Tandem Catalysis for Selective Oxidation of Methane to Oxygenates Using Oxygen over PdCu/Zeolite

Abstract

Selective oxidation of methane to oxygenates with O2 under mild conditions remains a great challenge. Here we report a ZSM‐5 (Z‐5) supported PdCu bimetallic catalyst (PdCu/Z‐5) for methane conversion to oxygenates by reacting with O2 in the presence of H2 at low temperature (120 °C). Benefiting from the co‐existence of PdO nanoparticles and Cu single atoms via tandem catalysis, the PdCu/Z‐5 catalyst exhibited a high oxygenates yield of 1178 mmol g−1Pd h−1 (mmol of oxygenates per gram Pd per hour) and at the same time high oxygenates selectivity of up to 95 %. Control experiments and mechanistic studies revealed that PdO nanoparticles promoted the in situ generation of H2O2 from O2 and H2, while Cu single atoms not only accelerated the activation of H2O2 for the generation of abundant hydroxyl radicals (⋅OH) from H2O2 decomposition, but also enabled the homolytic cleavage of CH4 by ⋅OH to methyl radicals (⋅CH3). Subsequently, the ⋅OH reacted quickly with the ⋅CH3 to form CH3OH with high selectivity.

The selective oxidation of methane to methanol using in situ generated H2O2 over palladium-based bimetallic catalysts

“…Bimetallic AuPd catalysts, known to offer high activity towards H 2 O 2 production, 13 have been widely studied for a range of oxidative transformations via in situ production of the oxidant, including for methane valorisation. [14][15][16][17][18][19] Recently, in an attempt to overcome reagent diffusion limitations and improve oxidant utilisation Jin et al investigated the modification of the external surface of a AuPd@ZSM-5 catalyst, with a hydrophobic organosilane layer, which was found to both promote the localised concentration of reagents near active sites and confine the synthesised H 2 O 2 near the AuPd nanoparticles for subsequent methane activation. 20 In contrast to the reaction mechanism proposed for CuFe-ZSM-5 materials 21 (another class of materials widely studied for methane oxidation when used in conjunction with H 2 O 2 ), AuPd catalysed methane oxidation using H 2 O 2 has been shown to proceed via the activation of the methane C-H bond through a hydrogen abstraction pathway, which is mediated by reactive oxygen species (ROS, ˙OOH, ˙OH and ˙O2 − ), which are generated from H 2 O 2 over AuPd surfaces, and the resulting formation of a methyl radical (˙CH 3 ).…”