Reaction mechanisms of propionic acid pyrolysis, hydrolysis, and H2 production during the thermal decomposition in various alcohol environments: a ReaxFF molecular dynamics.

Abstract

In this study, ReaxFF molecular dynamics (MD) simulations were employed to investigate the pyrolysis process and thermal decomposition of propionic acid (CH3CH2COOH) in various environments such as ethanol, methanol, propanol, and water. There were two reaction pathways in propionic acid pyrolysis through CH3CH2COOH → H + CH3CH2COO and CH3CH2COOH → OH + CH3CH2CO, leading to CO2 and CO products. Furthermore, propionate radicals are generated via the reaction C3H6O2 + C3H6O2 → H2O + C3H5O2 + C3H5O, when acid-acid molecular interactions occur. The primary cross-reaction channels between propionic acid and alcohols are C3H6O2 + CH4O → CH3 + H2O + C3H5O2, C3H6O2 + C3H8O → H2O + C3H7 + C3H5O2, C2H6O + C3H6O2 → H2O + C2H5 + C3H5O2 resulting in ethyl, methyl, and propyl propionates as well as water production. During thermal decomposition of propionic acid in various alcohol environments, the productions of CO and H2 increase, while the production of CO2 decreases. Conversely, the lowest H2 production is seen in water environment. Arrhenius equation was used for calculation of activation energies (Ea) for all systems. The activation energies of propionic acid were calculated as 163(8) kJ mol-1, 165(3) kJ mol-1, 172.58(12) kJ mol-1, and 193.05(25) kJ mol-1 in ethanol, methanol, propanol, and water solutions, respectively. Molecular dynamics (MD) simulations were performed using the Amsterdam Modeling Suite (AMS) software version 2026.103. The ReaxFF all-atom force field was used, and the parameters for carbon, hydrogen, and oxygen were taken from the CHO force-field parameter set. The numerous reactive events, reaction mechanisms, intermediate species, and product distributions were extracted from the MD trajectories by using ChemTraYzer2 developed by AMS tools. The simulations were equilibrated under NVT conditions, and the activation energies of the decomposition processes were determined using the Arrhenius equation based on the simulation data, enabling a quantitative comparison of the kinetic behavior in ethanol, methanol, propanol, and water.

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