Single-phase mixed molybdenum-tungsten carbides: Synthesis, characterization and catalytic activity for toluene conversion

Abstract

A series of single-phase mixed metal carbides of molybdenum and tungsten (Mo2-xWxC, 0 < x < 2) was synthesized. Precursors with intimately mixed metals were prepared by flash-freezing aqueous solutions of molybdenum and tungsten salts in liquid nitrogen and subsequent freeze-drying. The powders of freeze-dried samples had small particles that facilitated reduction and carburization. Direct carburization of freeze-dried precursors at a final temperature of 650 °C resulted in formation of single-phase metal carbides with hexagonal structures and CO adsorption capacities of 15–27 μmol/g. All carbides were catalytically active for conversion of toluene at a total pressure of 21 bar, and a H2:toluene molar ratio of 33. At a reaction temperature of 250 °C, only methylcyclohexane formed at TOFs of 0.13 (Mo0.5W1.5C) ― 2.1 s−1 (Mo2C). Activation energies for ring hydrogenation were in the range of 40–60 kJ/mol. At 400 °C, catalysts deactivated for 24 h before stabilizing. At steady state, methylcyclohexane and small alkanes formed via ring hydrogenation and hydrogenolysis, indicating the presence of sites with metallic character. Disproportionation of toluene to benzene and xylenes indicated the presence of weakly acidic sites. An excess of benzene was attributed to the presence of carbon vacancies on the surface, which formed more readily in the tungsten-containing carbides.