(344h) Quantification of Intraporous Hydrophilic Binding Sites in Lewis Acid Zeolites and Consequences for Sugar Isomerization Catalysis

Lewis acid sites catalyze aqueous-phase sugar isomerization at higher turnover rates when confined within low-defect hydrophobic zeolite micropores, because intraporous silanol groups serve as hydrophilic binding sites that stabilize extended water networks that entropically destabilize isomerization transition states. Silanol defect density varies widely among Sn-Beta zeolites prepared by hydrothermal and post-synthetic routes, leading to unintended or unknown consequences for reactivity. We developed a suite of methods to characterize and quantify intraporous silanol groups in Sn-Beta zeolites, and link the number of such groups to glucose isomerization rate constants. Sn-Beta zeolites with varying silanol density were prepared by grafting Sn to different extents into various dealuminated Beta supports, synthesized from parent Al-Beta zeolites with varying initial Al content (0.4–3.2 Al (unit cell)^-1) and residual defects by the mineralizing agent used (fluoride, hydroxide). Intrapore packing densities of methanol were quantified from single-component adsorption isotherms by comparing the relative uptakes of methanol (293 K) and dinitrogen (77 K) at the point of micropore filling, and decreased systematically from 0.98 to 0.07 among Sn-Beta zeolites with increasing SiOH density. IR spectra show that methanol (P/P₀<0.2, 303 K) arranges in isolated clusters within micropores of Sn-Beta with low silanol densities (<1 silanol (unit cell)^-1), but form extended hydrogen-bonded networks within micropores with high silanol densities (>1 silanol (unit cell)^-1). Total SiOH groups were quantified by H/D isotopic exchange with D₂ TPSR (500-873 K), while intraporous SiOH groups were quantified from strongly H-bound CD₃CN in IR spectra (2275 cm^-1). Aqueous-phase first-order glucose isomerization rate constants (per defect-open Sn, 373 K) were 4× higher on Sn-Beta prepared post-synthetically from dealuminated Beta supports that were initially low-defect (parent <0.6 Al (unit cell)^-1) than high-defect (parent >0.6 Al (unit cell)^-1). These findings suggest minimizing intraporous defect density in Lewis acid zeolites allow increasing sugar isomerization turnover rates.