Experimental Study On Chemical Looping Oxidative Coupling Of Methane Into Hydrocarbons By Using Mn-based Oxygen Carriers

Chemical Looping Oxidative Coupling of Methane（CLOCM）for the production of olefin has attracted extensive attention due to its novelty,high efficiency and economy and environmental protection.The technology is achieved through the cyclic alternating reaction of oxygen carriers between the two reactors.Oxygen carriers play a dual role of oxygen transfer and catalysis in this process.Methane can be highly selectively converted to C₂H₆ and C₂H₄,etc.Oxygen carriers are then recycled through air oxidation to restore their oxidizing capacity.In this paper,perovskite-type metal oxides with metal Mn at site B were selected as oxygen carriers,and the metal Mg,Ca,Ba,La at site A were selected to coordinate with the metal Mn,to investigate the influence of different metal coordination on the reaction performance of Mn-based perovskite oxides.The oxygen carriers were characterized by XRD,XPS,O₂-TPD,H₂-TPR and SEM,and the reaction performance of oxygen carriers in the process of chemical looping oxidative coupling of methane was investigated by fixed-bed experiment.The preliminary screening of oxygen carriers was carried out.The results show that LaMnO₃ and Mg₆MnO₈ oxygen carriers have higher reactivity to methane.Select low Li Na and alkali metal to doping LaMnO₃,preparation of x-Na/Li-LaMnO₃（x=0,0.14,0.16,0.18,0.20）in the body of carrying oxygen.Compared with Li,Na doped oxygen carriers show a higher yield of C2 hydrocarbons.Due to the close radius of Na element and La element,oxygen carriers can be better modified.The doping of Na results in a higher proportion of Mn⁴⁺/Mn³⁺and oxygen vacancy in the LaMnO₃ carriers compared to the unloaded ones.The results of XPS,O₂-TPD and continuous pulse experiments show that the doping of Na leads to the formation of many insufficiently reduced O^α-（0＜α＜2）,which is an oxygen species that selectively generates C2 hydrocarbons from methane.At 825℃and 1125mL/min*g,018Na-LaMnO₃ has the highest yield of C2 hydrocarbons close to 20%.The oxygen carrier has maintained good cycling stability after 20 cycles of experimental reactions.Then,Mg₆MnO₈ oxygen carrier was studied and modified with alkali metals Na and Na₂WO₄ alkaline earth metals.The Mg₆MnO₈ oxygen carrier has high temperature stability,cheapness and abundant O^α-（0＜α＜2）oxygen species which are not fully reduced on the surface.However,its oxygen release rate does not match the activation rate of methane.By doping the alkali metal Na,the oxygen vacancy concentration of oxygen carrier can be increased,thus reducing the adsorption energy of methyl radical intermediates.The results show that 10Na-Mg₆MnO₈ exhibits 43%methane conversion,54%C2 selectivity and 23%C2 hydrocarbon yield at the temperature of 2625mL/h*g at 850℃.Mg₆MnO₈ is loaded with Na₂WO₄ alkali metal salt to form Na₂WO₄ inclusion layer on the surface of oxygen carrier,which is conducive to regulating the release rate of reactive oxygen species and matching the activation of methane,thus improving the selectivity of target products.At 800℃and 1125mL/h*g,the oxygen carrier supported by Na₂WO₄ alkali metal salt shows 46.7%methane conversion rate,43.1%C2selectivity and 20.1%C2 hydrocarbon yield.After 20 cycles,the oxygen carrier maintains a high conversion rate and C2 selectivity,as well as a yield of C2hydrocarbons close to 20%.Through the characterization and reaction performance of the above oxygen carriers,the mechanism of activating methane and improving the yield of C2hydrocarbons has been preliminarily clarified.It provides a simple and effective way to prepare oxygen carriers suitable for chemical looping oxidative coupling of methane in the future.