Selective Oxidation Reactions: A Chemical Looping Approach

Abstract

The production of chemicals is a key contributor to industrial energy demand and greenhouse gas emissions. Chemical looping (CL) offers a novel route to improve energy efficiency and environmental performance, hence investigated in this Dissertation for two selective oxidation reactions: (i) the epoxidation of ethylene to ethylene oxide, and (ii) the oxidative dehydrogenation (ODH) of ethane to ethylene. CL approach depends on the redox cycling of a solid oxygen carrier, typically a metal oxide, which donates oxygen at typical reaction conditions, eliminating the need of a gaseous feed of molecular oxygen. The oxygen carrier is then regenerated by exposure to air to be ready for reuse in the principal reaction. By eliminating the cofeeding of gaseous oxygen and flammable reactants, the CL approach can render reactions safer, and, potentially, more selective and intensive. In the reactions of interest, a catalyst was supported on the oxygen carrier: a critical requirement is to synthesise carriers such that inter al. donation of oxygen to the catalyst occurs at the optimum temperature for the catalyst. For CL-epoxidation, Ag catalyst was supported on SrFeO3 as an oxygen carrier. The influence of oxygen carrier composition was investigated by generating carriers containing different ratios of the perovskite phases SrFeO3 and Sr3Fe2O7. Maximum yield of ethylene oxide was obtained for the sample with 1:1 SrFeO3: Sr3Fe2O7 ratio, leading to an increase in the yield of ethylene oxide around 4 times, compared to only SrFeO3. Despite the improved yield of ethylene oxide, the catalytic activities of the investigated catalysts were still too low to enable CL to be feasible for the commercial production of ethylene oxide. It was found that multiple factors might be hindering the activity of CL-epoxidation catalysts, including: (i) the depletion of the oxygen reservoir of the carrier, (ii) the contamination of the surface of the oxygen carrier with carbon-based species, such as strontium carbonate and coke, and (iii) the combustion of ethylene and ethylene oxide on the surface of the oxygen carrier during the reduction cycle. For CL-ODH, MoVTeNbO multimetallic mixed oxides were prepared and investigated in CL mode for the production of ethylene from ethane. The main premise of this study was that MoVTeNbO catalysts have enough available oxygen to serve the dual function of an oxygen donor and a catalyst. The results showed that the catalyst, in CL mode, can achieve 50% ethylene yield at 475°C, with selectivity towards ethylene and ethane conversion around 81% and 62% respectively. Process modelling and techno-economic analyses for CL-ODH have revealed that the CL approach can offer 100% decrease in energy consumption and 31-120% increase in the net present value at the end of plant lifetime compared to steam cracking. In summary, the catalyst studies at the laboratory scale and techno-economic assessments presented in this Dissertation indicate the conceptual feasibility of CL-epoxidation and CL-ODH, which merits further investigation for the two processes.