Glucose conversions catalyzed by zeolite Sn-BEA:synergy among Na⁺ exchange, solvent, and proximal silanol nest as well as critical specifics for catalytic mechanisms

Sn-BEA zeolite shows excellent catalytic performances for biomass transformation, and herein periodic density functional theory calculations accounting for the effect of zeolite framework are conducted to address the mechanistic aspects of glucose catalytic conversions. It is the synergistic effects of Na+ exchange, proximal silanol nest, and solvent (water/methanol) that cause the epimerization path via the Bilik mechanism to occur facilely at ambient conditions, and each of them plays a definite while disparate role. Na+ exchange reverses the relative stabilities of critical intermediates for the epimerization vs isomerization paths and drives the reaction toward the epimerization path with production of mannose. The proximal silanol nest participates in the Bilik reactions through the synchronous proton transfer to the sugar fragment, which is indispensable to promote the reaction thermodynamics and reduce the activation barriers. The activation barriers are generally lowered with increase of solvent (water/methanol) contents, and water (n = 4-6) achieves comparable catalytic results as methanol (n = 3). The difference between water and methanol solvents lies mainly in their divergent interactions with zeolite framework. Methanol rather than water constructs multiple methyl-H and lattice-O pairs and shows higher capability to retain Na+ ions, which account for its superior catalytic performances. At any solvent (water/methanol) content, the perfectly tetrahedral Sn(IV) site shows an apparently inferior catalytic effects than defect with the proximal silanol nest, owing the absence of the synchronous proton transfer.