| Summary: | 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 refonning in the commercial Fischer-Tropsch, methanol to gasoline and Shell
middle distillate synthesis processes, a direct method of CH₄ conversion to higher
hydrocarbons is very desirable. In direct conversion of CH₄ to higher hydrocarbons in the
presence of O₂ (e.g. oxidative coupling and partial oxidation), deep oxidation of CH₄ to CO
and CO₂ is a major drawback. In the two-step homologation of CH₄ in the absence of O₂, CH₄,
is first activated on a reduced transition metal catalyst at high temperature (e.g. 450 °C) to
produce H₂ and carbon species on the catalyst. The carbon species are then hydrogenated in
the second step at a lower temperature (e.g. 100 °C) to produce CH₄ and higher
hydrocarbons.
In the present study of the two-step homologation of CH₄, Si0₂ 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, H₂ chemisorption and Co re-oxidation. Carbon species deposited in
the activation step were recovered by isothermal hydrogenation at 100 °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 CH₄, activation step and the isothermal
hydrogenation to C₂₊ 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 CH₄ on
Co-SiO₂ catalysts was developed. In the kinetic model, gas phase CH₄ is first activated on Co
to produce adsorbed H and CH₃ species. Migration of some of the CH₃ species from the metal
to the support liberates Co sites for further reaction. H₂ is generated by further
dehydrogenation of CH₃ 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 CH₄ activation reaction.
Metal-support interactions in the Co-Si0₂ system play an important role in CH₄ 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 C₂₊ production in isothermal hydrogenation. It was shown that
C-C bond formation occurs to a great extent before the isothermal hydrogenation step.
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