Ind. Eng. Chem. Res. 1997, 36, 553-558
553
Membrane-Assisted Two-Step Process for Methane Conversion into Hydrogen and Higher Hydrocarbons Olivier Garnier, Jun Shu, and Bernard P. A. Grandjean* Department of Chemical Engineering and CERPIC, Laval University, Quebec, Canada G1K 7P4
In this work, methane conversion was carried out over a group VIII metal in a Pd-Ag membrane reactor to produce hydrogen and higher hydrocarbons in a nonoxidative atmosphere. A twostep reaction sequence operated at two different temperatures was adopted to pass around the thermodynamic barrier of low-temperature methane conversion. Methane was first activated as a result of the dissociative deposition over a Ru catalyst, followed by rehydrogenation to form higher hydrocarbons. The use of the Pd-Ag membrane reactor significantly enhanced the methane conversion into hydrogen and carbonaceous species in the first step and the yield of higher hydrocarbons upon rehydrogenation in the second step. The repartition of different carbonaceous species was monitored by temperature-programed surface reaction (TPSR). The effective methane conversion at low temperatures in the membrane reactor favored the formation of CR species which were selective to produce higher hydrocarbons. A C2+ yield of about 16% was obtained by conducting methane decomposition at 300 °C and rehydrogenation at 100-120 °C in the Pd-Ag membrane reactor. Introduction Methane conversion into higher hydrocarbons has received growing interest in order to valorize the copious natural gas reserve. Currently, only a small portion of it (7%) is used as a chemical feedstock to produce syngas which can be further converted into more valuable petrochemicals via methanol conversion or FischerTropsch synthesis. Though successfully industrialized, these indirect processes are generally energy intensive and costly. On the other hand, a dramatic increase of hydrogen production is expected with the demand for lighter and cleaner products in refineries and the new utilization of hydrogen as fuel for fuel-cell vehicle applications. As natural gas is still the cheapest source for the production of hydrogen at the present time, direct conversion of methane into hydrogen and higher hydrocarbons is potentially more economical than indirect processes (Steinberg and Cheng, 1987). However, the direct conversion of methane into hydrogen and higher hydrocarbons is thermodynamically unfavorable due to its highly stable molecular configuration. Temperatures in excess of 1200 °C are required to obtain a practical level of conversion of methane into ethylene (Figure 1), while the direct conversion of methane into ethane is thermodynamically forbidden due to a positive change of Gibbs free energy for temperature below 1223 °C. However, it has long been recognized that methane can be dissociatively chemisorbed on many transition metals, accompanied by hydrogen release (Frennet, 1974; Kemball, 1959). With the use of group VIII metal catalysts, the complete conversion of methane into deposited carbon and hydrogen could be reached at 800 °C. In 1962, Universal Oil Products developed a process named Hypro in order to produce hydrogen for refinery uses from methane at 870 °C over 7 wt % Ni/Al2O3 (Pohlenz and Stine, 1962). The carbon deposited on the catalyst was then burned to recover the catalyst and to produce the energy needed for the methane decomposition. In the early 1990s, Koerts et al. (1991, 1992) practiced a two-step process * Author to whom correspondence should be addressed. E-mail:
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Figure 1. Gibbs free energy change as a function of temperature at 1 bar for reactions relevant to the two-step methane conversion process.
to overcome the thermodynamic barrier of the direct formation of low alkanes from methane: methane was first activated on such supported transition-metal catalysts as Co, Ru, and Rh to produce hydrogen and carbonaceous intermediates between 175 and 527 °C, followed by a rehydrogenation of those surface species, leading to the formation of higher hydrocarbons up to C5 at a temperature range of 27-127 °C. Thus, they achieved a 100% conversion of methane in the first step at a long contact time and a total C2+ yield of 13% on 5 wt % Ru/SiO2 upon rehydrogenation at a low temperature. As can be seen from Figure 1, the former step is thermodynamically favored at high temperatures (>550 °C) whereas the latter at low temperatures (
