Homogeneous catalysts, immobilization zeolites

Table 11.6 adapted from Doraiswamy (2001) provides a partial list of these categories. The main features and applications of these two categories of catalysts are discussed in more detail by Doraiswamy (2001). Of these zeolites, TS-1 catalysts and homogeneous catalysts immobilized on solid supports deserve special mention. 

Another way of immobilizing catalyst complexes might be to trap them in the pores of solid particles, for instance by synthesizing the complex inside the pores of a zeolite ( ship in a bottle ). Another method could be to trap catalyst complexes in porous materials and deposit a membrane at the outer. surface. These methods of immobilizing a homogeneous catalyst do not involve chemical linkage between the catalyst and the carrier. The fixation is the result of steric hindrance. 

Although zeolite-encapsulated metal complexes were known for some time, the principle that such complexes could act as a new type of immobilized homogeneous catalysts was probably first demonstrated only in 1985 [37]. This achievement opened a new and fruitful area of research in immobilizing homogeneous catalysts. These catalysts are named appropriately ship-in-the-bottle (SIB) catalysts (see Fig. 42.5). General overviews of zeolite-encapsulated metal complexes were given [38, 39]. 

POMs can he immobilized onto anion-exchange resins and surface-modified metal oxides with quaternary ammonium cation- or amino-functional groups via anion-exchange. Jacobs and coworkers tethered Venturello s catalyst [P04(W0(02)2)4]3-on a commercially available nitrate-form resin with alkylammonium cations and have carried out the epoxidation of allylic alcohols and terpenes with this supported catalyst [166, 167]. The regio- and diastereoselectivity of the parent homogeneous catalysts were preserved in the supported catalyst. For bulky alkenes, the reactivity of the POM catalyst was superior to that of Ti- 3 zeolite with a large pore size. The catalytic activity of the recycled catalyst was maintained completely after several cycles. 

Abstract Immobilized metallic and bimetallic complexes and clusters on oxide or zeolite supports made from well-defined molecular organometaUic precursors have drawn wide attention because of their novel size-dependent properties and their potential applications for catalysis. It is speculated that nearly molecular supported catalysts may combine the high activity and selectivity of homogenous catalysts with the ease of separation and robustness of operation of heterogeneous catalysts. This chapter is a review of the synthesis and physical characterization of metaUic and bimetallic complexes and clusters supported on metal oxides and zeohtes prepared from organometaUic precursors of well-defined molecularity and stoichiometry. 

New types of mesoporous molecular sieves (their first synthesis opened a new subfield of molecular sieve chemistry) have been prepared over the last ten years by new synthetic approaches, different from those known for zeolites. The variety of the synthetic procedures described and the differences in the textural properties due to different synthetic procedures, as well as to the high temperature treatment, give evidence that mesoporous molecular sieves of different chemical compositions are very interesting materials not only in materials science. They could be important also for the application as heterogeneous catalysis, support for immobilization of homogeneous catalysts, adsorbents or materials for synthesis of new types of inclusion compounds. 

Even so, undesirable effects, such as deactivation and loss of selectivity for heterogeneous catalysts, are unavoidable for some specific reactions. Hence, separation of homogeneous chemical catalysts by membrane retention or immobilization is emerging as a new application field, which in fact demands supports that combine thermal and mechanical stability with chemical resistance under reactive conditions. As a first approach, zeolite membranes can act as molecular filters able to retain the homogeneous catalyst, that is, as a simple separator. MFI zeolite membranes have been applied by Turlan et al. [347] in the Heck reaction to allow the permeation of... 

The example for the immobilization of homogeneous catalysts with the aid of zeolites comes from the area of zeolite-catalyzed oxidation reactions, which have attracted wide attention over the past few years. 

The form and phase of the active catalyst in all of the examples above for liquid-phase carbonylation remain in some doubt. Rates and selectivities appear to be generally similar to the homogeneous system, but there are few comparative data. Convincing evidence for catalyst immobilization under liquid-phase conditions is currently lacking for carbon and zeolite supports. Such evidence could be provided by continuous flow experiments, by direct sampling vmder reaction conditions, or by using in situ techniques such as high pressure infrared spectroscopy in a batch reactor study. 

Silica-supported hydrogenation catalysts show less substrate selectivity than polystyrene-supported catalysts (Michalska and Webster, 1975 Ichikawa, 1976 Sinfelt, 1977). The rates of hydrogenation with silica-supported catalysts are high, although they are still lower than the equivalent homogeneous catalysts. It also appears that, unlike polystyrene, the reactive groups are confined only to the surface of silica. Ruthenium has also been immobilized in a zeolite matrix (Coughlan et al., 1977). 

Various successful attempts to improve the catalyst selectivity have been made by encapsulation of the Mn-tmtacn complex in zeolites [77]. Immobilization of the tria-zacyclononane ligand on an inorganic support resulted in a new class of heterogeneous manganese catalysts with increased epoxidation selectivity [78]. The conversions were usually lower than with the homogenous catalysts [78]. 

Immobilization of chiral complexes in PDMS membranes offers a method for the generation of new chiral catalytic membranes. The heterogenization of the Jacobsen catalyst is difficult because the catalyst loses its enantioselectivity during immobilization on silica or carbon surfaces whereas the encapsulation in zeolites needs large cages. However, the occlusion of this complex in a PDMS matrix was successful.212 The complex is held sterically within the PDMS chains. The Jacobsen catalyst occluded in the membrane has activity and selectivity for the epoxidation of alkenes similar to that of the homogeneous one, but the immobilized catalyst is recyclable and stable. 

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