Preparation Of Hydrotalcite-based Acid-base Bifunctional Oxides And Its Catalytic Performance For The Synthesis Of Ester Carbonates

Dimethyl carbonate（DMC）and ethyl methyl carbonate（EMC）can be used as a class of environmentally friendly chemical materials due to its higher biodegradability,lower toxicity and bioaccumulation.Various methods have been developed,such as phosgenation,oxidative carbonylation,synthesis from CO₂and transesterification method.Among the synthesis methods,transesterification route is a very competitive method,which has the advantages of less equipment corrosion,good safety,and high yield.However,the current industrial transesterification route is still a homogeneous catalytic system represented by sodium alkoxide,which has difficulty in product separation and produces a large amount of solid alkali and carbonized wastewater.Therefore,the development of efficient and stable heterogeneous catalysts has theoretical significance and practical application value.In recent years,hydrotalcite-based metal oxides have been widely used for catalytic field due to their unique properties such as controllable composition,high dispersion,and adjustable acid-base properties.Two different types of hydrotalcite-based composite oxides were prepared and evaluated in the following two reaction systems,respectively:（1）the transesterification of propylene carbonate（PC）and methanol（Me OH）to synthesize DMC;（2）the transesterification of DMC and ethanol（Et OH）to synthesize EMC;The above two reactions also represent two models of transesterification:the transesterification of cyclic carbonates and alcohols;the transesterification of chain carbonates and alcohols.First,a series of Mn-Al hydrotalcite-based oxide（Mn _XAl O）was prepared by a co-precipitation method,and used as catalysts for the transesterification of PC and Me OH to synthesize DMC in fixed bed reactor.From the characterization results of XRD,XPS and NH₃/CO₂-TPD,it could be seen that changing Mn/Al molar ratio of the precursors can adjust the composition,structure and surface acid-base property of the catalyst.The Mn₁Al O catalyst showed the highest PC conversion（55.2%）and DMC selectivity（98.9%）at 90 ^oC,n（Me OH/PC）=10:1 and 1.5 h^-1,which also could be used for 300 h without significant deactivation.Combining the characterization results with the catalytic activity,it was found that the larger pore size was beneficial to reduce the mass transfer resistance and speed up the reaction rate.Moreover,transesterification was a process of synergistic catalysis between the medium acidic sites and medium basic sites,and then a possible reaction mechanism was propose.Secondly,the hydrotalcite-based Mg-Al oxides（Mg _XAl O-SF）were prepared by mechanically grinding,hydrothermal,and calcination of metal nitrates and alkalis in the solvent-free condition and used as catalysts for the transesterification of DMC and Et OH to produce EMC.Compared with the co-precipitation method,the samples prepared by solvent-free method had larger particle size and higher specific surface area.Among them,Mg₂Al O-SF prepared by solvent-free method exhibited the best catalytic performance（Et OH conversion 68.9%,EMC selectivity 76.5%）at 80 ^oC,n（Et OH/DMC）=0.8:1 and 2.4h^-1.Moreover,the long-term experiments proved that the catalyst could be used for at least1800 hours without deactivation.The relationship between the acid-base property of the catalyst and the catalytic performance was studied.A catalytic mechanism of synergistic effect of weak acid sites and medium basic sites was proposed.