Table Of Content1991 International Conference
on Coal Science Proceedings
Edited by the
International Energy Agency Coal Research Ltd
Proceedings of the International Conference on Coal Science
16-20 September 1991
University of Newcastle-upon-Tyne
United Kingdom
U T T E R W O R TH
I N E M A N N
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Electron Transfer in Coals
John W. Larsen, Robert A. Flowers II, Peter Hall, Bernard G.
Silbernagel & Layce A. Gebhard
Chemistry Dept., Lehigh University/Exxon Res. & Engr. Co.
Most coals containing tetracyanoquinodimethane (TCNQ) at a level of one
TCNQ per aromatic unit in the coal are bright blue. Coals containing
similar amounts of tetracyanoethylene (TCNE) are bronze colored. The only
exceptions we have discovered are low volatile bituminous coals and
anthracites. We will present evidence here that these striking and
unusual colors are due to the formation of valence bands in coals
resulting from the addition of these two good electron acceptors.
A significant amount of electron density is transferred to TCNQ and TCNE
in coals. Both of these materials are good organic oxidants. It has been
demonstrated that there is a linear relationship between the amount of
electron density transferred from a donor to TCNQ and the shift in the
TCNQ CN infrared stretching frequency which occurs upon electron
transfer[1]. Electrons added to the TCNQ must enter its lowest unoccupied
molecular orbital (LUMO). This is an anti-bonding orbital and its
occupation weakens bonds throughout the structure. It is this bond
weakening that results in a shift in the CN stretching frequency. This
shift is 44 cm"1 for transfer of one electron. Transfer of one electron
to TCNE gives a 47 cm-1 shift. It is thus possible to use infrared
spectroscopy to determine the extent of electron transfer from coals to
these acceptors. TCNQ in the following coals at several molar levels
corresponding to between 3% and 100% of the concentration of aromatic
structures gives the full 44 cm-1 shift: Beulah-Zap Lignite (74%), Wyodak
Sub-bituminous (76%C), Illinois No. 6 (81%C), Pittsburgh No. 8 (85%C),
Upper Freeport (88%C). The concentrations of aromatics were calculated
from the data of Solum[2]. This shift indicates that the LUMO of TCNQ
contains one electron and is thus half occupied. Similar behavior was
observed with TCNE. If there also exists a linear relationship between
extent of electron transfer and the CN shift for TCNE, the observed 36 cm"1
shift corresponds to a LUMO which is 30% occupied. It is striking and
important that all of the TCNQ or TCNE added to these coals is shifted.
There is no evidence for a CN absorption band at its normal position and
there is no CN having an intermediate value for the stretching frequency.
ESR results demonstrate that the TCNQ radical anion is not formed. The
ESR spectra do show that there is a small increase in the spin population
in the doped coals. These data are presented in Table 1 together with the
calculated spin population if all the added TCNQ formed the radical
anion. The spectra themselves do not show any of the lines expected for
the radical anion.
Aromatic molecules of the types found in the coal are not individually
capable of transferring significant electron density to coal. This is in
1
accord with theory and is demonstrated by data which appear in Table 2. All
of the aromatic systems examined, a sampling of which appears in Table 2,
bring about shifts in the CN stretch between 3 cm"1 and 5 cm"1. This
demonstrates conclusively that individually these molecules are not capable
of bringing about the behavior observed with coals.
Our experiments have established that the TCNQ LUMO is half occupied in many
coals, that this occupation does not result from the formation of the
radical anion, and that individual aromatic structures of the types found
in coals cannot populate the LUMO producing the large IR shifts observed.
Similar experiments with coal extracts demonstrate that the observed effects
are not due to the presence of mineral matter. The most reasonable
explanation for these data is that the addition of the electron acceptors
to coal results in the formation of extended electronic valence bands.
These are well established in solid mixtures of good electron donors and
acceptors and there exists an extensive literature dealing with them[3].
Their existence in an amorphous system such as coal is surprising and
noteworthy. The data are consistent with the formation of such bands by the
extended overlap of the aromatic Π systems with those of the added oxidants.
The LUMO of the oxidants must form part of this band structure which is half
occupied in the case of TCNQ and 30% occupied in the case of TCNE. The fact
that electronic transitions occur at infrared frequencies for anthracites
and other high rank coal, demonstrates that extended Π structures occurs
naturally in these materials[4]. This is not the case for the lower rank
coals, though we seemed to have induced even more extensive Π structures by
the addition of the oxidants.
Our explorations of the electronic and physical structure of these inter-
esting materials continues and we are also exploring the role of internal
electron transfers in the chemistry of coals.
ACKNOWLEDGEMENTS
We thank the U. S. Department of Energy for support of the Lehigh University
portion of this work.
REFERENCES
1 Chappel, J.S., Bloch, A.N., Boyden, W.A., Maxfield, M., Poehler, T.O.,
Cowan, D.O., J. Am. Chem. Soc.. 1981, 103, 2442.
2 Solum, M.S., Pugmire, R.J., Grant, D.M., Energy & Fuels. 1989, 3, 187.
3 Soos, Z.G., Klein, D.J., Charge Transfer in Solid State Complexes in
"Molecular Association", Vol. (Ed. R. Foster) Academic Press, New York
1-109,1981
4 Brown, J.R., J. Chem. Soc. (London), 1955,744-52.
2
Table 1. EPR Spin Densities (Measured and Calculated)
of Coals Containing TCNQ
Calculated
Spin Density
Coal or Coal Spin Density x 10"19 for Complete
Derivative spins/gram Transfer x 10~19
Beulah-Zap Lignite 0,. 12
Beulah-Zap Lignite 0,. 74 5.90
w/3% TCNQ
Beulah-Zap Lignite 1,. 91 29.4
w/20% TCNQ
Beulah-Zap Lignite 1,. 18 120
w/100% TCNQ
Illinois No. 6 0,. 34
Illinois No. 6 2,. 70 115
w/100% TCNQ
Lewiston-Stockton 0,. 91
Lewiston-Stockton 0,. 80 5.90
w/3% TCNQ
Lewiston-/Stockton 7,. 03 130
*Spin Density EPR Measurements were obtained by
Dr. Bernard Silbernagel and Layce Gebhard, Exxon
Research & Engineering Company
Table 2. Changes in CN Stretching Frequency in Formation
of Aromatic Complexes with TCNQ in CHCI3
Compound (donor) Δ v (cm"1)
Fluoranthene 3.2
Pyrene 4.4
Phenanthrene 3.5
4-Phenylphenol 3.7
3,5-Dimethoxyphenal 3.6
2-methoxynaphthalene 3.7
3
EXAMINATION OF MACROMOLECULAR NETWORK OF COAL
BY DIFFERENTIAL SCANNING CALORIMETRY
Kouichi Miura, Kazuhiro Mae, Shoji Takebe and Kenji Hashimoto *
Research Laboratory of Carbonaceous Resources Conversion Technology,
*) Departmant of Chemical Engineering,
Kyoto University, Kyoto, 606 JAPAN
1. INTRODUCTION
Several attempts[l-4] have been made to estimate the non-covalent bond in coal from
calorimetric measurements during the solvent swelling, in which the heat of wettability of
the strong polar solvent was mainly measured using the micro calorimeter. However, the
information on the non-covalent bond could not be successfully extracted from these studies
because of the strong interaction between the polar solvent and the coal. Non-polar or weak
polar solvents should be used to study carefully the macromolecular structure of coal through
the solvent swelling.
From this viewpoint we used the non-polar solvent for the swelling, but the swelling was
performed at the temperatures as high as 220 °C. Then we tried to estimate the heat required to
break the non-covalent bonds in coal by measuring the DSC and TG profiles of this swollen
coal.
2. EXPERIMENTAL
The properties of five coals used are given in Table 1. These coals were ground into the
particles of less than 74 μπι in diameter, then dried in vacuo at 110 °C for 24 h before use. Four
solvents, tetralin (Tet.), 1-buthanol (1-BuOH), ethanol (EtOH) and quinoline (Q) were used
for the swelling. Tetralin was mainly used because of its non-polarity. The swelling of coal
was performed by simply mixing coal and solvent by the ratio of 1 to 0.6 by weight in a closed
tube. It was performed at 30 °C for 24 h for the polar solvent, and was performed at 100 to 220 °C
for 1 h under IMPa of N2 for the non-polar solvent. The swollen coal (STC) was evacuated at
70 °C for 24 h to completely remove the solvent from the STC. The swelling ratio of the STC
and thus prepared vacuum-dried coal (VDC) was measured by the volumetric technique [5].
Tobk 1 Prypçrti&QfÇQQl
Proximate Analysis (wt%) Ultimat e Analysis (wt%daf) Swelling Ratio
FC VM ASH C H N S O STC VDC
Morwell (MW) 48. 2 50. 3 1. 5 67. 1 4. 9 0. 6 0. 3 27. 1 1. 34 1. 00
Jacobsranch(JR) 48.8 39. 4 11. 8 68. 3 5. 5 0. 9 0. 5 24. 8 1. 32 1. 24
Taiheiyo (TC) 43. 2 45. 8 11. 0 74. 5 6. 0 1. 3 0. 2 18. 0 1. 33 1. 28
Illinois #6 (IL) 57. 8 34. 8 7. 4 77. 1 5. 6 1. 5 3. 9 11. 9 1. 22 1. 01
Liddel (LD) 57. 4 34. 5 8. 1 83. 4 5. 5 2. 2 0. 6 8. 3 1. 20 1. 04
To examine the change of non-covalent bond of the coai during the swelling by
calorimetrically, the DSC profile and the TG curve of the raw coal, the STC, and the solvent
4
were measured under a constant heating rate of 5 °C/min by use of a differential scanning
calorimeter (Shimadzu Co., DSC 50) and a thermobalance (Shimadzu Co., TGA 50),
respectively.
3. RESULTS AND DISCUSSION
The swelling ratio of TC treated with tetralin and that of the VDC are shown against the
swelling temperature. TC began to swell at around 70 °C, and seemed to reach an
equilibrium swelling ratio of ca. 1.35 over 150 °C. The swelling ratio of the VDC, ^VDC» is
shown by the closed key in Fig. 1. At the swelling temperatures lower than 100 °C the ^VDC
value is exactly unity. This means the swollen then vacuum dried coal returned to the raw
coal as far as the volume is concerned at these temperatures. On the other hand, the ^VDC
value does not return to unity at the swelling temperatures of 150 and 220 °C, and
interestingly ^VDC is very close to ^STC at 220 °C. This indicates that the swelling at 220 °C
is almost irreversible. This is also the case for JR as shown in Table 1. The irreversibly
swollen VDC is utilized later to estimate solvent-coal interaction solely.
Hf
HS
HC
HC
he
0
HS-C
0 50 100 150 200 250 ii nil nnii iipn
Swelling Temperature (°C) Coal Solv. Coal STC STC
25°C 25°C 220°C 25°C 220°C
Fig.l Changes of the swelling ratios Fig.2 Schematic enthalpy level of
of the STC and its VDC various state of coal and STC
We have clarified that the swelling by tetralin affects solely to the non-covalent bonding of
coal as far as the swelling is performed at a temperature lower than a critical temperature,
T. The critical temperature, T , which varied with coal type was estimated to be around 230
c c
°C for TC, for example. In the solvent swollen coal some non-covalent bonding are broken,
and the macromolecular network is expected to be altered from the raw coal. In this work we
have tried to estimate the energy required to break the non-covalent bonding through the TG
and DSC measurements during the heat up of raw coal, solvent, and the STC from 25 to 220
°C.
Figure 2 shows schematically the enthalpy levels of the raw coal heated to 220 °C (state A), the
STC at 25 °C (state B), and the STC heated to 220 °C (state C) on the basis of the raw coal and the
solvent at 25 °C. The enthalpy at state A, He', is easily measured. In the STC at state B some
non-covalent bonding are broken, and the solvent-coal interaction exists. Therefore, the
5
enthalpy at state B, Hj, is the sum of the energy to break the non-covalent bonding, HNC ana
that deriving from the solvent-coal interaction, Hs-C· The value of HNC is expected to be
positive, whereas that of Hs-C is negative. The enthalpy at state C, Hf, consists of the
enthalpy of the solvent, Hs, and the coal, He. Hs is easily obtained from the DSC
measurement of the solvent. The enthalpy difference ΔΗ (=Hf -Hj) can be obtained from the
DSC and TG measurements of the STC. Then we can extract the value of Hi from the
measurable quantities if we assume that the coal at state C is the same as that at state A,
namely Hc=Hc', by
Hi = Hf - ΔΗ = He' + Hs - ΔΗ (1)
If we could further estimate the value of Hs-C, we can determine the enthalpy required to
break the non-covalent bonding by
HNC = Hi-Hsc (2)
It was possible to determine the value of Hg-C for the coal swollen by tetralin by measuring
the DSC and TG profiles during the desorption of the tetralin from the irreversibly swollen
VDC which adsorbed tetralin. Figure 3 shows the TG and DSC curves for tetralin and the TC
swollen at 100 °C and 220 °C. The endothermic heats integrated over 25 to 220 °C correspond to
Hs for tetralin , H for the coal and ΔΗ for the STC, respectively.
c
0 50 100 150 200 250 Jet. 1-BuOH EtOH Q
Temperature (°C) 100°C 30°C 30°C 30°C
Fig.3 TG and DSC profiles ofSTCs Fig.4 Effect of solvent type on H
t
and tetralin
Figure 4 shows the values of Hi of TC estimated using several solvents. Hi decreased with
the increase in the polarity of solvent, and it was large negative value in the case of the
swelling by quinoline as mentioned in other earlier works [1-4]. This is because the polar
solvent interacts with the functional groups in the coal and the strong coal-solvent hydrogen
bonding is formed in place of the coal-coal hydrogen bonding during the swelling, leading
to IHS-CI>HNC·
6
Figure 5 shows the changes of the values of Hs-C and HNC against the swelling ratio. All the
values were estimated by utilizing tetralin as the solvent. Hs_c decreased with the increase
of the swelling ratio. This means that the solvent interacts strongly with the stronger non-
covalent site in the coal at higher swelling ratio. HNC increased with the increase in the
swelling ratio, and reached to 110 kj/kg-coal at the swelling ratio of 1.3. Since Hs-C is much
smaller than HNC» we can safely say that the enthalpy to break the non-covalent bonding
was successfully measured by use of tetralin as a solvent.
Figure 6 shows the effect of coal type on the value of HNC· The values of Hs-C were estimated
from the data in Fig. 5. HNC increased with the decrease in coal rank, because the lower
rank coal contained more oxygen and many non-covalent bondings. If we further assume
that the dissociation energy of all the non-covalent bonding are 28 kJ/mol, we can roughly
estimate that about 35 % of oxygen of coal are broken through the swelling for these three
coals. This is just a rough estimation, but we are expecting that the method proposed here
will give a clue for studying the non-covalent bonding, namely the macromolecular
network of coal.
150 1 1 1 1 T 1.6 Tet.100°C °5TC
TC-Tet. ■S U «s* VDC
& 1.2
100 HNC .eJn 1.0
50 /
o100
a
o
~ "" - - -Δ- - _
^ Hs-C o
V 50
-50 1 1 1 1 Ï 0
1.0 1.1 1.2 1.3 1.4 1.5 x MW TC LD
Swelling Ratio (- )
Fig.5 Changes of Hpjç and HS_Q Fig.6 Effect of coal type on Htfç
through the solvent swelling
4. CONCLUSIONS
We presented a method to estimate the heat required to break the non-covalent bonding in
coal using a differential scanning calorimetry (DSC) and a thermogravimetry (TG).
Utilizing DSC and TG profiles of three samples: the coal swollen by a solvent, the raw coal,
and the solvent measured by heating them up to 220 °C, we could successfully estimate the
enthalpy which is related to the strength and the amount of non-covalent bonds in coal.
PREFERENCES
(l)Tempy,G.K. et al., Energy & Fuels 1988, 2, 787-793.
(2)Fowkes,F.W.,Jones,K.L.,Li,G.,Loyd,T.B. Energy & Fuels 1989 3, 97-105.
(3)Groszek,A.J.,Templer,C.E. Fuel 1988, 67, 1658-1661.
(4)Hollenhead,J.B. et al. Energy & Fuels 1988 2, 121-124.
T
(5)Green,T.K.,Kovac,J.,Larsen,J.W. Fuel 1984, 63, 935-938.
7
COAL-WATER MOLECULAR INTERACTIONS
Leo J. Lynch, Wesley A. Barton and David S. Wesbter
CSIRO Division of Coal and Energy Technology
PO Box 136, North Ryde 2113, Australia
1. INTRODUCTION
Water sorbed or otherwise intimately associated with solid-like materials such as coals has
properties which differ somewhat from those of the normal thermodynamic states of bulk
water. These differences can be appreciated and understood best at the molecular level and,
in particular, if the microdynamic processes or molecular dynamics of the water are
considered. An experimental technique which allows such interpretations is proton nuclear
magnetic resonance ( H NMR) spectroscopy. There are well developed theories relating
H NMR relaxation data to molecular dynamic models of the system under study [1]. In
particular, the measured H NMR transverse relaxation often allows distinction of rigid
from mobile molecular structures/lattices on the basis of whether the molecular
reorientation rates are below or above -10 Hz respectively. Modifications to the water
properties result from molecular interactions between water and substrate and thus to an
extent are determined by the nature of the substrate. For coals the extent of polar
interactions is greatest for species rich in oxygen and other electronegative atoms.
Hydrophobie effects related to apolar regions of the coal, are also likely to contribute to
perturbation of the water. That vicinal water is perturbed with respect to bulk water leads to
the concept of interacting as distinct from non-interacting water and allows definition of the
maximum capacity of a material to interact with water, i.e., its saturation water content
(swc). The swc is expected to be equivalent to the equilibrium water content (ewe) at 100%
relative humidity (rh).
The substrate also is likely to be affected by interaction with water, but most models of
sorbed water systems imply that the substrate is inert. Clearly this is not so for brown coals
which are altered irreversibly when dried from the bed moist condition [2]. Higher rank
coals are less affected, but the extent can still be important for understanding these coal-
water systems. Water vapour sorption isotherms often are used to characterise water-
substrate systems - the isotherm shape can be related to models of the system [3] and the
swc estimated as the rh approaches 100%. However, the accuracy of this detennination is
limited by instability of the system near 100% rh. In this paper a refinement of a previously
described [4] *H NMR method to characterise and estimate the swc of coal-water systems
will be discussed and applied to a high inertinite, high moisture content coal.
2. EXPERIMENTAL
Crushed (-212 μιη) samples of the Australian coal studied (82.6%C, 4.5%H, 10.7%O (diff)
(daf); 7.5% ash (ad); 28% vitrinite, 69% inertinite (vol. % mmf)) were prepared with ewe's
8
