Molecular Interactions between Orinoco Belt Resins - Energy & Fuels

Article pubs.acs.org/EF

Molecular Interactions between Orinoco Belt Resins Olga Castellano,*,† Raquel Gimon,‡ Carlos Canelon,† Yosslen Aray,§ and Humberto Soscun*,§,∥ †

Gerencia Departamental de Investigación Estratégica RIIE, ‡Gerencia Departamental de Refinación e Industrialización, Gerencia General de Refinación, Petróleos de Venezuela (PDVSA)-Intevep, Los Teques, Estado Miranda, Apartado 76343, Caracas 1070-A, Venezuela § Centro de Quimica, Instituto Venezolano de Investigaciones Científicas (IVIC), Altos de Pipe, Caracas 1020-A, Venezuela ∥ Laboratorio de Química Inorgánica Teórica (LQIT), Departamento de Química, Facultad Experimental de Ciencias, La Universidad del Zulia, Apartado 526, Maracaibo 4001-A, Venezuela ABSTRACT: This paper describes the characterization of structural, energetic, and electric properties of the molecular complexes between Orinoco belt resins through the application of computational molecular mechanics (MM), semi-empirical parametrization (PM6), and density functional theory (DFT) (PW91 and HCTH) in conjunction with the double numeric and polarized (DNP) basis set. The resin sources for the studied molecules are Orinoco belt vacuum residues (Carabobo, Hamaca, Merey, Merey-Mesa, and Zuata). Molecular structures of these compounds were proposed from analysis of experimental characterization. The study of molecular interactions has shown that these resins are able to form stable van der Waals complexes, where their computed stability has a large dependence upon the applied theory level. A qualitative description of the formation of these complexes could be obtained with these methodologies. In particular, MM overestimates resin interaction energies when compared to PM6 and DFT (PW91 and HCTH) results. However, the HCTH/DNP approach leads to interaction energy values for resin−resin complexes that lie in the range from −2.39 to −7.09 kcal/mol, in better agreement with literature reports and chemical expectations than PW91/DNP interaction energy values. The stability of these complexes and the strength of the resin self-association can be rationalized considering their chemical nature and the induced electric properties (dipole moment and polarizabilities) by molecular interactions. Additionally, inclusion of dispersion in the DFT calculations of the resin molecular complexes improves the energetic pattern of the studied molecules significantly.

1. INTRODUCTION Petroleum is a complex mixture of organic and inorganic materials, in which the chemical composition depends upon the source, and their components are operationally classified in terms of solubility through the saturates, aromatics, resins, and asphaltenes (SARA) separation method.1 The petroleum stability in only one fluid phase is governed by a delicate combination of molecular interactions between their polar and nonpolar components, leading to a dynamic physical chemistry equilibrium.2 In general, saturate (S) and aromatic (A) components are nonpolar compounds, while the polarity in petroleum is introduced by the presence of low- and highpolar compounds, such as resins (R) and asphaltenes (A), respectively. Because of the great variety of forms, sizes, and chemical complexities of petroleum components, a detailed description of their structure is a very difficult experimental and theoretical challenge. Asphaltenes are the heaviest and most complex components of petroleum, which can be separated by using paraffinic solvents of low molecular weight, such as n-pentane and n-heptane compounds, which are often referred as A(n-C5) and A(n-C7) asphaltenes, respectively. The structure of these compounds, soluble in toluene and other organic solvents, is characterized by the presence of aromatic fused rings with terminal aliphatic chains of different lengths and the presence of heteroatoms (N, S, and O) in different coordination modes.3 These heteroatoms can be chemically coordinated into either the aromatic rings or the alkyl groups. With regard to the structural shape and molecular weight (MW) of asphaltenes, there has been a large controversy in the literature. Recently, © 2012 American Chemical Society

the employment of high-definition experimental techniques, such as diffusion and mass spectroscopy, has allowed for the determination that asphaltene structures are characterized by island-shape models with molecular weights of about 750 Da,4 which are independent of the petroleum origin source. Maltenes, referred to as the fraction of oil that remains in solution after the asphaltene separation, contain the A, S and R (insoluble in propane) components of oil. In particular, the shape and chemical composition of resins resemble those of asphaltenes but with smaller molecular structures and relatively longer aliphatic side chains, where these groups are responsible for their higher solubility in aliphatic solvents.5 It has recently been shown that different source resins can be chemically analyzed using electrospray ionization−mass spectrometry (ESI−MS), where the corresponding MWs are in the range of 367−423 Da.6 Resins are sticky, dark brown materials, where their properties lie between those of asphaltenes and the rest of the oil components. Structurally, the H/C resin ratio is in the 1.4−1.5 range, suggesting less aromatic structures and more alkyl and cycloalkyl groups than asphaltenes. Polarity is a chemical property that characterizes resin and asphaltene compounds, where resin polarity is intermediate Special Issue: 12th International Conference on Petroleum Phase Behavior and Fouling Received: September 26, 2011 Revised: January 30, 2012 Published: March 2, 2012 2711

dx.doi.org/10.1021/ef2014452 | Energy Fuels 2012, 26, 2711−2720

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and RR model compounds using quantum mechanics methods of first principles is due to Alvarez-Ramirez et al.11 at a density functional theory (DFT),12 where the interaction potential curves of these systems were calculated. More recently, we have demonstrated using DFT techniques that asphaltenes and resins are stabilized through noncovalent molecular interactions, which are dominated by London dispersion forces with the subsequent formation of stable van der Waals (vdW) complexes.13a Currently, it is well-recognized that stability calculations of noncovalent interactions significantly depend upon the medium- and long-range electronic correlations, identified with the dispersion interactions that occur in vdW complexes.14 In this context, DFT methods are able to treat structures and properties of large isolated molecular systems. However, in their standard implementation, they are not capable of taking into account dispersion effects in weakly bound complexes. To correct these deficiencies, DFT−vdW- and DFT−dispersioncorrected approaches have appeared recently in the literature.15 It is worth noting that standard DFT families, such as BLYP16a and B3LYP,16b are only able to give repulsive dimeric interactions, whereas PW9116c pure functional is able to give a correct description of the intermolecular potential for a great variety of molecular dimers.11,13a In this context, we have recently demonstrated that this particular functional gives a good description for σ−π and π−π interactions in a series of heteroaromatic monocyclic molecular complexes of benzene, pyridine, and thiophene molecules, where the binding energies are compared to results obtained with more sophisticated methods, such as MP4 and CCSD. These results were extended to real asphaltene−resin interactions of crude oil.13a In the present work, a study based on computational chemistry methods about the molecular interaction properties of five vacuum residue resins from Orinoco belt crude oils, named Carabobo (C), Hamaca (H), Merey (M), Merey-Mesa (MM), and Zuata (Z), and the corresponding implications in the crude stability is performed. The molecular structures of Orinoco resins employed in this research were proposed from experimental characterization of corresponding crude oils through elemental analysis, 13C nuclear magnetic resonance (NMR), and size-exclusion chromatography (SEC), and MW determination.17 The main goal of this work is to investigate the molecular interaction properties of characterized resin structures using molecular mechanics (MM),18 semi-empirical parametrization (PM6),19 and DFT. At this point, because of the consistency of the binding energy results about A−R interaction properties, the stability of R−R molecular complexes was investigated in this work using the PW91 functional. Furthermore, the performance of the HCTH20 functional for evaluation of resin properties is also investigated. For these resin complexes, the molecular structures, interaction energies (Eint), and electric properties, such as dipole moment (μ) and dipole polarizability (α) effects, were investigated in concordance to the findings about the A−R interaction.13a The results allowed us to obtain a structural, energetic, and electric description of the nature of the molecular interactions that occur in isolated resins and their implications in the stability of crude oil components.

between the nonpolar oil species, such as aromatic and saturates, and the highly polar asphaltene compounds. This resin property is of fundamental importance to understand the petroleum stability, where the self-aggregation tendency of asphaltenes is the driving force that is capable of breaking the equilibrium that regulates oil stability. In fact, resins have been associated with the asphaltene stabilization in crude oil through a peptization mechanism, which is able to explain how resins prevent agglomeration and stabilize the asphaltene particles through steric dispersing effects.7a More recently, it has also been proposed that oil stability can be rationalized in terms of the molecular interactions that occur between these species and asphaltene nanoaggregate particles.4 On the other hand, the development of advanced techniques for petroleum characterization, such as centrifugation7b and ultrafiltration,7c,d has demonstrated that the effective role of resins in asphaltene self-aggregation is relatively limited. For instance, centrifugation studies on live crude oil indicate that only small fractions of the bulk resins are involved in asphaltene aggregate formation.7b In fact, on the basis of large concentration gradient differences between asphaltenes and resins, it was shown that resins are not able to behave as surfactant species to avoid the asphaltene nanoaggregation. The results also show that only the colored resins (4% of the bulk resins) interact directly with asphaltene.7b Additionally, ultrafiltration studies have shown only a little evidence for the asphaltene− resin interaction7c when asphaltenes are separated by n-pentane and slightly stronger evidence for the asphaltene−resin interaction when n-heptane is used, indicating that the extent of the resin−asphaltene interaction varies with conditions, being on the order of 15% by mass.7d Furthermore, microcalorimetry results indicate that the asphaltene−resin interaction exists, where the strength of these interactions are in range of 2−4 kJ/mol7f. A great scientific and technologic effort has been made during the last 10 years to describe accurately the structure and properties of oil components. In particular, asphaltene research has motivated a large number of publications based on experimental characterizations, where the petroleomics (referred to as petroleum science) give deep insights about this subject.4,8 Despite these attempts to understand these relationships between the structure and properties of asphaltenes, not too many reports have been published about the structure and behavior of resins.9 It is important to emphasize that experimental chemical characterization of asphaltene and resin structures can be satisfactorily supported by the complementary employment of theoretical and computational techniques, where the corresponding properties and molecular interactions between these compounds can also be determined.10,11 To understand the asphaltene behavior in oil and the corresponding role of resins, it is important to investigate the fundamental properties that at a molecular level govern the stability of both polar fractions, which can be physically represented in terms of the energetics of asphaltene−asphaltene (AA), asphaltene−resin (AR), and resin−resin (RR) molecular interactions. The investigation of these systems at a theoretical level, of either isolated molecules or interacting molecules, is limited because of the scarce knowledge of the corresponding structures and their sizes, preventing the employment of advanced techniques of molecular quantum mechanics calculations. Most of the published research in this context corresponds to classical molecular mechanics and molecular dynamics applications. The first representative paper about the computation of molecular interactions in AA, AR,

2. THEORY 2.1. Supermolecule Model. When two neutral molecules (A and B) approach each other, a physical interaction occurs and an adsorption process can be established with the 2712


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polarizability α of the A and B monomers, according to the following equation:

formation of a weak molecular complex (AB), known as the vdW complex. This complex is energetically described by the molecular interaction energy (Eint) (binding BE). For a molecular system A + B = AB, this property describes their stability and is defined as E int = EAB − (EA + EB)

Edisp = −C(αA αB)/R6

where C is the dispersion coefficient, R is the distance between the A and B molecules in the AB complex, and αA and αB are the individual average polarizabilities of isolated A and B monomers, respectively. 2.2. Electric Properties of Molecular Complexes. Relevant to this work is the investigation of the electric properties [dipole moments (μ) and polarizabilities (α)] of isolated resin molecules and those induced by molecular interactions for the formation of stable R−R vdW complexes within the supermolecule approach. The induced dipole moment (μint) and induced interaction polarizability (αint) are the induced electric properties that interest us. In the frame of an external perturbation, the presence of a electric field (ℱ) induces a perturbation in the electronic charge density of a molecular system, where the total energy (E) can be represented as a power series in terms of the components of the field.13b This series can be written as

(1)

where EAB is the molecular energy of the AB complex and EA and EB are the energies of the isolated A and B molecules, respectively. Different energetic contributions to Eint are responsible for the stability of the AB complex, such as electrostatic (Eelect), polarization (Epol), exchange (Eexch), and dispersion (Edisp), which depend upon the chemical nature of the isolated A and B components.13a In fact, Eint can be partitioned in the form E int = Eelect + E pol + Eexch + Edisp

(3)

(2)

Electrostatic interactions are due to the interactions between net charges, dipole moments, and multipole moments on monomers. Polarization forces are due to the induction of dipole moments, quadrupole moments, etc. on one monomer by permanent multipoles on the other. These induced moments interact with the permanent multipole moments of the other monomers, giving the Epol contribution. The third component of Eint, the exchange energy (Eexch), also referred to as Born repulsion, is a short-range term that decreases exponentially with an increasing intermolecular distance R and is originated from the electron tunneling from one monomer to the other, as represented by the antisymmetric quantum mechanics effect on the wave function. Dispersion (Edisp) or London−vdW forces have a quantum mechanical origin and are due to the correlation between the electron motions on different monomers. In the sense of charge penetration, dispersion is an interaction of short range. However, dispersion is similar to electrostatic and induction interactions that are also long-range forces in the sense that they decrease with the inverse power of the intermolecular distance R. The objective of this research is not taken into account for the individual contributions of these components to Eint for the studied molecules but interpret Eint in terms of molecular electric properties of the interacting neutral species. However, to the end of characterizing the energy partition in molecular interactions, a series of methodologies based on energy decomposition analysis (EDA) are being implemented in our laboratory to rationalize the electronic origin of these forces in polyaromatic interactions of oil components and other important fluids. On the other hand, molecules interact through both their permanent multipolar moments and the correlated instantaneous fluctuations of their created dipole dispersion interactions. In this context, the electrostatic, polarization, and dispersion energies may all be expressed in terms of electric properties of the free molecules. Hence, a detailed knowledge of molecular charge distributions and polarizabilities is essential to the understanding of the intermolecular forces at large separations of molecular components of petroleum. In particular, the strength of the dispersion interaction depends upon the polarizability of the interacting species and is the energetic parameter that dominates the stability of the studied molecules. This property is directly related to the dipole

x

E(-) = E(0) −

∑ μe, i-i − i=x

1 2

z

z

∑ ∑ αij-i- j + ... i=x j=x (4)

where E(ℱ) and E(0) are the energies in the presence of the field and absence of the field, respectively, μe,i is the jth Cartesian component of the dipole moment, and the six αij quantities define the dipole polarizability symmetric tensor, where the invariant average polarizability (α) is defined as α = (1/3)(αxx + α yy + αzz)

(5)

where αxx, αyy, αzz are the components of α in the principal axis system. In the supermolecule approach (eq 1), the interaction electric properties are defined as the interaction dipole moment (μint) and the interaction polarizability (αint) as follows: ⎛ ∂E ⎞ μ int = −⎜ int ⎟ = μAB − (μA + μB) ⎝ ∂Fi ⎠ F → 0

(6)

⎛ ∂ 2E ⎞ int ⎟ α int = −⎜⎜ = αAB − (αA + αB) F Fj ⎟⎠ ∂ ∂ i ⎝ F→0

(7)

These properties, defined similar to Eint of eq 1, can give valuable information about the strength of the interaction between the resin molecules during the complex formation and the nature of the induced dipoles by effects of molecular interactions. Also, the interaction dipole polarizability can be a measure of the dispersion mechanisms that operate for the stability of these complexes, related to the volume reduction and orbital penetration that occurs by the effect of the A and B molecular interaction.

3. METHODOLOGY 3.1. Construction of Resin Molecular Structures. The employed structures for the five Orinoco belt vacuum residues, named MM, M, H, Z, and C, were experimentally characterized by elemental analysis, 13C NMR spectra, and MW determination from SEC.17a The corresponding asphaltenes and resins were characterized using the same standard procedures, and the obtained SEC MWs were proportionally fitted to the most likely values reported in the literature 2713


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individual components (x, y, and z) of μint and αint calculations are as follows:

for these compounds.4,6 From this characterization, a series of molecular structures were proposed for these compounds.17b,c We have considered that selected resin structures for the present research that are shown in Figure 1 are representative for each kind of Orinoco

⎛ E (+ Fi) − E int(− Fi) ⎞ μ int(i) = − ⎜ int ⎟ + 0(Fi3) 2Fi ⎝ ⎠

(8)

⎛ E (+ F ) + E (− F ) − 2E (0) ⎞ i int i int ⎟⎟ + 0(Fi 4) α int(ii) = ⎜⎜ int Fi 2 ⎠ ⎝

(9)

In addition to these properties, the electrostatic potential distribution (EPD) has been calculated for the isolated and resin complexes, using the HCTH/DNP approach with HCTH/DNP optimized geometries.20−22

4. RESULTS The results are organized as follows: (a) section 4.1 deals with details about the construction of studied resins; (b) section 4.2 is dedicated to the results of structural and energetic parameters of resin complexes; and (c) section 4.3 presents molecular and induced electric properties of isolated resins and complexes of resins. 4.1. Isolated Resin Molecular Structures. Five resins from MM, Z, M, H, and C oil vacuum residues of Orinoco belt (VRO) were characterized from experimental data of elemental analysis, 13C NMR, and MW determinations. These resins were separated and characterized in the Petróleos de Venezuela (PDVSA)-Intevep report by Canelón et al.17a and modeled by Castellano and Gimon.17b,c The models of these structures17 fully optimized with the MM, PM6, PW91/DNP, and HCTH/ DNP methods are displayed in Figure 2 (HCTH/DNP). It is worth noting that only slight differences are found between compositional parameters of experimental data and those from structural modeling resin results, when the H/C ratio, MW, and aliphatic and aromatic carbons are compared. Table 1 displays these experimental parameter results and the corresponding theoretical models used for this research (Mn, molecular weight number; H/C, hydrogen/carbon ratio; Cal, number of aliphatic carbon atoms; CH, CH2, and CH3, number of CH, CH2, and CH3 groups, respectively; and Fa, aromaticity factor). 4.2. R−R Dimers. It is well-known that molecular resins are able to form stable vdW dimers11 in a way similar to the polyaromatic compounds,24 whose interactions are dominated by electrostatic, induction, exchange, and dispersion forces. In the present work, the proposed structures of Orinoco belt resins were put in contact and left to interact at different levels of theory: MM, PM6, PW91/DNP, and HCTH/DNP approaches. The performance of these methodologies for studying the molecular interactions between Orinoco resins is shown in Figure 3, where the potential energy surface (PES) (Eint versus R contact distance) for M resin dimer is displayed in the range of 3−9 Å. It can be observed that MM, PM6, PW91, and HCTH levels of theory are able to give a qualitative or semi-quantitative description for this interaction, where the region of attraction is observed. However, MM gives an optimized contact distance for the M dimer that is too short when compared to the rest of the approaches. Furthermore, MM also gives values of Eint for this dimer that are too high with regard to the PM6 and DFT results. In this sense, the equilibrium contact distance and the interaction energies are underestimated and overestimated by MM methods, respectively. Additionally, the PES calculated at the MM level, is a multimodal curve that shows unphysical oscillations for contact distances larger than 5 Å, where a global minimum (more stable conformation)

Figure 1. Molecular resin structures proposed from experimental characterization. residue. However, further experimental and theoretical research is being performed in our laboratories to obtain an appropriated description of the behavior of the complete series of asphaltene and resin structures determined from Orinoco crude oils. 3.2. Calculations. MM, PM6, and DFT levels were employed for calculations. MM and PM6 methods were performed using the Discover program with a COMPASS consistent force field18 and the MOPAC2009 software program,19 respectively. DFT calculations were carried out using the self-consistent generalized gradient approximation (GGA) with the PW9116c and Hambrecht−Cohen−Tozer− Handy (DFT GGA−HCTH)20 exchange-correlation functionals and the double numeric and polarized (DNP) basis set, as implemented in DMol3 4.0 software.21,22 The results reported in ref 13a with the PW91 functional encourage us to evaluate other functionals contained in the DMol3 module of Materials Studio software. It was found that the HCTH/DNP approach was able to give more representative interaction energy (Eint) values for polyaromatic and resin dimers than PW91/DNP, which is one of the main achievements of this paper. The isolated geometries of the proposed resins and the corresponding structures of molecular dimers were fully investigated at C1 symmetry, following the parallel conformations, as reported by Ortega-Rodriguez et al.10a and Alvarez-Ramirez et al.11 It is expected, that, despite the lack of dispersion correction in the PW91/DNP and HCTH/DNP DFT approaches, the energetic and electric properties calculated for the resin complexes be qualitatively good enough to interpret their behavior at the oil scale. The electric properties of isolated and dimer resins were calculated using PM6 and HCTH/DNP methods in the frame of finite field (FF) methodology,13b,23 with perturbations of single electric field intensities of ±0.005 atomic unit (au) in the Cartesian axis, with MOPAC2009 and DMol3 (version 4.0), respectively. The FF expressions for the 2714


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To solve the problems associated with the multimodal behavior of PES at the MM level, a simulated annealing22c was performed for the dimer of M resin, using the COMPASS force field. In fact, a series of 14 different configurations were found for this dimer, where the most stable was located when the aliphatic group of each molecule is oriented 180° in a opposite way. However, the stability differences of this dimer with the rest of the configurations are almost 1 kcal/mol. In the set of MM configurations from the simulated annealing, a conformational dimer structure was found that is similar to the most stable structure found at the DFT level for M resin, as shown in Figure 2. The difference between both conformations lies in the fact that the aliphatic fractions are more separated in the new MM conformation than in the DFT conformation, preventing important steric interactions. The PES curve for this new conformation shows a behavior that is similar to PM6 and DFT approaches. Table 2 shows the calculated interaction energies (Eint) of all resin dimers in parallel conformation at different levels of theory: MM, PM6, and DFT. For C vacuum residue, two kinds of resins were studied, where the first structure includes the S atom as a thiol group, denoted by (RC) (Figure 1e), and the other structure presents a thiophene ring, referred to as RC(T) (Figure 1f). For the rest of the proposed resin structures, no heteroatoms were considered because of the low S and O atom content. However, it is important to mention that more structures for each type of resin were proposed, but in this work, only one representive for each kind of crude residue was selected. When results of Table 2 were analyzed, we found that the interaction energies (Eint) for resin dimers in parallel configurations at the MM level range from −12.51 kcal/mol for the RZ−RZ dimer to −18.00 kcal/mol for the RC−RC dimer, with equilibrium distances of 5.18 Å for the RZ−RZ dimer to 3.75 Å for the RC(T)−RC(T) dimer, respectively. PM6 values range from −2.66 kcal/mol (RMM−RMM) to −6.25 kcal/mol (RC(T)−RC(T)) at 5.25 and 4.34 Å, respectively. For DFT, Eint values range from −2.37 kcal/mol (RC(T)−RC(T)) to −7.09 kcal/mol (RM−RM) at 4.79 and 4.83 Å for the equilibrium distances, respectively, which are in concordance with the values reported in the literature (∼4.0−7.5 kcal/mol)11 at PW91 DFT. It can be seen that RC(T) dimers show the lower interaction energies because of the presence of the S atom in the thiophene form that leads to the higher repulsive interactions that give a particular chemical feature for these structures. However, when they interact in anti position, the repulsion decreases and Eint increases to −4.09 kcal/mol. A similar behavior is observed for the C resin (RC). Results of Table 2 indicate that MM methods considerably overestimate Eint when they are compared to PM6 and standard DFT, but the equilibrium distances are the shortest of the three methods employed here. Considering Eint DFT results, C resin dimers are more stable when the S atom is present as a thiol group (RC) (Eint = −4.85 kcal/mol) than the species with the thiophene moiety (RC(T)) (Eint = −2.37 kcal/mol). These results indicate that, for the RC(T) resin, the electrostatic repulsion because of the interaction between the thiophene groups is stronger than that in the RC resin, where the equilibrium distances are 4.86 and 4.79 Å, respectively. No significant diferences were found when the Eint of C vacuum residue resin dimers were compared to the rest of the studied dimers, where the calculated stabilities lie in the expected range of interaction energies for similar molecular complexes. Results obtained in the present work

Figure 2. Optimized geometries of isolated resins and dimers. The EPD of resin monomers and dimers are mapped on the optimized structures. Calculations at the HCTH/DNP DFT.

of −13.68 kcal/mol is found at 3.82 Å. These results are not in correspondence with those obtained at a higher level of theory, such as DFT methods. It is worth noting that the unphysical oscillations in the MM PES curve predict several local minima and saddle points that are known as multiple minima problems, which are mainly associated with the parametrization of the force field employed in the MM approaches.22b A similar behavior was found for the rest of the studied systems, where the corresponding interaction energies (Eint; kcal/mol) and optimized equilibrium contact distances (R; Å) are displayed in Table 2 for MM, M, Z, C, and H complex resins. 2715


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Table 1. Theoretical and Experimental Data of Resins from Different Orinoco Belt Vacuum Residues MM experiment

Z model

experiment

M model

H

experiment

model

experiment

C model

experiment

model C

a

a

C(T)b

property

C19H25

C21H26

C21H28

C23H30

C20H26

C24H32

C20H27

C24H32

C21H28S

C24H32S

C22H30S

Mn H/C Caro Cal CH CH2 CH3 Fa

274.7 1.30 8 11 2 7 2 0.40

278.4 1.24 10 11 2 7 2 0.48

299.3 1.34 8 13 3 6 3 0.39

306.5 1.30 10 13 3 7 3 0.43

284.7 1.33 7 13 3 7 3 0.35

320.5 1.33 10 14 3 8 3 0.42

292.0 1.33 8 13 2 8 3 0.38

320.5 1.33 10 14 3 8 3 0.42

304.7 1.33 7 14 2 9 3 0.35

352.6 1.33 10 14 3 8 3 0.42

326.5 1.36 8 14 3 8 3 0.36

C corresponds to the structure with S in the thiol form. bC(T) corresponds to the structure with S in the thiophene ring.

Figure 3. PES (Eint versus R contact distance) for M resin dimers using MM, PM6, and DFT (PW91 and HCTH) levels of theory.

Table 2. Interaction Energies (Eint; kcal/mol) and Equilibrium Distances (R; Å) of Resin Dimers Studied at Different Levels of Theory MM

a

PM6

PW91/DNP DFT

HCTH/DNP DFT

dimers

Eint (kcal/mol)

R (Å)

Eint (kcal/mol)

R (Å)

Eint (kcal/mol)

R (Å)

Einta (kcal/mol)

R (Å)

RMM RZ RM RH RC(T) RC RC anti RC(T) anti

−17.48 −12.51 −13.68 −13.65 −16.91 −18.00 −16.91 −14.11

4.18 5.18 3.82 4.18 3.93 3.75 3.74 4.14

−2.66 −4.96 −5.37 −3.29 −6.25 −5.10 −6.36 −4.22

5.25 4.54 5.28 5.09 4.34 4.34 5.55 7.62

−2.27 −2.27 −3.99 −2.78 −2.66 −2.36 −2.19 −1.77

5.05 4.94 5.41 5.07 4.88 5.08 5.23 5.78

−5.15 (−20.75) −4.98 (−20.16) −7.09 (−27.46) −6.92 (−26.87) −2.37 (−11.13) −4.85 (−19.71) −5.00 (−20.23) −4.09 (−17.08)

5.11 4.81 4.46 4.84 4.79 4.86 4.84 4.86

Values between parentheses correspond to the correlation of eq 10.

demonstrate that resins are able to form stable dimers that are structurally and energetically stabilized by molecular interactions. These results are in agreement with recent reports about the stability of these systems, which are charaterized by strong π−π electronic interactions.13a It is worth mentioning that these calculations were carried out in vacumm for the dimeric structures of resins, and at this point, it is interesting to evaluate the effect of solvents on the interaction energies of the studied complexes. In fact, taking the M compound as prototype of Orinoco oil resins, the solvent

effects were investigated using toluene, heptane, and pentane and the COSMO22d method with the HCTH/DNP DFT approach. The corresponding results are −5.41, −4.49, and −4.65 kcal/mol, respectively. It is clear from these results that the nature of the solvent systematically affects the resin association. However, these preliminary results are not enough to establish a definitive approach about the impact of solvents on the molecular interaction in resins. Further research is being performed in our laboratory to gain insights about this subject using resin clusters of larger dimensionality. 2716


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Table 3. PM6 and HCTH/DNP DFT Dipole Moments (Debyes) and Molecular Polarizabilities (α) for Resin Monomers (α(mono)) and Dimers (α(dimer)) in Atomic Units (au) PM6

HCTH/DNP DFT

resins

μ(mono)

μ(dimer)

α(mono)

α(dimer)

μ(mono)

μ(dimer)

α(mono)

α(dimer)

RMM RZ RM RH RC RC(T)

1.11 0.75 0.77 1.36 2.10 1.88

2.00 1.04 1.33 2.36 4.05 2.70

251.2 278.1 290.5 289.4 328.2 290.9

493.1 541.5 566.3 567.1 643.9 580.0

1.44 1.22 1.25 1.91 3.75 2.00

2.11 1.71 1.58 3.30 6.17 5.10

235.1 261.4 270.2 271.1 310.5 269.2

451.7 504.7 523.0 524.6 599.3 520.0

Despite HCTH Eint values being more representative than those of PW91, the validity of these DFT methods should be considered cautiously if they are compared to MM results. In fact, to validate the calculation methodology employed for the selected resin interactions, benzene, naphthalene, anthracene, dibenzanthracene, pyrene, and coronene dimers were studied in the face−face (parallel) conformation with the HCTH/DNP approach. The corresponding Eint values for these complexes correlate linearly with recent binding energies calculated using the general geometry (gg)-DFT-vdW-corrected functional, where the correlation coefficient is R2 = 0.9824 and the fitted linear correlation equation is as follows:

molecular complexes, where the negative (yellow) regions are susceptible to interact with molecular species containing positive (blue) EPD regions. When resin complexes are formed, it can be seen as the electrostatic potential that is redistributed within the interaction regions, where the positive regions dominate the external part of these complexes and the attraction region is characterized by the EPD that represents the aromatic part of resin monomer. This pattern is followed for all of the studied resin dimers. 4.4. Electric Properties: Dipole Moments and Molecular Polarizabilities Induced by Molecular Interactions. In Table 3, the monomer (μ(mono)) and dimer (μ(dimer)) dipole moments of studied resins are reported. The dipole moment values for resin monomers are in the range of 0.75−2.10 D and 1.22−3.75 D using PM6 and DFT/HCTH, respectively, where a linear relationship was found between μint at both levels of theory. These ranges are similar to those experimental values reported in the literature for a great variety of resins (2.0−4.0 D).5 In fact, these results show the lower polarity of resins with regard to asphaltenes whose dipole moments lie around 7.0 D.25 The effect of dimerization induces the significant changes in the electronic molecular distribution of resins that are reflected in their dipole moments, but no definite behavior can be withdraw by simple inspection. The molecular polarizabilities of monomers (α(mono)) and dimers (α(dimer)) are also shown in Table 3. Such polarizability values show the same tendency as dipole moments at both levels of studied (PM6 and HCTH) theories (R2 = 1.00). These results indicate that, from the studied resins, RC is the most polarizable, while RMM is the least polarizable. The higher polarizability of RC with respect to RC(T) is due to the presence of the S atom as sulfide in the former structure, because in the second S atom of thiophene is the least polarizable moiety (see Figure 2). According to the performance of these theoretical methods, electric properties can be theoretically determined reasonably at both PM6 and DFT (HCTH) approaches, for monomers and dimer complexes of petroleum resins of different sources. Similar to how it occurs for dipole moments, the dimer formation induces significant changes in the polarizability of complexes, where the total polarizability of these dimers is linearly correlated to the monomer polarizability, with a correlation coefficient of R2 = 1.00. Figure 4 shows the relationship between the total dimer polarizability (αtotal) in terms of the resin monomer polarizability (HCTH/DNP level), indicating that the driving force for the stability of resin dimers is originated from the monomer polarizability. Table 4 reports the results summary of Eint, μint, and αint calculated at both levels of theory, PM6 and HCTH/DNP DFT methods. The negative value of the dipole moment and polarizability of the interaction is a measure of the induced dipoles by the effect of the molecular interaction between resin species.

E int (gg DFT vdW) = −2.93 + 3.46E int (HCTH/DNP) (10)

This correlation allows us to propose more realistic values for Eint of Orinoco resin dimers that are as follows: MM, −20.75 kcal/ mol; Z, −20.16 kcal/mol; M, −27.46 kcal/mol; H, −26.87 kcal/ mol; C(T), −11.13 kcal/mol; C, −19.71 kcal/mol; Canti, −20.23 kcal/mol; and C(T)anti, −17.08 kcal/mol. These values are included in Table 2 between parentheses. The interaction energies obtained through the fitting relation eq 10 indicate that the dispersion is very important for these complexes, as recently reported by Mackie and DiLabio.15 These authors have demonstrated the paramount importance of the dispersion in molecular interactions of asphaltene dimers, a situation that is similar for the resins of Figure 2. In fact, the stability of these complexes results from a balance between the different noncovalent interactions involved in the molecular interactions described in the Theory, and the dispersion importance increases with the size of the molecular system. The interaction energies (Eint) reported by Mackie and DiLabio15 vary according to the MW of the dimers in the range of approximately 11−28 kcal/ mol. The scaled results for Eint of resin dimers of Figure 2 are in good agreement with the energies for similar MWs of structures and chemical features of studied compounds by DFT vdW methods.15 4.3. EPD of Isolated and Resin Complexes. Figure 2 displays the EPD of monomer and dimer resins, characterized by the solid color and dots that contain information on charge distributions and bonding preferences for the HCTH/DNP optimized structures. The positive and negative values of these EPDs are represented by blue and yellow regions, where the positive regions are associated with the H atoms and the corresponding aliphatic framework of resins that are characterized to interact preferentially through electrophilic interactions. The negative regions are associated with the electronic distribution of the aromatic part of these resins that is susceptible to interact preferentially with nuclephilic agents. The interaction between molecular monomers can be rationalized in terms of the association between these regions to give stable 2717


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Figure 4. Profile of resin dimer total polarizability as a function of the monomer resin polarizability values (au) in parallel conformation using HCTH/DNP DFT (R2 = 1.00).

Table 4. PM6 and HCTH/DNP DFT Interaction Energy (Eint; kcal/mol), Interaction Dipole Moment (μint; Debyes), and Interaction Polarizability (αint; au) of Resin Dimers PM6

polarizability (αint), in parallel conformation using DFT results (R2 = 0.90). This property can also be interpreted in the same context of orbital penetration when these resins are stabilized in the formation of the dimer structure. It is worth noting that a linear relationship was found when PM6 and DFT αint results are compared (R2 = 0.83). Despite this correlation not being so good, a multilinear correlation fitting was performed between Eint in terms of μint and αint, where a correlation coefficient of R2 = 0.95 was obtained for the following equation:

HCTH/DNP DFT

dimer

Eint

μint

αint

Eint

μint

αint

RMM RZ RM RH RC RC(T)

−2.66 −4.96 −5.37 −3.29 −6.25 −5.10

−0.22 −0.46 −0.22 −0.37 −0.15 −1.06

−9.31 −14.7 −14.7 −11.8 −12.5 −5.6

−5.15 −4.98 −7.09 −6.92 −4.85 −2.37

−0.67 −0.69 −0.92 −0.56 −1.34 −1.11

−18.9 −18.2 −17.2 −17.8 −18.5 −21.8

E int (HCTH/DNP)

In terms of orbital overlap, these induced dipoles are almost constant in the resin series, as inferred from the values of αint. Figure 5 displays the relationship between the interaction energy of resin dimers as a function of the interaction

= −23.60 − 0.863μ int (D) − 0.940α int (au)

(11)

This correlation indicates that the energetic stability of resins as polyaromatic hydrocarbons of low polarity can be quantitatively

Figure 5. Plot of the resin dimer interaction energy (Eint; kcal/mol) as a function of the interaction polarizability (αint; au) in parallel conformation at the HCTH/DNP DFT level of theory. 2718


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(6) Porter, D. J.; Mayer, P. M. Analysis of petroleum resins using electrospray ionization tandem mass spectrometry. Energy Fuels 2004, 18, 987−994. (7) (a) Nellensteyn, F. J. The colloidal structural of bitumen. The Science of Petroleum; Oxford University Press: London, U.K., 1938; Vol. 4, pp 2760−2763. (b) Indo, K.; Ratulowsli, J.; Dindoruk, B.; Gao, J.; Zuo, J.; Mullins, O. Asphaltene nanoaggregates measured in a live crude oil by centrifugation. Energy Fuels 2009, 23, 4460−4469. (c) Zhao, B.; Shaw, J. Composition and size distribution of coherent nanostructures in Athabasca bitumen and Maya crude oil. Energy Fuels 2007, 21, 2295−2804. (d) Zhao, B.; Becerra, M.; Shaw, J. On asphaltene and resin association in Athabasca bitumen and Maya crude oil. Energy Fuels 2009, 23, 4431−4437. (e) Merino-Garcia, D.; Andersen, S. Application of isothermal titration calorimetry in the investigation of asphaltene association. In Asphaltenes, Heavy Oil and Petroleomics; Mullins, O., Sheu, E., Hammami, A., Marshal, A., Eds.; Springer: New York, 2007; Chapter 13. (f) Merino-Garcia, D.; Andersen, S. Thermodynamic characterization of asphaltene−resin interaction by microcalorimetry. Langmuir 2004, 20, 4559−4565. (8) (a) Rogel, E.; Carbonagni, L. Density estimation of asphaltenes using molecular dynamics simulations. Energy Fuels 2003, 17, 378− 386. (b) Speight, J. The Chemistry and Technology of Petroleum; Marcel Dekker: New York, 1980; pp 401−471. (c) Murgich, J.; Abanero, J. A.; Strausz, O. P. Molecular recognition in aggregates formed by asphaltene and resin molecules from the Athabasca oil sand. Energy Fuels 1999, 13 (2), 278−286. (d) Acevedo, S.; Escobar, O.; Echevarria, L.; Gutierrez, L. B.; Mendez, B. Structural analysis of soluble and insoluble fractions of asphaltenes isolated using the PNP method. Relation between asphaltene structure and solubility. Energy Fuels 2004, 18, 305−311. (e) Groenzin, H. G.; Mullins, O. C. Molecular size and structure of asphaltenes from various sources. Energy Fuels 2000, 14 (3), 677−684. (f) Groenzin, H.; Mullins, O. C. Asphaltene molecular size and structure. J. Phys. Chem. A 1999, 103 (50), 11237− 11245. (9) (a) Daxi, W.; Yuqiu, P.; Hongye, Z. A quantum chemistry on structural properties of petroleum resin. Pet. Sci. 2007, 4 (4), 89−93. (b) Murgich, J.; Rodriguez, J.; Aray, Y. Molecular recognition and molecular mechanics of micelles of some model asphaltene and resins. Energy Fuels 1996, 10 (1), 736−742. (c) Rogel, E. Simulation of interactions in asphaltene aggregates. Energy Fuels 2000, 14 (3), 566− 574. (d) Spiecker, P. M.; Gawrys, K. L.; Trail, C. B.; Kilpatrick, P. K. Colloids Surf., A 2003, 220, 9−27. (10) (a) Ortega-Rodriguez, A.; Lira-Galeana, C.; Ruiz-Morales, Y.; Cruz, S. A. Interaction energy in Maya oil asphaltenes: A molecular mechanic study. Pet. Sci. Technol. 2001, 19, 245−256. (b) Merdrignac, I.; Espinat, D. Physicochemical characterization of petroleum fractions: The state of the art. Oil Gas Sci. Technol. 2007, 62 (1), 7−32. (11) Alvarez-Ramirez, F.; Ramirez-Jaramillo, E.; Ruiz-Morales, Y. Calculation of the interaction potential curve between asphaltene− asphaltene, asphaltene−resin, and resin−resin system using density functional theory. Energy Fuels 2006, 20, 195−204. (12) (a) Hohenberg, P.; Kohn, W. Inhomogeneous electron gas. Phys. Rev. [Sect.] B 1964, 136, 864−871. (b) Kohn, W.; Sham, L. Selfconsistent equations including exchange and orrelation effects. Phys. Rev. [Sect.] A 1965, 140, 1133. (c) Burke, K.; Werschinik, J.; Gross, E. K. U. Time-dependent density functional theory: Past, present and future. J. Chem. Phys. 2005, 123, 062206. (13) (a) Castellano, O.; Gimón, R.; Soscun, H. Theoretical study of the σ−π and π−π interactions in heteroaromatic monocyclic molecular complexes of benzene, pyridine, and thiophene dimers: Implications on the resin−asphaltene stability in crude oil. Energy Fuels 2011, 25 (6), 2526−2541. (b) Soscun, H. Ab initio and DFT study of the static dipole (hyper)polarizabilities of benzaldehyde and thio-benzaldehyde molecules in gas phase. J. Comput. Methods Sci. Eng. 2010, 10 (3−6), 587−597. (14) Grimme, S. Density functional theory with London dispersion corrections. Wiley Interdiscip. Rev.: Comput. Mol. Sci. 2011, 1, 211−228.

interpreted in terms of the induced electric properties (interaction dipole moment and interaction polarizability) by effects of molecular interactions.

5. CONCLUSION The molecular structures of resins proposed in this paper from five vacuum residues of the Orinoco belt are in agreement with the experimental data previously reported for resins of different sources. Results obtained in the present work demonstrate that the studied petroleum resins are able to form stable vdW complexes (R−R dimers) that are structurally and energetically stabilized by molecular interactions. The energetic stability of these complexes drastically depends upon the theoretical level employed for calculations. However, MM, PM6, and DFT (PW91 and HCTH) methodologies are able to describe qualitatively the dominant interactions that stabilize these complexes, despite higher correlation terms that represent dispersion not taking into account their formulations. Despite the fact that both PW91/DNP and HCTH/DNP DFT approaches include correlation energy, the former does not accurately describe the energetics of studied complexes. In fact, the HCTH functional gives interaction energies (Eint) at the equilibrium point whose values are in the range from −2.4 to −7.1 kcal/mol. These results are in agreement with recent reports about the stability of these systems, characterized by strong π−π electronic interactions.13a The scaling of these results in terms of vdW-corrected DFT approaches lead to realistic interaction energies for the Orinoco belt resin system, which ranged between −11.1 kcal/mol for C resin and −27.5 kcal/mol for M resin. These values are consistent with recent reports for asphaltene dimer interactions.15 In addition to the investigation of energetic and structural stability, we have demonstrated that the strength of oil resin systems can also be interpreted in terms of their interaction electric properties, dipole moment (μint) and polarizability (αint), that are induced by molecular interactions. These findings are being extended to real aspahltenes and A−R complex systems of Orinoco belt crude oils in gas and fluid phases.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] and/or [email protected] (H.S.); [email protected] (O.C.). Notes

The authors declare no competing financial interest.

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REFERENCES

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