Insights into the Microstructure and Diffusion Mechanism of Co ²⁺ /Mn ²⁺ in Acetic Acid–Water Solutions

The catalytic performance of Co/Mn/Br in liquid-phase oxidation of methyl aromatics is highly dependent on the solvent environment. However, the microstructure of metal species under reaction conditions remains unclear. Here, solvation structures and diffusion behaviors of Co2+ and Mn2+ in HOAc–water mixtures are studied using molecular dynamics simulations. As the water content increases, water molecules progressively displace HOAc and OAc– ligands in the first solvation shell around Co2+/Mn2+. Although diffusion coefficients increase with water content, the decrease in characteristic diffusion length scales implies more restricted short-range mobility for Co2+/Mn2+. Their transport involves a mixed mode mediated by the solvent. The radial distribution of Br– and ArCH2OOH away from Co3+ and Co2+, respectively, suggests a catalytic mechanism through solvent-mediated long-range interactions. The reduction in hydrogen bond lifetimes and catalytic accessibility under high water content provides a molecular-level explanation for the experimentally observed decline in metal ion catalytic reactivity.