Pore Structure of Imogolite Computer Models

Abstract

This study analyzes computer models of the microporous material imogolite. The purpose of this work is to validate computational methods developed for extracting pore size, pore volume, and surface area measurement of molecular level computer models. We accomplished this by comparing model properties with experimental data derived from N2 adsorption isotherms and by simulating CH4 and N2 adsorption and He/SF6 diffusion. Pore volume and pore size are easily determined and fit experimental data within reason for validation purposes. Surface area calculations are at first greater than those determined experimentally. Correcting for the curvature of the pore enables duplication of experimental data within the combined error of both methods. Simulations of adsorption and diffusion fit available experimental results reasonably well and along with previous conclusions allow identification of the most probable model structures. The computer-based method studied should be adequate for analyzing other silicate models such as sol−gel derived microporous membranes, aerogels, and zeolites.

The Surface Chemistry of Imogolite

“…Imogolite displays several unusual features that have not been satisfactorily explained from the present knowledge of the imogolite structure: (1) Imogolite never occurs as single fibers but always in bundles in close hexagonal packing (Farmer et al, 1983;Pohl et al, 1996;Hoshino et al, 1996). (2) Although some imogolite samples have a very low reactivity towards phosphate, imogolite always sorbs chloride in appreciable amounts (Clark and McBride, 1984).…”

“…For imogolite synthesized at 100~ these distances are greater (i.e., --23 and 11 .~) owing to the incorporation of more molecular imogolite units in one unit cell of the tube. The values given above are "average" values from several sources (Cradwick et aL, 1972;Farmer et al, 1983;Wada, 1989;Pohl et al, o Si ee AI O0 0 Oe OH Figure 1. Top: A cross-section of the structure of a 14-molecular unit imogolite tube (from Goodman et al, 1985).…”

“…1996). A unit cell of a tube contains ten molecular units of imogolite (i.e., ten Si atoms and 20 AI atoms) for natural imogolite (Cradwick et aL, 1972;Pohl et aL, 1996), whereas synthetic imogolite is thought to contain either 12 or 14 molecular units (Farmer et al, 1983;Pohl et al, 1996). The formation of imogolite is believed to occur in a gibbsite sheet through the displacement of three hydroxyl groups surrounding a vacant octahedral site by an orthosilicate anion.…”

“…Later research found that imogolite has a widespread occurrence in Andosols and in the B horizon of Spodosols worldwide (Wada, 1989;Gustafsson et al, 1999). It has a large surface area, in the order of 1000 m 2 g-J as measured by ethylene glycol (Egashira and Aomine, 1974) and --300-400 m 2 g-1 as measured by BET (N2) adsorption (Brunauer et al, 1938) at elevated temperatures (Ackerman et al, 1993;Pohl et aL, 1996). Furthermore, imogolite was shown to be reactive towards anions and cations (Farmer et al, 1983;Wada, 1989).…”

Nanotubular Aluminosilicates: A Case Study for Science and Industry

“…One of the first modifications was proposed by Wada and Wada (1982), consisting of partial substitution of Si by Ge, forming aluminogermanates, which have a diameter of 3.3 nm, 50% wider than that determined for imogolite, while its length is 15 nm, 7 times shorter than that obtained for imogolite 10,30,38,56 . Further research on these aluminogermanates indicates that this type of nanotube formation processes are faster and their packing is monoclinic and hexagonal as imogolite 30 .…”