Evolution of the crystallographic structure and physicochemical aspects of rectorite upon calcination

Rectorite (Rec) with exchangeable cations has a wide range of potential applications, including adsorbents, catalyst supports, high-temperature binders, and flame retardant additives. The understanding of the variation of rectorite upon thermal treatment is vital for exploiting its application potential. This paper reports on the thermal decomposition and structural evolution of Rec in the temperature range of 25–1200 °C. Thermogravimetric (TG) analysis, in-situ X-ray diffraction (XRD), and in-situ Fourier transform infrared (FTIR) spectroscopy were used to trace real-time variations in rectorite upon calcination. Then, pristine rectorite and its calcined samples were systematically characterized by 29Si and 27Al magic-angle sample spinning nuclear magnetic resonance (MAS NMR), Brunauer-Emmet-Teller (BET), Zeta potential, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) techniques. The results showed that Rec went through the following four stages after thermal treatment: dehydration at room temperature (RT) to 300 °C, dehydroxylation at 500–700 °C, structural collapse at 1000 °C, and formation of a new phase above 1100 °C. Furthermore, SiO4 tetrahedral units and AlO6 octahedral units in Rec were distorted around 1000 °C, so that the maximum contents of active SiO2 and Al2O3 could be simultaneously obtained. The atomic-level structural evolution mechanism of Rec upon calcination is proposed.