The isomerization of glucose to fructose is a critical step in the catalytic conversion of biomass to valuable chemicals, such as 5-hydroxymethylfurfural (HMF) and levulinic acid, and is currently the largest immobilized biocatalytic process worldwide because of the demand for high-fructose corn syrups. Base catalysts can isomerize glucose to fructose, but with only low selectivity because of competing degradation side reactions, while enzymatic catalysts are sensitive to feed impurities, maintain reactivity only within narrow pH and temperature ranges, and undergo irreversible deactivation with time.
In 2010, the Davis group first reported that Sn and Ti centers located in the framework of zeolite beta (a 12-membered-ring zeolite with pores ~0.7 nm in diameter) were highly active, selective and recyclable catalysts for glucose isomerization to fructose in aqueous solvent. These zeolites were able to rapidly convert concentrated glucose solutions (up to ~45 wt% in water) to near-equilibrium conversions at elevated temperatures (413 K). They also maintained reactivity in acidic media (pH ~1), in saturated aqueous salt solutions and in biphasic systems with organic solvents, which enabled integrating glucose isomerization to fructose with fructose dehydration to HMF in a “single-pot” reactor.
Mechanistic studies showed that framework Sn sites behave as Lewis acid centers that catalyze isomerization reactions via intramolecular hydride shifts along the carbon backbone of ring-opened sugars. Interestingly, this hydride shift mechanism involves elementary steps that are analogous to the steps involved in isomerization pathways catalyzed by enzymes (e.g., D-xylose isomerase).
Remarkably, framework Lewis acidic Sn sites in Sn-Beta maintain reactivity in water, which is known to inhibit catalysis by Lewis acids. One critical feature in the synthesis and crystallization of Sn-Beta is the use of fluoride anions as mineralizing agents, which form zeolites with hydrophobic properties that can exclude bulk water from entering microporous channels.
Román-Leshkov, Y; Moliner, M; Labinger, JA; Davis, ME; (2010) Angew. Chem. Int. Ed. 49 8954-8957.
Nikolla, E.; Román-Leshkov, Y; Moliner, M; Davis, ME (2011) ACS Catal. 1 408-410.
Bermejo-Deval, R; Assary, RS; Nikolla, E; Moliner, M; Román-Leshkov, Y; Hwang, S-J; Pallsdottir, A; Silverman, D; Lobo, RF; Curtiss, LA; Davis, ME; (2012) Proc. Natl. Acad. Sci. USA 109 9727-9732.