Surface sites multicenter

These results seem to offer a foretaste of possibilities for tailoring multicenter catalytic sites on support surfaces, but this field is still in its infancy. 

For facile reactions on metals the entire catalyst surface or all surface atoms may be presumed to be active. Multicenter sites of a proper configuration (composed of identical atoms) may thus be accessible for reactant molecules to form... 

The use of alumina and silica to promote Diels-Alder reactions was first reported by Hudlicky [24]. The reactions of 1-acetoxy-l,3-butadiene with tetrafluoro-p-benzoquinone or / -benzoquinone were considerably accelerated when they occurred inside the pores of the adsorbents. Several other silica- or alumina-catalyzed Diels-Alder reactions, dealing either with investigation of standard model reactions [25] or of reactions of more synthetic utility, were subsequently described. For example, neutral alumina catalyzed the intramolecular reaction of a tetraene to the verrucarol skeleton [26] (Scheme 3). The catalytic action was observed in an attempt to isolate the tetraene by column chromatography. A variety of Diels-Alder reactions were accelerated by silica or the more basic Florisil (Mg0.Si02) in the absence of solvent [27-30]. Table 3 gives some examples of these reactions, which are thought to be catalyzed via multicentered donor-acceptor interactions of the substrates with the active sites of the surface of the adsorbents. 

The multicentered nature of adsorption on catalysts surfaces was discussed in section 2.5.6 and it was indicated that the number of adsorption sites on which a molecule (especially in organic catalysis) is adsorbed is not equal to unity as is frequently (and tacitly) assumed. Moreover, in some reactions (for example hydrogenation) there could be a different number of adsorption sites for different substances (for hydrogen and say benzene). This results in a competition for sites between participating molecules. 

Design stable multicenter catalytic sites on surfaces (e.g., by use of metal cluster precursors) for bifunctional and promoted catalysts. 

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