Aluminum phosphate precipitate, composition

If catalysts are prepared by coprecipitation, the composition of the solutions determine the composition of the final product. Often the composition of the precipitate will reflect the solution concentrations, as was shown for CuO/ZnO catalysts for methanol synthesis [18], but this is not necessarily the case. For al-minum phosphates it was found that at low P A1 ratios the precipitate composition is identical to the solution composition, but if the P A1 ratio in the solution comes close to and exceeds unity, the precipitate composition asymptotically approaches a P A1 ratio of 1 [19]. Deviations from solution composition in coprecipitation processes will generally occur if solubilities of the different compounds differ strongly and precipitation is not complete or, if in addition to stoichiometric compounds, only one component forms an insoluble precipitate this the case for the aluminum phosphate. 

Variation in the solution pH values was only slight (Table I) the values did not appear to be affected significantly by the precipitation reaction. The precipitates were amorphous and dissolved in IN HCl almost instantaneously no gibbsite or any crystalline phase of aluminum phosphate was observed with x-ray diffraction or electron microscope. No change in composition, crystallinity, and morphology was noticed during aging up to at least six months. 

There are several methods to prepare amorphous silica by precipitation acidification of an aqueous solution of sodium silicate [9] or gelification of a silicon alkoxide solution [10]. As we want to use the same method to get the simultaneous coprecipitation of all the ions in the whole composition range, we choose to neutralize, by ammonia, a clear acid solution containing aluminum, phosphate and partially hydrolyzed tetraethylorthosilicate (TEOS). 

Tnteraction of aluminum and phosphate has long been a problem of interest to soil scientists and water chemists. Many different precipitates have been prepared by mixing solutions of aluminum salts and phosphate (4, 5, 6, 17). Some of these precipitates are crystalline, but many others are amorphous. Some have definite stoichiometric compositions, but many others do not. Some are dissolved in IN HCl almost instantaneously, but others can be dissolved only after prolonged heating. On the other hand, the formation of soluble complexes of aluminum and phosphate has been suggested (3y 8,14). 

Composition of Precipitate. Following the reasoning described, the precipitation of phosphate becomes the following simple neutralization of the net positive charge of aluminum polymers by phosphate ... 

Table II. Data Illustrating the Precipitation of Aluminum by Phosphate and the Composition of Precipitate from the Reaction Between AICI3 and NaH2P04. ...

The chemical composition of the samples has been compared with the composition of the initial solution. The precipitate contains the same relative amounts of aluminum and silicon as in the solution before precipitation. But the phosphate content shows significant variations which can be correlated with the overall composition of the powder. 

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