Structural insights of asphaltenes through mild oxidation reactions and MID infrared spectroscopy

Mild oxidation reactions were induced over four different asphaltenes. Oxidation products were characterized via infrared spectroscopy to propose structural descriptors, taking advantage of the high sensitivity for detecting carbon oxygen vibrations in the infrared spectrum. Although no quantification over functional groups was performed, a good comparison between feedstock and products was achieved, after de-convoluting the FTIR spectra. It was confirmed that methyl groups are useful moieties on tracing the position of alkyl topologies over the n-heptane insoluble structures throughout oxidation reactions, as oxidation is more suitable on α-H of side alkyl chains. Considering this, it was inferred that the esters were the main functional groups formed under such oxidation conditions. The formation of lactones over aromatic nuclei was monitored through the slope of a parity plot with band intensity ranging between 1730cm-1 and 1230cm-1. TGA-MS and 1H-NMR analysis of raw n-heptane insolubles rendered valuable complementary information to confirm the structural attributes of the samples obtained by FTIR. The results from this research prove that handling oxidation selectivity over such complex hydrocarbon mixtures could have broader relevance for structural characterization purposes.