Gas‐liquid chromatographic analysis of geometrical and positional octadecenoic and octadecadienoic acid isomers produced by catalytic hydrogenation of linoleic acid on Ir/Al2O3

Abstract

AbstractCatalytic hydrogenation of linoleic acid was studied on Ir/Al2O3. A detailed analysis of geometrical and positional isomers of octadecenoic acid (18:1) in the products was performed by capillary gas‐liquid chromatography with a new capillary column coated with isocyanopropyl trisilphenylene siloxane (TC‐70). Well‐resolved peaks of 10 species of 18:1 were observed in the product. In addition to monoenoic acid isomers, four species of trans‐dienoic isomers and conjugated dienoic isomers were found. From the distribution of 18:1 isomers, it was found that the double bond closer to the methyl end (Δ12) showed higher reactivity than that closer to the carboxyl end (Δ9) for hydrogenation. Because cis‐8 18:1 and trans‐8 18:1 were not observed but cis‐10 18:1 and trans‐10 18:1 were observed in the products, the double‐bond Δ9 did not migrate to the carboxyl end but migrated to the methyl end. On the other hand, the Δ12 bond migrated to both methyl and carboxyl ends. From the distribution of 18:1 isomers in the reaction pathway, the hydrogenation of linoleic acid proceeds via half‐hydrogenation states. Cis‐18:1 isomers were produced predominantly in the initial stage of the reaction, while trans‐18:1 isomers were produced during progress of the reaction. The cis/trans and positional isomerization took place by readsorption of 18:1 produced by the partial hydrogenation of linoleic acid.