Kinetics of Decomposition Reactions of Acetic Acid Using DFT Approach

Excessive amount of oxygen content in unprocessed bio-oil deteriorates the quality of bio-oil which cannot be used in transportation vehicles without upgrading. Acetic acid (CH₃COOH) is a vital component of ‘acids’ catalogue of unprocessed bio-oil produced from thermochemical conversions of most of biomass feedstocks such as switchgrass, alfalfa, etc. In this study, the decomposition reactions of acetic acid are carried out by two reaction pathways, i.e., decarboxylation and dehydration reactions. In addition, the reaction rates of decomposition are analysed in a wide range of temperatures, i.e., 298-900 K and at atmospheric pressure.

All quantum chemical calculations are performed in the gas phase using two DFT functionals, B3LYP and M06-2X, with 6-31g(d) basis set. The dehydration reaction of acetic acid proceeds directly from ground state structure of acetic acid, whereas, decarboxylation reaction forms an unstable intermediate of acetic acid to initiate the proton migration. Barrier height and kinetics study for both reactions and theories are different and illustrated in the reaction pathway and rate profiles, respectively. Furthermore, both levels of theories offer similar structural configurations but they differ slightly in energetics.

The reaction kinetics of both reactions is linearly fitted and the Arrhenius equations corresponding to each decomposition mechanism are generated by fitting the data from line equation.