Characterization of Catalytic Materials through a Facile Approach to

Article pubs.acs.org/JPCC

Characterization of Catalytic Materials through a Facile Approach to Probe OH Groups by Solid-State NMR Andrey S. Andreev* and Vincent Livadaris Total Research and Technology Feluy (TRTF), Zone Industrielle C, B-7181 Feluy, Belgium S Supporting Information *

ABSTRACT: A facile NMR approach based on dipolar filtering (DF) and spin echo (SE) is proposed to study commercially available dealuminated USY zeolites, such as H-CBV760, HCBV720, and NH4-CBV712 of nominal Si/Al ratio of 30, 15, and 6, respectively. The proposed 1H DF-SE magic angle spinning (MAS) NMR approach provides the substantial suppression of water signal intensity in partly hydrated samples providing an opportunity to obtain the signal of other surface groups. 1H DF-SE MAS NMR technique has been demonstrated its usability for quantitative (semiquantitative) analysis of dried, i.e., partly hydrated, samples. It is essential to use this approach when calcination under vacuum used as a reference procedure leads to drastic surface changes. Moreover, the technique is applicable for qualitative analysis of fully hydrated samples. This method is found to be extremely sensitive to the residual ammonium content in zeolite structure even in transformed to H form by calcination. Finally, the framework stacking faults species are found to be more pronounced in 1H DF-SE MAS NMR spectra in hydrated state as ∼1 ppm peak that can be crucial for understanding of relationships of structure and performance. Additionally, the “standard” 27Al and 29Si MAS NMR approaches are also discussed in both hydrated and dried states of zeolites. 29Si MAS NMR spectra demonstrate that a dependence on hydration state and the highest quantity of crystalline part is achieved in dried samples, whereas the best resolution of 27Al MAS NMR spectra is obtained in a fully hydrated state. Finally, a local order of Si framework given by full width at half-maximum parameter of crystalline Q40 peak correlates with increasing relative Al content, which is responsible for the distortion of zeolite structure.

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version of 1H magic angle spinning (MAS) NMR, which is crucial for surface analysis especially for catalytic supports,9 coupled with well-known 27Al and 29Si MAS NMR tools as a facile approach for catalytically important materials.10−12 The main advantage is the absence of sample pretreatment is that it is generally very time-consuming for 1H MAS NMR of high specific surface area materials. The surface OH group investigation by solid-state 1H MAS NMR is widely applied for many materials, such as alumina,13,14 silica,15 aerogels,16 zeolites,17−21 titania,22,23 heteropolyacids,24 fiberglasses,25,26 and vanadia.27 The main drawback of 1H NMR for surface OH groups is a sophisticated sample treatment prior to the experiment. Generally, the pretreatment requires high temperature dehydration under primary vacuum for several hours (4−12 h). The sample must be hermetically sealed using gas burner of a homemade vacuum installation. The following stage has two principally different approaches. The first consists in the sample spinning performed in a specially designed ampule, inserted into the NMR rotor.18,28 This approach

INTRODUCTION Aluminosilicate zeolites are framework materials based on silicon, oxygen, and aluminum atoms. The sophisticated cagechannel structure with very strong acidic sites determines their properties as catalytic and adsorption performances. A main drawback of widespread implementation of zeolites in technology is the mass-transfer restriction of cracking molecules due to very small channel sizes. To overcome these limitations postsynthesis treatments, such as dealumination and/or desilication, have been proved to be extremely convenient and efficient.1−6 The preparation of hierarchical FAU zeolites by base treatment is challenging since the framework stability is sensitive to the Si/Al ratio.7 The mesoporization of low Al content zeolites with low framework density as highly dealuminated USY zeolites requires mild conditions to preserve the zeolite framework from uncontrolled dissolution and the complete loss of zeolitic properties.8 Thorough surface and bulk structural analysis is essential to follow the transformation changes during various treatments. A deep insight into the structure could result in improved postsynthesis treatments enhancing the desired properties of the transformed materials. Therefore, the authors propose a combination of a “simplified” © 2017 American Chemical Society

Received: March 10, 2017 Revised: June 7, 2017 Published: June 9, 2017 14108

DOI: 10.1021/acs.jpcc.7b02283 J. Phys. Chem. C 2017, 121, 14108−14119

Article

The Journal of Physical Chemistry C

Figure 1. (a) Rotor-synchronous dipolar-filter (RSDF) sequence as described in ref 36 with explicit phase cycling; (b) RSDF coupled with SE excitation, which subtracts the signal from baseline and gives additional T2 water relaxation. P12 is hard π/2 proton pulse for dipolar filter, and d2 is dipolar filter interpulse delay which should be synchronized with rotation according to the equation 12(p12 + d2) = l1 × Trot, where Trot is 1/υrot one rotation period and l1 is integer. The total filtering time is determined by integer number (l2) of applied RSDF sequences, i.e., l2 × τRSDF. The delay d6 in echo sequence is also synchronized with rotation.

= 6), which have been shown their utility in mesoporization procedures.7,8,19,37 The aspects of sample hydration/dehydration are also emerged opening the discussion on a necessity of the pretreatment by dehydration. The zeolite structure is essentially responsible for the nature of OH groups. Many studies have been devoted to influence of hydration−rehydration on 27Al NMR spectra.38−41 Gaining better resolution of 27Al NMR spectra requires hydration of the zeolites, which is accepted as a best practice prior to acquisition of 27Al spectra of these materials. However, to study OH groups by 1H NMR the samples should be dehydrated at elevated temperature and under vacuum conditions. It is generally assumed that temperature, vacuum, and hydration− dehydration treatments do not influence the zeolite framework. Therefore, all conclusions made using 1H of dehydrated samples are matched with 27Al/29Si NMR data obtained in fully hydrated samples (saturated at high relative humidity). Nevertheless, the influence of sample hydration on 29Si NMR experiments is not well studied. In this work, we try to establish a link between the results on various nuclei (1H, 27Al, and 29Si) as a function of hydration state. The full hydration at 100% relative humidity and sample dying at 110 °C are discussed hereinafter. A discussion on 1H NMR data is made possible by the implementation of proposed DF-SE approach.

requires high operator qualification for sample preparation, particularly in glass soldering part, and it is limited in rotation frequencies to 5−7 kHz due to the ampule fragility. The last limitation is more important for high-field NMR spectrometers that require higher rotation frequencies for the same applications. The second method is the sample transfer from the treated ampule, which should be broken, and its packing into NMR rotor in a glovebox under inert flow.19,25 This approach has no spinning speed limitations; however, the additional sample transfer can lead to water adsorption (or other contamination) in the case of high specific surface area of the material. Therefore, being less experimentally sophisticated, the latter approach does not ensure the complete absence of the exchange with ambient atmosphere. A facile NMR approach based on dipolar filtering (DF)29−31 is proposed, which is widely used in pulse sequences as the magic and polarization echo (MAPE),32 and the rotorasynchronous dipolar-filter (RADF) sequence33−35 to filter broad dipole−dipole coupled peaks so as to substitute the conventional 1H NMR approaches for surface OH group research. A new rotor-synchronous dipolar-filter (RSDF) sequence has been recently developed by Liu et al.,36 having the advantage of being rotor-synchronized, thus facilitating an experimental set up (see Figure 1a). Moreover, RSDF does not suffer from destructive interferences between radio frequency and MAS averaging, being applicable for fast MAS spinning. The surface OH groups are very mobile species; in contrast, the adsorbed water can possess very strong dipole−dipole interaction. Therefore, RFDF sequence especially coupled with spin echo (SE) excitation (see Figure 1b) can be used for rigid water signal filtering, which mainly hinders the observation of surface OH. Such approach gives semiquantitative determination of many surface species without complicated sample pretreatments revealing the opportunity of fast sample screening for OH group determination, providing crucial information for catalysis. This paper aims the detailed description of RSDF approach application to the study of surface OH groups of commercial dealuminated USY zeolites CBV760 (H-form, nominal Si/Al = 30), CBV720 (H-form, nominal Si/Al = 15), and CBV 712 (NH4 form, nominal Si/Al

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EXPERIMENTAL SECTION The commercially available dealuminated USY zeolites from Zeolyst as CBV760 (H-form, nominal Si/Al = 30), CBV720 (H-form, nominal Si/Al = 15), and CBV 712 (NH4 form, nominal Si/Al = 6) were used as samples for study. The samples were examined in both fully hydrated and dried states. The first required filling the zeolite pores with water by means of at least 24 h exposure in a desiccator containing distilled water. The second state required drying the sample in a furnace at 110 °C for at least 4 h and its packing inside NMR rotor under ambient conditions. The symbol H was added to the end of sample name for hydrated sample, and D designates the dried ones. 14109


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The Journal of Physical Chemistry C Table 1. Relative Peak Ratio Obtained from the Decomposition of 15 kHz 27Al MAS NMR Spectra in Figure 2 relative intensity (a.u.)

a

sample

AlT-1 ∼62.5 ppm

AlT-2 ∼59.7 ppm

Al5 39.5 ppm

Alextra ∼0 ppm

AlO-1 ∼5.0 ppm

H-CBV760D H-CBV760H H-CBV720D H-CBV720H NH4-CBV712D NH4-CBV712H

22.4 42.0 26.6 37.7 12.7 18.7

34.8 19.4 40.1 35.7 29.3 30.2

26.1 20.4 20.7 11.9 24.6 13.1

3.8 3.4 2.9 1.7 0.9 0.9

12.9 14.7 9.7 13.0 21.5 23.4

AlO-2 ∼8.5 ppm

AlT/AlO ratioa

11.0 13.8

4.4 4.2 6.9 5.6 1.3 1.3

AlT/AlO ratio is calculated by dividing a sum of all AlT peaks, i.e., AlT-1 + AlT-2, by a sum of octahedral AlO ones, i.e., AlO-1 + AlO-2.

Table 2. Relative Peak Ratio Obtained from the Decomposition of 15 kHz 29Si MAS NMR Spectra in Figure 2 fwhm (Hz)b

relative intensity (a.u.) sample

Q20 −91 ppma

Q22 −96.5 ppm

Q31 −101.5 ppm

Q40 cryst −108 ppm

Q40 amorph −111 ppm

Si/Al ratio

Si/Al ratio “framework”

Q40 cryst −108 ppm

H-CBV760D H-CBV760H H-CBV720D H-CBV720H NH4-CBV712D NH4-CBV712Hc

1.3 2.1 1.2 1.6 0.7 2.6

1.3 0.9 4.0 3.7 4.3 6.2

16.3 20.5 16.3 19.5 17.7 20.3

68.8 61.0 69.2 65.0 68.5 65.0

12.3 15.5 9.3 10.2 8.8 6.0

21.1 18.0 16.4 14.8 15.2 12.3

18.5 15.2 14.9 13.3 13.9 11.5

106.5 122.9 125.7 152.5 158.6 199.1

a

QNK where N stands for the number of Si and K stands for Al atoms in silicon coordination sphere. bFull width at a half-maximum (fwhm) of this peak is linked to local order of Si zeolite framework, i.e., to the crystallinity of the sample (narrower peak responsible for higher crystallinity). cNote that NH4-CBV712 is not calcined thus the comparison of the acidity may be incorrect.

Table 3. Relative Peak Ratio after the Decomposition of 1H DF-SE 15 kHz MAS NMR Spectraa real content of NH4+ (atom %)c

relative OH group content without water and NH4+ (atom %)

sample

AlOH ∼1 ppm

SiOH 1.4−1.9 ppm

AlOH 2.0−2.2 ppm

H-CBV760D H-CBV720D NH4-CBV712D

21.8 0.7 0.0

5.0 28.4 33.5

18.4 15.3 8.6

b

bridge SiOHAl 2.9 ppm 14.5 18.9 35.0

bridge SiOHAl 3.3−3.9 ppm

organic residuals ∼9 ppm

NH4+ 6.7 ppm

24.2 34.9 13.0

16.2 1.8 9.9

10.9 83.0 81.2

a

The values in the table are given in atom % 1H. bThis OH group is linked to Al, it can be both terminal AlOH or bridge OH of small EFAl clusters or zeolite stacking faults. cThe NH4+ excluding water contribution from consideration.

The 1H NMR spectra were registered using Bruker Avance III HD 500 MHz (magnetic field is 11.7 T) equipment by means of the broad band MAS probe using 4 mm outer diameter rotors at the spinning speed of 15 kHz (operating frequency is 500.13 MHz). The pulse sequences were onepulse, two-pulse excitation (Hahn echo p1-d6-2*p1-d6-acquisition) of p1 = 3 μs (π/2) duration; the delay between the pulses τ was varied (to obtain the relaxation of absorbed water signal since it relaxes much faster than other peak in the spectra). The delay of sequence repetition was 5 s, and the number of transients was 16−256. DF coupled with SE contained a sequence of 12 (3 μs) 90° 1 H pulses with a delay which was varied to reach synchronization with rotation. The actual synchronization was performed with 5 rotor periods (spinning speed 15 kHz); the sequence was repeated several times depending on the studied sample. After DF, the sequence contained a usual spin echo (Hahn echo) part described above. The DF-SE 1H 15 kHz MAS spectra of CBV760D, CBV720D, and CBV712D zeolites are acquired with rotation synchronization with 5 rotor periods (l1 = 5, i.e., 333.3 μs), DF sequence repetition for 12, 6, and 3 times (total DF time 4000, 2000, and 1000 μs for CBV760D, CBV720D, and CBV712D, respectively), and finally 333.33 μs

SE time (interpulse delay). The technical aspects of DF sequence set up are given in Figures S1 and S2. The 27Al 15 kHz MAS NMR spectra were registered using Bruker Avance III HD 500 MHz (magnetic field is 11.7 T) equipment by means of the broad band MAS probe using 4 mm outer diameter rotors at the spinning speed of 15 kHz (operating frequency is 130.31 MHz). The pulse sequence contained one single excitation pulse of 0.5 μs (π/8) duration, the interpulse delay was 0.5 s, and the number of transients was 4096. The 29Si NMR spectra were registered at the operating frequency of 99.36 MHz by means of the broad band MAS probe using 4 mm outer diameter rotors at the spinning speed of 15 kHz. The hpdec sequence was used with the high power proton decoupling (SPINAL64). The 29Si excitation pulse was 5.1 μs (π/2), the decoupling pulse power was 50 μs (the single decoupling pulse duration was 9.5 μs), and the delay between the pulse sequences was 30 s. The acquisition required one night (∼1800 transients). The fitting of 27Al MAS NMR spectra was carried out using Dmfit software42 by implementation of Czjzek model (SzSimple) for simulation of quadrupole constant distribution. The 27Al MAS NMR spectra have obviously no perfect quadrupole peak shape of crystalline materials;43 thus, a correct 14110


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Figure 2. MAS NMR spectra (15 kHz 27Al) of dried (red) and hydrated (blue) CBV760 (a), CBV720 (c), and CBV712 (e) and 15 kHz 29Si MAS NMR spectra of CBV760 (b), CBV720 (d), and CBV712 (f). The spectra of hydrated and dried samples are normalized on number of scans. The most affected by hydration-drying treatment in 27Al MAS NMR spectra is FAl AlT-1, and in 29Si MAS NMR, it is the zeolite crystalline Q40 site.

66.66 μs synchronized with rotation and a recycle delay of 0.5 s, that is, a total experiment time of about 24−96 h. The sample of NH4-CBV712 was chosen due to its highest Al content. An appropriate signal-to-noise ratio of H-CBV720 sample could not be obtained in a reasonable time whereas H-CBV760 did not show a signal due to low Al content. 1 H−29Si dipolar heteronuclear multiquantum correlation (DHMQC) spectra with 1H detection were recorded using SR421 recoupling.46 The recoupling sequence was also applied to 1H channel. The 1H radiofrequency amplitudes for the 90 and 180° pulses and for the SR421 recoupling were equal to 110 and 30 kHz, respectively. The 90° conversion 29Si nuclei pulses were of 4 μs duration corresponding to a radiofrequency field amplitude of 62.5 kHz. The total dipolar recoupling time was 1200 μs. The 2D spectra resulted from the accumulation of 256 (512) transients for each of 256 (512) t1 increments with a step of 66.66 μs synchronized with rotation and a recycle delay of 1.5 s, that is, a total experiment time of about 24−96 h for each experiment.

peak ratio quantification requires a use of Czjzek model. In this case, a simple model is sufficient as the authors do not measure NMR parameters,44 whereas a more detailed spectra analysis requires the extended Czjzek model.45 The decomposition of 29 Si and 1H DF-SE MAS NMR spectra was performed using the same software by a mixture of Gauss/Lorentz peaks, which was 0.6 and 0.3 for 29Si and 1H NMR spectra, respectively. The isotropic chemical shifts of all peaks used in the decomposition are given in the Tables 1−3. 27 Al−1H dipolar heteronuclear multiquantum correlation (DHMQC) spectra with 27Al detection were recorded using SR421 recoupling.46 The details on this sequence can be found elsewhere.47−51 The 1H radiofrequency amplitudes for the 90° pulses and for the SR421 recoupling were equal to 83.3 and 30 kHz, respectively. The central transition selective pulses on 27Al nuclei were 10 and 20 μs (90 and 180° pulses, respectively) corresponding to a radiofrequency field amplitude of 25 kHz. The total dipolar recoupling time was 533.33 μs. The 2D spectra resulted from the accumulation of 2000 (4000) transients for each of 64 (128) t1 increments with a step of 14111


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RESULTS AND DISCUSSION

Bulk Structure of CBV Zeolites. 27Al MAS NMR. Prior to discussion of dealuminated USY zeolite surface, a general view on their structure should be done. 29Si and 27Al MAS NMR spectra are employed to characterize the series of CBV zeolites. Figure 2 shows 15 kHz MAS 27Al NMR spectra normalized on number of scans. The spectra display 5 main peaks (except of CBV712 displaying 6 peaks) of different Al sites: tetrahedral aluminum sites AlT-1 (∼62.5 ppm) corresponding to framework (FAl) sites in “perfect” FAU zeolite; AlT-2 (∼60.2 ppm) arising from distorted FAl tetrahedra; Al-5 (39.5 ppm) is 5coordinate Al sites (surface AlT with one additional hydrolyzed bond); Alextra (∼0 ppm) is an extaframework aluminum (EFAl) in the form of Al ions; and octahedral aluminums AlO-1 (∼5.0 ppm) are hydrolyzed Al sites most probably related to EFAl. Finally, only CBV712 contains an additional AlO-2 (∼8.0 ppm) peak most probably corresponding to another from zeolite aluminum-containing phase (as Al hydroxide, for example).19,38 The peak attribution and spectra decomposition is performed with respect of 27Al 3QMAS spectra (an example is shown in Figures S3 and S4). As mentioned in the Introduction, 27Al NMR spectra of zeolites are very sensitive to hydration state of the material (see for example).38,40 In the present work, three different commercially available CBV zeolites are investigated in fully hydrated state and after drying at 110 °C. Figure 2 displays all spectra in both hydrated and partly dehydrated (dried) states. The spectra are normalized on the number of transients. In contrast, a correction on sample mass can be hardly applied due to obvious difference of sample mass due to various water contents in hydrated and dried samples. Nevertheless, the Al visibility should not be drastically affected due to large residual water content observed in 1H MAS NMR spectra (see Figure 3).38 27 Al MAS NMR spectra in Figure 4 exhibit a clear distinction between dried and fully hydrated sample. First, the FAl AlT-1 sites show substantial broadening and intensity decrease; thus,

Figure 4. Comparison of DF-SE 1H 15 kHz MAS spectra of CBV760D (bottom), CBV720D (center), and CBV712D (top) zeolites. Different types of OH groups are emphasized. High unexpected content of NH4+ in CBV720D may be caused by insufficient calcination time during transformation from NH4+ to H zeolite form.

AlT-1 site population difference is observed after spectra decomposition of hydrated and dried sample (see Table 1). Additionally, with increased Al content from CBV760 to CBV720 and finally to CBV712, the contribution of 5coordinate Al5 becomes more pronounced. The unambiguous attribution of ∼39.5 ppm peak to 5-coordinate Al (not to distorted AlT) is made using 3QMAS spectrum of CBV712D (see Figures S3 and S4). The contribution of AlO-1 generally observed in USY zeolites is not significantly changed during the drying process in all samples as the octahedral-tetrahedral transformation which occurs only above 122 °C.52 However, the observed 5coordinate Al sites are drastically increased after partial dehydration process. The latter could be associated with different content of AlT in hydrated and dried zeolites, which tends decreasing after drying. This observation becomes more pronounced with decreasing Si/Al ratio in a row of CBV760, CBV720, and CBV712, respectively. A hypothesis made based on a recent work53 where the authors have demonstrated by DFT calculations that EFAl AlOH2+ ions (a form of EFAl ions) may change into tetracoordinated status by adsorbing one water molecule and coordinating with two framework oxygen atoms. However, it is clearly seen in Table 1 that EFAl content is mainly conserved in both dried and hydrated samples. Therefore, it cannot compensate increasing values of AlT after hydration. Another hypothesis consists in the creation of three-coordinate Al (Al3) sites mainly from Al tetrahedral. The latter should Al3 possess the similar chemical shifts as 5-coordinated Al. Generally, 3coordinate Al is observed to form in zeolites but at higher temperatures (above 400 °C).52 However, dealuminated CBV zeolites can possess more defective AlT sites that can lose their coordination more facilely than normal FAU zeolites. 29 Si MAS NMR. The humidity influence on 27Al MAS NMR spectra of zeolites has been widely observed in the literature.19,38−41 However, their effect on 29Si MAS NMR is not thoroughly reviewed. Some studies compare dehydrated and nonhydrated zeolites, thus the intermediate state is out of the research scopes. Nevertheless, Figure 2 clearly displays that silicon structure is also sensitive to sample humidity, i.e., the

Figure 3. Comparison of onepulse 1H 15 kHz MAS (top, blue) and 1 H DF-SE (l1 = 5, l2 = 12, and l4 = 5) 15 kHz MAS (bottom, black) NMR spectra of CBV760D zeolite. Obvious water suppression is reached in these NMR acquisition conditions without substantial change of intensities of the rest of the peaks (for example, the ratio of ∼1.1 and 3.8 ppm peak is conserved). The DF-SE approach allows detection of residual NH4+ content and the traces of organic treatment during commercial predealumination. 14112


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the ratio of ∼1.1 and 3.8 ppm peak is conserved). The SE part of the sequence advantages for improving several spectral features. First, it subtracts the broad contribution from the NMR probe shown in Figure S2, which is more beneficial than manual baseline correction that can cause a loss of broad peaks in spectral edge (as ∼9 ppm organic peak, for example). Second, SE forces an additional T2 relaxation of water signals being considerably shorter than for other 1H NMR peaks (in dried samples). The SE intensity dependence on T2 relaxation time is expressed by equation as I(2τ) = I(0) exp(−2τ/T2), where I(0) is a “true” intensity, I(2τ) is the intensity of SE experiment, and τ is the interpulse delay in the pulse sequence (denoted as d6 in Figure 1b). A short interpulse delay τ does not influence the signal intensity, except the water peak, due to longer relaxation times of surface OH groups. The T2 relaxation of dipolar spins (S = 1/2) is directly connected to fwhm of the peak (in the absence of paramagnetic nuclei) as T2 (μs) = 106/ (π fwhm(Hz)). The values of measured fwhm of water peak and recalculated T2 relaxation times are listed in Table S1. The difference in T2 relaxation times between dried and hydrated samples is obvious. The shorter relaxation time is more appropriated for DF due to substantial contribution of dipolar broadening to fwhm of water in small zeolite channels. On the basis of results of Table S1, one can state that the water molecules are more mobile in the series CBV760, CBV720, and CBV712, thus hinting to larger pore size (or less pore blockage by extra framework species) in this series. In general, the attribution of 1H MAS NMR spectra signals in Si−Al zeolites is described as follows. The high-intensity peak (seen from one-pulse spectrum in Figure 3) arises from residual physically adsorbed water in samples (4.2−4.8 ppm). This attribution is not valid for fully dehydrated samples which are hermetically sealed, thus no water should be observed in these conditions. The group of peaks in the range 0.7−1.1 ppm is attributed to terminal Al−OH groups bonded to different Al sites. Some authors assign these peaks to OH groups bonded to EFAl species.41,55 These chemical shifts are observed in recent papers,38 but the discussion of their origin is generally omitted. The peaks between 1.4−1.9 ppm (depending on zeolite type and chemical structure) are attributed to terminal Si−OH groups bonded to different Si sites.18,56 In general, the 2.1−2.5 ppm region of chemical shifts corresponds to bridge-type OH groups; however, in zeolites it can belong to terminal Al−OH (stacking faults of zeolites) or to OH groups of EFAl species as alumina small clusters, aluminum hydroxide, or amorphous alumosilicates.18,55−58 However, assuming only Alextra (∼0 ppm) and AlO-2 (∼8.0 ppm) peaks in 27Al MAS NMR spectra to be EFAl species,38,54 this peak should show much weaker intensity in CBV760 and CBV720 zeolites (which do not show AlO-2 contribution in 27Al MAS NMR spectra in Figure 2), but this does not occur according to Figure 2. Therefore, this peak can be attributed to hydrolyzed AlO-1 or partly hydrolyzed Al5 sites of FAl. The widths of AlO-1 and Al5 peak are very large, indicating high distortion level of these sites. Finally, assuming AlO-1 as EFAl, being of non-negligible amount according to Table 1, should provoke an observation of rather broad peak in 1 H MAS NMR spectra in the case of Al hydroxide (only for noncalcined samples)59 or the narrow intense peak at 0 ppm in the case of alumina-like EFAL species,60 which is not detected. Therefore, 2.1−2.5 ppm of OH groups in 1H MAS NMR spectra in Figure 4 could be attributed to stacking faults of zeolite framework. The next peaks at ∼2.9 and 3.6−4.0 ppm in 1 H MAS NMR spectra unambiguously correspond to acidic

drying at 110 °C mainly affects the Si zeolite framework atoms (Q40 cryst). Note that the spectra are not normalized for sample mass due to obvious difference in water content in dried and hydrated samples. The 15 kHz 29Si MAS NMR spectra displayed in Figure 2 exhibit 5 peaks corresponding to different Si environment that can be denoted as QNK where N stands for the number of Si and K represents the number of Al atoms in the silicon coordination sphere, i.e., Si(OSi)N(OAl)K(OH)4−N−K (N + K = 4; N, K = 0, 1, 2, 3, and 4). The narrowest peak at −108 ppm arises from Q40 environment, being attributed to FAU zeolite crystalline part with no Al atoms in proximity. The amorphous part, either amorphous silica−alumina or silica, is represented by −111 ppm broad peak (Q40). The Al atom incorporation into Si coordination sphere causes the changes of electron density on Si nuclei giving ∼5−6 ppm shift of the resonance peak of each Si replaced by Al40 relatively to the initial Si shift (Q40 at ca. −108 ppm). Therefore, Q31 and Q22 Si sites are found at −101.5 and −96 ppm, respectively, and these peaks are attributed to 1Al and 2Al atoms in the zeolite structure, accordingly. Finally, the peak at −91 ppm corresponds to Q20 Si environment, which has no Al in the coordination sphere, and is probably associated with outer surface defects. The “bulk” Si/Al ratio can be extracted from 29Si NMR data using the following equation taken from ref 54: 4

Si/Al =

4

∑ ISi(i Al)/∑ (i/4)ISi(i Al) i=0

i=0

(1)

where i is a number of Al atoms in Si coordination sphere, and ISi(iAl) is the integral intensity of corresponding peak. A slight modification of eq 1 can be done to extract “framework” Si/Al ratio. In this case, a contribution of amorphous silica is excluded of consideration, i.e., it does not contribute to numerator of eq 1. The analysis of both framework and bulk Si/Al ratios as well as the full width at a half-maximum (fwhm) parameter of both dried and hydrated states of zeolites facilitates the understanding of many structural features. First, the larger difference between two types of Si/Al ratio of the same sample states the higher amount of amorphous (silica or alumosilica) part in the zeolite as seen from the contribution of Q40 amorph peak. Another surprising fact is that the higher ratio of Si/Al values of dried samples, which can be connected with the partial hydrolysis of some Si−O−Al bounds. Another parameter better illustrating these changes is the fwhm of Q40 crystalline peak, which responsible for the local order of Si framework atoms. Indeed, the fwhm of all hydrated samples is significantly larger (15, 21, and 25% higher fwhm of CBV760, CBV720, and CBV712, respectively) than those of dried ones, which may indicate structural strains of the zeolite in the excess of water, i.e., at 100% relative humidity. Finally, a correlation between Si/Al ratio and fwhm can be drawn, i.e., decreasing Si/Al ratio leading to the relative increase of Al content in zeolite framework results in higher local disorder in zeolite framework, thus contributing to fwhm values of crystalline Q40 peak. Surface Structure of CBV Zeolites Probed by DF-SE 1H MAS NMR. The surface OH groups in this work have been examined by modified DF approach described in ref 36 by adding SE echo part as shown in Figure 1. Obvious water suppression is reached under these conditions without substantial change of intensities of the rest peaks (for example, 14113


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The Journal of Physical Chemistry C Brønsted bridge SiOHAl sites18,56−58 The ∼6.8 ppm peak is due to residual ammonium content in the samples. Finally, the peaks at 8.5−9.0 ppm indicate the residual parts of most probably organic groups, which were not sufficiently removed during calcination treatments. However, some authors attribute these peaks to rigid water in EFAl phases.41 The DF-SE 1H 15 kHz MAS spectra of CBV760D, CBV720D, and CBV712D zeolites using DF-SE technique are displayed in Figure 4. Longer DF time is required to suppress a higher initial amount of water in the samples (see one-pulse spectra in Figure S5). Nevertheless, appropriate water suppression is observed for all samples gaining the spectral resolution due to water signal elimination. This helps distinguishing all other types of OH groups mentioned hereinbefore. High unexpected content of NH4+ in CBV720D may be caused by insufficient calcination time during transformation from NH4+ to H zeolite form. A second noticeable feature is the peak of terminal OH groups at 1 ppm in CBV760D. Despite the fact that water is mainly suppressed as a special sequence its physical presence is inevitable. Therefore, a significant amount of hydrated EFAl species or zeolite stacking faults may cause this intense signal. To understand the number of spectral peaks a variation of SE time has been implemented (see Figures S6−S8) as described in ref 14. These spectra can be decomposed into separate components to estimate the contribution of various OH groups under a certain number of restrictions. First, this approach is not fully quantitative due to a possibility of relaxation of some peaks during DF and SE, but in the present work this factor is almost negligible. Second, the overlap of some bridge OH groups with water signal in the 4.5−6 ppm region is inevitable; thus, this group is out of quantification. Despite these constraints, the DF-SE approach keeps the zeolite conditions unchanged since the drying procedure at 100−120 °C is usually considered in synthesis procedure. Moreover, this approach does not require special equipment and complicated manipulations with samples, facilitating its implementation to follow the synthesis route. Finally, it ideally fits the demands of fast surface OH group screening of samples of the same origin with different pretreatments since the experimental set up conditions do not need to be changed or adjusted. The spectra decomposition values are given in Table 3. Note that for the first part the OH groups have been normalized excluding water and ammonium signals since they predominate in the spectra in Figure 4. The real contribution of ammonium excluding water content is given in the second part of Table 3. The high content of ammonium may be responsible for occupation of some site belonging to acidic SiOHAl groups in CBV720 and CBV712 zeolites. Moreover, the peaks at ∼9 ppm state the presence of residual organic groups that are not well eliminated during sample preparation. The increasing content of terminal SiOH groups (which are suggested as internal zeolite stacking faults) with decreasing Si/Al ratio state the higher defectiveness of low Si/Al zeolites compared. This observation also supports 29Si MAS NMR data on local Si framework order given by fwhm of crystalline Q40 peak. Another correlation with decreasing Si/Al values is the growing contribution of acidic bridge SiOHAl groups; however, the comparison of NH4-CBV712 is not correct since this zeolite in the ammonium form compared to H-CBV760 and H-CBV720, which nominally have proton form (however, experimental evidence claims an incomplete transformation of CBV720).

Finally, only H-CBV760 shows very narrow SiOHAl, stating a high order of such type OH groups. Moreover, this is the only sample showing relatively low content of residual ammonium. This approach is considered to be extremely useful to probe different sample pretreatments in catalysts. A clear demonstration of the usability of DF-SE approach is demonstrated here. It is noteworthy that the DF-SE approach can provide some intensity distortion due to peak relaxation. This should mainly affect the peak with larger fwhm as they have shorter relaxation times. Finally, an idea of additional experiments on hydrated samples is shown in Figure 5. These spectra are neither

Figure 5. Comparison of DF-SE 1H 15 kHz MAS spectra of CBV760H (center), CBV720H (top), and CBV712H (bottom) zeolites. Different types of OH groups are emphasized.

quantitative nor semiquantitative due to long SE times (for example, 2933.3 μs interpulse delay is comparable with T2 relaxation time of several peaks). These experiments can help in understanding the nature of the changed of Al sites during hydration-dehydration. Despite the application of DF sequence, the main mechanism decreasing the water signal intensity is SE. Moreover, at long SE times some dipole−dipole interactions can be reintroduced.61 The most noticeable fact in these spectra is an increased amount of ∼1 ppm peak of terminal OH groups. Moreover, instead of a single peak in dried samples, the fully hydrated zeolites exhibit 4−6 different signals that may be bonded to different Al sites of FAl or to EFAl species. Another, unexpected feature of these terminal Al−OH groups is their facile disappearance during simple drying procedure at 110 °C. Nevertheless, it is known, for example, in γ-Al2O3 that terminal Al−OH groups are still observable after 500 °C calcination under vacuum.48 Probably, these defects in zeolite framework or additional extra-framework phases play a crucial role in zeolite properties. The general feature of 1H MAS NMR spectra of hydrated samples is an emphasis of peaks which are connected to somehow to Al. Indeed, no signal of terminal SiOH in the spectra of hydrated sample is observed. The nature of these peaks at ∼1 ppm is discussed hereinafter. Link between Bulk and Surface Structure. To support the observations on bulk (27Al, 29Si) and surface (1H) structure, it is essential to establish a link between them via X−1H heteronuclear correlations. Despite the numerous techniques exist for establishing a proximity between heteronuclei, the through space correlation between 1/2 spin and quadrupolar nuclei (spin > 1/2) are still challenging and time-consuming. 14114


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Figure 6. D-HMQC MAS NMR (15 kHz 27Al−1H) spectra of hydrated (top) and dried (bottom) NH4-CBV712.

projection at ∼40 ppm of the spectrum is supposed to arise from a rather poor signal-to-noise ratio. The spectrum of hydrated NH4-CBV712 establishes several important correlations of all Al species observed in 1D 27Al MAS NMR spectrum. First, FAl (49 and 57 ppm) exhibits a proximity with 3.3, 6.7, and 8.8 ppm protons corresponding to AlT linked to bridge OH, NH4+, and the residual organic OH, probably forming carboxylic groups upon the hydration, respectively. In contrast, EFAl represented by AlO correlates with ∼2.4 and ∼3.7 ppm bridge OH, 6.5 ppm ammonia peak, and the 8.3 ppm peak discussed in the previous paragraph.

Recently, a robust technique named dipolar heteronuclear multiquantum correlation (D-HMQC) has been developed to provide a through space linkage of studied atoms.50,51,62 The main advantage compared to common heteronuclear correlations (HETCOR) via cross-polarization (CP) mechanism is a broadband behavior of D-HMQC compared to CP-HETCOR when applied to quadrupolar nuclei as 27Al. The D-HMQC sequence has been applied to both hydrated and dried samples of NH4-CBV712 as shown in Figure 6. The choice of CBV712 is governed by relatively low sensitivity of DHMQC; thus, taking a sample of the highest Al content is suggested to be reasonable. A hump observed on 27Al 14115


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Figure 7. HMQC MAS NMR (15 kHz 1H−29Si) spectra of dried (top) and hydrated (bottom) NH4-CBV712.

Finally, Al5 displays the same correlations (except 2.4 ppm peak) as AlO sites. The spectrum of dried NH4-CBV712 sample exhibits slightly dissimilar 2D contours; first, the vanishing of 8.8 ppm of OH group correlates with Al sites. This observation supports the assumption that this peak is strongly related to the effect of sample hydration. Another general observation is the same broadening of AlT sites as observed in 1D 27Al MAS NMR spectra in Figure 1. A noticeable difference with hydrated sample is a correlation of 2.7 ppm bridge OH peak with all Al species in the zeolite. Moreover, the 4.9 ppm OH group peak for all All species as well is not observed in the hydrated state of the same sample. The most important observation is the (5.0;

1.1) correlation peak, which supports the assumption made on the analysis of 1H MAS NMR spectra of hydrated samples where ∼1 ppm peak is pronounced that it belongs to some Al− OH interactions. The implementation of 1H−27Al D-HMQC spectrum of hydrated sample helps to assign this peak to OH groups bonded to EFAl (AlO) sites. The implementation of HMQC sequence with SR4 recoupling to a combination of 1H and 29Si nuclei is displayed in Figure 7.The numerous proximities are evidenced from this 2D spectrum. First, it is not surprising to observe the 1.7 ppm Si−OH group peak in proximity with all possible Si sites (including a correlation (1.5; −90.0) and (1.3; −98.5)) observed in simple 1D 29Si MAS NMR spectrum as the dried 14116



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sample still contains residual water leading to a possible hydrolysis of some bonds (including outer surface of zeolite particles). The most intensive peak is (1.7; −102.0), hinting to an easier hydrolysis of bridge SiOHAl groups as −102.0 29Si NMR peak contains 1 Al in the nearest proximity. The same correlation picture is observed for the 6.3 ppm NH4 peak, which is present almost on all Si species excepting −90.0 Q20 Si sites. The bridge OH groups at 2.9 ppm are slightly shifted downfield compared to those of the 1H MAS NMR spectrum, and they display proximity with Q40 and Q31 Si sites. The peak at 4.4 ppm of 1H NMR projection might correspond to the residual water in the sample that has large dipolar coupling, i.e., water molecules quenched in small micropores. Finally, we observed in the DF-SE 1H NMR spectra of hydrated samples that the ∼1 ppm peak is more pronounced in this 15 kHz 1 H−29Si HMQC MAS NMR spectrum, which is evidenced by the (0.8; −109.0) peak. The application of both 1H−29Si and 27 Al−1H HMQC spectra states that ∼1 ppm of OH groups displayed in Figure 5 represent a combination of Si−OH and Al−OH groups. The 1H−29Si HMQC spectrum of hydrated NH4-CBV712 is presented in Figure 7 (bottom). There is only one correlation (0.8; −109.0) excepting the water peak at 4.4 ppm of 1H projection appearing in hydrated state of the zeolite corresponding to some structural defects of zeolite structure that are evidenced by hydration procedure.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b02283. Technical aspects of experimental set up, 27Al 3QMAS spectra, water signal relaxation times, one pulse 1H NMR spectra, 1H DF-SE NMR spectra with variable SE time, NMR spectra of dehydrated sample (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Andrey S. Andreev: 0000-0003-0664-8820 Vincent Livadaris: 0000-0002-6031-9908 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Zeolyst Company for providing the samples for this study. The authors thank Ph.D. Julien Trébosc and Ph.D. Olivier Lafon from UCCS - UMR CNRS 8181 Université de Lille for the help with D-HMQC sequence. The consulting on pulse programming of Ph.D. Gerhard Althoff from Bruker is also acknowledged. No financial support from funding institutions is declared.

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CONCLUSIONS This work proposes a comprehensive analysis of different Al ratio zeolite depending on two types of hydration state, such as full hydration at 100% relative humidity and drying at 110 °C. The structural analysis is done on 27Al and 29Si nuclei, whereas the surface structure information is made possible by the application of a simple approach of a combination of dipolar filter (DF) and a spin echo (SE) sequences, which has never been applied to a probing the surface OH groups in high surface area materials such as zeolites. The link between individual nuclei spectra information has been established via 2D D-HMQC spectra of 27Al−1H or 1H−29Si correlations. In the present paper, DF-SE NMR technique has been demonstrated to have usability for a semiquantitative analysis of dried , i.e., partly hydrated, samples of zeolites of CBV series of nominal Si/Al ratios of 30, 15, and 6. Moreover, the technique is applicable for a qualitative analysis of fully hydrated samples. The DF-SE 1H NMR method is demonstrated to be extremely sensitive to ammonium in zeolite structure. A residual content of NH4+ is found in all samples even in those transformed to H form by calcination. Finally, the important structural stacking faults of FAl or EFAl species are found to be more pronounced in 1H DF-SE MAS NMR spectra in hydrated state as ∼1 ppm peak that can be crucial for understanding of relationships of structure and performance. These peaks represent a combination of both Si−OH and Al−OH species as follows from 2D DHMQC data. 29 Si MAS NMR spectra demonstrate a dependence on hydration state. The highest quantity of crystalline part is achieved in dried samples, whereas the best resolution of 27Al MAS NMR spectra is obtained in a fully hydrated state. Finally, a local order of Si framework given by fwhm parameter of crystalline Q40 peak correlates with increasing relative Al content, which is responsible for the distortion of zeolite structure.

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ABBREVIATIONS NMR, nuclear magnetic resonance; DF-SE, dipolar filter spin echo; MAS, magic angle spinning; FAl, framework aluminum; EFAl, extraframework aluminum

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REFERENCES

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