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

ABSTRACT: Imogolite, protoimogolite and two synthetic allophanes (A and B) with very different Si/A1 ratios have been pyrolysed over a range of temperatures, and the reactions followed by high-resolution solid-state ZVAI and 29Si nuclear magnetic resonance (NMR) spectroscopy. The Si(OAloct)3OH groups of protoimogolite are destroyed at relatively low ( ~ 200 ~ temperatures, whereas the same structures in imogolite are stable to ~ 300 ~ During pyrolysis of both materials there is little change in the observed coordination of AI at temperatures of < 500 ~ but at 500 ~ or higher, ~25 ~ of the NMR-visible A1 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 27A1 and 295i NMR, allophanes A and B both appear to produce products imilar to protoimogolite on pyrolysis. Poorly-ordered non-crystalline aluminosilicates are important constituents of many soils and are precursors to many catalysts. Naturally-occurring poorly-ordered aluminosilicates are known to consist of thread-like particles called imogolite and a less well-defined material called allophane. Imogolite has a molecular structure of hollow tubes in which isolated S