Origin of Water-Induced BrÃ¸nsted Acid Sites in Sn-BEA Zeolites - The

Subscriber access provided by Iowa State University | Library

C: Surfaces, Interfaces, Porous Materials, and Catalysis

Origin of Water-Induced Brønsted Acid Sites in Sn-BEA Zeolites Vitaly L. Sushkevich, Pavel A. Kots, Yury G. Kolyagin, Alexander Vyacheslavovich Yakimov, Artem V. Marikutsa, and Irina I Ivanova J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b12462 • Publication Date (Web): 05 Feb 2019 Downloaded from http://pubs.acs.org on February 6, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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

The Journal of Physical Chemistry

Origin of Water-Induced Brønsted Acid Sites in Sn-BEA Zeolites Vitaly L. Sushkevicha,b,*, Pavel A. Kotsa, Yury G. Kolyagina,c, Alexander V. Yakimova, Artem V. Marikutsaa and Irina I. Ivanovaa,c,* a Department

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

Scherrer Institute, 5232, Villigen PSI, Switzerland

c

A.V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of Science, Leninskiy prospect 29, 119991 Moscow, Russia

Abstract Transformation of the Sn-BEA sites structure during the interaction with water has been investigated by means of FTIR and NMR spectroscopy and catalytic experiments. It is shown that Lewis and Brønsted acid properties of Sn-BEA zeolite before and after adsorption of water change significantly. New surface OH groups exhibiting different structure are observed after adsorption, while the tin oxide supported on Si-BEA is inactive in this transformation. It is demonstrated that formed bridged OH groups possess strong Brønsted acidity, thus enabling the protonation of pyridine. It is suggested that adsorption of water occurred over tin Lewis acid sites followed by hydrolysis of Si-O-Sn bonds and the formation of Si-OH and Sn-OH surface species. In this process, tin atoms change their coordination from 4-coordinated to 6-coordinated, possessing different kinetics for different types of Sn sites observed by NMR spectroscopy. Formation of additional catalytically active acid sites through water adsorption on Sn-BEA is demonstrated in situ in the course of isobutene dimerization reaction.

Keywords: tin beta, infrared spectroscopy, NMR spectroscopy, Lewis acid sites, water, ammonia __________________________________________ *Corresponding author: Tel.: +7(495)939-3570; Fax: +7(495)939-3570; E-mail address: [email protected], [email protected]

1

ACS Paragon Plus Environment

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

Page 2 of 34

1. Introduction The understanding of the structure of the active sites of a heterogeneous catalyst that are present under the real reaction conditions is one of the most important challenges faced by the scientific community. Since it is highly important for the design of new effective catalysts for various chemical processes, different catalysts evolution is extensively studied by different techniques, including in situ and operando approaches.1-4 In the most of the cases, the initial catalysts undergo modification under reaction conditions by coke deposition5-7, water dissociation8,9, formation of new catalytically active phases10, partial reduction11 etc. Therefore, for reliable establishment of structure-to-activity relationship one should consider that active sites of the “working” catalyst might be different from those of the fresh catalyst. These aspects are highly important for the catalytic reactions in solution, where the solvents or/and the initial reagent could significantly influence the catalysts performance. An excellent example of such transformations is the conversion of biomass-related compounds over Sn-BEA zeolite in watercontaining media. This material was shown to be the best catalyst for Meerwein–Ponndorf–Verley, Bayer-Villiger, and Oppenauer oxidation reactions12,13, aldol condensation14,15, Diels–Alder reaction16, sugar isomerization,17-22 epimerization23 and many others.24-28 Although Sn-BEA zeolites are known to catalyze numerous reactions of sustainable chemistry in aqueous media, the role of water has not been investigated yet. Most of the researchers consider the unique properties of Sn-BEA materials from the point of view of its hydrophobicity and moderate Lewis acidity of tetrahedral tin atoms incorporated in zeolite framework12-28. However, the presence of water and other solvents could dramatically change the structure of surface sites of Sn-BEA zeolite. For instance, different coordination number of tin atoms observed by

119Sn

MAS NMR of dehydrated and

hydrated Sn-BEA samples was already reported by different groups29-34. In our recent papers we have shown that similar transformations are valid for the adsorption of different alcohols, aldehydes and nitriles over dehydrated Sn-BEA resulting in formation of 5- and 6-fold tin atoms in BEA structure34. In some cases, water coordination might participate in the reaction mechanism and govern the material

2


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


stability.35, 36 While the coordination of tin is well understood, the overall structure of the sites has not yet been reported. In view of these studies, it appears that a more systematic characterization of tin sites of SnBEA catalysts in the presence of water, which is essential for establishing any structure-activity correlation, is required. Therefore, we aim the present study to clarify the nature of tin sites present on hydrated Sn-BEA catalyst. Combining in situ FTIR and NMR spectroscopies as well as catalytic experiments to study hydrothermally synthesized Sn-BEA as compared to SnO2 supported on pure siliceous BEA, we showed for the first time the formation of strong Brønsted acid sites after the adsorption of water. Importantly, these sites can be generated in situ hence enabling the fine tuning of the catalyst activity and selectivity in either the biomass-related transformations or synthesis of fine chemicals. 2. Experimental 2.1 Catalysts preparation Synthesis of Sn-BEA was described elsewhere37-39.

119Sn-BEA

sample enriched with

119Sn

isotope were prepared by changing the tin source to H2119SnCl6 obtained via dissolution of metallic tin enriched with

119Sn

isotope32. The molar composition of initial gel was selected as described

previously37. After the crystallization, the obtained solid product was filtered, washed with deionized water, dried at 373K and calcined at 823 K for 6 h on air in static conditions. For comparison, SnO2/SiBEA catalyst was prepared by incipient wetness impregnation of Si-BEA with aqueous solution of SnCl4⋅5H2O to reach Si/Sn = 200 followed by drying at 373K and calcination at 773K for 3 hours in a flow of air. 2.2 Catalysts characterization and in situ measurements 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 sorptiondesorption isotherms were measured at 77K using a Micromeritics ASAP-2000 automatic surface area

3



Page 4 of 34

and pore size analyzer. Prior to the measurements, the samples were evacuated at 623K. Powder X-ray diffraction (XRD) patterns were recorded with a Bruker D2 diffractometer, applying Cu Kα radiation at 1.5456 Å. SEM images were obtained with a scanning electron microscope Hitachi TM3030 at 5 kV. UV−vis spectra (DRUV) were recorded on Evolution 600 spectrometer using BaSO4 as the standard in the spectral region of 190−320 nm. For graphical representation, Kubelka−Munk units were used. 1H, 29Si

and 119Sn solid-state mafic angle spinning (MAS) NMR was performed using AVANCE-

II 400WB (Bruker) spectrometer with magnetic field 9.4 T. A triple-resonance 4 mm MAS DVT probe (Bruker) was used for detection, the MAS frequency being 12 kHz. For registration of spectra on 1H and

29Si

nuclei 90o pulses were used; the chemical shifts are reported relative to TMS. For the

registration of spectra on

119Sn

nuclei direct polarization combined with CPMG echo-train acquisition

(DP-CPMG) was used as a main pulse sequence32. Spectra of dehydrated samples were measured by applying 600 echoes without any proton decoupling. For the hydrated samples and samples with adsorbed water, high-power 1H decoupling was used [SW-TPPM, τ = 8 μs, φ = 15°, ω(B1) = 105 kHz] with only 25 echo in CPMG echo-train. The

119Sn

NMR chemical shifts were calibrated by SnO2 as

secondary external reference (−604.3 ppm)40. For spectra processing, the TopSpin 2.1 (Bruker) software was used. All reported spectra correspond to the Sn-BEA sample synthesized with

119Sn

isotope enrichment. The samples were dehydrated under dynamic vacuum with ramping 5 K∙min-1 to 673 K and dwell for 3 hours. For hydration, a fixed dose of water was adsorbed on the sample (H2O/Sn = 4). The excess of water was used to model the pseudo zero-order reaction rate on water with observation of reaction kinetics for tin atoms conversion. After preparation procedure the samples were sealed in 4mm MAS rotor in glove-box under dry air atmosphere. For spectra processing, the TopSpin 2.1 software was used. The deconvolution of NMR spectra was performed by DMFIT software by using Lorenz (for 1H) and mixed Lorenz/Gauss (119Sn) lineshapes with ±15% accuracy. FTIR spectra in transmission mode were recorded using Nicolet Protégé 460 spectrometer equipped with a MCT detector. Room temperature cell was used for water, acetonitrile-d3 and pyridine (Py) adsorption. About 12 mg of the sample was pressed into the self-supported wafer. The sample was

4


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


then placed into the IR cells and activated as follows: ramping 5 K∙min-1 to 673 K and dwell for 3 hours. When the activation procedure was finished, the sample was cooled down to ambient temperature and a reference spectrum was recorded (128 scans at a 4 cm-1 resolution). Water, acetonitrile-d3 or pyridine (99.5%, Aldrich) calibrated aliquots were then gradually introduced into the cell and IR spectra subsequently recorded. Difference spectra were obtained by subtracting the reference spectra from the spectra of the samples with the adsorbate. The subtraction of the spectra was carried out using OMNIC 7.3 package. DRIFT spectra were measured using Perkin Elmer Frontier spectrometer equipped with Pike Tech DiffusIR in situ cell with heating flow test chamber and ZnSe optics. Spectra were registered in 4000– 1000 cm−1 region, accumulating 30 scans with automatic H2O/CO2 correction. Powders (50 mg) were placed in the alumina crucible (5 mm diameter). For sample dehydration, it was heated in a flow of dry Ar (100 ml·min-1) at 673 K for 1 h (3 K∙min-1). DRIFT spectrum of fully dehydrated sample was acquired at 673 K. Then, the dehydrated sample was slowly cooled down in a flow of wet Ar (~ 50 ppm H2O) while measuring DRIFT spectra, which were further subtracted from the spectrum of dehydrated at 673 K sample. For ammonia sorption, initially hydrated sample (stored for about 1 month at ambient humidity) was heated in a flow of NH3/Ar (100 ppm NH3) with simultaneous measurement of IR spectra. Resulting spectra were further subtracted from the spectrum of hydrated sample. 2.3 Catalytic experiments Catalytic experiments were carried out in a flow-type fixed-bed reactor under atmospheric pressure. In a typical experiment, 100 mg of catalyst (fraction 0.25-0.5 mm) was packed into the quartz tubular reactor and purged with helium at 823K for 1.5 h. Isobutene (Linde, 99.8 %) was used as a feed. The WHSV was 30 h-1, the reaction temperature was 420K. During the reaction 0.2 µl pulse of water was introduced by Hamilton syringe. Gaseous products were analyzed on line on Crystal 2000M gas chromatograph using 50m SE-30 column and mass spectrometer RGA200 (Stanford Scientific Systems). 3. Results and discussion

5



Page 6 of 34

3.1. Catalysts structure and composition The chemical and textural characteristics of the catalysts are presented in Table 1. They are typical of BEA zeolite synthesized via hydrothermal route in fluoride media. The amount of SnO2 incorporated into Sn-BEA and loaded onto Si-BEA was close to the expected values. Nitrogen adsorption-desorption data point to high pore volume of zeolitic catalysts (0.2 cm3/g, Fig. S1). The powder XRD pattern of Sn-BEA sample show typical features of well-crystallized BEA zeolite consisting of two polymorphs A and B, evidenced by asymmetry of the peak at 7-9°.41 No peaks due to crystalline SnO2 or any other crystalline impurity phases are detected for Sn-BEA (Fig. S2). In contrast, for SnO2/Si-BEA the presence of small features due to the crystalline SnO2 is observed at 26.57 and 33.84°. UV spectroscopy confirms the formation of SnO2 on the SnO2/Si-BEA sample revealing the broad line at 280 nm which is attributed to SnO2 (Fig. S3).25 A similar line was detected on the Sn-BEA pointing to the traces of SnO2 impurities formed during the synthesis procedure. SEM microphotographs of Sn-BEA materials show well-crystalline materials with the size of 5-10 µm typical for the zeolites synthesized in fluoride media (Fig S4). 29Si MAS NMR spectra of Sn-BEA and SnO2/Si-BEA (Fig. S5) show resolved signals at ca. -111.3, -111.9, -112.8 and -115.5 ppm in the Q4 range that can be assigned to Si(OSi)4 species occupying different crystallographic sites of BEA structure. No Q3-resonance indicative of Si(OSi)3OH structural defects was observed hence confirming the high crystallinity of materials studied. Overall, the synthesized samples show the same structure, crystallinity, crystal size, morphology, and quality typical of Sn-BEA materials.12-30, 39 To study the acid sites of both Sn-BEA and SnO2/Si-BEA samples, an infrared spectroscopy of adsorbed pyridine, which is widely applied for characterization of Sn-BEA synthesized by different procedures, was used42-44. Fig. 1 compares the IR spectra within the range of 1400-1650 cm-1 after the adsorption of 0.1 torr of pyridine at ambient temperature on dehydrated Sn-BEA and SnO2/Si-BEA samples. Introduction of Py into the cell leads to the appearance of the bands at 1610 and 1451 cm-1 corresponding to Lewis acid sites presented on Sn-BEA (Fig. 1). The bands at 1596 and 1443 cm-1 could be assigned to the pyridine H-bonded with OH groups and bands at 1582 and 1439 cm-1 are due

6


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


to the physically adsorbed pyridine, respectively.42 The presence of Lewis sites is associated with tin atoms, incorporated in zeolite BEA framework and confirmed by FTIR spectroscopy of adsorbed acetonitrile-d3 (Fig. S6).42 No bands typical to protonated pyridine species were detected, which is in line with previous studies42-44. Similar bands for pyridine adsorption on OH groups and physically adsorbed Py were observed over SnO2/Si-BEA with respect to Sn-BEA sample. The main difference arises from the position of a vibration mode at 1600-1610 cm-1 in the spectra which is very sensitive to the strength of Lewis sites42,45. SnO2/BEA sample reveals the presence of this band with the frequency of 1607 cm-1 which is lower than that observed over Sn-BEA (1610 cm-1). Thereby it suggests that Lewis sites on Sn-BEA possess higher acidity with respect to SnO2/Si-BEA: in Sn-BEA tin incorporated into the framework have tetrahedral symmetry in Sn(OSi)4 fragments, which is thermodynamically less favorable with respect to the octahedral tin in SnO2 leading to different electron properties and lower Lewis acidity of supported tin oxide. Summarizing the catalysts characteristics, it can be concluded that SnO2/Si-BEA sample consists of SnO2 phase distributed on the surface of high crystalline siliceous BEA. Its preparation procedure does not lead to formation of any framework tin sites. Both dehydrated Sn-BEA and SnO2/Si-BEA catalysts possess Lewis acid properties while the strong Brønsted acid sites are not observed by FTIR of adsorbed pyridine. The tin atoms in Sn-BEA framework possess stronger Lewis acid sites with respect to tin in SnO2/Si-BEA catalyst. 3.2 NMR study of dehydration/hydration of Sn-BEA The effect of water to the structure of tin sites is of special interest, since aqueous phase reactions are a coherent part of biomass conversion using Sn-zeolite based catalysts. Tin sites in catalysts are usually studied (e.g. using NMR or IR spectroscopy or temperature-programmed desorption of probe molecules) in dehydrated state, and changes in catalysts structure upon hydration could account for a number of observations responsible for catalytic activity.22, 29, 46-48 119Sn

MAS NMR spectroscopy was proven to be the excellent technique to follow the changes

in tin coordination during the dehydration/hydration process29-31. Our group has recently developed the

7



Page 8 of 34

MAS NMR technique based on CPMG pulse sequence, which allows significant decrease of acquisition time required for the recording of

119Sn

NMR spectrum of Sn-BEA32-34. Fig. 2 shows the

spectra acquired for hydrothermally synthesized Sn-BEA material. In the spectrum of the hydrated Sn-BEA, two signals at ca. -689 and -703 ppm are observed. These signals were reported by other groups and were attributed to hexa-coordinated tin atoms29-34. After dehydration, the 119Sn NMR spectrum of Sn-BEA reveals the presence of signals at -422 and -443 ppm, which corresponds to different tin sites with coordination number of 4.33 Such an interconversion of hexa-coordinated tin atoms into tetra-coordinated during dehydration/hydration is associated with coordination of water molecules to Sn atoms. Unfortunately, the acquisition of

119Sn

NMR spectrum

using CPMG pulse technique was not possible for SnO2/Si-BEA sample due to the low T2 relaxation time of tin atoms in SnO2 phase. 1H

MAS NMR spectroscopy was used to follow the changes in proton surrounding of tin sites

during hydration/dehydration process. 1H MAS NMR spectra of dehydrated Sn-BEA (Fig. 3a) and SnO2/Si-BEA (Fig. S7) reveal the presence of three similar peaks in the range of 1.8-3.2 ppm corresponding to different types of silanol groups of BEA framework. Position of the signal at 1.8 ppm is typical for terminal surface silanols, whereas peaks at 2.0 and 3.2 are generally assigned to vicinal and bridged silanols, respectively49. Upon hydration of Sn-BEA, the intensities of these bands become lower due to the formation of H-bonds and broadening of the spectrum peaks. Simultaneously, the major peak at 4.0 ppm corresponding to physically adsorbed water appears, together with the distinct signals at 5.8 and 6.4 ppm pointing to coordination of water to tin sites. Time-resolved 1H NMR spectra (Fig. 3b) revealed different kinetic behavior of new signals: appearance of the signal at 6.4 ppm is observed immediately after the introduction of water to the dehydrated Sn-BEA sample, whilst the intensity of the signal at 5.8 ppm progressively develops over the duration of the experiment (~70 h). At the same time the intensity of signal of adsorbed water decreases gradually. It should be noted that all the protons in hydrated Sn-BEA are extremely mobile as evidenced from the low intensity spinning side bands (Fig. S8).

8


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


To assign the 1H signals at 5.8 and 6.4 ppm and establish their relationship to framework tin atoms, a time-resolved 1H and 119Sn NMR spectra were collected after exposure of dehydrated Sn-BEA sample to water vapors. The evolution of normalized 1H signals together with

119Sn

signals at -581, -

689 and -703 ppm is shown in Fig. 4. Immediately after introduction of water, the signals at -422 and 443 ppm vanish from the

119Sn

NMR spectra and the signals at -581, -689 and -703 ppm develop

pointing to the coordination of water to tin atoms, hence changing their environment from tetracoordinated to the mixture of penta-coordinated and hexa-coordinated states (Fig. S9). In the end of the experiment, the intensity of the signal at ca. -581 ppm decreases from 80 to 30%, whereas the intensity of the signal at ca. -690 ppm increases from 0 to 50%. The signal at -703 ppm remains intact (Fig. S10). Interestingly, both 1H signal at 6.4 ppm and

119Sn

signal at -703 ppm immediately appear and

demonstrate stable intensity over 70 h. This indicates the fast hydration of tetrahedral tin sites (119Sn signal at -422 ppm) into octahedral state and results in the appearance of the signal at 6.4 ppm in 1H spectra, directly associated with119Sn signal at -703 ppm. In contrast, the relative intensity of the signal at 5.8 ppm in 1H NMR spectra was found to precisely follow the one for 119Sn signal at -689 ppm (Fig. 4), demonstrating very slow reaction kinetics. This correlation associates the proton signal at 5.8 ppm with the tin sites revealing the signal at -689 ppm with octahedral coordination state of tin sites. The latter is formed via further hydration and hydrolysis of penta-coordinated tin sites (-581 ppm) formed at the very beginning of the reaction with water. The attribution of the signals is also confirmed by comparison of integrative intensities of signals: both the intensities of 1H signals at (6.4 and 5.8 ppm) and 119Sn signals at (-422 and - 443 ppm) relate to each other as 15 to 85 (I5.8 : I6.4 = I-422 : I-443= 15 : 85). Considering the absence of any 1H signals with kinetic behavior similar to -581 ppm tin signal, we conclude that the formation of pentacoordinated tin does not involve the hydrolysis of Si-O-Sn bonds and formation of OH groups. Aside from the kinetic correlation of 119Sn signals with 1H signals, a chemical shift of the latter provides the important information about the chemical environment and structure of tin sites. Typically, the 1H signals within the 5-7 ppm range arise from the H-bonded bridged acidic sites in zeolite.53

9



Page 10 of 34

However, these signals are extremely sensitive to the presence of any access of water leading to signal’s broadening, which was not observed in the case of Sn-BEA (Fig. 3). Another interpretation of 1H

signals at 5.8 and 6.4 ppm is associated with water molecules coordinated to tin atoms. For instance,

the interaction of water with 6-coordinated Al sites was deeply studied for partially dealumunated zeolites51-53. It was shown that water coordinated to Lewis acid sites gives narrow 1H NMR signals at 6.5-6.7 ppm, whilst interaction of water with Brønsted acid sites leads to signal broadening.51-53 Importantly, the observation of water molecules chemically bonded to Sn atoms in 6-coordinated environment is coherent with recent findings by Qi et al. who, using two-dimensional proton-detected 1H/119Sn

correlation MAS NMR, made an assignment of these signals.50 Taken together, this data

allows to assign the 1H signals at 5.8 and 6.4 ppm in the spectrum of hydrated Sn-BEA to water molecules coordinated to tin atoms. The presence of two different signals can be rationalized by the existence of several hydrated states of 6-coordinated tin in Sn-BEA. 3.3 FTIR study of hydration of Sn-BEA and SnO2/Si-BEA In further experiments we followed the transformation of water upon adsorption over Sn-BEA using infrared spectroscopy. In the present study we used DRIFT spectroscopy due to its high sensitivity towards OH groups in the case of large zeolite particles, where conventional transmission FTIR suffers from low signal-to-noise ratio54. First, the surface sites evolution through water adsorption on Sn-BEA and SnO2/Si-BEA samples at different temperatures was studied (Fig. 5). Adsorption of small doses of water over SnBEA leads to immediate changes in the spectra within the range of OH vibrations (Fig. 5a). Introduction of wet stream results in the appearance of new bands at 3745, 3668, 3594 cm-1 accompanied by broad shoulders at 3220 and 3420 cm-1 indicating water dissociation over Sn-BEA sample. Decrease of temperature leads to the further adsorption of water and higher intensities of all bands. Based on frequencies of new bands it can be suggested that the band at 3745 cm-1 is due to O-H vibrations in isolated silanol groups and broad bands at 3220 and 3420 cm-1 refer to H-bonded surface Si-OH groups interacting with excess of water42,

55.

10

The band at 3668 cm-1 might correspond to


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


vibrations in Sn-OH groups, the position of this bands is close to the those observed for hydrated SnO2 samples and Sn-BEA.56 The frequency of the band at 3594 cm-1 is typical of bridged OH groups similar to those present on aluminum-substituted BEA materials49. Adsorption of water over SnO2/Si-BEA sample does not lead to formation of any new OH bands in comparison with Sn-BEA catalyst (Fig. 5b). Instead, an intensive negative peak at 3735 cm-1 develops accompanied by the appearance of the broad bands at 3220 and 3420 cm-1 due to the Hbonding of the surface silanols to water molecules. This observation confirms previous findings and suggests that tin oxide is not able to interact with water and the formation of new surface OH species occurs on the tin atoms of the framework. Taken together with NMR results, this data allows considering two distinct processes with different rates involving two different types of tin sites taking place during the interaction of Sn-BEA with water. The first corresponds to the stepwise hydration of tin sites having 119Sn NMR signature at 443 ppm into penta-coordinated tin and, slowly into hexa-coordinated tin with water in coordination sphere giving the 1H NMR signal at 5.8 ppm (Scheme 1). The second involves fast hydrolysis of Sn-OSi bonds leading to the formation of new bridged OH groups at 3594 cm-1. The rate of this process points to the involvement of tin sites with -422 ppm 119Sn NMR chemical shift into this transformation (Scheme 2). The difference in the reaction rates might be associated with the tin atoms location within the BEA framework. As such, tin atoms siting in crystallographycally different T-positions have different spectroscopic signatures33,55 and a priori might possess different reactivity; however, more work is required in this direction. We believe that both types of tin sites can form bridged OH groups after the complete reaction with water and formation of octahedral tin species. However, the reaction time in FTIR experiments (~1 h) was significantly lower as compared to NMR measurements (70 h), which does not allow observing and quantifying all the OH groups formed. Importantly, no NMR signals associated with infrared band at 3594 cm-1 were found in 1H spectra. We associate this observation with strong interaction of these bridged OH groups with the excess of water present in NMR experiment, leading to

11



Page 12 of 34

signal broadening, in contrast to IR measurements, where small doses of water were used to induce the formation of new OH groups. 3.4 Acid properties of the formed OH groups We have shown that adsorption of water leads to the formation of new OH groups on the surface of Sn-BEA. IR band at ~3600 cm-1 appeared during the water adsorption over Sn-BEA and attributed to bridged OH groups may possess strong Brønsted acid properties similar to the ones observed over aluminum zeolites. In general, these strong sites could be easily measured by FTIR spectroscopy of adsorbed pyridine. Interaction of water specifically with framework tin Lewis sites was confirmed in water adsorption experiments, where Sn-BEA was preliminary saturated with pyridine and outgassed at 373K. Fig. 6 displays subtraction spectra in experiment with increasing water coverage. They show that gradual adsorption of water leads to the appearance and progressive development of negative peaks at 1610 and 1451 cm-1 indicating the decrease of the amount of tin Lewis acid sites available for pyridine. Simultaneously, the intensity of the bands attributed to H-bonded pyridine (1596 and 1443 cm-1) and protonated pyridine (1639 and 1550 cm-1) increases. This observation clearly shows that adsorption of water over Sn-BEA leads to generation of new bridged OH groups that exhibit strong Brønsted acidity. Pre-adsorption of water with subsequent probing of acid sites with pyridine gives identical results (Fig. S11). Dose by dose adsorption of pyridine over the Sn-BEA sample with pre-adsorbed water leads to the appearance of negative peaks within the range of 3750-3600 cm-1 indicating the interaction of different OH groups, discussed above, with pyridine. Simultaneously, new bands due to vibrations of Py adsorbed over different sites develop. Besides the bands found on fresh Sn-BEA without pre-adsorbed water, a new band at 1550 cm-1 was observed and assigned to Py adsorbed over strong Brønsted sites. Comparing with SnO2/Si-BEA sample, it suggests that only framework tin Lewis sites are able to interact with water. To study the formation of Brønsted acid sites in situ, during the hydration/dehydration of SnBEA, we used infrared spectroscopy with gaseous ammonia fed directly into the DRIFTS cell. The

12


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


selection of this probe molecule is based on its strong base properties (pKa(NH4+) = 4.7, pKa(Py-H+) = 5.2), small diffusion radius and high volatility, which excludes the contamination of the equipment and samples during the measurements. Subtracted DRIFT spectra in Fig. 7a show gradual changes upon heating of hydrated Sn-BEA in a flow of ammonia. Broad bands at ca. 3387, 3287 and 3186 cm-1 could be assigned to ν(NH) stretching vibrations in ammonium cations57. Treatment of SnO2/Si-BEA sample in a flow of ammonia (Fig. 7b) does not lead to any spectral features due to ν(NH) indicating the absence of strong acid sites over this sample. Therefore, Sn-BEA material, which is generally postulated as hydrophobic Lewis acid catalyst for the transformation of different substrates in aqueous media, shows the formation of new Brønsted acid sites upon hydration. These sites a priori might be active either in the main reaction or/and in the process of conversion of initial reagents into by-products, which was already discussed in recent publications demonstrating the selectivity variation by addition of alkali into the reaction mixture.56,59 As such, in the processes catalyzed by Sn-BEA, which still acts mostly as the Lewis-acid catalyst, the addition of water or variation of its concentration can be used to control selectivity and activity. Presumably, this applies to the reactions, normally carried out in water-free media, while the biomassbased processes which already has an excess of water in the reaction, can hardly be controlled in this way. 3.5 Oligomerization of isobutene over Sn-BEA in presence of water Isobutene oligomerization is perfectly catalyzed by proton acids, such as aluminum-containing zeolites, ion-exchange resins and some other catalysts60,61. Lewis acid sites are less active in oligomerization62,63, so this reaction can be used as a benchmark probing newly generated Brønsted acid sites on catalytic properties of Sn-BEA. As such, the formation of acid sites in situ was studied in the isobutene dimerization reaction performed in a fix-bed microreactor system at 420K. In this reaction, the reacting feed and products do not contain water or alcohols that might lead to spontaneous conversion of tetra-coordinated tin atoms in zeolite framework. Moreover, the reaction temperature in

13



Page 14 of 34

the absence of water keeps all tin atoms in 4-coordinated state, whilst the addition of small dose of water will results in fast formation of Brønsted acid sites according to Scheme 2. Also, to avoid the influence of water impurities from the reacting feed, isobutene was additionally dried by passing through absorber filled with NaOH + CaO mixture. After the equilibration, the conversion of isobutene is set to 1.5% to avoid kinetic limitations and fast deactivation of the catalyst which can impact into the conversion measurements. Only two products, 2,2,4-trimethylpentene-3 and 2,2,4-trimethylpentene-4, were observed. Small pulses of water were introduced with micro-syringe in the feedstock to generate new acid sites (Fig. 8). Immediately after the H2O pulse, the conversion of isobutene decreases due to the competitive adsorption of water excess over Lewis sites of Sn-BEA. However, after certain time the conversion of isobutene increased and reached 2.0%, which is higher than initial conversion observed over dry Sn-BEA. It points to the formation of new sites, active in the process of isobutene dimerization. Selectivity does not change after water adsorption, which is explained by relatively low reaction temperature, which is not sufficient for the cracking of C8 products. Summarizing, the formation of additional Brønsted acid sites through water dissociative adsorption over framework tin sites of Sn-BEA zeolite occurs under the real reaction conditions. The formed acid sites are active in the reaction of isobutene dimerization hence confirming their high acid strength. 4. Conclusions The formation of new acidic OH groups has been established during the hydration of Sn-BEA materials in contrast to SnO2 supported over pure silica BEA. At least two kinetically different processes have been observed: i)

fast hydrolysis of tetrahedral tin sites characterized by NMR signal at -422 ppm to form octahedral tin sites with water molecules in coordination sphere characterized by 119Sn

signal at -703 ppm and 1H signal at 6.4 ppm. Simultaneous formation of bridged

OH groups was observed in IR spectra as a band at 3594 cm-1;

14


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


ii)

fast conversion of tetrahedral tin sites at -443 ppm into penta-coordinated sites via the water coordination giving 1H signal at 5.8 ppm with subsequent slow hydrolysis of SiO-Sn bonds into hexa-coordinated tin species.

It has been showed that bridged OH groups characterized by vibration frequency of 3594 cm-1 reveal strong Brønsted acid properties, enabling protonation of pyridine and ammonia. Furthermore, these acid sites can be generated in situ and lead to the increase of the catalyst activity as was shown for the isobutene dimerization reaction. The obtained results have promising practical implications in understanding the effect of water on the catalytic properties of Sn-containing materials, since they clearly demonstrate that water can modify the active Sn sites. However, the exact nature and quantity of the tin sites formed upon the reaction with water might differ for different tin-containing materials and therefore require further investigations. 5. Acknowledgements V.L.S., P.A.K., Y.G.K., A.V.Y., and I.I.I. thank the Russian Science Foundation for the financial support (Grant 14-23-00094). A.V.Y. and P.A.K. acknowledges Haldor Topsoe A/S for PhD fellowship. We gratefully acknowledge Dr. P. N. R. Vennestrom and Dr. S. Tolborg for the fruitful discussion. 6. Supporting information Supporting Information Available: nitrogen adsorption isotherms, XRD patterns, SEM images and UV-vis and MAS NMR spectra of studied materials. This material is available free of charge via the Internet at http://pubs.acs.org 7. References (1)

M.A. Banares, “Operando methodology: Combination of in situ spectroscopy and simultaneous activity measurements under catalytic reaction conditions”, Catal. Today, 2005, 100, 71-77.

(2)

H. Topsoe, “Developments in operando studies and in situ characterization of heterogeneous catalyst”, J Catal., 2003, 216, 155-164. 15



(3)

Page 16 of 34

R. Schoonheydt, “UV-VIS-NIR spectroscopy and microscopy of heterogeneous catalysts”, Chem. Soc. Rev., 2010, 39, 5051-5066.

(4)

F. Zaera, “New advances in the use of infrared absorption spectroscopy for the characterization of heterogeneous catalytic reactions”, Chem. Soc. Rev., 2014, 43, 7624-7663.

(5)

U. Olsbye, S. Svelle, M. Bjrgen, P. Beato, T. V. Janssens, F. Joensen, S. Bordiga, K.P. Lillerud, “Conversion of methanol to hydrocarbons: How zeolite cavity and pore size controls product selectivity”, Angew. Chem. Int. Ed., 2012, 51, 5810-5831.

(6)

P. Tian, Y. Wei, M. Ye, Z. Liu, “Methanol to olefins (MTO): From fundamentals to commercialization”, ACS Catal., 2015, 5, 1922-1938.

(7)

I. Ivanova, V. Sushkevich, Yu. Kolyagin, V. Ordomsky, “Catalysis by coke deposits: synthesis of isoprene over solid catalysts”, Angew. Chem. Int. Ed., 2013, 52, 12961-12964.

(8)

I. Beta, B. Hunger, W. Böhlmann, H. Jobic, “Dissociative adsorption of water in CaNaA zeolites studied by TG, DRIFTS and 1H and

27Al

MAS NMR spectroscopy”, Micropor.

Mesopor. Mater., 2005, 79, 69-78. (9)

V. Sushkevich, V. Ordomsky, I. Ivanova, “Synthesis of isoprene from formaldehyde and isobutene over phosphate catalysts”, Appl. Catal. A.: Gen., 2012, 441, 21-29.

(10)

A. Khodakov, W. Chu, P. Fongarland, “Advances in the development of novel cobalt FischerTropsch catalysts for synthesis of long-chain hydrocarbons and clean fuels”, Chem. Rev., 2007, 107, 1692-1744.

(11)

R. Kopelent, J. van Bokhoven, J. Szlachetko, J. Edebeli, C. Paun, M. Nachtegaal, O. Safonova, “Catalytically Active and Spectator Ce3+ in Ceria-Supported Metal Catalysts”, Angew. Chem. Int. Ed., 2015, 54, 8728-8731.

(12)

H. Y. Luo, D. F. Consoli, W. R. Gunther, Y. Román-Leshkov, “Investigation of the reaction kinetics of isolated Lewis acid sites in Beta zeolites for the Meerwein–Ponndorf–Verley reduction of methyl levulinate to γ-valerolactone”, J. Catal., 2014, 320, 198–207.

16


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

(13)


A. Corma, L. T. Nemeth, M. Renz, S. Valencia, “Sn-zeolite beta as a heterogeneous chemoselective catalyst for Baeyer–Villiger oxidations”, Nature, 2001, 412, 423–425.

(14)

S. Van de Vyver, C. Odermatt, K. Romero, T. Prasomsri, Y. Román-Leshkov, “Solid Lewis acids catalyze the carbon−carbon coupling between carbohydrates and formaldehyde”, ACS Catal., 2015, 5, 972–977.

(15)

D. Lewis, S. Van de Vyver, Y. Román-Leshkov, “Acid-base pairs in Lewis acidic zeolites promote direct aldol reactions by soft enolization”, Angew. Chem. Int. Ed., 2015, 54, 98359838.

(16)

J.J. Pacheco, M.E. Davis, “Synthesis of terephthalic acid via Diels-Alder reactions with ethylene and oxidized variants of 5-hydroxymethylfurfural”, Proc. Nat. Acad. Sci. USA, 2014, 111, 8363-8367.

(17)

M. Moliner, Y. Román-Leshkov, M. E. Davis, “Tin-containing zeolites are highly active catalysts for the isomerization of glucose in water”, Proc. Natl. Acad. Sci. USA., 2010, 107, 6164–6168.

(18)

Y. Román-Leshkov, M. Moliner, J. A. Labinger, M. E. Davis, “Mechanism of Glucose Isomerization Using a Solid Lewis Acid Catalyst in Water”, Angew. Chem., Int. Ed., 2010, 49, 8954–8957.

(19)

M. S. Holm, Y. J. Pagan-Torres, S. Saravanamurugan, A. Riisager, J. A. Dumesic, E. Taarning, “Sn-Beta catalysed conversion of hemicellulosic sugars”, Green Chem., 2012, 14, 702–706.

(20)

R. Bermejo-Deval, R. Gounder, M. E. Davis, “Framework and Extraframework Tin Sites in Zeolite Beta React Glucose Differently”, ACS Catal., 2012, 2, 2705–2713.

(21)

R. Gounder, M. E. Davis, “Monosaccharide and disaccharide isomerization over Lewis acid sites in hydrophobic and hydrophilic molecular sieves”, J. Catal., 2013, 308, 176-188.

(22)

J. Dijkmans, D. Gabriels, M. Dusselier, F. de Clippel, P. Vanelderen, K. Houthoofd, A. Maliet, Y. Pontikes, B. F. Sels, “Productive sugar isomerization with highly active Sn in dealuminated β zeolites”, Green Chem., 2013, 15, 2777–2785.

17



(23)

Page 18 of 34

W. R. Gunther, Y. Wang, Y. Ji, V. K. Michaelis, S. T. Hunt, R. G. Griffin, Y. Román-Leshkov, “Sn-Beta zeolites with borate salts catalyse the epimerization of carbohydrates via an intramolecular carbon shift”, Nat. Commun., 2012, 3, 1109.

(24)

E. Taarning, S. Saravanamurugan, M. S. Holm, J. Xiong, R. M. West, C. H. Christensen, “Zeolite‐Catalyzed Isomerization of Triose Sugars”, ChemSusChem, 2009, 2, 625–627.

(25)

C. Hammond, S. Conrad, I. Hermans, “Simple and Scalable Preparation of Highly Active Lewis Acidic Sn-β”, Angew. Chem., Int. Ed., 2012, 51, 11736-11739.

(26)

M. Dusselier, P. Van Wouwe, A. Dewaele, E. Makshina, B. F. Sels, “Lactic acid as a platform chemical in the biobased economy: the role of chemocatalysis”, Energy Environ. Sci., 2013, 6, 1415–1442.

(27)

J. D. Lewis, M. Ha, H. Luo, A. Faucher, V. K. Michaelis, Y. Román-Leshkov, “Distinguishing Active Site Identity in Sn-Beta Zeolites using 31P MAS NMR of Adsorbed Trimethylphosphine Oxide”, ACS Catal., 2018, 8, 3076-3086.

(28)

B. Tang, W. Dai, G. Wu, N. Guan, L. Li, M. Hunger, “Improved postsynthesis strategy to SnBeta zeolites as Lewis acid catalysts for the ring-opening hydration of epoxides”, ACS Catal., 2014, 4, 2801−2810.

(29)

S.-J. Hwang, R. Gounder, Y. Bhawe, M. Orazov, R. Bermejo-Deval, M.E. Davis, “Solid state NMR characterization of Sn-Beta zeolites that catalyze glucose isomerization and epimerization”, Top. Catal., 2015, 58, 435−440.

(30)

R. Bermejo-Deval, R. Assary, E. Nikolla, M. Moliner, Y. Roman-Leshkov, S.-J. Hwang, A. Palsdottir, D. Silverman, R.F. Lobo, L. Curtiss, et al. “Metalloenzyme-like catalyzed isomerizations of sugars by Lewis acid zeolites”, Proc. Natl. Acad. Sci. USA, 2012, 109, 9727−9732.

(31)

R. Bermejo-Deval, M. Orazov, R. Gounder, S.-J. Hwang, M.E. Davis, “Active sites in Sn-Beta for glucose isomerization to fructose and epimerization to mannose”, ACS Catal., 2014, 4, 2288−2297. 18


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

(32)


Y. Kolyagin, A. Yakimov, I. Ivanova, S. Tolborg, P.N.R. Vennestrøm, “Application of CPMG MAS NMR for fast characterization of Sn Sites in zeolites with natural

119Sn

119Sn

isotope

abundance”, J. Phys. Chem. Lett., 2016, 7, 1249−1253. (33)

Y. Kolyagin, A. Yakimov, S. Tolborg, P.N.R. Vennestrom, I. Ivanova, “Direct Observation of Tin in Different T-Sites of Sn-BEA by One- and Two-Dimensional

119Sn

MAS NMR

Spectroscopy”, J. Phys. Chem. Lett., 2018, 9, 3738-3743. (34)

A. Yakimov, Y. Kolyagin, S. Tolborg, P.N.R. Vennestrom, I. Ivanova, “119Sn MAS NMR Study of the Interaction of Probe Molecules with Sn-BEA: The Origin of Penta- and Hexacoordinated Tin Formation”, J. Phys. Chem. C, 2016, 120, 28083−28092.

(35)

D. Padovan, L. Botti, C. Hammond, “Active Site Hydration Governs the Stability of Sn-Beta during Continuous Glucose Conversion”, ACS Catal., 2018, 8, 7131-7140.

(36)

G. Yang, L. Zhou, “Glucose Conversions Catalyzed by Zeolite Sn-BEA: Synergy among Na+ Exchange, Solvent, and Proximal Silanol Nest as Well as Critical Specifics for Catalytic Mechanisms”, ACS Catal., 2018, 8, 6691-6698.

(37)

A. Yakimov, Y. Kolyagin, S. Tolborg, P.N.R. Vennestrom, I. Ivanova, “Accelerated Synthesis of Sn-BEA in Fluoride Media: Effect of H2O Content in the Gel”, New J. Chem. 2016, 40, 4367−4374.

(38)

S. Valencia, A. Corma, “Stannosilicate Molecular Sieves”, US6306364 B1, 1999.

(39)

S. Tolborg, A. Katerinopoulou, D. Falcone, I. Sádaba, C. Osmundsen, R. Davis, E. Taarning, P. Fristrup, M. Holm, “Incorporation of tin affects crystallization, morphology, and crystal composition of Sn-Beta”, J. Mater. Chem. A., 2014, 47, 20252-20262.

(40)

K. Chaudhari, T. Das, P. Rajmohanan, K. Lazar, S. Sivasanker, A. Chandwadkar, “Synthesis, characterization, and catalytic properties of mesoporous tin-containing analogs of MCM-41”, J. Catal., 1999, 183, 281−291.

(41)

M.A. Camblor, A. Corma, S. Valencia, “Synthesis in fluoride media and characterization of aluminosilicate zeolite beta”, J. Mater. Chem., 1998, 8, 2137-2145.

19



(42)

Page 20 of 34

V. Sushkevich, I. Ivanova, A. Yakimov, “Revisiting Acidity of SnBEA Catalysts by Combined Application of FTIR Spectroscopy of Different Probe Molecules”, J. Phys. Chem. C, 2017, 121, 11437-11447.

(43)

J.W. Harris, M. Cordon, J. Di Iorio, J. Vega-Vila, F. Ribeiro, R. Gounder, “Titration and quantification of open and closed Lewis acid sites in Sn-Beta zeolites that catalyze glucose isomerization”, J. Catal., 2016, 335, 141-154.

(44)

W. N. van der Graaff, G. Li, B. Mezari, E. Pidko, E. Hensen, “Synthesis of Sn ‐ Beta with Exclusive and High Framework Sn Content”, ChemCatChem, 2015, 7, 1152-1160.

(45)

V. Sushkevich, A. Vimont, A. Travert, I. Ivanova, “Spectroscopic evidence for open and closed Lewis acid sites in ZrBEA zeolites”, J. Phys. Chem. C., 2015, 119, 17633-17639.

(46)

R. Otomo, R. Kosugi, Y. Kamiya, T. Tatsumi, T. Yokoi, “Modification of Sn-Beta zeolite: characterization of acidic/basic properties and catalytic performance in Baeyer–Villiger oxidation”, Catal. Sci. Technol., 2016, 6, 2787-2795.

(47)

S. Roy, K. Bakhmutsky, E. Mahmoud, R.F. Lobo, R.J. Gorte, “Probing Lewis Acid Sites in SnBeta Zeolite”, ACS Catal., 2013, 3, 573-580.

(48)

T.D. Courtney, C.-C. Chang, R.J. Gorte, R.F. Lobo, W. Fan, V. Nikolakis, “Effect of water treatment on Sn-BEA zeolite: Origin of 960 cm−1 FTIR peak”, Micropor. Mesopor. Mater., 2015, 210, 69-76.

(49)

A. A. Gabrienko, I. G. Danilova, S. Arzumanov, A. Toktarev, D. Freude, A. G. Stepanov, “Strong acidity of silanol groups of zeolite beta: Evidence from the studies by IR spectroscopy of adsorbed CO and 1H MAS NMR”, Micropor. Mesopor. Mater., 2010, 131, 210-216.

(50)

G. Qi, Q. Wang, J. Xu, Q. Wu, C. Wang, X. Zhao, X. Meng, F. Xiao, F. Deng, “Direct observation of tin sites and their reversible interconversion in zeolites by solid-state NMR spectroscopy”, Commun. Chemistry, 2018, 1, 22.

20


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

(51)


S. Li, A. Zheng, Y. Su, H. Zhang, L. Chen, J. Yang, C. Ye, F. Deng, “Brønsted/Lewis Acid Synergy in Dealuminated HY Zeolite: A Combined Solid-State NMR and Theoretical Calculation Study”, J. Am. Chem. Soc., 2007, 129, 11161-11171.

(52)

S. Li, A. Zheng, Y. Su, H. Fang, W. Shen, Z. Yu, L. Chen, F. Deng, “Extra-framework aluminium species in hydrated faujasite zeolite as investigated by two-dimensional solid-state NMR spectroscopy and theoretical calculations”, Phys. Chem. Chem. Phys., 2010, 12, 38953903.

(53)

M. Hunger, D. Freude, H. Pfeifer, “Magic-angle Spinning Nuclear Magnetic Resonance Studies of Water Molecules adsorbed on Brønsted- and Lewis-acid Sites in Zeolites and Amorphous Silica-Aluminas”, J. Chem. Soc. Faradey Trans., 1991, 87, 657-662.

(54)

I. Halasz, B. Moden, A. Petushkov, J. Liang, M. Agarwal, „Delicate Distinction between OH Groups on Proton-Exchanged H-Chabazite and H-SAPO-34 Molecular Sieves”, J Phys. Chem. C, 2015, 119, 24046-24055.

(55)

P. Wolf, M. Valla, F. Núñez-Zarur, A. Comas-Vives, A. Rossini, C. Firth, H. Kallas, A. Lesage, L. Emsley, C. Copéret, I. Hermans, “Correlating Synthetic Methods, Morphology, AtomicLevel Structure, and Catalytic Activity of Sn‑β Catalysts”, ACS Catal., 2016, 6, 4047-4063.

(56)

A. Bhattacharjee, M. Ahmaruzzaman, A. Sil, T. Sinha, “Amino acid mediated synthesis of luminescent SnO2 nanoparticles”, J. Ind. Eng. Chem., 2015, 22, 138-146.

(57)

T. Bučko, J. Hafner, L. Benco, “Adsorption and vibrational spectroscopy of ammonia at mordenite: Ab initio study”, J. Chem. Phys., 2004, 120, 10263-10277.

(58)

S. Tolborg, I. Sádaba, C. M. Osmundsen, P. Fristrup, M. S. Holm, E. Taarning, “Tin ‐ containing Silicates: Alkali Salts Improve Methyl Lactate Yield from Sugars”, ChemSusChem 2015, 8, 613-617.

(59)

S. G. Elliot, S. Tolborg, R. Madsen, E. Taarning, S. Meier, “Effects of Alkali ‐Metal Ions and Counter Ions in Sn ‐Beta ‐Catalyzed Carbohydrate Conversion”, ChemSusChem, 2018, 11, 1198-1203. 21



(60)

Page 22 of 34

R. Quann, L. Green, S. Tabak, F. Krambeck, “Chemistry of olefin oligomerization over ZSM-5 catalyst”, Ind. Eng. Chem. Res., 1988, 27, 565-570.

(61)

M. Harmer, Q. Sun, “Solid acid catalysis using ion-exchange resins”, Appl. Catal. A: Gen., 2001, 221, 45-62.

(62)

C. O'Connor, M. Kojima, “Alkene oligomerization”, Catal. Tod., 1990, 6, 329-349.

(63)

M. Kojima, M. Rautenbach, C. O'Connor, „Acidity characterization of ion-exchanged mordenite: II. Infrared spectroscopic investigation”, J. Catal., 1988, 112, 505-515.

22


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


Table 1. Catalysts characteristics Sample SnBEA SnO2/BEA

Chemical composition Si/Sn Sn, µmol∙g-1 211 78 202 82

Total pore volume (cm3/g) 0.29 0.31

23


Micropore volume (cm3/g) 0.22 0.22


Page 24 of 34

Figure captures Fig. 1. FTIR spectra of Py adsorbed over SnBEA and SnO2/SiBEA dehydrated catalysts. Fig. 2. 119Sn NMR spectra of hydrated and dehydrated SnBEA. Fig. 3. a) 1H NMR spectra of dehydrated SnBEA before and after exposure to water vapors for 70 h; b) time-resolved 1H NMR spectra acquired during the hydration of Sn-BEA. Fig. 4. Normalized intensities of 1H and 119Sn NMR signals versus hydration time. Fig. 5. DRIFTS spectra in the range of OH vibrations observed after the water adsorption over a) SnBEA and b) SnO2/SiBEA. Fig. 6. FTIR spectra observed after the adsorption of water over SnBEA with pre-adsorbed pyridine. Fig. 7. DRIFTS spectra collected during the heating of a) SnBEA and b) SnO2/SiBEA samples in a flow of 100 ppm of ammonia in dry argon. Fig. 8. Conversion of isobutene and water concentration followed by mass spectrometry versus time during the water pulsing at 420K.

24


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


Fig. 1

25



Fig. 2

26


Page 26 of 34

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


Fig. 3.

27



Fig. 4

28


Page 28 of 34

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


Fig. 5

29



Fig. 6

30


Page 30 of 34

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


Fig. 7.

31



Fig. 8.

32


Page 32 of 34

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


1 119 119

119

Sn: -443 ppm

Si

O

Sn

O

Si

H 2O

Sn: -581 ppm

Si

O

Sn

O

H 2O

Si

slow

OH2 Scheme 1.

1 119 119

Sn: -422 ppm

Si

O

Sn

O

Si

H 2O fast

H: 6.4 ppm

Sn: -703 ppm OH2

Si

O H

Sn

O OH

Si

IR: 3594 cm-1 Scheme 2.

33


H: 5.8 ppm

Sn: -689 ppm OH2

Si

O H

Sn

O OH

Si


TOC artwork

34


Page 34 of 34

Origin of Water-Induced BrÃ¸nsted Acid Sites in Sn-BEA Zeolites - The

Recommend Documents