Structural investigations of perlite and expanded perlite using 1H, 27Al and 29Si solid-state NMR

Abstract

Industrial grade perlite and expanded perlite systems were investigated by multifield and multinuclear Solid State Nuclear Magnetic Resonance (SSNMR), Fourier Transform Infra-Red (FTIR) spectroscopies and Scanning Electron Microscopy (SEM). The SSNMR studies focussed on the 1H, 29Si and 27Al nuclei using single pulse excitation (SPE) and 1H, 29Si cross-polarization Magic-Angle-Spinning (CP-MAS) conditions, with 27Al Triple Quantum Magic-Angle-Spinning (3QMAS) measurements also being implemented. The resonances at ~ 52ppm detected in the 27Al MAS NMR spectra of the industrial grade and expanded systems are tentatively assigned to tetrahedrally coordinated aluminium, while the resonance at ~ 5ppm, which was only observed in the industrial grade perlite spectrum, is attributed to octahedrally coordinated aluminium. In neither case could the pentahedrally coordinated positions be identified using 27Al MAS and 3QMAS techniques. 27Al data suggests that tetrahedrally coordinated aluminium atoms do not have a significant role in the grain expansion process. The broad resonance envelopes found in the 29Si MAS NMR and FTIR data of both perlites were analyzed by the second-order derivative. The analyses confirmed complex Qn configurations for the perlitic structure suggesting the prevalence of Q3 and Q4 species. 1H-29Si CP-MAS spectra support the presence of Q2 units suggested by FTIR and SPE data. SEM micrographs revealed the honeycomb morphology of expanded domains in perlite after heat treatment.