The hydrogenation of a, -unsaturated aldehydes over modified metal catalysts

4 The hydrogenation of a,/3-unsaturated aldehydes over modified metal catalysts 

The promotion of Pt by the addition of Fe12,13 was followed by a further report indicating that a similar promotion was observed with Cu/chromia.16 These results were a forerunner for many systematic and detailed studies on promoted supported metal catalysts, resulting in some remarkably successful systems. 

Broadly speaking, the most successful catalytic systems have been achieved by use of supported metal catalysts modified using one of two distinct strategies (a) the addition of an electropositive additive or (b) an electronegative additive, which by its action on the adsorbate yields positive centres in the active phase. The principal for both routes is the formation of bonds between the aldehydic O-atom and the positive centre, resulting in electron depletion in the adjacent C-atom, and thereby its activation towards H-atom addition. 

The fact that Pt has a high alkene hydrogenation capability has lead to a considerable amount of research being undertaken on a less active metal, Cu. This, combined with the ability of sulphur to promote the formation of unsaturated alcohol by the formation of (5+ sites on the active surface as described earlier, lead to an in-depth study by Hutchings and Rochester.20-26 Some important points are illustrated in Table 9.2. 

The most important point to remember when considering vapour phase reaction data is that irrespective of the presence of promoters on the copper catalyst, the product distributions vary considerably with time on-stream. For example the first three entries in Table 9.2 show the drastic decrease in 

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