Zeolites encapsulated metal complexe

CoSalen Y carries oxygen as a cargo.72 The catalytic properties of the zeolite-encapsulated metal complexes depend mainly on the complexed metal atoms, which are used usually as oxidation catalysts but other applications are also beginning to emerge. The zeolite-encapsulated catalysts can be regarded as biomimetic oxidation catalysts.73 In liquid-phase oxidation reactions catalyzed... 

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]. 

There has been considerable debate about whether intrazeolitic species are elec-trochemically accessible and recent results on zeolite-encapsulated metal complexes are of interest in this regard. Before we discuss this topic, the role of ion exchange in electrochemical response is discussed. Electroactive transition metal ions can readily be ion exchanged into zeolites. Baker and co-workers reported that a Co +-zeolite Y-ZME resulted in reduction of cobalt ions if Li" was used as the supporting cation in the electrolyte solution, whereas the voltammetric response was absent in the presence of the larger tetrabutylammonium ion (TBA) [166]. Figure 32 compares the voltammetric data for Li+ and TBA. There are two ways to interpret these data ... 

Nuetral templates may not be bound to the zeolite surface but simply trapped. This is the basis of our zeolite synthesis method for the preparation of ship-in-a-bottle metal complexes, ie crystallization of the zeolite around a metal chelate complex. Zeolite encapsulated metal complexes have many applications, ranging from shape selective catalysis [9] to magnetic resonance imaging contrast agents [10]. 

Balkus KJ Jr, Gabrielov AG (1995) Zeolite encapsulated metal complexes. In Herron N, Corbin DR (eds) Inclusion chemistry with zeolites nanoscale materials by design. Kluwer, Dordrecht, p 159... 

A considerable amount of research has been devoted to the study of zeolite-encapsulated metal complexes. Zeolites are crystalline aluminosilicates with typical pore diameters varying between 0.4 and 1.4nm. Due to the confined space of the zeolite cavity the selectivity is improved by diffusion. It has been found that the regioselectivities improve over those obtained with typical homogeneous catalysts but the activities are lower than in homogeneous systems. 

The two oxygen-activating complexes [Co(L)j [L = salophen, tetra-tert-butylsalo-phen (55)] have been prepared and were also synthesized within dehydrated zeolite NaY using the intrazeolite ligand synthesis method [164]. These encapsulated metal complexes were shown to be capable of oxidizing hydroquinone and so were then used in a triple catalytic system to mediate the palladium-catalyzed aerobic 1,4-diacetoxylation of 1,3-dienes (Figure 5.28) [165]. The catalytic system involved [Pd(OAc)2], hydroquinone and the [Co(salophen)] complex in acetic acid (Co Pd diene hydroquinone LiOAc = 1 2.23 50 8.3 690, acetic acid, 25 °C,... 

A recent elegant example of the tailoring the chemical properties of encapsulated metal complexes is the work of Balkus etal. who prepared and studied perfluorinated phthalocyanine complexes of Fe, Co, Cu and Ru (Scheme 25)[230] in NaX. Perfluorinating the complexes enhances the stability and catalytic activity of the catalysts in the oxyfiinctionalisation of light alkanes. The rapid deactivation of the catalysts based on Fe, Co and Cu Fj Pc complexes was overcome by using Ru as the metal center. Similar catalysts, i.e.,Co-phthalocyanine (Co-Pc) encapsulated in zeolite Y, are active catalysts for cyclohexene and 1-hexene epoxidation (Scheme 27)[231]. Comparison of the activity of free and encapsulated Co-Pc has shown that the interaction with the zeolite stabilizes the complex. Co-Pc is still active after 24 hrs reaction whereas the free complex in solution is virtually inactive after 15 minutes. 

The capture of metal complexes is achieved in the synthesis of clusters within the porous network of zeolites, where the reactants are small enough to enter the large cavities, but the clusters formed are too large to escape ( ship- in-the-bottle synthesis). The cages limit the size of the cluster compounds that can be formed and the entrance to the porous channels prevents the departure from the cages. Other methods of encapsulating metal complexes utilize polymerization or polycondensation reactions such as the sol-gel process. The metal complex is dissolved in the medium to be polymerized and is therefore trapped in the matrix formed [93] (cf. Section 3.2.2). The limitations clearly arise from the porosity of the polymer formed. A pore structure with pores that are too wide cannot prevent the leaching of the complex, whereas a pore diameter that is too small results in mass-transfer limitations. 

The most important class of solid-state enzyme mimics is based on zeolites. Zeolites are solid materials composed of Si04 or AIO4 tetrahedra linked at their corners, affording a three-dimensional network with small pores of molecular dimensions. They possess a unique feature of a strictly uniform pore diameter. In particular, zeolites with encapsulated metal complexes are used as inimics of cytochrome P-450.An efficient enzyme mimic was obtained by encapsulating an iron phthalocyanine complex into crystals of zeolite Y, which were, in turn, embedded into a polydimethylsiloxane membrane acting as a mimic of the phospholipid membrane.With t-butylhydroperoxide as the oxidant, the system hydroxyl-ates alkanes at room temperature with rates comparable to those for the enzyme. It shows similar selectivity (preference oxidation of tertiary C-H bonds) and a large kinetic isotope effect of nine. 

Zeolite-encapsulated metal salen complexes have also been used for reactions other than biomimetic oxidations the Pd(II) salen complex, for example, has been used in the selective hydrogenation of carbonyl groups. ... 

Maurya, M., Chandrakar, A. andChand, S. (2007). Zeolite-Y Encapsulated Metal Complexes of Oxovanadium(VI), Copper(ll) and Nickel(II) as Catalyst for the Oxidation of Styrene, Cyclohexane and Methyl Phenyl Sulfide, J. Mol Catal A Chem., 274, pp. 192-201. 

The application of zeolite encapsulated metal chelate complexes in catalysis is a promising area of research. In particular shape selective oxidations catalyzed by metallophthalocyanines (MPc), shown in Figure 1, included in synthetic faujasite (FAU) type zeolites (2-10) appear to be competitive with other molecular sieve based catalysts that may have commercial potential. The restricted apertures ( 7.4 A) to the supercages (12A) in FAU type zeolites precludes removal of the large MPc complex unless the zeolite lattice is destroyed. Such physically trapped complexes have been termed ship-in-a-bottle complexes as well as zeozymes (to reflect the biomimetic reactivity that is often associated with these catalysts). 

The various strategies for preparation of zeolite encapsulated phthalocyanine complexes have largely involved the condensation of dicyanobenzene (DCB) around an intrazeolite metal ion to form the MPc complex. The efficiency of this template synthesis depends on the nature and location of the intrazeolite metal ion to be complexed. For example, metals have been introduced to the zeolite by ion exchange (7-13), metal carbonyls (14-19) and metallocene complexes (2-5,19-21) prior to reaction with DCB. Some of the advantages and disadvantages of these methods have been detailed by Jacobs (2). However, there are several problems that are inherent to the template synthesis in general. Often there is incomplete... 

There are a few examples of 02 oxidations catalyzed by zeolite-encapsulated complexes. Encapsulated CoPc was active in the oxidation of propene to aldehyde, whereas the free complex was inactive.76 A triple catalytic system, Pd(OAc)2, benzoquinone, and a metal macrocycle, was used to oxidize alk-enes with molecular oxygen at room temperature.77,78 Zeolite-encapsulated FePc79-81 and CoSalophen80,82 complexes were used as oxygen-activating catalysts. 

Metal macrocycles encapsulated in zeolites can be synthesized in different ways.50-53 The flexible ligand51 or intrazeolite complexation53 method involves the diffusion of a ligand into the pores of a metal-exchanged zeolite, where upon complexation with the metal ion, it becomes too large to exit. This... 

When supported complexes are the catalysts, two types of ionic solid were used zeolites and clays. The structures of these solids (microporous and lamellar respectively) help to improve the stability of the complex catalyst under the reaction conditions by preventing the catalytic species from undergoing dimerization or aggregation, both phenomena which are known to be deactivating. In some cases, the pore walls can tune the selectivity of the reaction by steric effects. The strong similarities of zeolites with the protein portion of natural enzymes was emphasized by Herron.20 The protein protects the active site from side reactions, sieves the substrate molecules, and provides a stereochemically demanding void. Metal complexes have been encapsulated in zeolites, successfully mimicking metalloenzymes for oxidation reactions. Two methods of synthesis of such encapsulated/intercalated complexes have been tested, as follows. 

Professor M. R. Maurya is currently heading the Department of Chemistry, IIT Roorkee. He has more than 26 years of teaching and research experience. He had worked in Loyola University of Chicago, USA, Iowa State University, Ames, Iowa, USA, National Chemical Laboratory, Pune, and Pune University Pune, before joining department of Chemistry at IIT Roorkee in 1996 and became full professor in 2008. His current area of research interests include structural and functional models of vanadate-dependent haloperoxidases, coordination polymers and their catalytic study, metal complexes encapsulated in zeolite cages and their catalytic study, polymer-anchored metal complexes and their catalytic study, and medicinal aspects of coordination compounds. So far, he has guided 21 doctoral and 7 Master s theses, co-authored more than 140 research papers in the international refereed journals. 

Transition metal complexes encapsulated in the cavities of zeolites and meso-porous materials exhibit enhanced catalytic activity, compared to their neat analogs. " We had earlier found that Cu(II)-acetate exhibited enhanced regiose-lective orf/zo-hydroxylation of phenols using atmospheric oxygen as the oxidant on encapsulation in molecular sieves Y, MCM-22 or VPI-5. Rao et al. had also found a similar enhancement for encapsulation in Al-MCM-48. 

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