A solid-state 15N NMR spectroscopic investigation of the origin of nitrogen structures in coal

Abstract

Solid-state 15N nuclear magnetic resonance (NMR) spectroscopy is a technique that has been available for the study of nitrigen compounds for many years but only recently has been applied to the study of nitrogen in coal due to the extremely low sensitivity of the 15N-isotope. Reported here is the application of 15N NMR to a maturation series of undegraded plant material, plant composts, sediments, and coal samples for the purpose of investigating the alterations of nitrogen functional groups during peat and coal formation. Spectroscopic parameters such as relaxation times and cross polarization dynamics were determined for 15N-enriched plant composts to establish the optimal NMR conditions to be used for natural abundance 15N measurements on samples not enriched in 15N. The results show that up to the peat stage most of the nitrogen occurs as amide nitrogen, which derives from biogenic precursors (presumably proteins). Upon increasing coalification, pyrrolic-N becomes the dominant form in the macromolecular coal network. Pyrrolic-N in coals may be derived from selective preservation of biogenic pyrroles or by rearrangement of amide chains during maturation. Pyridinic-N does not appear to be a major constituent of coal nitrogen of the coals studied here. Our successful application of solid-state 15N NMR to coal samples has provided new insights and understanding of coal-nitrogen chemistry.