Heterogeneously Catalytic Isomerization of Linoleic Acid over Supported Ruthenium Catalysts for Production of Anticarcinogenic Food Constituents

Abstract

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 , 0.000327 and 0.000153 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.