Methods of Impregnation

The simplest way to execute impregnation is by contacting a previously dried support, of pore volume Vpj, with a volume V = Vpj of solution containing the precursor of the active phase. The solution is drawn into the pores by capillary suction (hence capillary impregnation ) [12-14]. In the case of proper wetting no excess of solution remains outside the pore space and the procedure is also called dry or incipient wetness impregnation. 

The penetration of the liquid phase requires the elimination of air from the pores. If the pore radius is very small, capillary pressure is much larger than the pressure of the entrapped air compressed air dissolves or escapes from the solid through larger pores [13, 14]. Sometimes, the mechanical strength of the support is not sufficient to withstand forces imposed by the transitory formation of bubbles and the catalyst grains may burst [13]. This can be remedied by impregnation under vacuum or addition of a surfactant to the solution. 

Heat is generally released when a solid/gas interface is replaced by a solid/liquid interface. This might influence the quality of impregnation if the precursor solubility decreases when the temperature increases or if detrimental reactions involving the support take place due to a temperature rise. These drawbacks can be avoided by exposing the support to water vapor before impregnation and thus stabilizing the interface with a hydrous film. 

Impregnation can also be carried out in diffusional conditions, that is, by immersing a water-filled support in the precursor solution ( wet impregnation) [12-14]. Before water filling, air can be replaced by a water-soluble gas like ammonia to avoid gas bubbles in the pores [13]. Wet impregnation should be avoided when the interaction between precursor and support is too weak to guarantee the deposition of the former [15]. 

For powders, it can be convenient to suspend the support in an excess of solution and evaporate the solvent under regular stirring. Such a method can be used for the introduction of poorly soluble compounds, but leads to a dispersed phase only if seeds serving the subsequent growth of the particles have time to form all over the support surface before crystalKzation starts. It is thus advised to check by microscopic methods if the final distribution of the active phase is homogeneous at the scale of the support grain. 

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The process by which porous sintered plaques are filled with active material is called impregnation. The plaques are submerged in an aqueous solution, which is sometimes a hot melt in a compound s own water of hydration, consisting of a suitable nickel or cadmium salt and subjected to a chemical, electrochemical, or thermal process to precipitate nickel hydroxide or cadmium hydroxide. The electrochemical (46) and general (47) methods of impregnating nickel plaques have been reviewed. 

A better combination of fiber and polymer is achieved by an impregnation of [44] the reinforcing fabrics with polymer matrixes compatible with the polymer. Polymer solutions [40,45] or dispersions [46] of ]ow viscosity are used for this purpose. For a number of interesting polymers, the lack of solvents limits the use of the method of impregnation [44]. When cellulose fibers are impregnated with a bytyl benzyl phthalate plasticized polyvinylchloride (PVC) dispersion, excellent partitions can be achieved in polystyrene (PS). This significantly lowers the viscosity of the compound and the plasticator and results in cosolvent action for both PS and PVC [46]. 

A catalytic system Mo-V-Nb-W supported on alumina was prepared by impregnation and investigated for the selective oxidation of propane. The effects of the variation of each metal and of the catalyst preparation were analysed. The results show that Mo and V species supported on alumina can lead to catalysts with high selectivity to propene and reasonable selectivity to acrolein. The presence of Nb and W seems to have little effect. The catalyst can be affected by the method of impregnation. 

A method of impregnating textile fibres with latex. There is no natural affinity between the textile fibre and the latex particle this is overcome by making the latex slightly acid and the surface of the textile strongly alkaline. 

A family of techniques has been developed for the formation and creation of the binder phase. In Fig. 9.2 only the most common method of impregnation and curing is shown with steps 7 10. There are alternative methods that partly deliver much higher quality products but they are far more complex and energy-intensive in their application. Figure 9.3 summarizes the main methodologies. 

Table HI lists the THF solubilities of Illinois 6 obtained as a function of catalyst and method of impregnation. As shown in these dat2 the impregnation by the wet impregnation technique did little in terms of changes in THF solubility.

The method of impregnating liquid membranes has become more and more popular. By impregnating fine-pore polymer films with a suitable membrane liquid, relatively stable heterogeneous solid-liquid membranes are obtained. These membranes are shaped as thin, flat barriers or hollow fibers. Usually they are manufactured from oleophilic polymers, wettable by membrane liquid. The two interfaces, F/M and M/R, have equal or close areas which can be made very large by employing modules of spirally wounded flat membrane or bundles of hollow fibers. 

Fig. 15.14. Comparison of specific catalytic activities of Ni and Cu nanoparticles deposited onto a thermally oxidized silicon by laser electrodispersion (n m 5 x 1012cm-2) with those of catalysts prepared by the method of impregnation and reduction (1% Ni/Si02, 1% Cu/Si02). Reaction of carbon tetrachloride addition to olefins.

Two applications of ML are heterogeneous catalyst and sorbent synthesis. In these cases it is necessary to distribute in regular intervals very small amounts of substance on a surface. It is usually enough to have from 1 to 4 monolayers. In this case each atom in a monolayer can take part in reaction. This is not obtained using traditional methods of impregnation. 

Some experimental data obtained by Smith et al. (18) will be used to illustrate the method of approach. These data were obtained with a pressurized all glass flow reactor at a temperature of 399°. The catalyst used was 0.05% platinum on -alumina prepared by impregnating the alumina with chloroplatinic acid. The chlorine introduced by this method of impregnation was removed to reduce the acidity of the catalyst so that the isomerization of cyclohexene to methyl cyclopentane was suppressed. The amount of conversion was varied by changing the amount of catalyst in the reactor. The catalyst bed in the reactor consisted of 15 ml of crushed vycor (60-100 mesh) in which the desired amount of finely crushed (100-200... 

Silica-supported cobalt catalysts were prepared from cobalt nitrate (Co(N03)2), lanthanum nitrate (La(N03)3) and commercially available silica gel (Fuji Davison, ID gel, 270 m /g) using conventional methods of impregnation [14]. The composition of the catalyst was Co La Si02 = 20 6 87 by weight. The catalyst precursor was dried in air at 120°C and then calcined at 450 °C for 3 h to form supported metal oxides. It was then exposed to hydrogen at 400 °C for 12 h. The mean pore diameter of the catalyst was 8.7 nm. 

Chromia-alumina catalysts are often prepared by procedures other than the method of impregnation. A precipitated chromia was prepared as follows y-alumina was suspended in 25 per cent ammonium hydroxide solution. The mixture was stirred rapidly while chromic nitrate solution was added from a buret. The resulting mixture was then dried, ignited, and reduced in the same manner as for impregnated samples. A total of four samples was prepared. The susceptibility isotherm for this series is of the same general form as for the impregnation series except that point I is virtually absent. But the most striking... 

Resin systems have developed into extremely complex multi-ingredient formulations in an effort to ensure the maximum property benefit from the fiber. Normally there are four methods of impregnation (1) solution dip (2) solution spray (3) direct hot-melt coat and (4) film calendaring. 

The Institute of Natural Fibres in Poznan, Poland has developed new barrier fabrics, both woven and non-woven. The first was a cotton weave containing a small addition of polyester. The latter was made of flame retardant flax and hemp fibres. The flame retardant and its application method were developed by the institute. The flame retardant, called FOBOS M2T, was based on urea polyborates and the sodium salt of alkyl-aryl sulfonic acid. The method of impregnation is cheap and simple. It can be carried out on typical finishing equipment and increase manufacturing costs by only 10-15%. 

Other methods of impregnating a substrate involve depositing an aliquot of solution containing the catalyst precursor onto a substrate and allowing it to air dry... 

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