Toward the Asphaltene Structure by Electron Paramagnetic

Article pubs.acs.org/EF

Toward the Asphaltene Structure by Electron Paramagnetic Resonance Relaxation Studies at High Fields (3.4 T) G. V. Mamin,† M. R. Gafurov,*,† R. V. Yusupov,† I. N. Gracheva,† Yu. M. Ganeeva,‡ T. N. Yusupova,‡ and S. B. Orlinskii† †

Kazan Federal University, Kremlevskaya Street 18, Kazan 420008, Russian Federation A.E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Center, Russian Academy of Sciences, Arbuzov Street 8, Kazan 420088, Russian Federation

‡

S Supporting Information *

ABSTRACT: A series of 12 asphaltene samples extracted from heavy oils and the oxidized bitumen of different origin has been studied with high-frequency W-band (94 GHz) pulsed electron paramagnetic resonance (EPR) spectroscopy. Transverse (T2e) and longitudinal (T1e) relaxation times of the free radical (FR) and the vanadyl porphyrin (VO2+) were measured for each sample. A significant contribution of the spectral diffusion to T2e has been revealed and ascribed to the dipole−dipole interaction between the FR and VO2+. This indicates that the distance between the FR and VO2+ does not exceed a few nanometers, which means, in turn, that VO2+ can participate in construction of the asphaltene aggregates via the intermolecular interactions.

1. INTRODUCTION Heavy oils and natural bitumen comprise a significant amount of asphaltenes, the high-molecular-weight compounds with a strong tendency toward aggregation.1−3 Asphaltenes are apparently the most complicated fraction of oil. The basic criterion for assignment of the substances to the family called ”asphaltenes” is their similarity in solubility in definite solvents, namely, insolubility in n-alkanes and solubility in the toluene. Elemental composition of asphaltenes includes mostly carbon (80−86%), hydrogen (6−8%), oxygen (0.5−2%), nitrogen (0.5−2%), sulfur (2−9%), and metals (Ni and V).1−3 Nickel and vanadium are present mainly within the porphyrin complexes.4,5 Although asphaltenes are intensely studied by a broad variety of methods and techniques,6,7 important details of the aggregation mechanism(s) are not well understood. Thus, whether the vanadyl porphyrin (VO2+) complex is a constituent of the asphaltene molecule or the composite aggregates are formed as a result of the intermolecular interactions is still an open question. A new wave of interest for studying VO2+ complexes and other paramagnetic centers revealed in various oil-containing systems by electron paramagnetic resonance (EPR) and related methods has developed in the last few years.8−16 The EPR applications for a long time were restricted to the conventional X-band (microwave frequency νMW ≈ 10 GHz) EPR in the continuous wave (cw) mode that allows for essential extraction of only a little information on either the stable free radical (FR) localized within the polyaromatic condensed nuclei of the asphaltene molecules or VO2+ complex. Commercial availability and relative affordability of the high-field [high-frequency (HF)] EPR spectrometers (νMW > 70 GHz) with the possibility to exploit different pulsed techniques open new horizons in scientific research and routine EPR application as a result of their advantages, such as the enhanced spectral and temporal resolution and high sensitivity. Some applications of the HF © 2016 American Chemical Society

EPR approaches to the identification of different paramagnetic species in native oil-containing formations and their characterization were shown and reviewed in refs 9, 12, and 16. The main scope of this paper is to demonstrate further capabilities of modern HF (W-band, νMW ≈ 94 GHz) EPR spectroscopy for investigation of the structure and properties of asphaltene and vanadyl complexes, important also for their potential applications as spin qubits.13,14

2. MATERIALS AND METHODS The list of the studied samples is given in Table 1. Asphaltenes investigated in this work were precipitated from the raw material by

Table 1. List of the Studied Samples sample

raw material

source

1

technical (oxidized) bitumen technical (oxidized) bitumen crude oil taken from the Carboniferous deposits crude oil taken from the Carboniferous deposits

oil refinery 1 of Republic of Tatarstan, Russia oil refinery 2 of Republic of Tatarstan, Russia Akan oilfield (well 2023) of Republic of Tatarstan, Russia Akan oilfield (well 2262) of Republic of Tatarstan, Russia

2 3 4

investigated fractions of asphaltenes Ainit, A1, and A2

the addition of 40 mL g−1 of the petroleum ether [boiling point (bp) of 40−70 °C].6 Precipitated asphaltenes were washed in a Soxhlet apparatus with petroleum ether until the filtrate became colorless. Then, the asphaltenes with a filter were washed out with benzene, which was then evaporated. The asphaltenes thus obtained were named Ainit. Received: April 23, 2016 Revised: August 12, 2016 Published: August 16, 2016 6942

DOI: 10.1021/acs.energyfuels.6b00983 Energy Fuels 2016, 30, 6942−6946

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Energy & Fuels The A1 and A2 fractions of asphaltenes were obtained as follows.17,18 A total of 1 g of Ainit asphaltenes was first completely dissolved with 28 mL of toluene. Petroleum ether (bp of 40−70 °C) was then added in the amount of 52 mL. The resulting solution was kept in a dark place for 24 h and then filtered. Precipitated material was washed in a Soxhlet apparatus with toluene until the solvent became colorless, dried, and weighed until there was no change in the mass. The obtained fraction was called A1. Then, the petroleum ether was added to the supernatant in the amount of 228 mL and precipitated for 24 h. Using the operations described above, the fresh precipitated material was separated, washed with toluene in a Soxhlet apparatus, and dried. The A2 fraction of asphaltenes was recovered by evaporating the remaining solvent. The powders of the Ainit, A1, and A2 fractions with the typical particle size of about 20 ± 10 nm as followed from the microscopic analysis were studied at room temperature (RT) in pulsed mode of the commercial Bruker ELEXSYS E680 spectrometer in the microwave Wband (νMW ≈ 94.6 GHz). EPR spectra in the pulsed mode were detected via primary electron spin−echo (ESE) amplitude after the two-pulse echo sequence of π/2−τ−π with the π/2 pulse duration of 32 ns and the time delay τ = 240 ns while scanning a magnetic field B0 (field-swept ESE). Transverse relaxation T2e was studied by tracking the primary ESE amplitude with the same π/2−π pulse durations while varying τ. A longitudinal relaxation time constant T1e was extracted from inversion−recovery studies by applying the π−Tdelay−π/2−τ−π pulse sequence, where π pulse duration and τ were fixed (64 and 240 ns, correspondingly), while Tdelay was varied.19−21 The value of B0 during the relaxation time measurements was fixed at the particular value defined from the EPR spectra. To estimate the concentrations of the paramagnetic species, EPR spectra of the samples were obtained using a Bruker ESP 300 spectrometer operating at 9.6 GHz (X-band) in continuous wave mode. Concentrations were derived by comparing the integrated intensities of the spectra of species under investigation (double integration) and the reference one (Cu-DETC solutions) at RT in the double-cavity ER4105DR. Simulations of the EPR spectra were performed with the EasySpin subroutine module for MATLAB.22

factor and hyperfine A tensor of axial symmetry. The powder EPR spectrum of the vanadyl ions consists of 16 “lines” representing the 2 × 8 hyperfine patterns (the projection of I is allowed to take eight values: ±7/2, ±5/2, ±3/2, and ±1/2) for the parallel and perpendicular complex orientations.11 To define the EPR parameters of the existing paramagnetic species more accurately, the measurements at lower temperatures are performed (Figure 2). The simulation of the powder

Figure 2. W-band EPR spectrum of the Ainit fraction of sample 3 in pulsed mode at T = 50 K and short repetition time of 1 μs along with its simulation as a sum of VO2+ powder spectra with g|| = 1.964, g⊥ = 1.984, A|| = 16.8 mT, and A⊥ = 6.0 mT and the FR single line with g = 2.0036. Arrows of FR and VP mark the values of B0 at which the electronic relaxation times were measured for the FR and VO2+, correspondingly.

EPR spectrum of the vanadyl ion was performed assuming that the respective g and A components are collinear and their values roughly coincide with those obtained from the X-band studies.11 It gives g|| = 1.964 ± 0.003, g⊥ = 1.984 ± 0.003, A|| = 17 ± 2 mT, and A⊥ = 6.0 ± 1.4 mT. Petroleum porphyrins in oils exist in homologous manifolds of several structural classes and can contain different types of substitutions and binding moieties. These determine the wide dispersion of EPR parameters (especially A⊥ and A||) for not only different samples but even within each of them.23 The position of the FR varies from sample to sample with the g factors in the range from 2.0028 ± 0.0003 to 2.0040 ± 0.0003, while the line widths of the inhomogeneous single lines change in the range ΔH1/2 = 0.6−1.2 mT. It is usually assumed that FRs in the oil-containig species are mainly concentrated in asphaltenes and due to the delocalized π electrons of the aromatic rings and stable organic radicals of the side chains.4,6,18 The variations of EPR parameters can be caused by the diversity of different types of unresolved in EPR FRs, some of which are anisotropic.12 In this work, we measure the relaxation times only at the magnetic fields corresponding to the maximum of the ESE signal. An important consequence of the microwave frequency increase in comparison to the X-band measurements11 is a spectral resolution between the signals of the vanadyl ions and the FRs; even a small difference in the g-factor values is enough to avoid the overlap of the two main contributors to the spectra. This, in turn, allows for the performance of separate studies of the spectroscopic and relaxation properties of these paramagnetic species. The electronic relaxation time measurements for each sample were performed in the magnetic fields corresponding to the maximal values of ESE of the FR and the

3. RESULTS In Figure 1, ESE-detected EPR spectra of three fractions of sample 3 are presented, which are typical for the whole series of

Figure 1. RT W-band EPR spectrum of Ainit, A1, and A2 fractions of sample 3 in pulse mode. The spectra are due to the presence of the FR and VO2+ complexes. Arrows of FR and VP mark the values of B0 at which the electronic relaxation times were measured for the FR and VO2+, correspondingly.

samples studied. The W-band EPR spectra of the oil asphaltenes originate mainly from the paramagnetic vanadyl complex VO2+ in porhyrins (51V4+, 3d1, electronic spin S = 1/2, and nuclear spin I = 7/2) and from the carbon FRs.4,8,12 Atoms in VO2+ are arranged practically in a plane, thus defining the g 6943

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Energy & Fuels nearest VO2+ complex line (marked with arrows and denoted as FR and VP in Figure 1). We start with the relaxation properties of the FRs in asphaltenes. The transverse relaxation character T2e of the FR deviates significantly from the exponential decay (Figure 3). In

Figure 4. FR longitudinal relaxation times T1FR at RT for all of the investigated samples.

relaxation constants T1VP to the values of 1.2−1.5 μs indicates that the vanadyl complexes possess a similar structure in the studied samples; i.e., the treatment of the raw material did not modify significantly the local surrounding of the vanadyls and their structure. X-band concentration measurements allow for the estimation of the FR concentration in the range of (3−9) × 1018 spins per gram, while for VP, this value is in the range of (2−5) × 1018 spins per gram. Assuming that the molecular weight of asphaltenes is of 1000 amu, it implies one paramagnetic center per roughly 50−100 asphaltene molecules. No correlations between the relative and absolute concentrations of the FR and VP extracted from EPR with the investigated fractions as well as with the measured T2e times were found. It might be connected with the fact that the treatment of the fraction Ainit could lead to the disappearance of some certain types of FRs and to the creation of new types of FRs of alternate origin with the similar EPR parameters. Additionally, aggregation of the vanadyl complexes might play a role; aggregated and disaggregated VP could manifest in the same EPR spectra but different T2e. We have to note that combination−recombination/formation− annihilation/aggregation−dissagregation processes for the paramagnetic centers in the oil-containing systems are practically not described in the literature. These are the matter of our following publications.

Figure 3. Dependence of the primary ESE amplitude upon the delay τ between the two microwave pulses in the pulse sequence of sample 1 (fraction Ainit). Dots indicate the experimental data; a solid line is a result of the fit corresponding to eq 1; and the exponential decay (expected to be observed in the absence of the spectral diffusion) is shown with the dashed line.

our opinion, this difference (i.e., the bending of the T2e curve down) originates from the spectral diffusion, which has been considered in detail in refs 19, 21, and 24. Therefore, the following expression has been used to depict the dependence of the primary ESE amplitude upon the delay between the pulses in the applied pulse sequence: ⎛ 2t ⎞ Iecho = MFR exp⎜ − ⎟exp( −mτ 2) ⎝ T2FR ⎠

(1)

where T2FR is the transverse relaxation time of the FR, MFR is a factor proportional to the concentration of the FR, and m is a parameter accounting for the spectral diffusion. Equation 1 allows for the nice description of all of the observed T2FR relaxation curves; an example of the fit is shown in Figure 3 with a solid line. Relaxation values for all fractions and samples are presented in Table S1 of the Supporting Information. In our experiments, we did not observe any perceptible modulation of the T2e decay, which could be caused by the hyperfine interaction with 14N or 51V nuclei. As pointed out in ref 4, special thermal treatment of crude-oil-containing species can lead to manifestation of 14N hyperfine interactions even in the X-band EPR spectra. Therefore, we can conclude that the high-field ESE envelope modulation (ESEEM) techniques20 could also be potentially used for the investigations of asphaltenes. The relaxation constants in the T1e processes of the longitudinal relaxation defined under an assumption of its monoexponential character for all of the studied samples are presented in Figure 4 and given in Table S1 of the Supporting Information. Concerning the relaxation properties of the vanadyl complexes in asphaltenes, their relaxation times are significantly shorter than those for the FR. In all of the samples, a single exponential decay of the transverse magnetization is observed with the time constants T2VP within the range of 80−250 ns (Figure 4). Closeness of all of the measured longitudinal

4. DISCUSSION Let us now discuss in more detail the collected results. In comparison of the longitudinal relaxation times of the FR (Figure 4), it can be seen that T1FR in contrast to the vanadyl complexes changes significantly from sample to sample and from fraction to fraction. For all of the samples, fraction A2 is characterized by the longer T1FR than fraction A1. Taking into account that the molecules forming the asphaltenes in both fractions are of comparable size, the difference in the relaxation rates reflects the fact that the FR surrounding fractions A1 and A2 is different. We suppose that T1FR measurements (also at low frequencies, in X-band, for example) could be used to track the processes of polymerization, chemical or thermal treatments of oil-containing material, etc., in which the role of radicals is expected. Additional experimental and theoretical efforts are in need to understand the main mechanisms defining the longitudinal relaxation in asphaltenes. Spectral diffusion observed in the FR transverse magnetization decay can be induced by the presence in its nearest 6944

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Energy & Fuels surrounding of the VO2+ molecules or by other paramagnetic species (by other types of FRs, for example). We show below that the observed T2FR effect arises as a result of VO2+; vanadyl spin flips as a result of the electron−lattice relaxation causes the local magnetic field fluctuations at the FR. As a consequence, the FR resonance frequencies vary in a random manner. This results in an acceleration of the transverse relaxation. During the observation of the FR transverse relaxation process (ca. 1 μs), the VO2+ spins can change their orientations no more than once (T1VP of about 1 μs), which results in a slow spectral diffusion described by eq 1. According to ref 19, the spectral diffusion coefficient m is proportional to Δω/T1VP, where Δω is the shift of the FR resonance frequency as a result of the dipole−dipole interaction with the vanadyl electron spins. The value of Δω, in turn, is proportional to the concentration of VO2+ bonded with the FR.19 In this work, we suppose that Δω is proportional to the vanadyl transverse relaxation rate T−1 2VP. Figure 5 summarizes the experimental results for all of the studied asphaltene samples. As one can see, the dependence of

vanadyl ions and the spectral diffusion induced by the FR spins.21 Constant b is proportional to the concentration of the FR bonded with VP19 and, as we assume, to the FR transverse −1 relaxation rate T−1 2FR. Experimental T2FR rate values can be linearly fitted within the experimental uncertainty with the slope of −1 −1 T2FR ∝ (0.43 ± 0.08)T2VP

Spectral diffusion is observed in all of the studied samples, and therefore, we can conclude that the vanadyl complexes in asphaltene are bonded with the FRs, i.e., form the joint complex as a result of the intermolecular interactions. Recently, we have used the obtained influence of the doped (fast relaxing) Mn2+ ions on the relaxation of the intrinsic (slow relaxing) nitrogen centers in synthetic hydroxyapatite nanoparticles as a tool to establish whether the Mn2+ ions embed into the hydroxyapatite structure in the course of the synthesis route and to estimate the interatomic (interelectronic) distances.25,26 Accurate computation of distances between the paramagnetic centers from the relaxation measurements in the disordered nanoaggregated system with the undefined nature of one of those (FR) is quite a tricky task. Nevertheless, we can make some reasonable estimation. Different EPR approaches and their limitations to extract the electron−electron distances are nicely and briefly reviewed in ref 27. From the gathered statistics (see ref 27), it follows that the possibility to obtain an influence of one paramagnetic center on another paramagnetic center through the T2e acceleration corresponds to distances between them of 1.2−3.0 nm. Therefore, we can assume that the FR and VO2+ are located within a few nanometers from each other.

5. CONCLUSION Evidently, the transition from the traditional X-band to the HF W-band EPR allows for spectral resolution of the components of the asphaltene EPR spectra originating from the FRs and the vanadyl ions. It gives an opportunity to gain a deeper insight into the origin of the paramagnetic centers important both for fundamental research and industrial applications. Even besides the possibility to exploit the more elaborated EPR techniques, simply the line shape of the FR and intensity ratio between the FR and vanadyl signals, which served as the fingerprint of the hydrocarbon origin, can be determined much more accurately.12 Dynamical changes (dependence upon the pressure, temperature, treatment procedures and conditions, aging, etc.) can be traced easily. Relaxation characteristics can be used for these purposes as well; the relaxation times are not too short as expected for the HF EPR and appear in quite a comfortable range to measure them even at RT. The last gives an opportunity to apply different pulsed methods for the structure elucidation of the asphaltene-containing materials. With regard to the exact details presented in the paper, the interaction between the FRs and the vanadyl ion momenta has been revealed in all of the studied samples via the spectral diffusion process in the transverse magnetization decay. It shows that the typical distance between the FR and the vanadyl complex is of a few nanometers, and thus, VO2+ can actively participate in building the asphaltene aggregated by means of the intermolecular interactions. Experimental knowledge of the structural peculiarities of asphaltenes will be developed further

Figure 5. Results of the relaxation analysis of the whole asphaltene sample series: dependencies of the product of the vanadyl longitudinal relaxation constant to the spectral diffusion coefficient (filled squares, left axis) and the FR transverse relaxation rate (open circles, right axis) upon the vanadyl transverse relaxation rate. The straight lines are drawn according to eqs 2 and 4.

the product of vanadyl longitudinal relaxation time mT1VP upon the vanadyl transverse relaxation rate T2VP can be linearely fitted with a slope of −1 mT1VP ∝ (0.72 ± 0.05)T2VP

(2) 19

Such an agreement with the theory for all of the studied samples indicates that the spectral diffusion of the FR is caused by the VO2+ complexes. For all of the studied samples, the “backward” influence of the FR on the vanadyl relaxation is also observed. In ref 19, it has been shown that the organic radicals, being the source of the spin causing the spectral diffusion, lead to the exponential decay of the primary ESE amplitude Iecho R Iecho = MVP exp(− 2t /T2VP )exp( −bτ )

(4)

(3)

where MVP is the coeffcient proportional to the vanadyl concentration, TR2VP is a “true” transverse relaxation time, and b is the constant, accounting for the spectral diffusion in vanadyls. Clearly, there are two additive terms defining the measured relaxation rate T−1 2VP: the dipole−dipole interaction between the 6945

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(17) Tojima, M.; Suhara, S.; Imamura, M.; Furuta, A. Catal. Today 1998, 43, 347−351. (18) Yen, T.; Chilingarian, G. V. Asphaltenes and Asphalts. 1; Elsevier: New York, 1994; Developments in Petroleum Science, Vol. 40A, pp 459. (19) Salikhov, K. M.; Semenov, A. G.; Tsvetkov, Y. D. Electron Spin Echo and Its Applications; Nauka: Novosibirsk, Russia, 1976 (in Russian). (20) Schweiger, A.; Jeschke, G. Principles of Pulse Electron Paramagnetic Resonance; Oxford University Press: Oxford, U.K., 2001; pp 578. (21) Mims, W. B. Electron spin echoes. In Electron Paramagentic Resonance; Geschwind, S.. Ed.; Plenum: New York, 1972; pp 263−351, DOI: 10.1007/978-1-4899-5310-0_4. (22) Stoll, S.; Schweiger, A. J. Magn. Reson. 2006, 178, 42−55. (23) Smith, T. S., II; LoBrutto, R.; Pecoraro, V. L. Coord. Chem. Rev. 2002, 228, 1−18. (24) Klauder, J. R.; Anderson, P. W. Phys. Rev. 1962, 125, 912−932. (25) Gafurov, M.; Biktagirov, T.; Mamin, G.; Klimashina, E.; Putlayev, V.; Kuznetsova, L.; Orlinskii, S. Phys. Chem. Chem. Phys. 2015, 17, 20331−20337. (26) Gafurov, M. R.; Biktagirov, T. B.; Mamin, G. V.; Shurtakova, D. V.; Klimashina, E. S.; Putlyaev, V. I.; Orlinskii, S. B. Phys. Solid State 2016, 58, 469−474. (27) Berliner, L. J.; Eaton, G. R.; Eaton, S. S. Distance Measurements in Biological Systems by EPR; Kluwer: New York, 2002; Biological Magnetic Resonance, Vol. 19, pp 1−27. (28) Yavkin, B. V.; Mamin, G. V.; Orlinskii, S. B.; Gafurov, M. R.; Salakhov, M.Kh.; Biktagirov, T. B.; Klimashina, E. S.; Putlayev, V. I.; Tretyakov, Y. D.; Silkin, N. I. Phys. Chem. Chem. Phys. 2012, 14, 2246− 2249. (29) Gafurov, M. R.; Biktagirov, T. B.; Yavkin, B. V.; Mamin, G. V.; Filippov, Y. Y.; Klimashina, E. S.; Putlayev, V. I.; Orlinskii, S. B. JETP Lett. 2014, 99, 196−203. (30) Mukhambetov, I. N.; Lamberov, A. A.; Yavkin, B. V.; Gafurov, M. R.; Mamin, G. V.; Orlinskii, S. B. J. Phys. Chem. C 2014, 118, 14998−15003.

by means of electron nuclear double resonance (ENDOR) spectroscopy.8,28−30

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b00983. Relaxation parameters for FR and VP paramagnetic radicals derived from the W-band experiments at RT (Table S1) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A. A. Rodionov is acknowledged for the cw-EPR concentration measurements. This work was funded by the subsidy of the Russian Government to support the Program of Competitive Growth of Kazan Federal University among World’s Leading Academic Centers.

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DEDICATION The authors devote this work to Dr. I. N. Kurkin (Kazan) on the occasion of his 75th anniversary. REFERENCES

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DOI: 10.1021/acs.energyfuels.6b00983 Energy Fuels 2016, 30, 6942−6946