Zeolite-Entrapped Metal Complexes

The metal catalysts derived from the zeolite-entrapped metal cluster complexes have been studied because of the interest in a uniform distribution and a high degree of metal dispersion through the zeolite frameworks. Nevertheless, little information is available on the structural and chemical behavior of the entrapped metal cluster complexes, particularly on the retention of the cluster character under the reaction conditions, e.g., CO + H2, alkane hydrogenolysis and methane homologation re-... 

The second example demonstrated immobilization via ship in a bottle , ionic, metal center, and covalent bonding approaches of the metal-salen complexes. Zeolites X and Y were highly dealuminated by a succession of different dealumi-nation methods, generating mesopores completely surrounded by micropores. This method made it possible to form cavities suitable to accommodate bulky metal complexes. The catalytic activity of transition metal complexes entrapped in these new materials (e.g, Mn-S, V-S, Co-S, Co-Sl) was investigated in stereoselective epoxidation of (-)-a-pinene using 02/pivalic aldehyde as the oxidant. The results obtained with the entrapped organometallic complex were comparable with those of the homogeneous complex. 

The attachment and encapsulation of metals and metal complexes in the cavities of zeolites is an active area of research and provides a versatile method for the modification of these molecular sieves (39). Because of the enforced dispersion of the metal complexes in the zeolite, systems not readily observable in solution can be investigated in zeolites. For example, the mononuclear superoxo adduct of the cobalt(HI)-ammine system, [Co(NH3 )6(00-)]2+, which would be expected to dimerize in solution, could be observed entrapped in zeolite Y (40). 

In microporous supports or zeolites, catalyst immobilization is possible by steric inclusion or entrapment of the active transition metal complex. As catalyst retention requires the encapsulation of a relatively large complex into cages only accessible through windows of molecular dimensions, the term ship-in-a-bottle has been coined for this methodology. Intrinsically, the size of the window not only determines the retention of the complex, but also limits the substrate size that can be used. The sensitivity to diffusion limitations of zeolite-based catalysis remains unchanged with the ship-in-a-bottle approach. In many cases, complex deformation upon heterogenization may occur. 

An alternative method used for entrapment of large complexes into zeolite crystals is known as the so-called zeolite synthesis method .[67 701 In this method transition metal complexes are added to the synthesis mixture from which a faujasite zeolite is obtained. Therefore, the complex should be stable and dissolved in the medium in the conditions of zeolite synthesis, i.e. at elevated pH (> 12) and temperature (around 100 °C). It is not entirely clear whether occluded complexes are positioned in faujasite supercages or in cracks or defects of the crystals. To assure occlusion of isolated MePc complexes rather than of their clusters, the occluded amounts must be limited, implying the use of very active complexes. Ru and CoPcF17 complexes have been reported to show good activity and resistance to leaching.[67 701... 

At the basis of the application of zeolites in fine chemicals reactions is the rich variety of catalytic functions with which zeolites can be endowed. Bronsted acidity, Lewis acidity and metallic functions are well known from classical bifunctional chemistry but for specific reactions, unusual sites, e.g. Lewis acid Ti4+ centres, have been introduced into zeolites. Moreover, zeolites can acquire more or less weakly basic properties metal complexes can be entrapped in zeolite pores or cavities, and enantioselective reactions have been performed by decorating the zeolite surface with chiral modifiers. 

The choice of topics dealt with in this text refiects essentially the interests and experience of the author. It encompassed the applications of XPS and Auger spectroscopies to the elements constituting the zeolite lattice, to counter-ions and to probe molecules. This has left aside the very large applications of surface sciences to materials supported or occluded in the zeolitic pore lattice. These materials include highly dispersed metallic particles, finely spread oxidic phases, entrapped organo-metallic complexes or metallic clusters. To some extent however the analysis of the supported phase is not specific of the... 

This method relies on the size of the metal complex rather than on a specific adsorptive interaction. There are two different preparation strategies One, often called the ship-in-a-bottle approach, is based on building up catalysts in well defined cages of porous supports. Recently, enantioselective Mn epoxidation catalysts with different salen ligands have been assembled in zeolites. In zeoHte EMT [35] ees up to 88% and in zeolite Y [36] ees up to 58% were obtained with czs-P-methylstyrene. However, both entrapped catalysts were much less active than their homogeneous counterparts. Rh diphosphine complex were entrapped in the interlayers of Smectite [37]. The resulting catalyst was active for the enantioselective hydrogenation of N-acetamidoacrylic acid (ee 75%). 

The oxidation of dimethyl sulfide to the corresponding sulfoxide on different zeolites has been reported recently, using zeolite entrapped Cu-ethylenediamine ([Cu(en)2]2") complexes. Spectroscopic comparison between the neat and the NaY, KL, and NaBETA entrapped complexes, shows that the square planar complex undergoes distortion in the zeolite crystal [54-56], Changes in redox properties of the complexes in the zeolites are due to decrease of the HOMO / LUMO levels of the metal complexes upon encapsulation under influence of the electric field existing inside the zeolite [56]. The high activity in ZSM-5, however, points to the existence of extra-pore complexes, probably strongly adsorbed at the external surface. 

Zeolites are crystalline but versatile materials. They may be modified in many ways they can be tuned over a wide range of acidity and basicity, and of hydrophylicity and hydrophobicity, many cations can be introduced by ion exchange and isomorphous substitution is possible also allowing build-in of isolated redox centers (e.g. Ti) in the lattice. Moreover metal crystallites and metal complexes can be entrapped within the microporous environment. There is for instance much progress in enantioselective synthesis on chiral catalysts immobilized in microporous or mesoporous materials [16]. 

Many metal complexes and clusters are colored and have distinctive ultraviolet-visible spectra. [80] The method offers the advantage of ease of application, but it has been used only seldom in the characterization of zeolite entrapped oigano-metallics. The spectra may provide evidence of metal-metal bonds, as has been shown for carbonyl clusters of Fe, Ru, and Os, [81, 82] but there are hardly any data for zeolite entrapped clusters. The absorption bands of dusters are shifted to lower energy as the cluster nudearity increases. [83] Ultraviolet-visible spectroscopy has been used to detect the formation of [HFe3(CO)n] in NaY zeolite [50] and of clusters suggested to be [Pt,(CO),g] in NaY zeolite. [40-42] Since the spectra do not provide highly spedfic structural information, the method is of secondary importance. 

Aluminosilicate molecular sieves with the FAU structure have been crystallized in the presence of several metallophthalocyanines. A percentage of the complexes becomes included into the zeolites. The synthesis of NaX around the metal chelate represents a new method for encapsulating such complexes and modifying zeolite molecular sieves. The entrapped complexes were characterized by XRD, IR and UV-VIS spectroscopy. Preliminary results suggest the metal complexes may function as templates by modifying the gel chemistry. 

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). 

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