Kinetics and mechanism of the hydrogenation process – the state of the art

It is just over a century ago that the first industrial hydrogenation plant for edible oils was commissioned. Although at that time several fatty acids had been identified, the analytical characterization of these oils was mainly by physical properties like melting point, density and refractive index and chemical properties like saponification value and iodine value. Some of these properties were used to follow a hydrogenation reaction. Early kinetic studies of the hydrogenation reaction focused on triglycerides but with the advent of GC, fatty acid compositions became so readily available, that kinetic studies switched to the ‘common fatty acid pool’ concept. According to this concept, the rate of reaction of an unsaturated fatty acid in a triglyceride molecule does not depend on the chemical nature of the other fatty acid moieties in this molecule. The concept greatly simplified the understanding of what happens during a hydrogenation reaction and thereby stimulated research. It took more than 50 years before this concept was shown to be an incorrect assumption. Kinetic studies of the hydrogenation process have also been hampered by the lack of an instrument that can measure the hydrogen concentration in oil. That may well be the reason why this concentration has received rather little attention and that its effect on the relative rates of the various, simultaneously proceeding reactions has only fairly recently become clear. Accordingly, the present review will describe the mechanism of the various reactions that occur during the hydrogenation process and highlight the effect of the hydrogen concentration and the triglyceride composition rather than that of the fatty acid composition. It will discuss industrial process conditions rather than laboratory conditions and therefore limit itself to nickel catalysts. It will also paint the most simple picture that is consistent with generally accepted observations.