Palm oil methyl ester (POME) was obtained from Carotech Sdn. Bhd. (Ipoh, Perak, Malaysia). 2-ethylhexanol, sodium methoxide and other chemicals were purchased from Sigma-Aldrich Chemical Co. (St. Louis, MO).N,O-Bis (trimethylsilyl) trifluoroacetamide (BSTFA) was purchased from Fluka Chemie AG (Buchs, Switzerland). The fatty acid (FA) compositions of these samples were determined according to the standard methods for the analysis of oils and fats (15): POME (wt%): C12:0, 0.9; C14:0, 1.5; C16:0, 41.5; C16:1,0.3; C17:0, 0.1; C18:0, 2.7; C18:1, 40.6; C18:2, 11.9; C18:3,0.5; The calculated mean M.W. of the esters were determined from the FA compositions i.e. POME, 287.

The experiments were devised to determine the effect of temperature, POME/2-ethylhexanol (2EH) molar ratio, reaction time and catalyst concentration on the synthesis of 2-ethyl hexyl ester. Throughout the laboratory scale studies, the pressure (1.5 mbar) was kept constant. The reaction temperatures varied from 80, 100, 120 and 140 °C. The molar ratio of POME/2EH varied from 1:1.5, 1:2 and 1:2.2. The reaction time varied from 0.5, 1, 2, 3 and 4 h. The amount of catalyst varied from 1, 1.5 and 2% w/w.

The transesterification reaction was carried out according to the procedures described by Yunus et al. (2004). Briefly, 100 g of palm oil methyl ester (POME) were placed in a 500 mL three-neck flask equipped with a magnetic stirrer, thermometer, sampling port and reflux condenser. The three neck flask acted as a reactor and the flask was immersed in the silicon oil bath. The set up was connected to the vacuum pump with a relief valve and a vacuum trap from the condenser. POME was heated to slightly above 100 °C to remove all the water content in the solution as well as in the flask under vacuum pressure. After one hour, a certain amount of 2-ethylhexanol was added to the POME after which the temperature was dropped to below 100 °C. 2% w/w of sodium methoxide based on the total mass of the reactant was then added to the mixture. The reaction temperature had to be kept below 100 °C when the catalyst was added to the mixture. This was to avoid any spill over because sodium methoxide is highly reactive at high temperatures. The reaction product was purified by vacuum filtration and the methyl ester removal step. The vacuum filtration was used to separate the catalyst and the liquid product mixture. The liquid product mixture collected in the conical flask was purified further to remove the excess methyl ester. The liquid product was distilled at a temperature between 200–230 °C under vacuum to obtain the final purified product.

The analysis of the reaction product was done using Gas chromatography. According to Yunus (2003) one drop of sample of approximately 0.03±0.005 g was weighed into an auto sampler vial. 1 mL of ethyl acetate was added into the vial with 0.5 mL of N,O-Bis (trimethylsilyl) trifluoroacetamide (BSTFA) and swirled for a few minutes. The prepared sample was then heated in a water bath at 40 °C for 10 min to confirm that the sample was silylated. The sample was cooled down to room temperature before injection into the GC system.

Separation on the GC system was performed using a capillary column SGE 12 m x 0.53 mm, i.d. 0.15 µm (SGE, Melbourne, Australia). The oven temperature was set initially at 80 °C, held for 3 min and then increased at 6 °C min^-1 to 340 °C and held for another 8 min. The injector and detector temperature were set at 320 °C and 340 °C, respectively. Hydrogen gas was used as the carrier gas at a flow rate of 26.7 mL·min^-1 and a pressure of 4.13 psi. The inlet ratio was set at 1:1, and 1.0 mL of sample was injected into the GC system. This procedure provides a complete separation of the sample methyl ester and monoesters.

The kinematic viscosities at 40 °C and 100 °C were measured based on the ASTM D445 method. The flash point of the sample was measured using Semi-Automatic Pensky-Martens Flash Point Tester, while the pour point test used Petrotest PM4 according to the ASTM D 93 method.

Three different samples of each of the ethyl hexyl ester were analyzed independently in triplicate. Data are presented as mean values±SD of triplicate determinations.

The reaction time was varied at 0.5, 1, 2, 3 and 4 hours while other parameters were constant. The molar ratio of POME:2EH was 1:2, the temperature of reaction was 80 °C in the presence of a sodium methoxide catalyst (2% w/w). The pressure of the reaction was fixed throughout the study. Figure 1 depicts the results obtained at 0.5, 1, 2, 3 and 4 hours of reaction time, respectively. The results from the GC chromatograms at 0.5, 1, 2, 3 and 4 hour time reaction time showed exactly the same pattern in which all peaks appeared at the same time ranges and intensities compared to the results at 4 hours’ reaction time (Figure 2).

The final monoester conversion improves slightly with time until 2 hours of reaction, after which the improvement is marginal, as shown in Figure 1. Nevertheless, the difference in ester conversion was only 1.8% when the time was increased to 2 hours. The reaction can be considered almost complete after 1 hour of reaction time. Consequently, the reactions in the subsequent experiments were conducted in 1 hour. This is a marked improvement over the current method by Waskitoaji et al. (2012) who took 4 hours to successfully synthesize 95% ethyl hexyl oleate from palm oil oleic acid. In another study, Lamsa et al. (1994) obtained up to 90% conversion of 2-ethyl-1-hexanol esters from rapeseed oil in 8 hrs.

The temperature of the reaction varied from 80 °C, 100 °C, 120 °C and 140 °C. The temperature interval of 20 °C was selected because the improvement in the conversion is only observable if the interval is large enough. The other reaction parameters were similar to the previous experiments except for the fact that the temperature and the reaction time was fixed at 1 hour.

From the GC chromatogram, the results for 80 °C and 140 °C reaction temperature showed no difference in the peak positions and intensities. The ester conversions at 100 °C, 120 °C and 140 °C were 98.7, 98.9 and 98.8%, respectively. The results for temperature manipulated reactions are illustrated in Figure 3. Similar to the previous results, no marked difference was observed for the ester conversions at these temperatures. Therefore, it can be concluded that the reaction was already complete after one hour and the lowest temperature to attain this completion was 80 °C and could possibly be lower. However, according to a recent study on the esterification of oleic acid from palm oil with 2-ethylhexanol, the best operating temperature was 160 ^oC using 1.0% catalyst (Waskitoaji et al., 2012). Not only was the conversion obtained in this study higher than 98.9% but our operating temperature was also much lower at 80 °C.

Since POME is the limiting reactant, the amount of POME was fixed and the ratio of 2-ethylhexanol (2EH) to POME was manipulated between 1.5 and 2.2. The reaction temperature was fixed at 80 °C, 2% catalyst and 1 hour reaction time. The effect of molar ratio between POME and 2EH was studied because the equilibrium in the transesterification might be influenced by employing an excess of one reactant (Liu et al., 2006). Figure 4 shows the percentages of ester conversion according to reactant molar ratio.

The transesterification reaction is highly reversible, thus the alcohol should be in excess and that is why ratios 1:2, 1.5 and 2.2 were selected to study the effect of the reactant molar ratio. The results on the reaction conversion (2EH ester) showed only a slight improvement at molar ratio, 1:2.2, in which the 2EH ester percentage was 99.9%. For molar ratio 1: 1.5, 97.6% of 2EH ester was obtained, while at 1:2, the conversion was 98.8%, which is also very high. Comparatively, the ratio 1:2 is preferable because of the highly reversible nature of the reaction. A similar trend was observed by Waskitoaji et al. (2012) who found that the effect of molar ratio between oleic acid and alcohol has a marginal effect on ester conversion. Tan et al., (2006) reported a higher synthesis of 2-ethyl hexyl pamitate (>95%) at a molar ratio of alcohol/acid, 1:1.3 and when the reaction was carried out for 6–10 hrs.

A homogenous sodium methoxide catalyst (CH₃ONa) in methanol solution was used in the transesterification to produce 2EH ester. The amount of catalyst depends on the total mass of the reactant and is measured by weight percentage. In this study, 1%, 1.5% and 2% w/w of catalyst were used in the synthesis with 80 °C reaction temperature, the ratio POME:2EH was 1:2 and reaction time was 1 hour. The GC result showed that the 2EH conversion for 1% catalyst was 88% and 94% for 1.5% catalyst. These conversions are slightly lower than the conversion of monoester at 2% catalyst. However, the use of a large amount of catalyst may induce soap formation during the reaction. Fortunately, in this study, the amount of soaps formed by using 2%, 1.5% and 1% catalyst are almost similar, hence 2% catalyst was selected because a higher conversion of 2EH ester was achieved. The results are shown in Figure 5. The previous study using sulphuric acid as catalyst found the optimum catalyst to be 1% (Waskitoaji et al., 2012).

Consequently, further experiments were conducted at lower temperatures of 70 °C and 60 °C. The result for 70 °C reaction showed that the conversion of 2EH ester was 97%, which is very good, but at 60 °C reaction, only a small conversion was detected. To find the exact time required for the reaction to complete, the sample was taken at times 3, 5, 7, 10, 15, 20, 25, 30, 40, 50, 60, 120, 180 and 240 min. The condition of the experiment was fixed except for the temperature of the reaction varying from 70 °C to 100 °C as is given in Table 1.

The result of the time sampling in Figure 6 shows that there is no distinct difference in the percentage of ester conversion and also, the time taken to complete the reaction was only 3 minutes. Thus, the optimum conditions for the synthesis were: reaction temperature of 70 °C; reaction time below 10 minutes; 1:2, POME:2EH molar ratio; 2% w/w sodium methoxide catalyst and 1.5 mbar vacuum pressure. However, Waskitoaji et al., (2012) reported that the esterification of 2-ethyl hexyl oleate was best carried out at 160 ^oC in 4 h, with 1.0% catalyst. Most of the researchers reported the conversion of fatty acid esters of 2-ethyl-1-hexanol by using an enzymatic catalyst (Linko et al., 1994; Lamsa et al., 1994; He et al., 2002; Tan et al., 2006). The time taken to achieve 90% conversion was 8 hours and required at least 3.4% catalyst (Lamsa et al., 1994).

The purity of the 2-ethyl hexyl esters was determined by GC. The GC chromatogram revealed five distinct peaks with two different intensities, peaks at retention times 10.35, 13.0 and 15.89 (Figure 2) have less intensities (50 to 100 pA) compared to peaks at 18.8 and 20.9 minutes with intensities 2500 to 3000 pA. The GC chromatogram of fatty acid methyl ester standards depicted peaks at retention times between 12 to 15 minutes, while peaks appearing between18 to 21 minutes belong to monoesters. From this data, the transesterification process was considered successful because the majority of the reaction product consists of monoesters which were the desired products. Accordingly, the peak at 18.8 belongs to ethyl hexyl palmitate and the peak at 20.9 represents ethyl hexyl oleate. The total percentages of monoesters in this sample were 98.8% and 1.2% for unreacted palm oil methyl ester.

The basic properties of the ester oil were tested and the results show that the kinematic viscosity of the oil at 40 °C and 100 °C were 8.24 mm²·s^-1 and 3.03 mm²·s^-1, respectively. The flash point was >200 °C and the pour point was -15 °C.