Spectral characterization of liquefied products of Pakistani coal

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Fuel Processing Technology 85 (2003) 63–74

Spectral characterization of liquefied products

of Pakistani coal

M. Arsala Khan a, Imtiaz Ahmad a, M. Ishaq a, M. Shakirullah a,M. Tariq Jan b, Eid-ur-Rehman b, Ali Bahader a,*

aDepartment of Chemistry, University of Peshawar, Peshawar, NWFP, PakistanbDepartment of Chemistry, Islamia College, University of Peshawar, Pakistan

Accepted 1 April 2003

Abstract

Characterization of liquefied products (oil and asphaltene) of Pakistani coal was performed using1H and 13C nuclear magnetic resonance (NMR) spectrometry. It was noticed that bands

corresponding to aliphatic protons and carbons were sharp and well pronounced in all the profiles.

Peaks corresponding to aromatic hydrogens and carbons were featureless, which is suggestive of the

presence of configurations like aliphatic carbon chains and some alkyl substituents and absence of

configurations like hydro-aromatics, biphenyls, phenanthrene and anthracene in both of these

extracts.

D 2003 Elsevier Science B.V. All rights reserved.

Keywords: Coal liquids; Aliphatic; Aromatic configurations

1. Introduction

Conversion of coal into liquid fuel is an area of high economic importance due to the

day-by-day hikes in the price of petroleum-based oils and rapid depletion of the existing

reserves. Extensive research is under way worldwide in this regard [1–4]. Some

investigators have carried out detailed experimentation to decide whether coal liquids

are compatible with petroleum-based products or not [5–10]. Investigative tools are being

used for complete characterization of coal liquids like gas chromatography, GC-MS, IR,

FTIR, and NMR. It has been recognized that both 1H and 13C nuclear magnetic resonance

(NMR) spectrometries are useful tools for the characterization of coal-derived liquids.

0378-3820/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0378-3820(03)00100-0

* Corresponding author.

E-mail address: [email protected] (A. Bahader).

M. Arsala Khan et al. / Fuel Processing Technology 85 (2003) 63–7464

Brown and Ladner [11] andWilliam and Chamberlain [12] carried out some of the earlier

work for characterization of coal liquids using NMR technique. Subsequent researcher used

NMR in combination with other techniques [13,14]. Dorn and Wouton [15] worked on the

determination of C/H ratio for the aromatic alkyl carbons of their coal liquids. Schweighardt

et al. [16] used NMR for measurement of the ratio of aromatic to alkyl carbons.

The present work was under taken with a view to investigate for aliphatic and aromatic

hydrogen and carbon configurations of the liquefied products obtained from mild hydro-

genation of Pakistani coal in toluene and benzene.

2. Experimental

2.1. Liquefaction procedure

Ten-gram portion of coal understudy was taken in Pyrex made glass insert, slurried in

appropriate amount of solvent. The glass insert was fitted in to the magnedrive, motor-driven

autoclave (OSK-6505 Japan) bolted tightly, pressure tested and charged with hydrogen. The

autoclave was then heated at 5 jC/min to attain the desire temperature of 400F 10 jC in 1 h,

held there for the specified duration of time. After being contacted for 0.5 h, the autoclave

was allowed to cool. The contents of the glass insert were carefully collected. The solvent

was separated using rotavapor and the residual coal was soxhletly extracted with THF till

clearance in the thimble compartment. The extracts were combined and heated up to 50 jC to

attain reasonable fluidity while heating onwater bath and fractionated into oil and asphaltene

using the method described elsewhere [17]. Fifty milliliters of aliquot of n-pentane was

added and shook vigorously. The resultant precipitate was filtered. The filtrate was collected

as oil and the precipitate after being dissolved in THF as asphaltene.

A Bruker NMR Spectrometer (Avance 500 MHz) was used. Characterization of oil and

asphaltene was performed by dissolving each liquid fraction in CDCl3. Tetramethyl silane

(TMS) was used as internal standard.

3. Results and discussion

The current investigation was performed to characterize the liquefied products obtained

from mild hydrogenation of coal under given set of conditions (Table 1). The elemental

Table 1

Experimental conditions for liquefaction

Temperature 400F 10 jCHydrogen pressure (Cold) 15 kgf/cm2

Heating rate 5 jC/min

Coal particle size 250–212 AmResidence time 0.5 h

Rate of stirring 250–300 rpm

Coal/solvent ratio 1:4

Table 2

Proximate and elemental analysis (%) of degari coal

Proximate analysis Ultimate/elemental analysis

Moisture VM Ash FC C H S Cl Calorific value

(Btu/lb)

12.1 35.5 18.0 34.4 50.9 4.2 2.2 0.02 8326

Analyzed by: Holder Bank, Switzerland. Source: Chemical Consultants (Pakistan) 1984.

M. Arsala Khan et al. / Fuel Processing Technology 85 (2003) 63–74 65

analysis of the coal understudy is provided in Table 2. Based on proximate and ultimate

analysis, the coal is said to be lignite. The coal understudy is reluctantly used by the user

because of high volatile matter and low fixed carbon. An attempt was made to convert this

coal into liquid fuels (Syn-fuels) to provide a relatively clean burning fuel compared to

solid coal and to supplement dwindling supplies of petroleum-based oils.

Extraction was performed with toluene being a weakly polar solvent with a

permanent dipole and k electron system and benzene being a nonpolar solvent in order

to extract material having both nonpolar and weakly polar configurations from coal

understudy.

Fig. 1. 1H-NMR spectrum of oil fraction extracted with toluene.


3.1. Characterization of oil

3.1.1. 1H-NMR spectra

The proton NMR spectrum of the oil (extracted at 400 jC with toluene) is provided in

Fig. 1. The aliphatic region of the 1H-NMR spectrum features several signals. It can be

seen that there are certain sharp and well-pronounced signals within the range 0–3.0 ppm.

The signal between 0 and 1.0 ppm can be ascribed to methyl protons, 1.0–2.0 ppm to

methylene and methene protons and between 2.0 and 3.0 ppm to de-shielded aliphatic

protons such as those alphas to an aromatic system [18,19]. It can also be seen that some

signals are narrow and some are wide. The narrower the signal, the less branched is the

alkane or alkyl groups giving rise to it. The half height widths of the peaks may reflect the

degree of branching [20]. Some signals in the range 0.6–3.0 ppm are not very sharp, this

might probably be due to CH2 groups directly attached to aromatic rings, which may

belong to hydro-aromatics or to alpha CH2 groups of aryl-alkyl structure [21]. The range

6.0–9.0 ppm feature several signals centered in the region 7.0–7.4 and 6.9–7.0 ppm. The

prominent signals at 7.0–7.4 and 6.9–7.0 ppm are suggestive of biphenyl [22]. The

aromatic proton region spans the range where protons of phenanthrene and anthracene can

give signals. However, no signals are observed in this region thus suggesting the absence

of these species.

The general 1H spectrum of oil (extracted with benzene at 400 jC) is shown in Fig. 2.

From the aliphatic proton region, prominent signals can be seen mostly centered at 0.9

Fig. 2. 1H-NMR spectrum of oil fraction extracted with benzene.


ppm. This might be due to the presence of methyl protons. Another prominent signal

appearing at 1.27 ppm can be ascribed to methylene groups distant from a substituent [21].

These groups form a chain long enough to permit free rotation that ensures magnetic

equivalence of their protons, i.e., the signal originates from methylene chains present in n-

alkanes or n-alkyl group. There are resonances at 1.445 and 1.9 ppm. This can be

rationalized as methene shoulders from branched open chain alkanes. Signals at 2.2–2.3

suggest an n-propyl group attached to an aromatic ring [22], signal at 4.2–4.4 and 4.9–5.1

ppm can also be seen. These signals are assigned to hydrogen of bridged methylene groups

of alkyl-substituted aromatics, hydrogen attached to oxygen or nitrogen atom of hetero-

atomic molecules, or hydrogen alpha to ketone carbonyl groups [23].

Fig. 3. 13C-NMR spectrum of oil fraction extracted with toluene.


It can be seen from the aromatic proton region of the spectrum that there are prominent

peaks between 6.5–7.2 and 7.3–7.8 ppm. All these peaks are assigned to aromatic

hydrogens.

3.1.2. 13C-NMR spectra

The 13C-NMR spectrum of the oil (extracted at 400 jC with toluene) is shown in Fig. 3.

It is evident from the aliphatic region of the spectrum (12–40 ppm) that there are some

major resonance bands centered at 14–15, 20, 22, 23, 25, 26, 28, 29, 32, 33, 34, 37, 39

and 40 ppm. There are also some major resonances in the aromatic regions too, centered at

125–126 and 128–129 ppm. No signals are observed in the carbonyl region (150–210

ppm). The signals at 14–15 and 20 ppm are ascribed to methyl carbons. These can be

assigned to terminal methyl attached to methylene carbon at the end of an alkyl chain and

branch methyl attached to methene carbon in an alkyl chain.

The signal between 22 and 23 ppm can be assigned as methylene attached to terminal

methyl in alkyl chain. Signal at 30 ppm as methylene in long alkyl chain, more than three

carbons removed from the end of chain. Signal at 32 ppm as methylene, third carbon from

end of chain, and methylene h to aromatic system and signal at 37 ppm as methylene alpha

to aromatic system (benzylic methylene). The signal at 33 ppm is shown to be due to

methene, which can be assigned to a branch in an aliphatic chain [24,25].

Fig. 4. 13C-NMR spectrum of oil fraction extracted with benzene.


There are several features centered at 125–126 and 128–129 ppm in the aromatic

region of the spectrum. These complex bands are probably due to substituted aromatics

and polycyclic aromatic species [26]. The aromatic band spans the range where carbonyl

and phenolic carbons are found. No resolved lines are seen in this region (155–165 ppm),

which shows the absence of carbonyl carbons and phenolic carbons.

The general 13C spectrum of the oil (extracted with benzene at 400 jC) is shown in

Fig. 4. It is evident from the results that there are number of peaks centered at 14, 19–20,

22–23, 24–25, 27, 28, 29–31, 32, 33, 37–38 and 39–40 ppm. The peaks in the range

15–20 ppm are assigned to methyl carbons and between 21 and 40 ppm are assigned to

methylene carbon. It can also be seen that in the aromatic region, there are some signals

centered at 125–126, 127, l29, 131, 136 and 143 ppm. Some minor signals can be seen

through out the range. All these signals suggest the presence of aromatic carbons.

3.2. Characterization of asphaltene

3.2.1. 1H-NMR spectra

The 1H spectrum of the asphaltene (extracted with toluene at 400 jC) is shown in

Fig. 5. Several prominent peaks can be seen in the aliphatic region. Most of them

centered between 0.8–l.0, 1.0–1.2, 1.2–1.3, 1.5–1.7, 2.2–2.5, 3.2–3.8 and 4.l–4.3

ppm. All these signals suggest the presence of methyl, methylene and methene protons

such as those alphas to an aromatic system [18,19].

Fig. 5. 1H-NMR spectrum of asphaltene fraction extracted with toluene.


The aromatic proton region also exhibiting some features mostly dominated between

6.9–7.3, 7.4–7.6 and 7.6–7.8 ppm suggesting the presence of aromatic protons. No

signals can be seen in the range between 7.8 and 9.0 ppm suggesting the absence of

species like protons of phenanthrene and anthracene.

The 1H spectrum of the asphaltene (extracted with benzene at 400 jC) is shown in

Fig. 6. Several signals at 0.8–0.9, 1.2–1.5, 1.8–1.9, 2.2–2.3, 2.5, 3.7–3.8, 3.4–4.4, 5.0

and 5.5 ppm can be seen. All these signals reveal the presence of aliphatic protons like

methyl, methylene and methene. The broadened spectrum of aromatic region exhibiting

some evident features centered at 6.6, 6.8–7.0, 7.0–7.2 and 7.2–7.8 ppm. All these

signals reveal the presence of aromatic protons, however, some of these peaks are

featureless (Table 3).

3.2.2. 13C-NMR spectra13C spectrum of the asphaltene (extracted with toluene at 400 jC) is given in Fig. 7.

Some prominent signals centered at 11, 14, 17–18, 19–20, 22–24, 28–31, 31–32 and

39–40 ppm can be seen in the aliphatic region. Again the presence of these signals

suggests the presence of methyl, methylene carbons. The aliphatic region also displaying

some features at 68–69, 71, 71–73 and 75–76 ppm. All these signals suggest the presence

of aliphatic and cyclo aliphatic species and CH2 or CH3 groups associated with cyclic or

aliphatic ether. The spectrum exhibiting signals in the aromatic region, i.e., between 129–

Fig. 6. 1H-NMR spectrum of asphaltene fraction extracted with benzene.

Table 3

Results of 1H-NMR tests of liquefied products of coal

Signal (ppm) Configurations assigned

0.00–1.00 Methyl proton

1.27 Methylene group distant from a substituent

1.0–2.00 Methylene protons, methene protons

1.40–1.90 Methene shoulders from branched open chain alkenes

2.0–3.0 Deshielded aliphatic protons

2.2–2.3 Propyl group attached to aromatic ring

4.2–5.0 Hydrogen of bridged methylene groups of

alkyl-substituted aromatic and hydrogen alpha to

ketone carbonyl groups

6.9–7.6 Biphenyl and aromatic hydrogen/aromatic protons.


134 and 155–165 ppm reveal the presence of vinyl and phenolic carbons. The bands at

128–129 ppm are mainly due to substituted aromatics.13C spectrum of the asphaltene (extracted with benzene at 400 jC) is shown in Fig. 8.

It is evident from the displayed spectrum of the aliphatic region that there are peaks at 14,

19–20, 22–23, 28–31, 32, 33, 36–38 and 67–68 ppm regions. All these signals suggest

the presence of methyl carbons attached to aliphatic moieties, methyls attached to

aromatic species, h methylene in aliphatic structures and myriad of other possible

Fig. 7. 13C-NMR spectrum of asphaltene fraction extracted with toluene.

Fig. 8. 13C-NMR spectrum of asphaltene fraction extracted with benzene.


methylenes, carbons appearing at or adjacent to highly branched centers which have

paraffinic, ethylenic, or aromatic groups attached, CH2 or CH3 groups associated with

cyclic or aliphatic ethers [27]. In the aromatic region, some featureless signals can be seen

providing no evidence of the presence of aromatic carbons (Table 4).

Table 4

Results of 13C-NMR of liquefied products of coal

Signal (ppm) Configurations assigned

14.00–15.00 Methyl carbons attached to aliphatic moieties

20.0 Methyl attached to methene carbon in an alkyl chain

22.00–23.00 Methyl in long alkyl chain

30.0 Methyl in long alkyl chain

32.0 Methylene h to aromatic system

33.0 Methene in an aliphatic chain

37.0 Methylene a to aromatic system

39.00–40.00 Carbons appearing at or adjacent to highly branched paraffinic,

ethylenic groups associated with cyclic or aliphatic ethers

68.00–76.0 Aliphatic and cyclo aliphatic species and CH2 or CH3

groups associated with cyclic or aliphatic ethers

125.00–129.00 Substituted aromatics and polycyclic species

155.00–165.00 Vinyl and phenolic carbons


4. Conclusion

Coal is a three-dimensional aromatic lamellae, consisting of mono, di, tri and tetra

cyclic aromatic monomers which are connected by etheric, methylene, ethylene and

propylene bridges. These linkages are mainly a part of low rank coals. When such coals

are subjected to thermal shock, these linkages get cleaved resulting in the formation of free

radicals, which upon capping in hydrogen environment give rise to liquid products. In high

ranks, coals due to excessive maturation and the abstraction of methane during meta-

morphism leads to a more condensed aromatic structure. When such coals are subjected to

thermal shock, ring saturation, followed by ring opening and side chain removal lead to

the formation of liquid products. In the present work, low rank coal was liquefied under

mild condition, it was observed that liquefied products mainly comprised of aliphatic

configurations like methyl, methene and methylene protons and carbons which is

suggestive of the fact that etheric, methylene and propylene linkages got cleaved resulting

into liquid products having aliphatic and alkyl-substituted configurations. The products are

lean in material having aromatic or hydro-aromatic configurations like anthracene,

phenanthrene and biphenyls.

In order to extract materials having hydro-aromatic and aromatic configurations, more

severe conditions of liquefaction and the presence of external catalyst are suggested to

avoid not only retrogressive/capping reactions which otherwise leads to the formation of

more condensed products, but to assist also in the thermal fragmentation of the highly

condensed lamallaes or micellaes and to facilitate the extraction of host material from the

coal matrix.

Acknowledgements

The authors are grateful to National Science Research Development Board (NSRDB)

Islamabad, Pakistan for providing funds, HEJ Research Institute of Chemistry, University

of Karachi for providing laboratory facilities for some portion of this work and Mr.

Hidayat Shah, Department of Chemistry, University of Peshawar for computer graphics.

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Spectral characterization of liquefied products of Pakistani coal

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