Sequential precipitation

Both components are soluble in ethyl acetate at room temperature the resulting salt is not. By dissolving the components sequentially, precipitation of the salt is generally avoided. Should the salt form, however, it must be dissolved by gentle heating. 

Even though in the THOREX process 233U can be preferentially recovered from irradiated thorium fuel by using an extraction flowsheet based on 5% TBP n-dodecane as the extractant, further lowering of the concentration of TBP in the solvent has certain advantages in terms of reduced co-extraction of thorium and fission products (195, 196). Ramanujam et al. reported a sequential precipitation technique... 

If catalysts are prepared by coprecipitation, the relative solubilities of the precipitates and the possibility for the formation of defined mixed phases are essential. If one of the components is much more soluble than the other, there is a possibility that sequential precipitation occurs. This leads to concentration gradients in the product and less intimate mixing of the components. If this effect is not compensated by adsorption or occlusion of the more soluble component, the precipitation should be carried out at high supersaturation in order to exceed the solubility product for both components simultaneously. Precipitation of the less soluble product will proceed slightly faster, and the initially formed particles can act as nucleation sites for the more soluble precipitate which forms by heterogeneous precipitation. The problem is less crucial if both components form a defined, insoluble species. This is for instance the case for the coprecipitation of nickel and aluminum which can form defined compounds of the hydrotalcite type (see the extensive review by Cavani et al. [9] and the summary by Andrew [10]). 

Figure 3. Possible implementations of precipitation processes (after [14]). In the batchwise process (a) the pH and all other parameters except for the temperature change continuously during the precipitation due to consumption of the metal species. Coprecipitation should be carried out in the reversed arrangement by addition of the metal species to the precipitating agent to avoid sequential precipitation. In process (b) the pH is kept constant, but the batch composition and the residence time of the precipitate change continuously. In process (c) all parameters are kept constant.

Another problem with the use of coprecipitation to prepare supported catalysts is that this procedure places a relatively large amount of the active metal inside the particles of the support material and, therefore, unavailable for reaction. A more efficient arrangement is to have the active material on or near the surface of the support particles. One way of accomplishing this is through a sequential precipitation procedure in which, for instance, a precipitate of Ni(OH)2 is formed on a freshly prepared suspension of Al(OH)3.20 in coprecipitation the metal content of the precipitate is reasonably continuous throughout the precipitate. As illustrated in Fig. 13.2, though, in a sequential precipitation the first drops of... 

Ternary systems have been prepared in this way as well. The sequential precipitation of aluminum hydroxide, lanthanum hydroxide and, finally, nickel hydroxide gave, after calcination and reduction, a lanthamun activated Ni/Al203 catalyst which had smaller metal crystallites and was somewhat more active than a catalyst prepared by the simultaneous coprecipitation of the three... 

After isolation the supported precipitate is washed, dried and usually calcined to produce a supported oxide which is then reduced, commonly in a hydrogen stream. Reduction of these supported oxides generally proceeds more readily than the mixed oxides produced by coprecipitation since there is only a monolayer in which there is a direct interaction of the active component with the support. This monolayer can be considered to be a silicate or aluminate which is more difficult to reduce than the oxide or hydroxide found in the outer metal-containing layers.33 Precipitation-deposition gives catalysts having compositions similar to those produced by sequential precipitation as shown in Fig. 13.2. 

Colloidal Pt/RuO c- (C5 0.4nm) stabilized by a surfactant was prepared by co-hydrolysis of PtCU and RuCls under basic conditions. The Pt Ru ratio in the colloids can be between 1 4 and 4 1 by variation of the stoichiometry of the transition metal salts. The corresponding zerovalent metal colloids are obtained by the subsequent application of H2 to the colloidal Pt/Ru oxides (optionally in the immobilized form). Additional metals have been included in the metal oxide concept [Eq. (10)] in order to prepare binary and ternary mixed metal oxides in the colloidal form. Pt/Ru/WO c is regarded as a good precatalyst especially for the application in DMECs. Main-group elements such as A1 have been included in multimetallic alloy systems in order to improve the durability of fuel-cell catalysts. PtsAlCo.s alloyed with Cr, Mo, or W particles of 4—7-nm size has been prepared by sequential precipitation on conductant carbon supports such as highly disperse Vulcan XC72 [70]. Alternatively, colloidal precursors composed of Pt/Ru/Al allow... 

The conditions and procedures of precipitation play a significant role in catalyst morphology, texture, pore structure, physical strength, and consequently the performance (activity, selectivity, and stability) of the catalyst. By merely changing the sequence of solution addition, catalyst components can be precipitated simultaneously or sequentially. The methods of precipitation most often used are constant pH coprecipitation, sequential precipitation, acid-to-base precipitation, and base-to-acid precipitation. 

Sequential precipitation is used when a nonuniform distribution of catalyst components throughout a catalyst particle is desirable. In this method, the pH of the precipitation is controlled at specific levels at different... 

In another method, designated the sequential precipitation method, an aqueous solution of ruthenium trichloride is contacted with the ammoniacal hydrazine solution (3). The resulting precipitate is filtered out of solution and reslurried in water, after which a solution of copper nitrate is added to the slurry. On subsequent addition of ammoniacal hydrazine solution, further precipitation occurs in the presence of the original precipitate. The total precipitate is then dried and reduced in the same manner as the coprecipitated preparations. 

The pharmaceutical industry would prefer to make the gamma globulin/al-bumin separation with a UF membrane rather than Cohn fractionation (sequential precipitation with ethanol). But to do so, they would have to dilute the mixture way down to accomplish the separation. The processing of the large diluted volumes followed by concentration of the two fractions would make the membrane process more cumbersome and expensive than Cohn fractionation. 

Blood Plasma Processing. Purified therapeutic proteins are derived from human blood plasma via cryoprecipitation followed by sequential precipitations effected through increasing ethanol concentrations at controlled temperature, pH and ion composition ("Cohn precipitation"). As discussed in the section on fractionation of solutes, UF cannot be used to fractionate the various plasma... 

The above general remarks apply also to mixed precipitates of metal hydroxides as well as to silica—hydrated metal oxide mixed gels. As would be ejq)ected, products obtained by simultaneous precipitation of two (or more) compounds differ from those obtained by mechanical mixing of finished gels both of these types of gels are in turn different from mixed gels produced by sequential precipitation in the same solution. Finally, we should mention the so-called chalky silica gels (see p. 1656). 

Thus, there is in essence a critical supersaturation concentration below which nucleation is very slow and above which nucleation is very fast. The range between this critical supersaturation concentration and the solubility concentration is the domain of deposition-precipitation discussed elsewhere (Chapter 6), while impregnation (Chapter 4) is performed in the concentration range below the solubility limit. Especially for multicomponent systems, it is important to operate at high supersaturation levels so that the solubility product of all products is simultaneously exceeded, otherwise a possibly undesired sequential precipitation may occur. 

Figure 7.5 Effect of silicate addition temperature on pore size in nickel/silica catalyst made by sequential precipitation [21].

Fig. 9.2 Illustration of phase diagrams fw the sequential precipitation of various molecular weight fractions M with the increase of the interactimi parameter y...

As the pH increases in order to precipitate magnesium, the equilibrium solubility of silicate increases. In other words, the equilibrium concentration of the magnesium ion cannot be kept low by Eq. (73) to satisfy the demands of the downstream process and still be high to drive the reaction (72). The method outlined above therefore appears to be a nonequilibrium process that depends on the sequential precipitation of the two species. 

The preparation method significantly influence the crystallinity of the materials synthesised. Compounds synthesised under low supersaturation (LS) conditions are more ciystalline than by sequential precipitation (SP). Furthermore, the ciystallinity also increases with increase in atomic ratio. Hydrothermal treatments increased the ciystallinity of the material. This result is corroborated with the reduction in the surface area of the hydrothermally treated samples. However, hydrothermal treatments performed on Co-Fe-COg-HTlcs resulted in the... 

The crystallinity of the material is also dependent on the natiure of the trivalent metal ion present in the network. C0-AI-CO3-HT are more oystalline in comparison with Fe and Cr containing samples. In the case of C0- -CO3-HT, preparation by sequential precipitation yielded amorphous material whereas preparation under low supersaturation resulted in a better oystalline material. In our Co-Fe containing samples, it is not completely possible to exclude the presence of hexagonal Co(OH)2 prepared under sequential precipitation. These results indicated that presence of Arfavours the formation of crystalline HTlc phase which is in accordance with the results observed by Clause et al ftn nickel containing hydrotalcites [7]. TEM results showed spherical to hexagonal platelets of thin and wide nature characteristic of these materials [3]. 

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