Direct Synthesis Of Dimethyl Carbonate From CO₂ And Methanol Over CeO₂ Catalysts

Dimethyl carbonate （DMC） has been widely used as an environmentally benign intermediate in chemical industry for the replacement of phosgene for polycarbonate production and other processes. Over the past years, DMC has been considered as a ideal candidate for replacing methyl tert-butyl ether （MTBE） as the additive to gasoline and the utilization of DMC exhibits excellent prospects. Carbon dioxide, which contributes to the increase in global temperatures and the climate change due to the greenhouse effect, has attracted considerable attention for the chemical utilization. The process for direct synthesis of DMC from CO₂ and methanol not only produces economic chemicals with high value, but also benefits for the environment by using CO₂ as a recycled carbon resource. Direct synthesis of DMC from CO₂ and methanol over cerium oxide was studied systematically in this dissertation.Combining the NH₃/CO₂-TPD results with catalytic performance of CeO₂ calcined at different temperatures, it is suggested that the moderate acid sites with desorption peak of NH₃-TPD at 340℃exhibit an excellent catalytic formation of DMC and are the main active acid sites. The moderate acid sites with desorption peak at 260℃are unfavorable for the synthesis of DMC and the strong acid sites have no direct impacts on the catalytic activity. Compared to the base sites, the acid sites on the surface of CeO₂ are the main factors which affect the catalytic performance.The crystal of CeO₂ has the structure of cubic fluorite with the primary characteristic surfaces of （111）, （110） and （100）. Due to the differences in the packing of atoms in bulk structure and certainly at surface, the interaction forces between the surfaces and the reactants would be different. By controlling the concentration of alkali solution and the reaction temperature, CeO₂ with different morphologies was synthesized through the hydrothermal method. Combining with HRTEM and SEAD, the surfaces of CeO₂ with different morphologies were confirmed. Connected with the catalytic performance, it is suggested that nanorods exhibit the highest reactivity for the synthesis of DMC and （110） is the main active surface. Nanocubes show the lowest activity for the reaction indicating that （100） surfaces have little activity for the synthesis of DMC. The active sites of nanoparticles are provided by （111） surfaces with lower catalytic activity than （110）. Comparing the catalytic reactivity of the type of surfaces, it is showed that （110） is the main active surface for the synthesis of DMC.The catalyst lifetime of CeO₂ prepared by calcination and hydrothermal method was quite short as after the first cycle the activity decreased. It could be due to the disappearance of the active middle strong sites with desorption peak of NH₃-TPD at 340℃. On the other hand, the morphology of CeO₂ was changed during the reaction and the active （110） surfaces have a significant reduction in exposure, leading to the decrease of the catalytic performance.Quantum chemical study of the reaction mechanism for direct synthesis of DMC from CO₂ and methanol over CeO₂ catalyst was first explored using density functional theory （DFT） by Material Studio software. It is demonstrated that the formation of DMC from methanol and methoxycarbonyl is the rate-determining step during the reaction. The reaction energy barrier of the rate-determining step for （110） is lower than that for （111） suggesting that the formation of DMC is more readily compared to the （110） surface. The calculation results support experimental observation that （110） of CeO₂ is the main active surface for direct synthesis of DMC from CO₂ and methanol.