Lewis acid sites in zeolitic frameworks catalyze glucose-fructose isomerization at different rates when their surrounding pore environments contain silanol defects present in high (hydrophilic) and low (hydrophobic) densities, because hydrophilic pores stabilize extended networks of water molecules within them during aqueous-phase catalysis. Hydrophobic Sn-Beta zeolites are crystallized hydrothermally (Sn-Beta-HT-F) in fluoride media because non-polar siloxane framework bridges are preferentially formed. Sn-Beta prepared via post-synthetic grafting of tin precursors into dealuminated Beta supports (Sn-Beta-PS-OH), however, contain residual framework vacancy defects and silanol groups. As a result, glucose-fructose isomerization rate constants are lower (by ~15x, per open Sn site, 373 K) on Sn-Beta-PS-OH zeolites than on Sn-Beta-HT-F zeolites. Differences in first-order rate constants reflect Gibbs free energy differences between the 1,2-hydride shift transition state and two water molecules bound at open Sn sites. Solvent reorganization during transition state formation incurs entropic penalties that decrease rates in hydrophilic Lewis acid zeolites, relative to their hydrophobic analogs. Post-synthetic routes to prepare Sn-Beta (Sn-Beta-PS-F) from dealuminated Beta supports that initially contained a lower density of vacancy defects (parent Si/Al&gt;100) resulted in isomerization rate constants (per open Sn, 373 K) that were 4x higher than those for Sn-Beta-PS-OH. Methanol adsorption isotherms (293 K) collected on Sn-Beta-PS-F follow type V behavior, characteristic of hydrophobic Sn-Beta-HT-F. The packing density of methanol, assessed from methanol and nitrogen adsorption isotherms, decreased as the density of hydrophilic binding sites increased among Sn-Beta samples, assessed by H/D isotopic exchange in flowing D<sub>2 </sub>(303-873 K). IR spectra measured in flowing methanol (P/P<sub>0</sub>&lt;0.2, 293 K) provide evidence that methanol molecules arrange in clusters within the micropores of Sn-Beta-PS-F, but form extended hydrogen-bonded networks within the pores of Sn-Beta-PS-OH. These results highlight routes for the post-synthetic preparation of hydrophobic solid Lewis acids, the quantification of intraporous hydrophilic binding site density, and the impact of the structure of polar molecules within zeolitic micropores on sugar isomerization catalysis in liquid media.

Effects of hydrophilic binding site density in Lewis acid zeolites on glucose isomerization catalysis

2019 Spring National Meeting

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