Native Vanadyl Complexes in Crude Oil as Polarizing Agents for In Situ Proton Dynamic Nuclear Polarization

Abstract

The presence of paramagnetic species such as vanadyl complexes (VO2+) and free carbon radicals in petroleum disperse systems (PDSs) such as crude oil, bitumen, or kerogen causes significant interest of studying the structure of PDS, high-molecular weight components, and their effects on the physical and chemical properties of PDS products by magnetic resonance techniques. However, the lack of detailed studies keeps the exact structure, aggregation mechanism, and interaction with complex composites of the PDS still disputable. In this contribution, detailed electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) investigations, including advanced fast field cycling dynamic nuclear polarization, of heavy crude oil focused on vanadyl complexes are presented. A perceptible room-temperature 1H dynamic nuclear polarization (DNP) solid effect at the X-band (magnetic field of 300–400 mT corresponding to the EPR frequency of 9.5 GHz and NMR frequency of 14.6 MHz), with enhancement ±5, is observed at moderate microwave irradiation power in crude oil with a high concentration of VO2+, while no Overhauser DNP contribution is found. Using NMR T 2-encoding, DNP spectra and molecular dynamics, two components are distinguished, from which the one with slower dynamics exhibits higher DNP enhancement via VO2+ complexes. The observed difference is discussed in terms of electron–nuclear interaction and relative parts of hyperpolarized nuclear spins using an advanced model for DNP data simulation.

Quantifying Crude Oil Contamination in Sand and Soil by EPR Spectroscopy

“…While being rather viscous, it has an intermediate asphaltene fraction of 4.5% whereas some oil types exceed 15% of asphaltene (see Table 2). From [53] it is known (see Table 3) that the concentration of vanadyl complexes at 7.5 × 10 18 spins g −1 in asphaltenes is about ten times higher than that of free radicals (8.1 × 10 17 spins g −1 ). In resins, the content of which is about 26% in this oil, the amount is about 9 times lower.…”

“…Its main features are the free radical (FR) peak at g = 2.0030 with a width of approximately 6 G, and the asymmetric octet of the 51 V splitting in the VO 2+ porphyrin molecule. (See [53] and references therein for a detailed discussion of the vanadyl EPR spectrum). These two features are those most frequently encountered in heavy oils, although the relative contribution of VO 2+ varies significantly.…”

“…Concentrations of FR and VO 2+ (from simulation of experimentally obtained spectra in MatLab/ Easyspin and comparison with Cu-DETC reference) at T = 295 K[53] …”

“…FR refers to Free Radicals. The inset shows the vanadyl porphyrin which is expected to occur as part of the PAH of asphaltene molecules[53] 1 3Quantifying Crude Oil Contamination in Sand and Soil by EPR……”

Qualitative and Quantitative Analysis of Heavy Crude Oil Samples and Their SARA Fractions with 13C Nuclear Magnetic Resonance

“…Interest in the determination of SARA (saturates, aromatics, resins, and asphaltenes) components directly from the 13 C NMR experiments is also governed by possibilities to enhance the NMR signal in heavy oils through dynamic nuclear polarization (DNP) by applying EPR to the native for oil [6,[34][35][36][37] or artificially introduced [38] paramagnetic centers. It was shown that 1 H NMR signal enhancement depends not only on the type, electronic properties, concentration of paramagnetic species, and viscosity of oils but also on the SARA fraction [6,34] that can be selected in situ by choosing an appropriate repetition time of NMR experiment. To the best of the authors' knowledge, all of the oil DNP measurements to date have been conducted on 1 H nuclei.…”