Article pubs.acs.org/JPCB
Adsorption of Prototypical Asphaltenes on Silica: First-Principles DFT Simulations Including Dispersion Corrections Arturo Torres, Javier Amaya Suárez, Elena R. Remesal, Antonio M. Márquez, and Javier Fernández Sanz* Department of Physical Chemistry, Universidad de Sevilla, 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 1 order of magnitude. All of 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 the 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.
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 saturated and aromatic hydrocarbons, and resin and asphaltene 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 nonvolatile 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 the oil industry, especially since lighter conventional crudes are becoming depleted and the vast © 2017 American Chemical Society
reserves of heavy, extra heavy, and other conventional crudes are becoming the 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 the whole oil production chain. Some studies have established the interfacial activity of asphaltenes with high energy surfaces such as water.7−9 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 makes it challenging to obtain a holistic understanding of their properties.4,5 Historically, there had been some debate as to whether asphaltenes comprised several aromatic sections linked together by alkyl groups, known as the “archipelago” model, or if they were more like an “island” composed of a polycyclic aromatic core with pendant aliphatic side chains and paraffinic rings. Regarding molecular weight, there is not a unique range distribution for asphaltenes because Special Issue: Miquel B. Salmeron Festschrift Received: May 29, 2017 Revised: July 27, 2017 Published: July 31, 2017 618
DOI: 10.1021/acs.jpcb.7b05188 J. Phys. Chem. B 2018, 122, 618−624
Article
The Journal of Physical Chemistry B
Figure 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.
of its polydispersity 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 evidence of the presence of metalloporphyrins was 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 a greater 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 that in other phases.17 All of 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 interfacial 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. 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 have been a lot of research works devoted to unraveling the nature and properties of the interactions between silica surfaces and adsorbates.20−22
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 experimental26,27 and theoretical28−30 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,31−33 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 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
