The sulfur effect on conjugated linoleic acid isomer (CLA) formation during the combined hydrogenation/directed isomerization of safflower oil over a bifunctional (hydrogenation and isomerization) highly structured rhodium-based catalyst (Rh/SBA-15) was investigated either by direct addition of increased concentrations of 3-mercapto-1,2-propanediol to the reaction medium or by doping Rh/SBA-15 with the same sulfur-based compound yielding the sulfur-doped Rh-catalyst (S–Rh/SBA-15). These catalysts exhibited interesting activity, stability, and recyclability.
The maximum CLA contents obtained during the combined reactions with 0, 0.2, 1, 2, 5, and 10 ppm sulfur additions were 73, 99, 131, 110, 105, and 68 mgCLA goil−1, respectively, whereas the amount of harmful trans monoenes remained below 8 %. The safflower oil after partially hydrogenation under the same conditions over S–Rh/SBA-15 catalyst contained up to 110 mgCLA goil−1. These results showed clear evidence of the sulfur promotion effect on CLA formation during the dual hydrogenation/directed isomerization of safflower oil.
A mechanism for the sulfur promotion of the heterogeneous catalyst Rh/SBA-15 for the conjugated isomerization activity during hydrogenation/directed isomerization of safflower oil was determined by solid-state 1H NMR analysis of the fresh and spent catalysts. This was also confirmed by liquid-state 2H NMR analysis of deuterium-labeled product aliquots withdrawn throughout the reaction.
The sulfur promotion towards the double bond conjugation of linoleic acid to form CLA isomers could be explained mechanistically through the preferable formation of the more nucleophilic rhodium sulfide (RhSH) over that of the hydride (RhH). However, both types of Rh clusters constituted distinct catalytic sites leading to the formation of hydrogenation as well as conjugated and geometric isomerization products. The lumped kinetics model described the experimental data well and complied simply with the proposed mechanism.