Selective Oxidation of Methane to Methanol over Rh/Hydroxyapatite Catalysts Enabled by Strong Metal-Support Interactions.

Methane, the primary constituent of natural gas, represents a critical feedstock for the synthesis of high-value liquid fuels and chemicals. However, its practical conversion is often hampered by the high dissociation energy and low polarity of the C-H bond, as well as the susceptibility of methanol to overoxidation. In this study, we propose an effective strategy for methane oxidation utilizing atomically dispersed rhodium supported on hydroxyapatite (Rh/HAp), synthesized via a straightforward impregnation method. Experimental evaluations demonstrate that the 0.5Rh/HAp catalyst achieves a methanol yield of 3440 μmol·gcat -1·h-1 at 240 °C (under a CH4/O2/CO pressure ratio of 20:3:5 bar), maintaining a liquid-phase selectivity of >99% over a 1 h period. Characterization data suggest that the Rh species are atomically dispersed on the HAp surface, existing in a cationic state (Rhδ+) characterized by strong metal-support interactions. In situ infrared spectroscopy reveals the formation of thermally stable Rh-(CO)2 active sites. These sites appear to resist aggregation and overoxidation even under high-temperature and oxygen-rich conditions, which is likely attributable to the CO ligand effect and interactions with surface PO4 3- and OH- groups. Furthermore, in situ DRIFTS analysis suggests a mild, stepwise oxidation pathway, wherein methane is transformed into methanol through the formation of surface methoxy (*OCH3) intermediates.