VOLUME 19, NUMBER 1
JANUARY/FEBRUARY 2005
© Copyright 2005 American Chemical Society
Articles Activated Carbon-Catalyzed Hydrogen Transfer to r,ω-Diarylalkanes Lin-Bing Sun, Zhi-Ming Zong, Jia-Hui Kou, Guang-Feng Liu, Xi Sun, and Xian-Yong Wei* School of Chemical Engineering, China University of Mining and Technology, Xuzhou 221008, Jiangsu, China
Guo-Jiang Zhou School of Resources and Environmental Engineering, Heilongjiang Institute of Science and Technology, 1 Tangchang Street, Daowai District, Harbin 150027, Heilongjiang, China
Chul Wee Lee Advanced Chemical Technology Division, Korea Research Institute of Chemical Technology, P.O. Box 107, Yuseong, Daejeon 305-600, Korea Received August 13, 2004. Revised Manuscript Received October 12, 2004
Diphenylmethane (DPM), 9-benzylphenanthrene (BP), di(1-naphthyl)methane (DNM), 1,2-di(1-naphthyl)ethane (DNE), and 9-(1-naphthyl)phenanthrylmethane (NPM) were used as substrates and hydrogen-transfer reactions from molecular hydrogen to the substrates were examined over an activated carbon (AC) under pressurized hydrogen at 300 °C. The results show that the AC catalyzes monatomic hydrogen transfer to the substrates and that the reactivities of the substrates toward hydrocracking decrease in the following order: NPM > DNM > BP . DNE . DPM. These results can be interpreted by the differences in the hydrogen-accepting abilities of the ipso-carbons and the resonance stabilities of the leaving radicals.
Introduction Apart from their applications as absorbents and catalyst supports, carbon materials such as activated carbons (ACs) and carbon blacks can be used as catalysts, because of their extended surface area, microporous structure, and high degree of surface reactivity. Several studies have revealed that some of the carbonaceous catalysts promoted a variety of reactions * Author to whom correspondence should be addressed. E-mail:
[email protected].
such as chlorobenzene dechlorination,1 ethylbenzene and cyclohexanol dehydrogenations,2,3 ethanol dehydration,4 and hydrogen transfer from both hydrogen gas and tetralin to anthracene.5 (1) Santoro, D.; Jong, V.; Louw, R. Chemosphere 2003, 50 (9), 12551260. (2) Pereira, M. F. R.; Orfao, J. J. M.; Figueiredo, J. L. Appl. Catal., A 1999, 184 (1), 153-160. (3) Silva, I. F.; Vital, J.; Ramos, A. M.; Valente, H.; Botelho do Rego, A. M.; Reis, M. J. Carbon 1998, 36 (7), 1159-1165. (4) Szymanshi, G. S.; Rychlicki, G. Carbon 1991, 29 (4), 489-498. (5) Zhang, Z.-G.; Yoshida, T. Energy Fuels 2001, 15 (3), 708-713.
10.1021/ef0497964 CCC: $30.25 © 2005 American Chemical Society Published on Web 11/25/2004
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Table 1. Structures of the Products from the Hydrogenation and Hydrocracking of the Diarylalkanes (DAAs)
Farcasiu and Smith6 investigated the carbon blackcatalyzed decomposition of 4-(1-naphthylmethyl)bibenzyl (NMBB). They found that a specific bond in NMBB was effectively cleaved in the presence of the carbon black. Recently, Ni et al. reported that an AC selectively catalyzed the hydrocracking of di(1-naphthyl)methane (DNM), especially in the presence of sulfur.7 The high activity of the carbonaceous catalysts for the cleavage of specific C-C bonds is of interest, especially for coal liquefaction, because selective coal conversion may be possible with such catalysts. Our concern involves determination of the types of covalent bonds, especially C-C bonds, that can be selectively cleaved by hydrogen transfer over such catalysts. This concern prompted us to investigate the reactions of more coal-related model compounds over such catalysts. In the present study, we have used five R,ω-diarylalkanes (DAAs) as coal-related model compounds and investigated their reactions over an AC under mild conditions.
(dry basis) was also purchased from Aldrich Chemical Co., Inc., and dried under vacuum before use. General Procedure. A substrate (0.1 mmol), AC (0.05 g), and cyclohexane (10 mL) were placed into a 60-mL, stainlesssteel, magnetically stirred autoclave. After being pressured with hydrogen to 5 MPa at room temperature, the autoclave was heated to 300 °C within 10 min and kept at that temperature for a prescribed period of time (1-8 h). The autoclave then was immediately cooled to room temperature in an ice-water bath. The products and unreacted substrate in the reaction mixture that was taken out from the autoclave were identified by gas chromatography/mass spectroscopy (GC/ MS), using a system (Hewlett-Packard model HP 6890/5973) that was equipped with a capillary column coated with HP5MS (30 m × 0.25 mm inner diameter (i.d.), film thickness of 0.25 µm; Hewlett-Packard) and were quantified using a Hewlett-Packard model HP 6890 gas chromatograph that was equipped with a capillary column coated with HP-101 (50 m × 0.32 mm i.d., film thickness of 0.3 µm; Hewlett-Packard). Bromobenzene was used as an internal standard for the quantitation. Each experiment was repeatedly performed at least twice. The data reproducibilities were pretty good. The experimental error was BP . DNE. The reactions of the compounds include aromatic-ring (AR) hydrogenation and Car-Calk bond cleavage, affording hydrogenated DAAs and hydrocracked products, respectively. Figure 2 demonstrates that total selectivities of hydrogenated deriva-
Hydrogen Transfer to R,ω-Diarylalkanes
Figure 1. Time profiles of conversions of the R,ω-diarylalkanes (DAAs).
Figure 2. Time profiles of total selectivities of hydrogenated DAAs. HDNEs, HDAMs, HNPMs, HDNMs, and HBPs denote hydrogenated derivatives from 1,2-di(1-naphthyl)ethane (DNE), diarylmethanes (DAMs), 9-(1-naphthyl)phenanthrylmethane (NPM), di(1-naphthyl)methane (DNM), and 9-benzylphenanthrene (BP), respectively.
tives from NPM, DNM, and BP are 52%, implying that cleavage of the Car-Calk bond in the diarylmethanes (DAMs) is predominant. Active hydrogen species, including H+ and H•, are crucial for the Car-Calk bond cleavage.7,10-12 An acidic catalyst is necessary to produce H+ species,10,11 whereas other catalysts such as an AC7 and metal sulfides12 are responsible for the formation of the H• radical from H2. Keren and Soffer13 reported that pure carbon could split H2 to atomic form via hydrogen chemisorption. Zhang et al.14 studied AC-catalyzed hydrogenation of anthracene and noted that the AC was catalytically active for H2 dissociation. The addition of the dissociated H atoms (i.e., monatomic hydrogen transfer) to the ipsoposition in the DAAs should be an essential step for the
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Figure 3. Time profiles of the product distribution from the BP reaction.
Car-Calk bond cleavage. The hydrogen-accepting abilities (HAAs) of different ARs and different positions in the same AR may be quite different, which can be characterized by the corresponding superdelocalizability (Sr(R)) values,8,15-17 i.e., a C atom on an AR with a larger Sr(R) value accepts the H• radical more readily. Data in Table 2 indicate that the HAAs of ipso-carbon atoms in the DAMs used decrease in the order NPM (9-position) ) BP (9-position) > DNM (1-position) > DPM (1-position), which is the same as the order of conversions of the DAMs, except for BP. As Figure 3 illustrates, 1 and 12 are main products from BP reaction and 15 was not detected, indicating that the H• radical predominantly transfers to the 9-position in BP, which can be expressed in terms of a much larger Sr(R) value of the ipso-carbon atom in the 9-position than that in the 1-position in BP. H• transfer to the 9-position in BP affords a benzyl radical (B•) as the leaving group (Scheme 1), whereas the leaving group produced by H• transfer to the 1-position in DNM is the 1-naphthylmethyl radical (NM•; see Scheme 2). Resonance energies (REs) were used to characterize the resonance stabilities (RSs) of the leaving groups.16,18 The much smaller RE value of B• than that of NM• shown in Table 3 should be responsible for the smaller conversion of BP than that of DNM, although BP accepts the H• radical more readily. The resulting 12 can be hydrogenated to 10 and 11, whereas 1 is inert toward hydrogenation, because no methylcyclohexane was detected. The much-larger HAA of 12 than that of 1 leads to a smaller selectivity of 12 than that of 1 (see Figure 3). As Figure 4 displays, DNM is selectively hydrocracked to 6 and 3, whereas the difference in selectivity between 6 and 3 is ap-
Table 2. Sr(R) Values of Different Positions in Some Compoundsa
a
Data taken from refs 8 and 15.
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Scheme 1. Proposed Mechanism for Hydrogen Transfer to BP and Its Products Catalyzed by the Activated Carbona
Figure 4. Time profiles of product distribution from DNM reaction. Other HDNMs mainly include octahydro derivatives of DNM.
a HTIP and HTNIP denote hydrogen transfer to ipso and nonipso positions, respectively.
Scheme 2. Proposed Mechanism for Hydrogen Transfer to DNM and Its Products Catalyzed by the Activated Carbon
Figure 5. Time profiles of product distribution from the NPM reaction. Other HNPMs mainly include octahydro derivatives of NPM.
Table 3. Resonance Energy of Some Arylmethyl Radicalsa
a
radical
resonance energy, RE (kJ/mol)
B• NM• 9-phenanthrylmethyl
128.0 185.8 238.9
Data taken from ref 18.
preciably smaller than that between 1 and 12, because of a smaller difference in HAA between 6 and 3 than between 1 and 12. The inertness of DPM toward both hydrogenation and hydrocracking indicates that it is impossible for an (10) Olah, G. A.; Bruce, M. R.; Edelson, E. H.; Husain, A. Fuel 1984, 63 (10), 1432-1435. (11) Shimizu, K.; Karamatsu, H.; Inaba, A.; Suganuma, A.; Saito, I. Fuel 1995, 74 (6), 853-859. (12) Wei, X.-Y.; Ni, Z.-H.; Zong, Z.-M.; Zhou, S.-L.; Xiong, Y.-C.; Wang, X.-H. Energy Fuels 2003, 17 (3), 652-657. (13) Keren, E.; Soffer, A. J. Catal. 1977, 50 (1), 47-53.
isolated benzene ring (BR) to accept the H• radical under the reaction conditions. Therefore, as side reactions, hydrogen transfer to non-ipso positions (HTNIP) in BP affords, at most, one dihydro, two tetrahydro, and three octahydro derivatives, among which only the hydrocracking of the two tetrahydro derivatives may proceed via hydrogen transfer to the ipso position of the tetrahydrophenanthrene rings (see Scheme 1). Similarly, HTNIP in DNM produces, at most, three tetrahydro and three octahydro derivatives. Among the derivatives, cleavage of the Car-Calk bond only in 17 may proceed via hydrogen transfer to the 1-position of the naphthalene ring (NR), as shown in Scheme 2. Both smaller HAA of BR and smaller RS of the B• radical result in the inert nature of DPM toward hydrocracking, and, in contrast, both larger HAA of phenanthrene ring (PR) and larger RS of NM• lead to the largest reactivity of NPM toward hydrocracking among the DAAs. In addition, asymmetry and larger HAA of two ARs connected to methylene in NPM make hydrogen transfer to NPM very complicated. As Figure 5 demonstrates, 6 and 12 are main products from NPM, which suggests that hydrogen transfer preferentially proceeds to the 9-position in NPM. Larger HAA of PR than that of NR not only results in the preferential hydrogen transfer but also lead to the differences in selectivity between 6 and 12 and between 3 and 15. (14) Zhang, Z.-G.; Okada, K.; Yanamoto, M.; Yoshida, T. Catal. Today 1998, 45 (1-4), 361-336. (15) Yonezawa, T.; Nagata, C.; Koto, H.; Imamura, A.; Morakuma, K. Guide to Quantum Chemistry; Kagaku-Dojin Press: Kyoto, Japan, 1990; p 232. (16) Wei, X.-Y.; Ogata, E.; Niki, E. Bull. Chem. Soc. Jpn. 1992, 65 (4), 1114-1119. (17) Wei, X.-Y.; Ogata, E.; Niki, E. Sekiyu Gakksishi 1992, 35 (4), 358-361. (18) Herndon, W. C. J. Org. Chem. 1981, 46 (10), 2119-2125.
Hydrogen Transfer to R,ω-Diarylalkanes
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Scheme 3. Proposed Mechanism for Hydrogen Transfer to NPM and Its Products Catalyzed by the Activated Carbon
Similar to the reactions of BP and DNM, cleavage of the Car-Calk bond via hydrogen transfer to the ipso position occurs both in NPM and in some of its tetrahydro and octahydro derivatives (Scheme 3). DNE is thermally inert at temperatures up to 300 °C,16 although the dissociation energy (230.1 kJ/mol) of the Calk-Calk bond is much smaller than that of the CarCalk bond in DNE (405.8 kJ/mol).19 As Figure 6 displays, a series of decomposed compounds, including 2-9, are observed in the DNE reaction over the AC and the total selectivity of 3 and 9 is almost the same as that of 6,
Figure 6. Time profiles of product distribution from the DNE reaction. Other HDNEs mainly include octahydro derivatives of DNE.
implying that the AC catalyzes cleavage of the CalkCalk and Car-Calk bonds to almost the same extent. The catalysis of the AC in cleavage of the Calk-Calk bond is not clear. One possibility is that the AC induces the Calk-Calk bond homolysis to form chemisorbed NM• radicals, which then react with chemisorbed hydrogen. It deserves mentioning that the selectivity of 24 is significantly larger than that of 25, suggesting that hydrogen transfer occurs more preferentially to the methylene-substituted side, including the ipso position, than to another side of NR in DNE. Similar preferential hydrogen transfer to 6 over iron sulfides has been reported.20 The lowest conversion and the highest total selectivity of hydrogenated derivatives among the DAAs, except for DPM, indicate that cleavage of both Car-Calk and Calk-Calk bonds in DNE is difficult under the reaction conditions. Both DNM and DNE have two NRs; therefore, the HAA of DNE should be the same as that of DNM. Different from DNM, however, as Scheme 4 shows, cleavage of the Car-Calk bond in DNE affords the labile 2-(1-naphthyl)ethyl radical (NE•). The dif(19) Malhotra, R.; McMillen, D. F.; Tse, D. S.; John, G. A. S. Energy Fuels 1989, 3 (4), 465-468. (20) Wei, X.-Y.; Ogata, E.; Niki, E. Chem. Lett. 1991, (12), 21992202.
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Energy & Fuels, Vol. 19, No. 1, 2005 Scheme 4. Proposed Mechanism for Hydrogen Transfer to DNE and Its Products Catalyzed by the Activated Carbon
Sun et al.
whereas catalysis of the AC in cleavage of the Car-Calk bond in DNE requires further investigation. Conclusions The present investigation confirmed that activated carbon catalyzes monatomic hydrogen transfer to R,ωdiarylalkanes and induces the hydrocracking of diaryl methanes with at least one polycyclic aromatic ring at a relatively low temperature. The results suggest that, under mild conditions, selective cleavage of C-C bridges could occur during a process of hydrogen transfer to coals over an activated carbon.
ficulty in cleaving the Car-Calk bond in DNE should be ascribed to the lability of the NE• radical. Despite the difficulty, the Car-Calk bond cleavage should be related to hydrogen transfer to the ipso position in DNE,
Acknowledgment. We thank the Research Fund for the Doctoral Program of Higher Education (20020290007), National Natural Science Foundation of China (Projects 29676045 and 90410018), the Key Project of Chinese Ministry of Education (Project 104031), and the Brain-Pool Program of the Korean Federation of Science and Technology Societies. Dr. Chul Wee Lee acknowledges the financial support from the Basic Research Program of the Korea Science and Engineering Foundation (under Grant No. R012003-000-10069-0). EF0497964
