Aqueous-phase isomerization of d-glucose to d-fructose and l-sorbose is catalyzed in parallel by Lewis acidic Ti sites in siliceous frameworks.G lucose isomerization rates (per Ti,3 73 K) are undetectable when Ti sites are confined within mesoporous voids (Ti-MCM-41, TiO 2-SiO 2) and increase to detectable values when Ti sites are confined within the smaller 12-membered ring (12-MR) micropores of Ti-Beta. Isomerization rates decrease to lower values (by % 20)with further decreases in micropore sizeasT isites are confined within 10-MR pores (Ti-MFI, Ti-CON), likely because of intrapore reactant diffusion restrictions,a nd reach undetectable values within the 8-MR pores of Ti-CHA as size exclusion prevents glucose from accessing active sites.R emarkably,t he selectivity toward l-sorbose over d-fructose increases systematically as spatial constraints around Ti sites become tighter,a nd is > 10 on Ti-MFI. These findings demonstrate the marked influence of confinement around Ti active sites on the selectivity between parallel stereoselective sugar isomerization pathways.
Aqueous-phase isomerization of d-glucose to d-fructose and l-sorbose is catalyzed in parallel by Lewis acidic Ti sites in siliceous frameworks.G lucose isomerization rates (per Ti,3 73 K) are undetectable when Ti sites are confined within mesoporous voids (Ti-MCM-41, TiO 2-SiO 2) and increase to detectable values when Ti sites are confined within the smaller 12-membered ring (12-MR) micropores of Ti-Beta. Isomerization rates decrease to lower values (by % 20)with further decreases in micropore sizeasT isites are confined within 10-MR pores (Ti-MFI, Ti-CON), likely because of intrapore reactant diffusion restrictions,a nd reach undetectable values within the 8-MR pores of Ti-CHA as size exclusion prevents glucose from accessing active sites.R emarkably,t he selectivity toward l-sorbose over d-fructose increases systematically as spatial constraints around Ti sites become tighter,a nd is > 10 on Ti-MFI. These findings demonstrate the marked influence of confinement around Ti active sites on the selectivity between parallel stereoselective sugar isomerization pathways.
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