Study On The Catalytic Conversion Of Dimethyl Oxalate To Prepare Dialkyl Oxalate

In recent years,my country’s coal chemical industry has developed rapidly,and the coal-based oxygenated chemical industry is considered to be an effective way to achieve green and low-carbon development of the coal chemical industry.Among them,the synthesis of ethylene glycol with dimethyl oxalate(DMO)from coal has become a dominant industry in the coal chemical industry.The entire industry faces common problems such as severe product homogeneity competition,short industrial chain and low added value.Therefore,using DMO as a platform compound to develop a new synthetic route for downstream products has become a research hotspot in the chemical field,which is of great significance for improving the refinement development level of the coal-to-ethylene glycol industry.In this thesis,DMO was used as raw material to synthesize dialkyl oxalate by transesterification process.The basic thermodynamic parameters for the synthesis of dialkyl oxalate by transesterification were obtained by thermodynamic calculation,and then the structure-activity relationship of solid acid-base catalyst in the reaction process was studied,the active center of the reaction was clarified,and based on the above research conclusions a highly active ZnO catalyst was designed.The main findings are as follows:(1)Taking DMO and ethanol to synthesize diethyl oxalate(DEO)as a model reaction,the thermodynamic data of each substance in the transesterification reaction at 323-363 K were calculated by the Benson group contribution method and the Joback group contribution method,Δ_rH~θ,Δ_rS~θ,Δ_rG~θand K~θof each step reaction at the corresponding temperature were calculated by the relevant thermodynamic formula,and the basic thermodynamic parameters of the transesterification reaction were determined.The results show that at 323-363 K,theΔ_rG~θof the overall reaction equation is negative(-0.026～-0.005),and the reaction can proceed spontaneously.The enthalpy of the reaction between DMO and one molecule of ethanol to form methyl ethyl oxalate(MEO)becomes positive,the reaction is endothermic,and increasing the reaction temperature is beneficial to the formation of DEO.The equilibrium constant(0.53)of the further reaction of MEO and ethanol to form DEO is greater than that of the disproportionation reaction of MEO(0.28).The formation of DEO mainly depends on the further transesterification of MEO and ethanol.(2)Using common solid acid-base oxides as model catalysts,the catalytic performance of solid catalysts in the synthesis of DEO from DMO and ethanol was investigated,and it was found that MgO exhibited the best reactivity.The optimization results of process conditions showed that when the catalyst dosage was 2wt.%,the molar ratio of ester to alcohol was 1:10,the reaction temperature was 70℃,and the reaction time was 2 h,the DMO conversion and DEO selectivity reached 97.7%and 57.6%,respectively.After MgO was reused five times,the corresponding DMO conversion and DEO selectivity decreased from 95.8%and 57.7%to 80.5%and 21.1%,respectively.Moreover,the catalyst showed excellent substrate universality for the transesterification reaction of fatty alcohol and DMO.The morphology of MgO catalyst had no obvious effect on the reaction activity.(3)In order to solve the problem of low selectivity of transesterification reaction products,rod like,flower like,spherical and needle like ZnO were synthesized by different methods.The effect of ZnO morphology on the reaction performance of deo synthesized by DMO and ethanol transesterification was systematically investigated.The results show that the acicular ZnO has the best catalytic activity.When the catalyst dosage was 3 wt.%,the molar ratio of ester to alcohol was 1:18,the reaction temperature was 70℃,and the reaction time was 1.5 h,the DMO conversion and DEO selectivity could reach 97.7%and 77.0%,respectively.After 5 times of repeated use of ZnO,the corresponding DMO conversion and DEO selectivity gradually decreased to 53.3%and 12.1%,and the catalyst morphology collapsed.The morphology of ZnO was closely related to the reaction activity.When acicular ZnO was used as the transesterification catalyst,the activation energy of the reaction was 68.19 k J/mol.The acicular ZnO has excellent catalytic performance for the transesterification of fatty alcohols with DMO,and the high activity of acicular ZnO is mainly attributed to the abundant moderately strong basic sites and high specific surface area on the catalyst surface.