(520c) A Periodic DFT Study of the Influence of Lewis Acid Site Speciation on Ethanol Dehydration in Zeolites

Partially-hydrolyzed â??openâ?? sites in Sn-Beta have been implicated as the active site for Baeyer-Villiger oxidation (1) and glucose isomerization (2) in the liquid phase; however, few DFT studies exist which investigate the catalytic consequences of active site speciation on the kinetics and mechanism of gas phase reactions catalyzed by Lewis acid sites in zeolites. Here, the kinetics of ethanol dehydration are studied in the zeolite frameworks chabazite (CHA) and Beta. Experimental and theoretical studies of ethanol dehydration on γ-alumina support the conclusion that Lewis acid sites are the active sites (3-4), but site heterogeneity present in the material makes identification of structure-function relations difficult. Well-defined Lewis acid sites located within zeolitic frameworks, on the other hand, can be used here as a model system to assess the effects of site speciation and the confining environment on kinetic pathways.

Periodic DFT calculations were performed with the Bayesian Error Estimation Functional (BEEF-vdw) (5) to study the kinetics and thermodynamics of ethanol dehydration in Sn-Beta and Sn-CHA. Three types of sites are considered: a tetravalent Sn, a partially hydrolyzed Sn site formed by dissociation of water, and a partially hydrolyzed Sn site formed by dissociation of ethanol. Thermodynamic and kinetic barriers were calculated for the concerted and sequential pathways which form the experimentally measured products diethyl ether, ethylene, and acetaldehyde. At 404 K, ethanol partially hydrolyzed open sites are kinetically favorable over water partially hydrolyzed open sites in both Sn-Beta and Sn-CHA. Ethylene forms through a six-membered ring transition state on both closed and partially hydrolyzed Sn sites, while DEE can be formed either from an ethanol dimer or ethanol adsorbed on a partially ethanol-hydrolyzed Sn site. Acetaldehyde formation is suppressed at 404 K due to prohibitively high kinetic barriers in cleaving C^α bonds. Detailed comparisons of intrinsic barriers in Sn-CHA and Sn-Beta are presented for each pathway. In addition to reaction free energy diagrams, a detailed discussion of zeolite entropy calculations is included. Microkinetic modeling is used to differentiate the free energy pathways in conjunction with experimental measurements to develop a probe reaction. This probe reaction, once understood for Sn-CHA and Sn-Beta, can be extended to study the catalytic influence of metal atom identity and framework type.

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