Methane homologation by the two-step cycle on Co catalysts

Conversion of natural gas to liquid hydrocarbons upgrades a low density fuel to a valuable source of chemicals and liquid fuel. To eliminate the expensive intermediate step of methane-steam reforming in the commercial Fischer-Tropsch, methanol to gasoline and Shell middle distillate synthesis processes, a direct method of CH{dollar}sb4{dollar} conversion to higher hydrocarbons is very desirable. In direct conversion of CH{dollar}sb4{dollar} to higher hydrocarbons in the presence of O{dollar}sb2{dollar} (e.g. oxidative coupling and partial oxidation), deep oxidation of CH{dollar}sb4{dollar} to CO and CO{dollar}sb2{dollar} is a major drawback. In the two-step homologation of CH{dollar}sb4{dollar} in the absence of O{dollar}sb2,{dollar} CH{dollar}sb4{dollar} is first activated on a reduced transition metal catalyst at high temperature (e.g. 450{dollar}spcirc{dollar}C) to produce H{dollar}sb2{dollar} and carbon species on the catalyst. The carbon species are then hydrogenated in the second step at a lower temperature (e.g. 100{dollar}spcirc{dollar}C) to produce CH{dollar}sb4{dollar} and higher hydrocarbons.; In the present study of the two-step homologation of CH{dollar}sb4,{dollar} SiO{dollar}sb2{dollar} supported Co catalysts were prepared by incipient impregnation. The catalysts were characterized by BET surface area and pore volume measurement, powder X-ray diffraction, temperature programmed reduction, H{dollar}sb2{dollar} chemisorption and Co re-oxidation. Carbon species deposited in the activation step were recovered by isothermal hydrogenation at 100{dollar}spcirc{dollar}C, temperature programmed surface reaction and temperature programmed oxidation to account for the reactivity of different carbon species.; The effect of catalyst loading, activation time, activation temperature, carbon aging, reaction cycle and isothermal medium on both the CH{dollar}sb4{dollar} activation step and the isothermal hydrogenation to C{dollar}sb{lcub}2+{rcub}{dollar} hydrocarbons, were studied.; Based on the findings from deposition of more than a nominal monolayer carbon coverage on the supported metal, a semi-empirical kinetic model for the activation of CH{dollar}sb4{dollar} on Co-SiO{dollar}sb2{dollar} catalysts was developed. In the kinetic model, gas phase CH{dollar}sb4{dollar} is first activated on Co to produce adsorbed H and CH{dollar}sb3{dollar} species. Migration of some of the CH{dollar}sb3{dollar} species from the metal to the support liberates Co sites for further reaction. H{dollar}sb2{dollar} is generated by further dehydrogenation of CH{dollar}sb3{dollar} species on the metal and support, and desorption of adsorbed H.; The kinetic model and rate constants of different steps were used to interpret the effect of changes in operating conditions on the rate of different steps of the CH{dollar}sb4{dollar} activation reaction. Metal-support interactions in the Co-SiO{dollar}sb2{dollar} system play an important role in CH{dollar}sb4{dollar} activation and in determining the activity of carbon species.; With more than a nominal monolayer coverage of metal by carbon, a considerable amount of inactive carbon, which can only be removed by high temperature oxidation, is produced on the support. Hydrogen content and age of the carbon species were among the important factors affecting C{dollar}sb{lcub}2+{rcub}{dollar} production in isothermal hydrogenation. It was shown that C-C bond formation occurs to a great extent before the isothermal hydrogenation step.