Structural characterization of nitrogen-enriched coals

A subbituminous coal and a lignite were treated with some nitrogen reagents in several solvents with the aim of obtaining enriched coal samples which could behave as active carbon precursors. This paper deals with the structural characterization of the samples obtained with urea before and after activation. The attention is focused on the identification of the main functional groups able to react with urea, on the nature of the nitrogen functions introduced in the coal network, and on the behavior of these functions during the activation. The samples were investigated by infrared spectroscopy (DRIFT), X-ray photoelectron spectroscopy(XPS). It was shown that ester/lactone and carboxylic acid groups are mainly concerned in the reaction with urea, which is solvent dependent. The identification of nitriles proved the occurrence of amides in the first step of the reaction. The participation of ketones, though to a lesser extent, suggests the possibility of further heterocyclization. The nitrogen enrichment of the coals is evidenced by XPS. On the basis of Literature data, four nitrogen groups were characterized according to the binding energies. They were assigned to pyrrolic and several pyridinic forms. Their behavior during thermal treatments and steam activation is discussed.