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20 Desulfurization of Benzonaphthothiophenes and Dibenzothiophene with a Raney Nickel Catalyst Its Relationship to π-Electron Density Masatoshi Nagai , Hideo Urimoto , Kazuya Uetake , Noriyuki Sakikawa , and Richard D. Gonzalez 1,4
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Department of Chemical Engineering, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184 Japan Department of Industrial Chemistry, Nihon University, Chiyodaku, Tokyo 101 Japan Department of Chemical Engineering, University of Illinois at Chicago, Chicago, IL 60680 1
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Desulfurization
of polynuclear thiophenes was performed by contin
uous stirring with a Raney nickel catalyst in ethanol at 0—78.8 °C. The major products were biphenyl in the desulfurization of diben zothiophene,
α-phenylnaphthalene
benzo[b]naphtho[1,2-d]thiophene, desulfurization
in
the
desulfurization
and β-phenylnaphthalene
of benzo[b]naphtho[2,3-d]thiophene
benzo[b]naphtho[2,1-d]thiophene.
Observation
of these
of
in the and products
shows that the main reaction pathway is the extrusion of a sulfur atom to give the corresponding hydrocarbon. The π-electron densities of the sulfur atoms, which were calculated by using simple
Hückel
molecular orbital theory, are considered to be related to the adsorption of the sulfur compounds to the surface and consequently to the C-S bond-breaking rate.
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Author to whom correspondence should be directed Current address: Houkube Chemical Company, O'Hatano, Kanagawa 257 Japan
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0065-2393/88/0217-0357$06.00/0 © 1988 American Chemical Society
In Polynuclear Aromatic Compounds; Ebert, L.; Advances in Chemistry; American Chemical Society: Washington, DC, 1987.
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358
POLYNUCLEAR AROMATIC COMPOUNDS
H Y D R O D E S U L F U R I Z A T I O N O F H E A V Y P E T R O L E U M F E E D S T O C K S and coalderived liquids requires the conversion of high-molecular-weight compounds such as dibenzothiophene and the benzonaphthothiophenes (1-3). Several studies have investigated the mechanism of hydrodesulfurization of polynuclear thiophenes on cobalt or nickel molybdenum catalysts at high pressures (4-6). However, only a few investigations have related the chemical reactivity of these compounds to their electronic structure (7-9). The adsorption and hydrodesulfurization of the simple sulfur compound, thiophene, were stud ied recently by using quantum chemistry (10-12); however, thiophene can easily be decomposed even when it is not adsorbed at high temperatures and pressures. Nag et al. (7) noted that the reactivity of a polynuclear sulfurcontaining compound is not determined solely by the size of the molecule. In addition, Geneste and co-workers (8, 9) reported that the first step in the hydrotreating reaction of benzonaphthothiophene was related to the Cou lombic interaction term in the complete neglect of differential overlap (CNDO) calculations. Because competition occurs between the different processes (hydrogénation and desulfurization) during reaction, the relation ship between desulfurization and the electronic properties of the compounds under reaction conditions is difficult to understand. Furthermore, because the calculation of electronic structures necessarily involves many σ bonds of partially saturated rings (8, 9), as well as contributions from the d electrons of the sulfur atom and many ττ electrons of high-molecular-weight com pounds, the electronic structure of hydrogenated polynuclear thiophenes is difficult to estimate. Therefore, a catalyst and reaction conditions under which desulfurization takes place without hydrogénation should be selected.
Thiophenes are easily desulfurized to form hydrocarbons without hy drogénation on a Raney nickel catalyst by constant stirring in a solvent (13, 14). Because a Raney nickel catalyst has a strong affinity for electronegative atoms such as oxygen and sulfur (15), the ττ-electron density on a sulfur atom during desulfurization is possibly related to the adsorption of a sulfur atom to the catalyst surface during reaction. Simple Huckel theory can be used to calculate the ττ-electron density of a sulfur atom and other reaction indexes such as ΤΓ-bond order and the highest occupied molecular orbital ( H O M O ) and lowest unoccupied molecular orbital ( L U M O ) coefficients. In this study, such calculations are used because all four compounds considered have sim ilar ττ-electron configurations and the same number of ττ bonds being formed or broken: dibenzothiophene, benzo[fe]naphtho[l,2-d]thiophene, benzo[i>]naphtho[2,3-d]thiophene, and benzo[fc]naphtho[2,1-d]thiophene. In addition, thiophene and benzo[fc]thiophene were studied to compare their behavior with that of the four compounds just listed. The desulfurization studies were performed i n a batch reactor over a Raney nickel catalyst, and ethanol was used as the solvent. To avoid hydro génation, the catalytic studies were performed at low temperature with continuous stirring. The dependence of the rate of desulfurization on the
In Polynuclear Aromatic Compounds; Ebert, L.; Advances in Chemistry; American Chemical Society: Washington, DC, 1987.
20.
NAGAI ET A L .
Desulfurization of Benzonaphthothiophenes
359
electronic structure of the compounds was considered by using simple Huckel molecular orbital calculations. The mechanism of the adsorption and desulfurization of dibenzothiophene and the benzonaphthothiophenes is also discussed.
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Experimental Details The Raney nickel catalyst was prepared according to the procedure described by Adkins and Billia (16) for W - 7 Raney nickel. Following preparation, the catalyst was stored under ethyl ether and an inert atmosphere for about 1 month at 0 °C. D i benzothiophene (1), benzo[fr]naphtho[l,2-
