LOW-TEMPERATURE SWELLING OF ARGONNE PREMIUM COAL SAMPLES .2. MICROPORE WALL CHEMISTRY DURING SWELLING

An electron paramagnetic resonance (e.p.r.) spin probe method was used to study the micropore wall chemistry of coal during swelling at 298 and 333 K with polar and non-polar solvents. Argonne Premium Coal Samp le coals were swelled in the presence of small nitroxide molecules wit h R groups differing in their ability to hydrogen bond to coal. The re sults showed that unswelled coals contained small, spherical pores whi ch disappeared when coal was swelled in the presence of polar solvents . Nitroxides with polar R groups were retained in the micropore struct ure due to the ability of the R group to hydrogen bond with the coal. However, probe retention decreased with rank, emphasizing that hydroge n bonding was important primarily in low-rank coals. The presence of p olar solvents like nitrobenzene and pyridine decreased polar spin prob e retention due to competition for binding sites. Site competition was the greatest in pyridine. Finally, the results indicated the presence of hydrogen bonding sites in coal which vary in reactive strength.