Size Exclusion Chromatography for the Unambiguous Detection of

Energy & Fuels 2006, 20, 1165-1174

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Size Exclusion Chromatography for the Unambiguous Detection of Aliphatics in Fractions from Petroleum Vacuum Residues, Coal Liquids, and Standard Materials, in the Presence of Aromatics Eiman M. Al-Muhareb,† Fatma Karaca,† Trevor J. Morgan,† Alan A. Herod,*,† Ian D. Bull,‡ and Rafael Kandiyoti† Department of Chemical Engineering, South Kensington Campus, Imperial College London, London SW7 2AZ, U.K., and School of Chemistry, UniVersity of Bristol, Cantock’s Close, Bristol U.K. ReceiVed January 7, 2006. ReVised Manuscript ReceiVed March 2, 2006

A method has been developed using size exclusion chromatography (SEC) in heptane eluent that can detect aliphatics unambiguously without fractionation to remove aromatics. Spherical molecules such as colloidal silicas elute at the exclusion limit, while alkanes up to C50 elute through the porosity of the column. Detection of aliphatics was defined by use of an evaporative light scattering (ELS) detector with the simultaneous absence of UV absorbance at 300 nm. Alkanes smaller than C12 were not detected because the conditions of operation of the ELS caused their evaporation. All aromatics eluted after the permeation limit of about 25 min and were not detected until well after 45 min by their UV absorbance. The SEC method was applied to petroleum vacuum residues and coal liquids, and their fractions were soluble in pentane or heptane. High-temperature (HT) GC-MS confirmed the presence of alkanes in the pentane- and heptane-soluble fractions of petroleum vacuum residues, but did not elute any of the aromatics known to be present from SEC. Alkanes were examined in pentane-soluble fractions of a coal digest and a low-temperature coal tar; alkanes up to C40 were detected in the low-temperature tar and, although present in the digest, were masked by aromatics. No alkanes were detected by either SEC or HT GC-MS in fractions from a coal tar pitch. Aromatics in coal liquids and one petroleum residue were also examined by SEC using NMP as eluent and by UV fluorescence spectroscopy. The SEC method will find application to pentane- and heptane-soluble fractions of petroleum liquids and coal liquids where the alkanes are concentrated relative to the more abundant aromatics.

1. Introduction Our earlier investigation and application of size exclusion chromatography (SEC) involved the use of the solvent 1-methyl2-pyrrolidinone (NMP) as eluent. This enabled the examination of large-sized molecular material derived from coal liquids, petroleum residues, and humic acids.1-3 In that work, we examined material insoluble in pyridine that could not be examined by SEC using a solvent less powerful than NMP. However, NMP is a poor solvent for aliphatic materials, and the examination of petroleum vacuum residues using NMP only allowed the examination of some aromatic materials with aliphatic material remaining insoluble. This article describes a method for the examination of aliphatics. Initially, it was thought desirable to identify solvents that would allow the examination of samples containing both aliphatic (and alicyclic) and aromatic materials. Tetrahydrofuran (THF) can achieve the solution of both types of material. However, THF does not dissolve some of the larger molecular mass and/or more polar aromatic compounds. Furthermore, it * Corresponding author. E-mail: [email protected]. † Imperial College London. ‡ University of Bristol. § Present address: Marmara University, Engineering Faculty, Department of Chemical Engineering, Kadikoy-Istanbul, Turkey. (1) Herod, A. A.; Kandiyoti, R. J. Chromatogr., A 1995, 708, 143. (2) Karaca, F.; Islas, C. A.; Millan, M.; Behrouzi, M.; Morgan, T. J.; Herod, A. A.; Kandiyoti, R. Energy Fuels 2004, 18, 778. (3) Morgan, T. J.; Herod, A. A.; Brain, S. A.; Chambers, F. M.; Kandiyoti, R. J. Chromatogr., A 2005, 1095, 81.

has been shown that SEC in THF allows surface interactions, with elution of aromatics being delayed to times later than the permeation limit defined by polystyrene standards.1 In any case, SEC in THF elutes both types of materials, without providing a reliable method to observe aliphatic materials alone. Aromatics and related compounds show UV absorbance, while all molecular types would give a signal by ELS or refractive index detection. Carbognani4 used SEC on silica columns with toluene as eluent with operation at 45 °C and with injection valve and transfer lines at 60 °C. This system would probably elute aromatics, but this was avoided by the isolation of alkane concentrates. Large alkanes (>C30) are soluble in hot toluene but not in cold toluene, whereas they are soluble in cold heptane. Further work on the isolation of very large alkanes from crude oils5,6 indicated the presence of n-alkanes > C60. It was also reported to be likely that waxes were aromatics with long alkyl chains attached. Ku¨hn et al.7 used SEC in o-dichlorobenzene to examine technical waxes and found that the average molecular weights of several samples derived from SEC and MALDI-MS were in good agreement. Alkanes are well-known as components of crude petroleum and vacuum residues4 and may extend to C160. High molecular weight aliphatic hydrocarbons in crude oils from C40 to C120 (4) Carbognani, L. J. Chromatogr., A 1997, 788, 63. (5) Carbognani, L.; Orea, M. Pet. Sci. Technol. 1999, 17, 165. (6) Carbognani, L.; DeLima, L.; Orea, M.; Ehrmann, U. Pet. Sci. Technol. 2000, 18, 607. (7) Ku¨hn, G.; Weidner, St.; Just, U.; Hohner, G. J. Chromatogr., A 1996, 732, 111.

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1166 Energy & Fuels, Vol. 20, No. 3, 2006

have been detected by methods such as high-temperature (HT) GC-MS and field-desorption and field-ionization mass spectrometry.8 These aliphatics were not all n-alkanes9 but consisted of “paraffinic” waxes (mainly n-alkanes from 350 to 600 u) and “microcrystalline” waxes, which contained branched alkane and naphthenic ring structures with molecular weights ranging from 300 to 2500 u. These masses were determined by fielddesorption mass spectrometry. Aliphatic materials are known as components of coals, with methane (firedamp) as a volatile and dangerous hazard of coal mining.10 Higher alkanes are known in coals and can be evaporated at low temperatures from coals (and peat) when used as a gas chromatographic column packing.11,12 Aliphatic extracts of Chinese coals13 contained alkanes up to C33 with odd carbon number predominance, as well as tri- and tetracyclic diterpanes. High-temperature GC of waxes from New Zealand coals indicated alkanes greater than C40.14 When coal is pyrolyzed and the products examined by GC-MS, the major components detected are the n-alkane series and multicyclic terpane structures (see, for instance, articles in ref 15). Extracts from coals using super critical gas extraction and other methods16 gave alkanes up to C32 as determined by GC-MS in the extracted fractions. Hydropyrolysis and flash pyrolysis of low-rank coals17 gave alkanes from C13 to C33, but the presence of alkenes was considered to indicate that the aliphatics were generated by pyrolysis; the aliphatic signal observed by NMR methods was considered to reflect aliphatic groups attached to aromatic structures. The gas products from the high- and low-temperature coking of coals include methane and small alkanes,18 considered to result from the pyrolysis of alkyl groups attached to the coal structure. It is relevant then to consider where the free aliphatic materials known to exist in coals may appear in coal liquids produced by pyrolysis and liquefaction. Alkanes and cyclic aliphatics (mono-, di-, tri-, and tetracyclics) have been detected in a saturate fraction of a coal liquefaction recycle solvent19 by chemical ionization mass spectrometry. Series of alkanes have been detected through LC-MS work on hydropyrolysis tars20,21 showing alkanes up to C60 together with cyclic alkanes including pentacyclic triterpanes. NMR studies of coal tar pitch22,23 suggest (8) Huang, H.; Larter, S. R.; Love, G. D. Org. Geochem. 2003, 34, 1673. (9) Musser, B.; Kilpatrick, P. K. Energy Fuels 1998, 12, 715. (10) ICS Reference Library. Properties of Gases, Mine Gases, Mine Ventilation, Geology of Coal, Rock Drilling, ExplosiVes and Shot-Firing, Mine-Air Analysis, Geological Maps and Sections; International Correspondence Schools Ltd: London, 1920; Vol. 33A, section 31. (11) Herod, A. A.; Hodges, N. J.; Pritchard, E.; Smith, C. A. Fuel 1983, 62, 1331. (12) Herod, A. A.; Stokes, B. J.; Radeck, D. Fuel 1991, 70, 329. (13) Tuo, J.; Wang, X.; Chen, J.; Simoneit, B. R. T. Org. Geochem. 2003, 34, 1615. (14) Killops, S. D.; Carlson, R. M. K.; Peters, K. E. Org. Geochem. 2000, 31, 589. (15) Coal and Coal-Bearing Strata as Oil-Prone Source Rocks? Scott, A. C., Fleet, A. J., Eds.; Geological Society Special Publication No. 77; The Geological Society: London, 1994. (16) Bartle, K. D.; Jones, D. W.; Pakdel, H.; Snape, C. E.; Calimli, A.; Olcay, A.; Tugrul, T. Nature 1979, 277, 284. (17) Snape, C. E.; Ladner, W. R.; Bartle, K. D. Fuel 1985, 64, 1394. (18) Owen, J. The coal tar industry and new products from coal. In Coal and Modern Coal Processing: An Introduction; Pitt, G. J., Millward, G. R., Eds.; Academic Press: London, 1979; Chapter 9, pp 183-204. (19) Wilson, R.; Johnson, C. A. F.; Parker, J.; Herod, A. A. Org. Mass Spectrom. 1987, 22, 115. (20) Herod, A. A.; Ladner, W. R.; Stokes, B. J.; Berry, A. J.; Games, D. E.; Ho¨hn, M. Fuel 1987, 66, 935. (21) Herod, A. A.; Ladner, W. R.; Stokes, B. J.; Major, H. J.; Fairbrother, A. Analyst 1988, 113, 797. (22) Diaz, C.; Blanco, C. G. Energy Fuels 2003, 17, 907. (23) Millan, M.; Behrouzi, M.; Karaca, F.; Morgan, T. J.; Herod, A. A.; Kandiyoti, R. Imperial College London. Unpublished data, 2005.

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that the aromaticity is high, with only about 3% of aliphatic carbon, which is generally assumed to be mainly methyls attached to aromatic rings. Chinese coals extracted using CS2NMP mixed solvent and acetone at room temperature24 showed that aliphatics determined by GC-MS were in the acetone extract. Acetonitrile extracts of coals of different ranks25 were analyzed by GPC to determine molecular weight ranges and structures of aliphatics and indicated the range of mass to be from 110 to 2200 u, including aromatic molecules. Examination by GC-MS of organic matter from the Upper Silesian coal basin of Poland26 indicated aliphatics from C16 to C33 as well as terpene structures which originated from terrestrial plant matter. The aim of this work was to develop a method using standard materials and petroleum residues to allow the detection of aliphatics in coal liquids. Previous work has shown the presence of aliphatics in coal liquids,19-21 but rather than preparing aliphatic concentrates4 a method of detection without prior separation from the aromatics was thought desirable. We have already observed that the use of NMP as eluent in SEC tends to minimize surface interactions between aromatic species and the polymeric material (polystyrene/polydivinylbenzene) of the column packing. In contrast, the application of heptane as eluent tended to maximize the surface interaction between aromatics and the polymer packing. It was observed that the elution of aromatics was delayed until well after the permeation limit for aliphatics, allowing a complete separation of the types and an unambiguous identification of aliphatic material. The present article describes the experimental SEC setup and the calibration using alkane standards. Readily available aliphatic materials such as diesel fuel and candle wax as well as fractions of petroleum residues were examined. These were samples where aliphatics were concentrated but not separated from materials carrying aromatic chromophores. High-temperature GC-MS has been used to identify some of the aliphatic types, with pyrolysis GC-MS used to examine fractions soluble in pentane and toluene giving little or no signal in GC-MS. The methods have also been applied to three coal liquids, a coal tar pitch, a coal extract, and a low-temperature tar. Finally, fractions of the complex coal liquids and a petroleum residue have been examined by SEC using NMP as eluent and by UV fluorescence spectroscopy to attempt to gain structural information. 2. Experimental Section Samples. n-Alkanes with carbon numbers 13, 14, 16, 20, 22, 25, 30, 40, 44, 50, 60, and branched-C19 were obtained from Aldrich, UK, Polywax 500 was obtained from Greyhound Chemicals, UK, and Polywax 500, 655, and 1000 were obtained from Separation Systems (Gulf Breeze, FL). Diesel fuel and candle wax were from household suppliers. Petrox petroleum residue was from the Petrox refinery in Chile near Concepcion and has been described previously.27-30 Three vacuum bottom samples (labeled A, B, and (24) Wang, F.; Zhang, D.; Zhang, S.; Zao, Z. Chongqing Daxue Xuebao, Ziran Kexueban 2003, 26, 120, 129-132, CAN 140:359997. (25) Ye, C.; Feng, J.; Li, W.; Xie, K. Fenxi Huaxue 2004, 32, 622624. CAN 141:108597. (26) Fabianska, M. J.; Bzowska, G.; Matuszewska, A.; Racka, M.; Skret, U. Chem. Erde 2003, 63, 63. (27) Pindoria, R. V.; Megaritis, A.; Chatzakis, I. N.; Vasanthakumar, L. S.; Lazaro, M. J.; Herod, A. A.; Garcia, X. A.; Gordon A.; Kandiyoti, R. Fuel 1997, 76, 101. (28) Deelchand, J.-P.; Naqvi, Z.; Dubau, C.; Shearman, J.; Lazaro, M.J.; Herod, A. A.; Read, H.; Kandiyoti, R. J. Chromatogr., A 1999, 830, 397. (29) Suelves, I.; Islas, C. A.; Herod, A. A.; Kandiyoti, R. Energy Fuels 2001, 15, 429. (30) Suelves, I.; Islas, C. A.; Millan, M.; Galmes, C.; Carter, J. F.; Herod A. A.; Kandiyoti, R. Fuel 2003, 82, 1.

SEC for the Unambiguous Detection of Aliphatics C from Shell), a Forties vacuum residue, “Sample 2” residue28-30 as the heptane-soluble fractions, and a heptane asphaltene sample of Maya crude (a gift from Dr. Ancheyta, Mexican Institute of Petroleum) were also examined. Standard aromatic materials were examined: benzene, toluene, and fullerene (C60 and C72) from Aldrich Chemicals and polystyrenes from Polymer Labs (UK). Colloidal silicas of particle diameters 9, 12, and 22 nm, a gift from Nissan Chemical Industries Ltd, Houston Office, were also examined; they have been used previously with SEC calibration work for aromatic compounds.2 Coal liquids studied include: (a) Coal Tar Pitch. Tar from the high temperature coking of coal is distilled to leave pitch as residue. The present sample is a “soft” pitch, containing some light ends (from anthracene oil), such as phenanthrene. It has been used as our laboratory standard due to its homogeneity, chemical stability, and relative abundance. This sample has been investigated extensively.31-34 Its elemental composition was C 91.4%, H 4.1%, N 1.32%, S 0.76%, O by diff 2.4%. (b) Coal Liquefaction Extract or Liquefaction Extract. The coal liquefaction extract was from the former British Coal Point of Ayr Coal Liquefaction Pilot Plant. It corresponds to the extracted coal in recycle solvent stream, after filtration of undissolved solids and ash. This sample was of particular interest since it had suffered less thermal degradation than the coal tar pitch. The elemental composition of the sample was C 85.9%, H 6.77%, N 0.75%, O by diff 6.6. (c) Low-Temperature Tar. A low-temperature coal oil (LTT) from the Coalite process was produced by low-temperature distillation of coal to produce a smokeless solid fuel. The elemental composition of the sample was C 82.3%, H 7.83%, N 0.91%, O by diff 9.0%. This tar suffered the least thermal degradation of the three samples. Size Exclusion Chromatography. A Mixed-E column from Polymer Laboratories (Shropshire, UK) was used, heptane was used as the eluent, operation was at room temperature, and the flow rate was 0.5 mL min-1. The characteristics of this column type as listed by Polymer Labs are a particle size of 3 µm and a linear region relating elution volume or time and log10 molecular weight of polystyrene standards up to 30 000 u when using THF as eluent. The porosity range of the packing material, a copolymer of polystyrene/poly(divinylbenzene), is commercially confidential. Detection was by an evaporative light scattering (ELS 1000) detector (Polymer Laboratories) with a UV absorbance detector set at 300 nm. Operating conditions of the ELS were a sweep gas flow of nitrogen at 0.8 L min-1 at 4 bar pressure for heptane with a nebulizer temperature of 80 °C and an evaporation temperature of 105 °C. Because polystyrenes and other aromatic molecules did not elute from the Mixed-E column before the permeability limit, only n-alkanes were used to calibrate the mass range. SEC using NMP as eluent has been described in detail elsewhere.2 The ELS was operated at the flow rate as above, but with a nebulizer temperature of 150 °C and an evaporation temperature of 210 °C. The column chromatography fractions of Petrox residue and the coal liquids were examined using the Mixed-A column with NMP as eluent to examine aromatics in the different fractions. The polymer material of the Mixed-A column packing was the same as for the Mixed-E column, except that the particle size was larger, 20 µm, and the range of porosity allowed the linear range between elution time and log molecular mass to extend from 2000 to 40 million u in THF. The aromatic materials of heptane solubles and insolubles of vacuum residues were also examined using the Mixed-A column. Fractionation. Fractionation of the different samples was undertaken not to isolate pure aliphatic fractions but rather to simplify the aromatic contents and to investigate in which fractions (31) Domin, M.; Moreea, R.; Lazaro, M.-J.; Herod, A. A.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 1997, 11, 638. (32) Lazaro, M.-J.; Herod, A. A.; Kandiyoti, R. Fuel 1999, 78, 795. (33) Deelchand, J. P.; Naqvi, Z.; Dubau, C.; Shearman, J.; Lazaro, M.J.; Herod, A. A.; Read, H.; Kandiyoti, R. J. Chromatogr. 1999, 830, 397. (34) Herod, A. A.; Kandiyoti, R. J. Chromatogr. 1995, 708, 143.

Energy & Fuels, Vol. 20, No. 3, 2006 1167 the aliphatics were concentrated. Initial fractionations were achieved by planar chromatography28 on silica plates, using a solvent sequence pyridine, acetonitrile, toluene, and pentane. When aliphatics were detected in the pentane-eluted material of vacuum residues and coal liquids, the fractionation was changed to allow quantitative work. Quantitative separations were achieved using a column chromatography method on silica (Sigma silica gel, 1540-µm particle size and 60 Å average pore size), with elution by pentane (2 × 50 mL), toluene (100 mL), acetonitrile (100 mL), pyridine (100 mL), 1-methyl-2-pyrrolidinone (100 mL), and water (100 mL). This followed and extended the method used by Islas et al.35,36 to isolate the largest molecular fractions of coal liquids. In the present work, both pentane fractions and the toluene fraction of Petrox residue and coal liquids were examined using the heptane SEC column. All the fractions were examined using the Mixed-A column with NMP eluent to observe the aromatics and to compare whole samples with heptane insolubles. The petroleum vacuum residues were also extracted with heptane to prepare the heptaneinsoluble asphaltenes. The heptane-soluble extracts were examined using the heptane SEC column described in this article. The Maya (heptane-insoluble) asphaltene was extracted with heptane to produce heptane-insoluble asphaltenes by extraction of a small portion (1.3% of the asphaltene) of soluble material remaining from the original precipitation. The bulk of the vacuum residues were soluble in heptane, with asphaltenes representing only a few percent by weight at most. The Maya heptane asphaltene was 11.3% of the original crude;37 in this work, a heptane-soluble fraction of the asphaltene supplied was isolated. Others38 have extracted heptane solubles from nominally heptane-insoluble asphaltenes prepared by precipitation from toluene by excess heptane. Fractionation of Maya asphaltenes 39 using different proportions of heptane and toluene as mixtures has indicated that molecular weights of the fractions differed significantly but did not reveal the presence of any aliphatic components. Mass Spectrometry. The pentane and toluene fractions of Petrox residue, coal tar pitch, coal digest, and low-temperature tar were examined using a high-temperature GC column from SGE and supplied by Jones Chromatography, UK, a 25-m HT-5 column of diameter 0.32-mm id, film thickness of 0.1 µm, used with a Finnigan MAT TSQ700 mass spectrometer coupled via a heated transfer line to a Varian 3400 GC. The column temperature program was from 40° (held 4 min) to 350° at 10° min-1 (held 20 min), total time 53 min. Mass spectrometer settings were: scanning range m/z 50850, cycle time 1.5 s, electron energy 70 eV. Pyrolysis GC-MS was achieved using a CDS AS-2500 Pyrolysis Autosampler coupled to a Perkin-Elmer Turbomass Gold GC-MS. Pyrolysis details were: 610° C for 20 s. The column was a Chrompack CPSil-5CB 30 m × 0.32 mm × 3 µm film thickness. The temperature program was from 40 °C (held 4 min) to 320 °C at n min-1 (held 15 min), total time 75 min MS settings: scanning m/z 45-600, cycle time 0.48 s.

3. Results and Discussion Column Calibration. The elution times of n-alkanes are shown graphically in Figure 1 and listed in Table 1. The detection of the alkanes was by ELS since they have no UV absorbance and the absence of UV absorbance at 300 nm defines them as nonaromatic. Both the initial calibration data and more recent data are shown, with no significant differences. The separation of the alkanes up to C50 appears to follow a linear trend, but no signal was obtained for C60 alkane and it was (35) Islas, C. A. Ph.D. Thesis, University of London, 2001. (36) Islas, C. A.; Suelves, I.; Li, W.; Morgan, T. J.; Herod, A. A.; Kandiyoti, R. Fuel 2003, 82, 1813. (37) Ancheyta, J.; Centeno, G.; Trejo, F.; Marroquin, G.; Garcia, J. A.; Tenorio, E.; Torres, A. Energy Fuels 2002, 16, 1121. (38) Douda, J.; Llanos, Ma. E.; Alvarez, R.; Navarette Bolanos, J. Energy Fuels 2004, 18, 736. (39) Trejo, F.; Centeno, G.; Ancheyta, J. Fuel 2004, 83, 2169.


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Figure 1. Calibration of the Mixed-E column with alkanes. (2) C50; ([) and (9) represent calibrations at different times. Table 1. Elution Times of Alkane Standards from the Mixed-E Column in Heptane Eluent alkane

formula

n-pentacontane n-tetratetracontane n-tetracontane n-tricontane n-pentadocosane n-docosane n-eicosane branched C19 alkane n-hexadecane n-tetradecane n-tridecane

C50H102 C44H90 C40H82 C30H62 C25H52 C22H46 C20H42 C19H40 C16H34 C14H30 C13H28

elution time 1st data (min)

elution time 2nd data (min)

18.683 19.12-19.33 19.973 20.4 20.48-20.69 20.506 21.23-21.36 21.387 21.627

18.68 18.82-18.9 19.53-19.60 19.92 20.2-20.48 20.32

assumed the solubility was too low. The calibration of SEC columns with standard polymers using NMP as eluent showed the same linear trend of reducing elution time with increasing (log10) molecular mass up to the exclusion limit, when further increases of molecular mass produced no further reduction in elution time.2 The exclusion limit of the column was not established using alkanes because the higher alkanes were neither available nor soluble under the conditions used. The permeation limit was also not determined because the ELS detector evaporated alkanes smaller than C12. Initial results showed that an intense peak could be obtained in blank runs at about 11.8 min, probably corresponding to the exclusion limit of the column, but this came from the tip filters used to remove undissolved sample before injection; the material may be particulates from the plastic housing of the filters. The Polywax samples were dissolved in CS2 and diluted with heptane before examination, and their ELS chromatograms are shown in Figure 2a. The Polywax 1000 did not dissolve appreciably, and no chromatogram was obtained, while that for Polywax 650 was very weak. The range of alkanes in the Polywax samples given by the suppliers was: Polywax 500 C20-60, Polywax 655 C40-60, and Polywax 1000 C40-80. Mixtures of alkanes were examined to determine the separation capabilities of the system; Figure 2b shows a chromatogram of mixed C20 and C30 alkanes. There was a partial separation, but clearly the resolution was insufficient to resolve alkanes separated by one or two carbons. Other petroleum-derived and aliphatic materials were examined by this method. Figure 2c shows SEC chromatograms for diesel fuel and candle wax, and the elution times of the peaks corresponded to the expected alkane types for these products. No aliphatic materials eluted later than the small-molecule permeation limit, at about 24 min for this column. However, the small aromaticssbenzene and toluenesdid not elute before the permeation limit, and indeed,

Figure 2. Chromatograms on Mixed-E of (a) the Polywax samples, (b) alkane mixtures C20 and C30, and (c) candle wax and diesel.

they did not appear until about 45 min as a broad hump rather than a sharp peak. Similarly, polystyrene standards also eluted much later than the estimated permeation limit of the column. No alkenes were examined as standard materials, but the data (below) from high-temperature GC-MS indicated that the present samples did not appear to contain free alkenes; the characteristic alkene-alkane pairs of peaks in GC-MS chromatograms were only observed in results from pyrolysis GCMS. We have shown elsewhere40 that the terpenes and sesquiterpenes found in essential oils are soluble in NMP and could appear with the aromatic materials or in the aliphatic heptanesoluble fractions. Colloidal silicas of diameters 9, 12, and 22 nm all eluted at the exclusion limit of 11.95-12.0 min and were detected by the ELS; they were not added to the calibration graph since their masses (which are not known) are not relevant for the current purpose, which was to establish that known spherical particles would elute at the exclusion limit as found with Mixed-A and -D columns.2 Fullerene gave no peak before the permeation limit by ELS and no UV absorbance at 300 nm, showing that this molecule behaved as an aromatic molecule in this column in heptane solution. (40) Morgan, T. J.; Morden, W. E.; Al-Muhareb, E.; Herod, A. A.; Kandiyoti, R. Energy Fuels 2006, 20, 734-737.

SEC for the Unambiguous Detection of Aliphatics

Energy & Fuels, Vol. 20, No. 3, 2006 1169

Figure 3. Chromatograms on Mixed-E of aliphatics in heptane-soluble fractions of vacuum bottoms A, B, and C, Forties vac residue, Petrox residue, and Sample 2. Curves marked A, B, C, F, P, and 2, respectively.

Hence, these data suggest that the mechanism of size separation in the present column appears to apply only to aliphatic materials and not to aromatics. Although soluble in heptane, aromatics appear to elute very late, probably by a surface-adsorption mechanism. Heptane-soluble fractions and asphaltenes were prepared from several vacuum residues by extraction with heptane; a sample of heptane-insoluble asphaltenes from Maya crude was also extracted with heptane. Chromatograms of the extracts on the Mixed-E column are shown in Figure 3 and include vacuum bottoms A, B, C, Forties, Petrox, and sample 2 residues; differences in the shapes and ranges of the chromatograms are evident. The peaks for samples A and C at long times (22-25 min) and after the main peak are not n-alkanes since they would correspond to alkanes smaller than C12 that are lost with solvent in the ELS detector. They are likely to be multicyclic alkanes, but this has not been established. The heptane extracts contained aliphatics, while the heptane-insoluble fractions or asphaltenes contained no detectable aliphatics. Because the asphaltene precipitation method is incomplete when resulting from addition of excess heptane to a toluene solution of asphaltene, some material can be extracted into heptane from asphaltene samples. It appears, however, that the n-alkanes detectable using the Mixed-E column are all extracted into the heptane solution with no significant quantity remaining in the asphaltenes. Pentane and toluene fractions from the column chromatography fractionation of Petrox petroleum residue were examined using the Mixed-E column; their SEC chromatograms are shown in Figure 4a, where it is evident that both pentane extracts contained similar ranges of aliphatics as far as SEC is concerned. The range of Figure 4a compares with that of Petrox heptane solubles in Figure 3. The toluene fraction showed no detectable aliphatic signal. The reason for comparing the fractionation methods (heptane solubles/insolubles and column chromatography) using Petrox is that the heptane-soluble extraction is more generally used for petroleum work while the column chromatography method is more suitable for coal liquids. In vacuum residues, the proportion of toluene insolubles is normally insignificant, but in coal liquids, there are often significant proportions of toluene-insoluble materials. For comparison, Figure 4b shows the first pentane extract of the coal digest on the Mixed-E column and shows a smaller range of alkanes (of low intensity) than the Petrox fractions. The SEC chromatograms of the column chromatography fractions using the Mixed-A column and NMP as eluent are shown in Figure 5 for the Petrox sample (equivalent data for pitch, coal digest, and low-temperature tar can be found in the

Figure 4. SEC chromatograms of aliphatics in (a) Petrox two fractions soluble in pentane and (b) coal digest first pentane fraction (Mixed-E column).

Figure 5. SEC chromatograms on Mixed-A column of seven fractions from column chromatography of Petrox sample.

Supporting Information) and show a complex variety of aromatic molecules in each fraction with a shift to earlier elution as the solvent polarity increased; fraction weights recovered from the column chromatography are listed in Table 2. The vacuum residues and Maya asphaltene were also examined using the Mixed-A column to detect changes in the aromatic materials on fractionation. Figure 6 shows chromatograms for the whole vacuum bottoms A and the heptaneinsoluble fraction. Equivalent data for the Maya asphaltene and vacuum bottoms B and C can be found in the Supporting Information. In each case, the removal of heptane-soluble material shifted the retained peak to shorter elution times, indicating larger-sized molecules being isolated in the insoluble fraction, while the excluded peak also became more prominent. NMP does not completely dissolve the asphaltene samples, and


Al-Muhareb et al.

Table 2. Fraction Weights Recovered for the Petroleum Residue and the Three Coal Liquids fractionsa

petrox %

pitch %

PoA %

LTT %

pentane 1st pentane 2nd toluene acetonitrile pyridine NMP water SUM

50.7 18.0 5.7 1.7 2.5 6.2 11.3 97.0

3.8 17.4 26.5 5.2 15.3 15.1 2.8 86.1

12.4 29.6 10.1 3.1 17.5 8.5 5.1 86.4

10.5 25.7 19.6 13.7 4.0 4.3 6.6 84.4

a

Material soluble in the solvents shown by sequential elution.

Figure 6. SEC chromatograms on Mixed-A of heptane solubles and heptane insolubles of vacuum bottoms sample A.

the insoluble fraction, shown to have no fluorescence,41,42 may consist of large aromatics carrying large aliphatic groups.5,6 Molecular structures of asphaltene molecules showing little or no fluorescence may include oxygen, sulfur, and nitrogen atoms and allow the excitation energy from absorbance of a photon to transfer into vibrational energy rather than being emitted as a photon by fluorescent. The SEC method for aliphatics will find application to fractions soluble in pentane or heptane from petroleum liquids and coal-derived liquids, where the presence of aromatic molecules might obscure the determination of the aliphatics in other techniques. HT GC-MS and Pyrolysis GC-MS of Petrox Residue Fractions and Coal Liquids. The pentane- and toluene-soluble samples from column chromatography were examined, and chromatograms are shown in Figure 7a,b for the pentane fractions of Petrox. The first pentane fraction gave a series of alkanes from m/z 226 C16 to 408, C29 with others of higher mass to about C42, but with no molecular ions detected, superimposed on a broad hump of aliphatic material. There was no light material in the sample as would be expected from a residue from steam distillation. The second pentane fraction shows very few n-alkanes, but the unresolved hump of aliphatic material is shifted to later scans (higher mass) than that observed in the first fraction. The toluene fraction showed very little signal, indicating that the fraction contained very little material able to pass through the high-temperature column. It seems probable that these aliphatics in the second pentane extract might correspond to the microcrystalline waxes detected9 by fieldionization and field-desorption mass spectrometry. The aromatic components of these Petrox fractions were detected by SEC (41) Ascanius, B. E.; Merino-Garcia, D.; Andersen, S. I. Energy Fuels 2004, 18, 1827. (42) Morgan, T. J.; Millan, M.; Behrouzi, M.; Herod, A. A.; Kandiyoti, R. Energy Fuels 2005, 19, 164.

using the Mixed-A column in solution in NMP (Figure 5) but were not detected in the GC-MS as prominent components. Figure 7c shows the chromatogram of Polywax 500 under the same conditions. The pentane and toluene fractions of coal liquids were examined by HT GC-MS to determine the alkanes present. As with the Petrox residue, the toluene extracts of coal liquids contained no components that eluted through the GC column, and it is unlikely that the other fractions from column chromatography (acetonitrile, pyridine, and NMP solubles) would contain any small molecules able to pass through the column. No alkanes were detected in the coal tar pitch sample pentanesoluble fractions. The coal digest and the low-temperature tar had shown the presence of alkanes by normal-temperature GCMS up to C26 and up to C32, respectively;35 the alkane loading of the coal digest in the Pilot Plant43 could rise to 20% or so but does not appear to be so large in the present sample. Figure 8a,b shows the HT GC-MS chromatograms of the low-temperature tar first and second pentane-soluble fractions. In Figure 8a, the major peaks are the n-alkanes from C16 to about C42 with no significant aromatics, but in Figure 8b, the reverse is true and no significant aliphatics were detected. The chromatograms of the pitch and coal digest fractions are not shown; they contained aromatic and alkylated aromatics as expected with low-intensity alkanes in the coal digest first pentane fraction but none in the second pentane fraction or in the pitch fractions. The second pentane extract and the toluene extract samples were subjected to pyrolysis GC-MS. This was an attempt to examine the material that did not pass through the GC column initially. The chromatograms of the second pentane fractions are shown in Figure 9a-d, respectively, for Petrox, pitch, coal digest, and low-temperature tar. Figure 9a shows a series of alkene-alkane pairs from C8 to at least C18 at short retention times followed by aromatics that are mainly alkyl naphthalenes, fluorenes, diphenyls, phenanthrenes, fluoranthenes/ pyrenes, chrysenes, and other polycyclic aromatics up to m/z 252 benzopyrene isomers. These components were not observed without pyrolysis and are assumed to form from the macromolecules unable to pass through the column. Figure 9b for pitch shows the major aromatic compounds, phenanthrene m/z 178, fluoranthene and pyrene m/z 202, and their methyl derivatives at m/z 192 and 216, and chrysene isomers m/z 228. Figure 9c for the coal digest shows some of the major aromatics as fluorene m/z 166, phenanthrene m/z 178, and pyrene m/z 202 with only a very small peak for fluoranthene, with extensive series of alkylated derivatives of these aromatics. Figure 9d for the low-temperature tar shows some of the major aromatic components as fluorene m/z 166, phenanthrene m/z 178, pyrene m/z 202, and benzofluoranthenes and pyrenes m/z 252, but the major components are alkylated derivatives of the aromatics. No evidence for aliphatic materials was evident in the pyrolysis products of the coal liquid fractions. The pyrolysis chromatograms of the toluene soluble fractions are not shown, but they were very much less intense than those of the second pentane fractions and had fewer peaks. These chromatograms indicate that the second pentane and toluene fractions of all four samples contained relatively large components that could not pass through the GC column initially but on pyrolysis formed some small aromatic systems and, in the case of Petrox, some alkane and alkene fragments. In neither fraction did the alkanes become of any intensity after pyrolysis, and it is assumed that those detected originated by dealkylation of large alkyl aromatic (43) Walton, S. T. Fuel 1993, 72, 687.



Figure 7. HT GC-MS total ion chromatograms of Petrox fractions from column chromatography (a) first pentane, where C30 alkane is the most intense peak, with C18 alkane at 15 min, (b) second pentane, where peaks between 15 and 20 min are phenanthrene, methyl-, dimethyl-, and trimethylphenanthrenes, and (c) Polywax 500, where peaks at 17, 22, 25.5, 32.5, 37.5, and 49 min are, respectively, C20, phthalate impurity, C30, C40, C50, and C56 alkanes. Intensity vs elution time (min).


Al-Muhareb et al.

Figure 8. HT GC-MS chromatograms of (a) first pentane extract, where peaks at 14, 17, 25, and 31 min are C17, C20, C30, and C40 alkanes, respectively, and (b) second pentane extract of low-temperature tar, where all the components were aromatic with no aliphatics detected. Intensity vs elution time (min).

components rather than being free alkanes. Certainly, there was no indication that n-alkanes larger than those found in the first pentane fraction, up to about C41, were present in the subsequent two fractions since the GC conditions were capable of eluting n-C56 from a polywax sample at about 48 min. Also, there was no unresolved aliphatic peak as in the GC of the second pentane fraction, which may be presumed to have formed light alkane gas on pyrolysis. The evidence from the gas chromatograms of increasing molecular size and complexity with increasing polarity of the eluting solvent is supported by the UV fluorescence spectra of the fractions. Figure 10 shows synchronous UV fluorescence spectra of the column chromatography fractions of the Petrox sample. Equivalent data for the pitch, coal digest, and lowtemperature tar are available as Supporting Information. In each case, the synchronous UV fluorescence spectra shift to longer wavelengths as polarity of eluting solvent increases. However, there were some exceptions. For Petrox, fraction 4 (acetonitrile) fluoresced at wavelengths shorter than those of fraction 3 (toluene), suggesting that fraction 4 contained smaller aromatic groups than fraction 3. Similarly, fractions 6 (NMP) and 7 (water) showed shorter wavelength fluorescence than fraction 5 (pyridine). For pitch, coal digest, and low-temperature tar, the only exception to systematic shifts to longer wavelengths is fraction 7 (water) in each case. The types of molecules eluted by water with some NMP are not known, but the data of Table 2 suggest a significant proportion of Petrox eluted, with increasing quantities from pitch to low-temperature tar.

4. Conclusions A method of using size exclusion chromatography to detect aliphatic molecules (in the presence of aromatics) has been developed and tested using petroleum-derived products. The method distinguishes between aliphatic and aromatic using an evaporative light scattering detector and a UV absorbance detector in series. Aliphatics have no UV absorbance, while aromatics elute by a surface interaction mechanism, later than the permeation limit of the SEC column. Application of the method is likely to be to pentane-soluble fractions of petroleumderived and coal-derived liquids where the presence of aromatics may prevent or obscure the determination of aliphatics by other techniques. Standard n-alkanes < C50 elute with a linear relation between log10 molecular mass and elution time. Spherical colloidal silica standards were excluded from the column porosity. Candle wax and diesel fuel elute as expected for the alkane carbon numbers typical of these products. Mixtures of alkanes elute as partially resolved peaks at best, and polywax samples elute as a broad distribution with no separation of the small alkanes, with an upper limit at about the C50 alkane. Fractions of heptane-soluble materials from crude petroleum vacuum residues and a Maya asphaltene prepared by either column chromatography using pentane and toluene or solubility in heptane indicated differences in SEC behavior. The first pentane-soluble fractions contained only alkanes < C56, while the second pentane-soluble fractions and toluene fractions containing the Petrox residue contained mainly branched aliphatics. Those of coal liquids contained no significant



Figure 9. Py GC-MS chromatograms of the second pentane soluble fractions of (a) Petrox, where peaks before 25 min are alkene/alkane pairs, (b) pitch, where peaks at 24.5 min are phenanthrene; at 30 and 30.5, fluoranthene and pyrene; at 32-33 min, benzofluorenes; and at 36 min, chrysene isomers, (c) coal digest, where the major peak is pyrene, and (d) low-temperature tar. Intensity vs elution time (min).


Figure 10. Synchronous UV fluorescence spectra of Petrox residue sample. Fractions are: 1, first pentane; 2, second pentane; 3, toluene; 4, acetonitrile; 5, pyridine; 6, NMP; and 7, water. Intensities height normalized.

aliphatic components. Heptane-soluble fractions of vacuum residues gave SEC peaks in the retained region of the Mixed-E column and indicated differences in the ranges of alkanes from different samples. All of the seven fractions from Petrox and the coal liquids and the heptane-soluble and -insoluble fractions of vacuum residues contained aromatic components as determined by SEC in NMP using the Mixed-A column. These aromatics became more complex and of larger molecular size with increasing polarity of the solvent used to isolate the fraction. High-temperature GC-MS of pentane and toluene fractions indicated that n-alkanes could be detected in the first pentane fraction only, with none detected in the second pentane or toluene fractions of coal liquids; branched or cyclic aliphatics were detected in the second pentane fraction of the Petrox residue. Pyrolysis GC-MS of the second pentane and toluene

Al-Muhareb et al.

fractions failed to reveal any more free alkanes but did produce alkene/alkane pairs from the Petrox residue. These aliphatic fragments are presumed to arise from the fragmentation of aliphatic groups with up to 18 carbon atoms, attached to the aromatics. Pyrolysis GC-MS of the second pentane and toluene fractions of the coal liquids produced no aliphatics, and only aromatics were observed, although the aromatic fragments from the low-temperature coal tar were more highly alkylated than those from pitch. Synchronous UV fluorescence showed that the aromatic systems of each sample tended to become more complex with a shift in fluorescence maximum to longer wavelengths in general. These data indicate that the elution of different sets of compounds from column chromatography using a range of solvents of increasing polarity gave fractions in which the aromatic systems increased in size with increasing solvent polarity. This was true for the petroleum residue and coal liquids. Acknowledgment. We thank BCURA/DTI for financial support under Contract B53. The mass spectra were obtained at Kings College London and at Bristol University, and we thank NERC for supporting the Mass Spectrometry Unit at Bristol. We also thank Mahtab Behrouzi for some experimental work. Supporting Information Available: Figures showing SEC chromatograms on Mixed-A column of seven fractions from column chromatography of low-temperature tar, point of Ayr coal digest, and coal tar pitch; SEC chromatograms on Mixed-A of heptane solubles and heptane insolubles of Maya asphaltenes and vacuum bottom samples B and C; and synchronous UV fluorescence spectra of coal tar pitch samples, point of Ayr coal digest and lowtemperature tar. This material is available free of charge via the Internet at http://pubs.acs.org. EF0600098

Size Exclusion Chromatography for the Unambiguous Detection of

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