Reactions of neopentane, methylcyclohexane, and 3,3-dimethylpentane on tungsten carbides: The effect of surface oxygen on reaction pathways

Abstract

High surface area tungsten carbides with WC and β-W 2C structure were prepared by direct carburization of W0 3 in CH 4H 2 mixtures. Their surfaces appear devoid of excess polymeric carbon and adsorb between 0.2 and 0.4 monolayers of CO and H. These materials are very active in neopentane hydrogenolysis. Chemisorbed oxygen inhibits hydrogenolysis reactions and leads to the appearance of isopentane among the reaction products. Neopentane isomerization to isopentane occurs only on Pt, Ir, and An surfaces. Thus, oxygen-exposed tungsten carbides catalyze reactions characteristic of noble metal catalysts. 3,3-Dimethylpentane isomerizes much faster than neopentane on oxygen-exposed carbides; the isomer distribution suggests that isomerization proceeds via a methyl shift mechanism rather than through the C 5-ring hydrogenolysis pathways characteristic of highly dispersed Pt. The apparent involvement of 3,3-dimethyl-l-pentene reactive intermediates is consistent with carbenium-type methyl shift pathways. Secondary carbon atoms, capable of forming stable carbenium ions, are present in 3,3-dimethylpentane but not in neopentane; they account for the high 3,3-dimethylpentane isomerization rate and selectivity on oxygen-exposed tungsten carbide powders. Both dehydrogenation and isomerization reactions of methylcyclohexane occur on these carbide powders. These results suggest the presence of a bifunctional surface that catalyzes dehydrogenation and carbenium ion reactions typically occurring on reforming catalysts.