Variable-Temperature High-Resolution Proton NMR Study of Laboratory-Frame and Rotating-Frame Spin-Lattice Relaxation in Coals

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• 作者：Jincheng Xiong and Gary E. Maciel
• 刊名：Energy & Fuels
• 出版年：1997
• 出版时间：July 1997
• 年：1997
• 卷：11
• 期：4
• 页码：866 - 878
• 全文大小：560K
• 年卷期：v.11,no.4(July 1997)
• ISSN：1520-5029

文摘

We have carried out the first systematic in situvariable-temperature (25-180 C) high-resolution proton NMR study of laboratory-frame and rotating-frameproton spin-latticerelaxation of coal samples, based on the CRAMPS technique. Forcoal samples that have beenexposed to air, we confirmed the fact that paramagnetic oxygen is themain source of laboratory-frame proton spin-lattice relaxation (T₁).We demonstrate that paramagnetic oxygen trappedin coal can be used as a sensitive probe for monitoring structural anddynamical changes in coalas the temperature is varied. High-temperature spin-latticerelaxation experiments help toreveal the structural heterogeneity of coal because of reduced protonand electron spin-diffusionrates at high temperature. Large domains, on the order of200-800 Å, with distinctively differentparamagnetic oxygen concentrations, were found in all three coalsamples studied, consisting ofone low-volatile and two high-volatile bituminous coals from theArgonne Premium Coal bank.In particular, we found that aliphatic-rich domains with alength-scale larger than 500 Å existin Premium Coal 601. The observed dependences of therotating-frame ¹H spin-lattice relaxationtime T₁ on the strength of thespin-lock field and temperature support the view that themainrelaxation mechanism is time-dependent ¹H-¹Hdipolar interactions in coals. From thesedependences, we estimate that the correlation time of molecular motionresponsible for rotating-frame proton spin-lattice relaxation in coals is on the order of 5s, which is in agreement withconclusions drawn from previous proton dipolar-dephasing studies.Two T₁ values wereidentifiedfor each of the three coal samples studied, indicating the existence ofstructural heterogeneity incoal on a spatial scale of at least 50 Å. The sizes ofheterogeneous domains in coal are estimatedon the basis of measured spin-lattice relaxation times and theanalysis of proton spin-diffusionprocesses.