1
38
S. Nagashima et al. / Catalysis Today 164 (2011) 135–138
weak acid sites with a pKa less than 1.5, whereas the strong acid
sites with a pKa less than −3.0 are not active for the reaction [5].
Over high-silica MFI, the catalytically active sites are ascribed to
nest silanols with very weak acidity, which is undetectable by the
2.0
temperature-programmed desorption of ammonia (NH -TPD) [10].
3
Thus, the weak acid sites (H ≈ +1.5) of the hydroxo ligand on the
0
halide clusters are favorable for the selective Beckmann rearrange-
ment.
1.0
4. Conclusion
When the tungsten halide cluster, (H O) [(W Cl )Cl ]·6H O
3
2
6
8
6
2
0.0
(1), supported on silica gel was activated in a helium stream
+4
+3
+2
+1
0
◦
over the temperature range 250–350 C, catalytic activity for the
Acid strength (H0 units)
vapor-phase Beckmann rearrangement of cyclohexanone oxime
to -caprolactam developed. The maximum selectivity for -
caprolactam was 93%. The rearrangement of acetone oxime and
cyclopentanone oxime was similarly catalyzed by 1/SiO2. The sup-
ported niobium and tantalum clusters activated in the same way
also catalyzed the rearrangement of cyclohexanone oxime. Acid
titration shows that the activation of 1 developed a weak acid
Fig. 3. Butylamine titers vs. acid strength for (H3O)2[(W6Cl8)Cl6]·6H2O (1)/SiO2,
◦
◦
which was treated at 150 C (open squares) and 300 C (open circles) in a helium
◦
stream. The titer for SiO2 treated at 300 C (closed triangles) is also shown.
such as methanol, ethanol, 1-propanol, and water were used, the
decrease in conversion during the period 0.5–3 h was less than
5%, indicating that the deactivation was slight. The selectivity for
site with H ≈ +1.5 for each molecule. Vapor-phase Beckmann rear-
0
1
rangements can be catalyzed by weak solid acids; in contrast, strong
acids are less active and less selective for the reaction. Thus, the
activated clusters with weak acidity are favorable for the selective
Beckmann rearrangement.
-caprolactam was high (86%) for these protic solvents. On the
other hand, aprotic solvents such as benzene, toluene, acetone, and
acetonitrile exhibited a greater decrease in conversion during the
period (23–51%), and the selectivity for -caprolactam was much
lower (27–42%). In the Beckmann rearrangement over various zeo-
lites, alcohol and water solvents are suggested to remove the
produced -caprolactam from the acid sites before it is converted
to coke and other by-products such as nitriles, leading to both
prevention of deactivation and high selectivity for -caprolactam
References
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(
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[
[
0
0
◦
50 C, at which temperature the cluster changed to the aqua clus-
6
8
4
2
2
◦
[
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3
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0
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◦
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[
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(
0
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6
8
2
2
2
2
[
[
[
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[
[
[
25] H.A. Benesi, J. Am. Chem. Soc. 78 (1956) 5490.
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[
8]; however, HY and H-mordenite, having strong acid sites, exhibit
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2
2
[
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whereas strong acid sites on the catalysts accelerate formation of
by-products [4,6]. The rearrangement over HNaY is catalyzed by
2
[