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The unusual properties of high mass materials from coal-derived liquids☆
Fuel 82 (2003) 1813–1823 www.fuelfirst.com
The unusual properties of high mass materials from coal-derived liquidsq C.A. Islas1, I. Suelves2, W. Li3, T.J. Morgan, A.A. Herod*, R. Kandiyoti Department of Chemical Engineering and Chemical Technology, Imperial College London, University of London, South Kensington Campus, Prince Consort Road, London SW7 2BY, UK Received 31 May 2002; revised 20 March 2003; accepted 30 March 2003; available online 9 May 2003
Abstract This short paper highlights the unusual properties of the high-mass material of coal liquids isolated by their insolubility in pyridine and solubility in NMP. The separation has been achieved by a column chromatography method. One gram quantity have been processed and near quantitative recovery of the sample as fractions has been achieved. This fractionation permitted recourse to a broad range of analytical methods, including some (e.g. 13C NMR), which require large sample sizes. Multiple macro analyses have been undertaken, using elemental analysis, TGA proximate analysis, NMR and FT-ir in addition to the micro-analytical methods used previously—pyrolysis-gc-ms, SEC, UV – fluorescence, probe-ms and MALDI-ms. The fractions show increasing concentrations of large molecular mass material with increasing polarity of successive eluents used in the fractionation. Evidence from solid-state 13C NMR and UV – fluorescence spectroscopy show progressive structural changes with increasing apparent molecular mass. q 2003 Elsevier Science Ltd. All rights reserved. Keywords: Fractionation methods; Size exclusion chromatography; UV–fluorescence; NMR
1. Introduction Fractionation is usually a necessary first step in identifying the presence of special classes of materials, found in relatively low concentrations in complex mixtures. It has proved particular use in identifying high molecular mass materials in various heavy hydrocarbon liquids. In our more recent work, planar chromatography has been used as the preferred fractionation method. The analytical characterisation of the separated fractions has relied heavily on size exclusion chromatography (SEC) with 1-methyl-2-pyrrolidinone (NMP) as eluent, UV–fluorescence spectroscopy (UV–F) and several mass spectrometric methods, including heatedprobe-ms, gc-ms and MALDI-ms [1–9]. The present paper describes the fractionation of several coal-derived liquids by column chromatography. * Corresponding author. E-mail address: [email protected] (A.A. Herod). 1 Department of Chemistry, Lehigh University, Bethlehem, PA 180153172, USA. Tel.: þ44-207-5111; fax: þ 44-207-594-5604. 2 Instituto de Carboquimica (CSIC), Miguel Luesma Casta´n, 4, Zaragoza 50015, Spain. 3 State Key Laboratory of C1 Chemistry and Technology, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China. q Published first on the web via Fuelfirst.com—http://www.fuelfirst.com
We have used SEC with NMP as eluent, in characterising coal-derived fractions, since it has been shown to operate by a size dependent method with only a small interference from surface or structural effects. In the present work, the permeation (lower molecular mass) limit of the column has been observed to remain consistent for all samples examined, including biomass tars and petroleum derived materials [3,6,10 –18], indicating the absence of surface adsorption effects. Lafleur and Nakagawa [19] first used NMP as eluent for SEC but with an apparently different column packing. Our calibration study, undertaken in preparation for the present sample characterisation work, has been described [3,10] based on standard aromatic materials and fractions derived from a coal tar pitch by SEC [20,21]. UV – fluorescence spectra of unfractionated complex mixtures are similar to the spectra of the more abundant, lowest molecular weight fraction. The less abundant higher molecular mass fractions normally give lower fluorescence quantum yields and lower signal intensity; they are not generally observed without fractionation [1,3,5,6]. In addition, the fluorescence of coal-derived material shows an increasing red-shift with increasing apparent molecular mass (by SEC),
0016-2361/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0016-2361(03)00106-6
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sample was C 85.9%, H 6.77%, N 0.75%. 3. A low temperature coal oil from the Coalite process, an oil 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%. Of the three samples used in the study, this material was expected to show the least thermal degradation. This material has been referred to as low temperature tar (LTT).
decreasing mobility in planar chromatography and decreasing solubility in common solvents [1,4,5,7 – 9, 11,22,23]. The low fluorescence quantum yields are characteristic of large polynuclear aromatic ring systems; similarly, red-shifted fluorescence points to increasingly complex aromatic ring systems. Column chromatography was selected as able to separate large quantities of sample in a single run. This paper describes results from a column chromatographic method developed for separating heavy hydrocarbon liquid mixtures. The preliminary analytical characterisation of fractions from three coal-derived samples has been presented and shows some unusual properties of the largest sized fractions. The analytical results include 13C NMR solid-state spectra of the pitch fractions.
Elemental analyses of the fractions from column chromatography were obtained by micro-analytical methods by determination of C, H and N, with estimation of oxygen by difference, including sulphur. 2.2. Column chromatography
2. Experimental
A method to produce fractions up to 1 g of sample has been developed, based on the use of silica gel with sequential elution using acetonitrile, pyridine and NMP. Separate portions of 10 and 5 g of SIGMA Silica Gel, ˚ average pore size were 15– 40 mm particle size and 60 A weighed separately and heated in a vacuum oven at 200 8C overnight, cooled and stored. The sample (1 g) as a slurry (4 wt%) in acetonitrile was added to 10 g of silica gel and excess solvent removed by rotary evaporation under vacuum. The coated silica was added to a column (20 cm height £ 3 cm i.d.) already containing the other 5 g of the cleaned silica gel, held on a sintered glass plate. Elution of samples was by acetonitrile (2 £ 75 ml), followed by pyridine (2 £ 75 ml), followed by NMP (150 ml with vacuum elution). After elution, sample remained on the silica gel, indicated by a dark colour. Mass balances for the samples were determined and are shown in Table 1. The loss of material in each case was a combination of high mass material retained on the silica and low mass material lost with evaporation of solvent during distillation and in the vacuum oven.
2.1. Samples Three coal-derived samples were investigated. 1. 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 [1,3,4,6 – 8,10,24,25]. Its elemental composition was C 91.4%, H 4.1%, N 1.32%, S 1.76%. 2. Coal liquefaction extract or digest. 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 Table 1 Elemental analyses Sample
Fraction
Pitch Acetonitrile sols Pyridine sols NMP sols PoA liq extract Acetonitrile sols Pyridine sols NMP sols Low temp tar Acetonitrile sols Pyridine sols NMP sols
wt%
%C
%H
%N
Sum CHN
%Odiff
100 26.1 47.9 12.8 100 48.3 30.7 3.5 100 83.4 5 3
91.4 90.7 89.8 73.4 85.9 90.8 87.1 73.6 82.3 82.9 73.6 40.1
4.1 4.8 4.2 5.1 6.8 7.6 5.1 4.9 7.8 8.1 8.1 6.0
1.3 1.3 1.3 6.2 0.8 0.5 2.7 5.4 0.9 1.1 4.1 6.0
96.8 96.8 95.4 84.7 93.5 98.9 94.9 83.9 91.0 92.1 85.8 52.1
3.2 3.2 4.6 15.3 6.5 1.1 5.1 16.1 9.0 7.9 14.2 47.9
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2.3. Size exclusion chromatography Procedures for SEC have been described previously [26 – 31]. SEC using NMP as solvent was carried out using a 5 mm particle size polystyrene/polydivinylbenzene Mixed-D Column (Polymer Laboratories Ltd, Shropshire, UK). The whole extract sample and the fractions recovered from planar chromatographic separation have been examined by SEC with UV-absorbance detection at 280, 300, 350, 370 and 450 nm, with the column operating at 80 8C and a flow rate of 0.5 ml min21. The detectors were an Applied Biosciences Diode Array detector (supplied by Perkin – Elmer, Beaconsfield, UK) with a Perkin – Elmer LC290 variable wavelength detector in series linked to a data acquisition system. The solution injected was prepared to be less than 2 wt% to avoid band broadening caused by the overloading of the excluded region of the column. The porosity range of the Mixed-D column is such that the polystyrene molecular mass standards from 100 to , 300,000 u are retained by the column and elute with a linear relation between log10 molecular mass and elution volume or time. Larger molecular mass polystyrene standards up to 2 million u elute at shorter times with a different relation between molecular mass and time and are classed as excluded from the column porosity; the calibration curve has been shown elsewhere [3,10]. Evidence presented elsewhere [3,10,31] has shown that the polystyrene calibration of the column is a good indicator of molecular mass ranges of coal-derived materials, at least for the material retained by the column. 2.4. UV – fluorescence spectroscopy The procedure has been described in detail elsewhere [22]. The Perkin –Elmer LS50 luminescence spectrometer was set to scan at 240 nm min21 with a slit width of 2.5 nm; synchronous spectra were acquired at a constant wavelength difference of 20 nm. A quartz cell with 1 cm path length was used. The spectrometer featured automatic correction for changes in source intensity as a function of wavelength. Emission, excitation and synchronous spectra of the samples were obtained in NMP; only synchronous spectra are shown. Solutions were diluted with NMP to avoid self-absorption effects: dilution was increased until the fluorescence signal intensity began to decrease. However, it was necessary to examine the fluorescence from fractions eluted by NMP in relatively concentrated solutions because the fluorescence quantum yields were rather low; in these cases, sample was added until the fluorescence signal was significantly greater than the background fluorescence. 2.5. TGA analysis Thermogravimetric analysis of the samples and their fractions was performed to measure their volatility ranges, fixed carbon and ash contents. Although the ash content was
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expected to be small, less than 0.1 wt%, there was a possibility for NMP to dissolve some silica during extraction. In previous work, solubility of silica in NMP was observed at considerably higher temperatures [20]. The instrument was a Perkin – Elmer 7 TGA system, using He as sweep gas. The temperature program was initiated with an isothermal period of 10 min at 30 8C, followed by heating at 20 8C min21 to 105 8C and an isothermal step of 10 min; this was followed by a continuous ramp with heating at 100 8C min21 to 900 8C and holding for 45 min at peak temperature. At 900 8C, the sweep gas stream was switched to air after 35 min, to burn off residual carbon and determine the ash content. Weight changes were measured as volatiles below 100 8C, weight losses from intervals between 100 – 200, 200 –300, 300– 400 and 400 – 900 8C. ‘Fixed carbon’ was taken as the difference between the weight at 900 8C in He and air, with the residue in air as ‘ash’ content. Residual NMP would be expected to come off the sample in the ‘volatiles’ fraction or between 100 and 200 8C, but the relative weight losses at higher temperatures would be expected to reflect differences in structural features of the materials. 2.6. Solid-state
13
C NMR
A Bruker MSL300 spectrometer with a standard Bruker magic angle spinning mass probe at 75.5 MHz (7.05 T). The samples were studied as polycrystalline powder in zirconia rotors (7 nm external diameter) and MAS frequencies at 5 KHz. All spectra were recorded at ambient temperature. Single pulse excitation (SPE) was used with a repetition rate of 120 s to allow a more quantitative analysis. TTMS was used as standard to enable quantitative estimation of carbon content and aromaticity. The acetonitrile soluble fraction was a sticky liquid and it was coated onto silica to form a dry powder for analysis; the proportion of sample to silica was determined by TGA. The spectra were quantified by summing signal and spinning side bands for aromatics, for comparison with the aliphatic and reference signals, after smoothing at 5 or 100 Hz. In addition, 13C CPMAS TOSS and NQSTOSS spectra were obtained.
3. Results and discussion 3.1. The whole samples SEC profiles of the three samples are shown in Fig. 1a –c; corresponding UV – fluorescence spectra are shown in Fig. 2. In SEC, all three samples showed two groups of peaks, the first in the elution time range for material excluded from column porosity (8 – 11 min) and the other in the range where sample was resolved by molecular size. The ranges of elution times observed for the retained material (16 – 23 min) was similar for all three samples. However, the shapes of
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Fig. 1. SEC profiles of the three samples. (a) Pitch, (b) PoA extract, (c) low temperature tar. Curves are of UV absorbance at wavelengths (1) 280, (2) 300, (3) 350, (4) 370, (5) 450 nm. Relative intensity vs. elution time (min).
the distributions within these ranges shifted. The greatest intensity at longer elution times was found for the liquefaction extract (Fig. 1b) and the greatest intensity at the shorter time (within the 16 –23 min interval) in the LTT. The material in the liquefaction extract at the longest time is likely to correspond to the recycle solvent, drawn off as bottoms, during the distillation of the hydrocracked product. In preparing the pitch, the lightest material would have been
removed as creosote and anthracene oils. The LTT had not undergone any distillation apart from its preparation in a low temperature carbonisation process. For each sample, the UV absorbance at 280 nm was the most intense; the relative intensity of the 450 nm absorbance increased on going from the LTT to liquefaction extract, to pitch, reflecting the sequence of increasing severity of thermal treatment. The greatest intensity of excluded material was found in
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Fig. 2. Synchronous UV –fluorescence spectra of the three samples (1) pitch, (2) extract, (3) low temperature tar. Normalised intensity vs. fluorescence wavelength (nm).
the liquefaction extract and the least in the LTT. All three samples showed a valley of near-zero intensity between 11 and 16 min. For each sample, the relationships of chromatograms for different wavelengths observed in the excluded and retained regions were quite different. This observation
strongly suggests that aromatic chromophores observed in the two-elution time zones were structurally different. Aggregates of smaller molecules are not likely to behave in this fashion. The range of masses indicated by the polystyrene calibration for the retained peaks in Fig. 1a – c would be
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from 80 u at 21 min to 3400 u at 16 min; the uncertainty of about 1 min in elution time between standards and polystyrenes of similar masses, would introduce up to a factor two in these estimated masses. The retained peaks of Fig. 1a and b peak at about 20 min and correspond to masses of polystyrene of about 200 u, within the range of gas chromatography. The peak intensity of the retained peak in Fig. 1c at about 19 min corresponds to a polystyrene mass of about 400 u. Clearly, the masses of the bulk of the samples correspond to small molecules rather than to aggregates and reflect the situation indicated by fluorescence depolarisation studies [32] on petroleum residues and coal asphaltenes. The masses corresponding to the excluded peaks cannot be estimated from the polystyrene calibration except in that they elute as if larger than polystyrene of mass greater than 200,000 u. The UV –F profiles in Fig. 2 show that the LTT contains the smallest aromatic chromophores, while the pitch appears to contain the largest.
3.2. The fractions
3.3. Elemental analyses and TGA of fractions Elemental analyses of the samples and fractions are shown in Table 1. The hydrogen contents of the whole samples increased in the order: pitch (4.1%), to liquefaction extract (6.8%), to LTT (7.8%), reflecting the different thermal treatments. The analyses of the fractions suggest a decrease of carbon content in the solvent order acetonitrile . pyridine . NMP. Two explanations may be possible: (1) the fractions contained increasing quantities of inorganic material or (2) the fractions did not burn completely. The TGA results Table 2, shows no significant inorganic residue after complete combustion in air. However, the proportions of carbon involatile at different temperatures in inert atmosphere or fixed at 900 8C, increased with increasing solvent polarity. Therefore, the material of the NMP soluble fractions did not burn completely in the micro-analytical equipment, but formed char. These fractions contain materials which are not volatile or readily pyrolysed, but form char on heating and consist of large molecular mass components. 3.4. SEC profiles of fractions
The relative proportions of the fractions are listed in Table 2. The acetonitrile soluble fractions of the coal liquefaction extract and the LTT were both sticky liquids after solvent removal. The difficulty of completely removing NMP is well known; NMP and some pyrolysis products of NMP have been observed in the pyrolysis-gc-ms profiles of fractions from both planar and column chromatography [5,6,9,23]. The distribution of fractions in the different solvents reflects that, at least in part, the effect of high temperature treatment is to increase the abundance of material soluble only in NMP. This fraction was largest in the coal tar pitch and smallest in the LTT. We have shown elsewhere [16 – 18] that air blowing and thermal treatment of pitch both lead to increasing quantities of heavier materials, observed by larger SEC MM distributions and by the increasing quantities of NMP insoluble material in the samples.
Chromatograms of the fractions are shown Fig. 3. The profiles of the acetonitrile soluble fractions of each sample in Fig. 3 were similar to the chromatograms of the whole samples. For the pitch and the liquefaction extract samples, the NMP soluble fractions contained the greatest proportion of excluded material and the acetonitrile soluble fractions the least, indicating a shift in molecular size from acetonitrile to NMP. For the LTT however, the NMP soluble fraction appears to contain excluded material, which is of smaller size than that in the pyridine solubles. 3.5. UV –fluorescence of the fractions UV – F spectra of the three sets of fractions separated by column chromatography are shown in Fig. 4. The spectra have been height normalised; otherwise, the pyridine soluble fractions and the NMP soluble fractions would
Table 2 TGA data for volatiles, fixed carbon and ash contents of fractions soluble in acetonitrile, pyridine and NMP Sample (weight loss in temp ranges, %)
Vols , 100 8C 100 –200 8C 200 –300 8C 300 –400 8C 400 –900 8C Fixed carbon Ash , ld—less than limit of detection.
Pitch
Liquefaction extract
Low temperature tar
Acet sol
Pyr sol
NMP sol
Acet sol
Pyr sol
NMP sol
Acet sol
Pyr sol
NMP sol
4.9 43.8 26.7 16.8 6.9 1.3 ,ld
0.9 2.3 9.1 14.2 38.7 35.7 ,ld
6.1 7.5 11.9 18.2 21.5 37.5 ,ld
0.3 4.1 18.1 49.4 25.8 2.4 ,ld
0.5 2.3 5.2 6.0 34.3 51.5 ,ld
4.0 2.4 12.1 26.7 25.6 28.7 ,ld
2.0 15.8 36.3 35.5 7.1 3.2 ,ld
5.4 7.9 17.8 19.9 23.6 23.7 ,ld
3.7 5.3 19.3 33.0 26.1 13.5 ,ld
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Fig. 3. SEC profiles of column chromatography fractions, area normalised, 350 nm absorbance. Intensity vs. elution time; curves are: acetonitrile mobile, pyridine mobile, NMP mobile. (a) Pitch fractions, (b) extract fractions, (c) low temperature tar fractions.
have shown signal very close to baseline, when plotted with the acetonitrile solubles. The acetonitrile soluble fractions closely resembled the spectrum for the whole samples in Fig. 2. The pyridine and NMP soluble fractions show spectra with shifts of maximum intensity towards longer wavelengths. These shifts in fluorescence maxima with increasing molecular mass and size follow the same
trend as found [32] for petroleum and coal-derived asphaltenes. 3.6. Solid-state
13
C NMR spectra of pitch fractions
Data from single pulse experiments are shown in Table 3. The results indicate that the carbon contents of the whole
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Fig. 4. Synchronous UV–fluorescence spectra of column chromatography fractions: intensity vs. elution time; curves are: acetonitrile mobile, pyridine mobile, NMP mobile. (a) Pitch fractions, (b) extract fractions, (c) low temperature tar fractions.
pitch, the acetonitrile and pyridine solubles were not completely detected, but carbon in the NMP solubles was largely detected. Somewhat counter-intuitively, the aromaticity of the samples decreased and the aliphatic content increased in
the solvent sequence going from acetonitrile, pyridine and NMP. Fig. 5 shows CP-MAS TOSS spectra of the whole pitch and the NMP soluble fraction. The aliphatic component of the pitch structures has clearly been concentrated in the NMP solubles, as well as the carbonyl
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Table 3 Solid-state 13C NMR data for pitch and fractions, SPE spectra smoothed at 5 and 100 Hz, with peak areas quantified against TTMS for estimation of carbon contents of samples Sample
Total aroms.
Total aliph.
Aliph./arom.
Total signal
C (calc, mg)
Sample (mg)
%C
Aromaticity (%)
Pitcha Pitchb Acet moba Acet mobb Pyr moba Pyr mobb NMP moba NMP mobb
10.2169 11.0864 7.1052 9.9571 15.2173 14.3724 15.3542 18.5698
0.4380 0.4448 0.5107 0.8397 1.2171 1.2408 6.5519 6.7242
0.0429 0.0401 0.0719 0.0843 0.0800 0.0863 0.4267 0.3621
10.6549 11.5312 7.6159 10.7968 16.4344 15.6132 21.9061 25.2940
113.9 123.3 20.8 31.6 127.7 121.3 197.4 199.2
271 271 34.8 34.8 256.9 256.9 215.7 215.7
42 46 60 91 50 47 92 92
96 96 93 92 93 92 70 73
a b
Smoothing at 5 Hz. Smoothing at 100 Hz.
Fig. 5. Solid-state 13C CPMAS TOSS NMR spectra of (a) the whole pitch and (b) the NMP soluble fraction.
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peak. We have already observed a similar trend for planar chromatographic fractions of the same pitch [6]: aliphatic content increased with increasing molecular size as shown by SEC and relative immobility on the chromatographic plate. In addition, the carboxyl peak at about 175 ppm shift increased in relative intensity. The emerging model is an assembly of large polynuclear ring systems, held together by an unspecified network of aliphatic material. 3.7. General discussion Fractions from the separation of all three samples have been examined by pyrolysis-gc-ms [5,6,9,23]. The data for the three un-fractionated samples closely resembled data from their respective acetonitrile-solubles (or acetonitrile-mobile from TLC) fractions. By contrast, pyridine-soluble fractions and NMP-soluble fractions gave very few pyrolysis fragments, aliphatic or aromatic, which could pass through the GC column. The structures of these fractions appear to be entirely different from those envisaged from consideration of GC-MS data of the whole samples. A more complete examination of the fractions produced by column chromatography using methods such as 13C NMR in both solid and solution states, FT-ir by KBr pellet and total internal reflectance, and trace element analysis [20] has been carried out. Models of coal structure described by Ref. [33] and Shinn [34] do not include any large polynuclear aromatic material for which we are beginning to find evidence in these studies, e.g. which would not pass through a chromatographic column, such as material from the pyridine and NMP solubles. These could not be pyrolysed to give smaller fragments able to pass through the GC column. Neither were any aliphatic components incorporated into these models which were as long as the alkanes generated in pyrolysis-gc-ms (C12, C17) or similar to the possibly network of aliphatic material, which failed to pyrolyse into the gc-ms. These large molecules do not fluoresce and would therefore not be detected by fluorescence depolarisation methods in dilute solution [32]. Although more recent considerations [35] of coal structure indicate larger average structural units (400 –2000 u) than hitherto admitted (but within the range of the present work), there is no suggestion in the literature of large aliphatic groups or networks—as begins to be suggested by the current work. We have examined FT-ir and 13C NMR spectra of these fractions [20] and will report the details in subsequent publications. Trace element analyses have shown that the metallic elements concentrate preferentially in the largest molecular size fraction [20,36].
4. Conclusions A column chromatography routine has been developed and has been used to fractionate several coal-derived liquid samples. The method allows the recovery of larger sample fractions compared to planar chromatography; it also allows the closure of mass balances. Multiple macro analyses have been undertaken, using elemental analysis, TGA proximate analysis, NMR and FT-ir in addition to the micro-analytical methods used previously—pyrolysis-gc-ms, SEC, UV – fluorescence, probe-ms and MALDI-ms. The fractions show increasing concentrations of large molecular mass material with increasing polarity of successive eluents. Solid-state 13C NMR and UV – F show progressive structural changes with increasing apparent molecular mass. TGA indicated increasing fixed carbon with increasing molecular size. The fractions eluted with polar solvents did not combust completely in the microanalysis equipment but formed char. The NMP soluble fractions showed a greater propensity for trace metal elements than lighter fractions.
Acknowledgements The authors would like to thank the British Coal Utilization Research Association (BCURA) and the British Government Department of Trade and Industry for financial support under project B44. I. Suelves would like to thank the European Commission for a Marie Curie Fellowship. W. Li would like to thank the Royal Society for a Research Fellowship.
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[25] Herod AA, Zhang S-F, Carter DM, Domin M, Cocksedge MJ, Parker JE, Johnson CAF, John P, Smith GP, Johnson BR, Bartle KD, Kandiyoti R. Rapid Commun Mass Spectrom 1996; 10:171. [26] Herod AA, Kandiyoti R. J. Planar Chromatogr 1996;9:16. [27] Herod AA, Johnson BR, Bartle KD, Carter DM, Cocksedge MJ, Domin M, Kandiyoti R. Rapid Commun Mass Spectrom 1995;9: 1446. [28] Lazaro M-J, Herod AA, Cocksedge MJ, Domin M, Kandiyoti R. Fuel 1997;76:1225. [29] Herod AA, Shearman J, Lazaro M-J, Johnson BR, Bartle KD, Kandiyoti R. Energy Fuels 1998;12:174. [30] Herod AA, Zhang S-F, Kandiyoti R, Johnson BR, Bartle KD. Energy Fuels 1996;10:743. [31] Johnson BR, Bartle KD, Domin M, Herod AA, Kandiyoti R. Fuel 1998;77:933. [32] Groenzin H, Mullins OC. Energy Fuels 2000;14:677. [33] Davidson RM. Molecular structure of coal. Report No ICTIS/TR 08, IEA Coal Research, London; 1980. [34] Shinn JH. Fuel 1984;63:1187–96. [35] Krzesinska M. Energy Fuels 2001;15:930–5. [36] Herod AA, George A, Islas CA, Suelves I, Kandiyoti R. Energy & Fuels 2003;17:ef020267e on web.
The unusual properties of high mass materials from coal-derived liquidsq C.A. Islas1, I. Suelves2, W. Li3, T.J. Morgan, A.A. Herod*, R. Kandiyoti Department of Chemical Engineering and Chemical Technology, Imperial College London, University of London, South Kensington Campus, Prince Consort Road, London SW7 2BY, UK Received 31 May 2002; revised 20 March 2003; accepted 30 March 2003; available online 9 May 2003
Abstract This short paper highlights the unusual properties of the high-mass material of coal liquids isolated by their insolubility in pyridine and solubility in NMP. The separation has been achieved by a column chromatography method. One gram quantity have been processed and near quantitative recovery of the sample as fractions has been achieved. This fractionation permitted recourse to a broad range of analytical methods, including some (e.g. 13C NMR), which require large sample sizes. Multiple macro analyses have been undertaken, using elemental analysis, TGA proximate analysis, NMR and FT-ir in addition to the micro-analytical methods used previously—pyrolysis-gc-ms, SEC, UV – fluorescence, probe-ms and MALDI-ms. The fractions show increasing concentrations of large molecular mass material with increasing polarity of successive eluents used in the fractionation. Evidence from solid-state 13C NMR and UV – fluorescence spectroscopy show progressive structural changes with increasing apparent molecular mass. q 2003 Elsevier Science Ltd. All rights reserved. Keywords: Fractionation methods; Size exclusion chromatography; UV–fluorescence; NMR
1. Introduction Fractionation is usually a necessary first step in identifying the presence of special classes of materials, found in relatively low concentrations in complex mixtures. It has proved particular use in identifying high molecular mass materials in various heavy hydrocarbon liquids. In our more recent work, planar chromatography has been used as the preferred fractionation method. The analytical characterisation of the separated fractions has relied heavily on size exclusion chromatography (SEC) with 1-methyl-2-pyrrolidinone (NMP) as eluent, UV–fluorescence spectroscopy (UV–F) and several mass spectrometric methods, including heatedprobe-ms, gc-ms and MALDI-ms [1–9]. The present paper describes the fractionation of several coal-derived liquids by column chromatography. * Corresponding author. E-mail address: [email protected] (A.A. Herod). 1 Department of Chemistry, Lehigh University, Bethlehem, PA 180153172, USA. Tel.: þ44-207-5111; fax: þ 44-207-594-5604. 2 Instituto de Carboquimica (CSIC), Miguel Luesma Casta´n, 4, Zaragoza 50015, Spain. 3 State Key Laboratory of C1 Chemistry and Technology, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China. q Published first on the web via Fuelfirst.com—http://www.fuelfirst.com
We have used SEC with NMP as eluent, in characterising coal-derived fractions, since it has been shown to operate by a size dependent method with only a small interference from surface or structural effects. In the present work, the permeation (lower molecular mass) limit of the column has been observed to remain consistent for all samples examined, including biomass tars and petroleum derived materials [3,6,10 –18], indicating the absence of surface adsorption effects. Lafleur and Nakagawa [19] first used NMP as eluent for SEC but with an apparently different column packing. Our calibration study, undertaken in preparation for the present sample characterisation work, has been described [3,10] based on standard aromatic materials and fractions derived from a coal tar pitch by SEC [20,21]. UV – fluorescence spectra of unfractionated complex mixtures are similar to the spectra of the more abundant, lowest molecular weight fraction. The less abundant higher molecular mass fractions normally give lower fluorescence quantum yields and lower signal intensity; they are not generally observed without fractionation [1,3,5,6]. In addition, the fluorescence of coal-derived material shows an increasing red-shift with increasing apparent molecular mass (by SEC),
0016-2361/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0016-2361(03)00106-6
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sample was C 85.9%, H 6.77%, N 0.75%. 3. A low temperature coal oil from the Coalite process, an oil 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%. Of the three samples used in the study, this material was expected to show the least thermal degradation. This material has been referred to as low temperature tar (LTT).
decreasing mobility in planar chromatography and decreasing solubility in common solvents [1,4,5,7 – 9, 11,22,23]. The low fluorescence quantum yields are characteristic of large polynuclear aromatic ring systems; similarly, red-shifted fluorescence points to increasingly complex aromatic ring systems. Column chromatography was selected as able to separate large quantities of sample in a single run. This paper describes results from a column chromatographic method developed for separating heavy hydrocarbon liquid mixtures. The preliminary analytical characterisation of fractions from three coal-derived samples has been presented and shows some unusual properties of the largest sized fractions. The analytical results include 13C NMR solid-state spectra of the pitch fractions.
Elemental analyses of the fractions from column chromatography were obtained by micro-analytical methods by determination of C, H and N, with estimation of oxygen by difference, including sulphur. 2.2. Column chromatography
2. Experimental
A method to produce fractions up to 1 g of sample has been developed, based on the use of silica gel with sequential elution using acetonitrile, pyridine and NMP. Separate portions of 10 and 5 g of SIGMA Silica Gel, ˚ average pore size were 15– 40 mm particle size and 60 A weighed separately and heated in a vacuum oven at 200 8C overnight, cooled and stored. The sample (1 g) as a slurry (4 wt%) in acetonitrile was added to 10 g of silica gel and excess solvent removed by rotary evaporation under vacuum. The coated silica was added to a column (20 cm height £ 3 cm i.d.) already containing the other 5 g of the cleaned silica gel, held on a sintered glass plate. Elution of samples was by acetonitrile (2 £ 75 ml), followed by pyridine (2 £ 75 ml), followed by NMP (150 ml with vacuum elution). After elution, sample remained on the silica gel, indicated by a dark colour. Mass balances for the samples were determined and are shown in Table 1. The loss of material in each case was a combination of high mass material retained on the silica and low mass material lost with evaporation of solvent during distillation and in the vacuum oven.
2.1. Samples Three coal-derived samples were investigated. 1. 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 [1,3,4,6 – 8,10,24,25]. Its elemental composition was C 91.4%, H 4.1%, N 1.32%, S 1.76%. 2. Coal liquefaction extract or digest. 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 Table 1 Elemental analyses Sample
Fraction
Pitch Acetonitrile sols Pyridine sols NMP sols PoA liq extract Acetonitrile sols Pyridine sols NMP sols Low temp tar Acetonitrile sols Pyridine sols NMP sols
wt%
%C
%H
%N
Sum CHN
%Odiff
100 26.1 47.9 12.8 100 48.3 30.7 3.5 100 83.4 5 3
91.4 90.7 89.8 73.4 85.9 90.8 87.1 73.6 82.3 82.9 73.6 40.1
4.1 4.8 4.2 5.1 6.8 7.6 5.1 4.9 7.8 8.1 8.1 6.0
1.3 1.3 1.3 6.2 0.8 0.5 2.7 5.4 0.9 1.1 4.1 6.0
96.8 96.8 95.4 84.7 93.5 98.9 94.9 83.9 91.0 92.1 85.8 52.1
3.2 3.2 4.6 15.3 6.5 1.1 5.1 16.1 9.0 7.9 14.2 47.9
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2.3. Size exclusion chromatography Procedures for SEC have been described previously [26 – 31]. SEC using NMP as solvent was carried out using a 5 mm particle size polystyrene/polydivinylbenzene Mixed-D Column (Polymer Laboratories Ltd, Shropshire, UK). The whole extract sample and the fractions recovered from planar chromatographic separation have been examined by SEC with UV-absorbance detection at 280, 300, 350, 370 and 450 nm, with the column operating at 80 8C and a flow rate of 0.5 ml min21. The detectors were an Applied Biosciences Diode Array detector (supplied by Perkin – Elmer, Beaconsfield, UK) with a Perkin – Elmer LC290 variable wavelength detector in series linked to a data acquisition system. The solution injected was prepared to be less than 2 wt% to avoid band broadening caused by the overloading of the excluded region of the column. The porosity range of the Mixed-D column is such that the polystyrene molecular mass standards from 100 to , 300,000 u are retained by the column and elute with a linear relation between log10 molecular mass and elution volume or time. Larger molecular mass polystyrene standards up to 2 million u elute at shorter times with a different relation between molecular mass and time and are classed as excluded from the column porosity; the calibration curve has been shown elsewhere [3,10]. Evidence presented elsewhere [3,10,31] has shown that the polystyrene calibration of the column is a good indicator of molecular mass ranges of coal-derived materials, at least for the material retained by the column. 2.4. UV – fluorescence spectroscopy The procedure has been described in detail elsewhere [22]. The Perkin –Elmer LS50 luminescence spectrometer was set to scan at 240 nm min21 with a slit width of 2.5 nm; synchronous spectra were acquired at a constant wavelength difference of 20 nm. A quartz cell with 1 cm path length was used. The spectrometer featured automatic correction for changes in source intensity as a function of wavelength. Emission, excitation and synchronous spectra of the samples were obtained in NMP; only synchronous spectra are shown. Solutions were diluted with NMP to avoid self-absorption effects: dilution was increased until the fluorescence signal intensity began to decrease. However, it was necessary to examine the fluorescence from fractions eluted by NMP in relatively concentrated solutions because the fluorescence quantum yields were rather low; in these cases, sample was added until the fluorescence signal was significantly greater than the background fluorescence. 2.5. TGA analysis Thermogravimetric analysis of the samples and their fractions was performed to measure their volatility ranges, fixed carbon and ash contents. Although the ash content was
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expected to be small, less than 0.1 wt%, there was a possibility for NMP to dissolve some silica during extraction. In previous work, solubility of silica in NMP was observed at considerably higher temperatures [20]. The instrument was a Perkin – Elmer 7 TGA system, using He as sweep gas. The temperature program was initiated with an isothermal period of 10 min at 30 8C, followed by heating at 20 8C min21 to 105 8C and an isothermal step of 10 min; this was followed by a continuous ramp with heating at 100 8C min21 to 900 8C and holding for 45 min at peak temperature. At 900 8C, the sweep gas stream was switched to air after 35 min, to burn off residual carbon and determine the ash content. Weight changes were measured as volatiles below 100 8C, weight losses from intervals between 100 – 200, 200 –300, 300– 400 and 400 – 900 8C. ‘Fixed carbon’ was taken as the difference between the weight at 900 8C in He and air, with the residue in air as ‘ash’ content. Residual NMP would be expected to come off the sample in the ‘volatiles’ fraction or between 100 and 200 8C, but the relative weight losses at higher temperatures would be expected to reflect differences in structural features of the materials. 2.6. Solid-state
13
C NMR
A Bruker MSL300 spectrometer with a standard Bruker magic angle spinning mass probe at 75.5 MHz (7.05 T). The samples were studied as polycrystalline powder in zirconia rotors (7 nm external diameter) and MAS frequencies at 5 KHz. All spectra were recorded at ambient temperature. Single pulse excitation (SPE) was used with a repetition rate of 120 s to allow a more quantitative analysis. TTMS was used as standard to enable quantitative estimation of carbon content and aromaticity. The acetonitrile soluble fraction was a sticky liquid and it was coated onto silica to form a dry powder for analysis; the proportion of sample to silica was determined by TGA. The spectra were quantified by summing signal and spinning side bands for aromatics, for comparison with the aliphatic and reference signals, after smoothing at 5 or 100 Hz. In addition, 13C CPMAS TOSS and NQSTOSS spectra were obtained.
3. Results and discussion 3.1. The whole samples SEC profiles of the three samples are shown in Fig. 1a –c; corresponding UV – fluorescence spectra are shown in Fig. 2. In SEC, all three samples showed two groups of peaks, the first in the elution time range for material excluded from column porosity (8 – 11 min) and the other in the range where sample was resolved by molecular size. The ranges of elution times observed for the retained material (16 – 23 min) was similar for all three samples. However, the shapes of
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Fig. 1. SEC profiles of the three samples. (a) Pitch, (b) PoA extract, (c) low temperature tar. Curves are of UV absorbance at wavelengths (1) 280, (2) 300, (3) 350, (4) 370, (5) 450 nm. Relative intensity vs. elution time (min).
the distributions within these ranges shifted. The greatest intensity at longer elution times was found for the liquefaction extract (Fig. 1b) and the greatest intensity at the shorter time (within the 16 –23 min interval) in the LTT. The material in the liquefaction extract at the longest time is likely to correspond to the recycle solvent, drawn off as bottoms, during the distillation of the hydrocracked product. In preparing the pitch, the lightest material would have been
removed as creosote and anthracene oils. The LTT had not undergone any distillation apart from its preparation in a low temperature carbonisation process. For each sample, the UV absorbance at 280 nm was the most intense; the relative intensity of the 450 nm absorbance increased on going from the LTT to liquefaction extract, to pitch, reflecting the sequence of increasing severity of thermal treatment. The greatest intensity of excluded material was found in
C.A. Islas et al. / Fuel 82 (2003) 1813–1823
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Fig. 2. Synchronous UV –fluorescence spectra of the three samples (1) pitch, (2) extract, (3) low temperature tar. Normalised intensity vs. fluorescence wavelength (nm).
the liquefaction extract and the least in the LTT. All three samples showed a valley of near-zero intensity between 11 and 16 min. For each sample, the relationships of chromatograms for different wavelengths observed in the excluded and retained regions were quite different. This observation
strongly suggests that aromatic chromophores observed in the two-elution time zones were structurally different. Aggregates of smaller molecules are not likely to behave in this fashion. The range of masses indicated by the polystyrene calibration for the retained peaks in Fig. 1a – c would be
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from 80 u at 21 min to 3400 u at 16 min; the uncertainty of about 1 min in elution time between standards and polystyrenes of similar masses, would introduce up to a factor two in these estimated masses. The retained peaks of Fig. 1a and b peak at about 20 min and correspond to masses of polystyrene of about 200 u, within the range of gas chromatography. The peak intensity of the retained peak in Fig. 1c at about 19 min corresponds to a polystyrene mass of about 400 u. Clearly, the masses of the bulk of the samples correspond to small molecules rather than to aggregates and reflect the situation indicated by fluorescence depolarisation studies [32] on petroleum residues and coal asphaltenes. The masses corresponding to the excluded peaks cannot be estimated from the polystyrene calibration except in that they elute as if larger than polystyrene of mass greater than 200,000 u. The UV –F profiles in Fig. 2 show that the LTT contains the smallest aromatic chromophores, while the pitch appears to contain the largest.
3.2. The fractions
3.3. Elemental analyses and TGA of fractions Elemental analyses of the samples and fractions are shown in Table 1. The hydrogen contents of the whole samples increased in the order: pitch (4.1%), to liquefaction extract (6.8%), to LTT (7.8%), reflecting the different thermal treatments. The analyses of the fractions suggest a decrease of carbon content in the solvent order acetonitrile . pyridine . NMP. Two explanations may be possible: (1) the fractions contained increasing quantities of inorganic material or (2) the fractions did not burn completely. The TGA results Table 2, shows no significant inorganic residue after complete combustion in air. However, the proportions of carbon involatile at different temperatures in inert atmosphere or fixed at 900 8C, increased with increasing solvent polarity. Therefore, the material of the NMP soluble fractions did not burn completely in the micro-analytical equipment, but formed char. These fractions contain materials which are not volatile or readily pyrolysed, but form char on heating and consist of large molecular mass components. 3.4. SEC profiles of fractions
The relative proportions of the fractions are listed in Table 2. The acetonitrile soluble fractions of the coal liquefaction extract and the LTT were both sticky liquids after solvent removal. The difficulty of completely removing NMP is well known; NMP and some pyrolysis products of NMP have been observed in the pyrolysis-gc-ms profiles of fractions from both planar and column chromatography [5,6,9,23]. The distribution of fractions in the different solvents reflects that, at least in part, the effect of high temperature treatment is to increase the abundance of material soluble only in NMP. This fraction was largest in the coal tar pitch and smallest in the LTT. We have shown elsewhere [16 – 18] that air blowing and thermal treatment of pitch both lead to increasing quantities of heavier materials, observed by larger SEC MM distributions and by the increasing quantities of NMP insoluble material in the samples.
Chromatograms of the fractions are shown Fig. 3. The profiles of the acetonitrile soluble fractions of each sample in Fig. 3 were similar to the chromatograms of the whole samples. For the pitch and the liquefaction extract samples, the NMP soluble fractions contained the greatest proportion of excluded material and the acetonitrile soluble fractions the least, indicating a shift in molecular size from acetonitrile to NMP. For the LTT however, the NMP soluble fraction appears to contain excluded material, which is of smaller size than that in the pyridine solubles. 3.5. UV –fluorescence of the fractions UV – F spectra of the three sets of fractions separated by column chromatography are shown in Fig. 4. The spectra have been height normalised; otherwise, the pyridine soluble fractions and the NMP soluble fractions would
Table 2 TGA data for volatiles, fixed carbon and ash contents of fractions soluble in acetonitrile, pyridine and NMP Sample (weight loss in temp ranges, %)
Vols , 100 8C 100 –200 8C 200 –300 8C 300 –400 8C 400 –900 8C Fixed carbon Ash , ld—less than limit of detection.
Pitch
Liquefaction extract
Low temperature tar
Acet sol
Pyr sol
NMP sol
Acet sol
Pyr sol
NMP sol
Acet sol
Pyr sol
NMP sol
4.9 43.8 26.7 16.8 6.9 1.3 ,ld
0.9 2.3 9.1 14.2 38.7 35.7 ,ld
6.1 7.5 11.9 18.2 21.5 37.5 ,ld
0.3 4.1 18.1 49.4 25.8 2.4 ,ld
0.5 2.3 5.2 6.0 34.3 51.5 ,ld
4.0 2.4 12.1 26.7 25.6 28.7 ,ld
2.0 15.8 36.3 35.5 7.1 3.2 ,ld
5.4 7.9 17.8 19.9 23.6 23.7 ,ld
3.7 5.3 19.3 33.0 26.1 13.5 ,ld
C.A. Islas et al. / Fuel 82 (2003) 1813–1823
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Fig. 3. SEC profiles of column chromatography fractions, area normalised, 350 nm absorbance. Intensity vs. elution time; curves are: acetonitrile mobile, pyridine mobile, NMP mobile. (a) Pitch fractions, (b) extract fractions, (c) low temperature tar fractions.
have shown signal very close to baseline, when plotted with the acetonitrile solubles. The acetonitrile soluble fractions closely resembled the spectrum for the whole samples in Fig. 2. The pyridine and NMP soluble fractions show spectra with shifts of maximum intensity towards longer wavelengths. These shifts in fluorescence maxima with increasing molecular mass and size follow the same
trend as found [32] for petroleum and coal-derived asphaltenes. 3.6. Solid-state
13
C NMR spectra of pitch fractions
Data from single pulse experiments are shown in Table 3. The results indicate that the carbon contents of the whole
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Fig. 4. Synchronous UV–fluorescence spectra of column chromatography fractions: intensity vs. elution time; curves are: acetonitrile mobile, pyridine mobile, NMP mobile. (a) Pitch fractions, (b) extract fractions, (c) low temperature tar fractions.
pitch, the acetonitrile and pyridine solubles were not completely detected, but carbon in the NMP solubles was largely detected. Somewhat counter-intuitively, the aromaticity of the samples decreased and the aliphatic content increased in
the solvent sequence going from acetonitrile, pyridine and NMP. Fig. 5 shows CP-MAS TOSS spectra of the whole pitch and the NMP soluble fraction. The aliphatic component of the pitch structures has clearly been concentrated in the NMP solubles, as well as the carbonyl
C.A. Islas et al. / Fuel 82 (2003) 1813–1823
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Table 3 Solid-state 13C NMR data for pitch and fractions, SPE spectra smoothed at 5 and 100 Hz, with peak areas quantified against TTMS for estimation of carbon contents of samples Sample
Total aroms.
Total aliph.
Aliph./arom.
Total signal
C (calc, mg)
Sample (mg)
%C
Aromaticity (%)
Pitcha Pitchb Acet moba Acet mobb Pyr moba Pyr mobb NMP moba NMP mobb
10.2169 11.0864 7.1052 9.9571 15.2173 14.3724 15.3542 18.5698
0.4380 0.4448 0.5107 0.8397 1.2171 1.2408 6.5519 6.7242
0.0429 0.0401 0.0719 0.0843 0.0800 0.0863 0.4267 0.3621
10.6549 11.5312 7.6159 10.7968 16.4344 15.6132 21.9061 25.2940
113.9 123.3 20.8 31.6 127.7 121.3 197.4 199.2
271 271 34.8 34.8 256.9 256.9 215.7 215.7
42 46 60 91 50 47 92 92
96 96 93 92 93 92 70 73
a b
Smoothing at 5 Hz. Smoothing at 100 Hz.
Fig. 5. Solid-state 13C CPMAS TOSS NMR spectra of (a) the whole pitch and (b) the NMP soluble fraction.
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peak. We have already observed a similar trend for planar chromatographic fractions of the same pitch [6]: aliphatic content increased with increasing molecular size as shown by SEC and relative immobility on the chromatographic plate. In addition, the carboxyl peak at about 175 ppm shift increased in relative intensity. The emerging model is an assembly of large polynuclear ring systems, held together by an unspecified network of aliphatic material. 3.7. General discussion Fractions from the separation of all three samples have been examined by pyrolysis-gc-ms [5,6,9,23]. The data for the three un-fractionated samples closely resembled data from their respective acetonitrile-solubles (or acetonitrile-mobile from TLC) fractions. By contrast, pyridine-soluble fractions and NMP-soluble fractions gave very few pyrolysis fragments, aliphatic or aromatic, which could pass through the GC column. The structures of these fractions appear to be entirely different from those envisaged from consideration of GC-MS data of the whole samples. A more complete examination of the fractions produced by column chromatography using methods such as 13C NMR in both solid and solution states, FT-ir by KBr pellet and total internal reflectance, and trace element analysis [20] has been carried out. Models of coal structure described by Ref. [33] and Shinn [34] do not include any large polynuclear aromatic material for which we are beginning to find evidence in these studies, e.g. which would not pass through a chromatographic column, such as material from the pyridine and NMP solubles. These could not be pyrolysed to give smaller fragments able to pass through the GC column. Neither were any aliphatic components incorporated into these models which were as long as the alkanes generated in pyrolysis-gc-ms (C12, C17) or similar to the possibly network of aliphatic material, which failed to pyrolyse into the gc-ms. These large molecules do not fluoresce and would therefore not be detected by fluorescence depolarisation methods in dilute solution [32]. Although more recent considerations [35] of coal structure indicate larger average structural units (400 –2000 u) than hitherto admitted (but within the range of the present work), there is no suggestion in the literature of large aliphatic groups or networks—as begins to be suggested by the current work. We have examined FT-ir and 13C NMR spectra of these fractions [20] and will report the details in subsequent publications. Trace element analyses have shown that the metallic elements concentrate preferentially in the largest molecular size fraction [20,36].
4. Conclusions A column chromatography routine has been developed and has been used to fractionate several coal-derived liquid samples. The method allows the recovery of larger sample fractions compared to planar chromatography; it also allows the closure of mass balances. Multiple macro analyses have been undertaken, using elemental analysis, TGA proximate analysis, NMR and FT-ir in addition to the micro-analytical methods used previously—pyrolysis-gc-ms, SEC, UV – fluorescence, probe-ms and MALDI-ms. The fractions show increasing concentrations of large molecular mass material with increasing polarity of successive eluents. Solid-state 13C NMR and UV – F show progressive structural changes with increasing apparent molecular mass. TGA indicated increasing fixed carbon with increasing molecular size. The fractions eluted with polar solvents did not combust completely in the microanalysis equipment but formed char. The NMP soluble fractions showed a greater propensity for trace metal elements than lighter fractions.
Acknowledgements The authors would like to thank the British Coal Utilization Research Association (BCURA) and the British Government Department of Trade and Industry for financial support under project B44. I. Suelves would like to thank the European Commission for a Marie Curie Fellowship. W. Li would like to thank the Royal Society for a Research Fellowship.
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