Study On Water Vapor And Boron Nitride Aerogel Effect On Selective Catalytic Plasma Oxidation Of Methane

Methane is the most stable alkane molecule with high tetrahedral symmetry.It has high ionization energy,no electron affinity,no permanent electric dipole moment and low polarizability.The bond energy of C-H bond is as high as 439 kJ/mol.Therefore,although methane can be converted into formaldehyde,methanol,olefins,aromatics and other products at high temperature（600-1100 ℃）,the selectivity of the target product is poor,the yield is low,and the catalyst system has not made a breakthrough for a long time.The selective oxidation of methane under mild conditions by overcoming the constraints of reaction kinetics has become one of the important ways for the efficient utilization of methane.It is very difficult to activate methane C-H bond and construct C-O chemical bond under mild conditions,which has been highly concerned by academia and industry.In this paper,plasma was used to realize the catalytic conversion of methane at low temperature.The reaction path was optimized through the water vapor co-feed and boron nitride aerogel-plasma synergistic effect to improve the selectivity of methanol.Firstly,the optimal conditions of emputy tub reaction are explored.Using methane-air gas or methane-oxygen gas reaction system,the conversion of methane,the selectivity of methanol,the CH₃OH formationtion rate and energy efficiency are investigated by changing the reaction temperature,the total flow of feed gas,input power,the flow of water vapor and the molar ratio of oxygen to methane,so as to screen the optimal conditions.On this basis,the conversion of methane,the selectivity of methanol,the generation rate of methanol and the energy efficiency were investigated under the synergistic effect of plasma and aerogel gel catalyst.Finally,the morphology and structure changes of the catalyst before and after reaction were investigated by XRD,bet and SEM characterizations,and the stability was tested.The research results are as follows:（1）Compared with the activity under the best reaction conditions in the methane-air and methane-oxygen reaction systems,although the methane conversion of the methane-oxygen reaction system is lower,methanol selectivity is significantly improved,the atomic utilization rate of methane reaction is increased,and methanol yield is also slightly improved.Therefore,we choose the optimal reaction conditions of methane-oxygen system to screen suitable catalysts for cooperating with plasma to catalyze the selective oxidation of methane.（2）The co-feeding of water vapor improves the conversion of methane and methanol selectivity,which may be attributed to the dissociation of water vapor itself or the reaction between water and oxygen to produce more·OH radicals.On the one hand,the active·OH radicals can remove the H of methane and generate·CH₃ radicals to improve the conversion of methane;On the other hand,·CH₃ radical would further react with·OH radical to produce CH₃OH,which improves the selectivity of methanol.（3）Under the optimal reaction conditions of methane-oxygen reaction system,a series of parking catalysts,such as silicon oxide,zirconia,silicon nitride,titanium nitride,alumina,carbon nitride and boron nitride,were screened out.The super-light boron nitride nanoribbon aerogel has positive effect on selective oxidation catalysis with plasma.Compared with the catalytic activity of emputy tubes,the boron nitride aerogel and plasma co-catalytic reaction system can improve the methane conversion to 6.4%while maintaining the 40%methanol selectivity unchanged.It may be attributed to its three-dimensional multilevel pore structure,including stacked micron-macropores and mesopores,which is conducive to the plasma discharge and gas transport in holes.In addition,the BO_x and N defect sites generated by the oxidation of boron nitride aerogels to promote formation of surface active hydroxyl groups.