Molecular Dynamics and Proton Hyperpolarization via Synthetic and Crude Oil Porphyrin Complexes in Solid and Solution States

Abstract

The use of vanadyl porphyrins either in synthetic compounds or naturally occurring in asphaltenes is investigated as a source of proton hyperpolarization via dynamic nuclear polarization (DNP) in nuclear magnetic resonance (NMR) experiments. The features of dynamics and location of the vanadyl VO2+ complex in aggregates within the oil asphaltene molecules are studied by means of DNP, electron paramagnetic resonance (EPR), and NMR field cycling relaxometry. Both the solid effect and Overhauser DNP were observed for the asphaltene solution in benzene, as well as in the solution and solid states for synthetic compounds. By comparison with a solution of synthetic vanadyl porphyrins, it is shown that vanadyl porphyrins in asphaltene aggregates are localized outside of the interface of the asphaltene aggregates and more exposed to the maltene molecules than “free” carbon-centered radicals associated with the core of asphaltene molecules. The perceptible contribution of scalar interaction is observed in solutions for both synthetic and asphaltene vanadyl porphyrins.

Influence of High-Molecular n-Alkane Associates on Rheological Behavior of the Crude Oil Residue

“…Figure shows even more enlarged images of oil containing two large resin and asphaltene particles. It is known that asphaltene molecules tend to self-aggregate and can form both asphaltene nanoaggregates and clusters of asphaltene nanoaggregates. − According to the size of spherical formations seen in Figure , asphaltenes together with resins can form much larger conglomerates than previously thought. Apparently, paraffin hydrocarbons play a certain role in this process.…”

“…Due to the diversity of asphaltenes’ structures and the tendency to interact through various intermolecular forces (hydrogen bonds, acid–base, donor–acceptor, dipole–dipole, π-complex, exchange, etc. ), asphaltenes in good solvents and in crude oil can be observed both in the molecular state (dispersed asphaltenes) and in the form of supramolecular structures (aggregated asphaltenes). − It is known that asphaltenes tend to self-aggregation and can form asphaltene nanoaggregates of 6–10 molecules suspended in oil . Further association leads to the formation of clusters of asphaltene nanoaggregates. − The clusters consist of 8–10 nanoaggregates, the size of which is 2–5 nm. ,, The “continental” model of asphaltene structure assumes the formation of stacking structuresaggregates in the form of packs of several molecules with a plane-parallel arrangement of aromatic systems, held together with π–π and hydrogen bonds .…”

High-Field (3.4 T) Electron Paramagnetic Resonance, 1H Electron-Nuclear Double Resonance, ESEEM, HYSCORE, and Relaxation Studies of Asphaltene Solubility Fractions of Bitumen for Structural Characterization of Intrinsic Carbon-Centered Radicals

“…The first one (with a 16-line pattern for the powder spectrum) belonged to the VO 2+ complex with a g -factor of axial symmetry (nuclear spin I = 7/2 for 51 V nuclei, g || = 1.963, g ⊥ = 1.985, A || = 468 MHz, A ⊥ = 150 MHz). A detailed analysis of signal from VO 2+ complexes was given in our papers [ 20 , 44 ]. Another one was a single line from FR.…”

“…It was demonstrated not only that the concentration and line shape of FR was changed but also the electronic relaxation times T 1e and T 2e of asphaltenes FR were affected during the SCW conversion. Additionally, there is a surprisingly small number of publications reporting on the use of double resonance techniques such as electron–nuclear double resonance (ENDOR) and dynamic nuclear polarization (DNP) in which both EPR (microwave) and nuclear magnetic resonance (NMR, radiofrequency, RF) electromagnetic waves are applied to crude oils and their fractions (see [ 35 , 41 , 42 , 43 , 44 , 45 ]). The role of FR on aggregation processes and on changes of NMR parameters is undisclosed [ 10 ].…”

Fine-Tuning Simulation of the ESR Spectrum─Sensitive Tool to Identify the Local Environment of Asphaltenes In Situ

“…ESR spectroscopists traditionally use nitroxyl and trityl radicals as spin probes to study the local structure of biochemical systems and the dynamics of their structural fragments to determine local pH values, − oxygen and proton concentrations, ,− and so forth. In heavy oils, the VO 2+ (vanadyl) fragment of asphaltenes can serve as a natural spin probe to in situ monitor their dynamics not only under mild conditions − but also at elevated temperatures and pressure, − as well as the size evolution of asphaltenes caused by the aggregation/disaggregation processes. , Besides the spectrum of vanadyls, a single resonance line close to a g -factor of ∼2.00 is observed in the ESR spectra of oils which is usually related to hydrocarbon π systems , of oil components. The variations in spectral parameters of a single absorption line ( g -factor, linewidth, and integral intensity) are usually associated with the degree of aromaticity and the presence of heteroatoms in the hydrocarbon system. − In turn, the parameters of the vanadyl spectrum can be used to identify the complex type presented in the hydrocarbon system and, for example, to follow the ratio between porphyrin and nonporphyrin complexes during catalytic transformations. ,, …”