Selective oxidation of cyclohexene to adipic acid over silver supported tungsten oxide nanostructured catalysts

Article abstract of DOI:10.1039/c4gc00130c

We have developed a new synthesis strategy to prepare ～5 nm metallic silver nanoparticles (AgNPs) supported on tungsten oxide (WO₃) nanorods with diameters between 40 and 60 nm in the presence of a cationic surfactant, cetyltrimethylammonium bromide (CTAB). The catalyst was characterized by XRD, XPS, ICP-AES, FT-IR, Raman spectroscopy, EXAFS, SEM and TEM. The catalyst is very effective in liquid phase oxidation of cyclohexene to adipic acid with hydrogen peroxide as an oxidant. The synergy between the surface AgNPs and WO₃ nanorods plays the most vital role towards this very high catalytic activity. The reusability of the catalyst which is a prerequisite for practical applications was analysed and it was found that the catalyst exhibits no significant changes in its catalytic activity even after five cycles of reuse. A cyclohexene conversion of >99.9% with an adipic acid selectivity of ～94% was achieved over ～5 nm AgNPs supported on the WO₃ nanorod catalyst with a very high turnover frequency of ～12 h^-1. the Partner Organisations 2014.

Full text of DOI:10.1039/c4gc00130c

Green Chemistry
View Article Online
View Journal | View Issue
PAPER
Selective oxidation of cyclohexene to adipic
acid over silver supported tungsten oxide
nanostructured catalysts†
Cite this: Green Chem., 2014, 16,
826
2
a
a
a
a
Shilpi Ghosh, Shankha Shubhra Acharyya, Shubhadeep Adak,
a
b
L. N. Sivakumar Konathala, Takehiko Sasaki and Rajaram Bal*
We have developed a new synthesis strategy to prepare ∼5 nm metallic silver nanoparticles (AgNPs) sup-
ported on tungsten oxide (WO ) nanorods with diameters between 40 and 60 nm in the presence of a
3
cationic surfactant, cetyltrimethylammonium bromide (CTAB). The catalyst was characterized by XRD,
XPS, ICP-AES, FT-IR, Raman spectroscopy, EXAFS, SEM and TEM. The catalyst is very effective in liquid
phase oxidation of cyclohexene to adipic acid with hydrogen peroxide as an oxidant. The synergy
3
between the surface AgNPs and WO nanorods plays the most vital role towards this very high catalytic
Received 24th January 2014,
Accepted 6th March 2014
activity. The reusability of the catalyst which is a prerequisite for practical applications was analysed and it
was found that the catalyst exhibits no significant changes in its catalytic activity even after five cycles of
reuse. A cyclohexene conversion of >99.9% with an adipic acid selectivity of ∼94% was achieved over
DOI: 10.1039/c4gc00130c
−
1
www.rsc.org/greenchem
∼5 nm AgNPs supported on the WO nanorod catalyst with a very high turnover frequency of ∼12 h .
3
deposition etc. need drastic conditions high temperature,
hydrothermal treatment, and use of solvents, which make the
1
. Introduction
Particles in the nanosize domain have attracted considerable whole process not only environmentally unfriendly but also
attention not only for their fundamental scientific interest, but highly energy-intensive. Owing to these factors, considerable
also for many technological applications including catalysis research effort has been made to prepare supported Ag nano-
2
6–29
due to their very high accessible surface area and easy avail- structured materials in the solution phase.
ability of a large number of active sites.
Despite signifi-
In this nanosize cant research efforts, the controlled synthesis of dispersed
regime, the particles behave differently from their bulk silver nanoparticles in the aqueous phase under ambient con-
1
–4
1
–4
counterparts.
The catalytic efficiency of nanostructures is ditions is one of the most difficult challenges in catalysis. We
primarily governed by the size and shape of the nanocrystals report here the preparation of almost uniformly dispersed
5
and the composition of the metals and supports used. Noble ∼5 nm Ag nanoparticles (AgNPs) supported on WO
3
nanorods
metal nanoparticles such as silver and gold exhibit unique cat- with diameters between 40 and 60 nm and we found that this
6
–17
alytic, electrical and optical properties.
Silver nanoparticles catalyst is highly active for selective oxidation of cyclohexene to
(
AgNPs) have especially been of interest owing to their interest- adipic acid.
1
8–21
ing size dependent properties in various reactions.
Signifi-
Adipic acid (AA) is one of the most important compounds
cant research efforts have been made for the synthesis of silver among all aliphatic dicarboxylic acids produced industrially.
nanoparticles on different transition and non-transition metal With the global annual production of over 3.5 million tons,
2
2–25
supports such as titania, alumina, silica, etc.
The methods adipic acid is used predominantly as a raw material in the pro-
associated with the synthesis of supported Ag nanoparticles duction of a wide range of industrial and commercial products
such as sonochemical methods, microwave irradiation, vapour including synthetic fibers, nylon 6,6, polyurethanes, plastici-
zers, adiponitriles, low temperature synthetic lubricants and
3
0,31
so on.
Despite its immense importance, no catalyst has
a
Catalytic Conversion & Processes Division, CSIR-Indian Institute of Petroleum,
been reported to date which can be used industrially with
oxidizing agents like molecular oxygen or H O . It is still a
challenge to develop a clean technological process for the pro-
duction of adipic acid with the aid of environmentally benign
Dehradun 248005, India
2
2
b
Department of Complexity Science and Engineering, Graduate School of Frontier
Sciences, The University of Tokyo, Kashiwanoha, Kashiwa-shi, Chiba 277-8561,
Japan. E-mail: raja@iip.res.in; Fax: +91 135 2660202; Tel: +91 135 2525797
oxidants like molecular oxygen or H O . The method deployed
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
2 2
c4gc00130c
by DuPont involves the hydrogenation of benzene to cyclo-
2826 | Green Chem., 2014, 16, 2826–2834
This journal is © The Royal Society of Chemistry 2014

View Article Online
Green Chemistry
Paper
hexane, followed by aerial oxidation to obtain KA oil (a mixture wise addition of NH
4
OH solution. Then 0.68 g CTAB solution
of cyclohexanol and cyclohexanone), from where AA is being was added dropwise. The mixture was stirred for another
produced on further treatment of KA oil with HNO . The Asahi 10 minutes and 0.29 g NH NH was added. After 24 h of con-
3
2
2
Kasei process followed the selective hydrogenation path from tinuous stirring, the product was washed with ethanol and
benzene to cyclohexene, from where AA is being produced. dried in a 110 °C oven for 12 h, followed by thermal heating at
−1
Both these industrial processes are associated with the 500 °C for 6 h in the presence of helium (10 ml min ). The
coproduction of a substantial amount of environmentally anisotropic growth of the tungsten oxide nanorods can be
3
2,33
+
unfriendly, unwanted greenhouse gases like N O.
A
explained by the shape-controlled chemistry of the Ag ions
2
titania-based catalyst was found to catalyse cyclohexene to AA since we could not observe rod-like morphology in the absence
+
effectively, using tertiary butyl hydrogen peroxide (TBHP) as an of silver ions. Here, the Ag ions restrict the growth of tungsten
3
4
oxidant. But the use of TBHP is cost effective as compared to oxide except in one direction by selectively adsorbing onto
and suffers from environmental problems. Thus the certain crystal planes of tungsten, leading to preferential
2 2
H O
improvement of existing catalytic methods by means of envir- growth of tungsten oxide along one direction. Here CTAB acts
onmentally friendly molecular oxygen or H O2 is highly as a capping agent in controlling the size of silver nanoparti-
2
demanding. The hydrogen peroxide-based catalytic conversion cles, and tungsten oxide nanorods and hydrazine served as a
has received much attention; it has the high content of active reducing agent. The formation of AgNPs supported on the
oxygen species (47 wt%) as it generates water as the sole bypro- WO3 nanorod catalyst is shown in the schematic diagram
35
duct. Moreover, H
other peroxides and peracids.
cyclohexene to adipic acid using an environmentally benign
clean oxidant, such as H , is highly desirable from the A thorough characterization of the catalyst was done using
2 2
O is cheaper and safer to use than the (Scheme S1, ESI†).
3
6–39
So, direct conversion of
2.3 Catalyst characterization
2 2
O
industrial point of view. Noyori et al. demonstrated the direct different characterization techniques like XRD, SEM, TEM,
conversion of cyclohexene to AA using a tungsten-based homo- EXAFS, XPS, TGA etc. Powder X-ray diffraction patterns were
geneous catalyst in aqueous H
ary ammonium salts were used as phase transfer catalysts.
There have been reports on the production of adipic acid source. Diffraction patterns in the 2°–80° region were recorded
using different acids like H WO , H SO and H PO as catalyst at a rate of 0.5 degree (2q) per minute. Scanning electron
precursors in H medium. But all the processes suffer microscopy (SEM) images were taken on a FEI Quanta 200 F,
either from environmental issues or separation problems. So, using a tungsten filament doped with lanthanumhexaboride
the synthesis of AA in aqueous H medium is a topic of (LaB ) as an X-ray source, fitted with an ETD detector in high
2 2
O medium in which quatern- collected on a Bruker D8 Advance X-ray diffractometer fitted
3
1
α
with a Lynx eye high-speed strip detector and a Cu K radiation
2
4
0
2
4
3
4
3
2 2
O
2
O
2
6
potential interest and the exploration of a true heterogeneous vacuum mode using secondary electrons and an acceleration
catalyst with the ease of separability and recyclability is highly tension of 10 or 30 kV. Samples were analyzed by spreading
demanding for the production of AA. Herein, we report a cyclo- them on a carbon tape. Energy dispersive X-ray spectroscopy
hexene conversion of >99.9% with 94% adipic acid selectivity (EDX) was used along with SEM for the elemental analysis.
at 90 °C with H O as an oxidant over Ag nanoparticles sup- The elemental mapping was also performed with the same
2
2
ported on WO
3
nanorods.
spectrophotometer. TEM images were collected using a JEOL
JEM 2100 microscope, and samples were prepared by mount-
ing an ethanol-dispersed sample on a lacey carbon formvar
coated Cu grid. X-ray photoelectron spectroscopy (XPS) spectra
were recorded on a Thermo Scientific K-Alpha X-ray photo-
electron spectrometer and binding energies (±0.1 eV) were
determined with respect to the position of C 1s peak at 284.8
eV. Raman spectra were measured at 298 K using a laser
Raman spectrometer (JASCO, NRS-3100) with the 532 nm line
from a diode-pumped solid-state laser for excitation. Measure-
ments of the extended X-ray absorption fine structure (EXAFS)
at the Ag-K edge were carried out in transmission mode at
room temperature at the NW10A station of the Photon Factory
Advanced Ring for pulse X-rays at the Institute of Materials
Structure Science, High Energy Accelerator Research Organiz-
ation in Japan (KEK-IMSS-PF-AR). The electron storage ring
2
. Experimental
2
.1 Materials
H
2
WO , AgNO , cetyltrimethylammonium bromide, hydrazine,
4
3
HPLC grade (∼99.9%) cyclohexene, cyclohexene oxide, cyclo-
hexanediol, and acetonitrile were purchased from Sigma-
Aldrich Co. Hydrogen peroxide (50 wt% in water) was
purchased from Merck KGaA, Darmstadt, Germany. All the
chemicals were used without further purification. Double dis-
tilled water was used during the preparation of the catalyst.
2
.2 Preparation of the Ag/WO
Metallic AgNPs with a size of ∼5 nm supported on the WO
4
nanorod catalyst were synthesized using AgNO , H WO , CTAB was operated at 6.5 GeV. Synchrotron radiation from the
3
nanostructured catalyst
3
3
2
and NH NH (80% solution) maintaining a molar ratio of Ag : storage ring was monochromatized by a Si (311) channel cut
2
2
CTAB : H
2
O : NH
2
NH
2
= 1 : 0.80 : 400 : 2. An aqueous solution of crystal at the Ag K-edge. Ionization chambers, which were used
WO solution at 100 °C as detectors for incident X-rays (I ) and transmitted X-rays (I),
under vigorous stirring. The pH was adjusted to ∼9 by drop- were filled with 50% Ar–N2 mixture gas and 100% Ar gas,
0
.32 g AgNO
3
was added to 5.4 g H
2
4
0
This journal is © The Royal Society of Chemistry 2014
Green Chem., 2014, 16, 2826–2834 | 2827

View Article Online
Paper
Green Chemistry
respectively. The EXAFS raw data were analyzed with the
3
. Results and discussion
4
0
UWXAFS analysis package, including the background sub-
4
1
3.1 Catalyst characterization
for Ag was fixed Fig. 1 shows the XRD pattern of the prepared Ag/WO
at 0.95. The backscattering amplitude and phase shift were cal- The powder XRD pattern of the catalyst showed the peaks at 2θ
traction program AUTOBK and the curve fitting program
4
2
2
FEFFIT. The amplitude reducing factor S
0
3
catalyst.
4
3
44
culated theoretically using FEFF 8.4 code. ATOMS was values of 23.2°, 23.5°, 24.3°, 33.2° and 34.2° (2θ), which corre-
used to obtain the FEFF input code for crystalline materials. spond to the monoclinic WO (JCPDS no. 43-1035, space
3
Chemical analyses of the metallic constituents were carried group: P21/n). In addition, four diffraction peaks at 2θ values
out using an inductively coupled plasma atomic emission of 38.1, 44.3, 64.4 and 77.4° corresponding to the metallic Ag
spectrometer; model: PS 3000 UV (DRE), Leeman Labs, crystal faces of (111), (200), (220) and (311) respectively were
Inc. (USA). Fourier transformation infra-red (FTIR) spectra observed, which coincide well with the literature values (JCPDS
were recorded on a Thermo Nicolet 8700 (USA) instrument no. 04-0783). The metallic Ag does not change its phase after
−
1
with the operating conditions: resolution: 4 cm , scan: 36, the reaction as confirmed by XRD (Fig. 1e). The amount of
operating temperature: 23–25 °C and the frequency range: silver present in the Ag/WO nanostructured catalyst was esti-
3
1
4
000–400 cm− . Thermogravimetric analyses (TGA) of the mated by ICP-AES. SEM images showed that the samples pre-
uncalcined catalyst were carried out in a Pyris Diamond, pared by our own surfactant-assisted procedure exhibit a
Perkin Elmer Instruments, and technology by SII (Seiko Instru- nearly uniform rod-like structure with diameter around
ments Inc., USA) instrument-balance by heating 2.15 mg 40–60 nm (Fig. 2a, 2b). The high resolution SEM image
samples at 5 °C min− under flowing nitrogen.
1
(Fig. 2b) showed the presence of very small silver particles on
WO nanorods. EDS showed only the presence of silver, oxygen
3
and tungsten and no impurities could be observed in the
spectra (Fig. 2c). The samples prepared by the conventional
impregnation method show irregular and larger particles
2
.4 Oxidation of cyclohexene
Liquid phase oxidation of cyclohexene was carried out in a (Fig. 2d). High resolution transmission electron microscopy
double neck round bottom flask at 90 °C in an oil bath con- (HRTEM) revealed a one-dimensional rod-like morphology of
nected with a spiral condenser (length 60 cm) using 0.15 g tungsten oxide (Fig. 3a) with an average width of ∼50 nm de-
catalyst, 10 ml acetonitrile solvent and 1 g cyclohexene to posited by uniformly dispersed silver particles with average
which 4.15 g H
a period of 30 minutes in order to avoid the immediate to be about 0.26 nm and 0.22 nm, possibly due to the (022)
decomposition of H . The condenser was attached to a plane of WO and the (111) plane of metallic Ag, respectively
2 2
O (50% aq. solution) was added dropwise over size nearly ∼5 nm. The spacing of the lattice fringes was found
2
O
2
3
water circulating bath (Julabo, FC 600) maintaining a constant (Fig. 3b). The silver particle size distribution histogram is
temperature of 5 °C. Small aliquots of the sample were with- shown in Fig. 3d. To check the possible morphological
drawn from the reaction mixture at regular intervals for analy- changes of the catalysts during reaction, TEM measurements
sis. At the end of the reaction, the solid particles (catalyst) have been performed on the Ag/WO nanostructured catalyst
3
were separated by filtering during the hot conditions and pro- after 5 cycles of reuse. The mean diameter of AgNPs remained
ducts were analysed using a gas chromatograph (GC, Agilent ∼5 nm after the reaction, which confirms the stability of the
7
890) connected with an OV-1701 capillary column (30 m
length, 0.28 mm id and 0.25 µm film thickness) and a flame
ionization detector (FID). Conversion of cyclohexene was calcu-
lated based upon the GC-FID results, where cyclohexene con-
version = [moles of cyclohexene reacted]/[initial moles of
cyclohexene used] × 100 and the selectivity of products was cal-
culated by [total moles of product formed]/[total moles of
cyclohexene converted] × 100. During the GC analysis, diethyl
malonate was used as an internal standard. The individual
yields were calculated and normalized with respect to the GC
response factors. The product identification was carried out by
injecting authentic standard samples into GC and GCMS. For
few cases, we carried out column chromatography to get the
1
isolated yield and H-NMR was performed to identify the
product. The C-balance as well as material balance was carried
out for most of the experiments and it was found to be
between 98 and 102%. For the reusability test, after com-
pletion of the reaction, the catalyst was recovered from the
reaction mixture by filtration and washed thoroughly with
acetone and reused as such for multiple circles.
Fig. 1 XRD patterns of (a) metallic Ag(0), (b) Ag(I) oxide, (c) W(VI) oxide,
(d) fresh Ag/WO , and (e) spent Ag/WO
3
3
.
2828 | Green Chem., 2014, 16, 2826–2834
This journal is © The Royal Society of Chemistry 2014

View Article Online
Green Chemistry
Paper
3 3
Fig. 2 SEM image of the Ag/WO nanostructured catalyst: (a) low magnification, (b) high magnification, (c) SEM-EDAX spectra and (d) Ag/WO cata-
lyst prepared by the impregnation method.
ultra small metallic AgNPs on WO3 nanorods, presumably contribution at 0.2873 nm with a coordination number (C.N.)
resulting from the strong metal support interaction which also of 7.7 and the absence of Ag–O contribution indicate the for-
helps to resist the sintering of AgNPs (Fig. 3c and Fig. S9†).
mation of metallic silver in the fresh catalyst. For the spent
To gain deeper insight into the surface constitution and catalyst, an Ag–Ag bond at 0.2872 nm with a C.N. of 7.6 was
properties of the catalyst, Ag 3d, W 4f and O 1s binding observed. No Ag–O coordination shell was observed after the
energies were measured by X-ray photoelectron spectroscopy reaction. The observation indicated that the oxidation state of
(Fig. 4, Fig. S1, S2, ESI†). The 3d orbital of silver clearly metallic silver remained unchanged after the reaction. Raman
resolved into Ag 3d5/2 and Ag 3d3/2 contributions centred at spectra of the Ag/WO nanostructured catalyst are character-
3
3
68.4 eV and 374.4 eV respectively, which are in good agree- ized by well resolved sharp bands as shown in Fig. 5. The
4
5
−1
ment with the binding energies of metallic silver Ag(0). The highly intense band at 807 cm in the Raman spectrum corre-
absence of negative shift of the Ag 3d5/2 binding energy for sponds to the symmetric stretching vibration and the intense
silver oxides with respect to metallic silver (368.4 eV) clearly band at 718 cm− corresponds to the asymmetric stretching
supports that no silver oxide was present on the surface of the vibration of the O–W–O bond. The medium intense band at
catalyst. The W 4f5/2 and 4f7/2 spectra are attributed to the 272 cm− and weak bands at 245 and 328 cm−1 are due to the
binding energies 38.1 eV and 36.0 eV respectively suggesting bending vibrations of the O–W–O bond. After the reaction, the
1
4
7
1
+
6 46
that the tungsten in the tungsten oxide sample exists as W .
Raman spectrum of the spent catalyst remained unchanged,
The O1s spectra of the sample show a peak occurring at 530.8 reflecting the structural stability of the catalyst under the reac-
eV, which corresponds to WvO bonding modes. Ag-K edge tion conditions.
extended X-ray absorption fine structure (EXAFS) analysis of
the Ag/WO nanostructured catalyst revealed that no signifi- methyl ammonium bromide (CTAB) with the Ag–WO
cant changes occurred in the oxidation state of the silver the IR spectra of the Ag/WO catalyst were taken before and
In order to check the association of the surfactant cetyltri-
3
3
moiety,
3
species of the fresh and the spent Ag/WO3 nanostructured after thermal heating. The as-synthesized sample showed that
catalyst during the cyclohexene oxidation reaction. Table 1 the presence of the organic moiety in the catalyst strongly
shows the structural parameters of the catalyst by curve-fitting suggests the association of the surfactant CTAB with the cata-
of the Ag K-edge EXAFS data. It was observed that an Ag–Ag lyst. The peak at 1402 cm− corresponds to the symmetric
1
This journal is © The Royal Society of Chemistry 2014
Green Chem., 2014, 16, 2826–2834 | 2829

View Article Online
Paper
Green Chemistry
Fig. 3 TEM images of (a) fresh catalyst, (b) lattice fringes, (c) spent catalyst after 5 cycles of reuse and (d) particle size distribution of silver
nanoparticles.
thermal heating, the absence of the organic moiety reflects the
complete removal of CTAB during calcination (Fig. S4, ESI†).
The TGA spectrum showed a decomposition peak at ∼400 °C,
well consistent with the decomposition temperature of the sur-
factant cetyltrimethyl ammonium bromide (CTAB) (Fig. S5,
ESI†). A total 14% mass loss suggests that surfactant molecules
=
+
are strongly bound with the WO
4
and Ag crystals for the as-
synthesized sample. The morphology of the Ag/WO catalyst is
3
not affected by the removal of the surfactant during thermal
heating, thus showing the excellent stability of the Ag/WO
3
nanostructured catalyst at higher temperature which is attribu-
ted to the strong metal–support interaction between AgNPs
and WO nanorods.
3
3
.2 Activities of the catalysts
Table 2 shows the activities of the different Ag/WO
the selective oxidation of cyclohexene in the liquid phase
3
–N moiety. The frequencies using H O as an oxidant. The Ag/WO nanostructured catalyst
3
Fig. 4 Ag 3d3/2 spectrum of the Ag/WO catalyst.
3
catalysts in
+
stretching vibration of the CH
2
2
3
−
1
between 1600 and 3000 cm may be due to the CH sym- showed >99.9% cyclohexene conversion with ∼94% adipic acid
metric and antisymmetric vibration regions. However, after (AA) selectivity (Table 2, entry 6). The catalyst does not show
2
4
8
2830 | Green Chem., 2014, 16, 2826–2834
This journal is © The Royal Society of Chemistry 2014

View Article Online
Green Chemistry
Paper
3
Table 1 EXAFS curve-fitting parameters at Ag–K-edge Fourier-filtered k -weighted EXAFS functions
R (1 × 10− nm) DW (1 × 10−3 nm )
1
2
ΔK (1 × 10 nm−1
)
Δr (1 × 10−1 nm)
S
0
2
ΔE
0
(eV)
Catalyst
Shell
CN
f
R (%)
Fresh catalyst
Spent catalyst
Ag–Ag
Ag–Ag
7.7 ± 0.6
7.6 ± 0.6
2.873 ± 0.031
2.872 ± 0.032
9.4 ± 0.4
8.6 ± 0.4
3–14
3–16
1.0–3.0
1.5–3.0
0.95
0.95
1.8 ± 0.6
2.1 ± 0.7
1.04
1.06
pared by the conventional impregnation method exhibited
much lesser activity due to the larger particle size and exces-
sive leaching of the metals (as shown in a comparative study in
Fig. 6, Table 2, and entry 5). From this observation, it can be
concluded that the preparation method determines both the
3
size and morphology of the Ag/WO nanostructured catalyst.
WO nanorods with diameter ∼50 nm act as an excellent plat-
3
form for higher dispersion of very small AgNPs (∼5 nm). Being
very small in size, these ∼5 nm AgNPs possess very high
specific surface area which corresponds to higher dispersion
of silver and leads to a higher reaction rate due to the avail-
ability of more exposed surface active sites, where the catalytic
reaction takes place. Moreover, the superior accessibility of the
AgNPs’ surfaces for oxidants and substrates leads to higher
Fig. 5 Raman spectra of the Ag/WO
spent catalyst.
3
catalyst: (a) fresh catalyst and (b) catalytic activity than that of the impregnated one. The poor
catalytic activity of the impregnated Ag/WO catalyst may be
3
attributed to their irregular shape and larger size of Ag and
3
WO particles, which limit the accessibility of the catalyst
any activity at room temperature but with increasing tempera-
ture both the conversion of cyclohexene as well as the selecti-
vity of adipic acid increased. At 90 °C, we found that the
formation of AA was maximum. At higher temperature
towards the reacting substrates. We also noticed that the
impregnated catalyst showed leaching of Ag particles during
the cyclohexene oxidation reaction. It is very clear from the
experiment that the nanosize effect has a vital role in the high
(120 °C), the yield of adipic acid decreases (Fig. S6, ESI†). Most
3
activity and selectivity of the Ag/WO nanostructured catalyst.
probably at higher temperature the decomposition of H O is
2
2
The blank experiment (Table 2, entry 8, without any catalyst)
gave no conversion. Commercial silver or tungsten oxide alone
also does not show any activity (Table 2, entries 1, 2). Even
bare silver and tungsten oxide prepared by our surfactant-
assisted method also exhibit very poor yields (Table 2, entries
taking place. The effect of reaction time on cyclohexene con-
version and adipic acid selectivity is shown in Fig. 7. It is well
observed that with increasing time, initially the formation of
cyclohexane oxide (till 6 h) and cyclohexane diol (till 12 h)
increases, but after 12 h the formation of AA increases very
rapidly. In order to find out the reaction intermediates, we
took aliquots of the sample at regular intervals and analysed
them by GC. We observed that after 12 h of reaction, the con-
version of cyclohexene rose to almost 90%, but the selectivity
towards AA was low, ∼40%. This is due to the fact that most
probably the formation of AA follows the consecutive reaction
pathway and proceeds through the intermediates cyclohexene
oxide and cyclohexene diol. We performed parallel reactions
with cyclohexene oxide and cyclohexane diol as the respective
reactants under the same reaction conditions and found that
3
, 4). We believe that AgNPs and WO
3
nanorods both act syner-
nano-
gistically to enhance the catalytic activity of the Ag/WO
3
structured catalyst towards the cyclohexene to adipic acid
reaction. This synergistic effect is absent for the conventional
catalyst, so they are inactive for reaction. The catalyst showed
its activity in the acetonitrile solvent. Most probably in the
presence of a catalyst, the acetonitrile solvent forms per-
−
hydroxyl anions ( OOH, nucleophile) with hydrogen peroxide
which attacks the C–N group of acetonitrile to generate peroxy-
carboxyimidic acid, which is a very good oxygen donor
5
0
species. This explains the necessity of the solvent in the
reaction.
1
00% conversion of both cyclohexene oxide and cyclohexane
diol was achieved (Table 2, entries 9, 10). So, cyclohexene to AA
formation took place via cyclohexene oxide and cyclohexane
diol intermediates. The amounts of catalyst weight and Ag
3.3 Reusability of the catalyst
loading were found to be very important, since lower Ag To check whether the catalyst is truly heterogeneous or not,
loading and lower catalyst weight decrease the reaction rate the catalytic oxidation of cyclohexene was carried out with the
without affecting the adipic acid selectivity significantly. reused catalyst under the same reaction conditions. The
Although the self-decomposition temperature of H
around 73 °C, we can say that in the presence of tungsten not lead to any significant decline in its catalytic activity in
species in the catalytic system, the self-decomposition of H
terms of conversion and selectivity (Table 2, entry 7 and
2 2
O is activity of the recovered catalyst after 5 consecutive runs did
2 2
O
4
9
is restricted. It has to be noted that the Ag/WO
3
catalyst pre- Fig. S7, ESI†). After completion of the reaction, the solid cata-
This journal is © The Royal Society of Chemistry 2014
Green Chem., 2014, 16, 2826–2834 | 2831

View Article Online
Paper
Green Chemistry
a
3
Table 2 Activities of the different Ag/WO catalysts
c
Cyclohexene
Selectivity S
P
(%)
Turnover
conversion
frequency
TOF (h )
b
d
−1
Entry
1
Catalyst
Substrate
C
T
(%)
AA
Diol
3
Others
95
Yield Y
A
(%)
com
Ag
1.2
2
0.02
—
com
2
3
4
5
6
WO
3
4.7
3.4
3.5
3.3
5.2
4
91.3
92.7
82
0.16
0.01
1.2
—
us
Ag
—
us
3
WO
12
10
8
—
imp
3
Ag/WO
15
12
94
6
84
1.5
0.23
12.0
e
f
Ag/WO
3
3
nanostructured catalyst
>99.9
6
—
94
7
8
Ag/WO
nanostructured catalyst
>99.9
91
9
—
—
91
11.6
No catalyst
—
—
—
—
—
g
9
Ag/WO
3
3
nanostructured catalyst
>99.9
>99.9
95
5
—
—
95
99
ND
ND
h
10
Ag/WO
nanostructured catalyst
>99
—
a
Reaction conditions: solvent (acetonitrile) = 10 ml, substrate (cyclohexene) = 1 g, weight of the catalyst = 0.15 g, silver loading = 3.8 wt%,
b
reaction temperature = 90 °C; time = 18 h; cyclohexene : H
O
2 2
mole ratio = 1 : 5.
C
T
: conversion of cyclohexene based upon the FID-GC results =
P
moles of cyclohexene reacted/initial moles of cyclohexene used] × 100. S : selectivity of the product calculated by total moles of the product
d
c
[
e
formed/total moles of cyclohexene converted.
Y (Yield of adipic acid) = conversion × selectivity/100. ∼5 nm Ag nanoparticles supported on
A
f
g h
WO
3
nanorods. Catalyst after 5 cycles of reuse. Time = 12 h. Time = 8 h; com = commercial; us = bare Ag and WO
3
prepared by our method;
imp = impregnation method.
lyst was removed from the reaction mixture by filtration during
the hot conditions and the reaction was allowed to proceed
with the filtrate under the same conditions. The reaction was
completely stopped after the removal of the catalyst. The leach-
ing test was performed for Ag and W by ICP-AES analysis using
the filtrate and it was found that no Ag or W ions were present
in the filtrate. We also observed that the amount of Ag and W
present in the spent catalyst after 5 cycles of reuse is the same
as that of the fresh catalyst as estimated by ICP-AES. XPS analy-
sis also confirmed that the Ag concentration for the fresh and
the spent catalyst on the surface are the same (Table S1†).
TEM images of the spent catalyst showed an almost similar
particle size to that of the fresh catalyst (Fig. S9†). So the cata-
lyst is truly heterogeneous in nature. The Raman spectrum of
the reused catalyst is almost the same as that of the fresh cata-
lyst, which further suggests the structural stability of the
nanostructured catalyst under the reaction conditions, which
is a prerequisite for practical applications.
Fig. 6 Comparison of the activities of the Ag/WO
catalyst and the impregnated catalyst.
3
nanostructured
2832 | Green Chem., 2014, 16, 2826–2834
This journal is © The Royal Society of Chemistry 2014

View Article Online
Green Chemistry
Paper
Notes and references
1
2
R. B. Laughlin, Phys. Rev. Lett., 1983, 50, 1395.
J. Hu, T. W. Odom and C. M. Lieber, Acc. Chem. Res., 1999,
2, 435.
M. Law, J. Goldberger and P. Yang, Annu. Rev. Mater. Res.,
004, 34, 83.
3
3
2
4
5
A. K. Geim and K. S. Novoselov, Nat. Mater., 2007, 6, 183.
S. J. Tauster, S. C. Fung and R. L. Garten, J. Am. Chem. Soc.,
1978, 100, 170.
6
7
8
9
A. Henglein, Chem. Rev., 1989, 89, 1861.
P. Mulvaney, Langmuir, 1996, 12, 788.
M. P. Pileni, New J. Chem., 1998, 22, 693.
A. C. Templeton, W. P. Wuelfing and R. W. Murray, Acc.
Chem. Res., 2000, 33, 27.
1
1
1
1
0 M. A. El-Sayed, Acc. Chem. Res., 2001, 34, 257.
1 N. Toshima and T. Yonezawa, New J. Chem., 1998, 22, 1179.
2 M. Brust and C. Kiely, Colloids Surf., A, 2002, 202, 175.
3 K. L. Kelly, E. Coronado, L. L. Zhao and G. C. Schatz,
J. Phys. Chem. B, 2003, 107, 668.
Fig. 7 Effect of time on cyclohexene oxidation. [■] Conversion of
cyclohexene; [ ] selectivity to adipic acid; [ ] selectivity to cyclohexene
oxide; [ ] selectivity to cyclohexane diol. Reaction conditions: solvent =
14 P. V. Kamat, J. Phys. Chem. B, 2002, 106, 7729.
acetonitrile; cyclohexene
=
1
g; weight of catalyst
=
0.15 g; 15 D. Adams, L. Brus, C. E. D. Chidsey, S. Creager, C. Cruetz,
C. R. Kagan, P. V. Kamat, M. Lieberman, S. Lindsay,
R. A. Marcus, R. M. Metzger, M. E. Michel-Beyerle,
J. R. Miller, M. D. Newton, D. R. Rolison, O. Sankey,
K. S. Schanze, J. Yardley and X. Zhu, J. Phys. Chem. B, 2003,
cyclohexene : H mole ratio = 1 : 5; temperature = 90 °C.
2
O
2
1
07, 6668.
4
. Conclusions
1
1
6 P. V. Kamat, Pure Appl. Chem., 2002, 74, 1693.
7 T. K. George and P. V. Kamat, Acc. Chem. Res., 2003, 36,
888.
8 Y. Lei, F. Mehmood, S. Lee, J. Greeley, B. Lee, S. Seifert,
R. E. Winans, J. W. Elam, R. J. Meyer, P. C. Redfern,
D. Teschner, R. Schlögl, M. J. Pellin, L. A. Curtiss and
S. Vajda, Science, 2010, 328, 224.
We have described a facile method for the synthesis of ∼5 nm
AgNPs supported on WO3 nanorods with diameter around
5
0 nm for the direct conversion of cyclohexene to adipic acid
1
with hydrogen peroxide as an oxidant. The catalyst shows a
cyclohexene conversion of >99.9% with 94% adipic acid
selectivity after 18 h. The catalyst was characterized by XRD,
XPS, ICP-AES, FT-IR, Raman spectroscopy, EXAFS, SEM and
TEM. The activity of the catalyst depends on Ag loading and
the size of the Ag nanoparticles. The enhanced activity of
1
2
2
2
2
9 Z. Qu, M. Cheng, W. Huang and X. J. Bao, J. Catal., 2005,
2
29, 446.
0 R. A. Van Santen and H. P. C. E. Kuipers, Adv. Catal., 1987,
5, 265.
∼
5 nm AgNPs on WO
small size and the synergy between the well-dispersed AgNPs
and WO nanorods (40–60 nm) in the supported Ag/WO
3
nanorods is likely caused by their very
3
1 J. Lu, J. J. Bravo-Suárez, A. Takahashi, M. Haruta and
S. T. Oyama, J. Catal., 2005, 232, 85.
2 P. Wang, Z. Wang, J. Li and Y. Bai, Microporous Mesoporous
Mater., 2008, 116, 400.
3 A. B. Castle, E. Gracia-Espino, C. Nieto-Delgado,
H. Terrones, M. Terrones and S. Hussain, ACS Nano, 2011,
3
3
nanostructured catalyst. The catalyst did not show any leaching
after 5 cycles of reuse, confirming the true heterogeneity of the
catalyst.
5
, 2458.
4 S. E. Davis, M. S. Ide and R. J. Davis, Green Chem., 2013, 15,
7.
5 R. Grabowski, J. Słoczyński, M. Śliwa, D. Mucha,
R. P. Socha, M. Lachowska and J. Skrzypek, ACS Catal.,
2011, 1, 266.
2
2
1
Acknowledgements
S.G. thanks UGC, and S.S.A. thanks CSIR, India for the fellow-
ships. R.B. thanks CSIR, New Delhi for the financial support
in the form of 12 FYP Project (CSC-0125). The Director, 26 Y. Kobayashi, V. Salgueiriño-Maceira and L. M. Liz-Marzán,
CSIR-IIP, is acknowledged for his support and encouragement. Chem. Mater., 2001, 13, 1630.
The authors thank Analytical Section Division, CSIR-IIP. The 27 M. Epifani, C. Giannini, L. Tapfer and L. Vasanelli, J. Am.
XAFS measurements were performed at KEK-IMSS-PF with the Ceram. Soc., 2000, 83, 2385.
approval of the Photon Factory Advisory Committee (project 28 X. Ye, Y. Zhou, J. Chen and Y. Sun, Appl. Surf. Sci., 2007,
010G109). 253, 6264.
2
This journal is © The Royal Society of Chemistry 2014
Green Chem., 2014, 16, 2826–2834 | 2833

View Article Online
Paper
Green Chemistry
2
3
9 Y. Sun, Chem. Soc. Rev., 2013, 42, 2497.
0 Y. Wen, X. Wang, H. Wei, B. Li, P. Jin and L. Li, Green
Chem., 2012, 14, 2868.
41 M. Newville, P. Livins, Y. Yacoby, E. A. Stern and J. J. Rehr,
Phys. Rev. B: Condens. Matter, 1993, 47, 14126.
42 A. L. Ankudinov, B. Ravel, J. J. Rehr and S. D. Conradson,
Phys. Rev. B: Condens. Matter, 1998, 58, 7565.
3
3
1 K. Sato, M. Aoki and R. Noyori, Science, 1998, 281, 1646.
2 S. A. Montzka, E. J. Dlugokencky and J. H. Butler, Nature, 43 A. L. Ankudinov, A. I. Nesvizhskii and J. J. Rehr, Phys. Rev.
011, 476, 43. B: Condens. Matter, 2003, 67, 115120.
3 A. Popp, H. Lotze-Campen and B. Bodirsky, Glob. Environ. 44 B. Ravel, J. Synchrotron Radiat., 2001, 8, 314.
2
3
3
3
3
3
Chang., 2010, 20, 451.
4 G. Lapisardi, F. Chiker, F. Launay, J. P. Nogier and
J. L. Bonardet, Catal. Commun., 2004, 5, 277.
5 (a) B. Meunier, Chem. Rev., 1992, 92, 1411; (b) B. S. Lane 46 X. L. Li, J. F. Liu and Y. D. Li, Inorg. Chem., 2003, 42,
and K. Burgess, Chem. Rev., 2003, 103, 2457. 921.
6 P. T. Anastas and J. C. Warner, Green Chemistry: Theory and 47 R. J. Bose, R. V. Kumar, S. K. Sudheer, V. R. Reddy and
Practice, Oxford University Press, 1998. V. Ganesan, Appl. Phys., 2012, 112, 114311.
7 T. Anastas, L. B. Bartlett, M. M. Kirchhoff and 48 W. Cheng, S. Dong and E. Wang, Langmuir, 2003, 19,
45 V. I. Bukhtiyarov, M. Hävecker, V. V. Kaichev, A. Knop-
Gericke, R. W. Mayer and R. Schlögl, Phys. Rev. B: Condens.
Matter, 2003, 67, 235422.
T. C. Williamson, Catal. Today, 2000, 55, 11.
8 J. H. Clark, Green Chem., 1999, 1, 1.
9 R. A. Sheldon, Green Chem., 2000, 2, G1.
0 E. A. Stern, M. Newville, B. Ravel, Y. Yacoby and D. Haskel, 50 U. R. Pillai and E. Sahle-Demessie, New J. Chem., 2003, 27,
Phys. B: Condens. Matter, 1995, 117, 208. 525.
9434.
3
3
4
49 R. Ishimoto, K. Kamata and N. Mizuno, Angew. Chem., Int.
Ed., 2012, 51, 4662.
2834 | Green Chem., 2014, 16, 2826–2834
This journal is © The Royal Society of Chemistry 2014