Azo dyes encapsulated within aluminosilicate microporous materials

A novel synthetic method is presented for the encapsulation of azo dyes within a range of zeolites by in situ diazotisation and coupling reactions. The pH changes of methyl red, methyl orange and dimethyl yellow indicator dyes encapsulated in zeolites HY, NaY, ZSM5, mordenite and ultrastable Y were investigated by visual inspection and UV/visible spectroscopy. Changes depended on the concentration of hydrogen ions within the zeolite and, with ageing, equilibration with the external buffer solution produced colour change in the zeolite dye residue. Since the dye molecules were small enough to pass out of the pores of the zeolite, leaching was anticipated but was found to depend on the dye's polarity and nature of the solvent. The industrial application of these solid phase indicators is currently under investigation as is their potential to act as bleed‐resistant pigments. There is some evidence that methyl violet is formed in situ from the sorted coupling component N,N‐dimethylaniline.