Structural Changes by Low Temperature Oxidation of Coals

Investigation by Means of Solid State 13C-NMR and Pyrolytic Analysis

Abstract

Solid state ¹³C-NMR spectra of oxidized coals were measured in order to investigate changes of NMR behaviors of the coals through low temperature oxidation by air. By the oxidation for one week at 100-105°C, low rank coals showed increases in the signal intensity and spin relaxation time in the rotating frame (Tiρ^H). On the other hand, decreases in the intensity and T₁ρ^H were observed on high rank coals. Radical concentra-tion of the low rank coals decreased, but that of the high rank coals increased by the oxidation. Measurements of FT-IR spectra and pyrolysis reaction were also carried out for the oxidized coals, suggesting that various oxygen containing functional groups such as C=O and -OH are generated mainly by the oxidation of aliphatic carbons and that as a re-sults the value of the aromaticity (fa) increased during the oxidation. It was concluded from the results that the enhancement of T₁ρ^H values of the low rank coals is due to the in-crease in fa values which results in the restriction of the molecule motion and to the de-crease in radical concentration. On the contray, the lowering of T₁ρ^H of high rank coals is considered to be brought about predominantly by the increase in the radical concentration during the oxidation.