Preformed Supports

Entrapment Within a Porous Matrix Cells are allowed to diffuse into preformed porous matrices such as bricks, cordierite, and pore glass, in which they will grow and be trapped. The main advantages of this method are that the preformed support materials are more resistant to disintegration in packed beds or stirred vessels than other support materials, and the entrapment is not usually harmful to the cells. However, it is difficult to reach a high cell concentration due to the limited pore volume usually available for entrapment within typical support materials. 

To conclude, this method for metal catalyst preparation consists of the impregnation of a preformed support with metal precursors and subsequent calcination and reduction. 

It is desirable to begin this chapter with some general remarks about of use of terms to describe methods of preparation, because they are sometimes used loosely and inaccurately. The methods fall into two classes (i) the support and the gold precursor are formed at the same time and (ii) the gold precursor is applied to the preformed support. The procedure for method (i) is termed coprecipitation (Section 4.2.2), and although capable of many minor variations (e.g. as in the sol-gel method, Section 4.4.1), it does not have the same wealth of alternative procedures as does the second method. These may be listed as follows. 

The more common methods for preparing supported oxide catalysts involve the dispersion of the active species or its precursor onto the surface of a preformed support. This has been accomplished using procedures referred to as equilibrium adsorption, incipient wetness or dry impregnation.9,20... 

A preformed support may be impregnated with this HAuCU and then treated with a reducing agent. Alternatively, when the pH of a mixture of HAuCU and various metal ions is raised, gold hydroxide and metal hydroxide coprecipitate. Drying, calcination, and reduction of this mixture provide Au(0) supported upon a metal oxide. Suitable reducing agents include H2, BH4, citrate, and ascorbate. Permutations of... 

Ion exchange is the most important technique for the preparation of clay and zeolite catalysts from the preformed supports. It is used to make several catalysts described here, including montmorillonite-Fe3+, which is a useful Diels-Alder catalyst that can function in water and zeolite-Na+,H+ (i.e. partially proton-exchanged zeolite), which can catalyse some aromatic chlor-inations with shape selectivity.15,21,22... 

BM can be divided into three major processing categories (1) extrusion BM (EBM), which principally uses an unsupported parison (2) injection BM (IBM), which principally uses a preform supported by a metal core pin and (3) stretch BM, for either EBM or IBM, to obtain bioriented products, providing significantly improved cost-to-performance advantages. Almost 75 percent of processes are EBM, almost 25 percent are IBM, and about one percent use other techniques such as dip BM (2). About 75 percent of... 

Because of the very high supersaturation during the precipitation of most base metal hydroxides or carbonates, nucleation will be spontaneous, that is, homogeneous, which therefore explains why only limited deposition onto a preformed carrier occurs. Nucleation such as this by rapid mixing is generally much too fast for all the nuclei to diffuse into the pores and to deposit onto the internal surface of a preformed support. Therefore, significant amounts of precipitate may be formed separate from the support or on the external surface (Figure 7.2). 

If complete deposition onto a preformed support is important, different deposition methods like impregnation or deposition-precipitation should be considered (Chapters 4 and 6). 

The support is often coprecipitated simultaneously with the active metals in a single process step. This can be done by mixing in basic precursors of the support, such as sodium sihcate, sodium aluminate, or mixtures thereof, with the base precipitant. These can alternatively be added separately. Also, more acidic compounds like aluminum nitrate can be used as support precursors and, optionally, mixed in with the metal precursor. Compared to preformed supports, the amount of support precursor can be quite low and this is why coprecipitates may contain very high metal levels up to 80-90%, a level not attainable by impregnation. 20-30 mol% of silicate or aluminate relative to the metal is usually sufficient to obtain a high metal surface area after reduction. It may be debated whether the resulting metal silicate, aluminate, or alumina, are real supports or that names like stabilizer or spacer are more appropriate. 

Apart from soluble support precursors, preformed sohd supports can be introduced during coprecipitation. As discussed in Section 7.2 coprecipitation is too fast for quantitatively depositing metals onto the internal surface area of a support. Nevertheless, preformed supports are heing used, mostly because via a support dissolution/redeposition mechanism, catalysts are obtained quite similar in catalytic performance to those based on soluble support precursors. 

Impregnated catalysts starting from preformed supports... 

A patent from BP America (28) has claimed the preparation of catalysts for fluid-bed application by staged impregnation of preformed supports (suitable for fluid-bed operation) with a solution of metal alkoxide. The preparation is summarized in Scheme 9. 

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