MODIFICATION OF HALLOYSITE NANOTUBES FOR ENHANCEMENT OF GAS-ADSORPTION CAPACITY
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MODIFICATION OF HALLOYSITE NANOTUBES FOR ENHANCEMENT OF GAS-ADSORPTION CAPACITY SUNGHO LIM, SOOJI PARK, AND DAEWON SOHN * 1
Department of Chemistry and Research Institute for Convergence of Basic Science, Hanyang University, Seoul 04763, Korea
Abstract—Structure control and quantitative evaluation of porous materials are essential for many industrial and consumer applications of clay minerals, and nanotubular halloysite (HNT) has been used extensively for such purposes; performance enhancements are still needed, however. The objective of the present study was to improve the gas-adsorption capacity of HNT by controlling the particle size and porosity. This was accomplished through acid treatment and particle-size fractionation by centrifugation. Various particle sizes were obtained and porosities ranged from macropores to mesopores. Natural halloysite nanotubes were modified by sulfuric acid in various concentrations to selectively remove the alumina composition of the tubes. X-ray diffraction and energy dispersive X-ray spectroscopy were used to verify the mineralogical and compositional changes. Surface modification by the acid treatment increased the inner space volume of the tubes and decreased the mass of the nanotubes because of the elimination of alumina. The gas adsorption capacity of both natural and modified halloysite nanotubes was measured quantitatively using N2 adsorption and the Brunauer-Emmett-Teller (BET) method, and the morphology was determined from transmission electron microscopy (TEM) images. The results showed that the modified halloysite nanotube was 7.47 times more efficient at gas adsorption than pristine halloysite. Moreover, the dealumination of the surface increased the inner space. Greatly increased porosity characteristics, including gas adsorption and macroporosity, were obtained through modification by acid treatment. Key Words—Gas adsorption . Halloysite . Porous materials . Selective surface modification INTRODUCTION Halloysite is a natural aluminosilicate nanotube with the formula Al2Si2O5(OH)4, a kaolin mineral. This clay material has tube walls that are 20–30 layers thick, with each layer consisting of one alumina (Al-OH) sheet and one siloxane (Si-O-Si) sheet (a one-to-one ratio), thus forming multiple inorganic walls. The inner wall surface is composed of alumina units, and the outer surface area is made of siloxane units. The diameter of the lumen (hollow space or channel inside the tube) ranges from 20 to 30 nm and the outer diameter from 50 to 100 nm (Tazaki 2005; Kamble et al. 2012; Joo et al. 2013; Lvov et al. 2016). Halloysite nanotubes (HNTs) are of great interest due to their porosity as tubular structures, having specific mesoporosity (50 nm) characteristics (Shu et al. 2015). These nano spaces have great potential for use in the creation of nanocomposites in clay materials. HNTs are utilized frequently as additives to other materials (Mroczek et al. 2011; Hong et al. 2013). Owing to their tubular shape with a hollow inside, they can be used as carriers (Shi et al.
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