Size Exclusion Chromatography for the Unambiguous Detection Of

Energy & Fuels 2006, 20, 1165-1174 1165

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 has been shown that SEC in THF allows surface interactions, with elution of aromatics being delayed to times later than the Our earlier investigation and application of size exclusion permeation limit defined by polystyrene standards.1 chromatography (SEC) involved the use of the solvent 1-methyl- In any case, SEC in THF elutes both types of materials, 2-pyrrolidinone (NMP) as eluent. This enabled the examination without providing a reliable method to observe aliphatic of large-sized molecular material derived from coal liquids, materials alone. Aromatics and related compounds show UV 1-3 petroleum residues, and humic acids. In that work, we absorbance, while all molecular types would give a signal by examined material insoluble in pyridine that could not be ELS or refractive index detection. Carbognani4 used SEC on examined by SEC using a solvent less powerful than NMP. silica columns with toluene as eluent with operation at 45 °C However, NMP is a poor solvent for aliphatic materials, and and with injection valve and transfer lines at 60 °C. This system the examination of petroleum vacuum residues using NMP only would probably elute aromatics, but this was avoided by the allowed the examination of some aromatic materials with isolation of alkane concentrates. Large alkanes (>C30) are aliphatic material remaining insoluble. This article describes a soluble in hot toluene but not in cold toluene, whereas they are method for the examination of aliphatics. soluble in cold heptane. Further work on the isolation of very Initially, it was thought desirable to identify solvents that large alkanes from crude oils5,6 indicated the presence of would allow the examination of samples containing both n-alkanes > C60. It was also reported to be likely that waxes aliphatic (and alicyclic) and aromatic materials. Tetrahydrofuran were aromatics with long alkyl chains attached. Ku¨hnetal.7 (THF) can achieve the solution of both types of material. used SEC in o-dichlorobenzene to examine technical waxes and However, THF does not dissolve some of the larger molecular found that the average molecular weights of several samples mass and/or more polar aromatic compounds. Furthermore, it derived from SEC and MALDI-MS were in good agreement. Alkanes are well-known as components of crude petroleum 4 * Corresponding author. E-mail: [email protected]. and vacuum residues and may extend to C160. High molecular † Imperial College London. weight aliphatic hydrocarbons in crude oils from C to C ‡ University of Bristol. 40 120 § Present address: Marmara University, Engineering Faculty, Department of Chemical Engineering, Kadikoy-Istanbul, Turkey. (4) Carbognani, L. J. Chromatogr., A 1997, 788, 63. (1) Herod, A. A.; Kandiyoti, R. J. Chromatogr., A 1995, 708, 143. (5) Carbognani, L.; Orea, M. Pet. Sci. Technol. 1999, 17, 165. (2) Karaca, F.; Islas, C. A.; Millan, M.; Behrouzi, M.; Morgan, T. J.; (6) Carbognani, L.; DeLima, L.; Orea, M.; Ehrmann, U. Pet. Sci. Technol. Herod, A. A.; Kandiyoti, R. Energy Fuels 2004, 18, 778. 2000, 18, 607. (3) Morgan, T. J.; Herod, A. A.; Brain, S. A.; Chambers, F. M.; (7) Ku¨hn, G.; Weidner, St.; Just, U.; Hohner, G. J. Chromatogr., A 1996, Kandiyoti, R. J. Chromatogr., A 2005, 1095, 81. 732, 111.

10.1021/ef0600098 CCC: $33.50 © 2006 American Chemical Society Published on Web 03/28/2006 1166 Energy & Fuels, Vol. 20, No. 3, 2006 Al-Muhareb et al. have been detected by methods such as high-temperature (HT) that the aromaticity is high, with only about 3% of aliphatic GC-MS and field-desorption and field-ionization mass spec- carbon, which is generally assumed to be mainly methyls 8 9 trometry. These aliphatics were not all n-alkanes but consisted attached to aromatic rings. Chinese coals extracted using CS2- of “paraffinic” waxes (mainly n-alkanes from 350 to 600 u) NMP mixed solvent and acetone at room temperature24 showed and “microcrystalline” waxes, which contained branched alkane that aliphatics determined by GC-MS were in the acetone and naphthenic ring structures with molecular weights ranging extract. Acetonitrile extracts of coals of different ranks25 were from 300 to 2500 u. These masses were determined by field- analyzed by GPC to determine molecular weight ranges and desorption mass spectrometry. structures of aliphatics and indicated the range of mass to be Aliphatic materials are known as components of coals, with from 110 to 2200 u, including aromatic molecules. Examination methane (firedamp) as a volatile and dangerous hazard of coal by GC-MS of organic matter from the Upper Silesian coal basin 10 26 mining. Higher alkanes are known in coals and can be of Poland indicated aliphatics from C16 to C33 as well as evaporated at low temperatures from coals (and peat) when used terpene structures which originated from terrestrial plant matter. as a gas chromatographic column packing.11,12 Aliphatic extracts The aim of this work was to develop a method using standard 13 of Chinese coals contained alkanes up to C33 with odd carbon materials and petroleum residues to allow the detection of number predominance, as well as tri- and tetracyclic diterpanes. aliphatics in coal liquids. Previous work has shown the presence High-temperature GC of waxes from New Zealand coals of aliphatics in coal liquids,19-21 but rather than preparing 14 4 indicated alkanes greater than C40. When coal is pyrolyzed aliphatic concentrates a method of detection without prior and the products examined by GC-MS, the major components separation from the aromatics was thought desirable. We have detected are the n-alkane series and multicyclic terpane struc- already observed that the use of NMP as eluent in SEC tends tures (see, for instance, articles in ref 15). Extracts from coals to minimize surface interactions between aromatic species and using super critical gas extraction and other methods16 gave the polymeric material (polystyrene/polydivinylbenzene) of the alkanes up to C32 as determined by GC-MS in the extracted column packing. In contrast, the application of heptane as eluent fractions. Hydropyrolysis and flash pyrolysis of low-rank coals17 tended to maximize the surface interaction between aromatics gave alkanes from C13 to C33, but the presence of alkenes was and the polymer packing. It was observed that the elution of considered to indicate that the aliphatics were generated by aromatics was delayed until well after the permeation limit for pyrolysis; the aliphatic signal observed by NMR methods was aliphatics, allowing a complete separation of the types and an considered to reflect aliphatic groups attached to aromatic unambiguous identification of aliphatic material. structures. The gas products from the high- and low-temperature The present article describes the experimental SEC setup and coking of coals include methane and small alkanes,18 considered the calibration using alkane standards. Readily available aliphatic to result from the pyrolysis of alkyl groups attached to the coal materials such as diesel fuel and candle wax as well as fractions structure. of petroleum residues were examined. These were samples It is relevant then to consider where the free aliphatic where aliphatics were concentrated but not separated from materials known to exist in coals may appear in coal liquids materials carrying aromatic chromophores. High-temperature produced by pyrolysis and liquefaction. Alkanes and cyclic GC-MS has been used to identify some of the aliphatic types, aliphatics (mono-, di-, tri-, and tetracyclics) have been detected with pyrolysis GC-MS used to examine fractions soluble in in a saturate fraction of a coal liquefaction recycle solvent19 by pentane and toluene giving little or no signal in GC-MS. The chemical ionization mass spectrometry. Series of alkanes have methods have also been applied to three coal liquids, a coal tar been detected through LC-MS work on hydropyrolysis tars20,21 pitch, a coal extract, and a low-temperature tar. Finally, fractions showing alkanes up to C60 together with cyclic alkanes including of the complex coal liquids and a petroleum residue have been pentacyclic triterpanes. NMR studies of coal tar pitch22,23 suggest examined by SEC using NMP as eluent and by UV fluorescence spectroscopy to attempt to gain structural information. (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 2. Experimental Section Ventilation, Geology of Coal, Rock Drilling, ExplosiVes and Shot-Firing, Mine-Air Analysis, Geological Maps and Sections; International Cor- Samples. n-Alkanes with carbon numbers 13, 14, 16, 20, 22, respondence Schools Ltd: London, 1920; Vol. 33A, section 31. 25, 30, 40, 44, 50, 60, and branched-C19 were obtained from (11) Herod, A. A.; Hodges, N. J.; Pritchard, E.; Smith, C. A. Fuel 1983, 62, 1331. Aldrich, UK, Polywax 500 was obtained from Greyhound Chemi- (12) Herod, A. A.; Stokes, B. J.; Radeck, D. Fuel 1991, 70, 329. cals, UK, and Polywax 500, 655, and 1000 were obtained from (13) Tuo, J.; Wang, X.; Chen, J.; Simoneit, B. R. T. Org. Geochem. Separation Systems (Gulf Breeze, FL). Diesel fuel and candle wax 2003, 34, 1615. were from household suppliers. Petrox petroleum residue was from (14) Killops, S. D.; Carlson, R. M. K.; Peters, K. E. Org. Geochem. the Petrox refinery in Chile near Concepcion and has been described 2000, 31, 589. previously.27-30 Three vacuum bottom samples (labeled A, B, and (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. (24) Wang, F.; Zhang, D.; Zhang, S.; Zao, Z. Chongqing Daxue Xuebao, (16) Bartle, K. D.; Jones, D. W.; Pakdel, H.; Snape, C. E.; Calimli, A.; Ziran Kexueban 2003, 26, 120, 129-132, CAN 140:359997. Olcay, A.; Tugrul, T. Nature 1979, 277, 284. (25) Ye, C.; Feng, J.; Li, W.; Xie, K. Fenxi Huaxue 2004, 32, 622- (17) Snape, C. E.; Ladner, W. R.; Bartle, K. D. Fuel 1985, 64, 1394. 624. CAN 141:108597. (18) Owen, J. The coal tar industry and new products from coal. In Coal (26) Fabianska, M. J.; Bzowska, G.; Matuszewska, A.; Racka, M.; Skret, and Modern Coal Processing: An Introduction; Pitt, G. J., Millward, G. U. Chem. Erde 2003, 63, 63. R., Eds.; Academic Press: London, 1979; Chapter 9, pp 183-204. (27) Pindoria, R. V.; Megaritis, A.; Chatzakis, I. N.; Vasanthakumar, L. (19) Wilson, R.; Johnson, C. A. F.; Parker, J.; Herod, A. A. Org. Mass S.; Lazaro, M. J.; Herod, A. A.; Garcia, X. A.; Gordon A.; Kandiyoti, R. Spectrom. 1987, 22, 115. Fuel 1997, 76, 101. (20) Herod, A. A.; Ladner, W. R.; Stokes, B. J.; Berry, A. J.; Games, D. (28) Deelchand, J.-P.; Naqvi, Z.; Dubau, C.; Shearman, J.; Lazaro, M.- E.; Ho¨hn, M. Fuel 1987, 66, 935. J.; Herod, A. A.; Read, H.; Kandiyoti, R. J. Chromatogr., A 1999, 830, (21) Herod, A. A.; Ladner, W. R.; Stokes, B. J.; Major, H. J.; Fairbrother, 397. A. Analyst 1988, 113, 797. (29) Suelves, I.; Islas, C. A.; Herod, A. A.; Kandiyoti, R. Energy Fuels (22) Diaz, C.; Blanco, C. G. Energy Fuels 2003, 17, 907. 2001, 15, 429. (23) Millan, M.; Behrouzi, M.; Karaca, F.; Morgan, T. J.; Herod, A. A.; (30) Suelves, I.; Islas, C. A.; Millan, M.; Galmes, C.; Carter, J. F.; Herod Kandiyoti, R. Imperial College London. Unpublished data, 2005. A. A.; Kandiyoti, R. Fuel 2003, 82,1. SEC for the Unambiguous Detection of Aliphatics Energy & Fuels, Vol. 20, No. 3, 2006 1167

C from Shell), a Forties vacuum residue, “Sample 2” residue28-30 the aliphatics were concentrated. Initial fractionations were achieved as the heptane-soluble fractions, and a heptane asphaltene sample by planar chromatography28 on silica plates, using a solvent of Maya crude (a gift from Dr. Ancheyta, Mexican Institute of sequence pyridine, acetonitrile, toluene, and pentane. When ali- Petroleum) were also examined. phatics were detected in the pentane-eluted material of vacuum Standard aromatic materials were examined: benzene, toluene, residues and coal liquids, the fractionation was changed to allow and fullerene (C60 and C72) from Aldrich Chemicals and polysty- quantitative work. Quantitative separations were achieved using a renes from Polymer Labs (UK). Colloidal silicas of particle column chromatography method on silica (Sigma silica gel, 15- diameters 9, 12, and 22 nm, a gift from Nissan Chemical Industries 40-µm particle size and 60 Å average pore size), with elution by Ltd, Houston Office, were also examined; they have been used pentane (2 × 50 mL), toluene (100 mL), acetonitrile (100 mL), previously with SEC calibration work for aromatic compounds.2 pyridine (100 mL), 1-methyl-2-pyrrolidinone (100 mL), and water Coal liquids studied include: (100 mL). This followed and extended the method used by Islas et (a) Coal Tar Pitch. Tar from the high temperature coking of coal al.35,36 to isolate the largest molecular fractions of coal liquids. In is distilled to leave pitch as residue. The present sample is a “soft” the present work, both pentane fractions and the toluene fraction pitch, containing some light ends (from anthracene oil), such as of Petrox residue and coal liquids were examined using the heptane phenanthrene. It has been used as our laboratory standard due to SEC column. All the fractions were examined using the Mixed-A its homogeneity, chemical stability, and relative abundance. This column with NMP eluent to observe the aromatics and to compare sample has been investigated extensively.31-34 Its elemental com- whole samples with heptane insolubles. The petroleum vacuum position was C 91.4%, H 4.1%, N 1.32%, S 0.76%, O by diff 2.4%. residues were also extracted with heptane to prepare the heptane- (b) Coal Liquefaction Extract or Liquefaction Extract. The coal insoluble asphaltenes. The heptane-soluble extracts were examined liquefaction extract was from the former British Coal Point of Ayr using the heptane SEC column described in this article. The Maya Coal Liquefaction Pilot Plant. It corresponds to the extracted coal (heptane-insoluble) asphaltene was extracted with heptane to in recycle solvent stream, after filtration of undissolved solids and produce heptane-insoluble asphaltenes by extraction of a small ash. This sample was of particular interest since it had suffered portion (1.3% of the asphaltene) of soluble material remaining from less thermal degradation than the coal tar pitch. The elemental the original precipitation. The bulk of the vacuum residues were composition of the sample was C 85.9%, H 6.77%, N 0.75%, O soluble in heptane, with asphaltenes representing only a few percent by diff 6.6. by weight at most. The Maya heptane asphaltene was 11.3% of (c) Low-Temperature Tar. A low-temperature coal oil (LTT) the original crude;37 in this work, a heptane-soluble fraction of the from the Coalite process was produced by low-temperature distil- asphaltene supplied was isolated. Others38 have extracted heptane lation of coal to produce a smokeless solid fuel. The elemental solubles from nominally heptane-insoluble asphaltenes prepared by composition of the sample was C 82.3%, H 7.83%, N 0.91%, O precipitation from toluene by excess heptane. Fractionation of Maya by diff 9.0%. This tar suffered the least thermal degradation of the asphaltenes 39 using different proportions of heptane and toluene three samples. as mixtures has indicated that molecular weights of the fractions Size Exclusion Chromatography. A Mixed-E column from differed significantly but did not reveal the presence of any aliphatic Polymer Laboratories (Shropshire, UK) was used, heptane was used components. as the eluent, operation was at room temperature, and the flow rate Mass Spectrometry. The pentane and toluene fractions of Petrox was 0.5 mL min-1. The characteristics of this column type as listed residue, coal tar pitch, coal digest, and low-temperature tar were by Polymer Labs are a particle size of 3 µm and a linear region examined using a high-temperature GC column from SGE and relating elution volume or time and log10 molecular weight of supplied by Jones Chromatography, UK, a 25-m HT-5 column of polystyrene standards up to 30 000 u when using THF as eluent. diameter 0.32-mm id, film thickness of 0.1 µm, used with a Finnigan The porosity range of the packing material, a copolymer of MAT TSQ700 mass spectrometer coupled via a heated transfer line polystyrene/poly(divinylbenzene), is commercially confidential. to a Varian 3400 GC. The column temperature program was from Detection was by an evaporative light scattering (ELS 1000) 40° (held 4 min) to 350° at 10° min-1 (held 20 min), total time 53 detector (Polymer Laboratories) with a UV absorbance detector set min. Mass spectrometer settings were: scanning range m/z 50- at 300 nm. Operating conditions of the ELS were a sweep gas flow 850, cycle time 1.5 s, electron energy 70 eV. - of nitrogen at 0.8 L min 1 at 4 bar pressure for heptane with a Pyrolysis GC-MS was achieved using a CDS AS-2500 Pyrolysis nebulizer temperature of 80 °C and an evaporation temperature of Autosampler coupled to a Perkin-Elmer Turbomass Gold GC-MS. 105 °C. Because polystyrenes and other aromatic molecules did Pyrolysis details were: 610° C for 20 s. The column was a not elute from the Mixed-E column before the permeability limit, Chrompack CPSil-5CB 30 m × 0.32 mm × 3 µm film thickness. only n-alkanes were used to calibrate the mass range. The temperature program was from 40 °C (held 4 min) to 320 °C SEC using NMP as eluent has been described in detail at n min-1 (held 15 min), total time 75 min MS settings: scanning elsewhere.2 The ELS was operated at the flow rate as above, but m/z 45-600, cycle time 0.48 s. 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 3. Results and Discussion Mixed-A column with NMP as eluent to examine aromatics in the Column Calibration. The elution times of n-alkanes are different fractions. The polymer material of the Mixed-A column shown graphically in Figure 1 and listed in Table 1. The 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 detection of the alkanes was by ELS since they have no UV the linear range between elution time and log molecular mass to absorbance and the absence of UV absorbance at 300 nm defines extend from 2000 to 40 million u in THF. The aromatic materials them as nonaromatic. Both the initial calibration data and more of heptane solubles and insolubles of vacuum residues were also recent data are shown, with no significant differences. The examined using the Mixed-A column. separation of the alkanes up to C50 appears to follow a linear Fractionation. Fractionation of the different samples was trend, but no signal was obtained for C60 alkane and it was undertaken not to isolate pure aliphatic fractions but rather to simplify the aromatic contents and to investigate in which fractions (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.; (31) Domin, M.; Moreea, R.; Lazaro, M.-J.; Herod, A. A.; Kandiyoti, Kandiyoti, R. Fuel 2003, 82, 1813. R. Rapid Commun. Mass Spectrom. 1997, 11, 638. (37) Ancheyta, J.; Centeno, G.; Trejo, F.; Marroquin, G.; Garcia, J. A.; (32) Lazaro, M.-J.; Herod, A. A.; Kandiyoti, R. Fuel 1999, 78, 795. Tenorio, E.; Torres, A. Energy Fuels 2002, 16, 1121. (33) Deelchand, J. P.; Naqvi, Z.; Dubau, C.; Shearman, J.; Lazaro, M.- (38) Douda, J.; Llanos, Ma. E.; Alvarez, R.; Navarette Bolanos, J. Energy J.; Herod, A. A.; Read, H.; Kandiyoti, R. J. Chromatogr. 1999, 830, 397. Fuels 2004, 18, 736. (34) Herod, A. A.; Kandiyoti, R. J. Chromatogr. 1995, 708, 143. (39) Trejo, F.; Centeno, G.; Ancheyta, J. Fuel 2004, 83, 2169. 1168 Energy & Fuels, Vol. 20, No. 3, 2006 Al-Muhareb et al.

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 elution time elution time alkane formula 1st data (min) 2nd data (min)

n-pentacontane C50H102 18.683 n-tetratetracontane C44H90 18.68 n-tetracontane C40H82 19.12-19.33 18.82-18.9 n-tricontane C30H62 19.973 19.53-19.60 n-pentadocosane C25H52 19.92 n-docosane C22H46 20.4 n-eicosane C20H42 20.48-20.69 20.2-20.48 branched C19 alkane C19H40 20.506 20.32 n-hexadecane C16H34 21.23-21.36 n-tetradecane C14H30 21.387 n-tridecane C13H28 21.627 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 Figure 2. Chromatograms on Mixed-E of (a) the Polywax samples, (b) alkane mixtures C and C , and (c) candle wax and diesel. smaller than C12. Initial results showed that an intense peak could 20 30 be obtained in blank runs at about 11.8 min, probably corre- they did not appear until about 45 min as a broad hump rather sponding to the exclusion limit of the column, but this came than a sharp peak. Similarly, polystyrene standards also eluted from the tip filters used to remove undissolved sample before much later than the estimated permeation limit of the column. injection; the material may be particulates from the plastic No alkenes were examined as standard materials, but the data housing of the filters. (below) from high-temperature GC-MS indicated that the The Polywax samples were dissolved in CS2 and diluted with present samples did not appear to contain free alkenes; the heptane before examination, and their ELS chromatograms are characteristic alkene-alkane pairs of peaks in GC-MS chro- shown in Figure 2a. The Polywax 1000 did not dissolve matograms were only observed in results from pyrolysis GC- appreciably, and no chromatogram was obtained, while that for MS. We have shown elsewhere40 that the terpenes and sesquit- Polywax 650 was very weak. The range of alkanes in the erpenes found in essential oils are soluble in NMP and could Polywax samples given by the suppliers was: Polywax 500 appear with the aromatic materials or in the aliphatic heptane- C20-60, Polywax 655 C40-60, and Polywax 1000 C40-80. soluble fractions. Mixtures of alkanes were examined to determine the separa- Colloidal silicas of diameters 9, 12, and 22 nm all eluted at tion capabilities of the system; Figure 2b shows a chromatogram the exclusion limit of 11.95-12.0 min and were detected by of mixed C20 and C30 alkanes. There was a partial separation, the ELS; they were not added to the calibration graph since but clearly the resolution was insufficient to resolve alkanes their masses (which are not known) are not relevant for the separated by one or two carbons. Other petroleum-derived and current purpose, which was to establish that known spherical aliphatic materials were examined by this method. Figure 2c particles would elute at the exclusion limit as found with shows SEC chromatograms for diesel fuel and candle wax, and Mixed-A and -D columns.2 Fullerene gave no peak before the the elution times of the peaks corresponded to the expected permeation limit by ELS and no UV absorbance at 300 nm, alkane types for these products. No aliphatic materials eluted showing that this molecule behaved as an aromatic molecule later than the small-molecule permeation limit, at about 24 min in this column in heptane solution. s for this column. However, the small aromatics benzene and (40) Morgan, T. J.; Morden, W. E.; Al-Muhareb, E.; Herod, A. A.; toluenesdid not elute before the permeation limit, and indeed, 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 Figure 4. SEC chromatograms of aliphatics in (a) Petrox two fractions A and C at long times (22-25 min) and after the main peak soluble in pentane and (b) coal digest first pentane fraction (Mixed-E column). 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 Figure 5. SEC chromatograms on Mixed-A column of seven fractions contained similar ranges of aliphatics as far as SEC is concerned. from column chromatography of Petrox sample. The range of Figure 4a compares with that of Petrox heptane solubles in Figure 3. The toluene fraction showed no detectable Supporting Information) and show a complex variety of aromatic aliphatic signal. The reason for comparing the fractionation molecules in each fraction with a shift to earlier elution as the methods (heptane solubles/insolubles and column chromatog- solvent polarity increased; fraction weights recovered from the raphy) using Petrox is that the heptane-soluble extraction is more column chromatography are listed in Table 2. generally used for petroleum work while the column chromatography method is more suitable for coal liquids. In vacuum The vacuum residues and Maya asphaltene were also residues, the proportion of toluene insolubles is normally examined using the Mixed-A column to detect changes in the insignificant, but in coal liquids, there are often significant aromatic materials on fractionation. Figure 6 shows chromato- proportions of toluene-insoluble materials. For comparison, grams for the whole vacuum bottoms A and the heptane- Figure 4b shows the first pentane extract of the coal digest on insoluble fraction. Equivalent data for the Maya asphaltene and the Mixed-E column and shows a smaller range of alkanes (of vacuum bottoms B and C can be found in the Supporting low intensity) than the Petrox fractions. Information. In each case, the removal of heptane-soluble The SEC chromatograms of the column chromatography material shifted the retained peak to shorter elution times, fractions using the Mixed-A column and NMP as eluent are indicating larger-sized molecules being isolated in the insoluble shown in Figure 5 for the Petrox sample (equivalent data for fraction, while the excluded peak also became more prominent. pitch, coal digest, and low-temperature tar can be found in the NMP does not completely dissolve the asphaltene samples, and 1170 Energy & Fuels, Vol. 20, No. 3, 2006 Al-Muhareb et al.

Table 2. Fraction Weights Recovered for the Petroleum Residue using the Mixed-A column in solution in NMP (Figure 5) but and the Three Coal Liquids were not detected in the GC-MS as prominent components. petrox pitch PoA LTT Figure 7c shows the chromatogram of Polywax 500 under the fractionsa % % % % same conditions. pentane 1st 50.7 3.8 12.4 10.5 The pentane and toluene fractions of coal liquids were pentane 2nd 18.0 17.4 29.6 25.7 examined by HT GC-MS to determine the alkanes present. toluene 5.7 26.5 10.1 19.6 As with the Petrox residue, the toluene extracts of coal liquids acetonitrile 1.7 5.2 3.1 13.7 pyridine 2.5 15.3 17.5 4.0 contained no components that eluted through the GC column, NMP 6.2 15.1 8.5 4.3 and it is unlikely that the other fractions from column chroma- water 11.3 2.8 5.1 6.6 tography (acetonitrile, pyridine, and NMP solubles) would SUM 97.0 86.1 86.4 84.4 contain any small molecules able to pass through the column. a Material soluble in the solvents shown by sequential elution. No alkanes were detected in the coal tar pitch sample pentane- soluble fractions. The coal digest and the low-temperature tar had shown the presence of alkanes by normal-temperature GC- 35 MS up to C26 and up to C32, respectively; 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. Figure 6. SEC chromatograms on Mixed-A of heptane solubles and The second pentane extract and the toluene extract samples heptane insolubles of vacuum bottoms sample A. were subjected to pyrolysis GC-MS. This was an attempt to examine the material that did not pass through the GC column 41,42 the insoluble fraction, shown to have no fluorescence, may initially. The chromatograms of the second pentane fractions 5,6 consist of large aromatics carrying large aliphatic groups. are shown in Figure 9a-d, respectively, for Petrox, pitch, coal Molecular structures of asphaltene molecules showing little or digest, and low-temperature tar. Figure 9a shows a series of no fluorescence may include oxygen, sulfur, and nitrogen atoms alkene-alkane pairs from C8 to at least C18 at short retention and allow the excitation energy from absorbance of a photon times followed by aromatics that are mainly alkyl naphthalenes, to transfer into vibrational energy rather than being emitted as fluorenes, diphenyls, phenanthrenes, fluoranthenes/ pyrenes, a photon by fluorescent. chrysenes, and other polycyclic aromatics up to m/z 252 The SEC method for aliphatics will find application to benzopyrene isomers. These components were not observed fractions soluble in pentane or heptane from petroleum liquids without pyrolysis and are assumed to form from the macro- and coal-derived liquids, where the presence of aromatic molecules unable to pass through the column. Figure 9b for molecules might obscure the determination of the aliphatics in pitch shows the major aromatic compounds, phenanthrene m/z other techniques. 178, fluoranthene and pyrene m/z 202, and their methyl - - HT GC MS and Pyrolysis GC MS of Petrox Residue derivatives at m/z 192 and 216, and chrysene isomers m/z 228. Fractions and Coal Liquids. The pentane- and toluene-soluble Figure 9c for the coal digest shows some of the major aromatics samples from column chromatography were examined, and as fluorene m/z 166, phenanthrene m/z 178, and pyrene m/z 202 chromatograms are shown in Figure 7a,b for the pentane with only a very small peak for fluoranthene, with extensive fractions of Petrox. The first pentane fraction gave a series of series of alkylated derivatives of these aromatics. Figure 9d for alkanes from m/z 226 C16 to 408, C29 with others of higher mass the low-temperature tar shows some of the major aromatic to about C42, but with no molecular ions detected, superimposed components as fluorene m/z 166, phenanthrene m/z 178, pyrene on a broad hump of aliphatic material. There was no light m/z 202, and benzofluoranthenes and pyrenes m/z 252, but the material in the sample as would be expected from a residue major components are alkylated derivatives of the aromatics. from steam distillation. The second pentane fraction shows very No evidence for aliphatic materials was evident in the pyrolysis few n-alkanes, but the unresolved hump of aliphatic material is products of the coal liquid fractions. The pyrolysis chromato- shifted to later scans (higher mass) than that observed in the grams of the toluene soluble fractions are not shown, but they first fraction. The toluene fraction showed very little signal, were very much less intense than those of the second pentane indicating that the fraction contained very little material able fractions and had fewer peaks. These chromatograms indicate to pass through the high-temperature column. It seems probable that the second pentane and toluene fractions of all four samples that these aliphatics in the second pentane extract might contained relatively large components that could not pass 9 correspond to the microcrystalline waxes detected by field- through the GC column initially but on pyrolysis formed some ionization and field-desorption mass spectrometry. The aromatic small aromatic systems and, in the case of Petrox, some alkane components of these Petrox fractions were detected by SEC and alkene fragments. In neither fraction did the alkanes become of any intensity after pyrolysis, and it is assumed that those (41) Ascanius, B. E.; Merino-Garcia, D.; Andersen, S. I. Energy Fuels 2004, 18, 1827. detected originated by dealkylation of large alkyl aromatic (42) Morgan, T. J.; Millan, M.; Behrouzi, M.; Herod, A. A.; Kandiyoti, R. Energy Fuels 2005, 19, 164. (43) Walton, S. T. Fuel 1993, 72, 687. SEC for the Unambiguous Detection of Aliphatics Energy & Fuels, Vol. 20, No. 3, 2006 1171

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). 1172 Energy & Fuels, Vol. 20, No. 3, 2006 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 4. Conclusions no indication that n-alkanes larger than those found in the first A method of using size exclusion chromatography to detect pentane fraction, up to about C41, were present in the subsequent aliphatic molecules (in the presence of aromatics) has been two fractions since the GC conditions were capable of eluting developed and tested using petroleum-derived products. The n-C56 from a polywax sample at about 48 min. Also, there was method distinguishes between aliphatic and aromatic using an no unresolved aliphatic peak as in the GC of the second pentane evaporative light scattering detector and a UV absorbance fraction, which may be presumed to have formed light alkane detector in series. Aliphatics have no UV absorbance, while gas on pyrolysis. aromatics elute by a surface interaction mechanism, later than The evidence from the gas chromatograms of increasing the permeation limit of the SEC column. Application of the molecular size and complexity with increasing polarity of the method is likely to be to pentane-soluble fractions of petroleum- eluting solvent is supported by the UV fluorescence spectra of derived and coal-derived liquids where the presence of aromatics the fractions. Figure 10 shows synchronous UV fluorescence may prevent or obscure the determination of aliphatics by other spectra of the column chromatography fractions of the Petrox techniques. < sample. Equivalent data for the pitch, coal digest, and low- Standard n-alkanes C50 elute with a linear relation between log molecular mass and elution time. Spherical colloidal silica temperature tar are available as Supporting Information. In each 10 standards were excluded from the column porosity. Candle wax case, the synchronous UV fluorescence spectra shift to longer and diesel fuel elute as expected for the alkane carbon numbers wavelengths as polarity of eluting solvent increases. However, typical of these products. Mixtures of alkanes elute as partially there were some exceptions. For Petrox, fraction 4 (acetonitrile) resolved peaks at best, and polywax samples elute as a broad fluoresced at wavelengths shorter than those of fraction 3 distribution with no separation of the small alkanes, with an (toluene), suggesting that fraction 4 contained smaller aromatic upper limit at about the C50 alkane. groups than fraction 3. Similarly, fractions 6 (NMP) and 7 Fractions of heptane-soluble materials from crude petroleum (water) showed shorter wavelength fluorescence than fraction vacuum residues and a Maya asphaltene prepared by either 5 (pyridine). For pitch, coal digest, and low-temperature tar, column chromatography using pentane and toluene or solubility the only exception to systematic shifts to longer wavelengths in heptane indicated differences in SEC behavior. The first is fraction 7 (water) in each case. The types of molecules eluted pentane-soluble fractions contained only alkanes < C56, while by water with some NMP are not known, but the data of Table the second pentane-soluble fractions and toluene fractions 2 suggest a significant proportion of Petrox eluted, with containing the Petrox residue contained mainly branched increasing quantities from pitch to low-temperature tar. aliphatics. Those of coal liquids contained no significant SEC for the Unambiguous Detection of Aliphatics Energy & Fuels, Vol. 20, No. 3, 2006 1173

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). 1174 Energy & Fuels, Vol. 20, No. 3, 2006 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 Figure 10. Synchronous UV fluorescence spectra of Petrox residue compounds from column chromatography using a range of sample. Fractions are: 1, first pentane; 2, second pentane; 3, toluene; solvents of increasing polarity gave fractions in which the 4, acetonitrile; 5, pyridine; 6, NMP; and 7, water. Intensities height aromatic systems increased in size with increasing solvent normalized. polarity. This was true for the petroleum residue and coal liquids. aliphatic components. Heptane-soluble fractions of vacuum Acknowledgment. We thank BCURA/DTI for financial support residues gave SEC peaks in the retained region of the Mixed-E under Contract B53. The mass spectra were obtained at Kings column and indicated differences in the ranges of alkanes from College London and at Bristol University, and we thank NERC different samples. for supporting the Mass Spectrometry Unit at Bristol. We also thank All of the seven fractions from Petrox and the coal liquids Mahtab Behrouzi for some experimental work. and the heptane-soluble and -insoluble fractions of vacuum residues contained aromatic components as determined by SEC Supporting Information Available: Figures showing SEC in NMP using the Mixed-A column. These aromatics became chromatograms on Mixed-A column of seven fractions from column more complex and of larger molecular size with increasing chromatography of low-temperature tar, point of Ayr coal digest, polarity of the solvent used to isolate the fraction. and coal tar pitch; SEC chromatograms on Mixed-A of heptane High-temperature GC-MS of pentane and toluene fractions solubles and heptane insolubles of Maya asphaltenes and vacuum indicated that n-alkanes could be detected in the first pentane bottom samples B and C; and synchronous UV fluorescence spectra fraction only, with none detected in the second pentane or of coal tar pitch samples, point of Ayr coal digest and low- toluene fractions of coal liquids; branched or cyclic aliphatics temperature tar. This material is available free of charge via the were detected in the second pentane fraction of the Petrox Internet at http://pubs.acs.org. residue. Pyrolysis GC-MS of the second pentane and toluene EF0600098