Characterization of synthetic imogolite nanotubes as gas storage

Imogolite is a naturally occurring hydrated aluminumsilicate polymer that consists of a tubular structural unit having an external diameter of 2.3–2.7 nm and an inner diameter of ca. 1 nm. The tube length is from about 400 nm to several micrometers. The tube walls are composed of curved gibbsite-like sheets with SiOH groups on the inside and AlOH groups on the outside, having a composition (HO)3Al2O3SiOH [1]. Imogolite is common 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 investigations concerning natural imogolite have been published [4–6]. A synthetic route for production of imogolite was described in 1977 from dilute solutions of aluminum chloride and monosilicic acid [7]. Crystallization takes place at a lower concentration of the starting solutions and low pH under the boiling point treatment [8]. However, the above common process does not produce 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, which have great potential as materials with novel properties that are not found in conventional graphite or carbon fullerene [12]. For instance, since the suggestion of highly efficient storage of a natural gas with single walled carbon nanotubes, experimental determinations of the storage capacity and the mechanism of the storage have been performed extensively [13]. In the present work, the authors attempt to improve a synthetic method and to characterize an aluminosilicate mineral forming such hollow tubular structures for the purpose of gas storage. Imogolite nanotubes were synthesized from a highly concentrated starting solution under hydrothermal conditions. In a typical synthetic procedure, 66.7 ml of Na4SiO4 solution (concn. 100 mmol/l) was mixed with the same amount of 150 mmol/l AlCl3 solution, and then 1 mol/l NaOH was added dropwise (1 ml/min) into the above homogeneous solution under stirring