THE COMPARATIVE STUDY ON HYDROGEN TRANSFER FROM TETRALIN TO COAL, ITS TETRAHYDROFURAN EXTRACT AND RESIDUE

THE COMPARATIVE STUDY ON HYDROGEN TRANSFER FROM TETRALIN TO COAL, ITS TETRAHYDROFURAN EXTRACT AND RESIDUE Janusz Pajak Institute of Coal Chemistry, Polish Academy of Sciences 44-100 Gliwice, 1 Maja 62, POLAND INTRODUCTION Much work has been made to elucidate the details of hydrogen transfer from tetralin to coal; here we quote only examples of reviews [1,2]; yet the results are diverse. Our recent studies indicate substantial differences in hydrogen transfer to coal macérais [3,4]. The separation of coal into its extractable and non-extractable part should show, if any, differences in its ability and mechanism of accepting hydrogen from donor solvents. Here we describe the studies on the transfer of hydrogen from tetralin and its deuterated derivatives (isotopomers) to coal, its tetrahydrofuran extract and extraction residue. EXPERIMENTAL The coal came from Ziemowit mine in Poland. Its characteristic data (wt.%, daf basis) are: C, 81.4; H, 5.3; N, 0.9; and the pétrographie composition (vol.%) is: vitrinite, 57; exinite, 13; inertinite, 23; mineral matter, 7. The procedure of obtaining tetralin-o:-d4 is described elswhere [5], Tetralin - di2 and tetrahydrofuran were purchased from Aldrich. The powdered sample of coal was extracted with tetrahydrofuran in Soxhlet apparatus under nitrogen until extracting solvent was colourless. Extract solution was evaporated under reduced pressure till constant weight, and residue was also freed of solvent. Reactions of coal, extract and residue with tetralin and its isotopomers were carried out at 310°C. The reaction mixtures (ratio 1:1, usually 50 mg : 50//1) were sealed in glass tubes and placed in a thermostat controlled within + 0.3°C. Reaction products were extraced with hexane and analyzed by gas chromatography. RESULTS AND DISCUSSION The yield of tetrahydrofuran extract was 2.1 wt %. The transfer of hydrogen was followed by measuring the change of the naphthalene to tetralin ratio. The amount of miligrams of hydrogen transferred to 1000 mg of coal, its tetrahydrofuran extract or extraction residue can be calculated using eqn. (1). 247

HT = 4000/132 ·χ -T/M

(1) where T is the amount of tetralin (mg) in reaction mixture, M the amount of coal, extract or residue (mg) in reaction mixture, and x the molar fraction of naphthalene formed. Figure 1 shows the transfer of hydrogen to coal, extract and residue during 12 hours of reaction, and Figure 2,3 and 4 present the difference in hydrogen transfer resulting from the use of deuterated tetralins.

o tetralin

Δ

tetralin-«-dv

o tetralin-d 12

6 o

time [hours]

time [hours]

Fig.l. Reaction of coal, extract and residue with tetralin

Fig. 2. Reaction of coal with tetralin isotopomers

Studies at extended reaction times (24 hours) have led to the estimation of "complete" hydrogen transfer, which amounts (in mg H/1000 mg of sample) 7.4 for coal, 4.6 for extract and 7.9 for residue. The comparison of the relative times required for the same conversion led to the evaluation of H/D kinetic isotope effects (see Fig. 2-4). These values for tetralin-a-d4 and tetralin-di2, respectively, are: for coal 2.0 and 3.5; for extract 2.6 and 10.2; and for residue 1.9 and 4.1. It is clearly visible that residue accepts the highest amount of hydrogen, higher than the parent coal. When we kept the parent coal in boiling tetrahydrofuran and then removed the solvent, hydrogen acceptance remained unchanged. It shows that the removal of extract (here quite small amount), and not the increase of accessibility of tetralin by solvent action, is responsible for increase in hydrogen uptake. The most probable explanation is partial elimination of hydrogen-shuttling phenomenon, resulting in higher need for hydrogen from tetralin.

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6

8

0

1

2 1

2

time [hours]

4

6

8

10

12

time [hours]

Fig.3. Reaction of extract with tetralin isotopomers

Fig.4. Reaction of extraction residue with tetralin isotopomers

The kinetic isotope effects (KIE) values are different for the samples studied. For coal and for residue they may be interpreted as the result of symultaneous transfer of the pair of hydrogens, since KIE values for tetralin-a-d4 and tetralin-di2 are close to proportion X:X . However, the results for the extract are quite anomalous, since theoretical value for the transfer of the pair of hydrogens at 310 °C is about 6.7. The KIE values higher than calculated from classical mechanics are interpreted in literature as the result of "tunnel effects" [6]. Such effects are quite rare in organic reactions and temporarily we are even unable to state, what kind of compounds in THF extract is responsible for anomalous KIE effects. CONCLUSION The part of coal obtained by tetrahydrofuran extraction exhibits significant differences in comparison with the parent coal and extraction residue during hydrogen transfer reaction from tetralin. Its ability for hydrogen acceptance is much lower than that of the parent coal, and anomalous, very high values of KIE reveal the operation of another reaction pathway previously hidden by the dominance of more reactive, non-extractable part in overall coal/tetralin reaction.

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REFERENCES 1. Stock, L.M. In: Schlosberg, R.H.(Ed.), Chemistry of Coal Conversion, Plenum Press, New York, 1985 2. Gorin, E. In: Elliott, M.A. (Ed.), Chemistry of Coal Utilization, 2nd Suppl. Vol., Wiley Interescience, New York, 1981 3. Pajak, J., Fuel Processing Technol., 1989,21,245 4. Pajak, J., Fuel Processing Technol. 1989,23,39 5. Brower, K.R., J. Org. Chem., 1982,47,1889 6. Melander, L., Saunders, W.H. jr., Reaction Rates of Isotopic Molecules, Wiley, New York, 1980

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