Oxygen groups in coals and alginite-rich kerogen revisited

Abstract

Detailed Fourier Transform Infrared Spectroscopy (FTIR), chemical oxidation and reduction, and multivariate regression analysis have been applied to gain insight into the relative proportions and types of oxygen functional groups in coals of varying rank and in alginite-rich organic matter in oil shales and marine shale. FTIR spectra of demineralised and non-demineralised coals with a vitrinite reflectance below ~ 0.4% R o revealed that the spectra are significantly different in the carbonyl region, probably due to the configuration of weak conjugated oxygen bonds into a more rigid form during demineralisation. In coals and lignin, the relative FTIR response from oxygen groups occurs in the order hydroxyl ≥ ether > ketone/ester > carboxyl > quinone, where ester groups are related to cutinite and sporinite. A pronounced decrease in oxygen content occurs from ~ 0.43–0.50% R o , but the oxygen groups show different evolutionary trends during initial thermal maturation: (i) carboxyl group absorption increases significantly up to lignite/subbituminous C rank, followed by a rapid decrease and disappearance at ∼ 0.45% R o , (ii) ketone (carbonyl) group absorption is prominent at maturities < 0.45% R o , but at higher maturities the response declines gradually, (iii) ether group absorption increases to a maximum at ∼ 0.45–0.50% R o , from where it decreases gradually, (iv) at atomic O/C ratios > 0.2 (approximately < 0.45% R o ) hydroxyl group (principally phenolic) absorption is highly variable, but at increasing thermal maturity significant dehydroxylation occurs. Elimination of oxygen groups in the coaly organic matter thus occurs in the order quinone, carboxyl, ketone/ester, hydroxyl and ether. Both ether and hydroxyl are still present at an O/C ratio of 0.05. Oxygen in the alginite-dominated kerogen of the shales is entirely bound as ether, carbonyl (mainly ester) and hydroxyl. The presence of ether agrees with the dominant role of ether bonds cross-linking long-chain n -alkanes in algaenan. Hydroxyl may be related to phenolic units in for example Botryococcus algae. The relative FTIR response from oxygen groups in kukersite is ether > carbonyl (ketone) > hydroxyl, which generally reflects the structural models proposed for kukersite. Oxygen groups in the coals can overall be correlated to the least-altered huminite macerals textinite, texto-ulminite, and densinite, or to the associated cutinite and sporinite, whereas the variation of the oxygen group composition in the studied shales seems to be controlled by thermal maturity. Hydrogen Index (HI) values of chemically oxidised and/or reduced coal samples do not provide an unequivocal answer to whether HI values of immature coals are underestimated. Incorporation of carboxyl groups into the original coal kerogen does not produce a more immature coal manifested by decreased HI values.