Cu/SBA-15 is an efficient solvent-free and acid-free catalyst for the rearrangement of benzaldoxime into benzamide

Article abstract of DOI:10.1007/s10562-011-0713-0

Cu/SBA-15 catalysts with various Cu loadings in the range of 5-20 wt% were prepared by an impregnation method and characterized by N₂ adsorption, X-ray diffraction, temperature programmed reduction and X-Ray photoelectron spectroscopic techniques. Cu/SBA-15 catalysts are found to be highly active and selective for the Beckmann rearrangement of benzaldoxime into benzamide under solvent-free and acid-free conditions. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-011-0713-0

Catal Lett (2011) 141:1865–1871
DOI 10.1007/s10562-011-0713-0
Cu/SBA-15 is an Efficient Solvent-Free and Acid-Free Catalyst
for the Rearrangement of Benzaldoxime into Benzamide
•
Ganji Saidulu Narani Anand Kamaraju Seetha Rama Rao
•
•
•
Abhishek Burri Sang-Eon Park David Raju Burri
•
Received: 15 July 2011 / Accepted: 24 September 2011 / Published online: 15 October 2011
Ó Springer Science+Business Media, LLC 2011
Abstract Cu/SBA-15 catalysts with various Cu loadings
in the range of 5–20 wt% were prepared by an impregna-
tion method and characterized by N2 adsorption, X-ray
diffraction, temperature programmed reduction and X-Ray
photoelectron spectroscopic techniques. Cu/SBA-15 cata-
lysts are found to be highly active and selective for the
Beckmann rearrangement of benzaldoxime into benzamide
under solvent-free and acid-free conditions.
impediments in the Beckman rearrangement of aldoximes
into primary amides, the recent efforts in getting consid-
erable conversions and selectivities are worth noting. For
instance, metal catalyzed approach for the rearrangement
of aldoximes into primary amides involving the sequential
dehydration and hydration via nitrile intermediate forma-
tion exhibited substantial improvements [4]. Williams and
his co-workers revealed the successful conversion of al-
doximes into primary amides using Ru and Ir metal com-
plexes [4, 5]. It is proved that Rh complexes are good
candidates for the rearrangement of aldoximes to primary
amides [6–8]. However, Rh, Ir, and Rh containing catalytic
materials are highly expensive and in addition, usage of
these catalysts requires laborious workup procedure to
separate the product. Furthermore, use of environmentally
and biologically harmful solvents is one of the main dif-
ficulties in these chemical transformations.
Keywords Cu/SBA-15 Á Beckmann rearrangement Á
Benzaldoxime Á Benzamide
1 Introduction
Typically, amides are potential precursors for the synthesis
of various natural products and drug intermediates [1]. The
isomerization of oximes to amides (Beckmann rearrange-
ment) is one of the most straightforward synthetic routes
for the production of amides [2]. However, in the Beck-
mann rearrangement, migration of alkyl or aryl group is
facile rather than hydrogen. Hence it is not a general pro-
cess for the transformation of aldoximes into primary
amides [3]. Furthermore, Beckmann rearrangement (BR)
generally requires high reaction temperature and strong
acidic and dehydrating media [3]. Despite of these
In a recent report, solvent-free and acid-free Au/Ag
co-catalysts are disclosed for the effective conversion of
aldoximes to primary amides [9]. However, these Au/Ag
co-catalysts are homogeneous in nature. In one of the
reports, alumina supported rhodium hydroxide catalysts are
used successfully as heterogeneous catalyst in water sol-
vent [10], but rhodium is an expensive metal.
Recently, Williams and coworkers [11] reported a Cu
based catalyst for the rearrangement of aldoximes to pri-
mary amides, in which most of the examples are covered
by different homogeneous Cu2? and Cu1? salts. However,
the study reveals that Cu based catalysts are highly effec-
tive. Off late, design and synthesis of transition metal
active centers on molecular sieves has attracted increasing
attention. SBA-15 is a mesoporous silica material with
G. Saidulu Á N. Anand Á K. S. R. Rao Á D. R. Burri (&)
Catalysis Laboratory, Indian Institute of Chemical Technology,
Hyderabad 500 607, India
e-mail: david@iict.res.in
A. Burri Á S.-E. Park (&)
˚
uniform hexagonal channels ranging from 50 to 300 A,
˚
thick walls (31–64 A), and with higher hydrothermal sta-
Department of Chemistry, Laboratory of Nano-green Catalysis,
Inha University, Incheon 402-751, Korea
e-mail: separk@inha.ac.kr
bility than those of MCM-41 [12, 13]. Because of its
123

1866
G. Saidulu et al.
unique features, SBA-15 has been under intensive inves-
tigation as a support for a variety of active components
[14–22].
TPR system which consists of a quartz reactor placed in a
metal furnace equipped with a temperature programmer
come controller with a k-type thermocouple and a GC with
a thermal conductivity detector connected to the outlet of
the reactor. The data station with standard GC software
permits recording the profiles. About 50 mg of the catalyst
sample was placed at the center of the quartz reactor and
packed in between the two layers of quartz wool. It is
heated linearly at a rate of 5 °C/min from ambient tem-
perature to 700 °C and isothermal conditions were main-
tained for 30 min at 700 °C, while passing the reducing gas
mixture (11% H2 in argon). In between the out let of the
reactor and GC, a molecular sieve trap was placed to
remove the moisture. GC-17A with TCD (M/s. Shimadzu,
Japan) was used for analyzing and recording the profiles.
The surface analysis of the calcined catalysts was made
using a KRATOS Axis 165 photoelectron spectrometer
(XPS) equipped with Mg Ka radiation (1253.6 eV). The
binding energy correction was performed by using the C
1 s peak at 284.6 eV as a reference for all the elements
recorded. The relative intensities of the surface composi-
tion of different elements were corrected with their corre-
sponding atomic sensitivity factors using the vision 2
software in UNIX system.
So far, no heterogeneous catalyst for the complete
conversion of benzaldoxime into benzamide with 100%
selectivity has been reported without using acids or sol-
vents. Hence, herein for the first time, we report a new and
efficient Cu/SBA-15 heterogeneous catalyst.
2 Experimental
2.1 Catalyst Preparation
All the reagents and chemicals were obtained commercially
and used without further purification. The parent mesopor-
ous SBA-15 silica support was synthesized hydrothermally
under acidic medium using triblockco-polymer (P123) as a
template and Tetraethyl orthosilicate (TEOS) as a silica
source according original report of Zhao et al. [12] and our
previous publications [23–27]. To prepare Cu/SBA-15 cat-
alysts, the parent SBA-15 (calcined at 500 °C for 10 h) was
dried at 120 °C for 6 h prior to impregnation. The requisite
amounts of aqueous Cu(NO3)2Á3H2O solution likely to be
around 5, 10, 15, 20 wt% as Cu metal were impregnated.
These catalyst samples were dried at 100 °C for 12 h and
calcined at 450 °C for 6 h. A portion of each catalyst was
reduced at 280 °C for 3 h in the flow of H2 gas using a fixed
bed reactor and denoted as CS-X, where X denotes the
percentage loading of Cu by weight.
2.3 Catalyst Evaluation Test
In a typical catalytic activity test, 1 mmol of benzaldehyde
oxime and 25 mg of catalyst were taken in a 10 ml RB
flask and stirred at 100 °C for 3 h. After completion of
reaction, ethyl acetate was used to separate the products
from the catalyst. The products were identified by GC–MS
(QP-5050 model, M/s. Shimadzu Instruments, Japan)
equipped with DB-5 capillary column (0.32 mm dia. and
25 m long, supplied by M/s. J & W Scientific, USA). The
product analysis was made by a Shimadzu gas chromato-
graph (GC-17A, M/s. Shimadzu Instruments, Japan) using
a OV-1 capillary column (0.53 mm 9 30 m). The sepa-
rated catalyst was washed with ethanol and dried under
vacuum prior to reuse.
2.2 Catalyst Characterization
The X-ray diffraction (XRD) patterns were recorded at
room temperature using a Rigaku, Multiflex, diffractometer
with a nickel filtered CuKa radiation of wave length
˚
1.5418 A at a power of 40 kW and a current of 100 mA in
the 2h range of 0.5–58 and 5–808 for the verification of
SBA-15 structural ordering and Cu phase behavior
respectively. Low and wide angle XRD data were obtained
for unreduced and reduced CS-X catalysts respectively.
N2 adsorption–desorption isotherms were recorded for
calcined catalysts using a Tristar 3000 V 6.08A instrument
(M/S Micromeritics Instruments Corporation, USA) at
-196 °C. The samples were outgassed at 300 °C for 4 h
before the measurement. BET method was used to calcu-
late the surface areas using the amount of N2 adsorbed at
-196 °C. BJH method was used to calculate the pore-size
distribution considering the desorption branch of the iso-
therm. The pore volume was taken at a relative pressure
P/P0 = 0.989 (single point).
3 Results and Discussion
3.1 Catalyst Characterization
To assess the structural integrity of the catalysts, small-
angle XRD patterns of CS-X catalysts including parent
SBA-15 were recorded (patterns not shown). The parent
SBA-15 exhibited well-resolved diffraction peaks with a
sharp peak at about 0.89° and two weak peaks at about
1.56° and 1.80° on the 2h scale. Similar to that of parent
silica SBA-15 the calcined CS-X catalysts also exhibited
Temperature programmed reduction (TPR) studies of all
the calcined catalysts were performed on a home-made
123

Rearrangement of Benzaldoxime into Benzamide
1867
Table 1 Physico-chemical
Characteristics
SBA-15
CS-5
CS-10
CS-15
CS-20
characteristics of CS-X catalysts
and SBA-15 support
BET surface area (m2/g)
Micropore area (m2/g)
500
62
391
41
378
301
244
31
39
37
Total pore volume (cm3/g)
Micropore volume (cm3/g)
BJH adsorption volume (cm3/g)
BJH desorption pore dia. (nm)
XRD d(100)
0.75
0.03
0.77
4.10
9.90
11.43
7.34
–
0.69
0.65
0.02
0.66
4.72
9.62
11.1
6.38
30.81
0.103
0.025
0.53
0.02
0.53
4.88
9.71
11.2
6.43
34.54
0.159
0.042
0.58
0.02
0.70
4.84
9.44
0.01
0.58
6.39
9.53
Unit cell length (nm)
10.9
11.01
4.62
Pore-wall thickness (nm)
Cu crystallite Size (nm)a
Composition (Cu/Si)b
6.06
25.02
0.045
0.013
39.55
0.175
0.055
a
b
c
Determined by XRD
Determined by AAS
Determined by XPS
–
Surface composition (Cu/Si)c
–
three diffraction peaks positioned at 0.91–0.92°,
1.59–1.60°, and 1.83–1.85° on the 2h scale, revealing the
retention of mesoporous structural ordering. The position
of diffraction peaks are well matched with the reported
XRD patterns of SBA-15 [12]. However, the intensities of
the peaks are found to decrease with increase in Cu load-
ing. The estimated unit cell lengths for the calcined CS-X
catalysts are shown in Table 1, which are close or a little
lower compared to parent SBA-15 sample. The lower unit
cell lengths ruled out the incorporation of Cu in the silica
framework. In other words, Cu phase may be dispersed on
the surface of SBA-15 support.
The N2 adsorption–desorption isotherms of parent SBA-
15 and Cu/SBA-15 catalysts were generated at -196 °C.
The isotherms of Cu/SBA-15 exhibit type IV isotherms
with H1 hysteresis loops corresponding to the filling of
uniform mesopores with open cylindrical geometry [28].
However, the amount of N2 adsorbed is less in the iso-
therms of CS-X catalysts compared to the isotherms of
parent SBA-15. The phenomenon of low amount of N2
adsorption is an indication of lowering in the surface area
and pore volume. With increase in Cu loading, the average
pore diameter increases, which is due to the blockage of
smaller pores or the porous nature of Cu. Since the pore-
wall thickness is calculated from the difference between
unit cell length and average pore diameter, decrease in
pore-wall thickness with increase in Cu loading is obvious
as there is an increment in the average pore diameter. The
pore-size distribution curves (not shown) indicate that most
of pores are distributed in the range of 5–8 nm.
Fig. 1 XRD patterns of CS-X catalysts
characteristic of Cu (JCPDS 04-0836), which indicates that
the presence of metallic Cu in all the CS-X catalysts. It is
also observed two low intensity peaks corresponding Cu2?
.
Compared to diffraction peak intensity of Cu, the peak
intensity of Cu2? is very small. The gradual increase in the
peak intensity of metallic Cu (Fig. 1) and its crystallite
sizes (Table 1) with increase in Cu loading from 5 to
20 wt% Cu on SBA-15 is observed.
The reducibility of CuO/SBA-15 catalysts has been
measured by H2-TPR and the TPR profiles are depicted in
Fig. 2. The CS-5 gets reduced in two stages in the tem-
perature range of 200–320 °C. It is reported that the
reduction of dispersed CuO species to Cu1? species takes
place in low temperature region (below 300 °C) [29–31].
The high temperature signal in the temperature range of
320–480 °C can be attributed to the reduction of strongly
interacted CuO species with the support to Cu0 species or
due to reduction of Cu1?species to Cu0 species. Similar
kind of two-stage reduction of CuO is observed for CS-10
catalyst [32]. The low temperature peak maximum that
observed for the low CuO loading catalysts was not seen in
The CS-X catalysts were characterized by wide angle
XRD to determine the crystalline behavior and the sizes of
crystallites. The XRD patterns are displayed in Fig. 1 and
the crystallite sizes are depicted in Table 1. Figure 1 dis-
plays a strong diffraction peak at 2h of 43.48, followed by
two more intense diffraction peaks at 50.5 and 74.18
123

1868
G. Saidulu et al.
pores which results in lowering the BET surface area of the
catalyst (244 m2/g).
3.2 Catalytic Activity
The catalytic activity of CS-X catalysts for the transfor-
mation of benzaldoxime (oxime) into benzamide (amide)
through BR without using any acids or solvents has been
conducted in the liquid phase at atmospheric pressure. As it
is reported by several authors [6–11], benzaldoxime
transforms into benzonitrile (nitrile), benzaldehyde (alde-
hyde) and benzamide, of which benzamide is the main
(desired) product. Indeed, the transformation of ben-
zaldoxime into benzamide is a two-step process, wherein,
the selective dehydration of benzaldoxime leads to ben-
zonitrile, which on hydrolysis converts into benzamide.
Benzaldoxime also converts into benzaldehyde through
elimination of hydroxylamine followed by hydrolysis.
To attest the benzaldoxime rearrangement is a catalytic
process, an experiment was conducted without adding the
catalyst to the reaction mixture, no appreciable conversion
is observed (Table 2, run 1), which proves the particular
process is catalytic. In the next experiment SBA-15 support
was used as a catalyst and found that SBA-15 is capable of
converting benzaldoxime into benzonitrile and benzalde-
hyde but not into the desired benzamide product (Table 2,
run 2). To distinguish the catalytic activity of CuO and Cu
metal, two different experiments were conducted with and
without catalyst reduction (Table 2, run 3 and 4). The Cu
metal obtained through H2 reduction from the same bulk
CuO exhibited superior activity (Table 2, run 4), compared
to bulk unreduced CuO catalyst (Table 2, run 3). The
catalytic activity of Cu metal is higher not only in the
conversion of benzaldoxime but also in the selectivity of
benzamide. Similar trend is observed even in the case of
supported catalysts (Table 2, run 5 and 6).
Fig. 2 TPR of CuO/SBA-15 catalysts
the profiles of higher loading catalysts. It was observed
only one high temperature peak in the temperature range of
300–500 °C (CuO ? H2 ? Cu ? H2O). This peak may be
corresponding to strongly interacted CuO species with the
support [33]. However, there is a gradual shift in the high
temperature peak maximum with increase in CuO loading.
The high temperature peak maximum for CS-5 is at
400 °C, whereas for CS-20 the peak maximum is at
450 °C. Since the crystallite size (Table 1) of CS-20
(40 nm) is not too large compared to that of CS-5 (25 nm)
one can expects an interaction between CuO and support
which results in yielding a high temperature signal in the
TPR pattern.
The surface and bulk composition of CS-X catalysts were
determined by X-ray photoelectron spectroscope (XPS) and
atomic absorption spectroscopy (AAS) respectively and the
results were depicted in Table 1, wherein, the increase in Cu
loading on SBA-15 similar to that of total composition of Cu
from CS-5 to CS-20 (AAS) is observed. The lower Cu/Si
ratio (surface composition) of CS-20 catalyst as obtained by
XPS (Table 1) indicates presence of larger Cu-particles on
the surface of SBA-15 support. Because of these Cu-sites
whose size (35 nm) is larger than the average pore diameter
(*5 nm) of SBA-15, one can expect the blockage of surface
To find out the influence of water on the catalytic activity
of CS-20 catalyst in the rearrangement of benzaldoxime into
benzamide, two experiments were conducted. In the first
Table 2 Transformation of
benzaldoxime on different
catalysts
Catalyst
Run
no.
%Conv. of
benzaldoxime
%Sel. of
benzonitrile
%Sel. of
benzaldehyde
%Sel. of
benzamide
No catalyst
1
2
3
4
5
6
7
8
9
3.9
24.1
64.7
84.6
80.1
100.0
91.7
89.4
72.8
0.0
19.9
13.3
4.0
100.0
80.1
20.9
8.6
0.0
0.0
SBA-15
CuO (unreduced)
Cu (reduced)
74.8
87.4
70.3
100.0
74.1
96.0
92.0
CS-20 (unreduced)
CS-20 (reduced)
CS-20 (with H2O)
CS-20 (anhydrous)
CSG-20 (reduced)a
11.7
0.0
18.0
0.0
Reaction conditions:
Oxime = 1 mmol, catalyst
weight = 25 mg,
temperature = 100 °C,
pressure = 1 atm, time = 3 h
8.9
17.0
4.0
0.0
3.2
4.8
a
20 wt% Cu/Silicagel
123

Rearrangement of Benzaldoxime into Benzamide
1869
experiment 1 mmol of water was added to the reaction
mixture, i.e., benzaldoxime: water = 1:1, wherein, both the
conversion and the selectivity have been decreased, but not
so significantly. However, the activity of the catalyst is
lower compared to run 6, Table 2. In the second experiment
the molecular sieves were added to scavenge the water that
produced due to dehydration of oxime into nitrile. In the
conversion point of view, the influence of water is not so
significant, whereas, the selectivity of amide decreased
considerably in the presence of water, in its absence the
selectivity is 96% (Table 2, run 8), which is close to 100%
as in run 6, Table 2. While conducting the experiment using
the reduced CS-20 catalyst, N2 gas was used to prevent the
oxidation of Cu metal, where both the conversion and the
selectivity were almost 100% (Table 2 run 6), which reveals
that the transformation of benzaldehyde into benzamide is
expected to takes place via benzaldoxime rather than ben-
zoic acid. The conversion of alcohols and aldehydes into
amides via oximes has recently been reported [5, 34] using
iridium and palladium catalysts.
Fig. 3 Influence of Cu loading amount on SBA-15 for the transfor-
mation of benzaldoxime
10 min, three different products (benzamide, benzonitrile
and benzaldehyde) were formed. It seems that the isom-
erization of benzaldoxime proceeds through benzonitrile
and benzaldehyde intermediates. To confirm this observa-
tion, different experiments were conducted for a period of
20, 60 and 120 min. With increase in reaction time, both
the conversion of benzaldoxime and the selectivity of
benzamide were enhanced. When the reaction was con-
ducted for 180 min, both the conversion of benzaldoxime
and the selectivity of benzamide attained 100% (Fig. 4).
The results reveal that at low loading catalysts and at low
reaction timings the benzonitrile and benzaldehyde inter-
mediates are more, which gradually converts into amide.
The activity of CS-20 catalyst towards the rearrange-
ment of benzaldoxime into benzamide has been investi-
gated using 12.5, 25, 35 and 50 mg catalyst. When
12.5 mg catalyst is used the conversion of benzaldoxime
is 96% with the benzamide selectivity of 79%. At and
above 25 mg catalyst the conversion of benzaldoxime is
nearly 100% with high selectivity of benzamide, which
reveals that there are no considerable mass transport
limitations.
To attest role of mesopores in the conversion of ben-
zaldoxime to benzamide calcined silicagel (184 m2/g BET
surface area) supported copper (20 wt%) catalyst was
prepared by impregnation similar to that of CS-20 catalyst.
The surface area of CSG-20 is 155 m2/g. By using this
catalyst the activity was determined and the data was
depicted in Table 2, which implies that the porosity of the
support plays a prominent role in the conversion of ben-
zaldoxime and benzamide selectivity.
In general water present in the reaction mixture, inhibit
the process of dehydration of aldoximes to benzonitrile to a
greater extent. Unavailability of water in the reaction
mixture ceases at the stage of benzonitrile, its further
transformation into benzamide does not occur. The present
study elucidate that the influence of water on the activity is
negligible.
SBA-15 supported Cu metal catalysts with various Cu
loadings were studied to verify the influence of support and
the effect of Cu loading on the SBA-15 support for the
rearrangement of benzaldoxime, and the results obtained
are depicted in Fig. 3, which implies that Cu is capable of
catalyzing benzaldoxime into benzamide in a single step
even at a low loadings of Cu, i.e., CS-1. When the loading
of Cu increased from 1 to 2.5%, there is a significant
increment both in conversion and selectivity (Fig. 3). At
and above 5% Cu loading catalysts, the conversion attained
almost 100%. When CS-20 is used as a catalyst complete
transformation of benzaldoxime into benzamide occurred.
To understand the nature of reaction, different experi-
ments were conducted using CS-20 as a catalyst by varying
the period of reaction time keeping the other reaction
conditions constant and the data obtained were presented as
Fig. 4. When the reaction was conducted for a period of
Catalyst reusability is studied using CS-20 catalyst in 4
repeated cycles. The conversion of aldoximes over all the 4
cycles is nearly 100%, whereas the selectivity of benz-
amide decreased from cycle-1 to cycle-4. The reason for
decrease in the selectivity of benzamide in repeated cycles
is due to the presence of unconverted intermediate products
like nitriles and aldehyde. In other words the catalyst lost
some of its activity towards the conversion of intermediate
products (nitrile and aldehyde) into desired benzamide
product due to partial oxidation of Cu metal into CuOx
from cycle-1 to cycle-2 etc. Omission of reduction step in
the subsequent cycles leads to deactivation of the catalyst.
No deactivation of the catalyst is observed when the cat-
alyst is subjected to reduction in each repeated cycle.
123

1870
G. Saidulu et al.
nitrile intermediate is present in the product mixture [11].
In the present study, conversion and selectivity are nearly
100% over CS-20 catalyst. The present study implies that
the reaction is occurring through metal catalyzed approach
similar that of late transition metal catalyzed process rather
than the conventional Beckmann rearrangement. Based on
our catalytic results and literary reports a mechanism is
proposed (Scheme 1). In the proposed mechanism, ben-
zonitrile transforms into benzamide by abstracting the
required water molecule from the benzaldoxime leaving a
new benzonitrile intermediate, there by maximizing the
selectivity of benzamide.
4 Conclusions
Fig. 4 Effect of reaction time on the catalytic activity of CS-X
catalysts
CS-X is a highly efficient catalyst for the rearrangement of
benzaldoxime into benzamide selectively in the liquid
phase even in the absence of any acid or solvent. Isolated
distribution of Cu species on the surface of SBA-15 sup-
port, minimized diffusion limitations and rearrangement
through metal catalyzed transformation may be responsible
for the superior activity of this catalyst. CS-X is a first
example of inexpensive, highly efficient and eco-friendly
heterogeneous catalyst.
Acknowledgments The authors Mr. G. Saidulu and N. Anand
acknowledge the Council of Scientific and Industrial Research, New
Delhi for the award of research fellowship.
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