Article pubs.acs.org/EF
Physical Properties of Liquid Crystals in Athabasca Bitumen Fractions S. Reza Bagheri,† Brady Masik,† P. Arboleda,‡ Q. Wen,‡ K. H. Michaelian,‡ and John M. Shaw*,† †
Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2V4, Canada CanmetENERGY, Natural Resources Canada, Devon, Alberta T9G 1A8, Canada
‡
ABSTRACT: Naturally occurring amphotropic liquid-crystals were recently identified in unreacted hydrocarbon resources and resource fractions from around the world, including Athabasca bitumen. Liquid crystal forming constituents are present in both asphaltene and maltene fractions and appear to be an important class of materials that is missed entirely during conventional hydrocarbon characterization, i.e.: SIMDIST or SARA analysis. In this contribution, some physical properties of liquid crystals in Athabasca asphaltenes and maltenes identified using experimental methods as diverse as polarized light microscopy, differential scanning calorimetry (DSC), and mid- and near-infrared photoacoustic spectroscopy with depth profiling are reported. Liquid crystals, comprising materials with an aromaticity between that of maltenes and asphaltenes, form irreversibly on the surface of both asphaltene particles and maltene drops on heating. At higher temperatures the liquid crystals become isotropic but remain on particle surfaces. Liquid crystals do not reappear on cooling or subsequent reheating unless the samples are frozen and crushed between heating cycles. The interdependence of these phase properties on sample thermal and mechanical history may help explain unexpected and frequently deleterious surface and interfacial phenomena arising during Athabasca bitumen production and processing.
■
INTRODUCTION Liquid-crystals were recently observed in unreacted petroleum fractions including Athabasca, Maya, and Cold Lake pentane asphaltenes, Safaniya heptane asphaltenes and a fraction of Athabasca bitumen were extracted with supercritical n-pentane (SFE6).1 The liquid crystals form between 338 and 341 K for the pentane asphaltenes, 370 K for Safaniya heptane asphaltenes, and 316 K for SFE6. The liquid crystals disappear between 423 and 435 K for the pentane asphaltenes, 433 K for C7 Safaniya asphaltenes, and 373 K for SFE6. These materials also exhibit liquid crystalline domains in the presence of toluene vapor at room temperature.1 Compounds that exhibit liquid crystalline properties in a defined temperature range between the melting point and the transition to the isotropic liquid state are called thermotropic. Compounds that show liquid crystalline properties by the addition of solvents are called lyotropic. For this type of liquid crystal, concentration constitutes an additional degree of freedom. Compounds that exhibit both types of behavior are termed amphotropic or amphitropic.2 The liquid crystals observed in unreacted petroleum fractions belong to this latter group. Following these initial observations, detailed studies related to the thermophysical properties, physical structure, and chemical composition of the liquid crystals were initiated. For example, liquid crystal rich material extracted from Athabasca pentane asphaltenes comprises more than 10,000 different constituents. Their mean molecular size is smaller than asphaltenes, and they are enriched in heteroatoms relative to asphaltenes. Detailed chemical composition data are reported separately.3 This material is distinct from carbonaceous mesophase,4−9 a nematic discotic (disk-like) liquid crystal phase formed during petroleum coking6 and liquid crystal rich material separated from isotropic pitch using supercritical toluene10 and pentane.11 The liquid crystal rich material investigated here appears to share some characteristics with © 2012 American Chemical Society
liquid crystals observed at bitumen-water interfaces during production,3,12−14 but this link requires further elaboration. Naphthenic acids and their salts also form liquid crystals at oil− water interfaces.15 However, though present, such constituents appear to comprise a minor fraction of the liquid crystal forming material. Possible attributions are also found in cognate literatures. For example, plastic crystals exhibit crystal-like positional order with local rotational disorder and mobility,16 and Funaki et al.17 suggested that orientation and stressinduced plasticization can produce domains in polarizing optical microscopy that resemble Maltese crosses. Basic physical structures for liquid crystals are well established. They include the following: columnar liquid crystals, that have long-range order in two dimensions, e.g.: array of liquid tubes; smectic liquid crystals, that have quasilong-range order in one dimension, e.g.: liquid layers stacked on one another; and nematic liquid crystals, that have positional short-range order and orientational long-range order.18 Many individual molecules and classes of molecules are known to form liquid crystals: small elongated organic molecules, discoid organic molecules, and long helical rod like molecules, which may include single or multiple oxygen, nitrogen, and sulfur substitutions.18−20 Binary mixtures also form discotic 21 and nematic22 liquid crystals through cooperative interaction even if the individual components do not do so on their own. These phenomena have been known for some time.23 Liquid crystals involving multiple components, as would appear to be the case for asphaltenes and heavy oil fractions, is a less well-developed subject. The transition from solid to liquid crystal states is also known to be complex as are the resulting physical states and their properties.24 Metastable Received: February 27, 2012 Revised: June 23, 2012 Published: June 25, 2012 4978
dx.doi.org/10.1021/ef300339v | Energy Fuels 2012, 26, 4978−4987
Energy & Fuels
Article
and unstable states are observed, depending on how the phase boundaries are approached, and enthalpies of mixing, often negative, can swamp small positive phase transition enthalpies in dilute mixtures, if both are present.25,26 In this work, the phase behavior of unreacted Athabasca bitumen fractions is explored using diverse techniques with the objective of identifying details of reversible and irreversible phase transitions linked to the formation of liquid crystals. The goal is to discover more about the physical properties and structure of liquid crystals in unreacted petroleum fractions and methods for their isolation. This work will serve as a stepping stone for those engaged in the development of models describing the behavior of liquid crystals in petroleum and in the development of novel products, additives, separation techniques, and chemical reaction schemes related to petroleum production and refining.
■
Table 1. Summary of Athabasca Asphaltene Samples Used in This Work thermal/mechanical treatment
sample name C5 C5 C5 C5 C5 C5 C5 C5 C7 C7 C7 C7 C7 C7 C7 C7
EXPERIMENTAL SECTION
Materials. Organic solvents: heptane (Assay 99.5%, Fisher Scientific), pentane (Assay 99.6%, Fisher Scientific), and toluene (Assay 99.9%, Fisher Scientific) were used without further purification. 4-Isothiocyanatophenyl 4-pentylbicyclo[2,2,2]octane-1-carboxylate (C21H27NO2S) with a purity of 99%, purchased from Sigma Aldrich, served as a control for pure liquid crystalline behavior. Athabasca bitumen from Alberta, Canada was obtained from Syncrude Canada Ltd. The sample was characterized as a coker feed. It was derived from mined bitumen subjected to warm water extraction, naphtha dilution, and naphtha recovery by distillation between (523 and 623) K. Some volatile components initially present in the as-mined bitumen were lost during processing. A sample of an Athabasca bitumen vacuum residue fraction extracted using supercritical pentane11 was also available for this study. The vacuum residue was fractionated under the following conditions: the extraction section was maintained at a temperature of 473 K; the fractionation section was maintained at a temperature of 483 K; the pressure of the supercritical fluid began at 3.5 MPa and was increased linearly to 12 MPa over 8 h. The procedure resulted in the production of 10 fractions. Fraction #6, referred to here as SFE6, was extracted at pressures between 8 and 9 MPa. The samples were stored in airtight containers in a refrigerator and crushed prior to use. Athabasca C5 and C7 asphaltenes were precipitated from Athabasca bitumen by the addition of 40 mL of n-pentane/n-heptane per gram of feedstock. The mixture was agitated overnight at 400 rpm at room temperature and atmospheric pressure. Then the mixture was filtered in two steps using vacuum filtration: first through a Fisher brand filter paper Q2, with a pore size between 1−5 mm and then the permeate was filtered again using a 0.22 μm Millipore mixed cellulose ether membrane. In order to eliminate residual oil, the filtration membranes and the flask were washed with small volumes of n-pentane/n-heptane until the filtrate was colorless. The membranes with the precipitated material were placed overnight in a vacuum oven at 9 kPa and 333 K. “Solids free” asphaltenes, asphaltenes with reduced clay, sand, and adsorbed hydrocarbon contents were obtained by mixing the dried C5 or C7 participates above with toluene at a concentration of 10 g/L and agitating at 400 rpm for 1 h followed by centrifugation at 3500 rpm for 5 min at 293 K. The mixtures were then decanted. Supernatants were retained, and most of the toluene was evaporated using a rotovap. The residues were transferred to a beaker and placed in the oven at 373 K until the balance of the toluene evaporated. Aliquots of the four aphaltenes were subjected to heat treatment, 30 min in a vacuum oven at 473 K, and crushing using a mortar and pestle. The complete list of asphaltene samples, their definitions and abbreviations, is shown in Table 1. Polarized Light Microscopy. Ordered materials are easy to identify using cross-polarized light microscopy. Samples are placed between two orthogonal polarizing filters. Light first passes through one polarizer, illuminates the sample, and then passes through a second polarizer oriented perpendicular to the first.27 If the sample is isotropic, the initial polarization direction is unchanged, and the
asphaltenes asphaltenes asphaltenes asphaltenes asphaltenes asphaltenes asphaltenes asphaltenes asphaltenes asphaltenes asphaltenes asphaltenes asphaltenes asphaltenes asphaltenes asphaltenes
(solids (solids (solids (solids
free) free) free) free)
(solids (solids (solids (solids
free) free) free) free)
no treatment crushed heat treated heat treated then no treatment crushed heat treated heat treated then no treatment crushed heat treated heat treated then no treatment crushed heat treated heat treated then
crushed
crushed
crushed
crushed
abbreviation C5 C5 c C5HT C5HT c C5NSF C5NSF c C5NSFHT C5NSFHT c C7 C7 c C7HT C7HT c C7NSF C7NSF c C7NSFHT C7NSFHT c
material appears dark to the observer. Alternatively if the sample is anisotropic, as are liquid crystals, the direction of polarization changes as the light passes through the sample, and the material appears bright. Textures corresponding to different types of liquid crystalline domains are readily observed. In this work a Zeiss Axio-Observer inverted reflective microscope equipped with a custom controlled atmosphere hot-stage was used, but equivalent upright microscope arrangements can also be employed. The apparatus and operating procedures are described in detail in a recent publication.28 The microscope geometry and substrate properties play key roles in the observations. Briefly, liquid crystals in petroleum are observed in reflection. If the slide is transparent, samples must be observed from the sample-slide interface as shown in Figure 1a. If the slide is opaque and reflective, the liquid crystals are observed from the gas-sample interface as shown in Figure 1b. Liquid crystals are not observed from the gas-sample interface if a clear slide is used, even if they are present, Figure 1b. Differential Scanning Calorimetry (DSC). With this technique, heat flows to a sample are measured as a function of temperature. Phase transition enthalpies as well as heat capacities can be determined.29 In this work, enthalpies were measured using a TGDSC 111 (Setaram, France) differential scanning calorimeter. As the equipment has been used in numerous studies, calibration, operation, and procedure details, including ones related to asphaltenes and heavy oils, are available elsewhere.30,31 The uncertainty of the heat capacity measurements is less than 2% from 290 to 570 K. The combined use of polarized light microscopy and DSC measurement for liquid crystal identification was cross-validated using 4-isothiocyanatophenyl 4-pentylbicyclo[2,2,2]octane-1-carboxylate (C21H27NO2S). C21H27NO2S forms a nematic liquid crystal phase with reversible temperature bounds at 348 and 387 K.32 The temperature bounds were defined by deviations of heat flow measurements from a baseline. At higher temperatures C21H27NO2S is an isotropic liquid, and at lower temperatures it is a crystalline solid. The observed liquid crystal temperature bounds (Figure 2a) and texture (Figure 2b) both agree with the literature.32 There is a large endothermic peak centered at 350 K with an enthalpy of 55.4 J/g and a smaller endothermic peak centered at 388 K with an enthalpy of 2.7 J/g. The enthalpies for both transitions are above the minimum detectable value for the equipment, ∼0.1 J/g. The solid to liquid crystal enthalpy agrees with a prior report,32 and the enthalpy of the liquid crystal to isotropic liquid transition is similar to values reported for related liquid crystal forming compounds.33 The observed transition behaviors were reversible and repeatable as shown in Figure 2a where the apparent heat capacity curves for the same sample during successive heating cycles yield the same enthalpies of transition and 4979
dx.doi.org/10.1021/ef300339v | Energy Fuels 2012, 26, 4978−4987
Energy & Fuels
Article
Figure 1. Possible microscope arrangements for the observation of liquid crystals in heavy oil: (a) transparent slide, (b) opaque and reflective slide, and (c) transparent slide.
Figure 2. (a) Temperature dependence of the apparent specific heat capacity for C21H27NO2S during heating cycles 1 and 2, from room temperature to 473 K and (b) the nematic liquid crystal texture observed under cross polarized light at 360 K. comparable transition temperatures for the solid to liquid crystal and liquid crystal to liquid transitions. Photoacoustic Infrared Spectroscopy. Photoacoustic infrared (PA-IR) spectroscopy requires minimal sample preparation, is suitable for opaque materials, and is nondestructive. With this technique, modulated infrared radiation impinges on a sample; absorption gives rise to periodic heating, which causes a pressure wave in the carrier gas above the sample. This pressure (acoustic) wave is detected by a microphone, the signal then being Fourier-transformed to obtain an infrared spectrum. By varying the modulation frequency (spectrometer
mirror velocity) of the incident radiation, infrared spectra of different sample depths (thermal diffusion lengths) can be acquired. The spectra can be analyzed in a variety of ways. For example, by integrating regions of the spectra where specific bond types absorb, the ratio of clay (active from 852 to 1300 cm−1) to aliphatic hydrocarbon (2800−3000 cm−1) and the variation of this ratio with depth within a sample were obtained for an oil sand residue.34 General aspects of the technique and diverse applications are described in detail in a recently published handbook.35 4980
dx.doi.org/10.1021/ef300339v | Energy Fuels 2012, 26, 4978−4987
Energy & Fuels
Article
Standard (nondepth profiling) measurements were performed for the 16 samples listed in Table 1. A Bruker IFS 88 FT-IR spectrometer and an MTEC 200 PA cell were employed in this study, the scan velocity corresponding to a modulation frequency of 1.6 kHz (specified at the calibration laser wavelength). Dry nitrogen gas at ambient temperature and pressure was used to purge the spectrometer, thereby reducing interference caused by water vapor and carbon dioxide. Nitrogen also served as the carrier gas in the PA cell. Details related to the apparatus and operating procedures are available elsewhere.36 The aromatic C−H region of the spectrum at ∼3050 cm−1 was of particular interest. The ratio of the area of this region (3000−3100 cm−1) as a fraction of the total (aliphatic + aromatic) C− H stretching region from 2760 cm−1 to 3100 cm−1 was used to compare samples, as the structure of the molecules comprising the liquid crystals differs from the bulk asphaltenes and maltenes. For example, the mean H:C ratio of liquid crystal rich material isolated from Athabasca pentane asphaltenes (
