29Si, 27Al and 23Na solid-state nuclear magnetic resonance studies of combustion-generated ash

Abstract

Combustion-generated ash was synthesized, heated and analyzed by solid-state nuclear magnetic resonance spectroscopy with magic angle spinning (NMR–MAS). The structural information obtained from this study indicates that the ash contains different magnetic resonance environments for Al, Si and Na after combustion. The final ash material contains a major amorphous aluminosilicate glass phase with mullite and corundum crystals. 29Si NMR spectra showed a single peak at −110.5 ppm ( δ) for the initial powder sample due to SiO 2. When the sample was heated to 1500°C the peak shifts to lower field ( δ=−99 ppm) due to the formation of aluminosilicate amorphous glass containing alkali or alkaline earth metals. The 27Al NMR spectra of the initial sample gives a single peak at 10.1 ppm due to the AlO 6 (octahedral group) present in the Al 2O 3. When the sample is heated to 1500°C, a new peak appeared at 50 ppm due to the formation of AlO 4 tetrahedral represent the mullite and aluminosilicate compounds. The 23Na NMR spectra of both the initial and the heated samples (at 1500°C) give rise to a single peak at −14.3 and −28.7 ppm, respectively. The sharp peak at −14.3 ppm is contributed from the crystalline Na 2O and the broad peak around −28.7 ppm is due to the Na + present to compensate SiO − present in the amorphous aluminosilicate glass material. The presence of low amounts of heavy metals Cr (2800 ppm), Cd (2700 ppm) and Pb (2800 ppm) in the ash generally does not show any influence on the 27Al, 29Si and 23Na solid-state NMR spectra. On the other hand, the ash sample containing higher amounts of heavy metals (Cd=14210 ppm, Cr=13120 ppm and Pb=13140 ppm) showed significant effect on the 27Al NMR spectra. The replacement of Al 3+ from its octahedral sites by Cr 3+ leads to the decrease in the intensity of the AlO 6 octahedral peak. The presence of Cr 3+, Cd 2+ and Pb 2+ in the aluminosilicate matrix exerts a deshielding effect on 29Si resonance and thus shifts δ to low-field. The results clearly indicate an interaction between the heavy metals and the ash matrix compounds.