In this research project, we have tried to develop new catalysts, which can covert small alkane molecules such as methane, ethane and propane into higher hydrocarbons or aromatic compounds. First of all, we have investigated CHィイD24ィエD2-CO reaction over silica supported group VIII metal catalysts using closed gas circulation system, and found that benzene is formed selectively among hydrocarbons. Dependence of the initial rates of CHィイD24ィエD2-CO reactions as well as CHィイD24ィエD2 coupling and CO disproportionation upon the particle sizes of Rh was investigated by changing the loading amount of Rh on silica. The TOFs for benzene formation in CHィイD24ィエD2-CO reaction and CィイD22ィエD2 formation in CHィイD24ィエD2 coupling were increased abruptly by decreasing the loading amount of Rh. On the contrary, the TOF of COィイD22ィエD2 formation by CO disproportionation was decreased by decreasing the loading. These results suggest that smaller Rh particle may enhance the dissociation of methane C-H bond to supply surface hydrogen for benzene formation. We have also investigated the addition effect of rear-earth oxides to Rh/SiOィイD22ィエD2 catalysts, and found that NdィイD22ィエD2OィイD23ィエD2 is the effective additives for benzene acceleration for more than ten times.
On the other hand, we have constructed conventional flow reactor system and studied the methane coupling reaction over molybdenum carbides as well as supported group VIII metal catalysts. In the case of MoィイD22ィエD2C catalysts, benzene can be formed with the 10-20% conversions and about 60% selectivity. In the cases of silica supported Co, Ni, Ru, Pd, Os, Ir and Pt catalysts, small amount of benzene was detected over Rh/SiOィイD22ィエD2, Ir/SiOィイD22ィエD2 and Pt/SiOィイD22ィエD2 catalysts. However, the deactivation of the catalytic activity was so severe in any catalysts maybe because of the carbon accumulation. Further improvement of the catalytic systems are required for practical use of these catalysts.