Influence of depyritization on NMR relaxation parameters of Aleksinac oil shale kerogen
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Parallel proton and cross-polarization/magic-angle spinning (CP/MAS) nuclear magnetic resonance (NMR) relaxation measurements are reported on Aleksinac oil shale kerogen concentrate and pyrite-free kerogen concentrate. Possible relaxation mechanisms considered include proton-proton and C-13-proton interactions. The influence of depyritization on relaxation parameters is estimated. Depyritization prolongs the T-1H relaxation time constant by about five times. However, depyritization has no significant influence on relaxation in the rotating frame ( lt 5%). The probable explanation is that there is a certain motion of structural units with spectral density in the kHz region, which averages dipolar interactions between protons and pyrite. The repetition delay is found to be 150 ms for kerogen concentrate and 1-2 s for depyritized kerogen concentrate. The favourable value of contact time is identical for both samples, i.e., 1 ms. At the same time there are no significant changes observed i...n the CP/ MAS spectra of Aleksinac oil shale kerogen due to depyritization treatment, except for a small band (approximate to 60 ppm) assigned to alcoholic groups which appeared as a result of the partial reduction of carbonyl groups. Use of the pyrite-containing Bg sample in solid-state NMR investigations is found to be more favourable because of its shorter T-1H relaxation time and, consequently, shorter (by a factor of about five) experimental time. (C) 1998 Elsevier Science Ltd. All rights reserved.
Keywords:oil shale / kerogen / solid state NMR / depyritization
Source:Fuel, 1998, 77, 9-10, 1001-1003
- Elsevier Sci Ltd, Oxford