Crystalline supports

The same type of porphyrin-Ru complex was immobilized by coordina-tive adsorption on aminopropylsilicas (Fig. 26) as either amorphous or crystalline supports [79]. Mesoporous crystalline MCM-48 was the best support, as shown by the improved results obtained in the epoxidation of styrene with 2,6-dichloropyridine N-oxide (TON > 13 000 and 74% ee). The versatility of this catalyst was demonstrated in the intramolecular cyclopropanation of frans-cinnamyl diazoacetate. TON was ten times higher than that obtained in solution and 85% ee was observed. The solid was recycled and reused, although partial loss of selectivity occurred. 

There are no known examples of supported clusters dispersed in crystallo-graphically equivalent positions on a crystalline support. Thus, no structures have been determined by X-ray diffraction crystallography, and the best available methods for structure determination are various spectroscopies (with interpretations based on comparisons with spectra of known compoimds) and microscopy. The more nearly uniform the clusters and their bonding to a support, the more nearly definitive are the spectroscopic methods however, the uniformities of these samples are not easy to assess, and the best microscopic methods are limited by the smallness of the clusters and their tendency to be affected by the electron beam in a transmission electron microscope furthermore, most supported metal clusters are highly reactive and... 

Diffraction from Crystalline Supports. Although diffraction conveniently boosts particle contrast in ratio images,... 

The recent development of inorganic crystalline-supported metal catalysts for various liquid-phase oxidation reactions such as alcohol oxidation, epoxidation, Baeyer-Villiger oxidation and oxidation via C—H activation using molecular oxygen (02) or hydrogen peroxide (H202) as an oxidant are reviewed in this chapter. 

Impregnation has been used to prepare a number of catalysts having different metal support combinations. Highly loaded nickel catalysts supported on alumina, titania, silica, niobia and vanadium pentoxide were prepared by adsorption of nickel nitrate from an ammoniacal solution onto the support material. The supported salts were dried at 120°C and calcined at 370°C before reduction to the supported metallic nickel. It was found that the ease of reduction depended on the crystallinity of the support. Amorphous or poorly crystalline supports made the reduction of the nickel oxide more difficult than on crystalline supports. As examples of its generality, this procedure was also used to prepare... 

When the particles are in epitaxy with a crystalline support, the lattice of the small particles can be accommodated to the substrate lattice. If the lattice parameter of the support is larger than those of the particle, its lattice may expand. This has been observed by HRTEM for 2-nm Pd particles supported on MgO(lOO), where the expansion is 8% [34]. However, when the size of the particles increases, they progressively recover their bulk lattice parameter by the introduction of dislocations as shown experimentally [35] and simulated by molecular dynamics [36]. 

The lowest temperature assigned to the glass transition (1) has been termed the gamma process. Torsion pendulum measurements 1,5, 6, 7) suggest that this relaxation has two or more components. Dilatometry (8) resolved two discrete transitions at 108 and 152 K which have been labeled the 711 and 71, respectively (5,6, 7,9). Illers data on samples of various crystallinities support the conclusion that 711 and 71 arise from crystalline and amorphous phases, respectively (5, 6, 7). Several mechanisms for this transition have been discussed which involve local motions of three to five methylene units (5, 6, 7, 9). The interpretation of this relaxation as the glass transition is doubtful (2). 

On crystalline supports, the most useful phase contrast information can usually be extracted from metal particles which are... 

A different approach to the synthesis of structurally uniform metal complexes on a surface was reported by Goellner et al who used a crystalline support, attempting to prepare a complex bonded predominantly at a particular crystallographic site. The support was dealuminated Y zeoUte, and the catalyst was prepared by its reaction with Rh(CO)2(acac) (acac is acetylacetonate). The structure and... 

It can be concluded that the theoretical model describing the retention of injected samples on an amorphous polymeric stationary phase makes possible the prediction of the effect on the retention diagram of the following factors polymer film thickness, non-uniformity, crystallinity, support specific suriace area, carrier gas flow rate, glass transition and the difference between thermal expansion coefficients at T and Tg. 

The metal cations can also be immobilized onto supports by cation exchange. Already classic examples are those of metal-zeolites, metal-acidic clays, or metal-aluminophosphates (APO) [5,6,38,70-72]. Fe-substituted molecular sieves can be considered as very good examples for this variety of catalysts and the oxidation of pinacol to pinacolone evidenced the effect of the crystalline support structures on the catalytic activity. Thus, for a series of APOs and silicalites, the activity decreased in the following order APO-5> APO-11 > APO-8>VPI-5 > silicalite-1. Since the catalytic activity was independent of the pore diameter of these supports, the liquid-phase oxidation was considered to proceed mainly on the outer surfaces of the catalysts. The hydrophilicity of the aluminophos-phate surface was in the fevor of catalyzing the pinacol reaction, which in fact corresponds to the case of polar reactants and less polar products. Moreover, a high pinacol conversion was achieved by using solvents of low polarity [70]. 

Isotactic polymers of racemic 3,7-dimethyl-l-octene can be separated in fractions having optical activity of opposite sign by elution on an optically active crystalline support consisting of poly-[(S)-3-methyl-l penteneJ (2l). The separation degree F even for highly isotactic fractions is not higher... 

The examples of multistable anchorings and of anchoring transition reported above raise numerous fundamental questions concerning the structure of the interface between the nematic layer and the crystalline support. The existence of the anchoring transitions induced by variations of the water vapor pressure indicates clearly that adsorption of water molecules assists (or induces) variations of the anchoring direction. It must be so for following reasons. For a constant temperature T, only the chemical potential of water varies as a function of the partial pressure (variations of the partial... 

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