Adsorption of Prototypical Asphaltenes on Silica: First-Principles DFT

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Adsorption of Prototypical Asphaltenes on Silica: FirstPrinciples DFT Simulations Including Dispersion Corrections Arturo Torres, Javier Amaya Suárez, Elena Rodríguez Remesal, Antonio M. Márquez, Javier Fernández Sanz, and Cristina Rincon Cañibano J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b05188 • Publication Date (Web): 31 Jul 2017 Downloaded from http://pubs.acs.org on August 1, 2017

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The Journal of Physical Chemistry

Adsorption of Prototypical Asphaltenes on Silica: FirstPrinciples DFT Simulations Including Dispersion Corrections Arturo Torres, Javier Amaya Suárez, Elena R. Remesal, Antonio M. Márquez, Javier Fernández Sanz* Department of Physical Chemistry, University of Seville, 41012 Sevilla, Spain Cristina Rincón Cañibano Technology Centre of Repsol S.A., Madrid, Spain

Abstract In this work we explore the interaction between some prototypical asphaltene and porphyrin molecules with a fully hydroxylated (0001) surface of α-quartz by means of theoretical calculations based on the density functional theory (DFT) under periodic boundary conditions. The influence of dispersion forces, adsorption geometries and size of the side chain is analyzed. The inclusion of London dispersion forces is overriding as they increase the interaction by about one order of magnitude. All the considered molecules strongly interact with the hydroxylated surface and prefer to adsorb in a parallel position instead of vertically. It is also found that adsorption energy always increases with larger side chains because dispersion interactions also augment. Interestingly, in the case of porphyrin, the less stable isomer in gas phase is the preferred one after adsorption, which is substantiated by a differential stabilization induced by the surface. Finally, we present a comparative study of the adsorption of these model molecules in terms of energy per area unit and energy per interacting π electron.

*Corresponding autor: email: [email protected]

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1 Introduction Petroleum is well known to be the primary source of energy of the world. After the primary production, the remaining oil saturation in reservoirs is still too high, around 70% or more of the original oil in place. Small increases in recovery factor represent large increases in the oil production. In this concern, water-flooding and enhanced oil recovery (EOR) techniques become more and more important.1 In this regard, understanding the interfacial mechanisms that govern reservoir wettability is relevant to optimize the recovery. On the other hand, petroleum is an extremely complex system of mainly organic compounds that mostly contains saturate and aromatic hydrocarbons, and resins and asphaltenes fractions. These ones are the most complex fractions of the crude. Particularly, asphaltenes are described as a polydisperse mixture of molecules containing polynuclear aromatic, aliphatic, and alicyclic moieties with small amounts of dispersed heteroatoms like oxygen, nitrogen, sulfur and metal atoms.2,3 Asphaltenes are the heaviest and most surface reactive non-volatile petroleum fraction. They are characterized by a solubility regime: insoluble in n-alkanes, such as pentane or heptane, but soluble in aromatic compounds like toluene, benzene or pyridine.2,4,5 The asphaltene fraction is one of the topics that strongly concerns to the oil industry, especially since lighter conventional crudes are becoming depleted and the vast reserves of heavy, extra heavy and other conventional crudes are becoming main refining feedstock.4,6 Concern comes from asphaltene adsorption at solid surfaces. This adsorption is a ubiquitous, and generally undesirable, phenomenon that is found through whole oil production chain. Some studies have established the interfacial activity of asphaltenes with high energy surfaces such as water.7,8,9 Since asphaltenes are composed of a heterogeneous variety of chemical compounds, within a solubility regime, the exact molecular compositions of many of these species are still unknown. The variety of functionalities, molecular weights, and molecular architectures make it challenging to obtain a holistic understanding of their properties.4,5 Historically, there had been some debate as to whether asphaltenes


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comprised several aromatic sections linked together by alkyl groups, known as the “archipelago” model, or if they were more like an “island” composed by a polycyclic aromatic core with pendant aliphatic side chains and paraffinic rings. Regarding molecular weight, there is no a unique range distribution for asphaltenes because of its polidispersity and its tendency of self-aggregation,3 the last depending on the technique and solvent used in the MW determination analysis. For example, Groenzin and Mullins10 reported molecular weights ranging from 500 to 1000 g·mol-1, but Speight and Plancher11 concluded an average of 2000 g·mol-1. Porphyrins are another family of compounds present in oil, identified for the first time in 1934 by Alfred Treibs.12 In 1933, some evidences of presence of metalloporphyrins were found, along with tetrapyrrole compounds. Treibs’ postulate, confirmed in 1980 by Baker, concluded that petroporphyrins were derived from chlorophyll. Treibs’ scheme describes the biodegradation of chlorophyll (type A) to etioporphyrin.13 Metalloporphyrins also serve as indicators of oil maturation because young, heavy oils contain more quantity of vanadyl and nickel porphyrins than old, light oils.14 Oil recovery strongly relies on the wettability of reservoir rocks, primarily formed by carbonate, quartz grains and clay minerals. Most of the oil compounds are hydrophobic, so the adsorption of crude oil, especially asphaltenes, on mineral surfaces could be inhibited by the aqueous film usually present on mineral surfaces.2 According to previous studies, the adsorption and aggregation of asphaltenes on mineral surfaces would result in a thick and viscous Non-Aqueous Phase Liquid (NAPL).15 Coulon et al.16 indicated that the NAPL was the most dominant fraction for the distribution of oil hydrocarbons, and the retention time of Polycyclic Aromatic Hydrocarbons (PAHs) in the NAPL was at least 3-fold longer than in others phases.17 All these features are quite relevant in the tertiary oil recovery or EOR, one of the methods being based on the injection of chemicals that lower the surface tension of oil compounds on the rocks.18,19 Silicates are the most abundant minerals, since oxygen and silicon are the main elements in the Earth’s crust, in 46.6% and 27.7% percentages respectively. Silicon oxide is not only the origin of the most abundant families of materials but also of the most complex, which manifest in a large variety of silica polymorphs and silicate materials. Among polymorphs, the most abundant is rhombohedral α-quartz that accounts for more than 10% by mass of the earth’s crust. 3 Environment ACS Paragon Plus


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The way silica interacts with different materials, organic, inorganic or biologic, has been a major topic for more than a century. Mostly viewed from an adsorption point of view, there has been a lot of research works devotes to unravel the nature and properties of the interactions between silica surfaces and adsorbates.20,21,22 In the analysis of the interaction between a solid and an adsorbate, it should be taken into account that surfaces rarely are perfectly defined planes, but they incorporate imperfections of distinct nature. One of these defects is the presence of hydration water molecules in more or less extension over the surface. From a chemical point of view, the presence of water changes and determines the wetting properties of rock surfaces and, therefore, the capability to physisorb or chemisorb an adsorbate. Chemisorbed water, also called structural water, is commonly considered in the theoretical modeling of adsorption processes. With respect to asphaltene, recent studies23,24 have shown that most specimens consist of a central PAH core with pendant aliphatic side chains as proposed by the Yen-Mullins model.25 Instead of attempting to design a single molecular model that could include all relevant structural and chemical characteristics of asphaltenes, like has been done, with relative success, in recent experimental 26,27 and theoretical 28,29,30 work, we will use model molecules that represent significant fragments of asphaltene molecular systems. In this work, we present a theoretical research based on density functional theory (DFT) calculations on the absorption of some prototypical asphaltene and porphyrin molecules onto a hydroxylated (0001) surface of α-quartz in order to model the interaction between the heavy oil fraction and the rock present in the oil sandstone reservoirs.

2. Computational details and model Periodic three-dimensional (3D) calculations were carried out using the VASP 5.3 code,31,32,33 with the projector-augmented wave method (PAW).34,35 In these calculations, the energy was computed using the generalized gradient approximation (GGA) of DFT, in particular the exchange-correlation functional proposed by Perdew, Burke and Ernzerhof (PBE).36 To obtain a more appropriate description of the


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interaction, we used the approximations proposed by Tkatchenko and Scheffler (PBETS),37 and Grimme (PBE-D3)38 to account for the London dispersion forces not included in the original PBE functional. In all cases, electronic states were expanded using a plane-wave basis set with a cutoff energy of 400 eV. Forces on the ions were calculated through the Hellman-Feynman theorem, including the Harris-Foulkes correction to forces.39 Iterative relaxation of the atomic positions and lattice parameters was stopped when forces on the atoms were < 0.04 eV/Å. The Brillouin zone was sampled using the Monkhorst-Pack set of k points.40 Given the size of the unit cells employed, these calculations were undertaken using only the Γ point.

Fig. 1 Side (a) and top view (b) of the slab used to represent the hydroxylated (0001) α-quartz surface. Atom colors: Si, cyan; O, red; H, white.

The (0001) α-quartz surface is known to present silanol groups and, at near ambient temperatures is hydrophilic (see 20 and 41). Only after heating at 700-800 ºC the surface is progressively rendered hydrophobic, by condensation of surface hydroxyls to form siloxane bridges. For these reasons, we have used a fully hydroxylated (0001) α-SiO2 surface represented by a slab model spanned by a 14atomic-layer containing 3 SiO2 units as proposed by Remesal et al.42 First, the unit cell of α-quartz SiO2 bulk was optimized using a 4×4×4 kpoint grid. The resulting cell parameters were 5.023, 5.511, 1.625, 1.628 Å and 146.8° for a, c, the two Si–O bond distances and Si–O–Si angle respectively, in good agreement with the experimental values (4.916, 5.4054, 1.605, 1.614 Å and 143.73°).43 After cutting the (0001) plane of the surface, the top surface silanol groups were substituted by OH groups that will result from the dissociation of a water molecule per silanol group. As illustrated in Figure 1, capping hydrogen atoms were added to the lowest silicon and oxygen atoms to complete 5 Environment ACS Paragon Plus


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the coordination spheres and achieve the proper tetrahedral structures of quartz. During the calculations, the deepest four atomic layers were kept fixed to their bulk positions. A vacuum space of at least 12 Å was added in the c direction to avoid artificial interactions between images.

3. Results and discussion 3.1 Adsorption of model asphaltene As mentioned in the introduction, adherence between asphaltenes and rocks depends on involved interaction forces. We first considered the adsorption process of a model asphaltene onto the α-quartz surface, considering different plausible adsorption sites on the surface. Hydroxylated (0001) α-quartz surface shows three non-equivalent interaction sites (Fig. 1b): (a) large hydrogen bonds (2.38 Å), (b) short hydrogen bonds (1.80 Å) and disilanol groups. For the calculations showed in this section, we used a 23x15x30 Å3 supercell, resulting in a hydroxyl coverage of 0.29 nm-2. This size is large enough to avoid lateral interactions, the distance between images being more than 10 Å. The choice of an asphaltene model molecule is not an easy task because the molecular structure of these compounds can be very different; this is because they are polycyclic aromatic hydrocarbon with a large number of functional groups and heteroatoms. Schuler et al.24 considered a variety of synthetic asphaltene molecules through atomic-force microscopy (AFM) and scanning tunneling microscopy (STM). From this study, we chose the so-named CA21 as our model adsorbate (Fig. 2). CA21 molecule seems to be a good approximation because it contains the characteristic functional for asphaltenes. To include different functionalities, we considered a family of these molecules that result from switching the X heteroatom for CH2, NH, O and S, we also studied the influence of lateral substituents by adding alkyls motifs such as methyl and ethyl.

Fig. 2 Model asphaltene molecule CA21 where X = CH2, NH, O, S and R = H, Met, Et.


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To examine the role of London dispersion forces, we took into account the different functionals aforementioned (PBE-TS and PBE-D3). On the other hand, we also considered two adsorption geometries: a vertical interaction between the asphaltene and the surface where the angle is near 90 degrees and a totally parallel geometry where the adsorbate is lying on the surface as shown in Fig 3. In both cases, at least five different initial geometries were tested; only the more stable ones are presented.

Fig. 3 Adsorption geometries for CA21 with X = NH and R=H. (a) top view of the parallel geometry and (b) side view of the vertical geometry. Atom colors: Si, cyan; O, red; H, white; C, black; N, blue.

Adsorption energies were estimated according to the following expression: Eads = Eadsorbate + Esurface – Eadsorbate+surface , where Eadsorbate is the energy of the adsorbate molecule in gas phase, Esurface is the energy of the clean surface and Eadsorbate+surface is the energy of adsorbate/surface system. With this description of the adsorption energy, positive values mean favorable interactions. Table 1 shows the results obtained from these calculations. First, both PBE and PBE-TS values show that parallel geometries in general give rise to adsorption energies larger than vertical ones, except for the NH substituent for which the adsorbate orientation has little effect. This exception can be explained by means of the hydrogen bond formed when NH group interacts with the oxygen atoms of surface silanol groups. It is worth to note that favorable hydrogen bonds are only formed when X = NH, since



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when X = O and X = S, formation of hydrogen bonds partially weakens the surface OH network, which is energetically less favorable. While intramolecular distances are not very much affected by the inclusion of the dispersion forces correction (either PBE-TS or PBE-D3) the vertical distance between the surface and the average molecular plane in the parallel adsorption mode is greatly reduced. While the use of the uncorrected PBE functional results in distances of 4.0-4.2 Å, the TS corrected functional reduces this distance to 2.8-2.9 Å and the PBE-D3 to 2.9-3.2 Å, demonstrating that the inclusion of the dispersion forces correction is crucial to obtain an appropriate description of the adsorbate-surface interaction. Table 1 Adsorption energy values obtained with PBE, PBE-TS and PBE-D3 for model asphaltene CA21 with different substituent groups and adsorption geometries. Energy values expressed in kJ·mol-1.

PBE

PBE-TS

PBE-D3

CA21(X)

Eads,vert

Eads,paral

Eads,vert

Eads,paral

Eads,paral

X = CH2

0.7

8.3

16.4

97.7

56.5

X = NH

11.9

7.6

72.4

72.3

56.6

X=O

1.5

7.8

13.6

89.0

68.7

X=S

1.5

6.8

18.0

95.6

63.0

Energies calculated at the PBE-TS and PBE-D3 levels are higher than at the bare PBE due to the inclusion of London dispersion forces. As can also be observed, parallel adsorption always is more favorable than vertical because of the attractive interaction between the asphaltene conjugated π system and the disilanol groups of the surface. Again one exception is found in this trend when X = NH as vertical and parallel adsorption energies have almost the same value. This behavior shows that the interaction of only one relatively strong hydrogen bond can compete with four aromatic rings weakly interacting with the surface through dispersion forces. When PBE-D3 is used, only parallel geometries were considered because of the clear trend to be more stable than vertical ones. Comparing PBE-TS and PBE-D3, Grimme’s approximation yields interaction energies between 20 and 40 kJ·mol-1 lower, in agreement with the 8 Environment ACS Paragon Plus

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longer adsorbate-surface distances obtained, an indication of a better description of the van der Waals forces in the PBE-TS model.44 Hereafter only PBE-TS results will be presented.

Fig. 4 Adsorption geometry for CA21 with X = CH2 and R = Et in parallel position. Atom colors: Si, cyan; O, red; H, white; C, black.

To analyze the influence of the side chain R on the adsorbate/surface interaction, the energies for R=methyl, and ethyl have been computed and reported in Table 2. As can be seen, adsorption energy always rises when the size of the side chain increases. This trend can be easily explained inasmuch as the longer the side chain, the larger its interaction with the surface through London dispersion forces, so, the higher the adsorption energy. Beyond this general tendency it is worth to note that the increments to the interaction energy are not linear and, on the other hand, strongly depend on the chemical nature of the X group. Table 2 Adsorption energies (in kJ·mol-1) obtained with PBE-TS approach for the model asphaltene molecule CA21 with different side chains.

X

Eads(R=H)

Eads(R=Met)

Eads(R=Et)

CH2

97.7

100.3

112.9

NH

72.3

113.1

119.1

O

89.0

110.5

125.3

S

95.6

118.5

120.2



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3.2 Adsorption of model porphyrins The interaction between asphaltenes and α-quartz in terms of other model molecules, in this case, porphyrin and metalloporphyrins was studied using the models depicted in Figure 5, with different transition metals, X, located in the central hollow. In Figure 5b, X stands for Fe and Cu and vanadyl (VO) dipositive ions. Also, only parallel adsorption geometries were considered (Fig. 6) for these systems. Adsorption energies are shown in Table 3.

Fig. 5 Structures of (a) model porphyrin 21H-23H-porphyrin; (b) a metalloporphyrin where X = Fe, Cu, VO; (c) porphyrin isomer 21H-22H-porphyrin; and (d) porphyrin with side chains where R=Met, Et.

Fig. 6 Adsorption geometry for the metalloporphyrin with X = VO. Atom colors: Si, cyan; O, red; H, white; C, black; N, blue; V, orange.

As can be seen, adsorption energies were found to be of the same order than those estimated for model asphaltenes. The presence of a transition metal decreases the adsorption energies, revealing that porphyrin molecules interact stronger than metalloporphyrins regardless the metallic center. This effect might be understood bearing in mind that the metallic cation polarizes the electronic cloud towards the center


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of the molecule diminishing the dispersion interaction all over the π surface. It is worth to note that, for X = Fe, both singlet and triplet states were considered, finding that triplet state is the most stable according to literature.45 For both X = Cu and X = VO the energies of the doublets were computed using spin-polarized calculations too.

Table 3 Adsorption energy values obtained with PBE-TS for model porphyrin and metalloporphyrin molecules of Figure 5. Energy values expressed in kJ·mol-1.

Structure

R

Eads

5a

H

54.3

5c

H

108.5

5d

Met

84.4 / 146.4

5d

Et

97.8 / 160.6

5b, X=Fe

H

73.1

5b, X=Cu

H

69.4

5b, X=VO

H

69.8

The energetics of the adsorption process deserves a careful analysis since hydrogen shift gives rise to two isomers in pure porphyrin, depicted in Figure 5. The first is the 21H,23H-porphyrin (Figure 5a) and the second the 21H-22H-porphyrin (Figure 5c). To facilitate the discussion these isomers will be labeled 23H and 22H, respectively. Notice that no experimental data is available for 22H and only the 23H isomer has been found and synthesized in the laboratory. Relative stability, Erel, for these molecules in gas phase was calculated as follows: Erel = Egas,23H – Egas,22H. We found that isomer 23H is 25.1 kJ·mol-1 more stable than 22H, explaining why the 22H isomer has never been experimentally observed. Interestingly, Eads calculations show that 22H isomer adsorbs much stronger onto the surface than its isomer. This fact can be easily explained in terms of dipole-dipole interaction. Hydroxylated (0001) α-quartz surface has a surface dipole moment that comes from the disilanol groups and, while the 23H molecule is apolar, the 22H is not. In order to compute the dipolar moment of this



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molecule, we carried out molecular calculations using the Gaussian 09 code46 at the PBE/6-31g(d,p) level of theory. Because of the asymmetric arrangement of the hollow hydrogen atoms, the molecule turns polar and shows a dipole moment of 1.98 D. This dipole-dipole interaction, in addition to the London dispersion interaction that is already incorporated, makes the 22H molecule to adsorb stronger than 23H. Finally, we studied the variation of the adsorption energy when the side chain changes from H to methyl and ethyl. Figures 5d and 7 show the structure of the model molecules used for this study. The positioning for these substituent groups was chosen to mimic organic porphyrin-based organic molecules like heme group or chlorophyll. As can be observed in Table 3, the adsorption energy rises when the size of the side chain increases. This result is explained in the same way as for the CA21 asphaltene: when the side chain grows, the interaction through dispersion forces gets larger and the adsorption energy increases.

Fig. 7 Adsorption geometry for porphyrin with R = Et. Atom colors: Si, cyan; O, red; H, white; C, black; N, blue.

3.3 Comparison between asphaltene and porphyrin adsorption To finish this analysis we performed a comparison between the results obtained for the adsorption of the CA21 asphaltene with X = CH2 and porphyrin, both of them without any side chain. In this study, we introduced two new parameters: the interaction crosssection area (Aint) and the number of π electrons for each adsorbate. The interaction surface for porphyrin was calculated as the area of a square whose side is the distance 12 Environment ACS Paragon Plus

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between carbons number 2 and 7 in the porphyrin system, resulting in a value of 37.2 Å2. In the case of asphaltene model molecule CA21, the interaction cross-section was estimated as the sum of four regular hexagons and one regular pentagon whose sides are the interatomic carbon distance, giving an area of 24.3 Å2. For the model we assumed that each conjugated carbon and imine-type nitrogen contribute one π electron to the system, while the methylene (CH2) group and pyrrole-type nitrogen atoms contribute two electrons (18 π electrons for CA21 asphaltene and 26 for porphyrin). As shown in Table 4 the adsorption energy either by π e- or area unit shows that CA21 adsorbs stronger to the surface than porphyrin despite its lower surface and lower number of π electrons. This result is a consequence of the higher degree of charge condensation contained in the polyaromatic hydrocarbon.

Table 4. Comparison between adsorption energy values obtained with PBE-TS for porphyrin (average for isomers 5a and 5c) and CA21 asphaltene model molecule. Energy values expressed in kJ·mol-1 and kJ·mol-1·Å-2 for Eads/Aint.

Adsorbate

Eads

Eads/Aint

Eads/e- π

Porphyrin

81.4

2.2

3.1

CA21

97.7

4.0

5.4

4. Conclusions In this work, we have performed a computational study of the interaction between a rock, modeled through the (0001) surface of α-quartz, and a series of functionalized polycyclic hydrocarbons and porphyrins intended to model a variety of asphaltenes. The study is based on DFT periodic calculations and we have shown that the inclusion of long-range dispersion forces is crucial to render the interaction energy between the surface and the aromatic molecule. Two modes of adsorption have been considered: vertical and parallel, the second being, in general, more favorable. Functionalization through substitution of a ring carbon by N, O and S heteroatoms, often present in asphaltenes, moderately affects the interaction energy, with changes by at most ~0.15% in the case of the parallel mode. The preferred parallel adsorption might be understood by the larger long-range



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interaction between the aromatic π clouds and the surface. The exception is for the pyrrole derivative for which the vertical and parallel modes appear close each other due to the presence of hydrogen bonding. On the other hand, increasing the length of the side chain systematically increases the adsorption energy because of the enhancement of the dispersion forces contribution. Porphyrin adsorption energies are of the same order than those computed for polyaromatic models, CA21, here considered, although when they are referred to a per area unit or per π e- basis, CA21 adsorption is stronger in spite of its lower surface and lower number of π electrons. This result is a consequence of the higher degree of charge condensation contained in the polyaromatic hydrocarbon. On the other hand, the porphyrin adsorption shows a peculiar behavior inasmuch as the 21H-23H isomer is more stable in gas phase while the 21H-22H is more stable when adsorbed on the silica surface. This is due to the appearance of a dipole-dipole interaction between this isomer and the surface. Finally, whatever the metallic center here considered is, the interaction between metalloporphyrins and the surface is lower than that of bare porphyrin.

Acknowledgments This work was funded by Repsol S.A., Grant number 2264/0638.


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TOC Graphic


Adsorption of Prototypical Asphaltenes on Silica: First-Principles DFT

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