Fractional impregnation method

In the X-ray powder diffraction patterns of the composites, the disappearance of the broad band centered at 22 °20, typical of amorphous silica, indicates that the zeolitisation of the mineral fraction of the parent composite was complete. In no diffraction pattern any sign of crystallised chitosan could be found. The two methods in which the silica-polymer beads were extracted from the aluminate solution after impregnation (methods A and C) allowed the formation of the expected zeolite X, with traces of gismondine in the case of the method C. The method B, in which excess aluminate solution was present during the hydrothermal treatment, resulted in the formation of zeolite A. 

The method we applied is as follows The catalyst is sieved into a coarse and a fine fraction. The coarse fraction of the catalyst (PSD >75 yum) is impregnated homogeneously with about 4000 ppm vanadium fey a traditional pore volume impregnation method. 

The monometallic and bimetallic catalysts in this study were prepared either by the Institut Fran ais du Petrole (IFP) (1% Pt and 1% Pt-0.2% Rh) or in our laboratory (1% Rh) by impregnation of a y-alumina in pellet form of 2 to 4 mm in diameter. The precursor salts were H2PtCl6 and RhCl3, and the co-impregnation method was employed to prepare the bimetallic. Following evaporation and drying, the catalysts were calcined in air at 350°C for 4 hours and then reduced under hydrogen at 500°C for 8 hours. They were then crushed and sieved. The fraction of 80-250 pm was retained for the experiments. 

The catalyst surface area per unit volume, g(Ss), can be made to vary with pellet coordinate by choosing an appropriate impregnation method. Hence, this function represents not only the level of dispersion but also activity distribution. A partially impregnated (or equivalently hollow) pellet is a typical example of a pellet with a certain activity distribution. The motivation for making such a pellet becomes obvious if it is recognized that the reactant concentration becomes almost zero at some point in the pellet when the reaction is diffusion-limited. The fraction of the volume of the pellet for which the concentration is zero is not utilized at all. If this fraction is made hollow or inert, then the observed rate on a per pellet basis should be the same as the fully impregnated pellet. Let us examine this further. Suppose that a pellet is hollow or partially impregnated for a distance Li from the center. Consider a diffusion-limited, first-order reaction. The internal effectiveness factor for this hollow pellet is ... 

Phosphate must be applied as fertilizer to the soil. Ideally it is added in quantities sufficient to guarantee optimal yields, but not in excess in order to avoid P transportation into other compartments of the ecosystem. The amount added should be based on an accurate estimation of the plant-available fraction of P already present in a soil.This is an old and difficult task and a large number of extraction methods have been used since intensive land use was practised. Recently methods have been worked out in which a strip of filter paper impregnated with an Fe oxide (2-line ferri-hydrite) is dipped into a soil suspension and the amount of P adsorbed by the paper is taken as being plant-available (Sissingh,1988 Van der Zee et ah, 1987 Sharpley, 1993 Sharpley et ah,1994 Kuo and Jellum, 1994 Myers et ah 1997). Anion and cation resins extracted more P from four heavily fertilized soils than from goethite (Delgado Torrent, 2000). Other oxyanions adsorbed by soil Fe oxides are silicate, arsenate, chromate, selenite ( ) and sulphate. Adsorption of sulphate led to a release of OH ions and was substantially lowered once the Fe oxides were selectively removed (Fig.16.17). 

Some occlusion of the metal particles is not necessarily bad. In fact, unlike other methods such as impregnation and ion exchange, sol-gel preparation actually allows a certain amount of control over what fraction of a metal particle is accessible at the support surface. As long as there are exposed metal atoms available for catalysis, a partially buried metal particle may better resist deactivation due to sintering. For example, Lopez et al. [44] found that a Ru/SiC>2 sample prepared by sol-gel is more stable than one prepared by ion exchange. The sol-gel sample, which contains partially occluded Ru particles, does not sinter or volatilize under oxygen treatment at 723 K, whereas the ion-exchange sample does. 

Activity and selectivity of Pt/KL catalysts depend crucially on the method of their preparation. There seems to be a consensus that Pt/KL samples prepared by incipient wetness impregnation display a higher Pt dispersion and a higher aromatization yield than samples prepared by ion exchange. Temperature-programmed reduction shows that in samples prepared by impregnation, followed by calcination, a considerable fraction of the Pt is present as Pf ions, whereas Pt prevails in ion-exchanged samples after calcination (53). 

The americium is then stripped from the organic with water (five to seven contacts) using an A/0 0.2. The americium in the strip solution is precipitated with oxalic acid, and the americium oxalate is calcined to Am02. In the lab scale extraction chromatography method, the DHDECMP-impregnated Amberlite XAD-4 was loaded into a column 1.8 cm dia. x 23.5 cm and conditioned with 7M HNOj. The americium feed was fed to the column at 1.5 ml/min until a total of 300 ml had been fed. The column was washed with 200 ml of 7M HNO3 at 1.7 ml/min, then eluted with deionized water until elution was complete (determined radiometrically). Five to seven 50 ml fractions were required for elution. Samples were submitted for analysis. 

Figure 2 Influence of reaction time on the acrylonitrile content in the fraction of acrylonitrile plus acrolein (pnh3, inlet = 10 kPa T = 380 °C) over the base iron antimony oxide catalyst (Sb/Fe = 1) and the impregnated iron antimony oxide catalyst (method A 0.27 g Sb/g FeSb04)...

When the paper is impregnated with an organic liquid, the drying period is shortened to remove only the volatile mobile phase. The theory of unidimensional multiple development has been described [48], and the method has been used to analyze steroids [49] and amino acids [50] among other classes of compounds. Fractional chromatography is a special variation of multiple development in which a solvent of increasing polarity is employed for each subsequent development [51]. 

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