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.