Mechanism of glucose isomerization using a solid Lewis acid catalyst in water.

The conversion of glucose into fructose for the production of high-fructose corn syrups (HFCS) is the largest biocatalytic process in the world, and it recently has been considered as a key intermediate step in the conversion of biomass to fuels and chemicals. This reaction is typically catalyzed by an immobilized enzyme, xylose isomerase, that generates an equilibrium mixture of 42 wt% fructose, 50 wt% glucose, and 8 wt% other saccharides. The enzymatic process is highly selective, but it has several drawbacks that increase processing costs, including the use of buffering solutions to maintain pH, narrow operating temperatures, strict feed purification requirements, and periodic replacement of the enzyme due to irreversible deactivation. Inorganic bases can also catalyze this reaction, albeit with low yields due to the reduced stability of monosaccharides in the presence of basic catalysts. The mechanism of this aldose–ketose isomerization involves hydrogen transfer from C-2 to C-1 and from O-2 to O-1 of an a-hydroxy aldehyde to create the related a-hydroxy ketone (Scheme 1). This mechanism can occur either by a proton transfer (Scheme 1A) or by an intramolecular hydride shift (Scheme 1B). Several studies have shown that basecatalyzed isomerizations take place by a proton transfer mechanism through a series of enolate intermediates generated after the deprotonation of the a-carbonyl carbon in water. The xylose isomerase-catalyzed process is also thought to be mediated by enolate intermediates generated by histidine-directed base catalysis; however, recent studies have shown that metal centers in the enzyme are responsible for the stabilization of the sugar s open-chain form and the subsequent aldose–ketose isomerization by way of an intramolecular hydride shift. 7] Recently, we reported on a tin-containing zeolite (tin in the framework of a pure-silica analog of zeolite beta, denoted Sn-Beta) that is a highly active material for the isomerization of glucose into fructose in aqueous media. This catalyst was shown to be active over a wide temperature range (343– 413 K), in acidic solutions, and with glucose feeds as high as 45 wt% to give product yields equivalent to those obtained with xylose isomerase (at its preferred reaction conditions). Here, using H and C NMR spectroscopy on isotopically labeled glucose, we demonstrate that in the presence of Sn-Beta, the isomerization reaction in water proceeds by way of an intramolecular hydride shift. Although Lewis acidity is usually suppressed by the presence of water, verification of this mechanistic pathway confirms that framework tin centers in Sn-Beta act as Lewis acids in aqueous media. Previous reports have shown strong interactions between Lewis acid centers in zeolites and hydroxy/carbonyl moieties in reactants dissolved in organic solvents. For example, Corma et al. used Sn-Beta to catalyze the Meerwein–Ponndorf–Verley (MPV) reduction of carbonyl compounds in methanol, and Taarning et al. used Sn-Beta for the production of lactate derivatives from monosaccharides in methanol. The MPV reaction mechanism involves the simultaneous coordination Scheme 1. Glucose isomerization mechanisms by way of A) proton transfer and B) intramolecular hydride shift.