Dc. Doetschman et al., A STUDY OF COAL EXTRACTION WITH ELECTRON-PARAMAGNETIC-RESONANCE AND PROTON NUCLEAR-MAGNETIC-RESONANCE RELAXATION TECHNIQUES, Energy & fuels, 8(4), 1994, pp. 907-919
An electron spin and proton magnetic relaxation study is presented on
the effects of the solvent extraction of coal on the macromolecular ne
twork of the coal and on the mobile molecular species that are initial
ly within the coal. The eight Argonne Premium coals were extracted at
room temperature with a 1:1 (v/v) N-methylpyrrolidinone (NMP)-CS2 solv
ent mixture under an inert atmosphere. As much solvent as possible was
removed from extract and residue by treatment under vacuum oven condi
tions (approximately 10(-2) Torr at 145-150-degrees-C) until constant
weight was achieved. The extraction, without further washing with othe
r solvents, results in substantial uptake of NMP, apparently by H-bond
ing or acid-base interactions. The NMP uptake tends to be higher, and
the NMP tends to be more tightly bound in coal matter with higher hete
roatom (N, 0, S) content. The molecular material in the medium rank bi
tuminous coals is more aromatic and heteroatom-poor than the macromole
cular material and is mobilized by the extracting solvent. Likewise, t
he more aromatic and heteroatom-poor molecular free radicals are also
extracted. However, mobilization of the molecular free radicals by sol
vent and the exposure of free radicals in the macromolecular matrix to
solvent or species dissolved in the solvent result in preferential re
actions of the more aromatic and heteroatom-poor free radicals. Greate
r losses of extract free radicals, being the more aromatic, occur than
residue free radicals. As a consequence, the surviving extract radica
ls exhibit a greater heteroatom content than the original whole coals,
as determined from EPR g value changes. The electron paramagnetic res
onance (EPR) spin-lattice relaxation (SLR) of these coal free radicals
has previously been inferred (Doetschman and Dwyer, Energy Fuels 1992
, 6, 783) to be from the modulation of the intramolecular electron-nuc
lear dipole interactions of the CH groups in a magnetic field by motio
ns of the radical in the coal matrix. Such a modulation depends on the
flexing amplitude and frequency and to a lesser extent upon the elect
ron spin density at the CH groups in the radical. The observed EPR SLR
rates decrease with coal rank in agreement with the smaller spin dens
ities and the lower rocking amplitudes that are expected for increasin
g aromaticity with rank and increasing polycondensation at the highest
ranks. The EPR SLR rates are found to be generally faster in the extr
acts (than residues) where the molecular species would be expected to
be smaller and more flexible than in the cross-linked, polymeric, macr
omolecular matrix of the residue.