Study On Cerium-Based Solid Solution Catalyst For Direct Synthesis Of Dimethyl Carbonate From Methanol And Carbon Dioxide

The continuous depletion and combustion of fossil fuels have led to excessive emissions of carbon dioxide（CO₂）,resulting in serious environmental pollution.People urgently need to convert CO₂ into green chemical products.However,the C=O double bond in CO₂ molecule is difficult to be activated and thermodynamically stable,so its conversion efficiency and product selectivity are facing major challenges.Ce O₂ is an acid-base bifunctional catalyst,which can improve the catalytic performance by changing its crystal structure to regulate the surface oxygen mobility and oxygen vacancy content.When Ce O₂ is doped with some metal,some structural defects on its surface can change the surface acidity and oxygen vacancy content of the catalyst to achieve the purpose of changing the catalytic activity.In this paper,we mainly studied the binary and ternary Ce-La solid solutions doped with metal ions and the porous layered high entropy oxide（HEO）with high specific surface area of five elements,and discussed its catalytic performance for the synthesis of dimethyl carbonate（DMC）from methanol and CO₂.（1）The effects of different ratios of Ce_1-xLa_x solid solution and reaction conditions on the reaction activity were investigated.When Ce:La was 9:1,the reaction temperature was 140°C,the reaction time was 5 h,and the initial CO₂ pressure was 4MPa,the catalyst activity was the highest,and the yield of DMC was 200 mmol/g.（2）The ternary mesoporous Ce-La solid solutions doped with different metal elements（Co,Zr and Mg）were synthesized by citric acid coordination synergistic self-assembly method.Different metal ions doping did not significantly change the texture properties and crystal structure of the catalyst.The surface acidity and oxygen vacancy content of the catalyst were the main reasons affecting the catalytic activity.Studies have shown that Co-doped Ce-La solid solution catalyst has the best catalytic activity,and the yield of DMC reached 316 mmol/g.The doping of Co on mesoporous Ce-La solid solution helps to increase the specific surface area of the catalyst and promote the adsorption and activation of CO₂.The reaction of Ce⁴⁺+Co²⁺→Ce³⁺+Co³⁺on the catalyst surface helps to promote the formation of oxygen vacancies on the catalyst surface.（3）Ce_x（La Pr Sm Y）_1-xO_2-y holey fluorite-type HEO pentacerium-based solid solution with different calcination temperatures and metal molar ratios was prepared by anchoring method.The equimolar ratio of HEO catalyst calcined at 1000°C was used for the synthesis of DMC from CO₂ and methanol.The DMC yield was 7.2 mmol/g at140°C,4 MPa of initial CO₂ pressure and 2 h of reaction time.At the same time,the catalyst had a certain universality,and the DEC yield was 5.3 mmol/g under the conditions of reaction temperature 140°C,initial CO₂ pressure 4 MPa,and reaction time 2 h.The catalytic performance did not change significantly after four cycles.The p-type semiconductor property of high-entropy materials can enhance the electron transfer efficiency of methanol molecules to the catalyst.By adjusting the content of high-valent cation Ce⁴⁺in HEO to 50%,the positive potential vacancy can be increased,and more oxygen vacancies can be generated without damaging the crystal phase and structure of Ce_0.5（La Pr Sm Y）_0.5O_2-y catalyst,thereby improving the catalytic activity of the reaction.Under the same reaction conditions,the DMC yield can reach 7.6 mmol/g.The experiments show that the single fluorite crystal phase and porous layered structure of the high entropy oxide catalyst can significantly improve the catalytic reaction efficiency,which is essential for showing high catalytic performance.