Mechanistic Insights into Radical-Induced Selective Oxidation of Methane over Nonmetallic Boron Nitride Catalysts

Abstract

Boron-based nonmetallic materials (such as B2O3 and BN) emerge as promising catalysts for selective oxidation of light alkanes by O2 to form value-added products, resulting from their unique advantage in suppressing CO2 formation. However, the site requirements and reaction mechanism of these boron-based catalysts are still in vigorous debate, especially for methane (the most stable and abundant alkane). Here, we show that hexagonal BN (h-BN) exhibits high selectivities to formaldehyde and CO in catalyzing aerobic oxidation of methane, similar to Al2O3-supported B2O3 catalysts, while h-BN requires an extra induction period to reach a steady state. According to various structural characterizations, we find that active boron oxide species are gradually formed in situ on the surface of h-BN, which accounts for the observed induction period. Unexpectedly, kinetic studies on the effects of void space, catalyst loading, and methane conversion all indicate that h-BN merely acts as a radical generator to induce gas-phase radical reactions of methane oxidation, in contrast to the predominant surface reactions on B2O3/Al2O3 catalysts. Consequently, a revised kinetic model is developed to accurately describe the gas-phase radical feature of methane oxidation over h-BN. With the aid of in situ synchrotron vacuum ultraviolet photoionization mass spectroscopy, the methyl radical (CH3 •) is further verified as the primary reactive species that triggers the gas-phase methane oxidation network. Theoretical calculations elucidate that the moderate H-abstraction ability of predominant CH3 • and CH3OO• radicals renders an easier control of the methane oxidation selectivity compared to other oxygen-containing radicals generally proposed for such processes, bringing deeper understanding of the excellent anti-overoxidation ability of boron-based catalysts.

Mapping the Catalytic‐Space for the Reactivity of Metal‐free Boron Nitride with O2 for H2O‐Mediated Conversion of Methane to HCHO and CO

“…In path-D, the formation of HOO species is shown which is not in line with the experiment [26] available in literature. Hence, the occurrence of path-D during the methane oxidation is minimal.…”

“…In an in-situ synchrotron vacuum ultraviolet photoionization mass spectroscopy experiment available in the literature, [26] it was revealed that HOO radical formation does not take place in this reaction. Hence, path-C which demonstrates the presence of HOO radical, is not in-line with the experimental observation.…”

“…[25] In a more recent investigation by Wang and coworkers, h-BN catalyst at 600 °C depicts the oxidation of methane in presence of dioxygen to not only CO but also HCHO. [26] The group demonstrated that h-BN reacts with methane and dioxygen to produce methyl radicals. The conversion of CH 3 * to HCHO and CO was studied in the gasphase.…”

“…The conversion of CH 3 * to HCHO and CO was studied in the gasphase. [26] In a very recent study by Xie and coworkers, interesting surface chemistry of borocarbonitride (BCN) was explored for ODH of propane. [27] In brief, the reactivity of boron oxide for methane oxidation was investigated over the surface, whereas for h-BN, the reaction was majorly focused in the gas-phase.…”

Methane–H2S Reforming Catalyzed by Carbon and Metal Sulfide Stabilized Sulfur Dimers

“…Methane is a key energy carrier in the transition to sustainable production of fuels and chemicals. − Most approaches for methane utilization involve successive C–H bond cleavages to form surface carbons that are removed by co-reactant-derived oxygen species; these routes include steam and dry reforming (H 2 O or CO 2 as co-reactant − ), partial oxidation (O 2 as co-reactant − ), and autothermal reforming into CO x and H 2 , the simplest building blocks that can be converted into a wide variety of carbon-containing products. Strategies have also been devised for direct selective C–H bond activation of methane into functionalized compounds that may be further upgraded (including halogenations, − oxidative coupling, − and partial oxidation − ).…”