Effect of Promoters on Manganese-Containing Mixed Metal Oxides for Oxidative Dehydrogenation of Ethane via a Cyclic Redox Scheme

Ethylene is an important building block in the chemical industry; state of the art ethylene production (steam cracking) has multiple drawbacks, including high energy consumption, coke formation, and significant CO2 and NOx emissions. We propose a chemical looping oxidative dehydrogenation (CL-ODH) process to convert ethane into ethylene in a two-step, cyclic redox scheme. In this process, lattice oxygen in a metal oxide based redox catalyst is used to combust the hydrogen formed in ethane dehydrogenation, thereby enhancing ethylene formation while retarding coke formation. The oxygen-deprived redox catalyst is subsequently regenerated with air, releasing heat to balance the overall heat requirement. CL-ODH can realize a reduction of over 80% in primary energy consumption and pollutant emissions. The key to this process is an efficient redox catalyst with high selectivity and facile oxygen transport. Previously we determined that oxides with an Mg6MnO8 structure allow high lattice oxygen mobility and satisfactory oxygen-carrying capacity for the proposed redox reactions. However, unpromoted Mg6MnO8 exhibits poor ethylene selectivity, producing primarily CO2. In the current study, we examine the effects of various sodium-containing promoters on Mg6MnO8 CL-ODH activity and mechanism. Sodium tungstate promoted Mg6MnO8 was the most effective redox catalyst, showing an ethylene selectivity of 89.2% and yield of 68.2%, a significant improvement of thermal cracking (38.9% yield). Temperature-programmed reaction (TPR) experiments indicate that the reaction proceeds via gas-phase ethane thermal cracking in parallel with selective hydrogen combustion on the redox catalyst surface. XPS analysis indicates that the decreased ethane/ethylene oxidation activity on the sodium tungstate promoted redox catalysts results from the suppression of near-surface Mn4+. This is due to a combination of decreased surface manganese content and reduction in average Mn oxidation state. The suppression of Mn4+ results in a decrease in electrophilic surface oxygen species, inhibition of ethylene combustion, and enhanced ethylene yield.