Synthesis of Sucrose–Coconut Fatty Acids Esters: Reaction Kinetics

Article pubs.acs.org/IECR

Synthesis of Sucrose−Coconut Fatty Acids Esters: Reaction Kinetics and Rheological Analysis Priya S. Deshpande,† Tushar D. Deshpande,‡ Ravindra D. Kulkarni,*,‡ and Pramod P. Mahulikar† †

School of Chemical Sciences, North Maharashtra University, Jalgaon 425001, India Institute of Chemical Technology, North Maharashtra University, Jalgaon 425001, India

‡

ABSTRACT: The present study portrays the investigations on the optimization of the K2CO3-catalyzed synthesis of sucrose− coconut fatty acids esters. Determination of the hydroxyl value (HV) and the saponification value (SV), as well as Fourier transform infrared (FTIR) analysis, provided an understanding of the progress of transesterification, as a function of temperature (120−140 °C), holding period (30−150 min), catalyst concentration (1%−4%), molar ratio of sucrose to fatty acid methyl esters (FAME) (M = 0.364−6), and molecular weight of fatty acids. Bimolecular second-order kinetic model based on equal transesterification opportunity for all hydroxyl groups of sucrose/partial esters and that based on preferential transesterification of 6 −OH of the glucose unit in constant volume batch reactor were developed using integral and differential methods. The dependence of selective and nonselective kinetic models on the number and nature of hydroxyl groups in the sucrose were investigated. The changes in viscosity of sucrose ester solutions in dimethyl formamide (DMF) solvent were interpreted on the basis of solvent−ester/ester−ester interactions, hydrogen-bond density, and formation of mixed partial esters.

1. INTRODUCTION Sucrose, which is available in large amounts and at attractive prices, is used as the renewable feedstock for the selective functionalization with similarly renewable vegetable oils/fatty esters for the construction of perfect amphiphilic structures, which, because of their properties of being tasteless, odor-free, and nonirritating to the skin and eyes, as well as being nontoxic and biocompatible and having excellent biodegradability properties,1 hold promising applications in diverse areas such as detergents, bleaching boosters, cosmetics, medicines, fine chemicals, and food formulations.2 With six or more of the eight sucrose hydroxyl groups transesterified with fatty acids, sucrose esters attain physical and organoleptic properties similar to those of cooking and frying fats and are digested as a blend of sucrose and fatty acids in the stomach.3 Sucrose esters, with three or lesser fatty acids, are suitable as surfactants and food additives, because of their emulsifying, stabilizing, and conditioning properties.4,5 Use of sucrose ester as a template for the stabilized synthesis of nanoparticles has been demonstrated.6 Transesterification of fatty acid methyl esters (FAME) or triglycerides with sucrose, using a basic catalyst, is the only commercially feasible option for the synthesis of sucrose ester.7 However, the literature on this commercial process, in general, and the kinetics of its synthesis, in particular, is scant and in the form of patents.8−13 The main problem in synthesizing sucrose esters is related to the high functionality of the sucrose molecule with eight hydroxyl groups, which compete during the derivatization step and result in the formation of mixtures of monoesters, diesters, and even higher esters (see Figure 1). The transesterification may be oriented toward a specific substitution pattern by carefully choosing the reaction conditions. Therefore, the focus of the present study was toward elucidating the effects of the molar ratio of saccharose to fatty acids, K2CO3 catalyst and dimethyl formamide (DMF) solvent loading, and reaction temperature on the rate of formation of sucrose ester in specific substitution mode © XXXX American Chemical Society

(monoesters/diesters/polyesters). Since the medium-molecularweight fatty acids of coconut oil are used in a wide range of food, surfactant, and cosmetic products, the oil, which consists of mixed FAMEs derived from the oil and methyl esters of the principal coconut fatty acids (lauric and myristic acids), were chosen to investigate the influence of molecular weight and chain length on kinetics of transesterification. In tune with these process developments, investigations on kinetic modeling by integral and differential method of analysis of reaction data were conducted to predict the variation in rate constants as a function of experimental conditions, understand the selectivity of the process and provide the basis for the design of transesterification reactor. Since the flow characteristics of sucrose ester solutions are important in many applications, their rheological characterizations were performed and interpreted based on molecular interactions.

2. EXPERIMENTAL SECTION 2.1. Materials. Refined coconut oil (saponification value (SV) = 251, acid value (AV) < 0.5, density (ρ) @ 20 °C = 0.918) was procured from the local market. Its fatty acid composition, as determined using a Shimadzu Model GC 14 B system (packed columnRestek RTX wax, temperature programming = 180− 230 °C @ 2 °C/min and 230−250 °C @ 5 °C/min), was as follows: C8 = 10.5, C10 = 4.6, C12 = 48.6, C14 = 20.0, C16 = 11.1, C18 = 2.6, C18:1 = 2.6. The following chemicals were acquired from S D Fine-Chem Limited, Mumbai: sucrose (99%, ρ @ 25 °C = 1.587), lauric acid (99%), myristic acid (98%), methanol (98%), potassium carbonate (99%), acetic anhydride (98%), and toluene (99%). Dimethyl formamide (DMF) (99.5%, boiling Received: May 13, 2013 Revised: September 19, 2013 Accepted: September 27, 2013

A

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Figure 1. Product composition and equilibrium in the synthesis of sucrose esters by transesterification with FAME.

point (BP) = 153 °C @ 760 mm of Hg, ρ @ 25 °C = 0.9445) and butanol were obtained from Merck Specialties Ltd., Mumbai. All the chemicals and reagents were analytical grade and used as received. 2.2. Experimental Methodology. 2.2.1. Synthesis of Coconut Fatty Acid Methyl Esters (FAME) and Methyl Laurate/Myristate. 2.2.1.1. Preparation of Coconut Fatty Acids. Refined coconut oil was saponified using 30% NaOH solution at 90 °C, followed by acidulation using concentrated HCl. The resulting fatty acid layer, after separation, was washed repeatedly, to eliminate traces of mineral acids, and dried under an infrared (IR) lamp. 2.2.1.2. Preparation of Esters of Coconut Fatty/Lauric/ Myristic Acids. Methyl esters were obtained by p-toluene sulfonic acid (1%) catalyzed esterification of coconut fatty/ lauric/myristic acids with methanol (20% molar excess) under reflux conditions for 1 h, followed by the distillation of excess methanol and byproduct water. The esterification reaction was continued in the presence of additional methanol to reduce the AV to 1 ⎥ = C A′ 0(N − 1)k 2t ⎪ ln⎢ ⎩ ⎣ N (1 − XA ) ⎦

(2)

where k2 is the second-order bimolecular nonselective transesterification rate constant (expressed in units of L/(gmol min)), C′A0 is the initial hydroxyl concentration (given in units of gmol/L), and N denotes the ester functionality per unit hydroxyl functionalities:

N=

NB0 8NA 0

=

M 8

with M, the initial molar ratio of FAME to sucrose, being defined as M=

NB0 NA 0

XA is the fractional transesterification conversion for a given run: XA =

(initial HV) − (final HV) (initial HV)

(3)

2.3.1.2. Case IB: Reversible Transesterification Nonselective Kinetic Model. Irreversible kinetics is valid only for an initial holding period of 6′ OH ≫ 1′ OH, the transesterification is selective.17 Thus, when one initiates synthesis by providing M ≤ 1 (where M is the molar ratio of fatty ester to sucrose), the monosubstitution in sucrose takes place preferentially at C-6 in the glucose unit. Hence, in the present work, the second-order bimolecular transesterification kinetic model based on the selective reaction between the FAME and 6 −OH of the glucose unit, as given below, was proposed and evaluated. 2.3.2.1. Case IIA: Irreversible Selective Transesterification Kinetic Model. The irreversible selective transesterification kinetic model can be described by the expression

⎧ ⎡ ⎛ ⎞⎤ ⎪− ⎢M ln⎜ 1 − XA ⎟⎥ = C (1 − M )k ′t for M < 1 A 2 ⎪ ⎢⎣ ⎝ 1 − MXA ⎠⎥⎦ ⎪ ⎪ XA ⎨ = k 2′C A 0t for M = 1 ⎪ 1 − XA ⎪ ⎤ ⎪ ⎡ (M − XA ) = C A 0(M − 1)k 2′ ⎥t for M > 1 ⎪ ln⎢ ⎦ ⎩ ⎣ M(1 − XA )

where k2′ represents the irreversible selective rate constant and CA0 is the initial sucrose concentration (expressed in units of gmol/L). Recall that M is the initial molar ratio (M = NB0/NA0). 2.3.2.2. Case IIB: Reversible Selective Transesterification Kinetic Model. Combining the logic of selective modeling of case IIA with reversible selective reaction between sucrose−6OH and FAME, the bimolecular reversible second-order selective kinetic model takes the form proposed in eq 13:

k2

sucrose−6OH + RCOOCH3 → sucrose−6OCOR + CH3OH A

B

C

(12)

D

(11)

where sucrose−6OH represents the 6 −OH of the glucose unit of sucrose.

A+B↔C+D ⎧ [2Mk ′ + X (k ′ + Mk ′ ) + X ( (k ′ − Mk ′ )2 + 4Mk ′ k ′ )] ⎫ ⎪ ⎪ 2f A 2f 2f A 2f 2f 2 f 2b ⎬ = C A 0( (k 2′f − Mk 2′f )2 + 4Mk 2′f k 2′b )t ln⎨ 2 ⎪ [2Mk 2′ + XA (k 2′ + Mk 2′ ) + XA ( (k 2′ − Mk 2′ ) + 4Mk 2′ k 2′ )] ⎪ ⎩ ⎭ f f f f f f b

where k′2f and k′2b and represent the second-order selective transesterification forward rate and backward rate constants, respectively.

(13)

homogeneous kinetics for all second-order reactions. In heterogeneous kinetics (for example, for synthesis in the absence of DMF solvent), the experimental concentrations remain approximately constant during the initial period (within first hour) and then gradually decreases. The saponification value (SV) of the product sucrose ester is governed by the extent of monoester/diester/triester/polyester transesterification. The magnitude of SV of the product sucrose ester increased as the extent of transesterification increased. The SV of a particular product sucrose ester, consisting of a mixture of monoesters, diesters, and multiple esters, would be decided by its ester distribution. For example, a product richer in monoester will have a lower SV than that of a product which has diester as the major component. Fourier transform infrared (FTIR) spectroscopy was used to establish the existence of an ester linkage. Figure 2 presents the FTIR spectra of sucrose, FAME, and sucrose ester derived from mixed FAME (SE1−SE5). The characteristic functional group absorption bands to look for were as follows: 3420−3500 cm−1 (O−H stretch of free hydroxyl in sucrose); 1740−1750 cm−1 (ester CO); 1056, 1107 cm−1 (C−O stretch of C−O−C); 995 cm−1 (glycosidic bond stretch of sucrose); and 2847−2860, 2904−2945, 1460−1470 cm−1 (C−H stretch in CH3 and/or CH2). Apparently, the bands at 1747 and 995 cm−1 showed that the products were sucrose ester. The reduction in intensity of the hydroxyl band and the formation of strong bands corresponding to ester carbonyl and C−H stretch in CH3 and/or CH2 (1700− 1750 cm−1) were noticed in the FTIR spectra of products from different runs. The spectrum of polysubstituted sucrose ester (SE5) exhibited maximum carbonyl band intensity, in comparison to those in the spectra of SE1−SE4. Although transesterification is not regioselective, by choosing the specific molar ratio M, the reaction can be made selective. In all 24 runs, the reactant molar ratios (M = NB0/NA0, where B = FAME and A = sucrose) were chosen to provide diverse

3. RESULTS AND DISCUSSION 3.1. Synthesis of Sucrose−Coconut Fatty Acids Esters. Alkaline metal alkoxides (such as CH3ONa) as transesterification catalysts provide very high yields (>98%) in short reaction times (30 min), even at low molar concentrations (0.5%). However, these most active catalysts require substantially anhydrous reactants and an acid value of AV < 0.5, making them difficult to use industrially. Alkaline-metal hydroxides (KOH, NaOH) are less active than metal alkoxides and allow for equally high yields by increasing the catalyst concentration to 1%−2%. However, the reaction of the hydroxide with the alcohol will introduce some water in the system, which will hydrolyze some of the produced ester, with consequent soap formation. This reduces the final conversion. The use of potassium carbonate, in a concentration of 2%−3%, reduces the soap formation and permits increased yields. In fact, the addition of K2CO3 allows for the formation of bicarbonate instead of water, and the esters are not hydrolyzed. Hence, in the present investigations, K2CO3 was selected as a catalyst. Experimental investigations of 24 runs of synthesis of sucrose esters based on variation of the initial molar ratio of FAME to sucrose (M), the holding period (t, min), reaction temperature (T, °C), K2CO3 catalyst loading (%), and raw materials for medium-molecular-weight fatty esters (mixed methyl esters derived from coconut oil, methyl laurate, methyl myristate, and coconut oil) are reported in Table 1. The hydroxyl value (HV) of the reaction mixture represents the sucrose hydroxyl concentration available for the transesterification as shown in Table 1; HV exhibited the maximum drop within the first 30 min (reaction period), followed by retardation in the progress of the reaction, or sluggish reaction in the terminal period. This represents the essential feature of E

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hindrance, the fructose moiety of sucrose is less reactive and the reaction is rather selective on the glucose moiety. Thus, York et al.17 have reported that ∼80% of the fatty acid of the monoester is found on the glucose portion of the sucrose molecule, with the ratio of the substitution of the glucose to fructose moiety being 4:1 and the reactivity order being 6 OH > 6′ OH ≫ 1′ OH. Thus, when one initiated synthesis by providing M ≤ 1, the monosubstitution in sucrose would take place preferentially at C-6 in the glucose unit. In batch SE1, to keep excess sucrose well-dispersed, a higher amount of solvent (62%) was used. Also, lower catalyst dosing (1.1%) was used. The underlying assumption was that the surplus hydroxyl group in excess sucrose may provide the catalytic effect. However, the lowest specific nonselective (1.15 × 10−4) and selective (8.49 × 10−3) specific reaction rates were obtained. Hence, in the remaining batches (excluding those corresponding to coconut oil), ∼2.0% catalyst loading and ∼50% solvent were utilized, which provided a sufficiently rapid decrease in HV. However, the transesterification of coconut oil was extremely slow under these conditions. Hence, batches SE8 and SE9 were conducted at higher catalyst loading (3% and 4%, respectively) and reduced solvent usage (i.e., higher reactant concentration). Even with the use of high catalyst and reactant concentration, the coconut oil transesterification rate constants, as shown in Table 1, were 9.04 × 10−5 (batch SE8) and 1.24 × 10−4 (batch SE9), which were comparable or slightly better than that of batch SE1 (1.15 × 10−4) (the SE1 batch had the lowest catalyst loading of 1.1%). However, selective specific reaction rates of coconut oil are of lower magnitude, even at 4%. Since the magnitudes of the transesterification rate constant for coconut oil are low, the time required to attain equilibrium would have to be longer. Calculations based on the differential method of analysis (eqs 10 and 13) have indicated that the ratio of forward to backward rate constants (k2f:k2b) is >8. Thus, even after conducting the reaction for 2 h, the product concentrations were expected to display an insignificant effect on the kinetics. Hence, calculations for coconut oil based on the assumption of irreversible kinetics have fair accuracy and are reported in Table 1. 3.1.2. M = 1.5. Experiments were conducted for FAME (runs 7−9), methyl laurate (runs 15−18), methyl myristate (runs 19−22), and coconut oil (run 24). At M = 1.5, fatty esters are provided at 50% excess for the reaction with one hydroxyl of sucrose. As the molar ratio of fatty acid to sucrose increases to above 1, the yield of sucrose diesters and triesters is expected to increase, along with that of monoesters, i.e., with more availability of fatty esters (M just greater than 1), besides the 6 OH of the glucose unit, the 1′ OH and 6′ OH of the fructose unit would also be substituted to yield monoesters, diesters, and triesters, as presented by Chung et al.18 and Queneau et al.19 The further decrease in HV and increase in SV for batch SE3 (HV = 102.7, SV = 106.0, run 9), in comparison to those for SE2 (HV = 135.7, SV = 89.5, run 5) and SE1 (HV = 203.6, SV = 76.7, run 2) provides confirmation of the additional transesterification. The SV of batch SE3 was close to the theoretical SV of pure sucrose monolaurate: 103. Thus, transesterification may lead to the formation of at least sucrose monoesters. Diesters may be present, in part due to the substitution of the second hydroxyl by surplus fatty ester and in part due to the disproportionation of monoester into saccharose and dieseters. When one compares the extent of HV decrease for runs 7, 8, and 9 for FAMEsucrose esters, the reaction was observed to retard after 1 h.

Figure 2. FTIR spectra of sucrose, coconut oil FAME, and sucrose esters derived from mixed methyl esters (SE1−SE5).

transesterification opportunities for eight hydroxyl groups present in sucrose. Sucrose is a nonreducing disaccharide of unique structure, containing nine chiral centers. The eight hydroxyl groups (Figure 1) include three primary hydroxyls (at carbons 6, 1′, and 6′) and five secondary hydroxyls (at carbons 2, 3, 4, 3′, and 4′). The increase in M, from 0.73 to 6.0, augmented the opportunity for participation of more hydroxyls in transesterification and, thus, the activity contribution of each hydroxyl toward the average activity per hydroxyl group is increased as M increases. When fatty esters are supplied at less than the stoichiometric proportions (M < 1, excess sucrose), there is an increased chance of substitution of the three primary hydroxyl groups of sucrose. On the other hand, as M increases (M > 1.5, excess FAME), the more highly substituted derivatives (degree of substitution (DS) = 4−8) of sucrose are preferentially synthesized. Hence, the results and discussion on the 24 runs is primarily divided for three ranges of molar ratios: M < 1, M = 1.5, and M = 3−6. 3.1.1. M < 1. Experiments were conducted at following molar ratios: M = 0.364 for FAME (runs 1−3), 0.73 for FAME (runs 4−6), and 0.8 for coconut oil (run 23). Use of molar ratio M < 1 implies that FAME is available at quantities even less than that required for transesterification of one out of eight hydroxyls of sucrose. At M < 1, the competition between different hydroxyls in attacking the carbonyl group of fatty ester is created. Because of the abundant availability of hydroxyl group in relation to the ester, the substitution of, at the most, one hydroxyl in the majority of the sucrose molecules would be anticipated. Thus, out of the eight hydroxyl groups in sucrose, the three hydroxyl groups, viz, the 6 OH of the glucose unit and 1′ OH and 6′ OH of the fructose unit of sucrose molecule, being primary, are favored as the most reactive sites. Nevertheless, because of the steric F

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3.2.2. Influence of Molecular Weight/Chain Length on the Kinetics of Transesterification. The objective in choosing different raw materials of medium-molecular-weight fatty acids−mixed FAME derived from coconut oil (batch SE3, run 9, chain length = C6−C18), methyl laurate (SE6, run 17, C12), methyl myristate (SE7, run 21, C14), and coconut oil (SE9, run 24, triester) was to understand the influence of molecular weight and chain length on the kinetics of transesterification, with the other parameters being maintained constant. The comparison between these batches based on fraction conversion XA and rate constants indicated the influence of molecular weight and chain length on the extent of transesterification. The nonselective and selective forward rate constants were found to follow a descending order, relative to the ascending order of molecular weight: mixed FAME (3.07 × 10−3, 6.47 × 10−3) > methyl laurate (3.02 × 10−3, 6.24 × 10−3) > methyl myristiate (2.93 × 10−3, 5.87 × 10−3). Coconut FAME consists of a mixture of fatty acids ranging from C6 to C18: 1, with the principal fatty acids being C12 (lauric) and C14 (myristic). The presence of low-molecularweight fatty acids (C6, C8, and C10) in coconut FAME contributed to the 5%−10% increase in the magnitude of the rate constant over that of myristate. Because of its lower molecular weight (C12, MW 214), transesterification rate constant of methyl laurate was higher than those of methyl myristate (C14, MW 242) and coconut oil (triglyceride, MW 673.2). Coconut oil, being a triester, exhbiits the slowest rate of transesterification. 3.2.3. Series−Parallel Transesterifications and Their Influence on Overall Rate Constants. The specific reaction rates must be constant for a single reaction at a given temperature. However, we have observed that the rate constants varied as a function of molar ratio. Besides this, the decline in rate constants as a function of reaction period was noted. For example, the nonselective forward rate constants k2f for methyl laurate and methyl myristate, calculated for progressive conversions at different time intervals over the reaction period of 150 min, exhibited the following decreasing trend: 3.02 × 10−3, 2.07 × 10−3, 1.57 × 10−3, 1.25 × 10−3 and 2.93 × 10−3, 2.04 × 10−3, 1.55 × 10−3, then 1.25 × 10−3. Selective forward transesterification rate constants k2′ f also displayed a similar decline with time. On the other hand, nonselective and selective backward rate constants (k2b, k2′ b) exhibit an increasing trend. Thus, there appears to be a conflict between the results and the rate rule. These contradictions could be easily interpreted on the basis of the fact that the sucrose transesterification is not a single reaction but, rather, it involves series−parallel combinations of mono/di/ poly transesterifications. For example, in reference to sucrose, the mono/di/poly transesterifications follow series mode while in reference to FAME, the reactions follow parallel mode. Since all the hydroxyls other than 6 −OH on the glucose portion are less active and their participation will increase with retention time, the specific speed of reaction, under nonselective conditions, will decrease as the participation of these hydroxyls for transesterification with additional methyl esters increases. Moreover, the rate of monotransesterification is higher than that of ditransesterification, which, in turn, will be higher than that of tritransesterification, and so on. As the holding time increases, the opportunities for polytransesterifications will be augmented. Hence, one observes a drecrease in the magnitude of overall forward rate constants. On the other hand, backward rate constants will follow an opposite trend. 3.3.4. Influence of Temperature. The increase in reaction temperature from 120 °C to 140 °C caused only marginal

Similar retardations were observed for methyl laurate and methyl myristate. 3.1.3. M = 3−6. These molar ratios permitted the availability of fatty esters in sufficient excess to cause the transesterification of more than one hydroxyl present in sucrose in runs 10−12 (batch SE4) and runs 12−14 (batch SE4) for FAME. Under these conditions, the base-catalyzed transesterification may not allow regioselectivity and the degree of hydroxyl substitution on the sugar is uncontrolled. The product sucrose ester would then carry mixtures of monoesters, diesters, and even higher esters. The lowest hydroxyl value (18.3) and highest SV (205.8) observed in run 14 of batch SE5 confirmed the formation of polysubstituted sucrose ester, which is normally used as a noncaloric substitute for fat. 3.2. Kinetic Modeling of Synthesis of Sucrose−Coconut Fatty Acids Esters. The assumption of irreversible kinetics is valid only for the initial period, but it simplified the calculations and permitted the determination of forward rate constants via the integral method of analysis (i.e., by using eqs 2 and 12). With the availability of elaborate concentration−time data and good software backup for the differential method of analysis, forward and backward transesterification rate constants can be determined using eqs 10 and 13. The bimolecular secondorder nonselective (k2) and selective rate constants (k2′ ), calculated on the basis of irreversible conditions using eqs 2 and 12, respectively, are reported in Table 1. 3.2.1. Nonselective Kinetics versus Selective Kinetics. For the same source of fatty monoesters (batches SE2, SE3, SE4, SE5), identical catalyst loading (2%), and reaction temperature (120 °C), the magnitude of the nonselective rate constant k2 increases from ∼1.15 × 10−4 to ∼8.31 × 10−3 L/(gmol min) as M increases from 0.364 to 6.0 (or N increases from 0.0455 to 0.750). The nonselective rate constant k2 represents the average specific transesterification rate per hydroxyl group for all eight hydroxyls in sucrose and hydroxyl groups in partial esters. The increase in M from 0.364 to 6.0 caused more availability of fatty esters for reaction with −OH groups, which results in an increase in the average activity per hydroxyl group and led to the enhancement in magnitude of k2 with increasing M/N. On the other hand, when M < 1, the reaction would be rather selective, or oriented specifically toward the primary hydroxyl of the glucose moiety, because of its abundant availability in relation to the ester functionality. Hence, for M < 1, the selective rate constant k2′ (the specific reaction rate of the 6 −OH group) increased with M and attained the highest magnitude of 2.89 × 10−2 at M = 0.73/N = 0.0913. When M exceeds 1, esters happen to be available for reactions with other hydroxyls besides the 6 −OH groups. Thus, with increasing M, the transesterification becomes more nonselective. Correspondingly, one observes a decline in the selective specific reaction rate, from 1.93 × 10−2 (M = 1.5) to 1.23 × 10−2 (M = 6.0). Table 1 also presents the magnitudes of the forward (k2f and k′2f) and backward rate constants (k2b and k′2b), calculated using eqs 10 and 13. The ascending order of nonselective forward rate constants (k2f, t = 120 min) from 9.7 × 10−4 (M = 0.364/N = 0.0455) to 2.08 × 10−3 (M = 6.0/N = 0.75) is consistent with the rising trend exhibited by the nonselective transesterification rate constant (irreversible kinetics). The selective forward rate constants (k′2f) initially exhibited the rising trend (from 4.03 × 10−3 to 4.16 × 10−3) for M > 1 and declined thereafter and attained the lowest value of 2.82 × 10−3 at M = 6.0/N = 0.75. G

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acceleration in transesterification rate (e.g., run 1 vs run 3 for batch SE1, run 4 vs run 6 for batch SE2). The reaction was conducted in a batch reactor (i.e., no removal of byproduct methanol) for the purpose of kinetic modeling. In this case, one uses the combination of elevated temperature with the rapid removal of byproduct methanol (semibatch system), the increase in temperature would enhance equilibrium conversion through a shift of reaction equilibrium. The selective and nonselective reversible and irreversible kinetic modeling of synthesis of sucrose−coconut fatty acid esters were thus proved to be a facile and quantitative means to follow the specific substitutions occurring at the various OH positions within the sucrose as a function of degree of conversion and reactant molar ratio. It should be noted that the selective and nonselective kinetic models are not exclusive; rather, they complement each other. While models 1A and 1B present the average transesterification rate/overall kinetic activity of all eight hydroxyls in sucrose, Models 2A and 2B explain the transesterification activity of the 6 −OH group in sucrose. One needs knowledge of both k2f and k2′ f when evaluating the overall kinetics of transesterification, as well as when facilitating preferential synthesis of particular sucrose−monoesters/diesters/polyesters. 3.4. Rheological Characterizations of Sucrose−Coconut Fatty Acids Esters. The purifications after synthesis yielded yellowish, mild, waxlike hygroscopic sucrose ester powder with a softening temperature in the range of 70−90 °C. The surface tensions of aqueous solution of sucrose ester of batch SE3 (M = 1.5, run 9) at various concentrations ranging from 0.00012 g/L through 0.02 g/L, up to 0.1 g/L, were found to vary from 56.4 dyne/cm through 38.6 dyne/cm, up to 18.3 dyne/cm, respectively. The reduction in surface tension with the increasing concentration of sucrose ester confirmed their tensio-active properties. Since there are eight hydroxyl groups on sucrose, as M increased (e.g., M = 3−6), transesterification yielded numerous sucrose esters of higher molecular weights. The viscosity of sucrose esters would be increased with multiple substitutions and corresponding rise in molecular weights. Thus, the molar ratios of sucrose to FAME (M), varying between 0.364 and 6, which controlled the degree of substitution implicitly, could be correlated to the viscosity of the mixture. Thus, the objective was to investigate the structure−viscosity (η) relationships for sucrose esters derived from mixed FAME and coconut oil. In general, viscosity measurements were carried out in three concentration regions: dilute (9%), semidilute (40%), and concentrated (80%) regions. Although the viscosity of sucrose esters in the dilute regime is the same as that of DMF ( 1, the SV of this ester is close to the theoretical SV of monoester. However, transesterification in all probability yields a mixture of monoesters and higher esters. Hence, diversity of esters leads to weak sucrose ester molecular interactions, and this may be responsible for the decrease in viscosity. It should be noted that, for a similar DS (i.e., for similar initial molar ratios of M = 1.5), sucrose esters obtained from coconut oil and coconut FAME will have the same viscosity (see Table 2). At higher molar ratios (M = 3−6), the increase in DS gives an increase in molecular weight, as indicated by the increase in SV. Since, for sucrose esters obtained at M = 3−6, the viscosity is directly related to the degree of substitution, and with appropriate calibrations, the former could be taken as a direct measure of the latter.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +91-257-2257444. Mobile Tel.: +91-9404366700. Fax: +91-0257-2258403. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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NOMENCLATURE

Abbreviations

A = sucrose B = FAME DMF = dimethyl formamide FAME = coconut oil fatty acid methyl ester HV = hydroxyl value, mg KOH/g of oil SV = saponification value, mg KOH/g of oil

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CONCLUSIONS The K2CO3-catalyzed transesterification reaction between sucrose and coconut esters happens quite rapidly, with a large amount of hydroxyls being transesterified during the first 30 min. This represents the essential feature of homogeneous kinetics for all second-order reactions. For molar ratios of fatty acid to sucrose (M) above 1 (M = 1.5), the decrease in hydroxyl value (HV) and attainment of a saponification value (SV) close to the theoretical SV of pure sucrose monolaurate provided

Variables

CA0 = initial sucrose concentration, gmol/L CB0 = initial FAME concentration, gmol/L k2b = nonselective second-order transesterification backward rate constant, L/(gmol min) k2f = nonselective second-order transesterification forward rate constant, L/(gmol min) I

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k′2b = selective second-order transesterification backward rate constant, L/(gmol min) k2′ f = selective second-order transesterification forward rate constant, L/(gmol min) k′2 = selective second-order rate constant, L/(gmol min) k2 = nonselective second-order rate constant, L/(gmol min) M = initial molar ratio; M = NB0/NA0 N = functionality ratio; N = M/8 NA0 = initial sucrose moles NB0 = initial fatty ester moles t = reaction period, min T = reaction temperature, °C XA = fractional conversion for given run

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REFERENCES

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