Density Functional Theory Study On Reaction Mechanism For Biodiesel Synthesis With Waste Cooking Oils

Nowadays, biodiesel synthesis with waste cooking oils(WCOs) can not only achieve the cyclic utilization of rubbish, but also alleviate the increasingly-serious energy problem, which has drawn extensive attentions of researchers. At present, most investigations for the utilization of WCOs in biodiesel synthesis are experimental studies, however, the reaction mechanism, as a deeper theoretical aspect of a catalytic reaction, is still lack of discussion and reports. Therefore, it has become a burning question in the research of biodiesel synthesis with WCOs to clarify the reaction mechanism, obtaining the rate-determining step in reaction process, and putting forward effective ways to promote reaction, which will provide important theoretical basis for experimental study.In this paper, based on the physical and chemical characteristics of WCOs and the industrial technology of biodiesel manufacture, first-principle calculation using the density functional theory(DFT) method is conducted for three typical reactions in biodiesel synthesis with WCOs: free fatty acid(FFA) esterification reaction over acid catalyst, monoglycerides trans-esterification reaction over acid catalyst and pyrolysis reaction of triglycerides. The main conclusions are as follows:(1) The activation energy required for FFA esterification can be efficiently reduced over acid catalyst. The mechanism of FFA esterification reaction catalyzed by acid is that, H~+ firstly activates the carbonyl O in FFA molecule, and then the intermediate combines with methanol to form a tetrahedral structure that is the rate-determining step, and finally, the fatty acid methyl ester(FAME) is obtained after removing a water molecule. FFA esterification reaction is an exothermic process, and the reaction temperature plays an important role in ensuring the reaction rate. In addition, increasing the acid(H~+) content or methanol content should be more efficient to improve the yield of FAME for FFA elimination in utilization of WCOs for biodiesel synthesis.(2) The activation energy required for monoglycerides trans-esterification can be efficiently reduced over acid catalyst. Monoglycerides trans-esterification reaction mechanism catalyzed by acid is that, the H~+ firstly activates the carbonyl O in monoglycerides molecule to form a reaction intermediate, and then the intermediate combines with methanol to form a tetrahedral structure, in the next step, the tetrahedral structure dissociates to remove a glycerol molecule, which is the rate-determining step, and finally, the target product FAME is obtained after removing a hydrogen ion. Monoglycerides trans-esterification reaction is an exothermic process, and the reaction temperature plays an important role in ensuring the reaction rate, meaning that increasing the reaction temperature should be in favor of the dissociation of the tetrahedral structure which can accelerate the trans-esterification reaction efficiently.(3) Pyrolysis of triglycerides starts with the cleavage of C-O bond at lower reaction temperature, and long-chain fatty acids are the main pyrolysis products at this stage. The cleavage of C-C bond takes place at the temperature about 360 ℃, and the temperature for biodiesel synthesis by pyrolysis method should be higher than this temperature. With the increasing of pyrolysis temperature, the quality and yield of biodiesel product will be improved. Thermodynamic calculation results of triglycerides pyrolysis reaction are consistent with the literature reports, indicating that the method designed in this paper can provide a simple and feasible approach for further study of pyrolysis mechanism for biodiesel synthesis.