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

Abstract

We have carried out the first systematic in situ variable-temperature (25-180 °C) high-resolution proton NMR study of laboratory-frame and rotating-frame proton spin-lattice relaxation of coal samples, based on the CRAMPS technique. For coal samples that have been exposed to air, we confirmed the fact that paramagnetic oxygen is the main source of laboratory-frame proton spin-lattice relaxation (T 1 ). We demonstrate that paramagnetic oxygen trapped in coal can be used as a sensitive probe for monitoring structural and dynamical changes in coal as the temperature is varied. High-temperature spin-lattice relaxation experiments help to reveal the structural heterogeneity of coal because of reduced proton and electron spin-diffusion rates at high temperature. Large domains, on the order of 200-800 A, with distinctively different paramagnetic oxygen concentrations, were found in all three coal samples studied, consisting of one low-volatile and two high-volatile bituminous coals from the Argonne Premium Coal bank. In particular, we found that aliphatic-rich domains with a length-scale larger than 500 A exist in Premium Coal 601. The observed dependences of the rotating-frame 1 H spin-lattice relaxation time T 1p on the strength of the spin-lock field and temperature support the view that the main relaxation mechanism is time-dependent 1 H- 1 H dipolar interactions in coals. From these dependences, we estimate that the correlation time of molecular motion responsible for rotating-frame proton spin-lattice relaxation in coals is on the order of 5 μs, which is in agreement with conclusions drawn from previous proton dipolar-dephasing studies. Two T 1p values were identified for each of the three coal samples studied, indicating the existence of structural heterogeneity in coal on a spatial scale of at least 50 A. The sizes of heterogeneous domains in coal are estimated on the basis of measured spin-lattice relaxation times and the analysis of proton spin-diffusion processes.