Cooperative Effect of H+ and Solid Acids In the Process of Converting Acidic Oils Into FAME

Transesterification of triglycerides with methanol is an important reaction for synthesis of fatty acid methyl esters (FAME), which is used as biodiesel and important chemical intermediates. Conventionally, some homogeneous base catalysts such as NaOH and NaOCH₃ were employed. However, these homogeneous catalysts are corrosive. And removal of these catalysts after reaction causes a large amount of waste wash water and a long time for phase separation. Therefore a new heterogeneous catalytic process is desirable for biodiesel production, which has a low operation cost. Free fatty acids (FFA) are generally considered as an important factor for oil quality control during oil refinery process, and it widely exists in inexpensive oils, such as crude vegetable oil, waste cooking oil and trap grease. Many researchers have shown that this kind of inexpensive oils cannot be directly used in traditional biodiesel production process, as FFA will react with homogeneous catalysts (NaOH and NaOCH₃). Therefore, developing a series of catalysts that can convert FFA and triglyceride in inexpensive oils into FAME becomes attractive for present biodiesel plants.    

Our work discloses that a series of ZnO containing metal oxides can simultaneously catalyze both transesterification and esterification, and have triglyceride and FFA fractions converted into FAME. Catalytic mechanism for conversion process is discussed. Transesterification of triglyceride with methanol is catalyzed by both H+ originally from FFA and Lewis acid sites from solid catalysts. Our results show that there is a cooperative catalysis between H+ and solid acids. In presence of solid acids, the reaction rate of FAME formation in acidic oil is much higher than the case with pure triglyceride, and the case without solid acids addition. Modification of ZnO-based catalysts to change the electronic configuration of ZnO can greatly improve catalyst activity in both transesterification and esterification.