Niobium doped hexagonal mesoporous silica (HMS-X) catalyst for vapor phase Beckmann rearrangement reaction

Article abstract of DOI:10.1039/c3ra41840e

The synthesis of ε-caprolactam, a demanding monomer, through a heterogeneous catalytic pathway has remained a key area of research in the last decade. The Beckmann rearrangement reaction in the vapor phase using a solid acid catalyst was found to be very effective for producing ε-caprolactam. It is observed that niobium incorporated into mesoporous silica serves as a good catalyst for the Beckmann rearrangement reaction. Recently developed mesoporous silica, having an ordered honeycomb structure, is very useful as it can lead to effective diffusion of reactants and products for several reactions. In this study, Nb-doped mesoporous HMS-X nanocomposite materials with different Nb loadings were prepared by a one step hydrothermal synthesis and characterized by BET surface area and porosity measurements, wide and small angle XRD, SEM, HR-TEM, elemental mapping, FTIR, ²⁹Si-NMR and NH₃-TPD techniques. The activity of the Nb-HMS-X catalyst was evaluated for the vapour phase Beckmann rearrangement reaction. The catalyst characterization study shows that Nb is highly dispersed on the HMS-X matrix at lower Nb loadings. At higher Nb loadings it is present in the extra-framework position as revealed from XRD, HR-TEM and ²⁹Si-NMR studies. The NH₃-TPD result shows the presence of acidic sites on the catalyst surface, which are active sites for the Beckmann rearrangement reaction. Using the Nb-HMS-X catalyst (Si/Nb = 13) under vapour-phase reaction conditions [temperature = 350 °C, weight hourly space velocity (WHSV) = 15 h^-1, cyclohexanone oxime in benzene, cyclohexanone oxime:benzene weight ratio of 1:11] gave 100% cyclohexanone oxime conversion with 100% ε-caprolactam selectivity, with a space time yield of 1.4-1.6 × 10^-3 mol h^-1 gcat^-1. The catalyst was highly recyclable up to 9 times without significant loss of catalytic activity. The Royal Society of Chemistry 2014.

Full text of DOI:10.1039/c3ra41840e

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Niobium doped hexagonal mesoporous silica
HMS-X) catalyst for vapor phase Beckmann
rearrangement reaction†
(
Cite this: RSC Adv., 2014, 4, 845
a
a
a
b
Sandip Mandal, Chiranjit Santra, Rawesh Kumar, Malay Pramanik,
a
b
c
d
Sumbul Rahman, Asim Bhaumik,* Sudip Maity, Debasis Sen
and Biswajit Chowdhury*a
The synthesis of 3-caprolactam, a demanding monomer, through a heterogeneous catalytic pathway has
remained a key area of research in the last decade. The Beckmann rearrangement reaction in the vapor
phase using a solid acid catalyst was found to be very effective for producing 3-caprolactam. It is
observed that niobium incorporated into mesoporous silica serves as a good catalyst for the Beckmann
rearrangement reaction. Recently developed mesoporous silica, having an ordered honeycomb
structure, is very useful as it can lead to effective diffusion of reactants and products for several
reactions. In this study, Nb-doped mesoporous HMS-X nanocomposite materials with different Nb
loadings were prepared by a one step hydrothermal synthesis and characterized by BET surface area and
29
porosity measurements, wide and small angle XRD, SEM, HR-TEM, elemental mapping, FTIR, Si-NMR
and NH -TPD techniques. The activity of the Nb–HMS-X catalyst was evaluated for the vapour phase
3
Beckmann rearrangement reaction. The catalyst characterization study shows that Nb is highly dispersed
on the HMS-X matrix at lower Nb loadings. At higher Nb loadings it is present in the extra-framework
29
3
position as revealed from XRD, HR-TEM and Si-NMR studies. The NH -TPD result shows the presence
of acidic sites on the catalyst surface, which are active sites for the Beckmann rearrangement reaction.
Using the Nb–HMS-X catalyst (Si/Nb ¼ 13) under vapour-phase reaction conditions [temperature ¼
Received 17th April 2013
Accepted 18th September 2013
ꢁ1
ꢀ
3
50 C, weight hourly space velocity (WHSV) ¼ 15 h , cyclohexanone oxime in benzene, cyclohexanone
oxime : benzene weight ratio of 1 : 11] gave 100% cyclohexanone oxime conversion with 100%
DOI: 10.1039/c3ra41840e
ꢁ3
ꢁ1
ꢁ1
3
-caprolactam selectivity, with a space time yield of 1.4–1.6 ꢂ 10 mol h gcat . The catalyst was
www.rsc.org/advances
highly recyclable up to 9 times without significant loss of catalytic activity.
enormous production of ammonium sulphate, as high as
1
Introduction
2
1
.9 tons per ton of product, causing some ecological concerns in
The Beckmann rearrangement of cyclohexanone oxime to addition to an economical drawback. Starting with the optimi-
-caprolactam is an important process in the production of zation and reengineering of the old reaction pathways, important
3
polyamide 6, better known under the trade name ‘nylon 6’. Its producers nowadays focus their efforts on new synthetic routes. A
monomer, 3-caprolactam, is one of the y most important bulk solution to the problem, even for the world scale plants, could be
chemicals in the world, with an estimated annual production of to change from homogeneous to heterogeneous catalysis,
1
more than 4.4 million tonnes worldwide. The standard catalyst wherein, due to vapour phase reactions, the catalytic material
3
used in industrial plants for the oxidation and rearrangement is need not be separated and can be used aer recycling.
fuming sulphuric acid, where ammonia is used to neutralize the
Metal oxides can be used as catalysts, alone or in combination
product mixture. The disadvantage of this procedure is the with other materials during vapour phase catalytic reactions. It is
believed that a mixture of oxides brings out a combined effect or
4
a sort of synergistic behaviour enhancing the catalytic activity.
a
Department of Applied Chemistry, Indian School of Mines, Dhanbad, India. E-mail: Early studies showed that such diverse materials as tungsten
biswajit_chem2003@yahoo.com; Fax: +91-326-2296563; Tel: +91-326-2235663
b
5
6
oxide, and tantalum oxide are active for vapour phase Beck-
mann rearrangement. Recently Mao et al. reported B /TiO
Department of Material Science, Indian Association for Cultivation of Science,
O
2 3
2
–
Jadavpur, Kolkata, India
ZrO as an effective catalyst for vapour phase Beckmann rear-
2
c
Central Institute of Mining and Fuel Research, Dhanbad, India
7
rangement, suggesting that doped oxide catalysts are more
d
Solid State Physics Division, Bhabha Atomic Research Center, India
promising in catalytic reactions than pure metal oxides. Guo
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c3ra41840e
8
et al. demonstrated that silica-pillared layered niobic acid can be
1
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 845–854 | 845

View Article Online
RSC Advances
Paper
ꢀ
an efficient catalyst for the vapour phase Beckmann rearrange- was added and the whole mixture was stirred for 7 h at 120 C
ment reaction in the presence of 1-hexanol.
under Ar atmosphere. Aer complete stirring the material was
Aer the discovery of mesoporous silicious and non-silicious ltered and washed with anhydrous toluene along with anhy-
groups of materials, several catalytic systems were reported for drous ethanol.
9
–12
the synthesis of ne chemicals.
The report by Sch u¨ th et al.
13
showed that acid treated SBA-15 had an activity to produce
-caprolactam from cyclohexanone oxime and revealed the
2.2 Catalyst characterization
3
2
.2.1 BET surface area and porosity measurements. The
necessity of acidic sites for the reaction. The incorporation of
niobium into mesoporous silica has been widely reported for
Beckmann rearrangement, where more acidity can be generated
BET surface area and pore size analysis of the samples was
measured at liquid nitrogen temperature with a Quantachrome
Autosorb-1C-TPD at 77 K. Pre-treatment of the samples was
done at 473 K for 3 h under high vacuum. The surface area was
determined by the Brunauer–Emmett–Teller (BET) equation.
Pore size distributions were calculated using the NLDFT (Non-
linear Density Functional Theory) model of cylindrical pore
approximation.
14,15
resulting in better catalytic activity.
The recently developed ordered mesoporous material with a
16
hexagonal honeycomb structure and high thermal stability is
used for versatile catalytic applications. In this study we
assumed that incorporation of niobium into the mesoporous
silica framework would generate more acidic sites and might
result in better catalytic activity towards the Beckman rear-
2.2.2 X-ray diffraction (XRD). The small and wide angle
X-ray diffraction (SWAX) analysis was carried out by using a
Bruker AXS D-8 advance diffractometer operated at a 40 kV
voltage and 40 mA current and calibrated with a standard
silicon sample, using Ni-ltered Cu K
radiation.
14,17
rangement reaction.
We prepared Nb-doped HMS-X mate-
rials with three different Si/Nb ratios and characterized them by
BET surface area and porosity measurements, XRD, SEM, HR-
(l ¼ 0.15406 nm)
2
9
a
3
TEM, FTIR, elemental mapping, Si-NMR and NH -TPD tech-
niques. The catalytic activity for the Beckmann rearrangement
reaction was determined in a xed bed reactor operating at
atmospheric pressure. The catalyst characterization results have
been correlated with the activity studies, where the optimum
catalyst composition for a higher space time yield of 3-capro-
lactam was obtained.
2.2.3 Scanning electron microscopy (SEM). The SEM
images were obtained from a Hitachi S-3400N. The powder
samples were supported on lacey carbon and then coated with
gold by plasma treatment prior to measurement.
2.2.4 High resolution transmission electron microscopy
(
HR-TEM) and elemental mapping. The HR-TEM investigation
was done on a JEOL JEM 2100 microscope operated at a 200 kV
acceleration voltage using a lacey carbon coated Cu grid of 300
mesh size. The STEM-EDS used was from Oxford Instruments
2
Experimental
2.1 Catalyst preparation
(
model x-sight).
Hexagonally ordered mesoporous Nb–HMS-X nanocomposite
materials were synthesized by using triblock copolymer as a
template under acidic conditions. In this synthetic procedure,
Pluronic P123 (Aldrich) (4 g) was dissolved in 150 mL distilled
water and 7–8 g 35% HCl (Merck) by continuous stirring. Aer 4 h
of stirring a clear solution was obtained and then 4 g n-butanol
2.2.5 Fourier transform infrared spectroscopy (FTIR). The
FTIR measurements were carried out using a Perkin Elmer GX
spectrophotometer. The spectra were recorded in the range
4
ꢁ1
00–4000 cm using a KBr pellet.
2
.2.6 Magic-angle spinning nuclear magnetic resonance
2
9
(
MAS NMR). Si MAS NMR was recorded at 500 MHz on a
(Merck) was added to this solution and the stirring continued for
Bruker advanced II-500 spectrometer equipped with a magic
angle spin probe at room temperature.
1
h. Then tetraethyl orthosilicate (8.4 g) (TEOS, Acros) and an
ethanolic solution of niobium pentachloride (0.81 g) (Aldrich)
were added to this solution. The resulting gel composition of
2
.2.7 Temperature programmed desorption (TPD). TPD
proles of the samples were recorded with a ChemiSorb 2720
Micrometrics, USA) equipped with a thermal conductivity
the mixture was P123 : H
2
O : HCl : n-butanol : TEOS : NbCl
5
¼
(
0.017 : 200 : 5.4 : 1.325 : 1 : 0.075 (molar ratio) and it was stirred
detector (TCD). The TPD was carried out by rst heating under
He ow from room temperature to 350 C for 30 minutes. The
dried catalysts were cooled to ambient temperature. The NH
TPD prole was obtained by a temperature programme under
for 24 h at room temperature. Then the mixture was taken in a
ꢀ
ꢀ
closed polypropylene bottle and aged at 100 C for 24 h under
3
-
static hydrothermal conditions. Aer hydrothermal treatment
the material was ltered in hot conditions without washing and
ꢁ
1
4
% NH3 in He with a ow rate of 20 mL min raising the
ꢀ
then dried at 100 C for 12 h in air, followed by calcination at
ꢀ
ꢀ
ꢁ1
temperature from ambient to 1000 C at a rate of 10 C min
The amount of NH desorbed in the TPD peak area was evalu-
ated by pulses of NH in a ow of He.
3
.
ꢀ
5
40 C for 24 h. The nal material obtained aer calcination
3
was stored in a vacuum desiccator. The gel composition and
thermal treatment of the different samples are shown in
Table S1 (ESI†).
2
.3 Catalytic activity for the Beckmann rearrangement
The Nb–HMS-X was silylated following the procedure
18
reaction
reported in the literature. Before silylation, the Nb–HMS-X
ꢀ
material was dried at 200 C under vacuum. Aer vacuum The catalytic reaction was carried out in a xed bed catalytic
drying, 1 g Nb–HMS-X was suspended in 15 mL anhydrous reactor (quartz made, 1.5 cm inner diameter). The catalyst, Nb–
toluene. Then 0.652 g hexamethyldisilazane (HMDS, Aldrich) HMS-X (85 mesh size), was packed into the reactor and then pre-
846 | RSC Adv., 2014, 4, 845–854
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
treatment was carried out by owing air (moisture free, 30 mL [9.5 nm for HMS-X and 9.3 nm for Nb–HMS-X (Si/Nb ¼ 13)].
ꢁ
1
ꢀ
min ) at 500 C for 5 h. Aer pre-treatment the reactor was This suggests that niobium occupies some of the Si atom sites of
cooled to the desired reaction temperature and then N gas the porous architecture of HMS-X. Upon increasing the Nb
2
ꢁ1
(
purity 99.999%, 30 mL min ) was passed through the catalyst loading, a not sharp but steep inection at a lower pressure range
bed for 15 min. To prepare the feed mixture, cyclohexanone was observed in the case of Nb–HMS-X (Si/Nb ¼ 3 or 2), together
oxime (CHO, Acros) was dissolved in anhydrous benzene with a gradual decrease in the surface area and pore diameter.
(
CHO : benzene weight ratio of 1 : 11). The solution was injec- This indicates the formation of niobium oxide species at the
14
ted into the reactor by a piston pump (B/BRAUN) along with N
(
2
surface of the mesopores for the Nb-rich samples.
3.1.2 X-ray diffraction. The small angle XRD patterns of the
ꢁ
1
purity 99.999%) as a carrier gas (30 mL min ). The reactor
outlet was connected to a cooling trap, which was immersed Nb–HMS-X catalysts with different Nb loadings are shown in
into a salt–ice freezing mixture. The reactor effluent was Fig. 2. The XRD patterns show excellent structural order with
collected at different time intervals and analyzed by a gas
chromatograph (CIC, India) equipped with a ame ionization
detector and SE-30 column.
3
Results and discussion
3.1 Catalyst characterization
3.1.1 Nitrogen sorption isotherms. The BET surface area
and porosity measurement results are depicted in Fig. 1, and
show a decrease in surface area, pore volume and pore diameter
with an increase in Nb loading on the HMS-X matrix. The N
2
adsorption isotherm for the calcined siliceous HMS-X showed a
sharp inection at a relative pressure in the range of 0.6–0.75,
19
indicative of a highly mesoporous material. The shape of the N
2
adsorption isotherm of the Nb–HMS-X (Si/Nb ¼ 13) sample was
similar to that of the original HMS-X except for some reduction
in the height of the hysteresis loop and the surface area, and the
Fig. 2 Low angle XRD patterns of HMS-X and Nb–HMS-X catalysts
pore diameter remained almost the same as it is in HMS-X with different Nb loadings.
Fig. 1 Surface areas, pore volumes and pore dimensions of (a) Nb–HMS-X (Si/Nb ¼ 2), (b) Nb–HMS-X (Si/Nb ¼ 3), (c) Nb–HMS-X (Si/Nb ¼ 13) and
(
d) HMS-X only.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 845–854 | 847

View Article Online
RSC Advances
Paper
highly ordered hexagonal pore apertures for HMS-X and Nb–
20
HMS-X (Si/Nb ¼ 13). The structural ordering of HMS-X and
Nb–HMS-X (Si/Nb ¼ 13) was conrmed by the presence of basal
ꢀ
peaks corresponding to (100) at 2q ¼ about 1 with several
higher order reections corresponding to (110), (200), (210) in
ꢀ
19,21,22
the region 2q ¼ 1.5–2 .
Here we have used a Fourier-based
deconvolution/convolution method to determine the position of
the peak maximum. The unit cell values are determined through
REFLEX and CELSIZ unit cell tting programs. As the level of Nb
metal loading was increased up to (Si/Nb ¼ 2 or 3), the basal
diffraction peak (110) of the HMS-X material disappeared and
the peak at the lower 2q value broadens compared with the
undoped siliceous HMS-X and the Nb–HMS-X (Si/Nb ¼ 13)
materials. This decrease in basal plane spacing upon higher Nb
loading compared to the Nb–HMS-X (Si/Nb ¼ 13) analogue
conrms the presence of niobium on the outside of the silicate
framework. From HMS-X to Nb–HMS-X (Si/Nb ¼ 13), the Nb
atoms are incorporated inside the porous silicate framework by
Fig. 3 Wide angle XRD patterns of HMS-X and Nb–HMS-X catalysts
with different Nb loadings.
replacing the structural Si atoms of the porous architecture. HMS-X material with the lower Nb loading, the absence of
Hence, due to the large atomic size of Nb compared to Si, the d diffraction at higher 2q values, corresponding to niobium oxide,
spacing (which corresponds to the (100) plane) increases with indicated that niobium is present either well dispersed with less
the increment of the pore wall thickness (D). But when the than 2 nm size in the framework of HMS-X or attained as an
1
4
concentration of niobium increases to a very high level (Si/Nb ¼ XRD amorphous form outside the framework.
2
or 3), Nb is no longer incorporated into the mesoporous sili-
3.1.3 SEM and HR-TEM micrographs. SEM analysis was
cate framework and is rather deposited outside the porous carried out for all the catalysts and the SEM images are depicted
framework of the material. This is reected in the lower value of in Fig. 4. An interconnected network, with particles of similar
d (interplanar spacing) and higher pore wall thickness (D). The size (ꢃ2–3 mm) with a wormlike morphology, was observed in
results of the small angle powder XRD patterns of the Nb–HMS- the Nb–HMS-X (Si/Nb ¼ 13) catalyst, as depicted in Fig. 4(a). The
X samples are shown in Table 1. The Nb–HMS-X (Si/Nb ¼ 13) presence of interconnections between the hexagonal pore
sample showed the highest Nb incorporation as its unit cell is channels could be observed clearly. However, the shape of the
much higher than the other samples. For the Nb–HMS-X (Si/ particles in the Nb doped HMS-X materials depends on the Nb/
Nb ¼ 2 or 3) catalysts, the hexagonal order has been lost and the Si ratio during synthesis. Fig. 4(b) and (c) imply that on
wall thickness increases gradually with increasing Nb concen- increasing Nb loading the shape of the particles becomes much
tration. These results also support the BET analysis and HR- more irregular.
TEM analysis. For the HMS-X and Nb–HMS-X (Si/Nb ¼ 13)
The HR-TEM images of the Nb–HMS-X catalysts are depicted
samples, both the materials have type-IV isotherms with a in Fig. 5. They show an ordered hexagonal structure, which is
strong capillary uptake for the adsorbed gas at relatively high highly crystalline in nature. The parallel layer by layer assem-
pressure (0.6–0.75), suggesting the presence of ordered meso- blies are observed in the higher magnication HR-TEM images.
pores in the framework, but for the Nb–HMS-X (Si/Nb ¼ 2 or 3) These observations agree with the N
-adsorption and XRD
2
23
materials no such capillary condensation is observed and this results described earlier. With Si/Nb ¼ 2 loading, the dark
conrms that the hexagonal ordering of the pores is disturbed in spots observed at higher magnication might be due to the
the Nb–HMS-X (Si/Nb ¼ 2 or 3) samples.
2 5
presence of Nb O on the framework of the silica matrix. As
The wide angle XRD patterns of the Nb–HMS-X materials observed from the elemental mapping (Fig. 6) of a selected
with different Nb loadings are depicted in Fig. 3. For the Nb– region for the three different Nb–HMS-X materials, the Nb
Table 1 Quantitative analysis of the small angle XRD patterns of the Nb–HMS-X catalysts
2D-hexagonal planes
(
100)
(110)
(200)
(210)
a
Catalyst
2q
d
2q
d
2q
d
2q
d
a (nm)
Wall thickness (D) (nm)
HMS-X
0.87
0.82
0.86
0.89
10.15
10.77
10.39
9.93
1.54
1.43
—
5.737
6.178
—
1.76
1.64
—
5.08
5.39
—
2.38
2.12
—
3.712
4.167
—
11.72
12.43
11.99
11.46
2.22
3.13
3.99
5.76
Nb–HMS-X (Si/Nb ¼ 13)
Nb–HMS-X (Si/Nb ¼ 3)
Nb–HMS-X (Si/Nb ¼ 2)
—
—
—
—
—
—
a
Wall thickness (D) ¼ (2d100/O3) ꢁ pore diameter.
848 | RSC Adv., 2014, 4, 845–854
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
Fig. 4 SEM images of the (a) Nb–HMS-X (Si/Nb ¼ 13), (b) Nb–HMS-X (Si/Nb ¼ 3) and (c) Nb–HMS-X (Si/Nb ¼ 2) catalysts.
Fig. 5 HR-TEM images of (a) Nb–HMS-X (Si/Nb ¼ 13), (b) Nb–HMS-X (Si/Nb ¼ 3) and (c) Nb–HMS-X (Si/Nb ¼ 2).
Fig. 6 Elemental mapping (Nb) of (a) Nb–HMS-X (Si/Nb ¼ 13), (b) Nb–HMS-X (Si/Nb ¼ 3) and (c) Nb–HMS-X (Si/Nb ¼ 2).
ꢁ
1
25
element is distributed in an ordered pattern, which matches 817 cm is attributed to Si–O–Si symmetric stretching. The
ꢁ
1
with the HR-TEM images. A uniform distribution is also found gradual decrease of peak intensity at 817 cm with increasing
in the case of the Si and O elements (Fig. S1, ESI†). It is an loading of Nb might be signifying the successive formation of
ꢁ1
established fact that scanning transmission electron micros- Si–O–Nb moieties. The peak at 960 cm has been interpreted
dꢁ
26
copy (STEM) with a high-angle annular dark eld (HAADF) as the stretching vibrations of Si–OH and Si–O groups. Here,
mode is Z-contrasted, i.e. the contrast of the STEM image comes a simultaneous slight shiing of this band is observed in Nb–
from inelastic forward Rutherford electron scattering by the HMS-X at higher loadings, supporting the strong interaction of
2
dꢁ
27,28
nuclei of the elements. The scattering is proportional to Z , Si–O with Nb.
As found in the literature, the peak ascribed
24
where Z represents the atomic number of the element. Inter- in this region can be considered as evidence of the isomorphous
estingly, with an increase in the Nb loading, the crystallinity of substitution of Si by Nb metal ions, i.e. as Si–O–Nb connec-
the materials is more affected compared with lower loadings of tivity. This peak is also widely used to characterize the pres-
niobium in the silica matrix. ence of transition metal atoms near the silica framework, as the
.1.4 FTIR spectroscopy. The FTIR spectra of the HMS-X Si–O stretching vibration mode is perturbed by the neighbour-
materials with the three different Nb loadings are shown in ing metal ions. At higher loadings this peak ramies, probably
Fig. 7. The peaks at 1085 cm and 1195 cm (Fig. 7) are due to more extra-framework Nb in the silica matrix. In the
attributed to the asymmetric stretching vibration of framework hydroxyl region, the weak band at 1633 cm and the broad
Si–O–Si bridges, as observed in the Nb–HMS-X materials with band at 3500 cm can be attributed to a combination of the
29
3
3
0
ꢁ1
ꢁ1
ꢁ
1
ꢁ
1
15
the three different loadings of niobium. The strong peak at stretching vibration of silanol groups or silanol “nests” with
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 845–854 | 849

View Article Online
RSC Advances
Paper
5
+
(
Si/Nb ¼ 13) it is found that Nb incorporation increases the
density of weak acid sites in the HMS-X material, as well as
3
4,35
medium and strong acid sites.
It can be hypothesized that
5
+
expanding co-ordination of Nb increases the surface silanol
5
+
density, along with some co-ordinatively unsaturated Nb
species, which act as Lewis acid sites. Interestingly, there is no
signicant change in the amount of NH uptake with increase
3
in Nb loading, which emphasizes the formation of polyniobate
clusters at higher loadings.
3.2 Catalytic activity
The catalytic activities of the niobium doped and undoped
HMS-X materials towards the Beckmann rearrangement reac-
tion are shown in Table 2. The results indicate inherent acidity
in the HMS-X network, which is active upon longer reaction
time and can convert about 61% of cyclohexanone oxime into
product. This inherent acidity is capable of performing essen-
tial 1,2 alkyl shis that result in caprolactam formation. Similar
Fig. 7 FTIR spectra of the catalysts of (a) HMS-X, (b) Nb–HMS-X (Si/
Nb ¼ 13), (c) Nb–HMS-X (Si/Nb ¼ 3) and (d) Nb–HMS-X (Si/Nb ¼ 2).

ndings were observed in different studies based on MCM and
13,36,37
SBA-15 materials.
The Nb-doped HMS-X catalysts showed higher catalytic
activity compared to undoped HMS-X. The catalytic activity
decreased for higher niobium loading as shown in Table 2.
On increasing loading, the surface area and pore size were
decreased (as per the surface area and porosity measurements)
due to an increasing number of extra-framework niobium
cross hydrogen-bonding interactions and the H–O–H stretching
31
mode of physisorbed water.
2
9
29
3.1.5
Si-NMR spectroscopy. The Si-NMR spectra of the
Nb–HMS-X materials depicted in Fig. 8 show three different
signals at ꢁ91, ꢁ101 and ꢁ110 ppm, which are similar to those
obtained for doped mesoporous silica as reported in the liter-
2
9
32
species in the pores. From the Si-NMR studies it is found that
ature. The rst peak at around d ¼ ꢁ110 ppm can be assigned
3
4
the acidic vicinal silanol intensity (Q ) decreased with
to (–O–)
4
Si with no OH group attached to the silicon atom (Q ).
Si(OH)
with one OH group (Q ). The last peak at d ¼ ꢁ90 ppm, is
increasing Nb loading, which might be the reason for the lower
catalytic performance. Interestingly the amount of NH uptake
3
was low for higher Nb loadings because of the formation poly-
niobate clusters on the catalyst surface.
The second peak at d ¼ ꢁ102 ppm is assigned to (–O–)
3
3
2
33
2 2
assigned to (–O–) Si(OH) with two OH groups (Q ). A close
look at the Si-NMR spectra of the different niobium loaded
HMS-X materials reveals the decrease of Q peak intensity at
2
9
3
Therefore it can be argued that only framework niobium
(
i.e. Nb–O–Si linkages) is essential for high activity. While
comparing the prepared Nb-doped MCM-41 and HMS catalysts
ESI: catalyst preparation procedure†) we found that the cata-
higher loadings.
3
.1.6 NH -TPD proles. The surface acidic properties of all
3
(
four catalysts were determined by NH
3
-TPD studies (Fig. 9).
lytic activity was inferior to that of the Nb–HMS-X materials. All
the catalysts have a hexagonal pore structure, however the
MCM-41 has a unidirectional hexagonal pore structure and
While comparing the TPD proles of HMS-X and Nb–HMS-X
38,39
HMS-3 has a short range hexagonal pore structure.
This
2
9
Fig. 8
(
Si-NMR spectra of the catalysts; (a) Nb–HMS-X (Si/Nb ¼ 13),
b) Nb–HMS-X (Si/Nb ¼ 3) and (c) Nb–HMS-X (Si/Nb ¼ 2).
3
Fig. 9 NH desorbed from TPD profiles of the catalysts.
850 | RSC Adv., 2014, 4, 845–854
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
a
Table 2 Catalytic activities of different niobium loaded Nb–HMS-X catalysts towards the Beckmann rearrangement reaction
Selectivity (%)
ꢁ1
Catalyst
Conversion (%)
3-Caprolactam
Cyclohexanone
Aniline
WHSV (h
)
Mass balance (%)
HMS-X
61.47
96.17
100
76.56
75.64
84.06
89.62
51.97
100
100
78.97
74.85
82.94
97
44.59
0
0
15.33
24.13
4.05
1.55
3.42
0
0
5.69
1.01
13
15
15
15
15
15
15
15
99.79
100
100
100
100
100
100
Nb–HMS-X (Si/Nb ¼ 20)
Nb–HMS-X (Si/Nb ¼ 13)
Nb–HMS-X (Si/Nb ¼ 3)
Nb–HMS-X (Si/Nb ¼ 2)
Nb–HMS-3(Si/Nb ¼ 13)
Nb-MCM-41(Si/Nb ¼ 13)
1.44
a
ꢁ1
Reaction conditions: reactant (solution of cyclohexanone oxime in benzene, cyclohexanone oxime : benzene weight ratio of 1 : 11) ow ¼ 5 mL h
,
ꢁ1
ꢀ
carrier gas (N
2
) ow ¼ 30 mL min , catalyst pre-treatment temperature ¼ 500 C, catalyst pre-treatment time ¼ 5 h, TOS (time on stream) ¼ 1 h.
difference is probably due to the high dispersion of Nb species Nb–HMS-X (Si/Nb ¼ 13) catalyst is presented in Table 5.
in the hexagonally interconnected network of the HMS-X Desorption is not fast enough at lower temperatures causing
4
0
species.
enhancement of intrinsic residence time, which can result in
13
3
.2.1 Effect of pre-treatment time and temperature. The decomposition of cyclohexanone oxime. It is clear that at
effect of pre-treatment time and temperature on the conversion lower temperatures the hydrolysis product cyclohexanone is the
and 3-caprolactam selectivity over Nb–HMS-X (Si/Nb ¼ 13) are main product competing with the Beckmann product 3-capro-
shown in Tables 3 and 4. With longer pre-treatment times and lactam. At lower temperatures the mass balance was lower,
higher pre-treatment temperatures, sub-layer niobium oxide which may be due to different products, such as cyclohexanone,
species present in the mesopores diffuse to the surface gener- aniline, 5-hexenenitrile, 2-cyclohexen-1-one etc., sticking onto
14
ating more active sites, resulting in high oxime conversion and 3- the catalyst surface. Upon increasing the reaction temperature
ꢀ
caprolactam selectivity, even at the highest weight hourly space up to 350 C, the oxime conversion and 3-caprolactam selectivity
ꢁ1
velocity (WHSV) (15 h ). So, a longer pre-treatment time (5 h) are also increased, whereas the selectivity for cyclohexanone
ꢀ
ꢀ
and higher pre-treatment temperature (500 C) are favourable for decreases continuously. At a temperature of 400 C there is no
optimum catalytic performance in this present case. reaction and the catalyst bed turned black in colour, which
.2.2 Effect of reaction temperature. The effect of temper- emphasizes that thermal cracking products are formed on the
3
36
ature on the conversion and 3-caprolactam selectivity over the catalyst surface.
a
Table 3 Catalytic activity of Nb–HMS-X (Si/Nb ¼ 13) with different pre-treatment times towards the Beckmann rearrangement reaction
Selectivity (%)
Catalyst pre-treatment
time (h)
Conversion
(%)
Mass
balance (%)
ꢁ1
Catalyst
3-Caprolactam
Cyclohexanone
Aniline
WHSV (h
)
Nb–HMS-X
Nb–HMS-X
Nb–HMS-X
Nb–HMS-X
2
3
4
5
95.05
90.99
89.07
100
94.53
92.29
91.76
100
5.46
7.7
6.81
0
0
0
1.42
0
15
15
15
15
99.62
100
100
100
a
ꢁ1
Reaction conditions: reactant (solution of cyclohexanone oxime in benzene, cyclohexanone oxime : benzene weight ratio of 1 : 11) ow ¼ 5 mL h
,
ꢀ
ꢁ1
ꢀ
reaction temperature ¼ 350 C, carrier gas (N
2
) ow ¼ 30 mL min , catalyst pre-treatment temperature ¼ 500 C, TOS (time on stream) ¼ 1 h.
a
Table 4 Catalytic activity of Nb–HMS-X (Si/Nb ¼ 13) with different pre-treatment temperatures towards the Beckmann rearrangement reaction
Selectivity (%)
Pre-treatment
temperature ( C)
Reaction time
(min)
Conversion
(%)
Mass
balance (%)
ꢀ
ꢁ1
Catalyst
3-Caprolactam
Cyclohexanone
Aniline
WHSV (h
)
Nb–HMS-X
Nb–HMS-X
Nb–HMS-X
300
400
500
60
60
60
97.82
95.98
100
94.45
94.15
100
3.54
5.26
0
1.99
0.58
0
15
15
15
98.82
99.19
100
a
ꢁ1
Reaction conditions: reactant (solution of cyclohexanone oxime in benzene, cyclohexanone oxime : benzene weight ratio of 1 : 11) ow ¼ 5 mL h
,
ꢀ
ꢁ1
reaction temperature ¼ 350 C, carrier gas (N
2
) ow ¼ 30 mL min , catalyst pre-treatment time ¼ 5 h.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 845–854 | 851

View Article Online
RSC Advances
Paper
Table 5 Catalytic activity of Nb–HMS-X (Si/Nb ¼ 13) at 14 hꢁ1 WHSV at different reaction temperaturesa
Selectivity (%)
Reaction
Temperature ( C) (min)
Reaction time
Mass
balance (%)
ꢀ
ꢁ1
Catalyst
Conversion (%) 3-Caprolactam Cyclohexanone Aniline WHSV (h
)
Nb–HMS-X 250
Nb–HMS-X 300
Nb–HMS-X 350
Nb–HMS-X 400
60
60
60
60
81.7
95.21
99.28
No reaction
52.57
84.04
97.51
—
47.42
15.95
2.48
—
0
0
0
—
14
14
14
87.99
95.21
99.28
a
ꢁ1
Reaction conditions: reactant (solution of cyclohexanone oxime in benzene, cyclohexanone oxime : benzene weight ratio of 1 : 11) ow ¼ 5 mL h
,
ꢁ1
ꢀ
carrier gas (N
2
) ow ¼ 20 mL min , catalyst pre-treatment temperature ¼ 500 C, catalyst pre-treatment time ¼ 5 h.
3
.2.3 Effect of WHSV. The effect of WHSV and N ow rate
3.2.5 Catalytic regeneration. Catalyst deactivation remains
2
42
on cyclohexanone oxime conversion and product selectivity over a key problem in gas phase reactions. To study the regenera-
Nb–HMS-X (Si/Nb ¼ 13) are shown in Table 6. The WHSV is tion of the catalyst system, the reaction was carried out on a
varied in the present case keeping other reaction parameters deactivated Nb–HMS-X (Si/Nb ¼ 13) catalyst aer regenerating
2
constant. Though many authors reported decreasing conver- in the presence of a N atmosphere. The results based on oxime
sion, selectivity or both on increasing WHSV, which might be conversion and 3-caprolactam selectivity are depicted in Fig. 10.
17,22
due to faster deactivation of the catalyst,
2
the Nb–HMS-X The regeneration study under N showed almost constant
catalyst system retains high conversion and selectivity even at activity with high conversion and selectivity (more than 80% in
ꢁ1
1
5 h WHSV, which demonstrates the optimum production of each cycle) for ten consecutive cycles. When the catalyst was
ꢁ
1
ꢀ
3
-caprolactam on an industrial scale.
.2.4 Effect of silylation. Organic modication of meso- the original activity was completely recovered. The main reason
porous silica catalysts has been well practised for different for deactivation of the catalyst could be blockage of active sites
regenerated in owing N gas (30 mL min ) at 500 C for 4 h,
2
3
41
reactions. The Beckmann rearrangement reaction was carried by coke/oligomers. These oligomers, which are formed by ring
out under the same conditions (Table 7) with silylated Nb–HMS- opening reactions in the presence of acidic sites, can be elimi-
13
X (Si/Nb ¼ 13). The FTIR study of silylated Nb–HMS-X shows a nated with nitrogen at high temperature. The regeneration
ꢁ
1
band at 2800–3000 cm (Fig. S3, ESI†), which is due to C–H studies under N indicated that the deactivation is reversible.
2
species present on the surface, indicating the formation of Si–
Though there are several reports for the reaction mechanism
18
O–Si(CH
had lower productivity corresponding to its lower surface the reaction mechanism has been achieved so far. The active
3 2
) connectivity in the sample. The silylated catalyst of Beckmann rearrangement, no undisputed understanding of
4,43
37
silanol concentration. However, the substantial activity for the sites for the Beckmann rearrangement are thought to be the
silylated catalyst implies the presence of silanols in the surface Br ¨o nsted acid sites of the catalyst. It was suggested that
13
micropores.
the Beckmann rearrangement on the oxide surfaces proceeds
a
Table 6 Catalytic activity of Nb–HMS-X (Si/Nb ¼ 13) at different WHSVs and N
2
flow rates
Selectivity (%)
N
2
ow
Reaction
time (min)
Conversion
(%)
Mass
balance (%)
ꢁ1
ꢁ1
Catalyst
(mL min
)
3-Caprolactam
Cyclohexanone
Aniline
WHSV (h
)
Nb–HMS-X
Nb–HMS-X
Nb–HMS-X
10
20
30
60
60
60
96.07
99.28
100
94.78
97.51
100
5.21
2.48
0
0
0
0
12
14
15
99.30
99.28
100
a
ꢁ1
Reaction conditions: reactant (solution of cyclohexanone oxime in benzene, cyclohexanone oxime : benzene weight ratio of 1 : 11) ow ¼ 5 mL h
,
ꢀ
ꢀ
reaction temperature ¼ 350 C, catalyst pre-treatment temperature ¼ 500 C, catalyst pre-treatment time ¼ 5 h.
a
Table 7 Catalytic activity of silylated Nb–HMS-X (Si/Nb ¼ 13) material towards the Beckmann rearrangement reaction
Selectivity (%)
Mass
balance (%)
ꢁ1
Catalyst
Conversion (%)
88.76
3-Caprolactam
Cyclohexanone
8.89
Aniline
0.85
WHSV (h
15
)
Nb–HMS-X (silylated)
90.25
100
a
ꢁ1
Reaction conditions: reactant (solution of cyclohexanone oxime in benzene, cyclohexanone oxime : benzene weight ratio of 1 : 11) ow ¼ 5 mL h
,
ꢀ
ꢀ
reaction temperature ¼ 350 C, catalyst pre-treatment temperature ¼ 500 C, catalyst pre-treatment time ¼ 5 h.
852 | RSC Adv., 2014, 4, 845–854
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
and Si/Nb ¼ 2) materials. Due to the presence of agglomerated
niobium oxide species, the acid site density did not alter at
higher Nb loading. Therefore a mutual balance between the
niobium and silica ratio is a key factor for the surface acidity.
4
Conclusions
From this work the following conclusions are drawn:
ꢄ Nb–HMS-X catalysts with different Si/Nb ratios have been
synthesized and characterized by various spectroscopic tech-
niques. These analytical results provided strong evidence that
Nb has been incorporated into the mesoporous HMS-X silica
framework.
ꢄ The small angle XRD results revealed that all four catalysts
were mesoporous in nature with a hexagonal pore structure.
With increasing Nb loading, the crystal plans are badly affected
and the peaks become broader.
Fig. 10 Catalyst regeneration studies under the following reaction
conditions: reactant (solution of cyclohexanone oxime in benzene,
ꢄ N
2
sorption studies revealed that the Nb–HMS-X materials
have a very high surface area and porosity. With increasing Nb
content, partial loss of the mesoporous structure occurred. The
continuous decrease of surface area, pore volume and diameter
indicates formation of more Nb O , which might block the
ꢁ
1
cyclohexanone oxime : benzene weight ratio of 1 : 11) flow ¼ 5 mL h
,
ꢁ
1
ꢀ
carrier gas (N
2
) flow ¼ 30 mL min , reaction temperature ¼ 350 C,
ꢀ
catalyst pre-treatment temperature ¼ 500 C, catalyst pre-treatment
time ¼ 4 h, TOS (time on stream) ¼ 1 h.
2
5
mesopores.
ꢄ The HR-TEM image of Nb–HMS-X (Si/Nb ¼ 13) shows the
presence of a well-shaped, uniform, honeycomb-like surface
morphology with mesoporous channels. With increasing Nb
loading the channel like morphology of Nb–HMS-X was
destroyed.
via a fast step where a Br ¨o nsted acid site interacting with a
nitrogen atom is transferred to the oxygen atom of the oxime. In
the rate determining step, the transfer of the alkyl group and the
elimination of the hydroxyl group from the oxide are feasible to
produce caprolactam. Cyclohexanone, a major by-product, is
formed due to the hydrolysis of cyclohexanone oxime.
Based on the catalyst characterization results and catalytic
activity sites the reaction mechanism is depicted in Fig. 11. The
hydrogen atom bound to the bridging oxygen, i.e. the Si–O–Nb
linkage, is known to exhibit the strongest acidity. In the present
study we found from TPD measurement that Nb–HMS-X (Si/
Nb ¼ 13) has the highest acidity due to the presence of frame-
work Si–O–Nb moieties and major amounts of surface hydroxyl
groups compared to the other two Nb doped HMS-X (Si/Nb ¼ 3
44
ꢄ The FTIR spectroscopy results revealed a strong interaction
of Nb with silanol groups, as indicated by the presence of the Si–
O–Nb vibration mode. However at much higher loading this
peak ramied, probably due to the increased amount of extra-
framework Nb.
29
ꢄ The Si-NMR result indicated the presence of three
different types of silica species and Si–O–Nb linkage, conrm-
ing Nb incorporation into the Si framework.
ꢄ The NH
3
-TPD results revealed that with increasing Nb
loading, acidity decreases and the intensity of the surface
hydroxyl groups, Si(OH), are suitable for Beckmann
rearrangement.
ꢄ Rearrangement of cyclohexanone oxime carried out on Nb–
HMS-X materials with different Si/Nb ratios showed that the
catalysts are highly active with the highest Si/Nb ratios. Thus,
the catalytic activity mainly depends on the acidity of the cata-
lyst and the acidity of the samples is strongly dependent upon
the effect of niobium content in the sample.
ꢄ The WHSV studies suggested that a reaction temp. of
ꢀ
ꢁ1
3
5
50 C, a N
2
ow rate of 30 mL min , a pre-treatment temp. of
ꢀ
00 C and a pre-treatment time of 5 h are the optimum reaction
conditions for this reaction, obtaining 100% cyclohexanone
oxime conversion and 100% 3-caprolactam selectivity.
ꢄ The time on stream studies indicated that the catalyst
maintained constant activity for up to 40 h, aer which the
conversion started to decrease. Nevertheless, constant 3-capro-
lactam selectivity was observed during 1 h time on stream. This
indicates that the catalyst deactivates due to poisoning of the
catalyst active sites either by coke or by oligomers.
Fig. 11 Reaction mechanism for the vapor phase Beckmann rear-
rangement reaction.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 845–854 | 853

View Article Online
RSC Advances
Paper
ꢄ The successful regeneration of the Nb–HMS-X (Si/Nb ¼ 13)
and C. Minchev, Microporous Mesoporous Mater., 2008, 112,
327.
catalyst for ten cycles in the presence of nitrogen and the
ordered, thermally stable honeycomb mesoporous structure of 22 A. Prabhu, L. Kumaresan, M. Palanichamy and
Nb–HMS-X (Si/Nb ¼ 13) makes this catalyst viable as per
V. Murugesan, Appl. Catal., A, 2009, 360, 59.
23 D. Zhang, A. Aijun, Z. Zhao and C. Xu, J. Catal., 2010, 274,
industrial requirements.
273.
2
4 G. Etrl, H. Knozinger, F. Schuth and J. Weitkamp, Handbook
of Heterogeneous Catalysis, WILEY-VCH, Weinheim, 2nd edn,
2008, vol. 2, p. 810.
Acknowledgements
SR and BC acknowledge the nancial assistance from UGC,
Government of India [project no. F.39-802/2010 (SR)]. SM and 25 M. S. Morey, S. O’Brien, S. Schwarz and G. D. Stucky, Chem.
RK acknowledge ISM for providing research fellowships. CS
Mater., 2000, 12, 898.
acknowledges UGC, Government of India for a junior research 26 M. A. Camblor, A. Corma and J. P ´e rez-Pariente, J. Chem. Soc.,
fellowship.
Chem. Commun., 1993, 557.
27 T. Tsoncheva, L. Ivanova, J. Rosenholm and M. Linden, Appl.
Catal., B, 2009, 89, 365.
References
2
8 A. Ramanathan, T. Archipov, R. Maheswari, U. Hanefeld,
E. Roduner and R. Glaser, J. Phys. Chem. C, 2008,
112, 7468.
1
2
W. Eickelberg and W. F. Hoelderich, J. Catal., 2009, 263, 42.
H. Kath, R. Gl ¨a ser and J. Weitkamp, Chem. Eng. Technol.,
2001, 24, 150.
29 M. Selvaraj, B. H. Kim and T. G. Lee, Chem. Lett., 2005, 34,
3
4
W. F. H ¨o lderich, in Comprehensive Supramolecular Chemistry,
ed. G. Alberti and T. Bein, Pergamon, New York, 1996, vol. 7. 30 A. Bordoloi and S. B. Halligudi, J. Catal., 2008, 257, 283.
S. K. Pillai, O. Gheevarghese and S. Sugunan, Appl. Catal., A, 31 Y. Chen, Z. Guo, T. Chen and Y. Yang, J. Catal., 2010,
1290.
2009, 353, 130.
275, 11.
5
6
7
8
N. Kob and R. S. Drago, Catal. Lett., 1997, 49, 229.
T. Ushikubo and K. Wada, J. Catal., 1994, 148, 138.
D. Mao, Q. Chen and G. Lu, Appl. Catal., A, 2003, 244, 273.
32 S. Mandal, A. Sinhamahapatra, B. Rakesh, R. Kumar,
A. B. Panda and B. Chowdhury, Catal. Commun., 2011, 12,
734.
X. Guo, W. Hou, W. Ding, Y. Fan, Q. Yan and Y. Chen, 33 K. J. D. Mackenzie and M. E. Smith, Multinuclear Solid-State
Microporous Mesoporous Mater., 2005, 80, 269.
M. Pramanik, M. Nandi, H. Uyama and A. Bhaumik, Catal.
Sci. Technol., 2012, 2, 613.
NMR of Inorganic Materials, Elsevier Science Ltd., Pergamon,
1st edn, 2002.
34 T. Koranyi, E. Pfeifer, J. Mihaly and K. Fottinger, J. Phys.
Chem. A, 2008, 112, 5126.
9
1
1
1
1
0 M. Pramanik, M. Nandi, H. Uyama and A. Bhaumik, Green
Chem., 2012, 14, 2273.
35 H. G. Karge, V. Dondur and J. Weitkamp, J. Phys. Chem.,
1991, 95, 283.
36 K. Chaudhari, R. Bal, A. J. Chandwadkar and S. Sivasanker,
J. Mol. Catal. A: Chem., 2002, 177, 247.
37 V. R. R. Marthala, S. Rabl, J. Huang, S. A. S. Rezai, B. Thomas
and M. Hunger, J. Catal., 2008, 257, 134.
1 M. Nandi, J. Mondal, K. Sarkar, Y. Yamauchi and
A. Bhaumik, Chem. Commun., 2011, 47, 6677.
2 S. K. Das, M. K. Bhunia, A. K. Sinha and A. Bhaumik, ACS
Catal., 2011, 1, 493.
3 R. Palkovits, C. M. Yang, S. Olejnik and F. Sch u¨ th, J. Catal.,
2
006, 243, 93.
38 J. S. Beck, J. C. VartUli, W. J. Roth, M. E. Leonowicz,
C. T. Kresge, K. D. Schmitt, C. T.-W. Chu, D. H. Olson,
E. W. Sheppard, S. B. McCullen, J. B. Higgins and
J. L. Schlenker, J. Am. Chem. Soc., 1992, 114, 10834.
1
1
4 M. Anilkumar and W. F. Holderich, J. Catal., 2008, 260, 17.
5 M. Anilkumar and W. F. Hoelderich, Catal. Today, 2012, 198,
2
89.
1
6 S. Hudson, D. A. Tanner, W. Redington, E. Magner, 39 J. Li, Y. Shi, L. Xu and G. Lu, Ind. Eng. Chem. Res., 2010, 49,
K. Hodnett and S. Nakahara, Phys. Chem. Chem. Phys.,
006, 8, 3467.
5392.
2
40 S. Jun, S. Hoon Joo, R. Ryoo, M. Kruk, M. Jaroniec, Z. Liu,
T. Ohsuna and O. Terasaki, J. Am. Chem. Soc., 2000, 122,
10712.
1
1
1
2
2
7 L. Forni, C. Tosi, G. Fornasari, F. Trir `o , A. Vaccari and
J. B. Nagy, J. Mol. Catal. A: Chem., 2004, 221, 97.
8 J. M. R. Gallo, H. O. Pastore and U. Schuchardt, J. Catal., 41 B. Chowdhury, K. K. Bando, J. J. Bravo-Su ´a rez, S. Tsubota
006, 243, 57.
and M. Haruta, J. Mol. Catal. A: Chem., 2012, 359, 21.
9 F. Kleitz, S. H. Choi and R. Ryoo, Chem. Commun., 2003, 42 B. Chowdhury, J. J. Bravo-Su ´a rez, M. Date, S. Tsubota and
136.
M. Haruta, Angew. Chem., Int. Ed., 2006, 45, 412.
0 A. Vinu, S. Anandan, C. Anand, P. Srinivasu, K. Ariga and 43 Y. Ko, M. H. Kim, S. J. Kim, G. Seo, Y. S. Uh and M. Y. Kim,
T. Mori, Microporous Mesoporous Mater., 2008, 109, 398.
Chem. Commun., 2000, 829.
1 T. Tsoncheva, J. Rosenholm, M. Linden, F. Kleitz, 44 Y. Shinonara, S. Mae, D. Shouro and T. Nakajima, J. Mol.
2
2
M. Tiemann, L. Ivanova, M. Dimitrov, D. Paneva, I. Mitov
Struct.: THEOCHEM, 2000, 497, 1.
854 | RSC Adv., 2014, 4, 845–854
This journal is © The Royal Society of Chemistry 2014