Controlled Synthesis Of Perovskite-type Heterogeneous Catalyst And Catalytic Performances For Biodiesel Synthesis From High Acid Oil

Controlling fossil energy consumption and promoting the development of new energy is the best way to achieve the goal of "Carbon Peaking and Carbon Neutrality".Biodiesel is an excellent alternative to fossil fuels.However,the high cost of feedstocks has become the biggest challenge to achieve the large-scale industrialization of biodiesel in China.Therefore,using waste oil as feedstock for biodiesel production not only meets the demand of supply and environmental protection but also accords with the national conditions.However,the traditional method requires esterification to reduce the free fatty acid(FFA)content first due to the high content of FFAs in waste oil,which is complicated and costly.Thus,the development of heterogeneous bifunctional catalysts has become a research hotspot.In this paper,SrTiO3 and Fe-doped SrTiO3 perovskite bifunctional catalysts are prepared and applied in the transesterification reaction of palm oil with high acid value and methanol.On the one hand,through the experimental method,the preparation conditions of the catalysts are optimized,the physicochemical properties of the catalysts are analyzed by different characterization methods,the reaction parameters of the catalysts in transesterification are optimized,the acid and water resistance of the catalysts are discussed,and the reusability of the catalysts is evaluated.On the other hand,the mechanism of oxygen vacancy formation and adsorption characteristics of reactant molecules on catalyst surface before and after Fe doping are investigated based on density functional theory.(1)The catalytic performance of pure strontium titanate perovskite for biodiesel production is studied.A pure mesoporous SrTiO3 with a honeycomb pore structure is synthesized by the sol-gel method.The highest FAME yield of 93.14%is achieved in transesterification with the molar ratio of methanol to oil of 15:1 and catalyst concentration of 6 wt.%at 170℃ for 3 h.Besides,the catalyst is highly tolerant of FFAs and moisture,where the FAME yields of 83.80%and 86.50%are achieved even with the oleic acid and water addition of 10 wt.%and 5 wt.%.Meanwhile,the catalyst is found to be an effective catalyst for simultaneous transesterification and esterification.In addition,the prepared catalyst has good reusability,where the FAME yield can still reach 85.68%after three cycles.(2)The catalytic performance of strontium titanate type perovskite doped with B position by Fe element is investigated.The mesoporous SrTi1-xFexO3(x=0.05,0.10,0.15 or 0.20)catalysts are synthesized via the sol-gel method.The Fe dopant results in many oxygen vacancies in the perovskite structure.The formation of oxygen vacancies increases the electron density of the active sites on the surface and promotes the polarization of the perovskite structure,which improve the catalytic activity of the catalyst.The most oxygen vacancies and the best catalytic performance are observed in the SrTi0.85Fe0.15O3 catalyst,and the FAME yield of 97.52%can be achieved with the catalyst concentration of 5 wt.%and methanol to oil molar ratio of 18:1 at 150℃ for 3 h.Meanwhile,the catalyst maintains a high FAME yield(>80%)after 4 cycles.Also,the catalyst exhibits strong resistance to FFAs,where the FAME yield is still 93.58%even with oleic acid addition of 12 wt.%.Besides.the catalyst has catalytic activity for simultaneous esterification and transesterification.(3)The adsorption mechanism of reactant molecules on active sites in the simultaneous transesterification and esterification process of high acid value oil catalyzed by strontium titanate perovskite before and after Fe doping is revealed by molecular simulation.The calculated results illustrate that after Fe doping,oxygen vacancies are more likely to be formed on the surface of strontium titanate,leading to the increase of the electron density of the active sites on the surface,which improves the catalytic activity.Meanwhile,Fe doping is beneficial to the adsorption and the activation of methanol on the surface of the catalyst,thus promoting the transesterification reaction.In the reaction systems of the two catalysts,acetic acid tends to be adsorbed on the Ti active sites and activated,resulting in the esterification reaction.And the Sr active sites can be retained to catalyze the transesterification reaction of methanol.Therefore,both catalysts are highly tolerant of FFAs and can catalyze simultaneous esterification and transesterification.