VARIABLE-TEMPERATURE HIGH-RESOLUTION PROTON NMR-STUDY OF LABORATORY-FRAME AND ROTATING-FRAME SPIN-LATTICE RELAXATION IN COALS

We have carried out the first systematic in situ variable-temperature (25-180 degrees C) high-resolution proton NMR study of laboratory-fram e and rotating-frame proton spin-lattice relaxations or 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 so urce of laboratory-frame proton spin-lattice relaxation (T-1). We demo nstrate that paramagnetic oxygen trapped in coal can be used as a sens itive probe for monitoring structural and dynamical changes in coal as the temperature is varied, High-temperature spin-lattice relaxation e xperiments help to reveal the structural heterogeneity of coal because of reduced proton and electron spin-diffusion rates at high temperatu re. Large domains, on the order of 200-800 Angstrom with distinctively different paramagnetic oxygen concentrations, were found in all three coal samples studied, consisting of one low-volatile and two high-vol atile bituminous coals from the Argonne Premium Coal bank. In particul ar, we found that aliphatic-rich domains with a length-scale larger th an 500 Angstrom exist in Premium Coal 601. The observed. dependences o f the rotating-frame H-1 spin-lattice relaxation time T-1 rho on the s trength of the spin-lock field and temperature support the view that; the main relaxation mechanism is time-dependent H-1-H-1 dipolar intera ctions in coals, From these dependences, we estimate that the correlat ion time of molecular motion responsible for rotating-frame proton spi n-lattice relaxation in coals is on the order of 5 mu s, which is in a greement with conclusions drawn from previous proton dipolar-dephasing studies. Two T-1 rho values were identified for each of the three coa l samples studied, indicating the existence of structural heterogeneit y in coal on a spatial scale of at least 50 Angstrom, The sizes of het erogeneous domains in coal ape estimated on the basis of measured spin -lattice relaxation times and the analysis of proton spin-diffusion pr ocesses.