Isomerization and Epimerization of Glucose Catalyzed by Sn-Containing Mesoporous Silica

Abstract

Herein, we propose methodologies for the direct syntheses of Sn-containing MCM-41, SBA-16, MCM-48 with tin loadings between 1.5 and 4.5%. The catalysts were active in the isomerization of glucose to fructose, and the sample containing 3.0% of Sn showed the highest selectivity. Compared to the other structures, Sn-MCM-48 displayed a lower selectivity to fructose and a higher one to mannose, which is due to the presence of K+ remaining from the synthesis gel. Interestingly, the TOFFru is similar for Sn-MCM-41(3.0%) and K+/Sn-MCM-48(3.0). For K+/Sn-MCM-48(3.0%), the TOFMan and TOFFru were comparable. The presence does not affect the rate of fructose formation, but it promotes side reactions that involve glucose epimerization to mannose and, more importantly, the decomposition of glucose to unidentified products. Indeed, the ratio (TOFFru+TOFMan)/TOFGlu was 0.79, 0.37, and 0.81 for Sn-MCM-41, K+/Sn-MCM-48, and Sn-SBA-16(3.0%). The lowest TOF for fructose formation was observed for Sn-SBA-16(3.0%), which can be attributed to the high contribution of Sn in high coordination and extra framework tin oxide. For the direct conversion of glucose to HMF, Sn-MCM-41(3.0%), K+/Sn-MCM-48(3.0%), and Sn-SBA-16(3.0%) were combined with HCl, reaching selectivities of 77, 73, and 67%, respectively. These results are comparable or superior to those previously reported.