Kinetic experiments in selective isomerization of technical grade
(∼55%) 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 165 °C 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 (H2-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 165 °C 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.