3.2. Epoxidation and subsequent esterification of PKME for biolubricant production
The result of the effect of temperature and time at different times and temperatures, on the relative fractional conversion of the palm kernel methyl ester sample to oxirane is presented Fig. 9. The conditions used for this study were H2O2-ethylenic unsaturation molar ratio of 1.5:1, 2.5 wt% H2SO4 catalyst conc., and 1200 rpm stirring speed.
Temperature and time effects on the rate of epoxidation of the palm kernel methyl ester sample, was determined at 35, 45, 55, 65 and 75 °C. The result indicates that the relative fractional conversion to oxirane, increased directly with reaction time during the early stages of the reaction as could be seen in Fig. 9. However, it later started to decrease with additional increase in time of the reaction. This abnormality associated with additional increase in time was due to the commencement of oxirane ring opening [30-31]. Also, it was found that temperature increase favored peracetic acid formation. Hence, the resulting implication was not limited to accelerated epoxidation, but also in enhanced hydrolysis rate; that is oxirane ring opening of the product [32].
Furthermore, Fig. 9 shows the times necessary to achieve maximum relative conversion at different temperatures. As could be seen in the Figure, the time necessary to attain maximum relative conversion were 8, 7.9, 6, 6, and 4 hr., at temperatures of 35, 45, 55, 65 and 75 °C, respectively. From the Fig. 9, it was observed that smaller rates were noticed for the reactions at lower temperatures of 35 and 45 °C. Though, more stable oxirane rings were obtained at these temperatures, compared to those of higher temperatures 55, 65 and 75 °C, which showed higher rate, but more unstable oxirane ring, leading to higher degradation of the epoxide. It is important to state that at the higher temperatures of 55, 65 and 75 °C; the relative conversion of the PKME sample to oxirane attained maximum values in 6, 6 and 4 hr., respectively. Nevertheless, it was noticed that there was decrease in the relative conversion to oxirane with additional increases in time. For instance, at temperature of 75 °C, the observed decrease in the relative conversion to oxirane was very clear since it began after 4 hr. In the same way, at temperatures of 55 and 65 °C, their relative conversion started after 6 hr. Nevertheless, at 35 and 45 °C, more stable rings were obtained for the sample. This is because there was no decrease in the relative conversion to oxirane within the studied time intervals (see Fig 9).
Similar results were also obtained by Agu et al. [2], Goud et al. [32] and Cai et al. [33] for the epoxidations ofTerminalia catappa L. methyl ester, mahua and soyabean oils, respectively. The highest fractional conversion to oxirane obatined was 0.87, at 75 °C in 4 hr. as shown in Fig 9.
[CHART]
Fig. 9: Effects of temperature and time on relative fractional conversion to oxirane for the epoxidation of palm kernel methyl ester
Physicochemical properties of PKO and PKO biolubricants
The physicochemical properties of the PKO and the biolubricants produced by transesterification with trimethylolopropane (TMP) and epoxidation-esterification methods are presented in Table 1. The obtained oil yield of palm kernel was 49.82 %. This value was slightly higher than the 47.5 % and 49.2 % yields reported for palm kernel by Ibiam and Anosike [8] and Hossain et al. [34]. However, this value was lower than the 49.9 % yield, reported by Zaidul et al. [35]. This difference in the PKO yields could be attributed to the extraction method used [36].
A number of studies have shown that vegetable oils have short comings with respect to its use as biolubricant without modification of its structure [37-38]. These limitations include thermal, oxidative and hydrolytic instability as well as inadequate low temperature fluidity as a result of their high pour points [1]. These limitations are as a result of the glycerol moiety which is a major constituent in vegetable oil. Most of these problems are significantly reduced by chemical modification of the vegetable oil, in this case PKO. Hence, the reason for the modification of the PKO using the methods of transesterification with trimethylolopropane (TMP) [1,17] and epoxidation-esterification [37-40].
The synthesis of PKO biolubricant using two stage transesterifaction methods causes the removal of the β-hydrogen atom from the oil structure. Hence, provides an ester with high oxidative and thermal stability [41]. In this two stage transesterification process, the unstable hydrogen that is bared by the glycerol molecule is substituted by a more stable TMP. At the first stage of the transesterification reaction, the glycerol molecule is removed, while in the second stage, a more effective TMP molecule, replaces the glycerol. Thus, trimethylolpropane triester (TMPTE) that has more superior properties and performance is produced [1].
Similarly, the synthesis of PKO biolubricant was also achieved using the three step synthesis of the methyl ester that involves epoxidation, ring opening and esterification steps. The epoxidation process helps in the removal of unsaturation in the methyl ester by converting them into epoxy-groups that improves the oxidative stability. It is a known fact that the existence of double bonds in vegetable oils chains accelerates oxidative degradation. On the other hand, there are poor low temperature properties in the oil, leading to solidification at lower temperatures like 0 °C. These short comings, limit their application at low operating temperature, especially as biolubricants for automotive and industrial purposes [40]. Hence, suitable method to improve the low temperature flow properties is to attach branching sites at the epoxy carbons. This leads to the need for the ring opening step of this synthesis approach. This was achieved by careful carrying out esterification ring opening reaction using acid anhydride and boron trifluoro diethyletherate catalyst. The obtained branched methyl ester was used as precursors for the synthesis of modified triester-derivatives by acetylation with octanoyl chloride.
Conventional lubricant, the petroleum lubricant sample was analyzed using similar parameters as those determined for the biolubricant. This was to evaluate the properties nearness of the biolubricant samples to the conventional lubricant. Table 1 shows the physicochemical properties of the PKO, palm kernel biolubricat (PKBLT) synthesized by transesterification of trimethylolpropane (TMP), palm kernel biolubricat (PKBLE) synthesized by epoxidation-esterification of the methyl ester and the conventional petroleum lubricant.
Generally, it could be seen from the table that the biolubricants samples were more viscous and have higher weight than the petroleum lubricant counterpart. Hence, these properties give rise to the advantages of better mechanical load and thermal resistance of biolubricants over mineral lubricant [1]. Furthermore, the viscosity indices and pour points values of the studied samples were similar to the petroleum lubricant sample. The flash point values of the synthesized biolubricant samples were higher than the PKO sample. This is an indication that the synthesized samples had enhanced thermal resistance as a result of the transesterification with trimethylolpropane (TMP) and epoxidation-esterification reactions. The flash point values of the synthesized biolubricant samples were close to that of petroleum lubricant. Thus, this is an indication of their greater thermal stability.
Table 1: Physicochemical properties of PKO and modified PKO biolubricants