Sequential Extraction of Petroleum Asphaltenes with Magnesium Oxide: A Method To Reduce Complexity and Improve Heteroatom Identity

Abstract

Sequential extraction of an asphaltene sample with magnesium oxide nanoparticles provides an approach to remove selectively molecules from a complex fraction. This method of extraction relies, in part, on adsorption preferences for differing heteroatoms to leave weakly adsorbing sample constituents in a toluene solution. The extracted sample exhibits reduced complexity enabling more reliable identification of the remaining molecules in solution. Mass spectrometry (MS) data indicate a general bias for preferential removal of higher molecular weight species. This is supported by a shift in the average <i>m</i>/<i>z</i> ratio of the asphaltene distribution to lower <i>m</i>/<i>z</i>, as well as a decrease in the intensity observed for higher <i>m</i>/<i>z</i> ion signals, for higher extraction numbers. UV–vis absorption data corroborate MS data to provide an appreciable visual means to quantify sample uptake after several sequential extraction steps with MgO. Furthermore, both UV and MS data indicate a point of diminishing returns, after which subsequent extraction with MgO nanoparticles results in limited adsorption of remaining asphaltene constituents. The remaining asphaltene constituents can then be treated with NiO nanoparticles in order to identify molecules containing pyridyl functional groups. Implementation of a more exhaustive MgO extraction, prior to treatment with NiO, resulted in an improved method for profiling pyridyl-containing structures in a complex asphaltene mixture relative to previous work