Adsorption of Indole on Kaolinite in Nonaqueous ... - ACS Publications

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Adsorption of Indole on Kaolinite in Non-aqueous Media: Organoclay Preparation and Characterization, and 3DRISM-KH Molecular Theory of Solvation Investigation Jonathan Fafard, Olga Lyubimova, Stanislav R. Stoyanov, Gustave Kenne Dedzo, Sergey Gusarov, Andriy Kovalenko, and Christian Detellier J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp4064142 • Publication Date (Web): 09 Aug 2013 Downloaded from http://pubs.acs.org on August 13, 2013

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The Journal of Physical Chemistry

Adsorption of Indole on Kaolinite in Non-aqueous Media: Organoclay Preparation and Characterization, and 3D-RISM-KH Molecular Theory of Solvation Investigation Jonathan Fafard,a,b Olga Lyubimova,c,d Stanislav R. Stoyanov,c,e Gustave Kenne Dedzo,a,b Sergey Gusarov,c Andriy Kovalenko,c,d,* and Christian Detelliera,b,*

a

Department of Chemistry, University of Ottawa, 10 Marie Curie, Ottawa, Ontario, K1N 6N5,

Canada b

Centre for Catalysis Research and Innovation, University of Ottawa, Ottawa, Ontario, Canada

c

National Institute for Nanotechnology, National Research Council of Canada, 11421

Saskatchewan Drive, Edmonton, Alberta, T6G 2M9, Canada d

Department of Mechanical Engineering, University of Alberta, Edmonton, Alberta, Canada

e

Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta,

Canada

* Corresponding authors: E-mail: [email protected], tel: 1 613 562 5396 E-mail: [email protected], tel: 1 780 641 1716

ACS Paragon Plus Environment


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Abstract

Current oil sand mining operations in the Athabasca basin are predominantly

aqueous-based. Extracts containing large amounts of fines lead to the formation of stable organoclay suspensions in froths giving lower yields and greater tailing wastes, and making the development of more efficient extraction methods desirable from both economical and environmental perspectives. We examine an indole-kaolinite system as a model for these oil fines and their resistance to washing in non-aqueous solvents. The prepared organoclays show indole loading exclusively on the external surface of the clay. Micron-scaled vermicular structures, similar to natural kaolinite, are observed. Their formation is believed to be driven by strong adsorbate-adsorbate interactions. Indole is the primary adsorbate, as solvent adsorption is shown to be minimal based on both experimental and computational results. Isotherms are constructed and parameters calculated from linear regression analysis fitted to the BET equation. Monolayer quantities calculated match well to the theoretical amount calculated from surface areas measurements. Washing the organoclays with both toluene and isopropanol results in a 50% decrease of loaded organic material, leaving a monolayer equivalent of organic matter. The statistical-mechanical 3D-RISM-KH molecular theory of solvation is employed to perform full sampling of solvent orientations around a kaolinite platelet and gain insights into the preferred orientation and adsorption thermodynamics of indole on kaolinite in toluene and heptane solvents. In its preferred orientation, indole is hydrogen-bonded to one or two O atoms at the aluminum hydroxide surface of kaolinite. The calculated solvation free energy and potential of mean force for adsorption of indole and solvents on kaolinite in solution yield the increasing adsorption strength order of heptane < toluene < indole on the aluminum hydroxide surface. Multilayer adsorption profiles are predicted based on the integrated 3D distribution functions of indole, toluene and heptane.


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Keywords: clay minerals; non-aqueous extraction of bitumen; adsorption thermodynamics; multilayer adsorption; potential of mean force; organoclay



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Introduction The last three decades have brought a sharp increase in energy demand, mostly arising from the emerging economies of newly industrialized countries.1 The discovery of conventional sources is projected to decline2 and oil production is expected to shift towards non-conventional sources in the coming decades. Oil sands – a mixture of sand, clays, water and bitumen – are a very important unconventional source of oil. Canada’s oil sands hold the second largest hydrocarbon deposit after Saudi Arabia.3 Substantial research effort has been made to develop oil sands extraction methods based on aqueous extraction methods4 and centered mainly in the Athabasca Basin5,6 in Western Canada. Aqueous extraction requires approximately 12 barrels of water are required to produce 1 barrel of oil. Large amounts of water get incorporated into excessive, environmentally harmful tailing wastes.7 The ecological and land management consequences of these tailing wastes pose a great challenge for oil sands mining operations and require the development of alternative extraction and treatment methods. Fine organoclay particles are an important constituent of tailing wastes. The Athabasca Basin organoclays – predominantly kaolinite and illite8 – are hydrophobic in nature due to adsorbed organic matter,9,10 such as bitumen fixated during extraction, and form stable aqueous emulsions.11 Despite its inertness compared to smectitic clays, kaolinite features a modicum of reactivity due to the presence of the aluminum hydroxide layer. This property has been used extensively to prepare a variety of interesting materials through the intercalation12-14 and grafting of guest species.15,16 The formation of these organoclay fines is believed to be driven by strong dipolar and hydrogen bonding interactions between aluminum hydroxide layers and polar functional groups of adsorbed organics, especially asphaltenic materials. Due to the high proportion of heavy organics present in oil sands, asphaltenic materials are an important


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constituent of oil sand feedstocks. Models17 based on structural building blocks of asphaltenes that have been isolated and identified18,19 show an appreciable number of polar and hydrogen bonding functional groups, such as pyrroles, pyridines, thiophenes, and carboxylic acids present, making clay-asphaltene interactions very important. To date, two major theoretical approaches have been employed to study clay interactions with organic molecules, polymers and water: classical force field simulations and quantum chemistry methods. The 2006 review of Boulet et al. is focused on ab initio theoretical studies to predict hydroxyl group vibrational frequencies, hydration of clays, adsorption of molecules onto clays, catalytic activity and other properties.20 This review emphasizes the importance of bringing different levels of description and techniques to provide a more thorough view of materials structure and properties. A comparative quantum mechanics / molecular mechanics and density functional theory (DFT) with periodic boundary conditions (PBC) study of the adsorption of aromatic bitumen fragments on chabazite presented by us highlights the insights obtained using these surface modeling methods.21 Recently, Johnson et al. have reported the application of DFT with the exchange hole dipole moment dispersion model22 in periodic boundary conditions for exploration of the adsorption of benzene, n-hexane, pyridine, 2-propanol and water on kaolinite (001) surface.23 It has been found that adsorption on the aluminum hydroxide surface of kaolinite proceeds through OH-π interactions with benzene and CH-O and OH-C interactions with the aliphatic chain in n-hexane. Polar molecules, such as pyridine and isopropanol, tend to form strong hydrogen bonds with the alumina surface whereas the adsorption mode on the silicon oxide side is found to be similar to that of non-polar hydrocarbons. In all cases, adsorption on the aluminum hydroxide surface is calculated to be stronger than to the silicon oxide surface. 23 Molecular simulations of adsorption on kaolinite surfaces are limited to water adsorption24-28 and



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intercalation of small organic molecules, such as DMSO, formamide, methanol29 and urea.30 The results of molecular dynamics simulation of the adsorption of model asphaltenes and resins on kaolinite by Murgich et al. estimate the hydrogen bonding contribution to the interaction energy at 10% compared to 60-70% for van der Waals and 30-20% for Coulombic interactions.31 It should be noted that in this study the kaolinite layer has been kept entirely fixed, likely hindering the hydrogen bond formation between aluminum hydroxide groups and asphaltene heteroatoms. Molecular dynamics simulations to determine the partitioning behavior of organic compounds (benzo[a]- and benzo[c]carbazole) between water phases and inorganic surfaces of kaolinite and to

estimate

the

stability

of

four

different

configurations

of

the

three-phase

water/cyclohexane/kaolinite system are reported by van Duin and Larter.32 These authors show that a fully water-wet kaolinite is thermodynamically preferred over a fully cyclohexane-wet kaolinite and that the silicon oxide surface of kaolinite has a higher affinity for the water phase than the aluminum hydroxide surface. The last conclusion contradicts with previous waterkaolinite interaction studies in showing higher water affinity for the aluminum hydroxide surface. This contradiction can be attributed to the kaolinite structure atoms being fixed at crystallographic positions, keeping the hydroxyl groups vertical and pointed towards the absent silica surface of the next kaolinite layer. In the hypothetical isolated layer of kaolinite, the hydroxyl groups have both vertical and horizontal orientations.33,34 Recently, Ni and Choi have reported based on molecular dynamics simulations that multilayer thin films of water adsorb stronger than layers of n-heptane and pyridine,35 in agreement with microcalorimetry measurements.36 Modeling of solvation is crucial for understanding the adsorption of molecules on mineral surfaces and intermolecular interactions in the presence of solvents. A method successfully


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predicting solvation in complex molecular nanosystems based on the first principles of statistical mechanics is the so-called three-dimensional reference interaction site model theory with the closure approximation of Kovalenko and Hirata (3D-RISM-KH).37-42 The 3D-RISM-KH molecular theory of solvation properly accounts for hydrophobic and hydrogen bonding interactions, and reproduces structural and phase transitions in simple and complex liquids and solutions in a wide range of thermodynamic conditions43-45 and in nanoporous confinement. 44,46-48

The 3D-RISM-KH theory has been employed to study the adsorption of thiophenic

heterocycles on ion-exchanged zeolite surfaces in the presence of benzene solvent49 as well as for prediction of the effect of temperature on asphaltene aggregation in quinoline and 1methylnaphtahlene solvents.50 The effect of trace amounts of water in chloroform solvent on the free energy of aggregation of model asphaltene compounds containing polyaromatic hydrocarbon moieties tethered to pyridyl groups has been investigated using the 3D-RISM-KH theory and the contributions of hydrogen bonding and π-π stacking interactions to aggregation are evaluated based on electronic structure calculations.51,52 The 3D-RISM-KH theory has also been employed to predict the solvation structure and thermodynamics of electrolyte solutions 37 and ionic liquids,53 gelation activity,54-56 molecular boundary conditions of hydrodynamic flow,57 self-assembly and conformational transitions of synthetic organic supramolecular rosette nanotubular architectures,58-60 and molecular recognition in biomolecular nanostystems. 37,54,61-63 In this work, the interaction between the model asphaltene compound indole and the clay kaolinite is studied in organic solvents and the resultant organoclay materials are characterized using various physicochemical techniques to help further the development of non-aqueous solvent extraction methods as an alternative to water-based extraction methods in order to eliminate the need for aqueous tailing waste management. The solvents heptane and toluene are



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selected due to the high importance in crude oil refining, and their standard use in isolating and classifying asphaltenic materials. Moreover, the adsorption configurations and free energy of indole, toluene and heptane on kaolinite are investigated using the 3D-RISM-KH theory of solvation to help understand the mechanism and driving forces of multilayer adsorption of asphaltenic fragments on clays. Materials and methods Well-crystallized kaolinite (KGa-1b) was obtained from the Source Clay Repository of the Clay Minerals Society, Purdue University, West Lafayette, IN, USA. The < 2 µm fraction was isolated from the bulk material by sedimentation. Toluene and heptane (99%, HPLC grade) were obtained from Sigma Aldrich. Indole (99.9%) was obtained from Fischer Scientific. Preparation of organoclay aggregates. A known mass of indole was dissolved in 100 ml of solvent and the resulting solution was stirred magnetically until dissolution was complete. A 1g sample of kaolinite was added to the solution and the resulting mixture was dispersed by magnetic stirring for 48 hr at room temperature. The solid material was removed from solution by vacuum filtration and let dry under the vacuum for ½ hour before recovering. The organoclay was denoted KHI or KTI when heptane or toluene respectively was used as solvent. Organic content was quantified directly on the solids by elemental analysis (%CNS), using a VarioEL III. The organoclay used for desorption experiments (KHI) was prepared by suspending 1 g of kaolinite in 17.5 g L-1 indole in heptane. The mixture was then stirred for 48 hr and the solid recover by vacuum filtration and let dry under the vacuum for ½ hour. The organoclays used for characterization were prepared in toluene or heptane using similar procedure. Desorption of indole. A 100 mg sample of the organoclay (KHI) was dispersed in 100 ml of


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organic solvent for 48 hr. The resulting solution was vacuum filtered to remove the solid matter, dried under the vacuum for ½ hour and then the residual amount of organic matter determined by elemental analysis. Characterization. TGA curves were recorded using an SDT 2960 simultaneous DSC-TGA with a linear temperature ramp program (10°C min-1) under N2 (100ml min-1). X-ray diffraction patterns (XRD) of the organoclays were obtained using a Philips PW 3710 instrument equipped with Ni-filter and Cu-Kα radiation (λ = 0.15418 nm) operating at 45kV and 40mA. The 13C solid state nuclear magnetic resonance spectra (NMR) were obtained on a Bruker AVANCE 200 spectrometer using a 7 mm O.D. zirconia rotor spun at the magic angle spinning (MAS) of 4500Hz. A 1H → 13C cross polarization (CP) experiment was used to enhance the sensitivity of the 13C nucleus. Scanning electron microscope (SEM) images were taken on a JEOL JSM-7500F FESEM in low secondary electron imaging (LEI) mode with a 2kV acceleration voltage and a 10mm working distance. Computational Technique The 3D-RISM-KH molecular theory of solvation37-42 represents the solvation structure of a macromolecule or crystal layer in terms of 3D spatiial maps giving the probability density of finding site γ of solvent molecules at 3D space position r around the solute molecule, ργgγ(r), which is determined by the average number density ργ in the solution bulk times the 3D distribution function (normalized density distribution) gγ(r) of solvent site γ. The latter indicates site density enhancement when gγ(r)>1 or depletion when gγ(r)

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