Study of Catalysts and Mechanisms in Synthesis Reactions: Progress Report, January 1989-December 1989.

摘要

The role of surface-generated gas phase radicals is best illustrated in the catalytic oxidative dimerization of methane to ethane. We previously demonstrated that methyl radicals, formed on the surface of lithium-promoted magnesium oxide (Li(+)/MgO) and sodium- promoted calcium oxide (Li(+)/CaO) emanate into the gas phase where they couple to form ethane, which in turn is dehydrogenated to ethylene. The reactions of radicals with metal oxide surfaces is a largely unexplored area of surface chemistry, therefore it was of interest to determine first the relative activities of metal oxides with methyl radicals and ultimately the specific activities of selected oxides. Moreover, it was found that a very reactive surface could be modified by the addition of an alkali metal carbonate, and that such modifications caused a profound effect on the catalytic properties of the oxide. Other systems, such as NaMnO4/MgO have been examined to determine whether the coupling even occurs in the gas phase. This mixed-metal oxide is a selective oxidative dimerization catalyst, but it is ineffective in generating gas phase radicals. As a complementary technique to the matrix-isolation electron spin resonance (MIESR) system that was developed in our laboratory, a laser-induced fluorescence (LIF) spectrometer is being employed to study OH radicals that are formed during the oxidation of CH4. In addition to coupling, the surface-generated CH3 radicals become involved in chain-branching reactions which yield OH radicals. An extended goal of the project is to determine the vibrational and rotational temperatures of the radicals relative to the temperature of the surface.