| Abstract
| - A combination of XPS and solid-state 13C NMR techniques have been used to characterizeorganic oxygen species and carbon chemical/structural features in peats, pyrolyzed peats, lignites,and other coals. Both the 13C NMR and XPS results show the same ordering for the amount ofaromatic carbon, higher ranking coals > lignites > peats. In general the value for H/C decreasesas the percent of aromatic carbon increases. For pyrolyzed peats, the H/C level is higher thanlignites and other coals of comparable levels of aromatic carbon. This is likely due to significantdifferences in the carbon structural framework of these materials. A van Krevelen plot, based onelemental H/C data and XPS 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/Cvalues 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 percentaromatic carbon, fall below the values for lignites and other coals. These results indicate thatsimple pyrolysis does not appear to fully capture the chemical transformations encountered duringthe natural formation of coals. Both XPS and 13C NMR results are sensitive to the basic differencein the kinds of organic oxygen species found in peats and coals. The advantages of using acombination of XPS and 13C NMR along with the pitfalls of using a single technique for organicoxygen speciation are discussed. For peats, pyrolyzed peats, lignites, and other coals, XPS resultsfor the total amount of organic oxygen fall between upper and lower limit estimates based on 13CNMR 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 falO andfaOCH3 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. NMRresults show that the amount of phenolic and phenoxy carbon (faP) and aliphatic carbon−oxygensingle-bonded species (falO) are very similar for pyrolyzed peats, lignites, and other coals atcomparable levels of aromatic carbon. These results indicate that thermal decarboxylation/decarbonylation and demethoxylation pathways exist for peat and suggest that similar pathwaysoccur during natural coalification processes.
|
