Microscopic insights into the methanol oxidation mechanism in high-temperature supercritical water

Abstract

Although the methanol oxidation process in supercritical water has been studied in some previous experiments, the methanol oxidation rate is sensitive to the supercritical water concentration, and its influence on the methanol oxidation mechanism remains unclear. Moreover, while the presence of formic acid intermediate (HCOOH) has been reported, its detailed conversion pathways are still poorly understood, and experimental detection of such transient intermediates is difficult. To address the above shortcomings, this study employed reactive molecular dynamics simulations to explore the effects of supercritical water and oxygen concentrations on the methanol oxidation mechanism at the microscale, focusing on the conversion pathways of formic acid. The reaction rate constants and activation energies for the initial methanol oxidation were calculated using first-order kinetics theory to validate the accuracy of the CHO-S22 force field used. Adding supercritical water could decrease the activation energy of initial methanol oxidation and increase the reactivity. The evolution of various species under different ambient conditions was discussed. The results suggested that increasing water concentration could promote OH/H₂ production, enhance formaldehyde intermediate consumption, and inhibit CO production. The main methanol oxidation pathways were analyzed in detail. The simulations captured another important methanol supercritical water oxidation route involving formic acid: CH₃OH → CH₂OH/CH₃O → CH₂O → HCOOH → HCOO→ CO₂. Formic acid can be further oxidized to HCOO/COOH, but HCOO is predominant. HCOO was converted to CO₂ mainly by pyrolysis and reacting with OH, while COOH can be interconverted with CO. The reactions of formic acid, HCOO/COOH, HCO, and CO intermediates with OH were enhanced with increasing water concentration under stoichiometric conditions. Increasing oxygen concentration could promote the conversion of methanol to formaldehyde via O₂ and HO₂ but may inhibit the conversion of formic acid through OH radicals to HCOO/COOH. This study provides new insights into the reaction network of methanol oxidation in supercritical water from a microcosmic perspective.