High-resolution ¹ H MAS NMR spectra of water sorbed by various types of coals

Protons of coal-sorbed water were found to produce a continuous 1H NMR Hahn-echo spectrum over a chemical shift range of approximately 0 to 5 ppm. All experimentally obtained spectra were deconvoluted into three-peak resolution: a peak of free water at 4.8 ppm and two peaks displaced to the region of lower chemical shifts. Analysis of the obtained spectra and the intensities of deconvolution peaks on the moisture content of the samples showed that the behavior of various bands in the spectra of lignites and medium and highly metamorphosed coals is qualitatively different from each other. In the case of lignites and bituminous coals, an increase in humidity leads to a shift of bands in the spectra from −1 … +2 ppm (the most strongly bound water) to a position characteristic of free water (4.8 ppm). For coking coal, lean coal and anthracite, the direction of the shift is opposite: from ~4.8 ppm for the samples with low water content to ~3 ppm for the samples saturated with water. We attribute this behavior to the low binding energy of water molecules on hydrophobic graphite-like surfaces of highly metamorphosed coals and the strong shielding of water protons by π-electron clouds of aromatic surfaces.