Research Article Cite This: ACS Catal. 2019, 9, 6146−6168
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Distinct Catalytic Reactivity of Sn Substituted in Framework Locations and at Defect Grain Boundaries in Sn-Zeolites Jason S. Bates, Brandon C. Bukowski, James W. Harris, Jeffrey Greeley, and Rajamani Gounder* Charles D. Davidson School of Chemical Engineering, Purdue University, 480 Stadium Mall Drive, West Lafayette, Indiana 47907, United States
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S Supporting Information *
ABSTRACT: Measurements of turnover rates of gas-phase bimolecular ethanol dehydration to diethyl ether (404−438 K) on a suite of hydrophobic and hydrophilic Sn-zeolites (SnBeta, Sn-BEC, Sn-MFI) of varying Sn content, together with quantitative titration of active Sn sites by pyridine during catalysis, identify two types of Sn sites with reactivity differing by more than an order of magnitude (>20×). Apparent activation entropies to form bimolecular dehydration transition states from predominantly ethanol monomercovered sites are less negative (ΔΔS⧧app = 48 ± 22 J mol−1 K−1) at the more reactive subset of Sn sites, which are present in amounts equivalent to 17−26% of the Sn sites quantified by the peak centered at 2308 cm−1 in CD3CN IR spectra (Sn2308) but not correlated with that at 2316 cm−1 (Sn2316). Synthetic and postsynthetic treatments to prepare Sn-zeolites containing Sn sites hosted within diverse local coordination environments suggest that Sn2316 sites are not associated with Sn bound to residual fluoride anions or Sn sited at external crystallite surfaces, amorphous domains, or among the diverse T-site locations contained within CHA, MFI, BEC, and STT frameworks. Treating Sn-Beta in HF or NH4F solutions, which dissolve zeolitic domains preferentially at defect grain boundaries, decreased the number of Sn2316 sites but not Sn2308 sites. These data indicate that Sn2316 sites are preferentially located at stacking faults in zeolite Beta, which provide tetrahedral coordination environments for Sn in defect-open configurations ((HO)−Sn−(OSi)3) with proximal Si−OH groups that do not permit condensation to tetrahedral closed configurations (Sn−(OSi)4). A computational model was developed for stacking fault defect-open Sn sites, which predict apparent activation free energies for bimolecular ethanol dehydration that are 65−74 kJ mol−1 higher (at 404 K) than those at framework-closed Sn sites that are capable of stabilizing transition states via Sn site opening and closing as part of the catalytic cycle, consistent with the lower experimentally measured ethanol dehydration reactivity for Sn2316 sites. In contrast, defect-open sites possess Si−OH groups that preferentially stabilize hydride shift transition states involved in glucose−fructose isomerization catalytic cycles. These findings highlight the ability of a given zeolite framework to confer structural diversity to nominally site-isolated Lewis acid centers, thus generating configurations with distinct reactivity for different chemical transformations. KEYWORDS: Lewis acid, zeolite, ethanol dehydration, Sn-Beta, titration, pyridine, acetonitrile, stacking fault
1. INTRODUCTION The concepts of active sites1 and their turnover rates2 are foundational to understanding the reactivity of heterogeneous catalysts; yet, a priori knowledge of the structure of active sites is difficult to obtain.3 Site−time yields (STYs) often serve as a starting point for kinetic analysis, wherein initial assumptions of active site identity can lead to preliminary mechanistic insights that suggest new experiments to refine the structural descriptions of active sites.4 Even for catalytic solids containing sites that are presumed to be “well-defined”, such as those associated with single atoms, heterogeneous structural features often lead to differences in reactivity. Single metal atoms hosted on supports such as CeO2,5 graphene,6 and N-doped carbons7,8 enable maximum dispersion of active metals,9 yet potassium thiocyanate7 and benzoic acid8 titrations of Fe and Co atoms in © 2019 American Chemical Society
N-doped carbons reveal that 523 K) dehydration treatments under vacuum.24 In addition,
Scheme 1. Structurally Distinct Configurations of Sn Lewis Acid Sites in Zeolites Proposed in the Literature
interconversion among hydrolyzed-open and closed sites is thermodynamically accessible according to DFT calculations31,32 and has been observed by two-dimensional protondetected 1H/119Sn correlation NMR.33 Characterizations that detect multiple site configurations (e.g., CD3CN IR, 119Sn NMR) performed after high-temperature treatments under vacuum therefore would not sample hydrolyzed-open sites because they become equilibrated to closed configurations, supporting proposals for an additional site configuration called “defect-open” (Scheme 1c) on the basis of 119Sn DNP NMR and DFT calculations,34 wherein deletion of a neighboring Si atom generates an open site (HO)−Sn−(OSi)3 and three additional Si−OH groups that are not sufficiently proximal to the stannanol to reclose the site. A defect-open cluster model was also used in DFT calculations by Boronat et al.22 to rationalize the frequency shift of the 2316 cm−1 peak in CD3CN IR; however, it is unclear how defect-open sites are formed during crystallization, and there is a dearth of definitive experimental evidence for their existence. 2D 119Sn NMR has been used to propose that distinct Sn configurations exist at different T-sites in Beta frameworks,34,35 and multishell extended X-ray absorption fine structure (EXAFS) fittings have invoked additional site configurations such as three-coordinate Sn with a nearby charge-balancing SiO− as a frustrated Lewis pair36 and paired Sn sites at T5/T6 sites in 6-MR;37 however, these site proposals have not yet been verified by quantitative characterization and catalytic data. Identifying probe reactions sensitive to distinct site configurations is integral to this effort, as in the connections made between glucose−fructose isomerization turnover rates and Sn2316 sites identified by CD3CN IR.27 In our previous work31,38 studying the gas-phase bimolecular ethanol dehydration reactivity of Sn-Beta, theory and experiment together suggested that all Lewis acidic Sn sites, if speciated in the closed form, contribute to measured turnover rates. DFT calculations and microkinetic modeling indicated that interconversion between closed and hydrolyzed-open framework Sn sites was quasi-equilibrated during ethanol dehydration catalysis (404 K, 0.5−35 kPa C2H5OH, 0.1−50 kPa H2O), and that the most abundant reactive intermediate (MARI) species at steady state were adsorbed monomeric and dimeric intermediates at closed Sn sites, which accurately described measured STYs (per total Sn) and reaction orders in 6147
DOI: 10.1021/acscatal.9b01123 ACS Catal. 2019, 9, 6146−6168
Research Article
ACS Catalysis both H2O and C2H5OH.31 After ethanol dehydration catalysis, CD3CN titrants detected Sn sites predominantly covered by indistinguishable MARI species (ν(CN) = 2282 cm−1), whereas distinct Sn sites (Sn2316, Sn2308) were present in equivalent quantities before catalysis and after regenerative oxidation treatments (823 K) in flowing air.38 Complete coverage by indistinguishable MARI after ethanol dehydration is consistent with both types of Lewis acidic Sn contributing to measured turnover rates, as was assumed previously;38 however, as noted previously, this does not preclude the existence of a subset of intrinsically more reactive sites. The recovery of initial Sn configurations after catalysis indicates their distinct structural identities are retained and perhaps related to distinct local coordination environments. Here, the choice of gas-phase ethanol dehydration as a catalytic probe reaction of Lewis acid zeolites enables performing in situ pyridine titrations to quantify the densities of dominant active sites during catalysis on a suite of 11 Sn-Beta-F (i.e., hydrophobic, hydrothermally crystallized in fluoride media) and Sn-Beta-OH (i.e., hydrophilic, prepared via postsynthetic grafting of Sn into dealuminated supports) zeolites. Approximately 5−35% of the total Sn sites were found to contribute 70−90% of the overall bimolecular ethanol dehydration STY (404 K, per Sn) on Sn-Beta, prompting more detailed kinetic analyses and examination of hypotheses for structural features that lead to distinct sites (Sn2316) in Snzeolites. Targeted syntheses of different Sn-zeolite frameworks (CHA, MFI, STT, BEC) and postsynthetic treatments were used to test and eliminate hypotheses that Sn2316 sites were correlated with residual fluorine coordinated to Sn sites, Sn sites located at external crystallite surfaces, Sn located in minority amorphous regions of predominantly crystalline materials, and heterogeneities resulting from crystallographically unique T-site positions. After ruling out these possibilities, we propose that Sn2316 sites are defect-open Sn sites that are preferentially located at stacking fault grain boundaries in Sn-Beta, consistent with postsynthetic treatments with NH4F and HF that preferentially decrease the number of Sn2316 sites and etch zeolites at grain boundaries.39,40 We develop a computational model of stacking fault defect-open sites based on a 2 × 2 × 1 Beta supercell in order to compare their ethanol dehydration reactivity with that of closed sites31 and show that the apparent activation free energy for bimolecular ethanol dehydration (referenced to the ethanol monomer) is 65−74 kJ mol−1 higher at stacking fault defect-open sites than at closed sites because the former are unable to interconvert between open and closed configurations, thus requiring the reaction mechanism to proceed through higher energy transition states. The experimental and theoretical evidence indicate that the dominant active sites for ethanol dehydration are not the defect-open Sn sites (Sn2316), in contrast to glucose isomerization, and instead comprise 17−26% of the closed sites (Sn2308) as indicated by in situ pyridine titration of a diverse range of Sn-zeolites.
reported hydrothermal synthesis procedures are found in section S.1.1 of the Supporting Information. Si-STT-F was synthesized according to Camblor et al.44 and used as seed material for a Sn-STT-F crystallization under similar conditions. Tetraethyl orthosilicate (TEOS, SigmaAldrich, 98%) was dissolved in an aqueous solution of N,N,Ntrimethyl-1-adamantylammonium hydroxide (TMAdaOH, Sachem, 25 wt %) in a perfluoroalkoxy alkane (PFA) container and covered and stirred for 0.5 h. In the case of Sn-STT-F, a solution of SnCl4·5H2O (Sigma-Aldrich, 98%) in deionized water was added dropwise. The mixture was covered and stirred overnight at ambient conditions to completely hydrolyze TEOS. The mixture was uncovered to evaporate ethanol and partially evaporate water to achieve the desired water content. Aqueous HF (Sigma-Aldrich, 48%) was added, and the mixture was stirred with a PTFE spatula, yielding a thick gel with a molar composition of 1 SiO2/0.50 TMAdaOH/0.50 HF/x SnCl4/15 H2O, where x = 0 for Si-STT-F and x = 0.005 for Sn-STT-F. (Caution: when working with hydrofluoric acid, use appropriate personal protective equipment, ventilation, and other engineering controls.) For Sn-STT-F, air-treated Si-STT-F was added to the mixture (5 wt % of SiO2 contributed by TEOS) as seed. The gel was loaded into a PTFE liner, sealed within a stainless-steel autoclave (Parr Instruments), and heated in an isothermal tumbling oven (∼60 rpm, Yamato DKN-402C) at 423 K for 35 (Si-STT-F) or 31 days (Sn-STT-F). Attempts to crystallize SnSTT-F without seeds were abandoned when materials were amorphous after 95 days. Ge-BEC-F and Sn-Ge-BEC-F were synthesized according to the report of Zhang et al.45 TEOS was dissolved in an 11.5 wt % aqueous solution of N-isobutyl-N-methylpyrrolidinium hydroxide (iButOH, synthesis described in section S.1.3 of the Supporting Information) in a PFA container and stirred covered for 0.5 h. Germanium oxide (Sigma-Aldrich, 99.99%) was added to the gel and allowed to homogenize while being stirred. In the case of Sn-Ge-BEC-F, a solution of SnCl4·5H2O in deionized water (18.2 MΩ cm) was added dropwise. Then, the mixture was covered and stirred overnight at ambient conditions to completely hydrolyze TEOS. The mixture was uncovered to evaporate ethanol and partially evaporate water to achieve the desired water content. Aqueous HF (48%) was added, and the mixture was stirred with a PTFE spatula, yielding a thick gel with a molar composition of 1 SiO2/0.20 GeO2/0.50 iButOH/0.65 HF/x SnCl4/5 H2O, where x = 0 for Ge-BEC-F and x = 0.007 for Sn-Ge-BEC-F. The gel was loaded into a PTFE liner, sealed within a stainless-steel autoclave, and heated without agitation in an isothermal oven at 448 K for 38 h. As-made Sn-Beta-F zeolites were used as seed material for SiBeta-F syntheses with the intent to grow a Si-Beta shell around Sn-Beta. Briefly, a gel composition of 1 SiO2/0.54 TEAOH/0.54 HF/7.5 H2O was prepared following the same procedure described for Sn-Beta-F zeolites. Then, as-made Sn-Beta-F (SnBeta-F-116-2, Sn-Beta-F-169) was added to the gel as 30% of the mass of SiO2 contributed by TEOS, and the gel was homogenized with a PTFE spatula. The gel was loaded into a PTFE liner, sealed within a stainless-steel autoclave, and heated in an isothermal tumbling oven (∼60 rpm) at 413 K for 21 (SnBeta-F-169) or 25 days (Sn-Beta-F-116-2). These samples are denoted Si-Beta@Sn-Beta-F. After all hydrothermal zeolite syntheses, the solids were recovered by centrifugation, washed thoroughly with deionized water and acetone, and dried overnight in an oven at 373 K. The dried materials were then treated at 853 K (0.0167 K s−1) for 10
2. EXPERIMENTAL SECTION 2.1. Zeolite Synthesis and Postsynthetic Treatments. Sn-Beta-F zeolites were crystallized for different lengths of time starting from a single gel prepared following the procedure of Yakimov et al.41 A Sn-MFI-F sample was synthesized following “method D” of Mal et al.42 A second Sn-MFI-F sample was synthesized by modifying a procedure to synthesize Sn-MFI− OH from Boronat et al.43 Full descriptions of these previously 6148
DOI: 10.1021/acscatal.9b01123 ACS Catal. 2019, 9, 6146−6168
Research Article
ACS Catalysis h in air (UHP, 99.999%, Indiana Oxygen, 1.67 cm3 s−1 (g zeolite)−1) in a muffle furnace (Nabertherm LE 6/11). These samples are denoted M-ZEO-X-Y, where M is the metal incorporated within the siliceous framework (Sn, Ge, or Si if pure SiO2), ZEO is the three-letter code of the framework topology according to the International Zeolite Association (IZA)46 (“Beta” is written as such because its topology is illdefined as an intergrowth of two polymorphs A and B), X is the mineralizing agent in the gel (F, OH), and Y is the Si/M ratio quantified by atomic absorption spectroscopy. In one case, a final number (−1, −2) is added to distinguish separate zeolites with the same Sn content. Some of each sample was not subjected to the final air-treatment step and is denoted “asmade” for any other necessary characterizations (e.g., thermogravimetric analysis). We note here that Sn-Ge-BEC-F and Ge-BEC-F frameworks after air treatments to remove SDA were not stable when stored in ambient conditions for >1 year because of hydrolysis of Si−O−Ge bonds after hydration by ambient moisture,47 as indicated by semicrystalline X-ray diffraction (XRD) patterns after 520 days (Figure S42 patterns (d) and (e)). Relevant characterizations of Sn-Ge-BEC-F (CD3CN IR, N2 adsorption) were performed
