MoO₃/Al₂O₃ catalysts for chemical-looping oxidative dehydrogenation of ethane

MoO₃/γ-Al₂O₃ catalysts containing 0.3-3 monolayer (ML) equivalents of MoO₃ were prepared, characterized, and tested for ethane oxidative dehydrogenation (ODH) in cyclic redox and co-feed modes. Submonolayer catalysts contain highly dispersed (2D) polymolybdate structures; a complete monolayer and bulk Al₂(MoO₄)₃ are present at >1ML loadings. High ethylene selectivity (>90%) in chemical looping (CL) ODH correlates with Mo^+VI to Mo^+V reduction; CO_x selectivity is <10% under these conditions. Mo^+V and Mo^+IV species trigger CH₄ production resulting in much higher conversion albeit with <20% selectivity. In CL-ODH, submonolayer catalysts exhibit ethylene selectivities that decrease linearly from 96% at near-zero conversion to 70% at 45% conversion. >1ML catalysts provide higher conversions albeit with 10%-18% lower selectivity and greater selectivity loss with increasing conversion. In co-feed mode, ethylene selectivity drops to <50% at 46% conversion for a 0.6ML catalyst, but selectivity is virtually unaltered for a 3ML catalyst. We infer that at <1ML loadings, small domain size and strong Mo-O-Al bonds decrease 2D polymolybdate reducibility and enhance ethylene selectivity in CL-ODH.