Jc. Xiong et Ge. Maciel, VARIABLE-TEMPERATURE HIGH-RESOLUTION PROTON NMR-STUDY OF LABORATORY-FRAME AND ROTATING-FRAME SPIN-LATTICE RELAXATION IN COALS, Energy & fuels, 11(4), 1997, pp. 866-878
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.