Oxidative conversion of LPG to olefins with mixed oxide catalysts: Surface chemistry and reactions network

The catalyic performance of three mixed oxide catalytic systems V-Mo-, V-Mg and RE-Li-Halogen (RLH) in LPG oxidative conversion was measured at different O₂/LPG ratios, temperatures and WHSV. At high LPG conversions V-Mo-based catalysts yielded low olefins selectivity and high LPG combustion (CB), V-Mg - medium olefins selectivity by oxidative dehydrogenation (ODH) route and medium LPG CB selectivity, while RLH catalysts displayed high olefins selectivity by ODH and cracking (CR) routes at low CB. TP-reaction experiments and the effects of oxygen partial pressure on catalytic performance indicated a dynamic interaction of surface oxygen in the ODH, CB and CR routes. ESCA and TPD measurements detected three types of surface oxygen with different nucleophility and bonding strength. Their distribution correlated with LPG conversion selectivities. A correlation between catalysts acidity, the surface exposed metal cations concentration and the productivity by the CR route was derived. The surface basicity was also significant in olefins productivity by the ODH and CR routes. The selectivity of LPG oxidative reactions were attributed to different intermediates formed on the surface as a result of interaction of C₃-C₄ paraffins with oxygen atoms of different nucleophility. Both the redox balance of surface metal cations and the acidity-basicity balance are proposed to be significant.