Highly selective molybdenum-based catalysts for ring hydrogenation and contraction

The reduction and carburization of oxides of molybdenum, tungsten, niobium, and selected mixed oxides of the same elements in H-2/hydrocarbon atmospheres were investigated with the goal of generating non-noble metal catalysts capable of aromatic ring hydrogenation and contraction. Toluene served as a surrogate for heavy aromatics and as a diagnostic reactant for the nature of the surface sites. At 400 degrees C, a H-2:toluene molar ratio of 35:1, and a total pressure of 21 bar, all oxides except Nb2O5 were converted into active catalysts that produced methyl-cyclohexane, ethylcyclopentane, dimethylcyclopentane, benzene and xylene, and C1-C6 alkanes. Increasing hydrogenolysis selectivity with time on stream indicated slow carbide formation. At 350 degrees C, MoO3 was converted into a stable catalyst with about 35% and 55% carbon selectivity to methylcyclohexane and ring contraction products, respectively, and little C1-C6 formation. Selectivity was largely invariant to conversion. Post-reaction analysis of this material by powder diffraction indicated a mixture of MoO2 and a second phase characterized by cubic symmetry, which, while the sample contained a small amount of carbon, was best described as oxidic with a metal-to-oxygen ratio of roughly 1:2. Oxides containing two metal cations showed shorter induction periods and better long-term stability than MoO3, revealing further potential for optimization of this type of catalyst.