Selective Direct Synthesis of Ethane from Methane with Carbon Dioxide at Low Temperatures

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The present project focuses on a novel method to convert CH_4 directly to C_2H_6 including C_2H_4 as a secondary product at low temperatures by utilizing CO_2 as an oxidant. When Pr and TB oxides are used under ambient pressure with a fixed- bed quartz reactor, the presence of CO_2 remarkably enhances the selective conversion of CH_4 to C_2H_6 with a formation of almost the same amount of CO from CO_2 at 450-650℃. Yield of C_2H_6 increases with increasing W/F and reaches 8 mol% at the highest, though their catalytic activity is unstable. It is likely that oxygen vacancies in these oxides play key roles in this low -temperature reaction.In order to develop a composite oxide catalyst with high and stable performance, CeO_2, Cr_2O_3, orMnO_2 with high redox potential is modified with CaO with strong basicity by impregnating each oxide powder with Ca(NO_3) solution, followed air calcination .All of these binary catalysts show remarkable synergistic effects on C_2 formation at 850℃. C_2 selectivity and C_2 yield increase with increasing partial pressure of CO_2 , irrespective of the kind of catalyst, and the selectivity reaches 65-75% at 70 kPa. These catalysts provide stable performances with time on stream of 8 - 10 h. The TPD, XRD and XPS measurements strongly suggest that CO_2 first adsorbs on Ca^<2+> sites and the activation subsequently occurs on neighboring Ce^<3+>, Cr^<3+> or Mn^<2+> sites to yield active oxygen species, which work as the oxidant for selective formation of C_2 hydrocarbons.In may be expectable that the present achievements develop into a direct conversion process of low- grade natural gas containing a large amount of Co_2 to valuable chemical feedstock.