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Energy & Fuels 1996, 10, 1181-1186
1181
Base-Catalyzed Hydrogenation: Reduction of Polycyclic Aromatic Compounds Shiyong Yang and Leon M. Stock* Department of Chemistry, The University of Chicago, Chicago, Illinois 60637, and Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 Received June 18, 1996. Revised Manuscript Received July 29, 1996X
Dihydrogen can be activated by lithium and potassium organoamides, particularly the diisopropyl and bis(trimethylsilyl) derivatives, to reduce aromatic compounds at 1000 psig and 200 °C. Naphthalene was hydrogenated to tetralin in 100% yield by both reagents; anthracene was reduced with the bis(trimethylsilyl)amide catalyst to a mixture of the corresponding monocyclic aromatic derivatives, 1,2,3,4,4a,9,10,10a-octahydroanthracene (15%) and 1,2,3,4,5,6,7,8octahydroanthracene (84%); phenanthrene was reduced with this base to a mixture of 1,2,3,4,5,6,7,8octahydrophenanthrene (63%), 1,2,3,4,4a,9,10,10a-octahydrophenanthrene (33%), and 1,2,3,4tetrahydrophenanthrene (4%); chrysene was converted by the same reagent to 1,2,2a,3,4,5,6, 6a,9,10,11,12-dodecahydrochrysene (70%) and 1,2,2a,3,4,5,6,6a-octahydrochrysene (25%); and 1,2benzanthracene was hydrogenated to a mixture of dihydro- and dodecahydro-1,2-benzanthracenes. The reaction products show a striking selectivity for the preservation of an interior benzene ring. The catalytic properties of the strong bases depend on the nature of the organic ligands in the dialkylamide and the corresponding metal cations. The reactions proceed at modest pressures, about 500 psig, but require high temperatures, about 200 °C. The products of the reaction with dideuterium were investigated by magnetic resonance spectroscopy to define the reaction pathway. The results of these experiments and other available information suggest that hydrogen is transferred from an anionic dihydrogen-dialkylamide complex to the aromatic compound in the slow step of the reaction.
Introduction Despite the technological importance of aromatic hydrogenation, the subject has not received much attention compared with the many investigations of hydrodesulfurization,2-4 hydrodenitrogenation,5-7 and hydrodemetalation.8-10 The hydrogenation of naphthalene and other aromatic hydrocarbons with dihydrogen and heterogeneous catalysts is a challenging task because the reaction is reversible and the thermodynamic yields of the desired reduction products are quite low at low pressures and temperatures.1 Several attempts have been made to circumvent these difficulties. In 1979, Muetterties and co-workers11,12 discovered that the homogeneous hydrogenation of aromatic hydrocarbons was catalyzed by allylcobalt compounds liganded X Abstract published in Advance ACS Abstracts, September 1, 1996. (1) Stanislaus, A.; Cooper, B. H. Catal. Rev.-Sci. Eng. 1994, 36, 75. (2) Dautzenberg, J. G.; de Deken, J. C. Catal. Rev.-Sci. Eng. 1984, 26, 421. (3) Jacobson, A. C.; Cooper, B. H.; Hannerup, P. N. Presented at the 12th World Petroleum Congress, Houston, TX, 1987; p 97. (4) (a) Speight, J. G. The Desulfurization of Heavy Oils and Residua; Dekker: New York, 1981. (b) Ho, T. C. Catal. Rev.-Sci. Eng. 1988, 30, 117. (5) Perot, G. Catal. Today 1991, 10, 447. (6) Katzer, J. R.; Sivasubramanian, R. Catal. Rev.-Sci. Eng. 1979, 20, 155. (7) Ledoux, M. J. In Catalysis, Specialist Periodical Reports; The Royal Society of Chemistry: London, 1985; Vol. 7, p 125. (8) Quann, R. J.; Ware, R. A.; Huang, C.; Wei, J. Adv. Chem. Eng. 1988, 14, 95. (9) Mitchell, P. C. H. Catal. Today 1990, 439. (10) Toulhoat, H.; Szymanski, R.; Plumail, J. C. Catal. Today 1990, 7, 531. (11) Muetterties, E. L.; Bleeke, J. R. Acc. Chem. Res. 1979, 12, 324. (12) Bleeke, J. R.; Muetterties, E. L. J. Am. Chem. Soc. 1981, 103, 556.
with alkyl phosphites in a stereoselective manner at ambient temperature and low pressure. Fish and his associates showed that the triphenylphosphine complexes of rhodium(I) chloride and ruthenium(II) chloride were active catalyst precursors for the homogeneous hydrogenation of polynuclear heteroaromatic nitrogen compounds.13,14 Bennett and his colleagues found that bis(hexamethylbenzene)ruthenium(0) and the η-chloro and η-dihydrido dimers of hexamethylbenzeneruthenium with distorted aromatic structures were also efficient hydrogenation catalysts.15,16 We elected to pursue a different strategy that did not depend upon a metal reagent for the activation of the hydrogen molecule. As pointed out in the preliminary communication of this work,17 it has been known for a long time that simple basic molecules such as potassium tert-butoxide catalyzed the hydrogenation of benzophenone.18,19 Even earlier work established that hydroxide and amide ions catalyzed the exchange of dideuterium with water and ammonia, respectively.20 More recently, Rathke and his co-workers studied the role of simple bases on the reduction of carbon monoxide and reviewed the opportunities that were offered by base-catalyzed reduction.21-23 The evidence has been interpreted to (13) Fish, R. H.; Tan, J. L.; Thormodsen, A. D. J. Org. Chem. 1984, 49, 4500. (14) Fish, R. H.; Tan, J. L.; Thormodsen, A. D. Organometallics 1985, 4, 1743. (15) Bennett, M. A.; Huang, T. N.; Turney, T. W. J. Chem. Soc., Chem. Commun. 1979, 312. (16) Bennet, M. A. CHEMTECH 1980, 444. (17) Yang, S.; Stock, L. M. Energy Fuels 1996, 10, 516. (18) Walling, C.; Bolyky, L. J. Am. Chem. Soc. 1961, 83, 2968. (19) Walling, C.; Bolyky, L. J. Am. Chem. Soc. 1964, 86, 3750. (20) Dayton, C.; Wilmarth, W. J. Chem. Phys. 1950, 18, 759.
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1182 Energy & Fuels, Vol. 10, No. 6, 1996
mean that dihydrogen forms an adduct with the strong base and that the hydrogen atoms of the diatomic molecule exchange with the hydrogen atoms of the inorganic anion within the adduct rather than through a free hydride anion. The proposed intermediates, HOH2- and H2NH2-, have been detected as long-lived species in ion cyclotron resonance experiments.24,25 Theoretical work implies that the hydrogen-hydrogen bond is greatly weakened and polarized by complexation.26,27 Walling18,19 and Rathke21-23 and their colleagues have shown that unsaturated carbon-oxygen bonds in organic and inorganic molecules can be reduced with dihydrogen in the presence of hydroxide and alkoxide ions. The fact that dideuterium exchange proceeds more readily with amide ion than with hydroxide ion suggested that inorganic and organic amides could be more effective catalysts for the reduction reactions. Consequently, we investigated their use for the activation of dihydrogen and the hydrogenation of aromatic hydrocarbons that are commonly present in fossil fuels. This paper extends the work that was presented in a preliminary communication.17 Naphthalene was studied to establish the reaction conditions because it is relatively abundant in petroleum and coal liquids and it is also more difficult to reduce than other aromatic hydrocarbons. The scope of the reaction was examined by the study of other hydrocarbons with three and four aromatic rings. In addition, the reaction pathway was investigated to establish a possible course for the reduction reaction. Experimental Section Materials. The reagents used in this work including lithium diisopropylamide, lithium diethylamide, lithium dicyclohexylamide, lithium dimethylamide, lithium bis(trimethylsilyl)amide, potassium bis(trimethylsilyl)amide, lithium tertbutoxide, sodium tert-butoxide, potassium tert-butoxide, naphthalene (99%), anthracene (99%), acridine, phenanthrene (98%), chrysene, and 1,2-benzoanthracene were purchased from Aldrich Chemical Co. The alkali metal organoamides, e.g., lithium dimethylamide, are called organoamides to distinguish them from the alkali metal amides, e.g., sodium amide. All of the air-sensitive and moisture-sensitive chemicals were handled in a glovebox filled with nitrogen or argon. The solvents that were used for the reactions were carefully purified before use. For instance, hexane was purified by refluxing the commercial material over sodium hydride and then distilling it in nitrogen. General Methods. Gas chromatographic analysis was performed on a Perkin-Elmer Sigma 3B instrument with a 11.8 m × 0.32 cm column packed with 10% OV-101. The conditions for the analysis of mixtures of naphthalene and its reduction products were as follows: injector temperature, 300 °C; detector temperature, 300 °C; initial column temperature, 120 °C; final column temperature, 200 °C; ramp rate, 5 °C/min; initial time, 1 min; final time, 1 min. The products of some reactions were also analyzed by gas chromatography and mass (21) Klingler, R. J.; Krause, T. R.; Rathke, J. W. Adv. Chem. Ser. 1992, No. 230, 337. (22) Klingler, R. J.; Rathke, J. W. Prog. Inorg. Chem. 1991, 39, 131. (23) Klingler, R. J.; Krause, T. R.; Rathke, J. W. Catal. Lett. 1989, 3, 347. (24) Klingeld, J. C.; Ingemann, S.; Jalonen, J. E.; Nibbering, N. M. M. J. Am. Chem. Soc. 1983, 105, 2474. (25) Ingemann, J. C.; Klingeld, J. C.; Nibbering, N. M. M J. Chem. Soc., Chem. Commun. 1982, 1009. (26) Cremer, D.; Koraka, E. J. Phys. Chem. 1986, 90, 33. (27) Chalasinski, G.; Kendall, R. A.; Simons, J. J. Chem. Phys. 1987, 87, 2965.
Yang and Stock spectrometry with a Hewlett-Packard 5970 series mass detector (70 eV) connected to a 5890 gas chromatograph with an OV-101 capillary column, 15 m × 0.25 mm, that was programmed for operation between 100 and 180 °C at 5 °C/min. The 1H and 2H nuclear magnetic resonance spectra were obtained on GE Omega 500-MHz spectrometer. General Procedure for Hydrogenation. All of the reductions were performed in a Parr Instrument Co. Model 4576 HP/HT T316 autoclave system equipped with a Model 4842 temperature controller. In a typical experiment, naphthalene (6.4 g, 0.05 mol) and freshly distilled hexane (37.5 mL) were charged in the autoclave, and a suspension (10 wt %) of lithium diisopropylamide (12.5 mmol) in hexane (13.4 mL) was added to the autoclave under nitrogen. The sealed autoclave was purged with nitrogen and then hydrogen several times to replace the air. The reaction was performed with stirring at 200 °C for 4-18 h. The product mixture was hydrolyzed by adding it dropwise to 1 N aqueous ammonium chloride. The aqueous phase that separated from the organic phase was extracted with methylene chloride three times. Then the combined organic phase was washed with 1 N hydrochloric acid and water (three times) and dried over magnesium sulfate. The solvent was removed by rotary evaporation, and tetralin (6.2 g, 0.047 mol) was obtained. The identity of the product was established by microanalysis (Calcd for C10H12: C, 90.92; H, 9.18. Found: C, 90.90; H, 9.19) and by proton magnetic resonance and mass spectroscopy. Anthracene Hydrogenation. The reaction with lithium diisopropylamide provided 9,10-dihydroanthracene, and the reactions with other bases were examined. Potassium bis(trimethylsilyl)amide (2.0 g, 10.0 mmol) in toluene (20 mL) was added to a solution of anthracene (5.0 g, 28.0 mmol) in hexane in the autoclave under nitrogen. The reactor was pressurized to 1000 psig with hydrogen and heated at 250 °C for 18 h with stirring. The products were separated as described to provide a clear liquid (4.2 g) that subsequently crystallized. Gas chromatography showed that the solid was free of anthracene and that it contained two compounds. Microanalysis of the mixture (Calcd for C14H18: C, 90.26; H, 9.74. Found: C, 90.05, H, 9.80) indicated that the products were octahydroanthracenes. The most abundant component (84%) was established to be 1,2,3,4,5,6,7,8-octahydroanthracene (1) by its spectroscopic properties [MS, m/z 186 (C14H18+, 100), 158 (C12H14+, 95), 130 (C10H10+, 55); 1H NMR (CDCl3) 6.8 (s, 2H), 2.8 (t, 8H), 1.8 ppm (t, 8H)]. The second product, which was obtained in 14% yield, was shown to be 1,2,3,4,4a,9,10,10a-octahydroanthracene (2) by the distinctive mass spectrum [MS, m/z 186 (C14H18+, 100), 104 (C8H8+,100)]. Phenanthrene Hydrogenation. Phenanthrene was produced in 30% yield by the lithium organoamides. The reaction was therefore performed with potassium bis(trimethylsilyl)amide under the conditions used for the hydrogenation of anthracene. Chromatography showed that the reaction provided three components with no residual phenanthrene. Two of these compounds comprised 96% of the product. Microanalysis of the two component mixture (Calcd for C14H18: C, 90.26; H, 9.74. Found: C, 90.04; H, 9.72) indicated that the major products were octahydrophenanthrenes. The principal component (64%) was 1,2,3,4,5,6,7,8-octahydrophenanthrene (3) [MS, m/z 186 (C14H18+, 100), 158 (C12H14+, 75), 144 (C14H12+, 40); 1H NMR (DCCl3) 7.2 (s, 2H), 3.2 (t, 4H), 2.9 (t, 4H), 2.0 ppm (m, 8H)]. The mass spectrum of the component obtained in 33% yield [MS, m/z 186 (C14H18+, 100), 158 (C12H14+, 75), 145 (40), 129 (97), 117 (73), 104 (70), 95 (40), and 91(55)] indicated that it was an isomeric mixture of the 1,2,3,4,4a,9, 10,10a-octahydrophenanthrenes (4). The third component was 5,6,7,8-tetrahydrophenanthrene (5). Chrysene Hydrogenation. Potassium bis(trimethylsilyl)amide (2.0 g, 10.0 mmol) in toluene (20 mL) was added to the solution of chrysene (1.0 g, 4.4 mmol) in hexane (30 mL) in the autoclave under the protection of nitrogen. The reactor was pressurized to 1000 psig with hydrogen and heated at 250
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Reduction of Polycyclic Aromatic Compounds °C for 18 h with stirring. The reaction mixture was separated to provide a viscous liquid (1.2 g). The liquid was separated by thin layer chromatography on silica gel with chloroformhexane to give a white semisolid (0.74 g, 70%). Microanalysis (Calcd for C18H24: C, 89.94; H, 10.06. Found: C, 89.76; H, 10.12) indicated that the material was a dodecahydrochrysene. The spectroscopic properties, particularly the magnetic resonance spectrum with only two aromatic protons, suggest that the compound is 1,2,2a,3,4,5,6,6a,9,10,11,12-dodecahydrochrysene (6) [MS, m/z 240 (100), 212 (15), 197 (50), 183 (20), 141 (15); 1H NMR(CDCl3) 7.1 (m, 2H), 2.5-3.0 (m, 8H), 1.0-2.2 ppm (m, 14H)]. A second product (25%) for which the value of m/z for the parent ion was 236 and which exhibited a complex array of signals in the aromatic region of the NMR spectrum is a mixture of isomers of 1,2,2a,3,4,5,6,6a-octahydrochrysene (7). 1,2-Benzanthracene Hydrogenation. The reaction was performed in the same way. The structure of the principal product was assigned as 1,2,3,4,4a,5,6,6a,8,9,10,11,12b-dodecahydro-1,2-benzanthracene (8) on the basis of the microanalytical information (Calcd for C18H24: C, 89.94; H, 10.06. Found: C, 89.76; H, 10.12) and the spectroscopic information. The high-field region of the proton NMR spectrum was very complex in accord with the formation of a benzo compound in which every proton was magnetically distinct. Two other products with molecular weights of 230 were isolated and investigated by spectroscopic methods. These substances contained naphthalene fragments and were assigned as 5,6(9) and 7,12-dihydro-1,2-benzoanthracene (10). Reaction Pathway. A clear homogeneous solution was observed when the autoclave was opened after a reaction with lithium diisopropylamide, but a white solid slowly precipitated when the solution was exposed to the atmosphere. The white solid was collected, washed with hexane several times in a nitrogen atmosphere to remove tetralin and other impurities, and dried under vacuum at room temperature for 24 h. Elemental organic analysis (C, 4.87%; H, 3.84%; N, 0.91%) indicated that the solid was predominantly an inorganic material. The lithium contents of several different samples were determined gravimetrically by precipitation as trilithium phosphate28 to be between 31 and 38%. These results imply that the solid is primarily a mixture of lithium oxide and hydroxide (Calcd for Li in a 1:1 mixture: 37.7%), which was produced from the lithium diisopropylamide after exposure to air. The reaction between dideuterium and naphthalene was carried with lithium diisopropylamide under the same conditions except that dideuterium replaced dihydrogen. The reaction products were isolated in the same manner and investigated by 1H and 2H nuclear magnetic resonance and mass spectrometry. The mass spectrum of the tetralin obtained by the reduction of naphthalene with dideuterium is a mixture of labeled compounds in which the tetralin-d4 (m/z 136, 72%), the product that would be obtained in the absence of exchange, was accompanied by both deuterium-deficient species [tetralind3 (m/z 135, 55%), tetralin-d2 (m/z 134, 32%), tetralin-d1 (m/z 133, 22%), and tetralin-d0 (m/z: 132, 11%)] and deuteriumrich species [tetralin-d5 (m/z 137, 88%), tetralin-d6 (m/z 138, 100%), tetralin-d7 (m/z 139, 95%), tetralin-d8 (m/z 140, 77%), tetralin-d9 (m/z 141, 50%), tetralin-d10 (m/z 142, 27%), and tetralin-d11 (m/z 143, 9%)].
Results and Discussion This investigation was based on the concept that dihydrogen can be activated by strongly basic reagents through the formation of an adduct from which hydrogen can be transferred to an unsaturated organic compound. We readily repeated the work of Walling (28) Simmons, G. A., Jr. Ann. Chem. 1953, 25, 1386.
Energy & Fuels, Vol. 10, No. 6, 1996 1183 Table 1. Influence of the Structure of the Organoamides on Their Catalytic Activity for the Hydrogenation of Naphthalenea LiNR2
[LiNR2]/[substrate] (mol/mol)
LiN[CH(CH3)2]2 LiN[C6H11]2 LiN[CH3]2 LiN[CH2CH3]2 LiN[Si(CH3)3]2
0.25 0.25 0.25 0.25 0.25
NaN[Si(CH3)3]2 KN[Si(CH3)3]2
0.25 0.25
solvent
convrsn (%)
hexane hexane hexane hexane hexane toluene toluene toluene
100 95 3 7 16 3 51 100
a The reactions were carried out at 200 °C and 1000 psig of dihydrogen with a substrate to catalyst ratio of 4 to 1 in 50 mL of the solvent for 18 h.
and Bolyky,18,19 who first showed that benzophenone was reduced to benzhydrol by potassium tert-butoxide and dihydrogen.17 Our initial work indicated that aromatic hydrocarbons could not be reduced effectively with simple hydroxides and alkoxides. However, lithium amide catalyzed the reduction of naphthalene.17 This encouraging result led to the study of selected lithium, sodium, and potassium organoamides including the substances with dimethyl, diethyl, diisopropyl, dicyclohexyl, and bis(trimethylsilyl) substituents. The results in Table 1 for the lithium, sodium, and potassium salts of bis(trimethylsilyl)amide show the greater effectiveness of potassium, which converted naphthalene to tetralin in 100% yield. The conversion of naphthalene decreased to 51 and 3% when sodium and lithium bis(trimehylsilyl)amide were used as the catalysts, respectively, under the same conditions. The lithium diisopropyl and dicyclohexyl organoamides were especially effective catalysts compared to the dimethyl or diethyl derivatives. These results, which appeared to be more closely correlated with the size of the substituents than to their basic properties, suggested that the differences in reactivity probably originate in the differences in solubility of the bases in the solvent (hexane). The lithium organoamides with large aliphatic substituents are significantly more soluble in hexane than those with small substituents. It is pertinent to note that lithium diisopropylamide does not dissolve in hexane at room temperature; however, the solubility increases at the high reaction temperature, and clear solutions were observed when the autoclave was opened after the completion of the reaction. In any event, lithium dicyclohexylamide and potassium bis(trimethylsilyl)amide exhibit very active catalytic properties for the hydrogenation of naphthalene at 200 °C under 1000 psig of hydrogen by providing tetralin in more than 95% yield with no detectable byproducts. The success of the hydrogenation of naphthalene depends on the amount of the catalyst. As shown in Figure 1, the conversion of naphthalene increases as the molar ratio of [catalyst]/[substrate] increases, and conversion reaches 100% when the mole concentration of the catalyst to the substrate is more than 12.5%. Figure 2 shows the dependence of the conversion on the initial hydrogen pressure. The reaction proceeds very well with 80% conversion even at hydrogen pressures as low as 300 psig. The low yield at 150 psig is clearly a consequence of the insufficiency of dihydrogen in the apparatus; fully hydrogenating 0.05 mol of naphthalene to tetralin in a 200 mL autoclave requires about 200 psig of hydrogen at room temperature.
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1184 Energy & Fuels, Vol. 10, No. 6, 1996
Yang and Stock Table 4. Products of the Hydrogenation of Several Aromatic Compounds with Lithium Diisopropylamide at 200 °C convrsn (%)
product (% yield)
naphthalene anthracene phenanthrene
100 100 31
1-methoxynaphthalenea
100
tetralin (100) 9,10-dihydroanthracene (100) 1,2,3,4,4a,9,10,10a-octahydrophenanthrene (26) 1-methoxytetralin (63)
acridine
100
tetralin (12) 9,10-dihydroacridine (77)
compound
a
At 300 °C. Table 5. Hydrogenation of Aromatic Hydrocarbons Catalyzed by Potassium Bis(trimethylsilyl)amide
Figure 1. Effect of the lithium diisopropylamide to naphthalene molar ratio on the conversion of naphthalene under the experimental conditions shown in Table 1.
Figure 2. Dependence of the conversion of naphthalene on the initial hydrogen pressure in the reaction with lithium diisopropylamide under the experimental conditions shown in Table 1. Table 2. Effect of Temperature on the Reduction of Naphthalenea temp (°C)
convrsn (%)
temp (°C)
convrsn (%)
200 150
100 29
180 100
55 0.5
a The reactions were carried out with lithium diisopropylamide in hexane as described in Table 1.
Table 3. Effect of Reaction Time on the Conversion of Naphthalenea reaction time (h)
convrsn (%)
reaction time (h)
convrsn (%)
0.5 4.0
34 94
2.0 18.0
63 100
a The reactions were carried out with lithium diisopropylamide in hexane as described in Table 1.
The effects of temperature and time are shown in Tables 2 and 3. The conversion of naphthalene to tetralin decreases from 100% at 200 °C to about 0.5% at 100 °C, implying that the reaction has a significant activation energy. The reaction is not rapid under the experimental conditions; 4-5 h is needed for the complete reaction of naphthalene. The base-catalyzed reduction reactions differ in two significant ways from the reduction reactions of these substances with heterogeneous catalysts such as Ni-Mo/ Al2O3, Ni-W/Al2O3, or Co-Mo/Al2O3.29-31 First, the
compound
convrsn (%)
naphthalene anthracene
100 100
phenanthrene
100
chrysene
100
1,2-benzoanthracene
100
product (% yield) tetralin (100) 1,2,3,4,5,6,7,8-octahydroanthracene (84) 1,2,3,4,4a,9,10,10a-octahydroanthracene (14) 1,2,3,4,5,6,7,8-octahydrophenanthrene (63) 1,2,3,4,4a,9,10,10a-octahydrophenanthrene (33) 1,2,2a,3,4,5,6,6a,9,10,11,12-dodecahydrochrysene (70) 1,2,2a,3,4,5,6,6-octahydrophenanthrene (25) 1,2,3,4,4a,5,6,6a,7,12,12a,12b-dodecahydrobenzanthracene 5,6-dihydrobenzanthracene 7,12-dihydrobenzanthracene
heterogeneous hydrogenation reactions with these catalysts are reversible, and the unfavorable equilibria limit the conversion. The conversion of naphthalene to tetralin remains almost constant at about 35% and is unaffected by changes in catalyst composition, feed flow rate, and reaction temperature from 310 to 400 °C under 1000 psig of hydrogen.32 In contrast, the alkali organoamides irreversibly convert naphthalene to tetralin in 100% yield under milder conditions (200 °C, 1000 psig of hydrogen). Second, the heterogeneous catalysts require high pressures for the reduction of aromatic compounds. The organoamide catalysts enable the essentially complete conversion of naphthalene to tetralin at 500 psig of hydrogen. Hydrogenation of Polycyclic Aromatic Compounds. The hydrogenation reactions of several polycyclic aromatic compounds were investigated to gain perspective on the scope of the reaction. The results for lithium diisopropylamide and potassium bis(trimethylsilyl)amide are shown in Tables 4 and 5. The reduction of naphthalene with lithium diisopropylamide provided the desired tetralin, but the reductions of anthracene and acridine yielded the less completely reduced dihydro compounds, and phenanthrene was only partially reduced with this base. The reactions of naphthalene and the polycyclic aromatic hydrocarbons with potassium bis(trimethylsilyl)amide provided much more completely reduced compounds. Most of the products contained only one benzene ring. Anthracene was reduced to 1,2,3,4,5,6,7,8octahydroanthracene (1) and an unresolved mixture of two geometric isomers of 1,2,3,4,4a,9,10,10a-octahy(29) Prins, R.; De Beer, V. H. J.; Somorjai, G. A. Catal. Rev.-Sci. Eng. 1989, 31, 1. (30) Topsoe H.; Clausen, B. S. Appl. Catal. 1986, 25, 273. (31) Chianelli R. R.; Daage, M. Stud. Surf. Sci. Catal. 1989, 50, 1. (32) Patzer, J. F.; Farrauto, R. J.; Montagna, A. A. Ind. Eng. Chem. Process Des. Dev. 1979, 18, 625.
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droanthracene (2).
Similarly, phenanthrene was hydrogenated to a mixture of 1,2,3,4,5,6,7,8-octahydrophenanthrene (3), a mixture of geometric isomers of 1,2,3,4,4a,9,10,10aoctahydrophenanthrene (4), and a small amount of 1,2,3,4-tetrahydrophenanthrene (5).
Chrysene was converted in the presence of potassium bis(trimethylsilyl)amide to a mixture of the geometric isomers of 1,2,2a,3,4,5,6,6a,9,10,11,12-dodecahydrochrysene (6) and 1,2,2a,3,4,5,6,6a-octahydrochrysene (7).
1,2-Benzoanthracene was hydrogenated under the same conditions to the monoaromatic derivative, apparently 1,2,3,4,4a,5,6,8,9,10,11,12b-dodecahydro-1,2benzoanthracene (8). The remainder of the reduction product was a mixture of the two possible dihydro-1,2benzanthracenes, 5,6- (9) and 7,12-dihydro-1,2-benzanthracene (10).
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Energy & Fuels, Vol. 10, No. 6, 1996 1185
peripheral rings to form substances with interior benzene rings. Reaction Pathway. The reduction reactions of naphthalene with sodium and dihydrogen in tetrahydrofuran and with butyllithium and dihydrogen in hexane were carried out under conditions similar to those employed for the reactions with the lithium organoamide catalysts. Both reactions failed to produce tetralin. These observations imply that neither naphthyllithium nor lithium naphthalenide are on the principal reduction reaction pathway for the successful reactions with the organoamides. These results are therefore more compatible with the view of Walling and Rathke and their associates18,19,21-23 that dihydrogen is activated by the strong base to form an intermediate from which one activated hydrogen can be transferred to the aromatic compound to initiate the reduction. Dihydrogen, as noted in the Introduction, forms adducts with strong bases such as hydroxide and amide ion in the gas phase.24,25 Three different structural representations, A, B, and C, have been considered for these complex ions.26,27
Theory implies that the amide and hydroxide adducts with theoretical binding energies of 26 and 15 kcal mol-1, respectively, and H-H distances >0.9 Å, structure B for the amide, are more stable than the alternatives. The lengthening of the H-H distance imparts hydride character to the weakly bound hydrogen. Our work and the studies of Walling and Rathke and their co-workers indicate that ketones and carbon monoxide are more readily reduced than hydrocarbons. Thus, the available information is compatible with the view that the initial step in the reaction involves the transfer of the hydride hydrogen to the aromatic ring to form an ion pair.
Li+‚R2N‚H2- + C10H8 f Li+‚R2NH‚C10H9-
The results for the four polycyclic compounds indicate that the hydrogenation proceeds with a novel selectivity for the formation of 1, 3, 6, and 8. The reactivity pattern, as illustrated for phenanthrene,
The classic investigations of Cram and his students33 indicate that ion pairs of this kind collapse by proton transfer to provide the initial products including stable compounds such as 1,2- and 1,4-dihydronaphthalene and other unstable substances that are rapidly transformed into other more stable compounds by isomerization reactions with the strong base.
Li+‚R2NH‚C10H9- f
Neither 1,2- nor 1,4-dihydronaphthalene was detected among the products of the hydrogenation of naphthalene even when the reaction was quenched after 1 h. This result implies that the 1,2- and 1,4-dihydro derivatives, if present, must be rapidly converted to the final product, tetralin. exhibits a clear preference for the reduction of the
(33) Cram, D. J. Fundamentals of Carbanion Chemistry; Academic Press: New York, 1965.
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1186 Energy & Fuels, Vol. 10, No. 6, 1996
This feature of the reaction was also examined by carrying out the reduction of naphthalene with dideuterium. The 1H and 2H NMR spectra of labeled tetralins revealed that the resonances at 7.1 ppm (protons 5-8), 2.8 ppm (protons 1 and 4), and 1.8 ppm (protons 2 and 3) were almost 1:1:1 in both spectra. This ratio should be 2:1:1 (4H:2H:2H) in the 1H NMR spectrum and 0:1:1 (0H:2H:2H) in the 2H NMR spectrum if no deuteriumhydrogen exchange occurred during the reduction reaction. The mass spectrum of the same reaction products indicates that a broad array of isotopic molecules is present in the mixture, as expected from the magnetic resonance work, suggesting that exchange occurs readily. In comparison, when tetralin was treated with dideuterium gas under the conditions employed for the reduction reaction, i.e., at 200 °C for 18 h, the deuterium content of the tetralin is increased by only 8-10% protons at the four aromatic positions and by only 2-4% at the benzylic positions. This result indicates that the exchange in the catalytic reduction of naphthalene does not occur primarily before or after the hydrogenation process. Rather, the results suggest that the exchange reaction proceeds in parallel with reduction process through the reactive 1,2- and 1,4-dihydronaphthalenes. The explanation for the simultaneous exchange of the aliphatic and aromatic protons also emerges from the formulation of the reaction as an ion pair process. As shown in the last equation, the proton or deuteron bonded to the nitrogen atom in the ion pair may return at any position that possesses residual negative charge. Thus, deuterium enters both naphthalene rings during the reduction reaction. The selectivity of these hydrogenation reactions for the formation of compounds with the single remaining aromatic ring in the interior of the structure is apparently the consequence the subtle interplay of both kinetic and thermodynamic factors. These molecules are more thermodynamically stable than the other possible isomers, and the strongly basic reagents can certainly provide pathways for the interconversion of the reactive polyunsaturated intermediates, but kinetic
Yang and Stock
factors must also be important. The feature is illustrated by the fact that 9,10-dihydroanthracene with two isolated benzene ring is not readily reduced and converted to a tetrahydroanthracene that would lead to the more fully reduced octahydroanthracene. Rather, the reaction appears to follow a course in which there is a real kinetic preference for the delivery of hydrogen to the external rings. Summary and Conclusion Lithium diisopropylamide and potassium(trimethylsilyl)amide catalyze the reduction of naphthalene and other polycyclic aromatic compounds. The reduction of naphthalene to tetralin with lithium diisopropylamide was accomplished under conditions (200 °C, 1000 psi of hydrogen, 5 h) that are less severe than employed in heterogeneous catalysis. The catalytic properties of the alkali metal organoamides depend on the dimensions of the ligands and the inorganic ion. Potassium bis(trimethylsilyl)amide catalyzes the reduction of polycyclic aromatic compounds to mixtures of molecules with isolated benzene rings. The reactions of these substances exhibit a selectivity for the formation of reduction products with the isolated ring in the interior of the structure. The reductions apparently proceed by the transfer of hydrogen from a polarized adduct between the organoamide anion and the dihydrogen molecule to the aromatic compound to form a benzenanion, which is rapidly protonated to provide the initial reduction product. These reactive intermediates undergo exchange, isomerization, and reduction to complete the reaction. Acknowledgment. This research was carried out under the sponsorship of the Office of Fossil Energy of the U.S. Department of Energy. We are also indebted to Darryl Fahey of Phillips Petroleum Co., David Collum of Cornell University, and the Coal Science Group at Argonne National Laboratory for valuable conversations. EF960095J
