Structure and Thermal Transformations of Imogolite Studied by ²⁹Si and ²⁷Al High-Resolution Solid-State Nuclear Magnetic Resonance

Abstract

Abstract--Solid-state nuclear magnetic resonance (NMR) spectroscopy, thermal analysis, and X-ray powder diffraction data on the tubular, hydrous aluminosilicate imogolite were found to be fully consistent with a previously proposed crystal structure consisting of a rolled-up, 6-coordinate A1-O(OH) sheet, bonded to isolated orthosilicate groups. The calculated 29Si chemical shift of this structure agreed with the observed shift within 3 ppm. Thermal dehydroxylation of the A1--O(OH) sheet produced predominantly NMR-transparent 5-coordinate A1, but a few 4-and 6-coordinate sites and some residual hydroxyl groups may also have formed, as shown by NMR spectroscopy. Changes in the 29Si NMR spectrum on dehydroxylation suggest a condensation of the orthosilicate groups, but steric considerations rule out bonding between adjacent silicons. To account for these observations, an alternative mechanism to orthosilicate condensation has been proposed, involving the fracture and unrolling of the tubes, followed by the condensation of fragments to form a layer structure. The layer structure has a calculated z9Si chemical shift of -95.6 ppm, in good agreement with the observed value of -93 ppm.

The Structure and Thermal Transformations of Allophanes Studied by 29Si and 27Al High Resolution Solid-State NMR

“…The chemical analyses of all the allophanes are shown in Table 1. Thermal analyses of all samples were carried out as described by MacKenzie et aL (1989), and the results were used to determine the heating temperature in a series of sequential isothermal heating experiments. After each 15 min heat treatment, the samples were cooled in a desiccator to prevent rehydration, and examined by X-ray powder diffraction and high-resolution solid-state MAS NMR (see MacKenzie et aL, 1989).…”

“…They occur as a major component of soils derived from volcanic ash, and, less commonly, as naturally precipitated gels. The SIO2/ A1203 molar ratio of allophanes can vary from about 1:1 (as in the more crystalline tubular form imogolite (MacKenzie et aL, 1989), to about 2:1 (Wada, 1967). The total water content of allophanes from various sources is about 35.5 to 37.5 wt.%.…”

“…Another school of thought (Parfitt et aL, 1980) suggests that aUophane has a similar structural unit to tubular imogolite, which is based on a gibbsite-like hydroxy-aluminum sheet (MacKenzie et al, 1989). Where the SiOffA1203 ratio is approximately one, the structure is considered to contain imogolite-like isolated SiO4 units, whereas in aUophanes of SIO2/A1203 ratio of about two, the additional silicate units are thought to be condensed at one or both sides of the gibbsite sheet (Parfitt et aL, 1980).…”

“…High-resolution solid-state 298i and 27A1 NMR can provide structural information about poorly crystalline phases, especially when used in conjunction with thermal studies as in the case of imogolite (MacKenzie et al, 1989). Previous solid-state NMR studies of natural allophanes (Goodman et al, 1985;Shimizu et aL, 1985) indicated 29Si spectra containing a sharp imogolite-like peak at about -79 ppm, superimposed on a broad, higher-field hump ascribed to polymerized silicon species.…”

Synthesis and Characterization of Fe-Imogolite as an Oxidation Catalyst

“…Recently, imogolite has drawn a new attention as a new nano-material because of its unique nano-scale tubular structure similar to that of a single-walled carbon nanotube [3]. Studies on synthesis [4][5][6][7][8][9], mechanisms of formation [10], structural evolution [11,12], stability [13,14], electronic states [14,15] and application have been carried out. Proposed applications, such as a polymer composite [16,17], a fuel gas storage [18], an absorbent [19], an exchange material for heat pump system [20], a humidity-controlling material [6] and an antideweling material [6] have been discussed.…”