Study of the Beckmann rearrangement of acetophenone oxime over porous solids by means of solid state NMR spectroscopy

Article abstract of DOI:10.1039/b816276j

The Beckmann rearrangement of acetophenone oxime using zeolite H-beta and silicalite-N as catalysts has been investigated by means of ¹⁵N and ¹³C solid state NMR spectroscopy in combination with theoretical calculations. The results obtained show that the oxime is N-protonated at room temperature on the acid sites of zeolite H-beta. At reaction temperatures of 423 K or above, the two isomeric amides, acetanilide and N-methyl benzamide (NMB) are formed, and interact with the Bronsted acid sites of zeolite H-beta through hydrogen bonds. The presence of residual water hydrolyzes the two amides, while larger amounts inhibit the formation of NMB and cause the total hydrolysis of the acetanilide. Over siliceous zeolite silicalite-N, containing silanol nests as active sites, the oxime is adsorbed through hydrogen bonds and only acetanilide is formed at reaction temperatures of 423 K or above. In the presence of water, the reaction starts at 473 K, still being very selective up to 573 K, and the amide is partially hydrolyzed only above this temperature. the Owner Societies 2009.

Full text of DOI:10.1039/b816276j

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PAPER
www.rsc.org/pccp | Physical Chemistry Chemical Physics
Study of the Beckmann rearrangement of acetophenone oxime over
porous solids by means of solid state NMR spectroscopy
Ana Belen Fernandez, Ines Lezcano-Gonzalez, Mercedes Boronat, Teresa Blasco*
and Avelino Corma
Received 17th September 2008, Accepted 2nd March 2009
First published as an Advance Article on the web 2nd April 2009
DOI: 10.1039/b816276j
The Beckmann rearrangement of acetophenone oxime using zeolite H-beta and silicalite-N as
catalysts has been investigated by means of 15N and 13C solid state NMR spectroscopy in
combination with theoretical calculations. The results obtained show that the oxime is
N-protonated at room temperature on the acid sites of zeolite H-beta. At reaction temperatures
of 423 K or above, the two isomeric amides, acetanilide and N-methyl benzamide (NMB) are
formed, and interact with the Brønsted acid sites of zeolite H-beta through hydrogen bonds. The
presence of residual water hydrolyzes the two amides, while larger amounts inhibit the formation
of NMB and cause the total hydrolysis of the acetanilide. Over siliceous zeolite silicalite-N,
containing silanol nests as active sites, the oxime is adsorbed through hydrogen bonds and only
acetanilide is formed at reaction temperatures of 423 K or above. In the presence of water, the
reaction starts at 473 K, still being very selective up to 573 K, and the amide is partially
hydrolyzed only above this temperature.
as catalysts in the liquid-phase Beckmann rearrangement of
4-hydroxyacetophenone oxime to paracetamol.10–11
Acetanilide is an amide related to paracetamol which has
Introduction
The Beckmann rearrangement of ketoximes into amides is a
common reaction used in organic chemistry, and has been
also been synthesized by the Beckmann rearrangement of the
widely studied for many years.1 One of the most important
corresponding (acetophenone-) oxime using solid acids as
catalysts.12–14 Acetanilide possesses industrial interest as an
industrial applications of this reaction is the transformation of
cyclohexanone and cyclododecanone oximes into e-caprolactam
and o-laurolactam, respectively, which are raw materials in
the fabrication of fibers (nylon). The industrial production of
intermediate in the synthesis of pharmaceuticals and as an
additive in hydrogen peroxide, varnishes, polymers and
rubbers. However, there are only a few publications on the
lactams by the Beckmann rearrangement of cyclic oximes is
heterogeneous rearrangement of acetophenone oxime and
most of them in the liquid phase.12–14
The aim of this work is to investigate the reaction mechanism
classically catalyzed by liquid acids and much work has been
carried out with the objective of substituting this homo-
geneous process for an environmentally friendly, heterogenous
of the Beckmann rearrangement of acetophenone oxime using
zeolites containing active sites of differing nature as catalysts.
one.2–4 The research efforts of the last fifteen years have led to
the commercialization of a cleaner process by using silica-rich
This knowledge is essential to the design of catalysts with
MFI-type zeolite, consisting of a three-dimensional network of
adequate properties for the potential development of green
10-membered ring channels (average pore diameter 5.5 A), as
technologies to produce linear amides of pharmacological
catalysts. Sumitomo Chemical Co. has constructed a production
plant which has been working in Japan since 2003.5–7
Another potential industrial application of the Beckmann
interest, containing benzyl groups with different ring substituents.
For this purpose, we have studied the rearrangement of
acetophenone oxime over two different zeolites: Al-containing
rearrangement reaction is the synthesis of paracetamol
zeolite beta with Brønsted acid sites, and silicalite with silanol
(N-acetyl-p-aminophenol). Conventionally, this product is
commercially synthesized by acetylating p-aminophenol with
nests as active sites, combining in situ solid state NMR and
theoretical calculations. Zeolite beta possesses
a three-
acetic anhydride, but this process presents some difficulties
because of the occurrence of unwanted reactions. An alter-
native, new process involves the Beckmann rearrangement of
4-hydroxyacetophenone oxime with liquid acids,8 however, the
use of these catalysts requires extensive purification of the final
reaction product for human use.9 Therefore, environmentally
friendly solid acids such as zeolites have been successfully used
dimensional network of 12-membered ring channels with an
average pore diameter of ca. 6.7 A. Defective silicalite zeolite
was chosen because of its good catalytic performance in the
rearrangement of cyclic oximes.7,15,16
Experimental
Materials
´
´
Instituto de Tecnologıa Quımica (UPV-CSIC), Avda. de los Naranjos s/n,
46022, Valencia, Spain. E-mail: tblasco@itq.upv.es;
Fax: +34 96 387 9444; Tel: +34 96 387 7812
Zeolite H-beta (Si/Al = 12.5, crystal sizes of 0.1–0.2 mm),
produced by Zeolyst, is commercially available (CP811) and
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presents only minor amounts of extra-framework (octahedral)
aluminium as detected by solid state 27Al NMR. Silicalite was
synthesized following the method described in previous
publications,16,17 and subsequently submitted to basic
treatment to generate silanol nests (silicalite-N) as follows:
5 g of zeolite calcined at 823 K for 12 h were impregnated with
20 g of an aqueous solution of ammonia (25 wt%) and
ammonium nitrate (7.5 wt%) and heated in a stainless steel
Results and discussion
Theoretical calculations
The calculations on the adsorption of acetophenone oxime
over silanol and Brønsted acid sites in zeolite beta has been
previously reported by our group.18 Our results indicated that
the N-protonated oxime is readily formed upon adsorption at
Brønsted acid sites, whereas the oxime is adsorbed over silanol
groups forming H-bonds.18 These conclusions agree with
theoretical and experimental results reported by other
groups.25–27 Here, we focus our study on the interaction of
the reaction products of the Beckmann rearrangement of
acetophenone oxime with these same types of zeolite sites
i.e., silanols and bridging hydroxyl groups.
autoclave at 373
K
overnight.16 The final silicalite-N
(crystal sizes around 0.5 mm) catalyst was recovered after
washing with deionized water, filtering and drying at 383 K
for 4 h. (a-13C, 15N)-Acetophenone oxime was prepared
mixing 14.4 mmol of 15N-hydroxylamine hydrochloride
(98% 15N, Cambridge Isotope Laboratories) with 4.3 mmol
of a-13C-acetophenone, 24.4 mmol of sodium acetate and
222.2 mmol of water, and heating at 368 K for 1 h. Before
use, the 15N-acetophenone oxime was re-crystallized in ether
and identified by NMR.
The reaction product of the Beckmann rearrangement of
acetophenone oxime is acetanilide, which results from the
migration of the phenyl group in the anti position with respect
to the hydroxyl group. Although this reaction is highly
stereospecific, we have also considered the N-methyl
benzamide (NMB) isomer, as it has been reported to appear
as a by-product of the rearrangement of acetophenone
oxime.12 Accordingly, we have optimized the adsorption
complexes of the two amides, acetanilide and NMB, on silanol
(Acetanilide/Silanol and NMB/Silanol) and bridging
Si–OH–Al (Acetanilide/Brønsted and NMB/Brønsted) zeolite
groups, and obtained the models depicted in Scheme 1. The
15N and 13C (the 13C-carbonyl group) NMR chemical shifts
and adsorption energies, calculated using density functional
methods, are summarized in Table 1. The distances between
the amides and zeolite hydroxyl groups, depicted in the models
of Scheme 1, are consistent with the formation of hydrogen
bonds, which are shorter with Brønsted acid sites, resulting in
more stable complexes (models Acetanilide/Brønsted and
Solid state NMR spectroscopy
Solid state NMR spectra were recorded at room temperature
on a Bruker AV 400 WB spectrometer. Samples were
dehydrated at 673 K overnight reaching a final pressure of
10ꢁ5 mbar. To record 1H NMR spectra, a portion of the
sample was transferred into a rotor within a glove box under
an atmosphere of N2. The spectra were recorded with a BL4
probe spinning the sample at 10 kHz, by using 901 pulses of
5 ms and recycle delays of 15 s. To study the reaction, 300 mg
of catalyst outgassed at 673 K was mixed with 15 mg of
(a-13C, 15N)-acetophenone oxime and homogenized under an
inert atmosphere in a glove box. A portion of the mixture was
introduced into a glass insert, which was sealed after
outgassing at room temperature. The 1H to 15N (1H/15N)
cross-polarization (CP) MAS spectra were recorded with a
1H 901 pulse of 5 ms, a contact time of 5 ms and a recycle delay
of 5 s. The 1H to 13C (1H/13C) CP-MAS spectra were recorded
NMB/Brønsted in Scheme 1). Regarding chemical shifts,
15
the results summarized in Table 1 show that d
N
calc
of
acetanilide and NMB experience larger shifts to low field when
they interact with bridging hydroxyl groups, where they are
more strongly adsorbed. We must note that the 15N chemical
shifts calculated for NMB/Brønsted and the two acetanilide
complexes, i.e., Acetanilide/Brønsted and Acetanilide/Silanol,
are in the range ꢁ235 to ꢁ256 ppm, while displacement to high
field is calculated for NMB/Silanol (d 15Ncalc = ꢁ284.6 ppm, see
Table 1). The structural models calculated for acetanilide and
NMB suggest that neither of the two amides becomes proto-
nated on Brønsted acid sites (Scheme 1), in contrast with the
formation of O-protonated lactams (cyclic amides) reported
previously.28,29 This difference suggests that acetanilide and
NMB are less basic than cyclic caprolactam and o-laurolactam.
The presence of residual water, difficult to completely
remove from the reaction medium, can produce some
hydrolysis of the oxime and/or the amide, depending on the
catalyst properties. Accordingly, we have also theoretically
calculated the complexes resulting from the interaction of the
hydrolysis products of acetanilide (aniline and acetic acid) and
of NMB (benzoic acid and methylamine) with a zeolite
Brønsted acid site as depicted in Scheme 2. The simulation
of the interaction with silanol groups has been omitted,
since no noticeable changes on chemical shifts are expected
according to the results for the amides reported in Table 1.
1
with a 901 pulse for H of 5 ms, a contact time of 5 ms and a
recycle delay of 5 s.
Computational details
Silanol defects and Brønsted acid sites were simulated by
means of the Si(OSiH3)3OH and Al(OSiH3)3(OH)SiH3 cluster
models, respectively, as described in previous work.18 The
geometries of the two zeolitic clusters and the complexes
resulting from the adsorption of acetophenone oxime, and
of the possible reaction products (acetanilide, N-methyl
benzamide, aniline,. . .) were optimized using the density
functional B3PW91 method and the standard 6-31G(d,p) basis
set.19–21 In these calculations, the coordinates of all atoms
except the terminal H of the SiH3 groups were fully optimized.
Isotropic absolute chemical shielding constants were
calculated with the B3PW91/6-31G(d,p) method on geometries
optimized at the same level, using the gauge including atomic
orbitals (GIAO) approach.22,23 15N and 13C chemical shifts
were calculated as d = sref ꢁ s, and corrected with the
equations obtained from
a preliminary study of the
performance of B3PW91 functional.18 All calculations in this
work were performed using the GAUSSIAN98 computer
program.24
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Table 1 Adsorption energies calculated for the models depicted in Scheme 1. Calculated and experimental 15N and 13C isotropic chemical shifts
for acetanilide and NMB both isolated and in models (a)–(d) in Scheme 1, and for acetanilide interacting with water
15
N
13
C
15
13
C
Eads/kcal molꢁ1
d
/ppm
d
/ppm
calc
d
N
exp
/ppm
d
/ppm
exp
calc
Acetanilide
—
ꢁ247.8
ꢁ248.0
ꢁ235.9
ꢁ248.2
ꢁ293.2
ꢁ284.6
ꢁ255.7
165.5
174.2
175.6
168.2
165.8
175.1
177.3
—
176.0
173.0
177.0
176.0
Acetanilide/Silanol
Acetanilide/Brønsted
Acetanilide/H2O
NMB
NMB/Silanol
NMB/Brønsted
ꢁ9.1
ꢁ246.0
ꢁ230.0
ꢁ250.0
ꢁ22.2
—
—
166.0
a
a
ꢁ8.0
ꢁ21.8
ꢁ245.0
168.0b
a
b
NMB is not formed in pure silica solids. From a 13C NMR spectrum of NMB on H-beta zeolite, not shown.
The 15N and 13C (of the carboxylate group) NMR chemical
shifts calculated for the complexes of Scheme 2 by using
ab initio methods, together with the experimental values
reported in the bibliography are summarized in Table 2. We
must note that, according to Scheme 2, both amines, i.e., aniline
and methylamine, become protonated on the Brønsted acid
sites of the zeolite; though their d 15N values are only slightly
shifted when compared with the neat amine (see Table 2).
Si–OH–Al groups. These 1H signals are associated with the
infrared bands in the range 3725–3745 cmꢁ1 and 3605 cmꢁ1
,
respectively.30–32 Hydroxyl groups bonded to non-framework
aluminium species appear in the range 2.6–3.6 ppm
(see Fig. 1(b)), and are probably related to the infrared band
at 3780 cmꢁ1 30–32
.
The H NMR spectrum of the parent silicalite, which has
1
been included in Fig. 1 for comparison purposes, consists
mainly of a peak of isolated SiOH groups at 1.8 ppm
(Fig. 1(a)). The spectrum of silicalite-N, shown in Fig. 1(b),
evidences the creation of structural defects in the MFI zeolite
framework by submitting the sample to a basic treatment.
Besides isolated silanols (signal at 1.8 ppm), the spectrum of
Solid state NMR spectroscopy
Zeolite characterization by 1H NMR. The active sites present
in the zeolites H-beta and silicalite-N dehydrated at 673 K,
used here as catalysts, have been characterized in a previous
publication by infrared spectroscopy,31 and here by means of
1H NMR (see Fig. 1). The spectrum of acidic zeolite H-beta
(Fig. 1(c)) shows an intense peak at 1.8 ppm of isolated
silanols and a band at 4.0 ppm assigned to bridging hydroxyl
silicalite-N (Fig. 1(b)) shows
a shoulder at 2.2 ppm
usually assigned to geminal or vicinal SiOH groups
(infrared band at around 3690 cmꢁ1),16,31,32 and an additional
Scheme 1 Optimized structures of acetanilide (a, c) and N-methyl
benzamide (b, d) adsorbed on bridging Si–OH–Al (a, b) and silanol
zeolite groups (c, d). Bond distances are expressed in angstroms.
Scheme
2
Optimized structures of (a) aniline, (b) acetic acid,
(c) methylamine, and (d) benzoic acid interacting with bridging
Si–OH–Al zeolite groups. Bond distances are expressed in angstroms.
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Table 2 Calculated and experimental 15N and 13C isotropic chemical
shifts of the hydrolysis products of acetanilide and NMB adsorbed on
Brønsted acid sites depicted in Scheme 2
weaker, at ꢁ245 ppm, which are attributed to 15N-acetanilide
and 15N-NMB, respectively. This assignment is made by
comparison with the 15N chemical shifts calculated
theoretically for the two amides interacting with a bridging
hydroxyl group, listed in Table 1. It should be noted that the
signal at ꢁ245 ppm could also be attributed to 15N-acetanilide
interacting with silanol groups or with trace amounts of water
present in the system (see Table 1). However, the presence of
NMB in the products was confirmed by mass spectroscopy,
reinforcing the assignment of the ꢁ245 ppm signal to
15N-NMB adsorbed on a Brønsted acid site. Besides these
two peaks, the 15N spectrum (Fig. 2(b0)) displays weak signals
at ꢁ330 ppm and ꢁ360 ppm attributed to 15N-protonated
aniline and 15N-protonated methylamine on Brønsted acid
sites (see Scheme 2), according to the chemical shifts listed in
Table 2. These amines must come from the hydrolysis of
acetanilide (aniline) and NMB (methylamine) with residual
amounts of water present in the reaction system. The 13C
NMR spectrum recorded after heating at this same tempera-
ture (423 K, Fig. 2(b)) shows a band at 176 ppm due to
13CO-acetanilide and a weaker signal at 168 ppm from
13CO-NMB (see Table 1) with a shoulder at 182 ppm due
to 13COOH-acetic acid, again indicating hydrolysis of
acetanilide. The signal of benzoic acid (d 13C = 175.8 ppm)
coming from the hydrolysis of NMB must be overlapped in the
main broad resonance of the spectrum. Besides these signals, a
very weak 13CO-acetophenone signal appears at 193 ppm
pointing to some hydrolysis of the oxime.
15
13
15
13
d
ppm
N
/
calc
d
ppm
C
/
calc
d
N
exp
ppm
/
d
C
exp
ppm
/
Acetic acid/Brønsted
Benzoic acid/Brønsted
Aniline/Brønsted
—
—
ꢁ329.7
183.3
175.8
—
—
—
ꢁ330
182
—
a
Methylamine/Brønsted ꢁ360.7
—
ꢁ360
Appears in the amide region and, thus, is difficult to detect.
a
The increase of the reaction temperature of the aceto-
phenone oxime on zeolite H-beta to 473 K does not
appreciably change the 13C and 15N spectra, producing only
a better resolution of the peak of 15N-NMB at ꢁ245 ppm, and
an enhancement of the relative intensity of the anilinium
cation, as shown in Fig. 2(c) and (c0). Changes in the product
distribution are evident after the reaction temperature of
Fig. 1 1H MAS NMR spectra of zeolites dehydrated at 673 K
overnight: (a) silicalite, (b) silicalite-N and (c) H-beta.
15
623 K; the N NMR spectrum (Fig. 2(d0)) shows only some
band at 3.6 ppm that can be associated with the broad infrared
hydroxyl band centred at 3500 cmꢁ1, assigned to silanol nests
(see Fig. 1(b)).16,31
increase in the relative intensity of NMB, but new signals are
evident in the better resolved 13C spectrum (Fig. 2(d)).
The spectrum of Fig. 2(d) clearly shows the 13C peaks of
acetanilide (and probably benzoic acid) at 176 ppm, and of
NMB at 168 ppm, as well as that of acetic acid at 182 ppm and
the very weak peak of acetophenone at 193 ppm. Besides
these signals, which were already present at lower reaction
temperatures, a new one appears at 155 ppm (Fig. 2(d)). This
latter peak is attributed to a secondary product formed from
NMB and/or acetanilide at higher reaction temperatures
(above 573 K) on zeolites containing Brønsted acid sites;
however, we have failed in its identification by different
methods.
The main 15N and 13C NMR spectroscopic results of this
reaction over siliceous zeolite beta were reported in our
previous publication.18 The 15N NMR spectra are quite
similar to those depicted in Fig. 3 for the transformation of
(a-13C, 15N)-acetophenone oxime over silicalite-N. The
spectrum recorded after the adsorption of the oxime at room
temperature, displayed in Fig. 3(a), shows a very sharp peak at
ꢁ21 ppm due to unadsorbed oxime; however, inspection
of the spectrum shows a very weak, broad resonance centred
at ꢁ48 ppm from the 15N-oxime adsorbed on silanol groups.
15N and 13C NMR of the Beckmann rearrangement of
(a-13C, 15N) acetophenone oxime. Fig. 2 shows the 13C and
15N NMR spectra recorded for the Beckmann rearrangement
of (a-13C, 15N)-acetophenone oxime over zeolite H-beta,
containing Brønsted acid sites. The 13C NMR signals come
from the 13C-labelled a-carbons of the oxime, the 13CO
carbonyl groups of amides and the 13COOH carboxylate
groups of the acids produced by hydrolysis of the amide .
The oxime is adsorbed at room temperature by capturing
the zeolite acidic proton; it becomes 15N-protonated, giving
broad 13C and 15N peaks at 166.5 ppm and ꢁ148.5 ppm,
respectively (Fig. 2(a) and (a0)). A sharper and weaker 13C
component at 155 ppm in the spectrum of Fig. 2(a) can
indicate the presence of some amounts of non-interacting
oxime. Heating the reaction system at 423 K produces the
practical disappearance of the 13C and 15N signals of the
oxime and the appearance of new resonances as can be
observed in the spectra of Fig. 2(b) and (b0). The 15N NMR
spectrum of Fig. 2(b0) shows two peaks in the chemical shift
range typical of amides, one at ꢁ230 ppm and another,
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1
15
Fig. 2 1H/13C (a–d) and H/15N (a0–d0) CP-MAS NMR spectra of (a-13C, N)-acetophenone oxime: (a) + (a0) adsorbed on zeolite H-beta at
room temperature, and subsequently treated during 1 h at (b) + (b0) 423 K, (c) + (c0) 473 K, and (d) + (d0) 623 K.
Comparison of this spectrum with that recorded when the
same oxime is mixed with zeolite beta containing defective
silanol groups suggests a weaker interaction of the oxime
with silicalite-N,18 which can be due to a lower concentration
of connectivity defects. However, although the oxime enters
the pores of Al-containing zeolite ZSM-5 (spectrum
not shown) to become protonated, we cannot completely
discard the limitations of diffusion of the oxime into the
pores of silicalite.
As shown in Fig. 3(b), when the (a-13C, 15N)-oxime–
silicalite-N system is heated at 423 K, a new signal grows at
ꢁ250 ppm. Although the position of this peak is close to that
of 15N-NMB on zeolite H-beta, the d 15Ncalc of the two amides
interacting with silanol groups allows its assignment to
15
15N-acetanilide (d
Acetanilide/Silanol, Scheme 1, Table 1). When the reaction
N
calc
= ꢁ248 ppm for the model
temperature is increased to 473 K, the spectrum of Fig. 3(c) is
obtained, which shows
a unique signal of acetanilide
at d 15N = ꢁ250 ppm, indicating that the reaction is complete
at this temperature.
A remarkable result here is the formation of the amide
isomer NMB over Brønsted acid sites, which is not expected
according to the high stereospecificity of the migration of the
alkyl group in the anti position to the hydroxyl group, though
we must note that 5% of NMB has previously been reported in
the Beckmann rearrangement of acetophenone oxime on
zeolite HY.12 The formation of the NMB isomer probably
requires the prior isomerization of the oxime to place the
methyl group in the anti position with respect to the hydroxyl
group. This process involves rotation around the C–N bond
and must be hindered for the neutral oxime having a CQN
Fig. 3 1H/15N CP-MAS NMR spectra of (a-13C, 15N)-acetophenone
oxime: (a) adsorbed on zeolite silicalite-N at room temperature and
subsequently treated during 1 h at (b) 423 K, (c) 473 K and (d) 523 K.
double bond (see Scheme 3). However, this rotation can occur
to some extent in the N-protonated acetophenone oxime
possessing a single C–N bond. Accordingly, no NMB is
formed over siliceous zeolites, whereas some is formed
over acidic H-beta.
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two new signals appear, one at ꢁ250 ppm, in the amide region,
and another one at ꢁ332 ppm from 15N aniline protonated on
Brønsted acid sites (see Scheme 2 and Table 2). The corres-
ponding 13C NMR spectrum (Fig. 4(b)) shows a peak for
acetanilide at 176 ppm and another at 183 ppm due to acetic
acid. The presence of anilinium and acetic acid clearly indi-
cates the hydrolysis of acetanilide. The amide resonance is
slightly high-field shifted and appears at a similar position to
that of 15N-NMB on Brønsted acid sites (ꢁ250 ppm,
see Scheme 1 and Table 1). However, neither the 15N
13
nor the C spectra (Fig. 4(b) and (b0)) give any evidence for
the formation of NMB or any of its hydrolysis products.
This suggests that the 15N peak at ꢁ250 ppm in the spectrum
of Fig. 4(b0) must be assigned to acetanilide, and that its
chemical shift is changed by the presence of water. This
assignment is supported by the 15N chemical shift calculated
for the complex formed by the interaction of acetanilide with
one molecule of water (see Table 1). The absence of NMB
in the reaction products, further confirmed by mass
spectrometry, suggests that water inhibits the isomerization
of the oxime (or the migration of the syn-methyl group) during
rearrangement. Further increase of the reaction temperature
favours the hydrolysis of acetanilide, which is almost
complete at 473 K, as can be seen in the spectra shown in
Fig. 4(c) and 4(c0).
Fig. 5 shows the 15N NMR spectra obtained after heating a
mixture of zeolite silicalite-N without previous dehydration
and 15N-acetophenone oxime. The results reported in Fig. 5
indicate that the presence of water has no dramatic influence
on the Beckmann rearrangement reaction. The main effects of
water are: (i) to increase the temperature at which the
rearrangement starts to 473 K (from 423 K), and (ii) to
hydrolyse some acetanilide at reaction temperatures of
573 K or higher.
Scheme
3
Rotation of the C–N bond for (a) the neutral
acetophenone oxime, and (b) the N-protonated oxime.
Effect of water in the Beckmann rearrangement of 15N-
acetophenone oxime. As shown above, when the Beckmann
rearrangement is carried out using zeolite H-beta as catalyst,
residual water in the reaction system partially hydrolyzes
acetanilide and N-methyl benzamide. In order to better deter-
mine the effect of water, we have carried out the reaction by
mixing the oxime with the non-dehydrated zeolites. In these
conditions, the H2O/oxime molar ratios are 21 and 7 for
zeolites H-beta and silicalite-N, respectively.
Fig. 4 depicts the results obtained by 15N and 13C NMR
spectroscopy for the reaction of (a-13C, 15N)-acetophenone
oxime over zeolite H-beta. Despite the presence of water, the
acetophenone oxime adsorbed on the acidic zeolite becomes
N-protonated at room temperature, as evidenced in the 15N
NMR spectrum of Fig. 4(a0). As shown in the spectrum of
Fig. 4(b0), when the reaction temperature is raised to 473 K,
the 15N peak of the 15N-protonated oxime disappears, while
Fig. 4 1H/13C (a–d) and 1H/15N (a0–d0) CP-MAS NMR spectra of (a-13C, N)-acetophenone oxime: (a) + (a0) adsorbed on non-dehydrated
15
zeolite H-beta at room temperature and subsequently treated during 1 h at (b) + (b0) 423 K, and (c) + (c0) 473 K.
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Conclusions
The results obtained in the Beckmann rearrangement reaction
of acetophenone oxime over acidic zeolite H-beta and
silicalite-N, dehydrated and in the presence of water, have
led us to propose the reaction pathways shown in Schemes 4(a)
and (b).
N-protonated acetophenone oxime is readily formed on the
Brønsted acid sites at room temperature even in the presence
of water. After heating at 423 K or above, both acetanilide
and N-methyl benzamide (NMB) are formed, and a new
unidentified secondary product formed from the amides
appears at reaction temperatures above 573 K. Residual water
produces partial hydrolysis of acetanilide to give aniline and
acetic acid, and of NMB to give benzoic acid and methyl-
amine. Both amines (methylamine and aniline) are protonated
on the zeolite Brønsted acid sites. The presence of water
in the reaction medium inhibits the formation of NMB;
only acetanilide is formed, which is practically completely
hydrolyzed above 473 K.
Pure siliceous silicalite-N is more selective to acetanilide
than the acidic zeolite both in presence and absence of water in
the reaction medium. Only part of acetophenone oxime forms
hydrogen bonds with silanol groups at room temperature and
reacts above 423 K to selectively give acetanilide. When water
is present in the reaction medium (even if there is an excess
with respect to the oxime) there is only partial hydrolysis of
acetanilide above 523 K.
Fig. 5 1H/15N CP-MAS NMR spectra of (a-13C, 15N)-acetophenone
oxime: (a) adsorbed on zeolite silicalite-N at room temperature, and
subsequently treated during 1 h at (b) 373 K, (c) 473 K, (d) 573 K and
(e) 623 K.
The results presented here suggest that, as for cyclic oximes,
acid zeolites are more active and less selective catalysts than
siliceous zeolites containing structural defects, i.e. silanol
Scheme 4 Reaction pathway for the Beckmann rearrangement of acetophenone oxime into acetanilide on zeolite hydroxyl groups: (a) bridging
Si–OH–Al and (b) silanol.
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groups, in the Beckmann rearrangement of linear oximes
involving benzyl groups. However, they may require higher
reaction temperatures, and, therefore, it may be necessary to
carry out the reaction in the gas phase.
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Acknowledgements
The authors acknowledge the Spanish CICYT (MAT2006-
14274-C02-01 y CTQ2006-09358) for financial support.
I.L.-G. thanks UPV for a FPI-UPV grant.
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