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

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.

Confinement-Enhanced Selective Oxidation of Lignin Derivatives to Formic Acid Over Fe-Cu/ZSM-5 Catalysts Under Mild Conditions.

Aqueous-phase oxidation by H2 O2 , known as the Fenton-type process, provides an attractive route to convert recalcitrant lignin derivatives to valuable chemicals under mild conditions. The development of this technology is, however, limited by the uncontrolled selectivity, resulting from the highly reactive nature of H2 O2 and the thermodynamically favored deep oxidation to form CO2 . This study demonstrated that formic acid could be produced with a high selectivity (up to 80.3 % at 313 K) from the Fenton-type oxidation of guaiacol and several other lignin derivatives over a bimetallic Fe-Cu catalyst supported on a ZSM-5 zeolite. Combined experimental and theoretical investigations unveiled that the micropores of the zeolite support, which contained active metal sites, preferred to adsorb C2 -C4 intermediates over formic acid because of its stronger dispersive interaction with the larger guest molecules. This confinement effect significantly suppressed the secondary oxidation of formic acid, accounting for the uniquely high formic acid selectivity over Fe-Cu/ZSM-5.