Research ArticleHigh-Field (3.4T) ENDOR Investigation of Asphaltenes in NativeOil and Vanadyl Complexes by Asphaltene Adsorption onAlumina Surface

Marat Gafurov ,1 Georgy Mamin,1 Irina Gracheva,1 Fadis Murzakhanov,1 Yulia Ganeeva,1,2

Tatiana Yusupova,1,2 and Sergei Orlinskii1

1Kazan Federal University, 420008 Kazan, Russia2FRC Kazan Scientific Center of RAS, Arbuzov Institute of Organic and Physical Chemistry, 420088 Kazan, Russia

Correspondence should be addressed to Marat Gafurov; marat.gafurov@kpfu.ru

Received 23 October 2018; Accepted 12 February 2019; Published 16 May 2019

Academic Editor: Mohammad Sarmadivaleh

Copyright © 2019 Marat Gafurov et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Vanadyl porphyrin complexes in asphaltenes from heavy (Karmalinskoye) oil and in asphaltene films obtained as a result ofadsorption on the surface of aluminum oxide were studied by electron paramagnetic resonance (EPR) and double electron-nuclear resonance (ENDOR) in the W-band frequency range (microwave frequency of 95GHz, magnetic field of 3.4 T).Mims ENDOR spectra from 1H and 27Al nuclei are observed. ENDOR spectra are different for native oil and asphaltenesfrom one side and the adsorbed samples from the other side while no significant changes in X- (microwave frequency of9GHz) or W-band EPR spectra are found. The results allow supposing that vanadyl porphyrin complexes (at least in thestudied asphaltene films) participate in the formation of asphaltene aggregates through the functional groups rather than π− π interactions. The data show the feasibility of the commercial pulsed ENDOR approaches for the investigation of crudeoils and their constituents under external influence.

1. Introduction

The rising economic significance of heavy crude oilsdemands the application of different analytical tools for theircharacterization in situ and corresponding theoretical studiesof interactions between the different oil constituents. Heavypetroleum and natural bitumen includes several metallicchemical forms. Among them, vanadium and nickel are acommon cause of catalyst degradation and deactivation inhydrotreatment units. For the effective extraction of themetals, the molecular structure of the metal complexes andtheir location (binding) in the asphaltene-resin aggregatesare intensively studied [1–6]. These studies appear even moreinteresting in light of the data proposing vanadyl complexesas a part of quantum computers [7, 8] and even as a sensitiveprobe in biomedical applications [9].

Among the efficient ways of removal of asphaltenes andmetal complexes from oils and studying their structure, theadsorption (deposition) of the oils and their structural units

on the surface of different supports is widely used and is amatter of intensive studies [10–15]. Besides the rapid remov-ing of asphaltenes from the heavy oil making therefore theremaining fraction of oil transportable for conventionalprocessing, the mentioned supports (especially in the formof nanoparticles) could be employed as catalysts for upgrad-ing asphaltenes into light usable distillates [10]. Alumina-(Al2O3) based materials are one of the favorable choices forthese purposes due to the high adsorption capacity.

The majority of vanadium and nickel compounds arepresent in heavy petroleum as porphyrins and concentratedin asphaltenes. In general, petroleum porphyrins exist inhomologous manifolds of several structural classes and canmanifest great structural diversity. A series of papers nowa-days is devoted to the determination of various vanadylporphyrins (VP), to application of state-of-the-art mass-spectroscopic techniques for that purpose as well as to thedevelopment of different pathways of the effective vanadylextraction [1, 4, 16–23]. Dickie and Yen were the first to

HindawiGeofluidsVolume 2019, Article ID 3812875, 9 pageshttps://doi.org/10.1155/2019/3812875

suggest that, having many of the same interactive groups asasphaltenes, petroporphyrins could also play a role in asphal-tene self-assembly, adsorption, and phase separation [24]. Itis common knowledge that intermolecular interactions, suchas hydrogen bonding, dipole−dipole interactions, and π − πstacking, in asphaltenes lead to aggregation into nanoaggre-gates in the bulk oil phase and the formation of elastic filmsat organic−aqueous interfaces although the basic asphaltenestructure and mechanism of aggregation continue to bedebated [2, 17, 25]. The relationship between petroporphyr-ins and asphaltenes is largely unexplored [1, 4, 26–28].

Since practically all petroleum systems, especially high-molecular oil components (asphaltenes and resins) are para-magnetic [29–32], the presence of intrinsic paramagneticcenters (PC), primarily paramagnetic vanadyl-porphyrincomplexes and organic “free” radicals (FR), gives an opportu-nity to study the structure (organization) of petroleumcomplexes without additional processing, in situ [2]. Differ-ent aspects of application of various electron paramagneticresonance (EPR) and double resonance (ENDOR) tech-niques for the elucidation of the asphaltene architectureand the oil PC nature are covered in refs. [2, 19, 32–46].

The effectiveness of the use of the VP as a native probe forthe investigation of asphaltenes is determined by the resolvedanisotropic spectral parameters (g-factor and hyperfinecomponents, zero-field splitting, quadrupole tensors) in theEPR/ENDOR spectra, which makes VP to be orientation-selective even in disordered media [40–51]. Using X-band(microwave frequency νMW of 9GHz, magnetic field B0 of0.34T) and high-frequency (νMW of 95GHz, B0 of 3.4 T,W-band) conventional and pulsed EPR/ENDOR techniques,we have shown that, on the one hand, the EPR and ENDORspectra for VP in the initial oil samples of different genesisare similar while, on other hand, the relaxation parametersof VP (the electron longitudinal (T1e) and transverse (T2e)relaxation times) and the temperature dependences of theEPR spectra depend strongly on the nature of the sampleand the method of the fractionation procedure [49–52]. Itwas also shown that changes in the VP EPR spectra couldbe used to track the oxidation processes under the influenceof manganese [53]. In this paper, we investigate changes inEPR/ENDOR spectra during the adsorption of asphaltenesonto aluminum oxide particles by using an experience ofEPR/ENDOR alumina characterization described in previouspapers of our group [54, 55].

2. Materials and Methods

Samples of heavy oil from the Karmalinskoye oilfield (Volga-Ural Basin, Republic of Tatarstan, Russia) with a densityρ = 948 kg/m3 and a viscosity η = 580mPa s at room tem-perature were used to obtain samples of asphaltenes (As)in the form of a powder. Compositional analysis of theoil sample (SARA analysis) was conducted according tothe common technique [56] which includes asphalteneprecipitation by heptane and further chromatographic sep-aration of the asphaltene free oil through a column filledwith alumina. SARA (saturate, aromatic, resin, and asphal-tene) analysis data of the studied crude oil are presented

in Table 1. Organic solvents, such as toluene, heptane,dichloromethane, and methanol (purity of all solvents wasno less than 99.5%), were purchased from Component-Reaktiv and used without additional purification. Aluminumoxide (γ-Al2O3, particle size 0.05-0.15mm, neutral, surfacearea 155m2/g) were purchased from Sigma-Aldrich (CAS1334-28-1). Data on NMR investigation for the oil understudy are presented in ref [57].

To prepare asphaltene films, 1.000 g of Al2O3 was addedto dichloromethane solutions containing 2 × 10−3 g × ml−1

(sample Q10) or 2 × 10−2 g × ml−1 (sample Q1) of As inround-bottom flasks. The chosen concentration values weredetermined by the solubility of As (sample with the high Asconcentration—Q1) and the sensitivity of the EPR spectrom-eter (sample with the low As concentration—Q10). Obtainedmixtures were shacked for 30min, and then Al2O3 withadsorbed asphaltenes was filtered off and dried in a vacuumevaporator (20mbar, 40°C).

W-band EPR spectra at T = 100K were registered with aBruker ElexSys 680 EPR spectrometer by using standard elec-tron spin echo (ESE) π/2 – τ – π with the π/2 pulse durationof 32ns, time delay τ = 240 ns pulse sequence. The ability todetect ESE gives an opportunity to obtain ENDOR spectraby using Mims pulse sequence π/2 − τ − π/2 − T − π/2 withan additional radiofrequency (RF) pulse πRF = 18 μs (thepulse length is optimized for 1H nuclei) inserted betweenthe second and third microwave π/2 pulses (Figure 1). RF fre-quency in our setup could be swept in the range of (1-200)MHz. More details of the pulsed W-band EPR and ENDORmeasurements for oils and polymers with the correspondingreferences are given in our papers [50, 58].

X-band (9GHz) EPR and concentration measurementswere performed at room temperature by comparing the inte-grated intensities of the spectra of a test sample and areference sample (DPPH and a series of Cu-DETC solutions)in conventional (cw) mode. Spectral simulations are done inEasySpin package for MatLab [59]. Some of the experimentalresults were presented at the international Workshop “Ther-mal methods for enhanced oil recovery” ThEOR-2017 [60].

3. Results and Discussion

EPR spectra detected at the X-band (Figure 2) for oil, oilcomponents (resins and asphaltenes), and adsorbed asphal-tenes are very similar to each other and are defined by thepresence of stable “free” radical (FR, single line, electron spinS = 1/2, g ≈ 2 003) and paramagnetic vanadyl complexes(VP) while the initial Al2O3 samples are EPR silent withinthe sensitivity limit of our instruments. Origin and EPR ofFR (especially at the conventional X-band) in oils and oilasphaltenes are comprehensively described in the literature

Table 1: Results of SARA analysis for the crude oil sample.

SARA analysis (%)Saturate Aromatic Resin Asphaltene

18.4 44.8 28.4 8.4

2 Geofluids

(see [46] and references therein) and are not in the scope ofthe present study.

Assuming a nearly planar skeleton structure of a singleVP molecule (VO2+, S = 1/2, Figure 3), the EPR spectra couldbe satisfactory described by the g-tensor of axial symmetrywith g∥ ≈ 1 96, g⊥ ≈ 1 98, and anisotropic hyperfine interac-tion with a magnetic moment I = 7/2 (therefore, 8 projec-tions of I with the quantum numbers mI = ±7/2; ±5/2; ±3/2and ±1/2 are possible) for 51V nuclei (natural abundance of99.75%) with hyperfine structure constants A∥ ≈ 480MHz,A⊥ ≈ 157MHz (Table 2). In our notations, the values ofg, A ∣| correspond to the orientation perpendicular to theVO2+ plane (out of plane), and along the direction c andg, A ⊥ correspond to the orientation in the VO2+ plane, abplane (see Figure 3). As for the most VO2+-related investiga-tions, we suppose that the direction of the main axes coincidefor g and A tensors. Comprehensive description of VP spec-tra in the X-band, their spectral simulations, and spectralchanges with external treatment are given in [49, 52]. Forinstance, a fitting of the asphaltene EPR spectrum is shownin Figure 2. Results of the concentration measurements forthe investigated powders are presented in Table 3.

As it follows from Table 3, concentrations of both FR andVP of As/Al2O3 samples decrease with the lowering of the Asamount that could serve as additional proof of the proper(proportional to the As quantity) precipitation and samplerefining. The concentration of VP relative to FR also reduces.It could be assumed that asphaltenes with a more condensedstructure and lower VP content are adsorbed on alumina. Itagrees with the previously revealed trend of inverse depen-dence of the concentration of FR on the content of vanadylcomplexes (see papers [19, 21, 22] and references therein).Alternatively (or additionally), a formation/disappearing ofnew stable radicals, in which EPR spectra are indistinguish-able from the initial ones, as a result, for example, of chemicalbond opening/chemical bonding and oxidation processesupon adsorption [60, 61], could be supposed.

W-band EPR spectra in ESE mode at T = 100K (at roomtemperatures, the electronic relaxation times are too short toobtain ESE and ENDOR with the appropriate signal-to-noiseratio) are presented in Figure 4. Due to the difference invalues of g factors, W-band experiments allow separating

FR and VP contributions spectrally. Data for the componentsof g and A tensors for VP extracted from the simulations ofVP spectra in theW-band are presented in Table 2. In details,the nature and simulations of the W-band spectra arediscussed in refs. [45, 50, 51]. For clarity, Figure 5 presentsthe calculated electronic-nuclear energy levels of the VO2+

paramagnetic complex, EPR transitions, and the correspond-ing EPR absorption line with the values of the components ofg and A close to those obtained in the experiment.

As it is seen from Figures 1 and 4 and Table 2, no signif-icant changes of EPR spectra either in the X- or W-bandoccur with asphaltene extraction or deposition (except forsome broadening of the lines in the spectrum for samplesQ1 and Q10). It is often assumed that such kind of treatmentshould result in changes of the observed g values [62, 63]. Inthis case, the so-called B parameter has been widely used as asensitive indicator of structural distortions in the V=O bondlength which can be derived as follows:

B = Δg∥/Δg⊥, 1

where Δg∥ = g∥ − ge, Δg⊥ = g⊥ − ge, and ge is the free elec-tron g value of 2.0023.

For example, an increase in the B parameter is conven-tionally ascribed to a shorter V=O bond and/or a longerdistance to the nitrogen ligands, i.e., the higher electrondensity in the axial bond and more electron delocalizationto the V orbital. Absence of the g component shift or signif-icant changes of the A components in our study mean that nosignificant redistribution of the electronic density for VPsingle molecules occurs with the extraction or adsorption.

The same conclusion could be derived from the compar-ison of the oil and asphaltene 1H ENDOR spectra (Figure 6):the 1H ENDOR pattern (splittings along the ENDOR spec-trum) for the asphaltene sample is the same as that for theinitial oil. The obtained ENDOR is different for the paralleland perpendicular orientations, are in the range of 0.3-2.4MHz, and could be ascribed to the interaction with theprotons of the following structures of vanadyl porphyrinsidentified in natural hydrocarbons: etioporphyrin (VOEtio),deoxophylloerythroetioporphyrin (VODPEP), and benzoe-tioporphyrin (VOBenzo, see [50]).

In the case of a “free” nucleus, RF pulse applied at theLarmor frequency is

vLarmor = γB0 ≡ h−1 g I β I B0 , 2

where γ is a gyromagnetic ratio of the nuclear spin I, h is aPlanck constant, g I is a nuclear g factor and β I is a nuclearBohr magneton, which can change the state of the nuclearspin (the population of the nuclear sublevels). For protonwith I = 1/2, γ1H = 42 576MHz/T that results in νLarmor ≈144 76MHz for the magnetic field of B0 = 3 4 T.

In the case of a coupled electron spin S and a singlenuclear spin, such changing can modify the state of theelectron spin (can change the population of the energy levelscontributing to the EPR spectrum). For the hyperfine

MW�휋/2 �휋/2

�휋

�휋/2

�휏 �휏

RF

T

Stimulatedelectron spinecho

Figure 1: Mims pulse sequence at microwave and radiofrequenciesused to obtain the ENDOR spectra as a function of the stimulatedelectron spin echo amplitude from the frequency of RF pulse. Thevalues of time delays (τ, T) and pulse lengths (π/2 for MW and πfor RF) are given in the text.

3Geofluids

coupling constant A and simple electron-nuclear coupling(S = 1/2, I = 1/2), it can lead to the appearance of thecharacteristic features in the ENDOR spectrum at the RFfrequencies.

vENDOR = vLarmor ± A/2, 3

or

vENDOR = A/2 ± vLarmor, 4

depending on the ratio between A and νLarmor. In the caseof more than one nucleus coupled with electron spin, onecan observe the multiple signals around the correspondingLarmor frequencies or A/2 with different hyperfine cou-pling constants or νLarmor.

In [50], the 1H ENDOR spectra of the oil under studywere comprehensively investigated and the experimentalresults were compared with the density functional theory(DFT) calculations. The obtained ENDOR splittings, accord-ing to DFT, correspond to the interatomic distances betweenthe vanadium ion and the closest protons in the range of0.46-0.6 nm for the nearly plane geometry of the single VPmolecules mentioned above (see Figure 3).

400380360Magnetic field (mT)

FR

340320300

Q10(As/Al2O2)Asph

OilResin

X‑band cwEPRT = 300 K

(a)

Asph, fitting

400380360Magnetic field (mT)

340320300

Asph, experiment

(b)

Figure 2: (a) conventional X-band EPR spectra of oil, resins, and asphaltenes and deposited on alumina surface asphaltenes (sample Q10) atroom temperature. The signal belonging to FR is marked. Other EPR components originate from VP complexes. (b) example of theexperimental (black solid line) asphaltene EPR spectrum fitting (red dashed line) in the EasySpin package for MatLab [59] with theparameters given in Table 2.

VNOCH

B2 B1

cb

a

c

ba

Figure 3: Schematic representation of a single vanadyl porphyrin molecule. The directions of the external magnetic fields “in-plane” (B2) and“out-of-plane” (B1) for the W-band ENDOR measurements are shown (see Figures 4 and 5).

Table 2: EPR parameters for vanadyls derived from the MATLAB EasySpin approximations [59] of EPR spectra for two frequency ranges.Details of approximations are given in [49-52].

X-band W-bandg∥ g┴ A∥ (MHz) A┴ (MHz) g∥ g┴ A∥ (MHz) A┴ (MHz)

Oil 1.965 1.986 480 157 1.961 1.982 482 157

Asphaltenes 1.9644 1.986 480 157 1.961 1.9825 475 157

Q1, Q10 1.965 1.986 475 157 1.963 1.9818 460 157

Table 3: Concentrations (C) оf FR and VP in asphaltenes and theadsorbed samples from the X-band measurements.

Sample C VP , spin/g C FR , spin/g C VP /C FRAsphaltenes 1.4·1019 1.4·1018 10.0

Q1 4·1017 8·1016 5.0

Q10 1·1017 2.6·1016 3.8

4 Geofluids

In ref. [50], the choice of the external magnetic fieldsdenoted as B1 and B2, correspondingly, in which the ENDORspectra were acquired, was explained in details. Briefly, forconvenience such values of B0 were chosen that correspondto “pure” parallel and perpendicular orientations of theexternal magnetic field relative to VP geometry: the B1 valuecorresponds to B0 “in-plane,” perpendicular to the c-axis,while B2 for those “out-of-plane” for mI = 3/2 transitions(Figures 3 and 5). Besides being from the samemI (that sim-plifies the interpretation and theoretical analyses), the nextreasons were outlined in ref. [50]: (1) the sufficient ESEamplitudes to obtain a reasonable signal-to-noise ratio forthe appropriate time for both values/orientations, (2) absenceof overlapping with the shoulders of the FR signal, and (3)absence of the orthogonal contributions.

Comparison of the proton ENDOR for asphaltenes andthe adsorbed samples Q1 and Q10 are shown in Figure 7. Asignificant difference in the central part of the spectra of theproton ENDOR with adsorption is obtained. For conve-nience, the difference 1H ENDOR spectra (results of thesubtraction of the asphaltene ENDOR spectrum from theQ1 and Q10 ENDOR spectra, correspondingly) are pre-sented in Figure 8.

The narrow ENDOR peak symmetrically centered on the1H Larmor frequency (see Figure 8) has a linewidth on thehalf weight of Δf = 0 16 ± 0 02 MHz. It is due to the hyper-fine interaction of VP with a large number of the remoteprotons. In assumption of the simple electron-protonpoint-dipole approximation which is applicable for the largedistances between electron and nuclear spins, one can esti-mate the electron-proton distance r

Δf =gSgIβSβI 3 cos2Θ − 1

r3, 5

where gS, gI , βS, and βI are the g factors and Bohr’s magne-tons of the electron and nucleus, respectively, and Θ is thepolar angle between the direction of the applied magneticfield and the line connecting the S and I spins, which canbe taken as 0 degree in disordered systems for the minimal

3350 3400 3450 3500

B2

B1

FRQ10

ESE

ampl

itude

(a.u

.)

Magnetic field (mT)

OilAsph

Q1

VP

W‑band, two-pulse ESET = 100 K.

Figure 4: W-band spectra of the investigated samples in ESE modeat T = 100K. Position of FR line and VP spectra are marked byarrows. Values of the magnetic fields B1 and B2 in which 1HENDOR spectra were registered are shown.

3.35 3.40 3.45 3.50−50

−48

−46

46

48

50

E (G

Hz)

Magnetic field (T)

B2

B1

g||

ESE

inte

nsity

g⊥

−50

−48

−46

44

46

48

50

B⊥c

E (G

Hz)

mI = −7/2

mI = −7/2

mI = −7/2

mI = −7/2

mI = 7/2

mI = 7/2

mI = 7/2

mI = 7/2

ms = 1/2

ms = 1/2

ms = −1/2

ms = −1/2

B|| c

Figure 5: The energy levels (the top panel for the magnetic fielddirected along “out-of-plane” while the bottom one for themagnetic field “in-plane,” see Figure 3) and the correspondingabsorption EPR spectrum (central panel) for VO2+ complexcalculated for the microwave frequency ν = 94GHz, g∥ = 1 96, g⊥ =1 98, A∥ = 470MHz, and A⊥ = 150MHz. Particular contributionsfrom every EPR transition are color-marked. Calculations are donein EasySpin package for MatLab [59] in the assumption that themain axes of g and A tensors coincide. The arrows mark the valuesof the magnetic field equivalent to the values of g∥ and g⊥. Thevalues of external magnetic field that correspond to the EPRtransitions with mI = +3/2 are denoted as B1 and B2.

143 144 145 146RF Frequency (MHz)

Oila

145 146 147 148

145 146 147 148

Asph

Oil

Asph

b

B1 B2

Figure 6: Mims ENDOR for the initial oil and asphaltenes near theproton Larmor frequency measured at (a) B1 = 3395 3mT and (b)B2 = 3448 9mT. T = 20K, W-band.

5Geofluids

distance estimation. The obtained value of Δf corresponds toremote nuclei, e.g., to a distance of ~0.8 nm and largerbetween the paramagnetic centers and the protons, and isobserved for both (parallel and perpendicular) orientations.

The presence of the resolved, orientation-dependent (butconcentration-independent) ENDOR structure allows sug-gesting that the association of most of the paramagneticvanadyl porphyrins in the asphaltenes (or at least in theasphaltene films) of the oil studied is due to the interactionthrough the functional groups attached to the porphyrinring. Indeed, while only the central peak disappears inENDOR at the “out-of-plane” magnetic field B0 = B2(Figures 7(b) and 8(b)), in the magnetic “in-plane” field B1one can additionally observe that ENDOR lines with thesplittings of 1.3 and 2.2MHz, correspondingly, are comingout during the asphaltene adsorption (Figure 8(a)). In ouropinion, it means that the asphaltene adsorption leads notonly to the detaching of the remote protons (at the distanceof larger than 0.8 nm, isotropic interaction) but also to thebreaking of some hydrogen bonds in the porphyrin plane(anisotropic interaction) with the electron-proton distancein the range of 0.4-0.6 nm [50]. In the case of π − π interac-tion, it could be expected that one would observe someconcentration-dependent changes in 1H ENDOR spectrawith adsorption and mainly (or additionally) at B0 = B2.

Dickie and Yen were the first who proposed an aggrega-tion scheme for the macrostructure of petroleum asphaltenes,

within which petroporphyrins could stack to the aromaticsheets of asphaltene [24]. Their opinion as well as the abilitiesof EPR techniques to shed light on aggregation structure wascriticized by authors of [64]. In [64], solutions of porphyrincomplexes (i.e., model systems) were investigated by UV-Visand fluorescence spectroscopy. From the lack of detectablebinding of these model compounds, it was suggested that theassociation of the majority of vanadium and nickel petropor-phyrins in crude oils with the asphaltene fraction may bedue to other functionalities appended to the porphyrin ring,rather than favorable π − π stacking interactions of aromaticrings with the porphyrin core itself. Our ENDOR (i.e., EPR-based) positive experimental results (i.e., based not on thefailure of detection but on success of detection) carried outon real (i.e., not model) oil samples are in correspondence withthe title and conclusions of ref. [64].

For the adsorbed samples, we also have observed 27AlMims ENDOR spectra. Figure 9 shows for the Q10 sampleat T = 10K at the magnetic field B1 which corresponds tothe maximal ESE signal (see Figure 4) to enhance thesignal-to-noise ratio. Aluminum ENDOR shows that potro-porhyrins in the studied species are indeed adsorbed on thealumina surface, forming the electron-nuclear complexes.In this sense, ENDOR could be fruitfully utilized for studyingadsorption of oil constituents by substances containing thespecies with the nuclear spins like alumina and silica [65].From the other side, the value of ENDOR splitting of

−3 −2 −1 0 1 2 3

Q1Q10

AsphB1

�휈–�휈larm (MHz)

(a)

B2

Q1Q10

Asph

�휈–�휈larm (MHz)−3 −2 −1 0 1 2 3

(b)

Figure 7: Mims ENDOR near the proton Larmor frequency measured for asphaltenes and adsorbed samples Q1 and Q10 at (a) B1 =3395 3mT and (b) B2 = 3448 9mT. T = 20K, W-band.

1.3 MHz

Q1‑Asph

Q10‑Asph

2.2 MHz

−3 −2 −1 0 1 2 3�휈–�휈larm (MHz)

B1

(a)

Q1‑Asph

Q10‑Asph

−3 −2 −1 0 1 2 3�휈–�휈larm (MHz)

B2

(b)

Figure 8: 1H difference ENDOR spectra for samples Q1 and Q10 measured at (a) B1 = 3395 3mT and (b) B2 = 3448 9mT.

6 Geofluids

1.2MHz (~0.045mT) gives an additional sign that no signif-icant redistribution of the electronic density occurred duringthe asphaltene precipitation. It means that the presentedapproach allows determining VP complex formation withoutother presumption or sophisticated calculations. It also givesa key why the observed by 1H ENDOR changes are practi-cally not revealed by EPR—the changes are minimal andare hidden under the broadening of the EPR lines either inX- or W-band ranges.

We did not manage to obtain 27Al Mims ENDOR forB0 = B2. It might be connected with just the lower signal-to-noise ratio in the EPR spectrum compared to B1 (Figures 4and 5), and therefore, no deep conclusions from that factare done in this work.

4. Conclusion

While the EPR spectra either in X- or W-band do not revealthe changes caused by adsorption of oil asphaltenes on thesurface of γ-alumina, a perceptible difference in 1H ENDORspectra and appearance of 27Al ENDOR spectra are obtained.High-frequency EPR/ENDOR allows resolving the anisot-ropy of the magnetic properties of stable vanadyl complexesbeing a part of asphaltenes and, therefore, is orientation-selective for the space structure of VP complexes. From theanalysis of 1H ENDOR spectra, it could be supposed thatvanadyl porphyrin complexes participate in the formationof asphaltene aggregates (at least in the studied films)through the functional groups rather than π − π interactions.It appears that application of double resonance techniques,especially at high frequencies (high magnetic fields), couldsignificantly improve the understanding of the mechanismsof the aggregation (complex formation, self-organization)of various oil constituents and other high-molecular disor-dered systems of biogenic and synthetic origins [66, 67].

Data Availability

The data used in this study are available from the corre-sponding author on reasonable request.

Disclosure

Experimental results were presented in parts at the internationalworkshops “Thermal methods for enhanced oil recovery”ThEOR-2017 [60] and ThEOR-2018 (Kazan Russia andChengdu, China, correspondingly) and at Arbuzov memorialconference “Dynamic processes in the chemistry of organoele-ment compounds - 2018” (Kazan, Russia) [68].

Conflicts of Interest

The authors declare that there is no conflict of interestregarding the publication of this paper.

Acknowledgments

The work is supported by the Program of the competitivegrowth of Kazan Federal University (“5-100”). S. Orlinskiiacknowledges a financial support of the Russian ScienceFoundation, grant # 18-13-00149, for the possibility to carryout high-field ENDOR measurements on nanosized systems.The authors are thankful to Andrey Galukhin for providingthe oil and adsorbed samples, and Centre of the Shared Facil-ities of Kazan Federal University are acknowledged for thepossibility to use their equipment. Scientific Committees ofthe mentioned Events are also acknowledged.

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34 35 36 37 38 39 40 41RF frequency (MHz)

1.2 MHz

W‑bandT = 10 KB1 = 3.3953 T

�휈Larm = 37.67 MHz

Figure 9: 27Al ENDOR spectra for sample Q10 measured at B1 =3395 3mT.

7Geofluids

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