Low temperature swelling of Argonne Premium Coal Samples: 2. Micropore wall chemistry during swelling

Abstract

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 Sample coals were swelled in the presence of small nitroxide molecules with R groups differing in their ability to hydrogen bond to coal. The results showed that unswelled coals contained small, spherical pores which disappeared when coal was swelled in the presence of polar solvents. Nitroxides with polar R groups were retained in the micropore structure due to the ability of the R group to hydrogen bond with the coal. However, probe retention decreased with rank, emphasizing that hydrogen bonding was important primarily in low-rank coals. The presence of polar solvents like nitrobenzene and pyridine decreased polar spin probe 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.