Preparation and enrichment of linoleic acid from soybean oil, using as natural raw material for synthesis of synergist to insecticides

JOURNAL OF SCIENCE OF HNUE Chemical and Biological Sci., 2013, Vol. 58, No. 9, pp. 20-27 This paper is available online at PREPARATION AND ENRICHMENT OF LINOLEIC ACID FROM SOYBEAN OIL, USING AS NATURAL RAWMATERIAL FOR SYNTHESIS OF SYNERGIST TO INSECTICIDES Hoang Than Hoai Thu and Dao Van Hoang National Key Laboratory for Petrochemical and Refinery Technology, Vietnam Institute of Industrial Chemistry Abstract. Linoleic acid is an unsaturated 18-carbon fatty acid with relatively high content in vegetable oils, including soybean oil. Linoleic acid and its derivatives have interesting and valuable biological activities, including a synergistic effect as an insecticide component. For example, control of the rice brown plant hopper control with the use of the insecticide imidacloprid has been found to be enhanced when linoleic acid derivatives are added to the insecticide. In this study, linoleic acid is obtained by hydrolysis of soybean oil in ethanol/KOH, separation and enrichment using the urea-fatty acid complex formation method for the fractionation of fatty acids. The process involves the formation at 60 ◦C of a homogeneous solution of free fatty acid and urea in 95% ethanol, followed by cooling the resultant urea complex slurry to -5 ◦C and separation by filtration. The linoleic acid content obtained is about 87.9% and can be used as a synergistic component when added to the insecticide imidacloprid. Keywords: Linoleic acid, soybean oil, synthesis, imidacloprid. 1. Introduction Natural or synthetic substances are said to be synergistic when the addition of the substance increases the lethality and effectiveness of a currently available insecticide, and this is particularly useful when the problem of resistance has arisen. These substances that have such synergistic properties are by themselves considered to be nontoxic. In this respect, adding a synergist to a pesticide is an effective way to reduce environmental pollution. In recent years, natural-based synergists, such as synergistic components of Received November 12, 2013. Accepted December 16, 2013. Contact Hoang Than Hoai Thu, e-mail address: thuhth@gmail.com 20 Preparation and enrichment of linoleic acid from soybean oil, using as natural... vegetable oils, are being studied in hopes of finding components which could aid in plant protection [1, 2]. Linoleic acid is an unsaturated 18-carbon fatty acid contained in vegetable oils, such as soybean oil which has a relatively high linoleic content [3, 4]. Vegetable oils usually contain various saturated and unsaturated fatty acids as triglycerides, sterols, tocopherols, sterol esters and hydrocarbons [9]. To obtain a quantity of linoleic acid to be used as a synergistic component of pesticides, it’s necessary to separate it from the fatty acid mixture and do a subsequent enrichment [6-8]. 2. Content 2.1. Materials and method * Materials Commercial grade soy bean oil was purchased from the Vietnam Vegetable Oils Industry Corporation (VOCARIMEX) that had a density (at 20 ◦C) of 0.9116 - 0.926 g/mL, free fatty acids ≤ 0.1%, a saponification value of 190 and an iodine value of 125 - 135. * Method - An alkaline hydrolysis of soybean oil for obtaining free fatty acids mixture (FFA) in ethanol/KOH at 65 - 70 ◦C was carried out by boiling the soy bean oil under reflux for 3 hours. This was followed by removing most of the alcohol by distillation. Fatty acids were then liberated by the addition of mineral acid, usually with a 20% excess of HCl. - The separation and enrichment of linoleic acid from FFA was done using the urea-fatty acid complex formation method for fractionating fatty acids. To a solution of urea in hot ethanol (95%), FFA was added quickly and the mixture was stirred while being heated until there was complete dissolution. After cooling overnight at room temperature, the precipitate was filtered off at low temperature and discarded. The insoluble oil was dissolved in n-hexane and washed several times with water to remove the urea. Evaporation of the solvent yielded linoleic acid. The n-hexane layer was dried using anhydrous sodium sulfate and distilled under vacuum to recover an adducted concentration of linoleic acid. - To determination the FFA % contained from the hydrolysis of soybean oil, the method was used [5]. Approximately 50 mL of isopropanol was placed in a flask to which was added about 0.5 mL phenolphthalein. This was neutralized by adding sodium hydroxide (NaOH, 0.02 N) until a stable pink color was obtained. The neutralized isopropanol was added to 5 g of FFA which was in an Erlenmeyer flask and about 0.5 mL of phenolphthalein was added to that. After the mixture was shaken gently, the mixture was neutralized by adding 0.02 N NaOH solution, until a stable pink color was obtained. 21 Hoang Than Hoai Thu and Dao Van Hoang The FFA % was calculated using the following equation: %FFA = 28.2×N × V W where V: alkali volume (mL); N: alkali normality; W: sample weight (g). - A High Performance Liquid Chromatography (HPLC) analysis of the fatty acid was performenced on an Agilent C18 column. The parameters of the HPLC were set according to [10]. The samples were dissolved in 10 ml of the acetone/acetonitrile mixture before being injected into the HPLC. 2.2. Results and discussion 2.2.1. Preparation of free fatty acid by the hydrolysis of soybean oil Nowadays, a hydrolysis of oils or fats is usually carried out by refluxing, using KOH as the catalyst. There are two big advantages to doing this: the reaction is one-way rather than reversible and the products are easy to separate. Using a laboratory scale, a slight excess of KOH is used with the reaction mixture to be refluxed. The FFA was recovered after acidification with 20% excess HCl. Factors that affect the process of hydrolysis of soybean oil are ethanolic KOH concentration, temperature and time of reaction. Only aim of this study was to determine optimal hydrolysis conditions. Figure 1. Dependence of FFA % obtained on ethanol KOH concentration The results show the hydrolysis performance of the ethanolic KOH and its effect on the reaction when carried out under different laboratory conditons. Hydrolysis reactions when carried out at various ethanolic KOH concentration, from 1.5 M to 2.2 M. Figure 1 shows the effect of ethanolic KOH concentration on the FFA % obtained. The FFA % increased as the ethanolic KOH concentration increased. It can be clearly seen that the maximum FFA obtained at 1.9 M was about 101.1% The effect of the reaction temperature on the FFA % is shown in Figure 2. The FFA % increased as the temperature increased from 30 - 78 ◦C. At a temperature of 22 Preparation and enrichment of linoleic acid from soybean oil, using as natural... appoximately 70 ◦C, the FFA was 100.8%. The result indicates that reaction temperature is an important factor in the hydrolysis of soybean oil. Figure 2. Dependence of FFA % on reaction temperature The influence of reaction time on FFA % is studied at different times (30 mins to 180 mins). As seen in Figure 3, the trend was that the FFA % increased with the increase in reaction time and 2 hours was the time chosen to obtain the highest percentage of FFA (101.5%) Figure 3. Dependence of FFA % on reaction time In conclusion, optimal conditions for soybean oil hydrolysis were sought and determined. Hydrolysis occurs rapidly at 1.9 M ethanolic KOH concentration at a temperature of 70 ◦C in 2 hours. The FFA % obtained was nearly 100%. 2.2.2. Separation and enrichmentof linoleic acid from free fatty acid Urea complex formation is a classic method for fractionating fatty acids from seed and from other oils. The method’s simplicity, low cost, convenient scale and ecological friendliness suggest it could revolutionize modern fractionation. 23 Hoang Than Hoai Thu and Dao Van Hoang * Selection of crystallization temperature Crystallization temperature is an important parameter of urea inclusion. Crystallization temperature is not constant during the cooling state. In this study, maintaining unchanged conditions (crystallization time: 12 h, solvent/FA ratio: 7 : 1 and urea/FA: 3 : 1), the crystallization temperature was set from -10 ◦C to 10 ◦C to investigate the influence of temperature on the amount of linoleic acid obtained. As show in Figure 4, the linoleic acid content gradually increased with the decrease of crystallization temperature and achieved at -5 ◦C crystallization temperature. After this point, it started to maintain thermodynamic equilibrium with decrease of crystallization temperature. So, a crystallization temperature of -5 ◦C was adopted for this study. Figure 4. Determination of optimum crystallization temperature * Effect of urea/FA ratio (w/w) To maintainunchanged some conditions of crystallization (duration: 12 h, at -5 ◦C, solvent/FA ratio: 7 : 1) the urea/FA ratio was set at 1 : 1 to 5 : 1 for this study. The optimal ratio was found to be 3 : 1 which yielded a linoleic acid content of 87.5%. Figure 5. Determination of optimum urea/FFA ratio 24 Preparation and enrichment of linoleic acid from soybean oil, using as natural... * Effect of ethanol/FA ratio (w/w) In some expriments the ethanol to fatty acid ratio was set at 4 : 1 to 9 : 1, maintaining the urea to fatty acid ratio for 3 crystallization for 12 hours, with a crystallization temperature of -5 ◦C. Applying experiments with similar ways: maintaining unchanged some conditions of crystallization as above (time and temperature) and the ethanol/FA ratio was varied from 4 : 1 to 9 : 1 for studying its influence to content of linoleic acid obtained. The results are presented in Figure 6. Figure 6. Determination of optimum ethanol/FFA ratio As shown in Figure 6, when the ethanol/FFA ratio increased to greater than 6 : 1, less complex formed, suggesting that larger propotions of both urea and FFA remained in the filtrate. In addition, a greater amount of solvent promoted the partition of urea to the solvent phase than the solid complex phase. With an increase in solvent volume, the temperature required for solubilization of urea and FFA decreases. Fortuitously, when a small volume of ethanol is used, there is a increased complex urea formation and FFA. The most suitable ratio was found to be 6 : 1 in which the linoleic content was 87.7%. * Effect of water content in ethanol The protential for process recovery and reuse of ethanol suggested the need to investigate the effect of water content on process efficiency. Ethanol contained varying amounts (5 - 30%) of water. As expected, complex formation decreased as water content increased, enhancing urea solubility in the solvent phase. Ethanol water content should have a profound effect on the partitioning of FFA spices more likely to form complexes, so that removal of saturated FFA is strongly affected by the water content. When it is 30% water, no complexes formed. It was found that the optimal water content of ethanol is 5%. Under optimal conditions, a crystallization temperature of -5 ◦C, an alcohol to fatty acid ratio of 6 : 1 and a urea to fatty acid ratio of 3 : 1, the maximum content of linoleic acid, 87.9%, can be acheived. * High Performance Liquid Chromatography of fatty acid 25 Hoang Than Hoai Thu and Dao Van Hoang Figure 7. HPLC of FFA after hydrolysis of soybean oil with ethanolic KOH Figure 8. HPLC of linoleic acid after treatment with urea Figure 9. HPLC of linoleic acid standard 3. Conclusion To be used as raw material for subsequent synergist synthesis purposes, an FFA mixture was obtained from soybean oil hydrolysis carried out under optimal conditions. 26 Preparation and enrichment of linoleic acid from soybean oil, using as natural... Hydrolysis occurs rapidly (in 2 hours) in a 1.9 M ethanolic KOH solution at 70 ◦C, yielding 100.8% FFA. Linoleic acid was separated and enriched from the FFA mixture using the urea-fatty acid complex formation method for fractionating fatty acids. 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