Energy & Fuels 1994,8, 1039-1048
1039
UV-Fluorescence Spectroscopy of Coal Pyrolysis Tars Chun-Zhu Li, Fan Wu, Hai-Yong Cai, and Rafael Kandiyoti* Department of Chemical Engineering and Chemical Technology, Imperial College of Science, Technology and Medicine, University of London, Prince Consort Road, London SW7 2BY, U.K. Received February 2, 1994. Revised Manuscript Received May 9, 1994@
Changes in W-fluorescence spectroscopic properties of coal pyrolysis tars as a function of coal rank, maceral composition, and pyrolysis conditions have been investigated. The effects of excitedstate interactions (intramolecular energy transfer and excimer formation) on W-fluorescence spectroscopic properties of pyrolysis tars have also been evaluated. Changes in W-fluorescence spectroscopic properties with molecular mass have been studied by acquiring the spectra of successive SEC retention volume resolved fractions of the same tar sample. Fluorescence spectroscopic properties of coal pyrolysis tars have been found to correlate with the rank of the original coals; taken together with findings from FT-IR spectroscopy and size exclusion chromatography, these data show the increasing aromaticity of the tar samples with increasing coal rank. Synchronous spectra clearly show two characteristic peaks centred around 350 and 400 nm, the relative intensity of the first peak decreasing and that of the second peak increasing with increasing coal rank. The observed progressive increases in fluorescence intensity at longer wavelengths with increasing coal rank are interpreted as showing the presence of larger aromatic ring systems in increasing concentrations with increasing rank of the original coal samples, probably coupled with progressively shortened bridge structures connecting aromatic ring systems. Due to the observed changes in W-fluorescence spectroscopic properties with molecular mass, the observed trends with the whole tar samples have also been verified using SEC retention volume resolved fractions. Tars from the pyrolysis of different maceral concentrates of the same coal have been observed to show similar spectral profiles, although fluorescence and absorption intensities have been observed to change with maceral composition and pyrolysis conditions.
Introduction Despite the preponderance of aromatic structures in coals and coal-derived products, the reliable determination of aromatic ring system size distributions has generally proved a difficult task. In coals, the average size of fused aromatic ring systems is thought to increase with increasing rank, and a number of widely differing techniques have been used in attempts to determine distributions of aromatic ring systems in coals and coal-derived products. Statistical methods to derive the structural parameters, e.g., average sizes of aromatic clusters and average number of aromatic rings per structural unit, have been summarized by van Kreve1en.l A technique for determining many of the chemical structural features of coals and chars using 13Cnuclear magnetic resonance (13CNMR) was recently developed by Solum and co-workers.2 They have used 13C NMR based data to estimate average cluster molecular masses and average numbers of aromatic carbons per cluster in a set of coals and pyrolysis their findings suggest that the average number of aromatic carbons per cluster in pyrolysing solids does not change significantly during the pyrolytic process.
* To whom correspondence should be addressed.
Abstract published in Advance ACS Abstracts, July 1, 1994. (1)van Krevelen, D. W. Coal; Elsevier Publishing Co.: Amsterdam, 1961;pp 445-452. (2)Solum, M.S.;Pugmire, R. J.; Grant, D. M. Energy Fuels 1989, 3, 187. (3)Fletcher, T. H.; Kerstein, A. R.; Pugmire, R. J.; Solum, M. S.; Grant. D.M. Enerm Fuels 1992. 6. 414. (4)Fletcher, T.E.;Solum, M. S.; Grant, D. M.; Pugmire, R. J. Energy Fuels 1992, 6, 643. @
Among methods available for tar characterisation, W-absorption (W-A) and W-fluorescence ( W - F ) spectroscopies stand out as promising techniques for providing information on the relative concentrations and sizes of fused aromatic ring systems. Of the two techniques, W - F provides better resolution than UV-A for aromatic structures with overlapping absorption wavelengths. The requirement of only small amounts of sample by both techniques, furthermore, is well suited for characterising the limited amounts of tar produced in the wire-mesh pyrolysis experiments (about 0.7-3.5 mg per e ~ p e r i m e n t )described ~?~ below. W-fluorescence spectroscopy has already been used for the characterization of coal-derived products. Coaltar pitch fractions have been studied by Zander, Haenel, and co-workers (e.g. see refs 7-10). Aigbehinmua and co-workers have attempted to use “3D-stack plots’’ of W-fluorescence spectra to fingerprint extracts from Daw Mill coal.ll Clark and co-workers12and Mille and co-workers13 have also used synchronous W-fluorescence spectroscopy to fingerprint coal-derived liquid (5)Li, C.-Z.; Bartle, K. D.; Kandiyoti, R. Fuel 1993, 72, 3. (6) Li, C.-Z.; Bartle, K. D.; Kandiyoti, R. Fuel 1993, 72, 1459. (7)Zander, M.;Haenel, M. W. Fuel 1990, 69, 1206. (8)Zander, M.Fuel 1991, 70, 563. Haenel, M. W.; Zander, M. Fuel 1990, 69, 1226. (9)Boenigk, W.; (10)Zander, M.; Collin, G. Fuel 1993, 72, 1281. (11)Aigbehinmua, H.B.; Danvent, J. R.; Gaines, A. F. Energy Fuels 1987, 1 , 386. (12)Clark, E. R.;Danvent, J. R.; Demirci, B.; Flunder, K.; Gaines, A. F.; Jones, A. C. Energy Fuels 1987, 1 , 392. (13)Mille, G.; Kister, J.; Aune, J.-P. J.Chim. Phys. 1989,86(2), 277.
0887-0624/94/2508-1039$04.50/0 0 1994 American Chemical Society
Li et al.
1040 Energy & Fuels, Vol. 8, No. 5, 1994
Table 1. Properties of the Set of Rank-Ordered Coal Samples CWt% ash, wt % vitrinites, liptinites, vol % (daf) (daf) (daf) (dry) vol % (daf)
vM,Wt% sample Taff Merthyr Emil Mayrisch Heinrich Robert Santa Barbara Candin Bentinck Gedling Illinois No. 6 a
13.4 17.1 26.7 32.8 38.1 37.1 40.5 47.0
91.5 89.2 87.7 88.0 84.6 83.5 81.3 78.7
5.3 7.1 6.2 10.6 11.5
5.5 2.0 10.4
inertinites, vol % (daf)
81 74 78
0 0 4
19 26 18
&aa da da
da da da
da da da
66 77
13 8
21 15
d a : not available.
Table 2. Elemental and Petrographic Analyses of Point of Ayr Coal Derived Maceral Concentrate Samples C" H" N" 0a.c ashb Vitrinitesd Liptinitesd Inertinitesd Whole coal 85.2 5.2 1.9 5.9 1.9 a4 6 10 Vitrinite concentrate 84.8 5.0 1.9 5.7 2.3 91 5 4 Liptinite concentrate 85.7 6.5 1.5 6.4 2.8 30 61 9 Interinite concentrate 84.2 4.5 1.2 7.0 11.3 17 3 80 a Wt % daf basis. Wt % dry basis. By difference. Vol % dmmf basis.
products. More recently, Rathbone and co-workersl* pyrolysis tars to draw inferences regarding molecular have reported the use of W-fluorescence microscopy to structure and distributions of aromatic ring systems. characterize a "resid" from the liquefaction of coal. W-fluorescence spectroscopy of successive fractions A number of problems, however, complicate the eluted from a size exclusion chromatography (SEC) interpretation of W - F spectra of coal-derivedproducts. column ("SEC-retention-volume-resolved"fractions) has Among them are intramolecular energy transfer and been investigated to study possible changes in spectral excimer formation. Both are related to the high moproperties and molecular structure with molecular lecular mass nature of coal-derived products. As will mass. This type of experiment allows a measure of be described below, intermolecular energy transfer in information to be obtained on structural variations solution can largely be eliminated by the use of low accompanying changes in molecular size. Trends obsample bulk concentrations. Unlike individual polycyserved in W - F spectra of whole tars have been verified clic aromatic molecules in solution, however, aromatic by comparing equal SEC retention volume resolved ring systems embedded in large tar or extract molecules fractions of individual samples. The evaluation of the effect of excited-state interactions (i.e. intramolecular are not distributed randomly, nor can distances-and interactions-between them be diminished by dilution. energy transfer and excimer formation) on W-fluoresConsequently, their local concentrations within large cence spectral properties is thought to play an important molecules can be very high. The resulting intramolecupart in the interpretation of W - F based data and will lar energy transferlmigration between aromatic ring be addressed in this paper in some detail. systems has already been cited as an important decay route for excited states in molecules of coal-derived Experimental Section products.12 Intramolecular excimer formation, another Coal Samples. The rank-ordered set of whole coals were likely decay route for excited-state energy, must also obtained from the European Centre for Coal Specimens, SBN be evaluated carefully in the present context. (The Netherlands). The set of maceral concentrates derived Zander and Haene17have found correlations between bituminous) coal were prepared by from Point of Ayr (U.K. the positions of spectral peaks and the average molecuBritish Gas plc using methods presented e1se~here.l~ Properlar masses of pitch fractions. For pitches, which are ties of the rank-ordered set of coals and the Point of Ayr thought to have highly condensed structures, these derived maceral concentrates are given in Tables 1 and 2, findings may be interpreted as suggesting a relationship respectively. between aromatic ring size and molecular mass (MM). Pyrolysis of Coal Samples. The configuration of the heated wire-mesh rea~tol.5,~J~ enables capture and recovery of Coal pyrolysis tars and liquefaction extracts have broad l ~ under conditions minimizing extraparticle secondary MM-distributions extending to very high v a l u e ~ . ~ , ~ J ~ -tars reactions, structural comparisons are expected to bring out Distributions of fused aromatic ring systems in tars and features which might otherwise be masked by tar degradation extracts, at least in those prepared under conditions through spurious interactions. The experimental procedures limiting condensation and repolymerization reactions, used for producing pyrolysis tars under vacuum and at would depend more on the nature of the original sample atmospheric pressure have been described el~ewhere.~fJ~ The than the method of thermal treatment (in contrast to the case of coal tar pitch). Nevertheless, some effect of (14)Rathbone, R. F.; Hower, J. C.; Derbyshire, F. J. Fuel 1993,72, 1177. MM on the position of the spectral peak can be expected (15)Li, C.-Z. Ph.D. Thesis, University of London, 1993. for these samples: this is due t o the increases in the (16)Li, C.-Z.;Madrali, E. S.; Wu, F.; Xu, B.; Cai, H.-Y.;Guell, A. J.; sizes of aromatic ring systems with increasing molecular Kandiyoti, R. Fuel 1994,73, 851. (17) John, P.; Johnson, C. A. F.; Parker, J. E.; Smith, G. P.; Herod, mass observed in our study (see below). A. A.; Li, C.-Z.; Kandiyoti, R. Rapid Commun. Mass Spectrosc. 1993, The present paper focuses on factors contributing to 7 795 ., changes in the W-fluorescence spectra of coal pyrolysis (18)Gaines,A. F.; Li, C.-Z.;Bartle,K. D.; Madrali, E. S.;Kandiyoti, R. Proceedings of the International Conference on Coal Science, tars as a function of original coal rank, maceral comNewcastle-upon-Tyne, U.K. 16-20 Sept 1991; Butterworth-Heineposition, and pyrolysis conditions. The work aims to man: Oxford, U.K., 1991, pp 830-831. use changes in W - F spectroscopic properties of coal (19) White, A.; Davis, M. R.; Jones, S. D. Fuel 1989,68,511.
Energy & Fuels, Vol. 8, No. 5, 1994 1041
W-Fluorescence Spectra of Coal Pyrolysis Tars reactor configuration consists of a wire-mesh sample holder stretched between two electrodes, with the mesh also serving as resistance heater. In the present study, samples were heated at 1000 K s-l to 700 "C with 30 s (atmospheric pressure) or 5 s (vacuum) holding time at the peak temperature. Tar samples presented in Figure 10 were prepared by heating substrates at about 4000 K s-l to 700 "C with 30 s hold in atmospheric pressure helium. During atmospheric pressure experiments, a stream of gas (0.1 m 9-l) passed through the sample holding part of the mesh to remove volatiles away from the reaction zone into a liquid Nz cooled trap. In experiments under vacuum, released tar was exposed t o cooled surfaces and allowed to condense. Tar yields were determined from the weight uptake of the traps; tars were removed by washing with a 4:l mixture of HPLC grade ch1oroform:methanol. Concentrations of the resulting tar solutions were of the order of g mL-l, the actual value depending on the amount of coal sample used and the tar yield of the substrate under the particular pyrolysis conditions. Tar and total volatile yields determined during these experiments have been reported and discussed e l ~ e w h e r e . ~ ~ ~ J ~ , ~ ~ Size Exclusion Chromatography of Tars. Two 30-cm PL-gel 3-pm MMED-E analytical columns (Polymer Laboratory Ltd) were connected in series.20 A UV-absorptioncell (1 mm light path length) and an evaporative analyzer were used in tandem as mass detectors. Unstabilized tetrahydrofuran (THF)was used as the elution solvent. SEC-derived molecular mass distributions of tars from the Point of Ayr maceral concentrates were obtained from a system similar to the one described here and have been reported el~ewhere.~,'~ W-Absorption and W-Fluorescence Spectroscopies of Tars. UV-fluorescence spectra of tars were recorded with a Perkin-Elmer LS 50 luminescence spectrometer. The spectrometer features automatic correction for changes in source intensity as a function of wavelength and corrected excitation spectra were recorded directly. Emission and synchronous spectra were not further corrected for other factors, e.g., changes in emission photomultiplier response as a function of wavelength. Both constant wavelength and constant energy (i.e., constant wavenumber) synchronous spectra may be acquired with this instrument. A quartz cell of 1cm light pass length was used, with emission being detected perpendicular to excitation (90"). Unless otherwise stated, a scan speed of 240 nm min-' and a slit width of 2.5 nm were used. The reproducibilities of the spectroscopic measurement were found to be within the noise levels of the spectra presented. The spectrometer was also equipped with a transmittance accessory, allowing absorption spectra to be acquired. An HPLC flowcell accessory (0.14mm light pass length) allowed spectra of SEC elution fractions of short retention volume intervals (e.g., 0.16 mL) to be recorded directly when the spectrometer was used as an SEC detector, placed in tandem with the UVabsorption detector. Quantitative tar recovery from the traps allowed direct use of the tar weight determined during pyrolysis experiment for calculating tar concentrations in solutions. Tar solutions recovered from pyrolysis experiments were first diluted to 25.00 It 0.03 mL in a volumetric flask to calculate the tar concentrations. Resulting solutions were further diluted with CH30H (SpectrosoL grade, BDH) until a linear relationship between concentration and fluorescence intensity could be obtained, in order t o minimize effects due to self-absorption, intermolecular energy transfer, and excimer formation. Tar concentrations required for achieving this result were usually g mL-'. lower than 1.0 x Degassing by bubbling helium through sample solutions caused only about 5 % increase in fluorescence intensity, with spectral profiles remaining unchanged. Considering that the
Molecular mass, u Ih Illlnols No. 6 GL, Cedllng
2
c)
CD, Candin TM, Taff Merthyr
0.3
rr 0
c
.s 5 Ld
0.2
&
u
nnL "'"8
10
12
14
D. R.; Kandiyoti, R. Final Report to the Commission of the European Communities, Contract No. JOUF.0050.C(TT),August 1993.
18
20
22
1
Figure 1. SEC retention volume distributions of pyrolysis tars as a function of coalification rank of substrate. errors in the tar yield determinations were typically about 1.52%of daf ~ o a l ~(e.g., * ~ 6% J ~in 2 mg of tar obtained) and that the same solvent (CH30H) was used in this study (i.e. concentrations of dissolved oxygen are the same for all tar solutions), degassing was therefore found unnecessary. In a study of crude oils by UV-F, Zhu and MullinsZ1also noted little difference between measurements taken with or without NZ purging. Reasons for the insensitivity of tar spectra to oxygen quenching are not clear and may have resulted either from intramolecular energy transfer (see below) being a more rapid decay process than quenching by oxygen or from the high MM nature of these tars, oxygen being unable t o access aromatic ring systems embedded within large molecules during the lifetime of the excited states.
Results and Discussion
SEC Retention Volume Distributions of Tars as a Function of Coal Rank. Molecular mass distributions of tars from the pyrolysis of Point of Ayr maceral concentrates, based on a calibration with coal-derived p r o d u ~ t s , have ~~-~ been ~ presented elsewhere6. These data showed the existence of material with molecular masses extending to values beyond 5000 u. Observations of such high molecular mass material in pyrolysis tars have recently been independently confirmed by laser desorption mass ~ p e c t r o m e t r y the , ~ ~latter ~ ~ ~ experiments showing significant fractions of the material at molecular masses between 1000 and 5000 u, with traces extending up to 20 000 u. Figure 1presents evaporative analyzer (EA) detector derived SEC retention volume distributions of tars from the pyrolysis of Illinois No. 6, Gedling, Candin and Taff Merthyr coals, selected from the rank-ordered set used in the present study (Table 1). A molecular mass scale based on a column calibration by polystyrene standards is given at the top of the figure. Despite relatively small differences between MM distributions of Illinois No. 6, (21) Zhu, Y.; Mullins, 0. C. Energy Fuels 1992, 6, 545. (22) Bartle, K. D.; Taylor, N.; Mulligan, M. J.; Milles, D. G.; Gibson, C. Fuel 1983, 62, 1181.(23) Bartle, K. D.; Mills, D. G.; Mulligan, M. J.; Amaechina, I. 0.; Taylor, N. Anal. Chem. 1986,68, 2403. (24) Bartle. K. D.: Mulliean. M. J.: Tavlor. N.: Martin, T. G.: Snape, C. E. Fuel 19k4, 63; 1556.(25) Herod, A. A.; Parker, J. E.; Johnson, C. A. F.; John, P.; Smith, G. P.; Li, C.-Z.; Kandiyoti, R. Proc. 7th Int. Confi Coal Sci., BanK Alberta, Canada, 12-17 Sept 1993 1993,282-285. '
(20) Li, C.-Z.; Giiell, A. J. ; Madrali,E. S.;Cai, H.-Y .; Wu, F.; Dugwell,
16
Retention volume, ml
1042 Energy & Fuels, Vol. 8, No. 5, 1994
Li et al.
D
Wavelength, nm Figure 2. Excitation and emission spectra of Illinois No. 6 coal tar. Concentration: 8.1 x g mL-'. Each excitation spectrum is labeled with the emission wavelength used t o record it and each emission spectrum labeled with the corresponding emission wavelength. Gedling and Candin tars, these data are interpreted to indicate a shift towards smaller MMs with increasing coal rank. The trend was further confirmed using the tars from the full set of coals listed in Table 1. Similar trends were found in results obtained using the SEC W-absorption detector. A fuller account of observed changes in SEC-derived MM distributions of pyrolysis tars with increasing coal rank, using the set of coals in Table 1,is being prepared for publication. Fluorescence Spectra of Pyrolysis Tars as a Function of Coal Rank. Figures 2-4 present Wfluorescence spectra of tars from the pyrolysis of Illinois No. 6, Candin, and Emil Mayrisch coals, respectively; these data were selected as typical from a set obtained with tars from the rank-ordered coals (Table 1). In Figures 2-4, excitation and emission spectra have been presented on the same diagrams, with excitation spectra appearing at shorter wavelengths and labeled with the relevant emission wavelengths (in nm). Similarly, each emission spectrum has been labeled with the excitation wavelength used to record the spectrum. Common features of these spectra, including those of the other tars15 in the rank series (not shown), may be noted as follows: 1. With increasing coal rank, the relative intensities of excitation spectra increased significantly a t longer wavelengths. 2. In all emission spectra, very little emission was observed at wavelengths shorter than about 300 nm, irrespective of the excitation wavelengths used. 3. Emission intensities were observed to decrease with increasing excitation wavelength. Relatively little change was observed in the spectral profiles in the highwavelength region. Furthermore, emission intensities in the high-wavelength region of tars from higher rank coals were observed not to change significantly with increasing excitation wavelength from 260 to 300 nm (e.g., see Figure 4).
Wavelength, nm Figure 3. Excitation and emission spectra of Candin coal tar. Concentration: 8.0 x g mL-l. Each excitation spectrum is labeled with the emission wavelength used to record it and each emission spectrum labeled with the corresponding emission wavelength.
D
Wavelength, nm Figure 4. Excitation and emission spectra of Emil Mayrisch coal tar. Concentration: 8.7 x g mL-'. Each excitation spectrum is labeled with the emission wavelength used to record it and each emission spectrum labeled with the corresponding emission wavelength. The interpretation of results presented in Figures 2-4 requires consideration of a number of complicating factors. Intramolecular Energy Transfer. W-fluorescence spectra of complicated mixtures such as tars do not normally allow a precise size distribution of aromatic ring systems to be directly determined. However, the existence of a distribution of aromatic ring systems of varying size can readily be inferred. For example, the shapes of excitation spectra of pure compounds are
Energy & Fuels, Vol. 8, No. 5, 1994 1043
UV-Fluorescence Spectra of Coal Pyrolysis Tars 20.0
B
C 18
e
0
Wavelength, nm
IO
Wavelength, nm
Figure 6. Constant energy (-1500 cm-1) synchronous spectra of pyrolysis tars as a function of coalification rank of substrate. Concentration: 8.0 x lo-' g mL-1. TM, TaffMerthyr; EM, Emil Mayrisch; SB, Santa Barbara; HR, Heinrich Robert; CD, Candin; BT, Bentinck GD, Gedling; and IL, Illinois No. 6.
and the high molecular masses of these tars suggest normally independent of the emission wavelengths used that intramolecular energy transfer is one of the imto record them. In Figures 2-4, differences observed portant decay routes of the excited-state energy: W between the shapes of excitation spectra obtained at energy absorbed by individual ring systems may either different emission wavelengths clearly indicate the be emitted as fluorescent light or transferred to neighpresence of more than one type of aromatic ring system boring (larger) ring systems, t o be a t least in part rein each of these samples. Similarly, synchronous specemitted as longer wavelength (Le., lower energy) fluotra of pure compounds have been observed to show rescent light. By biasing the emission spectra to longer peaks of very narrow wavelength ranges in their wavelengths, intramolecular energy transfer thus comconstant wavelength or constant energy synchronous plicates the interpretation of W-fluorescence data. spectra;l3pZ6the synchronous (-1500 cm-l) spectra of tars from the rank-ordered coals in Figure 5 showed Observations similar to those on Figure 4 may be broad distributions centred around two peaks, indicatmade on the spectra shown in Figures 2 and 3 and ing the presence of a range of aromatic ring systems in spectra of other t a d 5 (not shown) in the present set. each of these tar samples. Clearly this is an expected In this respect, Figures 2-4 reflect trends found in result. However, SEC data in Figure 1also suggest the spectra of tars from the full rank-ordered set used in presence of very large molecules in these and similar the present study: pyrolysis tar samples: recent reports5~6~17~25~27~z~ have (i) The overlap between excitation and emission indicated the presence of large fractions with MMs spectra tends t o extend to progressively longer waveabove 1000 u. A number of aromatic ring systems of lengths with increasing coal rank. different sizes are thought t o be present within indi(ii) Changes in emission spectral profiles with invidual large tar molecules. creasing excitation wavelength were smaller in the This simple conclusion has a number of important longer wavelength region than in the shorter waveramifications. Within large tar molecules, aromatic ring length region: for tars from the highest rank coal systems are thought to be interconnected by different samples, emission intensities were very similar irtypes of bridge structures (e.g., aliphatic, etheric, alirespective of the excitation wavelengths between 260 cyclic). One immediate consequence of this picture of and 300 nm (see emissions a t wavelengths > 450 nm; tar molecules is the possible local high concentration of Figure 4). With very long excitation wavelengths (e.g., aromatic ring systems, irrespective of the degree of bulk 400 nm in Figure 31, significant emission, albeit of dilution achieved in solution. One of the important similar spectral profiles to the emission with shorter features of the spectra presented in Figures 2-4 is the excitation wavelengths (e.g., 260 nm), was also observed significant overlap between excitation and emission for the same tars. This is thought to be a clear spectra. indication of intramolecular energy transfer taking place when using shorter excitation wavelengths. For example, in Figure 4 excitation and emission spectra overlap between 300 and 450 nm; it is likely, With increasing coal rank, emission from pyrolysis furthermore, that the overlap extends to wavelengths tars is thus observed increasingly to take place from the longer than 450 nm. Taken together, spectral overlaps largest aromatic ring systems contained within samples. These data may be explained in terms of the presence of higher proportions of larger aromatic ring systems, (26) Vo-Dinh, T. Anal. Chem. 1978,50, 396. (27) John, P.; Johnson, C. A. F.; Parker, J. E.; Smith, G. P.; Herod, probably coupled to progressively shorter bridge strucA. A.; Gaines, A. F.; Li, C.-Z.; Kandiyoti, R. Rapid Commun. Mass tures connecting them and facilitating energy transfer Spectrosc. 1991, 5 , 364. (28) Li, C.-Z.; Gainee, A. F.; Kandiyoti, R. Proceedings of the between them. It would therefore be possible to view International Conference on Coal Science, Newcastle-upon-Tyne, U.K. the role of smaller aromatic ring systems, with their 16-20 Sept 1991; Butterworth-Heineman: Oxford, U.K., 1991, pp higher absorbance at shorter wavelengths, as sensitisers 508-511. ~
Li et al.
1044 Energy & Fuels, Vol. 8, No. 5, 1994 65.0
I
I
Wavelength, nm
Wavelength, nm
Figure 6. Emission spectra (excitation at 260 nm) of pyrolysis tar as a function of coalificationrank of substrate. Concentration: 8.0 x g mL-'. TM, Taff Merthyr; EM, Emil Mayrisch; SB, Santa Barbara; HR, Heinrich Robert; CD, Candin; BT, Bentinck GD, Gedling; and IL, Illinois No. 6. for the larger ring systems within the same tar molecule, the effect becoming more intense with shortening bridge connections between the aromatic ring systems. The ratios of emission intensities in response to different excitation wavelengths depend on, among other factors, the ratio of concentrations of smaller to larger aromatic ring systems. Effect of Coal Rank on Synchronous W-F Spectra. Figures 5 and 6 present constant energy (-1500 cm-l) synchronous and emission (excitation a t 260 nm) spectra, respectively, of tars prepared from the set of rank-ordered coals. Spectra in both figures were drawn on the basis of the same tar concentration in solution g mL-l); where actual concentrations did (8.0 x not precisely equal this value, linear relationships between W-fluorescence intensity and concentration were used to redraw the spectra. Results in Figure 6 showed increasing emission intensities with increasing coal rank for tar samples from coals ranging from Illinois No. 6 to Emil Mayrisch; moreover, emission intensities at longer wavelengths were observed to increase more (than at shorter wavelengths) with increasing coal rank. This is thought to suggest that the tars from higher rank coals may actually contain higher proportions of larger aromatic ring systems. Two characteristic peaks centered around approximately 350 and 400 nm may be observed in the family of synchronous spectra in Figure 5. While the overall intensities were observed to increase with increasing coal rank (except from Taff Merthyr), the relative intensity of the first peak was observed to decrease and that of the second peak to increase. This shift in relative intensity with increasing rank appears to reflect the increases in relative concentrations of larger aromatic ring systems with increasing rank, as suggested by the emission spectra. In the absence of corroborating evidence, some uncertainty must remain about the precise assignment of these relatively broad peaks to specific ranges of ring system sizes. It seems, at any rate, possible to relate the approximate rank of any given coal to the relative heights of the first and second peaks found in the synchronous spectra of its pyrolysis tar.
The trends in Figures 5 and 6 showing increasing fluorescence intensities with increasing coal rank, from Illinois No. 6 to Emil Mayrisch, were also confirmed15 with constant wavelength (4 nm) synchronous spectra and emission spectra excited at other wavelengths (280, 300,350, and even 400 nm). FT-IR spectra of the same set of tars have been presented elsewhere;20aromaticity values calculated from these spectra consistently increase with increasing coal rank, with the exception of the very high rank Taff Merthyr (also see below). Similar conclusions have been drawn from SEC data,20 where ratios of areas under W detector and evaporative analyzer (the two detectors were used in tandem) traces were calculated, suggesting that the tars become more aromatic with increasing coal rank, between Illinois No. 6 to Emil Mayrisch. It may be noted that the comparison of fluorescence intensities between the tars in the series has been undertaken on the basis of the same weight concentration (8.0 x g mL-l). Fluorescence quantum yields of similar sized larger aromatic ring systems (e.g., emissions at '400 nm) in higher and lower rank coal tars would be expected to remain approximately constant. The fluorescence quantum yields of smaller aromatic ring systems (e.g., emissions at 1400 nm) in tars would, if anything, decrease with increasing coal rank due to the possibly intensifying effects of intramolecular energy transfer. Figures 5 and 6 indicate increases in emission intensities in the longer wavelength regions in tar spectra with increasing coal r a n k these increases were not accompanied with concomitant decreases in emission intensities of shorter wavelength bands of the spectra. These findings are therefore thought to suggest increases in the concentrations of larger aromatic ring systems with increasing coal rank, without accompanying decreases in smaller ring system concentrations. Therefore, the observed increases in fluorescence intensity may, with reasonable confidence, be interpreted as arising from increases in aromaticity with increasing coal rank. Tar derived from Taff Merthyr appears as an exception in this series (Figures 5 and 6 ) . Comparison of EA and W-detector responses in size exclusion chromatography (to be reported separately) also suggested that
Energy & Fuels, Vol. 8, No. 5, 1994 1045
W-Fluorescence Spectra of Coal Pyrolysis Tars
Taff Merthyr tar was less aromatic than the Emil Mayrisch tar. No precise reason can be stated for this apparent anomaly. However, Taff Merthyr is a semianthracite which gave very low tar yields (9.7% of daf coal): it is likely that the observed tar characteristics simply reflect those of the more tractable parts of Taff Merthyr coal, the greater part of the coal mass not being susceptible of releasing much tar during pyrolysis. Intramolecular Excimer Formation in Large Tar Molecules. Excimers are defined as excited dimers, which, in the ground state, would dissociate in the absence of external restraint^.^^ The transfer/ migration of energy between monomers and excimer forming sites has been noted as an important excitedstate energy decay process in polymers (e.g., ref 30). In the present context, molecular configurations leading to the parallel alignment of aromatic ring systems with rather short distance between them would be required for the formation of singlet e x c i m e r ~ . ~ Due ~,~ t o~the fact that coal itself is highly cross-linked,it is reasonable to expect that the pyrolysis tars are also fairly crosslinked. It is difficult to see that the molecular configurations required for the intramolecular excimer formation would be greatly favored. As outlined above, emissions of similar spectral profiles were observed for the same tars with a wide range of excitation wavelengths, suggesting that intramolecular energy transfer, instead of excimer formation, dominated the decay processes of excited-state energy of smaller aromatic ring systems. In attempting to evaluate the effect of excimer formation on UV-F spectra of coal tars, synchronous spectra offer an interesting lead. The relationship between synchronous spectra and excitation and emission spectra has been well explained in the literature.26 The choice of an appropriate wavelength (or wavenumber) difference between the emission and excitation monochromators has proved to be a difficult task. A relatively small difference (e.g., 4 nm) would favor the emissions from aromatic structures with strong 0-0 transitions, whereas a relatively big difference appear to favor the emission from aromatic structures without strong 0-0 transitions. Based on the results from trial experiments, a difference of about -1500 cm-l seems to be a good compromise under these constraints. Emission from excimers usually takes place at wavenumbers of about 5000-6000 cm-l lower than those of the corresponding monomers (e.g., see ref 33). In acquiring synchronous spectra, a difference of -1500 cm-l (or a constant wavelength difference of 4 nm) used in the present study would therefore unlikely allow the observation of significant emission from excimers under these experimental conditions. Figures 5 and 6 show that tar fluorescence intensities in both emission and synchronous spectra increase with increasing coal rank. This comparison provides an upper limit to the effect of excimer formation on the spectra presented in this paper: while it seems difficult, in any case, to visualize (29) Birks, J. B. In Organic Molecular Photophysics; Birks, J. B., Ed.; John Wiley & Sons: London, 1975, p 495. (30) Semerak, S.N;Frank, C. W. Adu. Polym. Sci. 1984,54,31. (31)Lim, E. C.ACC.Chem. Res. 1987,20,8. (32) De Schryver, F.C.; Collart, P.; Vandendriessche, J.;Gcedeweeck, R.; Swinnen, A. M.; Van der Auweraer, M. Acc. Chem. Res. 1987,20, 159. (33) Birks, J. B. Photophysics of Aromatic Molecules; WileyInterscience: London, 1970; p 301.
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Wavelength, nm 1, Atmospheric pressure helium 2, Vacuum Figure 7. Comparison of constant energy (-1500 cm-l) synchronous UV-fluorescence spectra of tars from the pyrolysis of Point of Ayr maceral concentrates: heating at 1000 K s-l to 700 "C with 5 s (vacuum) or 30 s (atmospheric pressure) g mL-l. holding time. C.oncentration: 8.0 x
excimer formation as a dominant mode of energy decay in the present context, the evidence suggests that at least not all the shift to longer wavelengths with increasing rank of original coal is caused by excimer formation. Effects of Maceral Composition and Pressure on W-FluorescenceSpectra of Pyrolysis Tars. Figure 7 presents constant energy (-1500 cm-') synchronous spectra of pyrolysis tars from Point of Ayr maceral concentrates. The same tar concentrations of 8.0 x lo-? g mL-l were used. Comparison of the three sets of spectra shows (a)the same order of fluorescence intensities as their respective excitation and emission spectrai5, and, (b) the presence of two peaks as in Figure 5; a small shift was observed between the first and second peaks on going from the inertinite t o the vitrinite and the liptinite concentrate tars. Similar observations were made using maceral concentrate samples derived from another UK coa15J5 (Linby, low-rank bituminous). If evaluated in a manner analogous to results shown in Figure 5, the latter finding would suggest subtle shifts towards larger aromatic ring systems, on going from liptinite to vitrinite and to inertinite concentrate tars, although the differences involved are rather small.
Li et al.
1046 Energy &Fuels, Vol. 8, No. 5, 1994
Another significant feature of the spectra arises from differences in intensity found on going from vacuum t o atmospheric pressure pyrolysis tars: in the case of vitrinite and liptinite concentrate derived samples, a clear increase in intensity was observed on going from vacuum t o atmospheric pressure operation. The suppression of volatile and tar release by the effect of increasing ambient pressure has been extensively discussed in the literature (e.g., ref 34 and more recently refs 6, 15, 16, 35, and 36 1. Between vacuum and atmospheric pressure, a shift (of the order of 2-6%) from tar t o gaseous product has been observed, due apparently t o intensification of tar (or tar precursor) cracking reactions and the loss, primarily, of aliphatic (straight chain and alicyclic) structures.6 The increase in UV-fluorescence intensity with increasing pressure observed in Figure 7 , A and B, would appear to confirm findings suggesting the loss of aliphatic structure on going from vacuum t o atmospheric pressure. Qualitatively similar, but more intense, structural changes have recently been reported for tars produced between 2.5 and 70 bars.35 Spectra of inertinite concentrate tars do not appear to fit in this trend, although the greater experimental error possibly due to the relatively small amounts of tar (due to smaller tar yields) render these data somewhat less reliable; the smaller differences between the vacuum and atmospheric pressure tar spectra would also suggest the inertinite concentrate derived tars to be less prone to loss of aliphatic structure, as might have been expected from (the normally predominantly aromatic) inertinite structures.6J6,28 It may also be noted that as the pyrolysis pressure was decreased from atmospheric pressure to vacuum, the relative intensity of the second peak (at around 400 nm, compared to that of the first one at around 350 nm) appeared to increase, although overall intensities were observed to decrease with the same concentration. Detailed comparisons of corresponding emission spectra15 (vacuum vs atmospheric pressure tars of the same substrate) also suggest relative emissions at longer wavelengths to increase on going from atmospheric pressure t o vacuum. These relatively small changes provide some evidence of a higher relative concentration of larger aromatic ring systems in the vacuum tar than in the atmospheric pressure tar of the same substrate, particularly for the vitrinite and liptinite concentrates. These findings reflect the loss of some tar yield through charring (presumably of tar precursor carrying the larger aromatic structures) and some by cracking to gas (arguably of the more aliphatic components) on going from vacuum to atmospheric pressure pyrolysis. Effects of Maceral Composition and Pressure on UV-Absorption Spectra of Pyrolysis Tars. Figure 8A-C compare W-absorption spectra of the same set of Point of Ayr maceral concentrate pyrolysis tars. While these spectra show fewer structural features than analogous W-fluorescence spectra, they suggest, in parallel with Figure 7A-C, broad similarities in aromatic structures between tars from different maceral (34) Howard, J. B.In Chemistry of Coal Utilization; Elliott, M. A,, Ed.; Wiley: New York, 1981, Second Suppl. Vol., p 665ff. (35) Wu, F.;Guell, A. J.; Li, C-Z.; Madrali, E. S.; Cai, H.-Y.; Dugwell D.R.; Kandiyoti, R. Proc. 7th Int. Conf. Coal Sci., 12-17 Sept 1993, Banff, Alberta, Canada 1993,2,307-310. (36) Guell, A. J.;Kandiyoti, R. Energy Fuels 1993,7 , 943.
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concentrates derived from the same coal. Agreement between trends from W-F and W - A suggests, furthermore, a measure of confirmation that differences in intensity observed between vacuum and atmospheric pressure tars did not arise from differences in W - F quantum yields. Data in Figure 8 also indicate that the PoA vitrinite and inertinite concentrate tars absorb more UV energy than the corresponding liptinite tar of the same concentration. This would suggest that the sole use of a UV detector (set in the wavelength range indicated in the figure) during the size exclusion chromatography of coal pyrolysis tars would tend to give less signal for equal concentrations of liptinite tars compared t o vitrinite and inertinite tars. UV-FluorescenceSpectra of SEC Retention Volume Resolved Fractions. The HPLC flowcell accessory of the spectrometer used in the present study enables use of the instrument as a detector for size exclusion chromatography; the spectrometer was set up in tandem with an ordinary W-absorption detector. This arrangement enables acquisition of W - F spectra of successive retention volume resolved fractions of the same tar sample as the column eluent passes through the flow cell. In the present set of experiments, the highest available scan speed of the spectrometer (1500 nm min-l) was used, in order to record a spectrum within a short period (e.g., to scan 240 nm within 0.16 min), during which time, column eluent concentration in most cases could be considered as constant within experimental error (also see below). During these
Energy & Fuels, Vol. 8, No. 5, 1994 1047
W-Fluorescence Spectra of Coal Pyrolysis Tars 2ooo/
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Wavelength, nm Figure 10. On-line emission (excited at 254 nm) spectra of SEC retention volume resolved fractions of tars from the pyrolysis of Illinois No. 6 and Emil Mayrisch coals by heating at about 4000 K s-l to 700 "C with 30 s hold in atmospheric pressure helium. 1, Illinois No. 6, 18.3-18.5 mL; 2, Illinois No. 6,16.8-17.0 mL; 3, Emil Mayrisch, 18.3-18.5 mL; 4, Emil Mayrisch, 16.8-17.0 mL.
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Retention volume, ml Figure 9. (A) On-line UV-fluorescence emission spectra (excited at 254 nm) of SEC retention time resolved fractions of Point of Ayr vitrinite concentrate tar prepared by heating at 1000 K s-l to 700 "C with 30 s holding time in atmospheric pressure helium. (B)SEC chromatogram of the tar (UV detector set at 254 nm). Curve 1in Figure 10 corresponds to a retention volume range of 15.3-15.5 mL; curve 2 to 16.817.0 mL; curve 3 to 18.3-18.5 mL; curve 4 to 19.8-20.0 mL.
experiments, W - F spectra were recorded in unstabilized tetrahydrofuran (the elution solvent for the SEC system), rather than CH30H. Intensities shown in Figure 9A represent W - F emission intensity (excited at 254 nm) per unit absorbance in the W-absorption detector; in presenting W - F intensities divided by the W-absorption detector response, Figure 9A thus presents emission spectra, corrected for concentration differences, of retention volume resolved fractions of Point of Ayr vitrinite concentrate pyrolysis tar. The corresponding SEC chromatogram (W-absorption detector set at 254 nm) is presented in Figure 9B. While scanning speeds were the same in the acquisition of each of the spectra in Figure 9A, differences in sample concentrations and differences in rates of change of sample concentrations require that results presented in Figure 9A be viewed as semiquantitative. Some trends can nevertheless be discerned. Emission maxima
were observed to shift toward shorter wavelengths with increasing retention volume (decreasing molecular mass), in qualitative agreement with results by Zander and Haene17 on a set of coal-tar pitch samples. Results in Figure 9A suggest, fujhermore, that quantum yields decrease as the molecular mass increases: this is because intensities have been presented on the basis of equal W-energy absorption (i.e., in Figure 9A, intensity = W - F emissioW-absorption). Figure 10 presents the on-line emission (excited at 254 nm) spectra of SEC retention volume resolved fractions of tars from the pyrolysis of Illinois No. 6 and Emil Mayrisch coals by heating a t about 4000 K s-l to 700 "C with 30 s hold in atmospheric pressure helium. The two coals have been selected from the extremes of the present coal rank series. Comparing the spectra of SEC retention volume resolved fractions of the same tars (Figure 10 and other fractions not shown here) confirmed the observation made on the Point of Ayr vitrinite concentrate tar (Figure 9A): the increase in molecular mass caused emission maxima to shift to longer wavelengths and emission intensities per unit absorbance to decrease. While the limited data available would preclude a detailed explanation, it is thought that multistep intramolecular energy transferlmigration and/or the presence of some very large aromatic ring systems ("lowenergy" traps) in the larger molecules may provide a likely explanation for both observations. It should be noted that the increase in fluorescence intensity at longer wavelengths observed with increasing coal rank is not caused by the possible shift in molecular mass (Figure 1) with increasing coal rank. This is because the emission intensity per unit absorbance increases with decreasing molecular mass (Figures 9A and 10)more in the shorter wavelength region, whereas the emission intensity in Figures 2-4 and 6 was observed to increase more in the longer wavelength region. The data presented in Figure 10 also seemed
1048 Energy & Fuels, Vol. 8,No. 5, 1994
to confirm the trends observed with the whole tar samples. As was in the case of the whole tar samples, SEC retention volume resolved fractions from the Emil Mayrisch tar showed more intense emissions in longer wavelengths than the corresponding fractions from the Illinois No. 6 tar. Similar differences between the two tars were also observed for other SEC retention volume ranges. The data in Figure 10 furthermore suggest that the relative concentrations of larger aromatic ring systems are higher in the Emil Mayrisch fractions than in the Illinois No. 6 fractions of the same SEC retention volume ranges. Conclusions 1. Fluorescence spectroscopic properties of coal pyrolysis tars have been found to correlate with the rank of the original coals; taken together with findings from FT-IR spectroscopy and size exclusion chromatography, these data show the increasing aromaticity of the tar samples with increasing coal rank. 2. With increasing coal rank, emissions from pyrolysis tars were increasingly observed to take place from the largest aromatic ring systems contained within samples. This result may be explained by the presence of larger ring systems in higher proportions, probably coupled to progressively shortening bridge structures connecting the aromatic ring systems and facilitating energy transfer between them. 3. Intramolecular energy transfer and excimer formation cause emissions to be biased toward larger aromatic ring systems (emitting at longer wavelengths). This makes it difficult to arrive at quantitative distributions of aromatic ring system sizes in tars or indeed in any coal derived material.
Li et al. 4. Synchronous spectra of the tars clearly show two characteristic peaks centered around approximately 350 and 400 nm, the latter peak increasing in relative intensity with increasing coal rank. This result reflects the increases in the relative concentrations of larger aromatic ring systems. It appears possible to relate the approximate rank ordering of any coal t o the relative sizes of the first and second peaks in the synchronous spectra of its pyrolysis tar. 5 . W-fluorescence spectra of tars from different maceral concentrates of the same coal showed significant structural similarities; the W-fluorescence intensity, however, was observed t o be affected by maceral composition of the substrate and pyrolysis reaction conditions. 6. The use of W-fluorescence spectroscopy, in conjunction with size exclusion chromatography, has allowed to trace the changes in W-fluorescence spectroscopic properties of tars with increasing molecular mass. 7. The use of W-fluorescence and W-absorption spectroscopies has allowed comparison of aromatic structures present in coal pyrolysis tars. However, the present data do not allow precise determinations of upper limits t o sizes of fused aromatic ring systems present in the tar samples.
Acknowledgment. The authors thank Professor Alec F. Gaines of the Middle East Technical University (Erdemli, Turkey) for introducing them to W-fluorescence spectroscopy. The authors also thank SBN, the European Centre for Coal Specimens (The Netherlands), and British Gas plc for the provision of samples. Funding for this work by the Commission of the European Communities under Research Contract Nos. EN3V.0052.UK(H) and JOUF.0050.C(TT) is gratefully acknowledged.
