Monosaccharide and disaccharide isomerization over Lewis acid sites in hydrophobic and hydrophilic molecular sieves

Lewis acid sites isolated within low-defect, hydrophobic molecular sieves (Sn-Beta-F, Ti-Beta-F) catalyze monosaccharide (glucose–fructose) and disaccharide (lactose–lactulose) aldose–ketose isomerization reactions in liquid water at initial turnover rates (per total metal atom; 373 K) that are, respectively, ∼10–30 and ∼103–104 factors higher than sites isolated within highly defective, hydrophilic molecular sieves (Ti-Beta-OH) or amorphous co-precipitated oxides (TiO2–SiO2). Glucose-H2/glucose-D2 kinetic isotope effects of ∼2 (at 373 K) for intramolecular C2–C1 hydride shift isomerization to fructose indicate that glucose transport to active sites within Ti-Beta-F or Ti-Beta-OH does not limit turnover rates in liquid water or methanol, in spite of dramatic differences in the volumetric occupation of hydrophobic and hydrophilic void spaces by physisorbed solvent molecules. Glucose isomerization turnover rates (per total Ti; 373 K) in liquid water are first-order in aqueous glucose concentration (at least up to 1.5% (w/w)). The mechanistic interpretation of measured first-order isomerization rate constants indicates that they reflect free energies of kinetically relevant isomerization transition states relative to two bound solvent molecules, which adsorb competitively with sugars at Lewis acid sites and are the most abundant surface intermediates during steady-state catalysis. The lower isomerization rate constants on Ti centers in highly defective environments, in part, reflect stronger coordination of solvent molecules to Ti centers via additional hydrogen bonding interactions with proximal surface hydroxyl groups. The direct measurement of glucose isomerization rate constants in the liquid phase provides a rigorous and quantitative description of the catalytic differences prevalent among Lewis acidic silica-based solids with hydrophobic or hydrophilic properties, and their interpretation using a mechanism-based rate equation provides further clarity into the inhibition of catalytic turnovers at Lewis acid sites by solvent coordination.