Controlled Synthesis Of Carbon-Based Heterogeneous Catalyst And Catalytic Performances In Esterification For Biodiesel Production

Compared to the traditional fossil fuels,biodiesel has advantages in carbon emission reduction and pollutant control and is considered to be a typical renewable energy source.The main raw oils for domestic biodiesel production are waste cooking oils with high acid value,which will poison the base catalysts during the transesterification process and reduce the biodiesel yield.Therefore,the acid catalysts are needed to weaken the acid value of raw oils for further biodiesel production.Heterogeneous catalyst,for its high catalytic ability,easy separation,non-toxicity and non-pollution,has attracted more research attention in recent years.In this study,a series of heterogeneous catalysts are prepared from bamboo,bituminous coal and coal-baesd actived carbon via different methods and used in catalyzing esterification of oleic acid with methanol for biodiesel production.Conditions for catalysts preparation are optimized and the catalyst reusabilities are investigated respectively.The prepared catalysts are characterized by various techniques to explain the catalytic performances,including XRD,ATR-FTIR,N2 adsorption-desorption analysis,NH3-TPD,XPS,EA and acid density titration.Meanwhile,the esterification conditions are optimized via single-factor tests and response surface method(RSM).Moreover,molecular simulation is conducted to interpret promoted mechanism of acid catalyst in esterification based on the density functional theory(DFT),where the geometry of the reactants and products are optimized by the GGA-PW91 functional and transition state is searched through the complete LST/QST protocol by means of Dmol3 model.Finally,the molecular thermodynamic and kinetic parameters are calculated by frequency analyses.(1)A series of carbon-based solid acid catalysts are synthesized from bamboo through the approach of partial carbonization and sulfonation,where bamboo is chemically activated by phosphoric acid in advance.After carbonization at 350? for 2 h and sulfonation at 105? for 4 h,the catalyst gained amorphous carbon structure with large surface area of 1207.7 m2/g and strong acid density of 1.28 mmol/g.Due to the leaching of-SO3H groups,after fourth reused cycle of the catalyst,the biodiesel yield declines to 83.7%.The biodiesel yield reached 97.3%with catalyst dosage of 10 wt.%,molar ratio of methanol to oleic acid of 10,esterification temperature at 65?and 2 h.(2)Carbon-based heterogeneous acid catalysts are prepared from bituminous coal through the partial carbonization and sulfonation method.XRD results prove the catalyst an amorphous carbon structure.Further ATR-FTIR and XPS characterizations show that the-SO3H groups loading.Carbon catalyst,prepared by partially carbonized at 350? and sulfonated at 105?,catalyzes esterification and gains 98.7%yield with the conditions of catalyst dosage of 10 wt.%,molar ratio of methanol to oleic acid of 10,esterification temperature at 65 ? and 3 h.Finally,the activation energy of esterification(E?=54.69kJ/mol)and pre-exponential factors(A=8.93×108 min-1)are calculated through kinetic studies.(3)ZrO2-actived carbon heterogeneous acid catalysts are synthesized by the optimized conditions of Zr/AC mass ratio of 1:1,sulphuric acid concentration of 0.5mol/L,and the acid density achieve 5.46mol/g.Characterizations show that the catalyst possesses large specific surface area of 236.9 m2/g and plenty of ZrO2,where the-SO3H groups can easily be loaded.The esterification yield reduces to 95.85%after four reused cycles.Moreover,the yield reaches 99.41%with the optimized conditons of catalyst dosage of 6.82 wt.%,molar ratio of methanol to oleic acid of 22.02,esterification time of 55.19 min,which tallies good with the RSM prediction.(4)Two reaction routes are designed based on DFT and the molecular model reaction are established and optimized by Dmol3 model.Trasition state results show that the energy barrier of non-catalyzed route is 272.2kcal/mol.Finally,the thermodynamic and dynamic calculations are conducted according to the simulation results,which indicates that the esterification of oleic acid with methanol is mainly composed by nucleophilic reaction and intramolecular dehydration and the reaction is spontaneous and exothermic.The catalyst can effectively reduce the reaction barrier,increase the reaction rate and promote the esterification reaction.