Calcination-induced structural evolution and thermal degradation mechanism of ethylene glycol-grafted kaolinite

The thermal degradation behaviour of ethylene glycol-grafted kaolinite under an inert atmosphere remains poorly understood, despite the extensive literature on its synthesis and ambient stability.Here, it was systematically investigated using complementary characterization techniques and kinetic analysis.Ethylene glycol was grafted into kaolinite using a dimethyl sulfoxide (DMSO)-intercalated precursor, yielding a product with a basal spacing of 0.94 nm.Upon calcination, the basal spacing decreased stepwise from 0.94 nm to 2 0.86 nm at 450°C, and the (001) reflection disappeared at 500°C.Raman spectroscopy revealed the emergence of the D and G bands of amorphous carbon at 450°C, confirming the carbonization of the grafted ethylene glycol.Elemental analysis (CHNS) data showed that the carbon content decreased from 5.60 to 4.43 wt.% at 550°C, whereas hydrogen-dropped from 2.46 to 0.70 wt.% at 550°C, confirming a hydrogen depleted carbonaceous residue.The nearly constant intensity ratio of the D and G bands (I D /I G , 0.81-0.83),together with the decreases in specific surface area (8.51 to 5.45 m 2 /g) and pore volume (0.04 to 0.03 cm 3 /g), further indicated that carbonization was largely completed by 450°C and that the residue was dense and of low porosity.Kinetic analysis using the Kissinger-Akahira-Sunose (KAS) and Flynn-Wall-Ozawa (FWO) isoconversional methods yielded an average activation energy of ~155 kJ/mol, with the three-dimensional diffusion model (Jander, D3) as the most probable mechanism function.These findings indicated that the thermal degradation of ethylene glycol-grafted kaolinite proceeded through a multi-stage pathway: desorption, deintercalation, degrafting with carbonization, and framework dehydroxylation.