(458c) Kinetic and Mechanistic Roles of Hydrophobic and Hydrophilic Environments That Confine Lewis Acid Sites in Zeolites That Catalyze Glucose Isomerization
Here, we discuss our recent work on the synthesis and characterization of hydrophobic and hydrophilic Lewis acid BEA zeolites with different framework tetravalent metal (M) heteroatoms, different densities of open and closed Lewis acid sites (closed sites: M-(OSi)4; open sites: (HO)-M-(OSi)3) and different defect silanol densities. The numbers of open and closed Lewis acid sites were measured from infrared spectra of samples titrated with deuterated acetonitrile, which gave total Lewis acid site counts that were consistent with those measured by other Lewis base titrants (pyridine, deuterated acetonitrile, ammonia, n-propylamine) in ex situ infrared characterization and temperature-programmed desorption experiments. We compare ex situ quantification of open and closed sites to active sites counted in situ by titration during glucose isomerization catalysis, in order to determine the active site structures responsible for glucose isomerization on different M-BEA catalysts. Lewis acid zeolites were used in aqueous-phase batch reactor studies to measure kinetic rate constants of sugar isomerization reactions as a function of glucose concentration and temperature, in the presence and absence of base titrants and after rigorously accounting for possible corruption of measured rates by intrazeolite mass transfer limitations. We provide evidence that orders-of-magnitude (10-100x) higher effective first-order rate constants (per active M site, 373 K) prevail on hydrophobic than on hydrophilic Lewis acid zeolites, reflecting the weaker kinetic inhibition of framework Lewis acid centers by coordinated water molecules that are most abundant surface intermediates during catalysis in liquid water. The mechanistic interpretation of rate constants measured in different kinetic regimes provides new insights into the role of hydrophobic environments around active Lewis acid sites on the inhibition by coordinated water molecules. Finally, we discuss the consequences of different synthetic and post-synthetic preparations of Lewis acid zeolites on their crystallinity and active site structures and, in turn, on their reactivity for aqueous-phase glucose isomerization.