Fe-Doped SnO2 catalysts with both BA and LA sites: facile preparation and biomass carbohydrates conversion to methyl lactate MLA

Abstract

Fe-Doped SnO₂ solid acid catalysts with both Lewis acid and Brønsted acid sites were synthesized by a facile sol–gel method comprising hydrolysis of SnCl₄ and Fe(NO₃)₃ under the action of NH₄OH in aqueous solution, followed by 500 °C calcination. Characterization of the thus-prepared Fe-doped SnO₂ catalysts with HRTEM, XRD, Raman, and XPS demonstrate the insertion of Fe³⁺ into the SnO₂ crystal lattice. Based on the Py-IR, NH₃-TPD and BET characterization, the Lewis acid and Brønsted acid sites and acidity of the as-prepared catalysts could be adjusted by controlling the Fe doping amounts and calcination temperature. When evaluated as solid acid catalysts for the conversion of sugar to methyl lactate (MLA), the Fe-doped SnO₂ catalyst with a 0.2 Fe/Fe + Sn feed ratio exhibited a preferable MLA yield of 35% under the best conditions. The catalysts also showed good selectivity and reusability. The enhanced catalytic activity could be attributed to the cooperation of Lewis acid and Brønsted acid, and appropriate acid strength. In addition, when the substrates were sucrose and fructose, the yields of MLA reached 44% and 52%.