Self-supported catalyst, preparation

Laser electrodispersion (LED) method makes possible to fabricate dense nanostmctured catalysts with unique catalytic properties. In contrast to earlier laser ablation techniques, where nanoparticles were synthesized from vaporized matter, LED is based on the cascade fission of hquid metalhc drops. Fabricated catalysts consist of ensembles of nanoparticles that are uniform in size and shape, amorphous and stable to coagulatioa The catalytic activity of these self-assembled Pt, Ni, Pd, Au and Cu catalysts with extremely low metal content (<10 mass.%) in hydrogenation and hydrodechlorination is several orders of magnitude higher compared to that for separated metal clusters, highly loaded metal films and supported catalysts prepared by usual methods. 

In Figure 3 the FTIR-spectra of the Jacobsen ligand (A), the Jacobsen catalyst (B) and our immobilised catalyst salen 2 (C) are compared. While the spectra A and B have been done using standard KBr technique, the ship-in-a-bottle catalyst (C) has been prepared as a self-supported wafer. The bands at wavenumbers 1466 cm1, 1434 cm 1, 1399 cm 1... 

Self-assembly of molecules and nanoparticles to build well-defined structures, constitutes another approach to make model catalysts [33,34]. Here, nano-structured surfaces are made from nanoscale building blocks that are synthesized from atoms and molecules by chemical means. There has been a tremendous development in this field during the past decade, which includes a number of different strategies, including microemulsions [33], (micellar) block copolymers [35,36], and template CVD growth [37]. Relatively little work has, however, so far been directed toward heterogeneous catalysis in the sense described in this chapter, i.e., to make supported catalysts [38]. There are many reports on preparations but relatively much fewer on evaluations of catal3dic activity, trends, or reactivity versus particle size, etc. A main issue for model catalysts prepared by self-assembly is whether they maintain the well-defined character after, e.g., template removal and calcinations and other pretreatment steps, before they can be used as model catalysts. 

Catalyst preparations. Y zeolites with overall Si/Al ratio equal to 2.7, 15 and 45 were used as supports. The Y2>7 sample was a stabilized zeolite available from Union Carbide (LZY 82). This material when submitted to selfsteaming and acid leaching yielded HY4g. The HY15 sample derived from a commercial NaY (Union Carbide LZY 52) the transformation involved successive ammonium exchange, self-steaming and acid washing. 

X-ray dififaction studies were carried out with Phillips PW 1140 X-ray diffractometer, using Ni-filtered Fe K radiation. FTIR spectra of the catalysts were recorded on Nicolet-740 FTIR spectrometer. Self supporting discs were prepared from KBr and the catalyst mbrture applying pressure. These discs were used for recording FTIR spectra. 

Each of the prepared supported metal catalysts (40 mg) was pressed into self-supporting pellet of 20 mm in diameter, and was placed in an infrared cell, which was connected to an iso-volumetric system equipped with a vacuum line. The pressure of this system can be measured to the order of 1 x 10 Torr by a capacitance manometer. The catalysts were pretreated with O2 at 723 K for ten hours and then reduced under H2 at 723 K for ten hours, followed by evacuation at the same temperature for one hour before use. Adsorption of CO was carried out at 298 K. 

The method outlined applies equally well to supported oxides of transition metals. The familiar chromia-alumina catalyst is a good example. In such cases, the degree of attenuation of the supported oxide may be much greater than in the gel oxides, which may be considered to be self-supported. All the common paramagnetic oxides have been studied in this way, on a variety of supports, and as prepared by a variety of methods. A few oxides, such as molybdena, for one reason or another do not lend themselves to this method. But for most common catalyst components, the method has proved itself to be a useful supplement to x-ray diffraction. 

An unexpected result was the progressive apparent dechlorination of SiOTiCla. We have verified that this phenomenon was not related to the presence or absence of TiCU either physically adsorbed or in the gas phase. We could also observe the growth of the same IR bands between 1000 and 600 cm using a self-supporting disc. Therefore, the dechlorination of TiCU on silica and the eventual incorporation of Ti as a random mixed metal surface oxide is probably entropy driven. Although the initial chemisorption follows reaction (3) and (4), further dechlorination probably results in the formation of SiCl surface species. The vibrations of this near 7(X) cm would be impossible to detect with a thin film given the low extinction coefficient [15], and in any case, they would be masked by the much stronger SiOTi vibrations. Finally, the results have implications for mixed oxide catalysts which are prepared by chemical vapor deposition. Structural models which are based on the notion that only reactions like those depicted in schemes (3) and (4) occur are probably not valid. 

Self-supported chiral titanium clusters were prepared by mixing the ligand and titanium alkoxide in anhydrous toluene, followed by addition of a small amount of water (Scheme 7.22). In some cases, the cluster was completely insoluble in the reaction medium, especially in the case of 30. The cyanation works well with this heterogeneous catalyst, which was reused many times with no significant loss of activity. Additional experiments revealed that no appreciable amount of chiral catalyst had leached into the solution and the enantioselectivity came only from the solid catalyst. 

Scheme 5.6 The preparation of a self-supported, oligomeric, bifunctional catalyst system.

Scheme 5.9 The preparation of self-supported, chiral Mo catalysts 91a-d.

Relatively high iron contents are found for self-supported iron-polypyrrole catalysts which were prepared by spray pyrolysis. The authors proposed high site densities however, comparatively low current densities with respect to other Fe-N-C catalysts make it most probable that only a small fraction of the overall irrai is bonded in active sites. Nevertheless, the shape of these catalysts is interesting because porous carbon spheres with diameters of 100 up to 1,000 nm are formed [194],... 

Self-supporting pressed discs of the pure oxide powders are prepared for in situ characterisation studies by transmission/absorption IR spectroscopy. These samples are put onto the IR beam, in an appropriate cell allowing heating, cooling, and gas/vapour manipulation. Activation is mostly performed by outgassing at relatively high temperatures. In the case of diffuse reflectance infrared Fourier transform (DRIFT) experiments the pure catalyst powder is deposited on the sample holder, with smooth pressure, and activation is mostly performed by an inert, dry gas flow. 

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