1Faculty of Chemical Engineering, Isfahan University of Technology, P.O. Box 84156-83111 Isfahan, I.R. IRAN
2Catalyst Department, National Petrochemical Company, Research & Technology (NPC-RT), Tehran, I.R. IRAN
The Oxidative Coupling of Methane (OCM) over M-Na-Mn/SiO2 catalysts (M=W, Cr, Nb and V) was investigated using a continuous-flow quartz reactor at 775°C, 1 atm and 100 cm3min-1 gas flow rates, and correlated with the observed structure and redox properties.The interaction effects of the metal-metal and metal-support on the methane conversion and C2+ yield were investigated using X-Ray Diffraction (XRD), laser Raman spectroscopy, Fourier Transform Infrared Spectroscopy (FT-IR), and Temperature Programmed Reduction with H2 (TPR). The results revealed that the improvement of C2+ selectivity (or C2+ yield) follows the order W>Cr>Nb>V, while the catalytic conversion did not change significantly. XRD data indicated that Mn is well dispersed on the SiO2 support and also show that Mn2O3 and α-cristobalite were the predominant species in the surface catalysts. TPR data show that most of the Mn is present as Mn3+ and Mn2+. FT-IR analyses combined with the Raman results show that terminal M=O and bridging M–O–M species and the metal–metal and metal–support interactions, which take place due to the presence of sodium ion, depend on the transition metal that affect the catalyst performance. Results reveal that the interaction between metal oxide and sodium is required for high selectivity and control redox mechanism in transition metal oxide in OCM reaction.
 Fang X.L.S.L. J., Gu J., Yang D., Preparation and Characterization of W-Mn Catalyst for Oxidative Coupling of Methane. Chin. J. Mol. Catal., 6, p. 255 (1992).
 Kou Y. et al., Amorphous Features of Working Catalysts: XAFS and XPS Characterization of Mn/Na2WO4/SiO2 as Used for the Oxidative Coupling of Methane, Journal of Catalysis, 173, p. 399 (1998).
 Dingjun Wang, M.P.R., Jack H. Lunsford, Oxidative Coupling of Methane over Oxide-Supported Sodium-Manganese Catalysts, Journal of Catalysis, 155, p. 390 (1995).
 Sheng-fu Ji, T.-c.X., Shu-ben Li, Chuan-zhi Xu, Rui-ling Hou, Karl S. Coleman, Malcolm L.H. Green, The Relationship Between the Structure and the Performance of Na-W-Mn/SiO2 Catalysts for the Oxidative Coupling of Methane, Applied Catalysis A: General, 225, p. 271 (2002).
 Alejandra Palermo J.P.H.V., Adam F. Lee, Mintcho S. Tikhov, Richard M. Lambert, Critical Influence of the Amorphous Silica-to-Cristobalite Phase Transition on the Performance of Mn/Na2WO4/SiO2 Catalysts for the Oxidative Coupling of Methane. Journal of Catalysis, 177, p. 259 (1998).
 Utiyama M., Hattori H., Tanabe K., Exchange Reaction of Methane with Deuterium over Solid Base Catalysts, Journal of Catalysis, 53, p. 273 (1987).
 Effendi Widjaja J.T.S., Xuan Ding Han, Eunice Goh, Use of Raman Microscopy and Band-Target Entropy Minimization Technique to Differentiate Physical Mixture from Chemical Mixture in Mixed Metal Oxides, Catalysis Today, 131, p. 21 (2008).
 A. Klisin´ska S.L., Grzybowska B., Stoch a J., GresselI., Effect of Additives on Properties of V2O5/SiO2 and V2O5/MgO Catalysts II. Structure and Physicochemical Properties of the Catalysts and Their Correlations with Oxidative Dehydrogenation of Propane and Ethane, Applied Catalysis A: General, 309, p. 17 (2006).
 M.O. Guerrero-Pe´rez, M.C.H., I. Malpartida , M.A. Larrubia , L.J. Alemany , M.A. Ban˜ares, Operando Raman Study of Propane Oxidation over Alumina-Supported V-Mo-W–O Catalysts, Catalysis Today, 126, p. 177 (2007).
 Shengfu Ji, T.X., Shuben Li, Lingjun Chou, Bing Zhang, Chuanzhi Xu, Ruiling Hou, Andrew P.E. York, Malcolm L.H. Green, Surface WO4 Tetrahedron: the Essence of the Oxidative Coupling of Methane over M–W–Mn/SiO2 Catalysts, Journal of Catalysis, 220, p. 47 2003.
 Florina Buciuman, F.P., Radu Craciun, Dietrich R.T. Zahn, Vibrational Spectroscopy of Bulk and Supported Manganese Oxides, Phys. Chem. Chem. Phys., 1, p. 185 (1999).
 Maria Ziolek, I.S., Anna Lewandowska, Izabela Nowak, Piotr Decyk, Monika Renn, Beata Jankowska, Oxidative Properties of Niobium-Containing Mesoporous Silica Catalysts Catalysis Today, 70, p. 169 (2001).
 Arthur Dobley, K.N., Shoufeng Yang, Peter Y. Zavalij, M. Stanley Whittingham, Manganese Vanadium Oxide Nanotubes: Synthesis, Characterization, and Electrochemistry. Chem. Mater., 13, p. 4382 (2001).
 Carlo Resini, T.M., Guido Busca, Jih-Mirn Jehng, Israel E. Wachs, Comparison of Alcohol and Alkane Oxidative Dehydrogenation Reactions over Supported Vanadium Oxide Catalysts: in Situ Infrared, Raman and UV–Vis Spectroscopic Studies of Surface Alkoxide Intermediates and of Their Surface Chemistry, Catalysis Today, 99, p. 105 (2005).
 A. Malekzadeh, A.K., A. K. Dalai, M. Abedini, Oxidative Coupling of Methane over Lithium Doped (Mn+W)/SiO2 Catalysts, Journal of Natural Gas Chemistry, 16, p. 121 (2007).
 E.R. Stobbe, B.A.d.B., J.W. Geus, The Reduction and Oxidation Behaviour of Manganese Oxides, Catalysis Today, 47, p. 161 (1999).
 Maria Ziolek, I.S., Anna Lewandowska, Izabela Nowak, Piotr Decyk, Monika Renn, Beata Jankowska, Oxidative properties of Niobium-Containing Mesoporous Silica Catalysts, Catalysis Today, 70, p. 169 (2001).