Methane/water Adsorption Properties of Synthetic Imogolite Nanotubes
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Methane/water adsorption properties of synthetic imogolite nanotubes Fumihiko Ohashi, Shinji Tomura and Shin-Ichiro Wada1 Materials Research Institute for Sustainable Development, AIST Chubu, Shimo-Shidami, Moriyama, Nagoya 463-8560, JAPAN 1 Agricultural Department, Kyushu University, 6-10-1 Higashi, Hakozaki, Fukuoka 812-8581, JAPAN ABSTRACT Aluminosilicate nanotubes (imogolite) have been synthesized from highly concentrated inorganic solutions by hydrothermal treatment. These can be converted to microporous nanofibers with a pore radius in the range of 0.3-0.6 nm referring to the results from the nitrogen adsorption isotherm. The water vapor adsorption isotherms indicated that the natural imogolite plotted a proportional isothermal curve where the amount of adsorbed water increased in proportion to P/P0: the maximum amount of adsorbed water was ca. 60 wt%. The synthetic imogolite showed a rapid increase at 0.9-0.95 range of P/P0 and achieved a maximum of ca. 80 wt%, with a better methane storage property than that of the usual compressed natural gas storage. In order to obtain a high ratio of water adsorption and a large methane storage capacity, it is necessary to control the micro/meso porous structure and the hydrophilic/hydrophobic surface affinity. It is expected that the synthetic imogolite might become a multipurpose adsorbent. INTRODUCTION Imogolite is a naturally occurring hydrated aluminosilicate polymer consisting of tubular structural units with an external diameter of 2.3-2.7 nm and an internal diameter of ca. 1 nm. The tube length varies from 400 nm (approx.) to several micrometers. The tube walls are composed of curved gibbsite-like sheets with SiOH groups inside and AlOH groups outside, having a composition (HO)3Al2O3SiOH [1]. Imogolite is commonly observed in the clay fraction of soils derived from glassy volcanic ashes or pumice beds, and it has also been found in many podzols [2,3]. Several reports concerning natural imogolite have been published [4-6]. A synthetic pathway of imogolite was described in 1977, which explained the use of dilute solutions of aluminum chloride and monosilicic acid [7]. Crystallization takes place in the slightly concentrated starting solutions of low pH during the boiling point treatment [8]. However, this common process does not necessarily allow a high yield of well-crystallized imogolite tubes. Recently, nanotubes composed of various materials such as carbon, boron nitride, and oxides have been investigated [9-11]. In particular, carbon nanotubes have a great potential as a material with novel properties that are not found in conventional graphite or carbon fullerene, thus extensive studies are conducted on this material; for instance, the suggestion for a highly efficient storage of natural gas with single walled carbon nanotubes, experimental determinations of the storage capacity, and the mechanism of the storage [12,13]. In the present work, the authors attempt to improve a synthetic method and to characterize an aluminosilicate mineral that could form
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