Site-Isolated Heterogeneous Catalysts

In this chapter, several recent examples of site-isolated heterogeneous catalysts on solid support surfaces are highhghted. The coordination stractures of site-isolated supported metal complexes, their unique catalytic performance, and their 

Heterogeneous Catalystsfor Clean Technology Spectroscopy, Desi, and Monitoring, First Edition. 

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The liquid-phase autoxidation of cyclohexane is carried out in the presence of dissolved cobalt salts. A lot of heterogeneous catalysts were developed for this process but most catalysts lacked stability. The incorporation of cobalt ions in the framework of aluminophosphate and aluminosilicate structures opens perspectives for heterogenization of this process. CoAPO (cobalt aluminophosphate) molecular sieves were found to be active heterogeneous catalysts of this oxidation.133 Site isolation was critical to get active catalysts.134... 

Porphyrines and phthallocyanines suffer from oxidative degradation and oxidative dimerization [68]. The improved activity of the zeolitic systems is due to the effective site isolation within the pores, which prevents any bimolecular pathways to catalyst destruction [63]. Therefore, deactivation is more severe for the homogeneous catalysts than for the heterogenized TMPc. FePc itself is a poor catalyst for alkane oxidation with a high initial turn-over, but after less than 45 minutes it becomes completely inactive. On the contrary, FePc encaged in zeolite Y is stable for 24 hours [63]. 

A number of heterogeneous systems have been developed for oxidation reactions using H2O2 as oxygen source . In 1981, Taramasso, Notari and collaborators at Enichem opened new perspectives in this field with the discovery of the Ti-silicalite (TS-1) ° , a new synthetic zeolite of the ZSM family. In the TS-1 zeolite, titanium atoms are located in vicariant positions in the place of Si atoms in the crystalline framework . The remarkable reactivity of TS-1 is likely ascribable to the site-isolation of tetrahedral Ti(IV) in a hydrophobic environment. TS-1 has proved to be an efficient catalyst for the epoxidation of unfunctionalized short-chain olefins, especially terminal ones (equation 28). In addition, polyunsaturated compounds are mainly converted into the mono epoxides (equation 29). 

Ideally, the active sites should all be identical, and isolated from each other. This is seldom the case, because industrial heterogeneous catalysts are often amorphous, multicomponent and multiphase solids, containing many types of active sites. Nevertheless, there is one important exception zeolites. These highly crystalline materials can be synthesized to exacting specifications, including the setup ofidentical active sites at uniform distances. As we shall see, this is one of the key advantages of zeolites, which are the preferred catalysts today for many industrial processes. 

PCPs with well-defined pores and surface-isolated Lewis acid sites could potentially serve as size- or shape-selective heterogeneous catalysts, in a similar manner to zeolites.33 43 161-164 The two-dimensional PCP, [Cd(4,4 -bpy)2(H20)2] 2N03 4H20 , was the first example that showed catalytic properties for the cyanosilylation of aldehydes.33 Experimental data in the case of cyanosilylation of imines, which is also performed by the same compound, led to the conclusion that hydrophobic grid cavities bind to the substrate very efficiently to promote a rapid reaction, and that the heterogeneous reaction involves the selective activation of the imino nitrogen by the weak Lewis acid Cdn center.161 In this polymer, the NO3" anions exist in a coordination-free state. This situation contributes to increasing the Lewis acidity of the Cdn centers. 

Heterogeneous catalysts for hydrocarbon conversion may require metal sites for hydrogenation-dehydrogenation and acidic sites for isomerisation-cyclisation and these reactions may be more or less susceptible to the effect of carbonaceous overlayers depending on the size of ensembles of surface atoms necessary for the reaction. In reality we must expect species to be transferred and spilled-over between the various types of sites and if this transfer is sufficiently fast then it may affect the overall rate and selectivity observed. If there is spillover of a carbonaceous species [4] then there may be a common coke precursor for the carbonaceous overlayer on the two types of site. Nevertheless, the rate of deactivation of a metal site or an acidic site in isolation may be very different from the situation in which both types of site are present at a microscopic level on the same catalyst surface. The rate at which metal and acid sites deactivate with carbonaceous material may of course not be identical. Indeed metal sites may promote the re-oxidation of a carbonaceous species in TFO at a lower temperature than the acid sites would allow on their own and this may allow differentiation of the carbonaceous species held on the two types of site. 

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