Direct Measurement of Zeolite BrÃ¸nsted Acidity by FTIR Spectroscopy

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Direct Measurement of Zeolite Brønsted Acidity by FTIR Spectroscopy. Solid-State H MAS NMR Approach for Reliable Determination of the Integrated Molar Absorption Coefficients 1

Anton A. Gabrienko, Irina G. Danilova, Sergei S. Arzumanov, Larisa V. Pirutko, Dieter Freude, and Alexander G. Stepanov J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07429 • Publication Date (Web): 18 Oct 2018 Downloaded from http://pubs.acs.org on October 19, 2018

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The Journal of Physical Chemistry

Direct Measurement of Zeolite Brønsted Acidity by FTIR Spectroscopy. Solid-State 1H MAS NMR Approach for Reliable Determination of the Integrated Molar Absorption Coefficients Anton A. Gabrienko,*,†,‡ Irina G. Danilova,† Sergei S. Arzumanov,†,‡ Larisa V. Pirutko,† Dieter Freude,§ Alexander G. Stepanov*,†,‡ †

Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, Prospekt Akademika Lavrentieva 5, Novosibirsk 630090, Russia

‡

Novosibirsk State University, Faculty of Natural Sciences, Department of Physical Chemistry, Pirogova Street 2, Novosibirsk 630090, Russia §

Universität Leipzig, Fakultät für Physik und Geowissenschaften, Linnéstr. 5, 04103 Leipzig, Germany

ACS Paragon Plus Environment

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ABSTRACT: FTIR spectroscopy is broadly applied nowadays for probing concentration and the strength of acid sites in zeolite catalysts. Accuracy of direct determination of the quantity of different hydroxyl groups by FTIR method suffers from uncertainty of the integrated molar absorption coefficients, ε. The values of ε reported by different authors might differ by an order of magnitude. This paper provides an approach for reliable determination of the integrated molar absorption coefficients by combining 1H MAS NMR and FTIR spectroscopic techniques. The concentration of Brønsted acid sites for the series of H-ZSM-5 and H-ZSM-23 zeolite samples with different Si/Al ratio has been reliably established with 1H MAS NMR using methane and benzene as internal standards adsorbed on the studied samples. The data on the obtained concentration were further used to analyze same zeolite samples with FTIR spectroscopy and derive the information on the values of the integrated molar absorption coefficients. The coefficients ε have been reliably determined to be 3.06 ± 0.04 and 1.50 ± 0.06 cm µmol–1 for the IR bands at 3605–3615 cm–1 and 3740–3747 cm–1, respectively. ε values are similar for both HZSM-5 and H-ZSM-23 zeolites. It is also established that the ε values are constant with respect to the concentration of hydroxyl groups for H-ZSM-5 and H-ZSM-23 zeolites. The determined coefficients ε can be further used for reliable assessment of zeolite Brønsted acidity with the aid of the widely available and relatively simple methodology of FTIR spectroscopy.


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1. INTRODUCTION Zeolites are crystalline microporous materials, which are used for numerous commercial applications like detergents, adsorbents and catalysts. Application of zeolites in heterogeneous catalysis benefits from their unique microporous nature and strong Brønsted acidity. The combination of microporosity and acidity provides conversions of diverse compounds, including petroleum feedstock, synthesis gas, methanol and methane, to more valuable chemicals and motor fuels.1-3 The zeolite frameworks FER, MFI, MOR, BEA and FAU, referred to as the “Big Five”, are the most frequently used catalyst components for different processes.2 Recently, the catalysts based on ZSM-23 zeolite (framework type MTT) have demonstrated their excellent potential for the hydroisomerization of diesel fuels4-5 and heavier crude oil fractions.6 The acid properties of these zeolites were widely investigated. In particular, their Brønsted acidity due to bridging hydroxyl groups7-10 of Si–O(H)–Al type11-17 attracts much attention, since it determines the activity and selectivity of zeolite-based catalysts for acid-catalyzed reactions such as cracking, isomerization, oligomerization.10 Bridging hydroxyl groups are located inside the zeolite pores or channels between silicon and aluminum atoms of the zeolite framework. They can be considered as a negative charged AlO4 fragment and a positive ion H+. Recent NMR and FTIR experiments with BEA zeolites have revealed one more possible type of strong Brønsted acid sites (BAS), which has been related to silanol Si–OH groups within zeolites.18 Those strong BAS were detected by an IR band at ca. 3740 cm–1 and 1H NMR signal at ca. 2.1 ppm. The nature of strong Brønsted acidity of particular type of Si–OH groups remains unclear though a plausible explanation has been provided similar to that for amorphous silica-alumina.19-20


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For adequate prediction of zeolite catalytic properties, it is crucial to have a reliable and feasible approach to measure the quantity (concentration) of different BAS taking into account inhomogeneous properties of these surface sites.21

Several methods for zeolite acidity

characterization has been developed, including TPD, chemical methods, FTIR spectroscopy of adsorbed probed molecules (CO, NH3, pyridine) and solid-state 1H MAS NMR.10, 22 Each of the methods has its own advantages and disadvantages with respect to complex properties of zeolites and, particularly, BAS: inhomogeneous properties of the surface sites, their possible inaccessibility for probe molecules, presence of extra-framework species (Na+, alumina), adsorbed water. For example, TPD method can not distinguish between Brønsted and Lewis acid sites.23 Being able to characterize the nature, quantity and the strength of BAS, a powerful method of FTIR spectroscopy of adsorbed basic molecules (pyridine, NH3, etc.)24 meets specific challenges related to reliable assignment of the observed IR bands from adsorbed species like the identification of BAS which are different in chemical nature (i.e. Si–OH, Si–O(H)–Al or Al– OH), but similar in strength and accessibility of BAS for probe molecules.25-26 The solution for the issue of an adequate assessment of the Brønsted acidity of the zeolites is the use of a method which provides the opportunity to analyze the types of BAS present in the structure of a zeolite and to measure BAS concentration directly, i.e., without using probe molecules. Furthermore, such approach has to be broadly available and relatively simple for implementation. FTIR spectroscopy (in transmission mode) of zeolite hydroxyl groups, detecting the bands of corresponding stretching νO–H vibrations, matches these requirements and lacks disadvantages inherent to the indirect methods. FTIR spectroscopy has relatively high sensitivity and can detect various types of hydroxyl groups and, hence, BAS of the zeolites.9,

24, 27

The assignment of the νO–H bands, regularly


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monitored for different zeolites, to particular types of hydroxyl groups or BAS is quite well established and accepted.7, 28-36 The crucial point that the Beer-Lambert-Bouguer law has to be used for quantitative analysis based on the integrated intensities of the bands. This raises a task on the experimental determination of the values of corresponding integrated molar absorption coefficients ε. There have been several attempts to evaluate the values of the coefficients ε for the IR bands (3600–3650 cm–1) from Si–O(H)–Al sites in the structure of various zeolites.35-46 However, the range of obtained values is too broad for a proper analysis of BAS concentration: 3.1–19.9 cm µmol–1 for H-Y zeolite, 2.5–4.65 cm µmol–1 for H-MOR zeolite, and 3.7–11.2 cm µmol–1 for HZSM-5 zeolite of FAU, MOR and MFI framework types, respectively. To the best of our knowledge, there have been no efforts undertaken to determine the integrated molar absorption coefficients for the IR bands (3740–3745 cm–1) from silanol Si–OH sites of any zeolite yet, though the value of the coefficient ε3745 for silica has been obtained to be ca. 3 cm µmol–1.47 Such situation can be explained by complex, laborious and, which is more important, indirect methodology applied for the determination of BAS concentration. Therefore, the accuracy of the concentration measurements is not high enough and can vary significantly depending on the experimental conditions. So, it is not surprising that there is such large deviation for the obtained values of the coefficients ε since these values were, in some cases, calculated based on the imprecise reference, i.e. poorly measured BAS concentration. Hence, there is a need for reliable approach to be developed for the determination of the values of the coefficients ε for the IR bands of hydroxyl groups of the zeolites with high accuracy. To our understanding, this can be achieved by applying direct and reliable method to assess the concentration of different BAS in the structure of a zeolite.


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Solid-state 1H (MAS) NMR can provide credible data on the quantity of various hydroxyl groups. This advanced method proved to be capable of studying solid acid catalysts such as zeolites and metal oxides.17, 48-53 Moreover, the application of sealed glass ampules to record 1H MAS NMR spectra of zeolites with or without adsorbate helps to fully control the composition of the catalyst surface and adsorbed species on it.54 In particular, it is possible by using this NMR approach and different activation procedures (temperature, time, etc.) to obtain and study specific hydroxyl coverage of the zeolite surface.18, 55-57 Thus, 1H MAS NMR spectroscopy of a zeolite, activated and sealed inside glass ampule, provides the direct data on the type and quantity of hydroxyl groups. Hence, it is an interesting and exciting challenge, which has not been addressed yet, to apply 1

H MAS NMR and FTIR spectroscopy techniques jointly to study Brønsted acidity of the

zeolites in terms of a comprehensive determination of the integrated molar absorption coefficients ε for νO–H bands, based on reliable determination of the concentration of corresponding O–H groups by 1H MAS NMR. Present work aims at the demonstration of this opportunity to obtain complementary NMR and FTIR data on the concentration of BAS and the integrated molar absorption coefficient values, which provide the basis for direct and reliable measurement of zeolite Brønsted acidity by FTIR spectroscopy.

2. EXPERIMENTAL SECTION Zeolite samples and their characteristics. Zeolite H-ZSM-23 of MTT framework type was represented by three different samples. Sample MTT-1 was manufactured by Zeolyst Corp. (Si/Al = 15). Samples MTT-2 and MTT-3 were prepared by hydrothermal synthesis according to the procedure reported in Refs.58-59 The obtained zeolites were transformed into acid-form via the


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repeated ion exchange in NH4NO3 solution followed by calcination at 500 °С for 2 hours. HZSM-5 zeolite of MFI framework type (sample MFI-1, Si/Al = 13) was kindly provided by Tricat Zeolites. Two other H-ZSM-5 zeolites (samples MFI-2 and MFI-3 with Si/Al = 25 and 40, respectively) were purchased from Zeolyst Corp. as the brands CBV-5524G and CBV-8014, respectively. The MFI-type zeolites were calcined at 500 °С for 2 hours in air. Chemical analysis, ICP (Inductively Coupled Plasma) Spectroscopy data, has shown that the presence of sodium and iron in the samples was less than 0.002 and 0.004 wt. %, correspondingly. The structure and the absence of foreign phases for zeolite samples were confirmed by X-ray powder diffraction (XRD) data (see Figure S1). BET (Brunauer-EmmettTeller) surface area, SBET, was determined to be 190–280 m2 g–1 for ZSM-23 samples and 340– 430 m2 g–1 for ZSM-5 samples by low-temperature nitrogen adsorption at 77 K with automated static set-up ASAP-2400 (Micromeritics). The size and the shape of zeolite crystals and their agglomerates were evaluated by high-resolution transmission electron microscopy method with electron microscope JEM 2010 (JEOL, Japan). In addition, a set of samples were studied by scanning electron microscopy method using JSM 6460LV (JEOL, Japan) device (Figure S2). XRD data, SEM and TEM images are presented in the Supporting Information file. 29

Si and

27

Al MAS NMR spectra of the zeolites studied in this work are shown in Figure 1.

The structural Si/Alfr ratio for the studied zeolite samples (Alfr stands for framework aluminum atoms of tetrahedral coordination) was obtained with

29

Si MAS NMR by analyzing integrated

intensities of the observed signals according to equation V.6 in ref.60 The presence and the quantity of extra-framework aluminum species (octahedral aluminum atoms, Alexfr) were determined by the

27

Al MAS NMR analysis taking into account the intensities of the

corresponding signals (including their spinning side bands). The characteristics of synthesized


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zeolite samples as well as the concentrations of Si–O(H)–Al sites predicted for each sample from the compositions of their unit cells are summarized in Table 1. The composition of the unit cell for each zeolite was calculated on the basis of the Si/Alfr ratio.

MFI-1

MFI-2

×8

MFI-3

×8

MTT-1

×8

MTT-2

×4

MTT-3 –100

–110 δ / ppm

–120

100

50 δ / ppm

0


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Figure 1. 29Si (on the left) and 27Al (on the right) MAS NMR spectra of the samples under study. 29

Si MAS NMR spectra were deconvoluted into several symmetric lines. 2−4 lines (black

dashed lines) were necessary to fit the Q4 signal (SiO4) corresponding to silicon atoms without neighboring aluminum atoms. The red solid lines correspond to silicon atoms with one aluminum atom in outer coordination sphere.

27

Al MAS NMR spectra show the presence of both

framework Alfr and extra-framework Alexfr aluminum by the signals at ca. 54 ppm and 0 ppm, correspondingly.

Table 1. Characteristics of the zeolite samples under study. Characteristics Sample Si/Alfr *

Alexfr / %

Unit cell composition

Si–O(H)–Al quantity**/ µmol g–1

MFI-1

12

6.0

Aloct0.47 H7.38 Altetr7.38 Si88.62 O192

1280

MFI-2

26

2.2


615

MFI-3

36

0.3


450

MTT-1

15

1.8


1040

MTT-2

27

0.3


590

MTT-3

42

3.0


390

*

Estimated on the basis of 29Si MAS NMR spectrum analysis.

**

Estimated on the basis of the unit cell composition analysis.

Solid-state MAS NMR spectroscopy. Solid-state MAS NMR spectra were recorded in magnetic field of 9.4 T on a Bruker Avance-400 spectrometer equipped with a broad-band double-resonance 4-mm MAS NMR probe.


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Al MAS NMR spectra were obtained using following parameters: short radiofrequency pulse

of 1 µs duration (π/10), 10 000 scans with a 0.5 s recycle delay, and 15 kHz spinning rate.

29

Si

MAS NMR spectra were obtained with a π/2 excitation pulse of 5.0 µs duration and 60 s repetition time, 1000 scans were acquired for signal accumulation at spinning rate of 8 kHz. Both 27

Al and

29

Si NMR spectra were measured for zeolite powders loaded in to a 4-mm NMR

zirconia rotor. The samples of the zeolites were hydrated under moist atmosphere at ambient temperature prior to 27Al and 29Si MAS NMR spectra recording. 1

H MAS NMR spectra were recorded by the Hahn-echo pulse sequence: π/2–τ–π–τ–

acquisition with τ being equal to one rotor period. The excitation pulse length was 5.0 µs (π/2), 64 scans were accumulated with 60 s delay and 5 kHz spinning rate for each spectrum. The spectra were recorded at 296 K. The chemical shift was referenced to TMS as external standard for 1H and 29Si NMR spectra and to 0.1 M Al(NO3)3 aqueous solution for 27Al NMR spectra with an accuracy of ±0.5 ppm. We used fused glass ampules (3 mm outer diameter and 10 mm length) containing the zeolite material (ca. 30 mg) for the 1H MAS NMR experiments. Activation at 723 K for 24 h under vacuum with a residual pressure above the sample of less than 10–3 Pa removed the adsorbed water. Special attention has been paid to the parameters of high-temperature vacuum activation procedure of the zeolites to be sure that both NMR and FTIR methods provide complementary data. After the activation procedure, internal standard (methane or benzene or both) was adsorbed on the zeolite sample. The following methodology was applied for adsorption. 5.0–9.0 mbar of methane or benzene vapor was dosed in to the calibrated volume of 5.0 mL at ambient temperature. The amount of the standard to be adsorbed was measured with an accuracy of 0.1 mbar by the vacuum gauge (DVR 5, Vacuubrand, Germany). Then, methane or benzene from


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the

calibrated volume was adsorbed on the zeolite sample at liquid nitrogen temperature.

Completeness of the transfer of the standard species onto the zeolite sample was controlled with the vacuum gauge. If both standards were needed to be adsorbed, methane and benzene were adsorbed successively while keeping the sample at liquid nitrogen temperature. For a precise determination of BAS concentration, the amount of water present in the nonactivated zeolite samples has to been taken into consideration for the weight of “dry” zeolite. The final step of the preparation of NMR sample is the sealing of the glass ampule. It is performed by microtorch

flame while the sample is held in liquid nitrogen to prevent its

heating. Described approach helps to perform 1H MAS NMR analysis of the concentration of the zeolite BAS under controlled conditions without influence of ambient atmosphere moisture and other possible contaminants. To determine the concentration of different BAS, the 1H MAS NMR spectra of the zeolites were deconvoluted by DMFIT software.61 Measurements were performed several times to check the reproducibility of the result and estimate the experimental accuracy of the method. FTIR spectroscopy. The FTIR spectra of the zeolite samples were recorded on Shimadzu FTIR-8300 and Shimadzu FTIR-8400S spectrometers within the spectral range of 700–6000 cm– 1

with a resolution of 4 cm–1 and 400 scans for signal accumulation. The powder samples of the

zeolites were pressed in self-supporting wafers, 5–7 and 2–3 mg cm–2 density for ZSM-5 and ZSM-23, respectively, and activated in the IR cell at 723 K for 6 h under dynamic vacuum of less than 10–3 Pa. The wafer density was obtained as the ratio of wafer weight (mg) to wafer surface area (cm2). Each sample was analyzed twice using two different FTIR spectrometers to validate the accuracy of the absorbance measurement. In the recorded spectra, the absorbance was normalized to zeolite wafer density with the weight of “dry” zeolite being used similar to the


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NMR experiments. The spectra were recorded at 296 K. For integrated absorbance determination, the obtained FTIR spectra were deconvoluted to separate lines by DMFIT software.61 The Beer-Lambert-Bouguer law was applied in the form A = ε ⋅ С

(1),

where C is the concentration of hydroxyl groups (µmol g–1). A denotes the integrated absorbance of the corresponding IR band in the unit cm g–1, which is normalized to zeolite wafer density. ε is the integrated molar absorption coefficient, it means the integrated intensity of the absorption band for 1 µmol of the adsorbate per 1 cm2 cross-section of the light flux in the unit cm µmol–1.

3. RESULTS AND DISCUSSION Six samples of the acid-form ZSM-5 and ZSM-23 zeolites (framework types MFI and MTT, respectively) were investigated by FTIR and solid-state 1H MAS NMR spectroscopic methods. The integrated intensities of the IR bands have been related with the concentrations of the corresponding types of surface hydroxyls, determined by 1H MAS NMR. 1

H MAS NMR data. The 1H MAS NMR spectra of zeolite samples under study are shown

in Figure 2. The spectra demonstrate the presence of different hydroxyl groups, corresponding to various BAS in the structure of the zeolites. Silanol Si–OH groups exhibit two signals at 1.8 and 2.1 ppm; bridged Si–O(H)–Al groups, both isolated (have no interactions with framework oxygen atoms or hydroxyl groups) and H-bonded (experience H-bonding to neighboring hydroxyl groups or framework O-atoms), reveal the signal at 4.1 and the broad signal at 5.0–6.0 ppm, respectively; Al–OH groups of the extra-framework aluminum species are displayed as low-intense signals at 2.5–3.0 ppm located between more intense signals from Si–OH and Si–


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O(H)–Al groups. Detailed assignment of the signals usually observed in the 1H MAS NMR spectra of the zeolites can be found in the works by Hunger,53, Brunner.17, 52,

67

62-65

Freude,12,

16, 56-57, 66

and

The attribution of the signals in the 1H MAS NMR spectra of the zeolites to

different hydroxyl sites can be confirmed by the TRAPDOR experiments68-69 (for example, see Figure S3 for the sample MFI-1) which is capable of reliable differentiation of the signals from silanol Si–OH groups and aluminum associated Si–O(H)–Al and Al–OH groups.18, 49, 57 Taking into account previously reported TRAPDOR experiments for different zeolites as well as the data presented in Figure S3, it can be concluded that the broad signal observed at 5.0–6.0 ppm in the 1

H MAS NMR spectra of the studied zeolite samples belong to H-bonded Si–O(H)–Al sites,

whereas relatively narrow signal at ca. 4 ppm arises from isolated Si–O(H)–Al groups.

MFI-1

MTT-1

MFI-2

MTT-2

MFI-3

MTT-3

10

5 0 δ / ppm

10

5 0 δ / ppm

Figure 2. 1H MAS NMR spectra of the zeolite samples under study.


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Figure 3 shows the spectrum deconvolution for one of the studied samples of H-ZSM-5 zeolite with Si/Al = 36, referred to as MFI-3. The spectra of other zeolite samples were simulated in the same way, but not shown here. It appears that FTIR spectroscopy (see below) detects more bands in the FTIR spectra for these zeolite samples than the number of lines used for NMR spectra deconvolution (Figure 3). Hence, it can be argued that such deconvolution of the NMR spectra represents the real number and the quantity of the hydroxyl species in zeolites. However, the methodology of the quantitative analysis of the NMR spectra used in this work can be considered as viable taking into account the following reasons. Indeed, the lower sensitivity of NMR with respect to FTIR spectroscopy, regarding the number of hydroxyl species types that can be reliably observed, has to be taken into consideration. Not all hydroxyls monitored with FTIR technique are necessarily detectable with NMR. Low resolution of the 1H MAS NMR spectra of zeolite samples, low intensity and close location of some peaks to one another in the NMR spectra do not allow to reliably distinguish some types of zeolite hydroxyl species and thus unambiguous correlate the signals in NMR and FTIR spectra. Therefore, it is reasonable to use minimum number of lines in the 1H NMR spectrum for its deconvolution which could be considered as optimal. To demonstrate that the 1H MAS NMR approach does provide accurate quantitative data on the number of BAS in the zeolites, one could point to clear match between the concentration of Si–O(H)–Al groups predicted by the Si/Al ratio and that determined with the approach for all studied samples. However, it should be noted that the model samples of the zeolites, i.e. containing low amounts of extra-framework aluminum species and no extraframework cations (Na+, Ca2+, etc.), were intentionally selected for this purpose. For real zeolite based catalysts, Si/Al ratio for determination of the quantity of Si–O(H)–Al groups should be


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used with care, because of the possible presence of both extra-framework aluminum species and other cations. Si–OH 1.8 4.1

Si–O(H)–Al

2.1

2.6 5.4

Al–OH ×2

CH4

C6H6

10.0

5.0 δ / ppm

0.0

Figure 3. 1H MAS NMR spectra of H-ZSM-5 zeolite (sample MFI-3, Si/Al = 36): pure zeolite (top) and zeolite with adsorbed methane and benzene (bottom). Deconvolution into separate lines is presented for the spectrum of pure zeolite. Red lines at 4.1 and 5.4 ppm correspond to isolated and H-bonded bridged Si–O(H)-Al groups, respectively. The black line at 2.6 ppm arises from Al–OH groups of extra-framework aluminum species. Green lines at 1.8 and 2.1 ppm belong to silanol Si–OH groups.


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NMR concentration measurements require the comparison of signal intensities with an internal or external standard sample of known concentration. The internal standard is preferable, since an external standard can change the goodness of the radio frequency (RF) coil and, therefore, the RF power and signal sensitivity. We used methane and/or benzene as internal references for the concentration determination due to inert properties of these compounds with respect to probed hydroxyl groups and notable difference of the chemical shifts of the signals of methane/benzene and O–H groups in the 1H NMR spectrum. It has been found that evaluations based on methane or benzene as internal standards give nearly equal values for the hydroxyl groups concentration. It is also possible to use methane and benzene jointly for one experiment (Figure 3). Figure 3 shows the 1H MAS NMR spectrum of the sample MFI-3 with adsorbed methane and benzene. The deconvolution of such spectrum affords the signals from both various hydroxyl groups and adsorbed internal standards. The integrated intensities of these signals are directly proportional to the concentrations of corresponding species. This gives the quantitative data on the hydroxyl groups present in the zeolite samples. Results are shown in Table 2. At estimation of signal intensities not only the central signals have been taken into account (Figure 2 shows only the region of –3 ÷ +11 ppm), but also the corresponding spinning side bands, whose intensities were added to the corresponding signals.

Table 2. 1H MAS NMR data on the concentration of hydroxyl groups in the zeolite samples under study. Concentration of hydroxyl groups / µmol g–1 Sample

Si–OH

Al–OH

Si–O(H)–Al

Si–O(H)–Al

1.8, 2.1 ppm

2.6 ppm

(isolated and H-bonded)

(isolated only)


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4.1, 5.0–6.0 ppm

4.1 ppm

MFI-1

40 ± 8

96 ± 5

1287 ± 91

1043 ± 70

MFI-2

167 ± 40

122 ± 34

520 ± 85

438 ± 65

MFI-3

196 ± 24

71 ± 17

459 ± 44

312 ± 28

MTT-1

207 ± 9

152 ± 17

999 ± 51

504 ± 20

MTT-2

290 ± 32

300 ± 104

622 ± 65

328 ± 38

MTT-3

265 ± 38

94 ± 21

380 ± 39

315 ± 32

The concentration of different types of bridged Si–O(H)–Al groups, i.e. isolated and Hbonded, can be determined individually with high accuracy of 5–15 %. On the contrary, only a total concentration of two types of Si–OH groups could be determined accurately, because of overlapping the corresponding 1H NMR signals in the spectra, which causes low reliability for the determination of the integrated intensities of the individual signals. By a similar reason, relatively high experimental error (20–35 %) is obtained for the determination of the quantity of Al–OH groups of extra-framework aluminum species since the corresponding signals in 1H NMR spectra are hardly distinguishable against the background of intense signals of Si–OH and Si–O(H)–Al groups. Thus, the concentrations of different hydroxyl groups or corresponding BAS in the structure of the zeolites can be measured reliably using solid-state 1H MAS NMR spectroscopy which is capable of direct and selective detection of various proton sites of the zeolites. Having obtained these results as the reliable reference for O–H groups concentrations, one can further use these 1

H MAS NMR data to evaluate the integrated molar absorption coefficients ε for the IR bands of

corresponding O–H groups.


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FTIR spectroscopy data. Figure 4 demonstrates the FTIR spectra of the samples in the region of stretching νO–H vibrations. The absorbance of the bands detected for different types of surface hydroxyl groups varies and depends on their concentration. The assignment of the main bands is quite thoroughly discussed in the literature.7, 28-36 For better illustration of the individual components of the FTIR spectra of the zeolites as well as for the extraction of the integrated intensities of particular IR bands, the deconvolution of the spectra have to be performed similar to that for the 1H MAS NMR spectra. Since the FTIR spectra of studied samples in Figure 4 are similar, only the simulation of the spectrum of MFI-3 sample with separate lines is presented in Figure 5. Moreover, same colors have been used for the deconvolution lines for both 1H MAS NMR and FTIR spectra to demonstrate the same types

0.02 a.u.

0.02 a.u.

of hydroxyl groups detected with both spectroscopic methods.

MFI-1

Absorbance

MTT-1 Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 34

MTT-2 MFI-2

MFI-3 3000 3300 3600 Wavenumber / cm–1

MTT-3 3000 3300 3600 Wavenumber / cm–1

Figure 4. FTIR spectra of the zeolite samples under study. In the presented spectra, the absorbance was normalized to sample wafer density (g cm–2).


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The bands observed at 3611 cm–1 (3605–3615 for other zeolite samples) belong to isolated bridged Si–O(H)–Al groups or strong BAS. Notably, there is undoubtful assignment of the IR bands at 3605–3615 cm–1 and 1H NMR signals at 4.1 ppm to this particular type of zeolite BAS. The bands at 3740–3747 cm–1 are generally attributed to the presence of different types of isolated silanol Si–OH groups which are observed as the signals at 1.8 and 2.1 ppm in the 1H MAS NMR spectra of the zeolites. Moreover, these bands are poorly distinguishable due to close location to each other in the FTIR spectrum similar to the corresponding signals in the 1H MAS NMR spectrum. Hence, it is reasonable to consider the integrated intensities of these IR bands jointly as it has been done for the integrated intensities of the NMR signals. Extra-framework Al–OH species are detected by the bands at ca. 3660–3680 cm–1, which correspond to the signals at 2.5–3.0 ppm in 1H MAS NMR spectrum.

0.01 a.u.

isolated Si–O(H)–Al 3611

Absorbance

Al–OH 3684 3581 H-bonded OH groups

3480 3315

2800

3000

3200

3400

3600

3800

0.01 a.u.

Wavenumber / cm–1

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3690

isolated Si–OH

3742 3729

3746

3720 3750 3780 Wavenumber / cm–1


19


Page 20 of 34

Figure 5. FTIR spectrum of the MFI-3 sample of the zeolite H-ZSM-5 (Si/Al = 36). The deconvolution to separate lines is presented for the spectrum. Red line at 3611 cm–1 belongs to isolated bridged Si–O(H)-Al groups. The black line at 3684 cm–1 arises from Al–OH groups of extra-framework aluminum species. Green lines at 3742 and 3746 cm–1 are due to isolated silanol Si–OH groups. Light blue lines present the bands that are assigned to different H-bonded hydroxyl groups. The observation of the bands in the range of 3300–3580 cm–1 is attributed to the bridged Si– O(H)–Al and the silanol Si–OH groups perturbed by a strong H-bonding. The band at ca. 3730 cm–1 is assigned to either weakly H-bonded Si–OH groups or Si–OH groups on zeolite internal surface or silanol nests.70-71 Unfortunately, these assignments are not supported by any additional evidences and there are no solid understanding of the nature of these bands yet. However, the comparison of the FTIR and NMR spectra obtained in this work can give some interesting clues regarding the problem. No signal in the 1H MAS NMR spectrum that can be correlated with the band at ca. 3730 cm–1. It can be assumed that the concentrations of corresponding hydroxyl Si–OH groups are relatively low. Second, the result of the TRAPDOR experiment (1H{27Al} MAS NMR, see Figure S3) on the nature of the broad signal at 5–6 ppm shows that the IR bands at 3300–3580 cm–1 can be assigned to H-bonded Si–O(H)–Al sites. However, another explanation exists for a broad band at 3300–3580 cm–1. Due to its higher sensitivity FTIR spectroscopy could detect the H-bonding of different hydroxyl groups with trace amount of water that can be present in zeolites despite high-temperature activation. The integrated intensities of the IR bands from the particular types of hydroxyl groups can be easily derived from the FTIR spectra of the zeolite samples under study.


20

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Table 3 shows the integrated intensities for the bands of the isolated bridged Si–O(H)–Al groups at 3605–3615 cm–1 and isolated silanol Si–OH groups at 3740–3747 cm–1 obtained after the deconvolution of the FTIR spectra of all six samples performed in a similar way as for the spectrum in Figure 5. Further, the values of the integrated molar absorption coefficients ε for the IR bands can be obtained as the ratio of the integrated intensity of an IR band to the concentration of corresponding hydroxyl groups estimated by the analysis of the intensities of related signals in the 1H MAS NMR spectra. The set of the values of the coefficients ε obtained for the zeolite samples with different Si/Al ratios and different structural types is listed in Table 3. Both laboratory and commercially synthesized zeolite samples have demonstrated similar result on the values of the coefficients ε.

Table 3. FTIR spectroscopy data on the zeolite samples under study. Integrated absorbance / cm g–1

Coefficient ε / cm µmol–1

isolated Si–OH

isolated Si–O(H)–Al

isolated Si–OH

isolated Si–O(H)–Al

3740–3747 cm–1

3605–3615 cm–1

3740–3747 cm–1

3605–3615 cm–1

MFI-1

54 ± 2

3208 ± 80

1.35

3.07

MFI-2

224 ± 12

1392 ± 70

1.34

3.18

MFI-3

240 ± 6

1019 ± 25

1.22

3.27

MTT-1

311 ± 15

1502 ± 75

1.50

2.98

MTT-2

478 ± 24

944 ± 47

1.65

2.88

MTT-3

408 ± 41

907 ± 91

1.54

2.88

Sample


21


Page 22 of 34

The values of the coefficient ε3605–3615 determined for the bands at 3605–3615 cm–1 are similar. Therefore, it is plausible to assume that the value of the integrated molar absorption coefficient does not depend on the concentration of Si–O(H)–Al groups or Si/Al ratio for the studied range of the concentrations as well as on the type of zeolite structure, at least for MFI and MTT framework topologies. Similar behavior is detected for the value of the coefficient ε3740–3747 for the bands at 3740–3747 cm–1. In this regard, it is reasonable to have a plot in terms of the integrated intensity of the IR band versus the concentration of isolated bridged Si–O(H)–Al and silanol Si–OH groups or corresponding BAS. Figure 6 shows that there is obvious linear dependence between the integrated absorbance and the concentration of both types of hydroxyl groups. It is also important that this linear dependence is valid for two types of zeolite framework, MFI and MTT. These plots give the average values (the slopes of corresponding fitting lines) of the integrated molar absorption coefficients for the IR bands: ε3605–3615 = 3.06 ± 0.04 cm µmol–1 and ε3740–3747 = 1.50 ± 0.06 cm µmol–1. Thus, the value of the integrated molar absorption coefficient ε for the bands at 3605–3615 cm–1 has been obtained for H-ZSM-5 and H-ZSM-23 zeolites, which is close to that determined by Emeis for ZSM-5 zeolite.39 Note however, contrary to Emeis we directly measured the concentration of hydroxyl groups with

1

H MAS NMR and further used the obtained

concentration for determination of the coefficient ε. Moreover, the set of zeolite samples has been studied to reveal the possible dependence of the ε value on the Si/Al ratio and framework type. It should be also emphasized that the value of the integrated molar absorption coefficient ε for zeolite silanol groups has been obtained for the first time. The coefficients ε determined in the present work can be further used for the characterization of Brønsted acidity of the H-ZSM-5 and H-ZSM-23 zeolites with FTIR spectroscopy.


22

Page 23 of 34

Si–O(H)-Al 3500

Integrated absorbance / cm×g–1

ε = 3.06±0.04 сm×µmol–1 3000

H-ZSM-5 2500

2000

1500

H-ZSM-23 1000 200

400

600

800 1000

Hydroxyl concentration / µmol×g–1 Si–OH 600

ε = 1.50±0.06 сm×µmol–1 Integrated absorbance / cm×g–1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


500

H-ZSM-23 400

300

200

H-ZSM-5 100

0 0

100

200

300

Hydroxyl concentration / µmol×g–1

Figure 6. Integrated intensity of the IR band versus the concentration of hydroxyl groups for H-

ZSM-5 (red circles) and H-ZSM-23 (blue squares) zeolites: isolated bridged Si–O(H)–Al groups (top) and isolated silanol Si–OH groups (bottom).


23


Page 24 of 34

4. CONCLUSION

A reliable approach to the quantitative analysis of the Brønsted acidity of the zeolites has been developed. Its feasibility has been verified in this paper. The basis of the approach is the joint and complementary work of two techniques, 1H MAS NMR and FTIR spectroscopy. 1

H MAS NMR spectroscopy has been demonstrated to be capable of reliable assessment of the

concentration of different types of Brønsted acid sites via direct and selective observation of corresponding surface hydroxyl groups in the structure of the zeolites. Particularly, the quantity of bridged Si–O(H)–Al and silanol Si–OH groups for a set of acid-form ZSM-5 and ZSM-23 zeolites with different Si/Al ratio has been evaluated from the 1H MAS NMR spectra using internal standard methodology. Methane and/or benzene adsorbed on the studied samples with certain quantity have been used as internal standards for O–H group concentration determination. The same zeolite samples have been further studied with FTIR spectroscopy to obtain the integrated intensities for the νO–H bands of the corresponding hydroxyl groups of the zeolites. Further, the complementary data on the integrated intensities of the IR bands and the concentrations of the corresponding types of surface hydroxyls, defined by 1H MAS NMR, have been used to obtain the values of the integrated molar absorption coefficients ε. The values of the coefficients ε have been determined to be 3.06 ± 0.04 cm µmol–1 and 1.50 ± 0.06 cm µmol–1 for the IR bands at 3605–3615 cm–1 and 3740–3747 cm–1, respectively. It is established that the coefficients ε represent constant values with respect to varying concentrations of hydroxyl groups for two zeolite framework types, MFI and MTT. Precisely determined coefficients ε can be further used for reliable assessment of zeolite Brønsted acidity with the aid of widely available and simple method of FTIR spectroscopy. Moreover, the presented approach for the


24

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determination of the coefficients ε can be applied to analyze other possible zeolite framework types with regard to quantitative characterization of their Brønsted acidity. AUTHOR INFORMATION Corresponding Authors

Tel: +7 952 905 9559. Fax: +7 383 330 8056. E-mail: [email protected] (A.G. Stepanov) E-mail: [email protected] (A.A. Gabrienko) ASSOCIATED CONTENT Supporting Information

XRD data and TEM images for the samples of studied zeolites; 1H MAS NMR TRAPDOR spectrum for a sample of H-ZSM-5 zeolite (Si/Al = 12, MFI-1). Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources

Russian Foundation for Basic Research (grant no. 18-03-00189). RAS budget project no. AAAA-A17-117041710084-2 for Boreskov Institute of Catalysis. Notes

The authors declare no competing financial interest.


25


Page 26 of 34

ACKNOWLEDGMENT

This work was supported by Russian Academy of Sciences within the framework of budget project no. AAAA-A17-117041710084-2 for Boreskov Institute of Catalysis. This work was also supported, in part, by Russian Foundation for Basic Research (grant no. 18-03-00189).


26

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TOC Graphic

Intensity

NMR

Integrated Molar Absorption Coefficient

ε

cm/g µmol/g FTIR Absorbance


34

Direct Measurement of Zeolite BrÃ¸nsted Acidity by FTIR Spectroscopy

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