Manganese oxide as an alternative to vanadium-based catalysts for effective conversion of glucose to formic acid in water

Abstract

MnO_x catalysts were synthesized under hydrothermal conditions for conversion of glucose into formic acid (FA) in water with the objective to develop vanadium-free reaction systems. An FA yield of 81.1% was obtained with a MnO_x catalyst and glucose substrate in water at 160 °C and is almost twice the value obtained with vanadium-based heterogeneous catalysts. MnO_x materials prepared hydrothermally at 100 °C had higher Mn²⁺/Mn³⁺ ratios and adsorbed oxygen species than those prepared at higher temperatures and gave the highest FA yields among the catalysts evaluated. Mechanistic studies of glucose–MnO_x–water reaction systems revealed that two parallel reactions existed with arabinose being the intermediate in the α-scission route and glyoxylic acid being the intermediate in the β-scission route where CO₂ co-forms with FA. Small water-soluble carbohydrates (glucose, sucrose, cellobiose, maltose, xylose) afforded FA yields greater than 50%, while starch afforded FA yields greater than 20%, thus demonstrating the potential of MnO_x catalysts for converting biomass into FA.