Synthesis Of A New Diesel Additive Polyoxymethylene Dimethyl Ethers

In recent years,exhaust emission of vehicles has brought many problems,so it is urgent to solve the problem of exhaust pollution.Polyoxymethylene dimethyl ethers(PODEn)are excellent diesel additives,which are blended to the diesel fuel to improve the combustion of diesel,reduce the emissions of solid particles and carbon oxides and make the thermal efficiency of engine better.Therefore,PODEn have attracted more and more attention.However,there are some problems in industrial production,which are expensive paraformaldehyde with low degree of polymerization and difficult separation of PODEn.In order to solve the above problems,formaldehyde and methanol were used as new reaction materials,and new acidic catalysts were developed Furthermore,the synthesis rule of PODE,performance of catalysts and separation of PODE products were studiedFirst of all,the effects of mass ratio of methanol and formaldehyde,catalyst dosage,reaction temperature,reaction time,water content and gas atmosphere on the yield of PODE products and formaldyhede conversion were studied when H-SAR catalyst was used to catalyze the synthesis reaction of PODE.The results indicated that the reaction conditions(mass ratio of methanol and formaldehyde 2:4,amount of catalyst 10w%,reaction temperature 60?,reaction time 120 min,air atmosphere and anhydrous environment)were optimal.The yield of PODE1-9,PODE2-9 and PODE3-6 respectively reached 38.97w%.26.76w%and 15.6w%,and the formaldehyde conversion was 39.93%at above reaction conditionsSecondly,synthesis of PODE products from methanol and formaldehyde catalyzed by ZnC12 were investigated.The results indicated that the optimal conditions found in the catalytic reaction of ZnCl2 were methanol and formaldehyde mass ratio 2:4,reaction temperature 105?,reaction time 300 min and the amount of catalyst 3w%.The yields of PODE1-6,PODE2-6 and PODE3-6 were 29.48w%,19.9w%and 11w%at the above conditions,respectively.Compared with the yields of 37.86w%,23.83w%and 12.19w%with FeC13 as catalyst,they were slightly lower.Therefore,FeC13 was used to modify H-SAR catalyst.The effects of mass ratio of FeC13 and H-SAR catalyst,temperature,time and solvent type on activity of Fe-SAR catalyst were investigated.The results showed that mass ratio of FeC13 and H-SAR catalyst 0.5:1,modification temperature 70?,ethanol solvent and modified time 8 h were the optimal modification condition The effects of mass ratio of methanol and formaldehyde,amount of catalyst,reaction temperature,reaction time and water content on the yield of PODE products and formaldyhede conversion were studied.The results indicated that activity of the modified catalyst Fe-SAR was optimal at mass ratio of methanol and formaldehyde 2:4,dosage of catalyst 10w%,reaction temperature 70?,reaction time 240 min,and anhydrous environment.The yield of PODE1-9,PODE2-9 and PODE3-6 were 45.27w%,27.76w%and 14.82w%,respectively,and the conversion of formaldehyde was 43.40%at the above reaction conditionsNext,the structure,morphology,composition,acidity and stability of H-SAR and Fe-SAR catalysts were studied and compared by N2 isothermal adsorption-desorption,SEM,NH3-TPD,pyridine-IR and TG.And activity of the two catalysts were analyzed and compared.The results showed that the Fe-SAR catalyst had smaller surface area and pore volume,more developed pore structure,stronger acidity,less B acid,more L acid,and higher stability and catalytic activityThen,the reaction kinetics of H-SAR catalysis was studied.The reaction orders for formaldehyde,methanol,methylal and water were 0.32,0.45,0.42 and 0.86,respectively,which indicated that synthesis reaction would shift in the positive direction when concentrations of methanol and formaldehyde were increased or that of methylal and water were decreased;the formation and decomposition energies were 42.50 kJ·mol-1 and 45.73 kJ·mol-1 respectively,which demonstrated that the synthesis reaction was exothermic.The reaction mechanism of methanol and formaldehyde catalyzed by H-SAR and Fe-SAR catalysts were studied.It was concluded that PODE products could be produced by both chain growth and aldol condensation,and acetals with different degrees of polymerization could react with one another when the aldol condensation reaction was carried outFinally,the standard reaction enthalpy ?rHm,T,?,standard reaction entropy?rSm,T ?,standard Gibbs free energy ?rGm?,T,and standard equilibrium constant K?were calculated by group contribution method.The results indicated that the reaction of methanol and formaldehyde to synthesize PODE under acid catalysis was exothermic,and the improvement of temperature was unfavorable to the reaction.The liquid-liquid phase equilibrium data for six systems(water+methylal+p-xylene,toluene,n-heptane and water+PODE3+p-xylene,toluene,n-heptane)at 30? and 40? were measured by liquid-liquid equilibrium kettle method.The extraction ability of three kind of extractants were analyzed.Moreover,both NRTL and UNIQUAC thermodynamic model were used to correlate the experimental data for obtaining the binary interaction parameters.The results indicated that p-xylene,toluene andn-heptane had excellent extraction effects for separating methylal systems and PODE3 systems,and LLE data were correlated by NRTL and UNIQUAC models with good results in these six systems Thermodynamic data determined by liquid-liquid equilibrium kettle method can provide basic data for the extraction separation of PODE products and water.