Occlusion of Foreign Molecules in Primary Asphaltene Aggregates

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Occlusion of Foreign Molecules in Primary Asphaltene Aggregates from Near-UV−Visible Absorption Studies Igor N. Evdokimov,* Aleksey A. Fesan, and Aleksandr P. Losev Department of Physics, Gubkin Russian State University of Oil and Gas, Leninsky Prospekt, 65, B-296, GSP-1, Moscow 119991, Russia ABSTRACT: This study examined the states of vanadyl porphyrins in toluene solutions of n-heptane solid asphaltenes via measurements of near-UV−visible absorption spectra. Low intensity of the characteristic Soret absorption peak of porphyrins in most samples with various asphaltene concentrations CA indicated that porphyrins are bonded to individual basic (one to three ring) molecules of asphaltenes (at CA ≤ 0.5 mg/L), within porous supramolecular structures of most primary asphaltene aggregates (at CA = 0.6−30 mg/L) as well as in all colloidal-size complexes at higher asphaltene concentrations, up to CA = 1880 mg/L. However, porphyrin−asphaltene bonds appear to be weak and may be disrupted merely by dilution to some proper final asphaltene concentrations. Namely, for some specific values of CA (close to 4 and 12 mg/L) we observed sharp increases of porphyrin Soret absorption peaks attributed to the appearance of free, nonbonded, porphyrin molecules in these samples. We suggest that these specific dilutions of solid asphaltenes result in such equilibrium molecular systems where the active centers of asphaltenes are effectively engaged in internal bonds of primary asphaltene aggregates and, hence, are not available for aggregation with any foreign molecules.

1. INTRODUCTION Our recent experiments1−3 have shown that basic asphaltene molecules (sometimes referred to as “monomers”) include small, one to three ring, aromatic fluorophores and become predominant equilibrium species only after dissolution of solid asphaltenes in “good” solvents (benzene, toluene, and so on) to asphaltene concentrations CA ≤ 0.5−0.6 mg/L. In contrast, after dissolution to marginally higher concentrations (e.g., CA = 0.7−1.0 mg/L) predominant equilibrium species become polydisperse “primary” asphaltene aggregates which may contain up to 10−12 basic molecules associated mainly in a head-to-tail manner by noncovalent interactions. Furthermore, it has been experimentally verified that there is no simple “parental” relationship between the structures/properties of asphaltene aggregates obtained by dilutions to close concentrations. In particular, in the 4.3 mg/L solution asphaltene aggregates appear to be deficient in tight assemblies of one to two ring fluorophores and have smaller refractive indices as compared to aggregates in both 3.1 mg/L and 7.6 mg/L solutions.2,3 On the basis of these experimental results we concluded that the adequate structural description of primary asphaltene aggregates at concentrations up to CA ≈ 30 mg/L may be that suggested in ref 4. The authors of this publication described aggregates such as supramolecular assemblies of molecules, combining cooperative binding by Brønsted acid− base interactions, hydrogen bonding, metal coordination complexes, and interactions between cylcoalkyl and alkyl groups to form hydrophobic pockets, in addition to aromatic π−π stacking. They suggested a range of aggregate architectures, which almost certainly occur simultaneously, including porous networks and host−guest complexes. The latter may include organic clathrates, in which occluded guest molecules stabilize the assembly of a cage, as methane does in gas hydrates, as embodied by the “representative structure” in © 2017 American Chemical Society

Figure 1 (reproduced from Figure 3 in the original publication4). A typical “occluded guest” is a molecule of metalloporphyrin (MP), as illustrated in the lower right-hand corner of Figure 1. It should be noted that the negative impact of MPs on crude oil processing as well as the increasing use of heavy and extraheavy oils as a result of dwindling light oil reserves has promoted a significant research effort aimed at finding ways of removing or separating these contaminants from crude oil and related products. The association with asphaltenes is probably the reason behind the great difficulties in extracting or separating MPs from crude oils or residues. Although trapping/occlusion of MPs in colloidal asphaltenes is welldocumented,5−11 there is no general agreement on the underlying molecular mechanisms. In particular, a problem of MP trapping in primary asphaltene aggregates (in solutions with asphaltene concentrations below 30 mg/L) has never been studied experimentally. In this work we investigate this problem by examining characteristic Soret absorbance peaks of metalloporphyrins in dilute asphaltene solutions and conclude that the trapped MPs may be effectively liberated by diluting asphaltenes in toluene to some specific concentrations (e.g., those close to 4 and 12 mg/L).

2. EXPERIMENTAL SECTION 2.1. Materials. n-C7 (heptane insoluble) solid asphaltenes were separated according to the standard ASTM D6560 method12 from a heavy Russian crude oil (density at 20 °C, 955.3 kg/m3; viscosity at 20 °C, 1071 cSt; asphaltenes, 10.87 wt %; resins, 15.2 wt %; paraffins, 4.65 wt %; sulfur, 1.09 wt %; metals (V + Ni), 0.011 wt %; solids, 0.46 wt %). Several stock solutions of solid asphaltenes in toluene (“chemically Received: October 28, 2016 Revised: January 6, 2017 Published: January 10, 2017 1370

DOI: 10.1021/acs.energyfuels.6b02826 Energy Fuels 2017, 31, 1370−1375

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Energy & Fuels

Figure 1. Schematic representation of a supramolecular assembly composed of host and guest species in a representative asphaltene aggregate. Associations between molecules are color-coded: acid−base interactions and hydrogen bonding (blue), metal coordination complex (red), a hydrophobic pocket (orange), π−π stacking (face-to-face, dark green; within a clathrate containing toluene, light green). (Reproduced with permission from ref 4. Copyright 2011 American Chemical Society.) pure” grade) were prepared with asphaltene concentrations of ca. 2 g/ L. These stock solutions were stored in hermetically sealed glass vessels in darkness and at ambient pressure and temperature (24.5− 25.6 °C). In each sample of dilute solution, the concentration of asphaltenes (CA) was verified by weighing a certain volume (50−250 mL) of pure toluene before and after mixing with a predetermined volume of a stock solution (weighting precision of 0.1 mg). One of the reviewers recommended that we should explain in more detail why the solutions were weighed instead of performing the dilutions by volume. The reason was that in preparing the most dilute solutions we had to employ very small volumes of stock solutions, introduced into toluene by drops. For example, in preparation of the 0.4 mg/L solution one drop of the stock solution was introduced into ca. 250 mL of toluene. Volumes of individual drops cannot be determined with sufficient accuracy by any simple conventional techniques, not involving laborious procedures of sessile drop image processing. On the other hand, the mass of one drop in our experiments was 40−45 mg and could be measured with the accuracy not worse than 0.2%. Note that asphaltene dissolution/aggregation have recently been proven to be kinetic phenomena and can take several hours,13 days, or weeks in some cases.1−3,14 Consequently, optical tests (by our standard method of refractive index measurements 1−3) were performed at various times (from 1 h to 7 days) after preparation of representative samples (with CA = 1.03, 48.5, 184, and 493 mg/L), to ensure that the asphaltene aggregates in the studied dilute solutions no longer evolved. During the first day, strong kinetic effects were observed in all studied solutions. Regardless of asphaltene concentration in a sample, stable results were observed after 5 and 7 days of aging. In all samples the results virtually stabilized (at ≥96% of the stable values) after a period of 3 days. Hence, in order to reduce the overall experimental time, asphaltenes in solutions that were aged for this standard 3 days period were regarded to be in their equilibrium states. 2.2. Measurements of Optical Absorption. Optical absorbance spectra (A(λ)) were measured at the 270−1100 nm range using a Model SF-56 spectrophotometer (ZAO “OKB SPECTR”, SaintPetersburg, Russia). Standard 5−50 mm sample and reference (toluene) quartz cuvettes were used, and the measurements were performed at 24 ± 0.5 °C.

3. RESULTS AND DISCUSSION 3.1. Identification of Metalloporphyrins in Dilute Asphaltene Solutions. Due to the high molar coefficient of electronic absorption of near-UV−vis radiation by various porphyrins, vanadium and nickel porphyrins in asphaltenes can be identified and quantified by their characteristic near-UV−vis absorption peaks.15 As a representative example, Figure 2 shows

Figure 2. Characteristic porphyrin peaks in absorption spectra of dilute asphaltene solutions.

two parts of an absorption spectrum for toluene solution with asphaltene concentration CA = 12.6 mg/L. This group of absorption peaks may be reliably ascribed to a specific type of petroporphyrins. Namely, according to a previous spectroscopic investigation of purified porphyrin fractions,16,17 the peak at 410 nm corresponds to the Soret band of vanadyl porphyrins, while peaks at 573 and 533 nm correspond, respectively, to the so-called Qα and Qβ bands. Moreover, a clear predominance of Qα peak in Figure 2 allows classifying the discussed species more specifically as vanadyl etioporphyrins (VO-EP) with a molecular composition of C32H36N4OV.16,17 1371

DOI: 10.1021/acs.energyfuels.6b02826 Energy Fuels 2017, 31, 1370−1375

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Energy & Fuels It should be noted, however, that the low-intensity Qα and Qβ absorption peaks could be reliably registered only in some asphaltene solutions. Hence, in further studies of asphaltene concentration effects we measured the intensity of a more prominent Soret absorbance peak. 3.2. Effects of Asphaltene Concentration in Toluene Solutions on the Intensity of the Soret Absorption Peak of Porphyrins (First Data Set). In the first set of experiments with one of the stock solutions we employed a standard 10 mm sample and reference (toluene) cuvettes. To ensure that absorbances at 410 nm (position of the Soret peak) were within the recommended optimum range of 0.01−2.0,18,19 concentrations of asphaltenes in studied solutions were varied from 1.2 to 41.5 mg/L. The measured absorbance spectra in the neighborhood of porphyrin Soret peak are shown in Figure 3. In all spectra this peak is superimposed upon a featureless continuous background, typical for asphaltenes of any origin.6,20,21

Figure 4. Evaluation of relative intensity of the Soret absorbance peak of porphyrins (cf. text).

ACA, where AM and ACA are respectively the measured absorbance and the background absorbance from continuous approximation (cf. Figure 4). Complex effects of asphaltene concentration on the relative intensity of porphyrin absorbance peak at 410 nm are illustrated in Figure 5. The detailed discussion of the plausible

Figure 3. Soret peak of porphyrins in absorption spectra of asphaltene solutions in 10 mm sample cuvettes.

Figure 3 shows that at any fixed wavelength the continuous absorbance gradually increases with asphaltene concentration, as expected from the well-known Beer’s law.18,19 The relative content of porphyrins in the studied asphaltenes is not affected by toluene dilution; hence the Soret absorbance peak may have been expected to demonstrate a similar gradual increase in intensity with asphaltene concentration. However, our experimental data do not reveal such gradual concentration behavior. As may be seen from Figure 3, the Soret peak has appreciable intensity only in some specific asphaltene solutions (with concentrations of 3.5 and 12.6 mg/L) while it is hardly visible in samples with all other asphaltene concentrations. To our knowledge, such unusual concentration effects have never been reported before. For quantitative evaluation of these effects, on the basis of the data in Figure 3 for each solution we calculated the relative intensity of the Soret peak with respect to the continuous background by a procedure illustrated in Figure 4. First, from the measured absorbance spectrum we removed all data points constituting the Soret peak (in the range of 385−435 nm). Second, the remaining continuously falling parts of the absorbance spectrum in the range of 350− 500 nm were approximated by a single fourth order polynomial (solid line in Figure 4). Finally, at the wavelength of 410 nm we calculated the relative Soret peak intensity IREL = (AM − ACA)/

Figure 5. Effects of asphaltene concentration in toluene solutions on the relative intensity of porphyrin Soret peak at 410 nm in absorbance spectra measured in 10 mm cuvettes.

causes of these effects will be presented in section 3.4. Clearly seen is the low value of IREL in most solutions which we ascribe to a strong decrease of absorption (hypochromism22) of porphyrin chromophores bound within the respective structures of primary asphaltene aggregates (cf. an example in Figure 1). Accordingly, large values of IREL in samples with asphaltene concentrations of 3.5 and 12.6 mg/L may be ascribed to the appearance of free porphyrins in solutions where the equilibrium structures of primary asphaltene aggregates are deficient in active centers available for binding porphyrin molecules. Hypothetically, these active centers may be more effectively involved in internal interactions between basic asphaltene molecules within supramolecular complexes of the respective primary asphaltene aggregates.4 Note that specific structural properties of aggregates in solutions with asphaltene 1372

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are no longer involved in holding together the supramolecular structures of primary aggregates (cf. the discussion in section 3.4). Finally, in this series of experiments we investigated the longstranding problem of whether or not all petroleum metalloporphyrins are co-precipitated with n-heptane asphaltenes.31−33 Namely, we prepared several samples of the parent crude oil diluted with toluene to final asphaltene concentrations in the range of those in solutions of solid asphaltenes. As illustrated in Figure 6, the relative intensity of the Soret porphyrin peak in absorbance spectra of crude oil solutions was notably higher than that in solutions of solid asphaltenes; i.e., in the process of n-heptane precipitation a sizable quantity of crude oil vanadyl porphyrins remained in the liquid (maltenes) phase. 3.4. Hypochromism of the Soret Absorption Peak: A Reliable Indicator of Porphyrin Aggregation/Complexation. In the absence of other molecular species, porphyrins demonstrate the ability to self-assemble spontaneously into aggregates through noncovalent interactions.34,35 One of the well-proven consequences of self-aggregation is notable hypochromism (intensity decrease) of the Soret peak in nearUV−visible absorption spectra.36−38 In support of the interpretation of our data for more concentrated solutions in Figures 5 and 6 is a strong hypochromism of porphyrins incorporated into extended structures of various macromolecules (e.g., DNA) and into complex supramolecular assemblies.39−43 On the other hand, hypochromism is observed in noncovalent aggregates of porphyrins with individual molecules and nanoparticles (quantum dots, fullerenes, and carbon nanotubes).44−46 Of the immediate relevance to our results in most dilute solutions (with the predominance of nonaggregated one to three ring basic asphaltene molecules) are experimental and theoretical results reported in a recent series of publications by the research group of Prof. M. R. Gray.5,7,47,48 One of the problems investigated in these publications was an apparent “invisibility” of metalloporphyrins in crude oils and asphaltenesi.e., low intensity of porphyrin characteristic absorption peaks. The authors concluded that petroporphyrins that do not contribute a strong Soret band in near-UV−vis spectroscopy are bound to other asphaltene molecules/aggregates. In particular, they suggested that axial coordination (V−N bonds) and hydrogen bonding (O−H bonds) are the preferred aggregation modes of vanadyl porphyrins with nitrogen-containing pyridinic fragments of asphaltene aggregates and with individual pyridine molecules. In support of their theoretical results they present experimental absorption spectra7,48 for free molecules of vanadyl porphyrins in 1.5 μmol/L solution in heptane and for porphyrin−pyridine aggregates in a mixture of 0.5 mL of this solution with 1.00 mL of pure pyridinecf. Figure 7. Note strong hypochromism in the latter casethe Soret peak intensity for aggregates is only 39% of the intensity of this peak for free porphyrins. Remarkably, virtually the same quantitative manifestation of hypochromism was observed in our experiments illustrated in Figure 6. Namely, in solutions with CA = 4.3 mg/L (presumably containing free porphyrin molecules) the Soret peak intensity has its maximum value of IREL = 0.066. On the other hand, in the most dilute solutions with CA = 0.2 mg/L (presumably containing aggregates of porphyrins with individual basic asphaltene molecules) the Soret peak intensity decreases to IREL = 0.023, i.e., to 35% of the maximum value.

concentrations close to 4 and 12 mg/L have been revealed earlier by our steady state fluorescence measurements.2 3.3. Effects of Asphaltene Concentration in Toluene Solutions on the Intensity of the Soret Absorption Peak of Porphyrins (Second Data Set). To check the reproducibility of the observed concentration effects, we prepared from another stock solution (with CA = 1880 mg/ L) a new set of toluene solutions, with somewhat different asphaltene concentrations. This time absorption spectra were measured in cuvettes with various lengths1, 2, 10, 20, and 50 mm. This allowed extension of the range of asphaltene concentrations in the studied solutions to CA = 0.204−1880 mg/L. The observed effects of CA on the relative intensity of porphyrin Soret absorbance peak are illustrated in Figure 6.

Figure 6. Effects of asphaltene concentration in toluene solutions of solid asphaltenes and of the parent crude oil on the relative intensity of porphyrin Soret peak at 410 nm in absorbance spectra measured in cuvettes of various lengths: 50 mm (light blue squares), 20 mm (green circles), 10 mm (downward black triangles), 2 mm (red squares and upward red triangles), and 1 mm (dark blue circles and upward black triangles).

The data obtained with cuvettes of different lengths are denoted with different symbols. In good coincidence with the first data set (Figure 5) maxima of IREL, indicating the appearance of nonbound porphyrins, are observed in samples with asphaltene concentrations of 4.3 and 12.3 mg/L. The data of Figure 6 show that the low values of IREL characteristic for trapped/bonded porphyrins are observed not only in solutions with asphaltene aggregates described as “primary”1,2 (CA < 30 mg/L) but also in samples with asphaltene concentrations higher by almost 2 orders of magnitude. At such high concentrations asphaltene aggregates are known to form colloidal-size complexes including those with fractal geometry.23−26 Earlier, by various experimental techniques it was demonstrated that metalloporphyrins are effectively trapped in colloidal asphaltenes,5,6,27−30 while no covalent bonds or specific interactions appear to be required. It was proposed that trapping is the mechanical result of filling large fractal voids in colloidal asphaltenes with molecules of metalloporphyrins.30 Low values of IREL were also registered in samples with asphaltene concentrations below 0.5 mg/L where, according to previous fluorescence measurements, the predominant species are nonaggregated one to three ring basic molecules of asphaltenes.2 Apparently, this indicates that vanadyl porphyrins readily interact with individual active centers in asphaltenes that 1373

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REFERENCES

(1) Evdokimov, I. N.; Fesan, A. A.; Losev, A. P. New Answers to the Optical Interrogation of Asphaltenes. Monomers and Primary Aggregates from Steady State Fluorescence Studies. Energy Fuels 2016, 30 (6), 4494−4503. (2) Evdokimov, I. N.; Fesan, A. A.; Losev, A. P. New Answers to the Optical Interrogation of Asphaltenes. Complex States of Primary Aggregates from Steady State Fluorescence Studies. Energy Fuels 2016, 30 (10), 8226−8235. (3) Evdokimov, I. N.; Fesan, A. A. Multi-step formation of asphaltene colloids in dilute solutions. Colloids Surf., A 2016, 492, 170−180. (4) Gray, M. R.; Tykwinski, R. R.; Stryker, J. M.; Tan, X. Supramolecular Assembly Model for Aggregation of Petroleum Asphaltenes. Energy Fuels 2011, 25 (7), 3125−3134. (5) Dechaine, G. P.; Gray, M. R. Chemistry and Association of Vanadium Compounds in Heavy Oil and Bitumen, and Implications for Their Selective Removal. Energy Fuels 2010, 24 (5), 2795−2808. (6) Dechaine, G. P.; Gray, M. R. Membrane Diffusion Measurements Do Not Detect Exchange between Asphaltene Aggregates and Solution Phase. Energy Fuels 2011, 25 (2), 509−523. (7) Stoyanov, S. R.; Yin, C.-X.; Gray, M. R.; Stryker, J. M.; Gusarov, S.; Kovalenko, A. Density functional theory investigation of the effect of axial coordination and annelation on the absorption spectroscopy of nickel(II) and vanadyl porphyrins relevant to bitumen and crude oils. Can. J. Chem. 2013, 91, 872−878. (8) Acevedo, S.; Guzmán, K.; Labrador, H.; Carrier, H.; Bouyssiere, B.; Lobinski, R. Trapping of Metallic Porphyrins by Asphaltene Aggregates: A Size Exclusion Microchromatography With HighResolution Inductively Coupled Plasma Mass Spectrometric Detection Study. Energy Fuels 2012, 26 (8), 4968−4977. (9) Chacón-Patiño, M. L.; Vesga-Martínez, S. J.; Blanco-Tirado, C.; Orrego-Ruiz, J. A.; Gomez-Escudero, A.; Combariza, M. Y. Exploring Occluded Compounds and Their Interactions with Asphaltene Networks Using High-Resolution Mass Spectrometry. Energy Fuels 2016, 30 (6), 4550−4561. (10) Castillo, J.; Vargas, V. Metal porphyrin occlusion: Adsorption during asphaltene aggregation. Pet. Sci. Technol. 2016, 34 (10), 873− 879. (11) Liu, H.; Lin, C.; Wang, Z.; Guo, A.; Chen, K. Ni, V, and Porphyrinic V Distribution and Its Role in Aggregation of Asphaltenes. J. Dispersion Sci. Technol. 2015, 36 (8), 1140−1146. (12) Standard Test Method for Determination of Asphaltenes (Heptane Insolubles) in Crude Petroleum and Petroleum Products, ASTM D656000, IP 143/01; American Society for Testing and Materials (ASTM): West Conshohocken, PA, USA, 2000. (13) Sheu, E. Y.; Acevedo, S. A dielectric relaxation study of precipitation and curing of Furrial crude oil. Fuel 2006, 85 (14−15), 1953−1959. (14) Maqbool, T.; Balgoa, A. T.; Fogler, H. S. Revisiting Asphaltene Precipitation from Crude Oils: A Case of Neglected Kinetic Effects. Energy Fuels 2009, 23 (7), 3681−3686. (15) Zhao, X.; Xu, C.; Shi, Q. Porphyrins in Heavy Petroleums: A Review. In Structure and Modeling of Complex Petroleum Mixtures; Xu, C., Shi, Q., Eds.; Springer: New York, 2015; pp39−70, DOI: 10.1007/ 430_2015_189. (16) Doukkali, A.; Saoiabi, A.; Zrineh, A.; Hamad, M.; Ferhat, M.; Barbe, J. M.; Guilard, R. Separation and identification of petroporphyrins extracted from the oil shales of Tarfaya: Geochemical study. Fuel 2002, 81 (4), 467−472. (17) Czernuszewicz, R. S. Geochemistry of porphyrins: biological, industrial and environmental aspects. J. Porphyrins Phthalocyanines 2000, 4 (4), 426−431. (18) Hughes, H. K. Beer’s Law and the Optimum Transmittance in Absorption Measurements. Appl. Opt. 1963, 2 (9), 937−945. (19) Knowles, A., Burgess, C., Eds. Practical Absorption Spectrometry, Vol. 3; Chapman and Hall: London, U.K., 1984; DOI: 10.1007/97894-009-5550-9. (20) Mullins, O. C., Sheu, E. Y., Eds. Structures and Dynamics of Asphaltenes; Springer: New York, 1998.

Figure 7. Soret absorption peaks in heptane solutions with (1) free molecules of vanadyl porphyrins and (2) porphyrin−pyridine molecular aggregates. (Developed from data extracted from ref 7.)

4. CONCLUSIONS We performed a series of near-UV−visible absorption studies of toluene solutions with asphaltene concentrations down to CA = 0.2 mg/L. The presence of vanadyl porphyrin molecules was identified in all samples by their characteristic peaks in absorption spectra. The observed hypochromic behavior of the relative intensity of Soret absorption peaks IREL is consistent with binding of the porphyrins within porous supramolecular structures of primary asphaltene aggregates in most samples diluted to CA = 0.6−30 mg/L as well as in all colloidal-size complexes at higher asphaltene concentrations, up to CA = 1880 mg/L. Similarly, we propose that in solutions with CA ≤ 0.5 mg/L porphyrins aggregate with individual basic (one to three ring) molecules of asphaltenes. The most notable experimental result was that at some specific asphaltene concentrations (close to 4 and 12 mg/L) we observed sharp increases of IREL attributed to the appearance of free, nonbonded, porphyrin molecules in these samples. We suggest that these specific dilutions of solid asphaltenes result in such equilibrium molecular systems where the active centers of asphaltenes are effectively engaged in internal bonds of primary asphaltene aggregates and, hence, are not available for aggregation with other foreign molecules such as porphyrins. Distinctive structural properties of such primary asphaltene aggregates have been revealed also by measurements of steady state fluorescence emission.2 Taking into account a long time scale of equilibration in asphaltene solutions (up to several days) measurements of IREL could be used as a powerful kinetic tool to study the rates of association and dissociation of porphyrins with asphaltene molecules and aggregates.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. URL: http://eee.gubkin.ru. ORCID

Igor N. Evdokimov: 0000-0001-7806-0802 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge support from the Nedra-Test Research and Testing Establishment, LLC, Moscow, Russia in purchasing the experimental equipment. 1374

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DOI: 10.1021/acs.energyfuels.6b02826 Energy Fuels 2017, 31, 1370−1375