Molecular Characterization of Photochemically Produced Asphaltenes via Photooxidation of Deasphalted Crude Oils

The ability of molecular characterization to expose the chemical and structural composition of petroleum photooxidation products can aid future optimization of oil spill remediation techniques. Previous studies have documented molecular changes induced by photochemical reactions, their compositional/structural dependence, and thus revealed that they are sample-dependent. The work herein describes the photochemical transformation of nonasphaltenic petroleum compounds (maltenes) into asphaltenes. Pentane-soluble species (maltenes) were isolated from three geologically diverse crude oils and photooxidized in a solar simulator microcosm to investigate their transformation into pentane-insoluble molecules (asphaltenes), referred to as photochemically produced asphaltenes (PPA). All oils, photoproducts, and solubility fractions were characterized by positive-ion (+) atmospheric pressure photoionization (APPI) coupled to Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and tandem-MS to access their molecular composition and structural features. The oil-soluble photooxidation products from the three deasphalted oils exhibit asphaltene contents between ∼7–19 wt % after photoirradiation, which reveals the photogeneration of asphaltenes from oils initially devoid of asphaltenes. The variation in asphaltene yield after photoirradiation suggests that the quantity of PPA is sample-dependent. PPA are shown to have lower molecular weights, much higher oxygen content, and lower aromaticity than native asphaltenes from nonoxidized oils. Compositional trends for oxygen-containing photoproducts suggest that production of new asphaltenes in the environment might occur via concurrent photooxidation, photofragmentation, and photoinduced polymerization of native petroleum compounds.