Impact of Asphaltenes on the Adsorption Behavior of Petroleum Vanadyl Porphyrins: Kinetic and Thermodynamic Aspects

Abstract

In the present work, the first data on the adsorption behavior of petroleum vanadyl porphyrins in the presence and absence of asphaltenes are afforded. As adsorptives, N,N′-dimethylformamide extract of asphaltenes containing 9.88 wt % vanadyl porphyrins and high-purity vanadyl porphyrins preisolated from the extract by the sulfocationite-based chromatographic method were used. As adsorbents, two mesoporous silica gels distinguished in their ability to slow down the rate of diffusion of asphaltene nanoaggregates were chosen. Changes in adsorptive concentration upon adsorbing were monitored spectrophotometrically. Time- and concentration-dependent adsorption studies have been conducted, and particular features in the adsorption behavior of asphaltenes and vanadyl porphyrins have been revealed and used in modeling the intermolecular asphaltene–vanadyl porphyrin interactions occurring inside the adsorbent pores. An adsorption kinetic experiment showed that the asphaltenes diffuse inside the adsorbent pores in the aggregated form and reduce the diffusion rate of vanadyl porphyrins virtually up to the rate of the asphaltene nanoaggregates themselves. An equilibrium adsorption experiment revealed that when a Langmuir-type adsorption happens the asphaltene nanoaggregates adsorbed are composed of 5–8 monomers, which is well consistent with the Yen–Mullins model. No competition for free adsorption sites between asphaltenes and vanadyl porphyrins was observed, which indicates a coaggregation mechanism of vanadyl porphyrin uptake. This assumption was supported by an adsorption thermodynamic study displaying that enthalpy change of adsorption obtained by the van’t Hoff method takes positive values for both the asphaltenes and vanadyl porphyrins present in them (ΔH° > 0) but not for isolated vanadyl porphyrins (ΔH° < 0). An average molecular weight of asphaltene monomers required for thermodynamic calculations was derived from their matrix-assisted laser desorption/ionization (MALDI) mass spectrum mathematically simulated using a log–normal distribution function. The results of the present work contribute to better understanding the nature of asphaltene–petroporphyrin interactions responsible for aggregation and adsorption properties of these petroleum components.

Complexes of Transition Metals with Petroleum Porphyrin Ligands: Preparation and Evaluation of Catalytic Ability

“…It was reported in previous work [6] that petroporphyrin demetallation can be accompanied by changes in their component composition. According to the mass spectrum, the number average molecular weight (M n ) of demetallated porphyrins is equal to 500.6 g/mol, which is 65 g/mol less than that of the initial petroleum vanadyl porphyrins [19].…”