The effect of low-temperature swelling on the micropore structure of coal and model polymers as determined with a novel EPR-spin probe method.

摘要

The micropore structure of Argonne Premium Coal Samples (APCS) was studied under low temperature swelling conditions using an EPR - spin probe method. In this method, stable free radicals (nitroxides) were used as a spin probes. Spin probe molecules were imbibed into the micropore system of coal during swelling. After swelling, the pore structure was collapsed by drying and the coals washed with cyclohexane. The wash removed any nitroxides not trapped in pores of size and shape similar to that of the spin probe. Electron paramagnetic resonance spectroscopy was used to determine the concentration of spin probes retained in the coal.; Spin probes of different size and shape were used to study the effect of swelling on pore size and shape. Similarly sized nitroxides with R groups differing in hydrogen-bonding ability were used to study the chemistry of pore walls. The effect of swelling temperature, polarity of the swelling solvent, and coal rank were examined on the micropore structure of coal. Spin probe concentration decreased with rank under all conditions studied. Temperature and solvent polarity increased the severity of swelling. As swelling severity increased, pores became less spherical and increasingly elongated.; Spin probes had no interaction with coal through nitroxyl functionality. Nitroxides with R groups capable of hydrogen bonding did interact specifically with coal through the R group. Retention increased with increasing hydrogen bonding ability of the R group. Raising the temperature exposed more sites for specific interaction. Polar solvents reacted with these sites, displacing spin probes and decreasing retention.; Finally, the EPR-spin probe method was used to study the swelling of covalently cross-linked polystyrene-divinyl benzene copolymers, used as models of coal structural elements. The results were compared with swelling studies on APCS coals and confirmed results showing that coal was polymeric, that it had covalent cross-links increasing with rank, that it was structurally anisotropic, and that its swelling was anisotropic.