-
Investigation of a hydrothermally treated brown coal by i.r. and
chromatographic methods
Borbhla Zelei, Istvgn Hazai, G6bor Alexander, Tam&s Szbkely,
Henning Zobel, Hein Klare and Gerhard Keil Hungarian Academy of
Sciences, Research Laboratory for Inorganic Chemistry, H- 7 17 2
Budapest, Budatirsi ut 45, Hungary *Academy of Sciences of GDR,
Institute of Chemical Technology, DDR- 1199 Berlin, Rudower
Chaussee 5, GDR (Received 28 June 1985; revised 14 March 1986)
1.r. spectra of a brown coal (Merseburg, GDR) and two
hydrothermally treated samples, and their solvent extracts, were
studied. Difference spectra showed a significant increase in the
aliphatic absorptions, a decrease in hydroxy groups and changes in
the carbonyl, carboxylate and carbonate group contents as a
consequence of the treatment. Eight fractions of each extract
separated by column chromatography were also investigated, the
hydrocarbon fractions by gas chromatography and the other fractions
by i.r. spectroscopy. The non-uniform distribution of n-alkanes in
the original sample became uniform after the treatment.
(Keywords: brown coal; instrumental methods; structural
analysis)
1.r. spectroscopy allows both organic and inorganic constituents
in coal to be investigated in their natural form, without prior
separation or chemical treatment. The spectrum of a coal sample
consists of those of all its constituents superimposed1-3; its
complexity causes difficulties in the assignment of absorptions.
Except for some minerals4, no individual compounds, only functional
groups, can be identified.
The atoms in low rank coal occur in the average proportions: 100
C, 8&90 H, l&20 0,2-3 N and l-2 S in a great variety of
chemical environments. Consequently, the i.r. spectrum is dominated
by the bands of functional groups containing C, H and 0 atoms;
>C=O, COO- and 3COH groups give rise to especially intense
absorptions. Some minor, but important, constituents, e.g. organic
S and N compounds, may be masked by the complex band envelopes.
Because of the complexity reliable quantitative analysis requires
great care5-.
The inorganic constituents can be investigated separately, after
the low temperature ashing of coals4, and carbonization and
chemical reactions of coal can be followed by studying changes in
i.r. spectra.
By extraction with various solvents different amounts of low
molecular compounds trapped in the coal matrix can be removed. With
the same solvent (e.g. a toluene- ethanol mixture) the composition
of the extracts varies with coal rank and the yield decreases as
rank increases. Various hydrocarbons occur in small quantities in
coals and coal extracts, and the occurrence of some species, e.g.
straight chain hydrocarbons in lignites and sub- bituminous
coalslo, and substituted phenanthrenes in hard coals, can be
correlated with coal rank. By high resolution capillary gas
chromatography these hydro- carbon mixtures can be separated and
identified.
0016-2361/87/02018946$3.00 0 1987 Butterworth & Co.
(Publishers) Ltd.
This paper seeks to characterize the main structural changes in
a Merseburg coal sample and its extract after hydrothermal
treatment.
EXPERIMENTAL
Merseburg coal (C,) was subjected to hydrothermal treatment in
the absence and the presence of sodium formate, to produce samples
C, and C,, respectively. The characteristics of the samples and the
experimental details of the hydrothermal treatment and its effect
on the composition of the coal, have been described in a previous
paper12. The three materials were exhaustively extracted with 1 :l
toluene-ethanol (Soxhlet) to give respectively the extracts El, E,
and E,.
The extracts (1OOmg) were fractionated by open column
chromatography on a 30 x 1 cm Kieselgel column (0.063-0.2 mm grain
size) using Farcasius method 3 slightly moditied14. Eight fractions
(F,,_,,, F,,_,,, F,, _ 38) were eluted successively with 50ml
volumes of the following solvents (flow rate% 1 ml min- ): 1,
hexane; 2, hexane-benzene 85:15; 3, chloroform; 4, chloroform-
diethyl ether 97:3; 5, diethyl etherethanol 97:3; 6, methanol; 7,
chloroform-ethanol 97:3; 8, pyridine- ethanol 97:3. The successive
eluates were evaporated to dryness under vacuum at room
temperature, and the residues weighed. The first two fractions,
mainly aliphatic and aromatic hydrocarbons13, were redissolved in
n- hexane and analysed with a Perkin-Elmer F22 gas
chromatograph.
Transmission i.r. spectra were recorded between 4000
FUEL, 1987, Vol 66, February 188
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Hydrothermally treated brown coal: B. Zelei et al.
3000 1500
cm-
1000 500
Figure 1 Infrared spectra of Merseburg coal samples; C,,
original coal; Cz, hydrothermally treated coal; T=623 K, P=8.6 MPa;
Cj, coal hydrothermally treated with 1.5 M HCOONa. C,/C, and C,/C3
the difference spectra of the original and hydrothermally treated
coal samples. To emphasize the direction of the main compositional
changes a baseline is drawn arbitrarily
190 FUEL, 1987, Vol 66, February
-
and 200 cm- with a Perkin-Elmer 225 grating spectrophotometer,
with a resolution of about 4cm-. The coal samples and extracts were
examined as KI pellets (300mg KI and 3 mg sample). Difference
spectra (C,/C, and CJC,) were run very slowly in the regions of
intense absorption. The eluate fractions 3-8 were deposited as
films on CsI discs by evaporation of solutions in 1 :l
toluene+thanol.
Table 1 Characteristic group vibrational bands observed in the
i.r. spectra of Merseburg coal samples and their extracts4.5-7
Wavenumber (cm-) Approximate description
- 3400 v, br 3050 VW 2960 w, sh 2925 s 2900 w, sh 2870 w, sh
2855 m-s
-2650 v, br 2525 VW 1730 v 1700 v 1590 v 157ov 1460~ 1440v 1430
sh 1410 v 1375 m 1270 v
-12OOv, br 1170 v
-1loov
lloo-loo0 VW 870 v 810 w 740-720 v 710 v
- 550 v, br 31Ov(
v(OH .) H-bonded v(CH) aromatic vas(CHJ v,,(CH,) aliphatic and
alicyclic v(CH) aliphatic and alicyclic
vs(CHJ t*,(CH,) v(OH . . .) in carboxylic acids v(SH) v(C=O) in
aliphatic esters v(C=O) in carboxylic acids v(C=O .) in conjugated
ketones v,,(COO-) in carboxylates (humate salts)
t1G) -) in carbonate minerals $CH:) v&O0 -)
yS(CH2) and &(CH3) v,,(COC) in aliphatic esters v(CO)+/S(COH
. .) in phenols vas(COC) in ethers v(CO)+b(COH .) in alcohols v(CC)
aliphatic v,,(SiOSi) in silicate minerals y(CO:-) in carbonate
minerals y(CH) aromatic /&&Hz) and y(CH) aromatic y(COO-)
and fl,,(CO
-
Hydrothermally treated brown coal: B. Zelei et al.
I I I 1 1800 1600 1400
cm -1
Figure 3 The infrared spectrum of the extracts E,, E, and E, in
the range of the carbonyl and carboxylate stretching modes
0 .I F6 F8 : zo- B F3
F5
E F7
2
0" Fl F2
0 111 II I
123 123 123 123 123 123 123 123
Extract number
Figure 4 Mass distribution of the fractions F,,-F,,, F,,-F,, and
Far-F,, of the extracts obtained by sequential elutions in a liquid
chromatographic separation. Residues not eluted from the columns
were: R,=28%, R,=3% and R,=7%
relatively simple and does not resemble that of the parent coal;
it reflects a substantial amount of long aliphatic chains and small
amounts of
-
Hydrothermally treated brown coal: B. Zelei et al.
C
-.-. F35
. . . . . F 36
4000 3000 2000
Figure 6 The infrared spectra of the fractions F33-F36
Table 2 Functional groups and compounds types observed in the
fractions of E, extract (from g.l.c. and ir. spectra)
Fractions increasing polarity of eluant
Groups F 3, F,, F,, F,, F,, FX F,,
CH aliphatic 0 0 0 0 * * * CH aromatic 0 0 * - C-0-C ether _ _ :
i C=O ester _ _ : : : C=O acidic _ _ 0 0 * c=. conjugated _
carbonvl 0 0 * 0 *
COH alcoholic, phenolic, acidic _ _ 0 0 0 0 *
CPP- carboxylate anionic _ _ _ _ _ 0 -
0, a large amount; *,, medium quantity; 0, a small amount; -,
nil
The effect of the HCOONa additive was almost negligible (cJ Fzl,
F,,) and the change was most probably caused by the heat treatment;
it has been shown* that thermal alteration of organic sediments
eliminates any carbon numbered preference. The chromatograms of the
aromatic fractions F,,, F,,, Fs2 were also similar and it can be
concluded that the hydrothermal treatment has little effect on the
aromatic hydrocarbons, at least on those accessible to g.1.c. The
aromatic compounds were not studied further.
Fractions 3-8 were characterized by i.r. spectroscopy, (cJ
Painter et ~1.~). Thoseeluted with pyridine (F,,, F,,, F,,) showed
very complicated spectra, probably due to pyridine-carboxylic acid
complexes; pyridine is known to be retained in coal extracts. These
spectra were not further evaluated. Fractions 4 and 7 in the three
samples gave similar i.r. spectra.
With the exception of those eluted with methanol (F,,, F,,,
F,,), the fractions eluted from all those samples with the same
solvent gave similar spectra.
Because of the high yield of E,, fractions F,,-F,, were studied
more thoroughly. The i.r. spectra of fractions
1500 1000 500
cm -1
F,,-F,, are shown in Figure 6. It is evident that the more polar
solvents eluted more polar compounds (Table 2).
F,, contained carboxylic acids and esters with long, mainly
unbranched aliphatic chains, with a smaller quantity of conjugated
and H-bonded carbonyl compounds, probably quinones, absorbing at
1590 cm- .
The main components of the F,, fraction were long- chain
aliphatic esters, and aromatic compounds, e.g. phenols, were also
present. The fractions F, were obtained in the largest yield
(Figure 4); the amount was increased by hydrothermal treatment in
the absence, and decreased in the presence, of sodium formate.
Fraction F,, was rather heterogeneous, containing carboxylic
acids, alcohols and conjugated and H-bonded carbonyl compunds, as
well as aliphatic hydrocarbons.
The spectrum of fraction F36 was surprisingly similar to that of
the parent coal, C,. This fraction, the most polar, contained
carboxylates and also some carbonates.
The intensity of the c=o . . . conjugated carbonyl stretching
bands Increased in parallel with increasing polarity of the eluant.
This observation supports the assignment of the band appearing at
1590 cm - in the coal spectra to the stretching mode of conjugated
and H-bonded carbonyl groups, at least in the case of this low-rank
coal. Usually this is the most intense and broad band in coal
spectra and its assignment is much disputed in the
literature15J1.
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