We investigate the isomerization and epimerization of glucose to fructose and mannose in Sn-BEA and Na-exchanged Sn-BEA using density-functional theory calculations on periodic BEA crystals. We compare reaction pathways both in the absence and presence of water molecules in the vicinity of the active site and find that water effectively determines the selectivity in Na-Sn-BEA. We identify two competing epimerization pathways, one involving direct 1,2-carbon shift and the other involving 1,2-hydride shift via fructose. In Sn-BEA, the isomerization to fructose is the kinetically dominant pathway, while mannose is formed via the indirect 1,2-hydride shift epimerization pathway. In Na-Sn-BEA, the kinetically dominant pathway is epimerization to mannose via the direct 1,2-carbon shift pathway (Bilik mechanism) only in the presence of water solvent in the vicinity of the active site, whereas isomerization is preferred in the absence of water. We argue that polar water molecules that coordinate around the Na cation screen strong electrostatic interactions between Na⁺ and the glucose backbone that are responsible for the strong inhibition of the 1,2-carbon shift mechanism in the absence of water. In Sn-BEA, the presence of water does not influence the selectivity. Our calculations resolve for the first time the role of water and Na cations in the catalytic activity of Sn-BEA, and rationalize the experimental data.

我们使用周期性BEA晶体上的密度泛函理论计算，研究了Sn-BEA和Na交换的Sn-BEA中葡萄糖向果糖和甘露糖的异构化和差向异构化。我们比较了在活性位点附近不存在和存在水分子的反应途径，发现水有效地决定了Na-Sn-BEA的选择性。我们确定了两个竞争的差向异构途径，一个涉及直接1,2-碳转移，另一个涉及通过果糖1,2-氢化物转移。在Sn-BEA中，异构化为果糖是动力学上占主导地位的途径，而甘露糖则是通过间接的1,2-氢化物移位差向异构化途径形成的。在Na-Sn-BEA中，动力学上占主导地位的途径是通过直接1向差向甘露糖 2-碳转移途径（Bilik机制）仅在活性位点附近存在水溶剂的情况下进行，而在不存在水的情况下则优选进行异构化。我们认为，在Na阳离子屏幕周围配位的极性水分子会在Na之间强烈地产生静电相互作用⁺和葡萄糖主链在没有水的情况下对1，2-碳转移机理有很强的抑制作用。在Sn-BEA中，水的存在不会影响选择性。我们的计算首次解决了水和Na阳离子在Sn-BEA催化活性中的作用，并使实验数据合理化。