Impregnation, bead catalysts

The catalyst is made by impregnating the beads with aqueous solutions of salts of some rare earth metals and of salts of the desired precious metals such as Pt, Pd and Rh these impregnated beads are then dried and calcined. The distribution of precious metals over the bead radius must be achieved with care, to balance the mass transfer requirements with the poison resistance requirements (Figs. 24-26). The distribution of the active component over the pellet radius can be measured by an Energy Dispersive X-ray (EDX) scan on an individual pellet. However, since in the application a relatively broad distribution in diameters occurs, special procedures have been developed to determine some kind of average distribution of the active components over the pellet radius. The most common procedure is the attrition test, in which a known mass of pellets of known diameter distribution is immersed in a liquid that neither dissolves the active components nor the carrier. The pellets are stirred for a defined time, and are separated from the attrited powder. The powder mass is determined, and its chemical composition analyzed by sensitive methods. 

Pt/Al2C>3 and P1-S11/AI2O3 catalysts were prepared by impregnating y-alumina wiih different solutions of HjPtCla and H2PtCU + SnClj respectively. After impregnation, the catalysts were dried consecutively at 60°C and 120°C, and the residual chloride was then removed by steam treatment at 480 0, for 4 hours. The surface area, pore volume and diameter of the alumina beads were 150 m2/g, 1.43 ml/g and 1.5 mm, respectively. The Pt content of all catalysts used was 0.37%. 

Bead catalysts were based on typical supports such as Y, 5 or 0 alumina with surface areas of about 100 m /g. Alumina was easily impregnated with the platinum and palladium and the resulting catalysts were stable at temperatures up to 1000-1100°C before the phase change to a alumina took place. Alumina can be stabilized to a limited extent by the addition of other refiactory oxides but will still sinter at high temperature. 

FIGURE 12.19 The gases exhausted from an automobile engine pass through a catalytic converter where air pollutants such as unburned hydrocarbons (CjHy), CO, and NO are converted to C02, H20, N2, and 02. The photo shows a cutaway view of a catalytic converter. The beads are impregnated with the heterogeneous catalyst. 

Each bead represents one catalyst as a member of a library of solid catalysts. It may consist of an unporous material like a-Al2C>3 or Steatit or of typical porous support materials - such as A1203, Si02, Ti02, or the like. These beads can be subjected to different synthesis procedures and sequences like impregnation, coating etc. In addition, full mixed metal oxide catalysts can also be formed as spherical particles. 

In a column (such as a packed or fluidized bed) reactor, the reaction conversion is often limited by the diffusion of rcactani(s) into the pores of the catalyst or catalyst carrier pellets or beads. On the other hand, when the catalyst is impregnated or immobilized within membrane pores, the combination of the open pore path and the applied pressure... 

Two procedures for metal introduction in chitosan base were used impregnation and coprecipitation. According to the first procedure the metal deposition on chitosan micro beads was carried out from aqueous and alcohol solutions of NazPdCU, HzPdCU, RhCb, Rh2(CH3COO)4, ZnS04 and Pb(CH3COO)2. Pd and Pb/Zn in bimetallic catalysts was deposited by subsequent precipitation. Pd-Pb (Zn) atomic ratios were 1/1. Metal contents in the resulting samples were 0.5 - 4%. 

In general, activities of chitosan based catalysts prepared by impregnation method in hydrogenation of unsaturated organic compounds were comparable with those of traditional heterogeneous catalyst (as calculated per 1 mole of metal). It should be noted that the chitosan pretreatment influenced very much the catalyst activity. For instance, immediate Pd deposition from alcohol solution on dry chitosan fibers or micro beads led to almost completely inactive catalytic systems, regardless of the metal content. On the other hand, metal deposition on chitosan micro beads or fibers preliminary swollen in water dramatically improved the catalytic activity. 

The catalysts (MPcS chitosan) were obtained by impregnation of chitosan aerogel beads with an aqueous solution of sulfonated metal phthalocyanine. After impregnation, the solids were dried again under supercritical CO2 conditions. The textural properties are maintained and surface areas were greater than 140 m g (Table 4). 

As an example, Libanati et al studied catalyst deactivation by exposure to HMDS in a commercial catalyst reactor. The composition of the reactant mixtures was 88% ethanol and 12% n-propyl acetate. The total inlet concentration was approximately 1500 ppm (as Ci hydrocarbon). The design conditions were an inlet catalyst temperature equal to 616 K and a flow rate of 283 NmVmin. Individual catalyst beads of approximately 3.1 mm diameter were packed in a bed with 17.8 cm in depth. The catalyst was Pt-Pd/y-AbOs, impregnated to produce a 50-100 pm active metal eggshell layer on the outside of 3.1 mm beads. 

Hydrophilic phosphines can also be used to form palladium complexes, which can be absorbed on the surfaces of hydrophilic supports, such as silica. In this case, the catalyst is fully immobilized, though not chemically bonded, so it can easily leach to any polar solvent. Such an approach is referred to as glass bead technologyHere water is used only in the process of preparation of the catalyst, which is impregnated onto the support in aqueous solution. 

In the following, the different methods to create libraries of oxide materials will be discussed. We place only little emphasis on the synthesis of supported catalysts or modified zeolites by ion-exchange, wet impregnation, or other methods. Such procedures usually rely on custom manufactured prefabricated, typically oxidic support materials and oxides are not necessarily formed. To a much larger extent than in the carrier beads, which are used in the split-and-pool approach (see Section 12.3.6), the support material plays an essential role in the final materials, be it for high dispersion of a noble metal compound or by providing catalytically active sites as in a zeolite. 

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