2018 Spring Symposium
Jason S. Bates and Rajamani Gounder, Charles D. Davidson School of Chemical Engineering, Purdue University, West Lafayette, IN
Abstract — The different reactivity of Lewis acid sites (M) in zeolite frameworks, when confined within non-polar (hydrophobic) or polar (hydrophilic) secondary environments, can arise from differences in competitive inhibition by solvents, solvent-mediated mechanisms, and extended solvent structures.3 Framework Lewis acid centers also adopt open ((HO)-M-(OSi≡)3) and closed (M-(OSi≡)4) configurations that show different reactivity for Baeyer-Villiger oxidation, glucose isomerization, and aldol condensation.5 Here, we interrogate the reactivity of Sn centers isolated within Beta zeolites using bimolecular ethanol dehydration to diethyl ether (404 K). Sn sites in open and closed configurations, quantified from IR spectra of adsorbed CD3CN before and after reaction, convert to structurally similar intermediates during ethanol dehydration catalysis (404 K) and revert to their initial configurations after regenerative oxidation treatments (21% O2, 803 K). Dehydration rates (404 K, 0.5–35 kPa C2H5OH, 0.1–50 kPa H2O) measured on ten low-defect (Sn-Beta-F) and high-defect (Sn-Beta-OH) zeolites were described by a rate equation derived from mechanisms identified by DFT calculations, and simplified using microkinetic modeling to identify kinetically-relevant pathways and intermediates. Polar hydroxyl defect groups located in confining environments preferentially stabilize reactive (ethanol-ethanol) and inhibitory (ethanol-water) dimeric intermediates over monomeric ethanol intermediates. As a result, equilibrium constants (404 K) for ethanol-water and ethanol-ethanol dimer formation are 3–4× higher on Sn-Beta-OH than on Sn-Beta-F, consistent with
insights from single-component and two-component adsorption measurements. Intrinsic dehydration rate constants (404 K) were identical among Sn-Beta-OH and Sn-Beta-F zeolites; thus, measured differences in dehydration turnover rates solely reflect differences in prevalent surface coverages of inhibitory and reactive dimeric intermediates at active Sn sites. The confinement of Lewis acidic binding sites within secondary environments of different defect density confers the ability to discriminate surface intermediates on the basis of polarity, providing a design strategy to accelerate turnover rates and suppress inhibition by water.
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