Beckmann rearrangement of 15N-cyclohexanone oxime on zeolites silicalite-1, H-ZSM-5, and H-[B]ZSM-5 studied by solid-state NMR spectroscopy

Article abstract of DOI:10.1021/ja066392c

By means of solid-state ¹⁵N NMR spectroscopy, evidence for the formation of nitrilium ions as intermediates of the Beckmann rearrangement of ¹⁵N-cyclohexanone oxime to ε-caprolactam on silicalite-1, H-ZSM-5, and H-[B]ZSM-5 is reporte

Full text of DOI:10.1021/ja066392c

Published on Web 10/28/2006
Beckmann Rearrangement of 15N-Cyclohexanone Oxime on Zeolites
Silicalite-1, H-ZSM-5, and H-[B]ZSM-5 Studied by Solid-State NMR
Spectroscopy
V. R. Reddy Marthala, Yijiao Jiang, Jun Huang, Wei Wang,* Roger Gla¨ser, and Michael Hunger*
Institute of Chemical Technology, UniVersity of Stuttgart, D-70550 Stuttgart, Germany
Received September 4, 2006; E-mail: michael.hunger@itc.uni-stuttgart.de; wang.wei@itc.uni-stuttgart.de
To replace the conventional method for synthesizing ꢀ-capro-
lactam in concentrated sulfuric acid or oleum, the vapor-phase
Beckmann rearrangement of cyclohexanone oxime on solid acid
catalysts has received increasing interest as an environmentally
benign process.1,2 It was found that high-silica ZSM-5 zeolite
(silicalite-1)3 and boron-substituted zeolite ZSM-5 (H-[B]ZSM-5)4
are highly active and selective as catalysts for the transformation
of cyclohexanone oxime to ꢀ-caprolactam. In earlier studies, strong
Brønsted acid sites in zeolites were suggested to be crucial in
Figure 1. 15N CP/MAS NMR spectra of pure 15N-cyclohexanone oxime
catalyzing this reaction by protonating the oxime.1,2 However,
(a), 15N-cyclohexanone oxime physically mixed with silicalite-1 and heated
further investigations showed that weakly acidic silanol groups and
T-vacancies are responsible for the high activity and selectivity
toward the ꢀ-caprolactam.1,2
at 423 K (b), 15N-cyclohexanone oxime physically mixed with H-ZSM-5
(c), and 15N-cyclohexanone oxime physically mixed with H-[B]ZSM-5 and
heated at 423 K (d). All spectra were recorded at room temperature.
Asterisks denote spinning sidebands.
Recently, the possible reaction mechanism of the vapor-phase
Beckmann rearrangement has been studied experimentally5,6 and
theoretically.5,7,8 Utilizing 13C and 15N MAS NMR spectroscopy
together with theoretical calculations, Fernandez et al.5 investigated
the Beckmann rearrangement of acetophenone oxime on siliceous
and aluminum-containing zeolites Beta. In the present work, the
first solid-state NMR study of the Beckmann rearrangement of
cyclohexanone oxime on the zeolites silicalite-1, H-ZSM-5, and
H-[B]ZSM-5 and evidence for the formation of reaction intermedi-
ates and byproducts are reported.
Figure 1a shows the 15N CP/MAS NMR spectrum of pure 15N-
cyclohexanone oxime, which was synthesized via the reaction of
15N-hydroxylamine‚HCl and cyclohexanone. The single signal at
-55 ppm is due to the 15N-label in cyclohexanone oxime. The 15
N
Figure 2. 15N CP/MAS NMR spectra recorded at room temperature after
conversion of 15N-cyclohexanonoe oxime on silicalite-1 (left), H-ZSM-5
(middle), and H-[B]ZSM-5 (right). The reaction temperatures are indicated
in the Figure. Asterisks denote spinning sidebands.
CP/MAS NMR spectrum shown in Figure 1b was recorded after
preparing the physical mixture of 15N-cyclohexanone oxime and
silicalite-1 and heating at 423 K. In addition to the signal at -55
ppm due to pure 15N-cyclohexanone oxime, a new low-field signal
appeared at -46 ppm. This signal, which does not occur without
heating at 423 K, is due to 15N-cyclohexanone oxime interacting
with the SiOH groups of silicalite-1 via hydrogen bonding. A similar
low-field shift of about 10 ppm was observed by Fernandez et al.5
for the hydrogen bonding of 15N-acetophenone oxime with the SiOH
groups of siliceous zeolite Beta.
Studying the interaction of 15N-cyclohexanone oxime with the
Brønsted acid sites of zeolite H-ZSM-5, the 15N CP/MAS NMR
spectrum of the physical mixture shows a new high-field signal at
-160 ppm, which occurs already at room temperature (Figure 1c).
This signal was assigned to 15N-cyclohexanone oxime protonated
by the acidic bridging OH groups (SiOHAl) in the vicinity of
framework aluminum atoms in zeolite H-ZSM-5.5,6 In the 15N CP/
MAS NMR spectrum of the physical mixture of 15N-cyclohexanone
oxime and zeolite H-[B]ZSM-5 heated at 423 K, a high-field signal
occurs at -153 ppm (Figure 1d). As expected for silanol groups in
the vicinity of framework boron atoms (SiOH[B]) in zeolite H-[B]-
ZSM-5, which are characterized by a lower acid strength in
comparison with the SiOHAl groups in zeolite H-ZSM-5, the signal
of protonated 15N-cyclohexanone oxime is slightly shifted to lower
field.
Further experiments focused on the conversion of 15N-cyclo-
hexanone oxime on the zeolites silicalite-1, H-ZSM-5, and H-[B]-
ZSM-5 by stepwise heating the physical mixtures of the reactant
and the zeolite catalysts at temperatures between 423 and 523 K.
According to Figure 2a, the Beckmann rearrangement on silicalite-1
starts at about 473 K, which is indicated by new signals at -237,
-260, and -376 ppm. After raising the reaction temperature to
498 and 523 K (Figure 2b,c), the spectra consist of a single signal
at -260 ppm. This signal is caused by ꢀ-caprolactam, the final
product of the Beckmann rearrangement.5,6 The signal at -376 ppm
is a hint for the formation of an amine as a byproduct,6 while the
origin of the signal at -237 ppm is discussed in the following.
In the literature,1,2,5,7,8 the mechanism of the vapor-phase
Beckmann rearrangement of cyclohexanone oxime on zeolite
catalysts depicted in Scheme 1 is suggested. The chemical shift
values given in the scheme are the 15N NMR shifts observed
experimentally in the present study. The chemical shift values given
9
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J. AM. CHEM. SOC. 2006, 128, 14812-14813
10.1021/ja066392c CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1
supported by the simultaneous increase of both signals upon heating
the reaction mixture at 498 and 523 K (Figure 2h,i). As suggested
by Shouro et al.11 for the Beckmann rearrangement of oximes on
FSM-16-type materials, the preferred conversion of cyclohexanone
oxime to 5-cyano-1-pentene on H-[B]ZSM-5 may originate from
a reversible cleavage of the unstable -Si-OH-B- bridges in this
catalyst.
Formation of water on the working zeolite catalyst H-[B]ZSM-
5, for example, by dehydration of cyclohexanone oxime to 5-cyano-
1-pentene, can also cause hydrolysis of the reaction product
ꢀ-caprolactam. In the presence of water, ꢀ-caprolactam is converted
into ꢀ-aminocaproic acid, which is the first reaction step on the
route from ꢀ-caprolactam to polyamide.12 Because ꢀ-aminocaproic
acid has a predicted 15N NMR shift of -353 ppm,9 this species is
probably the reason for the 15N CP/MAS NMR signal at -364 ppm
observed in the spectra of H-[B]ZSM-5.
In summary, this solid-state NMR investigation of the Beckmann
rearrangement on MFI-type zeolite catalysts with different acid
strengths (see Supporting Information) shows that the conversion
of cyclohexanone oxime is catalyzed both by SiOH groups, as in
silicalite-1, and by acidic SiOH[B] and SiOHAl groups, as in
zeolites H-[B]ZSM-5 and H-ZSM-5. The protonation of the reaction
product ꢀ-caprolactam in the latter case leads to a strong adsorption
on the catalysts surface. This may result in a (i) reduced activity
of zeolites H-[B]ZSM-5 and H-ZSM-5 with respect to silicalite-1
and (ii) consecutive reactions, for example, formation of byproducts,
such as hydrolysis and polymerization products. These byproducts
can affect the selectivity for ꢀ-caprolactam and lead to catalyst
deactivation. In all zeolite catalysts under study, nitrilium ions occur
as intermediates of the vapor-phase Beckmann rearrangement
causing a 15N CP/MAS NMR signal at -237 ppm.
in parentheses were obtained via ref 9. The first steps of the
Beckmann rearrangement of cyclohexanone oxime on the zeolite
catalysts under study are the adsorption of the reactant molecules
via hydrogen bonding at SiOH groups in silicalite-1 (A) or the
N-protonation of the reactant by hydroxyl groups in zeolites
H-ZSM-5 and H-[B]ZSM-5 (B). Quantum chemical studies of
Bucko et al.8 indicate that the 1,2-H shift leading to O-protonated
cyclohexanone oxime C is followed by the formation of carbenium
ions D as intermediates. A more stable state of this intermediate is
the nitrilium ion E. The 15N NMR shift of -224 ppm predicted
for this intermediate indicates that species E may be responsible
for the signal at -237 ppm.9 This assignment is supported by the
decrease of the signal at -237 ppm, while the signals of nonpro-
tonated and protonated ꢀ-caprolactam (F and G, respectively) at
-260 and -347 ppm, respectively, are raised. In agreement with
Scheme 1, species E is consumed during formation of the final
product ꢀ-caprolactam.
Acknowledgment. Financial support by the Deutsche Fors-
chungsgemeinschaft is gratefully acknowledged.
The Beckmann rearrangement of 15N-cyclohexanone oxime on
zeolite H-ZSM-5 starts at about 423 K (Figure 2d). In addition to
the signal of protonated and noninteracting 15N-cyclohexanone
oxime at -160 ppm and -55 ppm, respectively, a signal at -347
ppm due to protonated ꢀ-caprolactam occurs. The spectrum in
Figure 2d is dominated by a signal at -237 ppm, which already
appeared in the spectrum of silicalite-1 heated at 473 K (Figure
2a).
Supporting Information Available: Procedures of sample prepara-
tion and synthesis of 15N-cyclohexanone oxime, parameters of 15N CP/
1
MAS NMR experiments, and the H MAS NMR characterization of
the hydroxyl coverage of the zeolite catalysts under study. This material
is available for free of charge via the Internet at http://pubs.acs.org.
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A more complicated situation was found for the Beckmann
rearrangement of 15N-cyclohexanone oxime on zeolite H-[B]ZSM-
5 (Figure 2g-i). The same signals as for the conversion of 15N-
cyclohexanone oxime on silicalite-1 and H-ZSM-5 were ob-
served: The signals of the nonconverted reactant at -55 ppm, of
the hydrogen-bonded reactant at -46 ppm, of nitrilium ions at -237
ppm, and of nonprotonated and protonated ꢀ-caprolactam at -260
and -347 ppm, respectively. In addition, side reactions lead to
byproducts causing signals at -199, -275, and -364 ppm. These
byproducts may be the reason for the loss of the ꢀ-caprolactam
selectivity of H-[B]ZSM-5 after a short time on stream in
comparison with silicalite-1 as described by Forni et al.10
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