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

An alternative approach to light paraffin dehydrogenation (thermodynamicall y limited) and oxydehydrogenation (thermodynamically not limited) is a comb ination 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. Whil e dehydrogenation is commercially practiced, the olefin yields are equilibr ium limited, and the process is endothermic, requiring large inputs of exte rnal heat, and cyclic, requiring frequent catalyst regenerations. Oxydehydr ogenation, while not equilibrium limited, suffers from low selectivity at h igh conversions with currently known catalysts and is therefore not practic ed 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 her e. We have discovered that certain metal oxides oxidize hydrogen with great preference to propane and propylene, making them good candidates for an ev entual 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 notewort hy that all of the good SHC catalysts contain elements which have a lone pa ir of electrons in their prevailing oxidation state at the start of the rea ction (redox mode) or under steady-state operating conditions (cofed mode). The experiments were carried out at atmospheric pressure and in the range o f 500 degrees C with gravimetric methods using a Cahn balance, and redox an d cofed methods using an automated microreactor. The best SHC compositions identified in this study constitute promising can didates for a proposed DH-SHC process and their viability in this context i s reported in a separate study (1). (C) 1999 Academic Press.