A multi-nuclear solid state NMR study of the bonding character in aluminosilcate inorganic polymers of varying nominal composition

Abstract

Aluminosilicate inorganic polymers were produced by activating metakaolinite with NaOH and studied using a variety of analytical techniques including 29Si, 27Al, 23Na and IH solid state NMR. The relationship is studied between the bonding characters of these materials and the chemical composition, and in turn the compressive strength. Magic-angle-spinning (MAS) NMR of 29Si spectra showed chemical shifts that systematically changed with the Si:Al ratio, indicating that the structural environment of the Si changes with Al content. Measurements of the temperature dependence of IH-29Si cross-polarisation MAS NMR and IH NMR spectra, together with IH fastMAS NMR spectra, were used to study the speciation and dynamics of hydrogen in the structure. From 27Al MAS NMR the aluminium speciation was found to be predominantly 4-coordinate, with small proportions of 5- and 6- coordinate sites being attributed to unreacted metakaolinite. The corresponding 23Na MAS and high-resolution MQMAS NMR experiments yielded spectra describing dominant amorphous character, however, some stoichiometrys contained additional well-defined components that can be attributed to regions with some short-range order which is not associated with zeolite or carbonate formation. These data suggest that the amorphous character of the inorganic polymers is due to disorder in the Si sublattice, as inferred from comparisons with the 29Si and 27Al spectral line widths for kaolinite. The structural information obtained is compared to previously-published models.