impregnated aluminum phosphate catalyst was reported to be
active in the dehydrogenation of cyclohexane at 400 °C.14
Although the active sites for this reaction are unclear as yet,
the high selectivity of VSB-1 for ethylbenzene highlights the
bifunctional role of VSB-1. Another important feature of VSB-
1 is its excellent stability. The catalytic performance is very
stable over several hours, and, significantly, no appreciable
deactivation is observed. The stability of VSB-1 is consistent
with a previous report that it is thermally stable in air to 550 °C,8
in contrast to most other open-framework transition metal
phosphates. In the case of NaX, severe catalyst deactivation was
noted as the reaction proceeded due to the formation and
deposition of oligomeric cokes.
In summary, our work shows that the nanoporous nickel(II
)
phosphate, VSB-1, has some interesting catalytic properties,
exhibiting good stability and high selectivity for ethylbenzene
in the dehydrocyclodimerization of 1,3-butadiene. The catalytic
performance is ascribed to the bifunctional role of VSB-1, i.e.
the dehydrogenation ability of Ni species and the concentration
effect of the nanoporous structure. Further studies are in
progress to elucidate the active sites of VSB-1 for the
cyclodimerization and dehydrogenation. In the light of earlier
work that demonstrated the dehydrogenation of EB to styrene at
temperatures as low as 250 °C,15 we are also exploring the
possibility that a single stage conversion of butadiene to styrene
might be possible.
A. K. C. thanks the Foundation de 1’Ecole Normale
Supérieure and the Région de 1’Ile de France for a Chaire
Internationale de Recherche, Blaise Pascal. We also thank the
CNRS for financial support and the provision of a Poste Rouge
for Q. G. A Korea Science and Engineering Foundation
Fellowship for J. S. C. is gratefully acknowledged. We also
thank the Korean Ministry of Science and Technology (Institu-
tional Research Program, KK-0005-F0) for supporting this
work and Mr Y. S. Choi for experimental support.
Fig. 2 The Diels–Alder cyclodimerization of 1,3-butadiene over (a) VSB-1
and (b) NaX. Reaction conditions: T = 400 °C, GHSV = 7800 h21, feed
gas = 1,3-butadiene–helium (3/10). Notation: BD; 1,3-butadiene, VCH;
4-vinylcyclohexene, EB; ethylbenzene.
Notes and references
1 M. E. Davis, Chem. Eur. J., 1997, 3, 1745 and references therein.
2 A. K. Cheetham, G. Fe´rey and T. Loiseau, Angew. Chem., Int. Ed., 1999,
38, 3268; G. Férey and A. K. Cheetham, Science, 1999, 283, 1125.
3 B. V. Vora, T. L. Marker, P. T. Barger, H. R. Nilsen, S. Kvisle and T.
Fuglerud, in Fourth International Natural Gas Conversion Symposium,
ed. M. de Pontes, R. L Espinoza, C. P. Nicolaides, J. H. Scholtz and M.S.
Scurrell, Elsevier, Amsterdam, Stud. Surf. Sci. Catal., 1997, 107, 87.
4 B. Notari, Adv. Catal., 1996, 41, 253; R. J. Saxton, Top. Catal., 1999,
43, 9.
5 V. Soghomanian, Q. Chen, R. C. Haushalter, J. Subieta and J.
O’Connor, Science, 1993, 259, 1596; T. Loiseau and G. Férey, J. Solid
State Chem., 1994, 111, 416.
6 D. R. Corbin, J. F. Whitney, W. C. Fultz, G. D. Stucky, M. M. Eddy and
A. K. Cheetham, Inorg. Chem., 1986, 25, 2280; H. M. Lin, K.-H. Lii,
W.-C. Jiang and S.-L. Wang, Chem. Mater., 1999, 11, 519.
7 J. Chen, R. H. Jones, S. Natarajan, M. B. Hursthouse and J. M. Thomas,
Angew. Chem., Int. Ed., 1994, 33, 639; P. Feng, X. Bu, S. H. Tolbert and
G. D. Stucky, J. Am. Chem. Soc., 1997, 119, 2497.
8 N. Guillou, Q. Gao, M. Nogues, R. E. Morris, M. Herview, G. Férey and
A. K. Cheetham, C. R. Acad. Sci. Paris, 1999, 2, 387.
9 D. A. Hucul, US Patent, 5 336, 822, 1994.
10 R. Hoffmann and R. B. Woodward, J. Am. Chem. Soc., 1965, 87, 4388;
D. Rowley and H. Steiner, Discuss. Faraday Soc., 1951, 10, 198.
11 R. M. Dessau, J. Chem. Soc., Chem. Commun., 1986, 1167.
12 P. Heimbach, P. W. Jolly and G. Wilke, Adv. Organomet. Chem., 1970,
8, 29.
reaction certainly involves the cyclodimerization of butadiene
to VCH followed by dehydrogenation to form ethylbenzene;
that is, the whole process is better described as a dehy-
drocyclodimerization. In contrast, NaX did not produce any
ethylbenzene or styrene. Interestingly, VSB-1 displays 2%
selectivity to styrene at 425 °C (data not shown), but catalyst
deactivation is observed at this temperature due to the
deposition of oligomeric products.
The Diels–Alder cylcodimerization of 1,3-butadiene to VCH
is a well-known thermally-initiated electrocyclic reaction, that
is kinetically second-order in butadiene.10 Neither acidic nor
basic catalysts have been used successfully for this reaction.11
In addition to homogeneous catalyst systems,12 the cyclodimer-
ization of butadiene has been found over large-pore molecular
sieves such as CuY,13 NaZSM-20,11 NaBeta11 and NaX.13 The
catalytic role of non-acidic zeolites in the Diels–Alder reaction
has therefore been understood in terms of the ability of zeolites
to concentrate hydrocarbons within their cavities.11 Thus, by
increasing the butadiene concentration inside the zeolitic pores
relative to that in the external gas phase, the zeolite can enhance
the rates of bimolecular reactions such as the Diels–Alder
reaction. In the present work, the high selectivity of VSB-1 for
the cyclodimerization may again be ascribed to the concentra-
tion effect within its large pore channel (8.8 Å). Moreover, the
high selectivity towards ethylbenzene over VSB-1 points to the
role of the Ni species within its open framework in the partial
dehydrogenation step. This type of selectivity has been seen
with other nanoporous materials containing transition metals,
but not with acidic or basic catalysts. For example, a Ni-
13 I. E. Maxwell, R. S. Downing and S. A. J. van Langen, J. Catal., 1980,
61, 485; I. E. Maxwell, Adv. Catal., 1982, 31, 1.
14 S. A. El-Hakam, A. A. El-Khouly and A. S. Khder, Appl. Catal. A: Gen.,
1999, 185, 247.
15 Y.-S. Choi, Y.-K. Park, J.-S. Chang, S.-E. Park and A. K. Cheetham,
Catal. Lett., 2000, 69, 93; R. Neumann and I. Dror, Appl. Catal. A: Gen.,
1998, 172, 67.
860
Chem. Commun., 2001, 859–860