Reversible colour changes in zeolite A treated with ammonium polysulfide

Abstract

A method for synthesis of ultramarine analogues, that consists of generation of the radical anion S₃˙^– inside zeolites has been developed, previously. Ammonium polysulfide has been used as the S₃^– radical precursor for modification of zeolites A, X and hydroxysodalite. It was found that modification of zeolite NaA results in a thermochromic effect; the product being blue at temperatures above 520 °C and almost colourless on cooling; the colour change is reversible. The sample preserves the LTA structure regardless of the thermal treatment. The original structure is also preserved in the other modified zeolites studied although hydroxysodalite turns blue whilst zeolite X remains colourless.