Efficacy of Catalysts in the Batch Esterification of the Fatty Acids of Thevetia Peruviana Seed Oil

The methyl, ethyl, propyl and butyl esters of the fatty acids of Thevetia peruviana seed oil were successfully prepared by the batch-esterification procedures. Various acid catalyst and various molar ratios of fatty acid to alcohol were investigated. H3PO4 was found to be ineffective to catalyze the esterification of the free fatty acid. HCl gas and conc. H2SO4 both proved very effective in catalyzing the reaction. Concentrated H2SO4 however, proved better of the two acids. Three molar ratios 1:1, 1:3 and 1:10 were investigated, 1:3 molar ratio of the free fatty acid to the alcohol was found to be the most efficient.

Thevetia peruviana, more commonly known as yellow oleander or milk bush, is an evergreen, dicotyledonous shrub which is believed to have originated from the forest of tropical America, precisely from central America, but has naturalized in the tropical and subtropical regions of the world.It is abundantly available in Nigeria, where it is mainly grown as an ornamental plant.The seed of the plant contains about 60% oil, which hydrolyses to give about 64.3% oleic acid, 6.3% linoleic, 17.1% palmitic, 11.8% stearic and 0.4% arachidonic acid (Jairo, 1981).The defatted seed cake is about 30% protein but rich in toxins.The work reported in this paper aims at preparing low alkyl esters of the fatty acids of the oil for possible use as oleochemicals.Oleochemicals provide competing alternative to petrochemicals.The oils of many oil-rich-seeds e.g soybean oil, rapeseed oil and other oilseeds have been converted to simple monobasic acid esters either by the hydrolysis-esterification procedure or by direct alcoholysis (transesterification) of the oil.(Charkrabarty, 1985;Chritie, 1992;Freedman et al., 1986).These low alkyl esters have found wide applications as oleochemicals -as textile fibre lubricants, rolling oils and cutting oils.They also find application as alternative fuel sources and as suppliments or even replacements to petroleum diesel.(Charkrabarty, 1985;Szpiz et al., 1985;Peterson, 1994;Howard, 1994;Gateau et al., 1985).The interest in oleochemicals is increasing in recent times because of the environmental friendliness, renewability and degradability of the products when compared with petrochemicals.(Staat and Rochietta, 1992;Baranescu, 1994;Morrison, 1995) This work therefore aims at finding an economic route to the preparation of low alkyl esters of thevetia seed oil with a view to finding economic uses for the plant, which has hitherto been used merely as an ornamental plant.

MATERIALS AND METHODS
The crude thevetia seed oil (TSO) used in this study was obtained by the solvent extraction of the oil from the crushed seed using light petroleum ether (b.p 60-80 0 C) as the solvent of extraction.The mixture was then distilled to recover the solvent and the oil was obtained as a residue of the distillation.The FFA was obtained from the oil by standard alkaline hydrolysis (AOCS, 1979).Adding dil.H 2 SO 4 to form an oily fatty acid layer then acidified the resultant solution.Four different alcohols -methanol, ethanol, 2propanol and 1-butanol were used for the esterification reaction, and three different molar ratios of fatty acid to alcohol -1:1, 1:3 and 1:10 were investigated for each of the alcohols.Different acid catalysts -dry HCl gas, conc.H 2 SO 4 and conc.H 3 PO 4 were also used with each of the alcohols and at the various molar ratios.For each reaction, a 250ml twonecked round bottom flask equipped with a reflux condenser and a magnetic stir bar was charged with the molar equivalent of the FFA and alcohol.The FFA was assumed to be predominantly oleic, thus the molar mass was taken to be about 282g, and this guided the measurements.To esterify 1:1 FFA: Methanol with H 2 SO 4 catalyst, 28g of FFA (about 0.1 mol) was reacted with 3.5g methanol (also about 0.1 mol) and 1.0ml conc.H 2 SO 4 as catalyst.The mixture was refluxed for about 2-3 hours.The reaction was monitored at regular intervals (15min) by a titration of aliquots with 0.1M NaOH solutions.The reaction was also studied by thin layer chromatography (tlc) using 1:2 chloroform: petroleum either as developing solvent.The developed plate was visualized in an iodine chamber.This procedure was repeated with the other molar ratios, catalysts and alcohols.

RESULTS AND DISCUSSION
The results obtained for the titration of the aliquots with 0.1M NaOH are shown in Figures 1-5 below.The slopes of the graphs are -0.0017min-1 , -0.0031 min -1 , -0.0076 min -1 , -0.00083 min -1 and -0.0082 min -1 respectively.Both HCl and H 2 SO 4 catalyze the esterification following the same trend but at different rates.Phosphoric acid result is not worthy of presentation because it is rather erratic.Catalysis by HCl served as control, and the results shows a steady decrease in titre values for each of the three preparations (1:1, 1:3, 1:10).The 1:10 ratio is adjudged to be the fastest.The esterification comes to equilibrium in 105 minutes.Furthermore, amount of acid that remains unreacted is lowest (16%).In all cases, there is the HCl involved in the titration, but the contribution by this is constant for every titration.There is a significant variation in the degree of conversion to the ester at zero time and when equilibrium is established.For the three preparations it is (100 -58) = 42%, (100 -35) = 65% and (100 -16) = 84% respectively for the 1:1, 1:3 and 1:10 preparations.
. For catalysis using H 2 SO 4 for 1:1 preparation, the equilibrium conversion rate amounts to (100 -70) = 30%, this performance is poorer compared with the 1:1 for HCl catalysis.The dibasicity of this catalyst (i.e.H 2 SO 4 ) is considered to account for the overall higher titre values.The 1:3 preparation catalyzed by H 2 SO 4 showed a much better result with conversion rate (100 -5.9) = 94.1%.Batch esterification using H 2 SO 4 as catalyst at ratio 1:3 and above is convincingly most efficient for the esterification of the fatty acids of thevetia peruviana seed oil.The titration results for the other preparations follow the same trend.Tlc gave an equally good picture of the reaction and the conversion of the fatty acid to the ester.This procedure was adopted for the studies of the esterification with other alcohols.Tlc provides supportive evidence to the idea that H 2 SO 4 is superior to HCl in the catalysis of esterification of the fatty acids of thevetia peruviana seed oil.Even though no quantitative analyses of the plates were done, qualitative analysis indicates that each preparation at equilibrium contains both the fatty acids and the esters.For the H 2 SO 4 catalysis, the proportion in all cases for the products outweighs the unreacted fatty acids.Whereas for HCl catalyzed reactions, there is about equal proportion for the products and the unreacted fatty acids.The ratio of the product to the unreacted fatty acids in both H 2 SO 4 and HCl catalyzed reactions show the same trend, which reflects a steady increase with time.The increase is however greater for the H 2 SO 4 catalyzed reactions.

Conclusion:
It is concluded that H 2 SO 4 catalysis of esterification of the fatty acids of thevetia peruviana seed oil would be tried for transesterification of the oil.Transesterification is more popularly adopted for the commercial production of alkyl esters of vegetable oils