5240
Ind. Eng. Chem. Res. 1997, 36, 5240-5245
Kinetics of Catalytic Transfer Hydrogenation of Soybean Lecithin Mateja Naglicˇ ,† Andrej S ˇ midovnik,*,† and Tine Koloini‡ National Institute of Chemistry, Laboratory of Food Chemistry, Ljubljana, Slovenia, and Faculty of Chemistry and Chemical Technology, University of Ljubljana, Slovenia
Catalytic transfer hydrogenation of soybean lecithin has been studied using aqueous sodium formate solution as hydrogen donor and palladium on carbon as catalyst. Kinetic constants and selectivity have been determined at intensive stirring. Hydrogenation reactions followed the first-order kinetics with respect to fatty acids. In addition to short reaction time, this method offers safe and easy handling. Hydrogenated soybean lecithin provides products with increased stability with respect to oxidation, mostly applied in cosmetics and pharmaceutical preparations. Introduction Lecithin is a complex, naturally occurring mixture of phospholipids containing two long hydrocarbon chains. Phospholipids possess emulsifying and stabilizing properties. Also, as primary constituents of biological membranes, they are vital building blocks of membrane lipids of cells. Lecithin has two long hydrocarbon chains and it spontaneously packs into bilayers, leading to a highly stable lamellar liquid crystalline phase in mixtures with water (Shinoda et al., 1993). Lecithin is widely used in foods and beverages, industrial coatings, animal health, and nutrition products. It also has importance in biological, pharmaceutical, and cosmetic applications (Weete et al., 1994), especially in the form of liposomes. The main advantage of the liposomal form used in cosmetics is to increase active compound concentration in the deeper layers of the skin. Phospholipids of animal (egg) or vegetable (soybean) origin can be used to prepare phospholipid liposomes. While animal phospholipids contain mainly saturated fatty acids, phospolipids from soybeans contain primarily polyunsaturated fatty acids (e.g., linoleic acid) that are designated as essential for healthy skin (Ghyczy et al., 1994). Soybean lecithin (SL) and hydrogenated soybean lecithin (HSL) are the most commonly used phospholipids in cosmetic commercial liposome formulation (Bonina et al., 1994). Brandl (1989) reported the usage of hydrogenated soybean lecithin in the case of encapsulated human hemoglobin. Lecithin from soybeans is a mixture of phosphatides (Figure 1) that consists mainly of phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylinositol (PI), and phosphatidic acid (PA). It seems easily understandable that the phospholipid mixture of soybean lecithin would not possess the optimum composition for every potential application. Individual phospholipids and other constituents in lecithin have diverse properties; consequently, fractions of the phospholipid compounds or modifications of one or more phospholipids will exhibit different effects in different applications (Ziegelitz, 1995). Hydrogenated lecithin (more than 60%) showed greater protective effect against the decomposition of tocoferols in olive oil than did the nonhydrogenated one (Kijimoto et al., 1987). * Author to whom correspondence should be addressed at the National Institute of Chemistry, Hajdrihova 19, 1000 Ljubljana, Slovenia. Phone: ++386 61 176-02-00. Fax: ++386 61 125-92-44, ++386 61 125-70-69. † National Institute of Chemistry. ‡ University of Ljubljana. S0888-5885(97)00135-8 CCC: $14.00
Figure 1. Phosphatidylcholine (PC), a principal component of soybean lecithin.
Of the sources available (including corn, cottonseed, peanut, and sunflower), the soybean represents the most abundant one. Crude soybean oil usually contains 2-3% phosphatides (Wan Nieuwenhuyzen, 1976). Sometimes, in phospholipid production, modification including hydroxylation, acetylation, hydrolyzation, and hydrogenation is needed. Hydroxylation, acetylation, and hydrolyzation reactions improve the emulsifying properties of lecithin. Hydrogenation of lecithin gives oxidatively stable products mostly applied in cosmetics and pharmaceutical preparations. We may predict that partially hydrogenated soybean lecithin obtained with catalytic transfer hydrogenation could give an excellent product for cosmetic commercial liposome formulation. Phospholipids from partially hydrogenated soybean lecithin still contain certain amounts of linoleic acid, a fatty acid which is designated as essential for healthy skin (Ghyczy et al., 1994); on the other hand the amount of linolenic acid, a fatty acid which is oxidized most easily, is minimized. Partially hydrogenated soybean lecithin is thus healthy for skin and at the same time has increased resistance to oxidation. Classical hydrogenation is usually carried out with deoiled lecithin in an alcoholic hexane solution after the addition of a catalyst (Ziegelitz, 1995). In the search of an optimal hydrogenation procedure, an alternate and new method for hydrogenation of lecithinsthe catalytic transfer hydrogenation (CTH)sis being developed. With a difference from the classical technique using molecular hydrogen, hydrogen donors as a source of hydrogen are used in a catalytic transfer reduction. This type of hydrogenation could be performed in aqueous media in the presence of various hydrogen donors (Sˇ midovnik et al., 1992, 1994). The © 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 5241 Table 1. Composition of the CTH-Hydrogenated Lecithin composition (%) untreated soybean lecithin Emulpur N
partially hydrogenated lecithina (120 min)
27.5 21.4 14.8 3.9 32.4
27.2 21.8 14.3 4.8 31.9
20.0 3.9 9.2 57.0 8.0 1.9
20.3 32.0 34.4 10.7 1.3 1.3
phosphatide classes phosphatidylcholine (PC) phosphatidylethanolamine (PE) phosphatidylinositol (PI) phosphatidyic acid (PA) other phosphatides fatty acid palmitic acid stearic acid oleic acid linoleic acid linolenic acid other acids rate constant (min-1) kOl kL kLe
saturation selectivity
0.009 0.020 0.028
SOl SL
1.40 2.22
a Reaction conditions: 10 g of lecithin, 2.78 M HCOONa, 0.25 g of 10% Pd/C, mechanical stirrer at 600 rpm, 70 °C.
following generalized equation, in which A represents hydrogen acceptor and D represents hydrogen donor, represents this process: catalyst
DH + A 98 D + AH
(1)
The present paper deals with the kinetics of the CTH reactions of lecithin (as hydrogen acceptor) and aqueous sodium formate solution (as hydrogen donor) in the presence of Pd/C catalyst. The effects of temperature, catalyst concentration, and mixing on the reaction rate and selectivity are presented. Experimental Procedures Materials. Hydrogenation was carried out with deoiled soybean lecithin Emulpur N (Lucas Mayer, Germany). Aqueous sodium formate (Fluka, Switzerland) solution was used as hydrogen donor, and E 101 NN/D 10% palladium on activated carbon (Degussa), specific surface area according to ASTM D3683 950 m2/ g, was used as the catalyst. Methods of Analysis. Fatty acid (FA) contents were determined as fatty acid methyl esters (FAME), prepared by IUPAC method II.D.19 (1979). For analysis, an SP-2380 fused silica capillary column (30 m × 0.20 mm inside diameter, 0.20 µm film thickness) was used in a Varian 3400 gas chromatograph equipped with an all-glass splitter system and flame-ionization detector. The gas chromatograph was operated at 150-200 °C, with a heating rate of 3 °C/min and a helium carrier gas flow rate of 1.2 mL/min. The separation and identification of phospholipids were accomplished by thin layer chromatography (TLC). Kieselgur 60 HPTLC plates (Merck, Germany) were used. The plates were developed once in an unsaturated glass flat bottom chamber (Camag) in a solvent system chloroform:methanol:water (65:25:4 by vol). Detection was carried out with 10% phosphomolybdenic acid in ethanol. The phospholipids were identified by comparing samples with Lucas Mayer standard (Table 1). The integration was done on a densitometer Camag TLC
Figure 2. Hydrogenation procedure.
scanner II in absorption mode using a W-lamp at 560 nm (wavelength of maximum absorbance). Hydrogenation Procedures. Soybean lecithin and the donor solution were homogenized with Ultraturrax (Janke & Kunkel) at 4000 rpm. Then catalyst was added, and the mixture was agitated in a 250 mL roundbottom flask. The mechanical stirrer with a 3-cm round-shaped Teflon blade was used. A water bath was used for flask termostation. The progress of the hydrogenation reaction was monitored by determining the fatty acid composition of the samples removed periodically during the process. Analyses were carried out by gas chromatography. After the hydrogenation process, lecithin was separated using hexane. Hexane was added to the hydrogenation mixture which was stirred vigorously for few minutes. After separation into two layers, the upper layer containing lecithin solution was decanted and pure hydrogenated lecithin was obtained with hexane evaporation. The extraction was then repeated two times (Figure 2). Results and Discussion To optimize the CTH procedure, the effects of several chemical and physical parameters on the reaction rate were examined. Effect of Lecithin Concentration. As mentioned above, lecithin has two long hydrocarbon chains and spontaneously packs into bilayers, leading to a highly stable lamellar liquid crystalline phase in a mixture with water (Shinoda et al., 1993). Thus, choosing the appropriate concentration of lecithin for the hydrogenation procedure is essential. At small concentrations of
5242 Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997
Figure 3. Effect of donor (HCOONa) concentration on the hydrogenation. Reaction conditions: 10.00 g of soybean lecithin, 250 mg of 10% Pd/C, 600 rpm, 70 °C.
lecithin dispersed in water, soft foam is formed and causes problems during the process. This lecithin foam cannot be stirred homogeneously throughout the whole volume, so the process cannot be controlled completely. When a larger concentration of lecithin is chosen, a compact, easily mixable dispersion is obtained. In this way, the process is completely controlled, and the products obtained show good reproducibility. For this reason, a concentration of 10 g of lecithin in 50 g of water was chosen in all other experiments. Effect of Hydrogen Donor Concentration. CTH with a donor in aqueous medium proceeds in a complex three-phase system (lecithin-water-solid catalyst). Several authors (Arkad et al., 1987; Sˇ midovnik et al., 1992 and 1993; Brigas and Johnstone, 1992) suggested that the reaction proceeded via competitive adsorption of water and formate on identical active sites on the catalyst surface. When sodium formate solution is used, it is believed that not only sodium formate but also water are hydrogen donors in the reaction. The following equation illustrates this process (Sˇ midovnik et al., 1992 and 1994):
HCOO- + H2O + A f HCO3- + H2A
(2)
In the previous studies of the CTH of vegetable oils (Sˇ midovnik et al., 1994), an acceptable concentration of aqueous sodium formate hydrogen donor was determined to be 2.78 M. At higher concentrations, a substantial decrease of the reaction rate was observed and was attributed to the salting-out effect on the emulsifying agent Mayodan 612. Mayodan 612 was added to the reaction mixture to stabilize the dynamic oil-water interfacial area on the catalyst surface where CTH occurs. In Figure 3, the hydrogenation process of lecithin at 2.8, 4.15, and 2.1 M concentrations is presented. The reaction rate was again the highest at 2.8 M concentration of the hydrogen donor, but the dependence of reaction rate on the concentration is weak. This can be explained by the fact that lecithin itself is an excellent emulsifier, which prevents the decrease of the interfacial area essential for the CTH. Constant addition of small amounts of formic acid increases the rate of reaction because it reduces foaming (Figure 4). Effect of Agitation. It is to be expected that, for a surface catalytic reaction taking place in a three-phase
Figure 4. Constant addition of HCOOH accelerates hydrogenation rate. Reaction conditions: 10.00 g of soybean lecithin, 2.78 M HCOONa-water solution, 250 mg of 10% Pd/C, 600 rpm, 70 °C.
Figure 5. Effect of agitation. Reaction conditions: 10.00 g of soybean lecithin, 2.78 M HCOONa-water solution, 250 mg of 10% Pd/C, 70 °C.
system (lecithin-water-catalyst), agitation has a considerable influence on the reaction rate until a certain value of agitation rate is reached (Figure 5). Additional increasing of agitation rate has no further influence on the reaction rate because the kinetics regime is achieved. Effect of Catalyst Concentration. For the hydrogenation of lecithin, a palladium on carbon catalyst was used. Experiments with different amounts of catalyst confirmed the expectations of the linear relationship between the amount of catalyst and the reaction rate (Figure 6). Kinetics of CTH. Soybean lecithin is a phospholipid containing two chains of fatty acids (FA). Some of these fatty acids are saturated. Palmitic (P), stearic (S), and the others are unsaturated: oleic (Ol) has one double bond, linoleic (L) has two double bonds, and linolenic (Le) has three double bonds. During the hydrogenation reaction, unsaturated fatty acids compete with hydrogen donors through adsorption for the “active sites” on the catalyst surface where they are gradually converted to the saturated state. Hydrogenation is an extremely complex series of saturation and isomerization reactions of the double bonds of unsaturated fatty acids. The complete scheme of the process is very complicated, and that is why simplified
Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 5243
dCLe ) -kLeCLe dt
(4)
dCL ) -kLCL + kLeCLe dt
(5)
dCOl ) -kOlCOl + kLCL dt
(6)
the rate constants are calculated using experimental data (Table 1). From the rate constants, saturation selectivities can be calculated as suggested by Coenen (1976). Saturation selectivities are defined as ratios of the relevant rate constants Figure 6. Effect of the catalyst amount. Reaction conditions: 10.00 g of soybean lecithin, 2.78 M HCOONa-water solution, 600 rpm, 70 °C.
SLe ) kLe/kL
(7)
SL ) kL/kOl
(8)
and
These ratios should be as high as possible to reach high saturation selectivity. The rate constants exhibit an Arrhenius relationship to the temperature in accordance with the equation
ln k ) ln ko -
Figure 7. Time course of fatty acid concentration; experimental data are compared with predicted values. Reaction conditions: 10.00 g of soybean lecithin, 2.78 M HCOONa-water solution, 250 mg of 10% Pd/C, 600 rpm, 70 °C.
E1 RT
By the application of the Arrhenius equation to the experimental data (Figure 8), the values of the appropriate Arrhenius constants, namely the pre-exponential factor ko and the activation energy E, were obtained. The first-order kinetics of all hydrogenation reactions considered was found to be a sufficient approximation of the reaction rate of the CTH process. Various schemes were used to represent the CTH reaction mechanism. One of the simplest is presented in eqs 10 and 11: k1
models are used. The simplified set of reactions proposed by Bailey (1949) is based on the assumption of a first-order and irreversible reaction, and it is in satisfactory agreement with the experimental data at various operating conditions. This was verified by Albright and Okkerse (Albright, 1965; Okkerse et al., 1967). The following equation represents this process: kLe
kL
kOl
Le 98 L 98 Ol 98 S
(3)
where Le, L, Ol, and S indicate the concentration of linolenic, linoleic, oleic, and stearic acids, and kLe, kL, and kOl represent the rate constants for the catalytic transfer hydrogenation of linolenic, linoleic and oleic acids (Figure 7). In the proposed chemical model, geometrical or positional isomers that always occur during hydrogenation are not considered. Sˇ midovnik et al. (1992, 1993, and 1994) reported good agreement of this simplified model and experimental data when this set of reactions was used to describe the catalytic transfer hydrogenation of soybean oil. With the simultaneous solving of differential equations derived from the simplified Bailey’s model (eq 3)
(9)
H2Dn + Pd 98 PdH2 + Dn
(10)
k2
PdH2 + R2CdCR2 98 R2CH-CHR2 + Pd (11) The rate at which the surface of the catalyst is covered with hydrogen is determined by the rate of donor adsorption and the rate of surface reaction. At the quasi steady state, eq 12 applies:
d[PdH2] ) k1(1 - θ)[H2Dn] - k2θ[R2CdCR2] ) 0 dt (12) The surface coverage θ, which is the fraction of catalytic surface covered by hydrogen, is defined by eq 13:
θ)
[PdH2] [Pd]
(13)
Equation 14 is another way of expressing eq 12:
θ)
k1[H2Dn] k1[H2Dn] + k2[R2CdCR2]
(14)
5244 Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997
useful alternative for the hydrogenation of lecithin because this method offers safe and easy handling. Acknowledgment This investigation was supported by the Ministry of Science and Technology of Slovenia (J2-7242-0104-95). We thank Dr. Jozˇe Kobe and Dr. Matjazˇ Kranjc for valuable advice. Nomenclature
Figure 8. Effect of temperature. Activation energies for fatty acids are obtained from experimental data: Ea(Ol) ) 25.7 kJ/mol, Ea(L) ) 31.4 kJ/mol, Ea(Le) ) 35.0 kJ/mol. Reaction conditions: 10.00 g of soybean lecithin, 2.78 M HCOONa-water solution, 250 mg of 10% Pd/C, 600 rpm.
The rate of surface reaction can therefore be written in the form of eq 15:
r)
k1k2[R2CdCR2][H2Dn] k1[H2Dn] + k2[R2CdCR2]
(15)
Because of the high concentration of hydrogen donor and low hydrogenation rates,
k1[H2Dn] . k2[R2CdCR2]
Literature Cited
Equation 15 can be therefore simplified into
r ) k2*[R2CdCR2]
θ ) surface coverage k1 ) adsorption rate constant (min-1) k2 ) reaction rate constant (min-1) k2* ) overall reaction rate constant (min-1) A ) hydrogen acceptor D ) hydrogen donor Le ) linolenic acid L ) linoleic acid Ol ) oleic acid S ) stearic acid kLe ) overall rate constant of linolenic acid (min-1) kL ) overall rate constant of linoleic acid (min-1) kOl ) overall rate constant of oleic acid (min-1) r ) reaction rate (mol/min L) SLe ) linolenic saturation selectivity SL ) linoleic saturation selectivity [PdH2] ) catalyst surface covered with hydrogen (m2/m3) [Pd] ) all catalyst surface available (m2/m3)
(16)
The first-order relationship is valid for all hydrogenation reactions considered. The linear relationship between the catalyst concentration and the reaction rate is also in agreement with the assumption made in deriving eq 16. Hydrogenated Products. The compositions of untreated and partly hydrogenated soybean lecithin are presented together with the reaction rate constants and saturation selectivity (Table 1). A high degree of hydrogenation of the linolenic and linoleic acids can be observed, whereas the distribution of phospholipid classes remains practically unchanged. This is an explanation as to why the emulsifying properties of hydrogenated lecithin are unchanged (Holman and Mahfouz, 1989). Conclusion Catalytic transfer hydrogenation of the examined soybean lecithin in a solution of sodium formate follows the simplified Bailey’s model of the first-order kinetic reaction with respect to the fatty acid compound when the kinetic regime is achieved by intensive stirring. During the catalytic transfer hydrogenation, the high degree of hydrogenation of linolenic and linoleic acids can be observed, whereas the distribution of phospholipid classes remains practically unchanged. Catalytic transfer hydrogenation using a solution of sodium formate as the hydrogen donor is a simple and
Albright, L. F. Quantitative Measure of Selectivity of Hydrogenation of Triglycerides. J. Am. Oil Chem. Soc. 1965, 42, 250. Arkad, O.; Wiener, H.; Garti, N.; Sasson, Y. Catalytic Transfer Hydrogenation of Soybean Oil Methyl Ester Using Inorganic Formic Acid Salts as Donors. J. Am. Oil Chem. Soc. 1987, 64, 1529. Bailey, A. E. J. Am. Oil Chem. Soc. 1949, 26, 596. Bonina, F.; Montenegro, L.; La Rosa, C.; Gasparri, F.; Leonardi, R. Comparison of Different Separative Techniques in the Quantitative Determination of Active Compound Enclosed in Liposomal System. Int. J. Cosmet. Sci. 1994, 183. Brandl, M.; Becker, D.; Bauer, K. H. Hemoglobin-Liposomes as Blood Replacement Fluid. Drug Dev. Ind. Pharm. 1989, 15, 655. Brigas, A. F.; Johnstone, R. A. W. Metal-Assisted Reactions. Part 24. The importance of solvents in liquid-phase heterogeneous catalytic transfer reduction. Tetrahedron 1992, 48, 7735. Coenen, J. W. E. Hydrogenation of Edible Oils. J. Am. Oil Chem. Soc. 1976, 53, 382. Ghyczy, M.; Gareiss, J.; Kovats, T. Cosmet. Toiletries 1994, 109, 75. Holman, R. T.; Mahfouz, M. M. Dietary Fats and Health, Lawson & Hill: New York, 1989. International Union of Pure and Applied Chemistry, Standard Methods for the Analysis of Oils and Derivatives, 6th ed.; Pergamon: Oxford, 1979; Part I (Sections 1 and 22). Kijimoto, G.; Yoshida, H.; Shibahara, A. Effects of Soy Lecithin with Different Degrees of Hydrogenation on the Colouring and Decomposition of Tocopherol in Heated Oils. J. Jpn. Soc. of Nutr. and Food Sci. 1987, 40, 497. Okkerse, C., de Jonge, A., Coenen, J. W. E., Rozendall, A. Selective Hydrogenation of Soybean Oil in the Presence of Copper Catalysts. J. Am. Oil Chem. Soc. 1967, 44, 152. Shinoda, K.; Shibata, Y.; Lindman, B. Interfacial Tensions for Lecithin Microemulsions Including the Effect of Surfactant and Polymer Addition. Langmuir 1993, 9, 1254. Sˇ midovnik, A.; Sˇ timac, A.; Kobe, J. Catalytic Transfer Hydrogenation of Soybean Oil. J. Am. Oil Chem. Soc. 1992, 69, 405.
Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 5245 Sˇ midovnik, A.; Plazl, I.; Koloini, T. Kinetics of Catalytic Transfer Hydrogenation of Soybean Oil. Chem. Eng. J. 1993, 51, B51B56. Sˇ midovnik, A.; Kobe, J.; Leskovsˇek, S.; Koloini, T. Kinetics of Catalytic Transfer Hydrogenation of Soybean Oil. J. Am. Oil Chem. Soc. 1994, 71, 507. Van Nieuwenhuyzen, W. Lecithin Production and Properties. J. Am. Oil Chem. Soc. 1976, 53, 425. Weete, J. D.; Betageri, S.; Griffith, G. L. Improvement of Lecithin as an Emulsifier for Water-in-Oil Emulsions by Thermalization. J. Am. Oil Chem. Soc. 1994, 71, 731.
Ziegelitz, R. Lecithin Processing Possibilities. INFORM 1995, 6, 1224.
Received for review February 7, 1997 Revised manuscript received August 12, 1997 Accepted August 12, 1997X IE970135M X Abstract published in Advance ACS Abstracts, October 15, 1997.
