Changes in Structure, Morphology, Porosity, and Surface Activity of Mesoporous Halloysite Nanotubes Under Heating

Abstract

Abstract-The objective of the present study was to investigate changes in the structural, textural, and surface properties of tubular halloysite under heating, which are significant in the applications of halloysite as functional materials but have received scant attention in comparison with kaolinite. Samples of a purified halloysite were heated at various temperatures up to 1400ºC, and then characterized by X-ray diffraction, electron microscopy, Fourier-transform infrared spectroscopy, thermal analysis, and nitrogen adsorption. The thermal decomposition of halloysite involved three major steps. During dehydroxylation at 500À900ºC, the silica and alumina originally in the tetrahedral and octahedral sheets, respectively, were increasingly separated, resulting in a loss of long-range order. Nanosized (5À40 nm) g-Al 2 O 3 was formed in the second step at 1000À1100ºC. The third step was the formation of a mullite-like phase from 1200 to 1400ºC and cristobalite at 1400ºC. The rough tubular morphology and the mesoporosity of halloysite remained largely intact as long as the heating temperature was <900ºC. Calcination at 1000ºC led to distortion of the tubular nanoparticles. Calcination at higher temperatures caused further distortion and then destruction of the tubular structure. The formation of hydroxyl groups on the outer surfaces of the tubes during the disconnection and disordering of the original tetrahedral and octahedral sheets was revealed for the first time. These hydroxyl groups were active for grafting modification by an organosilane (g-aminopropyltriethoxysilane), pointing to some very promising potential uses of halloysite for ceramic materials or as fillers for novel clay-polymer nanocomposites.

From platy kaolinite to aluminosilicate nanoroll via one-step delamination of kaolinite: Effect of the temperature of intercalation

“…7). These exotherms are attributable to the formation of γ-Al 2 O 3 and the phase transformation from γ-Al 2 O 3 and amorphous SiO 2 to mullite, respectively, according to previous reports on the structural changes of nanotubular halloysite under calcination (Smith et al, 1993;Yuan et al, 2012b). The exotherm at approximately 996°C of natural kaolinite or halloysite has been comprehensively discussed in the literature; it is commonly understood that this reaction is triggered by the removal of the last hydroxyl groups and accompanied by the formation of a distinct alumina-rich phase.…”

“…Through a comprehensive 29 Si and 27 Al MAS NMR study, Smith et al (1993) proposed that the exotherm at 996°C of halloysite should be attributed to the formation of γ-Al 2 O 3 on a very fine scale (b5 nm). This assumption is supported by Yuan et al (2012b) in a study on the calcination of natural halloysite.…”

Natural halloysite nanotubes as mesoporous carriers for the loading of ibuprofen

“…This observation agrees with our recent results showing that the tubular morphology of halloysite remained largely intact when the heating temperature did not exceed 900°C [46].…”

Mineralogy and Physico-Chemical Data of Two Newly Discovered Halloysite in China and Their Contrasts with Some Typical Minerals

“…The double peaks (peak 1 and 2) correspond to the smaller inter-layer pores and larger tube lumens in the samples, respectively. Peak 1 in the smaller size side (2~8 nm) is due to the internal/surface pores, artificial pores caused by the tensile strength effect and the newly formed mesopores during dehydration of tubular halloysite, including spaces between the overlaps of folded halloysite nanotubes [60]. The sharp peaks at 7.5 nm for Hal-LF and Hal-TL should also be regarded as the longitudinal pores created by the tightly connected rolled tubular halloysites in the drying or dehydration process, confirming the regularly arranged nanotubes in these two samples [61].…”