Spectroscopic Evidence for Open and Closed Lewis Acid Sites in

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Spectroscopic Evidence for Open and Closed Lewis Acid Sites in ZrBEA Zeolites Vitaly L. Sushkevich, Alexandre Vimont, Arnaud Travert, and Irina Igorevna Ivanova J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 08 Jul 2015 Downloaded from http://pubs.acs.org on July 8, 2015

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The Journal of Physical Chemistry

Spectroscopic Evidence for Open and Closed Lewis Acid Sites in ZrBEA Zeolites Vitaly L. Sushkevicha, Alexandre Vimont,b Arnaud Travertb and Irina I. Ivanovaa a

Department of Chemistry, Lomonosov Moscow State University, Leninskye Gory 1, bld. 3, 119991 Moscow, Russia b

Laboratoire Catalyse et Spectrochimie, CNRS-ENSICAEN-Université de Caen, 6 Boulevard Marechal Juin, Caen Cedex 14050, France

Abstract The Lewis acid properties of a series of isolated and well-defined Zr centers in a series of ZrBEA zeolites with different Si/Zr ratio and ZrO2 supported on silicious BEA have been investigated by means of IR spectroscopy of adsorbed probe molecules. Different types of Zr centers have been detected and assigned to (i) four-fold coordinated Zr atoms in framework positions that weakly interact with Lewis bases (closed Lewis site), and (ii) stronger Lewis acid sites associated with four-fold coordinated Zr centers with one Zr–OH hydroxyl group resulting from the hydrolysis of Zr–O–Si bond (open Lewis site). It has been shown that deuterated acetonitrile specifically probes open Lewis sites in ZrBEA, while the use of CO allows distinguishing between the two types Lewis sites. The relative amount of open and closed sites does not linearly depend on the Si/Zr ratio in the sample, which can point to different mechanisms of Zr incorporation from Zr-rich and Zr-poor gels.

Keywords: CO, Zr beta, infrared spectroscopy, Lewis acid sites, CD3CN, Py, DTBPy

__________________________________________ *Corresponding author: Tel.: +7(495)939-3570; Fax: +7(495)939-3570; E-mail address: [email protected].

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1. Introduction The development of isolated and well-defined single-site Lewis acid solid catalysts that are active and selective for new environmentally friendly chemical processes is an urgent need and a challenging scientific target.1 A very successful example of such materials is SnBEA, a catalyst for a series of reactions whose implementation represented an important breakthrough in the use of Lewis acids as heterogeneous catalysts.2 A more recent example is ZrBEA, which contains Zr(IV) sites introduced in the β-zeolite framework.3 The uniform distribution of isolated Zr Lewis acid sites, in combination with unique porous structure of the material, result in unravalled catalytic performances in such catalytic processes as Lewis acid-catalyzed transformation of levulinic acid into γ-valerolactone4, etherification of alcohols5, the Meerwein–Ponndorf–Verley–Oppenauer (MPVO) oxidation-reduction reaction6,7, and butadiene synthesis from ethanol.8 While these recent developments show that ZrBEA zeolites have very high potential as catalysts, the structure and the environment of its active sites is poorly understood with respect to that of SnBEA or TiBEA catalysts. It is well accepted that the latter possess two types of Lewis active sites – co-called open and closed sites.9-11 Closed sites represent transition metal ions fully incorporated in the zeolite framework, having four M-O-Si linkages and open sites correspond to partially hydrolyzed M sites with three M-O-Si linkages and one M-OH group:

Si

Si

O O Si

O O

M

O Si

Si

O M

O

H

O Si

Si Open site

Closed site Scheme 1

It is important to mention that for SnBEA and TiBEA it has been demonstrated that open and closed sites exhibit different catalytic behavior in various catalytic reactions.2,9-11 In particular open


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sites were found to be much more reactive with respect to closed sites. This effect was attributed to the stronger Lewis acidity of the latter and to their better accessibility for bulky molecules4-11. However, to the best of our knowledge, no study has evidenced the presence of closed and open Lewis acid sites on ZrBEA zeolites. For instance, adsorption of pyridine, well-studied by different authors4,6,12, showed the presence of only one type of Lewis acid sites, whereas no Bronsted sites were detected. In view of these few studies, it appears that a more systematic characterization of active sites of ZrBEA catalysts, which is mandatory for establishing any structure-activity correlations or for assessing the quality of the catalyst preparation, is lacking. The objective of the present study was to establish a characterization technique that would allow the identification and quantification of ZrBEA catalyst active sites. To achieve this goal, complementary probe molecules were used. Pyridine (Py) and ditertbutylpyridine (DTBPy) for probing Bronsted acid sites of strong to medium and weak strength, respectively. Deuterated acetonitrile CD3CN and carbon monoxide CO were employed for Lewis sites. Although the use of deuterated acetonitrile has been shown efficient for discriminating the Lewis sites of Sn-containing catalysts10,11 and is now generally used for this purpose10,11,13, it will be shown here that it is not fully applicable to Zr-containing zeolites: while CD3CN allows discriminating Zr atoms in zeolitic framework from polymeric, extra-framework zirconium species, it does not allow to distinguish open from closed Zr sites. As an alternative probe, carbon monoxide has been selected in this study since it is very sensitive to different coordination spheres of the metal centre and it can be monitored easily by IR spectroscopy.14

2. Experimental 2.1 Catalysts preparation ZrBEA catalysts were synthesized using the fluoride medium procedure described elsevwere.12 ZrOCl2 was used as Zr source. The final gel composition was 1.0 SiO2 : 0.02 Zr : 0.56 TEAOH : 8 H2O : 0.56 HF for Si/Zr = 100. Si/Zr ratio was varied from 100 to 400 by the variation of the composition of the final gel. Crystallization was carried out in a Teflon-lined autoclave at 413K for 20



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days. The solid product obtained was filtered, washed with deionized water, dried at 373K and calcined at 823K for 4 h. For comparison ZrO2/SiBEA catalyst was prepared by incipient wetness impregnation of silica BEA with aqueous solution of ZrO(NO3)2 followed by drying at 373K and calcination at 773K for 3 hours in flow of air.

2.2 Catalysts characterization The elemental analysis was performed using inductively coupled plasma mass spectroscopy (ICPMS). Prior to analysis the samples were dissolved in a mixture of HF, HCl and HNO3, followed by neutralization with B(OH)3. This solution was diluted by distilled water to obtain a concentration of measured ions of about 1 mg/L. Solutions for calibration were prepared from the standards with addition of the corresponding amount of acids to level the matrix effect. All measurements were performed on an ELAN ICP-MS machine (Perkin Elmer). Sorption–desorption isotherms of nitrogen were measured using an automated porosimeter (Micrometrics ASAP 2000). Prior to the measurements, the samples were evacuated at 673K. Powder X-ray diffraction (XRD) patterns were recorded with a DRON-3M diffractometer, applying Cu Kα radiation at the wavelength of 1.5456 Å. FTIR measurements were carried out on a Nicolet 6700 spectrometer equipped with a DTGS detector. Room temperature cell was used for acetonitrile-d3 adsorption and low temperature vacuum cell cooled with liquid nitrogen was used for CO adsorption measurements. About 20 mg of the samples was pressed to yield a self-supported wafer. The sample was then introduced in a glass cells specially designed for in situ FTIR spectroscopy and enabling preliminary thermal treatments. The activation procedure always started with a heating step (5 K min-1) under a vacuum from room temperature to 723 K with a 3 h plateau at 723 K. When the activation procedure was finished, the sample was cooled down to room temperature and a reference spectrum was recorded (128 scans at a 4 cm-1 resolution). Deuterated acetonitrile CD3CN (99.5% supplied by Fluka) or carbon monoxide (99.99997% Air Liquide) calibrated aliquots were then gradually introduced into the cell and IR spectra


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subsequently recorded. The procedure was repeated until the evolution of adsorbed species indicated that CO saturation of the surface was reached. Adsorption of pyridine and 2,6-ditertbutylpyridine (99.5%, Aldrich) was performed at 423K for 1h followed by cooling at room temperature and evacuation at increasing temperatures. Spectra were collected after cooling the wafer to the ambient temperature. Difference spectra were obtained by the subtraction of the reference spectra with the spectra of the samples with the adsorbate. The subtraction and curve-fitting of the spectra was carried out using OMNIC 7.3 package.

3. Results and discussion 3.1. Catalyst structure and composition The chemical and textural characteristics of the catalysts are presented in Table 1. The amount of ZrO2 incorporated into ZrBEA and loaded onto siliceous BEA was close to the targeted values. Nitrogen adsorption-desorption data point to high pore volume of zeolitic catalysts (0.2 cm3/g). All ZrBEA samples had reversible Type-I adsorption/desorption isotherms with a steep rise at p/p0 < 0.01, typical for microporous solids. The structure of the catalysts was analyzed by XRD techniques (Fig. 1). The powder XRD pattern of ZrBEA sample show typical features of well-crystallized BEA zeolite. The asymmetry of the peak in the range of 2θ = 7◦–9◦ indicated the presence of two isostructures, polymorphs A and B.12 No peaks due to crystalline ZrO2 or any other crystalline impurity phases were detected.

3.2 FTIR study of adsorbed pyridine and ditertbutylpyridine The chemisorption of bases such as pyridine, ammonia, and substituted pyridines is frequently used to characterize solid acid catalysts and to correlate their catalytic activity with the concentration of a particular type of acid site. Pyridine has been the most widely used base for characterization of acid properties purposes, due to its ability to interact with both Brønsted and Lewis sites, revealing specific IR absorption bands. The spectra of adsorbed pyridine within the region of 1400-1600 cm-1 are shown at Fig. 2. The spectrum after pyridine outgassing at 298K reveals the presence of the bands corresponding to Lewis



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acid sites (1445 and 1607 cm-1) and pseudo-liquid pyridine (1440 cm-1). Desorption at higher temperatures leads to vanishing of the bands corresponding to weakly adsorbed Py and to a slight decrease of the intensity of the bands attributed to Lewis sites pointing to the moderate strength of Lewis sites. The moderate strength is confirmed by the position of the band at 1607 cm-1, which is a good indicator of the Lewis acid strength, and is similar to that observed for bulk zirconia (1605 cm-1). Hence, the presence of Lewis sites could be assigned to Zr atoms, incorporated in zeolite framework leading to formation of open or closed sites possessing Lewis acidity. Calculation15 of the amount of Lewis sites from the Py spectra after evacuation at 423K gives the value of 98 µmol·g-1 which is close to the Zr content in ZrBEA(100) (122 µmol·g-1) and points on the full accessibility of all Lewis sites for the pyridine molecule. No bands typical of pyridinium species were detected, in line with previous studies6,12, where it was shown that no Brønsted sites strong enough to protonate pyridine are present on ZrBEA materials. Pyridine, however, is not a very strong base (pKa = 5.4) and stronger base have to be used to probe weak acid sites possibly present on ZrBEA. According to Corma et al.16, 2,6-ditertbutylpyridine (DTBPy) access 100% of Brønsted acid sites in AlBEA and can be used for the determination of weak Brønsted acid sites due to its higher basicity with respect to pyridine (pKa = 6.9). DTBPy adsorption was performed on the ZrBEA(100) sample, with sequential evacuation at 323 and 373K for 1h. Spectra in the range of 2800-4000 and 1350-1650 cm-1 before and after evacuation are shown in Fig. 3. The bands at 3370, 1615 and 1530 cm-1, persisting after outgassing, correspond to vibrations of protonated DTBPy.16 A perturbation of hydroxyl groups is associated to these protonated species with the simultaneous appearance of a negative ν(OH) band at 3670 cm-1. The dramatic and simultaneous decrease of the intensity of all these bands after sample evacuation at 423 K allows assigning the ν(OH) band at 3670 cm-1 to Brønsted acid sites strong enough to protonate DTBPy but not pyridine. The weak thermal stability of DTBPyH+ species also points to the weak strength of these Brønsted acid sites. Weak Brønsted sites were also reported for titanium silicalite catalysts17 and predicted for other M4+-doped zeolites (M = Ti, Zr, Ge, Sn, Pb)18. They were attributed to M-OH defect


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sites (Scheme 1), which could be H-bonded to Si-OH groups formed by hydrolysis of M-O-Si bridges.17,18 Besides that, for the bulk ZrO2, the band at 3667 cm-1 has been detected and attributed to Zr-OH vibrations.19 All these findings suggest that in the present study such sites could be represented by the Zr-OH groups in open sites of ZrBEA. In summary, the adsorption of Py and DTBPy on ZrBEA reveals the presence of both Brønsted and Lewis acidity associated with Zr atoms incorporated in the BEA framework.

3.3. FTIR study of adsorbed acetonitrile-d3 Deuterated acetonitrile has been found to be a suitable molecule for differentiating among Lewis acid sites of different strengths.10,11,20-22 In previous studies devoted to SnBEA it was shown that this probe molecule can be used for the qualitative and quantitative analysis of two types of Lewis sites in SnBEA.10,11 It should be mentioned that no IR bands due to CD3CN adsorbed on Lewis acid sites of SnO2 supported on silica BEA10 were detected, indicating that acetonitrile is selective for Sn framework Lewis acid sites. Fig. 4a shows typical IR spectra recorded upon deuterated acetonitrile adsorption on calcined ZrBEA(100). The bands in the 2260–2340 cm-1 range are associated with ν(C≡N) stretching vibration, while a weak band also appears at 2116 cm-1 that is due to δs(CD3) vibration. In the ν(C≡N) range, three bands at 2303, 2275, and 2268 cm−1 are detected, and no significant shift in the band maxima was observed when acetonitrile coverage is increased.

The band at 2303 cm-1 could be assigned to

acetonitrile coordinated with Lewis acid sites, bands at 2275 and 2268 cm-1 are due to acetonitrile Hbonded to silanol groups and physisorbed acetonitrile, respectively.20-22 Thereby, the infrared spectra of adsorbed CD3CN over ZrBEA(100) reveal only one band attributed to Lewis sites. Fig. 4b compares the IR spectra in the ν(C ≡ N) stretching vibration after the evacuation of excess of acetonitrile at ambient temperature on all ZrBEA samples as well as, for comparison, on ZrO2 highly dispersed on pure silica BEA. For all ZrBEA samples, only one band is observed at 2303 cm-1, which contrasts with SnBEA systems for which two bands, indicative of two types of Lewis acid sites, were detected.10,11



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Table 2 reports the relative amount of Lewis acid sites probed by acetonitrile on the various ZrBEA sample as obtained from the area of the band at 2303 cm-1. Table 2 and Fig. 4b show that the amount of Lewis acid sites increases with the increase of Zr content in the sample, which points to the good quality of the materials and confirm that hydrothermal synthesis allows the framework incorporation of rather high amount of Zr atoms without the formation of ZrO2 bulk phase. However, the decrease of the Si/Zr ratio from 400 to 200 does not reveal a proportional increase of the amount of Lewis acid sites as determined by FTIR of adsorbed CD3CN. ZrO2/SiBEA sample (Fig. 4b) does not show the bands, which can be attributed to Lewis acid sites and it could be concluded that acetonitriled3 selectively interacts only with framework Zr atoms.

3.4. FTIR study of adsorbed CO FTIR spectroscopy of adsorbed CO is a well-established technique for the characterization of the acidity of various catalysts.14 The adsorption of CO at low temperature (~100 K) leads to the formation of H-bonds with OH groups and to the coordination of CO to Lewis acid sites through σ donation. Due to this interaction, the ν(CO) vibration band of adsorbed CO shifts to higher wavenumbers with respect to the band of pseudo-liquid CO (2138 cm-1). This shift is characteristic for the nature (Lewis or OH) and strength of the site. Fig. 5 shows the spectra obtained after CO adsorption on ZrBEA(100). In this typical experiment, CO was gradually adsorbed dose per dose until the complete saturation of adsorption sites and the appearance of the band corresponding to pseudo-liquid CO. Introduction of the first doses of CO into the cell leads to the appearance of the band at 2188 cm-1, which progressively shifts towards lower frequencies at increasing CO coverage. At low CO doses, no significant changes in the ν(OH) vibrations are observed, which suggests that the band at 2188 cm-1 can be assigned to adsorption of CO on strong Lewis sites. The gradual increase of CO coverage leads to the appearance of two new bands at 2163 and 2156 cm-1 in the range of CO vibrations. Simultaneously, in the OH vibration region of the difference spectra, a negative peak with a low frequency assymetric tail develops at 3745 cm-1 together with two


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broad positive peaks at 3655 and 3500 cm-1. The band at 2156 cm-1 have the position typical for CO adsorption on OH groups, and could be assigned to the vibration of CO H-bonded with Si-OH surface groups, the latter being shifted from 3745 to 3655 cm-1 upon CO adsorption, hence explaining the corresponding negative and positive bands, respectively. The ν(CO) band at 2163 cm-1 growing simultaneously with the broad ν(OH) band at 3500 cm-1 suggests their assignment to the formation of OH···CO complexes. The position of the band at 2163 cm-1 with respect to the ν(CO) bands characteristic of CO interacting with silanol groups (2156 cm-1) or with Brønsted acid sites in AlBEA (2174 cm-1)23, points on the relatively weakly acidity of the corresponding OH groups. This is in line with DTBPy adsorption experiments which revealed the presence of weak acidic OH groups characterized by a band at 3670 cm-1 (Fig. 3). It should be mentioned here that this ν(OH) band is close to the ν(OH) band of perturbed silanol groups at 3655 cm-1 which grows upon CO adsorption, hence explaining why no perturbation of a band at ~ 3670 cm-1, if any, can be observed in such conditions (Fig. 5). Assuming that the ZrOH groups corresponding to the band at 3670 cm-1 lead to the broad band at ~3500-3520 cm-1 (Fig. 5) gives a ∆ν(OH) frequency shift of 150-170 cm-1 upon CO adsorption which is fully consistent ν(CO) band at 2163 cm-1,24,25 characteristic of Brønsted acid sites of weak to medium strength. Upon increasing of the CO loading above ~ 1 mmol g-1, a new band appeared at ~ 2176 cm-1. The appearance of this band did not lead to significant changes in the ν(OH) range of the IR spectra (3300-3800 cm-1), suggesting that this band can be assigned to CO coordinated to Lewis acid sites. The fact that this band appears only after the introduction of significant amount of CO on the sample and has a relatively low frequency14, indicates that the corresponding sites have weak acidity, which, by analogy with Ti- or Sn-BEA systems, would be the case of closed Zr sites.

3.5. Adsorption of CO after DTBPy pre-adsorption In order to confirm the assignment of the ν(CO) band at 2163 cm-1 to the weak Zr-OH Brønsted sites evidenced by DTBPy adsorption and characterized by the ν(OH) band at 3670 cm-1, CO adsorption experiments were performed over the ZrBEA(100) sample on which DTBPy was 9 Environment ACS Paragon Plus


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preliminary adsorbed and outgassed at 373 K. The corresponding spectra are shown in Fig. 6. They show that preadsorption of DTBPy prevents the appearance of the band at 2163 cm-1 in comparison with fresh ZrBEA sample (Fig. 5). Taking into account the fact that in such conditions only protonated DTBPyH+ species are evidenced and that DTBPy cannot coordinate to Lewis sites due to steric limitations16, the complete absence of the ν(CO) band at 2163 cm-1 is in line with its assignment to CO adsorbed on the acidic Zr-OH groups. Moreover, DTBPy pre-adsorption also led to a dramatic decrease of the intensity of the band at 2185 cm-1, which was attributed to CO adsorption on strong Lewis sites, while the band at 2176 cm-1, assigned to weak, closed Lewis acid sites is much less affected. This suggests that the strong Lewis acid sites characterized by the band at 2185 cm-1 are sterically hindered by DTBPyH+ species and are thus neighboring the ZrOH Brønsted sites of ZrBEA. Overall, these observations are a strong indication that strong Lewis sites (ν(CO) band at 2185 cm-1) and medium-weak Brønsted sites (ν(CO) band at 2163 cm-1) of ZrBEA are intimately related pointing to a structure analogous to those proposed for open sites on Sn- or Ti-BEA materials (Scheme l).

3.6. Adsorption of CO after CD3CN y pre-adsorption We have shown that adsorption of acetonitrile-d3 over all ZrBEA samples leads to the appearance of the only band corresponding to the interaction of CD3CN with Lewis sites, which contrasts with SnBEA systems for which two bands, indicative of two types of Lewis acid sites, are observed.10,11 On the other hand, CO adsorption does not indicate the heterogeneity of Lewis acidity of these materials with the presence of at least two types of Lewis acid sites: strong, open Zr sites (ν(CO) at 2185 cm-1) and weak, closed Zr sites (ν(CO) at 2176 cm-1). Such a discrepancy could be a priori explained by several reasons: i) non selective interaction of CD3CN with both types of sites, ii) selective interaction of CD3CN with only on type of Zr sites, or iii) broadening of the IR bands which leads to impossibility to separate two ν(CN) bands in the spectra. In order to answer this question, CO was adsorbed over the ZrBEA(100) sample on which CD3CN was preliminary adsorbed at saturation and outgassed at 298 K for 1 hour to remove all weakly bonded CD3CN (physically adsorbed and part 10 Environment ACS Paragon Plus

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of H-bonded CD3CN). After this procedure, the wafer was cooled down with liquid nitrogen and carbon monoxide was gradually adsorbed on the sample with preadsorbed acetonitrile. No shift of the band position was observed during the CO adsorption in comparison with bare ZrBEA(100). To compare the intensities of all bands observed over ZrBEA samples, a curve-fitting of the ν(CO) spectra was carried out in the 2080-2230 cm-1 range with Gauss-Lorentz peaks with fixed FWHM. The area of peaks at 2188 and 2176 cm-1 was further used for assessing the relative amount of open and closed acid sites. Fig. 7 compares the spectra obtained after CO adsorption on bare ZrBEA(100) and ZrBEA(100) with preadsorbed acetonitrile at similar CO coverage. The relative amounts of open and closed Lewis sites are given in Table 2. Fig. 7 and Table 2 show that acetonitrile preadsorption leads to a dramatic decrease of the amount of open acid sites (ν(CO) at 2185 cm-1), which decreases by ~80% upon CD3CN pre-adsorption. Interestingly, the band at 2163 cm-1 attributed to CO adsorption over Zr-OH groups of these open sites diminished by the same order of magnitude after CD3CN pre-adsorption (~ 80%). This latter decrease can be explained either by steric blocking of Zr-OH groups by acetonitrile adsorbed over open Lewis sites (as observed for DTBPy preadsorption) or by direct interaction of another CD3CN molecule with Zr-OH. On the contrary, the pre-adsorption of CD3CN had almost no effect on the band at 2176 cm-1, its area being diminished by 10%, a value close to the accuracy of the curve-fitting procedure. Hence, these results clearly show that CD3CN preadsorbtion leads to blocking of only open Lewis sites, with almost no effect on the closed sites, which indicates that deuterated acetonitrile does not interact with closed sites, suggesting that this probe molecule can only be used for probing of the open active sites in ZrBEA.

3.7. Influence of Zr content Fig. 8 shows the IR spectra obtained after CO adsorption on the three ZrBEA samples (Si/Zr = 100, 200 and 400). Adsorption of CO over ZrBEA(200) and ZrBEA(400) leads to the appearance of the same bands as for ZrBEA(100): 2186, 2176, 2163, 2156 and 2138 cm-1. Slight shifts within 1-2 cm-1 depending on the sample could be associated with the deformation of zeolite framework due to the incorporation of different amount of Zr and slight changes in the electron structure of adsorption sites.



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In the range of OH vibrations, the spectra were similar to that of ZrBEA(100) (not shown). In order to compare the amount of different Lewis sites observed over ZrBEA samples with different Si/Zr ratio the collected spectra were subtracted and curve-fitted by the procedure described above. The results are shown in Fig 8 and in Table 2. The amount of closed sites (ν(CO) band at 2176 cm-1) correlates with the amount of Zr atoms incorporated in the zeolite framework and gradually decreases from ZrBEA(100) to ZrBEA(400). The band at 2188 cm-1, characteristic of open sites, exhibits a more complex behavior with the variation of the Zr content in the samples. The increase of Si/Zr molar ratio from 100 to 200 leads to significant decrease of the intensity of this band, but further increase of Si/Zr ratio to 400 has only slight effect on its area. Interestingly, this trend is confirmed by CD3CN adsorption data. In particular, the relative amount of Lewis acid sites probed by CD3CN on ZrBEA materials satisfactorily correlates with the amount of open sites probed by CO, which also confirms that CD3CN specifically probes open Zr Lewis sites. Hence both CO and CD3CN adsorption points out that the amount of open Lewis acid sites is not a linear function of Zr content. This behavior could be explained by the different mechanisms of Zr incorporation during ZrBEA synthesis from Zr-rich and Zr-poor gels.

4. Conclusions The Lewis and Brønsted acid properties of ZrBEA zeolites with different Si/Zr ratio have been investigated by means of IR spectroscopy of adsorbed pyridine, ditertbutylbyridine, deutirated acetonitrile and carbon monoxide. Two types of Zr sites have been detected and assigned to

•

four-fold coordinated Zr atoms in the framework positions, which interact with Lewis bases and do not interact with Brønsted bases (closed sites);

•

four-fold coordinated Zr centers with one Zr–OH hydroxyl group resulting from the hydrolysis of Zr–O–Si bond (open sites); these sites exhibit strong Lewis and weak Brønsted acidic properties.

It has been shown that deuterated acetonitrile specifically probes open sites in ZrBEA, while the use of CO allows distinguishing between open and closed sites. The relative amount of


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open and closed sites does not linearly depend on the Si/Zr ratio in the sample, which can point to different mechanisms of Zr incorporation during ZrBEA synthesis from Zr-rich and Zr-poor gels.

5. Acknowledgements Vitaly Sushkevich and Irina Ivanova thank the Russian Science Foundation for the financial support (Grant №14-23-00094). Vitaly Sushkevich gratefully acknowledges French government for Metchnikov scholarship.

6. References (1) Román-Leshkov, Y.; Davis, M. E., Activation of Carbonyl-containing Molecules with Solid Lewis Acids in Aqueous Media ACS Catal. 2011, 1, 1566−1580. (2) Moliner, M., State of the Art of Lewis Acid-containing Zeolites: Lessons from Fine Chemistry to new Biomass Transformation Processes Dalton Trans., 2014, 43, 4197-4208. (3) Zhu, Y.; Jaenicke, S.; Chuah, G.-K., Al-free Zr-zeolite beta as a Regioselective Catalyst in the Meerwein-Ponndorf-Verley Reaction Chem. Comm., 2003, 21, 2734–2735. (4) Wang, J.; Jaenicke, S.; Chuah, G.-K., Zirconium-Beta Zeolite as a Robust Catalyst for the Transformation of Levulinic Acid to γ-Valerolactone via Meerwein-Ponndorf-Verley Reduction

RSC Adv. 2014, 26, 1348–13489. (5) Corma, A.; Renz, M., A General Method for the Preparation of Ethers Using Water-Resistant Solid Lewis Acids Angew. Chem. Int. Ed. 2006, 46, 298–300. (6) Zhu, Y.; Chuah, G.-K.; Jaenicke, S. Selective Meerwein-Ponndorf-Verley Reduction of α, βunsaturated Aldehydes over Zr-zeolite beta J. Catal. 2006, 241, 25-33. (7) Sushkevich, V.; Ivanova, I.; Tolborg, S.; Taarning, E., Meerwein–Ponndorf–Verley–Oppenauer Reaction of Crotonaldehyde with Ethanol over Zr-containing Catalysts J. Catal., 2014, 316, 121–129. (8) Sushkevich, V.; Ivanova, I.; Taarning, E. Ethanol Conversion into Butadiene over Zr-containing Molecular Sieves Doped with Silver Green Chem., 2015, 17, 2552-2559. 13 Environment ACS Paragon Plus


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Table 1. Catalysts characteristics

Sample ZrBEA(100) ZrBEA(200) ZrBEA(400) ZrO2/SiBEA(100)

Chemical composition ZrO2 Si/Zr (wt%) 1.52 134 0.76 263 0.44 455 1.66 121

Total pore volume (cm3/g) 0.29 0.29 0.29 0.31

Micropore volume (cm3/g) 0.20 0.20 0.20 0.22


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Table 2. Relative amount of Lewis acid sites determined from the spectra of adsorbed CO and acetonitrile

Sample ZrBEA(100) ZrBEA(200) ZrBEA(400) ZrBEA(100) with preadsorbed CD3CN

CD3CN adsorption, Lewis sites, a.u. 1 0.63 0.58 -

CO adsorption Open sites, a.u. Closed sites, a.u. 1 1 0.43 0.43 0.40 0.20 0.22


0.90


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Figure captures Fig. 1. XRD patterns of Zr-containing catalysts Fig. 2. FTIR spectra of Py adsorbed over ZrBEA(100) after evacuation at 298, 373 and 423K Fig. 3. FTIR spectra of DTBPy adsorbed over ZrBEA(100) after evacuation at 298, 323 and 373K Fig. 4. FTIR spectra of CD3CN adsorbed over a) ZrBEA(100) b) over all Zr-containing samples Fig. 5. FTIR spectra of CO adsorbed over ZrBEA(100) Fig. 6. FTIR spectra of CO adsorbed over ZrBEA(100) after DTBPy preadsorption Fig. 7. Deconvolution of FTIR spectra obtained after CO adsorption on ZrBEA(100) with and without preadsorbed CD3CN Fig. 8. Deconvolution of FTIR spectra obtained after CO saturation of a) ZrBEA(100), b) ZrBEA(200) and c) ZrBEA(400) samples


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Fig. 1.



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Fig. 2


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Fig. 3



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Fig. 4


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Fig. 5



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Fig. 6.


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Fig. 7.



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Fig. 8


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Table of contents artwork



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Spectroscopic Evidence for Open and Closed Lewis Acid Sites in

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