Optimizing conditions for sepiolite-based Maya blue Pigments: Insights into thermal treatment and coloration

Abstract

Thermal treatment is a key step in synthesizing Maya blue pigments. While palygorskite-based Maya blue has been extensively studied, the preparation of sepiolite-based pigments remains underexplored. This study synthesized sepiolite-based Maya blue by heating indigo-sepiolite mixtures at 100–500 °C for 1–72 h, systematically investigating the effects of temperature and duration on pigment color and stability. Mechanistic insights were provided using XRD, surface area and micropore analysis, HRTEM, and FTIR. The optimal preparation conditions were identified as 100–200 °C with heating durations under 8 h. Below 200 °C, increasing temperature and time enhanced the greenish-blue hue and improved chemical and photostability due to the conversion of indigo oligomers to monomers and their diffusion into sepiolite channels. No evidence of strong interactions, such as hydrogen bonding or coordination, was found between indigo and sepiolite. Indigo encapsulation within sepiolite's channels likely explains its stability, while the conversion of intermolecular to intramolecular hydrogen bonds accounts for its coloration. Above 300 °C, indigo degradation caused pigment darkening. HRTEM showed no significant structural changes in sepiolite after heating. Sepiolite's nanostructure, microporosity, and diverse surface groups make it ideal for clay-dye hybrid pigments. These findings advance the understanding and application of sepiolite-based organic-inorganic hybrid materials.