A new hydroxide-based synthesis method for inorganic polymers

Abstract

A new synthesis method is described to produce aluminosilicate inorganic polymers by direct reaction of aluminium hydroxide with amorphous silica under alkaline conditions. XRD and 27Al and 29Si MAS NMR spectroscopy suggest that the reaction mechanism involves the dissolution of the aluminium hydroxide in the alkali. The best results are obtained with more reactive gibbsite of small particle size and X-ray amorphous ρ-alumina containing 5-fold coordinated Al, and with compositions in the previously reported optimum range (SiO2: Al2O3 = 3.0, M2O: SiO2 = 0.34, H2O: M2O = 9.4). Unreacted aluminium hydroxide occurs where there is insufficient silica for complete inorganic polymer formation but the reaction is less sensitive to variations in the K2O: Al2O3 and K2O: SiO2 ratios. Zeolite formation was not observed in any of the present samples. More complex alumina sources such as thermally activated hydrotalcite can also be used to form an inorganic polymer, but under the alkaline synthesis conditions the unused alumina reacts with the poorly crystalline MgO present to re-form hydrotalcite, presumably containing atmospheric carbonate as the interlayer anion. This synthesis method can also potentially be extended to hydroxides other than those of aluminium.