Atomically Synergistic Sn-O-Al Active Sites in (Sn, Al)-H-Beta Zeolite Catalysts for Enhanced Furfural Production

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Abstract
Furfural is a key platform chemical in biorefining, accessible through the direct conversion of xylose. However, its production is notably influenced by the synergistic effect between the density and spatial distribution of Brønsted acid (BA) and Lewis acid (LA) sites in solid acid catalysts. In this work, a novel fluoride-free quasi-solid-phase synthesis method was developed to fabricate (Sn, Al)-H-Beta catalysts featuring adjacent framework Sn-O-Al active sites. The ratio and amount of BA and LA sites can be effectively controlled by varying the framework Sn content in the catalysts. Meanwhile, the adjacent Sn-O-Al active sites achieve the spatial proximity of BA and LA sites, reducing the side reactions along the diffusion path of intermediates. In situ FTIR and DFT calculation results demonstrated that the Sn-O-Al active sites promote the adsorption and isomerization of xylose by activating the C=O bonds in its aldehyde groups and the adjacent C-O/C-H bonds. The optimal QSP-Beta-1.6Sn catalyst could achieve 76.7% furfural yield with a 99% conversion at 190 °C for 2 h in the water/GVL system. This study provides a rational design strategy for solid acid catalysts with synergistic acid sites in cascade biorefining.

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