Study On Optimized Preparation And Catalytic Performance Of Heterogeneous Catalyst For Biodiesel Synthesis From Waste Cooking Oils

To vigorously promote the development of biodiesel industry in our country seems helpful to environmental protection and energy security.At present in our country,though,there is still an obvious gab,where the high cost of feedstocks becomes the biggest challenge to achieve the large-scale industrialization of biodiesel.Based on the national conditions and development guideline,application of waste cooking oils(WCOs)as feedstock to produce biodiesel has become an effective strategy,which meets the supply of raw oils and environmental-treatment demands at the same time.Currently,the most-developed way is the homogeneous alkali-catalyzed transesterification,which is quickly with considerable weaknesses,though,for example,poor reusability,separation difficulty,equipment corrosion and-waste water production.Besides,the high content of free fatty acids badly deactivate the alkali catalysts,which makes it impossible for the direct application of WCOs.There are two reaction routes if WCOs are hegeneously catalyzed for biodiesel production.The First one is the one-pot synthesis catalyzed by solid acid.And second,a two-step process is referred,where the WCOs are catalyzed by solid acid to reduce the FFAs content below 1 wt.%and by solid alkali to proceed transesterification in succession.Generally the second route occurs with milder conditions.The carbon-based solid acid catalysts have gained lots of attentions due to the advantages of cheap raw materials,high catalytic activity and environmental-friendly properties.Application of bamboo for carbon-based solid acid synthesis helps to achieve a high value utilization,but has rarely been reported so far.For the alkali catalysts,the supported catalysts and metallic oxide are usually mentioned.Dolomite is a natural mineral,of which the main component,CaMg(CO3)2,can be calcined to generate CaO-MgO.The binary CaO-MgO has been proved to be effective for catalyzing transesterification.In this study,a two-step synthesis of biodiesel was put forward,from WCOs heterogeneously catalyzed by bamboo-based solid acid and the modified dolomite in succession.The preparation,characterization,performance and mechanism were systematically studied to optimize the synthesis,where several data analysis methods were applied,such as single factor,response surface methodology(RSM)and artificial neural network(ANN).It mainly included the following four aspects.(1)Taking the esterification of oleic acid with methanol as a probe reaction,the effects of different preparation methods on catalytic capacity were carefully revealed,where several characterization methods were applied,such as XRD,FTIR,SEM,EA,XPS,Hammett indicator and acid density determination.The bamboo-based solid acid obtained by the partial carbonization followed by sulfonation possessed surface acid density 1.80 mmol/g and a amorphous carbon structure with low graphitization degree,while the catalyst obtained by the H3PO4 activation followed by impregnation of H3PW12O40 method possessed surface acid density of 2.02 mmol/g with a high graphitization structure.The single factor experiments were conducted to optimize the esterification parameters,and the results showed that the former catalyst could achieve a high conversion under the more moderate conditions.The acid catalyzed esterification could be strengthened by the eternal fields,such as agitation,microwave and ultrasonic.Therein,a combination of microwave and agitation behaved the best.Besides,the bamboo-based solid acid was also able to catalyze both esterification and transesterification of WCOs simultaneously.During the usage of catalyst,the inevitable loss of active sites was the main reason for the decrease of catalytic activity.(2)Taking NaAlO2 as active substance,the catalysts were synthesized by impregnation with different supports and then characterized by XRD,Hammett indicator and N2 adsorption-desorption to study the influence of support types and impregnation methods on the catalytic performance.The RSM with CCD and BBD models was applied to optimize the alkali-catalyzed transesterification parameters.Compared with the inert carriers,such as MCM-41,SiO2,?-Al2O3,the halloysite nanotube(HNTs)helped the obtained catalysts behave better activity and stability.Besides,for the SA/HNTs catalysts,the cavitation phenomenon caused by ultrasonic profited the uniform distribution of NaAlO2.For the dolomite as an active carrier,the hydration-impregnation-calcination method helped to achieve better pore structure,performance and stability,which brought more value in industrial application.(3)By means of XRD,N2 adsorption-desorption and CO2-TPD,the influences of calcination,hydration treatment and modification with alkaline earth metal oxide SrO on the catalytic performance of dolomite were detailedly studied.The reaction mechanism of binary CaO-MgO generation from pyrolysis of dolomite at high temperature was revealed,where the calcination temperature had a significant effect on the catalytic activity.The hydration treatment of dolomite could develop its pore structure,increase the alkali density and strength on the surface,and thus enhance its catalytic activity.It was found that the modified impregnation method significantly improved the activity and stability of SrO/CD catalysts.Due to the formation of lamellar hydroxides during hydration,Sr was wrapped and strongly interacted with'CaO to effectively realize the mutual fixation.The three-layer ANN model optimized by genetic algorithm(GA)was established to train and predict the performance of SrO/CD catalyzing transesterification.The predicted extreme value of biodiesel yield was achieved to 99.15%with 4.8%catalyst loaded amount,8.2 methanol/oil molar ratio and 65.4? reaction temperature,which was very close to the experimental mean yield of 99.09%,indicating the accuracy of the model(4)The modified impregnation method was once again applied to synthesize an acid-base androgynous catalyst ZnO/dolomite to catalyze WCOs for the one-pot production of biodiesel where characterizations and the density functional theory were applied,respectively,to analyze the feasibility from the view of experiment and mechanism.After optimizing the reaction parameters with ANN method,it was found that with the Zn/Ca molar ratio of 1,the ZnO/CD successfully catalyzed the reactions to achieve 98.92%biodiesel yield.Based on the DFT theory,a CaO-Zn model was set up,by which the adsorption configuration of methanol and acetic acid was calculated.Results showed that the doping of ZnO helped to increase the stability of catalyst.And in the reaction system,methanol tended to be adsorbed at the Ca atom,while acetic acid was more likely to be adsorbed on the Zn atom and the C=O was activated.The conclusion were mutually supported by the experimental data.