Heterogeneously Catalytic Isomerization of Linoleic Acid over

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Ind. Eng. Chem. Res. 2003, 42, 718-727

Heterogeneously Catalytic Isomerization of Linoleic Acid over Supported Ruthenium Catalysts for Production of Anticarcinogenic Food Constituents Andreas Bernas,† Pa1 ivi Ma1 ki-Arvela,† Narendra Kumar,† Bjarne Holmbom,‡ Tapio Salmi,† and Dmitry Yu. Murzin*,† Laboratory of Industrial Chemistry and Laboratory of Forest Products Chemistry, Process Chemistry Group, Åbo Akademi University, FIN-20500 Åbo/Turku, Finland

Although research interest on anticarcinogenic and other physiological effects of the cis-9,trans11 and trans-10,cis-12 positional and geometric conjugated dienoic isomers of linoleic acid (cis9,cis-12-octadecadienoic acid) on animals and humans has been growing explosively for the past decade, no strikingly efficient process for conjugated linoleic acid (CLA) synthesis has been reported. After development of a new heterogeneously catalytic pathway for isomerization of linoleic acid, several supported metal catalysts have been screened, and in present paper the conjugation reaction of linoleic acid to cis-9,trans-11- and trans-10,cis-12-CLA over 5 wt % Ru/C and 5 wt % Ru/Al2O3 catalysts in a diluted system was studied in the temperature range 76165 °C. Catalyst characterization was done by X-ray powder diffraction, hydrogen temperatureprogrammed desorption (H2-TPD), and nitrogen adsorption techniques. Reactions taking place were isomerization of linoleic acid to CLA, hydrogenation of linoleic acid and CLA to monounsaturated octadecenoic acids (oleic acid, elaidic acid, and cis- and trans-vaccenic acid), and further hydrogenation of monounsaturated acids to stearic acid (n-octadecanoic acid), with conjugation and hydrogenation being two competing parallel reactions. Rate enhancement was obtained by applying a technique of catalyst preactivation under hydrogen, but increased coverage of hydrogen on the Ru surface also restrained the isomerization selectivity. At similar conditions, Ru/C showed a higher turnover frequency than Ru/Al2O3, 3.27 × 10-4 and 1.53 × 10-4 s-1, respectively, and H2-TPD measurements indicated that the former had a higher hydrogen storage capacity than the latter. Ru/Al2O3, on the other hand, exhibited a higher selectivity for the cis9,trans-11- and trans-10,cis-12-CLA isomers than Ru/C. The selectivities were very sensitive to the reactant to catalyst mass ratio, and experiments over varied particle size ranges indicated that the conjugation reactions were close to the intrinsic kinetic regime. Hydrogenation selectivity and competitive adsorption between reactant and solvent were minimized by the use of nonpolar solvents such as n-nonane and n-decane, whereas protic solvents such as 1-propanol and 1-octanol exhibited lowered selectivity toward CLA. The CLA isomer composition was studied for the use of pure linoleic acid and cis-9,trans-11-, trans-10,cis-12-, and trans-9,trans-11-CLA isomers as reactants. The surface area of Ru/C decreased slightly from 841 to 749 m2/g while repeating the isomerization reaction over the same catalyst sample five times, and the catalytic performance did not indicate any deactivation. Introduction In recent years, there has been increased interest in applying heterogeneous catalysts to conventional organic reactions, which used to be performed under homogeneous conditions. Among such reactions, isomerization of linoleic acid (cis-9,cis-12-octadecadienoic acid) to conjugated linoleic acid (CLA) is found. CLA, first positively identified in 1987,1 is a collective term describing the positional and geometric conjugated dienoic isomers of linoleic acid. Linoleic acid (C18:2) has double bonds located on carbons 9 and 12, both in the cis configuration, whereas CLA has either the cis or trans configuration or both located along the carbon chain. Chemical shifts for 20 different CLA isomers have * To whom correspondence should be addressed. E-mail: [email protected]. Telephone: +358 2 215 4985. Fax: +358 2 215 4479. † Laboratory of Industrial Chemistry. E-mail: [email protected]. ‡ Laboratory of Forest Products Chemistry.

been identified by 13C NMR based on the signals for the four olefinic carbon atoms of CLA. These are cis,cis, trans,trans, cis,trans, and trans,cis isomers of the 7,9, 8,10, 9,11, 10,12, and 11,13 C18 diene acids. Conjugated dienoic isomers of linoleic acid are naturally found in meat and dairy products, especially those from ruminant sources where it is synthesized from linoleic acid by rumen bacteria.2 The major isomer of CLA in milk fat is cis-9,trans-11, which represents 8090% of the total CLA content.3 CLA is of great interest in food and health research. They have been found to affect insulin sensitivity, immunomodulation, and body composition alteration, and they can prevent or cure atherosclerosis and mammary, stomach, skin, colon, and prostate cancer. A significant body of literature describes the physiological effects of CLA on animals and humans.1-9 Recent findings suggest that not only does CLA affect in many different pathways but also individual isomers of CLA act differently. Several studies have demonstrated that the cis-9,trans-11-CLA isomer

10.1021/ie020642q CCC: $25.00 © 2003 American Chemical Society Published on Web 01/14/2003

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is responsible for the anticarcinogenic effects of CLA.1,3 It has also been concluded that the trans-10,cis-12-CLA isomer produces the changes in body composition (increased lean mass and/or reduced adiposity) that have been observed routinely in mice, rats, and pigs.1 Additionally, studies have indicated that the trans-10,cis12 isomer is responsible for the antiatherogenetic effects of CLA.2 CLA has been produced by the reaction of linoleic acid with strong alkali bases using tert-butyl alcohol, dimethyl sulfoxide, dimethylformamide, water, ethylene glycol, propylene glycol, and glycerol as solvents.10-12 Homogeneous catalysts tris(triphenylphosphine)chlororhodium13 and arenechromium carbonyl complexes14 enable lower reaction temperatures than 180-200 °C, which are necessary for noncatalyzed systems, but a disadvantage of alkali isomerization of linoleic acid is the use of excess sodium methoxide11 or strong basic hydroxides of lithium, sodium, or potassium, which are not environmentally friendly and difficult to separate. By the use of a heterogeneous catalyst, which is easy to filter and reuse, environmental problems and separation difficulties can be avoided. The double-bond migration reactions over a supported noble metal catalyst reported in the literature consider the isomerization of methyl linoleate (the ester of linoleic acid) in the temperature range 200-270 °C, which has been performed over Rh/C, Ru/C, and bimetallic Ru-Ni on γ-Al2O3, SiO2Al2O3, C, and MgO. These reactions were carried out in solvents that are supposed to be hydrogentransfer agents.15-17 The reason for this multistep route could be the slower isomerization rate for carboxylic acids.18 The use of linoleic acid as a raw material is a more attractive alternative to the use of methyl linoleate, but reactions with carboxylic acids are often difficult to perform. According to literature,19 geometric isomerization occurs readily on noble metal catalysts, particularly when hydrogen is present, which lowers the reaction temperature and favors cis isomerization. Generally, in the presence of hydrogen, double-bond migration proceeds via half-hydrogenated intermediates.20 Different mechanistic proposals have been advanced, and it has been demonstrated that hydrogen can promote the double-bond migration reaction because it is an astoichiometric component. It is, however, obvious that if double-bond migration occurs in a hydrogen atmosphere, the selectivity toward isomerization products is essentially diminished because of the double-bond hydrogenation reactions. In our previous papers, a new heterogeneously catalytic pathway has been developed for isomerization of linoleic acid to CLA at mild reaction conditions in a diluted system under a nitrogen pressure with hydrogen preadsorbed on the catalyst surface.21,22 It was shown that isomerization and hydrogenation of linoleic acid are two competing parallel reactions and that these reactions are influenced by the concentration of chemisorbed hydrogen. The pathway for the isomerization and hydrogenation of linoleic acid is demonstrated in Figure 1. Linoleic acid does react not only to the cis-9,trans11-, trans-10,cis-12-, and trans-9,trans-11-CLA isomers and oleic acid but also to several other CLA isomers and other monounsaturated fatty acid isomers (elaidic acid and cis- and trans-vaccenic acid), and CLA isomers also undergo hydrogenation to monoenoic species. Further, isomerization of monoenoic acids occurs. Isomerization experiments under hydrogen resulted in hydrogenated

Figure 1. Reaction scheme for isomerization and hydrogenation of linoleic acid in the presence of Ru catalyst.

end products and experiments under nitrogen resulted in low catalytic activity, but the isomerization reaction in a nitrogen atmosphere could be enhanced by catalyst preactivation under hydrogen. The isomerization properties of several supported metal catalysts have been studied. Investigated catalysts were Ru, Ni, Pt, Pd, Rh, Ir, Os, and bimetallic Pt-Rh supported by activated carbon, Al2O3, SiO2, SiO2Al2O3, MCM-22, H-MCM-41, Y, and β. After some catalyst screening experiments, it was concluded that Ru, Ni, and Pt have good properties for isomerization of linoleic acid. It became apparently clear that the activity and the isomerization selectivity were sensitive to the surface structure and hydrogen adsorption capacity of the metal-support combinations. Metals with high hydrogen storage capacities such as Pd showed high activity and high selectivity for the double-bond hydrogenation reaction, whereas most of the other tested metals, especially Ru, Ni, and Pt, favored double-bond migration. Very few catalysts, including Ru/C, Ru/Al2O3, and Ni/H-MCM-41, exhibited feasible conversion. Although the double-bond migration reaction of linoleic acid to CLA can be carried out over a wide range of supported noble metal catalysts, metal-modified acidic zeolites H-Y, H-β, H-MCM-41, or MCM-22 are not convenient materials for CLA synthesis because of their low selectivity toward functional foods and high selectivity toward several nondesired conjugated dienoic isomers, especially trans-9,trans-11-CLA, thus destroying the possible utilization of a potentially efficient process in practice. In contrast to zeolites, increased selectivity toward the desired products trans-10,cis-12CLA and cancer inhibitor cis-9,trans-11-CLA could be achieved by the use of ruthenium on Al2O3 or ruthenium on carbon catalysts. The Ru metals activity for isomerization is due to the fact that it has vacant d orbitals which can interact with π bonds of fatty acids as well as activating an adjacent C-H bond, which is a necessary step for a double-bond migration. In addition, the deactivation of Ru/C was very slow compared to the rapid deactivation of Ni/H-MCM-41. Therefore, the Ru/C and Ru/Al2O3 catalysts were selected for further studies. In the present paper kinetic experiments of the doublebond migration reaction of linoleic acid to CLA are conducted over Ru/C and Ru/Al2O3 catalysts in order to optimize several reaction parameters, such as solvent, reaction temperature, and catalyst surface coverage of hydrogen.

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Method The 5 wt % Ru/C and 5 wt % Ru/Al2O3 catalysts were supplied by Fluka (Switzerland) and Johnson Matthey (United Kingdom), respectively. Linoleic acid (C18H32O2) of 99% purity was supplied by Fluka (Switzerland). The trans-10,cis-12-, cis-9,trans-11-, cis-11,trans-13-, cis9,cis-11-, and trans-9,trans-11-CLA isomers (C18H32O2) of high purity were supplied by Matreya, Inc. The solvents 95% pure n-decane (C10H22) and 99.5% isooctane (C8H18) were supplied by Merck (Germany). 99.5% 1-octanol (C8H18O), 99% cyclohexane (C6H12), 99% 1-propanol (C3H8O), and 99% n-nonane (C9H20) were supplied by Sigma-Aldrich (Germany), Lab-Scan (Ireland), J. T. Baker (Holland), and Fluka (Switzerland), respectively. X-ray powder diffraction (XRD) measurements on Ru/C were performed using a diffractometer (Philips PW 1800) to determine the Ru crystallite size. The measuring conditions were as follows: generator voltage, 50 kV; generator current, 40 mA; Cu tube anode, automatic divergence slit; irradiated sample length, 12 mm; receiving slit, 0.2 mm; step size, 0.025°; counting time, 5.0 s/step. Hydrogen storage capacities of the Ru/C and Ru/Al2O3 catalysts were investigated by performing hydrogen temperature-programmed desorption (H2-TPD) measurements using a volumetric flow apparatus (Autochem 2910, Micrometrics) with nitrogen-argon (N2 99.5%, Ar 0.5%, AGA) as a carrier gas. In the H2-TPD measurements, the Ru catalysts were heated in situ from room temperature to the H2 adsorption temperature 100 °C with a heating rate of 10 °C/min under a 50 mL/min 100% hydrogen flow (AGA), kept at 100 °C for 1 h, and cooled to 40 °C with a rate of 3 °C/min under flowing hydrogen. Thereafter, the Ru catalysts were flushed with 50 mL/min N2/Ar for 45 min. The H2 desorption heating rate was 10 °C/min from 40 to 650 °C and the catalysts were kept at 650 °C for 30 min. The analyses of the desorbed gases were carried out continuously with a quadrupole mass spectrometer (QTMD, Carlo Erba Instruments). For quantitative measurements, the mass spectrometer was calibrated for the hydrogen signal. The pore size distributions and the Brunauer-Emmett-Teller (BET) specific surface areas of the carbon and Al2O3 support materials were measured with an automatic physisorption-chemisorption apparatus (Sorptometer 1900, Carlo Erba Instruments), and the catalyst particle size distributions were measured with sieves. In the catalytic linoleic acid isomerization experiments, the catalyst was charged into a 200 mL stirred glass batch reactor operating at atmospheric pressure. The reactor was provided with a reflux condenser system and a heating jacket using silicone oil as a heattransfer fluid. In a typical experiment, the catalyst was preactivated in situ for 1 h (including heating time) at the activation temperature 100 or 180 °C under 100 mL/ min flowing 100% H2 (AGA). Isomerization experiments were also conducted without H2 preactivation. Linoleic acid was mixed with 70 mL of a solvent in a glass tube. n-Decane, 1-octanol, cyclohexane, 1-propanol, isooctane, and n-nonane were tested to evaluate the effect of the adsorption strength of the solvent. The initial linoleic acid concentration and the ratio between the linoleic acid mass and the catalyst mass were typically 0.010 mol/dm3 and 1, respectively, but different solvent concentration and reactant to catalyst mass ratio were also tried by varying the reactant and catalyst masses. Air

above the reaction mixture and oxygen dissolved in the liquid phase were purged out by a 100% N2 (AGA) flow of 100 mL/min through the reactant solution. A N2 flow of the same flow rate was also fed through the reactor to get an inert atmosphere while adjusting the reactor to the reaction temperature. The catalytic experiments were carried out in a temperature range of 76-165 °C, and in some cases the reaction temperature was close to the boiling point of the solvent. The liquid phase containing linoleic acid and the solvent was fed into the reactor after a purging time of 20 min, and the reaction time was initialized to zero. A 100 mL/min N2 flow was fed through the reactor during the experiment in order to have an inert atmosphere, and the outlet was locked by a fluid to prevent oxygen from diffusing into the reactor. The temperature of the reflux condensers cooling medium was set to -20 °C. Stirring baffles were used inside the reactor, and the diluted system was stirred at 800 rpm in all experiments to keep the catalyst uniformly dispersed in the reaction medium and also to eliminate external mass-transfer effects. For calculation of the concentration versus time dependence, samples were withdrawn from the reactor at certain intervals through a sampling valve. The isomerization reaction was tested at different ranges of particle sizes of the Ru/C catalyst to determine the presence of limitations by internal diffusion. The CLA isomer composition was studied for the use of pure linoleic acid and cis-9,trans-11-, trans-10,cis-12-, and trans-9,trans11-CLA isomers as reactants. The deactivation of the Ru/C catalyst was investigated by performing consecutive experiments over the same catalyst sample six times. After each isomerization experiment, the catalyst was separated from the fatty acid solution by vacuum filtration, washed with acetone in a stirred tank, dried overnight at 100 °C, and rereduced at 400 °C under 100 mL/min flowing 100% H2 (AGA). The samples from the batch reactor were silylated and analyzed by a temperature-programmed gas chromatograph (GC) using a 25 m HP-5 column (inner diameter, 0.20 mm; film layer, 0.11 µm) and a flame ionization detector operating at 290 °C. In the silylation procedure, 10 µL of samples from the reactor was added into glass tubes that had been washed with methyl tert-butyl ether (MTBE, C5H12O). A total of 50 µL of a 0.5 mg/mL C17:0 fatty acid solution was added to each sample to serve as an internal standard for GC. MTBE and solvent were evaporated in a stream of nitrogen with water as the heating medium, and the samples were further dried in a vacuum desiccator at 40 °C for 20 min. The samples were dissolved in 20 µL of pyridine. To the residue were added 80 µL of N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) of 98% purity and 40 µL of trimethylchlorosilane (TMCS) of 98% purity, both supplied by Acros Organics. The solutions were kept in an oven at 70 °C for 45 min and were thereafter ready for analysis by GC. The evaporation and silylation operations were conveniently performed with the entire series of samples from one isomerization reaction in parallel in order to eliminate the effects of decomposition. Samples of 1 µL were injected into the GC with an autosampler unit using the injector temperature 260 °C. Helium served as a carrier gas with a flow rate of 0.9 mL/min and a split ratio of 1:20. The column temperature was initially 150 °C. A total of 0.5 min after the injection, the temperature was increased from 150 to 230 °C with a rate of 7 °C/min and from 230 to 290 °C with a rate of


Figure 2. H2-TPD pattern of (I) 5 wt % Ru/C and (II) 5 wt % Ru/Al2O2. Table 1. Pore Size Distribution of 5 wt % Ru/C and 5 wt % Ru/Al2O3 Catalysts relative volume (%) pore ranges (nm) 0-0.9 0.9-1.0 1.0-1.5 1.5-2.0 2.0-5.0 5.0-10.0 10.0-100.0

5 wt % Ru/C

5 wt % Ru/Al2O3

0 2.6 5.9 4.2 20.8 18.1 48.4

0 0 0 0 3.2 25.0 50.6

10 °C/min. Thereafter, the column was purged at 290 °C for 10 min. Peaks corresponding to conjugated dienoic isomers of linoleic acid were identified by injecting the authentic samples. Peak identities were verified by analysis with a GC-mass spectrometer (GC/ MS) applying the same GC conditions.

hydrogen, especially for Ru/Al2O3. A hydrogen molecule in the gas phase is first dissociated on the Ru surface to atomic hydrogen and then migrates onto the surface of adjacent support particles. Influence of Internal Diffusion on the Isomerization Rate. Activity and selectivity data on the investigated catalysts are reported in Table 2. In our previous study,21 the effect of external mass transfer on the isomerization rate was determined to be negligible. The isomerization reaction was carried out at stirring rates 400, 600, 800, and 1000 rpm, otherwise using the same reaction conditions. It was clear from the results that the diluted system is well mixed, and it was obvious that the isomerization experiments are performed on the plateau of the initial rate against the stirring rate. Rate limitation from resistance of internal diffusion of reagent and products was investigated by performing isomerization experiments at 165 °C in n-decane over 200 mg of hydrogen-preactivated Ru/C catalyst samples with different particle diameter intervals. Conversion and the initial rate decreased from 48% and 2.78 × 10-7 mol s-1 g-1 to 26% and 1.19 × 10-7 mol s-1 g-1, respectively, as the catalyst particle diameter was increased from the 0-45 µm interval to over 180 µm, otherwise using similar conditions (Table 2, entries 1-6), indicating the presence of rate suppression by internal diffusion for bigger catalyst particles. As the particle diameter decreases toward smaller sizes, the initial rate is first inversely proportional to the particle size and thereafter the rate approaches a constant value and a plateau of the initial rate against particle diameter, the chemical regime, appears.23 The corresponding catalyst particle size can be determined by calculation of the internal effectiveness factor η, defined as the ratio between the actual rate influenced by internal diffusion resistance and intrinsic kinetic rate, for a pseudo-firstorder reaction with respect to linoleic acid in a spherical catalyst pellet, which is related to the Thiele modulus φ by

Results and Discussion Characterization of Ru/C and Ru/Al2O3 Catalysts. The BET specific surface areas of the 5 wt % Ru/C and 5 wt % Ru/Al2O3 catalysts were determined to be 841 and 156 m2/g, respectively. The pore size distributions are given in Table 1. The supports are porous materials with the majority of the pore sizes in the range 10-100 nm, and Ru/C also has a small relative pore volume fraction of less than 2 nm. The catalyst powders consisted of particles smaller than 200 µm. The catalyst particle sizes of Ru/C were distributed around 90 µm, and the finest particles had a diameter of less than 45 µm. The Ru crystallite size of the Ru/C catalyst was determined to be 50 nm, as measured by XRD, indicating that most of the Ru crystallites are located on the outside of the micropores. Ru/C had a higher H2 storage capacity than Ru/Al2O3. H2-TPD patterns of the carbon and aluminum oxide supported Ru catalysts are shown in Figure 2. For both the Ru/C and Ru/Al2O3 catalysts, as much as 90% of the total amount of chemisorbed hydrogen remained on the catalyst surface after an increase of the temperature to 100 °C, which was used as a typical preactivation temperature. The hydrogen amounts left on the catalyst surface after a temperature increase to 180 °C was 50% for the Ru/C catalyst. Both catalysts exhibited a maximum H2 desorption rate at 125 °C. The TPD patterns shown in Figure 2 indicate the presence of spillover

3 (φ coth φ - 1) φ2

(1)

φ ) Rxk/De

(2)

η) where

where R, k, and De denote the particle radius, kinetic constant, and effective diffusivity, respectively. Because φ is proportional to R for fixed other conditions (surface kinetics, De, etc.), one compares the observed rates for two particle sizes, denoted by subscripts 1 and 2.

φ2 R2 ) φ 1 R1

(3)

After a combination of eqs 1-3, one arrives at the following expression:

-r1R12 2

-r2R2

)

φ1 coth φ1 - 1 φ2 coth φ2 - 1

(4)

After eq 4 is solved with respect to a Thiele modulus, corresponding effectiveness factors are obtained from eq 1, and particle sizes for other effectiveness factors can be obtained similarly by using eq 1 and the radius ratio given by eq 3. The intrinsic kinetic rate was

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Table 2. Linoleic Acid Isomerization Activity Data on Ru/C and Ru/Al2O3 Catalysts for Varied Solvent, Hydrogen-Preactivation Temperature, Reaction Temperature, Catalyst Quantity, Catalyst Particle Size, and Solvent Concentrationa

N

catalyst

solvent

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

Ru/C Ru/C Ru/C Ru/C Ru/C Ru/C Ru/C Ru/C Ru/C Ru/C Ru/C Ru/C Ru/C Ru/C Ru/C Ru/C Ru/C Ru/C Ru/C Ru/C Ru/C Ru/C Ru/C Ru/C Ru/Al2O3 Ru/Al2O3 Ru/Al2O3

n-decane n-decane n-decane n-decane n-decane n-decane cyclohexane 1-propanol isooctane n-nonane 1-octanol 1-octanol 1-octanol n-decane n-decane n-decane n-decane n-decane n-decane n-decane n-decane n-decane n-decane n-decane n-decane n-decane n-decane

H2 ads. reaction catalyst temp temp quantity (°C) (°C) (mg) 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 180 180 100 180 180 100 100 100 100

165 165 165 165 165 165 76 92 95 145 100 120 120 120 120 145 165 120 165 165 120 165 165 165 120 165 165

200 200 200 200 200 200 200 200 200 200 50 50 200 200 200 200 200 200 200 400 400 400 200 50 200 200 400

particle size interval (µm) 0-45 45-63 63-71 90-125 125-180 >180 0-180 0-180 0-180 0-180 0-180 0-180 0-180 0-180 0-180 0-180 0-180 0-180 0-180 0-180 0-180 0-180 0-180 0-180 0-180 0-180 0-180

initial LA concn conv conj hydr (mol dm-3) (%) (%) (%) 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.0025 0.01 0.01 0.01

48 43 41 44 33 26 13 59 40 31 28 38 75 72 11 33 49 48 49 63 73 66 45 34 44 31 45

42 38 36 38 30 23 8 24 18 28 18 28 33 47 10 30 42 35 41 55 51 57 42 31 29 28 40

6 5 5 6 4 3 5 35 22 3 9 10 42 24 1 3 7 13 8 8 22 8 3 3 16 3 5

SI

SH

SD

0.87 0.88 0.89 0.87 0.89 0.89 0.65 0.41 0.45 0.90 0.66 0.73 0.43 0.66 0.89 0.91 0.85 0.73 0.83 0.87 0.70 0.87 0.94 0.91 0.65 0.89 0.89

0.13 0.12 0.11 0.13 0.11 0.11 0.35 0.59 0.55 0.10 0.34 0.27 0.57 0.34 0.11 0.09 0.15 0.27 0.17 0.13 0.30 0.13 0.06 0.09 0.35 0.11 0.11

0.35 0.36 0.38 0.36 0.43 0.45 0.23 0.04 0.12 0.56 0.26 0.27 0.06 0.24 0.64 0.57 0.37 0.26 0.29 0.29 0.15 0.27 0.52 0.50 0.28 0.58 0.56

initial overall reaction rate (×10-7 mol s-1 g-1) 2.78 1.88 1.78 1.98 1.39 1.19 2.97 3.67 4.36 5.15 4.16 4.06 0.30 2.18 3.76 3.07 1.83 3.12 1.83 0.99 1.29 3.27 1.29 1.14

a Conditions: raw material, linoleic acid; solvent quantity, 70 mL; catalyst metal loading, 5 wt %; hydrogen adsorption time, 1 h; reaction time for Ru/C, 3 h; reaction time for Ru/Al2O3, 6 h; reaction pressure, 1 atm of nitrogen; stirring rate, 800 rpm.

Figure 3. Evaluation of diffusional and intrinsic chemical reaction kinetic regimes with the catalyst particle size. The solid line is calculated from eqs 1 and 4.

determined to be 2.87 × 10-7 mol s-1 g-1. The average particle diameter of the Ru/C catalyst was 90 µm, and the initial rate for the unsieved Ru/C catalyst was 2.18 × 10-7 mol s-1 g-1 (Table 2, entry 17). Hence, the internal effectiveness factor for unsieved Ru/C is η ) 0.76. The dependence of the initial rate on the particle diameter is presented in Figure 3. The effectiveness factor was equal to 0.95 for particle diameters of 25 µm, but although retardation by internal diffusion takes place, the overall isomerization selectivity and the selectivity toward the desired isomers are not affected (Table 2, entries 1-6). Influence of Solvent on Catalytic Activity. The catalytic isomerization reaction was tested at similar

conditions in six solvents from varied groups of organic compounds in order to evaluate the effect of the physiochemical properties of the solvent, such as polarity, adsorption strength, and solvation of linoleic acid. For cyclohexane, 1-propanol, isooctane, n-nonane, and ndecane, the isomerization reaction was carried out close to the boiling point of the solvent. When the reaction was conducted at 76 °C in a nitrogen atmosphere over 200 mg of 5 wt% Ru/C preactivated at 100 °C for 1 h under hydrogen using 200 mg of linoleic acid as the reactant and 70 mL of cyclohexane as a solvent, the total converted amount of linoleic acid at 3 h was only 13% (Table 2, entry 7). Here, the reason for the low conversion is the lowered reaction temperature, but anyhow the effects of quite low solvent polarity on the product selectivities can be observed. The selectivity toward double-bond migration was 65% and the selectivity toward double-bond hydrogenation was 35%, including both monounsaturated acids and stearic acid. The selectivity for the cis-9,trans-11- and trans-10,cis-12CLA isomers (desired products) was 23%. Polymerization, cracking, and geometric or skeletal isomerization of fatty acids were not observed at these conditions. No other reactions were detected except for isomerization of linoleic acid to CLA and hydrogenation of C18:2 fatty acids to C18:1 and C18:0 fatty acids. Hence, in the following discussion, the term isomerization refers to double-bond migration to form a conjugated system and the sum of the isomerization and hydrogenation selectivities is 1. When the reaction, on the other hand, was carried out in solvents with higher polarities such as alcohols 1-propanol and 1-octanol, otherwise using the same reaction conditions, the isomerization selectivity de-


creased dramatically. Both alcohols showed approximately 40% isomerization selectivity (Table 2, entries 8 and 13), and the reaction temperatures of 92 and 120 °C for 1-propanol and 1-octanol, respectively, did increase the conversion. The conversion was 59% for 1-propanol and 75% for 1-octanol. For these protic alcohols, the selectivity toward the desired isomers was only 4-6%. Experiments at 95 °C using slightly protic isooctane as a solvent resulted in slightly higher isomerization selectivity than that for 1-propanol. The conversion and isomerization selectivity were 40% and 45%, respectively (Table 2, entry 9) and the selectivity toward the desired CLA isomers was 12%, which is higher than that for 1-propanol and 1-octanol. It appears that the effect observed while using different solvents may be manifested by a dependence of the rates on the adsorption strength of the solvent. The reason for decreased conversion and increased isomerization selectivity by the use of isooctane instead of 1-propanol is competitive adsorption of the solvent. When the reaction is conducted in 1-propanol, the fractional coverage of linoleic acid on the Ru surface will be lower than that when isooctane is used as a solvent. This, in turn, will lead to a lower coverage of a key intermediate on the catalyst surface, and a low ratio between intermediate coverage and hydrogen coverage favors the double-bond hydrogenation reaction. It can be concluded that a nonpolar solvent is preferred for increased isomerization selectivity and the conversion can be increased by the use of high boiling solvents, which allow a higher reaction temperature. Isomerization experiments at 145 °C in nonpolar highboiling solvents such as n-nonane and n-decane resulted in identical catalytic activity. For both solvents, the conversion and isomerization selectivity were 30% and 90%, respectively, and the selectivity toward the desired CLA isomers was 56-57% (Table 2, entries 10 and 16). Therefore, n-decane was selected as a solvent for further investigations. Effect of Catalyst Preactivation under Hydrogen. The influence of catalyst preactivation under hydrogen on catalytic activity and the role of H2 coverage on the Ru surface were investigated by performing isomerization reactions with and without preactivation over 200 mg of Ru/C catalyst in n-decane at two different reaction temperature levels, 120 and 165 °C. Experiments at 120 °C resulted in an increase of the 3 h conversion and the initial overall reaction rate (based on total converted amount of linoleic acid after 30 min and catalyst mass) from 11% and 0.30 × 10-7 mol s-1 g-1 to 72% and 4.06 × 10-7 mol s-1 g-1, respectively, and a decrease of the isomerization selectivity from 89% to 66%, when the catalyst was preactivated at 100 °C under hydrogen (Table 2, entries 14 and 15). The selectivity toward the desired isomers decreased from 64% to 24%. The same dramatic increase of conversion and the initial rate by hydrogen preactivation could not be observed when the isomerization reaction was conducted at 165 °C. Here, the initial rate increased from 0.99 × 10-7 to 2.18 × 10-7 mol s-1 g-1 when the catalyst was hydrogen preactivated at 100 °C, but the overall conversion after 3 h increased only slightly from 45% to 49% (Table 2, entries 17 and 23). Also at 165 °C reaction temperature, the isomerization selectivity decreased after a preactivation similar to that at a reaction temperature of 120 °C. The overall isomerization selec-

Figure 4. Influence of catalyst preactivation under hydrogen on the isomerization properties of 5 wt % Ru/C at reaction temperatures (a) 120 °C and (b) 165 °C. Solid line ) hydrogen-preactivated catalyst, dotted line ) no preactivation, 9 ) linoleic acid, 2 ) monoenoic acids and stearic acid, and b ) CLA. Conditions: stirring rate, 800 rpm; solvent, 70 mL of n-decane; raw material, 200 mg of linoleic acid; catalyst quantity, 200 mg; catalyst preactivated at 100 °C for 1 h under hydrogen.

tivity decreased from 94% to 85%, and the selectivity toward desired products decreased from 52% to 37%. It is apparently clear that an increased concentration of chemisorbed hydrogen on the Ru metal surface increases the activity but suppresses the isomerization selectivity. The reason for the more dramatic increase of the activity at a lower reaction temperature is that hydrogen desorbs from the catalyst surface while flushing the catalyst with N2 and heating the reactor to the reaction temperature. By applying the technique of catalyst preactivation under hydrogen, one is balancing on the edge between increased catalytic activity and decreased isomerization selectivity. The observed effect is illustrated in Figure 4. The mole fraction in the figures refers to the concentration of the specific fatty acid compound relative to the total concentration of fatty acids. A typical pattern for these isomerization reactions is that the yields of hydrogenated fatty acids initially grow with reaction time and thereafter reach a stable value. The yields of CLA, in turn, are typically slightly increasing continuously with time, as shown in Figure 4. Hence, the isomerization selectivity increases with conversion and the hydrogenation selectivity decreases with conversion, as demonstrated in Figure 5, indicating

724


Figure 5. Dependence of selectivities on conversion for isomerization of linoleic acid at (a) 120 °C and (b) 165 °C over 5 wt % Ru/C preactivated under hydrogen. O ) selectivity toward doublebond migration, × ) selectivity toward hydrogenation, and + ) selectivity toward cis-9,trans-11- and trans-10,cis-12-CLA isomers (conditions are the same as those in Figure 4).

that the coverage of chemisorbed hydrogen on the catalyst surface affects the isomerization and hydrogenation rates. When the hydrogen coverage on the catalyst surface is high (as in the beginning of the reaction), the linoleic acid hydrogenation consecutively proceeding via monounsaturated acids to stearic acid increases. When, on the other hand, the concentration of chemisorbed hydrogen is low (as at the end of the reaction when most of the preadsorbed hydrogen has been consumed), isomerization is preferred over hydrogenation. For a hydrogen-preactivated catalyst, the overall reaction rate is high in the beginning of the reaction and decreases with the conversion. The consumption rate of linoleic acid is almost zero after a certain period of time, and the concentrations of the fatty acids are reaching approximately constant values. Conversion and selectivities at this point are through a complex relation influenced by the total amount of hydrogen generated on the catalyst surface in the hydrogen-preactivation step, the reaction conditions, availability of active vacant sites on the catalyst surface, and competitive adsorption of solvent and reaction products. When the preadsorbed hydrogen is consumed, the consumption rate of linoleic acid is limited to hydrogen derived from the solvent and reaction pathways in which H2 does not take part as an astoichiometric component.

In essence, the availability of hydrogen and the activation temperature used in the preactivation step influence the hydrogen coverage on the catalyst surface. This, in turn, affects the activity and selectivity pattern. The molecular H2 required for the hydrogenation observed in the experiment presented in Figure 4a is 17.8 mmol/gRu. The H2 quantity chemisorbed on the Ru surface according to monolayer adsorption is 9.89 mmol/ gRu, suggesting that the solvent generates hydrogenchemisorbed sites or that spillover H2 takes part in the reaction. The isomerization reaction of linoleic acid to CLA is believed to take place through several routes. If the first step involves C-H bond cleavage, an allylic intermediate is formed on a supported metal atom or on an acidic site. Subsequent hydrogenation at a different carbon atom results in double-bond migration. If linoleic acid adsorbs molecularly, it forms a π complex on the surface, with a CdC bond coordinated to a Lewis acid site. If Brønsted acid sites are present, protonation of linoleic acid can occur, resulting in a carbenium ion intermediate where the CdC character has been lost. Subsequent loss of a proton from a different carbon atom results in double-bond migration.24 Acid-catalyzed isomerization may also occur on oxidic metal sites. The double-bond migration over a hydrogen-preactivated catalyst is thought to occur via the Horiuti-Polanyi mechanism25 describing hydrogenation and isomerization of olefins. Influence of Reaction Temperature, Reactant to Catalyst Mass Ratio, and Solvent Concentration on Isomerization Selectivity. In our previous study,21 it was shown that identical conversions and selectivities could be obtained either by increasing the catalyst mass or by decreasing the reactant mass by the same factor, which could be expected because the ratio between the amount of chemisorbed hydrogen and the amount of linoleic acid remains constant. The total amount of chemisorbed hydrogen can be manipulated by both reaction temperature and catalyst mass. When the double-bond migration reaction was carried out at 100 °C over 50 mg of Ru/C preactivated at 100 °C for 1 h under hydrogen using 70 mL of 1-octanol as a solvent, the 3 h conversion was 28% and the isomerization selectivity was 66% (Table 2, entry 11). Conversion and isomerization selectivity increased from 28% and 66% to 38% and 73%, respectively, when the reaction temperature was elevated from 100 to 120 °C (Table 2, entry 12), and the initial rate increased from 4.36 × 10-7 to 5.15 × 10-7 mol s-1 g-1. The selectivity toward the desired CLA isomers remained approximately constant at 26-27%. Higher conversion and initial rate can be explained by elevated reaction temperature, and higher isomerization selectivity is caused by lowered hydrogen coverage. The conversion increased further from 38% to 75% when 200 mg of catalyst was used instead of 50 mg, otherwise using the same conditions, and the isomerization selectivity decreased from 73% to 43% because of the overall higher amount of chemisorbed hydrogen. The initial rate decreased slightly from 5.15 × 10-7 to 4.16 × 10-7 mol s-1 g-1 as 50 mg of Ru/C was changed to 200 mg, but the concentration of linoleic acid as a function of the reaction time decreased more rapidly for the higher catalyst quantity (Table 2, entries 12 and 13). The selectivity toward the desired products decreased dramatically from 27% to 6%. In comparison, the reaction was tested at varied reaction temperature and catalyst mass levels using

Ind. Eng. Chem. Res., Vol. 42, No. 4, 2003 725 Table 3. 20 h Fatty Acid Composition Obtained by Using Different Linoleic Acid Isomers as Reactantsa composition (mol %)

catalyst

reactant

linoleic acid

Ru/C Ru/C Ru/C Ru/C Ru/Al2O3

linoleic acid c9,t11-CLA t10,c12-CLA t9,t11-CLA linoleic acid

32.12 0.00 0.00 0.00 37.85

c9,t11-CLA

t10,c12-CLA

c9,c11-CLA

10.37 19.90 8.33 15.77 22.51

8.58 5.63 18.67 4.87 12.56

3.50 4.10 4.14 3.40 2.32

t9,t11-CLA

C18:1 fatty acids

C18:0 stearic acid

other isomers

27.77 39.37 44.13 53.12 13.10

5.17 6.66 7.42 5.61 5.68

0.62 8.41 3.22 2.12 0.91

11.87 15.92 14.08 15.10 5.07

a Conditions: solvent quantity, 70 mL; catalyst metal loading, 5 wt %; hydrogen adsorption time, 1 h; hydrogen adsorption temperature, 100 °C; reaction temperature, 165 °C; reaction pressure, 1 atm of nitrogen; stirring rate, 800 rpm; reactant mass/catalyst mass for Ru/C, 1; reactant mass/catalyst mass for Ru/Al2O3, 0.5.

n-decane as a solvent. When the reaction was carried out over 200 mg of Ru/C preactivated at 100 °C for 1 h under hydrogen using n-decane as a solvent, the same phenomenon as that for 1-octanol could not be observed regarding the initial rate and the conversion when the reaction temperature was elevated. At similar conditions, the overall conversion and the initial rate decreased from 72% and 4.06 × 10-7 mol s-1 g-1 to 49% and 2.18 × 10-7 mol s-1 g-1, respectively, as the reaction temperature was increased from 120 to 165 °C (Table 2, entries 14 and 17). However, the isomerization selectivity increased from 66% to 85%, and the selectivity toward the desired CLA isomers increased from 24% to 37%. The reason for decreased rate and conversion and increased isomerization selectivity is the lower amount of chemisorbed hydrogen on the catalyst surface. It appears that the double-bond migration is no longer diminished by the hydrogenation reaction at this temperature level, which is 40 °C higher than the temperature corresponding to the maximum hydrogen desorption rate indicated by the H2-TPD measurements on Ru/C. The catalyst mass affected the conversion and initial rate similarly as in the case of using 1-octanol as a solvent. The conversion increased from 49% to 63% when 400 mg of catalyst was used instead of 200 mg, and the initial rate decreased slightly from 2.18 × 10-7 to 1.83 × 10-7 mol s-1 g-1, but in this case the overall isomerization selectivity remained approximately constant at 85-87% because of the high reaction temperature and low hydrogen coverage (Table 2, entries 17 and 20). Increased conversion and constant isomerization selectivity did not result in higher yields of cis9,trans-11- and trans-10,cis-12-CLA. As the catalyst mass was increased, the selectivity toward these desired isomers decreased from 37% to 29% because of formation of the trans-9,trans-11-CLA isomer. Formation of trans-9,trans-11-CLA at elevated temperature was also the reason for decreased selectivity toward the desired isomers with conversion, as shown in Figure 5b. When the reaction was conducted at 165 °C over Ru/C using two different catalyst quantity levels, 200 and 400 mg, no changes in conversion or selectivities could be observed as the hydrogen-preactivation temperature was increased from 100 to 180 °C (Table 2, entries 17, 19, 20, and 22), but high preactivation temperature and low reaction temperature resulted in lowered isomerization selectivity (Table 2, entries 18 and 21) because of higher hydrogen coverage. Ru/C is a more active catalyst than Ru/Al2O3 in isomerization of linoleic acid. Experiments at 120 °C reaction temperature in n-decane over 200 mg of 5 wt % Ru/Al2O3 preactivated at 100 °C under H2 resulted in 44% conversion at a reaction time of 6 h, 65% overall isomerization selectivity, and 28% selectivity toward cis-

9,trans-11- and trans-10,cis-12-CLA (Table 2, entry 25). Similar to that for the Ru/C catalyst, the overall conversion and initial rate decreased from 44% and 3.27 × 10-7 mol s-1 g-1 to 31% and 1.29 × 10-7 mol s-1 g-1, respectively, as the reaction temperature was elevated from 120 to 165 °C (Table 2, entry 26), and the overall isomerization selectivity increased from 65% to 89% because of the lower amount of chemisorbed hydrogen. The selectivity for the desired products increased from 28% to 58%. Here, the 3 h turnover frequency was 1.53 × 10-4 s-1. At similar conditions, the corresponding value for Ru/C was 3.27 × 10-4 s-1. The catalyst mass affected the conversion, initial rate, and overall isomerization selectivity similarly as in the case of using Ru/ C. The conversion increased from 31% to 45%, the initial rate decreased slightly from 1.29 × 10-7 to 1.14 × 10-7 mol s-1 g-1, and the isomerization selectivity remained constant at 89% as 200 mg of Ru/Al2O3 was changed to 400 mg. Interestingly, in contrast to the Ru/C catalyst, no decrease in the selectivity toward the cis-9,trans-11and trans-10,cis-12-CLA isomers was observed as 400 mg of Ru/Al2O3 was used instead of 200 mg (Table 2, entry 27). At similar conditions, the conversion and initial rate decreased from 49% and 2.18 × 10-7 mol s-1 g-1 to 34% and 1.29 × 10-7 mol s-1 g-1, respectively, as the linoleic acid mass and the catalyst mass both were decreased simultaneously by a factor of 4, and the isomerization selectivity remained high (Table 2, entries 17 and 24); hence, a rather low solvent concentration might be preferable. When isomerization experiments are repeated over the same catalyst sample and the catalyst is regenerated as described in the Experimental Section, the deactivation of the Ru/C catalyst was found to be almost nonexistent. The BET area decreased only slightly from 841 to 749 m2/g after five uses, and the catalytic performance did not indicate any deactivation. No changes in the pore volumes of Ru/C were observed. CLA Isomer Composition. Several positional and geometric conjugated dienoic isomers are formed as intermediates in the consecutive isomerization of linoleic acid to trans-9,trans-11-CLA because of the geometry of linoleic acid, the cis-9,cis-12 form. The CLA isomer composition was studied for the use of pure linoleic acid and cis-9,trans-11-, trans-10,cis-12-, and trans-9,trans11-CLA isomers as reactants over hydrogen-preactivated Ru/C and Ru/Al2O3 samples. In all cases, the reaction temperature was set to 165 °C in order to diminish the double-bond hydrogenation and the reaction time was 20 h. The reactant to catalyst mass ratios for Ru/C and Ru/Al2O3 were 1:1 and 1:2, respectively. The 20 h fatty acid composition is given in Table 3. trans-9,trans-11-CLA was the dominating isomer in all

726


over Ru/Al2O3. Conversion and selectivity toward the desired isomers after 20 h were 62% and 56%.

Figure 6. Isomerization of linoleic acid over (a) 5 wt % Ru/C and (b) 5 wt % Ru/Al2O3. 9 ) linoleic acid, b ) cis-9,trans-11- and trans-10,cis-12-CLA, [ ) trans-9,trans-11-CLA, O ) other CLA isomers, and 2 ) monoenoic acids and stearic acid. Conditions: reaction temperature, 165 °C; stirring rate, 800 rpm; solvent, 70 mL of n-decane; raw material, 200 mg of linoleic acid; catalyst quantity, (a) 200 mg and (b) 400 mg; catalyst preactivated at 100 °C for 1 h under hydrogen.

experiments over Ru/C. When CLA isomers were used as the reactants, the 20 h sample did not contain any trace of linoleic acid. In contrast to Ru/C, the Ru/Al2O3 catalyst favored formation of cancer inhibitor cis-9,trans11-CLA. A comparison of the isomerization properties of the catalysts is given in Figure 6. When the isomerization reaction was carried out over Ru/C, the concentration of linoleic acid decreased with time until the conversion was 68%. The concentrations of cis-9,trans-11- and trans-10,cis-12-CLA initially increased with time and reached a stable value of 20 mol % after 2 h. The concentration of trans-9,trans-11-CLA increased monotonically to 28 mol %, but this compound started to build up later than the desired isomers as in a typical consecutive reaction; hence, the selectivity toward the desired isomers decreased with time. The selectivity toward the desired isomers after 20 h was 28%. As described earlier, the Ru/Al2O3 catalyst diminishes formation of the trans-9,trans-11-CLA isomer. Although the linoleic acid to catalyst mass ratio was lower than that in the case of Ru/C, the formation rates of all isomers and the selectivities toward all fatty acids were conversion independent as the reaction was conducted

Conclusions The heterogeneously catalyzed double-bond migration reaction of linoleic acid to CLA isomers over hydrogenpreactivated carbon and Al2O3-supported ruthenium catalyst samples was studied in a batch reactor between 76 and 165 °C in several solvents. The reactions taking place were isomerization of linoleic acid to CLA isomers, hydrogenation of linoleic acid and CLA to monoenoic acids (oleic acid, elaidic acid, and cis- and trans-vaccenic acid), and further hydrogenation of monoenoic acids to stearic acid. The competing isomerization and hydrogenation reactions were strongly affected by chemisorbed hydrogen on the Ru surface. The isomerization rate was enhanced by catalyst preactivation under hydrogen, but increased hydrogen coverage on the Ru surface also restrained the isomerization selectivity. For both catalysts, the overall isomerization selectivity as well as the selectivity toward the desired cis-9,trans11- and trans-10,cis-12-CLA isomers increased as the reaction temperature was increased because of the lower amount of chemisorbed hydrogen at higher temperatures. The isomerization and hydrogenation selectivities were very sensitive to the reactant to catalyst mass ratio, and the selectivities were found to be influenced by the solvent used. Nonpolar solvents such as n-nonane and n-decane exhibited higher isomerization selectivity than protic solvents such as 1-propanol and 1-octanol. At similar conditions, Ru/C exhibited a higher initial rate and higher conversion than Ru/Al2O3 because of the higher hydrogen adsorption capacity. Both catalysts showed approximately the same overall isomerization selectivities, but the selectivity toward the cis-9,trans11- and trans-10,cis-12-CLA isomers was always higher for Ru/Al2O3 than that for the Ru/C catalyst. These isomers are obtained as intermediates in the consecutive isomerization of linoleic acid to the trans-9,trans-11-CLA isomer. Retardation by internal diffusion in the Ru/C catalyst particles did not affect the overall isomerization selectivity, and the catalyst did not deactivate (with respect to the overall activity); however, the BET area and the isomerization selectivity had a tendency to decrease slightly as the catalyst was used and regenerated repeatedly. Acknowledgment This work is part of the activities at the Åbo Akademi Process Chemistry Group within the Finnish Centre of Excellence Programme (2000-2005) by the Academy of Finland. Financial support from the Raisio Group Research Foundation is gratefully acknowledged. The authors express their gratitude to Ensio Laine for his contribution to the XRD analyses. Notation SI ) selectivity toward isomerization ) conjugation/total conversion SH ) selectivity toward hydrogenation ) hydrogenation/ total conversion SD ) selectivity toward desired CLA isomers ) mole fraction of c9,t11 and t10,c12 CLA isomers/total conversion

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Received for review August 21, 2002 Revised manuscript received December 3, 2002 Accepted December 4, 2002 IE020642Q

Heterogeneously Catalytic Isomerization of Linoleic Acid over

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