Chemistry and Structure of Coals: Hydrogen Bonding Structures Evaluated by Diffuse Reflectance Infrared Spectroscopy

Abstract

Coal formation is a dynamic process that transforms plant residues into a rigid matrix of carbonaceous materials over the eons at somewhat elevated temperatures and pressure. Diffuse reflectance infrared spectroscopy (DRIS) is very informative in monitoring the maturation process by studying representative samples of given geologic maturity. Qualitative examination of the spectra is a facile and reliable means of ranking the coal. Further analyses of the DRIS data reveal that hydrogen bonding is a major force binding coals together. Five distinct types of hydrogen bonding exist, corresponding to the number of electron donor oxygen (proton acceptor) atoms involved. Rigidification of the cellulosic structures develops stronger fields, giving rise to a generally stronger set of hydrogen bonds. The successively stronger hydrogen bonds are noted to give rise to lower frequencies of resonance and widths of bands, and the stronger dipoles have greater extinction coefficients. Infrared spectroscopy is the most informative means of exploring the intricacies of the hydrogen bonds in coals. This preliminary work shows the merits of the DRIS technique, in that the studies are performed with neat samples (free from chemical and physical artifacts associated with support media and dilution agents).