Conversion of CO2 and small alkanes to platform chemicals over Mo2C-based catalysts

Abstract

The performance of Mo₂C-based catalysts in CO₂ assisted oxidative dehydrogenation (CO₂-ODH) of ethane was evaluated. Mo₂C on SiO₂ was synthesized via three different techniques: wet impregnation (WI), hybrid nanocrystal technique (HNC) and sol–gel method (SG) and exposed to the same carburization conditions. In terms of characteristic properties, the allotrope composition was the most affected, with the SG sample containing MoO_xC_y and the WI and HNC samples containing β-Mo₂C. The two different allotropes were suggested to follow different reaction pathways, leading to small differences in the catalytic performance. However, overall, all three catalysts showed a decrease in activity (below 6%) and an increase in C₂H₄ selectivity (from 60 to 80 C%) with time on stream (TOS). The deactivation mechanism was suggested to be mainly due to oxidation of the carbide to MoO_x and carbon deposition. Mo₂C was also supported on various metal oxide materials via the wet impregnation technique. Mo₂C supported on Al₂O₃ and ZrO₂ increased initial activity (about 8% C₂H₆ conversion) but a faster deactivation with TOS was observed. Mo₂C/Ga₂O₃ favoured the direct dehydrogenation reaction achieving high C₂H₄ selectivities (above 80 C%), but deactivation with TOS due to carbon deposition was significant. Mo₂C supported on CeO₂ and TiO₂ had lower activity (about 3% C₂H₆ conversion). Oxidation to MoO₂ and carbon deposition is again suggested to be the main deactivation mechanism. H₂ co-feeding, on Mo₂C/SiO₂ and Mo₂C/ZrO₂, increased the stability of the catalysts but C₂H₄ yield was affected (from 5 to 2%). At 17 vol% H₂ co-feeding, Mo₂C/ZrO₂ showed promising catalyst stability over a 20 h period, paralleled by a stable C₂H₄ yield.