Controlled Synthesis Of Heterogeneous Base Catalyst And Its Performance Optimization And Mechanism Study On Biodiesel Production From High Acid Value Oil

The development of biodiesel plays a significant role in China’s energy structure transformation,energy security,and economic development,due to its renewable nature,biodegradability,and environmental friendliness.However,the industrialization of biodiesel faces a major obstacle in the form of high production costs,primarily attributed to the cost of refined oils such as sunflower seed oil,soybean oil,and rapeseed oil.To address this issue,utilizing high-acid value feedstocks like microalgae,waste oils,and Mahua oil can effectively reduce production costs while ensuring food security in China.Currently,the commonly used homogeneous alkali-catalyzed method in the industry exhibits limitations such as poor catalyst reusability,complex separation and purification processes,and short equipment lifespan.Moreover,the free fatty acids present in high-acid value feedstocks undergo saponification with the alkali catalyst,leading to catalyst poisoning and deactivation.The use of bifunctional solid catalysts can overcome the detrimental effects of free fatty acids and convert both free fatty acids and triglycerides into biodiesel through esterification and transesterification reactions,respectively.Al2O3 possesses a rich porous structure favorable for mass transfer,effectively reducing internal diffusion resistance of large molecular reactants.It also has a high surface area for dispersing a large number of active sites,and the strong interaction between alumina and the active component can enhance catalyst stability.Therefore,alumina exhibits potential as a solid catalyst support for biodiesel production.Transition metal oxides ZrO2 and perovskite have both acidic and basic properties.By adsorbing free fatty acids onto acidic sites,the acid resistance of the catalyst is enhanced,enabling it to maintain activity in reactions involving high-acid value feedstocks.This paper proposes the research approach of using bifunctional solid catalysts for simultaneous transesterification and esterification of high-acid value feedstocks to produce biodiesel.The research focuses on catalyst preparation methods,component regulation,physicochemical properties,reaction conditions,and catalytic mechanisms.Response surface methodology,artificial neural networks,and convolutional neural networks are employed to conduct regression analysis on reaction conditions.The research is systematically conducted,considering four aspects:catalyst activity,stability,acid resistance,and performance in simultaneous esterification and transesterification.The research mainly includes the following aspects:(1)The effects of different active components on the activity and stability of aluminasupported catalysts were investigated.Solid base catalysts were prepared using alkali metal oxide NaAlO2 and alkaline earth metal oxides SrO and CaO.The transesterification between palm oil and methanol was employed as a probe reaction.Various characterization techniques,including TG,XRD,N2 adsorption-desorption,SEM-EDS,CO2-TPD,XPS,ATR-FTIR,and ICP-AES,were utilized.It was found that leaching of active components during the transesterification was the main reason for the reduced reusability of alumina-supported catalysts.The formation of solid solutions between active components and the support enhanced the stability of the loaded active components.In the NaAlO2/y-Al2O3 catalyst,the active components reacted with methanol to generate Na+ and AlOOH,leading to Na+leaching and rapid decline in catalytic activity during repeated use.The formation of Ca5Al6O14 and Ca12Al14O33 in the SrO-CaO-Al2O3 catalyst,which exhibited strong interaction with CaO,effectively addressed the issue of Ca2+leaching from the catalyst,resulting in good reusability.The developed porous structure of Al2O3 ensured sufficient contact between reactant molecules and active sites,with a predominance of mesopores in the prepared catalysts.Furthermore,the Al-O-Ca groups on the surface also served as active sites for the transesterification,thereby providing excellent catalytic activity for CaO-Al2O3.(2)A solid catalyst was prepared by modifying SrO-Al2O3 with transition metal zirconium,and various characterization techniques were employed to investigate the effects of different preparation methods and zirconium loading on the acid resistance mechanism of the solid catalyst.The reaction parameters were optimized using a backpropagation neural network combined with the genetic particle swarm optimization algorithm.It was found that the catalyst prepared by co-precipitation exhibited the highest activity in the reaction with high-acid value feedstocks.The catalysts prepared by solid-phase,hydrothermal,and impregnation methods contained three components:SrO,ZrO2,and Sr3Al2O6,while the catalyst prepared by coprecipitation mainly consisted of SrZrO3.SrO effectively promoted the transesterification,but it was prone to saponification reaction with free fatty acids,resulting in catalyst deactivation.Therefore,catalysts with SrO as the main active component showed lower yields in the production of biodiesel from high-acid value oils.SrZrO3 exhibited unique acid-base properties.Compared to pure SrO catalyst,the presence of acidic sites in SrZrO3 effectively prevented the poisoning of Sr active sites by free fatty acids and facilitated the esterification of free fatty acids.This allowed the catalyst to maintain high catalytic activity under high-acid value reaction conditions.Additionally,the interaction between Sr and Zr provided effective stabilization of the catalyst.(3)To achieve simultaneous esterification and transesterification,a bifunctional perovskite catalyst,SrZr1-xFexO3,was prepared by substituting Zr elements with transition metal Fe in SrZrO3 catalyst.Various characterization techniques were employed to investigate the effects of calcination temperature and Fe substitution level on the catalytic performance of the catalyst.Additionally,a convolutional neural network was utilized to explore the mapping relationship between reaction parameters and fatty acid methyl ester yield.Higher calcination temperature promoted the crystalline growth of SrZrO3 catalyst.As the calcination temperature increased,the grain size of SrZrO3 increased and lattice strain decreased.The catalyst exhibited the highest crystallinity and biodiesel yield when the calcination temperature reached 1000℃.After partial substitution of Zr with Fe,the surface charge density of Sr2+and Zr4+ increased due to the different ionic radii and valence states of the two elements.This accelerated electron transfer between different valence B-site cations and generated a large number of oxygen vacancies.Oxygen vacancies can activate oxygen molecules into chemically active oxygen species,providing additional active sites for the reaction,thereby enhancing the catalytic activity of the catalyst for esterification and transesterification.The SrZr0.75Fe0.25O3 catalyst demonstrated the ability for simultaneous esterification and transesterification,achieving a fatty acid methyl ester yield of 98.5%with an addition of 10 wt.%oleic acid,meeting the standard for fatty acid methyl ester content in biodiesel.(4)To provide theoretical support for simultaneous esterification and transesterification,molecular simulation studies were conducted to investigate the adsorption mechanisms of reactant molecules(methanol,acetic acid,and methyl acetate)on different active sites of SrZrO3 and SrZr-xFexO3 models.The computational results demonstrated that Fe substitution enhanced the performance of SrZrO3 in simultaneous esterification and transesterification.When using SrZrO3 catalyst for biodiesel production from high acid value oil,the reactant molecules,including methanol,acetic acid,and methyl acetate,preferentially adsorbed on Zr active sites,leading to competitive adsorption and hindrance to efficient reaction progression.However,for the SrZr1-xFexO3 catalyst,methanol preferentially adsorbed on Sr sites,acetic acid preferentially adsorbed on Zr or Fe sites,and methyl acetate preferentially adsorbed on Fe sites.This alleviated the competitive adsorption of reactant molecules on Zr sites and enhanced charge transfer between the carbonyl oxygen atom of methyl acetate and the catalyst surface,thereby increasing the electro positivity of the carbonyl carbon atom and facilitating the activation of methyl acetate molecules.This,in turn,favored efficient subsequent transesterification and esterification reactions.After Fe substitution,the adsorption energy of water molecules on the SrZr1-xFexO3(110)surface at Sr and Zr sites significantly decreased.Fe,as an additive,weakened the adsorption capacity of water molecules at Sr and Zr sites,thereby enhancing the catalyst’s resistance to water.