(632g) Combining Experiment and Theory to Elucidate the Active Site and Catalytic Behavior of Aqueous-Phase Sugar Isomerization in Hydrophobic and Hydrophilic Lewis Acid Zeolites

Lewis acid sites (e.g., Sn⁴⁺, Ti⁴⁺) substituted in zeolite frameworks catalyze stereospecific rearrangements of sugars via mechanisms that involve intramolecular hydride or carbon shifts within ring-opened intermediates in isomerization and epimerization pathways, respectively. In the case of glucose-fructose isomerization on Sn-zeolites, experimental evidence suggests that dominant active sites are tetrahedrally-coordinated defect-open structures ((HO)−Sn−(OSi)₃) with proximal Si−OH groups that do not permit condensation to closed (Sn−(OSi)₄) configurations. Relative to other zeolite frameworks, defect-open Sn sites are more pronounced in Beta zeolites, whose crystallites contain high densities of defect grain boundaries, and a computational model of Sn substituted at a stacking fault grain boundary in Beta was developed to probe its structure and catalytic behavior. Sugar isomerization occurs with higher turnover rates when Lewis acid sites are confined within hydrophobic than hydrophilic micropores, the latter of which stabilize extended hydrogen-bonded water networks during catalysis. Experimental and theoretical techniques are combined to elucidate the effects of co-adsorbed water structures on the stability of kinetically relevant glucose isomerization transition states and most abundant reactive intermediates (MARI). Turnover rates transition from a first-order to zero-order dependence on glucose thermodynamic activity as the MARI transitions from water to glucose-derived species, consistent with intermediates identified from modulation excitation spectroscopy during in situ attenuated total reflectance IR experiments. First-order and zero-order isomerization rate constants are systematically higher (by ~10×, 368−383 K) when active sites are confined within hydrophobic micropores because of higher activation entropies, the mechanistic origins of which are interpreted using DFT calculations. These findings provide molecular insights into the effects of hydrophobic and hydrophilic pockets on the stability of co-adsorbed water structures and in turn on sugar isomerization transition states, and suggest design strategies that modify the polarity of microporous environments to influence turnover rates and zeolite structural stability in liquid water.