CaMn0.9Ti0.1O3 based redox catalysts for chemical looping – Oxidative dehydrogenation of ethane: Effects of Na2MoO4 promoter and degree of reduction on the reaction kinetics

Abstract

Reduction kinetics and stability of 20 wt% Na2MoO4-promoted CaMn0.9Ti0.1O3 were investigated for its applications in Chemical Looping – Oxidative Dehydrogenation (CL-ODH) of ethane, a potential alternative for ethylene production with higher efficiency and lower emissions. The present work reports a kinetics model and parameters for a Na2MoO4-promoted, Ti-doped CaMnO3 (CaMn0.9Ti0.1O3) redox catalyst under H2 and C2H4. A first-order reaction model provides the best fit for the reduction of Na2MoO4/CaMn0.9Ti0.1O3 under H2, while the C2H4 reduction is well described by an Avrami–Erofe’ev model. The activation energy for C2H4 oxidation is approximately three times higher than that for H2 conversion, showing that the activation of C2H4 is significantly more difficult on the surface of the redox catalyst. The reduction rate of Na2MoO4/CaMn0.9Ti0.1O3 under H2 at 750 °C is more than two orders of magnitude greater than that under C2H4, while the reduction rate of unpromoted CaMn0.9Ti0.1O3 is comparable under H2 and C2H4, showing that the addition of Na2MoO4 effectively suppresses C2H4 combustion relative to H2 oxidation. The kinetics results for Na2MoO4/CaMn0.9Ti0.1O3 confirm its excellent selectivity towards hydrogen combustion, making it a promising candidate under CL-ODH. Additionally, the stability of the CaMn0.9Ti0.1O3@ Na2MoO4 core-shell structure, which was the underlying reason for the excellent selectivity, was examined under both shallow and deep reductions. It was determined that deep reduction of the redox catalyst, e.g. higher than 80% solid conversion, would lead to loss of sodium and hence to decreased selectivity for hydrogen combustion. In contrast, the core-shell structure was well-maintained, exhibiting excellent performance after 50 redox cycles when deep reduction of the redox catalyst was avoided. This study offers a basis for both the CL-ODH reactor design and redox catalyst optimizations.