Carbon isotope analysis and a solid state 13C NMR study of Mongolian lignite: Changes in stable carbon isotopic composition during diagenesis

The results of a combined carbon isotope analysis and NMR study of a set of lignite samples from Mongolia show that a correlation exists between δ13C value and the ratios of oxygenated carbon (O-aliphatic, carboxyl/carbonyl and O-aromatic carbons) over protonated and carbon substituted aliphatic and aromatic carbons. The results indicate that the δ13C values of the lignite are controlled by two mechanisms: (i) Depletion in 13C as a result of loss of isotopically enriched O-bonded carbons and (ii) enrichment in 13C caused by a loss of isotopically depleted CH4 from aliphatic and aromatic carbons. At the rank of lignite, coal is enriched in 13C because the amount of isotopically enriched CO2 and CO, released from coal as a result of change in chemical structure, is less than that of isotopically depleted CH4.