DEVELOPMENT OF NOVEL IONIC LIQUID CATALYSTS FOR BIODIESEL PRODUCTION FROM CRUDE PALM OIL

The transesterification of triglyceride (TG) and esterification of free fatty acid (FFA) with methyl alcohol either by alkaline or acidic catalysis are the fundamental reaction for biodiesel production. Today’s commercial processes use mainly homogeneous alkaline catalysts when the feedstock is a well treated refined oil. However, the use of acidic-H2SO4 catalyst becomes important when dealing with cheap feedstocks that contain high amounts of FFA, such as crude vegetable oils (unrefined oils) or restaurant waste oils. Unfortunately, the use of H2SO4 necessitates the installment of expensive, corrosion resistant equipment. In addition, there are several other problems associated with this technique that makes it neither technically nor commercially feasible.
The main objective of this research is to design novel green catalysts that capable of replacing traditional acid or alkaline catalysts. Ionic liquids (ILs) are considered as a clean chemical that have advantageous properties over acid or alkaline homogeneous catalysts with the prospect of improving the technical, economic and environmental aspects of traditional biodiesel production processes. The IL features such as ease of separation, catalyst reusability, less waste water generation, and non corrosive nature offer several advantages that allows for direct and continuous processing, and several process design possibilities. In particular, acidic IL catalysts have the ability to catalyze both transesterification and esterification reactions simultaneously. Most importantly, glycerol with a higher purity can be easily recovered without complicated processes, which can contribute approximately 6% saving to the production cost.
Twenty four thermally stable ILs were investigated with the objective of finding suitable catalyst and the optimum reaction conditions for biodiesel synthesis. The results suggest that the nature of the hydrogensulfate (HSO4–) anion and active sites for Brønsted IL-acidic catalysts, if its imidazolium, pyrazolium or ammonium based cations, determine the acidity of ILs. Among those ILs, imidazolium based cation is found to be a more superior acidic catalyst than pyrazolium or ammonium based cations. IL catalyst type, IL concentration, reaction temperature, molar ratio of methanol to crude palm oil and mixing intensity are found to be the influential variables affecting the percentage of esters conversion. A maximum esters yield of 94.6% was obtained using 5.0 % (wt/wt) of 1-butyl-3-(4-sulfobutyl) imidazolium hydrogensulfate (BSBIMHSO4) at methanol to crude palm oil molar ratio of 12:1, mixing intensity of 800 rpm, and temperature of 170°C for 7 hours.
In this study the IL-catalyzed transesterification is found to follow the 2nd order reaction kinetic. The reaction mechanism does not follow neither acidic nor basic catalyst. Hence, a new reaction mechanism is proposed. The presence of trace amount of water in IL has detrimental effect on the transesterification reaction beyond the scope of quenching the methanolysis reaction; trace amount of moisture absorbed by the IL could lead to deactivation of the catalyst. Generally, commercial processes involve low reaction temperatures i.e. near the boiling point of methanol to keep the alcohols in liquid phase. However, the IL catalyzed transesterification requires high temperature to overcome the mass transfer limitation of the three-phase CPO/methanol/IL, and to increase the solubility between the viscous CPO and the highly viscous IL-methanol mixture in order to produce higher esters yield.
The investigated ILs showed ability to be easily recovered by simple treatment since they are not soluble in both feedstock and ester product. Besides, these ILs can be reused for many cycles of catalytic reactions. This work has shown experimentally that the ILs can be recycled eight times without any loses in the yield and can be recovered almost 100 % from each transesterification reaction.
The results have shown that acidic ILs can effectively catalyze transesterification reaction of feedstocks containing high FFA. Although BSBIMHSO4 exhibits similar activity as sulfuric acid (H2SO4) mineral catalyst, it offers other advantages such as ease of separation, recoverability, recyclability, ease of glycerol separation, uses less amount of alcohol, does not require expensive equipment, and subsequent wastewater treatment is also not required. Owing to these advantages, it is believed that ILs are the potential green catalysts to replace conventional alkaline or acid catalysts.