Study On The Synthesis Of Propylene Carbonate From Carbon Dioxide And 1, 2-Propylene Glycol Over Acid Catalysts

The industrialization of the synthesis of dimethyl carbonate by transesterification of propylene carbonate(PC) with methanol has found a new application field for PC.In this process,however, a lot of 1,2-propylene glycol(PG) is generated as a by-product.Besides the feed PC is a petrochemical and its supply is controlled by petrochemical industry inevitably.CO₂ is a main greenhouse gas and how to decrease its emission to the environment is a world-wide problem.So making PG convert back to PC by the reaction of PG and CO₂ can not only make effective use of the greenhouse gas to improve the ecological environment but also can solve the two problems existing in the transesterification process simultaneously.Thermodynamic analysis of the reaction system for synthesizing PC from CO₂ and PG was studied.The result showed that the reaction was exothermic and itsΔ_rG was more than zero, which suggested that the reaction was thermodynamicsly unfavorable.Among a series of acetates,zinc acetate showed the highest catalytic activity in the reaction for the synthesis of PC from CO₂ and PG.The optimal reaction conditions for the synthesis of PC from CO₂ and PG over zinc acetate were as follows:reaction temperature 433 K,weight percentage of catalyst 2.5%,reaction time 2 h,molar ratio of acetonitrile(AN) to PG 1.8:1 and initial pressure of CO₂ 3.0 MPa,the conversion of PG,the yield of PC and the selectivity of PC were 19.2%,12.3%and 64.1%,respectively.The main side-reactions were presumed in detail by the analysis of GC-MS and XRD techniques.The mechanism for the reaction of CO₂ and PG to PC over zinc acetate catalyst was elucidated through in-situ IR.Some important side-reactions are confirmed as follows:the hydrolysis of AN to acetamide and a successive reaction of acetamide with PG to 1,2-propylene glycol acetate,the reaction of zinc acetate with PG to 1,2-propylene glycol acetate and the reaction of acetic acid with PG to 1,2-propylene glycol diacetate.A possible mechanism for the synthesis of PC from CO₂ and PG over zinc acetate catalyst was elucidated.Anhydrous zinc acetate first attacks a hydroxyl of PG molecule to form a coordinated complex.The oxygen atom in the coordinated complex can attack the carbon atom in CO₂ molecule to eliminate Zn²⁺ cation.The new formed coordinated complex can combine with H⁺ cation to form PC by elimination of a water molecule.The Zn²⁺ cation is transformed to zinc acetate through the recombination with acetate anion from acetic acid formed in the reaction.Supported zinc acetate catalysts were prepared by incipient impregnation and sol-gel methods to facilitate separation process and reuse of the catalyst.The catalytic performance of the supported zinc acetate catalyst for the synthesis of PC from CO₂ and PG was studied and the results showed that SiO₂-supported zinc acetate prepared by incipient impregnation method was a better one.However,it had a poor stability.The result of AAS analysis showed that the zinc atom content of the catalyst deceased from 6.29%to 1.53%before and after reaction.The specific surface area of the catalyst decreased a little while the pore volume and the pore width increased after reaction.The pore volume of the reused catalyst with pore width between 7nm to15nm was larger than that of the fresh catalyst,which suggests that the pore width of the lost zinc acetate was between 7nm to15nm.Since the reaction was thermodynamicly unfavorable,we separately added propylene oxide (PO) and urea to the reaction system in order to break the reaction equilibrium constraint.When PO was added,the yield of PC increased 30.1%;if urea was added,the selectivity of PC increased 7%.The solvent and catalytic performance of ionic liquids in the reaction for synthesis of PC from CO₂ and PG has been studied.When AN was replaced by ionic liquids[bmim]BF₄ as solvent,the conversion of PG,the yield of PC and the selectivity of PC were 19.9%,11.5%and 57.8%,respectively.The reuse of the ionic liquid was not satisfactory because the color of the recovered ionic liquids became dark and its viscosity increased.When Lewis acid ionic liquids [emim]Br-FeCl₃ was used as the catalyst for synthesis of PC,the conversion of PG,the yield of PC and the selectivity of PC were 35.5%,23.7%and 66.8%,respectively.However,the recovered ionic liquids could not be reused.Therefore the recovery and the reuse of the ionic liquids should be emphasized for the further research work.