Direct Low-Temperature Oxidative Conversion of Methane to Acetic Acid on Rhodium-Modified Zeolites

Abstract

Oxidative conversion of methane to acetic acid at 150°С and a pressure of 6.5 MPa with the assistance of carbon monoxide on rhodium-modified ZSM-5 zeolites with different SiO2/Al2O3 molar ratios was studied. A decrease in SiO2/Al2O3 from 300 to 30 leads to a 2.7-fold increase in the acetic acid yield. Such behavior of the catalytic system correlates with an increase in the acidity, primarily with an increase in the density of Bronsted acid sites on the zeolite surface. Introduction of 0.1–0.5 wt % rhodium into the zeolite increases the acetic acid yield by a factor of more than 3. The highest acetic acid yield (0.53 mmol/gcat) is attained on the zeolite with SiO2/Al2O3 = 33 modified with 0.5 wt % Rh. The methane transformation to acetic acid occurs while involving both the acid sites of the zeolite and the sites formed by reactive rhodium particles.