Relationship between Lewis acid sites and carbohydrate reactivity over Sn-β catalysts†

Abstract

Sn-β is a promising Lewis acid zeolite for carbohydrate conversion. This material can be prepared either directly by hydrothermal synthesis or by a dealumination – metal incorporation sequence starting from a pre-made zeolite (post-synthesis modification). The synthesis method and the metallic precursors significantly influence the formation of Lewis acid sites in the zeolite, which is the primary factor determining the activity of the catalyst in many reactions. We synthesized various materials through post-synthesis modifications and a hydrothermal method using three different precursors. Pyridine adsorption monitored by FTIR spectroscopy shows that Sn-β samples synthesized by solid state insertion with tin chloride as a precursor feature a concentration of Lewis acid sites proportional to the tin content (up to 1.5 wt% of tin) without forming an oxide phase. Hydrothermal synthesis leads to a sample exhibiting weaker acid sites. The catalyst yields three major products in glucose conversion: fructose, mannose, and lactic acid. The high yield of lactic acid (≈30%) indicates a faster ketose retro-aldolization compared to aldose (no C4 or C2 products detected).