Role of Alkali Metal Cations for the Selective Formation of CH3COOH over Cu-X-ZSM-5 from Co-activation of CH4 and CO2; A Theoretical Insight

Abstract

We employed the ONIOM model to investigate the role of alkali metals in the conversion of CO₂ and CH₄ into CH₃COOH on CuX-ZSM-5 (X = alkali metal). This hybrid model achieves high efficiency by dividing the computational system into layered regions, where key regions undergo high-accuracy quantum mechanical (QM) calculations, while the remaining regions are addressed with molecular mechanics (MM). The direct synthesis of CH₃COOH involves three sequential reaction steps: the heterolytic cleavage of the CH₄ bond, the formation of acetate from –CH₃ and –CO₂ species, and the subsequent desorption of CH₃COOH. Theoretical model reproduced the experimental kinetics trend for alkali metal doped CuX-ZSM-5 catalysts (K⁺ > Na⁺ > Li⁺ > H⁺). A synergistic effect of X-cations was observed, with its intensity increases as one moves down the group. The CuK-ZSM-5 and CuNa-ZSM-5 have the lowest energy barriers and maintain functionality for a specific period. The remarkable performance of the K⁺ containing catalyst in comparison to the other alkali metals cations, arises from an amalgamation of binding affinities and judiciously balanced metal size. When the alkali metal cations are too small, it coordinatively saturated and fail to activate CO₂ effectively. However, the desorption of CH₃COOH over CuX-ZSM-5 requires a significant amount of energy. This high energy demand leads to the saturation of the catalyst surface with –CH₃COO⁻ species over time, ultimately causing catalyst deactivation. It is critical to enhance the desorption process. The impressive selectivity of CuNa-ZSM-5 and CuK-ZSM-5 in the reaction is attributed to the blocking of active sites by Na and K-cations because of larger size, enabling only single C–H bond breakage. The CuNa-ZSM-5 and CuK-ZSM-5 catalysts exhibit exceptional atomic economy by generating no waste. All atoms from CO₂ and CH₄ are fully incorporated into valuable industrial products. Thus, these catalysts offer an economical, environmentally friendly, and practical approach for converting greenhouse gases into acetic acid with 100% atomic utilization.

Graphical Abstract