COMPLEXES INSIDE ZEOLITE, PREPARATION

The first position can be safely excluded since a high temperature calcination, causing the removal of Fe atoms from the lattice, remarkably increases the a>site concentration [27]. Besides, a-sites can be prepared via the impregnation of a ready zeolite matrix [28], when the probability for Fe atoms to incorporate into the lattice is very low. a-Sites do not occupy also the 3rd type position deactivation of the outer zeolite surface by its covering with an inert Si02 layer affects neither catalytic activity no a-site concentration [29]. Thus, we may deduce that the active iron occupies the second type position in ZSM-S matrix and is either isolated Fe ions or small complexes inside the micropore zeolite space. 

To avoid the limitations of the small apertures and cavities provided by zeolites, mesopores have been created inside zeolites X, Y and DAY.[140] By dealumination of the zeolite structure, mesoporous regions that are completely surrounded by micropores were obtained, and these intrazeolitic cavities were then used as the space in which to assemble metal complexes. The preparation of a cobalt-salen-5 complex provided a catalytic material that shows improvements in the conversion,... 

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. 

Bessel and Rolison reported the electrochemical behavior of [Co(SALEN)]2"1" and Fe(bpy)3]2+ in zeolite Y.[1571 They prepared an electrode using the complex-zeolite composite and carbon powder, and tested the electrochemical behaviors of the electrode and the composite dispersed in a solution. It was found that the electrochemical behaviors of these two materials differ to a great extent. After several cycles, the former loses all the electrochemical signals, whereas the latter continuously shows the signals. They believed that the electrochemical signals arise from the complex attached onto the zeolite external surface (defects or external supercages), whereas the complex inside the zeolite channel does not participate in electron transfer of the electrochemical process. In fact, there has been dispute on whether the electrochemical signals arise from electron transfer in zeolite channels or from those complexes on the zeolite external surface. Both views can find experimental support.1158 1591... 

In order to prove the presence of the Pt-salen complex inside the zeolite FTIR-spectra of the free and the occluded complex were recorded and compared (figure 4).The result shows that the Pt-salen complex is indeed present in the case of the catalyst prepared according to the ship-in-the-bottle approach. The non zeolitic IR-bands at 1630, 1550 and 1450cm in the spectmm b are simelar to the bands of the fi ee complex in spectmm a. To make sure that the platinum salen complex is occluded in the zeolite supercage some tests were carried out. First the zeolite in which the platinum salen complex is occluded (CAT 2) is washed with methylene chloride to extract the complex fi-om the outer surface of the zeolite. After extraction the... 

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. 

Metal phthalocyanines are easily synthesized by vapor-phase condensation of four molecules of dicyanobenzene in the presence of molecular sieves such as faujasites or A1PO-5 (123-126). This results in direct entrapment of the macrocycle inside the molecular sieve s channels and cages. There are also reports of ship-in-a-bottle synthesis of porphyrins in zeolites, but since porphyrin synthesis requires a mixture of pyrrole and an aldehyde instead of a single compound, porphyrin synthesis is a much less clean process than phthalocyanine preparation (127). Alternatively, soluble porphyrins or phthalocyanines can be added to the synthesis gel of, for example, zeolite X. This also results in entrapped complexes (128). 

Zeolites have been used for years as supports for metal catalysts [1-5]. Such catalysts are typically made by impregnation of the zeolite with an aqueous solution of a metal salt, followed by calcination and reduction in hydrogen. Because the metal particles in such catalysts are typically extremely small and nonuniform in size and shape, often being present both inside and outside the zeolite pore structure, their structures are not well understood. This structural complexity provides a fundamental motivation for preparing and investigating structurally simple zeolite-supported metals, those that are so small and uniform as to be nearly molecular in character and located almost entirely within the zeolite pores investigations of well-defined... 

The ship-in-a-bottle technique is perhaps the most common method for encapsulation of transition metal complexes. In this way the tetradentate Schiff base ligand SALEN (bis-salicylidene) ethylenediamine can diffuse through the 12 MR windows of faujasite. Then, when complexed with a previously exchanged metal ion, nearly square planar coordination geometry is formed inside the a-cages [97-100], Mn complexes with a chiral ligand, prepared by the ship-in-a-bottle technique inside Y and EMT zeolites, have enantioselectively carried at the epoxidation of olefins [101,102]. 

Long-range electron transfer is postulated to occur from ferrocene to tris(bipyridine)iron(III) constructed within the pores of a NaY zeolite. The iron bipyridine complex is too large to move throughout the faujasite pores to the surface, thus requiring the long-range transfer. The asymmetric catalyst, titanium tartrate, has been prepared inside NaY and used as an immobilized catalyst for the epoxidation of cinnamyl alcohol. ... 

Metal phthalocyanine complexes (MPc M = V, Co and Cu) encapsulated in zeolite-Y were prepared by in-situ ligand synthesis and characterized by chemical and thermal analyses and FT-IR, diffuse reflectance UV-vis and EPR spectroscopic techniques. The studies provided evidence for the encapsulation of MPc inside the supercages of zeolite-Y. The Pc moiety distorts from square planarity as a consequence of encapsulation. The encapsulated complexes exhibited enhanced styrene epoxidation activity with /e/-/-butylhydroperoxide compared to the neat complexes in homogeneous medium. The activity and product selectivity of the encapsulated complexes varies with the central metal atom. 

The Co-Na-MOR and Co-H-MOR were prepared by ion exchange of Na-MOR and H-MOR with Co(acetate)2 solutions at 350 K [10P2]. In the Co-Na-MOR and Co-H-MOR samples exchanged to various extents with cobalt (Co/Al = 0.1 0.5), isolated Co was the most abundant species, whereas the residual cobalt was present as [Co-O-Co]. The FTIR results demonstrated that in all Co-MOR samples, irrespective of the cobalt content, cobalt is almost exclusively located inside the mordenite chaimels. In the presence of monovalent cobalt complex with EDMA, zeolites have been synthesized from aluminosilicate gels in hydrothermal conditions [llKlj. Three kinds of zeolites, i.e., MOR-, MFI-, and ANA-types were obtained. The cobalt atoms have been shown to be located in the MOR-framework, through Co-atom substitution and not in micropores as a complex. 

SCHEME 10.3 Some examples of encapsulated species successfully prepared by in situ assembly of precursors inside the pores of zeolites, (a) Metal phthalo-cyanines (b) ruthenium tris-bipyridine (c) pyrylium cations (d) metal salen complexes and (e) heteropoly acid. The brackets generically represent the pore system of the solid host. 

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