Plasma-catalytic Reforming Of CH₄ And CO₂ To Lower Alcohols: Investigation Of Cu-based Catalysts

Methane(CH4)and carbon dioxide(CO2)are not only the main greenhouse gases in the atmosphere,but also the carriers of carbon resources.The efficient utilization of CO2 and CH4 is helpful for our country to develop natural gas chemical industry and achieve “carbon peak”and “carbon neutrality” in the near future.Production of lower alcohols from CH4 and CO2 is one of the most ideal ways to utilize carbon resources.Currently,plasma-catalytic CH4 and CO2 to alcohols has been successfully achieved at mild conditions,but the selectivity of alcohols is very low and the active site of catalyst toward alcohol formation is still unknown.In this thesis,copper was selected as the active component to investigate the performance of metal oxide supported and zeolite supported Cu-based catalysts in plasma catalytic CH4/CO2 reforming to lower alcohols.In addition,the structure and physicochemical properties of the catalysts have been characterized in detail to discuss the catalysis of Cu-based catalysts.Following results and conclusions have been achieved:(1)Using metal oxides as supports,a series of Cu-based catalysts were prepared by excessive impregnation method and their reaction performances were evaluated.It has been found that among the oxide supports(Al(OH)3,Mg(OH)2,Si O2,Ti O2 and HZSM-5)investigated,Cu/Al(OH)3 catalyst shows the best selectivity to lower alcohols,i.e.,17.9%.Catalyst characterization results(XRD,H2-TPR,NH3-TPD,XPS and HRTEM,etc.)reveal that the weak interaction between oxide supports and Cu O species,as well as high concentration of Cu2+ species in the Cu-based catalysts are beneficial to produce lower alcohols in CH4/CO2 plasma.(2)Using zeolites as supports,a series of Cu-based catalysts were prepared by ion exchange method,and their reaction performances were evaluated.It has been found that among the zeolites supports(13X,SSZ-13,β and HZSM-5)investigated,Cu/13 X catalyst shows the highest selectivity to lower alcohols and the acid amounts of Cu/zeolite catalysts significantly affects the selectivity of lower alcohols based on(NH3-TPD,XPS,HAADF-STEM,BET,XRD and H2-TPR)The Cu/13 X catalyst,with most abundant acid amounts originating from the lowest Si O2/Al2O3 ratio,shows the best selectivity of lower alcohols(75.2%).In addition,the positive function of acid amounts of Cu/zeolite catalysts on lower alcohols production has also been verified by steam passivation experiments.Catalyst characterization results show that acid amounts have been significantly reduced through steam passivation,and accordingly,the Cu/13 X catalyst treated through steam passivation shows lower selectivity for production of lower alcohols.(3)Using 13 X as support,Cu/13 X catalysts with varied loadings were prepared by the excess impregnation method,and their catalytic performance were explored.It is found that the loadings of Cu/13 X catalysts are linearly proportional to the selectivity of lower alcohols,and the selectivity of lower alcohols reached as high as 88.3% at 20 wt.% Cu loading.Furthermore,Cu/HZSM-5 and Cu/S1 catalysts have been prepared with similar excessive impregnation method,and they were used for control experiments to explore the effect of zeolite type on catalyst activity.It has been confirmed again that,in addition to the amount of copper,the acidity of zeolites is the crucial factor affecting the selectivity of lower alcohols.