Energy & Fuels 1987, 1 , 392-397
392
Fluorescence of Pyridine Extracts of Coals E. R. Clark,+J. R. Darwent,t B. Demirci,s K. Flunder,? A. F. Gaines,*t and A. C. Jones11 Solid Fuel Research Group, University of Aston, Birmingham B4 ?ET, U.K., Department of Chemistry, Birkbeck College, London W C l E 7 H X , U.K., Ataturk Universitesi Fen ve Ed. Fakultesi, Erzurum, Turkey, and The Royal Institution, London W1X 4BS, U.K. Received March 23, 1987. Revised Manuscript Received July 16, 1987
The emission spectra, excitation spectra, quantum yields, and lifetimes of the fluorescence of pyridine extracts of coals have been measured. The excitation spectra show broad but resolved peaks in the 340-, 390-, 440-,and 470-nm regions for each of the three coals studied (Yallourn brown coal and Gedling and Cresswell bituminous coals). Emission quantum yields were on the order of 6%. Time-resolved measurements showed that the emissions were a multiexponential process with lifetimes in the range 0.1-10 ns. Both of these results are consistent with the emission resulting from an aromatic polymer, in which most of the light energy is lost through radiative and nonradiative energy transfer. Increased proximity of the aromatic systems in polished blocks of coals leads to increased energy transfer, and consequently, fluorescence is diminished and observed a t longer wavelengths.
Introduction Fluorescence and reflectance measurements are used routinely by geochemists and petrologists to characterize coals.lP2 Polished coal surfaces show broad emission spectra from 500 to 700 nm. Of the three main maceral groups, exinites fluoresce most strongly and vitrinites also fluoresce but a t longer wavelengths whereas inertinites, which have the highest aromatic content, show no emission. Recent observations have shown that fluorescence from vitrinites can be related to their thermoplastic behavior and hence to their ability to form cokes.3 Nevertheless, relatively little is known about the chemical phenomena that are responsible for coal fluorescence. The optical behavior of solid coal is complicated by the very heterogeneous nature of the material and the general difficulties associated with the fluorescence of solids? By studying the fluorescence from coal extracts in organic solvents, it is possible partially to overcome these problems. Drake et al.5 observed that carbon disulfide extracts gave well-resolved emission spectra at low temperature, and these could be assigned to specific aromatic compounds. Carbon disulfide, however, extracts only a small proportion of coals, and the results are unlikely to be representative of the parent material. Pyridine can extract about 25% by weight from a bituminous coal,6 and the extracts are thought to provide a representative microcosm of the whole coal. They contain a mixture of simple molecules including alkyl benzenes, phenols, and naphthalenes together with more complex polymeric material' in which some of the simpler structures are thought to be entrapped.s Work in our laboratory using dynamic laser light scattering has shown that dilute pyridine extracts embrace a wide variety of material including large molecules with radii distributed around 100 nm and small particles with radii distributed around 10 ~ m . ~ Retcofsky, Brendel, and Friedello observed that pyridine extracts will fluoresce when excited with ultraviolet light to give an emission with maximum intensity near 440 nm.lo University of Aston.
* Birkbeck College.
Ataturk Universitesi Fen ve Ed. Fakultesi. Royal Institution.
11 The
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Table I. Atomic Hydrogen/Carbon Ratios of Fuels Used in These Experiments Gedling" 0.80 Frickley vitrinite" 0.80 Tun&ilekd 0.96 Cresswell" 0.81 Frickley exhitee 1.05 Seyitomerd 1.05 Yallournb 0.89 Manvers vitrinite' 0.76 Elbistand 0.86
Manvers exhitee 0.93 Bituminous, Carboniferous coal. IS0 classification: Gedling 733; Cresswell 634. *Australian brown coal. Maceral concentrate from lithotypes of bituminous coals; vitrinites and exinites were mainly telocollinites and sporinites, respectively. Turkish lignite.
Similar observations were made by Hombach.ll No excitation spectra have been reported, although these should provide valuable information about the electronic absorption spectra of coals. Lin and co-workers3have shown that dried pyridine extracts form solids which have emission properties similar to, though not identical with, those of the parent vitrinites. Thus, extracts contain a large fraction of the parent coals and their fluorescence spectra should relate to the fluorescence of simple molecules and to the emission from polished surfaces. The present investigation reports the fluorescence spectra of pyridine extracts of brown and bituminous coals and studies the parameters that characterize the spectra in order to determine the extent to which the fluorescence gave information about the aromatic species present. Experimental Section The solid fuels investigated consisted of two medium-volatile, British bituminous coals, bituminous vitrinite (telocollinite) and (1)Stach's Textbook of Coal Petrology, 2nd ed.; Gebruder Borntraeger: West Berlin, 1975. (2) Bensley, D. F.; Crelling, J. C.; ACS Symp. Ser. 1984 No. 252, 33. (3) Rui Lin; et al. Prepr. Pap.-Am. Chem. SOC.,Diu.Fuel Chem. 1985, 30(4),315 and references therein. (4) Parker, C. A. Photoluminescence of Solutions; Elsevier, Amsterdam, 1968. (5) Drake, H. A. G.; et al. Fuel 1978,57, 663. (6) Dryden, I. G. C. In Chemistry of Coal Utilisation;Lowry, H. H. Ed.; Wiley: New York, 1963; Suppl. Vol. 1, p 232 ff. (7) Davis, M. R.; Abbott, J. M.; Gaines, A. F. Fuel 1985, 64, 1362. (8) Given, P. H.; et al. Fuel 1986, 65, 155 and references therein. (9) Brown, G., personal communication,to be sumitted for publication, 1986. (10) Retcofsky, H. L.; Brendel, T. J. Friedel, R. A. Nature (London) 1972, 240, 18. (11) Hombach, H. P. Thesis, University of Munster, 1972.
0 1987 American Chemical Society
Energy & F u e l s , Vol. 1, No. 5, 1987 393
Pyridine E x t r a c t s of Coals
Table 11. Quantum Yields of Pyridine Extracts (Excitation at 354 and 370 nm) a t Room Temperature" Frickley Frickley Manvers Manvers Cresswell Gedling Y allourn vitrinite extinite vitrinite exinite 0.058 0.02 0.077 0.083 0.067 0.11 0.064 a
Standard error: =t0.006.
Table 111. Stern-Volmer Quenching Constants (dm3 mol-' s-l at 22 "C; Excitation at 420 nm) fuels" pyridine extract from G C Y E S T A N methanol b b b c c c b b 2-propanol b b b c c c b b KOH/methanol 400 200 6 c c c 80 e triethanolamine 1.6 0.9 1.2 2.1 2.8 3.3 9.2 170 diethanolamine 2.1 1.3 0.7 c c c c c triethylamine 0.2 0.8 b c c c c c N,N-diethylaniline 11 16 13 21 20 14 41 d
:--'-
--
,I J
6
100
Em-Ex.30 nm
b
8ol
t
40
20
o % k e i a k %
3 4 0 320 4 0 0 480 520 Excitation wavelength (nm)
0
Emission wavrlerqth (nm 1
" G = Gedling coal; C = Cresswell coal; Y = Yallourn brown coal; E = Elbistan lignite; S = Seyitomer lignite; T = Tunpbilek lignite; A = Anthracene; N = naphthalene. bNo quenching. CNoobservations. About 50 000. e Anomalous, fluorescence varied with time: excitation for anthracene, 370 nm; excitation for naphthalene, 320 nm.
100 Ex.456nn
40
exinite (sporinite) concentrates whose analyses have been given in detail elsewhere,' and brown coals from Australia and from Turkey, which generally gave lower yields of pyridine extracts than the bituminous samples. Analyses of the fuels are given in Table I. T h e pyridine used was either of spectroscopic quality or was AnalaR grade further purified by column chromatography. Pyridine extracts were obtained by refluxing coals crushed to -212 63 km with pyridine for a t least 24 h in the absence of air until further extracts were colorless. Uncorrected and corrected fluorescence spectra were measured on Aminoco-Bowman and Perkin Elmer LS5 spectrofluorometers, respectively. Slit widths were maintained constant a t 2.5 nm. Stern-Volmer quenching constants were measured at room temperature by determining the emission spectra of a series of mixtures consisting of constant volumes of pyridine extract and quencher (dissolved in pyridine). The concentration of the pyridine extract was maintained constant throughout the series, but the concentration of quencher was varied. The ratios of the maximum emission intensities in the unquenched and quenched states were plotted against the concentrations of quencher and the Stern-Volmer quenching constant calculated from the slope of the initial linear portion of the graph. Quencher concentrations were selected to give some five points over the linear region. Further experimental detail is given in Table 111. Solutions were excited by a synchronously pumped Spectrophysics cavity-dumped dye laser operated at 339 nm, and emission at selected wavelengths was monitored by single-photon counting after each 10-ps pulse of excitation. Pulses were repeated a t 4 MHz. The dye laser was pumped by a mode-locked, Spectrophysics Model 171 argon ion laser. The experiment was continued until a total of about 30 000 "counts" had been registered. The resulting decay of fluorescence, freed from instrumental parameters (Figure 4 is a typical example) was simulated by a sum of exponential decays whose lifetimes and preexponential factors were chosen by computerized least-squares fitting. Photon counting permitted life times on the order of nanoseconds to be measured. Thus a distribution of fluorescence lifetimes was obtained. T h e detailed experimental procedure is discussed in ref 12. Quantum yields relative to anthracene [d = 0.301 were obtained at room temperature by comparing the emission intensities of dilute [absorbance < 0.11 solutions in pyridine. Corrections were made for the small self-absorption that occurred. Several measurements were made on each solution by using 354- and 370-nm
+
(12) Ghiggino, K. P.; Roberts, A. J.; Phillips, D. Adu. Polyn. Sci. 1981, 40, 69. (13) Becker, R. S. Theory and Interpretation of Fluorescence and Phosphorescence; Wiley: New York, 1969; p 118 and references therein.
20
Emission wavelength (nm)
F i g u r e 1. Yallourn pyridine extract fluorecence spectra: (a) corrected excitation spectrum, emission 520 nm; (b) synchronous spectrum, A = 30 nm; (c) emission spectrum, excitation 340 nm; (d) emission spectrum, excitation 390 nm; (e) emission spectrum, excitation 445 nm; (f) emission spectrum, excitation 456 nm. All spectra were obtained a t the same concentration. Em-Ex-3Onm
I
so
5
60-
I
I
J d
J 40-
40
2 0-
20
L J
A'
0 t 2 b O ' 320'460' 5d3 Excitation Wavelength (nm)
%O
3 2 0 4 0 0 480 563 Emission wavelength (nm)
100 Exs460nm 80
I
40b2oP I
C
Y 6
40
20
40 IDt
20
400
480
U
4 0 0 480 560 480 Emission wavelength (nm)
560
U 480
560
F i g u r e 2. Gedling pyridine extract fluorescence spectra: (a) corrected excitation spectrum, emission 520 nm; (b) synchronous spectrum, A = 30 nm; (c) emission spectrum, excitation 335 nm; (d) emission spectrum, excitation 390 nm; (e) emission spectrum, excitation 440 nm; (f) emission spectrum, excitation 460 nm. All spectra were obtained a t the same concentration. light for excitation, emission being monitored at wavelengths at which anthracene gave maximum emission. Crushed (-63 fim) Cresswell, Gedling, and Yallourn coals were sonicated [14 wm vibration at 23 kHz] for 2 h with a cationic detergent [CTAB, hexadecyltrimethylammonium bromide) so that the particles were
394 Energy & Fuels, Vol. 1, No. 5, 1987 a
Em=520nm
Clark et al. I
1 0 1
Em-Ex.2Onm
b
sol
60
I
40t
Excitation wavelength (nm)
Emission wavelength (nm)
E m z 5 2 0 nm
, , , , , ,
601-
b
A
40
20
'240 320 4 0 0 480 '320 4 0 0 480 5 6 0 Excitation Wavelength Inm) Emission wavelength tnm)
too Ex. 465 nn
,Z
Channels
Figure 4. Typical decay curve yielding fluorescence lifetimes of the pyridine extract of Gedling coal. Table IV. Fluorescence Lifetimes, T,and Preexponential Factors, A , of Pyridine Extracts emission wavelength, coal nm A, T,, ns A, T,.ns A, T,.ns Cresswell 410 0.5 0.8 0.4 3.2 0.1 12.5
60
c
Gedling
4
5
40
4 0
20
20
Y allourn 200
480
560 4 0 0 480 56 280 Emission wavelength (nm)
560
6;
Figure 3. Cresswell pyridine extract fluorescence spectra: (a) corrected excitation spectrum, emission 520 nm; (b) synchronous spectrum A = 20 nm, 1/20 dilution of the solution used for spectrum a; (c) concentration as used for spectrum b, corrected excitation spectrum; emission 520 nm; (d) concentration as used for spectrum b, emission spectrum,excitation 330 nm; (e) concentration as used for spectrum b, emission spectrum, excitation 390 nm; (f) concentration as used for spectrum b, emission spectrum, excitation 435 nm; (g) concentration as used for spectr um b, emission spectrum,excitation 465 nm. broken up and the resulting solution was able to pass through a 0.45-wm filter.
Results The emission, excitation, and synchronous spectra of pyridine extracts of the three coals are shown in Figures 1-3. Unlike absorption spectra, the corrected excitation spectra show clearly resolved features with a number of peaks and shoulders between 300 and 500 nm. It was not possible to record spectra below 300 nm because of the strong absorption of pyridine in this region. The spectra were similar but not identical: all three extracts showed excitation maxima near 340,390,440, and 470 nm. There was little evidence of fluorescence for excitation wavelengths above 550 nm. Synchronous spectra were also measured. In these experiments, the emission and excitation spectra were simultaneously recorded with a fixed separation of 30 nm between the excitation and emission wavelengths. The results are shown in Figures lb, 2b, and 3b. As with crude oils, the synchronous spectra provide a characteristic fingerprint of the coal extracts. Emission spectra in Figures 1-3 corresponded to excitation at the maxima in the excitation spectra. All the
brown
448 510 575 410 450 514 575 410 440 475
0.5 0.4 0.7 0.5 0.5 0.4 0.7 0.7 0.6 0.7
0.4 0.2 0.2
0.6 0.4 0.2 0.1 0.3
0.3 0.2
0.4 0.4 0.2 0.4 0.4 0.4 0.2 0.2 0.3 0.2
2.7 3.3 2.9 2.9 2.6 3.0 2.7 2.5 2.4 2.2
0.1 0.2 0.1 0.1 0.1 0.2 0.1 0.1 0.1 0.1
10.0 9.4 8.7 11.7 9.6 9.0 8.5 11.7 9.4 8.0
emission spectra were broad and superficially centered around a single maximum, although it seems probable that more than one emission peak was present. Figures 1-3 demonstrate that emission and absorption wavelengths overlap more or less continuously through the 300-550-nm range. The emission quantum yields, measured relative to anthracene, are collected in Table 11. These values are substantially less than those obtained for aromatic compounds in pyridine such as anthracene (4 = 0.3) or isopropylnaphthalene (4 = 0.45). The quantum yield was higher for the exinite concentrates than the vitrinite concentrates or bituminous coals. Yallourn brown coal had the smaller quantum yield. Table I11 shows Stern-Volmer quenching constants for pyridine extracts of the three coals, together with results obtained for three lignites from Turkey. A cavity-dumped dye laser and single-photon detection were used to measure the fluorescence lifetimes of Cresswell, Gedling, and Yallourn extracts. Typical results are shown in Figure 4. Had the extracts consisted of a single fluorescing structure moving randomly in a homogeneous solvent, emission intensity (counts, Figure 4) would have decreased exponentiallywith time (channels, Figure 4) with Table IV) defined by the rea fluorescence lifetime (T, ciprocal of the rate constant. In fact, when the extracts were excited by light a t 339 nm, the fluorescence decay curve (Figure 4) showed a complex behavior that was described best by the sum of at least three exponential decays. The fitting procedure using three exponentials should not be over interpreted but at least indicates that there were a number of emission processes with lifetimes in the range 0.1-10 ns. The distribution of lifetimes was
Pyridine E x t r a c t s of Coals
Energy & Fuels, Vol. 1, No. 5, 1987 395
Discussion Fluorescence of Vitrinite and Exinite Concentrates. Figures 1-3 show marked differences between the emission properties of the bituminous coals and those found for the Yallourn brown coal. The differences are most apparent in the synchronous spectra. The two bituminous coals were rich in vitrinite and prompt the question as to the difference between the fluorescence spectra of vitrinite and exinite. Concentrated vitrinites (telocollinites) and exinites (sporinites) were prepared from Carboniferous coals,' and experiments showed that the emission spectra of their pyridine extracts were qualitatively similar. Excitation spectra of both vitrinites and exinites had maxima in the 340-,390-, 440-, and 470-nm regions mentioned above, but as yet, too few samples have been tested for trends to appear, for example in terms of coal rank (carbon content). The effects of concentration on the fluorescence spectra are illustrated in Figure 5. The variation of fluorescence with concentration is perfectly n0rma1.l~ As the concentration of the vitrinite or exinite extracts increased, the fluorescence became more intense and shifted to longer wavelengths. At relatively high concentrations the fluorescence collapsed. This phenomenon is consistent with self-absorption by the extracts where the absorption is most intense at the shorter wavelength^.'^ All the results show that exinite and vitrinite extracts of Carboniferous coal fluoresce similarly. No explanation can yet be offered of the structural differences indicated by Figures lb, 2b, and 3b to exist betweeen Yallourn and the other coals. Fluorescence Quenching. The absence of quenching by 2-propanol and methanol and the rapid quenching by amines (Table 111)was consistent with the expected aromatic photochemistry and the absence of carbonyl phot0~hemistry.l~ The Stern-Volmer constant for the reaction between N,N-diethylaniline and naphthalene was exceptionally
high, so it was possible to choose a concentration of the amine that completely quenched the emission due to naphthalene but only slightly reduced the emisson due to higher homologues such as anthracene. In fact it was possible to use N,N-diethylaniline to quench the 520-nm emission of pyridine extracts in the 300-340-nm excitation region and make no significant change to the excitation spectrum above 350 nm. Thus, not only were the quenching experiments consistent with the emission originating from polynuclear structures in the coals but also the emission resulting from excitation in the 300-340-nm region could be tentatively assigned to naphthalene structures. It should be emphasized that emission may also result from material such as porphyrins, present in trace quantities,16and from ground-state or excited-state complexes that may be formed in the polymeric structure of the extracts. The observed maxima in the excitation spectra were broad and poorly resolved due to the overlap of many absorption bands for different molecular structures. Further work is needed before it will be possible to assign specific fluorophors to the excitation or synchronous maxima. Naphthalene structures are the dominant species present in pyridine extracts of carboniferous vitrinites,' and it is clear that they account for most of the emission when the extracts are excited by light in the 300-340-nm region. The intensity of this emission is likely to be reduced by absorption by the other structures present, and the excitation is restricted to this relatively high wavelength region by absorption by the solvent, which is strong below 300 nm. Table I11 shows that methanolic potassium hydroxide quenched the fluorescence of the extracts very efficiently. The quenching rate was proportional to the concentration of the potassium hydroxide but independent of the methanol concentration. Replacing methanol with 2propanol changed the quenching constant. Fluorescence Lifetimes. The complex behavior shown in Figure 4 is perhaps not surprising in view of the heterogeneous nature of the material. The fluorescence spectra were broad (Figures 1-3) and result from the overlap of emissions from several components. The most striking observations in Table IV are the similarity of the values and the rather short lifetimes that were detected. The fluorescence lifetimes of naphthalene and anthracene are about 100 and 6 ns, respectively, in fluid solution;13however, the values can be as low as 1 ns when these compounds are incorporated into aromatic p o l y m e r ~ . ' ~ J ~InJ ~polymeric systems, the short lifetimes are thought to result from energy transfer between aromatic structures and low energy traps. In some systems energy "hops" along the polymer until the excitation reaches two aromatic residues, which are close together and in a favorable geometry for excimer f ~ r m a t i o n . ~Such ~~"~~~ a mechanism may also apply in the pyridine extracts; however, their heterogeneous nature could lead to other types of energy trapping. For example, the energy may migrate along the polymer until it reaches a site where electron transfer can occur. The distribution of lifetimes in Table IV cannot distinguish between these possibilities, although an excimer model usually leads to longer emission lifetimes at the higher wavelengths. The range of lifetimes in Table IV is entirely consistent with a polymeric model for the pyridine extracts; the short lifetimes indicate that
(14) Kolthoff, I. M. Treatise on Analytical Chemistry, 2nd ed.; Seitz, W. R., Ed.; Wiley: New York 1981; Part 1, Volume 7, p 161 ff. (15) Barltrop, J. A.; Coyle, J. D. Excited States in Organic Chemistry; Wiley: New York, 1975; p 101 ff.
(16) Bonnet, R.; Czechowski, F. Nature (London) 1980,283, 465. (17) Holden, D. A.; Guillet, J. E.Macromolecules 1980, 13, 289. (18) Holden, D. A.; Wong, P. Y.-J.; Guillet, F. E., Macromolecules 1980, 13, p 295.
Emission wavelength (nm)
Figure 5. Emission spectra (excitation at 375 nm) of pyridine extracts of Frickley exinite. The numbers on the spectra give dilution in powers of 2. Thus spectrum 7 is of the extract with spectrum 0 diluted 27 times with pyridine, which gave a concentration of approximately 1 mg/dm3.
fairly evenly spread and showed little change when the emission wavelength was increased. This is shown in Table IV, where the lifetimes are summarized.
396 Energy &Fuels, Vol. 1, No. 5, 1987
Clark et al. 100
"
300
350
460
b
0
540
Em9520 nm
300
380 460 Excitation wavelength (nm)
540
so
360
400
440
450
O%E%E-?&
520
ok?Tz-%doo
Emission wavelength inml
Figure 6. Fluorescencespectra of Cresswell particles stabilized in aqueous CTAB: (a) synchronousspectrum, A = 30 nm; (b) corrected excitation spectrum, emission = 520 nm; (c) emission spectrum, excitation 330 nm; (d) emission spectrum, excitation 395 nm; (e) emission spectrum, excitation 435 nm; (f) emission spectrum, excitation 455 nm.
there should be at least one aromatic system within every Forster r a d i ~ s . ' ~ J ~This * ' ~implies that in the dilute pyridine extract the aromatic compounds must be either associated as ground-state complexes or covalently linked to a polymer framework. Further investigation of the fluorescence lifeti-mes of coal extracts might give a more detailed explanation of their structure and physical chemistry; the possibility of intramolecular excimer formation is of interest since it could be used to provide information about the flexibility of the polymer segments.12 Fluorescence: Mechanism of Amine Quenching. Amines are known to react with the excited states of aromatic compounds by electron transfer and exciplex f0rmati0n.l~ The bimolecular rate constant for quenching will be given by dividing the Stern-Volmer constant by the fluorescence lifetime. From this, the results in Tables I11 and IV suggest that, for all of the pyridine extracts, the bimolecular rate constants lie in the range (0.02-1.6) X lo9 (lifetime 10 ns) to (2-160) X lo9 (lifetime 0.1 ns) dm3mol-' s-l. The Debye estimate of the diffusion-limited rate constant for reactions in pyridine at 300 K is (6-7) X lo9 dm3 mol-' s-'. This is likely to be an overestimate since the extracts contain aromatic structures that are embedded in moderately high molecular weight material. Thus the results in Tables I11 and IV are consistent with diffusion-controlled quenching by the amines for emissions with the longer average lifetimes. This also explains why the Stern-Volmer constants are lower for the extracts than for pure naphthdene; the difference in the Stern-Volmer constants clearly reflects the difference in the excited-state lifetimes of molecular naphthalene and the coal extracts. For the shorter lived species (ca. 0.1-ns lifetimes) the quenching by N,N-diethylaniline will be at a greater rate than the diffusion-controlled limit and may involve ground-state complexation to give static quenching. In all cases quenching by alkaline methanol was at a rate greater than the diffusion limit, and this implies that the quenching results from a complex formation or a thermal (19) Reference 15, p 112 and references therein.
reaction between the coal and the reagent. Fluorescence of Surfactant-SolubilizedCoal. Figure 6 shows typical absorption and fluorescence spectra for the solubilized coals. The excitation and to a lesser extent the emission spectra in these figures show broad but resolved peaks that may be attributed to molecular structures. The excitation spectra show maxima at wavelengths similar to those observed for the corresponding pyridine extracts, and preliminary measurements show that the fluorescence lifetimes were also similar. At this stage the nature of the solubilized coal is not known. It is not clear to what extent the material is present as solid particles or polymeric material. The limited amount of data available suggests that it resembles the pyridine extracts rather than the bulk material. Optical Properties of Extracts and Solids. It is now possible to postulate a mechanism to explain the emission from polished surfaces of coals. A key experiment in this context was that reported by Lin et al.,3 who dried out pyridine extracts to form solid materials, which emitted fluorescence similar to that of the parent coal. The marked overlap of absorption and emission shown in Figures 1-3 and 6 implies that the overlap integrals between the ground and excited states were relatively close to unity and conditions were almost ideal for transfer of electronic energy. The two major nonradiative mechanisms of energy transfer are Forster transfer (sometimes known as dipole-dipole transfer) and electron-exchange transfer. These require the interacting molecules to be less than 7 and 2 nm apart, re~pectively.'~~'~~'~ As a result the energy-transfer efficiency increases greatly when the concentration of the aromatic species increases. The rate of emission is, in contrast, independent of concentration, so as the concentration of aromatic species increases, the nonradiative energy transfer gains relative to the normal fluorescence. The concentration of aromatics in the polished surfaces of coals is evidently such that almost all of the absorbed light can be transferred nonradiatively from one aromatic group to another. This will result in loss of energy, since the transfer steps must be at best isoenergetic and are more
Energy I%Fuels 1987,1, 397-401 often exothermic, until the energy becomes trapped in a low-energy fluorophor or is lost in nonradiative processes such as electron transfer. The low-energy fluorophor may emit the radiation, but this will now be at a lower energy than would be seen in the extracts where energy transfer is less efficient and emission can still occur from the higher energy molecules, such as the naphthalene derivatives. The model explains why exinites fluoresce more strongly than vitrinites. The vitrinites have a higher aromatic content than exinites and consist of simple molecules entrapped in an aromatic framework, whereas the exinite matrix is predominantly aliphatic. In bituminous sporinites the matrix has about nine aliphatic carbons for every aromatic ring (assuming the aromatic structure is bemenoidam The nonradiative transfer will be facilitated by the high concentration of aromatic species in the vitrinites, so they w illtend to emit less strongly and at longer wavelengths than exinites. This description has major implications for fluorescence studies in coal petrology.
Conclusions The excitation spectra of pyridine extracts showed a number of resolvable peaks, which in bituminous Car~
(20) Davis, M. R. Ph.D. Thesis, University of Aston, 1985.
397
boniferous samples occurred in the regions 340,390,440, and 470 nm. Light above 550 nm did not lead to appreciable emission. The quantum yields for the emission were ca. 0.06. Quenching experiments confirmed that the fluorescence was derived from aromatic species. There was considerable overlap between the absorption and emission spectra, so that energy transfer could occur. The fluorescence lifetimes were in the range 0.1-10 ns, in line with values observed for aromatic polymers. Similar behavior was observed for coals solubilized in a cationic surfactant, where the material is expected to be more solid. Energy transfer probably occurs by the same mechanism as in organic polymers, and it is expected to increase when the local concentration of aromatic species increases. This will be especially true for the polished surfaces of coals where nonradiative energy transfer will lead to a loss of electronic energy until it is ultimately trapped and reemitted from the lowest energy fluorophor that is accessible. The high aromatic content of vitrinites, especially in the polymer matrix, will favor energy transfer compared to the less aromatic exinites, and as a result, the vitrinites will fluoresce less strongly.
Acknowledgment. This work was supported by the SERC and the London University Central Research Fund. We are also grateful to Professor D. Phillips of The Royal Institution for help in measuring fluorescence lifetimes.
Low-Temperature Coal Liquefaction Using n -Butylamine as a Solvent Hideyuki Tagaya,* Jouji Sugai, Masatoshi Onuki, and Koji Chiba* Faculty of Engineering, Yamagata University, 4-3-16 Johnan, Yonezawa, Yamagata 992, Japan Received March 11, 1987. Revised Manuscript Received June 1, 1987
To attain a more effective coal liquefaction process, low-temperature (200-300 OC) coal liquefaction using n-butylamine as a solvent was carried out. Conversions of five kinds of coals including 0alkylated Taiheiyo coal in n-butylamine were far higher than those in tetralin, and the conversion of Taiheiyo coal a t 300 "C for 24 h reached 93%. The substitution reaction of n-butylamine with hydroxyl groups was confirmed in the reaction of n-butylamine with phenol. Such substitution reactions may cause the rupture of noncovalent bonds in coal. The prevention of undesirable reverse reactions such as condensation is attained at low temperature. Furthermore, the ability of n-butylamine to cleave C-0 bonds in esters and ethers coupled with the ability of n-butylamine to undergo substitution reactions with hydroxyl groups in coal may contribute to the high coal conversions.
Many approaches to direct coal liquefaction employ high pressures and temperatures to achieve in one reactor the highest possible coal conversion and distillate yield. High-severity processing also leads to undesirable regression reactions. Undesirable condensation or polymerization reactions of coal fragments can be avoided by liquefaction at low temperatures.' However, with only a few exception^,^,^ not much research has been carried out in (1) Moroni, E. C. F'repr. Pap.-Am. Chem. SOC., Diu. Fuel Chem. 1986, 31(4), 1-4. (2) Kazimi, F.; Chen, W. Y.; Chen, J. K.; Whitney, R. R.; Zimny, B. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1985, 30(4), 402-413. (3) Larsen, J. W.; Mohammadi, M. US Dep. Energy [Rep.],DOEIPC 1983, DOE/PC/50789-T2 1-14.
0887-0624/87/2501-0397$01.50/0
Table I. Analyses of Coals (wt
%)
ultimate anal. (daf) coal Yallourn Taiheiyo Taiheiyo-AC Wandoan Miike
C 67.5 77.5 77.0 77.6 82.2
H
N
5.7 6.4 7.6 6.2 6.2
0.6 1.0 2.0 0.8 1.0
prox. anal. (dry)* ash mois FC VM 26.2 (7.2) 2.2 13.2 41.0 43.6 4.7 35.3 44.9 15.1 (6.8) 15.1 2.8 14.1 57.3 25.8 13.4 (3.8) 7.3 9.2 38.6 44.9 15.4 (7.3) 9.5 2.2 44.4 43.9 10.6 (3.6) Odiff
(OH")
a Content of hydroxyl oxygen. *Key: mois, moisture; FC, fixed carbon; VM, volatile matter. 0-alkylated Taiheiyo coal.
this area because of low extraction yields. It is well-known that basic nitrogen compounds such as quinoline are in@ 1987 American Chemical Society
