Characterization of Organically Bound Oxygen Forms in Lignites, Peats, and Pyrolyzed Peats by X-ray Photoelectron Spectroscopy (XPS) and Solid-State ¹³C NMR Methods

Abstract

A combination of XPS and solid-state 13C 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 13C 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 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/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 13C 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 13C 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 13C 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 falO and faOCH3 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 (faP) and aliphatic carbon−oxygen single-bonded species (falO) 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.

Pyrolysis of superfine pulverized coal. Part 2. Mechanisms of carbon monoxide formation

“…XPS and 13 C NMR), which indicates the lost of carboxyl, hydroxyl, carbonyl, methoxyl groups etc. with increasing coalification [22,23]. Therefore, the amounts of CO evolved during pyrolysis of NMG coals are all higher than those from SH coals.…”

“…Therefore, the ether and hydroxyl functionalities are the precursors for the high-temperature CO evolution during coal pyrolysis. In addition, the cleavage of heterocyclic oxygen-containing structures might also contribute to the release of this late CO [11,23,28]. These structures split off CO only at high temperatures (%700°C) [15].…”

Evaluation of alkaloids extract of Retama monosperma (L.) Boiss. stems as a green corrosion inhibitor for carbon steel in pickling acidic medium by means of gravimetric, AC impedance and surface studies

“…The first and most intense, at 284.9 eV, is mainly attributed to the presence of contaminant hydrocarbons and to the C-C, C=C and C-H bonds related to the different constituents of AERST [49]. The second component at 286.5 eV may be associated with the presence of C=O groups in the AERST molecules (Ammodendrine, Anagyrine, Nmethylcytisin...) [50]. The last component at higher binding energy (288.7 eV) can be also associated with the presence of carbonyl type groups and/or probably due to the C-N + [51], resulting from the protonation of the nitrogen atoms in the AERST molecules.…”

Effects of alkali-oxygen oxidation temperature on the structures and combustion properties of Shengli lignite

“…The C1s spectra demonstrated the presence of ve carbon species on the surface of coal samples, which were aromatic units with their alkyl substituent groups (C-C/C-H, 284.6 eV), vacancy defects on functional groups (C*-C*, 285.3 eV), hydroxyl or ether group (C-O-, 286.3 eV), carbonyl (C]O, 287.6 eV), and carboxyl (COO-, 289.1 eV), respectively. 30,31,54,55 Table 4 summarized the relative amounts of different C species on lignite surface.…”