2. Mechanism

The mechanism of hydrogenation is thought to be the reaction between unsaturated liquid oil and atomic hydrogen adsorbed onto the metal catalyst surface. In the first place a metal complex is formed at each end of the double bond (a). This complex then reacts with an atom of catalyst-adsorbed hydrogen to form an unstable half-hydrogenated state (b or c) in which the olefin is attached to the catalyst by one link only, permitting it to rotate freely. This can now react with another hydrogen atom and separate itself from the catalyst to yield the saturated product (d) or lose a hydrogen atom to the nickel catalyst in order to restore the double bond. This regenerated double bond may find itself in the same position as in the unhydrogenated compound or in a positional and/or geometric isomer of the original double bond (e, f).

Generally it is accepted that the concentration of the hydrogen adsorbed on the catalyst is the factor that determines selectivity and isomer formation. If the catalyst is hydrogen saturated, most of the active sites hold hydrogen atoms and the probability is higher that two atoms are in position to react with any double bond upon approach. This will nevertheless result in low selectivity, because saturation of any double bond approaching the two hydrogens will ocurr promptly. However, if there are only a few hydrogen atoms adsorbed, it is likelier that only one hydrogen atom react with the double bond, producing the half-hydrogenation-dehydrogenation sequency which increases probability of isomerisation. Hereafter, operating conditions (hydrogen pressure, intensity of agitation, temperature, type and concentration of catalyst) influence selectivity by their effect on the ratio of hydrogen to catalyst sites. For example, an increase in temperature increases the speed of the reaction and produces a faster removal of hydrogen from the catalyst, thus increasing selectivity.

The possibility of being able to change the SR by altering the processing conditions permits processors considerable control over the properties of the final oil. To exemplify, a more selective hydrogenation decreases linoleic acid and improves stability, reducing the formation of fully saturated compounds and preventing excessive hardness. However, a more selective reaction will enhance the formation of trans isomers, which may present concern to those nutritionally concerned. In the past years, the search for a hydrogenation process that minimises isomerisation at the same time as it avoids the formation of excessive amounts of fully saturated material has been a main objective for manufacturers, and circumventing this problem may only be acomplished with resource to total hydrogenation plus interesterification with non-hydrogenated oil.