Metal zeolite-entrapped

The induction of steric effects by the pore walls was first demonstrated with heterogeneous catalysts, prepared from metal carbonyl clusters such as Rh6(CO)16, Ru3(CO)12, or Ir4(CO)12, which were synthesized in situ after a cation exchange process under CO in the large pores of zeolites such as HY, NaY, or 13X.25,26 The zeolite-entrapped carbonyl clusters are stable towards oxidation-reduction cycles this is in sharp contrast to the behavior of the same clusters supported on non-porous inorganic oxides. At high temperatures these metal carbonyl clusters aggregate to small metal particles, whose size is restricted by the dimensions of the zeolitic framework. Moreover, for a number of reactions, the size of the pores controls the size of the products formed thus a higher selectivity to the lower hydrocarbons has been reported for the Fischer Tropsch reaction. 

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 characterization of zeolite-entrapped metal clusters by chemisorption of hydrogen or carbon monoxide provides valuable information, requiring, however, careful interpretation. When metal particles in zeolites approach monoatomic dispersion, the ratio of chemisorbed hydrogen to metal (H/M) fails to provide reliable information on metal dispersion. A first convincing example that the H/M ratio can actually decrease with increasing metal dispersion has been provided by determining the H/M ratio and the EXAFS profiles of reduced Pd/NaY and Pd/HY. It was found that the H/M ratio in Pd/HY is significantly smaller than that in Pd/NaY, particularly for small Pd particles when reduction was performed at temperatures below 500°C... 

Recent uses of EXAFS, X-ray scattering, and FTIR spectroscopies have revealed that exposure of zeolite-entrapped Pd or Rh often induces thorough reorganizations of the metal atoms. The combination of CO and zeolite protons can sometimes even lead to changes of the oxidation state of the entrapped metal. 

These results illustrate, as shown in Fig. 31, the advantages of zeolite-entrapped clusters as precursors for well-defined metal catalysts. The trapped clusters appear to be stable to cycling though oxidation and reduction without forming crystallites on the external zeolite surface. 

The wide possibilities to synthesize and/or to modify all kinds of zeolites such as they present channel networks which differ by their spatial distribution (allowing more or less the circulation of hydrocarbons) and by their size, such as they may bear active sites at different locations (acidic sites, catalytically active transition metal ions, entrapped metallic particles or clusters...) make the zeolite future look bright. At the present time it... 

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. 

Metal catalysts derived from zeolite-entrapped metal clusters... 

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-... 

Gentle treatment under a CO atmosphere is sufficient to convert some transition metal ions in zeolites into zeolite entrapped metal carbonyl clusters. Such clusters are usually formed under milder conditions in faujasite cages than in solution. This comparison illustrates the excellent solvating abilities of the cages. 

Nuclear magnetic resonance (NMR) spectroscopy has found only little application in the study of zeolite entrapped metal carbonyl clusters. With modern high-field instrumentation, however, increased attention to this method is... 

EXAFS spectroscopy is one of the most powerful methods for determining the structures of zeolite entrapped metal carbonyl dusters because it gives quantitative structural information. EXAFS spectroscopy can, in prindple, be used to characterize samples in reactive atmospheres. However, most of the reported results have been obtained for samples in unreactive atmospheres, and usually at liquid nitrogen temperature where the signal-to-noise ratio is markedly greater than that at room temperature. 

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. 

Zeolite entrapped metal carbonyl clusters prepared by the methods described above are potentially more stable and selective CO hydrogenation catalysts... 

In the preparation of zeolite-entrapped CdS, considerable attention has been paid to the ncd/nj ratio. A marked non-stoichiometry of zeolite-hosted metal sulfide particles has been found for CdS nanoparticles by X-ray photoelectron spectroscopy [345]. After sulfidation of a zeolite X sample that is partially ion-exchanged with Cd ions, the binding energies of Cd 3ds/2 electrons decrease by about 0.3 - 0.5 eV in dependence on the diameter of the CdS nanoparticles formed. The shift originates from the replacement of ionic interactions between the Cd + ions and the zeolitic framework oxygen by more covalent (Cd -S ) bonds. However, due to the larger effective masses of the electrons and holes in CdS (m, eff = 0.42 nie, mn, eff = 0.18 m ) [339], the absorption of CdS clusters in the pores of zeohtes is less affected by the zeoUte framework than that of PbS clusters. However, the effect of the zeolite framework on the excited-state relaxation processes, i. e., the luminescence behavior of the CdS clusters, can be very large. 

Obviously, the catalytic future of zeolite-occluded clusters and organometallics is two-fold. They can be used as coordination-type catalysts or act as zeolite-entrapped metal precursors. In both cases, the zeolite will provide its solid solvent property, its bifunctional capability (in a broad sense, either its acidic or macroanionic properties), and its unique shape-selective properties. 

Scale prevention methods include operating at low conversion and chemical pretreatment. Acid injection to convert COs to CO2 is commonly used, but cellulosic membranes require operation at pH 4 to 7 to prevent hydrolysis. Sulfuric acid is commonly used at a dosing of 0.24 mg/L while hydrochloric acid is to be avoided to minimize corrosion. Acid addition will precipitate aluminum hydroxide. Water softening upstream of the RO By using lime and sodium zeolites will precipitate calcium and magnesium hydroxides and entrap some silica. Antisealant compounds such as sodium hexametaphosphate, EDTA, and polymers are also commonly added to encapsulate potential precipitants. Oxidant addition precipitates metal oxides for particle removal (converting soluble ferrous Fe ions to insoluble ferric Fe ions). 

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... 

Scheme 10.7 Co-tetramerization of pyrrole and aldehydes in the presence of transition metal exchanged Y zeolite (MeY), yielding entrapped metalloporphyrin (after Jacobs1131).

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