The role of the spatial arrangement of single rhodium sites on ZSM-5 in the oxidative methane carbonylation to acetic acid

Single atom rhodium catalysts based on microporous ZSM-5 zeolite (SiO2/Al2O3 = 30) were studied in the reaction of oxidative methane carbonylation to acetic acid. For highly dispersed rhodium distribution on the zeolite surface, nitrogen-containing polymers, namely chitosan hydrochloride, polyethyleneimine, and polyvinylpyrrolidone, were used. Using XAS spectroscopy, all rhodium was found to be as single atom metal sites in the zeolite structure, which contributes to the acetic acid formation. The catalytic properties of rhodium zeolite catalysts in the oxidative methane carbonylation to acetic acid were revealed to depend not only on the single atom rhodium distribution and the catalyst acidity, but also on the spatial rhodium atom arrangement in the zeolite, which was established by the XPS. According to DFT calculations in combination with EXAFS modeling, rhodium anchored at the zeolite channel intersections was shown to be coordinated with four zeolite oxygen atoms and one hydroxyl group oxygen atom and be the dominant catalytic species in the acetic acid formation.