Chemistry Letters Vol.34, No.9 (2005)
1233
100
O
NH
Conv.
BPO4
Selectivity
N
N
Selectivity
8
6
4
2
0
0
0
0
OH
T
HO
N
N
OH
OH O
O
O
Silicalite-1
T
T
T
T
T
Conv.
BR
a
b
Figure 2. Possible mechanism for catalytic cycle completion
over BPO4.
GBR. Theoretical and spectroscopic approaches are ongoing to
understand the nature of the ꢂT–O–Tꢂ sites for the BPO4.
0
60 120 180 240 300 360
Time on stream [min]
H.T. thanks Drs. T. Aoshima, M. Tezuka, Y. Kawaragi, and
T. Okoshi (MCRC) for fruitful discussions.
Figure 1. Change in conversion and selectivity of e-caprolac-
tam with time on stream in the gas-phase Beckmann rearrange-
ment of cyclohexanone oxime using anhydrous benzene as a
diluent. Reaction conditions are given in Table 1 footnote.
References and Notes
1
a) G. Dahlhoff, J. P. M. Niederer, and W. F. H o¨ lderich, Catal. Rev.—
Sci. Eng., 43, 381 (2001). b) H. Ichihashi and H. Sato, Appl. Catal., A,
2
(
21, 359 (2001). c) A. Corma and H. Garcia, Chem. Rev., 103, 4307
2003).
1
:55) rivaled silicalite-1 (Si/Al2 ¼ 1250, supplied from the
Catalysis Society of Japan) in the selectivity to CL. The activity
normalized to the unit surface area was much higher for the
BPO4 than silicalite-1. The OXM conversion and CL selectivity
with time on stream are shown in Figure 1 for the BPO4 (P/B ¼
2
3
a) G. P. Heitmann, G. Dahlhoff, and W. F. H o¨ lderich, J. Catal., 186, 12
(1999). b) H. Ichihashi, M. Ishida, A. Shiga, M. Kitamura, T. Suzuki,
K. Suenobu, and K. Sugita, Catal. Surv. Asia, 7, 261 (2003).
a) H. Sato, H. Yoshioka, and Y. Izumi, J. Mol. Catal. A: Chem., 149, 25
(
1999). b) J. Peng and Y. Deng, Tetrahedron Lett., 42, 403 (2001).
c) R.-X. Ren, L. D. Zueva, and W. Ou, Tetrahedron Lett., 42, 8441
2001).
1
:56) and silicalite-1. The conversion decreased rapidly with
time for silicalite-1. High performance of the BPO4, on the other
6
(
hand, was kept after reaching a steady state level. The decrease
4
5
A. Tada, H. Suzuka, and Y. Imizu, Chem. Lett., 1987, 423.
First, we examined the acid strength distribution for BPO with a P/B
ratio above 1 by using Hammett’s reagents. The H0 values were ꢄ8:2
in conversion was within 5% for 6 h, while coke formation on the
catalyst was visually observed after the reaction. When ethanol
was used as a diluent, the performance of BPO4, however, was
significantly damaged as opposed to that of silicalite-1 for which
4
to ꢄ5:6 for BPO4 (1 < P/B < 1:9). No correlation was, however, con-
firmed between the CL selectivity and the acid strength distribution of
the catalyst. The reason for the superior performance of the BPO4 with
P/B ratio around 1.5 can not be attributed to acidity of the catalyst.
More than 20 years ago, Haber and Szybalska concluded that BPO4
with a P/B ratio above 1 shows no activity for the GBR on the basis
of their results obtained with a pulse reactor. In the present study,
material balance between the inlet and outlet of the reactor was poor
for several ten minutes just after feeding the reactant. We guess that
they measured the catalytic performance at an unstable state during
the initial period even if they turned their attention to the dehydration
of the catalysts prior to the catalytic examination: J. Haber and U.
Szybalska, Faraday Discuss., 72, 263 (1981).
7
the promotion effects using alcohol were reported (see: Table 1).
This indicates that the nature of the active sites is not consistent
with those for the catalysts comprising Si and O. For the BPO4 of
dehydration ability, the surface alkoxide and/or hydroxyl group
generated in the interaction with alcohol would lead to undesired
side reactions.
With respect to the mechanism on the BPO4 surface, we ex-
pect that the properties of the main group elements of the late 3rd
period (T ¼ Si, P, and S) could be advantageous to completion
of the catalytic cycle. On the analogy of the mechanism of ho-
mogeneous BR, the BPO4 of strong dehydration ability should
initiate the rearrangement via the OXM-phosphoric acid ester in-
6
7
8
a) M. Kitamura and H. Ichihashi, Stud. Surf. Sci. Catal., 90, 67 (1994).
b) L.-X. Dai, R. Hayasaka, Y. Iwaki, K. A. Koyano, and T. Tatsumi,
Chem. Commun., 1996, 1071. c) A.-N. Ko, C.-C. Hung, C.-W. Chen,
and K.-H. Ouyang, Catal. Lett., 71, 219 (2001).
Sato et al. pointed out the possibility of OXM-silyl ether and CL-silyl
ether as the intermediates for the GBR over silicalite-1, although this
dehydrated type of intermediate has not been approved afterward in
the mechanism proposed for the GBR at silanol nests: H. Sato, K.
Hirose, and Y. Nakamura, Chem. Lett., 1993, 1987.
8
termediate (a in Figure 2) under anhydrous condition. The bond
between the main group elements of the late 3rd period (T ¼ Si,
P, and S) and O or N involves a dꢁ–pꢁ orbital interaction orig-
inating from low energy level for 3d orbital of T and the lone
pair of electrons of O and N. Thereby, it is possible for the sur-
face T sites to interact more preferably with O than N.10,11 In ad-
dition, nucleophilic bases are more likely to attack and substitute
on the T sites than to coordinate to the T sites, which is different
from those for so-called Lewis acid sites consisting of early pe-
riod elements. If a similar approach is applicable to the present
9
9
1
F. A. Cotton and G. Wilkinson, ‘‘Advanced Inorganic Chemistry,’’ 4th
ed., John Wiley & Sons, N.Y. (1980).
0
For example, as is well known for the reactivity of a silylating reagent,
an active hydrogen can be protected as a silyl derivative; the order of
reactivity is reported as OH > NH > CONH. Also, the order of
hydrolysis of silyl derivatives is as follows: CON{Si > N{Si >
O{Si: a) T. W. Greene, ‘‘Protective Groups in Organic Synthesis,’’
John Wiley & Sons, N.Y. (1981). b) G. van Look, G. Simchen, and
J. Heberle, ‘‘Silylating Agents,’’ 2nd ed., Fluka Chemie AG, Buchs
ꢁ
system at 300 C, the OXM nucleophile is possible to kinetically
substitute the CL formed by BR on the T sites (CL-phosphoric
acid ester, b in Figure 2), thus completing the catalytic cycle.
In summary, we found that the BPO4 of strong dehydration
ability is an effective catalyst for the GBR. The results suggest
that the acid anhydride sites play a major role for the catalytic
(
1995).
1
1
Both the silylation agents such as silazane and P2O5 exhibit activity for
the dehydration of aldoxime or amide to nitrile of which mechanism is
similar to BR: W. E. Dennies, J. Org. Chem., 35, 3253 (1970).
Published on the web (Advance View) August 6, 2005; DOI 10.1246/cl.2005.1232