Estimation of Aromaticities by IR Spectra and Oxgen Contents of Coals

Abstract

Six kinds of coal were extracted with pyridine in a Soxhlet extractor. Petroleum ether insoluble and chloroform soluble parts in the pyridine extracts were extracted successively with ethyl ether, acetone and benzene. These solvent extracts were used as samples. The relations between K₂₉₂₀ and Hal, and K₁₆₀₀/Ca and Ox were discussed, where K₂₉₂₀ and K₁₆₀₀ are the specific extinction coefficients in IR spectrum, Hal, Ca and Ox are the weight fractions of aliphatic hydrogens, aro-matic carbons and oxygens, respectively, obtained from ¹H-and quantitative ¹³C-NMR spectra and ultimate analysis.
K₂₉₂₀ was recognized to be proportional to Hal and there was a linear relation between K₁₆₀₀/Ca and Ox. Considering the latter relation, the 1600 om^-1 band of aro-matic rings was found to be enhanced by oxygens bonded to them. The ratio of aro-matic ring carbons affected by oxygens to total aromatic carbons was proportional to Ox. Consequently, the following formula was derived.
^*Hal/^*Ca=7.6· (D₂₉₂₀/D₁₆₀₀) · (Ox+0.023)
where, ^*Hal/^*Ca is the atomic ratio of aliphatic hydrogens and aromatic carbons. D₂₉₂₀ and D₁₆₀₀ are the optical densities of 2920 and 1600 cm^-1, respectively.
Assuming that ^*Cal/^*Hal molar ratio of alkyl groups was 1/2 and using the above formula, the aromaticities of many original coals could be estimated by D₂₉₂₀/D₁₆₀₀ and Ox (wt. fraction, daf coals) obtained from their IR spectra and ultimate analyses. These estimated aromaticities agreed fairly well with the published values. In par-ticular, the appropriate aromaticities could be calculated for low rank coals having carbon contents below 70‰ for which it was difficult to estimate the correct aromaticities by the previous method.