Microwave-Assisted Hydrogen Transfer to Anthracene and

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Energy & Fuels 2009, 23, 638–645

Microwave-Assisted Hydrogen Transfer to Anthracene and Phenanthrene over Pd/C Yu-Miao Ma,† Xian-Yong Wei,*,†,‡ Xiao Zhou,† Ke-Ying Cai,† Yao-Li Peng,† Rui-Lun Xie,† Ying Zong,† Yan-Bin Wei,† and Zhi-Min Zong† School of Chemical Engineering, China UniVersity of Mining and Technology, Xuzhou 221008, Jiangsu, China, and Key Laboratory of Coal ConVersion and New Carbon Materials, Hubei ProVince, Wuhan UniVersity of Science and Technology, Wuhan 430081, Hubei, China ReceiVed September 24, 2008. ReVised Manuscript ReceiVed December 31, 2008

Microwave-assisted hydrogen transfer to anthracene and phenanthrene over Pd/C was investigated under mild conditions. The effects of reaction temperature, initial hydrogen pressure, and reaction time on the reactant conversions and product selectivities were examined. The results show that the hydrogenations of both reactants proceed at much lower temperature and hydrogen pressure under microwave irradiation than those by conventional heating and that related reactions include both mon- and biatomic hydrogen transfer. The reactivities of both reactants and their products toward hydrogenation and/or dehydrogenation are closely related to superdelocalizability and resonance energy values of the related species.

Introduction Polycyclic arenes (PCAs) are widely present in heavy fossil resources (HFRs), such as coal and heavy petroleum (HP). They not only pollute the environment when emitted from incomplete combustion of the HFRs but also decrease the cetane number and diminish the overall quality of the diesel oil derived from the HFRs.1,2 Catalytic hydrogenation of PCAs is important for upgrading the coal- and HP-derived oils.3-5 The hydrogenations of PCAs have been extensively investigated. Zhang et al.6 examined catalytic hydrogenation of anthracene under different conditions. They found that anthracene can be completely converted to octahydroanthracenes (OHAs), 9,10-dihydroanthracene (9,10-DHA), and 1,2,3,4-tetrahydroanthracene (THA) over Ni/C at 300 °C under 3 MPa of H2. Kotanigawa et al.7 investigated catalytic hydrogenations of naphthalene and phenanthrene at 430 °C under 10 MPa of initial hydrogen pressure (IHP) using a series of Ru-containing catalysts. They found that Ru/Mn203 (0.19)-NiO (0.81) showed the best performance for the reaction, yielding 33 mol % of decalins from naphthalene and 49 mol % of perhydroanthracene from anthracene. Nelkenbaum et al.8 reported their investigation on the metalloporphyrin-catalyzed hydrogenation of naphthalene, anthracene, and phenanthrene in aqueous solutions under * To whom correspondence should be addressed. E-mail: wei_xianyong@ 163.com. † China University of Mining and Technology. ‡ Wuhan University of Science and Technology. (1) Tao, S.; Li, X. R.; Yang, Y.; Coveney, R. M.; Lu, X. X.; Chen, H. T.; Shen, W. R. EnViron. Sci. Technol. 2006, 40 (15), 4586–4591. (2) Copper, B. H.; Donnis, B. B. L. Appl. Catal., A 1996, 137 (2), 203– 223. (3) Stanislaus, A.; Copper, B. H. Catal. ReV. Sci. Eng. 1994, 36 (1), 75–123. (4) Song, C. S.; Klein, M.; Reynolds, J. Catal. Today 1996, 31 (1-2), 1–2. (5) Mochida, I.; Sakanishi, K.; Korai, Y.; Fujitsu, H. Fuel 1986, 65 (8), 1090–1093. (6) Zhang, Z. G.; Okada, K.; Yamamoto, M.; Yoshida, T. Catal. Today 1998, 45 (1-4), 361–366. (7) Kotanigawa, T.; Yamamoto, M.; Yoshida, T. Appl. Catal., A 1997, 164 (1-2), 323–332. (8) Nelkenbaum, E.; Dror, I.; Berkowitz, B. Chemosphere 2007, 68 (2), 210–217.

ambient conditions. Their results show that a long time is needed for high conversion of the PCAs; e.g., it took 180 h to convert ca. 75% of anthracene. The above techniques for the hydrogenations of PCAs have some shortcomings, such as high reaction temperature and hydrogen pressure or too long of a reaction time along with the use of too expensive catalysts, such as metalloporphyrins. Microwave irradiation (MWI) is well-documented to significantly accelerate a wide range of organic reactions under mild conditions.9,10 Many reactions that do not occur by conventional heating can proceed under MWI.10 Moreover, MWI heating has the following advantages compared to conventional heating for organic reactions: no direct contact between the energy source and the reacting chemicals,11 reducing heat-transfer problems,12 energy efficiency,13 easy automation, and incident power control.14 In numerous organic reactions, rapid heating, selective heating, and enhancements of yield and purity are possible by MWI.15,16 Thus, such a technology was introduced to numerous hydrogenation reactions and achieved rapid realization under mild conditions.17-21 However, to our knowledge, no reports have been issued on the hydrogenation of PCAs under MWI. (9) Lidstro¨m, P.; Tierney, J.; Wathey, B.; Westman, J. Tetrahedron 2001, 57 (45), 9225–9283. ´ .; Moreno, A. Chem. Soc. ReV. 2005, (10) Hoz, A. D. L.; Dı´az-Ortiz, A 34 (2), 164–178. (11) Gabriel, C.; Gabriel, S.; Grant, E. H.; Kalstead, B. S. J.; Mingos, D. M. P. Chem. Soc. ReV. 1998, 27 (3), 213–223. (12) Mingos, D. M. P.; Baghurst, D. R. Chem. Soc. ReV. 1991, 20 (1), 1–47. (13) Enquist, P. A.; Nilsson, P.; Larhed, M. Org. Lett. 2003, 5 (25), 4875–4878. (14) Bonnet, C.; Estel, L.; Ledoux, A.; Mazari, B.; Louis, A. Chem. Eng. Process. 2004, 43 (11), 1435–1440. (15) Baghurst, D. R.; Mingos, D. M. P. J. Chem. Soc., Chem. Commun. 1992, 9, 674–677. (16) Galema, S. A. Chem. Soc. ReV. 1997, 26 (3), 233–238. (17) Banik, B. K.; Barakat, K. J.; Wagle, D. R.; Manhas, M. S.; Bose, A. K. J. Org. Chem. 1999, 64 (16), 5746–5753. (18) Vanier, G. S. Synlett 2007, (1), 131–135. (19) Chapman, N.; Conway, B.; O’Grady, F.; Wall, M. D. Synlett 2006, (7), 1043–1046.

10.1021/ef800808t CCC: $40.75  2009 American Chemical Society Published on Web 02/02/2009

MicrowaVe-Assisted Hydrogen Transfer

Energy & Fuels, Vol. 23, 2009 639

Table 1. Comparison of Reactivities among Benzene, BP, and DPM toward Hydrogenationa

substrate

reaction temperature (°C)

IHP

conversion (%)

product

benzene BP DPM DPM

100 80 80 100

0.70 0.62 0.62 0.62

0 16.7 2.6 8.3

none cyclohexylbenzene (cyclohexylmethyl)benzene (cyclohexylmethyl)benzene

a

Reaction time ) 5 min.

In the present study, we investigate the reaction mechanisms for hydrogen transfer to anthracene and phenanthrene and examine the effects of the reaction temperature, IHP, and reaction time on the microwave-assisted hydrogenation of anthracene and phenanthrene over Pd/C. Experimental Section Materials. Substrates anthracene, phenanthrene, 9,10-DHA, 9,10dihydrophenanthrene (9,10-DHP), diphenylmethane (DPM), biphenyl (BP), and benzene are commercially purchased analytical pure reagents. The former three ones were purified by recrystallization with ethanol. DPM was purified by silica-gel column chromatography (SGCC) using hexane as the eluent and subsequent recrystallization with hexane. BP was purified by SGCC using ethanol as the eluent and subsequent recrystallization with ethanol. Benzene was purified by SGCC and subsequent distillation. Catalyst Pd/C (10% Pd) was provided by the Nikki Chemical Company. Solvent tetrahydrofuran (THF) was also commercially purchased and distillated before use. General Procedure. A substrate (0.5 mmol), Pd/C (0.1 g), THF (20 mL), and a magnetical stirrer were added into an 80 mL CEM discover microwave reactor. After the air in the reactor was replaced with nitrogen 3 times and the reactor was subsequently pressurized with hydrogen to a prescribed IHP at room temperature, the reactor was heated to an indicated temperature and kept at that temperature for 1-9 min. Then, the reactor was immediately cooled to room temperature by purging compressed air. The reaction mixture was taken out from the reactor and filtrated through a membrane filter with 0.45 µm of pore size. The filtrate was analyzed with a HewlettPackard 6890/5973 GC/MS equipped with a capillary column coated with HP-5MS (cross-link 5% PH ME siloxane, 30 m × 0.32 mm i.d., 0.25 µm film thickness) and a quadrupole analyzer and operated in electron impact (70 eV) mode. The column was heated from 100 to 160 °C at 15 °C/min, then heated to 230 °C at 5 °C/ min, and held the temperature for 5 min. The mass range scanned was from 30 to 500 amu. Data were acquired and processed using Chemstation software. The compounds were identified by comparing mass spectra to NIST05 library data. The conversion of each substrate was calculated as the molar ratio of the substrate remained in the reaction mixture to that added (i.e., 0.5 mmol), and the selectivity of each product was calculated as molar ratio of the product to all of the products.

Results and Discussion Hydrogenation of the Benzene Ring. As Table 1 shows, benzene was not converted at all even at 100 °C under 0.70 MPa of IHP for 5 min, indicating that the benzene ring itself is inert toward hydrogenation under the reaction conditions. Under the same conditions (IHP, 0.62 MPa; reaction time, 5 min; reaction temperature, 80 °C), the conversion (16.7%) of BP is much higher than that (2.6%) of DPM. Even at 100 °C, only 8.3% of DPM was hydrogenated. The much larger reactivity (20) Quai, M.; Repetto, C.; Barbaglia, W.; Cerewa, E. Tetrahedron Lett. 2007, 48 (7), 1241–1245. (21) Wang, T. X.; Zong, Z. M.; Zhang, J. W.; Wei, Y. B.; Zhao, W.; Li, B. M.; Wei, X. Y. Fuel 2008, 87 (4-5), 498–507.

of BP than DPM toward hydrogenation should be ascribed to a much larger Sr value (0.910) of the 2 position in BP than that (0.833) in the benzene ring from DPM, as shown in Table 2.22 Direct hydrogenation of the benzene ring in DPM should be very difficult, because benzene hydrogenation does not proceed at all under the reaction conditions. As Scheme 1 illustrates, the formation of the diphenylmethyl radical and its subsequent conversion to the (E)-6-benzylidenecyclohexa-2,4-dien-1-yl radical could be crucial steps for DPM hydrogenation. Effect of the Reaction Temperature. When the IHP and reaction time were fixed to 0.62 MPa and 5 min, respectively, the hydrogenations of anthracene and phenanthrene at different reaction temperatures were compared. As Figure 1 shows, anthracene was almost completely converted even at 60 °C, whereas phenanthrene conversion is only 31.6% at the same reaction temperature and increased to near 100% at 100 °C. The change in product distribution with raising the reaction temperature is also significantly different, as exhibited in Figures 2 and 3. These results can be interpreted in terms of superdelocalizability (Sr)22 and resonance energy (RE)23 shown in Tables 2 and 3, respectively. Anthracene is known to be much more reactive toward photobromination24 and activated carbon (AC)-catalyzed hydrogenation25 than phenanthrene because of much larger Sr value (1.314) of the 9 and 10 positions of anthracene than those (0.998) in phenanthrene. The much larger reactivity of anthracene than phenanthrene toward Pd/C-catalyzed hydrogenation under MWI can be also attributed to the significant difference in the Sr value of the 9 and 10 positions between anthracene and phenanthrene. Anthracene was hydrogenated to 9,10-DHA and THA as main products with 93.9% of total selectivity along with 1,2,3,4,5,6,7,8octahydroanthracene (sym-OHA) and three 1,2,3,4,4a,9,9a,10octahydroanthracenes as minor products with 6.1% of total selectivity at 60 °C, as shown in Figure 2. The three 1,2,3, 4,4a,9,9a,10-octahydroanthracenes are cis-OHA, i.e., (4aS,9aR)1,2,3,4,4a,9,9a,10-octahydroanthracene (S,R-OHA), which is the same as (4aR,9aS)-1,2,3,4,4a,9,9a,10-octahydroanthracene (R,SOHA), and trans-OHAs, including (4aS,9aS)-1,2,3,4,4a,9,9a,10octahydroanthracene (S,S-OHA) and its isomer (4aR,9aR)1,2,3,4,4a,9,9a,10-octahydroanthracene (R,R-OHA). The total selectivity of the main products decreased, while that of the minor products increased with raising the reaction temperature. It is noteworthy from Figure 2 that both the decrease in 9,10DHA selectivity and the increase in sym-OHA selectivity with raising the reaction temperature, especially from 90 to 100 °C, are the most remarkable. This result can be interpreted in terms of the lability of 9,10-DHA and stability of sym-OHA. Pd/C consists of metallic Pd and AC. Metallic catalysts catalyze biatomic hydrogen transfer (BAHT),26-28 whereas AC promotes monatomic hydrogen transfer (MAHT).25,29 Therefore, Pd/C may catalyze both MAHT and BAHT to anthracene, phenanthrene, and their hydrogenated products. (22) Yonezawa, T.; Nagata, C.; Koto, H.; Imamura, A.; Morokuma, K. Guide to Quantum Chemistry; Kagaku-Dojing Press: Kyoto, Japan, 1990; p 232. (23) Herndon, W. C. J. Org. Chem. 1981, 46 (10), 2119–2125. (24) Zong, Z. M.; Zhang, W. H.; Jiang, Q.; Lu, J.; Wei, X. Y. Bull. Chem. Soc. Jpn. 2002, 75 (4), 769–771. (25) 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 (5), 1500– 1504. (26) Wei, X. Y.; Ogata, E.; Niki, E. Chem. Lett. 1991, (12), 2199–2202. (27) Wei, X. Y.; Ogata, E.; Zong, Z. M.; Niki, E. Fuel 1993, 72 (11), 1547–1552. (28) 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.

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Ma et al. Table 2. Sr Values of Some Unsaturated Hydrocarbons

Scheme 1. Proposed Pathways of DPM Hydrogenation under MWI over Pd/C

Because of the much larger Sr value of the 9 and 10 positions than those (1.073 and 0.922) in the 1 and 2 positions of anthracene, hydrogen transfer to the 9 and 10 positions to produce 9,10-DHA is much easier than to other positions during anthracene hydrogenation. As Scheme 2 shows, such hydrogen transfer should be MAHT, because the much larger distance (270.4 pm) between the 9 and 10 positions of anthracene than that (74.2 pm) between two hydrogen atoms in H2 makes BAHT impossible but favors MAHT because the larger distance reduces the possibility for abstracting hydrogen in the 9 position during MAHT to the 10 position. On the other hand, the dehydroge-

Figure 1. Anthracene and phenanthrene conversions at different reaction temperatures for 5 min.

Figure 2. Distribution of the products from anthracene hydrogenation at different reaction temperatures for 5 min.

nation of 9,10-DHA also proceeds much easier than those of other hydrogenated anthracenes (HAs) because of the much higher reactivity of 9,10-DHA than those of other HAs toward benzylic hydrogen abstraction.30-32 Alternatively, the much higher reactivity of 9,10-DHA than those of other HAs toward dehydrogenation can be interpreted by comparing the stabilities, in terms of RE, of the related benzylic radicals (RBRs), i.e., benzyl, naphthyl-2-methyl, and diphenylmethyl radicals, the RE values (kJ mol-1) of which increase in the order benzyl (128.0) < naphthyl-2-methyl (174.5) < diphenylmethyl (228.4), as listed in Table 3. Because of the high stability of 9-hydroanthracen10-yl radical (9-HAR), which is analogous to diphenylmethyl radical, 9,10-DHA dehydrogenation was accelerated at higher reaction temperatures, especially over 90 °C, leading to a decrease in 9,10-DHA selectivity with raising the reaction temperature. 10-Hydroanthracen-4a-yl radical (10-HAR) is a tautomer of 9-HAR, through which OHAs can be formed. As Table 4 lists, when using 9,10-DHA as the reactant, anthracene, THA, sym-OHA, cis-OHA, and trans-OHAs were also produced, but the HAs should result from anthracene hydrogenation rather than from 9,10-DHA hydrogenation. Much higher selectivity (7.6%) of cis-OHA than that (0.4%) of trans-OHAs indicates that BAHT to the 4a and 9a positions of 1,2,3,4,9,10hexahydroanthracene (1,2,3,4,9,10-HHA) is predominant. THA should be produced mainly via BAHT to the 1-4 positions of anthracene and minorly via MAHT to the 1 and 4

Figure 3. Distribution of the products from phenanthrene hydrogenation at different reaction temperatures for 5 min.



Table 3. HAs, Hydrogenated Phenanthrenes (HPs), and RE Values (kJ mol-1)23 of the RBRs

Scheme 2. Proposed Pathways of Anthracene Hydrogenation under MWI over Pd/C


Ma et al.

Table 4. Reaction of 9,10-DHA at 80 °C under 0.62 MPa for 5 min selectivity (mol %) conversion (%) anthracene THA sym-OHA cis-OHA trans-OHAs 58.9

1.4

46.4

44.1

7.6

0.4

positions followed by BAHT to the 2 and 3 positions of the resulting 1,4-dihydroanthracene (1,4-DHA), because MAHT proceeds preferentially to the 9 and 10 positions of anthracene. THA hydrogenation proceeds via both BAHT and MAHT. MAHT occurs more readily to the 5 and 8 positions than to the 9 and 10 positions of THA, because R-hydrogen atoms are much closer to the 9 and 10 positions than to the 5 and 8 positions,

so that there is a much higher possibility for abstracting R-hydrogen atoms to form H2 during MAHT to the 9 and 10 positions than to the 5 and 8 positions. As Scheme 2 displays, MAHT to the 5 and 8 positions of THA affords 1,2,3,4,5,8hexahydroanthracene (1,2,3,4,5,8-HHA), which is readily converted to sym-OHA via both MAHT and BAHT because hydrogen transfer to the 6 and 7 positions (Sr value of ca. 1) of 1,2,3,4,5,8-HHA is easier than to the 5, 8, 9, and 10 positions (Sr value of ca. 0.994) of THA. In a like manner, 1,2,3,4,9,10HHA results from MAHT to the 9 and 10 positions of THA and is converted to both cis-OHA via subsequent BAHT and trans-OHAs via subsequent MAHT. MAHT to afford cis-OHA

Scheme 3. Proposed Pathways of Phenanthrene Hydrogenation under MWI over Pd/C



Table 5. Reaction of 9,10-DHP at 80 °C under 0.62 MPa reaction selectivity (mol%) time conversion (min) (%) phenanthrene THP sym-OHP cis-OHPs trans-OHPs

0.5 1 5

9.9 18.7 41.5

16.1 1.0 0

38.5 62.6 17.6

33.0 25.2 57.0

9.6 10.3 23.0

2.8 0.9 2.4

is difficult, because the second hydrogen tends to abstract the first one. trans-OHAs should be S,S-OHA and R,R-OHA, but separation of the two isomers is very difficult using the present GC/MS. The most remarkable increase in sym-OHA selectivity among the selectivities of the OHAs with raising the reaction temperature could be ascribed to its highest stability among the HAs and higher possibility for THA hydrogenation to sym-OHA than to cis-OHA and trans-OHAs. A much smaller Sr value (0.833) of any position in BR than that of the 9 and 10 positions of anthracene leads to the difficulty in the hydrogenation of the HAs, including sym-OHA but except for THA, and a much smaller RE value of benzyl radical than that of diphenylmethyl radical results in the difficulty in the dehydrogenation of symOHA. In contrast, THA is much more reactive than the OHAs, especially sym-OHA. One reason is that a much larger Sr value (0.994) of R-naphthalene ring (NR) than that in BR enables THA to be much more reactive toward hydrogenation than the OHAs. Another is that a much larger RE value of naphthyl-2methyl radical than that of benzyl radical makes THA much more reactive toward dehydrogenation than the OHAs. The significant decrease in THA selectivity with raising the reaction temperature displayed in Figure 2 should be attributed to the much larger reactivities of THA toward both hydrogenation and dehydrogenation than those of the OHAs. An appreciably larger Sr value (0.998) in the 9 and 10 positions than other positions makes hydrogen transfer preferentially occur to the 9 and 10 positions of phenanthrene, affording 9,10-DHP as the main product as shown in Figure 3. As Scheme 3 exhibits, 9,10-DHP should be predominantly produced by BAHT to the 9 and 10 positions of phenanthrene, because either of the two hydrogen atoms in the 9 position of 9-hydrophenanthren-10-yl radical (9-HPR) tends to be abstracted during MAHT to the 10 position of 9-HPR. Data in Table 5 show that the selectivity of phenanthrene dramatically decreased, while those of sym-OHP and cis-OHPs remarkably increased with the reaction time, indicating that hydrogenation of the resulting phenanthrene from 9,10-DHP dehydrogenation significantly proceeded. 9,10-DHP has a similar structure to BP. Hence, direct hydrogenation of 9,10-DHP is also possible, and MAHT to the 4 position, affording 4-(4,9,10-trihydrophenthren-1-yl) radical [4-(4,9,10-THP)R], should be the first step for 9,10-DHP hydrogenation because of much larger Sr value in the 4 position than those in other positions. THP was produced via either BAHT to the 1-4 positions of phenanthrene or MAHT to the 1 and 4 positions followed by BAHT to the 2 and 3 positions of the resulting 1,4-dihydrophenanthrene (1,4-DHP). THP hydrogenation proceeds via both BAHT and MAHT, theoretically affording 1,2,3,4,5,6,7,8-octahydrophenanthrene (sym-OHP), cis-OHPs, i.e., (4aR,9aR)-1,2,3,4,4a,9,9a,10octahydrophenanthrene (R,R-OHP) and (4aS,9aS)-1,2,3,4,4a,9, (29) Ni, Z. H.; Zong, Z. M.; Zhang, L. F.; Sun, L. B.; Liu, Y.; Yuan, X. H.; Wei, X. Y. Energy Fuels 2003, 17 (1), 60–61. (30) Poutsma, L. M. Energy Fuels 1990, 4 (2), 113–131. (31) Wei, X. Y.; Zong, Z. M. Energy Fuels 1992, 6 (2), 236–237. (32) Zong, Z. M.; Wei, X. Y. Fuel Process. Technol. 1994, 41 (1), 79– 85.

Figure 4. Effect of the IHP on anthracene conversion at 70 °C and phenanthrene at 90 °C for 5 min.

9a,10-octahydrophenanthrene (S,S-OHP), and trans-OHPs, i.e., (4aS,9aR)-1,2,3,4,4a,9,9a,10-octahydrophenanthrene (S,R-OHP) and (4aR,9aS)-1,2,3,4,4a,9,9a,10-octahydrophenanthrene (R,SOHP), but trans-OHPs were not detected when using phenanthrene as the reactant. As listed in Table 5, when using 9,10DHP as the reactant, trans-OHPs were detected as minor products. It is not clear why trans-OHPs were not produced from phenanthrene hydrogenation but produced from the reaction of 9,10-DHP. Both cis-OHPs and trans-OHPs should consist of two isomers, but separation of the two couples of chiral isomers is not successful, similar to the case for the separation of trans-OHAs. Quite different from a monotonous decrease in 9,10-DHA selectivity with raising the reaction temperature, almost no change in 9,10-DHP selectivity is observed with raising the reaction temperature from 60 to 70 °C, but 9,10-DHP selectivity decreased to ca. 49.7% from ca. 59.3% with raising the reaction temperature from 70 to 80 °C and then increased to ca. 59.5% with raising the reaction temperature to 100 °C, as illustrated in Figure 3. The difference could be related to lower reactivities of phenanthrene and 9,10-DHP in comparison to those of anthracene and 9,10-DHA, respectively. The lower reactivity of 9,10-DHP than that of 9,10-DHA resulted in a lower selectivity of octahydrophenanthrenes (OHPs, including symOHP and cis-OHPs) than that of OHAs. MAHT to the 4a and 10a positions of 1,2,3,4,9,10-hexahydrophenanthrene (1,2,3,4,9,10-HHP) should be much easier than to those of 1,2,3,4,9,10-HHA by comparing the Sr value (1.266, β position) of styrene with that (1.000) of ethylene as listed in Table 2. However, dehydrogenation of the resulting asymmetrical OHPs is also much easier than that of asymmetrical OHAs because the hydrogen atom in the 4a position of the asymmetrical OHPs is both a benzylic and tertiary hydrogen. Effect of the IHP. When the reaction time was fixed to 5 min, Figure 4 demonstrates the effect of the IHP on the hydroconversions of anthracene at 70 °C and phenanthrene at 90 °C. Under 0.41 MPa of IHP, ca. 98.4% of anthracene was hydrogenated at 70 °C, while phenanthrene conversion was only 75.4% at 90 °C. This result further confirms the significant difference in reactivity between anthracene and phenanthrene toward Pd/C-catalyzed hydrogenation under MWI. Phenanthrene conversion appreciably increased with increasing IHP from 0.41 to 0.48 MPa and dramatically increased with further increasing the IHP. Because of its very high conversion (98.4%) even under low IHP, the change in anthracene conversion with increasing IHP is not significant. As Figure 5 illustrates, 9,10-DHA selectivity decreased to 46.4% from 51.6% with increasing IHP from 0.41 to 0.55 MPa but increased to 48.8% with further increasing the IHP to 0.69 MPa, while THA selectivity was almost not changed with


Figure 5. Effect of the IHP on the product selectivity from anthracene hydrogenation at 70 °C for 5 min.

Figure 6. Effect of the IHP on the product selectivity from phenanthrene hydrogenation at 90 °C for 5 min.

Figure 7. Time profiles of anthracene conversion at 70 °C and phenanthrene at 90 °C.

increasing IHP from 0.41 to 0.55 MPa but decreased from 39.5 to 29.9% with further increasing the IHP to 0.69 MPa. The selectivity of each OHA, especially sym-OHA, monotonously increased with increasing IHP. These results suggest that increasing the IHP favors THA hydrogenation to OHAs, especially to sym-OHA. As Figure 6 illustrates, 9,10-DHP selectivity monotonously decreased with increasing IHP from 0.41 to 0.62 MPa but was not changed with further increasing the IHP, while THP selectivity largely decreased with increasing IHP from 0.41 to 0.62 MPa but significantly increased with further increasing the IHP. In contrast, the selectivity of each OHP increased with increasing IHP from 0.41 to 0.62 MPa but decreased with further increasing the IHP, implying that the dehydrogenation of the OHPs to THP became significant by increasing the IHP from 0.62 MPa. The reason for the dehydrogenation of the OHPs to THP accelerated by increasing the IHP is not clear. Effect of the Reaction Time. When the IHP was fixed to 0.62 MPa, we examined the effect of the reaction time on the hydroconversions of anthracene at 70 °C and phenanthrene at 90 °C. As demonstrated in Figure 7, both anthracene and phenanthrene conversions monotonously increased with prolonging reaction time.

Ma et al.

Figure 8. Time profiles of the product distribution from anthracene hydrogenation at 70 °C.

Figure 9. Time profiles of the product distribution from phenanthrene hydrogenation at 90 °C.

As Figure 8 exhibits, 9,10-DHA selectivity largely decreased with prolonging reaction time, but the decrease is much less than that in THA selectivity. Corresponding to a significant decrease in THA selectivity, the selectivity of each OHA remarkably increased with prolonging reaction time. These facts suggest that THA hydrogenation to the OHAs is predominant. A remarkable change in the product distribution from phenanthrene hydrogenation occurred between 3 and 9 min, as exhibited in Figure 9. The obvious decrease in 9,10-DHP and THP selectivities and the significant increase in sym- and cisOHP selectivities can be seen between 3 and 5 min, while further prolonging the reaction time leads to the increase in 9,10-DHP and THP selectivities and the decrease in sym- and cis-OHP selectivities. Conclusions Pd/C-catalyzed hydrogenations of anthracene and phenanthrene can significantly proceed at low temperatures and hydrogen pressures under MWI. The related reactions involve both mon- and biatomic hydrogen transfer. Superdelocalizability and resonance energy proved to be proper parameters to interpret the related reaction mechanisms. Acknowledgment. This work was subsidized by 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), and the State Key Laboratory of Materials-Oriented Chemical Engineering.

Nomenclature PCA ) polycyclic arene HFR ) heavy fossil resource HP ) heavy petroleum OHA ) octahydroanthracene 9,10-DHA ) 9,10-dihydroanthracene THA ) 1,2,3,4-tetrahydroanthracene IHP ) initial hydrogen pressure MWI ) microwave irradiation

MicrowaVe-Assisted Hydrogen Transfer 9,10-DHP ) 9,10-dihydrophenanthrene DPM ) diphenylmethane BP ) biphenyl THF ) tetrahydrofuran BR ) benzene ring BPR ) biphenyl ring Sr ) superdelocalizability RE ) resonance energy AC ) activated carbon sym-OHA ) 1,2,3,4,5,6,7,8-octahydroanthracene cis-OHA ) S,R-OHA ) R,S-OHA S,R-OHA ) (4aS,9aR)-1,2,3,4,4a,9,9a,10-octahydroanthracene R,S-OHA ) (4aR,9aS)-1,2,3,4,4a,9,9a,10-octahydroanthracene trans-OHAs ) S,S-OHA and R,R-OHA S,S-OHA ) (4aS,9aS)-1,2,3,4,4a,9,9a,10-octahydroanthracene R,R-OHA ) (4aR,9aR)-1,2,3,4,4a,9,9a,10-octahydroanthracene BAHT ) biatomic hydrogen transfer MAHT ) monatomic hydrogen transfer HA ) hydrogenated anthracene RBR ) related benzylic radical 9-HAR ) 9-hydroanthracen-10-yl radical 10-HAR ) 10-hydroanthracen-4a-yl radical 1,2,3,4,9,10-HHA ) 1,2,3,4,9,10-hexahydroanthracene

Energy & Fuels, Vol. 23, 2009 645 1,4-DHA ) 1,4-dihydroanthracene 1,2,3,4,5,8-HHA ) 1,2,3,4,5,8-hexahydroanthracene NR ) naphthalene ring 9-HPR ) 9-hydrophenanthren-10-yl radical 4-(4,9,10-THP)R ) 4-(4,9,10-trihydrophenthren-1-yl) radical 1,4-DHP ) 1,4-dihydrophenanthrene sym-OHP ) 1,2,3,4,5,6,7,8-octahydrophenanthrene R,R-OHP ) (4aR,9aR)-1,2,3,4,4a,9,9a,10-octahydrophenanthrene S,S-OHP ) (4aS,9aS)-1,2,3,4,4a,9,9a,10-octahydrophenanthrene cis-OHPs ) R,R-OHP and S,S-OHP S,R-OHP ) (4aS,9aR)-1,2,3,4,4a,9,9a,10-octahydrophenanthrene R,S-OHP ) (4aR,9aS)- 1,2,3,4,4a,9,9a,10-octahydrophenanthrene trans-OHPs ) S,R-OHP and R,S-OHP OHP ) octahydrophenanthrene 1,2,3,4,5,8-HHP ) 1,2,3,4,5,8-hexahydrophenanthrene 1,2,3,4,9,10-HHP ) 1,2,3,4,9,10-hexahydrophenanthrene Supporting Information Available: Mass spectra of the hydroarenes detected. This material is available free of charge via the Internet at http://pubs.acs.org. EF800808T

Microwave-Assisted Hydrogen Transfer to Anthracene and

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