High‐Temperature Speciation Studies of Al‐Ion Hydrolysis

Abstract

Preparation of industrially important aluminium oxide, aluminium hydroxide and aluminium salt materials is often based on the hydrolysis of aluminium-ion solutions. [1] One of the frequently employed processes is neutralisation of Al-ion solutions with alkali or acid. [2] A number of reaction conditions affect the structure and properties of the resulting Alcontaining materials, such as pH, temperature, ionic strength, type of anion, etc. [1,2] Better control of materials synthesis through aqueous routes can be obtained by known speciation of Al-ions. The key problem of Al aqueous speciation in solutions with noncomplexing anions (chloride, nitrate, etc.) is the presence of large Al polycations of the Keggin type ± Al 13 -mer (I) and Al 30 -mer (II). [3,4] These polycations are used in a number of material science applications including clay pillaring, [5] preparation of Al 2 O 3 nanoparticles, [6] antiperspirant actives, [7] catalysts, [8] composite materials, [9] etc. The Al 13 -mer has been studied for decades, [10] whereas the structure of the Al 30 -mer has been identified recently. [3,4] Thermodynamic and kinetic data on the Al 30 -mer are not available in the literature.Aim: Preliminary results of a multi-technique study for Al-speciation are presented for model aqueous systems at high temperature and medium Al concentrations for the formation of large aluminium polycations. Information on the reaction pathway may aid in the development of routes to solutions containing single species that can be used in model studies to investigate the effects of aluminium in the environment as well as in the generation of materials from molecular scale precursors.Results and Discussion: The pH-metric titration curves reconstructed from multi-batch measurements (Fig. 2) show all major inflexions characteristic of the analogous systems at room temperature: [11] (1) h < 0.2 ± hydrolysis of Al monomers; (2) 0.2 < h < 2.6 ± formation of Al polycations including Al dimers/trimers, Al 13 -mers and Al 30 -mers; (3) 2.6 < h < 3.0 ± collapse of Al polycations and formation of the metastable colloidal phase of Al hydroxide; (4) h~3.0 ± bulk precipitation of Al hydroxide; (5) h > 3.0 ± gradual dissolution of Al hydroxide with the formation of Al(OH) 4 ± . At prolonged hydrolysis times the early hydrolysis stages (inflexion 1) did not change profoundly. The greatest time-dependent change was observed at stages (3), (4) and (5) in Figure 2. The major inflexion (4) moves to slightly higher hydrolysis ratios (from h~2.8 at 0 h to h~3.0 at 48 h). Other inflexions retained their position during 48 h of experiment. 27 Al solution NMR spectroscopy revealed a number of Al species present in the sample batches. Assignment of the NMR signals and corresponding Al species is given in Table 1. 27 Al solution NMR spectra of the sample solutions of hydrolysed Al-ions (Fig. 3) indicate all species listed in COMMUNICATIONS 836

Effect of thermal treatment on the formation and transformation of Keggin Al13 and Al30 species in hydrolytic polymeric aluminum solutions

“…Comparing the data listed in Tables 1 and 2, the changing tendency of Al species derived from Al-Ferron method is consistent with that obtained from 27 Our other research indicates that appropriate thermal treatment can enhance the stability of HPA solutions, and prevent gelling and precipitating. A clear solution almost cannot be obtained without thermal treatment or synthesized at high temperature especially for the samples with Al T higher than 1.0 mol L −1 .…”

The static anion exchange method for generation of high purity aluminium polyoxocations and monodisperse aluminium hydroxide nanoparticles

“…As has been shown previously in experiments on Al ion hydrolysis at 90 uC, 15,16 two major processes occur in Al ion solutions at high temperature in the vicinity of the optimum hydrolysis ratio for Al 30 formation (h y 2.40): conversion of the Al 13 to the Al 30 involving consumption of Al monomers and the breakdown of the Al 13 into Al monomers. The increasing concentration of the Al monomer observed in the dynamic speciation diagram (Fig.…”

Large Aqueous Aluminum Hydroxide Molecules

“…The Allouche and Taullele 79 supposition is consistent with the Son et al 87 Remarkably, Fu ¨rrer found that a solution of -Al 13 is metastable for at least 12 years at ambient conditions but that over this time scale, the -Al 13 converts to the Al 30 plus some protons [see Figure 8 in ref 81]. Clearly the elevated temperatures that are most commonly used in synthesis 64,[68][69][70] accelerate the conversion of the -Al 13 to the Al 30 , but the same pathways act at ambient conditions, albeit more slowly. It would involve a substantial set of experiments, but the pH dependence of the conversion could considerably advance the field.…”