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.