Oxidative carbonylation of methane to acetic acid over micro-mesoporous rhodium-modified zeolites

It was first demonstrated that under liquid-phase conditions, an increase in the mesoporosity of ZSM-5 zeolite boosts a rhodium-containing catalyst activity in the oxidative carbonylation of methane to acetic acid (the acetic acid yield is 570 μmol gcat−1). To increase the mesoporosity of zeolite, two approaches were used: ultrasonic treatment and desilication of zeolite. The ultrasonic pretreatment leads to an increase in the acetic acid yield with a decrease in the total yield of oxygenates (methanol/acetic acid molar ratio is 0.2). On the contrary, preliminary desilication contributes to an increase in the total yield of oxygenates, mainly due to methanol (methanol/acetic acid molar ratio is 1.8). The combination of ultrasonic pretreatment and desilication of zeolite improves the total yield of oxygenates with an increase in the acetic acid yield. Such a change in the catalytic properties is attributed both to the acidity of the parent zeolite and the local atomic environment of rhodium. XAS spectroscopy showed that rhodium can be present on the zeolite surface either as nanoclusters giving rise to methanol or as isolated rhodium atoms, which promote the acetic acid formation.