Impregnation diffusional

In order to overcome the main limitations of the impregnation processes, connected to the limited solubility of the compounds in the supercritical fluids, Perman [68] proposed an alternative method. A supercritical impregnation process was coupled with a liquid solvent (preferentially water) to enhance the drug solubilization. The system composed of a liquid drug solution and the polymeric support was pressurized with the supercritical fluid. Consequently, the swelled polymer allows rapid diffusional transport of the solute into the polymeric substrate. In different examples, bovine serum albumin microspheres were impregnated with insulin, trypsin and gentamicin (see Table 9.9-5). 

From these data one can deduce that there exists a proportionate relationship between the concentration of (ri6-C6H6)2V in the pentane phase and the amount of intrazeolitic (n6-CGH6)2V. Clearly under these impregnation conditions there appears to exist no appreciable diffusional barrier to the entry of (u6-C6H )2V into the supercages of NaY. 

Two cases can be distinguished, depending on whether the pore space of the support contains only ambient air at the start or whether it is already filled by the solvent from the impregnation solution (usually water) or by another liquid. Impregnation is said to be capillary in the former case and diffusional in the latter. 

The principle of diffusional impregnation is much simpler than when a precursor-support interaction occurs. Two cases may be distinguished ... 

Diffusional impregnation is almost never used for preparing catalysts when there is no appreciable interaction between the precursor and the support. 

The role of adsorption kinetics and the diffusion of surfactants is especially important in controlling capillary impregnation. According to studies by N.N. Churaev, the solution impregnating the capillary quickly loses its dissolved surfactant due to adsorption of the latter on capillary walls, so the rate of impregnation may be limited by the diffusional transport of surfactant from the bulk of the solution to the menisci in the pores. 

Because of the small pores in zeolitic catalysts, reaction rates may be controlled by rates of diffusion of reactants and products. Beecher, Voorhies, and Eberly (4) studied hydrocracking of mixtures of n-decane and Decalin with mordenite catalysts impregnated with palladium. They found that acid leaching of the mordenite produces an aluminum-deficient structure of significantly higher catalytic activity. At least part of this improvement appears to be caused by the decrease in diffusional resistance. They observed that with this type of catalyst, the effective catalyst pore diameter appears to be smaller than calculated owing to the strong interaction or adsorption of hydrocarbon molecules on the pore walls. 

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]. 

Basically, impregnation has been rationaKzed following two physical models. For the sake of clarity, their presentation starts with the simplest one (wet/diffusional impregnation) that will be further completed to describe dry impregnation. 

In diffusional impregnation, the distribution of the solute inside the wet porosity of the pellet is assumed to be governed by two phenomena [24-27] (Figure 4.1a) the diffusion of the solute into the pores of the pellet, described by Pick s law, and the adsorption of the solute onto the support, which depends on the adsorption capacity of the surface and on the adsorption equilibrium constant. These two parameters are experimentally determined from adsorption isotherms, but the final distribution is not fundamentally changed if equilibrium is not reached and adsorption is ruled by kinetics [28]. 

Impregnation entails wetting a solid support material (typically of high surface area) with a liquid solution containing the dissolved surface oxide precursor. Subclassification of impregnation methods depends upon the relationship between impregnating liquid volume (V ) and support pore volume (Y ) [6] Capillary, dry, or incipient wetness (IW) is often used to describe the process when Vp whereas equilibrium adsorption, diffusional, or ion exchange is preferred when (Section 11.2.2). 

During impregnation, the precursor is deposited on the support from a liquid solution, which in most cases is water-based. If the support surface is hydrophobic or if hydrolysis of the support must be avoided, a nonaqueous solution is used [11]. Typically, the support is immersed in a solution that contains the inert precursor as a metal salt. In the case of capillary impregnation, the support is initially dry, whereas during diffusional impregnation, the support is initially filled with the liquid solvent... 

The calcination step is also called oxidation. In diffusional impregnation the support is saturated with water or with acid solution, and immersed into the acqueous solution containing the metal compoimd. That compoimd subsequently diffuses into the pores of the support through the aqueous phase. Figure 18 shows an example of catalyst finishing (impregnation). 

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