Effect of homogeneous alkaline catalyst type on biodiesel production from soybean [Glycine max (L.) Merrill] oil

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1 Indian Journal of Biotechnology Vol 15, October 2016, pp Effect of homogeneous alkaline catalyst type on biodiesel production from soybean [Glycine max (L.) Merrill] oil A Saydut 1 *, A B Kafadar 2, F Aydin 2, S Erdogan 2, C Kaya 2 and C Hamamci 2 1 Mining Engineering Department, Engineering Faculty and 2 Chemistry Department, Science Faculty Dicle University, TR Diyarbakir, Turkey Received 15 April 2015; revised 28 August 2015; accepted 7 September 2015 Transesterification or alcoholysis is the most commonly applied method for biodiesel production. A catalyst is needed to improve the transesterification reaction and yield. The present study used soybean oil as the raw oil to mix with methanol and four strong alkali catalysts (NaOH, KOH, CH 3 ONa & CH 3 OK) to undergo a transesterification reaction. Transesterification was carried out using 100% excess alcohol, i.e., molar ratio of alcohol to soybean oil was 6:1, and catalyst concentration of 1% at 60 o C. Alkali metal alkoxides were found to be more effective transesterification catalysts compared to hydroxides. Sodium methoxide was the most efficient catalyst, although KOH and NaOH could also be used because they are cheaper and are used widely in large scale processing. Keywords: Biodiesel, catalyst, soybean, transesterification Introduction Soybean [Glycine max (L.) Merrill] is a protein rich oilseed crop, which is presently the leading edible oil source throughout the world. Though soy protein ranks high compared to other vegetable proteins, it is still of poor nutritional quality compared to animal proteins. Sulphur containing amino acids like cysteine and methionine are the most nutritionally limiting amino acids in soybean protein. Soybean protein mainly consists of globulins 1-3. The major unsaturated fatty acids in soybean oil triglycerides are: 7% alphalinolenic acid (C-18:3); 51% linoleic acid (C-18:2); and 23% oleic acid (C-18:1). It also contains the saturated fatty acids, 4% stearic acid and 10% palmitic acid 4. Soybean and its products are economically valuable because of their nutrient and phytochemical characteristics, which also classifies it as a food of high nutritional value and functional claims. Its low cost and useful health benefits are improving its use even to animal or human nutrition in different groups, in order to reduce risk factors for chronic diseases like diabetes mellitus, cancer, cardiovascular disease, osteoporosis and others 5. Biodiesel is currently the most widely accepted alternative fuel for diesel engines due to its technical, Author for correspondence: Tel: ; Fax: saydut@dicle.edu.tr environmental and strategic advantages 2. Biodiesel, derived from vegetable oil or animal fats, is recommended for use as a substitute for petroleumbased diesel, mainly because it is a renewable and domestic resource with an environmentally friendly emission profile and is readily biodegradable 6-8. The choice of raw materials depends mainly on its availability and cost 9. The plant oils usually contain free fatty acids, phospolipids, sterols, water, odorants and other impurities. Therefore, the oil cannot be used as fuel directly. To overcome these problems the oil requires slight chemical modification 10. Transesterification (also called alcoholysis) is the reaction of a fat or oil with an alcohol to form esters and glycerol. A catalyst is usually used to improve the reaction rate and yield. Excess alcohol is used to shift the equilibrium toward the product because of reversible nature of reaction. For this purpose primary and secondary monohybrid aliphatic alcohols having 1-8 carbon atoms are used Transesterification seems to be the best choice as the physical characteristics of biodiesel are very close to those of diesel fuel and the process is relatively simple. In the esterification of an acid, an alcohol acts as a nucleophilic reagent, while in the hydrolysis of an ester, an alcohol is displaced by a nucleophilic reagent. This alcoholysis (cleavage by an alcohol) of an ester is called transesterification. A catalyst is usually used to improve the reaction rate and yield. Because the reaction is

2 SAYDUT et al: BIODIESEL FROM SOYBEAN OIL 597 reversible, excess alcohol is used to shift the equilibrium to the product side 14,15. The transesterification mechanism is given in Fig. 1. The main purpose of the present work was to study the influence of homogeneous alkaline type of catalyst (NaOH, KOH, CH 3 ONa & CH 3 OK) against soybean oil on the quality of biodiesel synthesized. Materials and Methods Soybean oil was supplied from a commercial firm in Turkey. All the chemicals used were of analytical grade, unless otherwise stated. Methanol, sodium hydroxide, potassium hydroxide, sodium methoxide and potassium methoxide were purchased from Merck (Darmstadt, Germany). Production Procedure for Biodiesel Four different processes were carried out using soybean oil as the raw material and four homogeneous alkali catalysts (NaOH, KOH, CH 3 ONa or CH 3 OK) were used in the experiments. Biodiesel derived from soybean oil was prepared by reacting 300 g of oil, 60 g CH 3 OH (approx 6:1 molar ratio) and 3 g catalyst. The reaction was carried out for 2 h at reflux temperature while maintaining stirring. The reaction was carried out using 100% excess methanol, i.e., molar ratio of methanol to oil 6:1, and catalyst concentration of 1% at 60 o C. The reactor was equipped with a reflux condenser to Fig. 1 Mechanism of base catalyzed transesterification condense back the methanol escaping from the reaction mixture. The reaction mixture was then allowed to stand overnight and the methyl ester layer was separated from the glycerol layer using a reparatory funnel. After completion of the reaction, crude glycerol was separated by gravity, and the catalyst was removed by hot water washings. The complete removal of the catalyst was checked by phenolphthalein indicator. Traces of moisture and unreacted methanol were removed by vacuum distillation. The distillation was continued until the loss in weight of ester was constant. The crude methyl ester was further purified by distilling off the unreacted methanol under normal atmospheric pressure, washing several times with water, centrifugation and drying with vacuum desiccator. After completion of the transesterification, the reaction mixtures were allowed to cool down to room temperature to produce two phases: crude ester phase and glycerol phase. This phase separation generally occurred quickly and can be observed within the first 10 min of settling, but the ester layer was opaque, indicating that the separation was incomplete. Experimental results showed that given enough time for complete settling, the opaque ester phase could turn crystalline and transparent. This complete separation could take as long as 8-18 h. In fact, during the settling, the transesterification process was still going on. Therefore, the longer the settling time, the more favorable are the separation and the conversion. After that, weight of the ester was taken for product yield calculation. When the reaction temperature closes or exceeds the boiling point of methanol, the methanol will vaporize and form a large number of bubbles, which in turn inhibit the reaction. Furthermore, all materials should be substantially anhydrous because water causes soap formation, which consumes the catalyst and reduces catalyst efficiency. The resulting soap causes an increase in viscosity, formation of gels and makes the separation of glycerol difficult. Characterization of Biodiesels Biodiesel is a domestic, renewable fuel for diesel engines derived from natural oils like soybean oil, which meets the specifications of ASTM D Kinematic viscosity of soybean oil methyl ester was obtained using Koehler Kinematic Viscosity Bath Model K at 40 o C (ASTM D-445). Pour point (ASTM D-97) and cloud point (ASTM D-2500) were determined simultaneously using a Tanaka

3 598 INDIAN J BIOTECHNOL, OCTOBER 2016 Mini-Pour/Cloud Point Tester Model MPC- 101A/101L. For flash point (ASTM D-93), a Tanaka Automatic Flash Point Tester Model APM-6T-A was used. Heating value (ASTM D-2015) was determined using an IKA Calorimeter System C 2000 basic control calorimeter. Density was measured at 15 o C by a Metler-Toledo densimeter (ASTM D-941). C and S were determined by a Carlo Erba 1108 Model elemental analyzer and an Eltra CS 500 Carbon Sulfur Determinator. The iodine number, cetane number was calculated, while the acid value was obtained by titration 6,16,17. Results and Discussion Transesterification of vegetable oils to obtain biodiesel consists of replacing the glycerol of triglycerides with a short chain alcohol in the presence of a catalyst. Transesterification reaction can be catalyzed by both homogeneous (basic or acidic) and heterogeneous (basic, acidic or enzymatic) catalysts 18. In transesterification method, we used four homogeneous alkaline catalysts. The reaction mechanism for alkali catalyzed transesterification was formulated in three steps as explained in Fig. 1. The first step is an attack on the carbonyl carbon atom of the triglycerides molecule by the anion of the alcohol (methoxide ion) to form a tetrahedral intermediate, which reacts with an alcohol (methanol) to regenerate the anion of alcohol (methoxide ion). In the last step, rearrangement of the tetrahedral intermediate results in the formation of a fatty acid ester and a diglyceride 16,17. When NaOH, KOH, K 2 CO 3 or any other similar catalyst is mixed with alcohol, the actual catalyst, the corresponding alkoxide group is formed. These bases show almost the same catalytic activity with conventional homogeneous NaOH catalyst under the optimized reaction conditions. For an alkali catalyzed transesterification, glycerin and alcohol must be substantially anhydrous because water makes the reaction partially change to saponification, which produces soap 6,8. Lower amount of catalyst results in incomplete reaction, whereas higher amount of catalyst causes soap formation. Hence, an optimum amount of catalyst is a very important parameter in the transesterification reaction. In our study, transesterification was carried out using an optimum catalyst concentration of 1%. The amount of catalyst used in the process is another variable that should be taken into account, not only because it determines the reaction rate, but also because it can lead to hydrolysis and saponification 9. Soybean oil lies in a group of vegetable oils those contain considerable amounts of linolenic acid (C18:3), which is responsible for the development of an offflavour problem known as flavour reversion. Standards and quality control of manufacturing and distribution of biodiesel are being developed to assure that reliable and consistent fuels are supplied to users 4. As an alternative fuel, biodiesel has physical and chemical properties qualifying to the operation of diesel engines. These properties play a vital role in quality control in the petroleum-based diesel fuel industry. The fuel characteristics of the synthesized alkyl esters were evaluated according to ASTM standard methods 8,15,19. This standard identifies the parameters the pure biodiesel must meet before being used as a pure fuel or being blended with petroleum-based diesel fuel. EN is an international standard that describes the minimum requirements for biodiesel produced from vegetable oil fuel stock. Biodiesel specifications (ASTM D-6751) and (EN 14214) are given in Table 1 14,19. Table 1 Biodiesel specifications according to ASTM D-6751 and EN standards Property ASTM D EN % C % S (ppm) Kinematic viscosity (40 C, mm 2 s -1 ) Heating value (MJ/kg) - - Density (15 C, g cm -3 ) Flash point 130 o C min (ASTM D-93) >101 Iodine number max Neutralization number (mg KOH/g) 0.50 max 0.50 max Pour point ( C) - - Cloud point ( C) Not specified - Cetane number 47 min (ASTM D-613) 51 min (EN ISO 5165) Ester content % (m/m) min (EN 14103)

4 SAYDUT et al: BIODIESEL FROM SOYBEAN OIL 599 Table 2 Fuel properties of biodiesels produced from refined soybean oil using four alkaline catalysts Biodiesel/ Soybean oil methyl ester Diesel Catalyst NaOH KOH CH 3 ONa CH 3 OK % C Kinematic viscosity (40 C; mm 2 s -1 ) Heating value (MJ/kg) Density (15 C, g cm -3 ) Flash point ( C) >55 Iodine number Pour point ( C) Cloud point ( C) Cetane number Ester content (% mass) Since biodiesel is produced in quite differently scaled plants from vegetable oils of varying origin and quality, it is necessary to install a standardization of fuel quality to guarantee engine performance without any difficulties. Fuel properties of methyl esters of soybean oil compared well with ASTM D-6751 and EN biodiesel standards. The properties of soybean oil methyl ester were also very close to the diesel 20. An inverse relationship exists between the oxidation stability and cold temperature properties of the biodiesel. Better stability of biodiesel is achieved with oil having more content of saturated fatty acids, which is attributed to their resistance to autooxidation, whereas presence of more percentage of unsaturated fatty acids is favored for low temperature flow properties of the fluid, such as, cloud point and pour point. The flash point of the ester is higher than that of diesel, which requires higher compression ratio and modifications in fuel injector to ignite the fuel in a smooth pattern The reduction in calorific value for the biodiesel and its blends compared to diesel was due to the presence of oxygen in the biodiesel. Base-catalyzed transesterification is the most widely used method for biodiesel production. Sodium hydroxide (NaOH) and potassium hydroxide (KOH) are the catalyst commonly used for alkaline transesterification. However, in the present study, sodium methoxide (CH 3 ONa) and potassium methoxide (CH 3 OK) gave better yield compared to NaOH and KOH. As shown in Table 2, high biodiesel yield was obtained by using the sodium and potassium salts of methoxide with and 98.93% yields, respectively because they only contain the methoxide group necessary for saponification as a low proportion impurity. However, when sodium or potassium hydroxides were utilized as catalysts, biodiesel yields decreased to and %, respectively. Methoxides are also stronger bases and nucleophiles in base-catalyzed transesterification systems. Generally, potassium-based catalysts gave higher yields compared to the sodium-based catalysts and methoxide catalysts gave better yields compared to the hydroxide bearing catalysts. Conclusion Raw materials contribute to a major portion in the cost of biodiesel production. Among the commodity fats and oils, soybean oil is the most produced vegetable oil in the world. Recently, use of soybean oil has significantly increased due to its nutrition values and wide applications not only in food industry but in other industries. For food and animal feed, soybean is the good resource of oil and protein. It is also a good source of mineral and vitamins. In the present study, alkoxides are reported to be the more effective transesterification catalysts compared to hydroxides. Further, sodium alkoxide is the most efficient catalyst, although KOH and NaOH can also be used. About 98.93% conversion of soybean oil into alcohol was obtained under the optimum reaction conditions. References 1 Manjaya J G, Suseelan K N, Gopalakrishna T, Pawar S E & Bapat V A, Radiation induced variability of seed storage proteins in soybean [Glycine max (L.) Merrill], Food Chem, 100 (2007) Qi D H & Lee C F, Influence of soybean biodiesel content on basic properties of biodiesel-diesel blends, J Taiwan Inst Chem Eng, 45 (2014) Santos E M, Piovesan N D, Barros E G & Moreira M A, Low linolenic soybeans for biodiesel: Characteristics, performance and advantages, Fuel, 104 (2013) Farhoosh R, Einafshar S & Sharayei P, The effect of commercial refining steps on the rancidity measures of soybean and canola oils, Food Chem, 115 (2009)

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