Catalysis surface fragmentation

There are only a few weU-documented examples of catalysis by metal clusters, and not many are to be expected as most metal clusters are fragile and fragment to give metal complexes or aggregate to give metal under reaction conditions (39). However, the metal carbonyl clusters are conceptually important because they form a bridge between catalysts commonly used in solution, ie, transition-metal complexes with single metal atoms, and catalysts commonly used on surfaces, ie, small metal particles or clusters. 

In 2002 Mehnert and co-workers were the first to apply SILP-catalysis to Rh-catalysed hydroformylation [74], They described in detail the preparation of a surface modified silica gel with a covalently anchored ionic liquid fragment (Scheme 7.7). The complex N-3-(3-triethoxysilylpropyl)-4,5-dihydroimidazole was reacted with 1-chlorobutane to give the complex l-butyl-3-(3-triethoxysilylpropyl)- 4,5-dihydroimidazolium chloride. The latter was further treated with either sodium tetrafluoroborate or sodium hexafluorophosphate in acetonitrile to introduce the desired anion. In the immobilisation step, pre-treated silica gel was refluxed with a chloroform solution of the functionalised ionic liquid to undergo a condensation reaction giving the modified support material. Treatment of the obtained monolayer of ionic liquid with additional ionic liquid resulted in a multiple layer of free ionic liquid on the support. 

Heterogeneous catalysis is primarily a molecular phenomenon since chemical bonds are created and/or broken (between the molecule and the surface) this implies that surface organometallic fragments are intermediates in any catalytic reaction on a surface. If one can design and synthesize surface organometallic fragments and study their reachvity, especially elementary steps, then one possesses in principle a crihcal tool to better understand the mechanisms of heterogeneous catalysis. 

The concept of site isolation is important in catalysis. On metal particles one usually assumes that ensembles of metal atoms are necessary to activate bonds and to accommodate the fragments of molecules that tend to dissociate or to recombine. We present here three examples of such effects the dehydrogenation of decane into 1-decene, the dehydrogenation of isobutane into isobutene and the hydrogenolysis of acids or esters into aldehydes and alcohols. In most cases the effect of tin, present as a surface alloy, wiU be to dilute the active sites, reducing thereby the yield of competitive reactions. 

In terms of atomistic description of the phenomenon of catalysis, the border between these two approaches is determined by the rate of attenuation of the electron perturbation in a solid with the distance from an adsorbed molecule as well as by the degree of similarity between electron structures of the whole surface of a solid and of a fragment considered as a model of the adsorption or of an active site. For a long time this problem lacked an unambiguous solution. Therefore both approaches were equally widely used to describe catalytic phenomena, often without proper regard to specific features of the systems considered. Thus, active sites on metal catalyst surfaces were frequently modeled by individual metal atoms. On the contrary, catalysis on insulator oxide surfaces was sometimes discussed in terms of their cooperative electron properties. 

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