E.s.r. study of electron acceptor doped coals

E.s.r. study of electron Yuzo

Sanada,

Haruo

Kumagai

and

acceptor Masahide

Metals Research Institute, Faculty of Engineering, Kita-ku, Sapporo 060, Japan Government Industrial Development Laboratory, (Received 9 November 1992; revised 5 October l

doped coals

Sasaki”

Hokkaido Hokkaido, 1993)

University, Sapporo

Kita- 13, Nishi-8, 06.2, Japan

Non-covalent bond interactions play important roles in coal structure. They serve as virtual crosslinks which help to hold the network together. This paper concentrates, particularly, on the structure of coal with respect to charge transfer interaction between coal and electron acceptors. Iodine and tetracyanoquinodimethane (TCNQ) as electron acceptors interact with coal molecules producing charge transfer complexes. A good correlation is obtained between the spin concentration, N,, of coalLiodine complexes and the Bloch decay value (carbon aromaticity, f,) of parent coal. The N, value increases with the increase of aromaticity in coal. On the other hand, TCNQ is a molecule with strong electron accepting ability, as is iodine. The N, value for TCNQ doped coal increases with decreasing coal rank. Interpretation of the above fact is that the TCNQ molecule moves to sites associated with oxygen containing functional groups, which are able to form hydrogen bonds. (Keywords: coal; reactivity; structure)

The inter- and intra-molecular association forces have long been acknowledged as being of fundamental importance to the overall physical properties of coal. There are several types of non-covalent interactions, such as van der Waals forces, and their relative population changes with coal rank. It is well known that there are three different types of specific non-covalent interactions, i.e. ionic bonds, hydrogen bonds and aromatic 71-n interaction. The solubility, viscosity (fluidity), apparent molecular weights and solvent swelling of coal are influenced by hydrogen bonding and other secondary bonding present in the macromolecular structure’. Hydrogen bonding in coal and model compounds has been investigated by means of infrared’-‘, nuclear magnetic resonance (n.m.r.) spectroscopy6,7 and calorimetric methods’-lo. In previous papers”-15, the interaction of coal molecules with selected guest molecules, such as iodine and TCNQ, has been investigated. This paper provides a deeper understanding of charge-transfer interaction between coal and the electron acceptor molecule by means of electron spin resonance (e.s.r.) measurement. EXPERIMENTAL Samples

Coal samples tested are mostly chosen from Argonne Premium coal samples. The analytical data for these samples are shown elsewhere16. Electron

acceptor

as a guest substance

Iodine and 7,7,8,8-tetracyanoquinodimethane (TCNQ) were selected as electron acceptors. These had a purity of >98% (guaranteed reagent), and were used without further purification. OO16-2361/94/06/084&03 c 1994 Butterworth-Heinemann

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Preparation

of electron

acceptor

doped coal

A known amount (10 wt%) of the guest substance dissolved in a given solvent was added to the coal. The solvent used was chloroform. The suspensions of coal in the solvent were placed in an ultrasonic bath for 10min and stirred for 1 h at room temperature. The solvent was then evaporated off, and the coals were kept at 40°C in a vacuum oven overnight. E.s.r. measurement

Samples were degassed at 10m4 Pa and sealed in e.s.r. glass tubes. Spectra were obtained using a Varian model El09 spectrometer. The value of spin concentration was calibrated by l,l-diphenyl-2-picrylhydrazyl and was reproducible within 2%.

RESULTS

AND

DISCUSSION

Change of spin concentration coal

in electron

acceptor

doped

It becomes clear that strong interaction between iodine and coal molecules results from the formation of charge-transfer complexes”. Good correlation between the spin concentration of polynuclear aromatic hydrocarbon-iodine complexes and the ring compactness factor has been established. The larger the ring condensation of aromatic hydrocarbon, the higher the spin concentration value in the complex becomes”. It has already been shown that the values of spin concentration for coals, Nso, and that of iodine or TCNQ doped coal, N,, vary as a function of the coal carbon content (rank)12. Good correlation is obtained between spin concentration and rank. A remarkable increase in N, values for the iodine doped coal and decrease in N,

Electron acceptor values for the TCNQ doped coals have been recognized with increasing rank, respectively. Iodine and TCNQ are both electron acceptors. TCNQ, as a guest, plays a particularly important role in low rank coal. Taking into account that: 1. the aromatic molecule is an electron donor, and 2. aromaticity and size of the aromatic ring increase monotonically with coal rank. Then, for iodine as a guest, the difference in spin concentration with rank is mostly positive. Figure 1 shows the relationship between the N,-N,, value and the aromaticity of coal, by means of solid state n.m.r. measured by Muntean and Stocki7. N,-N,, values increase monotonically with increasing coal rank. Therefore, an insight into the aromatic structure of coal, through e.s.r. measurement of iodine doped coal, was obtained. The mechanism of charge transfer complex formation, between tetralin or methyl naphthalene and iodine, has also been studied by means of high temperature n.m.r.. The iodine molecule is associated mainly with aromatic nuclei and does not interact with the cycle paraffinic ring and aliphatic side chain, with respect to the change of the n.m.r. absorption line over the temperature range 2@25O”C (unpublished data). On the other hand, the difference in spin concentration decreases with increasing rank by the addition of TCNQ, which also acts as a strong electron acceptor. Another mechanism for the change in spin concentration of TCNQ doped coal must be considered. In the system of coal-TCNQ, the TCNQ molecule associates with the sites of functional groups which are able to form hydrogen bonds. Table I shows the spin concentration of aromatic hydrocarbon-TCNQ complexes. TCNQ accepts electrons from aromatic hydrocarbon molecules and from formed charge-transfer complexes. N, values observed are small for aromatic hydrocarbons without functional groups. On the other hand, compounds with functional groups do show a fair amount of spin concentration, as illustrated in Table I. Anthrafravic acid, two aromatic rings attached with quinone and hydroxy groups, exhibits the largest N, value so far studied. By means of FT-i.r. and free energy change measurements, strong interaction has been confirmed between the 26

I

doped coals: Y. Sanada et al.

Table 1 Spin concentration of aromatic compound-TCNQ (host molecule:TCNQ, 2O:l molecular ratio)

Compound

Spin cont. (x lOI spins mol-t)

Naphthalene Fluoranthene Anthracene Pyrene Anthraquinone 9-anthracene carboxylic Anthraflavic acid

0.113 0.296 0.904 0.414 0.739 1.271 5.960

acid

COOH

9-anthracene

carboxylic

acid

Anthraflavic

fa*(Bloch

Relation

between N,-N,,

decay

method),

value andf, values”

of parent coal

acid

5

.

II

z

4

6

8

10

12

14

content

of

OlOH+COOHl,%

Figure 2 Relation parent coal

between

N,-N,,

values

and

oxygen

TCNQ molecule and carbazole or phenol, which are able to form hydrogen bonds 13,r4 Thus, a relationship can be expected between the enhanced spin concentration of TCNQ doped coal and the oxygen content of the parent coal. Figure 2 shows the relationship between the enhanced spin concentration, N,-NsO, and the phenolic and carboxylic oxygen content of the parent coal measured by Jung et al.“. From this figure, an approximately linear relationship is shown to exist between N,-N,, values and the phenolic and carboxylic oxygen contents, except for the WyodakkAnderson coal. Therefore, interaction of TCNQ with these particular sites in the coal structure may result in the change of spin concentration of coal. E.S.R. SPECTRAL SHAPE OF ACCEPTOR DOPED COAL

Figure 1

complexes

ELECTRON

Figure 3 illustrates spectra obtained for Blind Canyon and Pittsburgh No. 8 high volatile bituminous coals and their electron acceptor doped coals. The spectrum of Blind Canyon coal consists of one broad line. While, Pittsburgh No. 8 coal consists of two distinct lines, one narrow and the other broad. The structural model of bituminous coal recently proposed by Kovac and Larsen’9*20 is very well adapted to the interpretation of

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Electron acceptor Blind

Canyon

doped coals: Y. Sanada et al.

l80.7C%)

Pittsburgh#8183.2Cl)

The intermolecular association of non-covalent bonds, which apparently act as covalent bonds, is broken by electron acceptor molecules. CONCLUSIONS

iodine

doped

In conclusion, the nature of charge transfer complexes of coal-TCNQ and coal-iodine systems are quite different. TCNQ interacts with oxygen containing functional groups, and iodine interacts with aromatic rings in the coal structure. Guest and host interaction, using e.s.r. techniques, give further insight into non-covalent interaction in the coal molecular structure. The approach described above will provide one of the means of identifying the nature of non-covalent bonding in coal. When the mechanism of coal interaction with iodine and TCNQ systems is elucidated, the strength and population of non-covalent and hydrogen bonds will be clarified.

coal

ACKNOWLEDGEMENTS The authors are grateful to Dr S. Vorres, Argonne National Laboratory, for supplying coal samples.

l.O”,T

Figure 3

E.s.r. spectra

of coal and electron

acceptor

doped

REFERENCES I

coal ?

e.s.r. signals. Their model involves a molecular (M) phase and a macromolecular (MM) phase. The MM phase forms a three-dimensional skeleton, consisting of macromolecular fragments, connected by cross-bonds. The M phase is distributed in the pores of the MM phase, or on its edges. The analysis of relaxation times led to the result that the paramagnetic centre related to the narrow line are attached to the latticed macromolecular phase MM, whereas the centres related to the broad line are distributed throughout the molecular phase M”. Presumably, non-covalent bonds bind together the macromolecular network with the molecular phase. The line width of coal increases by addition of the iodine or TCNQ molecules. An enhanced broad line is due to the increase in M phase in the coal. The electron acceptor molecule, such as iodine and TCNQ, may attack non-covalent bonds in the coal. Interaction between the coal molecule and the electron acceptor molecule probably results in an increase in the M phase in the coal. Gino et ~1.~~found that carbon disulfide-i\i-methyl-2pyrrolidinone (NMP) mixed solvent gives high extraction yields for many bituminous coals at room temperature. After solvent evaporation part of the CS,-NMP soluble fraction became insoluble in the solvent. This insolubilization behaviour may be ascribed to the formation of non-covalent bonding between CS,-NMP soluble fractions. However, the insolubilization behaviour of the CS,-NMP soluble fraction can be inhibited by addition of electron acceptor molecules, such as iodine or TCNQ. This result also suggests that electron acceptor molecules may disrupt non-covalent bonding in the CS,-NMP soluble fraction.

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