Xinjiang University team: defective UiO-66 catalyst turns methane into acetic acid with 84% selectivity
A UiO-66 catalyst with ligand defects oxidizes methane to acetic acid in one step with H2O2 at 150 °C in the study; the industrial route needs three steps.
A research team led by Prof. Haibao Huang, Dr. Hao Guo and Dr. Di Hu at Xinjiang University (Urumqi) and Sun Yat-sen University (Guangzhou) has designed a UiO-66-based catalyst with abundant ligand defects. According to a release published on eurekalert.org, the catalyst converts methane directly into acetic acid using hydrogen peroxide under mild conditions. eurekalert.org
eurekalert.orgeurekalert.orgOpen source ↗ At the optimal temperature of 150 °C it reached an acetic acid selectivity of 84% and a yield of 1,691 μmol·g cat.^-1. The results were published in the Chinese Journal of Catalysis (DOI: 10.1016/S1872-2067(26)65127-3).
Direct conversion of methane to C2 products
The current industrial process for making acetic acid from methane is indirect. It takes three steps, methane reforming, methanol synthesis and methanol carboxylation, and consumes large amounts of energy. Direct synthesis of C2 products is difficult because it requires activating the C–H bond of methane and controlling C–C coupling. The authors point to metal-organic frameworks, with their tunable structure, adjustable pores and well-defined metal sites, as a platform for regulating the microenvironment around active sites.
Ligand defects in UiO-66 as active sites
The catalyst, designated UiO-66-XT, was synthesized through the competitive complexation effect of trifluoroacetic acid. It has a higher specific surface area and more defect sites than the starting framework. The ligand defects expose unsaturated Zr4+ sites, and the d-band center of the Zr 3d orbital at these sites shifts upward, which promotes decomposition and C–C coupling.
The authors report the following mechanism. The exposed sites promote adsorption of methane and hydrogen peroxide. Efficient charge transfer at these sites then facilitates C–H bond activation, while defect-stabilized intermediates catalyze C–C coupling to form acetic acid. eurekalert.org
eurekalert.orgeurekalert.orgOpen source ↗
The team also observed a volcano-shaped correlation between the degree of ligand defects in UiO-66 and acetic acid selectivity. This means selectivity did not grow steadily with the number of defects but passed through a maximum. According to the authors, the catalyst also showed exceptional stability.
The authors say the findings add to the understanding of defect engineering in materials and of the partial oxidation of methane. The release gives results for a single reaction, methane to acetic acid with hydrogen peroxide at 150 °C.
