Learning from ancient Maya: Preparation of stable palygorskite/methylene blue@SiO2 Maya Blue-like pigment

Abstract

A facile method for the preparation of stable palygorskite/methylene blue@SiO2 (PAL/MB@SiO2) Maya Blue-like pigment was reported. The PAL/MB pigment was prepared via adsorption of MB by PAL, which was further coated with a layer of SiO2 by polycondensation of tetraethoxysilane (TEOS) with acetic acid (HAc) as the catalyst to form the PAL/MB@SiO2 pigment. The weight ratio of MB to PAL, ball milling time and heating temperature play important roles in affecting stability of the PAL/MB pigment. The MB content in the PAL/MB pigment is up to 12%, which is higher than all the state-of-the-art Maya Blue-like pigments. The MB molecules can only be adsorbed onto the external surface, the grooves and the openings of the channels of PAL, but cannot enter the channels according to the BET, zeta potential, FTIR and XRD analyses. Owing to the fact that the stability of the PAL/MB pigment is not very high, a layer of SiO2 is introduced to shield the MB molecules and further improve the stability. The PAL/MB@SiO2 pigment shows excellent stability against elution, thermal aging and intensive UV irradiation. A TEOS/HAc/H2O molar ratio of 1/2/140 in forming the SiO2 layer is greatly helpful to improve the stability.