Synthesis of Glycerol 1-Monooleate by Condensation of Oleic Acid

Ind. Eng. Chem. Res. 2009, 48, 6949–6956

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Synthesis of Glycerol 1-Monooleate by Condensation of Oleic Acid with Glycidol Catalyzed by Anion-Exchange Resin in Aqueous Organic Polymorphic System Ze´phirin Mouloungui,* Vony Rakotondrazafy, Romain Valentin, and Bachar Zebib UniVersite´ de Toulouse-UMR1010 Chimie Agro-Industrielle, ENSIACET, INPT, INRA, 118 Route de Narbonne, F- 31077 Toulouse Cedex 4, France

The condensation of technical-grade oleic acid (OA) (65%, 85%, and 90% oleic acid) with glycidol (Gly) was carried out in an aqueous medium in the presence of a macroporous anion-exchange resin containing ammonium groups. The reaction was optimized by a unifactorial method and by a 2n-1 fractional factorial plan. The effects of the following main parameters were quantified from experiments based on a 2(5-1) ) 24 factorial design: OA/Gly molar ratio, concentration (mmol of OA/mL of H2O), catalytic efficiency (mequiv of X/mmol of OA), and temperature. The conditions were optimized for a discontinuous process in a stirred reactor for selective synthesis of glycerol 1-monooleate (1-GMO) in a polymorphic system (solid resin/ emulsion or microemulsion) consisting of OA/Gly/H2O/1-GMO. The catalytic role of the Ambersep 900-X resin was demonstrated by conducting the reaction using the resin in the different functional forms Ambersep 900-OH- and Ambersep 900-HCO3- and in the nonfunctionalized form Ambersep 900-Cl-. The highest yield of 1-GMO (97%) was obtained with the nonfunctionalized form Ambersep 900-Cl- at 70 °C. Introduction In view of their polyfunctional nature and their emulsifying, complexing, and lubricating properties, fatty acid monoglycerides find application in the food,1,2 cosmetics, pharmaceutical,3-5 and textile and fiber industries.6,7 In certain applications, their efficacy hinges on the incorporation of pure monoglycerides (>90%) as additives.8 However, to our knowledge, few industrial processes give rise to pure monoglycerides. All of the conventional methods for the preparation of monoglycerides, either by direct esterification of glycerol by fatty acids or by hydrolysis or transesterification of oils, lead to mixture of glycerides. Pure monoglycerides are generally obtained by molecular distillation, which raises their cost. An alternative chemical method is to prepare the monoglycerides from glycidol according to Scheme 1. In general, this reaction is conducted in the presence of a basic catalyst in a homogeneous medium: amines and/or quaternary ammonium salts,9 metal alcoholates,10,11 or bases.12 Although the yields of the glycerol monoesters are relatively satisfactory, the operating conditions in the homogeneous phase require phase-transfer agents and solvents that are able to withstand high temperatures. Furthermore, 1-monoglycerides are not readily extractable from these media, and the catalysts cannot be immediately recycled.13,14 In heterogeneous catalysis, amines supported on mesoporous materials present good results, especially with hydrophobic surfaces.13 Similar catalysts can be recycled 11 times without loss of activity.15 The process described herein for the synthesis of pure monoglycerides, glycerol 1-monooleate in the present case, involves the direct condensation of oleic acid with glycidol in aqueous medium in the presence of an anion-exchange resin as a recyclable catalyst.8,16,17 The new objective in organic chemistry is the development of reactions using water as the solvent instead of organic liquids. Of course, glycidol is a solvent of oleic acid, and glycidol and water are also miscible. 1-GMO and water produce emulsions.

The condensation reaction presented herein is an unusual outcome in water or in aqueous medium. Experimental Section Materials. Technical-grade oleic acid (68-85% GC) was obtained from Fluka (L’Isle d’Abeau Chesnes, France). The impurities detected were essentially linoleic stearic and palmitic acids. Glycidol (2,3-epoxypropan-1-ol, 96%) was supplied by Aldrich Fluka (L’Isle d’Abeau Chesnes, France). The anionic resin, Ambersep 900, was purchased from Rohm & Haas (Lauterbourg, France). The solvents used in processing the reaction medium were obtained from SDS (Peypin, France) and were of synthetic grade, and HPLC-grade solvents were employed for the quantitative analyses by thin layer chromatography with flame ionization detection (TLC/FID). Pretreatment of Resin. Anionic resins are usually supplied in chloride form and contain water. They need to be suitably functionalized before use. To this end, a volume V (in milliliters) of resin, previously swollen in deionized water, was placed on a column equipped with a sintered filter of porosity 0. The residual impurities were removed by washing with a volume 2V of deionized water. The column was percolated with 5V of aqueous 2 N sodium hydroxide or potassium bicarbonate through the resin bed to replace the Cl- ions with OH- or HCO3- ions. The resin was then washed with deionized water until neutral pH and subsequently washed with technical ethanol (2V) and ethyl ether (2V) to remove the water. The resin was dried under a vacuum for 10 min. Scheme 1. Mechanism of Glycidol Epoxide Ring-Opening by Fatty Acid to Obtain 1-GMO, Catalyzed by Anion-Exchange Resin

* To whom correspondence should be addressed. Tel.: +33 5 62 88 57 24. Fax: +33 5 62 88 57 30. E-mail: zephirin.mouloungui@ ensiacet.fr. 10.1021/ie900101k CCC: $40.75  2009 American Chemical Society Published on Web 07/02/2009

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Ind. Eng. Chem. Res., Vol. 48, No. 15, 2009

The Ambersep 900-OH- resin now marketed under the name Ambersep 900 OH is a macroporous anion-exchange resin based on polystyrene containing type I quaternary ammonium groups. The Ambersep 900-X resins have the following characteristics: mean pore size, 40-70 nm; contact surface area, 25-30 m2/g; porosity, 0.5 mL pore/mL of beads; moisture retention, e73% (HO- form) or 2.44 mequiv/g (HCO3- form); coefficient of uniformity, e1.35; reversible swelling, 25% from Cl- to HOform. In view of their uniform particle size and the existence of large channels in the particles, the Ambersep 900 macroporous resins readily adsorb fatty acids. Experimental Setup. For each experiment, 2 g of anionic resin was placed in a 100-mL three-neck flask fitted with a cooling system and mechanical stirrer. Oleic acid (16 mmol) was added, and the mixture heated at 70 °C for 15 min to allow complete adsorption. At time t ) 0, an aqueous solution of glycidol (4.3 mmol/mL) was then added, and the reaction mixture was kept at 70 °C on a thermostatted bath. The mixture was stirred (500 rpm) for the whole 4 h of the reaction. Because the rate of the process was controlled by the chemical reaction, stirring was not an important factor. At the end of the reaction, the reaction medium was filtered, and the resin was washed on a column by eluting first with 100 mL of technical ethanol and then with 100 mL of a mixture of hexane/ethanol (50:50 by volume) containing 0.1 N acetic acid. The solvent was then removed in a rotary evaporator. The crude reaction mixture was analyzed by TLC/FID. Iatroscan TLC/FID Analysis. The sample for analysis was dissolved in chloroform at a concentration of 20 mg/mL, and 1 µL of this solution was deposited on the SII chromarods in an Iatroscan MK5 apparatus. The rods were run in a hexane/ethyl ether/formic acid mixture (75:25:0.04 by volume) for a migration time of 20 min. The hydrogen and air flow rates at the detector were adjusted to 140 mL/min and 1.8 L/min, respectively. The rods were swept with the flame at a rate of 35 s/chromarod. The spectra were recorded and quantitation was carried out using Boreal software (Bionis, France). The yield of the reaction (% 1-GMO) was calculated as the ratio of the number of moles of 1-GMO formed to the initial number of moles of either one of the reactants (OA or Gly).30 Microscopy. The mixtures were examined using a polarization light microscope (Olympus CH2) equipped with a rotating polarizer and analyzer and a thermostatted plate to monitor the effect of temperature on the phase behavior. The samples were placed between the slide and the coverslip and examined at a magnification of ×40. Preparation of Pseudoternary Phase Diagrams for the OA/Gly/1-GMO/H2O System. A series of samples were prepared with different water contents, and after being weighed, the tubes were incubated in a thermostatted water bath at 35 °C (1-GMO is a fluid at this temperature). The tubes were vortexed (speed 5) for 2 min, and their contents were then examined under a microscope. The tubes were then returned to the water bath at 35 °C, and their contents were re-examined 2 and 24 h later. The other mixtures of OA and 1-GMO (B and H) were treated in the same way. The compositions of the mixtures are listed in Table 1, and the phase diagrams are presented in Figures 1 and 2.

Table 1. Compositions of Reaction Mixtures mixture OA/Gly (mg) 1-GMO (mg) OA (%) Gly (%) 1-GMO (%) A B C D E F G H

39.7 63.7 84.0 122.3 142.5 196.1 232.2 256.1

651.2 492.0 496.9 474.7 436.7 414.4 386.0 353.6

16.6 21.3 23.7 28.6 31.9 38.0 42.4 46.0

0.4 0.8 1.0 1.4 1.8 2.3 2.7 3.0

83.0 77.9 75.3 70.0 66.3 59.7 54.9 51.0

of 1-GMO fell somewhat if OA and Gly were both added to the resin at the same time. No reaction occurred if the glycidol was added before the oleic acid. These differences in behavior can be explained in terms of differences in the polarities of the species present. Glycidol is thought to have a higher solvating capacity for the resin than either OA or an OA/Gly mixture. As a result, if glycidol is added first, the active sites on the resin are made inaccessible to oleic acid, and no reaction takes place. On the other hand, upon contact with the first molecules of oleic acid, the matrix of the resin becomes hydrophobic. A hydrophobic film is formed on the surface of the hydrophobic matrix, and the anion-exchange resin acquires both chemical and mechanical stability. The lipochemical reaction takes places via mass-transfer-facilitated affinity and the structural similarities between the “hydrophobic” resin and the fatty acid.18,8 The adsorbed fatty acid is thus activated, generating new reaction centers and thereby propagating the reaction. Determination of Optimal Operating Conditions by Factor Study. The results presented here reflect the yields after recovery of product. The reaction parameters employed for optimization of operating conditions thus consisted of the catalytic efficiency, expressed in milliequivalents of OH- per millimole of OA; the OA/Gly molar ratio, the initial amount of water present, reflected by the condensation of OA (mmol of OA/mL of water); the temperature T; and the reaction time t. We first employed a factor-by-factor method to evaluate in a qualitative way the influence of each of these reaction parameters on the yield of 1-GMO with respect to a reference experiment carried out under the following experimental conditions: OA, 16 mmol; OA/Gly molar ratio, 1:1; Ambersep resin 900-OH,

Results Effect of the Reactant Addition Mode. We investigated the effect of the order of addition of the reactants on the synthesis of glycerol 1-monooleate. Prior adsorption of oleic acid onto the resin enhanced progress of the reaction, whereas the yield

Figure 1. Pseudoternary phase diagram of the oleic acid/glycidol/1-GMO/ water system at 35 °C after 24 h. Visual observations: 1, oil; 2, inhomogenous translucent solid (gel appearance); 3, white fatty solid; 4, translucent solid and white fat/white solution; 5, white fatty solid/milky solution (cloudy solution); 6, oil/water; 7, oil/milky solution.


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Table 2. Experimental Domain for Studying the Effects of Different Factors (Xi) levels i 1 2 3 4 5

Figure 2. Pseudoternary phase diagram of the oleic acid/glycidol/1-GMO/ water system at 35 °C after 2 h. Visual observation: 1, oil; 2, inhomogeneous translucent solid gel (gel appearance); 3, with fatty solid; 4, translucent solid and with fat/milky solution (cloudy solution); 5, white fatty solid/ milky solution; 6, with solution/water; 7, oil/milky solution; 8, white foam; 9, white gel.

0.75 mequiv of OH-/mmol of OA; OA concentration, 0.64 mmol of OA/mL of H2O; temperature, 50 °C; reaction time, 6 h. Effect of Resin Amount. In the absence of resin, the reaction hardly progressed. Excess resin was also detrimental to the synthesis of 1-GMO. The amount of Ambersep 900-OH- resin catalyst must thus be chosen carefully. Over a range of catalyst concentrations from 0.25 to 1.25 mequiv of OH-/mmol of OA, the yield of 1-GMO ranged from 0% to 8%. The maximum yield was attained at a catalyst concentration of 0.5 mequiv of OH-/mmol of OA. Effect of Counterion. The catalytic performance of the anionic resin was also investigated with respect to the nature of the counterion: the strong anionic form OH-, the weak anionic form HCO3-, or the original chloride anionic form Cl-. Effect of Molar Ratio. Zlatanos et al.19 showed that the monoglyceride yield of the condensation reaction of fatty acid with glycidol catalyzed by tetraethylammonium iodide can be improved by adjusting of the molar ratio of the reactants. We observed that an excess of either of the reactants (OA/Gly molar ratios of 4:1 and 1:2) led to a significant improvement in the yield of 1-GMO (14% and 12%, respectively). Effect of Water. With respect to the initial amount of water, over a range of OA concentrations from 0.21 to 2.29 mmol of OA/mL of water, it was apparent that the more dilute media did not favor the formation of 1-GMO. This finding is in line with that observed with excess reactants. A maximum yield of 15% was obtained. Effect of Temperature. Temperature was also found to be an important parameter. Heat was found to be favorable for the formation of 1-GMO. From room temperature (25 °C) to 70 °C (maximum operating temperature of Ambersep 900 resin in its OH- form), the yield rose from 2% to 12%. Effect of Time. Furthermore, a study of the reaction progress with time showed that prolonged reaction times (under the reference conditions) led to a drop in yield of 1-GMO at the expense of byproducts, the corresponding glycerol di- and trioleates, indicating that the duration of the reaction needs to be controlled. The factor-by-factor study produced a maximum 1-GMO yield of 15%, indicating the feasibility of synthesizing 1-GMO

-1

factor -

catalytic efficiency (mequiv of OH /mmol of OA) 0.25 OA/Gly (molar ratio) 4 concentration (mmol of OA/mL of H2O) 0.32 duration (h) 3 temperature (°C) 30

0

1

0.63 0.47 0.70 13.5 50

1.00 0.25 1.07 24 70

in an aqueous medium. In addition, this study showed that the condensation of oleic acid with glycidol is a catalytic reaction. The anion-exchange resin acts as a “triphase catalyst”, that is, a catalyst that is basically supported and used in a reaction in a two-phase system (aqueous-organic). Thus, there is a great scope to highlight the appropriate reaction mechanism and associated reaction parameters. Optimization by Factorial Design of Experiments. We chose an empirical model for predicting the effects of the parameters on the response in the form of a second-degree polynomial Y ) R0 +

∑RX

i i

+

∑R XX

ij i j

+

∑R X

2

ii i

where Y is the 1-GMO yield, Xi corresponds to the main effects, XiXj corresponds to the interaction effects, and Xi2 corresponds to the squared terms. We employed the same reaction parameters as described above and limited the experimental domain by upper and lower cutoffs (Table 2). Table 3 summarizes the experimental results obtained. The significance of the effects calculated with respect to the experimental error was obtained from analysis of the repeatability of the experimental results (Table 4). The values of the coefficients R (Table 5) were computed using the NEMROD statistics program. The parameters favorable to the reaction emerging from this analysis did not differ significantly from those identified in the previous factor-by-factor study. Specifically, use of catalytic amounts of the anion-exchange resin, marked effect of an excess of glycidol and high concentration of oleic acid in water, and favorable effect of heat on the formation of 1-GMO. The main effects and the effects of interactions between the various parameters were obtained from this experimental analysis. The isoresponse curves reflecting the effect of each of the variables as a function of time (Figure 3) showed the predominant effects of reactant concentration and temperature. Optimization of these parameters raised the yield to 25%. An increase in yield to 45% was obtained by analysis of the isoresponse curves reflecting the interaction of these two terms (Figure 4). The predicted yield was thus comparable to the synthetic yield. To validate this predictive model, an experiment was conducted under the operating conditions defined by the plan: OA, 16 mmol; OA/Gly molar ratio, 1:4; Ambersep resin 900-OH-, 0.25 mequiv of OH-/mmol of OA; OA concentration, 1.07 mmol of OA/mL of H2O; temperature, 70 °C; and reaction time, 4 h. Under these conditions, the maximum yield of the 1-GMO was increased from 45% to 71%, which confirms the validity of the model. Analysis of Parameters Acting on the Progress of the Reaction. In view of the complexity of the triphasic system, the study of the reaction progress was conducted in a discontinuous manner. Each point on the plots of the kinetics

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Table 3. Optimization by 2 × 2 Factorial Plan [2(5-1)] expt

X1

X2

X3

X4

X5

1-GMOa (%)

GDOb,c

GTOc,d

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

-1 +1 -1 +1 -1 +1 -1 +1 -1 +1 -1 +1 -1 +1 -1 +1 -1 +1 0 0 0 0 0 0 0 0 0

-1 -1 +1 +1 -1 -1 +1 +1 -1 -1 +1 +1 -1 -1 +1 +1 0 0 -1 +1 0 0 0 0 0 0 0

-1 -1 -1 -1 +1 +1 +1 +1 -1 -1 -1 -1 +1 +1 +1 +1 0 0 0 0 -1 +1 0 0 0 0 0

-1 -1 -1 -1 -1 -1 -1 -1 +1 +1 +1 +1 +1 +1 +1 +1 0 0 0 0 0 0 -1 +1 0 0 0

+1 -1 -1 +1 -1 +1 +1 -1 -1 +1 +1 -1 +1 -1 -1 +1 0 0 0 0 0 0 0 0 -1 +1 0

trace 0 trace 1.4 0 1.7 45.5 1.7 trace trace 30.6 4.4 24.3 trace 13.4 0 10.1 7.6 5.3 4.3 16.8 32.0 5.6 6.3 10.8 22.4 21.9

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ++ 0 ++ 0 0 ++ 0 ++ ++

0 0 0 0 0 0 0 s 0 s 0 0 s s s + s 0 s 0 s -

a 1-GMO, glycerol 1-monooleate. trioleate.

b

GDO, glycerol dioleate.

c

s, trace; -, low content; +, medium content; ++, high content.

Table 4. Reproducibility of the Experimental Results expt

X1

X2

X3

X4

X5

1-GMO (%)

28 29 30

-0.35 -0.35 -0.35

-0.60 -0.60 -0.60

-0.16 -0.16 -0.16

-0.71 -0.71 -0.71

0 0 0

3.9 4.1 4.2

corresponds to an experiment carried out for a defined reaction time. Three parameters were analyzed: the OA/Gly molar ratio, the initial amount of water, and the temperature. Molar Ratio. The results for the effect of the OA/Gly molar ratio (Figure 5) exhibited two distinct types of reaction progress. Similar kinetics were obtained for reactions carried out with oleic acid in excess or in stoichiometric proportions. The induction period was prolonged, and the yield did not exceed 30%. On other hand, reactions conducted with excess glycidol gave rise to quite different kinetics, with a maximum 1-GMO yield of 71%. Molar Composition. We conducted further studies using these proportions of reactants, but at different initial concentrations (Figure 6) and different temperatures (Figure 7). The progress of the reaction was found to depend of these conditions, and the zone of greatest influence (80% 1-GMO) corresponded to a concentration of 2.13 mmol of OA/mL of H2O for a temperature of 70 °C and a reaction duration between 2 and 3 h. The reaction was found to be slower and to have a longer induction period at other values of these parameters. Temperature. In light of these results, we carried out similar experiments using different counterions. Preliminary studies showed that the chloride and hydrogen carbonate forms of the Ambersep 900 had comparable catalytic efficiencies. Temperature was found to have markedly different effects on the kinetics of the reactions in the presence of these different forms of the catalyst (Figures 8 and 9). Although there was little relationship between reaction progress and temperature, the kinetics were accelerated at higher temperatures. In this qualitative study, the optimal conditions for a 97% yield of 1-GMO were found to be as follows: OA, 16 mmol;

d

GTO, glycerol

OA/Gly molar ratio, 1:4; Ambersep resin 900-OH-, 0.25 mequiv of OH-/mmol of OA; OA concentration, 2.13 mmol of OA/mL of H2O; temperature, 70 °C; and reaction time, 3 h. Structured Phases and Phase Diagram. Phase diagrams of the OA/Gly (7% by weight of Gly)/1-GMO/H2O system are shown in Figures 1 and 2. These diagrams were established from mixtures with reconstituted compositions (Table 1) close to those of the crude reaction mixtures. The samples were examined microscopically at different times at 35 °C and higher temperatures. Samples were heated to 70 °C and then cooled to 35 °C, after which they were reheated to 100 °C and cooled to 35 °C. Depending of the water content, the solid mixtures were homogeneous or inhomogeneous. Upon heating from 35 °C, the mixture formed a hexagonal crystalline structure at around 45 °C, and with additional gradual heating, droplets were observed at 79 °C. Upon cooling from 100 °C, the hexagonal crystalline structure was again observed at 65 °C. We observed hexagonal crystalline, isotropic liquid, and reverse crystalline hexagonal phases. The microemulsion phases appeared as oils and were characterized by a perfectly homogeneous structure. They were stable and transparent, with water as the continuous phase. All other mixtures were emulsions. Discussion The condensation of oleic acid with glycidol to form glycerol 1-monooleate is heterogeneously catalyzed by anion-exchange resins. The following processes are involved in the overall reaction:20(a) diffusion of reagents to the catalytic sites of resins, (b) adsorption of reagents in the catalytic sites of the resin, (c) reaction at the surface between adsorbed reagents, (d) diffusion of the reaction products from the pores to the external layer of the resin, and (e) desorption of the reaction products into the liquid phase. After the reaction has been initiated, stages a and b no longer govern the overall kinetics. We found that the product and unreacted starting reactants were strongly bound in the poly-


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Table 5. Coefficients of the Polynomial Model i

Ri

standard error

ii

Rii

standard error

ij

Rij

standard error

ij

Rij

standard error

0 1 2 3 4 5

14.6 -6.0 +3.8 +3.6 +1.2 +5.3

5.7 0 0 0 0 0

11 22 33 44 55

-4.7 -8.5 +10.8 -7.3 +2.9

2.7 2.7 2.7 2.7 2.7

12 13 14 15 23

-3.7 -3.4 -1.4 -5.6 -0.1

0 0 0 0 0

24 25 34 35 45

-2.8 +2.8 -2.8 +1.8 -0.6

0 0 0 0 0

meric matrix of the resin. Because desorption did not occur automatically, stage e was not considered as limiting to the overall kinetics of the reaction. Therefore, the reaction on the surface (process c) is assumed to be the sole stage determining the kinetics. Hydrophobic-hydrophilic interactions of the molecules of the organic reactants were involved in the pseudoternary aquiorganic system.21 Based on observations of the evolution of the resin and the monooleate glycerol during the reaction, two types of adsorption phenomenon can explain the interactions between glycerol monooleate and resin on one hand and oleic acid and resin on the other hand: physisorption of glycerol monooleate in the outer layer of resin and chemisorption of oleic acid. The former

phenomenon is demonstrated by the easy retrieval of the product by a simple solvent extraction. In the latter case, desorption of the resin was achieved through the ion-exchange reaction of oleate ions with acetate ions. The extractant used was an ethanol/ hexane mixture containing acetic acid. The latter was chemically active because acetate ion presented a greater structural affinity for the anionic resin matrix than did oleate ion. We confirmed the classification determined by the affinities of the anions with respect to the active center of heavy anionic resins22 chloride > bicarbonate > acetate > oleate > hydroxyl. This explanation is validated by the fact that oleic acid led to more swelling of the OH- form of the resin than did contact with water (increase in volume of 50-55% with respect to that of the dry resin at 25 and 70 °C versus 28-30%). The marked

Figure 3. Effects of interactions of the predominant parameters as functions of reaction time. Figure 5. Effect of the OA/Gly molar ratio on the progress of the reaction catalyzed by Ambersep 900-OH- resin. OA, 16 mmol; Ambersep 900-OH- resin, 2 g; water, 15 mL; temperature, 70 °C.

Figure 4. Main effect of each of the parameters as a function of reaction time.

Figure 6. Effect of the initial concentration of OA on the progress of the reaction in the presence of Ambersep 900-OH- resin.

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Figure 7. Effect of temperature on the progress of the reaction in the presence of the Ambersep 900-OH- resin. OA, 16 mmol; OA/Gly, 1:4; Ambersep 900-OH- resin, 2 g; water, 15 mL.

Figure 9. Effect of temperature on the progress of the reaction in the presence of Ambersep 900-HCO3- resin. OA, 16 mmol; OA/Gly, 1:4; Ambersep 900-HCO3- resin, 2 g; water, 7.5 mL.

Figure 8. Effect of temperature on the progress of the reaction in the presence of Ambersep 900-Cl- resin. OA, 16 mmol; OA/Gly, 1:4; Ambersep 900-Cl- resin, 2 g; water, 7.5 mL.

Figure 10. Reaction progress in the presence of the three types of Ambersep 900 resins.

swelling of the anionic resin with oleic acid indicates that the fatty acid penetrated the resin and that oleic acid was in contact with the active sites, where it was adsorbed and activated. We also noted a marked elimination of water during the phase of preadsorption of oleic acid onto the Ambersep 900-X (X ) OH-, Cl-, HCO3-) anion-exchange resins. Oleic acid thus appears to effectively desiccate the polymeric lattice, thereby enhancing the organohydrophobic nature of the solid polymeric catalyst. Then, adsorption of the fatty acid is governed by van der Waals-type forces between the fatty hydrophobic hydrocarbon chains of the acid and the cross-linked polystyrene matrix, giving rise to a strong adsorbing capacity. The long-term stability of the resins is thus increased, with an enhanced catalytic system. We believe with a good degree of confidence that the same phenomenon occurs with the other studied resins. The bulky nature of oleic acid and its rigid structure suggest that adsorption takes place in the macroporous sites as well in the microporous sites (gel) both on the surface and within the resin.23 The reaction takes place in the zone where there are both macroporous active sites and micropores, which favor

condensation between the adsorbed oleic acid and the soluble glycidol. By preadsorbing oleic acid onto the resin, the stage of transport and diffusion (stage a) of the fatty acid is no longer a limiting step in the overall reaction. The ensuing condensation between oleic acid and glycidol in an OA/Gly molar ratio of 1:1 or 4:1 is a surface reaction. It takes place between reactants within the droplets dispersed in the continuous organic hydrophobic pseudophase within and on the surface of the macroporous particles and gels. The kinetics appeared to obey those described by the Langmuir-Rideal model24,25 for the surface reaction between oleic acid and glycidol in an aquiorgano hydrophobic resin system providing the transport of reacting droplets in the hydrophobic organic pseudophase for an OA/Gly molar ratio of 1:1 or 1:4. The catalytic efficiency of the active sites of the hydrophobic resin is probably favored by the level of hydration around the hydroxide, chloride, or bicarbonate ions of the resins. On the other hand, under conditions in which glycidol is present in excess (OA/Gly ) 1:4), the organic pseudophase becomes strongly organophilic. Water and glycidol are quite miscible, and at the start of the reaction, mixtures consisting of hydrophobic anionic resin/oleic acid/glycidol/water are triphasic.


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Figure 11. Physicochemical evolution of the condensation reaction of glycidol with oleic acid in the presence of Ambersep 900-Cl- resin, (w/o, water/oil; o/w, oil/water; µe, microemulsion). Resin before reaction (hydrophilic resin). Adsorption of oleic acid on resin (hydrophobic resin). Contact of glycidol/ water solution with resin (hydrophilic/hydrophobic resin). Resin surrounded by polymorphic system (w/o system, o/w system, or microemulsion system).

This system can be considered as an emulsion or a microemulsion supported on a solid catalytic phase and presents a large surface exchange in the system, which is favorable to an increase in the catalytic efficiency by contacting largely all of the components. As the reaction progresses (Figure 10), 1-GMO, itself a surface-active agent, is formed, and we found that its concentration outside the particles of Ambersep 900-OH- resin increased, whereas the particles themselves contained undissociated and unreacted oleic acid. The presence of molecules of water and glycidol in the pores of the resin effectively expelled 1-GMO from the resin particles. The internal layer of the resin is thus not completely hydrophobic, but is a living layer that can be a water-in-oil emulsion, an oil-in-water emulsion, or a microemulsion depending on the reaction progress. Indeed, examination of the crude reaction mixture containing oleic acid/ glycidol/water/glycerol monooleate by polarizing microspcopy indicated the existence of organized media consisting of direct and reverse hexagonal liquid crystal phases in stable microemulsions and emulsions. The formation of direct and reverse hexagonal liquid crystal phases with increasing temperature stems from the requirement of a larger cross-sectional area of the hydrocarbon chain compared to the area of the polar headgroup with increasing thermal mobility of the chain tail.26 These phase structures are formed by self-aggregation as a result of hydrophobic interactions between the hydrocarbon tails and water and the favorable interactions between polar head groups and water or water/glycidol.27 The alkyl chains in the aggregates are highly mobile and liquid-like.26,28 The pure 1-GMO formed in situ gives rise to thermodynamically stable structured phases, such as microemulsions, that favor solubilization of the fatty acid and accelerate condensation of the oleic acid with glycidol, so that the reactants are well dispersed to the catalytic sites in the ion-exchange resin. The

Ambersep 900-Cl- resins produced the highest yield of 1-GMO (97%), and the high catalytic activity of this nonfunctionalized form of the resin indicates that the reaction is essentially catalyzed by quaternary ammonium cations within the polystyrene lattice. We propose the scheme in Figure 11 to describe the overall reaction. These solid catalysts thus behave as cationic surfactants whose properties depend on the nature of the associated anion. Cl- is an excellent counterion for generating aqueous micelles in which the carboxylic groups of oleic acid are strongly bound to the ammonium cations of the surfactant.29 The latter co-micellizes with glycidol whose hydroxyl group is in contact with the aqueous pseudophase, whereas the acyl hydrocarbon chain of the fatty acid remains in the hydrophobic environment of the polystyrene polymer. Conclusions To understand the mechanism of this type of condensation reaction and to optimize the yield of 1-GMO, it is of major importance to characterize the different phases of the system. Because of the existence of hexagonal and microemulsion, isotropic liquid, inverse liquid crystal hexagonal, and microemulsion phases, the hydrophobic effect appears to be dominant. The interaction between all surface-active agents, which are also the chemical reactants, appears of major importance. The interactions of oleic acid (reagent) and/or glycerol 1-monooleate (product) molecules at the solid/liquid interface determine the stability and catalytic activity of the polystyrene-polymersupported ammonium ions. In the surrounding liquid, the glycidol is an alcohol that acts as a reactant, a solvent, and a pseudocomponent of the reaction medium. An increase of the glycidol concentration leads to a decrease of the interaction energy of the interface with water. Consequently, the glycidol

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is a cosurfactant, and the interactions of glycidol at the liquid/ liquid and liquid/solid interfaces are balanced at a higher temperature and a higher mass fraction. Then, the use of new water-containing systems made up of incompatible components reveals the ability to produce new reaction media in the field of green chemistry. Hydrophobic and hydrophilic reagents are compatibilized at the liquid/liquid and solid/liquid interfaces by emulsification phenomena enhanced by synthesis products such as monoglycerides. Consequently, organic and toxic solvents can be substituted by microemulsion or emulsion catalytic media acting as solvents for organic molecules, increasing mass transfer in continuous separation systems. Acknowledgment The authors acknowledge the financial support from ONIDOL through J.P. Jamet, the Region Midi-Pyrénées Languedoc Roussillon, and the European Community. Literature Cited (1) Hartunian-Sowa, S. M.; White, P. J.; Batres, L. V. Influence of Monoglycerides of Different Chain Lengths on Texture and Flavor of Breads Made with Waxy Cornstarch. Starch-Sta¨rke 1990, 42 (2), 53–56. 1990, 42, 2, 53. (2) Fujimura, M.; Yamauchi, H.; Matsushita, T.; Oshima, M.; Oya, K. Shortenings Containing Glycerides and Amylase and/or Protease and Their Manufacture. Japanese Patent JP 03 292 848, 1991. (3) Rieger, M. Glyceryl stearate, chemistry and use. Cosmet. Toiletries 1990, 105, 51. (4) Kabara, J. J.; Lie, M. S. F. Antimicrobial Lipids: Natural and Synthetic Fatty Acids and Monoglycerides. Lipids 1977, 12 (9), 753. (5) Kato, N. Antimicrobial Activity of Fatty Acids and Their Esters against a Film-Forming Yeast in Soy Sauce. J. Food Safety 1981, 3 (2), 121. (6) Kamei, T.; Teraoka, T.; Hirano, H. Antistatic Acrylic Polymer or Polycarbonate Laminate Sheets. Japanese Patent JP 02 276 636, 1990. (7) Mc Intire, R. T. Fatty Acids in Industry; Marcel Dekker: New York, 1989. (8) Mouloungui, Z.; Gauvrit, C. Synthesis and influence of fatty acid esters on the foliar penetration of herbicides. Ind. Crops Prod. 1998, 8, 01. (9) Lok, C. M. Glyceride Esters. U.S. Patent 4,234,498, 1980. (10) Burgos, C. E.; Ayer, D. E.; Johnson, R. A. A new, asymmetric synthesis of lipids and phospholipids. J. Org. Chem. 1987, 52 (22), 4973. (11) Caron, M.; Sharpless, K. B. Titanium isopropoxide-mediated nucleophilic openings of 2,3-epoxy alcohols. A mild procedure for regioselective ring-opening. J. Org. Chem. 1985, 50 (9), 1557. (12) Tamura, T. Preparation of Higher Fatty Acid Monoglycerides from Fatty Acids and Glycidol. Japanese Patent JP 04182451, 1992.

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ReceiVed for reView January 20, 2009 ReVised manuscript receiVed April 16, 2009 Accepted June 17, 2009 IE900101K

Synthesis of Glycerol 1-Monooleate by Condensation of Oleic Acid

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