S. R. Kelemen

A combination of solid-state 13C NMR, X-ray photoelectron spectroscopy (XPS) and sulfur X-ray absorption near edge structure (S-XANES) techniques are used to characterize organic oxygen, nitrogen, and sulfur species and carbon chemical/structural features in kerogens. The kerogens studied represent a wide range of organic matter types and maturities. A van Krevelen plot based on elemental H/C data and XPS derived O/C data shows the well established pattern for type I, type II, and type III kerogens. The anticipated relationship between the Rock−Eval hydrogen index and H/C is independent of organic matter type. Carbon structural and lattice parameters are derived from solid-state 13C NMR analysis. As expected, the amount of aromatic carbon, measured by both 13C NMR and XPS, increases with decreasing H/C. The correlation between aromatic carbon and Rock−Eval Tmax, an indicator of maturity, is linear for types II and IIIC kerogens, but each organic matter type follows a different relationship. The average aliphatic carbon chain length (Cn‘) decreases with an increasing amount of aromatic carbon in a similar manner across all organic matter types. The fraction of aromatic carbons with attachments (FAA) decreases, while the average number of aromatic carbons per cluster (C) increases with an increasing amount of aromatic carbon. FAA values range from 0.2 to 0.4, and C values range from 12 to 20 indicating that kerogens possess on average 2- to 5-ring aromatic carbon units that are highly substituted. There is basic agreement between XPS and 13C NMR results for the amount and speciation of organic oxygen. XPS results show that the amount of carbon oxygen single bonded species increases and carbonyl−carboxyl species decrease with an increasing amount of aromatic carbon. Patterns for the relative abundances of nitrogen and sulfur species exist regardless of the large differences in the total amount of organic nitrogen and sulfur seen in the kerogens. XPS and S-XANES results indicate that the relative level of aromatic sulfur increases with an increasing amount of aromatic carbon for all kerogens. XPS show that the majority of nitrogen exists as pyrrolic forms in comparable relative abundances in all kerogens studied. The direct characterization results using X-ray and NMR methods for nitrogen, sulfur, oxygen, and carbon chemical structures provide a basis for developing both specific and general average chemical structural models for different organic matter type kerogens.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTQuantification of Nitrogen Forms in Argonne Premium CoalsS. R. Kelemen, M. L. Gorbaty, and P. J. KwiatekCite this: Energy Fuels 1994, 8, 4, 896–906Publication Date (Print):July 1, 1994Publication History Published online1 May 2002Published inissue 1 July 1994https://pubs.acs.org/doi/10.1021/ef00046a013https://doi.org/10.1021/ef00046a013research-articleACS PublicationsRequest reuse permissionsArticle Views964Altmetric-Citations269LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose Get e-Alerts

The present work explores the potential of Raman spectroscopy to provide maturity information about catagenesis stage kerogens and coals. The first-order Raman spectra of coals and kerogens show a broad amorphous A band between 1310 and 1360 cm-1 and a graphite-like G band near 1580−1600 cm-1. As vitrinite reflectance (R0) increases, the Raman A band becomes narrower and shifts to lower frequencies while the absorption strength of the A band relative to the G band decreases. The area ratio of the A band to G band (R = fA/fG) decreases from 3.2 to 2.4 in going from the least mature sample up to R0 = 2.0%. Similar trends were observed for maturation suites of Type II and other Type III kerogens. Laboratory thermolysis produced a range of samples with well-defined laboratory R0. The changes in the Raman A and G bands from the laboratory matured samples directionally paralled the natural samples. These results demonstrate that the Raman spectra of catagenesis stage samples vary in regular ways with sample maturity. The underlying chemical and structural changes associated with changes in the Raman A and G bands are discussed.

A combination of XPS and solid-state 15N NMR have been used to characterize the nitrogen forms in a variety of different carbonaceous samples having high natural nitrogen abundance. It is currently difficult to unequivocally interpret and quantify individual 15N NMR and XPS spectra. The advantages of using a multiple technique approach for nitrogen speciation as well as the limitation of XPS nitrogen (1s) and 15N NMR spectroscopy are discussed. XPS and 15N NMR results show that pyridinic and pyrrolic nitrogen are present in quinoline and isoquinoline pyrolysis chars. Pyridinic nitrogen is the most abundant form in quinoline pitch while isoquinoline pitch produced the most pyrrolic nitrogen. A small amount of quaternary nitrogen (∼10 mol %) appeared in the XPS spectrum of the chars, however, no additional nitrogen forms were identified above the noise level in the 15N NMR spectra. The 15N NMR spectrum of Green River kerogen shows a single broad feature centered around −245 ppm in the 15N NMR, consistent with the position of pyrrolic nitrogen forms. XPS spectra of Green River kerogen show a large peak at 400.2 eV and the peak at this position in the curve-resolved nitrogen (1s) spectrum is consistent with pyrrolic nitrogen forms. Other XPS peaks were consistent with and attributed to pyridinic quaternary and amine nitrogen species. Acid treatment of Green River kerogen resulted in an increase in the quaternary nitrogen feature in both the XPS and 15N NMR spectra with a concomitant decline in the XPS pyridinic and amine nitrogen features. Pyrolysis of kerogen resulted in the appearance of a peak near −80 ppm in the 15N NMR spectrum attributed to pyridinic nitrogen forms. XPS spectra also show an increase in pyridinic nitrogen upon pyrolysis. XPS results show that the major peak in the curve-resolved nitrogen (1s) spectrum of both fresh and pyrolyzed peats appears at 400.2 eV. Both amide and pyrrolic nitrogen appear at this energy position and it is impossible to distinguish between these two species based on XPS data alone. For fresh peat, the major peak in the 15N NMR spectrum occurs at chemical shift positions consistent with the presence amide nitrogen forms. The main peak in the 15N NMR spectrum of pyrolyzed peats broadens and shifts toward higher field. This change is associated with the loss of amide forms and the appearance of pyrrolic nitrogen forms