Molybdenum Carbide Nanoparticles in Silicalite-1 Zeolite for CO2-Assisted Propane Dehydrogenation

Abstract

Molybdenum carbides (MoC_x) have been shown to be effective catalysts in a number of challenging industrial-relevant reactions. Their promising performance combined with their natural abundance and low cost make them valuable alternatives to the use of noble metals. However, an often too limited exposure of active sites in micrometer-sized MoC_x particles combined with their strong acid features depletes the ultimate catalytic performance for C–H bond activation in alkane dehydrogenation. In this work, we have synthesized highly dispersed molybdenum carbide nanoparticles (NPs) embedded into the framework of silicalite-1 zeolite (MoC_x/Si-1) to be employed in direct and/or CO₂-assisted propane dehydrogenation reaction. The as-synthesized MoC_x/Si-1 exhibits enhanced CO₂-assisted oxidative propane dehydrogenation (CO₂-ODHP) performance with rates up to 172.8 μmol g_cat^–1 min^–1 with C₃H₆ selectivity > 94% (61.5% C₃H₆ selectivity for bulk β-Mo₂C), which is attributed to the moderate catalyst acidity, better H-spillover property, and the appropriate propane adsorption or propylene desorption mechanisms on such micropore systems. Extensive characterization data prove that a high concentration of exposed molybdenum oxycarbide (MoC_xO_y) sites was crucial for CO₂-ODHP. Propane is converted on MoC_xO_y active sites to propylene and H₂O, while the MoC_xO_y are reduced into MoC_x sites. Subsequently, the reduced MoC_x sites are reoxidized by CO₂ into MoC_xO_y active sites and CO gas. This work designs molybdenum carbide nanoparticles embedded in Si-1 zeolite, demonstrates high activity and propylene selectivity in CO₂-assisted propane dehydrogenation, and provides guidance for the application of carbides in alkane dehydrogenation.