Chemical and structural changes in vitrinites and megaspores from Carboniferous coals during maturation

Chemical and structural changes occurring in kerogen upon thermal alteration are identified and analysed based on a set of naturally matured Carboniferous coals from the Ruhr Basin (Germany). For this purpose, handpicked vitrinite from eleven samples comprising a maturity range from 0.55 to 2.86% VRr was analysed using attenuated total reflectance infrared spectroscopy (ATR FT-IR) and Curie Point pyrolysis gas chromatography/mass spectroscopy (CP-Py-GC/MS) at two pyrolysis temperatures. Additionally, reflectance μFT-IR was used to assess variations in the proportions of functional groups in megaspores from five oil mature coal samples. Infrared spectra of the vitrinites show a clear decrease in aliphatic CHx absorbance in favour of aromatic CH absorbance, pointing out an increase in aromaticity with increasing maturity. Spectra of megaspores are dominated by the absorbance of the aliphatic CHx stretching region and reveal the loss of C=O groups with increasing maturity, while the degree of aromaticity (γCH/νCHx) increases slowly compared to that of the vitrinite spectra. Vitrinites pyrolysed at 590 °C show higher yields in aliphatic hydrocarbons than those pyrolysed at 764 °C, while at the higher pyrolysis temperature the yields in aromatic compounds, including phenols and sulphur-containing aromatics are higher. The aromatic fraction of the pyrolysates, in particular the relative amount of polyaromatics increases upon maturation, while the phenolic fraction decreases in favour of benzenes. Major processes leading to these structural and chemical changes in vitrinites and megaspores are defunctionalisation of oxygen-containing groups, the loss of aliphatic compounds and the formation of monoaromatic molecules. These prevail over the condensation of aromatic ring-structures, which is, however, evidenced by increasing proportions of polyaromatic fractions in the pyrolysed vitrinites.