Manganese silicate based redox catalysts for greener ethylene production via chemical looping – oxidative dehydrogenation of ethane

Abstract

The current study investigates manganese silicate based redox catalysts for ethane to ethylene conversion in a chemical looping oxidative dehydrogenation (CL-ODH) process. Facilitated by a two-step cyclic redox scheme, CL-ODH has the potential to overcome the drawbacks of traditional steam cracking including high energy consumption, coke formation, and significant CO2 and NOx emissions. In CL-ODH, lattice oxygen in manganese silicate based redox catalysts is used to combust the hydrogen formed from ethane dehydrogenation, enhancing ethylene formation and suppressing coke formation. The oxygen-deprived redox catalyst is subsequently regenerated with air, releasing heat to balance the overall heat requirement. The key to this process is an efficient redox catalyst with high selectivity and facile oxygen transport. In this study, redox catalysts with combined manganese and silica phases were tested. We report that redox catalysts with high manganese content are more effective for CL-ODH due to their higher oxygen capacity at reaction temperatures. Sodium tungstate was used as a promoter due to its effectiveness to suppress COx formation. Among the redox catalysts investigated, sodium tungstate promoted (1.7 wt.% Na) manganese silicate (Mn:Si molar ratio = 70:30) was the most effective, showing an ethylene selectivity of 82.6% and yield of 63.3%. Temperature programmed reaction (TPR) experiments indicate that the sodium tungstate promoter inhibits ethane activation on the surface of the redox catalyst and is selective towards hydrogen combustion. XPS analysis indicates that the manganese silicate redox catalysts have a smaller amount of near surface Mn4+ than previously studied manganese containing redox catalysts, leading to higher ethylene selectivity on the un-promoted redox catalysts. XPS also indicates that the reduction of the un-promoted redox catalysts leads to the consumption of silica and formation of inosilicate species. ASPEN Plus® simulations of the CL-ODH scheme using manganese silicate based redox catalysts indicate significant energy and emissions savings compared to traditional steam cracking: the overall energy consumption for ethylene production can potentially be reduced by 89% using the manganese silicate based redox catalyst in the CL-ODH process. Resulting from the significant energy savings, CO2/NOx emissions can be reduced by nearly one order of magnitude when compared to traditional steam cracking.