(140d) Effects of Structural and Surface Promoters on Manganese-Containing Oxides in Cyclic Redox Reactions

Mn-containing oxides are frequently investigated as low cost and environmentally benign oxygen carriers for chemical looping combustion. The current study investigates Mn-based mixed oxides for two distinct redox applications, i.e. chemical looping with oxygen uncoupling (CLOU) for coal combustion and Chemical Looping based Oxidative Dehydrogenation (CL-ODH) for ethylene production from ethane.

In terms of CLOU, perovskite-structured oxygen carriers with a general formula of A_xAâ??_1-xMn_yB_1-yO₃ (A/Aâ?? = Sr, Ca, Ba; B = Fe, Ni) are investigated. Although CaMnO₃ has the tendency to lose its oxygen carrying capacity through irreversible transition to spinel and Ruddlesden-Popper phases, small amount of Sr or Fe dopants enhances the redox stability of CaMnO₃. Moreover, the initial oxygen release temperature is significantly lowered, enabling facile oxygen donation and more effective coal combustion. DFT calculations were performed to interpret the dopant effect. It was determined that the aforementioned dopants changes the spatial locations, charge and vacancy formation energy for oxygen anions. Satisfactory correlations among vacancy formation energy, energy barrier for vacancy migration, free energy for phase transition, and redox activity and stability of Mn-containing oxygen carriers are observed.

With respect to CL-ODH, a number of Mn-containing model oxides are investigated. In this cyclic redox scheme, a Mn-based redox catalyst is used to for ethane ODH. Since air is used to regenerate the redox catalyst, CL-ODH avoids the air separation plant. Mg₆MnO₈ based model redox catalysts are investigated in the current study. The effect of various promoters are determined along with the corresponding reaction pathway and underlying mechanisms. It was determined that unpromoted Mg₆MnO₈ redox catalysts are highly selective for CO₂ formation, making it a potentially suitable oxygen carrier for chemical looping combustion. In comparison, promoters such as Na and W significantly improve the selectivity and ethylene yield for the ODH reaction. The overall ODH reaction proceeds through parallel gas phase cracking reaction and selective H₂ combustion on the surface of the promoted redox catalysts.