Lewis acidic Ti4+ centers substituted in pure-silica frameworks have been reported to catalyze the isomerization of D-glucose to D-fructose and L-sorbose in parallel via 1,2-hydride shift and 1,5-hydride shift transition states, respectively. Here, we assess the dependence of glucose-sorbose isomerization turnover rates (per M, 373 K, 1% (w/w) glucose in water) on the size of the confining environment and on metal heteroatom identity (M4+ = Ti, Zr, Hf). Glucose isomerization turnover rates (per Ti, 373 K) are &gt;200x lower when tetrahedrally-coordinated Ti4+ sites are confined within mesoporous voids (Ti-MCM-41) or unconfined surfaces (TiO2-SiO2), compared to sites confined within microporous voids (Ti-Beta). This finding is consistent with van der Waals interactions between confining voids and kinetically-relevant isomerization transition states that become stronger with decreases in void size and, in turn, result in lower apparent Gibbs free energies of activation. Glucose isomerization turnover rates (per Ti, 373 K) are ~20x lower when Ti active sites are sequestered within the 10-MR channels of CON and MFI (~0.5-0.6 nm diam.) than within the 12-MR channels of Beta (~0.7 nm diam.), consistent with restricted intrazeolite transport of glucose (~0.7 nm kinetic diam.) that decreases measured reaction rates. Sorbose-to-fructose selectivities, however, are higher on 10-MR Ti-CON and Ti-MFI zeolites (&gt;2) than on 12-MR Ti-Beta zeolites (~0.6), suggesting the preferential stabilization of reaction coordinates that form sorbose within more constrained environments. Glucose isomerization turnover rates (per Ti, 373 K) become undetectable on Ti-CHA zeolites, because intracrystalline glucose transport is prevented by 8-MR CHA windows (~0.4 nm diam.). Glucose-sorbose isomerization rates were observed on Zr-Beta and Hf-Beta, but with lower sorbose-to-fructose selectivities (&lt;0.4) than Ti-Beta, suggesting the 1,2-hydride shift transition state is preferentially stabilized compared to the 1,5-hydride shift transition state at these heteroatoms. These results reveal additional details about the effects of Lewis acid site identity and confining environment on turnover rates and selectivities of glucose isomerization to fructose or sorbose on Lewis acid zeolites.

Active site and confining environment requirements for glucose-sorbose isomerization in microporous Lewis acids

2019 Spring National Meeting

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