Experimental investigations on the thermochemical oxidation of n-alkane and alcohol compounds by MnO2 and Fe2O3 at temperatures up to 325 °C

Abstract

Thermochemical oxidation of organic components (TOOC) coupled to the reduction of Mn(IV) or Fe(III) can occur in a spectrum of geologic settings. Examples of geologic case studies implied that the TOOC can steadily occur at temperatures below 200 °C. However, existing relevant experimental work was conducted at temperatures above 350 °C. Therefore, uncertainties remain over the reaction at lower temperatures. This study mainly adopted Raman spectroscopy combined with fused silica capillary capsules (FSCCs) to explore the thermochemical oxidation of methane, n-alkanes (here C13H28) and alcohol compounds (here CH3OH and C2H5OH) by MnO2 or Fe2O3 at relatively low temperatures. The results showed that the TOOC reaction can proceed efficiently at temperatures below 200 °C, which, to a large extent, decreased the temperature gap between experiments and geological cases. The onset temperature of TOOC is largely dependent on the species of organic matter. Expectedly, long chain n-alkane (C13H28), methanol (CH3OH) and ethanol (C2H5OH) show higher reactivity than CH4 and thus were readily oxidized by oxidants at lower temperatures. The results also implied that either the oxidation of organic components or the reduction of MnO2 proceeds by steps, during which a series of intermediate products are generated. The activation energy of the thermochemical oxidation of CH4 by manganese oxides was also estimated based on quantitative Raman spectroscopic analyses. Finally, we discussed the potential contributions of TOOC to the alteration of organic components, the modification of reservoir properties, the carbon cycles and other geological processes.