ABSTRACT
Marinelli et al. (2) have discussed the thermodynamic constraint in selectively hydrogenating the C=O group in cinnamaldehyde in presence of the C=C group, stressing the fact that thermodynamics favors the C=C bond hydrogenation. As a result, they rightly point out that replacing one metal by another will have no substantial effect in selectively hydrogenating the carbonyl group. One of the ways to achieve high C=O hydrogenation selectivity, then, is to manipulate the kinetics of the process. This may be achieved by taking advantage of electronic and steric effects (stemming from some unique properties of some metals or combinations thereof) and also by choosing suitable supports (2, 3). Tuley and Adams (4) were the first to show the beneficial effect of iron chloride (as a co-catalyst) on Pt for increased selectivity for the C=O group in the liquid phase hydrogenation of cinnamaldehyde. Various other workers (5-22) have since confirmed the general phenomenon of enhancement in the C=O bond hydrogenation selectivity by metals like Pt and Ru with other metals like Sn and Fe acting as promoters. Poltarzewski et al. (5) reported that tin oxide, when incorporated into a Pt/Nylon catalyst, increases the C=O bond hydrogenation selectivity. They explained this augmented selectivity as an effect of Lewis acidity imparted to the active metal Pt by Sn. Similar electronic effects on C=O bond hydrogenation selectivity have been found by others using Pt-Fe (6); Pt-Sn (la); PtGe (7b); and Ru-M (8) where M=Sn,Fe,Zn,Ge and Sb. In the catalysts used by the above workers, enhanced C=O bond hydrogenation selectivity has been attributed to electron transfer from the promoting metals (M) to Pt or Ru. A similar effect of Lewis acidity, imparted by Snx+ to Ru, in Ru-Sn catalysts, has also been reported (9).
