Integrated Processes For Synthesizing Propylene Carbonate And Dimethyl Carbonate Starting From Glycerol

A large surplus of glycerol is formed as a by-product from biodiesel production processand how to convert it into value-added derivatives has become a word-wide research topic.The present research aims to realize integrated processes for synthesizing propylenecarbonate (PC) and dimethyl carbonate (DMC) starting from glycerol, thus to develop newtechnologies to utilize glycerol effectively. It covers the reaction integration of glycerolhydrogenolysis and CO2alcoholysis (or urea alcoholysis) to PC, and the reactionintegration of urea alcoholysis and transesterification to DMC. Some single-step reactions,such as CO2alcoholysis, urea alcoholysis and glycerol hydrogenolysis are also studied inthis work.Firstly, an exploratory study was performed for the reaction integration of glycerolhydrogenolysis and CO2alcoholysis to PC.(1) The reaction for PC synthesis from CO2and propylene glycol (PG) over supportedZn(OAc)2catalyst was studied on a fixed-bed reactor for the first time. The effect ofsupporter and Zn(OAc)2loading on the catalytic performance and the effect of reactionconditions on PC synthesis reaction were investigated. The results indicates thatZn(OAc)2/AC with loading of40wt.%shows a better catalytic performance. The suitablereaction conditions were obtained as follows: n(PG): n(acetonitrile): n(CO2)=1:1.8:11,reaction temperature of160℃, CO2pressure of4.0MPa, Zn(OAc)2/AC volume of2mLand LHSV=0.9h-1. The yield and selectivity of PC were6.3%and49.0%, respectively.(2) The catalytic mechanism of Zn(OAc)2in the reaction of CO2and PG to PC waselucidated through in-situ IR and designed experiments. The chemical adsorption betweenCO2and Zn(OAc)2is weaker while it is much stronger between PG and Zn(OAc)2.Chemical bond can be obviously detected between PG and Zn(OAc)2, so Zn(OAc)2canactivate PG in this reaction.Secdonly, the reaction integration of glycerol hydrogenolysis and urea alcoholysis toPC was studied.(1) The effect of preparation conditions on catalytic performance of Cu/γ-Al2O3for thehydrogenolysis of pure glycerol to PG was investigated. The physicochemical properties ofCu/γ-Al2O3catalyst prepared by incipient impregnation method were studied by means of H2-TPR, XRD and CO chemisorption. The suitable conditions for the preparation ofCu/Al2O3catalyst were as follows: Cu loading of15wt.%, reduction atmosphere of purehydrogen, temperature-rising rate of2℃/min, reduction temperature of250℃, andreduction time of2h. The conversion of glycerol was100%, and the selectivity of PGreached92.9%. The characterization results indicate that:①Cu particles are highlydispersed on the surface of γ-Al2O3;②the particle size and dispersion of Cu are the mainfactors influencing the catalytic performance of Cu/γ-Al2O3: the smaller particle size andhigher dispersion of Cu result in better catalytic performance of the catalyst. The study onthe reaction pathway via acetol intermediate shows that the conversion of glycerol to acetolrequires both acid sites and metal sites and the hydrogenation of acetol to PG is the ratecontrolling step.(2) The reaction of urea alcoholysis to PC catalyzed by Zn-Al oxide was performed onthe fixed-bed reactor. The suitable preparation conditions of Zn-Al oxide were as follows:Zn(NO2+3+3)2and Al(NO3)3(n(Zn):n(Al)=3) as zinc source and aluminum source, NaOHand Na2CO3aqueous solution as the precipitant, using parallel-flow precipitation method,pH=9.5, aging at40℃for24h,and calcination at500℃for4h. Characterization resultsshows that specific surface area is the main factor influencing the catalytic performance ofZn-Al oxide. Then the catalytic performance of Zn-Al oxide for the synthesis of PC viaurea and PG was evaluated. Under the reaction conditions of n(PG):n(urea)=6:1, Zn-Aloxide volume of3mL, reaction temperature of140℃and LHSV=0.8h-1, the yield andselectivity of PC could reached87.4%and96.8%, respectively. Furthermore, the yield ofPC was no less than85%during the60h stability test, demonstrating that Zn-Al oxidecatalyst exhibits a pretty stable activity. Additionally React-IR was employed to investigatethe reaction pathway of PC synthesis from urea and PG. The results show that the reactionfor synthesis of PC proceedes in two steps: hydroxypropyl carbamate is formed from ureaand PG at the first step and then PC is produced by elimination of an ammonia fromhydroxypropyl carbamate molecule, which is the rate-controlling step.(3) The reaction integration of glycerol hydrogenolysis and urea alcoholysis to PC wasimplemented over both Cu/γ-Al2O3and Zn-Al oxide catalysts, which were separately filledin two sections of the fixed-bed reactors. The suitable operating conditions were as follows:n(H2): n(glycerol)=60:1, n(urea): n(glycerol)=1:3, urea concentration of0.10g/mL DMF,LHSV of glycerol=0.20h-1, temperature of the upper stage=190℃, temperature of thelower stage=150℃. Under these conditions, the conversion of glycerol was100%, theselectivity of PG was81.2%, and the yield of PC was16.3%. The selectivity of PC was 60.5%based on the PG amount formed in glycerol hydrogenolysis. In addition, the reactionpathway of this reaction integration was proposed.Lastly, the reaction integration of urea alcoholysis and transesterification to DMC wasinvestigated.(1) A new technology for DMC synthesis using urea and methanol as starting materialand PG as a recycle agent was implemented over Ca-Zn-Al oxide catalyst. The suitablepreparation conditions of Ca-Zn-Al oxide were as follows:①Zn(NO3)2and Al(NO3)3(n(Zn2+):n(Al3+)=3:1) as zinc source and aluminum source, NaOH aqueous solution as theprecipitant, using parallel-flow precipitation method, pH=9.5. After aging, filtration andwashing, Zn-Al hydroxides was obtained.②Ca(OH)2was introduced into Zn-Alhydroxides (n(Ca2+):n(Al3+)=0.2:1). After stiring, aging and drying, the cake was calcinedat650℃for4h to form Ca-Zn-Al oxide. The activity evaluation and characterizationresults indicate that the catalytic performance of Ca-Zn-Al oxide for the first step isaffected by its specific surface area while it is constrained by the basic amount for thesecond step.(2) The suitable operating conditions were as follows: n(PG): n(urea)=2:1, Ca-Zn-Alcatalyst weight percentage of2.7%, reaction temperature of170℃for the first step and73℃for the second step, reaction time of2h for the first step and8h for the second step, theflowrate of methanol=0.6mL/min. Under these conditions, the yield of DMC could reach82.9%.