Encaged metal catalysts

Volatile transition metal complexes have been widely used for the production of zeolite-encaged metal particles or organometallic compounds (222,223). The resulting catalysts are active for a wide range of reactions (224) and in some cases are superior to other preparations. For those metals... 

These zeolite-encaged metal complexes are of importance as catalysts, too. From this point of application they possess two particular features each catalytic centre are separated, and the stability of the complex is enhanced (since the zeolite- cage protects the molecule from decomposition). Due to these features the zeolite encaged metal complexes resemble in a certain extent to enzymes, as well, where the catalytic centre might be a transition metal ion, and the stability and steric constraints are provided by the protein. In both systems the complexes of multivalent transition metal ions can catalyze the process of oxygen transfer for mild oxidations. 

These results also illustrate the advantages of using zeolite encaged dusters as precursors for well defined metal catalysts. Some encaged clusters appear to be stable to cycling through oxidation and reduction without forming crystallites on the external zeolite surface. 

Zeolite supported metal clusters are important catalysts. The zeolite matrix not only imposes steric constraints on reacting molecules (shape selective catalysis) and provides addic sites, but it also apparently affects the electronic properties of the encaged metal clusters. 

Zeolites seem to be promising catalysts for the conversion of fine chemicals and organic intermediates [1-3]. Metallo-phthallocyanines encaged in zeolites Y have been proposed as enzyme mimics [4-7]. Zeolites can replace the protein portion of natural enzymes and modify the reactivity in the same way as enzymes do by imposing steric constraints on the environment of the active metal ion site. 

For transition metals, such migrations are accompanied by ligand replacement. The chemistry and catalysis of transition metal complexes in zeolites have been recently reviewed by Lunsford (156). In these catalysts, it can be desirable to replace, for example, ammonia by pyridine. Zeolite-encaged cobalt complexes, for example, exhibit high potential for oxygen activation. 

This principle appears amenable to generalization active sites and catalyst promoters can be positioned in the same cage in order to systematically study catalyst promoter effects due to direct interaction of metal particles and metal ions. Quantum chemical calculations by van Santen et al. have resulted in detailed predictions, e.g., of the effects of Mg ions, that are in direct contact with zeolite-encaged Ir4 tetrahedra, on the adsorption of H2 (i72) or CO 373) on these clusters. These theoretical results should be verified experimentally, as they could form a basis for general predictions on the action of ionic promoters on chemisorbing transition metals. 

For PtCu/NaY, however, most of the coke is presumably not in intimate contact with metal clusters, its oxidation therefore takes place at a higher temperature. Since splll -over of active oxygen from the oxidized metal over finite distance can be excluded, the oxidation of this coke will not be catalyzed by Pt or Cu. In the reduced form of the catalysts the Ft ensembles at the surface of the encaged bimetal particles are diluted with Cu and additional protons of hi Brensted acidity were formed in the reduction of each Cu ion to Cu°. The combination of both phenomena, small Pt ensembles and hl concentration of protons of strong Brensted acidity, results in an Increase of the RE/RO activity ratio as observed and reported(S). It is therefore reasonable to attribute the TPO peak at higher temperature to the coke deposited on the acid sites of the zeolite via carbenium ion formation and polymerization. The results are thus quite similar to those observed and... 

Not only metals but some oxide catalysts are active in diene hydrogenation ZnO modified by Sn(CH3)4 afforded 1-butene in hydrogenation of butadiene at room temperature. Reduced and sulfided moUbdena on alumina catalyst hydrogenated butadiene and cyclohexadiene selectively5 . When the transition metal complex Mo(CO)6 was encapsulated in NaY zeolite cages, it converted //wnv-l, 3-pcnladicnc to cA-2-pentene and 1,4-pentadiene to c -l,3-pentadiene at 150°C °. Cr(CO)3 encaged in LiX or NaX zeolite was efficient and selective in butadiene hydrogenation to cw-2-butene . ... 

Transition metal complexes of phthalocyanine encaged in faujasite type zeolites have been reported as efficient catalysts in the oxidation of alkanes at room temperature and atmospheric pressure [6-13]. These catalysts constitute potential inorganic mimics of remarkable enzymes such as monooxygenase cytochrome P-450 which displays the ultimate in substrate selectivity. In these enzymes the active site is the metal ion and the protein orientates the incoming substrate relative to the active metal center. Zeolites can be used as host lattices of metal complexes [14, 15]. The cavities of the aluminosilicate framework can replace the protein terciary structure of natural enzymes, thus sieving and orientating the substrate in its approach to the active site. Such catalysts are constructed by the so-called ship in a bottle synthesis the metal phthalocyanine complexes are synthesized in situ within the supercages of the zeolite... 

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