Structural characterization of the kerogen from Youssoufia phosphate formation using mild potassium permanganate oxidation

Abstract

A 10 step alkaline KMnO 4 degradation of a Type II kerogen from Moroccan Youssoufia phosphate rock was carried out under mild conditions to give a 61 wt% yield of oxidation products. These consisted mainly of diethyl ether-insoluble, high molecular weight (HMW) acids and diethyl ether-soluble acids. Detailed gas chromatography (GC) and gas chromatography–mass spectrometry (GC–MS) analysis of the soluble acids and products of further controlled KMnO 4 degradation of the HMW acids allowed quantitative estimation of the contribution (based on gravimetry and chromatographic area) of the various types of products. The acid oxidation products are dominantly aromatic (63.5 wt% of oxidation products) and aliphatic acids (36.2 wt%). The products suggest a prevalently aromatic nature for the complex matrix of Youssoufia kerogen, with a significant contribution of aliphatic structures. Detailed analysis of the acids indicates that the network is formed by aromatic structures, mainly of naphthalene type, with aliphatic cross linked chains. The basic network is substantially substituted by aromatic moieties and alkyl chains acting as monosubstituents (giving monocarboxylic acids on oxidation). However, the matrix contains a high amount of tightly trapped hydrocarbons which can be released when it is partly or completely destroyed. If these hydrocarbons are included in the total balance of oxidation products, the proportion of aromatic structures in the Youssoufia kerogen is 32%. This value is consistent with the aromaticity factor ( f a = 0.29) determined using cross polarization-magic angle spinning 13C nuclear magnetic resonance spectroscopy (CP-MAS 13C NMR).