Analysis of carboxyl group in coal and coal aromaticity by Fourier transform infrared (FT-IR) spectrometry

Thirty-five SS coals as well as two Australian lignites were analyzed by Fourier transform infrared (FT-IR) spectrometry using the KBr tablet method; the SS coals are a series of standard coals produced in various countries. A selected zone 1800–1500 cm−1 of the FT-IR spectrum obtained was deconvoluted, and the peak area for COOH group in coal was obtained. The content of COOH group in coal was calculated from the FT-IR analysis with the aid of the molar absorption coefficient, which had been derived using various aromatic carboxylic acids. On the other hand, the conventional chemical analysis (ion-exchange method) for the determination of COOH group in coal was carried out. When the 37 coals were tested, the COOH contents obtained from the FT-IR analysis were in good agreement with those from the chemical analysis. The zones of 3100–ca. 2990 cm−1 and ca. 2990–2750 cm−1 are assigned to aromatic CH and aliphatic CH, respectively. The ratio of peak area for the two peaks are appreciably correlated to the fraction of aromaticity (fa) obtained from the solid-state 13C NMR analysis. In addition, the ash content could be roughly estimated from the crystal water peak in the FT-IR spectrum.