Outer-Sphere Stabilized Sn Active Site in Î²-Zeolite

Research Article pubs.acs.org/acscatalysis

An Inner-/Outer-Sphere Stabilized Sn Active Site in β‑Zeolite: Spectroscopic Evidence and Kinetic Consequences Jan Dijkmans,† Michiel Dusselier,† Wout Janssens,† Maarten Trekels,‡ André Vantomme,‡ Eric Breynaert,† Christine Kirschhock,† and Bert F. Sels*,† †

Center for Surface Chemistry and Catalysis, Katholieke Universiteit Leuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium Nuclear and Radiation Physics Section, Katholieke Universiteit Leuven, Celestijnenlaan 200D, 3001 Heverlee, Belgium

‡

S Supporting Information *

ABSTRACT: A highly active Sn site with Lewis acid properties is identified in post-synthetically synthesized Sn/ DeAlβ catalyst, prepared by liquid-phase Sn grafting of a dealuminated β-zeolite. Though apparently similar Sn activesite structures have been reported for the post-synthetic and the conventional hydrothermal Snβ, detailed study of the electronic structure and redox behavior of Sn with EXAFS, XANES, DR UV−vis, and TPR clearly reveals dissimilarities in geometry and electronic properties. A model of the active Sn site is proposed using a contemporary interpretation of inner-/outer-sphere coordination, assuming inner-sphere coordination of SnIV with three framework SiO− and one outer-sphere coordination by a distant charge-balancing SiO−, resulting in a separated Lewis acid−base pair. Stabilization of this geometry by a nearby water molecule is proposed. In comparison with active Sn sites in a hydrothermally synthesized Snβ, those in the grafted dealuminated material are sterically less demanding for substrate approach, while the low inner-sphere coordination of Sn leads to a stronger Lewis acidity. Proximate silanols in the active-site pocket, identified by FTIR, 29Si MAS NMR, 1H−29Si CP MAS NMR, DR NIR, and TGA, may impact local reagent concentration and transition states stabilization by hydrogen bonding. The structural dissimilarity of the active Sn site leads to a different kinetic behavior. Kinetic experiments using two Lewis-acid-catalyzed reactions, Baeyer−Villiger and Meerwein− Ponndorf−Verley, show differences that are reaction-type dependent and have different entropic (like sterical demand and hydrogen bonding) and enthalpic contributions (Lewis acid strength). The active-site model, containing both inner- and outersphere ligands with the zeolite framework, may be considered as a general model for other grafted Lewis acid single sites. KEYWORDS: Lewis acid, single-site catalysis, Sn β-zeolite, post-synthetic synthesis, outer-sphere coordination, Meerwein−Ponndorf−Verley, Baeyer−Villiger, frustrated Lewis pair significant reduction of the synthesis time can be achieved.9 Other recent alternatives omit the hydrothermal step of the procedure, required to build up the zeolite framework, by introducing Sn into preformed, commercially available zeolite frameworks. These materials are first dealuminated with acid to accommodate Sn atoms inside the zeolite structure. Several procedures to introduce Sn have been described, such as reacting the acid-treated zeolite with gaseous or dissolved SnCl4, or using solvent-free exchange procedures.4a,10 In the category of post-synthesis procedures, we recently reported the synthesis and characterization of a Sn-grafted dealuminated β-zeolite.4a,11 After dealumination of the zeolite in the presence of HNO3, leaving silanol nests at the removed Al sites, Sn4+ from SnCl4·5H2O is introduced into these nests during a grafting procedure in dry isopropanol. This procedure results in a dealuminated β-zeolite material with Sn attached to

1. INTRODUCTION The concept of single-site catalysis has proven paramount to the comprehension of reaction kinetics and mechanistic pathway phenomena. Such knowledge allows rational design and development of catalytic materials by providing strategic principles for atomic environment modifications leading to optimal catalytic performances.1 The concept is especially useful in zeolites research. These materials are stable porous materials which can enclose exceptionally active and welldefined metal sites.2 One famous example is the well-defined Snβ zeolite, a Sncontaining β-zeolite frequently used in Lewis-acid-catalyzed reactions involving hydride shifts and redox chemistry.3 In particular, its use in the catalytic conversion of biomass-related feedstock has grown steadily during recent years.4−7 The material is traditionally synthesized by placing a Sn-containing gel in hydrothermal conditions, allowing nucleation and growth of the zeolite crystals.8 Next to this traditional procedure, several alternatives have been reported. By using modified seeding procedures or steam conversion of Sn-containing gels, © 2015 American Chemical Society

Received: August 18, 2015 Revised: October 26, 2015 Published: November 10, 2015 31

DOI: 10.1021/acscatal.5b01822 ACS Catal. 2016, 6, 31−46

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ACS Catalysis

activity of the partially hydrolyzed site originates from the extra flexibility, provided by the SiOH lattice defect, and that the group plays only a minor role in the mechanistic scheme.19 Later, Pidko et al. calculated a similar activity of the closed and open Sn sites, but the presence of silanol groups in the vicinity of the Sn site (at defects) causes multidentate activation of the substrate, favoring the rate-determining H-shift.20 For glucose epimerization reactions, no participation of the silanol group was reported in water.18 However, upon the addition of borate salts, it was found that the silanol group does participate in this reaction, acting as a Brønsted acid.5c,21 From all these results it is obvious that not only the openness of the Sn site could influence the reaction rate, but also the direct environment, in particular the proximity of silanol groups, could play a significant role in the catalytic mechanism. This contribution demonstrates a clear difference between the post-synthetically treated and hydrothermally synthesized material in the kinetics of both BV and MPV reactions. A detailed structure-sensitive characterization study, using FTIR, DR NIR, and site-specific physicochemical characterization tools such as EXAFS, XANES, DR UV−vis, TPR, and FTIR of adsorbed cyclohexanone, is employed to identify the subtle differences in the Sn active site in order to explain the catalysis’ dissimilarities. Depending on the reaction type, both the Sn active-site geometry and proximity effects in the active-site pocket seem to play a crucial role in the Lewis acid single-site catalysis.

the framework, further denoted as Sn/DeAlβ in this article. Up to 2 wt% of isolated tetrahedral Sn can be introduced into the internal nests of the dealuminated material, whereas higher Sn loadings result in the formation of extraframework Sn species, blocking transport in the micropores of the zeolite.11 Aldo− keto isomerization of sugars in the presence of Sn/DeAlβ have been documented recently. The activity per Sn site of the Sn/ DeAlβ in glucose isomerization surpassed that of the original hydrothermally synthesized Snβ.4a The activity increase was tentatively explained by the small crystal size of the postsynthesis material, when compared to the hydrothermal Snβ, promoting faster diffusion, which is of importance especially when dealing with large substrates like hexoses. A similar explanation was suggested for an observed drop in activity in the conversion of dihydroxyacetone to methyl lactate for hydrothermal Snβ catalysts for crystal sizes larger than 7 μm.12 In contrast to the higher activity of Sn/DeAlβ in glucose isomerization, we recently also found comparable activity of the post-synthetic Sn/DeAlβ and hydrothermal Snβ in a hydrogen shift reaction of pyruvic aldehyde to lactate in alcoholic media. In the multistep conversion of 1,3-dihydroxyacetone to lactate, we establish that the presence of a Brønsted acid framework is key to efficiently catalyze an initial dehydration step.4b,13 As the available characterization information on both materials suggests the presence of identical Lewis acid tetrahedral Sn+IV (pyridine-probed FTIR, 119Sn Mössbauer, XPS, and DR UV− vis) inside the zeolite framework, that is, a closed Sn coordination site, viz. Sn(OSi)4 (based on deuteratedacetonitrile-probed FTIR and 119Sn MAS NMR), a difference in activity, especially when reactions are carried out in the chemical regime, is therefore not expected. Despite several efforts, the nature of the single active site in both hydrothermal Snβ and post-synthetic Sn/DeAlβ zeolites is still under debate. Boronat et al. suggested that the catalytically active Sn site for Baeyer−Villiger (BV) and Meerwein− Ponndorf−Verley (MPV) reactions is a partially hydrolyzed (open) Sn species, viz. Sn(OSi)3(OH).14 Yet, since a recent stability study of Snβ in a multi-step transfer hydrogenation and etherification reaction showed a stable transfer hydrogenation activity, despite a transformation of the Sn site from a fully framework connected (closed) to a partially hydrolyzed (open) coordination, also the closed Sn sites can be assumed to be catalytic sites in MPV reactions.5b Furthermore, for a carbonylene reaction, a reasonable activity was found in the absence of SnOH groups, again indicating a catalytic contribution of the closed Sn framework sites.15 Also for sugar isomerization reactions, the activity of open or closed Sn sites is debated elaborately. The similarity of the open Sn site and the active cleft of the xylose isomerase enzyme, combined with a match between experimental and computed activation enthalpies, supported the belief that the partially hydrated Sn site is the active species in this reaction.16 This result was further strengthened by another theoretical study; for the isomerization of glyceraldehyde to dihydroxyacetone, which has an identical mechanism as the glucose isomerization, the activation energy on an open site was found to be significantly lower than that on a closed site.17 Later, results from density functional theory (DFT) calculations on the glucose isomerization on an open Sn site advocated a silanol group, adjacent to the open Sn site, that participates in the rate-determining H-shift. When a silanol group was placed close to the open Sn site, a lower activation energy was found, pointing to an active partaking of the silanol group.18 Yet other simulations suggested that the enhanced

2. MATERIALS AND METHODS Material Synthesis. The catalytic materials were synthesized as described in the literature.4a Commercial β-zeolite (CP814e, Zeolyst International, SiO2/Al2O3 = 25) was dealuminated by stirring the zeolite powder overnight in an aqueous HNO3 solution (55 mL per gram of zeolite, at 353 K), an acid concentration of 7.2 M was used. Afterward, the powders were filtered, washed with water, and dried at 333 K. Before grafting, the powders were dried at 423 K to remove excess of physisorbed water. The dealuminated zeolite was contacted with SnCl4·5H2O (27 mmol·g−1 of zeolite) in dried isopropanol (100 mL·g−1 of zeolite) and placed in a reflux setup under N2 atmosphere. After 7 h, the mixture was filtered in air, rinsed with dry isopropanol, dried at 333 K, and calcined (3 K· min−1 to 473 K, dwell 6 h, 3 K·min−1 to 823 K, dwell 6 h). The material is denoted as Sn/DeAlβ. Hydrothermal Snβ was synthesized following procedures described in the literature.22 Catalytic Tests. Catalytic tests were performed in magnetically stirred and closed 10 mL glass reactors, which were placed in a copper heating block. Temperature control was carried out in a reference glass reactor containing only solvent. For BV reactions, 1.11 mmol of ketone was added to 50 mg of catalyst in 5 mL of dioxane, and then 50 wt% aqueous H2O2 solution was added in a H2O2/ketone molar ratio of 2 (0.114 mL). Ethylcyclohexane was used as the internal standard for chromatographic analysis and quantification. For reactions with peracid, 2.22 mmol pf m-chloroperoxybenzoic acid (mCPBA) was used as the oxidant instead of H2O2. The catalytic reactions were performed at 363 K. MPV reactions were performed at 373 K, in which 15 mg of catalyst was added to a 2-butanol solution containing 1 mmol of ketone. The solvent was used in a 2-butanol/ketone ratio of 50 (4.6 mL). 1,4-Dioxane was used here as the internal standard. For each reaction, aliquots of the sample were taken at regular time intervals through a rubber septum and were quantitatively 32


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calculated using FEFF6.25 EPR spectra were recorded at room temperature and 120 K while operating in X band with a microwave power of 20 mW using a Bruker ESP 300E instrument in a rectangular TE104 cavity. A TPR MS analysis was performed in a home-made oven with temperature control, coupled to a Pfeiffer Omnistar Mass spectrometer. The catalyst powder was pelletized, and 150 mg was placed inside a quartz tube. The sample was pretreated at 823 K in N2 for 2 h and cooled to 373 K before starting the measurement. Next, the sample was heated at 10 K·min−1 in 14.8 mL·min−1 of a 5% H2 in N2 flow. The m/z of 2 (H2) and 18 (H2O) were monitored with MS. Chemisorption was used to quantify the consumption of H2 at 1023 K. A loop with known volume, containing 8.16327 × 10−6 mol of H2, was pulsed over the catalyst, and the H2 consumption at 1023 K was monitored with MS. Calibration of the MS was performed before the actual measurement. The sample was pretreated at 823 K in N2 before measurement.

analyzed with an Agilent 6850 GC, equipped with a HP-1 column and FID detector. Identification of these products was based on retention time analysis and confirmed by GC-MS (Agilent 6890 GC with HP5-MS column and Agilent 5973 mass-selective detector). Productivity was calculated as gproduct· h−1·gcatalyst−1 and turnover frequency (TOF) as molproduct·h−1· molSn−1. To exclude substrate-deficiency limitations, productivity and TOF were determined at initial conversion points after 30 min of reaction (15 min for homogeneous peracid reactions). Selectivity was determined near full conversion (reached after 6 h). Material Characterization. FTIR measurements were performed on a Nicolet 6700 Spectrometer equipped with DTGS detector. Samples were pressed into self-supporting wafers and degassed at 673 K in vacuo before measurements. Lewis acid sites were probed with cyclohexanone as a probe molecule, which was adsorbed at room temperature. The samples were exposed to 6.5 mbar of probe molecule until saturation. Spectra were recorded at different temperatures in vacuo after equilibration of 10 min at each heating step. Direct excitation and cross polarization (1H−)29Si MAS NMR spectra were recorded on a Bruker AMX300 spectrometer (B0 = 7.0 T) equipped with a 4 mm MAS probe head. At this field, the resonance frequency of 29Si is 59.6 MHz. 5400 scans were accumulated with a recycle delay of 10 s. The length of the 1H 90° pulse for the CP experiment was 5.0 μs; the length of the contact pulse was 5.0 ms. The sample-rotation rate was 3000 Hz. Tetramethylsilane was used as chemical shift reference. The Sn contents of the materials were determined by electron probe microanalyzer (EPMA) analysis conducted on a JEOL JXA8530F field emission microprobe using WDS. Samples were embedded in a resin, the surface was ground, polished, and coated with carbon before measurement. The microprobe was operated at 10 kV with a probe current of 1.5 nA. The Sn Lα1 signal was detected using a PETH crystal, and the Sn concentration was quantified with a cassiterite standard. ZAF (Z represents the atomic number correction, A the absorption correction, and F the characteristic fluorescence correction) was used for the matrix correction method. Mössbauer spectroscopy was measured in transmission geometry, using a gas proportional counter at 23.88 keV nuclear resonance. Nominal activity of the 119Sn source was 370 MBq (matrix CaSnO3). The line-width of the radiation emitted from the source was determined to be 0.75 ± 0.01 mm/s fwhm. Measurements of the samples and standard materials like SnO2, SnO, and Sn0 were performed at room temperature. A Lorentzian line shape model was used to fit the results. The sample was placed in a quartz tube and reduced at 1023 K in a flow of H2. Afterward, the powder was transferred into an air-closed container under N2-atmosphere before measurement. Diffuse reflectance (DR) measurements in the NIR and UV−vis regions were recorded on an Agilent Cary 5000 spectrophotometer. Samples were placed in a quartz tube with a window and dried to 823 K before measurement, if indicated. Thermogravimetric analyses were carried out on a TA Instruments TGA Q500. Samples were dried in situ in a dry N2 flow at 573 K, cooled in dry N2 to 323 K, and exposed to a humid N2 flow. Room-temperature Sn K-edge XAS data were recorded in transmission mode on the DUBBLE beamline (BM26a)23 at the ESRF (Grenoble, France) using a Si(111) double-crystal monochromator. Data processing and analysis were performed using IFEFFIT in combination with the open source programming interface Demeter.24 Theoretical phase shifts and amplitudes were

3. RESULTS AND ANALYSIS Structural Characterization of the Zeolite Support. The crystal size of post-synthesized Sn/DeAlβ zeolites differs significantly from that of the hydrothermally synthesized Snβ. Sn/DeAlβ contains agglomerates of small 10−30 nm crystals, whereas the hydrothermally synthesized Snβ has particle sizes of 1000−1500 nm (as measured with SEM). Due to the change in particle size, slightly lower micropore volumes were found for the grafted material in a N2-sorption experiment (0.16 vs 0.19 mL·g−1 for grafted and hydrothermal material, respectively). This volume is similar to that of the Sn-free dealuminated β-zeolite, viz. 0.17 mL·g−1, indicating absence of non-framework Sn species inside the zeolite pores of the materials used in this work. For materials with higher Sn loadings, formation of non-framework Sn species blocking the zeolite pore, was described recently.11 The small particles of Sn/DeAlβ do create a higher amount of interparticle mesopores (0.44 vs 0.06 mL·g−1).4a The higher external surface area of the smaller particles affords a higher concentration of silanol end groups, as ascertained by FTIR spectroscopy. A spectrum of Sn/DeAlβ in Figure 1 displays presence of different types of silanol groups. The presence of external silanol groups is evidenced by

Figure 1. FTIR of the silanol region of (a) Sn/DeAlβ sample and (b) hydrothermal Snβ. Signals are scaled to weight. 33


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ACS Catalysis the signal at 3740 cm−1, whereas the vibrational signal at 3730 cm−1 is associated with internal silanol groups.26 While the former species originate from crystal surfaces and large defects, the latter only arise from small defects and upon acid removal of Al from the zeolite framework. The internal silanol groups are used for anchoring of Sn, but the presence of the 3730 cm−1 vibration in Sn/DeAlβ shows that not all internal SiOH groups are consumed in the grafting procedure, and residual holes, also called silanol nests, remain in the zeolite structure. The broad shoulder spanning to 3400 cm−1 is caused by hydrogen-bonded silanol groups and indicates the silanol groups’ potential to interact with polar guest molecules. The hydrothermally synthesized Snβ is free of such silanol nests, and therefore this material can be considered more hydrophobic. Indeed, FTIR spectroscopy indicates a very limited amount of silanol groups, as can be concluded from the low intensity of the SiOH signals (Figure 1, spectrum b). The presence of residual silanols in Sn/DeAlβ is also evident from 29Si MAS NMR and 1H−29Si CP MAS NMR. The spectra of the original Alβ zeolite and the dealuminated form (DeAlβ) can be found in Figure S1. The spectra of a Sn-grafted zeolite (Sn/DeAlβ) and a hydrothermally synthesized Snβ are

silanols, in agreement with IR spectroscopy. The parent Alβ zeolite shows only a very weak silanol signal at −102 ppm, attributed to minor local structural defects and crystal surfaces. A weak signal at −92 ppm was also detected, which can be assigned to the presence of Si(OSi)2(OH)2 species. In addition to the signal at −102 ppm, the signal at −110 ppm also gains intensity in the 1H−29Si CP MAS NMR spectrum. Though this CP effect is unexpected, we tentatively explain the observation on the basis of the close spatial vicinity of Si(OSi)4 to the abundantly present silanols and sorbed water molecules inside the partially destructed zeolite pore system. The grafting of Sn causes a significant drop in intensity of the −102 ppm signal, due to reaction of the Si(OSi)3(OH) group with SnCl4. Yet, as was established with IR spectroscopy, part of Si−OH remains present after the grafting procedure, pointing to the presence of unreacted silanol groups. Comparison of the Si(OSi)3(OH) concentrations (obtained through deconvolution, Figure 2C) of a calcined dealuminated Sn-free material and a (calcined) Sncontaining sample indicates only a small drop of 2.7% for Si(OSi)3(OH), corresponding to consumption of 0.43 mmol of SiOH per gram of material, or 3.3 SiOH per Sn atom present in the material (1.57 wt% Sn, EPMA analysis; see Supporting Information for calculations). As full framework connectivity of the Sn atom requires consumption of four SiOH groups, a lower oxygen coordination of the Sn atom is suggested. The hydrothermally synthesized Snβ shows several sharp signals at positions around −111 ppm, which are assigned to a total of nine crystallographically different Q4-silicon species.27 The observed pattern is typical for pure silica β-zeolites.28 The narrow nature of the signals and the absence of any signal downfield from −111 ppm indicate a well-defined crystalline structure and a silanol-free nature of the material, respectively. The latter was confirmed by the absence of signals in the 1 H−29Si CP MAS NMR spectrum, suggesting fully frameworkconnected Sn atoms. The presence of physisorbed water and its interaction with the silanol groups are displayed in Figure 3. The displayed NIR spectra show the overtone and combination bands of hydrated and dried Sn/DeAlβ and hydrothermally synthesized Snβ. Hydrated Sn/DeAlβ shows various vibrational absorption bands, all assigned to silanol or siloxane groups interacting with water molecules: (i) a combination band of the stretching and bending of water molecules, hydrogen-bonded to vicinal and isolated silanol groups, at 5270 and 5119 cm−1, respectively; (ii) a combination band of the silanol OH stretch and bending, hydrogen-bonded to water, and a band of the stretch vibration of these silanol groups combined with the siloxane stretch, at 4400−4500 cm−1; and (iii) the overtone signals at 6860−7225 cm−1 of silanol groups hydrogen-bonded to water molecules, water molecules hydrogen-bonded to silanol groups, and water−water hydrogen-bonding.29 Once the sample was dried in a dry N2 flow at 823 K, the majority of water-related signals disappeared, and the NIR spectrum now showed the overtone band of the OH stretch of isolated silanol groups at 7296 cm−1 and the combination bands of stretching and bending modes of isolated silanols, and also a combination of a silanol stretching vibration with a siloxane stretch, around 4536 cm−1.30 Upon close inspection, a weak and broad signal around 5270 cm−1 was found, suggesting the presence of strongly bound adsorbed water molecules which are not removed during the drying process. Evidence of adsorbed water molecules after drying is also provided by a FTIR analysis (Figure S2). A broad signal around 1640 cm−1 can be attributed

Figure 2. 29Si MAS NMR (top) and 1H−29Si CP MAS NMR (bottom) of (a) grafted zeolite Sn/DeAlβ and (b) hydrothermally synthesized Snβ. (c) Normalized 29Si MAS NMR spectra of calcined dealuminated Sn-free (red) and Sn-grafted (black) materials. The table on top shows the relative area of the Q3 (−102 ppm) and Q4 (−110 and −114 ppm) species, obtained by deconvolution of the spectra. The inset visualizes the difference between both materials.

presented in Figure 2. Signals at −110 and −114 ppm in the 29 Si MAS NMR spectra, present in the parent Alβ zeolite, corresponding to Si(OSi)4 species at crystallographically different sites, shift slightly upfield (0.7 ppm) as a result of Al removal, but the shift is unchanged after Sn grafting. The shape and distribution of the two signals remain identical to those of the signals for the starting material. The signal of Si(OSi)3(O(H)Al), around −104 ppm, disappears in the dealumination procedure, and a new signal arises at −102 ppm.4a A significantly increased intensity in 1H−29Si CP MAS NMR proves that this signal is due to the formation of Si(OSi)3(OH) 34


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Figure 4. Thermogravimetric analysis of water adsorption at 323 K on hydrothermally synthesized Snβ (black) and Sn/DeAlβ (red). Samples were dried in situ in a dry N2 flow at 573 K, cooled to 323 K, and exposed to a room-temperature saturated humid N2 flow. The amount of water adsorbed is relative to the materials dry weight. Afterward, the water was successfully removed by heating the sample in dry N2 flow, emphasizing the physical nature of the adsorption.

experiments showed a higher concentration of stronger Lewis acid Sn sites in Sn/DeAlβ.4a Despite the enormous similarity of the Sn active site, we unexpectedly noticed a different redox behavior between the post-synthetically modified and hydrothermally synthesized catalyst in a temperature-programmed reduction experiment under hydrogen atmosphere (H2-TPR). The results of the TPR analysis, presented as hydrogen consumption rate in function of temperature, are displayed in Figure 5. The dealuminated βzeolite without Sn shows little H2 consumption, as expected by the chemical inertness of the support. However, a significant H2 consumption signal was observed for both dried Sn-containing zeolite samples. Whereas Sn/DeAlβ consumes H2 from 700 to 1100K with a maximum around 943 K with release of equimolar amounts of water (despite drying before analysis), reduction of SnIV in the hydrothermal Snβ requires a much higher temperature, with a maximum H2-consumption rate around 1257K, while no water release was observed. Both materials show one feature in the reduction profile, pointing to a dominant single reduction step of SnIV, likely a two-electron reduction to SnII. A quantitative chemisorption experiment of the consumed H2 was performed by using calibrated pulses of H2 at 1023 K (Figure 5-2). A total molar H2 consumption of 0.107 mmol·g−1 was determined, corresponding to a H2:Sn ratio of 0.84:1 (for a sample containing 1.57 wt% Sn), suggesting a 84% reduction of SnIV, assuming formation of SnII. This value corresponds to the value found when integrating the TPR profile up to 1023 K (84% of the total surface, Figure S4). Proof of SnII formation is provided by 119Sn Mössbauer spectroscopy. The spectrum of Sn/DeAlβ, after treatment at 1023 K in H2, is displayed in Figure 5-3. Two signals are found. The first one has an isomer shift (IS) of −0.24 and quadrupole splitting (QS) of 0.50 mm/s, which equates to a tetrahedral SnIV species in highly oxygeneous environment,4b,32 and a second signal with IS = 3.05 and QS = 2.08, a shift and splitting profile characteristic of SnII species.33 Fitting of the data suggests that 81% of SnIV is reduced to SnII, a value comparable to that of the chemisorption measurement. Finally, no signs of Sn0 (IS = 2.69, QS = 0.0) were found after reduction in H2. The possibility of reduction of framework Sn species has been described for MFI and MEL zeolites before.33 Similary, SnIV to SnII reduction processes have been described for SnO2. In comparison, large SnO2 particles show SnIV to SnII reduction

Figure 3. DR NIR spectrum of (a) dehydrated and (b) hydrated Sn/ DeAlβ material and (c) dehydrated and (d) hydrated hydrothermally synthesized Snβ. (e) White standard measurement. Samples were dehydrated at 823 K in a dry N2 flow and corrected with BaSO4. The inset shows a zoom of 5000−5500 cm−1 of the dehydrated samples (indicated with a dotted box).

to the bending frequency of water molecules, strongly adsorbed onto the catalyst surface,31 despite prior drying to 672 K in vacuo. Similar FTIR and NIR signals were observed for Sn-free dealuminated materials, probably by water molecules adsorbed in silanol nests. A similar exercise with a hydrothermal Snβ zeolite shows a slightly different trend in NIR and FTIR, with all water-related signals disappearing after drying and no signals found at 5270 or 1640 cm−1, but a very small signal of unknown origin was detected at 5324 cm−1. Though a quantitative comparison of the reflected NIR signals is delicate since a correction for sample weight is difficult, the surface ratio of the integrated signals assigned to water in the hydrated samples (5270 + 5119 cm−1) and those related to silanol groups in the dried samples (4400 + 4500 cm−1) is higher for the grafted material (1.8 vs 1.3 for the hydrothermal material), pointing to a higher ratio of water to silanol. Because of the higher concentration of silanol groups in Sn/DeAlβ materials, as ascertained by FTIR and 29Si MAS NMR, a higher water content in this material is concluded from DR NIR spectroscopy. Thermogravimetric analysis indeed demonstrates a 9 times higher water uptake from a room temperature water saturated N2 flow at 323 K on a dried Sn/DeAlβ in comparison to the hydrothermal Snβ (2.86 vs 0.32 wt% of water adsorbed, respectively; Figure 4). Sn/DeAlβ is thus clearly more hydrophilic than hydrothermal Snβ. Characterization of the Sn Active Site. Characterization of the Sn active site in both hydrothermal Snβ and postsynthetic Sn/DeAlβ was reported earlier, and the studies indicated similar active sites; that is, Sn occurs mainly in closed configuration in the framework and predominantly has a +IV oxidation state, whereas no signals of reduced Sn species (Sn2+ or Sn0) were observed, and also no dominant bulk SnO2 phase is present.4b FTIR temperature-controlled pyridine desorption 35


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Figure 6. DR UV−vis spectra of (a) hydrated and (b) dehydrated Sn/ DeAlβ and (c) hydrated and (d) dehydrated hydrothermally synthesized Snβ. The dotted lines display the result of the signal deconvolution. The maximum of each signal is indicated in the plot. Drying of the samples occurred at 573 K in a dry O2 flow. Sn/DeAlβ samples are corrected with a DeAlβ spectrum, and Snβ is corrected with a BaSO4 spectrum (a Snβ spectrum corrected with DeAlβ is shown in Figure S3).

Figure 5. (1) H2 TPR profiles of dry (a) Sn/DeAlβ, (b) DeAlβ, and (c) hydrothermally synthesized Snβ. The vertical dashed lines indicate the temperature at maximal H2 consumption. (2) Quantitative H2 chemisorption of Sn/DeAlβ performed at 1023 K. Relative uptake of the pulses with known H2 concentration is given above each pulse. (3) 119 Sn Mössbauer spectra of Sn/DeAlβ after reduction at 1023 K.

maximum absorbance below 200 nm (Δλ > 21 nm). In comparison, DR UV−vis of the hydrated hydrothermal Snβ shows an absorption with two features, deconvoluted into signals with maxima at 217 and 242 nm. Absolute quantification of the obtained signals is arduous, as the weight of the sample and the extinction coefficients of the signals are unknown. Relative comparison is impossible as these coefficients are known to change with varying wavelengths and crystal structures.36 Drying of the sample leads to the same twofeature profile with a slight shift of the high-energy signal to 215 nm toward the UV (Δλ = 2 nm) and a red shift of the lowenergy signal to 248 nm (Δλ = 6 nm). The opposing shifts indicate complex bonding changes of the Sn active site upon drying. The substantially higher absorption energy of the charge transfer in Sn−O in case of Sn/DeAlβ (99

824

78

>99

0 0

0 0

/ /

Baeyer−Villiger Reactions 0 0 0 0 1.57 742

/ / 49

/ / 98

1.26

37

96

33.7 78.7

33.7 78.7

448 999 0

3.9 0

88 /

Productivity and TOF were calculated after 30 min of reaction. MPV reactions were performed at 373 K, BV reactions at 363 K.

material, showing high activity and lactone selectivity.3b,51 Therefore, BV of cyclohexanone with H2O2 to form εcaprolactone, a precursor for caprolactam, is used here as model reaction. The catalytic results are presented in Table 2. No sign of activity was observed for the parent β-zeolite, CP814E, its acid-dealuminated precursor and bulk SnO2 (entries 7, 8, and 12), whereas the Sn chloride salt is only moderately catalytically active. Both the Sn-containing zeolites are very active, emphasizing the need of the proper Sn active site for BV catalysis. A productivity of 742 gcaprolactone·h−1· kgcatalyst−1 was for instance detected with the grafted Sn/DeAlβ for the oxidation of cyclohexanone with H2O2 (entry 9). For comparison, reaction in the presence of hydrothermal Snβ produces 448 gcaprolactone·h−1·kgcatalyst−1, the latter value being similar to reported ones (entry 10),3b while the lactone selectivity is excellent (98 and 96%, respectively) for both materials. The TOF of 49 molcaprolactone·molSn−1·h−1 for Sn/ DeAlβ is substantially higher than the 37 molcaprolactone.molSn−1· h−1 in the presence of hydrothermal Snβ. To comprehend the observed differences in activity a kinetic study with cyclohexanone of the catalytic materials was performed for MPV and BV reactions. Such kinetic studies unravel differences in enthalpic and entropic contributions, which are at the origin of the activity differences, and they could be linked with the observed spectroscopic differences in geometry of the active Sn sites. However, since the crystal size of hydrothermal and postsynthetic Snβ is considerably different, involvement of diffusion processes in the kinetic experiments, especially for the large hydrothermal Snβ, should be excluded first. As synthetic control of both Sn sites as well crystal sizes with the current state-of-the-art is not straightforward, for the post-treatment method nor the hydrothermal synthesis, an alternative, indirect strategy based on deduction, was applied to exclude mass transport limitation. 39


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and turnover frequency (per Sn site) (Figure 10A). Nevertheless, the MPV Arrhenius plots show similar observed activation energies, corresponding to 51.2 and 52.3 kJ·mol−1, for Sn/DeAlβ and hydrothermal Snβ, respectively, slightly lower than the reported 63 kJ·mol−1 for hydrothermally synthesized Snβ.14a Such high values confirm absence of (pore) mass transfer for the crystals used in this work. According to Eyring−Polanyi analysis, 53 a comparable activation energy means that a transition state with a similar enthalpic level is formed during MPV reaction at the Sn active site of both materials. The comparable enthalpy values of the transition state tentatively indicates that the strength of the Lewis acidity is not really a determining factor in the MPV reaction. Although the observed difference in reaction rate could in theory be caused by the content and accessibility of Sn sites, similar intensity (per Sn normalized) of the FTIR cyclohexanone adsorption band (Figure 9) reveals that Sn in both materials is equally accessible, despite the substantial difference in observed TOF. In accord with a recent study of Hermans et al., correlation between the adsorbed cyclohexanone amount and catalytic activity of Sn is not straightforward, and thus the local properties of the single Sn site are essential.54 Therefore, we ascribe the higher MPV rate predominantly to entropic reasons, correlated to the preexponential factor, suggesting that the Sn active site in the grafted Sn/DeAlβ catalyst forms the required substrate orientation more efficiently to undergo fast MPV reaction. It is thus likely that steric hindrance of the Sn active site plays a dominant role in Snβ catalysis for bimolecular MPV. The Arrhenius plot for BV (Figure 10B) shows a different kinetic behavior. BV is also faster with the grafted catalysts, both in terms of productivity and turnover frequency, in the usual temperature range, but the hydrothermal Snβ becomes more active at higher temperature with an isokinetic point at about 373 K. An activation energy of 30.3 kJ·mol−1 is calculated in the presence of grafted Sn/DeAlβ, whereas the hydrothermal Snβ reaction gives rise to a considerably higher energy barrier of 49.4 kJ·mol−1, very similar to values found in the literature, viz. 46 kJ·mol−1.14b The higher activation energy found for the hydrothermally (larger) synthesized Snβ materials, when compared to the (smaller) post-synthetic one, again confirms the absence of diffusion limitations, as a lower activation energy would be expected in case of diffusion limitations in the large crystals. The clear difference in enthalpy of the transition state between the two materials suggests a different Sn active-site geometry, in favor of the grafted catalyst giving it a higher activity at lower temperatures. In addition, because of the isokinetic point, the two catalysts have different pre-exponential factors, and thus they have different entropic contributions to BV catalysis. The lower entropic value of the grafted material advocates a more stabilized transition state during the course of the BV reaction, as if additional proximity effects such as hydrogen bonding or other interactions occur. As a control, the Oppenauer oxidation of 2-ethylcyclohexanol and the BV oxidation of 4-tert-butylcyclohexanone (Figure 10C,D) were performed, and the same trends for observed activation energy and pre-exponential factors were found. This similarity indicates a analogous transition state in both reactions. As discussed above, a higher reaction rate was found in BV reactions for the bulkier 4-tert-butylcyclohexanone in comparison to unsubtituted cyclohexanone. Comparison of the pre-exponential factor of both cyclohexanone and 4-tertbutylcyclohexanone substrates indicates lower entropic values

Therefore, several BV reactions were performed with a small (cyclohexanone) and a bulky (4-tert-butylcyclohexanone) substrate at different temperatures, in the range of 353−393 K, and the initial reaction rate was determined. If diffusion limitation was present in the conversion of cyclohexanone, physical transport of the molecule to and from the active site would be the slowest step in the catalytic cycle. If this was the case, the observed reaction rate of the bulkier 4-tert-butylcyclohexanone should be at best equal, but more likely lower, than this of cyclohexanone, due to the lower diffusivity of the bulky molecule. However, such expected drop in reaction rate is not in accord with the kinetic observations. For both heterogeneous materials, a significantly higher conversion rate of the bulky 4-tert-butylcyclohexanone was found (respectively 20.7 and 6.0 mmol·h−1·g−1 for the grafted material and 18.0 and 5.5 mmol·h−1·g−1 for the hydrothermal material), in agreement with the confinement-free reactivity order of both substrates found in the literature and verified in reference homogeneous peracid reactions.52 This observation excludes diffusion control of the reactions with cyclohexanone. Furthermore, the ratio of the reaction rates is comparable for both materials, despite the considerable difference in crystal size. Arrhenius plots of the data are presented in Figure 10 (more information in SI). In case of MPV, irrespective of the temperature, the catalytic reactions between cyclohexanone and 2-butanol are always faster in the presence of the grafted catalyst, in terms of both space time yield (per catalyst weight)

Figure 10. Arrhenius plots of (A) cyclohexanone MPV, (B) cyclohexanone BV, (C) 2-ethylcyclohexanol Oppenauer oxidation, and (D) 4-tert-butylcyclohexanone BV reactions, with the calculated activation energies. Black squares = Sn-grafted material (a), red circles = hydrothermal Snβ (b). For reactions with cyclohexanone, averages and standard deviation based on four individual reactions (using different catalyst batches) are shown; calculations are in the SI. 40


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Figure 11. Tentative visualization of the proposals for the Lewis acid sites in hydrated hydrothermal Snβ (A) and post-synthetic Sn/DeAlβ (B) and the materials dried at 823 K in N2hydrothermal Snβ (C) and post-synthetic Sn/DeAlβ (D)in the BEB framework. Green atom = Sn, red atom = framework O, blue atom = water-related O, and white atom = H. Covalent bonds are displayed as thick links, electrostatic interactions are thin lines. BEB was chosen over BEA as 56% B-polymorph was found in materials similar to the Al-containing starting zeolite.55 The water molecule found in the dried post-synthetic Sn/DeAlβ is not depicted as its location is uncertain, but its present is probably necessary for stabilization of the Sn site.56

for the bulky substrate (respectively 16 × 104 and 2.2 × 104 mmol·h−1·g−1 for the grafted and 66 × 106 and 3.3 × 106 mmol· h−1·g−1 for the hydrothermal material), as can be expected for a bulky substrate. In the MPV reaction, a similar conclusion is reached; a lower pre-exponential factor was found for the steric demanding 2-ethylcyclohexanol (respectively 1.8 × 108 and 13 × 108 mmol·h−1·g−1 for the grafted and the hydrothermal material), but a similar observed activation energy suggests minimal influence of the 2-ethyl substitution on the enthalpic value of the MPV transition state.

framework, a compensating inner-sphere hydroxyl ligand being present in the latter case. In addition to that, the open-site configuration experiences a proximate silanol. Currently, the role of the silanol in catalysis with Snβ is a matter of debate. Where some authors argue its role, others suggest a beneficial effect in BV and sugar isomerization or calculated that such proximity effect is energetically most favorable with a more distant silanol.14b,18,20b Furthermore, interconversion of the closed and open Sn sites is also under discussion; depending on the reaction studied, the Sn site coordination was shown to be either unaffected or changed.5b,6a,16,58 In this work, Sn is isolated in the β-zeolite and in oxidation state +IV, irrespective of the synthesis method. Based on 119Sn MAS NMR and acetonitrile-d3-probed FTIR spectra of hydrothermal Snβ and grafted Sn/DeAlβ materials, we previously proposed an apparently closed Sn site with four

4. DISCUSSION Previous studies on hydrothermal Snβ identified two Sn active sites, a closed-site and an open-site configuration.14b,57 The two-site model places Sn in a tetrahedral coordination with either four or three connectivity points with the zeolite 41


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ACS Catalysis Si−O ligands for the grafted material.4b Additional characterization now revealed differences in the coordination of Sn for the material synthesized via grafting, thereby requiring to adapt the previously proposed model. It is now demonstrated that Sn4+ is strongly coordinating three framework O ligands instead of the four previously proposed. Similar 3-coordinated framework species have been described frequently for Lewis acid Al 3+ sites, and their existence has been proven experimentally.59 Figure 11 shows the proposed Sn active site inside a β-zeolite, next to the commonly proposed closed species of hydrothermal Snβ. A schematic representation of the Sn site proposals including the bond distances ascertained by EXAFS can be found in Figure 12.

synthesis protocol. Perfect substitution of Sn4+ in the BEA topology is energetically difficult.38a Less perfect substitutions are more easily attained, and stabilization by outer-sphere coordination is not uncommon in biological systems or crystallization processes.64 The EXAFS data of the dry material did reveal a long-distance oxygen that fits with a long-distance water and is confirmed by removal of a water molecule per Sn during a reductive treatment at high temperature. The water molecule possibly interacts with the SiO− (Figure 12), or allows the Sn-atom to relax into a tetrahedral geometry (Figure S16), but its exact nature is unknown. The distance for this distant oxygen species (2.47 Å) found in this work is comparable to values found for other structural water molecules on metal complexes or metal oxihydroxides (2.35−2.5 Å), affirming the possible presence of such structural water molecule with the Sn center.65 Recent reports support the possibility of such strongly adsorbed water molecules in zeolites as active-site stabilizer.66 Furthermore, theoretical calculations on various model Sn active sites reported a stabilizing effect of a distant water molecule of otherwise too-energetic Sn sites.56 Next to a different Sn site, a more hydrophilic environment is found for the grafted materials due to presence of unoccupied silanol nests in the zeolites framework (FTIR, NIR, TGA, (1H)-29Si MAS NMR). These sites are also displayed in Figure 11, and have been suggested to play a significant role in substrate−product stabilization.20a Trace amounts of adsorbed water remain present in these silanol nests, even after drying at elevated temperatures. Both the closed site and the grafted site allow the coordination of water molecules on Sn, a higher degree of Sn-coordinated water being present in the latter case. In contrast to literature, our data do not support the commonly accepted presence of two water molecule ligands for the hydrothermal sample. Instead, the data suggest that the hydrated closed site contains only one water molecule, whereas two water molecules coordinate the grafted Sn active site, albeit differently. The hydrated models are presented in Figure 11. Dehydration of both materials lowers the number of nearest neighbors, in accordance with the downfield shift in 119Sn MAS NMR spectra,4b,67 but leads to stronger interaction of the Sn core with the zeolite framework, as pointed out by the shorter Sn−O bond lengths (see EXAFS). In the hydrothermal material, three of the four Sn−O bonds of the closed site become stronger upon drying, while one Sn−O bond is enlarged. This observation respectively agrees with the blue and red shifts of signals in UV−vis spectroscopy upon drying the hydrothermal Snβ. No elongation of a Sn−O bond is observed in a dehydrated grafted Sn site, supported by the observed UV absorption shifts only to higher energies. The exact location of Sn in the beta framework is difficult to assess. A limitation to the substitution of Sn in the dealuminated zeolite, despite presence of ample silanol nest grafting sites, suggests a T-site-specific adsorption procedure.11 Though requiring further research, the location may be suggested indirectly from the preferred Al locations. A recent combination of EXAFS, 27Al MAS NMR, and DFT study indicated that, in β-zeolites with a Si/Al ratio of 25, identical to the ratio used here before dealumination, Al3+ preferentially resides at T2 and T7 positions.68 Calculated 27Al NMR chemical shifts for these sites are 56.7 and 52.9 ppm, respectively, similar to experimental values found by us for the Al species in the parent zeolite used in this work.4b The final position of Sn atoms in the grafted materials is linked to

Figure 12. Schematic representation of the tentative proposals for the Lewis acid sites in the hydrothermal (left) and the post-synthetic Sn/ DeAlβ (right) material in both hydrated (top) and dry (bottom) states. The red numbers indicate the Sn−O and Sn−Si distances as obtained in the EXAFS analysis. Distances are given in Å.

In accordance with Werner’s inner-/outer-sphere coordination theory,60 one can formulate the Sn active site in the grafted Sn/DeAlβ as a three-fold inner-sphere Si−O−Sn complex, the tetrahedral coordination being stabilized by an outer-sphere anionic SiO−, like in [(|SiO)3Sn]+···−(OSi|). In the hydrated state, an extra water molecule is in acid−base equilibrium with the near silanol. Similar long distance outersphere charge-balancing interactions are not uncommon in inorganic and organometallic chemistry.61 In the material, (OSi)3Sn+ and SiO− behave as Lewis acid and base, respectively. Since they are sterically separated by their fixed position in the zeolite framework, the acid−base adduct cannot be formed. Such species are known as frustrated Lewis pairs and often show high catalytic activity as both acid and base interact simultaneously with reagent molecules. Although most described frustrated Lewis pairs are (metallo-) organic complexes,62 recently heterogeneous examples were described on the edge of graphane-graphene nanoribbons and in a NaY zeolite, proving that such pairs can exist in solid materials.63 One could ask why the newly proposed site does not evolve into an open-site configuration with an actual (SiO)3Sn(OH) formation, but EXAFS analysis, 119Sn MAS NMR, and acetonitrile-d3-adsorbed FTIR exclude this structure. Likely a zeolite framework relaxation upon Al removal leads to such specific sites geometry, emphasizing the importance of the Snβ 42


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ACS Catalysis the stability of the Al3+ sites during the dealumination treatment. As Al3+ atoms in T2 positions are known to resist acid dealumination quite well and more severe dealumination did not result in more incorporated Sn atoms in an adsorption process or higher activity,4b,26,69 we speculate a preferential incorporation of the Sn atoms in the T7 site of the β-zeolite. For hydrothermally synthesized Snβ zeolites, the T2 site was determined to be the most thermodynamically favorable position for Sn substitution in the beta framework.70 An EXAFS study of this material, on the other hand, pointed to T5 and T6 substitution.38b This difference can be explained by kinetic factors playing a role during the zeolite synthesis, which are not considered in the thermodynamic studies. Nonetheless, the most likely Sn sites in the hydrothermal material differ from the site suggested for the grafted material. Though immature, there are some literature reports suggesting reactivity differences depending on the T site.71 For Al3+ substitutions in zeolites it is known that a different T site influences its Brønsted acid strength.72 Similarly, thermodynamic calculations on Snβ show a correlation between Sn-site Lewis acid strength and the degree of tetrahedron distortion of the T site.70a The lowest acid strength has been found for T2 sites, whereas substitution at T7 sites seems to result in the highest Lewis acidity because of a pronounced site distortion. As we hypothesize a limited substitution of Sn at the T7 site, based on sorption experiments and the location of Al and its removal during acid treatment in the parent zeolite,11,68 the observed increased Lewis acid strength seems to be confirmed by such thermodynamics. Awaiting theoretical initiatives, our spectral key features and reactivity results support unique characteristics of the novel active Sn site. The structural analysis by EXAFS, the lowered SiOH connectivity found in 29Si MAS NMR, and the low UV wavelength absorption of Sn−O charge transfers in the electronic spectrum all indicate a lower inner-sphere coordination number of Sn. Accordingly, Sn in such a coordination is prone to reduce faster under action of hydrogen and should have a higher Lewis acid strength, both consistent with our TPR and IR (on chemisorbed cyclohexanone) experiments. The different local geometry and electronic state of Sn in the grafted Sn/DeAlβ site as compared to hydrothermal Snβ has interesting consequences for catalysis. The higher preexponential factor of bimolecular MPV reactions with the grafted Sn/DeAlβ is in line with the sterically less demanding geometry of the Sn active site. The electronic properties, like the Lewis acid strength or presence of SiO−the former initially important in the elementary reaction steps to activate cyclohexanone and 2-ethylcyclohexanol and to allow the hydride shift and the latter in alcohol deprotonationseem less important to the reaction rate, as both catalytic materials show similar activation energies. In contrast, we hypothesize that the stronger Lewis acidity of the grafted material accelerates the formation of the Criegee adduct14b during BV reactions by strong activation of the carbonyl group and attack of hydrogen peroxide on the more electrophilic carbonyl carbon atom. In addition, the formal Lewis base site near the active Sn may well assist deprotonation of the mildly acidic H2O2, increasing its reactivity to interact with the electrophilic carbonyl; as a consequence, the observed activation energy of BV in the presence of grafted Snβ Sn/DeAlβ is lower. Transition-state stabilization due to the proximity of SiO−, SiO−H−OH−, or SiOH (from nearby silanol nests) cannot be ruled out, and likely explains the higher reported conversion

rates of hydrothermal Snβ with the open site, when compared to the closed site.10d,14b Since entropic effects should also be in favor to accommodate the large transition state of the BV reaction, the measured lower pre-exponential factor is unexpected, but we speculate that stabilization by hydrogen bonding, for instance of the Criegee adduct, causes a retardation in the reaction rate. Therefore, depending on the reaction temperature, either grafted or hydrothermal Snβ shows the highest BV activity, the activity of hydrothermal Snβ being higher at high temperature and thus governed more entropically, whereas the activity of grafted Sn/DeAlβ, being more directed by enthalpic factors, is more of use at lower reaction temperatures. For BV, one should therefore be cautious before ranking catalysts. Based on the above considerations, the local organization of the zeolite lattice determines the geometry and electronic properties of the Sn active site, but also proximity effects influences the rate of the reaction. A detailed computational study of the grafted frustrated Lewis pair will be necessary to define and evaluate the unique catalytic properties of the site for different reaction types. If valid, the proposed site for the grafted Sn/DeAlβ might be a very useful model to investigate the stabilizing effects of outer-sphere coordinations and to understand its impact on the Lewis acid properties. Though the key role of outer-sphere coordination is widely appreciated in biology like in blue Cu proteins,64a these issues are less described in classic heterogeneous catalysis. Similar studies on other post-synthetic procedures10 could provide valuable insight in the activity of various Sn sites. The separate acid− base site proposal might be an alternative model to add to the understanding of the existence of other stable single site Lewis acid zeolites (Al, Zr). For example, in a recent comparison of post-synthetic and hydrothermally synthesized Zrβ materials,73 increased Lewis acid strength and water adsorption were observed for the post-synthetic material, similar to the conclusions found here. Taking the new insights established in this work into account, we suggest that the higher Lewis acidity may be caused by a similar low inner-sphere coordination geometry of the Zr centers.

5. CONCLUSIONS The Sn active site in grafted Sn/DeAlβ was identified as a unique catalytic center by characterization with a combination of spectroscopy and catalysis. The site shows similarities with the Sn active site in hydrothermal Snβ: Sn is isolated and exclusively present in oxidation state IV. However, there are subtle differences, indicating that the synthesis history is important to the final geometry and electronic properties of the supported single Sn site. Closed sites with four-coordinated Sn are found in hydrothermal synthesis in agreement with literature, but the locally distorted zeolite framework of a dealuminated β-zeolite is an appealing host to favor a lower inner-sphere Sn coordination. Steric constrains prevents fourfold framework connectivity of Sn and leads us to hypothesize the formation of a frustrated Lewis pair with outer-sphere coordination of SiO−. The altered Sn-site geometry in the postsynthetic material, with lower framework coordination and/or at a different T site, results in an increased Lewis acid strength. Furthermore, unoccupied silanol nests in the grafted materials result in a more hydrophilic material. The higher catalytic activity of the site can be associated with its electronic properties and higher Sn site accessibility for substrate approach, but depending on the reaction type, also proximity 43


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effects like hydrogen bonding could play a role in the kinetics. Detailed computational analysis of the Sn active site should elaborate the understanding of its occurrence and further develop the knowledge of frustrated Lewis pairs, outer-sphere coordinations, and their application in heterogeneous catalysis. The new site proposal might offer an alternative look at the geometry of other Lewis acid single sites in zeolites.

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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/acscatal.5b01822. Additional Sn-site schemes and results of 29Si MAS NMR, 1H−29Si MAS NMR, DR UV−vis, EXAFS, and FTIR analyses (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Dr. Dipanjan Banerjee is gratefully thanked for excellent user support during EXAFS measurements. E.B. and M.D. acknowledge a fellowship as Postdoctoraal Onderzoeker van het Fonds Wetenschappelijk Onderzoek-Vlaanderen. The Belgian government is acknowledged for financial support through IAP funding (Belspo). E.B. and C.K. acknowledge long-term funding in the frame of a Methusalem Grant (grant holder Johan Martens). Kristof Houthoofd and Pieter Magusin are acknowledged for the NMR measurements in this work. Prof. Robert Schoonheydt is thanked for valuable discussions on NIR and UV−vis.

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REFERENCES

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ACS Catalysis

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Outer-Sphere Stabilized Sn Active Site in Î²-Zeolite

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