Heterogeneous catalytic production of conjugated linoleic acid

Kinetic experiments in selective isomerization of technical grade (similar to55%) linoleic acid to cis-9, trans-11-conjugated linoleic acid (B) and trans-10, cis-12-conjugated linoleic (E) acid isomers were performed batchwise at 165degreesC over two series of supported metal catalysts, i.e., hydrogen preactivated and nonpreactivated. Activated carbon- and aluminium oxide-supported Ru, Pd, Os, Ir, and Pt-Rh catalysts with 5 wt% metal loading were screened. Catalyst characterization was done by X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy-energy-dispersive X-ray analysis (SEM-EDX), temperature-programmed desorption of hydrogen (H-2-TPD), direct current plasma atomic emission spectrometry- (DCP-AES), and nitrogen adsorption techniques. Over such catalysts, the reaction scheme involves six steps: (1) double bond migration of linoleic acid to conjugated linoleic acid, (2) positional and geometric isomerization of conjugated linoleic acid, (3) double bond hydrogenation of linoleic acid to monoenoic acids, (4) double bond hydrogenation of conjugated linoleic acid to monoenoic acids, (5) positional and geometric isomerization of monoenoic acids, and (6) double bond hydrogenation of monoenoic acids to stearic acid. Over Ru/C catalyst, chemisorbed hydrogen on the metal surface dramatically increased the linoleic acid isomerization rate in a diluted system as an astoichiometric component-enhancing double bond migration but decreased the isomerization rate by promoting deactivation in a solvent-free system. Over Ru/Al2O3 catalyst, on the other hand, dissociated hydrogen increased the isomerization rate for both the diluted and nondiluted systems. The effect of eliminating the solvent was an increase of the turnover frequency (TOF) at 165degreesC by a factor of 12 with respect to that shown in a diluted system. At the same conversion carbon- and aluminium oxide-supported Ru catalyst showed higher selectivity toward B and E than carbon- supported Pd, Os, Ir, and Pt-Rh catalysts.