Control of catalyst performance in selective oxidation oflight hydrocarbons: Catalyst design and operational conditions

Abstract

This paper is an attempt to summarize the situation with respect to the selective catalyticoxidation of light alkanes using heterogeneous catalysts. Methane oxidation reactions and the oxidation of butane to maleic anhydride will only be alluded to occasionally, because they have been reviewed in detail in a large number of papers. We shall first show that it is still far from clear which are the families of catalysts to be used for the various reactions: mainly oxidative dehydrogenation or oxidation to oxygen-containing molecules of ethane, propane or isobutane. Much research is still necessary for understanding the mechanisms leading to high selectivity. In this context, we shall suggest that many concepts inherited from the development in selective oxidation and ammoxidation of olefins are probably of little use. Conversely, much emphasis has to be laid on new data which opens promising perspectives, namely (i) the occurrence of cooperation effects between two (or several) separate phases and especially the role of spillover oxygen and the so-called “remote control” and (ii) the occurrence of homogeneous non-catalysed reactions which occur at temperatures only slightly higher than the catalytic ones and correspond to similar selectivities. This suggests that research on selective catalytic oxidation, to be effective, should be comprehensive; it should continue to involve a search for new active phases and efforts to improve the already known catalysts. But research should also include investigations on the role of spillover oxygen, the nature of this oxygen (more or less electrophilic), the donors that can generate it, and the way this spillover oxygen reacts with the catalytic surface. Research should also contemplate the problem of how homogeneous and heterogeneous reactions proceed simultaneously or consecutively. In parallel with these research lines, chemical engineering must develop new concepts and new reactors. Recent spectacular results in methane coupling or oxidative dehydrogenations show that considerable progress can be made if the problem of light alkane selective oxidation benefits from a multifacetted approach.