Characterization of organically bound oxygen forms in lignites, peats, and pyrolyzed peats by X-ray photoelectron spectroscopy (XPS) and solid-state 13C NMR methods

A combination of YPS and solid-state C-13 NMR techniques have been used to characterize organic oxygen species and carbon chemical/structural features in peats, pyrolyzed peats, lignites, and other coals. Both the C-13 NMR and XPS results show the same ordering for the amount of aromatic carbon, higher ranking coals > lignites > peats. In general the value for H/C decreases as the percent of aromatic carbon increases. For pyrolyzed peats, the H/C level is higher than lignites and other coals of comparable levels of aromatic carbon. This is likely due to significant differences in the carbon structural framework of these materials. A van Krevelen plot, based on elemental H/C data and Y-PS derived O/C data, shows the well-established pattern for peats, lignites, and other coals. In general, O/C decreases as the percent of aromatic carbon increases, with the expected magnitude ordering, peats > lignites > higher ranking coals. Most of the H/C values of pyrolyzed peats are higher than coals at comparable O/C. A range of O/C levels (0.23-0.13) were produced from pyrolysis of peats; however, these data, when plotted versus the percent aromatic carbon, fall below the values for lignites and other coals. These results indicate that simple pyrolysis does not appear to fully capture the chemical transformations encountered during the natural formation of coals. Both XPS and C-13 NMR results are sensitive to the basic difference in the kinds of organic oxygen species found in peats and coals. The advantages of using a combination of XPS and C-13 NMR along with the pitfalls of using a single technique for organic oxygen speciation are discussed. For peats, pyrolyzed peats, lignites, and other coals, XPS results for the total amount of organic oxygen fall between upper and lower limit estimates based on C-13 NMR derived parameters associated with different oxygen species. For lignites and other coals, there is a sharp drop in the number of carbonyl and carboxyl groups near 60% aromatic carbon. The amount of carbon-oxygen single-bonded species reflected in the NMR parameters fal(o) and fa(OCH3) and the XPS parameter C-O oxygen, decrease as the percent aromatic carbon increases. The highest levels of phenolic and phenoxy oxygen are found near 60% aromatic carbon. NMR results show that the amount of phenolic and phenoxy carbon (fa(P)) and aliphatic carbon-oxygen single-bonded species (fal(o)) are very similar for pyrolyzed peats, lignites, and other coals at comparable levels of aromatic carbon. These results indicate that thermal decarboxylation/ decarbonylation and demethoxylation pathways exist for peat and suggest that similar pathways occur during natural coalification processes.