Calcination-induced changes in structure, morphology, and porosity of allophane

Allophane with an Al/Si molar ratio of 1.6 was hydrothermally synthesized, followed by calcination at defined temperatures up to 1300 °C. The heated products were then characterized using a combination of techniques including X-ray diffraction, Fourier transform infrared spectroscopy, thermal analyses, nuclear magnetic resonance, transmission electron microscopy, and N2 physisorption. In allophane, the imogolite local structure (ImoLS) had a relatively low thermal stability, and the adsorbed water was removed at a low temperature. Five major steps were outlined for the thermal evolution process of allophane. (i) At 200 °C or lower, the loss of adsorbed water and partial dehydroxylation of SiOH accompanying with the formation of SiOSi resulted in the loss of ImoLS. (ii) As the temperature rose, further dehydroxylation of SiOH as well as disassociation of AlOH occurred, leading to decreases in the specific surface area and porosity due to the continued agglomeration of hollow spherules, whereas the spherical morphology of allophane was largely retained. (iii) At approximately 500 to 900 °C, the disconnection of AlO octahedra and SiO tetrahedra caused increased disintegration of the allophane structure and formation of amorphous alumina and silica. (iv) At approximately 1000 °C, nanosized mullite was crystallized by the reaction between the yielded amorphous alumina and silica. (v) Finally, further growth of the already-formed mullite crystals and formation of cristobalite occurred, with the consumption of excess silica.