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Energy & Fuels 2008, 22, 1824–1835
Optimization of 1H and 13C NMR Methods for Structural Characterization of Acetone and Pyridine Soluble/Insoluble Fractions of a Coal Tar Pitch Trevor J. Morgan,*,† Anthe George,† David B. Davis,‡ Alan A. Herod,† and Rafael Kandiyoti† Department of Chemical Engineering and Chemical Technology, Imperial College London, London SW7 2AZ, United Kingdom, and Department of Chemistry, Birkbeck College, UniVersity of London, London WC1E 7HX, United Kingdom ReceiVed NoVember 28, 2007. ReVised Manuscript ReceiVed January 20, 2008
1H and 13C high-resolution liquid-state NMR methods were used for the quantitative characterization of different molecular weight fractions of a coal tar pitch (CTP). Three fractions were studied: pitch acetone solubles (PAS), pitch pyridine soluble-acetone insolubles (PPS), and pitch pyridine insolubles (PPI). Standard liquid-state NMR methods were modified and calibrated for use with undeuterated quinoline or undeuterated 1-methyl-2-pyrrolidinone (NMP) as the solvent. This made it possible to calculate the average structural parameters for the higher molecular weight (MW) fractions of the coal tar pitch. Quantitative comparisons of structural differences between the solubility-separated fractions of the pitch are reported. The aromaticity and the average number of aromatic rings per polynuclear aromatic structure were both found to decrease with increasing solubility. Similarly, pericondensed and all other quaternary carbon species were found to decrease with increasing solubility. This suggests that “continental” type structures become more dominant as the solvent solubility of these coal derived fractions diminishes. The estimated average number of aromatic rings ranged from 1 to 2 rings in the PAS fraction, 4 to 21 rings in the PPS fraction, and 11 to 210 rings in the PPI fraction. These ring-numbers were directly related to the number average molecular mass (Mn) assigned to the particular fraction in the average structural parameter (ASP) calculations. The lower-limit of the Mn values was derived from the ASP calculations as 200, 450, and 6200 u for the PAS, PPS, and PPI fractions, respectively.
Introduction Coal tar pitch (CTP) is an increasingly important commodity with numerous high value end uses such as binders for copolymers, electrodes for aluminum processing and steel manufacture, supercapacitors for fuel cells, and in the production of carbon materials.1–3 In Europe, demand for pitch has been outstripping supply in recent years.1 Thus, the upgrading of waste materials, such as anthracene oil, into pitchlike materials has become an economically realistic possibility.3,4 At present, only about 20% of the components present in a typical CTP can be readily identified by standard analytical techniques such as gas chromatography5 (GC). The characterization of the heavier, more complex hydrocarbons has led to debate and to a level of uncertainty in the literature.5,6,8–15 Materials beyond the GC-range have been examined by using * To whom correspondence should be addressed. E-mail: t.j.morgan@ imperial.ac.uk. † Imperial College London. ‡ University of London. (1) Turner, N. R. JOM 1993, 45 (11), 39. (2) Wombles, R. H.; Kiser, M. D. J. Light Met. 2000, 537. (3) Research fund for coal and steel, Contract No. RFCR-CT-200500004. Ecopitch project. (4) Bermejo, J.; Fernández, A. L.; Granda, M.; Rubiera, F.; Suelves, I.; Menéndez, R. Fuel 2001, 80 (9), 1229. (5) Kandiyoti, R.; Herod, A. A.; Bartle, K. Solid Fuels and HeaVy Hydrocarbon Liquids; Elsevier: Oxford, U.K., 2006; Chapters 7-8. (6) Karaca, F.; Islas, C. A.; Millan, M.; Behrouzi, M.; Morgan, T. J.; Herod, A. A.; Kandiyoti, R. Energy Fuels 2004, 18 (3), 778. (7) Islas, C. A.; Suelves, I.; Li, W.; Morgan, T. J.; Herod, A. A.; Kandiyoti, R. Fuel 2003, 82, 1813.
a wide variety of techniques, including size exclusion chromatography (SEC), UV-fluorescence spectrometry (UV-F), nuclear magnetic resonance (NMR), FT-IR, and various mass spectrometric techniques, among which laser desorption/ionization mass spectrometry (LD-MS) has been found particularly useful.9,16–18 There are numerous issues that need to be resolved regarding the accuracy of these methods, particularly relating to the heavier and more insoluble fractions of CTPs. Nevertheless, taken together, these techniques are useful in showing significant qualitative differences between the structures and the molecular mass distributions of solvent separated pitch fractions. Increasing number average molecular mass (Mn) (as observed by SEC) (8) Morgan, T. J.; Millan, M.; Behrouzi, M.; Herod, A. A.; Kandiyoti, R. Energy Fuels 2005, 19, 164. (9) Millan, M.; Morgan, T. J.; Behrouzi, M.; Karaca, F.; Galmes, C.; Herod, A. A.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 2005, 19, 1867–1873. (10) Herod, A. A.; Kandiyoti, R.; Bartle, K. D. Energy Fuels 2007, 21 (4), 2176. (11) Herod, A. A.; Kandiyoti, R.; Bartle, K. D. Fuel 2006, 85 (12–13), 1950. (12) Mullins, O. C. Fuel 2007, 86 (1–2), 309. (13) Tanaka, R.; Sato, S.; Takanohashi, T.; Hunt, J.; Winans, R. Energy Fuels 2004, 18, 1405. (14) Acevedo, S.; Gutierrez, L. B.; Negrin, G.; Pereira, J. C.; Mendez, B.; Delolme, F.; Dessalces, G.; Broseta, D. Energy Fuels 2005, 19, 1548. (15) Hortal, A. R.; Hurtado, P.; Martinez-Haya, B.; Mullins, O. C. Energy Fuels 2007, 21, 2863. (16) Islas, C. A.; Suelves, I.; Millan, M.; Apicella, B.; Lazaro, M. J.; Herod, A. A.; Kandiyoti, R. J. Sep. Sci. 2003, 26, 1. (17) Herod, A. A.; Millan, M.; Morgan, T. J.; Li, W.; Feng, J.; Kandiyoti, R. Eur. J. Mass Spectrom. 2005, 11, 429–442. (18) Seki, H.; Kumata, F. Energy Fuels 2000, 14, 980.
10.1021/ef700715w CCC: $40.75 2008 American Chemical Society Published on Web 03/18/2008
Structural Characterization of PAS, PPS, and PPI
and a red shift in fluorescence spectra have been observed, with decreasing solvent-solubility of sample fractions. The pyridine insoluble fractions (up to 30% by weight) of CTPs show evidence for the presence of molecules above the >10 000 u level.9 However, above the GC-range,21,42 limited information is available on the actual structural features of these complex mixtures. NMR has been found useful in identifying and quantifying structural features in complex mixtures beyond the range of GC. This is best achieved through average structural parameter (ASP) calculations by using data from liquid-state NMR, combined with elemental analysis and measurements of the Mn.19,20 Liquidstate NMR methods are generally restricted to the analysis of samples soluble in common NMR solvents,6,20,22–29 such as chloroform, carbon disulphide, dimethyl sulfoxide (DMSO), or tetrachloroethane (TCE). These solvents are only capable of dissolving up to perhaps 60% of a typical pitch.30 In particular, they are unable to dissolve the high-mass, more complex materials in the heavier factions. The somewhat exotic solvent mixture S2Cl2/SO2Cl2 has been previously used in NMR analyses of materials insoluble in the more common solvents.31,32 However, this solvent mixture has been found to react with components of pitch, making the data difficult to interpret. Previous work in this laboratory has investigated the use of several solvents in attempting to overcome solubility issues. The solvents used include perdeuterated 1-methyl-2-pyrrolidinone (NMP-d9) or perdeuterated pyridine.6 Mixtures of NMP-d9 and deuterated chloroform or NMP-d9 and DMSO have also been attempted as NMR solvents. Results from these previous analyses are limited to the analysis of the aromatic part of the sample due to overlap from the solvent peaks in the aliphatic region of the NMR spectra. Hence, it is not possible to calculate aromaticity. Solid-state NMR has often been used to side step some of these solubility issues when examining the heavier fractions of pitchlike materials.6,33–36 However, these methods do not always show the levels of resolution or accuracy necessary for identifying differences between different fractions of a CTP. A dipolar dephasing method has been reported36 that can achieve higher levels of quantification. In such applications, however, experi(19) Rongbao, L.; Zengmin, S.; Bailing, L. Fuel 1988, 67, 565. (20) Dickinson, E. M. Fuel 1980, 59, 290. (21) Herod, A. A.; Islas, C.; Lazaro, M. J.; Dubau, C.; Carter, J. F.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 1999, 13, 201. (22) Randell, J. C. NMR and Macromolecules; ACS Symposium Series 247; American Chemical Society: Washington, DC, 1984, p 199. (23) Johnson, L. F.; Heatley, F.; Bovey, F. A. Macromolecules 1970, 3, 175. (24) Heatley, F.; Bovey, F. A. Macromolecules 1968, 1, 301. (25) Bovey, F. A.; Mirau, P. NMR of Polymers; Academic Press: San Diego, CA, 1996. (26) Diaz, C.; Blanco, C. G. Energy Fuels 2003, 17, 907. (27) Guillen, M.; Diaz, C.; Blanco, J. Fuel Process. Technol. 1998, 58, 1. (28) Cookson, D.; Smith, B. J. Magn. Reson. 1984, 57, 355. (29) Netzel, D. A. Anal. Chem. 1987, 59, 1775. (30) Guillen, M.; Blanco, J.; Canga, J.; Blanco, C. Energy Fuels 1991, 5, 188. (31) Greinke, R. A. Fuel 1984, 63, 1374. (32) Twigg, A. N.; Taylor, R.; Marsh, K.; Marr, G. Fuel 1987, 66, 28. (33) Strom, D. A.; Edwards, J. C.; Decanio, S. J.; Sheu, E. Y. Energy Fuels 1994, 8, 561. (34) Snape, C. E.; Axelson, D. A.; Botto, R. E.; Delpuech, J. J.; Tekely, P.; Gerstein, B. C.; Pruski, M.; Maciel, G. E.; Wilson, M. A. Fuel 1989, 68, 547. (35) Botto, R. E.; Wilson, R.; Winans, R. E. Energy Fuels 1987, 1, 173. (36) Andersen, J. M.; Luengo, C. A.; Moinelo, S. R.; Garcia, R.; Snape, C. E. Energy Fuels 1998, 12, 524. (37) Herod, A. A.; Islas, C. A.; Suelves, I.; Carter, J. F.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 2000, 14, 1766.
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mental times are long (>24 h). Furthermore, the resolutions achieved were not found to be sufficiently high for the purposes of the present study. The work outlined in this paper focuses on the development of liquid-state NMR methods that allow the less soluble fractions of a CTP (PPS and PPI) to be analyzed by using well established ASP calculations.19,20,26,27,37–39 The study focused on quantitative NMR measurements on four types of quaternary aromatic carbon: (i) aromatic carbon substituted with aromatic groups, (ii) aromatic carbon substituted with aliphatic groups, and (iii) catacondensed and (iv) pericondensed carbon moieties.19 These parameters are essential in understanding the most likely structural conformations taken on by components of different pitch fractions. At issue is whether this combination of methods can provide indications for the degrees of condensation (i.e., condensation indices) within sample fractions and whether they can help distinguish between “archipelago” and “continental” type structures.40 Two of the strongest solvents for pitchlike materials, NMP and quinoline,30 were found useful in carrying out the present work. Experimental Section Model Compounds. The following model compounds were used for the calibrations: fluorene (Fl), fluoranthene (Fl-An), acenapthene (Ace), methylcholanthrene (Meth), diphenyl methane (DPM), pyrene (Py), and coronene (Cor). These compounds have all been identified as present in the GC-MS spectra21 and quantitative chromatographic analyses41–43 of the pitch. They contain most of the carbon and hydrogen environments of interest in the present NMR study. Toluene (Tol), ethyl toluene (ET), and 9-methyl anthracene (9MA) were also used to investigate the effect of solvent on observed chemical shifts. Coal Tar Pitch (CTP). The pitches consist of the solid residue obtained after distillation of the tars from the pyrolysis of coal in a coke oven. The present sample is a “soft” pitch, containing some light components (anthracene oil fraction), such as phenanthrene, and has been extensively investigated.6,8,9,16,17,44–46 The pitch was fractionated by solvent-solubility, as described elsewhere.44 Contact with filtration media was avoided in order to maximize sample recovery and to avoid loss of high molecular mass material including trace-metal containing components.22 Three fractions were produced: the pitch acetone soluble (PAS) fraction, the pitch pyridine soluble-acetone insoluble (PPS) fraction, and the pitch pyridine insoluble (PPI) fraction, with each fraction making up about one-third of the whole.22 Liquid-State NMR Spectroscopy. NMR spectra were recorded on a Bruker AMX 600 spectrometer equipped with an 8 mm TXI probe for 1H and 13C analyses. Proton analysis was performed at 600 MHz, and 13C analysis was performed at 150 MHz, with a working temperature of 308 K. The “lock solvent” was 1,1,2,2tetrachloroethane-d2 (TCE-d2). The relaxation agent used was iron(III) pentane-2,4-dionate [Fe(acac)3]. For quantitative results, 1H pulse saturation (ZGPS) and inverse gated 13C (IG) Bruker (38) Brown, J. K.; Ladner, W. R. Fuel 1960, 39, 87. (39) Dereppe, J.-M.; Moreaux, C.; Castex, H. Fuel 1978, 57, 435. (40) Murgich, J. Mol. Simul. 2003, 29, 451. (41) Blanco, C. G.; Dominguez, A.; Iglesias, M. J.; Guillen, M. D. Fuel 1994, 73 (4), 510. (42) Guillén, M. D.; Iglesias, M. J.; Domínguez, A.; Blanco, C. G. J. Chromatogr. 1992, 591, 287. (43) Blanco, C. G.; Canga, J. S.; Domínguez, A.; Iglesias, M. J.; Guillén, M. D. J. Chromatogr. 1992, 607, 295. (44) Lazaro, M.-J.; Herod, A. A.; Kandiyoti, R. Fuel 1999, 78, 795. (45) Herod, A. A.; Kandiyoti, R.; Lazaro, M. J.; Dubau, C.; Richaud, R.; Shearman, J.; Card, J.; Jones, A. R.; Domin, M.; Suelves, I. Energy Fuels 2000, 14, 1009. (46) Millan, M.; Behrouzi, M.; Karaca, F.; Morgan, T. J.; Herod, A. A.; Kandiyoti, R. Catal. Today 2005, 109, 154.
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Morgan et al.
Table 1. Sample and Reference Concentrations for 1H and 13C NMR Analyses sample solution
proton
13carbon
sample weight/mg Fe(acac)3 weight/mg sample + solvent volume/mL sample concentration/% w/v Fe(acac)3 concentration/% w/v
200 e3.0 1.0 20 0.0-0.3
200 30 1.0 20 3.0
reference/lock solution
proton
carbon
reference material (TKS) weight/mg Fe(acac)3 weight/mg lock solvent volume (TCE-d2)/mL reference concentration/% w/v Fe(acac)3 concentration/% w/v
10 0.0 0.2 5 0.0
20 3.3 0.2 10 1.6
microprograms were used. QUAT48,49 and DEPT50 45°, 90°, and 135° microprograms were used to aid identification of structural features. All analyses were performed without spinning the sample tube. Quantitative analysis was performed by using an independent reference material, tetrakis-(trimethylsilyl)-silane (TKS), which was chosen over the more common tetramethylsilyl silane (TMS). The volatility of TMS (bp 300 K) makes its use unreliable in quantitative work. Coaxial NMR tubes (8 mm diameter) were used to maximize the amount of sample in the active area of the instrument. Solvents, concentrations, and sample preparation methods are discussed below. A summary of the optimal conditions, in terms of sample concentrations and instrumental parameters, is given in Tables 1 and 2. NMR Sample Preparation. An outline of the procedures will be presented here, and the more important details will be discussed in the Supporting Information associated with this paper. Solvents. For the NMR analysis of the PAS fraction, TCE-d2 was used as the solvent. For the PPS and PPI fractions (only partially soluble in TCE-d2), NMP and quinoline were used as the NMR solvents. NMP, which contains only aliphatic carbon and hydrogen alongside a carbonyl group, was used as the solvent for analyzing the aromatic part of the sample. Since quinoline is completely aromatic, it was used as the solvent in a second set of separate analyses to examine the aliphatic parts of the spectrum. See section S1 of the Supporting Information for factors that must be considered when using NMP or quinoline as the solvent for the NMR analysis of the CTP fractions. Sample Concentrations. For solution-state 13C analysis, a concentration of 20% w/v was found to be necessary in order to operate at an acceptable signal-to-noise ratio and to complete satisfactory measurements in a reasonable length of time,
