Zeolites as enzyme mimics

This concept of zeolites as enzyme mimics was used by Derouane and Vanderveken (59) to explain the selective aromatization of n-hexane on Pt/LTL catalysts confinement effects combined with the unique pore structure of LTL zeolite would be responsible for the fast and selective conversion of n-hexane to benzene. 

Another advance seems likely through the use of zeolites as enzyme mimics.This centers on the reactions of organometallics with zeolite internal surfaces. The best-known example is the production of a [Co (bipyridyl)(terpyridyl)] + complex inside the main cavity of zeohte Y that can selectively and reversibly absorb oxygen from the air. The catalytic potential of a Co phthalocyanine moiety prepared in the Y cavity has also been demonstrated. 

The following sections review 3 systems which we have developed to explore the generality of these concepts mimics of hemoglobin, cytochrome P450 and of iron-sulfur ferredoxins. While the use of zeolites as enzyme mimics is an under-studied area it is certainly not new as evidence a previous volume in this series (1). The intent here is to emphasize the general concepts of zeolite biomimicry by highlighting the key results we have obtained. Interested readers are referred to the... 

Herron, N., Toward Si-based life Zeolites as enzyme mimics, Chemtech, September, 542-548 (1989). 

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. 

Zeolites are well suited for the preparation of encapsulated complexes by virtue of the large supercages. Metallo-phthalocyanines encaged in zeolites have been proposed as enzyme mimics [7,8 Zeolite-encapsulated iron phthalocyanine catalysts have been used in hydrocarbon oxidations it was found that the resistance of the zeolite-encaged complexes against oxidative destruction by far exceeded that of free iron phthaTocyanines [9,10]. In the present work, zeolite-encaged phthalocyanine catalysts were studied in the triple catalytic oxidation of olefins. 

Organic Zeolite Analogues as Enzyme Mimics in Water... 

In ZEOlite based enZYME mimics (ZEOZYMES), the function of the protein mantle is replaced by the inorganic framework, which imposes geometric and steric constraints on the reaction of substrate molecules. Strictly speaking, ZEOZYMES are only mimics of enzymes with rigid protein mantle. Such materials are often denoted as ship-in-a-bottle" complexes (1). However, this notation is too limitative as it only refers to complexes physically encaged in bottle-type zeolite pores and does not take into account any physical and chemical interaction of the pore or cage wall with the complex. 

Since all of the CdS clusters reside in the sodalite cages of the zeolite Y framework, the larger supercages of the structure are still available for absorption of substrate molecules - in this case olefins for photo- oxidation via electron transfer. Colloidal CdS in free solution has been used for such oxidations previously(19) and in a competitive oxidation of styrene and 1,1-diphenylethylene we find that unconfined bulk CdS will effect oxidation in a ratio of 1 2 for these two olefins (irradiation at 365nm). In the zeolite confined system we find however that the ratio becomes 1 1 ie a slight shift in selectivity toward the smaller substrate as may be expected on the basis of size/diffusion effects. From the viewpoint of the enzyme mimic, we have here a system... 

Metal chelate complexes entrapped in zeolites have attracted increasing attention for appfication as oxygen carriers, enzyme mimics and catalysts [48,... 

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. 

Encapsulated Cu—chlorophthalocyanines oxidize hexane at C-l using 02 and at C-2 using H202 as oxidants. The dimeric structure of copper acetate is intact when it is incorporated into the zeolite. This is a regioselective aromatic hydroxylation catalyst, which mimics the specificity of the monooxygenase enzyme tyrosinase.82,89 Zeolite NaY catalysts made with a tetranuclear Cu(II) complex were synthesized and characterized.90... 

The FePcY-PDMS supramolecular catalyst resembles the architecture of natural enzymes. In this system the PDMS membrane takes over the role of the phospholipid double layer likewise, the zeolite imitates the protein and the FePc complex the Fe-protoporphyrin. Zeolite-encaged Cu-histidine complexes were also studied as mimics of natural Cu-enzyme complexes.173... 

One of the most smdied examples is the mimic of the enzyme cytochrome P-450 in the pores of a faujasite zeolite [196,204,225], The iron-phthalocyanine complex was encapsulated in the FAU supercage and is used as oxidation catalyst for the conversion of cyclohexane and cyclohexanone to adipic acid, an important intermediate in the nylon production. In this case the two step process using homogeneous catalysts could be replaced by a one step process using a heterogeneous catalyst [196]. This allowed better control of the selectivity and inhibited the auto oxidation of the active compound. In order to simulate a catalyst and the reaction conditions which are close to the enzymatic process, the so obtained catalyst was embedded in a polydimethylsiloxane membrane (mimics the phospholipid membrane in the living body) and the membrane was used to limit oxygen availability. With this catalyst alkanes were oxidized at room temperature with rates comparable to those of the enzyme [205]. 

Parton et al. [126] reported on the development of a synthetic system that mimics the cytochrome P-450 enzyme. They embedded zeolite Y crystallites containing encapsulated iron phthalocyanine complexes in a polymer membrane. Using tertiary-butylhydroperoxide as oxidant, this catalytic system oxidizes alkanes at room temperature with rates comparable to those of the real enzyme. 

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... 

The encapsulated MePc in Y zeolite can be considered as a formal mimic of cytochrome P450, the rigid zeolite environment becoming a substitute for the flexible amino-acid environment of the natural enzyme [17]. After embedding in a polydimcthylsiloxanc (PDMS) membrane, the MePcY is able to function in a membrane reactor, and mimic the performance of the natural cytochrome P450 in a cell membrane. 

The interactions of a reagent with a zeolite are governed by the shape-selective properties and the confinement effects previously mentioned. The cavities and pores of a zeolite can be considered as microreactors, containing stabilized active sites, which can be designed to have particular properties. This is comparable to the active site of an enzyme, where the catalytic activity depends on the three-dimensional arrangement of the functional groups on the side-chains of the amino acids of the protein. Several of these zeolite mimics of enzyme action have been developed, and three are worthy of mention here. 

Phthalocyanine complexes within zeolites have also been prepared by the ship-in-a-bottle method (see Section 6.6), and have subsequently been investigated as selective oxidation catalysts, where their planar metal-N4 centres mimic the active sites of enzymes such as cytochrome P450, which is able to oxidize alkanes with molecular oxygen. Cobalt, iron and ruthenium phthalocyanines encapsulated within faujasitic zeolites are active for the oxidation of alkanes with oxygen sources such as iodosobenzene and hydroperoxides. Following a similar route, Balkus prepared Ru(II)-perchloro- and perfluorophthalocyanines inside zeolite X and used these composites for the selective catalytic oxidation of alkanes (tert-butylhydroperoxide). The introduction of fluorinated in place of non-fluorinated ligands increases the resistance of the complex to deactivation. 

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