Assessment of Thermal Evolution of Kerogen Geopolymers with Their Structural Parameters Measured by Solid-State ¹³C NMR Spectroscopy

Abstract

The sidebands in the solid-state 13C NMR spectra of 13 polycyclic aromatic hydrocarbon compounds associated with kerogen structure were suppressed with cross polarization (CP), magic-angle spinning (MAS), and total sideband suppression (TOSS). The chemical shift values of these model compounds were obtained under various chemical circumstances, which were subsequently used to determine the chemical shifts of aliphatic and aromatic carbons in kerogen structure via CP/MAS/TOSS 13C NMR measurements. Dipolar dephasing (DD) was used to obtain the spectra of nonprotonated carbon, discriminating protonated and nonprotonated carbons in the aromatic cluster. Consequently, the structural parameters for different carbons were characterized. High-resolution solid-state 13C NMR measurements were conducted on a suite of kerogen macromolecules isolated from source rocks originated from the Kuqa depression, Awati River, and Kapusaliang River of the Tarim Basin in northwestern China to develop an NMR-associated method for monitoring the thermal alteration of kerogens. These rocks covered a broad maturity range, as indicated by the R o values of 0.52%−1.81%. Our results suggest that the aromaticity (f a) and aromatic cluster size (χb) are effective parameters for assessment of the thermal evolution of kerogens that resulted from the geological heating process undergone in the ancient sediments.

Chemical and nanometer-scale structure of kerogen and its change during thermal maturation investigated by advanced solid-state 13C NMR spectroscopy

“…Among the solid-state NMR structural parameters that have been proposed for assessing kerogen thermal maturity, aromaticity has been generally accepted as the most useful proxy (Wang et al, 1995;Wei et al, 2005;Werner-Zwanziger et al, 2005). Our results are in agreement with this evaluation.…”

“…Therefore, various groups have tried to quantify the fraction of bridgehead carbons, v bridge , by NMR. In some cases (Mann et al, 1991;Patience et al, 1992;Wei et al, 2005), it was simply assumed that all carbons that resonate around 130 ppm are bridgehead C, and that aromatic CH is limited to a narrow band around 108 ppm; the clear dipolar dephasing by >20% of the 130-ppm band in our spectra demonstrates the significant presence of C-H signal. Therefore, the increase in the bridgehead-carbon fraction claimed by Patience et al (1992) reflects mainly the increase in aromaticity and decrease in oxygen substitution of the rings.…”

“…Comparison with dipolar-dephased spectra shows that around 125 ppm, the signals of protonated aromatics are coincident with those of one of two nonprotonated aromatic bands. However, Patience et al (1992) and Wei et al (2005) as- signed all of this signal to (nonprotonated) bridgehead carbons and built their assessment of the degree of condensation on this assumption. Subspectra of CH 2 signals are shown in Figs.…”

“…NMR has clearly documented that kerogen aromaticity is enhanced with increasing maturity (Dennis et al, 1982;Miknis et al, 1982;Barwise et al, 1984;Solli et al, 1984;Smernik et al, 2006) and NMR-derived aromaticity is widely accepted as an eminently useful parameter for assessing thermal maturity (Wang et al, 1995;Wei et al, 2005;Werner-Zwanziger et al, 2005). Aromatic cluster size estimated from signals assigned to bridgehead carbons has also been used as a maturity parameter (Wang et al, 1995;Wei et al, 2005) using a semi-quantitative approach.…”

Characterization of biochar from fast pyrolysis and gasification systems

“…ply assumed that all carbons resonating around 130 ppm are bridgehead C, and that aromatic CÀ ÀH resonances are in a narrow band around 108 ppm [39,40]; the clear dipolar dephasing by >20% of the 130-ppm band in the spectra in Figure 2, however, demonstrates the significant presence of CÀ ÀH signal and thus shows that this assumption is false. A better approach was suggested by Solum et al, who focused on the signals of nonprotonated C (selected by dipolar dephasing) and assigned those between 135 and 90 ppm to bridgehead carbons [24,25].…”