Performance And Kinetics Of Cerium-Based Catalysts For The Preparation Of Dimethyl Carbonate From Carbon Dioxide And Methanol

CO₂ is an abundant C1 raw material in nature.The catalytic conversion of CO₂ into high value-added compounds is not only beneficial to environmental protection,but also of great significance to the recycling of resources.Due to its low toxicity,biodegradability and special chemical structure,dimethyl carbonate（DMC）can be used as methylation carbonylation reagent,gasoline additive,lithium battery electrolyte solution and precursor for the production of polycarbonate,etc.Dimethyl carbonate has a broad application market.However,the traditional production process of DMC has the disadvantages of large environmental pollution,easy corrosion of production equipment and high production cost.The new synthesis process of DMC,the direct synthesis from CO₂ and methanol,has the advantages of simple process,green process,low cost of raw materials and high atom utilization rate.It is a process route with great potential for development.In this paper,aiming at the direct synthesis of DMC from CO₂ methanol,rod-shaped CeO₂ was prepared by a hydrothermal method,and dried by various drying methods.Through various characterization methods,it is found that the change of drying environment has an important influ ence on the surface properties of CeO₂ solids.The lower the oxygen concentration in the dry environment,the higher the oxygen vacancy concentration and acid-base sites on the CeO₂ surface.Among them,the CeO₂-FD catalyst prepared by freeze-drying has the largest specific surface area,pore volume,acid-base sites and oxygen vacancy concentration due to the low oxygen concentration in the drying environment and the removal of water as a template-like agent.Catalytic performance experiments show that CeO₂-FD catalyst has the best catalytic activity,and the catalytic performance and surface oxygen vacancy concentration of CeO₂ catalyst are linearly related to the number of acidic sites.Under the optimal reaction conditions obtained by the single factor experiment,the CeO₂-FD+2-CP catalytic system made the DMC yield and selectivity as high as 873 mmol·g-1 cat and 99.6%,and the methanol conversion rate reached 51.6%.At the same time,the catalytic performance of CeO₂-FD catalyst was almost unchanged after five cycles under optimal conditions,which proved that CeO₂-FD catalyst has good stability.Subsequently,the catalytic reaction mechanism of the direct synthesis of DMC from CO₂ methanol in the CeO₂-FD+2-CP catalytic system was proposed,and a sequential catalytic reaction kinetic model was established combined with the Langmuir-Hin sh elwood mechanism.Catalytic reaction kinetics experiments were carried out,and MATLAB was used to fit kinetic data to obtain relevant kinetic parameters.It was found that the experimental values fit the model values well,with an ARD of 4.6%.The activation energy of the catalytic reaction is calculated by the Arrhenius equation,and Ea=20±3.3kJ·mol^-1.Finally,a series of CZn@CeO₂-FD catalysts were obtained by optimizing the CeO₂-FD solid surface using ZIF-8 as a precursor.Characterization analysis showed that the retention of ZIF-8 polyhedral carbon skeleton after high temperature calcination is beneficial to increase the incorporation of Zn2+into CeO₂ lattice,and the incorporation of Zn2+is beneficial to increase the surface oxygen vacancy concentration of CeO₂.The surface of CZn_0.5@CeO₂FD catalyst supported the most Zn species,which made the catalyst obtain extremely high oxygen vacancy concentration（I564/I462=254.94%）after high temperature calcination.The CZn_0.5@CeO₂-FD+2-CP catalytic system was applied to the reaction of CO₂ and methanol to synthesize DMC,achieving high selectivity（99%）of DMC at low pressure（1 MPa）.