Energy & Fuels 1996, 10, 927-931
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Hydrocracking of Aromatic Hydrocarbons over USY-Zeolite Metta Chareonpanich, Zhan-Guo Zhang, and Akira Tomita* Institute for Chemical Reaction Science, Tohoku University, Katahira, Sendai 980-77, Japan Received November 21, 1995X
Hydrocracking of 1-butylbenzene, tetralin, diphenylmethane, naphthalene, 1-methylnaphthalene, 9,10-dihydroanthracene, anthracene, and phenanthrene was investigated at 400, 500, and 600 °C under 5 MPa of hydrogen pressure. With metal-free ultrastable Y-zeolite catalyst and at a severe cracking temperature of 600 °C, accumulated yields of benzene, toluene, and xylene (BTX) and C1-C3 hydrocarbon gases were nearly 100%. The formation of tar and coke was almost negligible. On the basis of the product distribution pattern, the reaction pathways leading to BTX and light gases were postulated. The reaction for diphenylmethane, n-butylbenzene, and tetralin was initiated by the hydrocracking of a C-C single bond, whereas for other compounds the hydrogenation of the aromatic ring was the initial step followed by the hydrocracking reaction. Even in the case of 1-methylnaphthalene, the hydrogenation of the aromatic ring preceded the hydrocracking of the methyl group. The hydrocracking of anthracene and phenanthrene was likely to proceed via the cracking at the outer ring.
Introduction Catalytic hydrogenation and hydrocracking of heavy hydrocarbons from various sources have attracted much attention to increase gasoline yield from heavy hydrocarbon feedstocks. Conversion of polycyclic aromatic compounds to lower molecular weight species of high H/C ratio requires catalysts having both hydrogenation and hydrocracking activities. The effect of metal catalysts on the hydrogenation and hydrocracking of twoand three-ring aromatic compounds and alkyl aromatic compounds has been investigated by a number of investigators;1-12 however, the reactions have been carried out in most instances with metal-loaded catalysts at temperature around 300-400 °C and at hydrogen pressure of 5-15 MPa. Cracking reactions using metal-free zeolite have been reported in the absence of hydrogen.13-15 There have been relatively few detailed investigations on the reaction under severe conditions. Gra¨ber and Abstract published in Advance ACS Abstracts, June 1, 1996. (1) Sapre, A. V.; Gates, B. C. Ind. Eng. Chem. Process Des. Dev. 1981, 20, 68. (2) Haynes, H. W.; Parcher, J. F.; Helmer, N. E. Ind. Eng. Chem. Process Des. Dev. 1983, 22, 401. (3) Salim, S. S.; Bell, A. T. Fuel 1984, 63, 469. (4) Lapinas, A. T.; Klein M. T.; Gates, B. C. Ind. Eng. Chem. Res. 1987, 26, 1026. (5) Nishijima, A.; Shimada, H.; Yoshimura, Y.; Sato, T.; Matsubayashi, N. Stud. Surf. Sci. Catal. 1987, 34, 59. (6) Suzuki, T.; Yamada, H.; Sears, P. L.; Watanabe, Y. Energy Fuels 1989, 3, 707. (7) Ueda, K.; Matsui, H.; Song, C.; Xu, W.-C. Sekiyu Gakkaishi 1990, 33, 413. (8) Lemberton, J. L.; Touzeyidio, M.; Guisnet, M. Appl. Catal. 1991, 79, 115. (9) Bouchy, M.; Dufresne, P.; Kasztelan, S. Ind. Eng. Chem. Res. 1992, 31, 2661. (10) Guerzoni, F. N.; Abbot, J. J. Catal. 1993, 139, 289. (11) Ho, T. C. Energy Fuels 1994, 8, 1149. (12) Matsui, H.; Akagi, K.; Murata, S.; Nomura, M. Energy Fuels 1995, 9, 435. (13) Mostad, H. B.; Riis, T. U.; Ellestad, O. H. Appl. Catal. 1990, 63, 345. (14) Townsend, A. T.; Abbot, J. Appl. Catal. 1992, 90, 97. (15) Townsend, A. T.; Abbot, J. Appl. Catal. 1993, 95, 221. X
S0887-0624(95)00238-6 CCC: $12.00
Hu¨ttinger used a flow tube reactor to hydrogenate unsubstituted aromatics at temperatures between 800 and 1000 °C.16 Even without catalyst, light hydrocarbon fractions could be produced when the temperature was set at above 750 °C. Nelson and Hu¨ttinger pyrolyzed naphthalene at 650-1050 °C at various hydrogen pressures and found that more light products were obtained at higher temperature and higher hydrogen pressure.17 Our original objective was to convert polyaromatic molecules present in the coal volatile matter into high value-added products such as benzene, toluene, and xylene (BTX) through hydrogenation and hydrocracking reactions. First we examined the effect of NiMo and Co-Mo type catalysts and found some increase of BTX yield under a high-pressure hydrogen.18 In a previous paper, we reported that ultrastable Y-zeolite (USY-zeolite) significantly enhanced the BTX yield at a cracking temperature around 600 °C even in the absence of metallic component.19 The detailed results on the catalytic activity of metal-free USY-zeolite on the upgrading reaction are presented elsewhere.20 It has been suggested that under a high pressure of hydrogen and a relatively high reaction temperature, USY-zeolite has a hydrogenation activity as well as a hydrocracking activity. This study attempts to clarify the hydrocracking reaction of various polyaromatic hydrocarbons as model compounds of coal volatile matter. Aromatic compounds having one to three aromatic rings are used as reactants. It is possible to qualitatively speculate the reaction pathway for each reactant by examining the reaction products from these reactants. For example, if the product distribution with anthracene resembles that with dihydroanthracene, we may be able (16) Gra¨ber, W.-D.; Hu¨ttinger, K. J. Fuel 1982, 61, 499. (17) Nelson, P. F.; Hu¨ttinger, K. J. Fuel 1986, 65, 354. (18) Chareonpanich, M.; Takeda, T.; Yamashita, H.; Tomita, A. Fuel 1994, 73, 666. (19) Chareonpanich, M.; Tomita, A.; Nishijima, A. Energy Fuels 1994, 8, 1522. (20) Chareonpanich, M.; Zhang, Z.-G.; Nishijima, A.; Tomita, A. Fuel 1995, 74, 1636.
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to speculate that dihydroanthracene is the reaction intermediate in the hydrogenation of anthracene.
Chareonpanich et al. Table 1. BTX Yields from Various Compounds without and with USY-Zeolitea BTX yield (%, carbon basis)
Experimental Section Apparatus and Procedure. The experimental apparatus and techniques used in this study were similar to those used in the previous studies,18,19 and all experiments were performed by using a two-stage fixed bed reactor with independently controlled two-zone heaters. The reactor is made of SUS-316 tube (7.6 mm L). About 1-2 µg of aromatic hydrocarbon was placed in a small quartz boat in the middle part of the first stage. After the reactor was pressurized with hydrogen up to 5.0 MPa, hydrogen gas was allowed to flow at a rate of 200 mL (STP) min-1, which corresponded to a linear velocity of 1.5 mm/s at 5.0 MPa. The second stage of the reactor packed with 800 mg of catalyst was isothermally controlled at a temperature ranging from 400 to 600 °C. The aromatic compound in the first stage was then rapidly heated by an infrared furnace at a rate of 10 K/s to a temperature close to its boiling point. The vaporized reactant was introduced into the catalyst bed and hydrotreated therein. Thus, this apparatus is a kind of pulse reactor. The contact time of reactant with catalyst was about 10 s, if the linear velocity of reactant in the hydrogen stream was assumed to be the same as that of hydrogen gas. The reaction products were collected in a liquid nitrogen trap packed with quartz sand and Porapack P for liquid and gaseous products, respectively. After the depressurization, the collected products were heated to desorb from the trap and then subjected to gas chromatography for analysis. Hydrocarbon gases were analyzed by flame ionization detector (FID) with a Unibeads C-packed column, while the heavier products were analyzed by FID with a Chemipack PH column. The product yield is presented in terms of carbon conversion. For example, when 1 g of methane was obtained from 10 g of naphthalene, the carbon yield was 8% [) (1 × 12/16)/(10 × 120/128) × 100]. Reactants and Catalyst. Descriptions of the reactants used in this study are given in Table 1. The catalyst used was USY-zeolite mixed with Al2O3 (USY-zeolite/Al2O3 ) 60: 40), its size being 32-60 mesh. Prior to each series of experiments it was calcined at 500 °C in oxygen for 0.5 h.
Results Effect of USY-Zeolite Catalyst on the BTX Yield. Table 1 shows the BTX yield in the hydrocracking of eight aromatic compounds at 600 °C without and with USY-zeolite catalyst. In the absence of the catalyst, BTX yields were very low. The highest yield, 15% (on the carbon basis), was obtained with diphenylmethane, followed by butylbenzene and tetralin, which gave almost half of the yield from diphenylmethane. This result is understandable when it is assumed that a C-C single bond in a methylene bridge or in a naphthenic ring is more easily hydrocracked than an aromatic C-C bond. The breakage of a methylene bridge in diphenylmethane results in two molecules of either benzene or toluene, while butylbenzene and tetralin give only one molecule of BTX. This is related to the difference in BTX yield between diphenylmethane and the other two compounds mentioned above. The presence of catalyst significantly promoted formation of the BTX fraction. Again, diphenylmethane produced the largest amount of BTX (60%), and other aromatic compounds produced more or less similar amounts of BTX (28-42%). In every case with catalyst, the conversion of reactant was almost 100%. The rest of the products besides BTX were mainly light hydro-
bp (°C)
without catalyst
USYzeolite
butylbenzene
183
8
31
tetralin
207
6
35
naphthalene
218
3
36
245
1
42
diphenylmethane
264
15
60
9,10-dihydroanthracene
305
2
28
anthracene
342
1
32
phenanthrene
340
0
31
model compd
1-methylnaphthalene
a
structure
CH3
H2 pressure, 5 MPa; reaction temperature, 600 °C.
Figure 1. Reaction of diphenylmethane over USY-zeolite (H2 pressure, 5 MPa).
carbon gases, and this means that little tar and coke formation was produced under the present conditions. The BTX yield for anthracene and phenanthrene was much less than that for diphenylmethane and nearly equal to those for the naphthalene group. This implies that these three-ring compounds were not hydrocracked at the center ring but at the outer ring. Thus, only one molecule of BTX was produced from these three-ring aromatic compounds. The stoichiometric yield of benzene from anthracene is 43% [(6/14) × 100)]. The observed yield was somewhat lower than this value. Diphenylmethane. Figure 1 shows the effect of reaction temperature on the yields of light hydrocarbon products from diphenylmethane. The results at 400 °C are not presented here because of a poor reproducibility due to unknown reasons. The main products at 500 °C were benzene and toluene with a ratio of 1.8:1. Only small amounts of xylene and C1-C3 gases were obtained. At 600 °C, the total yield increased, but the yield of BTX substantially decreased with some increase of C1-C3 gases. The ratio of benzene to toluene increased to 3:1.
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Figure 2. Reaction of naphthalene group over USY-zeolite (H2 pressure, 5 MPa; temperature, 400-600 °C).
Figure 3. Reaction of butylbenzene over USY-zeolite (H2 pressure, 5 MPa).
Naphthalene Group. Figure 2 shows the product distribution pattern for the reaction of the naphthalene group at 400-600 °C. The accuracy of GC analysis was not so perfect that in some cases total carbon conversion exceeded 100%. However, they are shown without any correction. At a cracking temperature of 400 °C, only half of the reactant was converted to lighter products, while the total yield reached 100% at 600 °C. Methane yields were 0%, 5-10%, and 20-24% (carbon basis) at 400, 500, and 600 °C, respectively. It should be pointed out that the product distributions among the three reactants resemble each other, especially at 600 °C: about 20% of CH4, 50% of C2-C3, and 30% of benzene and toluene. In addition to the above three compounds, the reaction of n-butylbenzene was carried out, and the results are shown in Figure 3. This is one possible intermediate for the reaction of naphthalene. Interestingly, the product distribution at 600 °C was almost the same as for the naphthalene group; the yields of CH4, C2-C3, benzene, and toluene were again about 20%, 50%, and 30%, respectively. However, at a lower temperature (400 °C), the yield of C3-C4 compounds from butylbenzene was fairly large, as was anticipated; the C3-C4 yield was 42% in contrast with 10% obtained from naphthalene. The effect of hydrogen pressure on the product yields was examined in the case of 1-methylnaphthalene, and the result is shown in Figure 4. The increase of hydrogen pressure from 0.1 to 2 MPa resulted in the increase of the total yield. However, the increase between 1 and 2 MPa is mainly due to the increase of light gases. Beyond 2 MPa the pressure effect becomes rather small. The BTX yield was almost the same between 1 and 5 MPa. Therefore, excess hydrogen is not necessary for BTX production, at least in this case. Anthracene Group. The product distribution for the anthracene group is illustrated in Figure 5. The
Figure 4. Effect of hydrogen pressure on hydrocracking of 1-methylnaphthalene over USY-zeolite (reaction temperature, 600 °C).
product distribution pattern is quite similar to that for the naphthalene group. It is interesting to compare the result for anthracene with those for partially hydrogenated compounds. The results for 9,10-dihydroanthracene and 1,2,3,4,5,6,7,8-octahydrophenanthrene (not shown here) were essentially the same as that of anthracene. The similarity of the product distributions among these compounds implied that these partially hydrogenated compounds are, at least partly, the intermediates in the hydrotreatment of anthracene. Discussion Role of Zeolite Catalyst. It is obvious that zeolite catalysts have not only a high hydrocracking activity but also a high hydrogenation one, since the aromatictype bond is hardly cleaved without prior hydrogenation.5 Under high hydrogen pressure and a relatively high reaction temperature (g400 °C), Ebitani et al. have reported that Lewis acid sites on metal-free zeolite can interact with molecular hydrogen and form protonic acid sites.21 Thus, even without the metal component, zeolite itself can activate the hydrogenation reaction, which is the initial step of a series of hydrocracking reactions of polyaromatics. The hydrocracking activity of USY-zeolite was greatly influenced by the reaction temperature as shown in Figures 1-3 and 5. The yield of less branched hydrocarbons increased with increasing reaction temperature at the expense of branched aromatics. The ratios of B:T:X from anthracene shown in Figure 5 were 1:2:0.5, 1:1:0.2, and 1:0.25:0 at 400, 500, and 600 °C, respectively. This trend holds true for all other compounds. (21) Ebitani, K.; Tsuji, J.; Hattori, H.; Kita, H. J. Catal. 1992, 135, 609.
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Figure 5. Reaction of anthracene group over USY-zeolite (H2 pressure, 5 MPa; temperature, 400-600 °C).
Figure 6. Proposed reaction pathway for diphenylmethane.
For aliphatic hydrocarbons, long-chain aliphatics were similarly converted to shorter ones at high temperature; the ratios of C1:C2:C3 from anthracene were 1:8:27, 1:2: 4, and 1:2:1 at 400, 500, and 600 °C, respectively. Reaction Pathway. On the basis of the product distributions shown in Figure 1, the reaction pathway for diphenylmethane was deduced as shown in Figure 6. The reaction was assumed to start with the hydrocracking of the C-C single bond, resulting in the same amount of benzene and toluene. Wei et al. also reported a similar route for the reaction of diphenylmethane under somewhat different experimental conditions.22 A part of toluene was further cracked to benzene and smaller molecules, since the molar ratio of benzene to toluene was always larger than unity. Finally, a part of benzene was hydrocracked to light hydrocarbon gases, especially at 600 °C. As the temperature increased, the conversion of toluene to benzene as well as the conversion of benzene to C1-C3 increased. If it is assumed that all toluene was hydrocracked through benzene, we can estimate the fraction of toluene converted to benzene from the mass balance, and it was found to be 37% at 500 °C and 62% at 600 °C. On the basis of Figure 2, the reaction pathway for the naphthalene group is illustrated in Figure 7. Since the product distributions from naphthalene and tetralin were nearly the same, it can be deduced that the hydrogenation of naphthalene to tetralin occurred first. The tetralin would further be hydrocracked either to alkylbenzene or to decalin. In our case, the main reaction path would be the former one, since a fairly large amount of BTX (34%) was obtained. Alkylbenzene (22) Wei, X.-Y.; Ogata, E.; Zong, Z.-M.; Niki, E. Energy Fuels 1992, 6, 868.
Figure 7. Proposed reaction pathways for naphthalene and 1-methylnaphthalene.
could mainly produce BTX and C1-C4, while decalin would only produce aliphatic products such as C1-C4. It is interesting to compare the reaction path of 1-methylnaphthalene with that of naphthalene. Since the yield of methane from 1-methylnaphthalene was almost the same as that from naphthalene, the hydrocracking of the methyl group would not be the main path, but the hydrogenation reaction of one of the aromatic rings might precede. Bouchy et al. also reported that the hydrogenation of 1-methylnaphthalene to methyltetralin is the initial step over Ni-Mo catalyst.9 They determined the conversion rate as a function of contact time and found that the reaction at 400 °C is very fast and equilibrium was easily attained. In our case, the reaction condition is more severe and, therefore, further hydrogenation and hydrocracking of methyltetralin took place to produce BTX and C1-C4 as shown in Figure 2. The reaction path of anthracene is proposed as shown in Figure 8. Anthracene was first hydrogenated and then hydrocracked over USY-zeolite. The hydrogenation reaction could take place through two possible paths; one is hydrogenation of the outer ring, and the other is hydrogenation of the inner ring. Wiser et al. reported the hydrogenation of anthracene with a batchtype reactor at a temperature range of 220-435 °C and concluded that 9,10-dihydroanthracene was a reaction
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Energy & Fuels, Vol. 10, No. 4, 1996 931
Figure 8. Proposed reaction pathway for anthracene.
intermediate for the reaction of anthracene.23 Rosal et al. also clarified that with increasing reaction time anthracene was converted to 9,10-dihydroanthracene and then to tetrahydroanthracene over Ni and Ni-Mo catalysts.24 This pathway is also reasonable from theoretical considerations. Many reactivity parameters, including frontier electron density25 and hydrogen affinity,26 are much larger at the 9-position of anthracene than at the 1- or 2-position. Also in this study, most of the anthracene might be first converted to 9,10-dihydroanthracene. However, the direct hydrocracking of 9,10-dihydroanthracene in the center ring would not be the case. The cracking at the center ring should have produced diphenylmethane as a main product, and therefore if the reaction proceeds through this step, the product distribution in the reaction of anthracene should be similar to that of diphenylmethane. However, in fact, they were quite different, as indicated in Figures 1 and 5; for example, the yield of benzene from anthracene was around 25% at 600 °C, whereas that from diphenylmethane was 45%. Therefore, the hydrocracking reaction might take place after the isomerization and further hydrogenation to 1,2,3,4tetrahydroanthracene or 1,2,3,4,5,6,7,8-octahydroanthracene. The hydrocracking of these intermediates should lead to either alkylnaphthalenes or alkyltetralin. This is the reason why the reaction product patterns are very similar between anthracene and naphthalene groups. The proposed scheme (Figure 8) is very similar to that presented by Salim and Bell, who used ZnCl2 and AlCl3 catalysts at 325 °C.3 In the case of phenanthrene, Lemberton and Guisnet examined the reaction (23) Wiser, W. H.; Singh, S.; Qader, S. A.; Hill, G. R. Ind. Eng. Chem. Res. Dev. 1970, 9, 350. (24) Rosal, R.; Diez, F. V.; Sastre, H. Ind. Eng. Chem. Res. 1992, 31, 1007. (25) Fukui, K.; Yonezawa, T.; Shingu, H. J. Chem. Phys. 1952, 20, 722. (26) Stein, S. E.; Brown, R. L. J. Am. Chem. Soc. 1991, 113, 787.
product as a function of phenanthrene conversion, and they concluded that naphthalene derivatives are the intermediates in the hydrocracking reaction.27 This is consistent with the present observation that the product composition pattern for phenanthrene (Figure 5) resembles that for the naphthalene group (Figure 2). Conclusions The hydrocracking reaction of several aromatic hydrocarbons over USY-zeolite has been investigated under rather severe reaction conditions. At higher temperatures, such as 600 °C, even three-ring aromatic hydrocarbons were completely converted to BTX and lighter gases. The reaction paths were proposed on the basis of observed product distribution. The hydrocracking of the C-C single bond in diphenylmethane, tetralin, and butylbenzene was found to be the initial step of the reaction. On the other hand, the methyl group in methylnaphthalene was not hydrocracked but the hydrogenation of the aromatic ring took place as the first step. In the case of anthracene, the hydrogenation first took place perhaps in the inner ring, but instead of the hydrocracking of the thus-produced saturated ring, the hydrocracking of the outer ring after further hydrogenation or isomerization might be more probable as a reaction pathway. Acknowledgment. We thank Dr. A. Nishijima of the National Institute of Materials and Chemical Research, Tsukuba, for the supply of catalyst as well as many helpful discussions. Partial financial support from Nippon Steel Corp. and the Center for Coal Utilization Japan is also acknowledged. EF950238M (27) Lemberton, J. L.; Guisnet, M. Appl. Catal. 1984, 13, 181.
