Aspects of solid-state chemistry of fly ash and ultramarine pigments

Abstract

Ultramarine pigments are aluminosilicate-based and contain sulphur-based chromophores. Several samples from two batches of fine fly ash, a predominantly aluminosilicate waste product of coal combustion, were used successfully to synthesise ultramarine blue. This was confirmed by infrared, Raman and X-ray diffraction results. Fly ash had the advantage of being amorphous, whereas the traditional starting reagent, kaolin, needed to be heat-treated before the ultramarine synthesis to weaken its structure. A comparison of the scanning electron micrographs of fly ash, fly ash treated at 1 000°C, fly ash reacted with sodium carbonate at 860°C and the ultramarine products showed that sulphur had a structure-directing effect. The sulphur clusters found in ultramarine pigments were studied by Self-Consistent-Field Hartree-Fock theory extended by Møller-Plesset second order perturbation theory at the minimum energy with the 6-311G** basis set to determine the relative stability of S2 , S2-• , S22-, and S3, S3-•, S32-. The singly charged species were the most stable in both sets, supporting the hypothesis that the exothermic transition from green to blue ultramarine was the transformation of the doubly charged species to the singly charged species. The open, C2v, isomer was most stable for the S3-• molecule - the blue ultramarine chromophore. The S4 molecule was a likely chromophore in ultramarine red. A Woodward-Hoffmann analysis supported the concerted formation of the puckered square S4 , pyramidal S4 , and gauche S4 chain isomers. Other possible species for the red chromophore were S4 -, S3 , S3 Cl, S3 Cl-, S2 Cl, S2 O, and S2 O-. On the basis of their calculated vibrational spectra most of the species could be discounted as possible red chromophores. The best candidate chromophore was the cis S4 chain based on the computed electronic spectrum.