Vapor phase Beckmann rearrangement of cyclohexanone oxime over a novel tantalum pillared-ilerite

Article abstract of DOI:10.1039/b001466o

The vapor phase Beckmann rearrangement of cyclohexanone oxime has been studied using a novel tantalum pillared-ilerite as catalyst: the cyclohexanone oxime conversion rate reaches 97.1% and the selectivity for ε- caprolactam is up to 89.1% at 350 °C.

Full text of DOI:10.1039/b001466o

Vapor phase Beckmann rearrangement of cyclohexanone oxime over a novel
tantalum pillared-ilerite
Younghee Ko,a Myung Hun Kim,a Sun Jin Kim,a Gon Seo,b Mi-Young Kimb and Young Sun Uh*a
a Materials Chemistry Research Center, Korea Institute of Science and Technology, PO Box 131, Cheongryang,
Seoul 130-650, Korea. E-mail: uhyoung@kistmail.kist.re.kr
b Department of Chemical Technology, Chonnam National University, Kwangju 500-757, Korea
Received (in Cambridge, UK) 22nd February 2000, Accepted 4th April 2000
Published on the Web 27th April 2000
The vapor phase Beckmann rearrangement of cyclohex-
anone oxime has been studied using a novel tantalum
pillared-ilerite as catalyst: the cyclohexanone oxime conver-
sion rate reaches 97.1% and the selectivity for e-caprolactam
is up to 89.1% at 350 °C.
an additional 24 h. The H-ilerite was recovered by filtering off,
washing, and was dried at 40 °C. The sodium content of the
sample was 0.11 wt% as determined by atomic absorption
spectroscopy. Octylamine intercalation was performed to
increase the interlayer spacing of ilerite by allowing H-ilerite to
react with an excess of octylamine at room temperature.
The pillar precusor for tantalum oxide pillaring was prepared
by the hydrolysis of tantalum pentaethoxide.10 A small amount
of octylamine was added to the solution to increase the degree
of hydrolysis of tantalum pentaethoxide. The molar ratio of
Ta(OC2H5)5+H2O+octylamine was adjusted to 1+12+0.3. The
pillar precusor can be represented as TaxOy(OR)z. For compar-
ison, silicon pillared-ilerite was prepared according to the
procedure in the literature.9 The pillar precusor for silicon oxide
pillaring was prepared by mixing octylamine and TEOS in the
molar ratio of 2+5. The pillar precusor solution was added
dropwise to the octylamine-ilerite gel with continuous stirring.
The final suspension was allowed to stand for three days with
stirring at room temperature. The resultant product was washed
with ethanol, filtered off and dried at 100 °C. Samples were
calcined at 700 °C for 1 h to eliminate residual organic
molecules. The tantalum content of tantalum pillared-ilerite
samples, as determined by inductively coupled plasma (ICP)
spectroscopy, ranged from 3.8 to 5 wt%.
The as-synthesized metal pillared-ilerites have basal spacings
of 27.2 Å (Ta) and 29.4 Å (Si) much larger than those of Na- and
H-ilerites, (11.1 and 7.4 Å, respectively).9 Although the basal
spacings gradually decreased with increase in calcination
temperature, the structures of the metal pillared-ilerites were
preserved even after calcination at 700 °C (Fig. 1). Broad peaks
in the XRD powder diffraction patterns indicate that poorly
ordered structures are formed in metal pillared-ilerites. The N2
adsorption–desorption isotherm of tantalum pillared-ilerite after
complete elimination of organic molecules showed type IV
behavior according to the BDDT classification. At high relative
pressure (P/Po > 0.6), hysteresis is observed owing to capillary
condensation in mesopores, caused by pillaring of the layered
silicates.
Sulfuric acid as a catalyst plays an essential role in the industrial
mass production of e-caprolactam, a raw material for nylon-6.
However, it poses significant environmental and operational
problems such as generating undesirable salt and causing
equipment corrosion. To overcome these problems, a number of
heterogeneous catalysts for the vapor phase Beckmann re-
arrangement of cyclohexanone oxime have been proposed.
Among typical heterogeneous catalysts for this reaction include
solid acid catalysts such as boric acid, silica–alumina, zeolites
like Y, ZSM-5, TS-1, B-ZSM-5 and tantalum oxide on silica.1–4
Highly siliceous ZSM-5 and ZSM-5 modified with boron
showed high activity and selectivity in the Beckmann rearrange-
ment reaction.5 However, the catalysts gradually deactivated
with time. Solid acid catalysts of high activity with long
catalytic lifetime are still under investigation.
Various types of hydroxy groups in amorphous silica or
zeolites have been investigated as catalytically active sites for
the Beckmann rearrangement reaction. A majority of re-
searchers found that neutral or weakly acidic hydroxy groups
present on the external surface of zeolites are effective for the
Beckmann rearrangement reaction, whereas Brönsted acid sites
of zeolites accelerate the formation of by-products.6,7 This is
suggestive that an efficient solid acid catalyst for the rearrange-
ment reaction can be obtained if the catalyst is designed to
include not only a controlled acidity but also a sufficient number
of hydroxy groups.
In our study, an efficient solid acid catalyst for the vapor
phase Beckmann rearrangement reaction is designed using
layered silicates. A small amount of metal was intercalated
between the silicate layers by the usual pillaring process to
generate moderate acidity as a mixed oxide and to increase the
thermal stability of the layered silicate for use in catalysis.
Layered silicates, such as kanemite (NaHSi2O5·3H2O), maga-
diite (Na2Si14O29·11H2O), kenyaite (K2Si20O4·11H2O), maka-
tite (Na2Si4O9·5H2O) and ilerite (Na2Si8O17·xH2O) are com-
posed of tetrahedral silicate sheets only and the each silicate
sheet is terminated by hydroxy groups.8 Layered silicates,
therefore, possess abundant hydroxy groups oriented in a
crystallographically regular manner, and have a great potential
to act as new catalysts for the vapor phase Beckmann
rearrangement reaction.
The BET surface areas of samples calcined at 700 °C are 395
m2 g21 for tantalum pillared-ilerite and 358 m2 g21 for silicon
pillared-ilerite, much larger than those of the H- and Na-ilerites,
(20 and 40 m2 g21, respectively).
The acidic properties of the tantalum pillared-ilerite was
studied to identify the inherent acidity caused by incorporation
of tantalum oxide between the layers of ilerite. In addition, the
acidic properties of silicon pillared-ilerite was investigated to
compare inherent acidities caused by incorporation of Ta or Si
between the layers of ilerite. Acidities were determined by
carrying out NH3-TPD experiments. The TPD profile of the
tantalum pillared-ilerite exhibits a broad desorption peak with a
maximum at 220 °C. On the other hand, the silicon pillared-
ilerite shows no desorption peak. This implies that tantalum
pillared-ilerite contains a large number of acid sites of moderate
acidity, whereas silicon pillared-ilerite shows no acidity.
The Beckmann rearrangement of cyclohexanone oxime was
conducted using a continuous flow reactor under atmospheric
Here, the synthesis, characterization and catalytic application
to the Beckmann rearrangement reaction of layered silicate
catalysts pillared with tantalum oxide is presented.
Na-ilerite (Na2Si8O17·xH2O) was synthesized hydrother-
mally at 100 °C for two weeks from a suspension of Ludox-HS
40 and NaOH solution with a molar ratio of SiO2+NaOH+H2O
= 1+0.5+7.9 H-ilerite was prepared by acid titration of Na-
ilerite. A suspension of Na-ilerite in water was titrated with 0.1
M HCl to a final pH of 2.0 and allowed to stand with stirring for
DOI: 10.1039/b001466o
Chem. Commun., 2000, 829–830
This journal is © The Royal Society of Chemistry 2000
829

Table 1 Vapor phase Beckmann rearrangement of cyclohexanone oxime
over various metal pillared-ileritesa
FTIR spectra were measured for metal pillared-ilerite
samples to elucidate the reaction pathway in detail. FTIR
experiments were conducted using an in situ IR cell (Graseby,
specac). Metal pillared-ilerite pellets were preheated at 550 °C
under vacuum for 1 h. After the samples were cooled to room
temperature, FTIR spectra were recorded [Fig. 1(a)]. One drop
of a 1% ethanol solution of cyclohexanone oxime was added to
the pellet held in the in situ cell under He gas flow. After the
evacuation of physically adsorbed oxime at room temperature
for 1 h, FTIR spectra were recorded at increments of 20 °C
min21 [(Fig. 1(b)–(h)].
The IR spectrum of tantalum pillared-ilerite [Fig. 1(a)] shows
absorption bands of hydroxy groups11 at ca. 3700 cm21
.
However, these bands disappeared upon adsorption of cyclo-
hexanone oxime and new bands appeared at 1450 and 1663
cm21 due to characteristic vibrations12 of CH2 and CNN groups
of cyclohexanone oxime, respectively. This indicates strong
interaction between the hydroxy groups of the catalyst and the
reactant. For the temperatures above 100 °C, the characteristic
vibration modes12 of CNO and N–H of e-caprolactam at 1635
and 1505 cm21 were detected. However, with a stepwise rise of
evacuation temperature, the CNO and N–H absorptions of e-
caprolactam were weakened around 400 °C and completely
disappeared at 500 °C. The desorption temperature of the
produced e-caprolactam is shifted to a higher value than that
observed in the catalytic reaction and might be due to the fast
temperature increase rate in the FTIR experiment. IR spectra of
silicon pillared-ilerite taken at different temperatures were
similar to those of tantalum pillared-ilerite. However, the
intensities of bands at 1635 and 1505 cm21 due to the
characteristic vibrations of CNO and N–H groups of e-
caprolactam were lower than those on tantalum pillared-ilerite,
which implies that silicon pillared-ilerite has a low catalytic
activity.
From the FTIR data, it is seen that the rearrangement to e-
caprolactam from cyclohexanone oxime over the tantalum
pillared-ilerite takes place around 100 °C, while the desorption
of the produced e-caprolactam occurs at a higher temperature
( > 350 °C). Also this data strongly suggests that the hydroxy
groups are active sites as reported for other zeolite cata-
lysts.6,7
In conclusion, we report the synthesis and characterization of
a novel tantalum pillared-ilerite that is shown to be a new
advantageous catalyst for the vapor phase Beckmann rearrange-
ment of cyclohexanone oxime. The excellent catalytic perform-
ance of tantalum pillared-ilerite for the Beckmann rearrange-
ment reaction is ascribed to a large number of well dispersed
hydroxy groups over the interlayer surface of tantalum pillared-
ilerite and their relatively weak acidity.
pressure. Cyclohexanone oxime was dissolved in ethanol and
injected with syringe pump at a WHSV of 0.5 h21 under He
flow with reaction carried out at 350 °C. The product was
analyzed by GC using an SE-54 column.
The results of the rearrangement reaction over tantalum
pillared-ilerite are shown in Table 1. The catalytic activity of
tantalum pillared-ilerite was compared to Silicalite-1 (pure
silica ZSM-5), which is known as the most selective catalyst for
the vapor phase Beckmann rearrangement of oxime to lactam
(Table 1). The tantalum pillared-ilerite showed a higher
conversion of oxime than silicalite-1 while the selectivity for
lactam over tantalum pillared-ilerite was slightly lower than that
over silicalite-1. Also, the data was compared with that of
silicon pillared-ilerite to investigate the effect of the metal as a
pillar. For the tantalum pillared-ilerite catalyst, the cyclohex-
anone oxime conversion rate reaches 97.1% and the selectivity
for e-caprolactam is 89.1% at 350 °C. Tantalum pillared-ilerite
catalyst is superior to the silicon pillared-ilerite in terms of both
conversion and selectivity. Silicon pillared-ilerite shows quite
high conversion and selectivity initially owing to inherent
hydroxy groups from its layered features. However, it is rapidly
deactivated within 8 h, which may be explained by the fact that
it does not possess the moderate acidity required to catalyze the
rearrangement reaction (ammonia TPD).
Although the exact role of the metal is not yet fully
understood, it can be speculated that acidity of hydroxy groups
in the metal pillared-ilerite is controlled by the metal used as
pillar. The acid strength of the assumed species –Ta–O–Si–OH
generated by the pillaring process is adequate enough to
catalyze the rearrangement reaction.
This work was supported by the Korea Institute of Science
and Technology under contract 2E15210 and Brain Pool
Program of the Korean Government.
Notes and references
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Chem. Commun., 2000, 829–830