Formation of crystalline sulfates from Al13 polymer solutions: effect of washing on the transformation of type I to type II crystals

Abstract

The formation of the Al₁₃ tridecamer, AlO₄Al₁₂(OH)₂₄(H₂O)₁₂⁷⁺, from 0.25 mol dm^–3 Al³⁺ aqueous solutions progressively neutralized by 0.25 mol dm^–3 NaOH aqueous solution at 80 °C was followed by ²⁷Al NMR spectroscopy. By adding either Na₂SO₄ or (NH₄)₂SO₄ solution to these Al₁₃ tridecamer solutions and then ageing the resulting precipitates for 7–70 days at 20 °C, the crystalline sulfates of the Al₁₃ polymer were prepared, and characterized by means of XRD, SEM, TG–DTA–MS and ²⁷AI MAS NMR. It was found that a monoclinic sulfate crystal with a space group of P2/a (type I) was formed in the solution with a hydrolysis ratio, R, above 2.2 after a prolonged ageing of more than 7 days. The crystal was transformed into a new type of sulfate crystal (type II) by washing with redistilled water. Thermal behaviours of both crystals on heating were compared.