Brief Review of Catalytic Oxidative Coupling of Methane to Ethane and Ethene

Evan J. Granite
National Energy Technology Laboratory
Pittsburgh, PA 15236-0940

Abstract

Methane, the simplest hydrocarbon, is the major constituent of natural gas. Much of the methane produced worldwide is flared due to lack of pipelines. The prime use of methane is as a fuel for home heating and cooking, as well as electricity production. Because of its abundance, methane has the potential of becoming a major feedstock for petrochemical production. The problem is that methane is a stable molecule, and the direct conversion to useful chemicals is difficult.
In 1982 Keller and Bhasin1 reported that many metal oxides can catalyze the oxidative coupling of methane to the more valuable chemicals ethane and ethene. In the presence of the catalyst, methane reacts with oxygen and forms the higher hydrocarbons ethane and ethene during oxidative coupling, shown in reactions (1) and (2) below:
2 CH4 (gas) + ½ O2 (gas) â?? C2H6 (gas) + H2O(gas) (1)
2 CH4 (gas) + O2 (gas) â?? C2H4 (gas) + 2 H2O(gas) (2)
The catalytic deep oxidation of methane to carbon monoxide and carbon dioxide will also occur, shown as reactions (3) and (4) below:
CH4 (gas) + 3/2 O2 (gas) â?? CO (gas) + 2 H2O (gas) (3) CH4 (gas) + 2 O2 (gas) â?? CO2 (gas) + 2 H2O (gas) (4)
Unfortunately the yields of ethane and ethane obtained by Keller were small (5% maximum total C2 yield) and the process temperature required was very high (800oC). In the ensuing thirty two years, a vigorous search proceeded for an active coupling catalyst2-23.
The first breakthrough occurred in 1985, when Lunsford2 reported 19% C2 yields when Li doped MgO was used as a catalyst. In 1986 Otsuka3,4 reported rather spectacular C2 yields of
30% with LiCl-MnO2 and LiCl - NiO catalysts. In this case however, the catalyst was short lived, suggesting that chlorine in the catalyst is a reagent, and chlorine is known to inhibit the deep oxidation of methane.
Labinger18 suggested in 1988 that there is a fundamental limit on the C2 yield of 30%. The catalysts typically require high operating temperatures of 600 - 800oC. Unfortunately these factors work against commercialization of the catalysts.
In order for an oxidative coupling catalyst to be commercially viable, three criteria must be met18. First a total C2 yield (preferably as the more valuable ethene) of at least 30% is needed. Second the process should operate at lower temperatures to be less energy intensive. Finally the catalyst should have a long life. None of the metal oxide catalysts examined exhibits all of the necessary characteristics to be commercially viable. The discovery of such an oxidative coupling catalyst remains an elusive and sought after prize. The mechanisms by which the catalysts convert methane will be discussed and future research directions will be suggested.

Disclaimer

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

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