Mechanistic insights into the production of methyl lactate by catalytic conversion of carbohydrates on mesoporous Zr-SBA-15

• High yields of methyl lactate were produced with Zr-SBA-15 in near-critical methanol.
• Lewis acid sites on Zr-SBA-15 strongly catalyzed transformation of carbohydrates.
• Lewis acidity on Zr-SBA-15 varied with the Si/Zr molar ratio.
• Pore size of Zr-SBA-15 affected the formation of undesired humins.
• Methyl lactate yield was insensitive to carbohydrate biomass feedstocks.

The as-synthesized Zr-SBA-15 catalysts with tunable mesoporous structures showed excellent catalytic performance for the conversion of carbohydrates to methyl lactate in a “one-pot” process using near-critical methanol or methanol–water mixture as the solvents. The effects of reaction conditions, including temperature, reaction time, and catalyst loading amount, on the conversions of carbohydrates and the yields of methyl lactate were investigated. The high yields of methyl lactate, up to 41% and 44%, were produced from pentose and hexose, respectively, in the near-critical methanol at 240 °C. Moreover, the Si/Zr ratio of the Zr-SBA-15 catalysts profoundly affected the Lewis acidity and therefore the catalytic activity and selectivity to methyl lactate in the conversion of carbohydrates. The pore size of the Zr-SBA-15 catalysts, tuned by the synthesis temperature, strongly affected the formation of solid residues. The key intermediates such as glyceraldehyde, glycolaldehyde, and pyruvaldehyde were used as probe reactants to understand the mechanism. The role of the Zr-SBA-15 catalyst in the aldol- and retro-aldol condensation, isomerization, and Cannizzaro reactions of carbohydrates and their derivatives was discussed. Furthermore, 28% and 27% yields of methyl lactate were obtained from cellulose and starch, respectively, in methanol–water mixture (5 wt% water and 95 wt% methanol) at 240 °C. The Zr-SBA-15 catalyst was relatively stable in short term without regeneration.

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