Mechanistic Insight into the Quantitative Synthesis of Acetic Acid by Direct Conversion of CH₄ and CO₂: An Experimental and Theoretical Approach

Abstract

Conversion of CH4 and CO2 into value-added products has vital environmental and economic importance. Their direct conversion to acetic acid is challenging due to their high activation energy. Hence, kinetic and mechanistic information are crucial for the carbonylation of CH4 with CO2. Regarding this, single and dual component catalysts with different combinations of ZnO-, CeO2-, and MnO2- supported montmorillonite (MMT) were prepared and characterized by XPS, Raman, and XRD. Quick solid-state NMR, TGA, and FT-IR techniques were used and Langmuir-Hinshelwood model was considered to investigate mechanistic steps involved in the conversion of CH4 and CO2 to acetic acid. The obtained mechanistic and kinetic results were also theoretically proved by density functional theory (DFT) calculations. We found that ZnO and CeO2 dual active sites preferentially adsorb the CH4 and CO2, respectively that avoid surface adsorption competition. The rate of acetic acid formation was maximum when these sites exist at appropriate concentration (Ce: 0.44 wt%, Zn: 2.20 wt%). DFT calculations elucidated that the formation of acetic acid is strongly favored on ZnO catalyst with easier migration of the adsorbed CO2 from CeO2 to the ZnO side.