| In recent years,with the development of biomass ethanol industry,the production of bioethanol gives a very fast enhancement.The rational use of bioethanol to produce high value-added products have attracted extensive attention,in which synthesis of diethyl carbonate is an effective way for ethanol conversion.As an important synthetic intermediate and green solvent,diethyl carbonate is widely applied in the fields of fine chemicals,medicine with a broad application prospect.The traditional method for the diethyl carbonate production by phosgene suffers from a high toxicity and serious pollution of reactants.Recently,the synthesis of diethyl carbonate via reaction of urea and ethanol has attracted considerable interest because of the advantages of green economy,less by-products,and facile product separation.The acid-base synergistic catalyst is the most promising catalyst for the conversion of urea and ethanol to diethyl carbonate.However,two main problems remain unresolved in the study of acid-base synergistic catalysts:(1)How to effectively regulate the structure of acid-base active sites so as to improve reaction performance and catalyst stability;(2)How to deeply understand the regulation mechanism of active site structure of acid-base catalysts as well as the reaction mechanism.In this dissertation,based on the tunable properties of metal cation/ratio of layered double hydroxides(LDHs)and the structural topological transformation characteristics,mixed metal oxides(MMO)catalysts with acid-base sites were prepared and applied in the synthesis of diethyl carbonate from urea and ethanol.In addition,by virtue of a variety of experimental methods,the influence of catalyst composition on the structure of acid-base active sites was studied,and the corelation between acid-base sites with the yield of diethyl carbonate was investigated.This work provides a new method for the design and preparation,of acid-base synergistic catalyst.The detailed research contents and results are as follows:1.Studies on Mg2ZrxAl1-x-MMO acid-base synergistic catalyst for the synthesis of diethyl carbonateA series of Mg2ZrxAl1-X-LDH as precursors were prepared by co-precipitation method;subsequently,metal oxide catalysts(denoted as Mg2ZrxAl1-x-MMO)with acid-base sites were prepared via the topological transformation process of LDHs materials by regulating the composition of LDHs precursors.SEM and BET results show that the morphology of the Mg2ZrxAl1-x-MMO samples maintains the lamellar structure of LDH precursors,but with a mesopores structure.The characterizations of XRD and TEM confirm that MgO and ZrO2 are highly dispersed on the amorphous Al2O3 substrate in Mg2ZrxAl1-X-MMO samples.The catalytic performance shows that the Mg2Zr0.53Al0.47-MMO catalyst gives the best diethyl carbonate yield(37.6%)from urea and ethanol.Furthermore,the results of NH3-TPD and CO2-TPD indicate that both mediu basic sites and weak acid sites are present on the surface of Mg2ZrxAl1-x-MMO catalysts,which are provided by ZrO2、MgO and ZrO2,respectively.Studies on structure-activity correlation reveal that the medium basic sites and weak acidic sites synergistically catalyze the reaction process:medium basic sites serve as active sites to activate reactant ethanol;while weak acidic sites effectively activate urea and the intermediate ethyl carbamate,accounting for the enhanced catalytic performance.2.Studies on Zn2ZrxAl-MMO acid-base synergistic catalyst for the synthesis of diethyl carbonateBased on the above work,in order to further modulate the acidity and basicity of catalyst to improve catalytic performance,several Zn2ZrxAl-LDH precursors with various Zr contents were synthesized by hydrothermal method,followed by a calcination process to obtained Zn2ZrxAl-MMO samples based on topotatic transformantion.SEM and HRTEM images show that Zn2ZrxAl-MMO samples still maintain plate-like morphology of LDHs precursors with a particle size of 180-200 nm,and XRD results confirm the predominant presence of ZnO pahse.The catalytic performances show that Zn2Zr0.1Al-MMO catalyst gives a high yield of diethyl carbonate(42.1%),superior to the Mg2Zr0.53Al0.47-MMO catalyst.The CO2-TPD and NH3-TPD studies indicate that with the increase of Zr content,the concentration of both weak acid and medium strong basic site enhances significantly and reaches a maximal value in the sample of Zn2Zr0.1Al-MMO.In addition,the Zn2Zr0.1Al-MMO catalyst provides a decreased strength of medium-strong base sites compared with Mg2Zr0.53Al0.47-MMO catalyst,which is more favorable for the activation of ethanol,accounting for the enhanced yield of diethyl carbonate. |
