Heterogeneously Catalytic Isomerization of Linoleic Acid over Supported Ruthenium Catalysts for Production of Anticarcinogenic Food Constituents Andreas Bernas, † Pa 1 ivi Ma 1 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,trans- 11 and trans-10,cis-12 positional and geometric conjugated dienoic isomers of linoleic acid (cis- 9,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/Al 2 O 3 catalysts in a diluted system was studied in the temperature range 76- 165 °C. Catalyst characterization was done by X-ray powder diffraction, hydrogen temperature- programmed desorption (H 2 -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/Al 2 O 3 , 3.27 × 10 -4 and 1.53 × 10 -4 s -1 , respectively, and H 2 -TPD measurements indicated that the former had a higher hydrogen storage capacity than the latter. Ru/Al 2 O 3 , on the other hand, exhibited a higher selectivity for the cis- 9,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 m 2 /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 or- ganic reactions, which used to be performed under homogeneous conditions. Among such reactions, isomer- ization 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 (C 18:2 ) 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 been identified by 13 C 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 C 18 diene acids. Conjugated dienoic isomers of linoleic acid are natu- rally 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 80- 90% 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 de- scribes 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 indi- vidual isomers of CLA act differently. Several studies have demonstrated that the cis-9,trans-11-CLA isomer * To whom correspondence should be addressed. E-mail: dmurzin@abo.fi. Telephone: +358 2 215 4985. Fax: +358 2 215 4479. † Laboratory of Industrial Chemistry. E-mail: abernas@abo.fi. ‡ Laboratory of Forest Products Chemistry. 718 Ind. Eng. Chem. Res. 2003, 42, 718-727 10.1021/ie020642q CCC: $25.00 © 2003 American Chemical Society Published on Web 01/14/2003