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Thermal transformations of synthetic allophane and imogolite as revealed by nuclear magnetic resonance

Published online by Cambridge University Press:  09 July 2018

M. A. Wilson ,
K. Wada ,
S. I. Wada  and
Y. Kakuto
M. A. Wilson
Affiliation:
CSIRO Division of Coal Technology, PO Box 136, North Ryde, NSW 2113, Australia
K. Wada
Affiliation:
Faculty of Agriculture, Kyushu University 46, Fukuoka 812, Japan
S. I. Wada
Affiliation:
Faculty of Agriculture, Kyushu University 46, Fukuoka 812, Japan
Y. Kakuto
Affiliation:
Faculty of Agriculture, Kyushu University 46, Fukuoka 812, Japan
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Abstract

Imogolite, protoimogolite and two synthetic allophanes (A and B) with very different Si/Al ratios have been pyrolysed over a range of temperatures, and the reactions followed by high-resolution solid-state 27AI and 29Si nuclear magnetic resonance (NMR) spectroscopy. The Si(OAloct)3OH groups of protoimogolite are destroyed at relatively low (∼ 200°C temperatures, whereas the same structures in imogolite are stable to ∼ 300°C. During pyrolysis of both materials there is little change in the observed coordination of AI at temperatures of < 500°C but at 500°C or higher, ∼25% of the NMR-visible Al is converted to tetrahedral coordination. The mechanism of decomposition of both protoimogolite and imogolite is shown to involve the formation of highly branched Si-O-Si chains. As far as can be discerned by 27Al and 29Si NMR, allophanes A and B both appear to produce products similar to protoimogolite on pyrolysis.

Type
Research Article
Information
Clay Minerals , Volume 23 , Issue 2 , June 1988 , pp. 175 - 190
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1988

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