Study On Synthesizing Biodiesel Catalyzed By Solid Base Catalyst

Biodiesel is being developed as one of the most suitable substitutions of fossil fuels in solving energy problems as well as the relative environmental problems, since it is a green, safe and reproducible alternative energy. However, some problems still exist in the transesterification catalyzed by the most frequently used homogeneous base catalyst, such as the high requirement for the quality of materials, the severe erosion to the equipments, the difficulties in separations, and the large amount of wastewater produced during the processes of separation and purification, resulting in severe pollution to environment. As some solid base catalysts are considered to be environmental benign and may solve the above problems, it is of significance to make experimental and theoretical studies for synthesizing biodiesel catalyzed by solid base catalysts with high activity.In this work,two kinds of solid bases were studied for producing biodiesel with rapeseed oil and methanol. The thermodynamic calculation and the kinetics of transesterification catalyzed by solid base were also presented.The solid base catalyst KOH/γ-Al₂O₃ was prepared by impregnation method. The experimental results demonstrated that the catalyst KOH/γ-Al₂O₃ has high activity under the following optimized preparation conditions: the adding amount of KOH is 35 wt% referred to the amount ofγ-Al₂O₃, the calcination temperature is 500℃and preparation time is 6 h. With the prepared KOH/γ-Al₂O₃, the biodiesel yield can reach as high as 84.20% after 6 h reaction at 65℃, with a 15:1 molar ratio of methanol to oil, a catalyst amount of 2.5 wt%, and a stirring rate of 270 rpm. The prepared catalyst was characterized by Hammett indicator method, CO2-TPD, XRD, SEM, BET, IR and XPS. It can be found that the Al-O-K species is produced and both the basic strength and the basicity of the catalyst are increased afterγ-Al₂O₃ is loaded with KOH upon calcinations. Moreover, the heterogeneous base KOH/γ-Al₂O₃ is more tolerant to free fatty acid and water contents compared to homogeneous KOH.The solid base K/KOH/γ-Al₂O₃ was also prepared by impregnation followed with high temperature reaction. It was firstly used in catalyzing the transesterification of rapeseed oil with methanol to synthesize biodiesel. Both the catalyst preparation conditions and the reaction conditions of the transesterification were optimized. The results demonstrated that the catalyst K/KOH/γ-Al₂O₃ has high catalytic activity with the preparation conditions that the adding amounts of KOH and K are separately 20 wt% and 7.5 wt% referred to the amount ofγ-Al₂O₃ with temperature of 360℃and 240℃and the time of 2h and 1h, respectively. With the prepared K/KOH/γ-Al₂O₃, the biodiesel yield can reach as high as 84.52% after 1 h reaction at 60℃, with a 9:1 molar ratio of methanol to oil, a catalyst amount of 4 wt%, and a stirring rate of 270 rpm. The characterization results of catalyst show that after metal potassium is subsequently loaded onto KOH/γ-Al₂O₃ to form catalyst K/KOH/γ-Al₂O₃, not only the Al-O-K species increase, but also the surface color centers of Fs(+) are produced, causing the further decrease of the O1s binding energy. And thus, the surface of catalyst K/KOH/γ-Al₂O₃ has stronger electron donor sites, showing superbase property. It was also found that only 4.2% potassium was extracted from catalyst K/KOH/γ-Al₂O₃ after the first time use. With the recycled catalyst, the biodiesel yield reached 75.61%, suggesting that the catalyst has relatively high chemical stability.Thermodynamic calculation of the three consecutive transesterification reactions of triolein with methanol was carried out. The results demonstrated that in the temperature range of 30 65℃, the reaction enthalpy is around zero, the equilibrium constant is about 1, the reaction is close to thermodynamic equilibrium, and the standard Gibbs free energy change is positive for each step of the three consecutive reactions. The activity coefficient of main composition (i.e., methanol) is around 1.The kinetics of transesterification of triolein with methanol catalyzed by K/KOH/γ-Al₂O₃ was studied under the optimized reaction conditions. The kinetic model, which assumed surface reactions as rate-determining steps, was proposed. The values of activity energy, integral adsorption heat and pre-exponential factor were obtained by the kinetic parameters estimated from the model. The residual standard deviation between calculated results and experimental data for the biodiesel yield of transesterification at 60℃is 0.0196, demonstrating that the kinetic model is reliable enough to provide an important guidance for industry.