Occluded clusters

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. 

As a check to confirm that no extraneous non-polymer-attached catalytic species were present, the following experiment was performed. Polystyrene without attached cyclopentadiene was exposed to Co2(C0)e, extracted using a Soxhlet extractor and dried in vacuo in exactly the same manner as was used to synthesize 5. When used under the above Fischer-Tropsch reaction conditions, these treated, white polystyrene beads did not discolor, release any detectable species into solution, cause a CO/H2 pressure drop, or result in the formation of any detectable amounts of methane. These observations argue against the presence of small amounts of occluded Co2(C0)e or C04 (CO) 12 which could conceivably have been active or precursors to active species. It should be noted that the above clusters were reported to be essentially inactive under Fischer-Tropsch conditions (140°C, toluene, 1.5 atm., 3/1 H2/CO, three days) leading to mere traces of methane (11). The lack of products under our conditions also indicates that, at least in the absence of resin-bound CpCo(C0)2 or its derivatives, the polystyrene support did not degrade. 

Alkenes may also be used as reagents in side-chain alkylation. Zeolites with occluded alkali metal and alkali metal oxide clusters were tested in alkylation of toluene with ethylene439 440 Alkali metal clusters created by decomposing Na or... 

Difficulties that have been encountered in divergent synthesis can be attributed to the clustered nature and sequences of peptide dendrimers that form interchain hydrogen bonds and occlude coupling reactions. Solvent combinations such as DMSO and NMP, as well as coupling at elevated temperatures (50-60 °C),[94] have been used to overcome these side reactions and should be used if a single coupling fails to complete the reaction. It is best not to dry the peptide resin at any stage of the synthesis because re-solvation of dried peptide dendrimer resin is difficult. 

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... 

Milk powder particles <100 pm in size are often difficult to wet and tend to become lumpy when dispersed in water (Schubert, 1987). Hence, milk powder is agglomerated during manufacture to make the powder instantly wettable in warm (>45°C) water. Agglomeration is the process of forming porous clusters of powder particles to increase the volume of occluded air, thereby increasing its dispersibility and making the powder... 

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... 

Layered silica clays are recognized for their ability to form a variety of pillared intercalated derivatives with properties for occluding organic compounds and metal complexes, including organometallic clusters (158). These materials consist of positive charged Mg(OH)2-like layers separated by hydrated gallery anions. 

Inclusion or occlusion takes place when impurities or solvents are physically trapped within crystals. This simation is different from the case of solid compounds. In inclusion or occlusion, impurities can be occluded sporadically within crystals, whereas in solid compounds, impurities are distributed throughout the crystal lattice. While both inclusion and occlusion enclose solvent or impurities in the crystals, inclusion is more applicable to cases where solvents or impurities are trapped within crystal cavities during crystallization, whereas occlusion is more applicable to cases where surface liquid is trapped within crystal clusters or agglomerates during drying. In this book, the terms inclusion and occlusion will be used interchangeably. 

VPI-5 is an aluminophosphate framework with very large one-dimensional pores defined by 18-member ring[22]. The crystal structure report[7j of synthesized VPI-5 revealed the possible role of water molecules as templates. The use of VPI-5 as a molecular sieve and as a catalyst primarily depend on the removal of the occluded water molecules without the destruction of the framework structure. Prasad et al.[23] reported from their TGA experiment that the seven distinct types of water molecules could be desorbed from VPI-5 in a step-wise fashion, in the temperature range of 35 to 60°C. The cluster model calculations[24] have revealed the actual electronic interaction of water molecules with VPI-5 framework. CG technique also indicated that the void volume in VPI-5 could be controlled by the partial removal of water molecules. 

From another standpoint, it may be said that the mobility of the guest species within the channels is considerably lower than that in the bulk fluid that is, the occluded molecules have sustained significant decreases in translational, rotational, and vibrational degrees of freedom (entropy effects), compared with their vapors (12). Similar effects would be expected in relatively small pore ion-exchange gels, such as Dowex 50 or Amberlite I.R. 120, with aqueous or polar solvents systems (42). Thus, zeolitic water may be viewed as intermediate in mobility between the bulk liquid and ice (1). Further, clusters of molecules may exist in zeolite cavities. In faujasite, due to its openness, these clusters are not merely isolated, but form continuous filaments of dense zeolitic fluid (12). 

When TRAMs are made on dispersed probes, tc will reflect the speed of rotation of the fluorophore [20,46,60,76], which can be related to the microviscosity of the medium. In the context of probing the structure of PMAA, the fluorescent dyes have been occluded in the hypercoiled conformation [46,60,76] allowing an estimate of the size of the rotating, solubilizing cluster to be derived from the resultant tc. Alternatively, if the fluorophore is covalently attached to the polyelectrolyte in the form of a label then, depending on its mode of attachment, information concerning motion of the chain ends [46,60,76], the backbone [26,88,112,113], and chain substituents [26,88] can be derived from tc. 

The chemical composition of the zeolitic host also affects the basic properties. When the basicity of Cs-over-exchanged zeolites X and Y, with Si/Al ratios of 1.2 and 2.5, respectively, were measured by stepwise thermal desorption of CO, it was found that different cesium oxide species are present CS2O inside the CsNaX zeolite and (Cs20)2 inside the CsNaY zeolite [73,74]. The basicity of the occluded CS2O species was, moreover, greater than that of intrazeolitic clusters (Cs20)2, as deduced from catalytic activity and TPD results. 

In order to take benefit of the strong geometrical constraints of the zeolite voids, so as to improve the basic properties, basic moities insertion inside the pores was achieved by several authors [2-4,9,12,13]. The direct incorporation of alkali hydroxide during their synthesis by treatment of aqueous or methanolic solution [13] can lead to loss of crystallinity by Al-0 solvolysis. This phenomena can be avoided by using a mild method consisting of a thermal decomposition of neutral alkali salts previously occluded inside the zeolite void cavities [9]. In this respect. Martens et al. prepared supported sodium clusters on zeolites generated by impregnation of sodium azide followed by its thermal decomposition [2-4]. 

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