Daoyong Tan

CHANGES IN STRUCTURE, MORPHOLOGY, POROSITY, AND SURFACE ACTIVITY OF MESOPOROUS HALLOYSITE NANOTUBES UNDER HEATING

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 1400°C, 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 500–900°C, the silica and alumina originally in the tetrahedral and octahedral sheets, respectively, were increasingly separated, resulting in a loss of long-range order. Nanosized (5–40 nm) γ-Al2O3 was formed in the second step at 1000–1100°C. The third step was the formation of a mullite-like phase from 1200 to 1400°C and cristobalite at 1400°C. The rough tubular morphology and the mesoporosity of halloysite remained largely intact as long as the heating temperature was <900°C. Calcination at 1000°C 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 (γ-aminopropyltriethoxysilane), pointing to some very promising potential uses of halloysite for ceramic materials or as fillers for novel clay-polymer nanocomposites.

FTIR spectroscopy study of the structure changes of palygorskite under heating

Progressive heat treatment was applied to palygorskite, and the changes of the position and intensity of its infrared vibrations, particularly those in the low wavenumber region, were monitored by use of Fourier transform infrared spectroscopy. Moreover, the responses of the impurities in palygorskite under heating were also examined. Palygorskite is characterized by two bands at 1196 and 647 cm(-1), which are attributed to the asymmetric stretching vibration of the Si-O-Si group that connects the adjacent inverse SiO(4) tetrahedrons and the H(2)O-Mg-H(2)O stretching vibration in the MgO(6) octahedra at the edges of the channels, respectively. The band at approximately 680 cm(-1) is attributed to the overlapping symmetric stretching vibrations of Si-O-Mg and Si-O-Al (VI). In addition, the 865 cm(-1) band corresponds to the amorphous carbonate impurity.

Facile preparation of hierarchically porous carbon using diatomite as both template and catalyst and methylene blue adsorption of carbon products

Hierarchically porous carbons were prepared using a facile preparation method in which diatomite was utilized as both template and catalyst. The porous structures of the carbon products and their formation mechanisms were investigated. The macroporosity and microporosity of the diatomite-templated carbons were derived from replication of diatom shell and structure-reconfiguration of the carbon film, respectively. The macroporosity of carbons was strongly dependent on the original morphology of the diatomite template. The macroporous structure composed of carbon plates connected by the pillar- and tube-like macropores resulted from the replication of the central and edge pores of the diatom shells with disk-shaped morphology, respectively. And another macroporous carbon tubes were also replicated from canoe-shaped diatom shells. The acidity of diatomite dramatically affected the porosity of the carbons, more acid sites of diatomite template resulted in higher surface area and pore volume of the carbon products. The diatomite-templated carbons exhibited higher adsorption capacity for methylene blue than the commercial activated carbon (CAC), although the specific surface area was much smaller than that of CAC, due to the hierarchical porosity of diatomite-templated carbons. And the carbons were readily reclaimed and regenerated.

Methoxy-modified kaolinite as a novel carrier for high-capacity loading and controlled-release of the herbicide amitrole.

Methoxy-modified kaolinite was used as a novel carrier for loading and release of the herbicide 3-amino-1,2,4-triazole, known as amitrole (abbreviated here as AMT). The methoxy modification made the interlayer space of the kaolinite available for AMT intercalation. The AMT loading content in methoxy-modified kaolinite reached up to 20.8 mass% (twice the loading content by unmodified kaolinite). About 48% of this amount is located in the interlayer space. The release profiles of the AMT fit with the modified Korsmeyer-Peppas model. Due to the diffusional restriction of the intercalated AMT by the lamellar structure of the kaolinite and the strong electrostatic attraction between the intercalated AMT and the kaolinite, a slow release of AMT from the methoxy-modified kaolinite was achieved. These results show that the methoxy-modification is a facile method to make the interlayer space of kaolinite available for hosting other guest molecules. The methoxy-modified kaolinite is a promising candidate for high-capacity loading and controlled-release of other molecules such as drugs, agrochemicals, and biochemicals.

Effects of Inherent/Enhanced Solid Acidity and Morphology of Diatomite Templates on the Synthesis and Porosity of Hierarchically Porous Carbon

The inherent or enhanced solid acidity of raw or activated diatomite is found to have significant effects on the synthesis of hierarchically porous diatomite-templated carbon with high surface area and special porous structure. The solid acidity makes raw/activated diatomite a catalyst for the generation of porous carbon, and the porous parameters of the carbon products are strongly dependent on the solid acidity of diatomite templates. The morphology of diatomite also dramatically affects the textural structure of porous carbon. Two types of macroporous structures in the carbon product, the partially solid pillars and the ordered hollow tubes, derive from the replication of the central and the edge pores of diatom shell, respectively. The hierarchically porous carbon shows good capability for the adsorption of solvent naphtha and H(2), enabling potential applications in adsorption and gas storage.

Surface Modifications of Halloysite

Abstract Tubular halloysite is a naturally occurring clay mineral with a characteristic lumen surface, as well as an interlayer surface and an external surface. The chemical environment endows these surfaces with specific chemical reactivity, enabling site-specific surface modification of halloysite. The lumen surface, covered by Al–OH groups, can be modified by covalently grafting organic compounds such as organosilane and organophosphonic acid. This covalent grafting allows durable immobilization of the reactive organic groups on the lumen surface of halloysite. The negatively charged external surface can be modified by coating with positive cations. The interlayer surface can be modified by some specific guest molecules via direct or indirect intercalation, but it cannot be directly grafted with organic compounds because of interlayer hydrogen bonding and limited interlayer space. Using preintercalated halloysite as a precursor, covalent modification of the interlayer surface can be achieved under harsh conditions. Surface modifications can regulate the physical (solubility, dispersion, hydrophilicity/hydrophobicity, rheology, etc.) and the chemical (reactivity, biotoxicity, electrochemistry, etc.) properties of halloysite and improve the performance of halloysite in the applications of clay polymer nanocomposites, controlled release and pollution remediation.