Abstract
Author could prove that the catalyst species in the hydrogenation by iron pentacarbonyl is a homogeneous one by the, following experimental results that a glycerid and its methylester could be hydrogenated with same rate and selectivity and that the system which contains colloidal iron formed by thermal decomposition of iron pentacarbonyl in saturated fatty acid ester at 250°C in hydrogen atmosphere displayed no catalytic activity.
In view of the fact that monoene was reduced with same initial rate as diene and also the catalyst was decomposed more rapidly in monoene, it seemes reasonable to assume that the π-diene iron tricarbonyl complex is not an essential catalyst type but a stabilized type of catalyst species. In as much as the hydrogen-uptake by diiron enneacarbonyl and triiron dodecacarbonyl began at same temperature as iron pentacarbonyl, the reduction process which proceeds through unstableπ-mono-ene iron tetracarbonyl complex is deniable.
Great value in activation energy of about 60 kcal at 180-200°C suggests that the rate determining step in this hydrogenation is thermal- and partial-decomposition of stable iron pentacarbonyl.
The hydrogenation is strongly obstructed by addition of pyridine or free fatty acid and only isomerzation is accelerated. In the latter case the catalyst was decomposed into iron soap. Obstruction by hydroxyl group was not so remarkable.
Upon discussion of these data, it seemed reasonable to assume that the active catalyst species is a type of Fe (CO)3, which will be then thermally accelerated to takeup hydrogen from proper hydrogen sources, i.e. molecular hydrogen or active methylene group, and acts as iron corbonyl hydrid such as H2Fe (CO)3.
