Preparation Of Ce-based And La-based Rare-earth Oxides Catalysts For The Synthesis Of Dimethyl Carbonate

Dimethyl carbonate（DMC）is one of the most concerned green chemicals,which can be used as a gasoline additive to improve the octane value,but also to replace phosgene and dimethyl sulfate as carbonylation and methylation reagents.In addition,it is an excellent organic solvent and fine chemical synthesis intermediate.At present,DMC is produced mainly by transesterification of propylene carbonate（PC）and methanol in China and propropylene glycol（PG）,a by-product of this method,is also an important basic chemical.There are many catalysts for DMC synthesized by transesterification,but there are problems such as low catalytic activity and long reaction time.However,the direct synthesis of DMC from CO₂ and methanol is conducive to reducing the large amount of CO₂ emissions and transforming methanol with excess capacity.This process route has great development prospects.In this thesis,rare earth La-based and Ce-based composite oxide catalyst was prepared for the synthesis of DMC by transesterification of PC and methanol and carbonylation of CO₂ and methanol,respectively.1.Three different morphological La₂O₂CO₃ nanocatalysts（nanoparticles,nanorods and nanosheets）were successfully synthesized by precipitation method,hydrothermal method and solvothermal method.Comparing with nanorods and nanoparticles,La₂O₂CO₃ nanosheets showed the highest catalytic activity in the DMC synthesis from PC and methanol.After 8 h,the conversion of PC was 58%and DMC selectivity was 89%.Combined with the analysis of HRTEM and CO₂-TPD results,the mainly exposed（001）plane in La₂O₂CO₃ nanosheets is mainly composed of C and O atoms with the most basic sites,which further promotes the activation of methanol and leads to the improvement of catalytic activity.This work shows that the morphology change can adjust the surface basicity of the catalyst.2.The Ce-doped La₂O₂CO₃ nanosheets catalyst was successfully synthesized by hydrothermal method and tested for its catalytic performance in DMC synthesis from PC and methanol.When the molar ratio of La and Ce was 1:0.15,the highest catalytic activity was demonstrated,and after 6 h,PC conversion was 84%and DMC selectivity was 97%.The order of catalytic activity was:CeO₂<La₂O₂CO₃<La₁Ce_0.03<La₁Ce_0.3<La₁Ce_0.15.The characterization results prove that appropriate Ce doping generate substantial surface basic sites through defects associated with oxygen interstitial and vacancy and obviously enhance basicity of La₂O₂CO₃,which further leading to enhanced catalytic activity.3.Four different morphological CeO₂ catalysts（nanoparticles,nanorods,nanopencils and nanocubes）were successfully synthesized by precipitation method,hydrothermal method and solvothermal method.Among them,CeO₂ nanorods showed the best catalytic activity than other morphological CeO₂ catalysts in the direct DMC synthesis reaction from CO₂ and methanol,and the DMC yield reached 10.6 mmol _DMC/g_cat.According to the characterization results,higher activity of CeO₂nanorods is attributed to the highest acid-base sites and the most oxygen vacancy.The surface acid-base sites favors the adsorption activation of methanol to generate methoxy species,while the oxygen vacancy can activate CO₂,causing the high catalytic activity of CeO₂ nanorods due to the bifunctional synergy effect of the surface acid-base sites and oxygen vacancy.4.Zn-doped CeO₂ nanorood catalyst was successfully synthesized by hydrothermal method.When the molar ratio of Ce and Zn was 1:0.1,the highest catalytic activity was demonstrated,with a DMC yield of up to15.8 mmol _DMC/g_cat.According to the characterization results,the introduction of the appropriate amount of Zn increased the oxygen vacancy on the CeO₂ surface and did not change amount and strength of the acid-base sites,which indicating that the surface oxygen vacancy on the CeO₂ is the main cause of its high catalytic activity and the activation ability of CO₂ is the rate-determining step of the reaction.