868
Energy & Fuels 1992,6, 868-869
Effects of Hydrogen Pressure, Sulfur, and FeS2 on Diphenylmethane Hydrocracking Xian-Yong Wei,' Eisuke Ogata, Zhi-Min Zong, and Etsuo Niki Department of Reaction Chemistry, Faculty of Engineering, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113, Japan Received April 2, 1992. Revised Manuscript Received September 9, 1992
There has been much interest in studying the catalytic roles of iron sulfides and H2S in coal liq~efactionl-~ and model reactions.4* In most cases, the active form of ironsulfur systems under liquefaction conditions is believed to be pyrrhotite, but some alternative interpretations2J4 have been proposed implying that H2S is also the catalyst for coal liquefaction. In addition, a synergistic effect of H2S and pyrrhotite was suggestedby some researchers.lOJ However,very few answers in the published literature were given to the following question: how do iron sulfides and H2S work to promote the cleavage of relatively strong C-C bond such as C,-C& bond in diphenylmethane (DPM)? The answer to the above question is important to understand the mechanisms of iron sulfides and H2S for hydrocracking reactions. To clarify the mechanism of iron sulfides and H2S for hydrocracking reactions, we investigated the effects of hydrogen pressure, sulfur, and FeS2 on DPM hydrocracking. Diphenylmethane (DPM), dibenzyl, decalin (DHN), sulfur, and catalyst Fe (ultrafine particles of FeO, 200 A in diameter) were purchased commercially. Synthetic pyrite FeS2 was offered by Asahi Chemical Industry Co. Ltd. (Japan). A substrate (DPM or/and dibenzyl, 7.5 mmol), prescribed amounts of a catalyst (FeS2 or Fe) and sulfur, and 30 mL of DHN were put into a 90-mL, stainless steel, magnetically stirred autoclave. After being pressurized by hydrogen or/and nitrogen to 10 MPa at room temperature (20 "C), the autoclave was heated to 300 "C in 15 min or 400 "C in 18 min and kept at the temperature for a prescribed period of time. Then the autoclave was immediately cooled to room temperature in an ice-water bath. The reaction products were identified by GC-MS and quantified by GC. Table I shows the effects of hydrogen pressure, sulfur, and FeS2 on DPM hydrocracking. In the presence of H2 and FeS2, the reaction of DPM proceeded mainly via the cleavage of C,-Cd bond, affording benzene (PhH) and toluene (PhMe). Without H2 or FeS2, DPM was not convertedat all. In the presence of FeSz, DPM conversion increased with increasinghydrogen pressure. These results (1) Sondreal, E. A.; Willeon, W. G.; Stenberg, V. I. Fuel 1982,61,925. (2) Mukherjee, D. K.; Mitra, J. R. Fuel 1984, 63, 722. (3) Silver, H. F.; Frazee, W. S. Fuel 1985, 64, 743. (4) Stenberg, V. I.; Ogawa, T.; Willson, W. G.; Miller, D. Fuel 1983,62, 1487. (5) Hei, R. D.; Sweeny, P. G.; Stenberg, V. I. Fuel 1986,65,577. (6) Sweeny, P. G.; Stenberg, V. I.; Hei, R. D.; Montano, P. A. Fuel 1987,66, 532. (7) Lambert,J. M. Jr. Fuel 1982, 61, 777. (8) Baldwin, R. M.; Vinciguerra, S . Fuel 1983, 62, 498. (9) Hirschon, A. S.; Sundback, K.; Laine, R. M. Fuel 1985, 64, 772. (10) Ogawa, T.; Stenberg, V. I.; Montano, P. A. Fuel 1984,63, 1660. (11) Montano, P. A.; Stenberg, V. I.; Sweeny, P. J.Phys. Chem. 1986, 90,156.
0887-0624/92/2506-08668$03.00/0
Table I. Effects of Hydrogen Preeeure,Sulfur,and FeS, on DPM Hydrocracking. selectivity, mol % Hz IPP, MPa sulfur, g FeSz, g convn, % PhH PhMe BCH
0.05 0.5 0 0.05 0.5 37.8 99.2 99.2 0.8 0.05 0.5 59.1 98.0 97.2 2.4 0 0 0 0.05 0 0 0.50 0 0 0 0.5 53.0 99.5 99.5 0.5 0.20 0.5 71.1 99.1 99.1 0.9 0.80 0.5 74.8 99.1 99.0 0.9 DPM 7.5 mmol, DHN 30 mL, initial pressure (Hz + Nz)10 MPa, 400 O C , 1 h. PhH = benzene, PhMe = toluene, BCH = benzylcyclohexane. IPP = initial partial pressure. 0 5.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0
indicate that molecular hydrogen and FeS2 are significant in DPM hydrocracking. DPM is very stable thermally. DPM hydrogenolysis was reported not to proceed even at 430 "C under H2 in tetralin.12 The low reactivity of DPM toward C-C single bond scission could be related to the instability of phenyl radical resultingfrom direct cleavageof C-C bond between the benzene ring and the methylene linkage in DPM. FeS2 has been reported to facilitate the formation of free-radical intermediatessuch as H*and HS' duringita decomposition to Fe1-zS.13J4H-atom addition to the ipso positionof DPM must be an essential step in C,-C, bond scission, because the C,-C&, bond scission induced by H-atom addition yields benzene and benzyl radicalsrather than the unstable phenyl radical. The data in Table I also show that in the absence of FeS2, sulfur addition did not induce DPM hydrocracking but that, in the presence of molecular hydrogen and FeS2, DPM conversion increased with increasing sulfur feed. Stenberg et studied the roles of H2S and pyrrhotite in DPM hydrocracking at 450 "C. Their results indicate that DPM conversion increased with increasing H2S pressure even in the absence of a catalyst. The significant difference in the role of H2S between this study and that by Stenberg et al.4may be due to the significantdifference in reaction temperature. At a temperature as high as 450 "C, The thermal cleavage of H-SH to directly produce H* and 'SH may proceed and Fe from the autoclave may well be involved. Using H2S alone has been found to be insufficientto catalyzethe hydrogenation of quinolineand naphthalene.15 In the absence of added catalyst, H2S decomposition to H' and HS' should be very difficult at (12) Futamura, S.; Koyanagi, S.; Kamiya, Y. Fuel 1988, 67, 1436. (13) Thomas, M. G.; Padrick, T. D.; Stohl,F. V.; Stephens, H. P. Fuel 1982, 61, 761. (14) Srinivasan, G.; Seehra, M. S. Fuel 1982, 61, 1249. (15) Guin, J. A.; Curtis, C. W.; Kwon, K. C . Fuel 1983, 62, 1412.
0 1992 American Chemical Society
Energy & Fuels, Vol. 6, No.6, 1992 869
Communications
Table 111. Additive Effect of Sulfur on Fe-Catalyzed Reaction of DPM*
Table 11. H-Atom-InducedDecomposition of DPM* conversion, % additive
gas phase
DPM
none dibenzylb dibenzylb
H2 Nz Hz
0
temp, SIFe, convn, selectivity, mol % OC mol/mol % CH PhH MCH PhMe BCH DCHM .~~~~
dibenzyl
~
0
12.4
~
21.7 28.3
7.5 "01, DHN 30 mL, initial Hz or Nz pressure 10 MPa, 400 OC, 10 h. 7.5 "01. a DPM
400 "C because of the large bond dissociation energy (93 kcal/mol)' of the H-SH bond. Ogawa et al.'O investigated the roles of H2S, pyrrhotite, and pyrite in DPM hydrocracking. Their observations suggest that pyrite seems to be a more active promoter of coal liquefaction than is pyrrhotite under H2 and that pyrite formed from pyrrhotite increases with increasing H2S pressure. Because pyrite is rapidly decomposed to pyrrhotite above 300 OC13and sulfur to H2S at even lower temperature,16the promotional effect of sulfur addition in the presence of FeS2 may be ascribed to the fact observed by Ogawaet al.l0that increasing H2S pressure favors pyrite regeneration. Thomas et al.13 have suggestedthat active sulfur formed from FeS2 decomposition plays an important role in H-atom formation according to the following reactions:
FeS,
-
FeS + S
S + H, +H* HS'
-.
+ H2
+ HS'
H'
+ H,S
(1) (2)
(3)
We believe that the addition of H atoms resulting from reactions 2 and 3 to the ipso position of DPM should be a crucial step in DPM hydrocracking. In fact, the results in Table I1 indicate that DPM decomposition proceeded only when both molecular hyrogen and dibenzyl are present. In this case, molecular hydrogen stabilizes benzyl radicals resulting from dibenzylthermolysis, affordingfree H atoms." Therefore, the catalysis of FeS2 on DPM (16) Meyer, B. Chem. Rev. 1976, 76,367. (17) Vernon, L. W.Fuel 1980,59, 102.
300 300 300 300 300 4 0 0
ob 0.5b
1.ob 2.e 4,ob W
400
O.SC
400 400 400
l.W
2.W 4.W
58.4 0 0.8 3.5 6.4 79.3 1.0 2.9 4.9 9.2
0
0
100 0 100 0 99.7 2.9 0 0 99.9 0 100 0 100 0 99.9 0
0
0
86.6
13.4
0
99.5 99.6 99.5 0 99.6 99.5 99.9 99.5
0 0 0 50.7
0 0 0 37.4 0 0 0 0
0 0
8.7 0 0 0 0
0
0 0 0
a DPM 7.5 "01, DHN 30 mL, initial Hz preeeure 10 MPa, 1h. Fe 0.23 g. Fe 0.02 g. CH = cyclohexane, MCH = methylcyclohexane, DCHM = dicyclohexylmethane.
hydrocracking could primarily be attributed to H-atom formation caused by FeS2 decomposition. Compared to the reaction under nitrogen, dibenzyl conversion under hydrogen increased (Table 11). The increased conversion of dibenzyl is considered to be attributed to dibenzyl hydrogenolysis induced by the resulting free H atoms. Table I11 investigates the additive effect of sulfur on Fe-catalyzed reaction of DPM. Quite different from FeS2, in the absence of sulfur, Fe mainly catalyzed DPM hydrogenation. At 300 "C, DPM decomposition did not proceed at all. Even at 400 OC, the main products were hydrogenation products such as benzylcyclohexane (BCH) and dicyclohexylmethane (DCHM)rather than decomposition products. However, adding sulfur to the reaction system inhibited DPM hydrogenation. DPM conversion to benzene and toluene increased with increasing sulfur. These facts indicate evidently that, under hydrogen and in the presence of a iron catalyst (Fe or FeSz), sulfur addition promotes the cleavage of Cm-Ca bond in DPM. In conclusion, under pressurized hydrogen, FeSz plays a very important role in the formation of H atoms, the addition of which to the ipso position of DPM leads to the cleavage of the C,-Ca bond in DPM, whereas H2S alone does not result in DPM hydrocracking at 400 "C. Registry No. DPM, 101-81-5; S, 7704-34-9; FeS, 1317-37-9; HzS, 7783-06-4.
