-5/15; see Table 4). Again, the peaks at 43 and 30 ppm are due to crystalline and physisorbed TMPO, respectively. Thus, only five resonances with δ31P exceeding 53 ppm are associated with TMPO adsorbed on BA sites. It can be seen that, by increasing the Si/Al ratio of the

Table 4. 31P NMR Chemical Shift Assignments and Distribution of Brønsted Acid Sites of H-ZSM-5 Zeolites Loaded with TMPO and TBPO Probe Molecules56 TMPO δ P (Δδ) 31

a c,d

H-ZSM-5/15 H-ZSM-5/26c,d H-ZSM-5/75c,d δ31P (Δδ)a e

H-ZSM-5/15 H-ZSM-5/26e H-ZSM-5/75e

86(47)

75(36)

67(28)

63(24)

53(14)

43(4)b

30b

0.5 (−, 0.005) 6.9 (0.014, 0.010) 3.5 (0.003, 0.002)

22.4 (0.165, −) 45.4 (0.159, −) 69.8 (0.108, −)

37.5 (0.258, 0.017) 22.7 (0.067, 0.012) −(−, 0.002) TBPO

36.6 (0.242, 0.027) 25.0 (0.063, 0.025) 26.7 (0.032, 0.009)

3.0 (0.021, −)

X X X

X

92(45)

75(28)

71(24)

54(7)b

47b

10.5 22.3 15.4

35.0 25.2 17.6

54.5 52.5 67.0

X

X

a Data in bold denote the observed δ31P (±2 ppm); italic numbers in parentheses (Δδ) refer to the corresponding chemical shift differences with respect to crystalline TMPO (39 ppm) or TBPO (47 ppm). bThe X indicates appearance of the characteristic peak. cValue under slash at the end of notation represents sample Si/Al ratio. dValues on top represent relative concentration of acid sites (%); data in parentheses (int, ext) give the amounts of internal and external acid sites (±0.002 mmol g−1), respectively. eValues specifically represent relative concentration of external acid sites (%).

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calculations.61−63 Similar conclusions may also be drawn for TMPO/TBPO adsorbed on H-ZSM-5/26 and H-ZSM-5/75, as summarized in Table 4. Moreover, it is noteworthy that together with complementary results from elemental analyses, reliable quantitative information on distributions and absolute amounts of internal and external acid sites may be determined. Taking the case of TMPO-loaded H-ZSM-5/15 as an example, the relative concentrations of the BA sites corresponding to the characteristic resonances with δ31P at 86, 75, 67, 63, and 53 ppm were 0.5, 22.4, 37.5, 36.6, and 3.0%, respectively (Table 4). In the case of TBPO, the relative concentrations corresponding to the three 31P resonances at 92, 75, and 71 ppm were found to be 10.5, 35.0, and 54.5%, respectively. Further combining element analysis data from ICPMS, the precise amount of adsorbed TMPO or TBPO and hence the corresponding amounts of internal and external acid sites can readily be determined. This is accomplished by assuming a oneon-one adsorption scheme (i.e., each acid site can only accommodate one guest probe molecule). In practice, the amounts of external acid sites having different acidic strengths (i.e., different δ31P) were derived based on elemental analysis of the P element in TBPO. In view of the fact that the adsorbed TMPO is capable of probing both internal and external acidities, the amount of individual internal acid site (with different observed δ31P) can readily be determined by subtracting the corresponding external amount (determined above) from the corresponding total concentration obtained from analysis of TMPO-loaded sample. Note that the contributions from crystalline and physisorbed TMPO at 43 and 30 ppm (or TBPO at 54 and 47 ppm) must be excluded in advance to render accurate determination of internal and external acid amounts. For example, an amount of external acid site for the H-ZSM-5/15 was determined to be 0.005, 0.017, and 0.027 mmol per gram catalyst for the 31P resonance at 92, 75, and 71 ppm, respectively, based on the 31P-TBPO NMR measurements and the subsequent elemental analyses. Accordingly, the amounts of internal and external acid sites, denoted as (int, ext) in Table 4, for each characteristic 31P resonance at 86, 75, 67, 63, and 53 ppm were determined to be (null, 0.005), (0.165, null), (0.258, 0.017), (0.242, 0.027), and (0.021, null) mmol g−1, respectively. It is noteworthy that the total amount of external acid sites is only ca. 6% of the total acidity, as anticipated for micron-sized zeolite crystallites. Similarly, detailed distributions and amounts of acid sites for H-ZSM-5/26 and H-ZSM-5/75 catalysts may also be determined, as depicted in Table 4. By means of periodic molecular mechanical simulations and 31 P MAS NMR of adsorbed triphenylphosphine (PPh3, molecular size ca. 11.7 × 7.1 Å),172 Bao and co-workers173 revealed that the probe molecule tend to preferentially adsorb near the external surface pockets (size ca. 7.1 × 9.2 Å) of the 12MR structured MCM-22 zeolite,174 as illustrated in Figure 33. Thus, the 31P-PPh3 SSNMR approach is a practical technique for probing external acid sites on the surfaces of 12-MR zeolites. The 31 P MAS NMR spectrum of PPh3 adsorbed on MCM-22 zeolites (Si/Al = 64) is shown in Figure 34. The 31P resonance at −4.6 ppm was assigned to physisorbed PPh3, whereas the two downfield peaks at 11.1 and 14.8 ppm were both attributed to PPh3 bound at BA sites on external surfaces of the MCM-22 zeolite. By combining the above results with elemental analysis by X-ray fluorescence (XRF) spectroscopy, the amount of external BA sites was estimated to be ca. 6% of the total acid amount possessed by the MCM-22 zeolite.

Figure 33. Illustrative optimized adsorption structure of triphenylphosphine (PPh3) adsorbed on external side pocket of a MCM-22 zeolite unit cell. Adapted with permission from ref 173. Copyright 2004 American Chemical Society.

Figure 34. 31P MAS NMR spectrum of triphenylphosphine (PPh3) adsorbed on H-MCM-22 zeolite. Adapted with permission from ref 173. Copyright 2004 American Chemical Society.

4.2.5. Pore Confinement Effects. It is intriguing that 31P resonance with δ31P ≥ 86 ppm, a threshold value for superacidity,60−63 was observed for H-ZSM-5 zeolite based on the 31P-TMPO NMR approach (cf. Tables 3 and 4). The presence of superacidity in zeolitic catalysts indeed has been a subject of controversial debate. By using an alternative SSNMR approach for acidity characterization using 2-13C-acetone as probe, the threshold δ13C value for superacidity was found to be 246 ppm, as observed for HPW superacid.47−49 Nonetheless, the strongest acidic strength (or δ13C) observed for typical H-ZSM-5 zeolites using 2-13C-acetone as a probe seldom exceeds 223 ppm, revealing the presence of only BA sites with modest to strong acidity. As such, the origin of superacidity in H-ZSM-5 based on the 31P-TMPO NMR approach remains unresolved. In this context, theoretical prediction by DFT calculations has becoming an advanced and reliable method especially in investigating the adsorption structure (including bond distances and energies) and relevant NMR parameters of the guest−host system, namely probe molecule bounded on acid sites of the catalyst. As an illustration, optimized equilibrium structures of TMPO adsorbed on various predicted models of H-ZSM-5 zeolite containing 8−72 framework T atoms (termed as 8T−72T 12502


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Figure 35. Optimized equilibrium configurations of TMPO adsorbed on the (A) 8T, (B) 26T, (C) 38T, (D) 52T, (E) 58T, and (F) 72T models of the ZSM-5 framework viewing along the [100] face. Selected interatomic distances (in Å) are indicated. Adapted from ref 60. Copyright 2008 American Chemical Society.

cluster models) have been explored, as depicted in Figure 35.60 In other words, the zeolite frameworks were mimicked by a series of cluster models having sizes ranging from 8T to 72T. Note that a truthful zeolitic framework was largely simplified in the smallest 8T cluster model, which in principle only represents the local structure of a BA site, while the largest 72T cluster model readily consists of a complete framework skeleton with confined geometry in the proximity of the BA site. Moreover, BA sites were intentionally placed at the intersections of the straight and sinusoidal channels (i.e., Al12−O24H−Si12) of the ZSM-5 zeolite in these calculations to facilitate accessibility of reactant during the catalytic reaction. As shown in Figure 35, a notable increase in P−O bond length of the TMPO probe molecule with increasing cluster size from 1.549 Å observed for the 8T cluster to 1.553 Å (26T), 1.554 Å (38T), 1.562 Å (52T), 1.571 Å (58T), and finally reaching 1.576 Å for the 72T cluster model, respectively. While the O−H bond length between the acidic proton and oxygen atom of TMPO decreased consistently with increasing cluster size from 1.151 Å (8T) to 1.031 Å (72T), indicating a gradual enhancement in TMPO protonation level with increasing size of cluster model. This, of course, resulted in a monotonic downfield shift of the predicted δ31P of adsorbed TMPO from 65.0 ppm (8T), 70.4 ppm (26T), 71.0 ppm (38T), 77.9 ppm (52T), 80.2 ppm (58T), and finally to 86.7 ppm (72T). The above results verified the stronger interactions (higher δ31P value) between the adsorbed TMPO molecule and increasing complexity of the

microporous structure of zeolite.60 Therefore, it is conclusive that the apparent acid strength of zeolites could be enhanced by the electrostatics and van der Waals (vdW) interactions associated with the zeolite framework, which in turn improves their catalytic performances. Mordenite (MOR) zeolite is also one of the popular acid catalysts commonly used in commercialized processes.176 The structural framework of MOR is composed of a large 12-MR channels (pore aperture ca. 6.5 × 7.0 Å2) parallel to the c axis, which is interconnected with 8-MR side pockets (ca. 2.6 × 5.7 Å2) parallel to the b axis.177 As shown in Figure 36, the 31P MAS NMR spectrum of TMPO adsorbed on H-MOR (Si/Al = 30) zeolite may be deconvoluted into six characteristic resonances with δ31P at 88, 78, 71, 66, 60, and 50 ppm. These peaks were ascribed unambiguously to TMPOH+ complexes due to protonation of TMPO by BA sites with various acidic strengths.175 Apparently, the resonance at lowermost downfield with δ31P of 88 ppm has surpassed the threshold of superacidity (86 ppm). As discussed above, a similar resonance with δ31P at 86 ppm observed in H-ZSM-5 may be attributed to the confinement of TMPO in the 10-MR channels (with pore sizes of 5.3 × 5.6 Å and 5.1 × 5.5 Å). In this context, such confinement effect should be small in the relative larger 12-MR channels of H-MOR. Further theoretical calculations verified that the enhancement in Brønsted acidity in H-MOR was in fact caused by the intermolecular solvent effect in channel voids in which several 12503


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sites. For typical micron-size zeolite catalysts, the amount of acid sites on the external surfaces of the crystallite normally takes only less than 10% of the total acid sites. These issues may be unraveled at large part by using nanosized and/or mesoporous zeolites with hierarchical structures.135,149,180−188 Zeolites with nanoparticle164,169 or nanosheet180,181 morphologies have the advantage of high external surface area, hence more available acid sites desirable for catalytic reactions. Whereas hierarchical zeolites normally possess micropores and mesopores to render shape-selective catalysis for a wide range of molecule with sizes up to mesoscopic range.182−188 For examples, mesoporous ZSM-5 zeolite (termed as ZSM-5M) and its b-axis-aligned counterpart (termed as ZSM-5-OM) supported Ru nanoparticles (NPs) were found to be efficient catalysts for hydrodeoxygenation (HDO) of phenolic bio-oil to alkanes.186 The large amount of exposed acid sites available in the open mesostructure of ZSM-5-M and ZSM-5-OM were found to show superior activity for the cleavage of C−O bonds, which is the key step during HDO of phenols to alkanes. The acid properties of these mesoscopic ZSM-5 zeolites were characterized by using 31P-TMPO NMR approach.186 As shown in Figure 37, 31P NMR spectra of TMPO adsorbed on the H-ZSM-

Figure 36. (a) 31P MAS NMR spectrum of TMPO adsorbed on HMOR zeolite. The dotted curves represent peaks deduced from Gaussian deconvolution. (b) Optimized adsorption structure of three TMPO molecules confined in the 12-MR channel of H-MOR. Assorted interatomic distances (in Å) are indicated. Adapted with permission from ref 175. Copyright 2012 Royal Society of Chemstry.

TMPO molecules were confined in the micropores.175 In this case, a highly overlapping distribution of electron clouds in the confined geometry through van der Waals interactions may be inferred for the adsorbed TMPO molecules, which in turn, led to an enhancement in the extent of protonation for the TMPOH+ complexes, hence, the notable shift of the 31P resonance toward further downfield. Thus, it is conclusive that the confined micropores and/or nanochannels possessed by zeolitic catalysts may affect not only the configurations of the protonated adsorption complexes and electronic distributions but also the Brønsted acidities and catalytic activities. 4.2.6. Nano-Sized and Hierarchically Structured Zeolites. Zeolites, which possess unique microporous structures at nanoscale, have been extensively used as adsorbents for gas adsorption/separation or as heterogeneous shape-selective catalysts for various chemical/petrochemical reactions.178,179 However, hampered by steric constraints and diffusion limitation in the microporous pores/channels of zeolitic catalysts, adsorption and catalytic conversion of bulky molecules particularly those with sizes exceeding 1.5 nm are unlikely to occur. Moreover, mass transport limitations occur in the micropores tend to induce formation of cokes during catalytic reactions, leading to secondary or undesirable side reactions on the external surfaces of zeolite catalysts.90,178,179 In any case, catalytic reactions that take place on the external surfaces of zeolites are highly inefficient due to the lack of available active

Figure 37. 31P MAS NMR spectra of TMPO adsorbed on (a) H-ZSM-5, (b) H-ZSM-5-M, and (c) H-ZSM-5-OM catalysts. The asterisk denotes spinning sidebands. Adapted from ref 186. Copyright 2015 American Chemical Society.

5 zeolite and its mesoscopic H-ZSM-5-M and H-ZSM-5-OM counterparts typically exhibit multiple characteristic resonances with δ31P at 82−86, 76, 70, 65, and 51−53 ppm, revealing the presence of acid sites with a distribution of acid amounts and acidic strengths. As supported by results obtained from theoretical calculation, the resonance at 86 ppm observed for H-ZSM-5 should be associated with the Al12−O24−Si12 bridging hydroxyl site in the 10-MR structure.60 Note that this peak with δ31P of 86 ppm observed for microporous H-ZEM-5 with a relative concentration of ca. 14.3%, which represents acid sites with ultrastrong acidic strength close to superacidity, shifted toward upfield with a less acidic amount for mesoporous HZSM-5-M (85 ppm; 9.9%) and H-ZSM-OM (82 ppm; 4.1%). This indicates that a marginal decrease in guest/host interactions between TMPO and the porous zeolite adsorbent, while the corresponding pore structure of the support changed from micropores to mesopores. Meanwhile the relative concentration of the resonance corresponding to acid sites with weakest acidic strength at ca. 51−53 ppm increased notably from 16.4% for HZSM-5 to 25.0% and 36.5% for H-ZSM-5-M and H-ZSM-5-OM, 12504


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Figure 38. (A) 31P MAS NMR spectra of triphenylphosphine oxide (TPPO) adsorbed on (bottom, purple curve) Al-MCM-41 with Si/Al = 17, (middle, back curve) ordered mesoporous zeolite synthesized using the C18−6−6−18 surfactant with Si/Al = 15, (middle, blue curve), bulk BEA zeolite with Si/Al = 15, and (top, red curve) nanosponge BEA zeolite synthesized with N6-diphe surfactant with Si/Al = 14. (B) 31P MAS NMR spectra of TMPO, and (C) TBPO adsorbed on various samples, including Al-MCM-41 (MCM-41), nanosheet MFI zeolite with a thickness of 2.5 nm (NS-2.5), organosilanedirected MFI zeolite with a wall thickness of 10 nm (OS-10), desilicated MFI zeolite with a framework thickness of 20 nm (DZ-20), MFI zeolite nanoparticle with a thickness of 40 nm (NP-40), and conventional MFI zeolite with particle size exceeding 300 nm (CB-300). Adapted from ref 181. Copyright 2013 American Chemical Society.

gave rise to five overlapping resonances in the δ31P range of 55.7−34.0 ppm (top red curve, Figure 38A), which were notably more intense compared to the conventional microporous BEA zeolite (middle blue curve, Figure 38A). For the latter, since the size of the TPPO molecule is too large to enter the 12-MR pore aperture of microporous beta zeolite, the probe molecule can only be adsorbed on the external surfaces of the sample. In terms of acidic strength, it is indicative that the mesoporous BEA, which showed 31P resonance of TPPO with the highest δ31P at 55.7 ppm, surpassed that of mesoporous zeolite synthesized by the C18−6−6−18 surfactant (upper most δ31P at 47.3 ppm) and conventional AL-MCM-41 (44.3 ppm) with a similar Si/Al ratio. As such, these mesoporous zeolites are found to exhibit excellent catalytic activities for Friedel−Crafts alkylation of aromatics.181 The above results obtained from the 31P-TPPO NMR also revealed that TPPO is a suitable probe molecule for probing hierarchical zeolites comprising of both micro- and mesoporosities. However, in this context, the TPPO molecules are capable of probing only BA sites within the internal mesopores and on the external surfaces of the substrate rather than in the micropores of the zeolite walls. Moreover, the acid properties of a series of 10-MR mordenite framework inverted (MFI) zeolites (such as ZSM-5) with various wall thicknesses and particle sizes were also characterized by means of 31P-R3PO NMR approaches using TMPO and TBPO as the probe molecules.181 Again, for microporous zeolites with only 10-MR pore apertures, while TMPO probe molecule is capable of probing acid sites in both internal pore channels and external surfaces, the bulkier TBPO can only be adsorbed on external acid sites.56 Thus, by combining results obtained using TMPO and TBPO approaches, the relative distributions of internal and external acid sites may be determined, as shown in Figure 38 (panels B and C). The solid acid catalysts examined

respectively. The above results reveal that the H-ZSM-5-OM catalyst with aligned pore characteristics possessed a greater amount of exposed acid sites in the open mesopores than the HZSM-5-M and the conventional H-ZSM-5. This unique feature was found to facilitate accessibility of acid sites to bulky reactant molecules during catalytic conversions.186 As mentioned earlier, the catalytic activity of a reaction system is dictated not only by the diffusivities of the reactants in the catalyst but also by the accessibilities and acidic strengths of acid sites available for catalyzing the reaction. In this context, the syntheses of layered and/or nanosheet zeolites with hierarchical microporous and mesoporous structures, which possess highly stable structural frameworks and accessible active sites on the external surfaces of the catalysts to render efficient conversion of bulky reactants, have drawn considerable R&D interests.187−189 To afford acidity characterization of hierarchical zeolite, 31P SSNMR of triphenylphosphine oxide (TPPO, KD ca. 1.02 nm) and tributylphosphine oxide (TBPO; KD ca. 0.82 nm) have been exploited. As shown in Figure 38A, 31P NMR spectrum of TPPO adsorbed on conventional Al-MCM-41 (Si/Al = 17; bottom spectrum, purple curve) showed two main overlapping resonances with 31P at 44.3 and 32.9 ppm, both of which were attributed to TPPOH+ associated with framework BA sites.181 On the other hand, spectrum of TPPO adsorbed on a hexagonally ordered mesoporous zeolite synthesized by using C18H37−N+(CH3)2−C6H12−N+(CH3)2−C6H12−N+(CH3)2− C18H37 (termed as C18−6−6−18) surfactant as the structural directing agent revealed overlapping peaks at 47.3, 45.4, 43.1, and 34.7 ppm (middle black spectrum, Figure 38A), indicating a broad distribution of Brønsted acidity in mesoporous molecular sieve (MMS) materials, as confirmed by theoretical DFT calculations, while the spectrum corresponding to mesoporous beta (BEA) zeolite synthesized by using the N6-diphe surfactant 12505


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include a mesoporous Al-MCM-41 aluminosilicate (denoted as MCM-41), a nanosheet MFI zeolite with a thickness of 2.5 nm (NS-2.5), an organosilane-directed MFI zeolite with a wall thickness of 10 nm (OS-10), a desilicated MFI zeolite with a framework thickness of 20 nm (DZ-20), a MFI zeolite nanoparticle with a thickness of 40 nm (NP-40), and a conventional MFI zeolite with particle size exceeding 300 nm (CB-300).181 As shown in Figure 38B, four 31P resonances with δ31P at 66 (I), 68 (II), 76 (III), and 86 (IV) ppm were observed for TMPO adsorbed on BA sites of MFI zeolites with nanoparticle (NP-40) and nanosheet (NS-2.5) morphologies. Note that two additional peaks at 43 and 30 ppm due to crystalline and physisorbed TMPO (cf. Figure 32 and Table 4), respectively, were also observed. On the other hand, in the case of the TBPO probe, three peaks at 72, 74, and 92 ppm associated with external BA sites were observed in addition to the crystalline TMPO signal at ca. 52 ppm. Since an intrinsic δ31P offset of 8 ± 2 ppm is anticipated between the TMPO and the TBPO probe molecules when probing a given BA site,56,60−63 the three peaks observed at 72, 74, and 92 ppm for TBPO adsorbed on external BA sites should have acidic strengths comparable to sites I, II, and IV observed for the adsorbed TMPO. It is clear that, regardless of the size and morphology of the MFI zeolite, the 31P resonance observed for adsorbed TMPO at 76 ppm should be exclusively associated with internal BA sites. The above results are in excellent agreement with previous experimental and theoretical studies.56,61 Accordingly, the fraction of external versus total BA sites in each catalyst may be derived by their intensity ratio (i.e., Aex/Atot). Upon closer examination of results observed for various samples in Figure 38 (panels B and C), it is indicative that a consistent increase in the observed Aex/Atot ratio with decreasing thickness of the zeolite framework may be inferred. For example, the Aex/Atot ratio increased from ca. 5% of the CB300 to 32% of the NS-2.5. Moreover, the amount of external BA sites having the strongest acidic strength (corresponding to δ31P at 86 ppm for adsorbed TMPO and 92 ppm for adsorbed TBPO) in various samples follows the trend: NS-2.5 > OS-10 > NP-40 > DZ-20 > CB-300. Moreover, a strong correlation between the concentration of these strong external BA sites and catalytic activity during decalin cracking was observed over the nano-MFI zeolite.181 More recently, the acid properties of a new nanosheet zeolite called MIT-1, fabricated by one-pot synthesis using a rationally designed organic structure-directing agent (denoted as Ada-i-16; see Figure 39) of delaminated MWW zeolite have also been investigated by 31P-TMPO/TBPO NMR approaches.180 MWW zeolites are known to possess two independent 10-MR channels and 12-MR supercages. The MIT-1 nanosheet was found to show superior catalytic activity during the Friedel−Crafts alkylation of benzene with benzyl alcohol than conventional MWW zeolites (such as MCM-22 and MCM-56) by at least three-fold. As shown in Figure 40 (panels a−c), the 31P NMR spectra observed for TMPO adsorbed on MCM-22, MCM-56, and MIT-1 revealed multiple resonances with δ31P at 85, 72, 68, 63, and 53 ppm, along with two peaks responsible for crystalline (42 ppm) and physisorbed (31 ppm) TMPO. On the other hand, the corresponding spectra observed for the adsorbed TBPO also resonances ca. 75 and 73 ppm, together with the peaks for crystalline (52 ppm) and physisorbed (41 ppm) TBPO (Figure 40, panels d−f). An additional peak at ca. 57 ppm was also observed for the MCM-56 sample. By comparing the two sets of 31 P NMR results obtained from two different probe molecules, it is indicative that the peaks at 75 and 73 ppm of adsorbed TBPO

Figure 39. Schematic representation of synthesis route for the MIT-1 nanosheet zeolite. Adapted with permission from ref 180. Copyright 2015 Royal Society of Chemstry.

Figure 40. 31P MAS NMR spectra of TMPO adsorbed on (a) MCM-22, (b) MCM-56, (c) MIT-1, and TBPO adsorbed on (d) MCM-22, (e) MCM-56, and (f) MIT-1. A Lorentzian method was used for spectral deconvolution. Adapted with permission from ref 180. Copyright 2015 Royal Society of Chemstry.

correspond to that of 68 and 63 ppm of adsorbed TMPO. In this context, the other three peaks at 85, 72, and 53 ppm of TMPO should be solely due to internal acid sites. Note that the peak at 63 ppm has been previously attributed to TMPO adsorbed on LA sites (see Table 3).163 Moreover, by comparing the results obtained from TBPO-adsorbed MIT-1 with conventional MCM21, a notable increase in the intensity of the peak at 73 ppm is evident, while the peak at 75 ppm greatly decreases. Likewise, variations of peak intensities may also be inferred for the case of adsorbed TMPO. Further measurements by elemental analysis showed that the amount of external acid sites may be directly deduced from the peak distribution of the TBPO results after excluding contributions from crystalline and physisorbed TBPO. Accordingly, the amount of internal acid sites may readily be derived by subtracting the external amount obtained above from the corresponding total concentration of each peak obtained from adsorbed TMPO (after excluding contributions from crystalline and physisorbed TMPO). Accordingly, the total amounts of acid sites were found to be ca. 46, 32, and 33 × 10−5 mol g−1 for MCM-22, MCM-56, and MIT-1, respectively. In addition, an amount of external acid sites determined for MIT-1 12506


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Figure 41. Optimized configurations of TMP adsorbed on various Lewis structures in H-beta zeolites (see text), including type I: 3-fold-coordinated framework Al species (LI1, LI2, LI3, and LI4), type II: extra-framework Al species (LII1, LII2), and type III oxo-Al species (LIII1, LIII2, LIII3, and LIII4). Two energy minima were found for the Al(OH)3 species; they are denoted as LIII1 and LIII1′. Adapted with permission from ref 195. Copyright 2009 Elsevier B.V.

was 21 × 10−5 mol g−1, which is greater than that observed for conventional MCM-22 by more than three-fold (6 × 10−5 mol g−1) and ca. 2-fold compared to MCM-56 (13 × 10−5 mol g−1). In other words, the amount of external acid sites, which is responsible for the catalytic activity observed during the Friedel− Crafts alkylation reaction, increased notably in MIT-1 nanosheet zeolite, revealing the presence of acid sites in the 12-MR sidepockets where the reaction is most likely to occur.57,180 4.2.7. Characterization of Lewis Acidity. As discussed in Section 2, while the 31P-TMPO NMR is a useful and practical approach for characterization of BA sites in acid catalysts,61−63 the 31P-TMP NMR approach is more suitable for probing LA sites.52 Hierarchical porous materials, which possess prolific micro- and mesoporosities, are advantaged by the high accessibility and diffusivity available for the reactant/product molecules, hence, rendering enhanced activity during catalytic reactions.189 For example, hierarchical SSZ-13 zeolite, which possesses large chabazite cages (7.3 × 12 Å2) connected by small 8-MR pores (3.8 × 3.8 Å2), was found to show improved catalytic

performance during methanol-to-olefins (MTO) reaction.190 Since the molecule of TMP (KD ca. 0.55 nm) is considerably larger than the pore aperture of conventional SSZ-13 zeolite (0.38 nm), no 31P resonance was observed for the TMP-loaded SSZ-13, as expected. On the other hand, a 31P resonance signal at ca. −4.5 ppm due to TMP adsorbed on BA sites was observed for the hierarchical SSZ-13 zeolite, indicating the presence of larger pores.190 The hierarchical SSZ-13 was prepared by an one-step synthesis route using N,N,N-trimethyl-1-adamantanammonium hydroxide (TMAdOH) as the structure directing agent (SDA) and organosilane and a diquaternary ammonium-type surfactant (namely, C22−4−4Br2) as mesoporogens. Through spectral and quantitative analyses, the authors further concluded that the amount of Brønsted acidity in the mesopores of hierarchical SSZ13 is proportional to the ratio of the amounts of incorporated mesoporogens and SDA.190 In addition to the 31P-TMPO NMR approach, TMP has also been employed as a probe molecule for characterizing acid property of various solid acid catalysts, including microporous 12507


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zeolite (e.g., H-ZSM-5,191 H−Y,192,193 H-mordenite,51 H-beta,51 and TS76) and mesoporous aluminosilicates (such as Al-SBA1579 and Al-MCM-41194). For example, by combining the 31PTMP NMR approach with conventional 27Al and 1H MAS NMR, Luo et al.194 investigated the acid properties of Al-MCM-41. For pure siliceous MCM-41, the 31P spectrum of adsorbed TMP exhibited a single resonance at −59 ppm, which may be ascribed due to physisorbed TMP. On the other hand, the spectra of TMP adsorbed on a series of Al-MCM-41 with varied Si/Al ratios (from 16 to 80) revealed two distinct signals at −4 and −59 ppm. The intensity of the peak at −4 ppm was found to increase with increasing Al content in Al-MCM-41 materials, implying the progressive increase in the concentration of BA sites (see section 2.1). Note that no signal corresponding to TMP adsorbed on LA sites was present in Al-MCM-41. If the LA sites were present, additional 31P resonance with δ31P at around −32 to −58 ppm should be observed. The absence of Lewis acidity in Al-MCM-41 is supported by the 27Al MAS NMR result, which exhibited no extra-framework Al (EFAL) species in the samples. As mentioned earlier in section 2.1, the 31P-TMP NMR approach is inferior for differentiating BA sites, owing to the fact that the δ31P observed for the protonated TMPH+ adducts (at ca. − 5 ppm) is insensitive to Brønsted acidic strength. Moreover, in the absence of proton decoupling, during acquisition of the 31P spectrum, the peak at −4 ppm split into a well-resolved doublet arising from direct 31P−1H dipolar coupling of the TMPH+ adsorption complexes. Accordingly, a coupling constant, JP−H, of ca. 450 Hz was determined for the Al-MCM-41, which is slightly smaller than Al-SBA-15 (470 Hz)79 and H-ZSM-5 (480 Hz)75 and H-mordenite (493 Hz)51 zeolites. It has been proposed that the coupling constant associated with the 1H−31P coupling may be used to assess the average Brønsted acidity of solid acids.51,79 The larger the J coupling constant, the stronger the Brønsted acidity. In other words, the average Brønsted acidity of the following acid catalysts obeys the ascending order: Al-MCM-41 < Al-SBA-15 < H-ZSM-5 < H-mordenite. The acidic properties of H-beta zeolite have also been studied by Bao and co-workers195 using the 31P-TMP NMR approach in conjunction with theoretical calculations. To avoid chemical exchange among different TMPH+ adsorption complexes, the authors performed the experiment at −100 °C. In addition to the 31 P resonance with δ31P at a −4.5 ppm due to TMPH+ associated with BA sites, the 31P NMR spectrum of TMP adsorbed on Hbeta zeolite showed two additional weak resonances at −32.0 and −47.0 ppm. The latter two signals were attributed to TMP bound to LA sites in H-beta zeolite.50 To gain further information on the two different structures of Lewis acidities, theoretical DFT calculations were invoked. In this context, three different categories of LA sites in zeolites have been proposed previously,48,196,197 which are termed as types I, II, and III, respectively. Type I Lewis structures are associated with framework 3-fold-coordinated Al species, which may be formed through hydrolysis of the bridging hydroxyls (Si−O−Al).198 As illustrated in Figure 41, the 3-coordinated Al sites (i.e., Al5, Al8, and Al1) corresponding to Lewis structure and designated to be LI1, LI2, and LI3, respectively. An additional Al−OH species, in which the Al atom is jointly bonded with two framework oxygen and one hydroxyl (O−H), may also be formed through hydrolysis of the Si−O−Al group. This type of Lewis structure is referred to as LI4. Type II Lewis structures are linked to extraframework Al species (EFAL), which may be formed when zeolites are subjected to severe treatment conditions such as steaming or acid washing. As such, monovalent EFAL species

such as AlO+ and Al(OH)2+ both belong to type II Lewis structure and are denoted as LII1 and LII2, respectively.199 Moreover, type III Lewis structures involve oxo-Al species such as Al(OH)3, AlO+, AlOH2+, and Al(OH)2+, which normally show synergistic effects with BA sites48,64,200 and are termed as LIII1, LIII2, LIII3, and LIII4, respectively. All structural parameters as well as corresponding adsorption energies (Eads) for TMP adsorbed on various LA centers of H-beta zeolite during the DFT calculations are listed in Table 5. The predicted 31P chemical shift Table 5. P−H Distance, Adsorption Energy (Eads), and Theoretical (δcal) and Experimental (δexp) 31P Chemical Shifts Corresponding to TMP Adsorbed on Various Lewis Acid Sites of H-Beta Zeolitea Lewis site LI1 LI2 LI3 LI4 LII1 LII2 LIII1 LIII1′ LIII2 LIII3 LIII4

P−Al (Å)

Eads (kcal mol−1)

δ31Pcal (ppm)

2.428 2.428 2.435 2.437 2.442 2.432b 2.431 2.502 2.449 2.514 2.533

−37.7 −36.8 −36.5 −27.6 −28.0 −5.0 −47.0 −13.3 −36.9 −6.9 −15.0

−48.9 (−39.6) −48.8 −49.5 −50.2 −55.5 (−43.4)c −55.8 (−57.3)c −46.9 −45.3 −46.1 −35.2 −45.2

δ31Pexp (ppm) c

−47.0 −60.0 −47.0

−32.0 −47.0

The δcal values were obtained at the B3LYP/6-31G(d, p) and HF/ DZVP2 levels.195 bThe P−H distance (see Figure 41). cThe 31P chemical shifts obtained at the HF/DZVP2 level of theory.

a

(δ31Pcal) values for TMP adsorbed on respective Lewis acid sites (in parentheses) are −48.9 (LI1), −48.8 (LI2), −49.5 (LI3), −50.2 (LI4), −55.5 (LII1), −55.8 (LII2), −46.9 (LIII1), −45.3 (LIII1′), −46.1 (LIII2), −35.2 (LIII3), and −45.2 (LIII4) ppm, respectively. Thus, it is indicative that the 31P resonances observed with δ31Pexp at −47.0 ppm is most likely due to 3-foldcoordinated framework Al species (LI3) or extra-framework Al species interacting with BA sites such as Al(OH)3 (LIII1, LIII1′), AlO+ (LIII2), and Al(OH)2+ (LIII4),195 while the 31P resonance observed at −32.0 ppm should be associated with AlOH2+ species (LIII3). Apart from the presence of the Lewis acidity in conventional aluminosilicate zeolites, titanium silicalites, TS-1, which show prospective applications as efficient catalysts for selective redox reactions under mild conditions,201 it has been demonstrated that Lewis acid centers in TS-1 zeolites tend to form acid−base adducts with the oxidizing agents, which in turn enhance the electrophilicity of the oxidant to catalyze the oxidation reaction.202 By means of the 31P-TMP NMR approach, Bao and co-workers76,203−205 have make intensive efforts in investigating the role of Lewis acidity during oxidation reactions over TS-1 zeolite catalysts. Figure 42A displays the 31P MAS spectra of TMP-loaded TS-1 zeolite samples. The spectrum of freshly TMP adsorbed TS-1 revealed distinct resonances with δ31P at −4.8, − 32.0, − 34.2, and −59.8 ppm, as shown in Figure 42A(a). The peak at −4.8 ppm may be unambiguously assigned to TMPH+ species associated with BA sites in TS-1, while the peak at −59.8 ppm may be a priori ascribed due to weak physisorbed TMP, since it vanished completely when the sample was subjected to a thermal desorption (for 0.5 h) at temperatures beyond 373 K [Figure 42A(d−f)]. In addition, the authors 12508


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343 K (Figure 42B), the intensities of the peaks associated with LA sites (δ31P at −32.0 and −34.2 ppm) gradually decrease with the extent of oxygen exposure, meanwhile a new signal at 53.7 ppm emerged, indicating the oxidation of TMP to form TMPO.204 Similar results were observed when exposing the TMP-loaded TS-1 in urea hydrogen peroxide, which is a wellknown oxidizing agent consisting of equal amounts of hydrogen peroxide and urea.205 4.3. Functionalized Porous Organic Catalysts

Polystyrene sulfonic acid resins (PS-SO3H) are important solid acids in industry and have been widely used for catalyzing reactions such as esterification, olefin hydration, and alkylation of phenols.207−211 However, although PS-SO3H catalysts are known to possess abundant acid sites, they are drawback by low thermal stability and surface area. To unravel these critical issues, Yang and co-workers210,211 developed a synthesis route to fabricate a novel solid acid composite with a double-shell nanostructure (DSN) via the self-assembly of PS-SO3H in confined nanospace of hollow silica nanospheres (Figure 43a). By invoking the 31P-TEPO NMR approach, the authors demonstrated that the acidic strength of the composite catalyst may be reversibly manipulated through morphology transformation from aggregated to swelling state of PS-SO3H within the confined nanospace of hollow silica nanospheres. Similar to TMPO, the δ31P of protonated TEPO (i.e., TEPOH+) is sensitive to acidic strength of BA sites. As shown in Figure 43c, the δ31P observed for TEPO adsorbed on SO3H acid sites of commercial Amberlyst-15 catalyst was 89.4 ppm (bottom spectrum, Figure 43c), similar to the threshold value for superacidity (92 ppm),60,61,63,212 whereas the corresponding 31 P spectrum observed for TEPO adsorbed on the PS-SO3H@ mesosilicas DSNs revealed a broad resonance centering further downfield of 90.3 ppm, indicating a relatively stronger acidic strength than Amberlyst-15. The above results clearly indicate an

31

Figure 42. (A) In situ P MAS NMR spectra of TMP-loaded TS-1 zeolite (a) freshly TMP adsorbed sample at room temperature, and partially TMP desorbed samples for 0.5 h at (b) 333 K, (c) 353 K, (d) 373 K, (e) 393 K, and (f) 413 K, respectively. (B) Variations of 31P MAS NMR spectra of TMP adsorbed on TS-1 zeolite obtained for (a) freshly adsorbed sample at RT, and after exposing to air at 343 K for (b) 10, (c) 20, (d) 30, (e) 40, and (f) 50 min. The asterisk (*) represents spinning sidebands. Adapted with permission from ref 204. Copyright 2004 Elsevier. Adapted with permission from ref 206. Copyright 2008 Elsevier B.V.

attributed the 31P resonances observed at −32.0 and −34.2 ppm to TMP adsorbed on two types of LA sites, namely Ti(OSiO3)4 and (OSiO3)3Ti(OH) species with the Ti atom at the T12 site, as determined based on DFT calculations.203 As the sample was subjected to a thermal desorption treatment for 0.5 h at different temperatures (333−413 K), in addition to the desorption of physisorbed TMP (δ31P at −59.8 ppm), the intensities of the peaks associated with LA sites (−32.0 and −34.2 ppm) gradually decrease with increasing temperature of desorption while the peak for BA sites (−4.8 ppm) remained practically unchanged at temperatures lower than 393 K.204 The above results reveal the stronger adsorption of TMP on BA than LA sites. Moreover, as the TMP-loaded TS-1 sample was progressively exposed to air at

Figure 43. (a) Schematic illustrations of the synthesis procedures for PS-SO3H@mesosilicas DSNs. (b) Variations of acidic properties of the composite material with different extent of sulphonation. Curve a, total sulfur content; curve b, acidic-exchange capacity; and curve c, molar ratio of sulfur to acidic exchange capacity. (c) 31P MAS NMR spectra of TEPO adsorbed on the PS-SO3H@mesosilicas DSNs (top spectrum a) before and (middle spectrum b) after the base-treatment by DMF, and (bottom spectrum c) corresponding spectrum of TEPO adsorbed on commercial Amberlyst-15. Adapted with permission from ref 210. Copyright 2014 Nature Publishing Group. 12509


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Figure 44. Illustrations of (A) chemical procedure for preparing sulfated MOF-808-2.5SO4 and (B) three possible types of BA sites in MOFs (c) 31P MAS NMR spectra of TMPO adsorbed on (a) MOF-808-P, (b) MOF-808-0.65SO4, (c) MOF-808-1.3SO4, (d) MOF-808-2.5SO4, and (e) MOF-8082.5SO4 exposed to atmospheric moisture. Experimental spectra are shown in black, while deconvoluted peaks are in red; their sums are in green. The asterisk denotes spinning sidebands. Adapted from ref 220. Copyright 2014 American Chemical Society.

on PDVB-SO3H shows two resonances with δ31P at 72 and 80 ppm due to the presence of BA sites with strong acidity. Upon further treatment by HSO3CF3, the resultant PDVB−SO3H− SO2CF3 composite resulted in a 31P spectrum of adsorbed TMPO with δ31P at 83 ppm, revealing the presence of much stronger BA sites. On the basis of correlation between the observed δ31P of TMPO with DPE (or PA) value shown in eq 8, the values of DPE for the above resonances may be quantified, which follow the descending order: 284 (72 ppm), 264 (80 ppm), and 256 kcal mol−1 (83 ppm). Since the smaller the DPE (or PA) value, the easier the Brønsted acidic protons may be deprotonated, thus, the stronger the acidic strength. Hence, the acidic strengths of the PDVB-SO3H were readily enhanced by grafting strong electron-withdrawing CF3 moieties. Such PDVBSO3H−SO2CF3 composite was found to show excellent catalytic activities for conversions of biomass.100 By the same token, enhanced acidity and catalytic performances were also observed for nitrogen (N)-doped porous carbons functionalized with acidic ionic liquids or sulfonic groups for the productions of biodiesel and fine chemicals.215−217 Sulfated metal−organic framework (MOFs) materials represent a novel class of solid acid catalysts with rich structural diversity, crystalline structure, and tunable porosity.218−220 Most stable MOF-based catalysts were prepared by utilizing sulfonated organic linkers or hydroxyl ligands coordinated to the metal sites in the framework.219 Owing to the poor thermal stability of MOFs, conventional acidity characterization method such as NH3-TPD is not feasible due to the fact that sample pretreatment at elevated temperatures may lead to disintegration of the organic framework. In a recent study, Jiang et al.220 utilized the 31PTMPO NMR approach for characterizing the acid properties of Zr(IV)-based MOF-808 microcrystalline powder (denoted as

effective enhancement in Brønsted acidic strength by confining the highly concentrated acidic SO3H in nanospace.210 However, after a base-treatment by N,N-dimethylformamide (DMF), the morphology of the composite sample changed from DSN to hollow nanostructures (denoted as DMF-PS-SO3H@mesosilicas HNs), while a slight decrease in δ31P toward upfield (88.0 ppm) was observed. This confirms an effective lowering of acidic strength caused by the base treatment. By further combining the 31 P-TEPO NMR results with that obtained from 1H NMR, the authors were able to confirm that the hydrogen-bonding interactions tend to increase with decreasing acidic strength of the composite catalyst. More recently, it was demonstrated that the acidic strengths of these solid acid composites may be manipulated by varying the structural morphology of the confined nanospace (hence the aggregation degree of PSSO3H) of the [email protected] As monitored by the 31PTEPO NMR approach, the authors concluded that the PSSO3H@SiO2 yolk−shell nanospheres show the strongest acidic strength, while its hollow nanostructured counterpart gave rise to the weakest acidic strength. Acid catalysts containing strong electron-withdrawing elements (such as fluorine in superacidic CF3SO3H and HF-SbF5) normally exhibit relatively stronger acidity compared to other mineral acids such as H2SO4 and HCl.213,214 Along the same track, Xiao and co-workers100 designed and fabricated a hydrophobic stable mesoporous polymeric solid acid with ultrastrong acid strengths by grafting a SO2CF3 group onto a network of mesoporous sulfonated poly(divinylbenzene), which was denoted as PDVB-SO3H. As evidenced by the 31P-TMPO NMR approach, a considerable enhancement in Brønsted acidic strength was found after treating the PDVB-SO3H with HSO3CF3. The 31P MAS NMR spectrum of TMPO adsorbed 12510


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Figure 45. 31P MAS NMR spectra (red curve) and deconvoluted resonances (black curve) of TEPO adsorbed on the pure carbon CT-1 and various carbon-silica materials (CST-y) prepared with varied H2SO4/organic precursor ratios. Adapted with permission from ref 222. Copyright 2014 Royal Society of Chemstry. 31

MOF-808-P) and sulfated MOF-808-P (denoted as MOF-808xSO4, where x represents the number of sulfate groups per secondary building units, see Figure 44A). The 31P spectra of TMPO adsorbed on the pristine MOF-808-P and the mildly sulfated MOF-808-0.65SO4 are rather similar (spectra a and b, Figure 44C); except for the peak at ca. 43 ppm due to physisorbed TMPO, two 31P resonances with δ31P at 62 and 56 ppm were observed, which may be assigned to TMPO adsorbed on two different weak BA sites. Upon increasing the amount of the SO42− sulfate group, a new resonance at 69 ppm emerged for the MOF-808-1.3SO4 and the MOF-808-2.5SO4 catalysts (spectra c and d, Figure 44C), indicating the formation of new BA sites with stronger acidic strength, as also evidenced by the superior catalytic performances observed in these two samples. The authors hypothesized that there are three different types of BA sites in MOFs, namely (1) encapsulated Brønsted acid molecules within the pores of MOFs, (2) ligated Brønsted acid groups on metal sites of secondary building units (SBUs), and (3) covalently bound Brønsted acid functional groups of organic linkers (see Figure 44B). However, upon exposing the MOF808-2.5SO4 sample to atmospheric moisture (spectrum e, Figure 44C), notable decreases in peak intensities of both resonances at 69 and 55 ppm were observed, indicating that the acidic strength of sulfated MOF materials is sensitive to water. Sulfonated porous carbon catalysts, which may be fabricated by a facile procedure at low cost, represent metal-free, highly stable solid catalysts with desirable acidities. For example, Hara et al.221 reported the synthesis of sulfonated carbon catalysts through sulfonation and carbonization of polycyclic aromatic hydrocarbons. The sulfonated carbon-based catalysts were found to possess a considerable amount of surface active sites with different functionalities and acidic strengths such as −COOH, −SO3H, and −OH bound ligands. By chemical activation of ptoluenesulfonic acid with H2SO4, Russo et al.222 prepared sulfonated carbon (CT-1) and carbon-silica materials (CST-y; where y is the mass ratio of H2SO4/organic precursor). The reported procedures allow for tailoring of surface compositions, texture, and acid properties of the materials to warrant stability of the solid acid catalysts against leaching issues. On the basis of the

P spectra of TEPO adsorbed on CT-1 and various CST-y catalysts shown in Figure 45, it is clear that the acid properties of the CT-1 and CST catalysts were largely influenced by the preparation conditions. Up to six 31P resonances with δ31P spanning over the range from ca. 50 to 100 ppm were observed for all CST materials. These resonances from the adsorbed TEPO may be categorized to acid sites with weak (55−65 ppm), medium (65−75 ppm), strong (75−92 ppm), and ultrastrong (≥92 ppm) acidic strengths. It is noteworthy that for the 31PTEPO NMR approach, the threshold δ31P value for superacidity should be ca. 92−94 ppm.61−64 Apparently, all sulfonated CSTs were found to possess BA sites with superacidic strengths. Moreover, the distribution and relative concentrations of BA sites with various acidic strengths may readily be obtained by the spectral deconvolution method. For example, for the mildly sulfonated CST-0.1, a small amount of BA sites associated with resonances with δ31P centering at 97 ppm should be superacidic, whereas more than 50% of the BA sites were found to possess only weak acidic strengths (61 ppm). On the other hand, while superacidic BA sites (at ca. 100 ppm) were also found in the CST-1 material, a substantial amount of ultrastrong BA sites (at ca. 88 ppm; 58%) were formed. It is obvious that increases in both the overall acidity and amount of BA sites with stronger acidic strengths with increasing H2SO4/organic precursor mass ratio (i.e., the y value) may be inferred, since the acidic strength of the functional groups on the surfaces of the carbon-based materials should follow the trend: OH < COOH < SO3H. The resonances at ca. 61 and 72 ppm were most likely associated with TEPO coupled with relatively weaker OH and COOH acid groups, respectively. While, those observed in the δ31P range of 75−85 ppm were assigned to TEPO interacting with the stronger SO3H groups, whereas those with δ31P exceeding 92 ppm are attributed to TEPO adsorbed on sulfuric ester groups with superacidity stronger than SO3H. Sulfonic acid functionalized mesoporous organosilicas have also drawn considerable R&D attention lately.223−226 For example, through the functionalization of mesoporous benzenesilicas with propylsulfonic acid, Siegel et al.224 reported the synthesis of water-tolerant Ph-PMO-PrSO3H solid acid catalysts, 12511


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In particular, in addition to the 31P resonances centering at 60.4 and 72.9 ppm representing the presence of weak and medium BA sites, an additional peak at 94.2 ppm was observed for all TEPOloaded PMO-1 samples, revealing the presence of superacidic BA sites. It is noteworthy that the signal intensity of the resonance at 94 ppm remained practically unchanged upon varying the SO3H loading. On the other hand, notable increases in the acid sites corresponding to medium acidic strengths (at 72.9 ppm) were observed with increasing loading of propylsulfonic acid. The above results reveal that while the presence of surface SO3H moieties warrants the formation of superacidic BA sites, increasing its concentration leads only to an overall increase in acid sites with medium strengths. As shown above, the incorporation of sulfonic moieties onto the inert silica or carbon tends to greatly improve the acid properties, hence the catalytic activity of the functionalized inorganic catalysts. To further optimize the thermal and mechanical stability of the functionalized inorganic solid acids, several new strategies have been developed, for example, by fluoridation of sulfonic polymers227 or through an organic− inorganic hybridization scheme.228 Perfluorinated Nafion-silica nanocomposite represents one of the nice example for the entrapment of acidic organosulfonic functionalities in the mesostructured silicas.229 An alternative approach is to incorporate the organosulfonic functional groups on porous carbons, which can be achieved by direct high-temperature carbonization of sulfoaromatic hydrocarbons under inert environment.221 To compare the performances of various functional groups with different chemical characteristics, the acidic properties of various sulfonated solid acid catalysts were characterized by the 31 P-TEPO NMR approach. As shown in Figure 47, catalysts such

which showed superior activity during dehydration of fructose in neat water with high turnover frequencies compared to regular sulfonated periodic mesoporous organosilicas (PMOs) such as SBA-SO3H, MCM-SO3H, and HMS-SO3H. Again, the 31PTEPO NMR approach was invoked to probe subtle differences in local acid properties of various sulfonated PMOs, as shown in Figure 46.225 Unlike the sulfonic acid-free Ph-PMO, which exhibited only a single 31P resonance with δ31P at ca. 58.5 ppm due to TEPO adsorbed on weakly acidic silanol groups, the surface modified PMO-1 functionalized with different amounts of propylsulfonic acid revealed a broad overlapping 31P resonances with δ31P spanning from 60 to 95 ppm, indicating the presence of an inhomogeneous distribution of sulfonic sites.

Figure 47. Illustrations of typical sulfonated solid acid catalysts. Adapted from ref 230. Copyright 2015 Elsevier Inc.

as Nafion-silica nanocomposite (SAC-13) with a Hammett acidity function (H0) of −11 to −13, Dowex 50W × 2 (D50) and Amberlyst-15 (A15), propylsulfonic silica (PSS) and Deloxan ASP I/9 (DI9), sulfonated hydrothermal carbon (SHTC), and sulfonated carbon nanofibers (SCNF) were examined.230,231 Among them, D50 and A15, both of which have an H0 value of ca. −2.2, represents functionalization of aromatic organosulfonic groups on polystyrene gel-type resin with a cross-linking degree of divinylbenzene of 2% and 20%, respectively, while PSS and DI9, which both have a H0 value of ca. −3, represent surface functionalization of inorganic silica by aliphatic sulfonic groups. SHTC and SCNF were prepared by sulfonating hydrothermal carbon (HTC) derived from glucose and carbon nanofibers, respectively.216

Figure 46. (Top) Schematic representations of the procedure for preparing the sulfonic acid functionalized PMO-1 materials. (Bottom) 31 P MAS NMR spectra of TEPO adsorbed on the parent Ph-PMO, PMO-1a−c functionalized with 1.11, 0.56, and 0.36 mmol g−1 of propylsulfonic acids, and siliceous Si-PMO-1c with a propylsulfonic acid loading of 0.34 mmol g−1. Adapted with permission from ref 225. Copyright 2014 Royal Society of Chemstry. 12512


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Figure 48. 31P MAS NMR spectra of TEPO adsorbed on various solid acid catalysts prepared by adsorption of probe molecule in the presence of different solvent solutions: (A) C6H14 on (a) silica gel, (b) PSS, (c) DI9, (d) D50, (e) A15, and (f) SAC-13; (B) CH2Cl2 on (a) SElor, (b) HTC, (c) CCR, and (d) SHTC; and (C) MeOH on (a) SElor, (b) HTC, (c) CCR, (d) D50, and (e) SHTC. Adapted from ref 230. Copyright 2015 Elsevier Inc.

(51.7), HTC (57.5), CCR (72.1; 55.3), and SHTC (58.0), respectively (Figure 48B). Among them, SElor and CCR represent sulfonated activated carbons and carboxylicpolyacrylic gel-type resin, respectively. Surprisingly, sulfonated carbons such as SElor, HTC, and SHTC only revealed weakly adsorbed TEPO on weak surface hydroxyls, leading to resonances with δ31P smaller than that observed for silica gel (62.3 ppm) and CCR (72.1 ppm). A possible reason for the weak acidity observed for these sulfonated carbons is the limited accessibility for the TEPO probe molecule to the acid sites in micropores of the carbon substrates when adsorbed in the presence of nonpolar solvent, as illustrated in Figure 49. On the other hand, when TEPO is adsorbed onto various catalysts in the presence of a polar solvent such as MeOH, notable differences in the corresponding 31P NMR spectra were observed for: SElor (48.2), HTC (63.7), CCR (64.6), D50 (88.3), and SHTC (88.2; 68.8), respectively (Figure 48C). In particular, 31P resonances corresponding to TEPO adsorbed on strong BA sites were observed for D50 and SHTC. Meanwhile, a broad shoulder peak centering at 68.8 ppm was also observed for the SHTC catalyst, indicating the

The effect of solvent for adsorption of probe molecule onto various sulfonated solid acids during acidity characterization by means of the 31P-TEPO NMR approach was investigated by Fraile et al.230 The authors examined the 31P NMR spectra of various TEPO-loaded solid catalysts prepared by using nonpolar hexane (C6H14) and dichloromethane (CH2Cl2) and polar methanol (MeOH) solvent solutions, as shown in Figure 48. In the presence of C6H14 solvent, the adsorption of TEPO onto various solid acids resulted in 31P NMR spectra with respective δ31P (in parentheses) ranging from weak (55−65 ppm) and medium (65−75 ppm) to strong (75−92 ppm) acidity: silica gel (62.3 ppm), PSS (74.6), DI9 (76.4), D50 (87.8), A15 (88.0), and SAC-13 (89.5), as shown in Figure 48A. Among them, the 31P resonance observed at 62.3 ppm was assigned to the TEPO bound to silica gel through weak hydrogen-bonding with surface hydroxyl (−OH) groups, whereas the resonances observed in other sulfonated solid acids were attributed to TEPOH+ through adsorption of the probe molecule on BA sites. Likewise, for adsorption of TEPO in nonpolar CH 2Cl2 solvent, the corresponding δ31P observed for various solids are SElor 12513


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Figure 49. Illustrations of solvent effect on adsorption of TEPO and corresponding formations of (left) ultramicropores in the presence of nonpolar solvent and (right) open pores in the presence of polar solvent (methanol). Adapted with permission from ref 231. Copyright 2014 Elsevier Ltd.

adsorption of TEPO onto weak carboxylic and/or phenolic groups on the surfaces of the sulfonated carbon substrate. Note that, the 31P resonance signal observed for TEPO adsorbed on HTC carbon in the absence of sulfonic functional groups remained unchanged regardless of the solvent used. Thus, it is indicative that the presence of polar solvent during adsorption of TEPO onto sulfonated carbon catalyst tends to provoke pore opening to render accessibility of surface sulfonic groups by the probe molecule, as also illustrated in Figure 49. This result is in agreement with the hydrophilicity of the solid acid catalyst, as confirmed by water adsorption.231 In this context, the adsorption of TEPO on SElor is clearly not affected by the presence of polar solvent; this is likely due to the stable microporosity possessed by the solid catalyst. The above results indicate that the presence of solvent during adsorption of probe molecule is crucial for acidity characterization of sulfonated carbon catalysts. The presence of polar solvent during adsorption of R3PO probe molecules may lead to breakage of hydrogen-bonded network of the sulfonated solid catalyst to render improved accessibility of sulfonic acid sites, hence, the enhanced catalytic performance of the acid catalyst.

Figure 50. 31P MAS NMR of TMPO adsorbed on H3PW12O40 (HPW) calcined at various temperatures. All spectra were obtained with a loading of ca. 3.0 TMPO per Keggin-unit, followed by a sample baking treatment at 150 °C prior to the NMR measurement. The asterisk represents spinning sidebands. Adapted with permission from ref 72. Copyright 2014 Elsevier Inc.

treatment (at temperature ca. 150 °C) are essential in achieving a homogeneous adsorption of TMPO over the available BA sites in HPW. Nevertheless, the effect of calcination temperature rather than baking treatment temperature is discussed here. It is noteworthy that there are three available protons (H+) in the Keggin-type HPW (H3PW12O40), these protons should be eliminated progressively with increasing calcination temperature.92 As shown in Figure 50, for TMPO-loaded samples calcined at temperatures below 350 °C, 31P resonances with δ31P exceeding 86 ppm were observed, indicating the presence of BA sties with superacidic strengths.60−63 Further increasing the calcined temperature led to consecutive removal of protic sites, hence, lowering of their acidic strengths. Eventually, as the calcination temperature reached 500 °C, degradation and collapse of Keggin-structured HPW is accompanied by complete diminishing of acidity, as anticipated. By incorporating the experimental NMR data with relevant NMR parameters obtained from theoretical DFT calculations, a detailed peak assignment of the seemingly complex 31P NMR spectra of adsorbed TMPO may be resolved, as demonstrated earlier for the system of 12-tungstophosphoric acid (H3PW12O40; HPW)92,232 and 12-molybdophosphoric acid (H3PMo12O40; HPMo).95 The 31P spectra of TMPO adsorbed on HPMo pretreated at different baking temperatures are shown in Figure 51. For calcined HPMo (at 250 °C) loaded with 2.5 TMPO per Keggin unit, the 31P NMR spectrum obtained after the baking treatment at 120 °C revealed 31P resonances mainly at different regions: region I (60−65 ppm) and region II (80−90 ppm), together with three additional peaks at 47, 57, and 77 ppm. While the peak at 47 ppm may be ascribed to physisorbed TMPO, the peaks at 57 and 77 ppm were most likely due to TMPO interacting with BA sites; however, their adsorption sites remained uncertain. Upon increasing the baking temperature to 160 °C, diminishing of signals at 47, 57, and 77 ppm were

4.4. Heteropolyacid-Based Catalysts

Keggin-type heteropolyacids (HPAs) are highly acidic heterogeneous/homogeneous catalysts comprising of heteropolyanions (XM12O40n−) with countercations (H+, H3O+, H5O2+, etc.), where X represents heteroatoms (e.g., P5+, Si4+, B3+, etc.) and M denotes metal addenda atom (e.g., Mo6+, W6+, V5+, etc.).232 HPA catalysts have been extensively employed as solid acid catalysts in various homogeneous solutions, liquid−solid, and gas−solid heterogeneous reactions due to their ultrastrong Brønsted acidity, high solubility in polar solvents, and pseudoliquid characteristics.232,233 Figure 50 displays the 31P MAS NMR of TMPO adsorbed on tungstophosphoric acid (H3PW12O40, denoted as TPA or HPW) calcined at various temperatures.72 The 31P resonances arising from the bare PW polyanions (PW12O403−) are located at the δ31P range of −10 ∼ −20 ppm, where the resonances in the range of 50−95 ppm are attributed to complexes of TMPO interacting with BA sites in HPW.92 On the basis of theoretical DFT calculations, the predicted δ31P values for TMPOH+ adducts arising from adsorption of TMPO on available Brønsted acidic proton sites of HPW are confined in the region of ca. 75−95 ppm, whereas 31P resonances occurring in the range of 50−75 ppm were attributed to adsorption of more than one TMPO probe molecule on one protic site on the surfaces of the HPW.92 Moreover, it has been demonstrated that an adequate amount of guest adsorbate as well as a sample baking 12514


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evident, while notable increases in signals over regions I and II were observed. Further increasing sample treatment temperature to 200 °C, signals in region II appeared to increase at the expense of those in region I. Possible adsorption structures of TMPO on HPMo have been investigated theoretically, and their corresponding δ31P were also predicted. On the basis of theoretical DFT calculations shown in Figure 52, the 31P resonances in region II were attributed to one and/or two TMPO molecules adsorbed per Keggin unit (KU) of HPMo [i.e., (TMPOH+)/KU and/or (TMPOH+)2/KU], whereas the signals in region I are assigned strictly due to (TMPO)2H+/KU in which two TMPO probe molecule are adsorbed on each BA site. Moreover, it was also confirmed that the peak observed at 47 ppm was due to physisorbed TMPO, while peaks at 57 and 77 ppm were assigned due to three TMPO adsorbed per Keggin unit [i.e., (TMPOH+)3/KU].

Figure 51. 31P MAS NMR spectra of TMPO adsorbed on H3PMo12O40 (loading: 2.5 TMPO per KU) under different sample baking treatments (at 120, 160, and 200 °C) for 8 h. Prior to the adsorption of TMPO, the HPMo catalyst was subjected to a calcination treatment at 250 °C to remove crystalline water. The asterisk represents spinning sidebands. Adapted from ref 95. Copyright 2010 American Chemical Society.

4.5. Homogeneous Acid Catalysts

As demonstrated above, the 31P-R3PO NMR approaches are versatile techniques for probing detailed acid properties of heterogeneous solid catalysts.60−64 Likewise, the 31P-TMPO

Figure 52. Optimized adsorption structures of 1, 2, and 3 TMPO (I, II, and III) adsorbed on different Brønsted acid sites based on the O1b2c and O2b1c models of H3PMo12O40 and that of 2.0 TMPO adsorbed on each Brønsted acid (IV) based on the O1b2c model. Selected interatomic distances (in Å) are depicted. Adapted from ref 95. Copyright 2010 American Chemical Society. 12515


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chemical shift observed at extremely dilute TMPO loading (i.e., at ρ → 0). Note that the δ0 value represents the intrinsic acidic strength of the aqueous acid sample in the absence of influence from the base probe molecule and may readily be obtained by extrapolating the straight lines in Figure 54a to the vertical ordinate (i.e., the δ31P axis at ρ = 0). Accordingly, the dependence of δ0 value with acid concentration of the liquid acid may be laid out, as shown in Figure 54b for variations of δ0 with the concentration of H2SO4, expressed as [H2SO4]. It is clear that the intrinsic acidic strength (i.e., the δ0 value) observed for the acid solutions increased initially with increasing [H2SO4] and then gradually leveled off at ca. 85−90 ppm. Similar observations was also observed for other liquid Brønsted acids, covering from weak, medium, strong, to superacids such as acetic acid (CH3COOH), phosphoric acid (H3PO4), nitric acid (HNO4), perchloric acid (HClO4), and triffic acid (CF3SO3H) etc.71 Interestingly, for the case of H2SO4, the observed δ0 value at 86 ppm (i.e., the threshold value for superacidity)60,61 corresponds to a [H2SO4] concentration of ca. 13 M, which is slightly larger than general cognition. Moreover, at extremely diluted acid system (e.g., [H2SO4] → 0), a δ0 value of 53.8 ppm was derived, which coincided with the extrapolated δ31P value observed for TMPO dissolved in water.71 It is noteworthy that the same extrapolated δ0 value (53.8 ppm) was observed for all acid sample systems mentioned above, as expected. The results illustrated above clearly revealed that the 31P-R3PO NMR approaches implemented originally for solid acids are not only applicable for liquid acids but also with enhanced sensitivity and extraordinary resolution and accuracy surpassing the traditional acidic scales measured by titration (pH) and color indicator (Hammett acidity function, H0). The 31P-TMPO NMR approach has also been applied for probing Lewis acid catalyst systems such as metal halides (e.g., AlCl3, BF3, and TiCl4), which play important roles in catalyzing homogeneous reactions involving carbon−carbon bond formations such as Friedel−Crafts alkylation and acylation of the aromatics.234,235 However, most metal halide Lewis acids tend to decompose into their corresponding metal hydroxides when in the presence of water, leading to the undesirable loss in original reactivity. Thus, the development and implementation of watertolerant Lewis acid catalysts are demanding for chemical industries.236−239 Utilizing the 31P-TMPO NMR approach, Koito et al.236 studied the formation of Lewis acidity in water for a series of trifluoromethanesulfonates (OTf) and chlorides containing various metal species such as Sc, Y, La, Lu, In, and Zn. For comparison, 31P spectra of TMPO dissolved in typical liquid Brønsted acids such as H2SO4 and H3PO4 were also acquired. With a fixed Lewis (or Brønsted) acid to TMPO concentration ratio (i.e., [acid]/[TMPO]) of 0.25, the 31P resonance observed for TMPO dissolved in D2O revealed a singlet peak with δ31P at 53.5 ppm (Figure 55a). Similarly, the spectra observed for TMPO dissolved in Lewis acids such as Y(OTf)3, La(OTf)3, Lu(OTf)3, Zn(OTf)2, YCl3, and LaCl3 all showed a sharp signal within the δ31P of 53−55 ppm, so do that observed for Brønsted acids such as H2SO4 and H3PO4. The above results indicate that, for these Lewis acid systems, the base TMPO probe molecules prefer to coordinate with D2O molecules than those metal OTf and metal chlorides. On the other hand, the corresponding δ31P values observed for TMPO dissolved in Sc(OTf)3, In(OTf)3, and ScCl3 were 64.5, 61.9, and 63.4 ppm, respectively, indicating a notable downfield shift in the 31P resonance. Taking Sc(OTf)3 as an example, the observed δ31P of TMPO increased rapidly with increasing acid concentration (i.e., [Sc(OTf)3]) initially and then

NMR approach has also been exploited for characterizing the acidity of homogeneous catalysts such as aqueous acid solutions69−71 and ionic liquid-based catalysts72,73 systems. It is noteworthy that, in the case of solid acids, the spectra obtained from the 31P-R3PO NMR approaches are capable of providing information not only on acid types but also detailed distribution of acid sites with various acidic strengths and concentrations through the measurements of δ31P and relevant peak areas via spectral analyses. In general, the 31P spectra obtained for R3PO dissolved in nonviscous liquid acids normally exhibit a singlet resonance due to rapid, random motions of the molecules, which in turn result in an effective average of dipolar interactions, hence, an averaged acidic strength. As such, unlike in solid acids for which broad, overlapping 31P resonances were commonly observed with a spectral resolution of ca. ± 2 ppm, a much better NMR sensitivity (i.e., higher signal-to-noise ratio) and spectral resolution (typically in hundredths ppm level) may be achieved for liquid acid systems. 4.5.1. Aqueous Acid Solutions. Taking sulfuric acid (H2SO4) as an example, all 31P spectra of TMPO dissolved in saturate (18.4 M) H2SO4 with varied probe molecule loadings indeed exhibited highly resolved singlet resonance. Moreover, the observed δ31P decreased monotonically from 89.37 to 86.51 ppm, while the loading of TMPO increased from 0.01 to 0.50 mol %, as shown in Figure 53.71 In other words, the overall acidity

Figure 53. High-resolution 31P NMR spectra of TMPO dissolved in saturated (18.4 M) sulfuric acid (H2SO4) with varied TMPO loadings (0.01−0.50 mol %; right). The sharp lines on the right-hand side with constant δ31P (0.72 ppm) indicate the reference signal of 85% H3PO4 in D2O, whereas that on the left-hand side represent the observed isotropic δ31P. Adapted with permission from ref 71. Copyright 2009 Chinese Culture University.

of the system decreased linearly with increasing concentration of the base TMPO probe molecule (see Figure 54), as expected. Moreover, the effects of probe molecular loading and acid concentration were further investigated, and the results are depicted in Figure 54a. Clearly, for H2SO4 with a given acid concentration, a linear correlation between the observed isotropic δ31P with the fraction of TMPO loading (i.e., TMPO/H2SO4) may be inferred. Such a linear correlation may be expressed by the linear equation: δ 31P(ρ) = δ0 + σsρ

(9)

where δ P(ρ) is the observed chemical shift at a given TMPO/ H2SO4 molar ratio (ρ), σs is the slope, and δ0 denotes the intrinsic 31

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Figure 54. (a) Variations of isotropic δ31P with varied amounts of TMPO dissolved in various aqueous H2SO4 solutions (0.15−18.4 M). (b) Correlation of the intrinsic chemical shift (δ0) with H2SO4 concentration. Adapted with permission from ref 71. Copyright 2009 Chinese Culture University.

Besides the observed δ31P of TMPO dissolved in aqueous LA system, which is capable of probing the exchange between TMPLA and TMP-water complexes, NMR line width analysis render determination of the exchange rates of TMPO with the LA and solvent (H2O) states at NMR time scale (typically, in the order of μs−ms).240 For example, Figure 57 displays the variabletemperature 31P NMR spectra of TMPO dissolved in aqueous Sc(OTf)3 and In(OTf)3 solutions with a loading ratio of [Sc(OTf)3]/[TMPO] = 1:4 and [In(OTf)3]/TMPO = 1:2, respectively, over the temperature range of 278−303 K. For aqueous Sc(OTf)3 LA system, the 31P spectrum of dissolved TMPO observed at 278 K revealed two resonances with δ31P at ca. 64 and 54 ppm, which may be assigned due to TMPOSc(OTf)3 and TMPO−D2O complexes, respectively [Figure 57a(A)]. For simplicity, the two resonances corresponding to the TMPO-Sc(OTf)3 and TMPO-D2O complexes are denoted as peak 1 and peak 2 with a chemical shift of δ1 and δ2, respectively. Upon increasing temperature from 278 to 303 K, notable line width broadenings were observed for both peaks 1 and 2, which are accompanied by a lowering of chemical shift difference (Δδ = δ1 − δ2) between them due to increases in the exchange rates (v12, v21) between TMPO-Sc(OTf)3 and TMPO-H2O, which may be defined as

Figure 55. 31P NMR spectra of TMPO dissolved in D2O solution of various Lewis and Brønsted acid catalysts at fixed acid concentration ([acid] = 0.25 M) and TMPO loading ([TMPO], [acid]/[TMPO] = 0.25): (a) neat TMPO in D2O in the absence of Lewis acid, (b) Sc(OTf)3, (c) Y(OTf)3, (d) La(OTf)3, (e) Lu(OTf)3, (f) In(OTf)3, (g) Zn(OTf)2, (h) ScCl3, (i) YCl3, (j) LaCl3, (k) H2SO4, and (l) H3PO4. Adapted with permission from ref 236. Copyright 2014 Elsevier.

v12

→ [(TMPO)m (OTf)x ] + n[H 2O]←[(TMPO)m (H 2O)n ]

level off at ca. 66.4 ppm when the Lewis acid to TMPO concentration ratio (i.e., [Sc(OTf)3]/[TMPO]) exceeded ca. 2.0 (see Figure 56). Similar conclusions may also be drawn for the In(OTf)3 Lewis acid system (Figure 56b). On the other hand, there were only marginal changes in δ31P of TMPO in Y(OTf)3, La(OTf)3, Lu(OTf)3, and Zn(OTf)2 Lewis acid (LA) systems. Thus, it is indicative that a strong affinity between TMPO probe molecule and LA may be inferred for Sc(OTf)3, In(OTf)3, and ScCl3 catalysts, forming stable TMPO-LA complexes in water. Such water-tolerant LA catalysis was found to exhibit superior catalytic performances in water.238,239 Apparently, the observed 31 P singlet resonance should be a weighted average of two interacting groups, namely the TMPO-solvent (D2O) and the TMPO-LA complexes, which are in fast-exchange with one another. Thus, the downfield shift in 31P resonance observed for the above three liquid LA systems with increasing acid concentration simply reflects that more weight was lean toward the TMPO-LA than the TMPO-solvent complex.

v21

+ [M(OTf)x ]

(10)

0 v21 = k 21 [(TMPO)m (H 2O)n ][M(OTf)x ]

= k 21[(TMPO)m (H 2O)n ]

(11)

where k12 and k21 are the apparent rate constants corresponding to the ligand exchange rate, which may be derived from the simulated NMR spectra based on the observed δ31P, resonance peak intensity, and ligand exchange rate. It has been shown that water content is an important parameter which may be used to characterize the catalytic activity of Lewis acid in the presence of water as solvent.239 Taking the experimental values of peak 1 and peak 2 with the respective chemical shift values of δ1 = 63.5 ppm and δ2 = 54.2 ppm and peak intensity 0.48 and 0.52 observed for the aqueous Sc(OTf)3 system at 278 K [Figure 57a(A)], the 12517


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Figure 56. (a) 31P NMR spectra of TMPO/D2O solution (0.25 M) (A) without and (B−F) with the presence of Sc(OTf)3 with different [Sc(OTf)3]/ [TMPO] ratios of (B) 1:2, (C) 1:1, (D) 2:1, (E) 4:1, and (F) 8:1. (b) Variations of δ31P of TMPO with varied molar concentration ratios of various [M(OTf)x]/[TMPO] (M = Sc, Y, La, Lu, In, and Zn). Adapted with permission from ref 238. Copyright 2014 Wiley-VCH.

Figure 57. 31P NMR spectra of TMPO/D2O solution in the presence of (a) Sc(OTf)3 with [Sc(OTf)3]/[TMPO] = 1:4 and (b) In(OTf)3 with [In(OTf)3]/TMPO = 1:2 at (A) 278 K, (C) 283 K, (D) 293 K, and (E) 303 K. Spectra (B) represent simulated spectra of (A). Adapted with permission from ref 238. Copyright 2014 Wiley-VCH.

values of k12 and k21 were derived to be 720 and 665 s−1, respectively, based on the simulated spectrum [Figure 57a(B)]. Similarly, corresponding values of k12 and k21 for the LA system of In(OTf)3 with [In(OTf)3]/[TMPO] = 1:2 were determined to be 2800 and 1575 s−1, respectively, at 278 K (see Figure 57(B)). The larger exchange rate constants observed for the In(OTf)3 LA system than Sc(OTf)3 indicates a slower substitution of D2O for Sc(OTf)3 with the interacting TMPO. For comparison, a typical exchange rate constant exceeding 105 s−1 was observed for the other metal triflates such as Y(OTf)3, La(OTf)3, Lu(OTf)3, and Zn(OTf)2, a value which is more than 2 orders of magnitude larger than those derived for Sc(OTf)3 and In (OTf)3 LA systems.

4.5.2. Determination of Gutmann Acceptor Number. It has been shown that the δ31P observed for liquid acids based on the 31P-R3PO NMR approaches are sensitive to the presence of an acceptor solvent.212,241,242 In this case, the oxygen atom on the trialkylphosphine oxides is prone to be affected by the solvent, resulting in an enhancement in positive charge of the neighboring phosphorus atom, hence, a downfield shift of the observed δ31P toward a larger value. Gutmann and coworkers241,242 performed a series of experiments on various solvents by using the 31P-TEPO NMR approach. The authors reported an empirical relationship for the acceptor properties of the solvent with the observed δ31P of dissolved TEPO, defined as the Gutmann acceptor number (AN; vide infra). From the 12518


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Figure 58. (a) Optimized structures of TEPO (Et3P = O)-adsorbed halogen and hydrogen-bonded complexes. Color codes: Cl (green), Br (red brown), I (purple), F (pale yellow), O (red), P (yellow), H (white), and C (gray). (b) Correlations between AN and the interaction energies (ΔE) calculated at different theoretical levels. Adapted with permission from ref 212. Copyright 2013 Elsevier B.V.

exothermic heat may be represented by the observed δ31P relative to a specific reference solvent. This concept has been verified by theoretical calculations based on several halogenated solvents and their perfluoro derivatives.212 Figure 58 displays the

viewpoint of thermodynamics, the AN number may be considered as the molar exothermic heat (in kJ mol−1) required for complexing the targeting solvent with the NMR probe molecule (i.e., TEPO) in a dilute solution. As such, the molar 12519


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example, Coffie et al.248 reported the synthesis of borenium ionic liquids (BCl3L) possessing Lewis superacidity by a solvent-free method utilizing two-electron L-type ligands (denoted as L) with differing aromatic nitrogen (N) donors such as pyridine (py), 3picoline (3pic), 4-pico-line (4pic), and 1-methylimidazole (mim), as illustrated in Figure 60a.248,249 Subsequently, the Lewis acidic strengths of these borenium ILs were characterized by the 31P-TEPO NMR approach.248 As shown in Figure 60b, Alcontaining mim-BCl3-2AlCl3 LAIL gave rise to two sharp 31P resonance with δ31P at 84.2 and 117.2 ppm, which were assigned to TEPO interacting with Al and borenium cations, and corresponds to an AN value of 96.2 ad 174, respectively. A similar 31P resonance at 116.3 ppm was detected for the [BCl3L]/2GaCl3 systems, which corresponds to an AN value of 173. The above results indicate the presence of Lewis superacidic sites in the tricoordinated borenium ILs [(mim)BCl3]/2MCl3 systems (M = Al and Ga). Moreover, it is found that the ratios of [BCl3L]/nMCl3 (n = 1−3; M = Al, Ga) strongly affect the Lewis acidic strength of the IL catalysts with the observed δ31P of TEPO spanning over the range of 94.5−119.2 ppm, corresponding to an AN of 120−172 depending on the loadings of metal halides (see Figure 60c and Table 6).248,249 Among them, the [BCl3L]/3GaCl3 systems, which showed the most downfield δ31P of TEPO at 119.2 ppm and a corresponding AN value of 182, represent borenium ILs with the strongest Lewis superacidity reported to date. By comparison, the other borenium IL systems such as [B(C6F5)2L]+ in CD2Cl2 (with δ31P at ca. 75−77 ppm and AN of 80−85) and [(cat)B]+ in C6D6 (δ31P at 106.9 ppm and AN = 155) show much lower Lewis acidities.251,252 Furthermore, it has been shown that couplings of the group 13 M(III) chlorides (M = aluminum Al3+, gallium Ga3+, or indium Id3+) with organic chloride salts (e.g., room temperature 1-ethyl3-methylimidazolium chloride IL) lead to the formation of acidic ILs, which are commonly used as solvents or acid catalysts.253−255 The effective molar fraction of monomeric metal chloride, χMCl3, defined as the ratio of metal chloride to organic chloride salts in ILs, plays a crucial role in affecting their acid properties.256 It is interesting that the intrinsic 31P chemical shift of TEPO, δinf (or δ0; see eq 9 in section 4.5.1), obtained at infinite dilute solution in MCl3 ILs (M = Al3+, Ga3+, and In3+), showed notably different dependence on χMCl3, as shown in Figure 61.257 In particular, a much higher δinf value was observed for the AlCl3 IL compared to its counterparts, GaCl3 and InCl3, at a given value of χMCl3 ≤ 0.5. However, for χMCl3 > 0.5, the δinf values observed for GaCl3 IL increased dramatically to reach a level similar to that of AlCl3. Whereas the δinf values observed for InCl3 IL system, through an increase gradually with increasing χMCl3 up to a value of ca. 0.50, it reached a plateau at χMCl3 > 0.5. Furthermore, on the basis of the δinf values observed for ILs containing different organic chloride salts, the corresponding AN values may readily be obtained for various IL systems, covering from weak, medium, strong, to superstrong Lewis acidity, as illustrated in Figure 62. Apart from the acidity of homogeneous Lewis acid (LA) catalysts, the AN values have also been exploited for determining the Brønsted acid-ionic liquid (BA-IL) mixtures (e.g., [C2mim][A]-HA, where A¯ = bistriflamide [NTf2]¯, triflate [OTf]¯, mesylate [OMs]¯, or acetate [OAc]¯, and [C2mim]+ = 1-ethyl-3methylimidazoliumcation).258 It was shown that the AN values so determined through measurement of δ31P based of dissolved TEPO based on the 31P-TEPO NMR approach were sensitive to the presence of anionic clusters, thus, representing a convenient approach for quantitative scaling of acidic strengths in IL catalysts.

optimized structures of TEPO-halogen (C−X···O = PEt3) and TEPO-hydrogen bonded (C−H···O = PEt3 ) complexes simulated by accurate MP2 and DFT calculations along with the correlations between AN and interaction energy (ΔE) predicted at different levels of calculations. Clearly, a linear correlation between ΔE and AN may be inferred regardless of the calculation methods, indicating that the AN value may readily be used as a quantitative scale for the acceptor ability of a solvent. As such, the stronger the interaction between the probe molecule (TEPO) and the solvent, the higher the observed δ 31P, hence the larger AN value. The effect of probe molecule loading on the observed δ31P of the TEPO-dissolved solvent systems has also been studied.242 As illustrated in Figure 59, the intrinsic 31P chemical shift, δ0, which

Figure 59. Variations of 31P NMR chemical shift of TEPO with its concentration in different acidic and alkaline-buffered melts: acidic (●), LiCl (■), NaCl (⧫), and KCl (×). Adapted from ref 242. Copyright 1997 American Chemical Society.

may be deduced by extrapolating the straight line observed for δ31P of TEPO versus its concentration (i.e., [TEPO]) at infinite dilution may be obtained for different solvents.242 Since AN is a dimensionless value, to correlate the acceptor properties of the solvent with the corresponding intrinsic chemical shift (δ0) observed through the 31P-TEPO NMR approach, Gutmann and co-workers defined the so-called Gutmann acceptor number (AN) by the empirical relation:243 AN =

100x(δ 31P)0 (δ 31P)SbCl5

= 2.348(δ 31P)0 (12)

The authors developed a scale for AN from 0−100 by taking the δ31P value obsereved for hexane (41 ppm) as 0 and that of SbCl5 (in 1,2-dichloroethane; 86.1 ppm) as 100. Later, this approach was originally implemented to offer a quantitative scaling for electrophilic characteristics of solvents and was further extended to include Lewis acids.244−248 As a result, liquid acids with AN > 100 are generally considered as Lewis superacids. Table 6 shows the assorted AN values and corresponding δ31P values for various solvents and liquid acid catalysts available in the reported literature. Thus far, scaling of acceptor ability of solvents and/or liquid Lewis acids by Gutmann acceptor number (AN) values, which may be inferred from measurements of intrinsic chemical shift (δ0) based on the 31P-TEPO NMR approach, has been wellestablished.241−243 Recently, some interesting works on Lewis acidic ionic liquids (LAILs) invoking borenium cations as Lewis metal centers have been reported and employed for C−H borylation and alkene hydroboration reactions.247,249,250 For 12520


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Table 6. 31P Chemical Shift (δ31P) Obtained by the 31P-TEPO NMR Approach and Corresponding Acceptor Number (AN) of Various Solvent and Lewis Acids

a

solvent/Lewis acid

δ31P

AN

reference

solvent/Lewis acid

δ31P

AN

reference

hexane (reference solvent) diethyl ether tetrahydrofuran (THF) benzene carbon tetrachloride (CCl4) diglyme hexamethylphosphoramide (HMPA) dioxane acetone N-methyl pyrrolidionone dimethylacetamide (DMA) pyridine nitrobenzene (NB) benzonitrile (BN) dimethylformamide (DMF) dichloroethylene carbonate pyridinium dichromate (PDC) CH3CN dimethyl sulfoxide (DMSO) CH2Cl2 nitromethane (NM) CHCl3 i-propanol ethyl alcohol formamide (FA) methyl alcohol acetic acid water SbCl5 in dichloroethane (reference LA) CF3COOH CH3SO3H CF3SO3H

− − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − −

0.0 3.9 8.0 8.2 8.6 10.2 10.6 10.8 12.5 13.3 13.6 14.2 14.8 15.5 16.0 16.7 18.3 19.3 19.3 20.4 20.5 23.1 33.5 37.1 39.8 41.3 52.9 54.8 100.0 105.3 126.1 129.1

244 244 244 244 244 244 244 244 244 244 244 244 244 244 244 244 244 244 244 244 244 244 244 244 244 244 244 244 244 244 244 244

(Me2N)3B (nBuO)3B (EtO)3B (MeO)3B (nBuO)3B3O3 BH3·THF (tetrahydrofuran) (MeO)3B3O3 BF3·Et2O BCl3 BBr3 Bl3 B(C6F5)3 AlCl3 BBr3 Me3Si(O3SCF3) F2B(O3SCF3) Et3Si[CbBr6] CatB(O3SCF3)a CatB[CbBr6]a py/AlCl3a py/GaCl3a 3pic/GaCl3a 3pic/AlCl3a 4pic/AlCl3a 3pic/2GaCl3a 4pic/2GaCl3a py/2AlCl3a 4pic/2AlCl3a 3pic/2AlCl3a mim/2GaCl3a mim/2AlCl3a mim/3GaCl3a py/3GaCl3a 3pic/3GaCl3a 4pic/3GaCl3a

45.1 46.3 48.7 51.4 70.3 75.8 76.4 80.9 88.7 90.3 92.9 76.6 80.3 90.3 92.8 84.6 91.2 85.4 106.9 94.5 94.7 95.5 95.5 96.2 99.6 101.7 109.6 112.5 113.2 116.3 117.2 117.4 119.2 119.8 119.9

9.1 11.8 17.1 23.1 65.0 >77.1 78.5 88.5 105.7 109.3 114.9 − − − − − − − − 120 121 121 124 124 135 140 162 170 170 173 174 175 180 182 181

247 247 247 247 247 247 247 247 247 247 247 248 248 248 248 249 250 250 250 251 251 251 251 251 251 251 251 251 251 251 251 251 251 251 251

Abbreviations: Cat = catecholato, C6H4O22−; py = pyridine; 3pic = 3-picoline; 4pic = 4-pico-line; mim = 1-methylimidazole.

hydrogen sulfate ions HSO4−, a maximum value of PPS/TPA ratio, y, of 3.0 is anticipated; however, a total occupancy of Brønsted acidic H+ in TPA by the HSO4− normally results in complete loss in catalytic acidity.72 To investigate the influence of the pretreatment temperature (x) and PPS/TPA ratio (y) on the acidity of PPS-TPA-x-y, 31P-TMPO NMR approach has been used. As shown in Figure 64, in addition to the sharp 31P resonance of TMPO dissolved in the PPS-TPA IL with δ31P of ca. 77−84 ppm due to the formations of TMPOH+ complexes in the presence of BA sites, two additional peaks at 0.71 and −14.7 ppm were observed. The latter two peaks, which are invariant with x and y, may be assigned to signals arising from internal reference (85% H3PO4 in D2O) and the PO43− tetrahedra of the Kegginstructured TPA, respectively. First, the effect of TPA calcination pretreatment temperature on Brønsted acidic strength was investigated. In the absence of calcination pretreatment, the 31P spectra observed for PPS-TPA-RT-2.5 (Figure 64a) and PPSTPA-RT-3.0 (Figure 64b) were nearly identical, revealing the invariant downfield δ31P resonance at 77.8 ppm regardless of the change in PPS/TPA ratio from 2.5 to 3.0. As mentioned earlier, a sample calcination pretreatment of HPAs at ca. 250 °C for at least several hours is required in order

4.5.3. Ionic Liquid-Heteropolyacid Hybrids. A new series of catalysts, which combine acidic ionic liquid (IL) with heteropolyacid (HPA) to form an organic−inorganic composites, have been developed and utilized for conversion of biomass.72,259−263 Owing to the “green solvent” nature of ILs254−256 and “pseudoliquid” and superacidic characteristic of HPA,264,265 the IL-HPA hybrid catalysts are highly soluble to water and other polar solvents. As such, these water-tolerable ILHPA hybrids may be utilized not only for heterogeneous catalysis but also for homogeneous reactions if carried out under proper solvents.259 For example, by immobilizing tungstophosphoric acid (H3PW12O40, TPA) with zwitterionic pyridinium propyl sulfobetaine (PPS) IL immerged in acetic acid (HOAc), Liu and co-workers have synthesized a series of PPS-TPA composite catalysts (see Figure 63) and exploited them for catalytic acetylation of glycerol (GL).72 The acid properties of these water-tolerable organic−inorganic hybrid catalysts were characterized by using the 31P-TMPO NMR approach. The 31P NMR spectra obtained for typical catalysts, denoted as PPS-TPA-x-y, where x represents the calcination temperature (in °C) and y denotes the PPS/TPA molar ratio, are shown in Figure 64. In view of the three protons on TPA available for bonding with the 12521


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Figure 61. Plot of intrinsic δ31P of TEPO, δinf, obtained at infinite dilute solution in MCl3 ILs incorporated with M = Al3+ (●), Ga3+ (⧫), and In3+ (▲). Dashed lines at χMCl3 = 0.33, 0.50, and 0.67 facilitate comparisons of the crucial compositions. Adapted with permission from ref 257. Copyright 2010 Royal Society of Chemistry.

Figure 62. Comparisons of AN values observed for assorted common solvents, acids, and 1-octyl-3-methylimidazoliumchlorometallate(III) systems. Adapted with permission from ref 257. Copyright 2010 Royal Society of Chemistry.

Figure 60. (a) Structures of borenium ILs consisting of a borenium cation with different anions, (b) 31P NMR spectra of TEPO dissolved in [(mim)BCl3]/2MCl3 (M = Al, Ga) borenium ILs with a TEPO loading of 1 mol %, and (c) Gutmann acceptor number (AN) for various borenium ILs synthesized with various ligands (L) and anions. Adapted with permission from ref 248. Copyright 2015 Wiley-VCH.

Figure 63. Schematic of the TPA-immobilized PPS composite catalyst.

to remove crystalline water,92,265 hence, to enhance the acid strength of the IL-TPA catalysts. As illustrated in Figure 64 (panels c−e), at a fixed PPS/TPA ratio (y = 2.0), the observed 31 P resonance responsible for BA site gradually shifted toward downfield from δ31P of 82.9 to 83.6 ppm as the calcination temperature (x) of TPA increased from 100 to 300 °C, indicating a gradual increase in acidic strength. On the other hand, the PPS/ TPA molar ratio also has a positive effect on δ31P, which in turn results in an increase in acidic strength. For the catalyst prepared with a fixed x = 300 °C while varying y from 2.0 to 3.0, the observed δ31P of the TMPOH+ complexes increased slightly from 83.6 to 83.9 ppm (Figure 64, panels e−g). Clearly, while both of the calcination pretreatment temperature (x) of TPA and PPS/TPA ratio (y) can mediate the acid strengths of PPS-TPA catalysts, the former is more prominent to determine their acidities. Moreover, the effect of water concentration ([H2O]) on acidic strength of the PPS-TPA hybrid pretreated at 100−300 °C with PPS/TPA ratio at 2.0 was also investigated, as depicted

in Figure 65a. A linear correlation between the observed δ31P with [H2O] may be inferred. The results observed for various samples nearly coincide with one another, indicating that [H2O] has an overwhelming effect over the acidic strength of the catalyst compared to the calcination pretreatment temperature of TPA. Moreover, by extrapolating the linear curve to nearly null [H2O] → 0, an intrinsic δ0 value of 83.6 ppm may be derived for the series samples. Comparing to the effect of [H2O], concentration of acetic acid ([HOAc]) clearly exhibit a much weaker dependence on acidic strength of the catalyst (Figure 65b). Aside from the study of IL-TPA hybrids, the same 31P-TMPO NMR approach has also been exploited to probe acid properties of IL liquids with coexisting Brønsted and Lewis acidities.73,266

5. SUMMARY AND OUTLOOK We have demonstrated herein various acidity characterization techniques by means of direct and probe molecule NMR 12522


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Figure 64. (a−g) 31P NMR spectra of TMPO dissolved in various PPS-TPA-x-y catalysts (TMPO/TPA = 0.1) prepared with varied pristine TPA calcination temperature (x = 100−300 °C) and PPS/TPA ratio (y = 2.0−3.0). For spectra (a) and (b), no calcination pretreatment was made for the pristine TPA (denoted as x = RT). All spectra were acquired with catalysts dissolved in HOAc. Adapted with permission from ref 72. Copyright 2014 Elsevier Inc.

of proton affinity or deprotonation energy) together with the wide detection range from weak, medium, strong, to superacidity, indeed is a versatile technique for qualitative and quantitative determinations of detailed acid features. Detailed acid properties, viz. type, concentration, distribution, and strength of acid sites may be simultaneously obtained, as illustrated herein for a wide variety of solid and liquid acid catalysts. In view of practical applications, while the 31P-R3PO NMR approach has been applied in many homogeneous catalysts systems, there lacks a comprehensive and robust methodology in incorporating the NMR data with some of the existing methods for acidity scaling by pH, pKa, and H0 values, which heavily rely on aqueous titration and color indicator methods. In view of the excellent signal sensitive and extraordinary spectral resolution warranted by high-resolution NMR spectroscopy, it is highly anticipated that such a unique, reliable, and versatile technique should shed new light in future perspective applications in acid−base reaction systems, particularly in homogeneous catalysis.

AUTHOR INFORMATION Figure 65. Correlations of δ31P of dissolved TMPO with varied (a) water and (b) acetic acid concentrations over assorted PPS-TPA catalysts. Adapted with permission from ref 72. Copyright 2014 Elsevier Inc.

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

methods. In view of the accuracy, reliability, and detectable range of the methodology invoked, and relevant operation complexity and cost, it is conclusive that 31P NMR chemical shifts of phosphorus probes represent a versatile approach for acidity characterization for both heterogeneous and homogeneous catalysts, surpassing most conventional methods, such as titration, Hammett acidity function, NH3-TPD, and FT-IR methods. Compared with other acidity characterization methods invoking NMR spectroscopy, such as 1H and 13C NMR of base organic probe molecules, the 31P NMR approach based on phosphorus probes is more practical in terms of signal sensitivity, resolution, and reliability. In particular, the 31P-R3PO NMR approach in which a linear correlation is present between the observed 31P chemical shift and acidic strength (scaled by means

Anmin Zheng: 0000-0001-7115-6510 Feng Deng: 0000-0002-6461-7152 Notes

The authors declare no competing financial interest. Biographies Anmin Zheng obtained his Ph.D. (2005) from the Wuhan Institute of Physics and Mathematics (WIPM), Chinese Academy of Sciences (CAS), under the supervision of Prof. Feng Deng and was affiliated with WIPM, CAS as an assistant research fellow in the same year. In 2007, he was a visiting research fellow in Prof. Shang-Bin Liu’s lab at Institute of Atomic and Molecular Sciences, Academia Sinica. He was promoted to full professor in 2012. His past and current research interests have been 12523


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DQ = double quantum DMF = N,N-dimethylformamide DMQ = dimethylquinoline DPE = deprotonation energy DSN = double-shell nanostructure EFAL = extra-framework aluminum FAL = framework aluminum FT-IR = Fourier-transformed infrared H0 = Hammett acidity function HDO = hydrodeoxygenation HETCOR = heteronuclear correlation HOAc = acetic acid HPA = heteropolyacid HPW = H3PW12O40 HPMo = H3PMo12O40 IL = ionic liquid INEPT = insensitive nuclei-enhanced by polarization transfer KD = kinetic diameter KU = Keggin unit LA = Lewis acid LG-CP = Lee−Goldburg-cross-polarization MAS = magic-angle spinning MCl3 = metal chloride MMS = mesoporous molecular sieve MOR = modenite zeolite M/SZ = metal-promoted sulfated zirconia MTO = methanol-to-olefins NP = nanoparticle OTf = trifluoromethanesulfonate PA = proton affinity PMO = periodic mesoporous organosilicas PPh3 = triphenylphosphine PPS = pyridinium propyl sulfobetaine py-d5 = deuterated pyridine py-IR = pyridine infrared REDOR = rotational echo double resonance R3PO = trialkylphosphine oxides RT = room temperature SDA = structure directing agent SQ = single quantum SSNMR = solid-state nuclear magnetic resonance SZ = sulfated zirconia TBPO = tributylphosphine oxide TEOA = triethanolamine TEPO = triethylphosphine oxide (O = PEt3) TMP = trimethylphosphine TMPO = trimethylphosphine oxide TOPO = trioctylphosphine oxide TPA = tungstophosphoric acid TPD = temperature-programmed desorption TPPO = triphenylphosphine oxide TRAPDOR = transfer of population in double resonance UV−vis = ultraviolet−visible VO = oxygen vacancy XRF = X-ray fluorescence

focusing on the studies of structure and reaction mechanism of solid acid catalysts by means of experimental solid-state NMR and theoretical quantum chemical calculations. Shang-Bin Liu is an emeritus research fellow and professor of Institute of Atomic and Molecular Sciences (IAMS), Academia Sinica (AS), Taiwan. He obtained his M.S. (1982) and Ph.D. (1985) in the Department of Physics, College of William and Mary in Virginia, under the supervision of Prof. Mark S. Conradi. After his postdoctoral research (1985−1987), which was jointly appointed by the Department of Chemistry, University of California, Berkeley (under Prof. Alexander Pines) and the Department of Geology, Stanford University (under Prof. Jonathan F. Stebbins), he was affiliated with IAMS, AS, Taiwan as an Associate Research Fellow (1987−1993) and Research Fellow (1993−2017). He was an adjunct professor in the Department of Chemistry, National Taiwan Normal University (2009−2016). His past research interests include developments of solid-state NMR techniques for porosity and acidity characterization of porous catalytic and adsorptive materials as well as synthesis, modification, and characterization of novel porous carbon materials and organic−inorganic composites as catalysts, supports, and/or adsorbents for applications in energy (fuel cells, supercapacitors etc.), environment (CO2 capture, de-NOx, biosensors etc.), and conversion of biomass. Feng Deng obtained his B.S. (1988) in Department of Chemistry, Sichuan University, M.S. (1991) and Ph.D. (1996) from Wuhan Institute of Physics and Mathematics (WIPM), Chinese Academy of Sciences (CAS), under the supervision of Profs. Chaohui Ye and Youru Du. After his postdoctoral research (1997−1998) in the Department of Chemistry, Texas A&M University (with Prof. James F. Haw), he has been affiliated with WIPM, CAS as a full professor (1999 to present). His research interests include solid-state NMR methods and their applications to heterogeneous catalysis and chemistry of functional materials.

ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (Grants 91645112, 21522310, 21473244, 21210005, and 21733013), Key Research Program of Frontier Sciences, CAS (Grant QYZDB-SSW-SLH026), and Ministry of Science and Technology, Taiwan (NSC-104-2113-M-001-019). SYMBOLS AND ABBREVIATIONS aB, aH+, aBH+ = activity of a base probe molecule (B), proton (H+), and BH+ complex, respectively [B], [BH+] = concentration of B and BH+, respectively ΔE = interaction energy Δδ = chemical shift range δ = chemical shift δiso = isotropic chemical shift δ0, δint = intrinsic chemical shift ρ = guest molecule loading γB, γBH+ = thermodynamic activity coefficient of B and BH+, respectively k12, k21 = apparent rate constants Ka = equilibrium constant v12, v21 = exchange rates AN = acceptor number BA = Brønsted acid BE = binding energy BEA = beta zeolite CP = cross-polarization CSA = chemical shift anisotropy DFT = density functional theory

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