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Size Exclusion Chromatography of Soots and Coal-Derived Materials with 1-Methyl-2-pyrrolidinone as Eluent: Observations on High Molecular Mass Material A. A. Herod,* M.-J. Lazaro, I. Suelves, C. Dubau, R. Richaud, J. Shearman, J. Card, A. R. Jones, M. Domin,† and R. Kandiyoti Department of Chemical Engineering and Chemical Technology, Imperial College, University of London, London SW7 2BY, U.K. Received February 15, 2000. Revised Manuscript Received June 1, 2000
A number of samples which eluted at unexpectedly short retention times during size exclusion chromatography have been characterized. Soot and tar samples likely to show similar behavior have been examined. Distinct peaks from about 6 min were observed, compared to 9-10 min for more usual samples. Molecular masses of the early eluting material appear to be large, although extrapolation of existing calibrations does not seem appropriate. A naphthalene mesophase pitch also gave peaks at short elution times. It appears reasonable to interpret chromatograms of this nonpolar material as a direct indication of the presence of large molecular mass material, and to infer that excluded peaks of SEC chromatograms do not necessarily result from the presence of clusters of polar molecules. GC-MS and probe-MS examination of the samples showed only very limited proportions of the samples to have small molecular masses. MALDI-MS spectra of the samples indicated the presence of signal up to 20 000 u. Taken together, data from SEC and the three MS techniques indicated the presence of very large molecular mass materials in this set of samples. Tar deposits recovered from entrained, combusting coal particles have also been examined, providing direct evidence for the presence of large molecular mass material in combustion environments. The observation contrasts with mathematical models of coal burners, where rates of combustion of volatiles are assumed similar to rates of combustion of methane. The nature of the early-eluting material is not known but repeated microfiltration and TEM indicate that it may correspond to molecular diameters in the region of 20 nm. The soots and other samples appear to be in true solution rather than in colloidal suspension.
Introduction Distributions of molecular masses in coal derived liquids are relevant to research in fields as diverse as catalytic hydrocracking and environmental pollution diagnostics. In attempting to develop tools for the characterization of “heavy” coal liquids, our recent work (summarized in ref 1) has led to the following observations: (i) in size exclusion chromatography, when NMP (1methyl-2-pyrrolidinone) is used as eluent in a polystyrene/polydivinylbenzene column, retention times of model compounds correlate closely with those of polystyrene standards and are almost entirely independent of structure up to at least 1000 u; (ii) in MALDI massspectrometry, reliable upper mass limit estimates in the vicinity of 100,000 u can be obtained for a coal tar pitch; (iii) in the structural examination of the heaviest fraction of the same coal tar pitch, the presence of aliphatic bridge units were observed, apparently linking * Corresponding author. E-mail address:
[email protected]. † Department of Pharmaceutical and Biological Chemistry School of Pharmacy, University of London, London WC1N 1AX, U.K. (1) Herod, A. A.; Lazaro, M.-J.; Domin, M.; Islas, C. A.; Kandiyoti, R. Fuel 2000 79, 323.
polynuclear aromatic structures which themselves appear too large to pass through a gas chromatographic column. During this investigation, a kerogen extract2 and tars recovered from coal particle combustion experiments in methane flames showed behavior that could be considered as anomalous: in SEC, peaks were observed at unexpectedly early elution times of 6-7 min, instead of the 9-10 min usually found for coal-derived extracts and tars. It was then felt that collateral evidence relevant to the behavior of large molecular mass materials in coal-derived liquids could be obtained by studying heavy carbonaceous materials that may contain very large molecular mass materials. The present paper describes results from the examination of coal tar pitch and several samples of combustion derived soot by size exclusion chromatography; results have been correlated with data from various mass spectrometric techniques and UV-fluorescence spectroscopy. Statement of the Problem. The atomic composition of, say, the coal tar pitch used in the present study cannot be arrived at by using the structures of polycyclic aromatic compounds identified in data from the exami(2) Herod, A. A.; Lazaro, M.-J.; Rahman, M.; Domin, M.; Cocksedge, M. J.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 1997, 11, 1627.
10.1021/ef000023z CCC: $19.00 © 2000 American Chemical Society Published on Web 07/08/2000
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nation of samples by GC-MS and probe-MS. Furthermore, conceptual extrapolation by the addition of benzo groups to polycyclic aromatic structures (identified by GC-MS and probe-MS) leads to C/H ratios significantly deficient in hydrogen compared with the overall pitch composition. Structural features other than the conceptual addition of benzo groups to three and four ring aromatics are necessary for arriving at the actual (measured) C/H ratios. This argument3 provides simple and powerful evidence against viewing “heavy” liquid mixtures as containing aggregates of small polar molecules rather than high-mass species. However, most of the detailed structural information available on the molecular makeup of coal-derived materials comes from analyses4,5 by GC-MS (20 nm was redissolved in NMP and passed through a clean filter; the filtrate was clear and the soot remained on the filter. The SEC profile of the >20 nm soot remained identical to curve 1 of Figure 4. This experiment indicates that the large particles were not simply aggregates of smaller molecules but were large, stable entities which we assume to be molecules. Within the framework of the present study, it appears difficult to distinguish between the soot being in true solution or its presence in colloidal suspension. However, the UV-fluorescence emission spectra of the candle soot contained sharp peaks caused by light scattering at twice the exciting wavelength, which were smaller in intensity than the equivalent sharp peaks produced by the cell with pure solvent alone; this finding (72) Haumaier L.; Zech W. Org. Geochem. 1995, 23, 191. (73) Goldberg M. C.; Weiner E. R. Fluorescence Spectroscopy in Environmental and Hydrological Sciences. In Fluorescence spectroscopy-New Methods and Applications, Wolfbeis, O. S., Ed.; SpringerVerlag: Berlin, 1993; Chapter 16, p 213.
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Figure 4. SEC chromatograms of (curve 1) fraction of sample which failed to pass through the 20 nm membrane and (curve 2) the fraction of sample which passed through the 20 nm membrane; chromatograms obtained by UV-absorbance at 350 nm. Permeation limit at about 23 min, exclusion limit at 10.5 min; area normalized profiles.
tends to indicate true solution. In addition, the procedure for recovery of soot from NMP solution was initially based on evaporation of the bulk of solvent followed by addition of excess water or methanol to dissolve the remaining NMP and precipitate the soot. However, this did not work rapidly as expected, but gave instead what appeared to be a true solution of soot in water or methanol. We interpret this to indicate the strong binding (solvation sheath) of NMP to organic molecules which cannot easily be detached by either water or methanol; the complete removal of NMP from high mass material has not yet been solved. We note that dissolved organic matter isolated from seawater by ultrafiltration in the size range 1-100 nm was described as being in the high molecular weight or colloidal fraction,74 while poly(ethylene oxide) of molecular mass 8000 u is thought to have a hydrodynamic radius of 100 nm in aqueous solution.75,76 For bitumens, the colloidal state was considered a property of the mixture of saturates, oils, resins and asphaltenes.77 It is clear, however, that the elution region of the SEC column corresponding to 6-7 min is equivalent in molecular dimensions to a diameter g 20 nm. UV-Fluorescence Spectroscopy of the Samples. Figure 5a presents UV-fluorescence spectra of the fractionated effluent from the SEC of coal soot, the 7 min peak, 8-11 min interval, and 15-22 min interval. The fluorescence profile for the 7 min peak was very weak and corresponded to the residual fluorescence observed from the NMP alone. However, the other two (74) Clark, L. L.; Ingall, E. D.; Benner, R. Nature 1998, 393, 426. (75) Adams, M.; Dogic, Z.; Keller, S. L.; Fraden, S. Nature 1998, 393, 349, (76) Devanand, K.; Selser, J. C. Nature 1990, 343, 739. (77) Loeber, L.; Muller, G.; Morel, J.; Sutton, O. Fuel 1998, 77, 1443.
fractions showed the expected trend for coal derived material,78 with the fluorescence profile for the smaller size (later) fraction shifting to shorter wavelengths compared with the excluded peak (8-11 min). This shift of the maximum intensity of fluorescence in these spectra is a clear indication that the aromatic systems producing the fluorescence are of different sizes, as indicated by SEC, rather than agglomerates of small molecules. The fluorescence of agglomerates would be expected to reduce in intensity with increasing agglomeration and concentration, but not to shift to longer wavelengths. Figure 5b shows synchronous spectra of the candle soot fraction from SEC corresponding to the 7 min peak and the whole soot. The 7 min peak showed no significant fluorescence despite being concentrated and the peak at about 290 nm corresponds to the residual fluorescence of NMP. The analogous UV-fluorescence data for the two fractions of candle soot following filtration though the 20 nm membrane are not shown because they matched those of Figure 5b, with the oversize material identical to that of the 7 min peak and the undersize material showing the profile of the whole soot. The data from the repeated filtration showed similar spectra except that the second filtrate from the solution of the initial >20 nm fraction showed no significant signal even after concentration. All samples showed particularly weak fluorescence intensities, suggesting the presence of relatively large polynuclear aromatic ring systems with low quantum yields, but the fraction of candle soot which filtered through the 20 nm (78) 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.
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Figure 5. Synchronous UV-fluorescence spectra of (a) the coal soot 7 min peak, (curve 1), the 8-11 min peak (curve 2) and the 15-22 min peak (curve 3) collected from the SEC fractionation, (b) the candle soot 7 min peak (curve 1) collected from the SEC fractionation, and the whole candle soot (curve 2). Profiles are height normalized.
membrane showed significantly more intense signal compared to the fraction which failed to pass through the membrane. These (albeit small) differences are consistent with the presence of a minor proportion of smaller molecular mass material in the filter solution observed by SEC (Figure 5b, curve 2). The variable proportions of UV-absorbance intensity
at different elution times in the SEC profiles for the range of samples examined in this work indicates that these samples are structurally very different from each other: the soot samples are unlike the combustion tars and demineralized coal. The latter has not, of course, been exposed to elevated temperatures or combustion conditions.
Chromatography of Soots and Coal-Derived Materials
Figure 6. TEM images of the soot as recovered: (a) after solution in NMP and (b) after passage through the SEC column.
Transmission Electron Microscopy. Figure 6a,b show TEM images of the candle soot as recovered (a) after solution in NMP and (b) as the 7 min peak after passage through the SEC column. In both pictures, the sizes and shapes of detected forms are similar although the sample recovered from simple solution contained quantitatively more material than that after passage through the SEC column. Clearly, however, dilution made no significant change to the size of detected forms as observed by TEM and made no difference to the elution point in SEC: the sample recovered as the 7 min peak from SEC eluted at the same time on reinjection although at much greater dilution. Pictures at greater magnification (not shown) indicated that the individual “particle” sizes were up to about 50 nm diameter. These soot fractions appeared to be soluble in NMP, rather than existing as colloidal particles, and the UVfluorescence spectra indicated a true solution on the basis of scattered light intensity. Difficulties encountered in their preparation also appear to indicate that we might indeed have been dealing with a solution. Attempts were made to precipitate this sample material from solution in NMP by dilution with water or methanol; neither method succeeded and the NMP was eventually evaporated by heating under vacuum. The X-ray electron diffraction patterns obtained for these samples indicated that they
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were amorphous and covalent carbon, rather than layered or fullerene type structures.79 Large Molecules or Aggregates of Small Molecules. A major problem remaining is the question of whether the early eluting signal in SEC consists of aggregates of small molecules or corresponds to large molecules. The evidence presented here favors large molecules rather than aggregates for the following reasons. (1) The standard polycyclic aromatics in NMP solution behave in SEC as molecules in terms of elution relative to polystyrene standards. (2) The MALDI mass spectra of the material retained by the SEC column indicate a similar range of mass as the polystyrene calibration. The available evidence suggests that the laser desorption of highly polar molecules such as proteins and peptides as well as polycyclic aromatics produces mainly the molecule ion rather than cluster ions, which show only relatively low intensities. (3) The synchronous UV-fluorescence spectra indicate that the large sized material is different from the small sized fractions; the shift of maximum intensity of fluorescence with increasing molecular size (as indicated by SEC) is indicative of increasingly complex aromatic systems rather than an increase of agglomeration of small molecules. (4) The process of collection of a fraction from the SEC column, reinjection of a dilute solution, and the observation27 of an unchanged elution time and even a shift to an earlier elution time with no production of later eluting material, indicates that increased dilution produced no disaggregation. In the present work, the solutions injected were dilute to avoid overloading the exclusion region of the column and for the candle soot, collection of a >20 nm fraction followed by resolution and a second filtration produced only >20 nm material and no disaggregation. (5) The transfer of insoluble material into solution in NMP with ultrasonic agitation for at least 30 min may be considered80 to result from the more powerful dipole/induced-dipole interaction between NMP and the previously aggregated, solid material, resulting in dispersion in NMP as individual molecules. The disruption of intermolecular forces in a solid by interactions between solvent and solute defines the formation of a solution. However, the work does not completely rule out aggregates. Summary and Conclusions This study attempted to characterize a number of samples which, during size exclusion chromatography, eluted at unexpectedly short retention times compared with those ordinarily found for coal derived extracts and tars. A number of other soot and tar samples likely to provide a source of material also eluting very early in size exclusion chromatography were examined. Samples have been characterized by GC-MS and probe-MS, as well as by size exclusion chromatography and MALDIMS. UV-fluorescence spectroscopy has been used to distinguish structurally between these unusual materials. Our findings may be summarized as follows. 1. With one exception, a distinct peak at about 6 min was observed as a common feature in all these samples, (79) Amaratunga, G. A. J.; Chhowalla, M.; Kiely, C. J.; Alexandrou, I.; Aharanov, R.; Devenish, R. M. Nature 1996, 383, 321. (80) Atkins, P. W. Physical Chemistry, 3rd ed.; Oxford University Press: U.K., 1986; Chapter 24, p 586.
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compared to the more usual 9-10 min for most coal derived samples. With the present SEC-column, the polystyrene standard of mass 1.84 million u eluted at 9 min. The molecular masses of this early eluting material thus appears to be very large, although it is not clear that the same calibration curve could be extrapolated to 6 min to give an estimate of molecular masses. 2. A sample of naphthalene mesophase pitch also gave peaks at short elution times. It appears reasonable to interpret this chromatogram as a direct indication of the presence of large molecular mass materialsand to infer that excluded peaks of SEC chromatograms (observed for any type of sample) do not necessarily result from the presence of clusters of polar molecules. 3. GC-MS and probe-MS examination of the samples gave results showing that only a very limited proportion of the samples were made-up of small molecular mass (polar or nonpolar) material. MALDI-MS spectra of the samples indicated the presence of signal up to 20,000 u, the upper limits shown in these diagrams. Taken together, data from SEC and the three MS-techniques indicated the presence of very large molecular mass materials in the set of samples. 4. Tar deposits have been recovered on cold stubs inserted into the paths of entrained and combusting coal particles. Characterization of these tars has provided direct evidence for the presence of large molecular mass material ejected into the vapor phase in a combustion environment. The observation contrasts with mathematical models of coal burners, where rates of volatiles combustion are assumed to be similar to the rate of combustion of methane. 5. All samples showed particularly weak fluorescence intensities, suggesting the presence of relatively large polynuclear aromatic ring systems with low quantum
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yields. The fraction of the candle soot which filtered through the 20 nm membrane, showed significantly more intense UV-fluorescence compared to the fraction which failed to pass through the membrane. However, the >20 nm material did not disaggregate on being dissolved in NMP and refiltered. These differences were consistent with the presence of an (albeit small) proportion of smaller molecular mass material observed by SEC, in the solution which passed through the 20 nm membrane. 6. The nature of the early-eluting material is not known but microfiltration and TEM indicate that it may correspond to a molecular diameter of about 20 nm. The exclusion limit of the column corresponds to a molecular diameter of about 1 nm. XRD indicated that components of the candle soot were not fullerenes or graphitic carbon, but covalent carbon structures. Acknowledgment. Support for this work by the British Coal Utilization Research Association (BCURA) and the U.K. Department of Trade and Industry under BCURA Contracts B32a and B44 is gratefully acknowledged. The authors thank the University of London Intercollegiate Research Service for the provision of mass spectrometry facilities at Kings College and the School of Pharmacy, the European Community (Nonnuclear Energy Program, MCRG) for postdoctoral research fellowships (MJL and I.S.), Trevor Morgan for preparing fractions recovered from SEC, and Graham Briers of the Materials Science Department (IC) for the TEM data. We acknowledge the helpful comments of the referees. EF000023Z
