Silanization of heat-treated halloysite nanotubes using γ-aminopropyltriethoxysilane

Abstract By X-ray diffraction, thermal analysis, diffuse reflectance infrared spectrometry, solid state nuclear magnetic resonance (1H and 29Si), and transmission/high-resolution transmission electron microscopy, silanization of heat-treated halloysite samples (300–1200 °C calcination) using γ-aminopropyltriethoxysilane through condensation reaction was investigated. Driven by calcining at higher temperatures, the crystalline structure of halloysite transformed to metahalloysite (500–900 °C), mixed phases of γ-alumina, silica, and primary mullite (~1000 °C, accompanied by an exothermic reaction), and secondary mullite (1200 °C), sequentially; while the lower calcination temperature 300 °C failed to change the crystalline structure except for dehydrating surface moisture. In the calcining processes, the tubular morphology of halloysite remained largely unchanged in the range 300–900 °C with additional surface mottling at temperatures ≥600 °C, but got damaged at higher temperatures ≥1000 °C. For the still tubular-shaped calcined samples, the hydroxyl distribution of the 300 °C-calcined sample was similar to that of raw halloysite; while for other samples from 600 to 900 °C calcination, the inner-surface Al-OH groups gradually decreased until completely diminished, and the external surface Si-OH groups reached maximum at 700 °C with decreasing trends as the temperature deviated from 700 °C. The 700 °C-calcined sample with maximum Si-OH groups acting as reactive sites was thus optimal for silanization, giving the highest silane loading of 5.87 mass %, which would depend on the main modification sites at the tube external surface without strict space limitation, and the unignorable oligomerization reaction of silane species besides their grafting reaction in this case. These results would be very helpful for surface properties optimization of halloysite nanotubes by heat-activation followed by silane modification, and hence for the development of halloysite-based advanced materials.