Energy Fuels 2009, 23, 4877–4882 Published on Web 08/27/2009
: DOI:10.1021/ef900398g
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Selective Hydrogen Transfer to Anthracene and Its Derivatives over an Activated Carbon Lin-Bing Sun,*,†,‡ Xian-Yong Wei,*,‡,§ Xiao-Qin Liu,† Zhi-Min Zong,‡ Wen Li,§ and Jia-Hui Kou
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† State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, China, ‡Key Laboratory of Coal Processing and Efficient Utilization, China University of Mining and Technology, Xuzhou 221008, Jiangsu, China, §State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China, and Ecomaterials and Renewable Energy Research Center (ERERC), Department of Physics, Nanjing University, Nanjing 210093, China
Received May 4, 2009. Revised Manuscript Received August 1, 2009
Hydrogenation reactions of three polycyclic arenes (PCAs), that is, anthracene, 9-phenylanthracene (PA), and 9,10-diphenylanthracene (DPA) were carried out under an initial hydrogen pressure of 5 MPa at 300 °C. An activated carbon (AC, a metal-free catalyst), was employed to catalyze the PCA hydroconversions. The results show that the AC can split gaseous hydrogen into atomic form and catalyze monatomic hydrogen transfer to aromatic rings. Interestingly, the AC selectively catalyzed the hydrogenation of the anthracene ring, and prevented the benzene ring from hydrogenation and the C-C linkage from cleavage. The reactivity of the PCAs toward hydrogenation over the AC decreased in the order of anthracene > PA > DPA. The hydrogen-accepting ability and steric hindrance effect are demonstrated to be responsible for the difference in reactivity.
used as supports to prepare highly dispersed catalysts. However, some investigations show that carbon supports have catalytic activity, for example, for 4-(1-naphthylmethyl)bibenzyl decomposition9 and chlorobenzene dechlorination.10 This means that carbon materials used as supports may not be innocent, but act as catalysts in some reactions. A double catalytic effect thus could be expected when a carbon-supported catalyst is employed under some circumstances. These results prompt us to explore catalytic performances of carbon materials themselves. As another type of metal-free catalyst, carbon materials possess quite different physicochemical characteristics from both phosphonium borates and transition metals, which may result in interesting catalytic properties on hydrogenation. Therefore, it is desirable to study the catalytic performance of carbon materials on hydrogenation and to clarify the hydrogen transfer mechanism over carbons. In the present investigation, we employed an activated carbon (AC) as a metal-free catalyst for hydrogenation. The reactivity of several polycyclic arenes (PCAs) over the AC was examined in detail. The reaction mechanism of AC-catalyzed hydrogenation was proposed. The PCAs investigated included anthracene and its derivatives, that is, 9-phenylanthracene (PA) and 9,10-diphenylanthracene (DPA). These PCAs are important coal-related model compounds and functional molecules with high fluorescence efficiency.11 Understanding their reactivities toward hydrogenation is significant for the efficient utilization of coal resource as well as for the design and development of new functional molecules.
1. Introduction Hydrogenation is the addition of hydrogen to unsaturated organic compounds. Such reactions are widely used for the production of diverse chemicals,1 the upgrading of crude oil,2 and coal conversion.3,4 The hydrogenated products are also proposed to act as hydrogen storage media for protonexchange membrane fuel cells.5 It is generally accepted that hydrogenation reactions are catalyzed by transition-metalbased catalysts.6 The literature concerning nonmetal-catalyzed hydrogenation are relatively scarce. Recently, Stephan and co-workers discovered a new kind of hydrogenation catalyst without metal, that is, phosphonium borates.7 These metal-free catalysts are effective for hydrogenating imines, nitriles, and aziridines to primary and secondary amines.8 Enlightened by their studies, researchers begin to pay attention to novel metal-free catalysts for the hydrogenation of various unsaturated organic compounds. Due to their high surface area, carbon materials such as carbon blacks and activated carbons have been extensively *To whom correspondence should be addressed. (L.B.S.) Telephone: þ86 (25) 8358-7177. Fax: þ86 (25) 8358-7191. E-mail:
[email protected]. (X.Y.W.) Telephone/Fax: þ86 (516) 8388-4399. E-mail: wei_xianyong@ 163.com. (1) de Vries, J. G.; Elsevier, C. J. The Handbook of Homogeneous Hydrogenation; Wiley-VCH: Weinheim, 2007. (2) Rana, M. S.; Ancheyta, J.; Maity, S. K.; Rayo, P. Catal. Today 2008, 130, 411–420. (3) Sun, L. B.; Zong, Z. M.; Kou, J. H.; Zhang, L. F.; Ni, Z. H.; Yu, G. Y.; Chen, H.; Wei, X. Y.; Lee, C. W. Energy Fuels 2004, 18, 1500– 1504. (4) Sun, L. B.; Zong, Z. M.; Kou, J. H.; Liu, G. F.; Sun, X.; Wei, X. Y.; Zhou, G. J.; Lee, C. W. Energy Fuels 2005, 19, 1–5. (5) Wang, B.; Goodman, D. W.; Froment, G. F. J. Catal. 2008, 253, 229–238. (6) Studt, F.; Abild-Pedersen, F.; Bligaard, T.; Sorensen, R. Z.; Christensen, C. H.; Norskov, J. K. Science 2008, 320, 1320–1322. (7) Welch, G. C.; Juan, R. R. S.; Masuda, J. D.; Stephan, D. W. Science 2006, 314, 1124–1126. (8) Chase, P. A.; Welch, G. C.; Jurca, T.; Stephan, D. W. Angew. Chem., Int. Ed. 2007, 46, 8050–8053. r 2009 American Chemical Society
2. Experimental Section 2.1. Materials. The AC (Darco KB-B, PA > DPA, that is, the introduction
(12) Stanislaus, A.; Cooper, B. H. Catal. Rev. Sci. Eng. 1994, 36, 75– 123.
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Figure 3. Time profiles of the product distribution from PA hydrogenation.
Figure 4. Time profiles of the product distribution from DPA hydrogenation.
but the yield of compound 4 was much higher than compound 5. 3.3. Hydrogen Transfer to PA. As illustrated in Figure 3, neither the hydrogenation of benzene ring nor the scission of the C-C bond connecting benzene and anthracene rings proceeds, indicating that the AC catalyzes the hydrogenation of the anthracene ring selectively. Four hydrogenated products were detected, including two dihydro derivatives (compounds 6 and 7), one tetrahydro derivative (compound 8), and one octahydro derivative (compound 9). The tetrahydro derivative was the most abundant among the products. Different from anthracene hydrogenation, only a small amount of PA was converted to 9,10-dihydro derivative. The selectivity of the 9,10-dihydro derivative from anthracene hydrogenation is 40.2% at 4 h, whereas that of the 9,10-dihydro product from PA hydrogenation is only 8.3%. The yield of the 1,4-dihydro derivative (compound 7) is very low, which is similar to the case of anthracene hydrogenation. Only the octahydro derivative was detected with a yield of 12.5% at 8 h. 3.4. Hydrogen Transfer to DPA. As Figure 4 displays, similar to PA hydrogenation, only the hydrogenation of the anthracene ring in DPA occurred. Five hydrogenated products, including three dihydro derivatives (compounds 10, 11, and 12), one tetrahydro derivative (compound 13), and one octahydro derivative (compound 14), were detected. The yield of tetrahydo derivative is the highest among the products. Interestingly, three dihydro derivatives were detected. One was identified as 1,4-dihydro-9,10-diphenylanthracene (compound 12). As Figure 5 shows, mass spectra of other two isomers (both of them are 9,10-dihydro-9,10diphenylanthracenes) are similar, but the relative abundances of their molecular ions are remarkably different. According to their structural stability, the 9,10-dihydro derivative with larger relative abundance of molecular ion should be trans-9,10-dihydro-9,10-diphenylanthracene (compound 11) and the other should be cis-9,10-dihydro9,10-diphenylanthracene (compound 10). As shown in Figure 4, the yield of compound 10 was obviously higher than that of compound 11, although the molecular structure of compound 11 is more stable. The selectivity of 9,10dihydro derivative from DPA hydrogenation is 7.4% at 4 h, which is appreciably lower than that (8.3%) from PA hydrogenation and much lower than that (40.2%) from anthracene hydrogenation.
sion of anthracene and its derivatives originated from thermal reaction can be excluded. Zhang and Yoshida13 investigated AC-catalyzed hydrogenation of anthracene using a downflow fixed bed reaction system. They reported that about 60% of anthracene was converted to dihydro and tetrahydro derivatives under a hydrogen pressure of 6 MPa at 300 °C. By using tetralin instead of gaseous hydrogen, Li et al. mentioned that AC can promote the hydrogen transfer from tetralin to anthracene.14 The conversion of anthracene can reach 40% in an autoclave reactor at 400 °C. These results suggest that AC is effective for hydrogen transfer to anthracene. Active hydrogen species, including proton and hydrogen atom, are crucial for the hydrogenation and hydrocracking of PCAs.15-17 An acidic catalyst is necessary to produce protons,15,16 whereas other catalysts such as metal sulfides15 are responsible for the formation of hydrogen atoms from gaseous hydrogen. Zhang et al.18 found that AC was catalytically active for hydrogen dissociation. Keren and Soffer19 demonstrated that pure carbon could split gaseous hydrogen into atomic form through hydrogen chemisorption. By comparing carbon with γ-Al2O3, Furimsky suggested that the former is less acidic, therefore the hydrogen transfer from the surface to reactant molecules should proceed via hydrogen atoms.20 The presence of unsaturated bonds in reactant molecules is necessary for such transfer to take place. According to these reports, the AC-catalyzed hydrogenation of PCAs should mainly proceed via monatomic hydrogen transfer to an aromatic ring, similar to metal sulfides. The reactivities of aromatic hydrocarbons toward hydrogenation are known to be dominated by their hydrogenaccepting ability. The hydrogen-accepting ability of different aromatic rings and different positions in the same aromatic ring may be quite different, which can be characterized by the corresponding superdelocalizability (Sr) values.21,22 A carbon atom on an aromatic ring with a larger Sr value accepts (13) Zhang, Z. G.; Yoshida, T. Energy Fuels 2001, 15, 708–713. (14) Li, M.; Wang, J. Q.; Deng, W. A.; Que, G. H. J. Fuel Chem. Technol. 2007, 35, 558–562. (15) Shimizu, K.; Inaba, A.; Saito, I.; Karamatsu, H.; Suganuma, A. Fuel 1995, 74, 853–859. (16) Olah, G. A.; Husain, A. Fuel 1984, 63, 1427–1431. (17) Wei, X. Y.; Ni, Z. H.; Zong, Z. M.; Zhou, S. L.; Xiong, Y. C.; Wang, X. H. Energy Fuels 2003, 17, 652–657. (18) Zhang, Z. G.; Okada, K.; Yanamoto, M.; Yoshida, T. Catal. Today 1998, 45, 361–366. (19) Keren, E.; Soffer, A. J. Catal. 1977, 50, 43–55. (20) Furimsky, E. Carbons and Carbon-Supported Catalysts in Hydroprocessing; RSC Publishing: Cambridge, 2008. (21) Wei, X. Y.; Zong, Z. M.; Qin, Z. H.; Cheng, C. The Chemistry of Coal Liquefaction; Science Press: Beijing, 2002. (22) Wei, X. Y.; Ogata, E.; Niki, E. Bull. Chem. Soc. Jpn. 1992, 65, 1114–1119.
4. Discussion Because blank tests demonstrate that no substrates were converted at all in the absence of a catalyst, the hydroconver4879
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Figure 5. Mass spectra of two isomers of 9,10-dihydro-9,10-diphenylanthracene.
hydrogen atom more readily. Table 2 lists the Sr values of different positions in some aromatic compounds. The first hydrogen atom should preferentially transfer to 9-position of anthracene, since the Sr value of 9-position is much larger than those of other positions in anthracene ring. However, to our knowledge, no data of Sr values of PA and DPA are available. It seems difficult to compare the reactivity of anthracene, PA, and DPA. Fortunately, some instructions emerge by analyzing the Sr values of an aromatic ring before and after substituting a group. As shown in Table 2, the Sr value of each position in benzene is 0.8333. After being connected with a benzyl group, the Sr value of the ipso-position decreases to 0.8194. An even lower Sr value of the ipso-position (0.7722) is observed after being connected with a phenyl group. Similarly, the Sr values of 1-position in naphthalene and 9-position in anthracene decline after being connected with a benzyl group. On the basis of these data, it is reasonable to deduce that the Sr value of 9-position in anthracene should decrease after being connected with a phenyl group (i.e., PA), and the Sr values of both 9- and 10-positions should decline after they are connected with a phenyl group (i.e., DPA). According to the foregoing analysis, the Sr values of the most active positions, that is, 9- and 10-positions, should decrease in the order of anthracene > PA g DPA and anthracene g PA > DPA, respectively. Hence, the reactivity toward hydrogenation over the AC should decrease in the order of anthracene > PA > DPA, which is in good agreement with the experimental results (Figure 1). The steric hindrance is considered to be another factor affecting the reactivity of the PCAs. Hydrogen transfer to 9or 10-positions in anthracene is easy due to the low steric hindrance in the positions. However, the existence of phenyl groups in 9- and 10-positions obstructs the addition of hydrogen atoms to ispo-positions in PA and DPA. The steric hindrance thus results in a lower conversion of PA and DPA
than that of anthracene. Therefore, the difference in reactivity of the PCAs can be ascribed to their different hydrogenaccepting ability and steric hindrance effect. Taking into consideration that computational methods could help to understand the behavior of the studied molecules,23 the molecular geometries were thus optimized by means of density functional theory using the B3LYP functionals in the Gaussian 03 program.24 Tables S1 and S2 in Supporting Information present the optimized geometries of PCAs and corresponding 9,10-dihydro derivatives. It is worthy to note that the absolute energy values of all the PCAs become higher after hydrogenation. The differences in the absolute energy value between the PCAs and corresponding 9,10-dihydro derivatives were calculated to be 35.498, 36.630, and 37.713 eV, respectively, for anthracene, PA, and DPA. This means that hydrogen transfers to anthracene readily, whereas that to DPA is relatively difficult. The computed data thus support the experimental results, indicating that the (23) Kukhta, A. V.; Kukhta, I. N.; Kukhta, N. A.; Neyra, O. L.; Meza, E. J. Phys. B: At. Mol. Opt. Phys. 2008, 41, 205701. (24) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A., Gaussian 03, Revision E.01; Gaussian, Inc.: Wallingford CT, 2004.
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Table 2. Sr Values of Different Positions in Some Arenes
Figure 6. Proposed pathway for hydrogen transfer to anthracene over the AC.
Figure 7. Proposed pathway for hydrogen transfer to PA over the AC.
reactivity of three PCAs toward hydrogenation decreases in the order of anthracene > PA > DPA. With prolonging the reaction time, a large amount of PCAs can be converted over the AC. Nonetheless, neither hydrogenation of benzene ring nor cleavage of C-C linkage occurs, indicating that the AC catalyzes the PCA conversion selectively. The present study has demonstrated that the AC, a kind of metal-free catalyst, is effective for the selective hydrogenation of PCAs. The selective hydrogen transfer to PCAs may make carbon materials useful as catalysts for synthesizing special chemicals as well as for obtaining valuable aromatics and hydroaromatics from coal, petroleum, and other raw materials. Figure 6 depicts the proposed pathway for the AC-catalyzed hydrogenation of anthracene. Hydrogen transfer to 9and 10-positions leads to the formation of compound 1, while hydrogen transfer to 1- and 4-positions results in the formation of compound 2. The intermediate 2 is quite active, which can be easily hydrogenated to compound 3. Hence, the yield of compound 2 is very low during the reaction. Two octahydro derivatives (compounds 4 and 5) are produced; they should derive from compound 3 rather than from compound 1, because the hydrogenation of a naphthalene ring is much easier than a benzene ring according to their Sr values (Table 2). A hydrogen atom can transfer to either the 8- or 9-position in compound 3; however, compound 4 is the main product from the hydrogenation of compound 3, implying that the hydrogen transfer occurs preferentially to the
8-position as an initial step in the hydrogenation of compound 3. A possible reason for the preferential hydrogen transfer is that, during its transfer to the 9-position in compound 3, a hydrogen atom has a tendency to extract a benzylic hydrogen atom from the 1-position in compound 3 to afford H2, because the 1-position is closer to the 9-position than to the 8-position. Figures 7 and 8 illustrate the proposed pathways for hydrogen transfer to PA and DPA over the AC. The selectivity of 9,10-dihydro derivatives from PA and DPA hydrogenation is much lower than that from anthracene hydrogenation. Two factors are considered to be responsible for this phenomenon. One is the decline of the Sr value of ipsopositions after the introduction of benzene rings as described above. Another is the steric hindrance effect of benzene rings on ipso-positions, which obstructs the addition of hydrogen atom to yield 9,10-dihydro products. These two factors are also believed to account for the formation of 1,2,3,4,5,6,7,8octahydro derivatives rather than 1,2,3,4,9,9a,10,10a-octahydro ones from PA and DPA hydrogenation. As shown in Figure 8, hydrogen atoms can be transferred both to the same side and to different sides of anthracene ring in DPA, leading to the formation of trans- and cis-isomers of 9,10-dihyro derivatives. The hydrogen transfer should preferentially occur at the same side, since the yield of compound 10 is higher than that of compound 11. Such preferential hydrogen transfer may be associated with the adsorption state of the DPA molecule on the surface of the catalyst. 4881
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Figure 8. Proposed pathway for hydrogen transfer to DPA over the AC.
5. Conclusions
Acknowledgment. The authors are grateful for financial supports from the Key Program in Major Research Plan for Energy of Western China, Natural Science Foundation of China (Project 90410018), the Special Fund for Major State Basic Research Project (Project 2004CB217601), the Program of the Universities in Jiangsu Province for Development of High-Tech Industries (Project JHB05-33), the Foundation of State Key Laboratory of Coal Conversion (Grant No. 09-301), State Key Laboratory of Materials-Oriented Chemical Engineering, the Major Basic Research Project of Natural Science Foundation of Jiangsu Province Colleges (No. 08KJA530001), and the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT0732).
Our investigation demonstrates that an AC, a metal-free catalyst, is effective for the hydrogenation of PCAs, and the reactions proceed via monatomic hydrogen transfer to an anthracene ring. The AC catalyzes the hydrogenation of anthracene ring selectively, and neither the hydrogenation of benzene ring nor the scission of C-C linkage occurs. The carbon materials with selective hydrogenation ability may provide a good candidate for synthesizing special chemicals as well as for obtaining valuable aromatics and hydroaromatics from coal, petroleum, and other raw materials. The reactivities of three PCAs toward hydrogenation over the AC decrease in the order of anthracene>PA>DPA. The difference in reactivity can be ascribed to their different hydrogen-accepting ability and steric hindrance effect.
Supporting Information Available: Optimized Geometries of anthracene, PA, and DPA as well as their hydrogenated derivatives. This information is available free of charge via the Internet at http://pubs.acs.org.
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