Ionic liquid (IL), or also known as molten salt, has emerged as one of the potential candidate in the effort of searching for greener catalyst in biodiesel production. It is considered to be superior to its homogeneous catalyst counterpart (i. e., NaOH, H2SO4) in terms of its properties, such as volatility and solubility with reactants. The out­standing volatility of IL is attributed by its low vapor pressure and high thermal stabil­ity. The ionic nature resulted in strong bond between the cation and anion of an IL.

It is considered greener solvent than common volatile organic compound that decomposes at high temperature and released to the atmosphere. Another important characteristic of an IL is its solubility with reactants, especially the poor solubility of IL in biodiesel phase (Fang et al. 2010). This helps to direct the transesterification towards the product side as a result of solubility difference between IL and biodiesel. IL can be synthesized by manipulating the combination of different cations and anions, and the physicochemical and thermal properties are influenced by the types of ions. Figure 9.3 represents some of cations and anions that are available for ionic liquid synthesis.

Cation

Fig. 9.3 Commonly used cations and anions in ionic liquids

Not only IL has excellent properties that portray its greenness than conventional organic solvents, it also has catalytic activity as good as conventional catalysts in biodiesel production.

In the transesterification of cottonseed oil catalyzed by Brqrnsted acidic ionic liquids, Wu et al. (2007) found that the best ionic liquid (1-(4-sulfonic acid) butyl- pyridinium hydrogen sulfate) had almost similar FAME content (81%) compared to the one produced using concentrated H2SO4 catalyst (86%) after 3 h reaction time. The high catalytic performance was contributed by its strong Brqrnsted acidity, the type of cation used, and also the length of carbon chain in IL catalyst. Meanwhile, Liang et al. (2009) used ethylammonium chloride ([EtNH3][Cl]) mixed with metal chlorides that provide Lewis acid sites for the conversion of soybean into FAME. The comparison shows that the novel catalyst resulted in highest biodiesel yield than other traditional conventional catalyst, such as sulfuric acid, phosphoric acid (H3PO4), and p-toluenesulfonic acid (PTSA).

The combination of its cation and anion governs the performance of an IL. Zhang et al. (2009) prepared several Brqrnsted acidic ionic liquids that were later used for esterification of FFAs. Based on the analytical productivity of ethyl oleate, they found that ILs with same cation (i. e., CH3SO3Hmim) but different anions resulted in different yields. [CH3SO3Hmim][CH3SO3] catalyst performed the best for the same cation. The stronger acidity of IL was believed to be influenced by the stronger acid used to acidize the inner salt MIMPS. On the other hand, [NMP][CH3SO3] stands out as the catalyst that produced the highest biodiesel yield (96.5%) for the same anion. Added with the economical cost of cation and easier catalyst preparation, the

catalyst seems to be a good candidate for conversion of FFAs and biodiesel

synthesis.

The catalytic activity of ILs can be enhanced by means of the inclusion of metal chlorides. Guo et al. (2011) studied the addition of metal chlorides in ILs to check their effect on esterification of oleic acid and transesterification of crude Jatropha oil (CJO) to produce biodiesel. After the screening of several commercial ILs for con­verting oleic acid to methyl ester, 1-butyl-3-methylimidazolium tosylate ([BMIM] [CH3SO3]) was further studied as it gave the highest conversion rate. Later, several divalent and trivalent metal chlorides were added to the IL and tested for the trans­esterification of CJO. Results shown that the IL with FeCl3 has the highest biodiesel yield (99.7%) among all the metal chlorides studied. They concluded that trivalent metallic ions showed higher activity due to their stronger Lewis acidity than biva­lent metallic ions.

Ionic liquids also have the edge over conventional catalysts in terms of their cata­lytic performance when different feedstocks and alcohols are used. In the work by Fang et al. (2010), the catalyst with the excellent activity ([TMEDAPS][HSO4]) was further studied to observe the effect using different fatty acids and alcohols. Neither the length of the alkyl chains of alcohols nor that of different fatty acids had a sig­nificant effect on the conversion of fatty acids. Plus, the esterification of mixed fatty acids with ethanol is indeed acceptable.

In the meantime, different patterns were observed in the experiment conducted by Ghiaci et al. (2011). The ILs contain sulfonic group, which provides Brqrnsted acid sites for the catalyst. The conversion gives 95.1 wt% biodiesel when methanol was used but decreased to 88.7 wt% as я-butanol was employed. Also, the vegetable oil having higher degree of saturation resulted in lower biodiesel yield compared to the feedstock with the lowest degree of saturation (i. e., canola oil).

Up until now, there is no commercial production of biodiesel using ionic liquid as catalyst. Commercial application of IL is hindered by its prices, which are very costly than conventional solvents. This is reflected by the price of ionic liquids that are usually 2 to 100 times more expensive than the cost of organic solvents (Plechkova and Seddon 2008). Nevertheless, the gap in prices can be bridged by effectively utilizing ionic liquids in the process. Ionic liquids need to be recovered and then used in the subsequent runs after the recycling step. The catalytic activity of ILs also must not deteriorate after the recycling in order to ensure the process becomes economical and viable at larger scale. The continuous use of the catalyst also reduced the problem related to disposal of spent catalyst.

There are several techniques available that have successfully recovered IL from reactants. The most commonly applied method for the recovery is distillation. After removal of glycerol from the lower phase of the biphasic system, vacuum distilla­tion is typically employed to remove other substances from the IL, such as alcohol and water. The recovered IL is later used directly in the new batch of process. The insolubility of IL in esters also helped the separation step becomes more effective. Other methods that can also be employed for the recovery purpose are the super­critical carbon dioxide extraction, membrane separation, and the addition of anti­solvent for crystallizing the IL. In the effort to efficiently use IL as the catalyst in

biodiesel synthesis, it is expected that there is insignificant change in the perfor­mance of recycled IL after appropriate separation and purification steps.

Table 9.2 sums up the changed in recycled IL activity for the esterification and transesterification processes. There was not much change in the catalytic activity of ILs, even after they have been reused for few times.

In some cases, the recycled IL maintained its performance after being recycled without significant change in its performance. Some factors that may have contrib­uted to the decline in the activity are slight loss of the catalyst after each cycle, acidity of the catalyst decreased, and also water content of the oil (Liang et al. 2009; Fang et al. 2010). In addition, the selectivity towards product in biodiesel production remains virtually unchanged when IL is used to catalyze the reaction.