Role of Silanol Group in Sn-Beta Zeolite for Glucose Isomerization and Epimerization Reactions

Density functional calculations are used to elucidate the role of the silanol group adjacent to the active site Sn metal center of the Sn-BEA zeolite in the isomerization and epimerization of glucose. We find that the silanol group plays an important role in the isomerization reaction, wherein hydride transfer and subsequent proton transfer occur in a single step with a lower energy of activation. Epimerization, on the other hand, proceeds via a mechanism similar to the Bílik mechanism and has lower activation barrier when the silanol group does not participate directly in the transition state. Our calculations indicate that cooperative effects, often encountered in enzymatic catalysis, promote hydride transfer in the isomerization reaction but not for the Bílik mechanism for epimerization.