Speciation of Metals in Asphaltenes by High-Performance Thin-Layer Chromatography and Solid–Liquid Extraction Hyphenated with Elemental and Molecular Identification

Abstract

Asphaltenes are among the most challenging components in petroleum processing because they contain high amounts of heteroatoms (i.e., S, N, O, V, and Ni) thought to be responsible for strong aggregation tendencies, precipitation, and fouling problems. The role of vanadium and nickelcontaining petroleum compounds (i.e., "petroporphyrins") in aggregation and fouling is not completely understood because asphaltene composition and structure is still a subject of debate in the petroleum chemistry community. Characterization of asphaltenes, namely, molecular analysis that employs no chromatographic separation, often fails to reveal their comprehensive composition. The work herein presents asphaltene fractionation by 1) solid/liquid extraction, which allows for separation of single-core ("island") and multicore ("archipelago) structural motifs, and 2) high-performance thin layer chromatography (HPTLC) with cellulose as the stationary phase and DCM/MeOH as the eluent, which facilitates access to petroporphyrins.Characterization is performed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and matrix-assisted laser desorption ionization Fourier transform ion cyclotron resonance mass spectrometry (MALDI FT-ICR MS). The results demonstrate that even with multiple separation steps, a large quantity of vanadyl porphyrins remains inaccessible for molecular analysis by MALDI FT-ICR MS, which raises the question of what portion of a complex sample of asphaltene can be revealed by ultrahigh resolution mass spectrometry. Furthermore, the results show that easily accessible porphyrins migrate with the solvent front in HPTLC. Thus, HPTLC can be used to isolate and identify "free" porphyrins, not locked into asphaltene aggregates; however, further development of separation methods is required to access the most difficult and problematic asphaltene fractions, which do not migrate and impose analytical challenges due to their stronger aggregation tendency.

Advances and Challenges in the Molecular Characterization of Petroporphyrins

“…81 High-performance thin-layer chromatography. Moulian et al 82 studied the contribution of porphyrins to asphaltene aggregation and reported an increase in the molecular weight of asphaltene aggregates in the presence of metalloporphyrins that interacted with asphaltene aggregate surfaces, and these species likely play a critical role in asphaltene aggregation, coprecipitation, and demetallation processes. Importantly, this study directly implicates porphyrins in the stabilization of noncovalent asphaltene aggregates, consistent with simulations reported more than a decade previously.…”

“…However, molecule-level characterization using MALDI FT-ICR MS revealed vanadyl porphyrins for only the TLC fraction eluted with acetone. 82 The combination of molecule-level (FT-ICR MS) and elemental analyses (ICP-MS) proved that abundant porphyrins are inaccessible to FT-ICR MS because of matrix effects and ionization efficiencies. 82 Importantly, these porphyrins are present in asphaltene fractions that contain abundant archipelago asphaltenes, which experience a disproportional aggregation tendency compared with island structural motifs.…”

“…82 The combination of molecule-level (FT-ICR MS) and elemental analyses (ICP-MS) proved that abundant porphyrins are inaccessible to FT-ICR MS because of matrix effects and ionization efficiencies. 82 Importantly, these porphyrins are present in asphaltene fractions that contain abundant archipelago asphaltenes, which experience a disproportional aggregation tendency compared with island structural motifs. Furthermore, Moulian et al 82 present a method to separate free porphyrins from asphaltenes based on high-performance thin-layer chromatography (HPTLC).…”

“…82 Importantly, these porphyrins are present in asphaltene fractions that contain abundant archipelago asphaltenes, which experience a disproportional aggregation tendency compared with island structural motifs. Furthermore, Moulian et al 82 present a method to separate free porphyrins from asphaltenes based on high-performance thin-layer chromatography (HPTLC).…”

Insights into the Self-Aggregation of Porphyrins and Their Influence on Asphaltene Aggregation

“…In addition, there is no clear understanding of the role of porphyrins on asphaltene aggregation during the formation of asphaltene supramolecular aggregates. Thus, exploring the roles of porphyrins on the aggregation of asphaltenes is significant for researching the aggregation of asphaltenes and solving the problems caused by asphaltenes. − …”