Metal oxides as selective hydrogen combustion (SHC) catalysts and their potential in light paraffin dehydrogenation

An alternative approach to light paraffin dehydrogenation (thermodynamically limited) and oxydehydrogenation (thermodynamically not limited) is a combination process of dehydrogenation (DH) with selective hydrogen combustion (SHC). By selectively combusting the hydrogen produced in the DH reaction, the overall combination process becomes thermodynamically not limited. While dehydrogenation is commercially practiced, the olefin yields are equilibrium limited, and the process is endothermic, requiring large inputs of external heat, and cyclic, requiring frequent catalyst regenerations. Oxydehydrogenation, while not equilibrium limited, suffers from low selectivity at high conversions with currently known catalysts and is therefore not practiced commercially. For these reasons we have launched a study of DH-SHC, The first task is the discovery and identification of good SHC catalysts, which are reported here. We have discovered that certain metal oxides oxidize hydrogen with great preference to propane and propylene, making them good candidates for an eventual DH-SHC process. Among the best are Bi2O3 (>99% sel.), Bi2Mo3O12 (99% sel.), and In2Mo3O12 (98.4% sel,). In stark contrast to these compositions stands V2O5, which attacks hydrocarbons with great preference to hydrogen, resulting in a hydrogen oxidation selectivity of only 6.1%. It is noteworthy that all of the good SHC catalysts contain elements which have a lone pair of electrons in their prevailing oxidation state at the start of the reaction (redox mode) or under steady-state operating conditions (cofed mode). The experiments were carried out at atmospheric pressure and in the range of 500 degrees C with gravimetric methods using a Cahn balance, and redox and cofed methods using an automated microreactor. The best SHC compositions identified in this study constitute promising candidates for a proposed DH-SHC process and their viability in this context is reported in a separate study (1). (C) 1999 Academic Press.