Stepwise conversion of methane over supported metal-boron amorphous alloy catalysts

The stepwise conversion of CH₄ to higher hydrocarbons over HMCM-22 zeolite supported metal-boron amorphous alloy catalysts has been investigated, including: (1) the influence of metals (Co, Ni, Pt, Rh and Ru) of the catalysts on the reaction; (2) the promotional effect of V on the catalytic behavior of the catalysts; (3) the influence of hydrogenation pressure and CH₄ decomposition temperature; and (4) the nature of carbon species. It is found that the yield of C₂⁺ hydrocarbons is strongly dependent on the metals. Good yields of C₂⁺ hydrocarbon are reached only on the supported NiB and CoB catalysts. The probability of C-C chain growth is increased by V promotion without seriously affecting the activities of CH₄ decomposition and hydrogenation. The ease of carbon removal via hydrogenation is strongly affected by the CH₄ decomposition temperature. Increasing hydrogenation temperature has a negative effect on the yield of C₂⁺ hydrocarbons. XPS measurements show that a carbide-(like) carbon species is active and selective for hydrogenation to produce higher hydrocarbons. Its activity/selectivity is greatly reduced at high CH₄ decomposition temperatures, mainly due to transition of the reactive carbidic to un-reactive graphitic form. Graphite/filamental carbons were found to be formed at high CH₄ decomposition temperature.