Ind. Eng. Chem. Res. 1993,32, 2065-2068
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GENERALRESEARCH Esterification of Oleic Acid by Methanol Catalyzed by p-Toluenesulfonic Acid and the Cation-Exchange Resins K2411 and K1481 in Supercritical Carbon Dioxide? Corinne Vieville, ZBphirin Mouloungui,’ and Antoine Gaset Laboratoire de Chimie des Agroressources, ENSCT, ZNP de Toulouse, 118, route de Narbonne, 31077 Toulouse Ceder, France
p-Toluenesulfonic acid @-TSA) and the cation-exchange resins K2411 and K1481 were found to be good catalysts for the esterification of oleic acid by methanol in supercritical C02. In the presence of solid p-toluenesulfonic acid monohydrate, the reaction took place in a pseudohomogeneous phase throughout the medium [Bl, whereas with the sulfonic acid resins, the reaction took place in the agitated medium [9] in a heterogeneous phase. Under the latter conditions, the reaction was found to depend on diffusion of oleic acid into the cationic resins and desorption of the methyl oleate from the resin. These effects can be circumvented using high pressures as in supercritical C02.
Introduction There have been a number of reports on the enzymatic catalysis of the transesterification, interesterification, and hydrolysis of fatty acid esters, or the esterification of fatty acids in supercritical fluids (Randolph et al., 1988; Hammond et al., 1985; Nakamura et al., 1986; Chi et al., 1988;Pasta et al., 1989;Nakamura et al., 1990;Adshiri et al., 1992). It has been found that free or immobilized lipase is both stable and active in supercritical carbon dioxide. Its activity can even be improved by optimizing the reaction conditions. French workers (Marty et al., 1990; 1992) have shown that the water content of the support for the lipase of Mucor miehei immobilized on a macroporous anionic resin is a determining factor in the catalytic activity of the enzyme for esterification. The study of enzymatic reactions in supercritical fluids has mostly concerned supercritical carbon dioxide (SC-Cod in view of its low viscosity,high diffusibility,lack of toxicity, and ease of separation from substrates and products. To our knowledge,there have been no reports of chemical studies (Subramaniam and McHugh, 1986)of equilibrium reactions in supercritical media. The esterification of carboxylicacids with alcohols is an example of a reversible reaction that has been widely studied in conventional media. To shift the equilibrium of the esterification towards the desired alkyl ester, one of the reactants, usually the alcohol, is in excess. This is both convenient and cost effective. In some cases, one of the constituents, generally water, is eliminated chemically, physically, or by pervaporation, although such methods tend to be rather burdensome. Esterification can be achieved at high temperatures with an adequate reaction duration and suitable catalyst. Numerous homogeneous and heterogeneous catalysts have been examined for these reactions, and esterification may be improved in heterogeneous phases by the use of ion-exchange resin catalysts. They are convenient to use, noncorrosive, and can be readily
+Part of this work was presented at the 2nd Colloque International sur les Fluides Supercritiques,Paris,16/17October, 1991.
recovered by filtration of the organic phase (Pietrzyck, 1976; Polyanskii and Sapozhnikov, 1977). In the present study, we evaluated the use of SC-C02 for esterification of oleic acid by methanol in the presence of p-toluenesulfonic acid monohydrate @-TSA) or the cationic exchange resins K2411 and K1481 employed as solid catalysts. We initially compared the behavior of p-TSA and resin K2411 on the nature of the catalytic process and their respective catalytic efficiencies. Our ultimate goal is to employ polymeric resins with functional groups as chemical catalysts (Riad, 1989; Asdih, 1990) in supercritical fluid media.
Experimental Section Pretreatment of the Resins. After being rinsed in water, the resin K2411 is permuted to the H+ form in a column by percolation with 2 N hydrochloric acid, washed with water until neutrality, washed with ethanol and with ether, and then dried at 313 K in an oven. The capacity is determined for a given weight of dried catalyst by percolation of an aqueous solution of 2 N sodium chloride until neutrality, followed by rinsing in water. The percolation and rinsing solutions are titrated with 0.1 N sodium hydroxide using phenolphthalein as indicator.This gives the capacity in mequiv of H+/g of dry catalyst. The required amount of dry resin K2411 is left to swell in methanol for 1h before the experiment, and the remaining methanol is then aspirated with a syringe. For the resin K1481, the procedure was the same as above but with vigorous stirring followed by filtration instead of percolation and rinses. The resin was then used in its dried form for the experiments. Procedure. The apparatus used in this study is illustrated in Figure 1. Oleic acid, methanol, and catalyst are placed in the sapphire reactor 191 (Top Industrie) in the amounts indicated in Tables II-IV as a function of the totalvolume of the system (62 cm3) or of that of the reactor itself (12 cm3) when the reaction was carried out in a closed system. (Numbers and letters in brackets refer to elements of Figure 1.) The reactor is then closed with filters and valves. After connectionofthe reactor, valve 161is opened
0 1993 American Chemical Society 0888-5885/93~2632-2Q65$Q4.OO/Q
2066 Ind. Eng. Chem. Res., Vol. 32, No. 9, 1993
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Figure 1. Apparatus used in this study: 111 COz cylinder, [21 cooler (268K), 131high-pressure pump, 141safetydiaphragm (500 bar m u ) , [SI thermostated bath (313 K),161 valve, [71 pressure sensor, [a] temperature probe, 191 sapphire reactor, [lo] sampling loop, [ H I recirculation pump, [A] thermostated chamber (313K), [B]reaction system.
and C02 is admitted at a pressure of 50-55 bar after preheating [51 to 313 K. The pressure is increased by pump 131 (Top Industrie). When the desired pressure is reached, valve [61 is closed and the circulating pump 1111 (Micropump 220 designed for pressures up to 300 bar) is switched on. Samplesremoved during the reaction from the sampling loop 1101 were flushed into 2 mL of methanol. At the end of the experiment, the reaction system was flushed into 30 mL of methanol, and the system was rinsed with this solution using a peristaltic pump to remove oleic acid, methyl oleate, and p-TSA when used as catalyst. The samples were analyzed by HPLC and TLC-FID. Analytical Method. The oleic acid and methyl oleate concentrationswere determined by HPLC using a Spectra Physics P1500 pump with a LDC refractometer detector on a NucleosilC18 5-pm column (25cm X 4.6 mm, Touzard et Matignon-France) at 313 K. The mobile phase was a mixture of methanol/AcOH 3 L at a flow rate of 0.7 mL/ min. The compounds were quantified from calibration curves established with oleic acid and methyl oleate standards. The results were compared to those from thinlayer rod chromatography using a Iatroscan MK5 flame : ionization detector (TLC-FID). The samples were deposited on quartz rods coated with silica (chromarods SIII), and the rods were run in a mixture of hexanelethyl ether/ AcOH (971311,V/V/V) for 20 min. The data were acquired and processed by computer using Boreal Software (FlotecFrance).
Experimental Results The preliminary results showed that without catalyst no condensation of oleic acid ([OAI, = 13 mM) with methanol ([MeOH10 = 500 mM) took place in SC-CO2 a t 313 K and 135 bar. The esterification reaction therefore needs to be catalyzed in this medium. pToluenesulfonic Acid as Catalyst. An homogeneous mixture was obtained with the reactants in the presence ofp-TSA a t the system pressure and temperature. Methyl oleate was obtained in excellent yield (see Figure 2) for relatively short reaction durations (160 min). p-TSA was thus an efficient catalyst for esterification in SC-CO2, and the yield obtained was higher than that in alcoholic medium assisted by pervaporation of water reported by Kita et al. (1987, 1988). From the experimental curves, the kinetics of esterification of oleic acid by methanol are better described by a first-order (see Figure 3) than a second-order model (see Figure 4). The first-order model gives a rate constant of 8.64 X lo9 min-l for a pressure (P,of 165 bar and a ratio r (number of moles of p-TSA to number of moles of oleic
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Figure 2. Oleic acid conversion versus time: catalyst p-TSA, T = 313 K,[OAI, = 13 mM, [MeOHl, = 200 mM.
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Figure 3. First-order model for the esterification of oleic acid with methanol catalyzed by p-TSA T = 313 K,[MeOHl, = 200 mm, [OAI, = 13 mM. LW
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Figure 4. Second-order model for the esterification of oleic acid with methanol catalyzed by p-TSA T = 313 K,[MeOHl, = 200mM, [OAI, = 13 mM.
acid) of 1/15 and a rate constant of 1.43 X le2 min-l for P = 190 bar and r = 115. These encouraging results with p-TSA prompted us to test the more convenient cationexchange resins. Catalytic Action of the Sulfonic Resins K2411 and K1481. To our knowledge, in SC-CO2, the cationic exchange resins in the H+ form have been successfully employed in the single case of absorption and extraction of alkaloids (Schaeffer, 1989). For the esterification reaction studied here, the catalytic properties of the sulfonic groups in these resins were exploited. The specificities of the sulfonic resins K2411 and K1481 are listed in Table I. Independently of pressure and temperature, two other parameters affect the reaction rate: the stirring conditions and the ratio r. Both resins produce a heterogeneous mixture in SC-CO2, and the rate of reaction is governed by adsorption of oleic acid and desorption of methyl oleate, as shown in Figure 5. The reactants and products were desorbed by rinsing with methanol and ether, and the extracts were quantified by HPLC after evaporation and dilution in methanol. The resin was found to be poisoned
Ind. Eng. Chem. Res., Vol. 32, No. 9, 1993 2067 Table 111. Effect of the Ratio r on the Esterification of Oleic Acid Catalyzed by K2411 Resin experimenta condition 1 2 ,a 4 1/4 P,bar 135 135 time, h 3.5 3.5 methyl oleate yield, % 90 25 I
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a Operating conditions: [OA], = 13 mM, [MeOHl, = 200 mM, T = 313K.* Ratio of the mole number of sulfonic sites to mole number of oleic acid.
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Figure 5. Mole number of methyl oleate in the SC-CO2phase during the esterification of oleic acid with methanol catalyzed by K2411 resin: T = 313 K,MA = 8 X lo-' mol, [MeOHI = 200 mM. Table I. Specificities of the K1481 Powder Resin and K24ll Resin resin characteristic K1481 K2411 gel porous type styrene styrene polymer 8 8 croea-liking level? % 25 porosity, % 400 pore size, A 25 surface area, m2/g
