Construction of vitrinite molecular structures based on 13C NMR and FT-IR analysis: Fundamental insight into coal thermoplastic properties

Abstract

Attempts were made to construct the molecular structures of vitrinite samples from four bituminous with different ranks and to investigate the correlation of molecular structures to thermoplastic properties of vitrinite by using a number of advanced analytical techniques combined with theoretical methods. The Gieseler plastometry and caking index showed that the Ewirgol vitrinite had a high fluidity (MF = 17675 ppm), wide plastic range (ΔT = 105.1 °C) and good caking properties (G = 99.32). Various analytical technique, e.g. Solid-state 13C nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction analysis and Raman spectroscopy, provided comprehensive structural information for establishing molecular models of vitrinite samples with Density Functional Theory. These molecular models showed that the Ewirgol vitrinite had more cyclic aliphatic structures that contribute significantly to the fluidity development and caking properties. These cyclic aliphatic structures could be precursors of the aromatic rings in liquid molecules during coking and provide transferable hydrogens that saturate the free radicals in the thermoplastic range. The cyclic aliphatic structures may facilitate the formation of such molecular structures with an appropriate molecule size that maintains the mobile phase in the thermoplastic range to develop fluidity.