Characterization of Heat-Treated Synthetic Imogolite by 27Al MAS and 27Al MQMAS Solid-State NMR

The structure of imogolite treated at 623, 773, and 973 K was characterized by Al-27 MAS and Al-27 multiple-quantum magic-angle-spinning (MQMAS) NMR. For comparison, Al-27 NMR spectra of imogolites, allophanes, and kaolinite heated at 623, 773, and 973 K were acquired to investigate the structural change of these clay minerals. From these spectra, the structural change of imogolite has been observed at 623 K as well as in the case of allophane, while that of kaolinite has been demonstrated at a higher temperature of 773 K. The formation of five-coordinated Al from the original six-coordinated Al in imogolite after heat-treatment was confirmed as well as in the case of kaolinite and allophane. Furthermore it is found that the peaks for five- and four-coordinated Al of imogolite shifted to lower field and that their quadrupolar products (P-Q) increased with treated temperature. It can be concluded that the five- and four-coordinated Al species are generated from the decomposition of six-coordinated Al. The chemical shifts of all peaks above 623 K indicate the increase of the structural strain in the heat-treated imogolite and the formation of amorphous phases by the decomposition of the original tubular structure.