Exploring the continuous cleavage-oxidation mechanism of the catalytic oxidation of cellulose to formic acid: A combined experimental and theoretical study

Abstract

Catalytic oxidation of cellulose using O2 in aqueous solution shows important potential in the production of formic acid, an important industrial chemical as well as a promising hydrogen storage material. However, the microscopic oxidation mechanism is unclear due to the challenges in observing the cleavage of chemical bonds and the formation of transient intermediates and radicals. In this work, a combination of experimental and theoretical methods has been used to explore the catalytic oxidation mechanism of cellulose to formic acid. In the oxidation of cellulose, hydrogen cation plays a catalytic role and a decrease in pH improves the transformation with 57.7% formic acid obtained. Based on the model compound experiments, intermediate detection, molecular dynamics simulation, and density functional theory calculation, a mechanism is proposed, in which polyhydroxy aldehydes generated by cellulose hydrolysis undergo a continuous carbon–carbon bond cleavage-oxidation reaction to form formic acid with aldehyde moiety as the direct precursor. Acid has a catalytic effect on the cleavage-oxidation reaction of carbon–carbon bonds, reducing the energy barrier of carbon–carbon bond cleavage through generating epoxy intermediates. The aldehyde moieties are kept at an appreciable concentration, thereby increasing the yield of formic acid. Moreover, it is confirmed that the carboxyl moiety obtained by the oxidation of aldehyde is the precursor of the by-product CO2. These findings provide an in-depth insight into the production of bio-chemicals through carbon–carbon bond cleavage and a theoretical support for further process optimization.