Zeolites zeolite-encapsulated complex

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. 

FIGURE 11.18. EPR spectra of (i) Cu(salen) and (ii) Cu(5-Cl-salen) at 77 K (a) neat polycrystalline sample, (b) frozen CHas/CHsCN (3 1) solution, (c) frozen DMF solution and (d) zeolite-Y-encapsulated complexes. Dotted line (e) is a blown-up trace of curve (d). 

Zeolite encapsulated complexes catalyse the oxygenation of alkanes with peroxides according to oxo chemistry, following a mechanism very similar to the oxygen-rebound mechanism encountered in monooxygenase enzymes. 

Still controversial and of significant importance is the electrochemistry with zeolite-encapsulated complexes. Because of the unusual steric constraints and the... 

It is fairly apparent that encapsulation of the RuFiePc complex in NaX dramatically alters the catalytic activity and selectivity, however, that in itself is not evidence for the intrazeolite location of the complex. Therefore, we examined the oxidation of the much larger cyclododecane using the same reaction conditions as for cyclohexane. We found the homogeneous RuFisPc catalyst had virtually no preference for either cycloalkane, showing approximately the same number of turnovers per day. In contrast, the RuFiePc-NaX catalyst exhibited relatively low activity ( 300 tumovers/day) for the larger cyclododecane. The acti dty of the zeolite encapsulated complex was nearly 10 times greater for the smaller cyclohexane. This shape selectivity is consistent with the active sites located inside the zeolite. 

Z. Gaillon, N. Sajot, F. Bedioui, J. Devynck, and KJ. Balkus, Jr, Electrochemistry of Zeolite Encapsulated Complexes. Part 3. Characterization of Iron and Manganese SALEN Entrapped in Y Faujasite Type Zeolite. J. Electroanal. Chem., 1993, 345, 157-167. 

The characterization of zeolite-encapsulated complexes is by no means simple and the same battery of techniques (see earlier) has been brought to bear [14] as with framework-substituted sieves. 

More recently, these systems have been elevated to a new level of biomimetic sophistication, namely by embedding the zeolite-encapsulated complex in a polydimethylsiloxane membrane their performance could be improved even further [81]. The hydrophobic membrane mimics the pho holipid membrane in which cytochrome P-450 resides and acts as an interface between two immiscible phases (cyclohexane and aqueous 70%TBHP). 

The discovery, in the mid-eighties, of the remarkable activity of TS-1 as a catalyst for selective oxidations with aqueous H2O2 fostered the expectation that this is merely the progenitor of a whole family of redox molecular sieve catalysts with unique activities. However, the initial euphoria has slowly been tempered by the realization that framework substitution/attachment of redox metal ions in molecular sieves does not, in many cases, lead to a stable heterogeneous catalyst. Nevertheless, we expect that the considerable research effort in this area, and the related zeolite-encapsulated complexes, will lead to the development of synthetically usefril systems. In this context the development of chiral ship-in-a-bottle type catalysts for intrazeolitic asymmetric oxidation is an important goal. Such an achievement would certainly justify the appellation mineral enzyme . 

No decrease in catalytic activity was observed during the catalytic reaction. The zeolite-encapsulated complex is easier to handle than the non-supported one, it can be removed from the reaction mixture by simple filtration and it can be reused in several subsequent catalytic runs with similar catalytic activity. 

Encapsulated rhodium complexes were prepared from Rh-exchanged NaY zeolite by complexation with (S)-prolinamide or M-tert-butyl-(S)-prolinamide [73,74]. Although these catalysts showed higher specific activity than their homogeneous counterparts in non-enantioselective hydrogenations, the hydrogenation of prochiral substrates, such as methyl (Z)-acetamidocinnamate [73] or ( )-2-methyl-2-pentenoic acid [74], led to low... 

Encapsulation of [Co(bpy)3]2+ within zeolite frameworks has also been shown to have a remarkable influence on the electronic spin state of the complex.240 Distortions imparted on the tris-chelate complex by the confines of the zeolite supercage are found to be responsible for stabilizing the unusual low-spin electronic ground state.241,242 The [Co(bpy)3]3+/2+ couple has been measured for the encapsulated complex and it has been found that the complexes remain within the zeolite and do not exchange with the bulk solution.243 Electrochemistry of [Co(bpy)3]3+/2+ immobilized within a sol-gel has also been studied.244... 

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... 

The physicochemical properties of Schiff-base complexes encapsulated in zeolite70 and the surface chemistry of zeolite-encapsulated CoSalen and [Fe(bpy)3]2+ catalysts were studied and published.71... 

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]. 

Knops-Gerrits, P.-P., De Vos, D., Thibault-Starzyk, F. and Jacobs, P. A. Zeolite-encapsulated Mn(II) complexes as catalysts for alkene oxidation, Nature, 369 (1994), 543-546... 

C. Bowers, and P. K. Dutta, Olefin oxidation by zeolite-encapsulated Mn(salen)" complexes under ambient conditions, J. Catal. 122, 271—279 (1990). 

C. R. Jacob, S. R. Varkey, and R. Ratnasamy, Zeolite encapsulated copper (X2-Salen) complexes, Appl. Catal. A 168, 353-364 (1998). 

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