Coprecipitation techniques

Ferric hydroxide coprecipitation techniques are lengthy, two days being needed for a complete precipitation. To speed up this analysis, Tzeng and Zeitlin [595] studied the applicability of an intrinsically rapid technique, namely adsorption colloid flotation. This separation procedure uses a surfactant-collector-inert gas system, in which a charged surface-inactive species is adsorbed on a hydrophobic colloid collector of opposite charge. The colloid with the adsorbed species is floated to the surface with a suitable surfactant and inert gas, and the foam layer is removed manually for analysis by a methylene blue spectrometric procedure. The advantages of the method include a rapid separation, simple equipment, and excellent recoveries. Tzeng and Zeitlin [595] used the floation unit that was devised by Kim and Zeitlin [517]. 

The coprecipitation technique was based on the dropwise addition of a synthetic polymer solution, in a solvent mixture, into an aqueous protein solution under magnetic stirring. The progressive interaction between the water insoluble polymer and the protein gave rise to the microsphere formation. The glycolipid was then added as an aqueous dispersion to the nanoparticle suspension. No sedimentation was observed after several weeks of storage at room temperature. 

Table 1. Some industrially relevant catalysts and supports obtained by precipitation or coprecipitation techniques. ...

Khoudiakov, M., Gupta, M. C. and Deevi, S. (2005). Au/Fe203 nanocatalysts for CO oxidation A comparative study of deposition - precipitation and coprecipitation techniques. Appl. Catal. A-Gen. 291(1-2), 151-161. 

The coprecipitation technique is also very applicable to this type of separation. Here centrifugation or suction systems are often used to speed up the filtration and washing time. 

In the coprecipitation method, trace metals are concentrated by adsorbing onto the surface of a precipitate. As the precipitate, hydroxides of Fe(III), Mn(IV), Al(IH), Bi(II), and Zr(IV) and sulfides of Co(II), Pb(II) and Fe(II) are frequently used. In AAS the use of hydroxide precipitates is more popular than that of sulfides. Coprecipitation using the hydroxide precipitates is generally unspecific, and many trace metals may be concentrated simultaneously. However, some limitations are often found in the coprecipitation technique [10],... 

DETERMINATION OF TRACE ELEMENTS IN SEAWATER AND RECOVERIES USING THE ZIRCONIUM-COPRECIPITATION TECHNIQUE... 

The membranes are designed to retain strontium specifically. The disk can retain 3 mg strontium without significant loss (against 8 mg for the Eichrom s 2 mL Sr-spec columns). Some information provided by the manufacturer and other users indicate possible interferences. Presence of cations such as Na", K", NH/ and Ca " can interfere with strontium fixing. If necessary, a preconcentration with the Ca-phosphate coprecipitation technique can be performed to eliminate some interfering elements. 

At a location in the Sargasso Sea (Station 5 of EN-073, 33° N, 72° W) a series of samples through the water column was processed to compare the solution-phase 8-HQ chelation shipboard analyses with the cobalt-APDC coprecipitation technique ashore after 2.5-months storage of the samples... 

The need for precise control of properties has resulted in modifications of the mixed oxide technique. These include chemical coprecipitation from various precursors , freeze-drying, and molten salt synthesis. The coprecipitation technique is particularly suited to large scale production. 

Cu-, Ni-, and noble metal-supported catalysts Fixed-bed reactor, 100-600°C H20/EtOH = 6-10 LHSV = 1.6-2.0 h"1 Catalysts were prepared by impregnation and coprecipitation techniques. Among the catalysts tested in the SRE reaction, the Cu0/Zn0/Al203 exhibited better performance. Sheng et al.266... 

Up to now several methods have been used to prepare iron molybdates, the most part of them based on coprecipitation techniques. Previous studies [11] have evidenced that the catalytic behavior of Mo-Fe oxides depends on many variables of the coprecipitation procedure starting compounds, concentration of parent solutions, pH and temperature of coprecipitation step, order of addition of parent solutions, ripening etc. In a typical preparation procedure iron molybdate is coprecipitated from solutions of ferric chloride or ferric nitrate and ammonium molybdate [8]. The control of all the above mentioned procedure variables, strongly difficult the preparation of these catalysts and deviations from the preparation recipe can have very adverse effects on the performances of the catalyst from the standpoint of activity, selectivity and stability. 

Sol-gel techniques provide interesting routes to prepare iron molybdates catalysts for selective oxidation of methanol. Catalysts prepared by this method have higher surfiice areas than catalysts prepared by coprecipitation techniques what allows to operate at lower tempo tures with the advantage of limiting the consecutive oxidation of formaldehyde to CO. 

PCL/CPE <= secondary crystallization coprecipitation technique + melt crystaUization [Defieuw and Groeninckx, 1989a]... 

When coprecipitation techniques are used to prepare a chromia-alumina catalyst the situation is somewhat more complicated in that not only must one insure a proper dispersion of the two oxide phases, but at the same time care must be taken to obtain the desired surface texture. The concentrations of metal ions in the precipitating solution, the pH and temperature at which the gel is precipitated, and the method of drying the gel are but a few of the many variables which influence the surface texture of the final catalyst. 

It would be expected that chromia-alumina catalysts prepared by coprecipitation techniques should differ from impregnated catalysts, and this difference has been demonstrated by Eischens and Selwood (5) who measured the magnetic susceptibility of a chromia-alumina catalyst (35 wt % Cr) prepared by coprecipitation with ammonium hydroxide from a solution of aluminum nitrate and chromium nitrate. The susceptibility of the reduced catalyst indicated a much greater dispersion of the chromium than was characteristic of the impregnated catalysts. This was attributed to the presence of a three-dimensional dispersion of the chromium in the coprecipitated catalysts, as compared to a two-dimensional dispersion in the case of the impregnated catalysts. 

The first of these new cobalt catalysts were made in 1986 by coprecipitation techniques using aqueous solutions with ammonium bicarbonate as the precipitant in a similar way to the methods used for methanol synthesis catalysts. The new catalysts were immediately found to be very active and selective catalysts for the conversion of syngas into hydrocarbons. A particularly attractive feature was their low methane make and tolerance of CO2 The CO2 tolerance was ascribed to the interplay between the support and the cobalt phase both in the oxidized and reduced forms. The general belief is that the support stabilizes the cobalt phase such that the catalyst can be operated at the higher temperatures, required to maintain activity despite competitive adsorption by CO2, without any loss in stability. Other investigators e.g. Shell have used similar strategies [2]. 

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