Nanostructured Catalysts in Activation of Light Alkanes

The activation of small alkane molecules originating from natural gas or renewable resources on metal oxide surfaces regains increasing scientific interest since direct utilization of saturated hydrocarbons will offer alternative pathways to higher value chemicals in a postpetroleum era. The focus of the project lies on oxidation catalysis of small alkane molecules including methane, ethane, propane and butane. Oxidative dehydrogenation of alkanes to the corresponding olefin or selective oxidation to functionalized monomers, like acrylic acid or maleic anhydride, generally suffer from insufficient selectivity due to very complex reaction networks that involve consecutive and parallel reactions leading to carbon oxides as the result of deep oxidation of intermediates or products. The core issue in the project addresses, therefore, the limits of selectivity in oxidation catalysis. Is the activation of C-H bonds in an alkane molecule compatible with high selectivity, or, in other words, are the same active sites or catalyst components responsible for alkane activation, oxygen insertion and total combustion or will it be possible to avoid consecutive reactions by intelligent catalyst design? What are the critical reaction parameters that determine the selectivity? We address these questions by following up a comprehensive approach that involves controlled synthesis of well-defined catalysts, detailed kinetic analysis of reaction networks, and the investigation of the molecular nature of the surface considering also collective properties of the solid state, like crystal and electronic structure of the catalyst under reaction conditions. From the materials point of view, we concentrate on vanadium and molybdenum oxides, which are synthesized as crystalline bulk catalysts or supported monolayers on meso-structured silica as an essentially inert support. Molybdenum and vanadium carbides and nano-structured carbons are included for the purpose of reference. In the oxidative coupling of methane, doped and non-doped alkaline earth oxides have been investigated as model catalysts. Highlights of our research are summarized in the following: