(47f) Understanding the Role of Silanol Groups in Sn-Beta Zeolite for Glucose Isomerization and Epimerization Reactions

Aldose to ketose isomerization is one of the key steps for converting cellulosic biomass to platform chemicals such as HMF. Epimerization, which involves alteration of stereochemistry at specific stereocenters, on the other hand, is the key to generating non-naturally abundant forms of sugars. Davis and co-workers [Bermejo-Deval et al. ACS Catal. 2012, 2, 2705-2713] have shown that framework Sn in Sn-Beta zeolite can very efficiently facilitate these two reactions upon changing the solvent employed in the process. The use of water as the solvent leads to isomer as the major product while methanol leads, primarily, to epimerized product. Numerous experimental and computational studies point to the open hydrolyzed Sn site to be responsible for the catalytic activity. However, the role of the silanol groups adjacent to the active Sn site remains unclear. In this work, we employ density functional calculations to elucidate the role of the silanol group in the isomerization and epimerization of open chain form of glucose.  We find that the silanol group plays an important role in the isomerization reaction, wherein hydride transfer and subsequent proton transfer occurs in a single step with a lower energy of activation.  Epimerization, on the other hand, proceeds via mechanism similar to Bílik mechanism and has lower activation barrier when the silanol group does not participate directly in the transition state.