Revisiting Acidity of SnBEA Catalysts by Combined Application of

Article pubs.acs.org/JPCC

Revisiting Acidity of SnBEA Catalysts by Combined Application of FTIR Spectroscopy of Different Probe Molecules Vitaly L. Sushkevich,* Irina I. Ivanova, and Alexander V. Yakimov Department of Chemistry, Lomonosov Moscow State University, Leninskye Gory 1, bld. 3, 119991 Moscow, Russia S Supporting Information *

ABSTRACT: The Lewis and Brønsted acid properties of SnBEA zeolites as well as SnO2 supported on silica BEA have been examined by means of IR spectroscopy of adsorbed pyridine, 2,6-ditertbutylpyridine, and deuterated acetonitrile. Three types of surface sites have been detected and assigned to (i) framework Sn centers possessing Lewis acid properties; (ii) weak Brønsted acid sites associated with framework tin atoms; and (iii) nonframework SnO2 particles which possess Lewis acidity. The total amount of Lewis sites can be determined using pyridine as a probe molecule, while the type of Lewis sites can be distinguished by FTIR of adsorbed acetonitrile. The band at 2316 cm−1 is attributed to strong framework sites, whereas the band at 2287 cm−1 is attributed to nonframework sites. Brønsted acid sites can be characterized using 2,6ditertbutylpyridine (bands at 3363, 1613, and 1530 cm−1) and deuterated acetonitrile (band at 2308 cm−1). The relative amount of Lewis and Brønsted acid sites on SnBEA can be varied by treatment in hydrogen at different temperatures.

1. INTRODUCTION The discovery of SnBEA catalyst has been proven to give a significant breakthrough in the application of solid Lewis acids in aqueous media.1−3 The studied transformations include Meerwein−Ponndorf−Verley, Bayer−Villiger, and Oppenauer oxidation reactions,4−6 aldol condensation,7,8 and Diels−Alder reaction.9 Besides that, SnBEA zeolite was successfully applied in several carbohydrate conversion reactions like sugar isomerization,10−15 epimerization,16 and Cannizzaro-type reactions17−19 showing the exceptional activity and selectivity. While these recent achievements show that SnBEA zeolites have very high potential as catalysts1−19 and a lot of procedures for SnBEA synthesis have been extensively evaluated18,20−27 the structure and the environment of their active sites is poorly understood. Based on FTIR studies of acetonitrile adsorption and theoretical calculations, it has been reported that there are two types of framework Sn sites, co-called open and closed sites.28,29 Closed sites represent tin atoms with four Sn−O−Si linkages incorporated into the zeolite framework, and open sites correspond to partially hydrolyzed Sn sites with three Sn− O−Si linkages and one Sn−OH group (Scheme 1). Furthermore, it has been predicted that open and closed sites of SnBEA exhibit different catalytic behavior in various catalytic reactions.22,31−34 In particular, open sites were found to be more active with respect to closed sites. This effect was

attributed to the stronger Lewis acidity of the open sites and to the synergism with neighboring silanol groups (Scheme 1).31−33 In general, for the identification of open and closed acid sites two experimental techniques are applied: (i) FTIR spectroscopy of adsorbed acetonitrile24,27−30,35 mentioned above and (ii) 119 Sn MAS NMR spectroscopy.23,36−38 In most of the publications, the latter approach allows to observe two different 119 Sn signals centered at ca. −424 and −445 ppm, which were attributed to open and closed acid sites of SnBEA, respectively.23,37,38 The attribution of −424 ppm signal to open sites was confirmed by 1H−119Sn CPMAS NMR experiments.37 However, the results of Davis group36 and our recent work39 showed the presence of multiple signals within the region of −410−450 ppm which require careful revision of 119 Sn NMR signals attribution. FTIR spectroscopy of adsorbed acetonitrile which was first used for the identification of open and closed sites generally shows the presence of two adsorption bands at 2316 and 2308 cm−1, which were attributed to open and closed sites, respectively.28,29 However, the presence of two bands in FTIR spectra of adsorbed acetonitrile was not confirmed by several recent reports.27,30,35 In the latter studies, the only the band at 2310 cm−1 was observed. This band was not resolved even after the evacuation of excess of CD3CN. The observation of two bands in the earlier studies was explained by solvation effects.30 The band with similar position was observed during the adsorption of deuterated acetonitrile over SnBEA samples

Scheme 1. Proposed Configurations of Open and Closed Sites in SnBEA

Received: March 8, 2017 Revised: May 15, 2017 Published: May 16, 2017 © 2017 American Chemical Society

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DOI: 10.1021/acs.jpcc.7b02206 J. Phys. Chem. C 2017, 121, 11437−11447

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The Journal of Physical Chemistry C obtained by post-synthetic treatment.26,27 In this case, the exceptional formation of closed sites during the post-synthesis was proposed to explain the presence of the only band at 2310 cm−1. In several reports, SnBEA was exchanged by alkaline ions or ammonia38,40 expecting the selective substitution of open sites and decrease of the only band at 2316 cm−1 in CD3CN spectra. But the spectra of ion-exchanged samples showed the decrease of both bands at 2316 and 2308 cm−1.38,40 It is important to mention that adsorption of pyridine, extensively studied by different groups15,21−23,27 showed the presence of the only one type of Lewis acid sites and the absence of strong Brønsted acid sites. The analysis of all these data points out that the results presented so far are rather contradictory. In view of that, it appears that a more systematic characterization of tin sites in SnBEA is needed to get deeper insight into the nature of tin sites, which is mandatory for the assessing the mechanism of tin sites performance and for establishing structure−activity relationships. The objective of the present study was to clarify the nature and strength of Sn-sites in SnBEA by combined application of infrared spectroscopy of different probe molecules. Pyridine (Py) and 2,6-ditertbutylpyridine (DTBPy) were used for probing strong to weak Lewis and Brønsted acid sites, respectively. The assignment of the bands observed in the spectra of deuterated acetonitrile CD3CN was carefully checked and revised. The correlation between the data obtained using different probe molecules has been established.

pore size analyzer. Prior to the measurements, the samples were evacuated at 623 K. Powder X-ray diffraction (XRD) patterns were recorded with a Bruker D2 diffractometer, applying Cu Kα radiation at the wavelength of 1.5456 Å. SEM images were obtained with a scanning electron microscope LEO EVO 50XVP (Zeiss). UV− vis spectra (DRUV) were recorded on Evolution 600 spectrometer using BaSO4 as reference standard in the spectral region of 190−320 nm. For graphical representation Kubelka− Munk units were used. 29 Si MAS NMR was performed using a Bruker Avance-400 spectrometer, operating at a resonance frequency of 79.46 MHz with a spinning rate of 10 kHz, pulse length of 3 μs, and recycle time of 20 s. The 29Si chemical shifts are reported relative to TMS. FTIR measurements were carried out on a Nicolet Protégé 460 spectrometer equipped with a MCT detector. Room temperature cell was used for acetonitrile-d3, pyridine (Py), and 2,6-ditertbutylpyridine (DTBPy) adsorption. In a typical experiment, 15 mg of a sample was pressed into a selfsupported wafer (2 cm2). A sample was placed in a glass FTIR cell and activated as follows: ramping 5 K·min−1 to 723 K and dwell for 3 h. After the activation the sample was cooled down to room temperature and the reference spectrum was recorded (128 scans at a 4 cm−1 resolution). Deuterated acetonitrile CD3CN (99.5% supplied by Cambridge Isotope Laboratories), pyridine (Aldrich), and 2,6-ditertbutylpyridine (99.5%, Aldrich) aliquots were gradually introduced into the cell, and IR spectra were subsequently recorded. Spectra of surface species were obtained by the subtraction of the reference spectra from the spectra of the samples with the adsorbate. The processing of the spectra was carried out using OMNIC 7.3 package. The extinction coefficient for DTBPy band at 3363 cm−1 was estimated from the comparison of Py and DTBPy adsorption spectra over AlBEA as shown in the Supporting Information (Figure S1).

2. EXPERIMENTAL SECTION 2.1. Catalysts Preparation. SnBEA was prepared according to the procedure described elsewhere.25 In brief, this procedure included mixing of tetraethylammonium hydroxide (TEAOH; 40 wt %, Sigma-Aldrich) with tetraethyl orthosilicate (TEOS, 98%, Reakor), stirring to obtain clear solution and addition of aqueous solution of SnCl4·5H2O (98%, SigmaAldrich). Then, the mixture was stirred to allow TEOS to hydrolyze and the formed ethanol to evaporate completely. Afterward aqueous solution of HF (40 wt %, Fluka) was added slowly, forming a white rigid gel. Water was evaporated to achieve H2O/SiO2 ratio of 6.8 as proposed in ref 41. The molar composition of the final gel was as follows: 1.0 SiO2/0.27 TEA2O/0.005 SnO2/0.54 HF/6.8 H2O. The gel was transferred into a Teflon-lined autoclave and crystallized at 413 K for 7 days. The solid product obtained was filtered, washed with deionized water, dried at 353 K and calcined at 823 K for 4 h in a flow of air. The sample was designated as SnBEA. Pure silica SiBEA sample was synthesized via the same procedure except with the addition of SnCl4 into the synthetic gel. Crystallization time was reduced to 3 days. SnO2/SiBEA catalyst was prepared by incipient wetness impregnation of SiBEA with aqueous solution of SnCl4 to reach Si/Sn = 200 molar ratio, followed by drying at 373 K and calcination at 773 K for 4 h in a flow of air. 2.2. Catalysts Characterization. The elemental analysis was performed using energy dispersive X-ray fluorescence spectroscopy (EDXRF). Prior to the analysis, the samples were mixed with B(OH)3 and pressed in self-supporting wafers. The wafers were analyzed using a Thermo Scientific ARL Perform’x WDXRF. N2 sorption−desorption isotherms were measured at 77K using a Micromeritics ASAP-2000 automatic surface area and

3. RESULTS AND DISCUSSION 3.1. Catalysts Structure and Composition. The chemical and textural characteristics of the catalysts are given in Table 1. Table 1. Catalysts Characteristics chemical composition sample SnBEA SiBEA SnO2/SiBEA

Si/Sn Sn, μmol·g−1 211 202

78 0 82

total pore volume (cm3/g)

micropore volume (cm3/g)

0.29 0.33 0.31

0.22 0.23 0.22

The amounts of tin incorporated into SnBEA and loaded onto siliceous BEA are close to the expected values. Nitrogen adsorption−desorption data point to high micropore volume of zeolitic catalysts (0.2 cm3/g). All SnBEA and SiBEA samples had reversible Type-I adsorption/desorption isotherms with a step at p/p0 < 0.01, typical for microporous solids. Deposition of SnO2 results in slight decrease of micropore volume of SnO2/BEA sample with respect to pure silica BEA. This decrease in volume indicates that a part of tin precursor has penetrated into zeolite pores forming small SnO2 particles. But these results do not exclude the deposition of tin oxide particles on the external surface of zeolite crystals. 11438


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The Journal of Physical Chemistry C The structure of the catalysts was examined by XRD technique (Figure 1). The powder XRD patterns of all samples

Figure 3. materials. Figure 1. XRD patterns of Sn-containing zeolites. For SnO2/SiBEA, sample SnO2 phase is marked with asterisks.

29

Si NMR spectra of SnBEA, SiBEA, and SnO2/SiBEA

local order caused by the incorporation of heteroatoms into the zeolite structure. No significant Q3-resonance signals indicative to Si(OSi) 3 OH structural defects are observed hence confirming high crystallinity of BEA materials used in the present study. 3.2. FTIR Study of Pyridine and 2,6-Ditertbutylpyridine Adsorption. The infrared spectroscopy of adsorbed pyridine and substituted pyridines is frequently used to characterize the nature and amount of acid sites over solid catalysts, in particular, zeolites. Due to its comparatively small kinetic diameter pyridine (Py) is able to enter the porous system of large pore and medium pore zeolites and to interact with both Brønsted and Lewis sites, revealing specific IR absorption bands. On the contrary, 2,6-ditertbutylpyridine (DTBPy) does not interact with Lewis acid sites due to the steric hindrance of tert-butyl groups. However, it is particularly useful for the characterization of weak Brønsted acid sites due to its higher basicity with respect to pyridine. The spectra of surface species observed over SnBEA sample at different Py coverages are shown in Figure 4. Introduction of first small dose of Py into the cell leads to the appearance of the bands at 1610, 1576, 1491, 1451 cm−1 and 1596, 1589, 1443 cm−1 which can be assigned to the Lewis acid sites21−23 and Hbonded pyridine, respectively. The bands at 1582, 1482, and 1439 cm−1 which appear at higher Py pressures correspond to the physically adsorbed pyridine. The presence of Lewis sites could be assigned to tin atoms, incorporated in zeolite BEA framework. No bands typical for protonated pyridine species are detected, which is in line with previous studies15,21−23,27 showing that no Brønsted sites strong enough to protonate pyridine are present on SnBEA materials. At the range of OH vibrations, the development of three negative peaks at 3745, 3736, and 3717 cm−1 is observed due to the interaction of pyridine with silanol groups. The position of the band at 3745 cm−1 is typical for terminal surface silanols, whereas bands at 3736 and 3717 cm−1 are generally assigned to vicinal silanols and surface H-bonded silanol chains, respectively.45 Figure 5a compares the IR spectra of SnBEA with those of SnO2/SiBEA in the range of 1400−1650 cm−1 after the adsorption of 0.1 Torr of pyridine at ambient temperature.

show typical features of well-crystalline BEA-type zeolite. No peaks due to crystalline SnO2 or any other crystalline impurity phases are detected for SnBEA, whereas in the case of SnO2/ SiBEA sample the reflections arising from tin oxide are detected. This result shows that deposition of tin by impregnation from aqueous solution results in deposition of SnO2 phase on SiBEA. DRUV spectra (Figure S2) confirm the presence of SnO2 phase in SnO2/SiBEA sample revealing the line at 250 nm, which is typical for tin oxide.15,19,26,27 SnBEA material also shows the presence of small peak at 245 nm due to the small amount of SnO2, which is not incorporated into zeolite structure during the synthesis. SEM microphotographs of SnBEA and SiBEA materials show well-crystalline materials with the crystal size of 5−10 μm typical for the zeolites synthesized in fluoride media (Figure 2). The shape of the crystals is in line with the results previously reported by Tolborg et al.25

Figure 2. SEM images of (a) SiBEA and (b) SiBEA samples. 29

Si MAS NMR spectra of SnBEA (Figure 3) show signals at ca. −111.3, −111.9, −112.8, and −115.5 ppm in the Q4 range, which can be assigned to Si(OSi)4 species occupying different crystallographic positions of BEA structure. The lines observed over SnBEA are slightly broader with respect to pure silica BEA.42 The line broadening has been reported for ZrBEA42,43 and for TiBEA materials44 and was explained by the decrease of 11439


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Figure 4. FTIR spectra of Py adsorbed over SnBEA within the pressure range of 0.01−0.3 Torr.

Figure 5. FTIR spectra of (a) pyridine, (b) 2,6-ditertbutylpyridine, and (c) acetonitrile-d3 adsorbed over SnBEA and SnO2/SiBEA.

To study Brønsted acidity of tin containing samples, DTBPy was selected as a probe molecule. DTBPy is stronger base than pyridine and can be used to probe weak acid sites. According to ref 48., all Brønsted acid sites in BEA zeolites should be accessible for DTBPy. DTBPy adsorption was performed on the SnBEA sample, with sequential evacuation at 323 K for 1h. The spectra obtained in the range of 1320−1680 cm−1 are shown in Figure 5b. The bands at 3363, 1613, and 1530 cm−1 correspond to vibrations of protonated DTBPy. In the range of OH vibrations, three negative peaks are observed (Figure S3) as in the case of pyridine adsorption. The SnO2/SiBEA sample does not show any Brønsted acid sites hence indicating the necessity of framework tin atoms for generating of Brønsted acid sites. Weak Brønsted acid sites were also reported for titanium silicalite catalysts,49 ZrBEA50 and predicted for other M4+doped zeolites (M = Ti, Zr, Ge, Sn, Pb)51 and were assigned to M−OH···OH−Si defect sites (Scheme 1). The existence of such defect sites is generally assumed for tin BEA materials and

Almost the same bands corresponding to pyridine adsorption on OH groups and physically adsorbed Py are detected for both catalysts. However, the band assigned to Py interaction with Lewis acid sites show different frequency: 1607 cm−1 in the case of SnO2/SiBEA and 1610 cm−1 in the case of SnBEA. The position of ν8a vibration mode in the spectra is very sensitive to the strength on Lewis sites,46 thereby it could be suggested that Lewis sites in SnBEA have higher acidity with respect to those in SnO2/SiBEA. Integration of the area of the band at 1451 cm−1 and calculation47 of the amount of Lewis sites gives the value of 65 μmol·g−1 for SnBEA. This amount is 20% lower than the Sn content in SnBEA, which could be due to the presence of extraframework Sn-sites in the sample studied, as confirmed by DRUV spectra (Figure S2). The amount of Lewis acid sites determined for SnO2/SiBEA sample (28 μmol·g−1) indicates that significant part of tin atoms is not accessible for Py and points to the low dispersion of SnO2 species on the surface of SiBEA zeolite. 11440


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Table 2. Relative Amount of Lewis and Brønsted Acid Sites Determined from the Spectra of Adsorbed Py and DTBPy sample

Sn, μmol·g−1

Py adsorption, Lewis sites, μmol·g−1

DTBPy adsorption, Brønsted sites, μmol·g−1

SnBEA SnO2/SiBEA

78 82

65 28

15 0

Figure 6. FTIR spectra of CD3CN adsorbed over SnBEA.

quantitative analysis of framework Lewis sites in SnBEA.28−30,54 However, no IR bands corresponding to CD3CN adsorbed on Lewis acid sites of extraframework SnO2 species supported on silica BEA were previously detected.30 Figure 6 shows IR spectra collected upon deuterated acetonitrile adsorption on activated SnBEA. The bands within the 2260−2340 cm−1 range are associated with ν(CN) stretching vibration, while a weak band at 2116 cm−1 is due to δs(CD3) vibration. The bands within the range of 3000−4000 cm−1 in the difference spectra are due to different types of surface OH-groups interacting with acetonitrile. In a typical experiment, CD3CN was gradually adsorbed dose per dose until the complete saturation of adsorption sites and the appearance of the band at 2268 cm−1 corresponding to physically adsorbed CD3CN. Introduction of the first doses of acetonitrile into the cell leads to the appearance of the bands at 2316, 2287, and 2275 cm−1 accompanied by slight changes in the ν(OH) vibrations region. The gradual increase of CD3CN coverage leads to the appearance of new band at 2308 cm−1 in the range of CN vibrations (Figure 6). The fact that this band appears only after the introduction of significant amount of CD3CN on the sample and has a relatively low frequency, indicates that the corresponding sites have weak acidity. Upon increasing of the acetonitrile loading above ∼0.1 mmol g−1, a band at 2268 cm−1 appear due to physically adsorbed CD3CN. The plot of the intensities of each band observed versus CD3CN coverage is shown in Figure S4. Referring the literature data,30,45,50,53 the band at 2275 cm−1 could be attributed to the ν(CN) vibrations of acetonitrile

assigned to open sites, formed by partial hydrolysis of Si−O− Sn linkages. For the quantification of weak Brønsted acid sites measured by DTBPy adsorption, an extinction coefficient for DTBPy band at 3363 cm−1 was estimated from the results on pyridine and DTBPy adsorption over AlBEA (for the details, see the Supporting Information (Figure S1)). Assuming pyridine extinction coefficient to be ε(1545 cm−1) = 1.54 cm·μmol−1,52 the value of DTBPy extinction coefficient was found to be ε(3363 cm−1) = 9.7 cm·μmol−1. It should be mentioned that the extinction coefficients were obtained by indirect measurement and therefore could be slightly different from the exact values. Calculation of the amount of Brønsted acid sites for SnBEA gives the value of 15 μmol·g−1, which is significantly lower than the Sn content (Table 2). This fact indicates that the only part of Sn atoms is involved in formation of weak Brønsted sites over SnBEA. In summary, the adsorption of Py and DTBPy on SnBEA reveals the presence of both Brønsted and Lewis acidic sites associated with Sn atoms incorporated in BEA framework. Tin oxide supported on SiBEA reveals only the presence of Lewis acid sites, which are weaker with respect to Lewis sites associated with Sn atoms incorporated into zeolite framework. 3.3. FTIR study of Acetonitrile-d3 Adsorption. Deuterated acetonitrile has been found to be a good probe molecule for examination of Lewis acid sites of different materials.53 In the previous studies devoted to SnBEA, it was suggested that acetonitrile-d3 can be used for the qualitative and 11441


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Figure 7. FTIR spectra of Py adsorbed over SnBEA after pretreatment in hydrogen at different temperatures for 40 min.

titration of tin atoms during pyridine adsorption and observation of two distinct bands in the region of ν8a vibrations mode (1610 and 1607 cm−1) over SnBEA. Assuming 100% of tin in SnO2/SiBEA sample form extraframework species, the relative content of SnO2 phase in SnBEA material can be estimated from FTIR spectra of adsorbed acetonitrile-d3 (Figure 5c). For the quantification of the area of the band at 2287 cm−1 for SnBEA and SnO2/SiBEA, a curve-fitting procedure with Gauss−Lorentz peaks with fixed fwhm was used. The obtained values showed that in the case of SnBEA up to 20% of all tin measured by elemental analysis forms extraframework SnO2 species. The analysis of the literature data on acetonitrile adsorption over SnBEA materials points that in most of the reports the band at 2287 cm−1 was not detected, probably due to high quality of the materials associated with low concentration of nonframework tin. Osmundsen et al.24 observed the similar band over SnMFI sample but have not assigned it. The presence of the band at 2287 cm−1 could also be missed in some of the previous studies due to the procedure used for acetonitrile adsorption, implying nonequilibrium desorption of CD3CN in dynamic vacuum from the saturated sample.28,29 In this case some kinetic effects could mask 2287 cm−1 band presence in the final spectra. Thus, our FTIR study of acetonitrile adsorption over SnBEA and SnO2/SiBEA confirmed the attribution of the band at 2275 cm−1 to silanol groups and the bands at 2316 and 2308 cm−1 to strong and weak acid sites generated by framework tin atoms and suggested that the band at 2287 cm−1 can be assigned to acid sites generated by extraframework SnO2 species. To verify further the attribution of these bands to Lewis or Brønsted sites we tried to correlate the results obtained for acetonitrile adsorption with those of Py and DTBPy adsorption. 3.4. Relationship between FTIR Data Obtained Using Different Probe Molecules. Summarizing the data obtained using different probe molecules, it can be concluded that, on the one hand, probing of active sites with pyridine and DTBPy reveals the presence of Lewis and Brønsted acid sites over SnBEA, whereas, on the other hand, adsorption of CD3CN points to the existence of extraframework Sn sites and two

H-bonded with silanol groups, The detailed revision of the OH vibrations region reveals the presence of the negative bands at 3717, 3736, and 3745 cm−1 (Figures 6 and S5), which interacts with acetonitrile CN group giving typical for H-bonded silanols broad band at 3430 cm−1. Interaction of all these silanols with acetonitrile proceeds simultaneously and leads to the appearance of the only one band of perturbed silanols indicating similar acidity of all Si−OH groups presented on SnBEA. The spectra obtained in this region are similar to those observed during pyridine adsorption on SnBEA (Figure 4). It is generally assumed that the bands at 2316 and 2308 cm−1 correspond to the adsorption of acetonitrile over open and closed sites.28,29 The band at 2287 cm−1 was recently speculated to be due to the double-hydrolyzed Sn open sites, but without any experimental evidence.54 Therefore, in further experiments, we tried to evaluate the assignment of this band by comparing the spectra of acetonitrile-d3 adsorbed over SnBEA and SnO2/ SiBEA (Figure 5c). The spectrum of SnO2/SiBEA shows an intensive peak at 2287 cm−1 with a shoulder at 2275 cm−1, whereas the bands at 2316 and 2308 cm−1 are not detected. Thereby, it could be suggested that the adsorption band at 2287 cm−1 is due to extraframework SnO2 species. To exclude grafting of Sn species on the surface of silica BEA in SnO2/ SiBEA sample, which can give double-hydrolyzed Sn open sites, FTIR spectra of acetonitrile-d3 adsorbed on the mechanical mixture of SnO2 and silica BEA were collected (Figure S6). The spectrum obtained is very similar to those observed for SnO2/ SiBEA, hence confirming the attribution of the band at 2287 cm−1 to the Lewis sites generated by the extraframework SnO2 species. Interestingly, no shift of the band position at 2287 cm−1 was observed for the samples studied thus the synthesis protocols used and, therefore, the dispersion of SnO 2 extraframework species might be completely different. Therefore, we assume that the difference in the dispersion for the studied samples in not sufficient to affect the position of FTIR bands of adsorbed probe molecules. Observation of the line at 2287 cm−1 over SnBEA suggests the presence of significant fraction of extraframework species in this sample. The formation of such SnO2 particles is in line with DRUV (Figure S2) measurements. It also explains incomplete 11442


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Figure 8. FTIR spectra of DTBPy adsorbed over SnBEA after pretreatment in hydrogen at different temperatures for 40 min.

Figure 9. FTIR spectra of CD3CN adsorbed over SnBEA after pretreatment in hydrogen at different temperatures for 40 min.

that tin-free SiBEA and SnO2/SiBEA samples does not reveal any changes after the reduction in hydrogen (Figure S10). In further experiments, the SnBEA sample was subjected to stepwise reduction at different temperatures directly in the IR cell prior to adsorption of probe molecules. After treatments at each temperature for 40 min followed by evacuation at 673 K, the acid properties were examined by FTIR measurements of adsorbed Py, DTBPy and CD3CN. Spectra of pyridine adsorbed over SnBEA samples reduced at 553−673 K are shown in Figure 7. The results point to gradual decrease of the intensity of the bands at 1610 and 1451 cm−1 indicating progressive loss of the Lewis acid sites upon reduction of Sn(IV) into Sn(II). Interesting trend is observed for the ν8a mode, the decrease of this band intensity is followed by the gradual shift of its position from 1610 cm−1 for untreated SnBEA to 1607 cm−1 for the sample treated at 673 K. It is noteworthy that the band at 1607 cm−1 was observed over SnO2/SiBEA sample (Figure 5a). Observation of this band after complete reduction of framework Sn sites suggests that

framework Sn sites. In attempt to correlate the results of Py, DTBPy, and CD3CN adsorption, we tried to vary the content of Sn(IV) framework sites, which induces the acidity of SnBEA. This variation was achieved by treatment of SnBEA with hydrogen at different temperatures. Such treatment was shown55,56 to convert Sn(IV) into Sn(II) without zeolite destruction. The temperature region was selected based on the results reported by Lázár et al.,56 who demonstrated that 80% SnIV in SnMFI can be reduced at 673 K as evidenced by Mössbauer spectroscopy. Using this approach, a series of samples with different Sn(IV) content was prepared. The preservation of BEA structure was confirmed by XRD (Figure S7). The results revealed that treatment of SnBEA in hydrogen at 673 K leads to the reduction of all framework Sn sites, as was determined by TPR in H2/He flow (Figure S8). Furthermore, it was demonstrated that the changes in Sn oxidation state are reversible and that treatment in oxygen at 673 K can result in reoxidation of Sn sites (Figure S9). It also should be mentioned 11443


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Figure 10. Relative intensities of acetonitrile adsorption bands versus H2 treatment temperature and (a) amount of Brønsted acid sites (band 3363 cm−1) estimated from the results of FTIR spectroscopy of adsorbed DTBPy and (b) amount of framework Lewis acid sites (band 1610 cm−1) estimated from the results of FTIR spectroscopy of adsorbed Py. The highest concentration of Lewis and Brønsted sites corresponds to SnBEA sample with Si/Sn = 100 and is shown for comparison.

procedure of all bands was carried out in the structure sensitive range with Gauss−Lorentz peaks with fixed fwhm. The intensity of the band at 2287 cm−1 does not reveal any correlation neither with Lewis nor with Brønsted framework Sn sites hence confirming the assignment of this band to the CD3CN adsorption over nonframework SnO2 particles, which are not reduced in hydrogen at 673 K.55 On the contrary, the band at 2308 cm−1 shows linear correlation both with Lewis and Brønsted acid sites measured by Py and DTBPy (Figure 10). The band at 2316 cm−1 also shows correlation with Lewis and Brønsted framework Sn sites for the samples pretreated at 553−593 K. At higher pretreatment temperature, all sites corresponding to the band at 2316 cm−1 are reduced and no correlation is observed. All these data suggest that the bands at 2308 and 2316 cm−1 are due to framework tin sites which can generate both Lewis or Brønsted sites. The similar dependences observed for Lewis and Brønsted sites (Figure 10) suggest that these sites can be interconnected. However, no definite attribution of the bands at 2316 and 2308 cm−1 to Lewis or Brønsted sites can be done on the basis of correlations obtained. For further verification of the assignment of these bands, the acetonitrile-d3 adsorption was carried out over SnBEA with preadsorbed DTBPy. 3.5. Acetonitrile Adsorption over SnBEA with Preadsorbed DTBPy. In our previous work,50 we have shown that preadsorption of DTBPy over the ZrBEA materials allows selective poisoning the Brønsted acid sites leaving the Lewis sites intact. In the present study, we used this approach to clarify the attribution of acetonitrile adsorption bands at 2316 and 2308 cm−1. Since both DTBPy and acetonitrile-d3 are stable and nonreactive toward each other under the measurements conditions and also have different nature of interaction with the acid sites of SnBEA, we were able to study the adsorption of acetonitrile over the sample with preadsorbed DTBPy. In a series of experiments, the fresh SnBEA sample was exposed to different equilibrium pressures of DTBPy within 0.01−0.2 Torr at 373 K for 1 h. After adsorption samples were evacuated at 323 K for 3 h to ensure that all weakly and unselectively bonded diterbutylpyridine was desorbed. After cooling to ambient temperature, the small doses of acetonitrile were adsorbed over the samples with preadsorbed DTBPy. The

hydrogen interacts only with framework tin sites and does not affect SnO2 species. Furthermore, SnO2/SiBEA sample does not reveal any hydrogen consumption in TPR experiment. Indeed, the reduction of bulk SnO2 was found to occur only at the temperatures higher than 900 K.57 The band at 1610 cm−1 can be therefore attributed to framework Lewis Sn sites, whereas the band 1607 cm−1 can be attributed to nonframework Lewis Sn sites. Results on DTBPy adsorption over H2-treated SnBEA samples are shown in Figure 8. The increase of the temperature of treatment leads to stepwise decrease of the intensity of the bands at 3363, 1613, and 1530 cm−1, corresponding to weak Brønsted acid sites, and their complete disappearance after reduction at 673 K. This result confirms that the origin of Brønsted acid sites is also related to presence of Sn(IV) sites in the framework of SnBEA. Acetonitrile-d3 adsorption spectra within the range of 2200− 2400 cm−1 after stepwise H2 treatment are summarized in Figure 9. The increase of pretreatment temperature leads to dramatic changes in the intensities of the bands at 2316 and 2308 cm−1, hence confirming their attribution to the acetonitrile adsorption over framework tin sites. The sites associated with the acetonitrile band at 2316 cm−1 interact with hydrogen much faster than those related to 2308 cm−1 band: the former band disappears from the spectra after the treatment at 573 K, whereas the later persists until 673 K. The behavior of the species associated with the 2316 cm−1 acetonitrile adsorption band confirms their higher reactivity with respect to the 2308 cm−1 band as it was suggested in previous reports.27,28,54 It should be noted that the bands at 2287, 2275, and 2268 cm−1 do not show any changes with hydrogen treatment, which is in line with our previous assignments discussed in sections 3.2 and 3.3. Thereby, the CD3CN adsorption bands exhibit different behavior during the H2 treatment of SnBEA sample at various temperatures. In an attempt to assign these bands to Lewis or Brønsted acid sites, their relative intensities were plotted versus the amount of framework Lewis (band 1610 cm−1) and Brønsted (band 3363 cm−1) sites determined by FTIR spectroscopy of adsorbed Py and DTBPy as shown in Figure 10. For all spectra obtained over SnBEA samples, a curve-fitting 11444


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Figure 11. FTIR spectra of CD3CN adsorbed over SnBEA with preadsorbed DTBPy at different coverage.

2308 cm−1 band to acetonitrile adsorption over weak Brønsted acid sites. The presence of weak Brønsted sites on SnBEA was recently confirmed by Chen et al.59

spectra of DTBPy adsorbed and corresponding spectra of acetonitrile-d3 are shown in Figure 11a and b, respectively. The results show that gradual poisoning of Brønsted acid sites with DTBPy (band at 3363 cm−1) is accompanied (Figure 11a) by the simultaneous decrease of the intensity of the band at 2308 cm−1 in the region of acetonitrile-d3 vibrations (Figure 11b). It is important to note that the intensity of the other bands at 2316, 2287, 2275, and 2268 cm−1 do not change significantly and the appearance of any additional bands was not detected. Linear correlation of the area of the band corresponding to protonated DTBPy (3363 cm−1) with the area of the band at 2308 cm−1 (Figure S11) suggests that the latter can be attributed to the Brønsted acid sites in SnBEA. Summarizing, preadsorption of DTBPy revealed that the band at 2308 cm−1 can be assigned to the weak Brønsted acid sites interacting with CN bond of acetonitrile, while 2316 cm−1 belongs to Lewis acid sites. The origin of these Brønsted acid sites is not clear. The analysis of the FTIR spectra in the region of OH-groups vibrations (Figures 4, 6, and S2) shows that no bands typical for bridging OH groups45 in the region of 3615−3595 cm−1 can be distinguished. The bands observed are all in the range of 3745−3717 cm−1 typical for isolated Si−OH groups. Therefore, we can assume that Brønsted acidity is associated with isolated silanol groups, which are somehow affected by Sn framework atoms. Unfortunately, the careful analysis of the intensities of the bands at 3745, 3736, and 3717 cm−1 does not reveal any correlation with the acetonitrile-d3 adsorption band at 2308 cm−1. More detailed study addressing this question is now in progress. Our attribution of the CD3CN bands at 2316 and 2308 cm−1 is in contradiction with generally accepted assignment of these bands to open and closed sites, which was proposed by Boronat and co-workers based on DFT calculations.28,29 However, no experimental confirmation of this assignment has been reported in the literature. The analysis of FTIR data on CD3CN adsorption over various SnBEA materials shows that two bands at 2316 and 2308 cm−1 are usually observed for the materials obtained by direct hydrothermal synthesis,24,28,38,54 whereas the materials synthesized by post-treatment procedure from dealuminated BEA zeolites show mostly the band at ca. 2308 cm−1.15,26,27 The comparison of this observation with the data obtained for ZrBEA,58 which suggest that post-treatment leads mostly to formation of open sites, supports our attribution of

4. CONCLUSIONS The combined application of IR spectroscopy of adsorbed pyridine, 2,6-ditertbutylpyridine, and deuterated acetonitrile is demonstrated to be a powerful tool for the characterization of different Sn sites over dehydrated SnBEA. The following three types of sites are identified: • framework Sn sites possessing strong Lewis acid properties; • nonframework SnO2 particles with weak Lewis acidity; • weak Brønsted acid sites associated with the framework tin atoms. The total amount of Lewis sites can be determined using Py as probe molecule, while the type of Lewis sites can be distinguished by FTIR of adsorbed acetonitrile. The band at 2316 cm−1 is attributed to strong framework sites, whereas the band at 2287 cm−1 is attributed to nonframework weak sites. Brønsted acid sites can be characterized using 2,6-ditertbutylpyridine (bands at 3363, 1613, and 1530 cm−1) and deuterated acetonitrile (band at 2308 cm−1). It has been demonstrated that the amount of framework Brønsted and Lewis sites in SnBEA can be varied by the treatment in hydrogen at different temperatures. The combined application of FTIR spectroscopy of adsorbed pyridine, 2,6-ditertbutylpyridine, and deuterated acetonitrile will allow one to benefit in determination of structure-toperformance relationship in numerous reactions catalyzed by SnBEA and to control the quality of synthesized catalysts.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b02206. Measurements of DTBPy extinction coefficient; DVUV spectra and FTIR spectra of the samples; XRD and FTIR spectra of adsorbed acetonitrile for reoxidized SnBEA (PDF) 11445


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AUTHOR INFORMATION

Corresponding Author

*Phone: +7(495)939-3570. Fax: +7(495)939-3570. E-mail: [email protected]. ORCID

Vitaly L. Sushkevich: 0000-0002-3788-8969 Irina I. Ivanova: 0000-0002-8742-2892 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS V.L.S. and I.I.I. thank the Russian Science Foundation for the financial support (Grant No. 14-23-00094). A.V.Y. acknowledges Haldor Topsoe for Ph.D. fellowship. The authors thank Dr. Andrey Popov for DTBPy extinction coefficient measurements. Dr. Peter N.R. Vennestrøm is gratefully acknowledged for the fruitful discussion during the preparation of the manuscript.

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REFERENCES

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Revisiting Acidity of SnBEA Catalysts by Combined Application of

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