Determination of the molecular structures of petroporphyrins has been crucial to understand the diagenetic pathways and maturation of petroleum. However, these studies have been hampered by their structural complexity and the challenges associated with their isolation. In comparison to the skeletal macrocyclic structures, much less is known about the substitutions, which are more sensitive to the maturation and diagenesis pathways. While these isolated vanadyl petroporphyrins largely consist of etioporphyrin and deoxophylloerythroetioporphyrin as expected, surprisingly, we find evidence that one or a few β hydrogens are present in petroporphyrins of low carbon numbers using a combination of ultraviolet–visible spectroscopy, Fourier transform ion cyclotron resonance mass spectrometry, and non-contact atomic force microscopy. Petroporphyrins with β hydrogens were not anticipated on the basis of their biological precursors. The data support dealkylation under catagenesis but not transalkylation or random alkylation of the β and meso positions, despite the fact that more complex porphyrin structures are formed.

Petroporphyrins contribute to many of the challenges encountered when producing, transporting, and refining heavy crude oil and bitumen. They are the source of heavy metals that poison catalysts and may facilitate the aggregation, deposition, and emulsion formation exhibited by asphaltenes. Here, they are extracted and enriched to ultrahigh purities from several sources: an Athabasca bitumen, a Canadian northern tier crude oil, and a North American heavy crude oil. Our motivation is to produce usable quantities that can be characterized and used in model studies to understand the molecular structure of asphaltenes and to probe asphaltene−petroporphyrin intermolecular interactions, in the bulk and at interfaces. Extraction is performed in a Soxhlet apparatus. The porphyrin-rich extract is then further purified using extrography (on silicapacked columns) and chromatography (on alumina-packed columns). The process yields purified petroporphyrins in unprecedented quantities (>100 mg). These purified petroporphyrins can be further refined to ultrahigh purities (>85% petroporphyrin by weight) using temperature and centrifugation to fractionate them into more and less soluble fractions. Petroporphyrins are characterized by ultraviolet−visible spectroscopy, X-ray fluorescence spectroscopy, and mass spectrometry (time of flight and Fourier transform ion cyclotron resonance). The majority of the petroporphyrins are simple etioporphyrin (407 nm Soret band) or deoxophylloerythroetioporphyrin (410 nm Soret band) types, but some are more functionalized compounds with highly broadened and shifted Soret bands.

No abstract

Small-angle neutron scattering (SANS) is used to characterize the aggregation of pure vanadyl petroporphyrins (VOPPs), as well as of asphaltenes mixed with either VOPPs or synthetic porphyrins. Simple VOPPs generally form small nanoaggregates, likely dimers and trimers, and have low solubility in toluene and form precipitates. More-complex VOPPs can form large, asphaltene-like nanoaggregates in toluene. Less-functionalized porphyrins are shown to inhibit asphaltene aggregation. More-functionalized porphyrins, potentially having one or more intermolecular interaction sites outside of the central macrocycle, have a tendency to promote aggregation and/or flocculation of asphaltenes at lower temperatures, but have little or no effect at or above room temperature. Highly functionalized porphyrins (petroleum-based and synthetic) are also shown to facilitate asphaltene aggregation, either through the formation of larger nanoaggregates or flocculation, while simpler porphyrins have a tendency to inhibit asphaltene self-assembly.

Vanadyl petroporphyrins purified from a North American petroleum vacuum residue are used in experiments probing the interfacial phenomena of the petroporphyrins as well as asphaltene–petroporphyrin mixtures. Water–oil emulsion stability as a proxy for film strength is measured using the centrifugal method. Interfacial tension and elasticity of asphaltene and petroporphyrin films are recorded over tens of hours using an interfacial dilational rheometer. Film thicknesses of asphaltene films with and without petroporphyrins are estimated by fitting multiple models to small-angle neutron scattering curves. UV–visible spectroscopy is used to compare petroporphyrin content in the interfacial and bulk material of emulsions. Petroporphyrins are shown to be surface active and have significant effects on asphaltene film development. Surface-active petroporphyrins form monolayers at the interface with very low tension and high dilatational elasticity. However, this high elasticity does not translate to film strength. Being more surface active than asphaltenes, petroporphyrins may displace the first layer of asphaltenes that adsorb at the interface. However, thick asphaltene films that kinetically stabilize the emulsified water droplets still form. Monodisperse populations of petroporphyrins are prone to precipitation and therefore do not stabilize the interface or form an elastic film.