Kinetics of Catalytic Transfer Hydrogenation of Soybean Lecithin

Mateja Naglic,† Andrej Smidovnik,*,† and Tine Koloini‡

National Institute of Chemistry, Laboratory of Food Chemistry, Ljubljana, Slovenia, and Faculty of Chemistryand Chemical Technology, University of Ljubljana, Slovenia

Catalytic transfer hydrogenation of soybean lecithin has been studied using aqueous sodiumformate solution as hydrogen donor and palladium on carbon as catalyst. Kinetic constantsand selectivity have been determined at intensive stirring. Hydrogenation reactions followedthe first-order kinetics with respect to fatty acids. In addition to short reaction time, this methodoffers safe and easy handling. Hydrogenated soybean lecithin provides products with increasedstability with respect to oxidation, mostly applied in cosmetics and pharmaceutical preparations.

Introduction

Lecithin is a complex, naturally occurring mixture ofphospholipids containing two long hydrocarbon chains.Phospholipids possess emulsifying and stabilizing prop-erties. Also, as primary constituents of biologicalmembranes, they are vital building blocks of membranelipids of cells. Lecithin has two long hydrocarbon chainsand it spontaneously packs into bilayers, leading to ahighly stable lamellar liquid crystalline phase in mix-tures with water (Shinoda et al., 1993).Lecithin is widely used in foods and beverages,

industrial coatings, animal health, and nutrition prod-ucts. It also has importance in biological, pharmaceuti-cal, and cosmetic applications (Weete et al., 1994),especially in the form of liposomes. The main advantageof the liposomal form used in cosmetics is to increaseactive compound concentration in the deeper layers ofthe skin. Phospholipids of animal (egg) or vegetable(soybean) origin can be used to prepare phospholipidliposomes. While animal phospholipids contain mainlysaturated fatty acids, phospolipids from soybeans con-tain primarily polyunsaturated fatty acids (e.g., linoleicacid) that are designated as essential for healthy skin(Ghyczy et al., 1994). Soybean lecithin (SL) and hydro-genated soybean lecithin (HSL) are the most commonlyused phospholipids in cosmetic commercial liposomeformulation (Bonina et al., 1994). Brandl (1989) re-ported the usage of hydrogenated soybean lecithin inthe case of encapsulated human hemoglobin.Lecithin from soybeans is a mixture of phosphatides

(Figure 1) that consists mainly of phosphatidylcholine(PC), phosphatidylethanolamine (PE), phosphatidyl-inositol (PI), and phosphatidic acid (PA).It seems easily understandable that the phospholipid

mixture of soybean lecithin would not possess theoptimum composition for every potential application.Individual phospholipids and other constituents inlecithin have diverse properties; consequently, fractionsof the phospholipid compounds or modifications of oneor more phospholipids will exhibit different effects indifferent applications (Ziegelitz, 1995). Hydrogenatedlecithin (more than 60%) showed greater protectiveeffect against the decomposition of tocoferols in olive oilthan did the nonhydrogenated one (Kijimoto et al.,1987).

Of the sources available (including corn, cottonseed,peanut, and sunflower), the soybean represents the mostabundant one. Crude soybean oil usually contains2-3% phosphatides (Wan Nieuwenhuyzen, 1976). Some-times, in phospholipid production, modification includ-ing hydroxylation, acetylation, hydrolyzation, and hy-drogenation is needed. Hydroxylation, acetylation, andhydrolyzation reactions improve the emulsifying proper-ties of lecithin. Hydrogenation of lecithin gives oxida-tively stable products mostly applied in cosmetics andpharmaceutical preparations.We may predict that partially hydrogenated soybean

lecithin obtained with catalytic transfer hydrogenationcould give an excellent product for cosmetic commercialliposome formulation. Phospholipids from partiallyhydrogenated soybean lecithin still contain certainamounts of linoleic acid, a fatty acid which is designatedas essential for healthy skin (Ghyczy et al., 1994); onthe other hand the amount of linolenic acid, a fatty acidwhich is oxidized most easily, is minimized. Partiallyhydrogenated soybean lecithin is thus healthy for skinand at the same time has increased resistance tooxidation.Classical hydrogenation is usually carried out with

deoiled lecithin in an alcoholic hexane solution after theaddition of a catalyst (Ziegelitz, 1995).In the search of an optimal hydrogenation procedure,

an alternate and new method for hydrogenation oflecithinsthe catalytic transfer hydrogenation (CTH)sisbeing developed. With a difference from the classicaltechnique using molecular hydrogen, hydrogen donorsas a source of hydrogen are used in a catalytic transferreduction. This type of hydrogenation could be per-formed in aqueous media in the presence of varioushydrogen donors (Smidovnik et al., 1992, 1994). The

* Author to whom correspondence should be addressed atthe National Institute of Chemistry, Hajdrihova 19, 1000Ljubljana, Slovenia. Phone: ++386 61 176-02-00. Fax: ++38661 125-92-44, ++386 61 125-70-69.

† National Institute of Chemistry.‡ University of Ljubljana.

Figure 1. Phosphatidylcholine (PC), a principal component ofsoybean lecithin.

5240 Ind. Eng. Chem. Res. 1997, 36, 5240-5245

S0888-5885(97)00135-8 CCC: $14.00 © 1997 American Chemical Society

following generalized equation, in which A representshydrogen acceptor and D represents hydrogen donor,represents this process:

The present paper deals with the kinetics of the CTHreactions of lecithin (as hydrogen acceptor) and aqueoussodium formate solution (as hydrogen donor) in thepresence of Pd/C catalyst. The effects of temperature,catalyst concentration, and mixing on the reaction rateand selectivity are presented.

Experimental Procedures

Materials. Hydrogenation was carried out withdeoiled soybean lecithin Emulpur N (Lucas Mayer,Germany). Aqueous sodium formate (Fluka, Switzer-land) solution was used as hydrogen donor, and E 101NN/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), pre-pared by IUPAC method II.D.19 (1979). For analysis,an SP-2380 fused silica capillary column (30 m × 0.20mm inside diameter, 0.20 µm film thickness) was usedin a Varian 3400 gas chromatograph equipped with anall-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 carriergas 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) wereused. The plates were developed once in an unsaturatedglass flat bottom chamber (Camag) in a solvent systemchloroform:methanol:water (65:25:4 by vol). Detectionwas carried out with 10% phosphomolybdenic acid inethanol. The phospholipids were identified by compar-ing samples with Lucas Mayer standard (Table 1). Theintegration was done on a densitometer Camag TLC

scanner II in absorption mode using a W-lamp at 560nm (wavelength of maximum absorbance).Hydrogenation Procedures. Soybean lecithin and

the donor solution were homogenized with Ultraturrax(Janke & Kunkel) at 4000 rpm. Then catalyst wasadded, and the mixture was agitated in a 250 mL round-bottom flask. The mechanical stirrer with a 3-cmround-shaped Teflon blade was used. A water bath wasused for flask termostation. The progress of the hydro-genation reaction was monitored by determining thefatty acid composition of the samples removed periodi-cally during the process. Analyses were carried out bygas chromatography.After the hydrogenation process, lecithin was sepa-

rated using hexane. Hexane was added to the hydro-genation mixture which was stirred vigorously for fewminutes. After separation into two layers, the upperlayer containing lecithin solution was decanted and purehydrogenated lecithin was obtained with hexane evapo-ration. The extraction was then repeated two times(Figure 2).

Results and Discussion

To optimize the CTH procedure, the effects of severalchemical and physical parameters on the reaction ratewere examined.Effect of Lecithin Concentration. As mentioned

above, lecithin has two long hydrocarbon chains andspontaneously packs into bilayers, leading to a highlystable lamellar liquid crystalline phase in a mixturewith water (Shinoda et al., 1993). Thus, choosing theappropriate concentration of lecithin for the hydrogena-tion procedure is essential. At small concentrations of

Table 1. Composition of the CTH-Hydrogenated Lecithin

composition (%)

untreatedsoybeanlecithin

Emulpur N

partiallyhydrogenated

lecithina(120 min)

phosphatide classesphosphatidylcholine (PC) 27.5 27.2phosphatidylethanolamine (PE) 21.4 21.8phosphatidylinositol (PI) 14.8 14.3phosphatidyic acid (PA) 3.9 4.8other phosphatides 32.4 31.9

fatty acidpalmitic acid 20.0 20.3stearic acid 3.9 32.0oleic acid 9.2 34.4linoleic acid 57.0 10.7linolenic acid 8.0 1.3other acids 1.9 1.3

rate constant (min-1) saturation selectivity

kOl 0.009 SOl 1.40kL 0.020 SL 2.22kLe 0.028a Reaction conditions: 10 g of lecithin, 2.78 M HCOONa, 0.25

g of 10% Pd/C, mechanical stirrer at 600 rpm, 70 °C.

DH + A98catalyst

D + AH (1)

Figure 2. Hydrogenation procedure.

Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 5241

lecithin dispersed in water, soft foam is formed andcauses problems during the process. This lecithin foamcannot be stirred homogeneously throughout the wholevolume, so the process cannot be controlled completely.When a larger concentration of lecithin is chosen, acompact, easily mixable dispersion is obtained. In thisway, the process is completely controlled, and theproducts obtained show good reproducibility. For thisreason, a concentration of 10 g of lecithin in 50 g ofwater was chosen in all other experiments.Effect of Hydrogen Donor Concentration. CTH

with a donor in aqueous medium proceeds in a complexthree-phase system (lecithin-water-solid catalyst).Several authors (Arkad et al., 1987; Smidovnik et al.,1992 and 1993; Brigas and Johnstone, 1992) suggestedthat the reaction proceeded via competitive adsorptionof water and formate on identical active sites on thecatalyst surface. When sodium formate solution is used,it is believed that not only sodium formate but alsowater are hydrogen donors in the reaction. The follow-ing equation illustrates this process (Smidovnik et al.,1992 and 1994):

In the previous studies of the CTH of vegetable oils(Smidovnik et al., 1994), an acceptable concentration ofaqueous sodium formate hydrogen donor was deter-mined to be 2.78 M. At higher concentrations, asubstantial decrease of the reaction rate was observedand was attributed to the salting-out effect on theemulsifying agent Mayodan 612. Mayodan 612 wasadded to the reaction mixture to stabilize the dynamicoil-water interfacial area on the catalyst surface whereCTH occurs. In Figure 3, the hydrogenation process oflecithin at 2.8, 4.15, and 2.1 M concentrations ispresented. The reaction rate was again the highest at2.8 M concentration of the hydrogen donor, but thedependence of reaction rate on the concentration isweak. This can be explained by the fact that lecithinitself is an excellent emulsifier, which prevents thedecrease 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

system (lecithin-water-catalyst), agitation has a con-siderable influence on the reaction rate until a certainvalue of agitation rate is reached (Figure 5). Additionalincreasing of agitation rate has no further influence onthe reaction rate because the kinetics regime is achieved.Effect of Catalyst Concentration. For the hydro-

genation of lecithin, a palladium on carbon catalyst wasused. Experiments with different amounts of catalystconfirmed the expectations of the linear relationshipbetween 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 thesefatty acids are saturated. Palmitic (P), stearic (S), andthe others are unsaturated: oleic (Ol) has one doublebond, 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 adsorptionfor the “active sites” on the catalyst surface where theyare gradually converted to the saturated state. Hydro-genation is an extremely complex series of saturationand isomerization reactions of the double bonds ofunsaturated fatty acids. The complete scheme of theprocess is very complicated, and that is why simplified

Figure 3. Effect of donor (HCOONa) concentration on thehydrogenation. Reaction conditions: 10.00 g of soybean lecithin,250 mg of 10% Pd/C, 600 rpm, 70 °C.

HCOO- + H2O + A f HCO3- + H2A (2)

Figure 4. Constant addition of HCOOH accelerates hydrogena-tion rate. Reaction conditions: 10.00 g of soybean lecithin, 2.78M HCOONa-water solution, 250 mg of 10% Pd/C, 600 rpm, 70°C.

Figure 5. Effect of agitation. Reaction conditions: 10.00 g ofsoybean lecithin, 2.78 M HCOONa-water solution, 250 mg of 10%Pd/C, 70 °C.

5242 Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997

models are used. The simplified set of reactions pro-posed by Bailey (1949) is based on the assumption of afirst-order and irreversible reaction, and it is in satisfac-tory agreement with the experimental data at variousoperating conditions. This was verified by Albright andOkkerse (Albright, 1965; Okkerse et al., 1967). Thefollowing equation represents this process:

where Le, L, Ol, and S indicate the concentration oflinolenic, linoleic, oleic, and stearic acids, and kLe, kL,and kOl represent the rate constants for the catalytictransfer hydrogenation of linolenic, linoleic and oleicacids (Figure 7).In the proposed chemical model, geometrical or posi-

tional isomers that always occur during hydrogenationare not considered. Smidovnik et al. (1992, 1993, and1994) reported good agreement of this simplified modeland experimental data when this set of reactions wasused to describe the catalytic transfer hydrogenation ofsoybean oil.With the simultaneous solving of differential equa-

tions derived from the simplified Bailey’s model (eq 3)

the rate constants are calculated using experimentaldata (Table 1).From the rate constants, saturation selectivities can

be calculated as suggested by Coenen (1976). Satura-tion selectivities are defined as ratios of the relevantrate constants

and

These ratios should be as high as possible to reach highsaturation selectivity.The rate constants exhibit an Arrhenius relationship

to the temperature in accordance with the equation

By the application of the Arrhenius equation to theexperimental data (Figure 8), the values of the ap-propriate Arrhenius constants, namely the pre-expo-nential factor ko and the activation energy E, wereobtained.The first-order kinetics of all hydrogenation reactions

considered was found to be a sufficient approximationof the reaction rate of the CTH process.Various schemes were used to represent the CTH

reaction mechanism. One of the simplest is presentedin eqs 10 and 11:

The rate at which the surface of the catalyst is coveredwith hydrogen is determined by the rate of donoradsorption and the rate of surface reaction. At the quasisteady state, eq 12 applies:

The surface coverage θ, which is the fraction ofcatalytic surface covered by hydrogen, is defined by eq13:

Equation 14 is another way of expressing eq 12:

Figure 6. Effect of the catalyst amount. Reaction conditions:10.00 g of soybean lecithin, 2.78 M HCOONa-water solution, 600rpm, 70 °C.

Figure 7. Time course of fatty acid concentration; experimentaldata are compared with predicted values. Reaction conditions:10.00 g of soybean lecithin, 2.78 M HCOONa-water solution, 250mg of 10% Pd/C, 600 rpm, 70 °C.

Le98kLe

L98kL

Ol98kOl

S (3)

dCLe

dt) -kLeCLe (4)

dCL

dt) -kLCL + kLeCLe (5)

dCOl

dt) -kOlCOl + kLCL (6)

SLe ) kLe/kL (7)

SL ) kL/kOl (8)

ln k ) ln ko - ER1T

(9)

H2Dn + Pd98

k1PdH2 + Dn (10)

PdH2 + R2CdCR2 98k2R2CH-CHR2 + Pd (11)

d[PdH2]dt

) k1(1 - θ)[H2Dn] - k2θ[R2CdCR2] ) 0

(12)

θ )[PdH2]

[Pd](13)

θ )k1[H2D

n]

k1[H2Dn] + k2[R2CdCR2]

(14)

Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 5243

The rate of surface reaction can therefore be writtenin the form of eq 15:

Because of the high concentration of hydrogen donorand low hydrogenation rates,

Equation 15 can be therefore simplified into

The first-order relationship is valid for all hydrogena-tion reactions considered. The linear relationship be-tween the catalyst concentration and the reaction rateis also in agreement with the assumption made inderiving eq 16.Hydrogenated Products. The compositions of un-

treated and partly hydrogenated soybean lecithin arepresented together with the reaction rate constants andsaturation selectivity (Table 1). A high degree ofhydrogenation of the linolenic and linoleic acids can beobserved, whereas the distribution of phospholipidclasses remains practically unchanged. This is anexplanation as to why the emulsifying properties ofhydrogenated lecithin are unchanged (Holman andMahfouz, 1989).

Conclusion

Catalytic transfer hydrogenation of the examinedsoybean lecithin in a solution of sodium formate followsthe simplified Bailey’s model of the first-order kineticreaction with respect to the fatty acid compound whenthe kinetic regime is achieved by intensive stirring.During the catalytic transfer hydrogenation, the high

degree of hydrogenation of linolenic and linoleic acidscan be observed, whereas the distribution of phospho-lipid classes remains practically unchanged.Catalytic transfer hydrogenation using a solution of

sodium formate as the hydrogen donor is a simple and

useful alternative for the hydrogenation of lecithinbecause this method offers safe and easy handling.

Acknowledgment

This investigation was supported by the Ministry ofScience and Technology of Slovenia (J2-7242-0104-95).We thank Dr. Joze Kobe and Dr. Matjaz Kranjc forvaluable advice.

Nomenclature

θ ) surface coveragek1 ) adsorption rate constant (min-1)k2 ) reaction rate constant (min-1)k2* ) overall reaction rate constant (min-1)A ) hydrogen acceptorD ) hydrogen donorLe ) linolenic acidL ) linoleic acidOl ) oleic acidS ) stearic acidkLe ) 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 selectivitySL ) linoleic saturation selectivity[PdH2] ) catalyst surface covered with hydrogen (m2/m3)[Pd] ) all catalyst surface available (m2/m3)

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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 theQuantitative Determination of Active Compound Enclosed inLiposomal System. Int. J. Cosmet. Sci. 1994, 183.

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Figure 8. Effect of temperature. Activation energies for fattyacids 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, 250mg of 10% Pd/C, 600 rpm.

r )k1k2[R2CdCR2][H2D

n]

k1[H2Dn] + k2[R2CdCR2]

(15)

k1[H2Dn] . k2[R2CdCR2]

r ) k2*[R2CdCR2] (16)

5244 Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997

Smidovnik, A.; Plazl, I.; Koloini, T. Kinetics of Catalytic TransferHydrogenation of Soybean Oil. Chem. Eng. J. 1993, 51, B51-B56.

Smidovnik, A.; Kobe, J.; Leskovsek, S.; Koloini, T. Kinetics ofCatalytic Transfer Hydrogenation of Soybean Oil. J. Am. OilChem. Soc. 1994, 71, 507.

Van Nieuwenhuyzen, W. Lecithin Production and Properties. J.Am. Oil Chem. Soc. 1976, 53, 425.

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Received for review February 7, 1997Revised manuscript received August 12, 1997

Accepted August 12, 1997X

IE970135M

X Abstract published in Advance ACS Abstracts,October 15,1997.

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