Structural effects of sample ageing in hydrocracked coal liquefaction extracts

A sample of Point of Ayr coal extract has been hydrocracked in a microbomb reactor with NiMo on alumina catalyst in tetralin as solvent and hydrogen donor and under hydrogen pressure. The product was separated from solvent and catalyst and then split into equal parts and stored either under nitrogen atmosphere in a freezer or in air at room temperature. Samples of the products were examined at 2 h frequencies for a day, then daily for a week, then at less frequent intervals for a year. Methods used for examination were size exclusion chromatography (SEC) and UV-fluorescence spectroscopy (UV-F), both using 1-methyl-2-pyrrolidinone as solvent. Ageing was assessed in terms of shifts to shorter elution times in SEC and parallel changes in UV-F spectra. Both stored products showed significant structural evidence of ageing over the first week of storage. After that time, changes observed were within the range of variability of the chromatography method based on polystyrene standards. The ageing was attributed to the presence of low-reactivity free radical species, which underwent recombination reactions during storage. These changes are likely to affect the viscosity and combustion characteristics of the hydrocracked product.