In situ infrared spectroscopic study of the effects of exchanged cations on thermal decomposition of a brown coal

The thermal decomposition of a brown coal with or without cations up to 500°C was followed by in situ diffuse-reflectance infrared spectroscopy. Part of the spectra obtained were deconvoluted using seven Gaussian absorption profiles. The characteristics of the infrared spectra after exchanging cations varied significantly with the species of cation exchanged. The changes in infrared spectra with increasing temperature also differed markedly. When cations were absent, carboxyl groups could be decomposed easily. Most of the functional groups decomposed and their absorption became small; however, some new functional groups appeared, e.g. aromatic CH groups and esters or anhydride groups. The absorption of the OH stretching mode varied with the valency of the cations exchanged. A shift in the peak position of the aromatic ring stretching mode to a lower wavenumber was observed regardless the presence of cations. The exchanged cations were proved to have a significant influence on the mode of decomposition.