CHARACTERIZATION, ACTIVITY, AND ADSORPTION-DESORPTION BEHAVIOR OF ALKALI-PROMOTED MOLYBDATE CATALYSTS FOR THE OXIDATIVE COUPLING OF METHANE

MnMoO4 and alkali-promoted (Li, Na, or K) MnMoO4 catalysts have been examined for the methane oxidative coupling reaction. The catalysts were characterized by XRD, XPS, LRS, TPR, and TPD. The molybdate structure appeared to remain intact, although the presence of the promoter ions was found to change the TPR and TPD profiles. Increased selectivity to C2 hydrocarbons has been observed for similar conversion levels (Driscoll, Zhang, and Ozkan, in ''Catalytic Selective Oxidation'' (S. T. Oyama and J. W. Hightower, Eds.), Vol. 523, p. 340. ACS, Washington, DC, 1993) through the addition of the alkali-promoter ions, especially potassium. Further examination of the catalysts at equal residence times revealed an increased activity due to the addition of the alkali-promoter ions, with no change in activity or selectivity when excess carbon dioxide was added to the feed. While the TPD experiments provided clues as to the nature and function of surface oxygen, laser Raman spectroscopy studies provided evidence of the existence of carbonyl groups on the surface. (C) 1994 Academic Press, Inc.