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.本研究采用密度泛函计算（Density functional calculations），阐明了Sn-BEA沸石（Sn-BEA zeolite）活性位点（active site）锡金属中心（Sn metal center）邻近的硅羟基（silanol group）在葡萄糖异构化（isomerization）与差向异构化（epimerization）过程中的作用。研究发现，硅羟基在异构化反应中发挥关键作用：氢转移（hydride transfer）与后续质子转移（proton transfer）以单步路径进行，且具备更低的活化能（activation energy）。另一方面，差向异构化则通过与比利克机制（Bílik mechanism）相似的反应路径进行，当硅羟基不直接参与过渡态（transition state）时，其活化能垒（activation barrier）更低。我们的计算结果表明，酶催化（enzymatic catalysis）中常见的协同效应（cooperative effects）可促进异构化反应中的氢转移，但对差向异构化的比利克机制并无此类促进作用。