CHAPTER 6: Recent Advances in the Production of CLA and Conjugated Vegetable Oils: Production of CLA and Conjugated Vegetable Oils via Metal Catalysis

Conjugated isomerization of free linoleic acid and polyunsaturated vegetable oils can be catalysed by transition metals in homogeneous and heterogeneous systems. The most efficient organometallic complexes for conjugated isomerization of linoleic acid and its esters, and vegetable oils, under mild conditions in homogeneous systems are based on rhodium [H-Rh((p-CH₃C₆H₄)₃P)₂] and ruthenium [Ru(η⁶-naphtalene)(η⁴-cycloocta-l,5-diene)]. They lead to a quantitative yield of conjugated products, where the two main CLA isomers are in configuration (9-cis,11-trans) and (l0-trans,12-cis). Moreover, when Ru and Rh organometallic complexes are dissolved in BMI.NTf2 or NBu4.Br ionic liquids, the products phase is simply collected by decantation. In heterogeneous catalytic systems, conjugated and cis–trans isomerizations of polyunsaturated fats are competing parallel reactions with hydrogenation. Nevertheless, gas–liquid mass transfer limitation conditions occurred at low hydrogen pressure, low stirring rate, high catalyst content, and high reaction temperature favored the production of CLAs. In these operating conditions, Ru/C, Ru/Cs-USY, S-Rh/SBA-15 and Ag/SiO2 exhibited an interesting conjugated isomerization activity. Complexation of the double bond with the metal is the key step in the conjugation. The alkyl mechanism occurs especially with unsaturated metal hydride complexes, and the allylic mechanism is typical of metals that preferably form π-allyl complexes.