Direct oxidative carbonylation of waste methane to acetic acid over Rh/zeolite catalysts

Direct oxidation of methane to oxygenates remains challenging in heterogeneous catalysis. In this work, direct oxidative carbonylation of low-concentration methane to acetic acid has been investigated over Rh/zeolite catalysts. Formation of acetic acid over Rh/ZSM-5 is shown to be far superior compared to a range of other Rh/zeolite catalysts, including Rh/Z-beta, Rh/Z-Y, Rh/Mor, Rh/Fer, and Rh/SSZ-13. The importance of the 3D channel structure of ZSM-5 is emphasised by a comparison with unidirectional Rh/ZSM-23 which showed much lower production of acetic acid. Acetic acid production is found to be maximum at a SiO 2 /Al 2 O 3 ratio corresponding to ZSM5–50. Isolated Rh is identified as the active site for liquid oxygenate production with 0.09 wt% Rh/ZSM5 giving the highest acetic acid production. Higher Rh loading leads to a drop in production due to the gradual formation of Rh nanoparticles. Acetic acid production is shown to be strongly pressure dependent consistent with 2nd or higher order apparent kinetics compared to apparent 1st order for C1 products. Competing direct CO oxidation to CO 2 was responsible for above 85 % of the total CO 2 production. These findings highlight the critical role of zeolite topology in enhancing selective methane valorisation over Rh catalysts, and provide insights into developing practical catalytic processes for low-carbon chemical manufacturing. • Rh/ZSM-5 gives superior acetic acid production compared to other Rh/zeolite catalysts. • Rh/ZSM-5 gives highest C2 selectivity in oxidative carbonylation of waste methane. • SiO 2 /Al 2 O 3 = 50 leads to the highest acetic acid production over Rh/ZSM5. • Acetic acid production shows greater pressure dependence than C1 products.