B-B Sites in Molten Boron Oxide are Superior to Cyclic B-O-B Sites for Methane Oxidation to Formaldehyde and Carbon Monoxide.

Boron oxide (B2O3) has recently emerged as a promising metal-free catalyst for the oxidative dehydrogenation (ODH) of methane, with remarkable selectivity for formaldehyde (HCHO) and carbon monoxide (CO) at 823 K. Since B2O3 melts at ∼725 K, the ODH reaction is likely to proceed in its molten state. In 2023, ab-initio molecular dynamics simulations by Rousseau and co-workers revealed that molten B2O3 comprises of 8-, 6-, 4-membered cyclic B-O-B sites and a B-B site. So herein, we carried out a detailed DFT-based mechanistic investigation for CH4 oxidation across these structural motifs with 1O2 and 3O2. Due to the multireference character of 1O2 and its associated species, CASSCF/NEVPT2 methodology was employed. Among all motifs, the B-B site exhibits superior catalytic activity toward HCHO formation (36.6 kcal mol-1), while cyclic B-O-B sites require significantly higher activation barriers (∼ 50 to 78 kcal mol-1). Orbital analysis reveals that electron transfer in cyclic B-O-B sites originates from oxygen lone-pair located in the ring, while at the B-B site, it originates from σ(B-B) bond. In addition, CO formation is thermodynamically more favorable than CO2, consistent with existing experimental observations. Overall, these mechanistic insights offer a foundation for designing efficient, metal-free methane oxidation catalysts.