Abstract
Controllable methane oxidation directly into higher-value-added products under mild conditions remains a challenge due to the stability of the C-H bond. To promote methane oxidation using metal-organic frameworks, it is still necessary to explore ways of stabilizing metal active sites on MOFs due to the leaching and near-complete degradation of the catalyst after exposure to highly oxidative environments. Herein, we report a structural engineering approach based on Ga(3+)-Fe(3+) complexes in biological systems to tailor the redox-cycle activity. It was imitated by tailoring Ga(3+) doping into Fe-MIL-88B. Thus, novel MOFs with differing compositions of Fe and Ga were synthesized and denoted as Fe (x) Ga (y) -MOF. Chemical stability tests in water and oxidative environments confirmed that the bimetallic MOFs indeed exhibited higher stability with reduced leaching of iron sites. Fe(0.3)Ga(0.7)-MOF was demonstrated to be the most stable material while being active and was selected for further catalytic evaluations. Several parameters for the methane oxidation reaction were optimized such as mass of catalyst, temperature, pressure, and others. Fe(0.3)Ga(0.7)-MOF exhibited a productivity of 29.9, 381.9, and 90.1 μmol g(cat) (-1) for methanol, formic acid, and acetic acid, respectively. Compared to the Fe-MIL-88B, the Fe(0.3)Ga(0.7)-MOF had an enhancement of 36% toward the selectivity of oxygenates and also reduced by almost 95% the undesired evolution of CO(2). This material demonstrated excellent stability, retaining its catalytic activity after three cycles with only 0.1% metal leaching, highlighting the effectiveness of the stabilization method. In contrast, Fe-MIL-88B showed poor stability, with 38.3% metal leaching after the first cycle. Mechanistic insights indicated a major role of reactive oxygen species in the formation of products.