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Ind. Eng. Chem. Res. 2009, 48, 10816–10819
Influence of Carbon Chain Length and Unsaturation on the Esterification Activity of Fatty Acids on Nb2O5 Catalyst Kalaru Srilatha,† N. Lingaiah,† Potharaju S. Sai Prasad,*,† B. L. A. Prabhavathi Devi,‡ R. B. N. Prasad,‡ and S. Venkateswar§ Inorganic & Physical Chemistry DiVision and Lipid Science and Technology DiVision, Indian Institute of Chemical Technology, Hyderabad 500607, India, and Osmania UniVersity, Hyderabad 500607, India
The influence of chain length and the extent of the saturation on the esterification activity of long chain carboxylic acids were studied on Nb2O5 catalyst. A series of individual fatty acids (caprylic, capric, lauric, myristic, palmitic, and steric acids) and their mixtures present in vegetable oils, namely, coconut oil, ground nut oil, olive oil, and soybean oil, were selected for esterification with methanol at different reaction temperatures. The esterification reactivity was found to be inversely proportional to alkyl chain length of the acid. The trend was discussed in terms of steric and inductive effects. The kinetic parameters estimated revealed a mild dependency of the activity on the extent of unsaturation. 1. Introduction Because of diminishing reserves of crude oil and the environmental impact of exhaust gases from petroleum derived fuels such as gasoline and diesel, biodiesel has attracted attention during the past decade as a renewable and green fuel.1 The American Society for Testing and Materials (ASTM) defines biodiesel as mono alkyl esters of long chain fatty acids derived from a lipid feedstock such as vegetable oil or animal fat.2 Biodiesel is manufactured by the transesterification of fats and oils. Triglycerides are readily transesterified in the presence of alkaline catalyst, for example sodium hydroxide, at atmospheric pressure and at a temperature between 60 and 70 °C with excess methanol. However, the relatively higher amounts of free fatty acids (FFAs) in the feedstock results in the production of undesirable saponified products in the presence of alkali catalyst. Therefore, it is necessary to remove FFAs before the transesterification for which esterification is the common option. In fact, commercial base-catalyzed processes often employ an acidcatalyzed pre-esterification reactor to convert FFAs.3 Many industrial esterification processes are carried out in the presence of strong Bronsted acid catalysts such as sulfuric acid or p-toluenesulfonic acid. However, such homogeneous acids are not environmentally benign and they require special processing in the form of neutralization involving costly and inefficient catalyst separation from product mixtures. This results in substantial energy and chemical waste. Replacing such conventional homogeneous Bronsted acids with recyclable solid acids is a most promising solution to this problem, and the potential of these solid acids has been studied extensively. Solid catalysts employed for the esterification comprise ion-exchange resin,4,5 zeolites,6 and heteropoly acids.7 A solid catalyst allows a much simpler down stream separation process, because it does not need any washing or neutralizing equipment. Thus, the process becomes economic as a pure product can be produced. So far, low-molecular-weight carboxylic acids have been used to address fundamental issues of esterification because of their * Corresponding author. Tel: +91-40-27193163. Fax: +91-4027160921. E-mail:
[email protected]. † Inorganic & Physical Chemistry Division, Indian Institute of Chemical Technology. ‡ Lipid Science and Technology Division, Indian Institute of Chemical Technology. § University College of Technology.
simplicity, availability, and ease of product analysis.4,6,8 Detailed investigations into the kinetics of free fatty acid (FFA) esterification are scarce. Having correlations between their structure and reactivity provides insight into differences in chemical behavior in the case of larger molecular weight carboxylic acids such as FAs. Some researchers have conceptualized and quantified the possible effects of substituents on the chemical reactivity. For example, the Hammett equation for aromatic systems and the Taft equation for aliphatic systems were the first models proposed that attempted to quantify polar, resonance, and steric effects of substituents in carboxylic acids.9,10 These models were improved,11 modified,12 and expanded to other reactions.13 Although most studies have been limited to the use of homogeneous catalysts, the qualitative conception and quantitative correlations reported in them provide a basis for understanding the probable structural effects of large carboxylic acids. These aspects have been hardly explored in the case of heterogeneous catalysts. Only few studies related to this topic can be found in the literature. Lilja et al.14 used ion-exchanged resins, Mochida et al.15 used sodium-poisoned silica-alumina, and Liu et al.16 used Nafion /silica composite solid acid catalyst to explain the influence of chain length on the esterification reactivity of small chain carboxylic acids, not longer than caprylic acid (C8 acid). Particularly, these studies were carried out on individual acids. Similar studies devoted to reaction performed on long chain fatty acids present in oils are rather scarce. Of late, niobium compounds have been selected as important catalysts for various reactions. Thus, the research and development on the catalytic application of niobium compounds has increased several folds. Hydrated niobium pentoxide, Nb2O5 · nH2O, which is called niobic acid, and niobium phosphate (NbPO4) exhibit strong acidic properties and are used as solid acid catalysts. The former, containing large amount of water, exhibits high catalytic performance for acid-catalyzed reactions. When calcined at relatively low temperatures (100-300 °C), it has a high acid strength (Ho ) -5.6) corresponding to 70% that of H2SO4, whereas the surface of niobic acid calcined at 500 °C is almost neutral.17 Niobic acid shows high catalytic activity and 100% selectivity for the esterification of ethyl alcohol with acetic acid. SO42-/ TiO2, one of the solid super acids, shows high initial activity but its activity rapidly
10.1021/ie900864z CCC: $40.75 2009 American Chemical Society Published on Web 11/02/2009
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a
Table 1. Composition (wt %) of Fatty Acids Derived from Vegetable Oils
coconut FA groundnut FA olive FA soybean FA a
C8:0
C10:0
C12:0
C14:0
C16:0
C18:0
C18:1
C18:2
7.9
5.8
47.4
20.2
8.6 14.6 13.8 11.2
2.46 4.2 2.8 4.7
6.25 41.7 74.4 27.5
1.5 34.1 8.8 50.2
C18:3
C20:0
C20:1
C22:0
C22:1
0.8
1.7
1.6
5.2
1.3 0.3 0.75
Cn:b represents carboxylic acid with “n” number of carbons and “b” number of double bonds.
decreases with time.18 Resin becomes black after 1 h of reaction, making its repeated use impossible. In this respect, the activity of niobic acid does not change even after usage of 60 h. For the esterification of acrylic acid with methanol, niobic acid shows 95% conversion of acrylic acid and 100% selectivity with a good stability.19 The present work reports on the esterification reactivity of long chain fatty acids present in different oils which is more relevant to the synthesis of biodiesel. The objective of this work is to study the reactivity of individual fatty acids of chain length varying from C8 to C18 and to compare this with that observed when they are in the combined form. Correlating the degree of saturation with the reactivity is yet another objective. 2. Experimental Section 2.1. Materials and Catalyst. Caprylic, capric, lauric, myristic, palmitic, and stearic acids and methanol (AR grade) were obtained from Sigma-Aldrich Co., USA. Coconut, groundnut, olive, and soybean oils were purchased from a local market. KOH used for hydrolysis of oils to produce their respective fatty acids was procured from Sigma-Aldrich Co. The Nb2O5 catalyst used was obtained by calcination of niobic acid (CBMM, HY 34O) in air at 200 °C for 4 h. 2.2. Esterification Reaction and Analysis. The reactor, Radleys parallel synthesizer (which simultaneously performs up to six, heated and stirred reactions), was provided with reflux condenser and a temperature indicator. In a typical experiment, an appropriate amount of fatty acid was weighed and taken into the round-bottom flask of parallel synthesizer. Required amounts of catalyst and methanol were also added to the fatty acid. The acid to methanol ratio was kept at 1:14 (mol/mol). The weight of the catalyst was varied between 2.5 and 20 wt % FA. After carrying the reaction for specific period, the product was separated by filtration and the unreacted methanol was recovered by drying in a rotary evaporator. The product was analyzed on a gas chromatograph (6890N,Agilent Technologies, USA) using a flame ionization detector and HP INNOWax column, 30 m × 0.25 mm × 0.25 µm (Hewlett-Packard Part no. 19091N133).
Figure 1. Conversion of fatty acid was found to increase with increase in catalyst concentration up to 15%, and there on it almost stabilized. On the basis of these observations, we made further studies using 15 wt % catalyst concentration. Esterification of different individual carboxylic acids (C8 to C18) with methanol was carried out on Nb2O5 catalyst at reflux temperature (65 °C) with an acid to alcohol molar ratio of 1:14. As revealed by Figure 2, the increasing chain length of acid had a retarding effect on reaction rate. The same trend was observed at 1, 2, and 3 h of reaction time. The study was extended to different mixtures of fatty acids. Coconut fatty acid was taken for this study, as it contained essentially saturated fatty acids of varying carbon chain length. The results on esterification activity are shown in Figure 3. The decreasing trend in carboxylic acid reactivity with increasing alkyl chain length is also seen, as shown in Figure 3. Thus, it is confirmed that as in the case of small chain alcohols, the reactivity of long chain fatty acids toward esterification also decreases with chain length. This is an important observation. Similar effects were reported in the literature. Lilja et al.14 observed decrease in
Figure 1. Influence of catalyst concentration on esterification activity of fatty acids: ( ×) coconut fatty acid, (2) groundnut fatty acid, (0) olive fatty acid, (b) soy bean fatty acid (reaction conditions: T ) 65 °C, time ) 3 h).
3. Results and Discussions Different feedstocks like waste cooking oils,5,7,20 high FFA rice bran oil,21 and sunflower oil22 have been used for biodiesel production. The present study also deals with edible oils. However, the composition of edible oil is not the same always. It depends on many environmental factors like the variety of seeds, soil, and climatic conditions. The oils chosen for the present study were first analyzed for their fatty acid composition and the data are shown in Table 1. It can be observed from the table that the saturation content of FFA decreases from coconut to soybean oil and proportionately the unsaturation content increases. To study the effect of catalyst concentration on the reactivity, the amount of catalyst was varied from 2.5 to 20 wt % based on the fatty acid taken. The activity results are summarized in
Figure 2. Reactivity of individual fatty acids: (+) 1 h, (0) 2 h, (2) 3 h (reaction conditions: T ) 65 °C, catalyst amount ) 15 wt % acid).
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Ind. Eng. Chem. Res., Vol. 48, No. 24, 2009
Figure 3. Reactivity of fatty acids present in coconut oil: (+) 1 h, (0) 2 h, (2) 3 h (reaction conditions: T ) 65 °C, catalyst amount ) 15 wt % acid).
reactivity with increase in the chain length from acetic to pentanoic acid. They reported this phenomenon on homogeneous as well as heterogeneous catalysts. Liu et al.16 also made the same observation on SAC-13 catalyzed esterification of carboxylic acids with chain length less than eight carbons and reported that steric effects dominate catalyst activity in acidcatalyzed esterification. Zaidi et al.23 suggested that there exists a relationship between the values of kinetic parameters and chain length of substrates and reported that an increase in the number of carbons of alcohol decreases the esterification reactivity. Alonso et al.24 studied the reason for lower reactivity of long chain or more voluminous carboxylic acids and esters during heterogeneous esterification and transesterification reaction. As biodiesel feedstock contains fatty acids of vegetable oils, the results obtained in the present work attain considerable significance. As the alkyl chain length increases, the ability of the acid to release electrons increases, which in turn increases the inductive effect, responsible for more energy hindered rate limiting nucleophilic attack by the alcohol. The steric component, the decisive factor for acid-catalyzed esterification, increases with increase in alkyl chain length of acid.9,12 Steric hindrance increases with molecular size, inducing electronic repulsion between nonbonded atoms of reacting molecules, lowering the electron density in the intermolecular region and disturbing the bonding interactions.25 Thus, the factors contributing to the diminished carboxylic acid reactivity are inductive and steric in nature.9 To understand the influence of unsaturation on the reactivity of fatty acids, we carried out the esterification on different vegetable fatty acids at different temperatures and reaction times and the results are given in Figures 4 and 5. Interesting results were obtained where the conversions were more for coconut FA and least for soybean FA. As can be seen from Table 1, the composition of unsaturated fatty acid increases as we move from coconut to soybean FA. The order of unsaturation is as follows: coconut FA < groundnut FA < olive FA < soybean FA. From Figures 4 and 5, it is evident that the unsaturation content of the fatty acid retards the esterification reactivity. Lilja et al.14 reported a decrease in the reactivity of various acids on branching of alcohol chain on smopex-101 catalyst. Saturated fatty acids are linear in structure, whereas unsaturation distorts the linearity of the molecule. Linear fatty acids can easily pass through the pores and adsorb on to the catalyst surface so as to be available for the reaction. In the case of unsaturated fatty acids, their nonlinearity might decrease the rate of diffusion,
Figure 4. Reactivity of vegetable fatty acids at 40 °C: (+) 1 h, (0) 2 h, (2) 3 h (reaction conditions: catalyst amount ) 15 wt % acid).
Figure 5. Reactivity of vegetable fatty acids at 50 °C: (+) 1 h, (0) 2 h, (2) 3 h (reaction conditions: catalyst amount ) 15 wt % acid).
Figure 6. Arrhenius plots of (b) coconut fatty acid, (0) ground nut fatty acid, (2) olive fatty acid, (+) soy bean fatty acid.
leading to a decrease in reactivity, as explained by Liu et al.16 and Alonso et. al24 The above results are transformed into kinetic relationships assuming a first order behavior. Energies of activation and frequency factors have been obtained using the Arrhenius equation. The respective plots for coconut, groundnut, olive and soybean fatty acids are shown in Figure 6. The decrease in reactivity at the same temperature reflects in increasing the activation energy. Table 2 discloses the values of activation energies against the extent of unsaturation. A moderate dependency of the values of activation energy and frequency factor on the unsatuation can be seen.
Ind. Eng. Chem. Res., Vol. 48, No. 24, 2009 Table 2. Pre-Exponential Factor and Activation Energy Values of Different Oils fatty acid
unsaturation content (wt %)
activation energy (kcal/mol)
pre-exponential factor
coconut ground nut olive soybean
7.75 81.2 83.4 84.1
12.53 13.38 13.73 14.16
14.39 15.53 15.96 16.54
Further work is under progress at authors’ laboratory to determine the properties of the catalyst and to check its reusability, along with a detailed kinetic modeling. Conclusions From the above study the following conclusions can be drawn: 1. As the chain length increases, the reactivity of fatty acids decreases. 2. The esterification reactivity of fatty acids in oils is greater when they contain higher amounts of saturated fatty aids. 3. The activation energy for the fatty acid esterification increases with the extent of unsaturation fatty acid. Literature Cited (1) Barnwal, B. K.; Sharma, M. P. Prospects of biodiesel production from vegetable oils in India. Renewable Sustainable Energy ReV. 2005, 9, 363. (2) Zhang, Y.; Dube, M. A.; McLean, D. D.; Kates, M. Biodiesel production from waste cooking oil: 1. Process design and technological assessment. Bioresour. Technol. 2003, 89 (1), 1. (3) Ramadhas, A. S.; Jayaraj, S.; Muraleedharan, C. Biodiesel production from high FFA rubber seed oil. Fuel 2005, 84, 335. (4) Mazzotti, M.; Neri, B.; Gelosa, D.; Krugov, A.; Morbidelli, M. Kinetics of liquid phase esterification catalyzed by acidic resins. Ind. Eng. Chem. Res. 1997, 36, 3. (5) Ozbay, N.; Oktar, N.; Tapan, N. A. Esterification of free fatty acids in waste cooking oils (WCO): Role of ion-exchange resins. Fuel 2008, 87, 1789. (6) Mehmet, R. A.; Alime, C. Kinetics study of esterification of acetic acid with isobutanol in the presence of Amberlite catalyst. Appl. Catal. A: Gen. 2003, 239, 141. (7) Cao, F.; Chen, Y.; Zhai, F.; Li, J.; Wang, J.; Wang, X.; Wang, S.; Zhu, W. Biodiesel production from high acid value waste frying oil catalyzed by superacid heteropolyacid. Biotechnol. Bioeng. 2008, 101 (1), 93. (8) Lilja, J.; Aumo, J.; Salmi, T.; Murzin, D. Y.; Arvela, P. M.; Sundell, M.; Ekman, K.; Peltonen, R.; Vainio, H. Kinetics of esterification of propanoic acid with methanol over a fibrous polymer supported sulphonic acid catalyst. Appl. Catal., A 2002, 228, 253. (9) Taft, R. W. Steric Effects in Organic Chemistry; Newman, M. S., Ed.; Wiley: New York, 1956; p 556.
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(10) Aranda, D. A. G.; Goncalves, J. A.; Peres, J. S.; Ramos, A. L. D.; Melo, C. A. R., Jr.; Antunes, O. A. C.; Furtado, N. C.; Taft, C. A. The use of acids, niobium oxide, and zeolites catalysts for esterification reactions. J Phys.Org.chem. 2009, 22, 709. (11) McPhee, J.; Panaye, A.; Dubois, J. Steric effects-I: A critical examination of the taft steric parametrer-ES. Definition of a revised, broader and homogeneous scale. Extension to highly congested alkyl groups. Tetrahedron 1978, 34, 3553. (12) Charton, M. Steric effects. I. Esterication and acid-catalysed hydrolysis of esters. J. Am. Chem. Soc. 1975, 97, 1552. (13) DeTar, D. F. Effects of alkyl groups on rates of SN2 reactions. J. Org. Chem. 1980, 45, 5174. (14) Lilja, J.; Murzin, D. Y.; Salmi, T.; Aumo, J.; Arvela, P. M.; Sundell, M. Esterification of different acids over heterogeneous and homogeneous catalysts and correlation with the Taft equation. J. Mol. Catal., A 2002, 182, 555. (15) Mochida, I.; Anju, Y.; Kato, A.; Seiyama, T. Elimination reaction on solid acid catalysts. IV. A δR LFER study of the esterification of alcohols with carboxylic acids over solid acid catalysts. Bull. Chem. Soc. Jpn. 1971, 44, 2326. (16) Liu, Y.; Lotero, E.; Goodwin Jr, J. G. Effect of carbon chain length on esterification of carboxylic acids with methanol using acid catalysis. J. Catal. 2006, 243, 221. (17) Tanabe, K. Niobic acid as an unusual acidic solid material. Mater. Chem. Phys. 1987, 17, 217. (18) Chen, Z.; Iizuka, T.; Tanabe, K. Niobic acid as an efficient catalyst for vapor phase esterification of ethyl alcohol with acetic acid. Chem. Lett. 1984, 1085. (19) Iizuka, T.; Fujie, S.; Ushikubo, T.; Chen, Z.; Tanabe, K. Esterification of acrylic acid with methanol over niobic acid catalyst. Appl. Catal. 1986, 28, 1. (20) Kulkarni, M. G.; Dalai, A. K. Waste cooking oil - An economical source for biodiesel: a review. Ind. Eng. Chem. Res. 2006, 45, 2901. (21) Zullaikah, S.; Chin Lai, C.; Vali, S. R.; Ju, Y. A two-step acidcatalyzed process for the production of biodiesel from rice bran oil. Bioresour. Technol. 2005, 96, 1889. (22) Berrios, M.; Siles, J.; Martin, M. A.; Martin, M. A kinetic study of the esterification of free fatty acids (FFA) in sunflower oil. Fuel 2007, 86 (15), 2383. (23) Zaidi, A.; Gainer, J. L.; Carta, G.; Mrani, A.; Kadiri, T.; Belarbi, Y.; Mir1, A. Esterification of fatty acids using nylon-immobilized lipase in n-hexane: kinetic parameters and chain-length effects. J. Biotechnol. 2002, 93, 209. (24) Alonso, D. M.; Granados, M. L.; Mariscal, R.; Douhal, A. Polarity of acid chain of esters and transesterication activity of acid catalysts. J. Catal. 2009, 262, 18. (25) Fujimoto, H.; Mizutani, Y.; Endo, J.; Jinbu, Y. Theoretical study of substituent effects. Analysis of steric repulsion by means of paired interactiong orbitals. J. Org. Chem. 1989, 54, 2568.
ReceiVed for reView May 26, 2009 ReVised manuscript receiVed October 9, 2009 Accepted October 17, 2009 IE900864Z
