Energy & Fuels 2008, 22, 3265–3274
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Calibration of Size-Exclusion Chromatography Columns with 1-Methyl-2-pyrrolidinone (NMP)/Chloroform Mixtures as Eluent: Applications to Petroleum-Derived Samples ´ lvarez, Marcos Millan, Cesar Berrueco, Silvia Venditti, Trevor J. Morgan, Patricia A Alan A. Herod,* and Rafael Kandiyoti Department of Chemical Engineering, Imperial College London, London SW7 2AZ, United Kingdom ReceiVed May 18, 2008. ReVised Manuscript ReceiVed July 6, 2008
This paper presents work on the size-exclusion chromatography (SEC) of petroleum-derived fractions, using a Mixed D column (Polymer Laboratories, U.K.), where pure 1-methyl-2-pyrrolidinone (NMP), as well as several NMP/CHCl3 mixtures were used as eluents. The column calibration was performed using a variety of polymeric molecular mass standards and model polycyclic aromatic hydrocarbon (PAH) and heterocyclic compounds. Mixing CHCl3 into the eluent was observed to reduce the structure dependence of elution times of sample polymers compared to NMP alone. Elution times of a set of PAHs and heterocyclic compounds gave a line with a different slope for these smaller mass model compounds (m/z 700), then it would not be detected by probe mass spectrometry. All of the solvents apart from hexane investigated15 in the sequential extraction of a bituminous coal (Pittsburgh no. 8), hexane, benzene, dichloromethane, acetonitrile, methanol, and pyridine, extracted some excluded material. Hexane gave no measurable extracted material from the coal, and benzene seems unlikely to extract very polar compounds. Our work on calibrating the present column and in earlier papers using either mixed solvents or NMP alone 1,3,4,7 showed that the highly oxygenated standards eluted earlier than the neutral parent compounds but not by more than 3 min. As shown in Tables 2 and 3, some acids eluted later than expected, by up to 3.3 min, in the present work. Polysaccharide standards also eluted earlier than PSs of similar mass as shown here, but the difference decreased in the mixed solvents. In all of our work involving fractionation of coal liquids and asphaltenes, we have observed material excluded from column porosity in SEC that could be classified as of high polarity, even in fractions soluble in heptane. We discussed this apparent high polarity or the presence of aggregates of small molecules in an earlier paper42 and demonstrated separately3 that of three fractions collected from the excluded peak of pitch with NMP as eluent, the first two eluted on reinjection at their times of collection, whereas the third fraction eluted at the collection time of the second fraction, providing evidence that the excluded region could easily overload with sample. There was no evidence of disaggregation with dilution. In an examination of fractions of a low-temperature tar in acetonitrile, pyridine, and NMP, followed by pyrolysis-GC-MS,43 the NMP solubles gave only peaks derived from the NMP, although containing the greater (42) Herod, A. A.; Johnson, B. R.; Bartle, K. D.; Carter, D. M.; Cocksedge, M. J.; Domin, M.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 1995, 9, 1446–1451. (43) Islas, C. A.; Suelves, I.; Carter, J. F.; Li, W.; Morgan, T. J.; Herod, A. A.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 2002, 16, 774–784.
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Figure 11. Synchronous UV-fluorescence spectra of crude B and crude B asphaltenes, NMP-soluble (NMP S) and -insoluble (NMP I) fractions in chloroform.
proportion of the excluded material. We have therefore rejected the notion that the excluded material is made up of aggregates, partly because we could not cause disaggregation by added salts44 or dilution3 or pyrolysis-GC-MS and partly because the increasing molecular mass of oligomers leads to insolubility in solvents capable of dissolving the smallest oligomers (an apparent increase of polarity). The synchronous UV-fluorescence spectra (Figure 11) indicated marked differences in range of chromophores detected in the two sets of fractions. As expected, the largest aromatic systems appeared to be concentrated in the NMP-insoluble fractions of the asphaltenes. Similar differences have been shown for a Kuwaiti asphaltene.5 The same anonymous reviewer also requested comment on a series of mass spectrometric results, all claiming to show the mass range of asphaltenes to lie between m/z 400 and 1000, with a maximum intensity around m/z 800. The present paper contains no mass spectrometric results; therefore, a detailed discussion is not warranted at present. However, the present SEC results show significant differences among crude petroleum samples, asphaltene samples, and NMP-insoluble fractions run in the mixed solvents. There is no obvious reason why all asphaltenes should show the same range of mass by mass spectrometry unless instrumental factors limit the observable mass range. Mass discrimination factors or insolubility may be at work, and fractionation before mass spectrometry is the only route to determine such effects. We have unpublished evidence to support this view. In 1986, a personal view45 of new analytical techniques was of a brash, confident road runner, taking everything in its stride, until it came up against fossil fuels, when it fell flat. Caution is still necessary in applications of new techniques to such complex and little understood materials as petroleum asphaltenes and coal liquids. 4. Summary and Conclusions During the SEC of coal-derived materials, NMP has been observed to largely block solute-packing interactions. However, NMP is a poor solvent for most petroleum-derived fractions. Mixing NMP with small amounts of CHCl3 (7:1, 6:1, and 5:1 ratios by volume) has already been shown to work well for these samples. This paper presents work on the SEC of petroleum(44) Herod, A. A.; Shearman, J.; Lazaro, M.-J.; Johnson, B. R.; Bartle, K. D.; Kandiyoti, R. Energy Fuels 1998, 12, 174–182. (45) Retcofsky, H. L.; Perry, M. Prepr. Pap.-Am. Chem. Soc., DiV. Fuel Chem. 1986, 31 (1), 1–6.
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derived fractions, using a larger porosity Mixed D column (Polymer Laboratories, U.K.), where pure NMP, as well as several NMP/CHCl3 mixtures were used as eluents. Mixing CHCl3 into the eluent has brought about reductions of the structure dependence of elution times of standard polymers. In pure NMP, polysaccharide samples showed the largest differences from the PS calibration, which increased with increasing mass. A factor of 2.5 difference (6000 units) was observed in molecular-mass estimates at PS masses of about 15 000 units. The use of NMP/CHCl3 mixtures as eluent has narrowed the gap at lower masses, so that differences of the order of a factor of 2.5 were found at PSAC masses of 100 000 units (corresponding to 250 000 units of PS mass). This is a significant improvement over the result obtained with pure NMP eluent. Elution times of a set of smaller PAHs and heterocyclic compounds (
