First-Principles Insights into the Direct Synthesis of Acetic Acid from CH4 and CO2 over TM-Si@2D Catalysts

Abstract

The direct synthesis of acetic acid from natural gases has attracted great attention. However, achieving selective C-C coupling remains a major challenge. We designed doped single-atom transition metal catalysts on 2Ds materials, guided by DFT calculations on the reaction pathways for acetic acid synthesis via CH₄/CO₂ coupling. Among the catalysts examined, Ni-Si@h-BN shows strong electron synergy in CH₄ activation and C-C coupling under the E-R mechanism, confirmed by kinetics.Transforming CH₄ and CO₂ into acetic acid offers a green route to convert greenhouse gases into value-added chemicals 1 . Understanding the reaction mechanism is essential for catalyst design: CH₄ first dissociates into CH₃ * and H * , after which CH₃ * couples with CO₂ to form CH₃COO * . This intermediate undergoes hydrogenation to yield CH₃COOH, which subsequently desorbs as acetic acid 2 . Homogeneous catalysts like Pd(OAc) 2 /Cu(OAc) 2 /K 2 S 2 O 8 /CF 3 COOH 3 , RhCl 3 4 and PdSO 4 5 have been used for this conversion but it is challenging to recycle the catalysts. More practical heterogeneous catalysts use supports like TiO 2 , Al 2 O 3 , SiO 2 and zeolites, with active sites such as Zn or Cu 6 . For instance, Cu-K-ZSM-5 zeolite 7 converts CH 4 and CO 2 to acetic acid at 500℃, achieving 5% CH 4 conversion and 100% acetic acid selectivity after 1 hour, while ZnO-CeO₂ supported on montmorillonite 8 achieves 8.33% CH₄ conversion and 100% selectivity at 300°C. Zn-based catalysts generally show higher activity than Cu-based ones but they still require stringent conditions for effective activation 9 . Theoretical calculations can underpin experimental efforts to improve catalyst activity by revealing reaction pathways and energy barriers. In particular, understanding the elementary steps of C-H activation and subsequent C-C coupling is especially important for the direct synthesis of acetic acid catalysts 10 . Langmuir-Hinshelwood (L-H) and Eley-Rideal (E-R) mechanisms are two commonly adopted mechanisms for CO₂-CH₃* coupling. Nie et al. 11 found that Fe/ZnO₂ catalysts activate COMMUNICATION