Study On The Catalytic Performance For Dimethyl Carbonate Synthesis From CO₂ And Methanol Over KIT-6 Supported CeO₂

With the burning of traditional fossil fuels(coal,oil and gas),the emissions of CO2 into the atmosphere continue to rise,resulting in a series of environmental and ecological problems.Moreover,CO2 is also regarded as a rich C1 resource,and the transformation of CO2 into high value-added chemicals and fuels can not only realize the resource utilization,but also effectively promote the reduction emission of CO2.Dimethyl carbonate(DMC)is considered as a green versatile chemical,which are widely applied in the electrolyte of lithium-ion battery,the intermediates of organic synthesis and the raw materials of polycarbonate.Thereinto,the direct synthesis of DMC from CO2 and methanol is deemed as the cleanest route due to the low price of raw material,the high rate of atom utilization and the safety of production.The surface of CeO2 catalyst has abundant acid and base active sites,which can effectively adsorb and activate CO2 and methanol molecules,and further transform them into DMC.The morphology of CeO2 significantly affects the catalytic activity,and nanorod CeO2 shows better catalytic performance than that of nanocubes and nanooctahedra.The number of surface acid and base active sites is further improved after doping of metal,and thus the catalyst activity is obviously improved.Nevertheless,the particle size of nanorod CeO2 is big.The diameter and length of particles were about 13 nm and 150 nm,respectively,resulting in the less exposed acid and base active sites and low utilization of CeO2.Metal species can be highly dispersed on the surface of order mesoporous silicon materials due to large specific surface area and abundant surface Si-OH groups,and thus improve the utilization of active species.Inspired by the mentioned above,in this thesis,a series of KIT-6 support with varied specific surface area were prepared by controlling the aging temperature with TEOS and P123 as silicon source and template,and the corresponding CeO2/KIT-6catalysts were prepared.The effect of KIT-6 specific surface area on the dispersion state and valence distribution of CeO2,surface acidity and basicity of the catalyst as well as the catalytic performance for synthesis of DMC from CO2 and methanol was investigated.On this basis,Al-doped KIT-6 supports were prepared with aluminum isopropanol as Al source and the corresponding catalysts were prepared.The effect of Al doping amount on the structure of KIT-6,the dispersion state and valence distribution of CeO2,acidity and basicity of the catalyst and the catalytic performance for synthesis of DMC from CO2 and methanol was explored.The main obtained conclusions include the following aspects:(1)KIT-6 prepared at different aging temperatures maintained ordered three-dimensional mesoporous structure.The specific surface areas of KIT-6 firstly increased and then decreased with the increase of aging temperature,which attained the maximum(683 m2/g)as the aging temperature was 100 oC.High specific surface area of KIT-6 was favorable for the improvement of the number of Si-OH,and thus improve the dispersion of nanoparticles CeO2 on surface.As the aging temperature was 100 oC,the dispersion of CeO2 nanoparticles reached the highest and the particle size was the smallest(5.9 nm).The highly dispersed CeO2 nanoparticles were beneficial to the increased medium base/acid adsorption sites and content of Ce3+/oxygen vacancy on surface,and thus improve the catalytic activity.As the aging temperature was 100 oC,the DMC yield was the highest,reaching 15.0 mmol/gCeO2.(2)The catalytic activity firstly increased and then decreased with the increasing reaction temperature,and attained the highest as the reaction temperature was 140 oC.In addition,the catalytic activity increased with the increase of reaction pressure.(3)After recycle six times,the DMC yield of CeO2/100-KIT-6 catalyst gradually decreased from 15.0 mmol/g CeO2 to 2.8 mmol/g CeO2.The decrease of catalyst activity was about 81.3%.The agglomeration CeO2 nanoparticles on the catalyst surface was the main reason for the decrease of catalytic activity.(4)KIT-6 remained ordered three-dimensional mesoporous structure after Al doping.With the increase of Si/Al ratio,the specific surface area firstly increased and then decreased,and attained the maximum(737 m2/g)as the Si/Al ratio was 40.High specific surface area of support and abundant Si-OH and Si-O-Al groups on surface enhanced the interaction between support and surface CeO2,and therefore promoted the dispersion of CeO2 on surface.As the Si/Al ratio was 40,the particle size of CeO2 on the surface of catalyst was the smallest.Highly dispersed CeO2 nanoparticles were beneficial to the increased medium base/acid adsorption sites and content of Ce3+/oxygen vacancy on surface.In addition,the number of Lewis acid sites on the surface of KIT-6 was significantly improved after Al doping,which also contributed to the improved catalytic activity.As the Si/Al ratio was 40,the catalytic activity was the highest,reaching 25.7 mmol/g CeO2.(5)After recycle six times,the DMC yield of CeO2/AK-40 catalyst gradually decreased from 25.7 mmol/g CeO2 to 8.7 mmol/g CeO2.The decrease of catalyst activity was about 66.1%.The enhanced interaction between KIT-6 support and surface CeO2 after Al doping inhibited the agglomeration of CeO2 nanoparticles and alleviated the deactivation of catalyst.