(530d) Effects of Hydrophilic Binding Site Density in Lewis Acid Zeolites on Glucose Isomerization Catalysis

Lewis acid sites located within the micropores of zeolitic frameworks catalyze glucose-fructose isomerization at different turnover rates (per open Sn, 373 K) when their microporous cavities contain silanol defects present in high (hydrophilic) and low (hydrophobic) densities, because hydrophilic micropores stabilize extended networks of water molecules within them during aqueous-phase catalysis [1]. Sn-Beta zeolites with hydrophobic micropores can be crystallized hydrothermally (Sn-Beta-HT-F) in fluoride media because non-polar siloxane framework bridges are formed preferentially. Sn-Beta zeolites prepared via post-synthetic grafting of stannic chloride into dealuminated Beta supports (Sn-Beta-PS-OH), however, contain hydrophilic binding sites because of the presence of silanol groups and residual framework vacancy defects. As a result, glucose-fructose first-order isomerization rate constants are lower (by ~15x, per open Sn site, 373 K) on Sn-Beta-PS-OH than on Sn-Beta-HT-F. Differences in first-order rate constants (per open Sn, 373 K) reflect Gibbs free energy differences between two water molecules, which are the most abundant surface intermediates, and the kinetically-relevant 1,2-hydride shift transition state bound at open Sn sites. Reorganization of water molecules around the formation of the 1,2-hydride shift transition state incurs entropic penalties that are more prominent in hydrophilic than hydrophobic Lewis acid zeolites and results in lower isomerization rate constants (per open Sn, 373 K). Post-synthetic preparations of Sn-Beta (Sn-Beta-PS-F) from dealuminated Beta supports with a lower starting density of framework vacancy defects (parent Si/Al > 100) resulted in materials with isomerization rate constants (per open Sn, 373 K) that were 4x higher than those measured on Sn-Beta-PS-OH. The intrapore methanol packing density, assessed from methanol and nitrogen adsorption isotherms, decreased among Sn-Beta zeolites with increasing density of hydrophilic binding sites, quantified by H/D isotopic exchange in flowing D₂ temperature-programmed surface reactions (303-873 K). IR spectra collected at low partial pressures of methanol (P/P₀ < 0.2, 293 K) provide evidence that methanol molecules arrange in isolated 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 post-synthetic routes to prepare hydrophobic solid Lewis acids, the quantification of their intraporous hydrophilic binding site density, and the impact of the structure and size of polar molecules within zeolitic micropores on sugar isomerization catalysis in liquid media.

[1] Cordon et al., J. Am. Chem. Soc., 140 (2018) 14244.