Coordinatively unsaturated Lewis acidic aluminum sites in zeolites for direct partial oxidation of methane

Abstract

In this study, we show that transition metal-free aluminosilicate-type zeolites (Al-zeolites) are active and highly selective for partial oxidation of CH₄ (POM) to CO at temperatures ranging from 600 to 800 °C by using O₂ as an oxidizing agent. Although the POM activities of Al-zeolites depend on their topologies, CO selectivity was high at around 80% even at CH₄ conversions above 20%, regardless of the topologies. CHA and Beta zeolites exhibit relatively high and stable catalytic performance over a reaction time of 6 h, while MFI zeolite was highly active at the initial period and then quickly deactivated with prolonged duration. MOR and FAU zeolites show low POM activity. The acid sites in Al-zeolites after the POM reaction were analyzed by infrared (IR) spectroscopy using CO as a molecular probe. The results illustrate that the Al-zeolites, with a greater abundance of coordinatively unsaturated Lewis acidic Al, result in higher POM activity. A mechanistic study demonstrates that the C−H bond was activated by a strong LAS derived from a coordinatively unsaturated Al atom via a four-centered transition state. Computational activation energy was revealed as 32 kcal mol⁻¹, being close to the experimental value (34 kcal mol⁻¹).