The isomerization of glucose to fructose has recently emerged as an important route for the conversion of cellulose to intermediate platforms. Catalysts based on Sn-Beta have been found active for transforming glucose into its isomer fructose, as well as its epimer mannose. The active sites in these heterogeneous materials are difficult to characterize, especially under reaction conditions. We developed a homogeneous catalyst model to isolate, characterize and test different types of active sites, and compare these to heterogeneous catalysts. These stannasilsequioxane catalysts, or Sn-Cubes, are active for glucose isomerization and epimerization, although they differ in selectivity from Sn-Beta.
We performed DFT calculations to investigate numerous isomerization and epimerization pathways on this Sn-Cube catalyst. We modelled glucose adsorption and ring-opening to produce the open chain form of glucose. From open-chain glucose, we considered three overall reactions to produce either fructose via a C2-C1 hydride transfer (HT), mannose via a subsequent C1-C2 hydride transfer, and mannose via a Bilik reaction, shifting C3 bonding from C2 to C1. There are three types of pathways available for each reaction: a stepwise mechanism through a chelate intermediate, and 2 possible concerted mechanisms. Finally, we considered fructose and mannose ring-closing, reprotonation, and desorption to complete the full catalytic cycle. The highest-barrier steps were found to be the hydride transfer and Bilik reactions. The complete reaction network was analyzed using the energetic span model to estimate the relative rates of isomerization and epimerization. We found agreement between our predictions and kinetic measurements, as well as 13C labelling experiments that distinguish among the different possible pathways for producing fructose and mannose. The findings from this work are compared to those of Sn-BEA in order to close the gap between molecular analogues of a heterogeneous catalyst and the actual material.