348
Energy & Fuels 2005, 19, 348-352
Effects of Pressure on Hydrogen Transfer from Tetralin to Coal Macerals Janusz Pajak*,†,‡ and Lukasz Socha‡,# Institute of Mathematics, Physics and Chemistry, Opole University of Technology, Luboszycka 5, 45-036 Opole, Poland, Institute of Coal Chemistry, Polish Academy of Sciences, Sowinskiego 5, 44-121 Gliwice, Poland, and Department of Physical Chemistry and Technology of Polymers, Faculty of Chemistry, Silesian University of Technology, Strzody 9, 44-100 Gliwice, Poland Received June 2, 2004. Revised Manuscript Received October 7, 2004
The influence of pressure (within a range of 5-100 MPa) on the rate of hydrogen transfer from tetralin to various coal macerals has been studied. Ten maceral concentrates (five vitrinites, three inertinites, and two liptinites) were used. The reactions were conducted at a temperature of 310 or 320 °C, depending on sample reactivity. The progress of reaction was followed by measuring tetralin dehydrogenation with gas chromatography. Vitrinite concentrates with high oxygen content are the most reactive, and the rate of their reaction with tetralin is accelerated by pressure. Pressure effects suggest a bimolecular transition state for most vitrinite/tetralin reactions. The reaction rate of inertinite concentrates is unchanged or even slowed by the change in pressure. These results suggest the increased contribution of the radical capping mechanism or the reaction path, in which tetralin reacts with low-energy coal radicals. As the pressure increases, the reaction rate of one liptinite sample is accelerated while the reaction of the other liptinite is retarded.
Introduction Much work has gone into elucidating the details of the reaction of coal with hydrogen donor solvents, such as tetralin, and, here, we cite only the examples of reviews.1,2 This reaction turned out to be very complex, and the results as well as explanations were quite disparate. Many researchers adopted the hypothesis of Curran, Struck, and Gorin, that the coal molecule undergoes thermal homolysis and then the free radicals abstract H atoms from tetralin.3 Structures that have a three-atom bridge connecting aromatic rings can react via a radical chain mechanism faster than structures that homolyze to benzylic or aryloxy radicals.4-6 It is also possible that radical intermediates are formed by a disproportionation reaction between the coal molecule and the tetralin. Reactions believed to exhibit this mechanism include the disproportionation of 1,2-dihydronaphthalene to naphthalene and tetralin7 and the * Author to whom correspondence should be addressed. E-mail address:
[email protected]. † Opole University of Technology. ‡ Institute of Coal Chemistry, Polish Academy of Sciences. # Silesian University of Technology. (1) Gorin, E. Fundamentals of Coal Liquefaction. In Chemistry of Coal Utilization, 2nd Suppl. Volume; Elliot, M. A., Ed.: WileyInterscience: New York, 1981; pp 1845-1918. (2) Stock, L. M. 1985. Hydrogen-Transfer Reactions. In Chemistry of Coal Conversion; Schlosberg, R. H., Ed.; Plenum Press: New York, 1985; pp 253-316. (3) Curran, G. P.; Struck, R. T.; Gorin, E. Ind. Eng. Chem. Process Des. Dev. 1967, 6, 166-173. (4) Gilbert, K. E.; Gajewski J. J. J. Org. Chem. 1982, 47, 48994902. (5) Poustma, M. L.; Dyer, C. W. J. Org. Chem. 1982, 47, 4903-4914. (6) Gilbert, K. E. J. Org. Chem. 1984, 49, 6-10. (7) Hessing, A.; Mullers, W. Chem. Ber. 1980, 113, 9-18.
disproportionation of cyclohexa-1,3-diene to benzene and cyclohexene.8 Ross and Blessing proposed that a hydride transfer mechanism may apply not only to the reduction of coal with alcohols but also to other types of reactants, such as tetralin.9 Virk proposed a pericyclic mechanism for hydrogen transfer reactions between coal and hydrogen donor solvent.10 The complexity and heterogeneity of the coal structure seems to be one reason for various results and explanations. On the other hand, the reactivity reflects the coal structure. To explain the details of the reaction, concentrates of maceral groups, which are more homogeneous than the whole coal, may be used.11 Vitrinites are regarded as the most representative coal macerals.12 The vitrinite/ tetralin reaction should provide a more homogeneous system than the whole coal/tetralin reaction. Moreover, the studies for macerals should make it possible to reveal details that are obscured by the complexity of the whole coal reaction or by the domination of the vitrinite reaction. The effect of pressure on reaction rate may provide information on the reaction mechanism.13 It allows us (8) De Mare, G. R.; Huybrechts, G.; Toth, M. J. Chem. Soc., Perkin Trans. 2 1972, 1256-1258. (9) Ross, D. S.; Blessing, J. E. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1979, 24, 125-133. (10) Virk, P. S.; Bass, D. H.; Eppig, C. P.; Ekpenyong, D. J. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1979, 24, 144-154. (11) Zilm, K. W.; Pugmire, R. J.; Larter, S. R.; Allan, J.; Grant, D. M. Fuel 1981, 60, 717-722. (12) van Krevelen, D. W. Coal. Typology-Physics-ChemistryConstitution; Elsevier: New York, 1993. (13) Isaacs, N. S. Liquid-Phase High-Pressure Chemistry; Wiley: New York, 1981.
10.1021/ef040053s CCC: $30.25 © 2005 American Chemical Society Published on Web 02/15/2005
Hydrogen Transfer from Tetralin to Coal Macerals
Energy & Fuels, Vol. 19, No. 2, 2005 349
Table 1. Composition of Maceral Samples and Their Purity Composition (wt %, daf) sample
C
H
N
S
Jaworzno vitrinite Ziemowit vitrinite Ziemowit liptinite Ziemowit inertinite Rydultowy vitrinite Rydultowy liptinite Rydultowy inertinite Bogdanka vitrinite Bogdanka inertinite Jastrzebie vitrinite
67.2 73.1 76.5 76.2 73.2 81.9 74.4 74.9 82.1 88.2
5.32 5.15 6.71 3.82 4.86 6.24 4.71 5.37 3.07 5.22
1.41 1.41 1.18 0.98 1.15 1.53 1.11 1.64 0.55 1.42
1.32 0.69 1.11 0.62 0.50 0.50 0.48 1.12 0.83 0.52
a
maceral Odiffa purity (vol %) 24.7 19.9 14.5 18.4 20.3 9.8 19.4 16.9 13.4 4.6
81 83.5 86 98 85 85 88 93 97 88.5
By difference.
to estimate the activation volume, that is, the change in volume on going from reactants to the transition state. Bond breaking in the transition state increases the volume, whereas bond making decreases the volume. A large negative activation volume is characteristic for ionization.13 Use of the effect of pressure on reaction rate seems to be well-suited to obtain information on reactions that cannot be described in detail, such as the reactions of coal. This method has been applied by Brower,14 to study the reaction of sub-bituminous coal with tetralin, and by Pajak and Brower,15 to study the reaction of bituminous coal with decalin. Recently, we applied this method to investigate the reaction of different coals with tetralin.16 Here, we used this method to study the reaction of various coal macerals with tetralin. Experimental Section Coal samples of different rank were used. The samples were ground to a size of
