Ion exchange in the charge-balancing sites of aluminosilicate inorganic polymers

Abstract

A simple ion-exchange method is reported for replacing the charge-balancing cations in aluminosilicate inorganic polymers with a range of other cations. Complete exchange of the Na+ in a conventional Na-inorganic polymer by K+, Ag+, NH4+ and Pb2+ was achieved by this method, with a lower degree of exchange by Li+ (82%), Cd2+ (78%) and Mg2+ (57%). The NH4+-compound can be completely exchanged by Ag+, but the exchange of NH4+ for other cations is less efficient in this compound than when using the Na-compound as the starting material. The X-ray-amorphous nature and Al and Si environment of the original Na inorganic polymer are unchanged by the ion exchange process. The K+, Li+ and NH4+-compounds prepared by ion exchange are suitable precursors for crystalline ceramics, forming leucite, spodumene and mullite respectively, upon heating at 1100 °C. The ammonium ion in the NH4+-exchanged compound was shown by FTIR and solid-state MAS NMR to decompose gradually on heating up to 600 °C, at which temperature, a significant proportion of the Al is in 5-fold coordination with oxygen. This suggests that the NH4+-exchanged compound heated at 600 °C may be an efficient catalyst for organic reactions such as the Friedel–Crafts alkylation. The efficient exchange capability of Na aluminosilicate inorganic polymer for Pb2+ and Cd2+ (but not Hg2+) suggests a potential application for this material for the cost-effective removal of heavy metal ions from wastewater streams, while the Ag+-exchanged compound was shown in this work to be a powerful antimicrobial agent against Staphylococcus aureus.