Silanization of heat-treated halloysite nanotubes using γ-aminopropyltriethoxysilane

摘要

By X-ray diffraction, thermal analysis, diffuse reflectance infrared spectrometry, solid state nuclear magnetic resonance (H-1 and Si-29), and transmission/high-resolution transmission electron microscopy, silanization of heat-treated halloysite samples (300-1200 degrees C calcination) using gamma-aminopropyltriethoxysilane through condensation reaction was investigated. Driven by calcining at higher temperatures, the crystalline structure of halloysite transformed to metahalloysite (500-900 degrees C), mixed phases of gamma-alumina, silica, and primary mullite (similar to 1000 degrees C, accompanied by an exothermic reaction), and secondary mullite (1200 degrees C), sequentially; while the lower calcination temperature 300 degrees 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 degrees C with additional surface mottling at temperatures >= 600 degrees C, but got damaged at higher temperatures >= 1000 degrees C. For the still tubular-shaped calcined samples, the hydroxyl distribution of the 300 degrees C-calcined sample was similar to that of raw halloysite; while for other samples from 600 to 900 degrees C calcination, the inner-surface Al-OH groups gradually decreased until completely diminished, and the external surface Si-OH groups reached maximum at 700 degrees C with decreasing trends as the temperature deviated from 700 degrees C. The 700 degrees 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.