Chemical Industry & Chemical Engineering Quarterly

Available on line at Association of the Chemical Engineers of Serbia AChE www.ache.org.rs/CICEQ

Chem. Ind. Chem. Eng. Q. 24 (3) 283−291 (2018) CI&CEQ

283

VLATKO KASTRATOVIĆ

MILJAN BIGOVIĆ

Faculty of Natural Sciences and Mathematics, University of

Montenegro, Podgorica, Montenegro

SCIENTIFIC PAPER

UDC 547.295:544:66

ESTERIFICATION OF STEARIC ACID WITH LOWER MONOHYDROXYLIC ALCOHOLS

Article Highlights • The highest content of fatty acid was converted to ester with primary alcohols • Secondary and tertiary alcohols and alcohols with branched chains react more slowly • The presence of the double bond in unsaturated fatty acids reduces the conversion of

acid to ester • The increase of the molar ratio of acid/alcohol increases the speed and yield • The increase of the number of carbon atoms in the alcohol decreases activation

energy Abstract

Esters play a significant role in everyday life but also in the chemical industry. The aim of this study is to investigate the influence of different parameters on the process of esterification of higher monocarboxylic acids with lower mono-hydroxylic alcohols. We examined the influences of the following variables: the type and amount of the catalyst, the structure of alcohols and fatty acids, the acid/alcohol molar ratio, and the temperature of the esterification process. The descending order of reactivity found alcohols is: 1-butanol > 1-propanol >2-methyl-1-propanol > ethanol > 2-butanol >2-propanol > 2-methyl-2-propanol. The results of this study show no significant effect of chain lengths of saturated fatty acids on the speed and yield of esterification. The presence of the double bond in unsaturated fatty acids reduces the acid to ester conversion. The high-est yield (99%) was obtained in the reaction of stearic acid and 1-butanol with an acid/alcohol/catalyst (H2SO4) mole ratio 1/15/0.75 and at a temperature of 65 °C.

Keywords: ester, esterification, fatty acid, alcohol.

Esterification of carboxylic acids with alcohols represents a well-known category of liquid-phase reactions that is commonly present in industry due to the enormous practical importance of organic ester products [1-3]. Esters constitute a group of con-venient chemical intermediates in the synthesis of various compounds such as amides, sulfonates and fatty alcohols. These compounds are also used as solvents, reagents in polymers production, as well as materials in textile, cosmetics, cotton, and pharma-ceutical industries [4].

Correspondence: V. Kastratović, Faculty of Natural Sciences and Mathematics, University of Montenegro, G. Washington Street, P.O. Box 5455, 81000 Podgorica, Montenegro. E-mail: vlatkok@ac.me Paper received: 27 March, 2017 Paper revised: 11 October, 2017 Paper accepted: 30 November, 2017

https://doi.org/10.2298/CICEQ170327040K

In the past decade esters of long chain fatty acids and alcohols have earned a special interest. There are several industrial possibilities for industrial use of fatty acid esters as natural compounds [5]. In the esterification process of vegetable oils, there is usually a reaction of oleic acid and other long chain fatty acids (12-20 carbon atoms) with selected alco-hols. The chain length of the alcohol substrate deter-minates the use of the ester product. Short-chain alcohols with low molecular weights can be used for production of biodiesel and biofuels [6]. Long-chain alcohols (5 to 12 carbon atoms) in the esterification process make lubricants [5]. Fatty acid esters with high molecular weight alcohols (C16-C30) are waxes. Besides the fuel production, they are used in food, detergents, cosmetics, and pharmaceutical industries [7].

Fatty acid esters may also be obtained from tri-glyceride hydrolysis and transesterification reaction in

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the presence of a base catalyst, most frequently NaOH and KOH [8-10].

The advantage esters show for their practical use is that they do not accumulate, do not pollute the environment, bear a low chemical risk, provide maxi-mum safety in use, and are mostly biodegradable. Raw materials for their production are easily found in vegetable oils and animal fats.

Testing for the production of these esters has become more and more intensive, especially since esterification is a balanced process. For this reason, a great deal of attention is dedicated to examining para-meters that move the balance of the reaction to the side of the desired product - ester. Conventional pro-duction requires a catalyst and a significantly greater amount of alcohol than the stoichiometric technology. The process is often expensive and requires unnec-essary electricity consumption, results in impure org-anic bio-compounds as side products, and the final product with changing quality. At present, many res-earchers examine the esterification reaction in order to obtain a process without waste, with minimal energy consumption, higher yields, and shorter ester-ification reaction time.

The key factors that influence the esterification process are: the type and amount of the catalyst, the size and structure of the alcohol and acid, the acid/ alcohol/catalyst mole ratio, and the reaction tempera-ture (Table 1). Studies have shown that combinations of different key factors can often have an antagonistic effect [11].

The aim of this work is to achieve an optimum esterification process with stearic acid and lower monohydric alcohols. The intention was to investigate the influence of the type and amount of catalyst, the structure of the alcohol and fatty acid, the acid/alcohol molar ratio and the temperature on the esterification process.

METHODS AND MATERIALS

In the experiment part of this work, esterification processes of fatty acids (in particular, stearic acid) with lower monohydroxy alcohols were carried out, using the sulfuric acid as a catalyst, where the inf-luences of the alcohol structure, possibilities for using strong organic acids as catalysts, acid/alcohol molar ratio, acid/catalyst molar ratio, and the reaction tem-perature were examined.

The reactions were carried out in a reaction con-tainer (Erlenmeyer flask) with a volume of 300 mL with a reflux condenser on the magnetic stirrer at a speed of 300 rpm. We used chloroform as the sol-vent. The experiments were done at atmospheric pressure and at a constant predefined temperature. The following various alcohols were reacting: ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, 2- -methyl-1-propanol and 2-methyl-2-propanol. In order to test organic acids as catalysts in esterification reactions, we experimented with saturated acids, here palmitic and stearic acid, and with unsaturated oleic acid. The experiments were done at 35, 45, 55 and 65 °C. The tested acid/alcohol mole ratios in this study were 1/15, 1/10 and 1/5, while the acid/catalyst ratios were 1/0.75, 1/0.50 and 1/0.25.

In order to determine the conversion degree of stearic acid during the esterification reaction, the tit-ration method was used as a simple and effective technique that does not require highly specialized ins-truments. Aliquots of 3 cm3 were taken from the react-ion mixture at 30 min intervals, to which few drops of phenolphthalein were added and which were titrated with 0.1 M (±0.0001) alcoholic KOH solution.

The degree of conversion of the stearic acid into ester is calculated with the formula:

−= 0

0

Conversion 100 tc cc

Table 1. Summary of esterification processes from some previous studies

Acid Alcohol Mole ratio of acid/alcohol wt.% of catalyst relative to acid Temperature, °C Reaction time, h Reference

Oleic Ethanol 1/6 5% H2SO4 70 1.5 [12]

Benzoic 1-Propanol Stoichiometric 10 mol% SnCl2 100 20 [9]

Fatty Glycerol 1/1 0.3 wt.% ZnCl2 195 – [13]

Acetic Methanol 1/3 10 wt.% Amberlyst-15 55 – [14]

Oleic Methanol 1/60 5% H2SO4 60 2 [8]

Stearic Methanol 1/20 10 wt.% oxalic acid 65 8 [15]

Oleic Methanol 1/14 10 wt.% TiO2-SiO2 120 3 [16]

Lauric Methanol 1/10 5 wt.% MgO-SBA-15 118 6 [16]

Oleic Methanol 1/5 2.5 wt.% Amberlyst-15 and ZrTiO 60 2 [17]

Acetic Ethanol 3/1 HSiW (7.5×10-4:1) 85 4.5 [18]

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where c0 - stearic acid concentration at the beginning of esterification process, and ct - stearic acid concen-tration at the point of time t.

RESULTS AND DISCUSSION

Influence of the catalyst

Figure 1 shows the influence of a strong inorg-anic acid (H2SO4) and several strong organic acids as catalysts on the conversion degree of stearic acid to ester. The following organic acids were chosen: picric acid (PA), dibromophenol (DBP), trifluoroacetic acid (TFA), and trichloroacetic acid (TCA). The strong min-eral acid H2SO4 as the catalyst in the esterification reaction has shown much faster kinetics and higher reactivity in comparison to the strong organic acid. The order of the activity potential with strong organic acid catalysts descends as well as their dissociation factor as follows: trichloroacetic acid > picric acid > trifluoroacetic acid > dibromophenol.

For commercial purposes, esterification of fatty acids with alcohols is most commonly done in the presence of strong inorganic acids (H2SO4, HCl, HI). These catalysts are inexpensive, stable, with a high activity potential and efficiency, and have relatively short reaction times. The downsides are that they adversely affect the environment, they are corrosive, creating large quantities of bio-products that are dif-ficult to treat and recycle, etc. [9-10,19-20]. In order to avoid adverse effects of acids as catalysts, a con-siderable number of researchers are examining the use of heterogeneous acid catalysts in the esterific-ation reaction. The tested solid catalysts in the pro-duction of esters are: ion-exchangeable organic resins, such as Amberlyst-15 [21-22], zeolites [23], silicate heteropolyacid, HPA/silica, [24], nafion-com-

posite solid catalyst SAC-13 [19], ZrO2 [25], niobium-phosphate [10]. However, they generally show limit-ations in use as esterification catalysts due to their low thermal stability (Amberlyst-15, < 140 °C), short-ness of mass transfer (zeolites) [23,26], loss of acid zones activity potential in a polar medium (HPA/silica) [24], and blockage of catalytic zones by long-chain acids [19].

Influence of catalyst quantity

Figure 2 shows that the increase in concentra-tion of the catalyst (H2SO4) is followed by the increase in the kinetics of the reaction during the first 30 min. After 90 min, the ester yields become almost identical with the number of moles ratios of H2SO4/stearic acid 0.5/1 and 0.75/1.

As the process continues the ester yield becomes similar (after 210 min) also for the 1/0.25 ratio, regardless of the increase in the concentration of the catalyst.

Altic [27] studied the effects of sulfuric acid cat-alyst concentration on the esterification reaction with free fatty acids in raw tallow with methanol. The speed and yield of the reaction are greatly increased with the range of sulfuric acid concentration between 0.3 and 1.0% of volume of the oil used. Outside this range, further addition of sulfuric acid does not lead to a significant further transformation of acid into ester as a result of a 15-min reaction in an ultrasonic bath.

Cardoso et al. [20] studied the effect of the quantity of tin(II) chloride dihydrate as a catalyst on the reaction of esterification of oleic acid with ethanol (1/120). The increase in the amount of the catalyst resulted in the increase yield of ethyl oleate during the reaction time. At a low molar ratio of acid and tin (II) chloride (1/0.01), 24% of ethyl oleate was obtained

Figure 1. Influence of the catalyst on the esterification of stearic acid with 1-butanol. Reaction conditions: acid/alcohol/catalyst mole ratio

1/10/0.25 at temperature of 45 °C.

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after 24 h. However, the use of a higher mole ratio (1/0.4) in the same time interval resulted in a con-siderably higher yield of the reaction (about 90%).

The influence of the amount of ZnCl2 as a cat-alyst (0.1, 0.2 and 0.3 wt.%) on esterification of glycerol with unsaturated fatty acids (1/1), and at 195 °C was investigated by Mostafa et al. [13]. The inc-rease in the concentration of the catalyst reduced the reaction time for reaching the maximum conversion. Thus, with 0.3 wt.% ZnCl2, the maximum yield of 99% was reached after 100 min, as opposed to non-cat-alyzed reactions, where the necessary reaction time was over 200 min. Further increase in the catalyst quantity did not result in the increase of the yield of esterification; however, it disturbed the regeneration of the catalyst.

Influence of alcohol type

For determination of effects of alcohol structure on the yield of the esterification reaction 7 alcohols

were studied: ethanol, 1-propanol, 2-propanol, 1-but-anol, 2-butanol, 2-methyl-1-propanol and 2-methyl-2- -propanol (Figure 3). Experiments were carried out with pure stearic acid and sulfuric acid as a catalyst at a temperature of 45 °C. The mole ratio of acid/alc-ohol/catalyst was 1/10/0.25. The solvent used in this reaction was chloroform (tk = 60 °C).

The descending order of reactivity of the tested alcohols was: 1-butanol > 1-propanol > 2-methyl-1- -propanol > ethanol > 2-butanol > 2-propanol > 2- -methyl-2-propanol.

Based on these results it can be concluded that the yield of esterification depends mainly on the phys-ical properties of alcohol. Out of the tested alcohols, 1-butanol has the highest boiling point (117.5 °C) and the lowest evaporation, that is, it is present in the largest quantity within the reaction mixture. Besides the physical properties of the alcohol (the chain length), esterification is also affected by the structure of the alcohol. Figure 3 shows the descending speed

Figure 2. Influence of the catalyst (H2SO4) amount on the kinetics of esterification of stearic acid with 1-butanol. Conditions: acid/alcohol

mole ratio 1/10; temperature 45 °C; acid/catalyst mole ratios: 1/0.25, 1/0.5 and 1/0.75.

Figure 3. Influence of the chain length and structure of the alcohol on esterification of stearic acid. Conditions: acid/alcohol/catalyst

(H2SO4) mole ratios: 1/10/0.25; temperature 45 °C.

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of esterification due to the steric factor: primary alcohol > secondary alcohol > tertiary alcohol.

Neji et al. [4] came to similar conclusions as they studied esterification with the same acid and alcohols used in this work, but in the presence of acidic clay as a catalyst. The effect of the alcohol structure was examined by Bassan et al. [10]. They carried out esterification of lauric acid with lower ali-phatic alcohols using niobium phosphate as a cat-alyst. The best results were achieved with 1-butanol.

Braga et al. [28] carried out the esterification of acetic acid with ethanol, butanol and iso-pentanol in the presence of a catalyst system Nb2O5/SiO2-Al2O3,

and found that reactivity increases starting with etha-nol up to isopentanol. Saravanan et al. [29] tested various alcohols in the reaction of transesterification of vegetable oil, catalyzed with sulfuric acid. Due to the applied higher temperature reflux, they received higher yields of esters with a long chain alcohol use, due to their higher boiling point.

Cho et al. [30] obtained a higher yield of the ester when the reaction was carried between satur-ated and unsaturated aromatic acid with propyl alco-hol compared to the secondary alcohol - isopropyl alcohol. They also observed that the conjugation of the double bond of the carbonyl group with double bonds of the aromatic ring does not affect the react-ivity of the carboxylic acid groups.

The fact that the length of the hydrocarbon series of alcohols affects the speeds and rates of conversion of fatty acids in the esterification reaction was concluded by Cardoso et al. [31]. In their work they present opposite conclusions to the ones above: that an increase in the number of carbon atoms in the alcohol leads to a decrease in the conversion of acid

ester, which is in accordance with the nucleophilicity of alcohol that decreases with an increasing number of carbon atoms.

Influence of acid/alcohol mole ratio

The acid/alcohol mole ratio is one of the most important variables which have a direct impact on a degree of conversion into a fatty acid ester. With an increase of the acid/alcohol mole ratio from 1/5 to 1/15, the speed and yield of esterification also grow (Figure 4). The maximum difference in the yield of the ester was achieved after 60 min reaction period, where 47.2% of the ester was obtained with the acid /alcohol mole ratio of 1/5, and 67.4% for the mole ratio of 1/15. With further increase of the reaction time the difference was fading. After 210 min from the reaction start the resulting yield of the ester amounted to 84.3 (1/5) and 90.1 (1/15).

The study of Marchetti and Errazu [32] resulted in conclusions very similar to the ones in this work. They found a maximum conversion of free fatty acids from the oil 0.96 in the following reaction conditions: the acid/alcohol molar ratio 6.126, the reaction tem-perature 55 °C, reaction time 240 min and 2.261 wt.% H2SO4.

Abbas and Abbas [12] examined the effect of varying mole ratios acid/alcohol (1/1 to 1/6) on ester-ification of oleic acid with ethanol. They also noticed an increase in the conversion of oleic acid to the ester of 0.61 with the mole ratio of 1/1 to 0.87, 1/6, after 180 min, with sulfuric acid as a catalyst (1 wt.%) at 70 °C. In the presence of larger quantities of the catalyst (5%), at the same temperature and for the same time period, the yield of the ester was 0.87 (acid/alcohol mole ratio 1/1) to 0.92 with mole ratio 1/6 in 90 min.

Figure 4. Influence of acid/alcohol mole ratio (1/5, 1/10 and 1/15) on the reaction speed in esterification of stearic acid with 1-butanol.

Conditions: acid/catalyst (H2SO4) mole ratio 1/0.25, temperature 45 °C.

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Mostafa et al. [13] examined the influence of glycerol/fatty acid mole ratio on the yield of esterific-ation reaction at a temperature of 195 °C. Esterific-ation process was conducted at molar ratios 1/1, 1/2, 1/3 and 3/1 (glycerol/fatty acid). Maximum conversion of fatty acids into esters was achieved in mole ratio 1/1.

In cases of a higher alcohol/acid mole ratio the results were somewhat unexpected, in particular with regard to the evacuation of water from the reaction mixture, whereby a certain amount of alcohol is also distilled. In the homogenous esterification with H2SO4 and heterogeneous esterification with solid catalyst mole ratios of above 20/1 and even up to 120/1 showed a similar reaction rate but not the yield [20,33,34].

Influence of fatty acids structure

Figure 5 shows the results of examination of the influence C-atoms chain length in saturated fatty acids and the presence of double bonds have on the speed and yield of the esterification reaction. Ester-ification reactions of stearic acid and palmitic satur-ated acids and unsaturated oleic acid with 1-butanol in presence of sulfuric acid at 45 °C were examined. The mole ratio of acid/alcohol/catalyst was 1/10/0.25.

Based on Figure 5, it can be concluded that there is no significant impact of saturated fatty acids chain lengths on the speed and yield of esterification. The presence of double bonds in unsaturated fatty acids reduces the conversion of acid into ester.

Reactivity of carboxylic acids in esterification reactions was determined by the steric factors of the linear alkyl series. In the work of Liu et al. [19] it was found that, with low acid forms from acetic to butanoic acids, reaction speed decreases with the increase in

number of carbon atoms. For higher acids a minor further effect on the reaction was observed with further increase in the number of carbon atoms. Fur-thermore, the results from other authors [4,9-10,35- -38] also show that the increasing number of carbon atoms in straight-chain saturated fatty acids slightly decreases the speed of esterification reaction. The presence of double bonds in the hydrocarbon chain acids (e.g., oleic acid) decreases the speed of ester-ification and yield of ester compared to the saturated acids.

Influence of temperature

In this work, the influence of the reaction tem-perature on esterification of stearic acid with 1-but-anol was examined. The esterification reactions were carried out at 35, 45, 55 and 65 °C, and with acid/alc-ohol/catalyst mole ratio of 1/10/0.25. The results are shown in Figure 6. With the increasing temperature the speed of formation and yield of obtained ester rise. After 30 min from the reaction start the largest difference in the conversion of acid into ester occurs, showing 0.272 at 35 °C and 0.680 at 65 °C. Further into the process, up to 210 minutes reaction time, the difference in the yield of ester is fading, especially in the range between 45 and 65 °C, while at 35 °C this difference fades significantly more slowly.

Maximum conversion of stearic acid to ester in this experiment amounted to 0.921. Abbas and Abbas [12] found the conversion of oleic acid into ester of 0.87, with mole ratio ethanol/oleic acid 6/1 using 1% sulfuric acid as a catalyst at 70 °C after 150 min react-ion time and 0.92 at the same temperature for 90 min but with 5% sulfuric acid as a catalyst.

Figure 5. Influence of the length of the alkyl line and degree of saturation of fatty acids on the speed and yield of the esterification

reaction.

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Ratnaparkhi and Waghmode [15] tested the same temperature intervals as in this work, during the esterification of stearic acid with ethanol (1/20) in the presence of oxalic acid as a catalyst (10 mass%). After 8 h from the esterification start they obtained the maximum conversion of acid into ester at 65 °C.

Bassan et al. [14] found that at the reaction temperature of 65 °C, there was a 35% conversion of lauric acid into methyl-ester after 4 h, with niobium phosphate as a catalyst.

The rate constant and activation energy

Literature data have attributed a first order dep-endence on fatty acid concentration in the majority of esterification reactions [20]. From this standpoint, in this present case we can assume that equation could be used to describe the substrate concentration variation with relation to time:

ln [stearic acid]t = -kt + ln [stearic acid]0

Table 2 shows the values for the rate constant (k) obtained at each temperature (Figure 6). As it was expected, the increase in the reaction temperature has caused a corresponding improvement on the reaction rate.

The Arrhenius equation is applied to consider the effect of reaction temperature on the forward rate constant.

k = Aexp(Ea/RT)

where A is the pre-exponential factor, Ea is the act-ivation energy, R is the ideal gas constant, and T is the reaction temperature. This equation can be rear-ranged as follows:

ln k = ln A – ( Ea/R)(1/T)

Figure 7 shows the lines by plotting ln k versus 1/T. The forward reaction activation energies were calculated from the slopes (-Ea/R) of the straight lines.

Table 2. Values of rate constant (k) and linear correlation coefficient (R2) for the H2SO4 catalyzed stearic acid esterific-ation with 1-butanol

Temperature, K k / min−1 R2

308 0.0089 0.99

318 0.0108 0.98

328 0.0117 0.93

338 0.0151 0.91

Based on the above, it was determined that the activation energy of the process was 14.89 kJ⸱mol-1. Abbas and Abbas [12] found the activation energy of 26.62 kJ⸱mol-1 for the esterification of oleic acid with ethanol. Berrios, et al. [8] found activation energy of 42.76 kJ⸱mol-1 for the reaction between oleic acid and methanol. It is evident that the increase in the number of carbon atoms in the alcohol decreases the activation energy for the esterification reaction of fatty acids.

CONCLUSIONS

Based on the alcohol structure, due to steric disturbances, the descending order of reactivity observed with alcohols in the esterification of fatty acids is as follows: primary alcohols > secondary alcohols > tertiary alcohols.

Perhaps unexpectedly, but principally due to the physical properties (boiling point), with the increasing number of carbon atoms the reactivity of the alcohol in the esterification of fatty acids increases.

Figure 6. Influence of temperature on the conversion of stearic acid into ester with 1-butanol. Conditions: acid/alcohol/catalyst (H2SO4)

mole ratio 1/10/0.25.

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The descending order of reactivity with the examined alcohols is as follows: 1-butanol > 1-pro-panol > 2-methyl-1-propanol > ethanol > 2-butanol > 2-propanol > 2-methyl-2-propanol.

With the increase in the acid/alcohol mole ratio from 1/5 to 1/15, in the beginning of the process the speed and yield of esterification also significantly rise. With further duration of the reaction, this difference is fading.

With the increase of temperature, in the first 30 min of the reaction the speed of production and yield of ester rise. Further into the process, up to 210 min reaction time, the difference in the yield of ester is fading, especially in the range between 45 and 65 °C, while at 35 °C this difference fades significantly more slowly.

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VLATKO KASTRATOVIĆ MILJAN BIGOVIĆ

Faculty of Natural Sciences and Mathematics, University of

Montenegro, Podgorica, Montenegro

NAUČNI RAD

ESTERIFIKACIJA STEARINSKE KISELINE NIŽIM MONOHIDROKSILNIM ALKOHOLIMA

Estri igraju značajnu ulogu u svakodnevnom životu ali i u hemijskoj industriji. Cilj ovog rada je ispitivanje uticaja različitih parametara na proces esterifikacije viših monokar-boksilnih kiselina sa nižim monohidroksilnim alkoholima. Ispitivani su uticaji sledećih varijabli na proces esterifikacije: tip i količina katalizatora, struktura alkohola i masnih kiselina, molski odnos kiselina/alkohol i temperatura. Opadajući niz reaktivnosti ispitiva-nih alkohola je: 1-butanol > 1-propanol > 2-metil-1-propanol > etanol > 2-butanol > 2--propanol > 2-metil-2-propanol. Rezultati ovog rada pokazuju da nema značajnijeg uti-caja dužina lanca zasićenih viših masnih kiselina na brzinu i prinos reakcije esteri-fikacije. Prisustvo dvostruke veze kod nezasićenih masnih kiselina smanjuje konverziju kiseline u estar. Najveći prinos estra (99%) je dobijen u reakciji stearinske kiseline i1-butanola sa molskim odnosom kiselina/alkohol/katalizator (H2SO4) 1/15/0.75 na temperaturi od 65 °C.

Ključne reči: estar, esterifikacija, masna kiselina, alkohol.