Synthetic methods to obtain conjugated linoleic acids (CLAs) by catalysis – A review

Abstract

The addition of synthetic CLA is a possible way in order to compose foods enriched with conjugated linoleic acids (CLAs). The most environmental friendly methods for CLA synthesis are based on microbial biosynthesis. With homogeneous catalysis using organometallic catalysts (Ru and Rh complexes) high (approximately 80%) yields were obtained with high selectivity related to bioactive isomers. The heterogeneous catalysis has the advantage that at the end of the reaction there is no need for a supplementary separation operation or recycling of the catalyst. In heterogeneous process, the maximum yield may even be higher than 90% and the selectivity remains quite high as the reaction conditions are optimized. The substrates for obtaining CLAs are, in general, linoleic acid or alkyl linoleates and the catalysis is acidic. The yield and the selectivity depend on the strength and the type of acidic sites, as well as on the size distribution of the particles. Beside the existing catalytic methods, a photocatalytic process with UV and visible light irradiation with iodine promoter can be applied.Keywords: conjugated linoleic acid, CLA, homogeneous catalysis, heterogeneous catalysis, photocatalytic process Recently many new health-benefi t effects of the conjugated linoleic acids (CLAs) were discovered. They decrease the body fat quantity and increase muscle mass, possess antiinfl ammatory and cancer preventive effects, exert benefi cial effects on the skeletal system, act as immunostimulants, and decrease the probability of asthma occurrence (HA et al., 1989;PARIZA et al., 2001).Theoretically 56 isomers exist, however, few of them (cis(c)-9,trans(t)-11; t-10,c-12-and t-9,t-11 CLA) have been shown to have signifi cant biologic activity. The bioactivity of the c-9,t-11 CLA isomer (Fig. 1) was discovered two decades ago (HA et al., 1989) and this isomer is predominant in the natural products used as foods. The main foods rich in CLA are milk, dairy products, meat and fat of the ruminants, as this isomer is produced by the bacteria of the rumen microfl ora.Nowadays, CLAs are obtained for industrial purpose from vegetable oils that are processed at high temperature (PHILIPPAERTS et al., 2011). The basic method in the industrial approach is the isomerisation of linoleic acid (LA) with basic catalysis by which the amounts of the two formed isomers (c-9,t-11 and t-10,c-12 CLA) are almost equal, and the yield is quite high. This synthetic method cannot be applied for the direct transformation of vegetable oils into CLA-rich triacylglycerols, because ester bonds of triacylglycerols hydrolyze to free fatty acids (or salts) and glycerol.

Revealing ruthenium and basicity synergetic effects in Ru–MgAl catalysts for isomerization of linoleic acid to conjugated linoleic acid

“…This requirement of halfhydrogenated intermediates led to the formation of unwanted hydrogenated by-products. 16 Though the production of CLAs by heterogeneous catalysts without hydrogen is a difficult and complicated process, it can reduce the production of unwanted by-products. Therefore, Bernas et al 17 further developed an isomerization catalyst; they used Ru/C catalyst for the isomerization of linoleic acid without the use of H 2 , but the CLA production rate was only 23 g(CLA) per g(metal) per h. Philippaerts et al 18 described H 2 -free production of CLA using Ruloaded zeolite catalysts with high CLA production rate; however, the process needed negative pressure during the reaction itself and the catalysts synthesis process was complex.…”

Synthetic Transformations for the Valorization of Fatty Acid Derivatives

“…Isomerization of unsaturated fatty acid derivatives may have two different objectives. On the one hand, the isomerization of linoleic acid derivatives to produce conjugated linoleic acids (CLA), 161,162 due to the interesting health benefits claimed for some of the conjugated isomers (Figure 4). The classical method involves reaction under strong basic conditions (KOH) in an organic solvent, such as ethylene glycol, at 180 °C, leading to more than 97% yield of CLA, with more than 92% yield of the two most interesting isomers, c9,t11 and t10,c12 (Figure 4).…”

Synthesis of conjugated linoleic acid by microwave‐assisted alkali isomerization using propylene glycol as solvent

“…In alkali isomerization, the positions of protons are changed along the hydrocarbon chain of LA to conjugated positions of the double bonds, that is, without methylene carbons between them . For this purpose, vegetable oils with a high LA proportion are heated at 180°C with a strong base as sodium or potassium hydroxide and ethylene glycol for a given period of time . However, the main problem in the alkali isomerization process is the difficulty to separate the homogeneous catalysts from the CLA products.…”

“…In contrast to the heterogeneous catalysis, microbiological methods possess a high specificity toward the production of bioactive CLA isomers, mainly toward the cis 9, trans 11‐CLA isomer . Unfortunately, the low conversion of LA to CLA and the long reaction times employed by the microbiological methods make them uncompetitive against alkali isomerization .…”