Selective allylic oxidation of cyclohexene over a novel nanostructured CeO2–Sm2O3/SiO2 catalyst

Article abstract of DOI:10.1007/s11164-018-3482-1

Abstract: Selective allylic oxidation of cyclohexene was investigated over nanostructured CeO₂/SiO₂ and CeO₂–Sm₂O₃/SiO₂ catalysts synthesized by a feasible deposition precipitation method. The CeO₂–Sm₂O₃/SiO₂ catalyst showed excellent catalytic efficiency with ~89?% cyclohexene conversion and ~90?% selectivity for allylic products (i.e., 2-cyclohexen-1-ol and 2-cyclohexene-1-one), while only ~50 and ~35?% cyclohexene conversion was observed, respectively, over CeO₂/SiO₂ and CeO₂ catalysts. Systematic characterization of the designed catalysts was undertaken to correlate their catalytic activity with the physicochemical properties using X-ray diffraction (XRD) analysis, Brunauer–Emmett–Teller (BET) surface area measurements, Raman spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and NH₃-temperature programmed desorption (TPD) techniques. The results revealed that doping of Sm³⁺ into the ceria lattice and simultaneous dispersion of resultant Ce–Sm mixed oxides on the silica surface led to improved structural, acidic, and catalytic properties. The better catalytic efficiency of CeO₂–Sm₂O₃/SiO₂ was due to high specific surface area, more structural defects, and high concentration of strong acid sites, stimulated by synergistic interaction between various oxides in the catalyst. The cyclohexene conversion and selectivity for allylic products depended on the reaction temperature, nature of solvent, molar ratio of cyclohexene to oxidant, and reaction time. Possible reaction pathways are proposed for selective allylic oxidation of cyclohexene towards 2-cyclohexen-1-ol and 2-cyclohexene-1-one products. Graphical Abstract: SiO₂-supported CeO₂–Sm₂O₃ nanocatalyst exhibited outstanding catalytic performance with superior selectivity for allylic products in liquid-phase selective oxidation of cyclohexene under mild reaction conditions.[Figure not available: see fulltext.].

Full text of DOI:10.1007/s11164-018-3482-1

Res Chem Intermed
https://doi.org/10.1007/s11164-018-3482-1
Selective allylic oxidation of cyclohexene over a novel
nanostructured CeO –Sm O /SiO catalyst
2
2
3
2
1
,2
1,3
1,2
•
Bolla Govinda Rao
M. Yugandhar Reddy
•
Putla Sudarsanam
•
•
P. R. G. Nallappareddy
1,2
•
1
T. Venkateshwar Rao
1
,2
Benjaram M. Reddy
Received: 24 January 2018 / Accepted: 21 May 2018
Ó Springer Science+Business Media B.V., part of Springer Nature 2018
Abstract Selective allylic oxidation of cyclohexene was investigated over nanostruc-
tured CeO /SiO and CeO –Sm O /SiO catalysts synthesized by a feasible deposition
2
2
2
2
3
2
precipitation method. The CeO –Sm O /SiO catalyst showed excellent catalytic effi-
2
2
2
3
ciency with *89 % cyclohexene conversion and *90 % selectivity for allylic products
i.e., 2-cyclohexen-1-ol and 2-cyclohexene-1-one), while only *50 and *35 %
cyclohexene conversion was observed, respectively, over CeO /SiO and CeO catalysts.
(
2
2
2
Systematic characterization of the designed catalysts was undertaken to correlate their
catalytic activity with the physicochemical properties using X-ray diffraction (XRD)
analysis, Brunauer–Emmett–Teller (BET) surface area measurements, Raman spec-
troscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy
(
XPS), and NH -temperature programmed desorption (TPD) techniques. The results
3
3?
revealed that doping of Sm into the ceria lattice and simultaneous dispersion of
resultant Ce–Sm mixed oxides on the silica surface led to improved structural, acidic,
and catalytic properties. The better catalytic efficiency of CeO –Sm O /SiO was due to
2
2
3
2
high specific surface area, more structural defects, and high concentration of strong acid
sites, stimulated by synergistic interaction between various oxides in the catalyst. The
cyclohexene conversion and selectivity for allylic products depended on the reaction
temperature, nature of solvent, molar ratio of cyclohexene to oxidant, and reaction time.
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11164-
018-3482-1) contains supplementary material, which is available to authorized users.
&
Benjaram M. Reddy
bmreddy@iict.res.in; mreddyb@yahoo.com
1
Inorganic and Physical Chemistry Division, CSIR–Indian Institute of Chemical Technology,
Uppal Road, Hyderabad 500 007, India
2
3
Academy of Scientific and Innovative Research, CSIR-IICT, Hyderabad, India
Center for Surface Chemistry and Catalysts, KU Leuven, Celestijnenlaan 200f, Box 2461,
3001 Louvain, Belgium
123

B. G. Rao et al.
Possible reaction pathways are proposed for selective allylic oxidation of cyclohexene
towards 2-cyclohexen-1-ol and 2-cyclohexene-1-one products.
Graphical Abstract SiO -supported CeO –Sm O nanocatalyst exhibited outstanding
2
2
2 3
catalytic performance with superior selectivity for allylic products in liquid-phase
selective oxidation of cyclohexene under mild reaction conditions.
OH
O
+
Cy-Ol Cy-One
o
TBHP, 70 C
CH CN
3
O
Cyclohexene
CeO -Sm O /SiO
2
2
2
3
Cy-Oxide
Keywords Cyclohexene Á Allylic oxidation Á Ceria Á Samarium Á Silica Á Acid
sites
Introduction
Selective oxidation of olefins has attracted significant research interest owing to the
numerous applications of the resultant oxygenated products in chemical industry
[
1–4]. In particular, selective oxidation of cyclohexene is a vital chemical
transformation, since it contains two reactive centers (allylic C–H and olefinic
C=C). Therefore, controlling the oxidation at one of these positions is a key step for
synthesis of desired products [5]. Direct epoxidation of cyclohexene at olefinic C=C
bond yields cyclohexene oxide, whereas 2-cyclohexen-1-ol and 2-cyclohexen-1-one
are obtained as products of allylic oxidation of cyclohexene. 2-Cyclohexen-1-ol and
2
-cyclohexene-1-one are important intermediates in production of spices, pesticides,
medicines, and insect pheromones [6–9]. Also, cyclohexene oxide can be used as a
monomer in synthesis of polymers and as a key intermediate in the coatings industry
[
10, 11].
N-tert-Butylphenylsulfinimidoyl chloride and 2-iodoxybenzoic acid are oxidizing
agents typically employed for selective oxidation of cyclohexene [12]. However,
these oxidants are highly toxic and generate large amounts of undesirable side
products and hazardous organic waste [13, 14]. Alternatively, some other oxidants
including H O , tert-butyl hydroperoxide (TBHP), and O have been explored,
2
2
2
yielding quite positive results. Generally, catalysts based on homogeneous systems
are reported to be efficient for oxidation of cyclohexene. However, application of
such liquid-phase catalysts on industrial scale is often hindered by various problems,
such as difficulty in catalyst separation from the reaction mixture and recycling, and
generation of large amounts of unwanted products which exhibit strong corrosion
1
23

Selective allylic oxidation of cyclohexene over a novel…
[
15], among others. To overcome these issues, a wide variety of heterogeneous
catalysts, especially based on metal nanoparticles dispersed on various supports,
such as SiO , TiO , graphite, polyoxometalates, carbon nanotubes, zeolites, and
2
2
metalorganic frameworks (MOFs), have been investigated [12, 16–21]. Among the
various catalysts examined, supported noble metals (e.g., Au, Pd, and Ru) have been
found to show excellent catalytic performance [22–24]. In spite of their high
catalytic activity, use of noble metals is strongly discouraged in chemical industry
due to their high cost and limited availability. Against this background, develop-
ment of cost-effective and highly efficient noble-metal-free catalysts is highly
desirable for oxidation of cyclohexene and related reactions.
Ceria has been recognized as a promising catalyst component in many organic
reactions, such as dehydration of alcohols, hydrolysis of nitriles, C–C coupling, C–
O cleavage, oxidation of olefins, oxidative coupling of amines, oxidation of
alcohols, etc. [25–30]. The widespread application potential of ceria is due to its
4
?
3?
facile redox couple (Ce $ Ce ) and abundant oxygen vacancies (structural
defects) [31]. In addition, dual acid–base nature is another characteristic feature of
ceria-based oxides. As is known from literature, the acidic property of catalysts
plays a crucial role in oxidation of cyclohexene [32]. It is therefore believed that, by
appropriately modifying its redox and acidic properties, the catalytic efficiency of
ceria for oxidation of cyclohexene could be improved. Several promising strategies
to tune the structure–activity relationship of ceria have been reported, including
doping of suitable cations into the ceria lattice, dispersion of the resultant mixed
oxide on a high-surface-area support, and tailoring of the oxygen vacancy
3
?
population [33, 34]. Incorporation of an aliovalent cation such as Sm is expected
to introduce strain into the ceria lattice due to the difference in ionic radius between
3
?
4?
˚
(1.08 A) and Ce
˚
(0.97 A) cations. Stimulation of lattice strain
the Sm
contributes to formation of unusual structural and redox properties in the final mixed
oxides, resulting in unusual catalytic performance [35]. Furthermore, homogeneous
dispersion of nanooxides on a high-surface-area support (e.g., SiO ) can lead to
2
enhanced textural and redox properties due to synergistic metal oxide–support oxide
interactions. It is therefore highly desirable to investigate the characteristics of ceria
after simultaneous incorporation of Sm cations and SiO support, and their ensuing
2
role in selective oxidation of cyclohexene. The present investigation was undertaken
against the aforesaid background. Accordingly, systematic structure–activity
relationship investigation is expected to provide useful information for development
of feasible and selective catalytic protocols for not only oxidation of cyclohexene
but also other industrially important chemical reactions.
In this work, nanosized CeO /SiO2 and CeO –Sm O /SiO catalysts were
2
2
2
2
3
synthesized and evaluated for oxidation of cyclohexene using TBHP as oxidant.
Physicochemical characterization of the developed catalysts was carried out using
X-ray diffraction (XRD) analysis, Brunauer–Emmet–Teller (BET) surface area
measurements, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS),
transmission electron microscopy (TEM), and ammonia-temperature programmed
desorption (NH -TPD) techniques. The effect of reaction temperature, nature of
3
solvent, molar ratio of cyclohexene to oxidant, and reaction time were also
investigated to optimize the reaction conditions. Efforts were also made to correlate
123

B. G. Rao et al.
the catalytic activity results with the physicochemical properties of the synthesized
catalysts.
Experimental
Catalyst preparation
CeO /SiO catalyst (CeO :SiO = 1:1 mol ratio based on oxides) was prepared by
2
2
2
2
deposition precipitation method. Briefly, estimated stoichiometric amounts of
colloidal silica (Ludox, 40 wt%, Aldrich) and Ce(NO ) Á6H O [Aldrich, analytical
3
3
2
reagent (AR) grade] were dissolved separately in deionized water then mixed
together under stirring. Subsequently, the precipitating agent, i.e., aqueous NH3
solution, was added dropwise to the above solution until the pH of the solution
reached *8.5. The resultant slurry was filtered off and washed several times with
double-distilled water until free from anion impurities. The obtained sample was
oven-dried at 120 °C for 12 h and finally calcined at 500 °C for 5 h at heating rate
-
1
of 5 °C min in air atmosphere.
CeO –Sm O /SiO catalyst (CeO :Sm O :SiO = 0.8:0.2:1 mol ratio based on
2
2
2
3
2
2
3
2
oxides) was prepared by deposition coprecipitation method. In a typical procedure,
colloidal SiO was dispersed in deionized water and stirred for 2 h. Desired amounts
2
of Ce(NO ) Á6H O (Aldrich, AR grade) and Sm(NO ) Á6H O (Aldrich, AR grade)
3
3
2
3 3
2
were dissolved in deionized water and mixed together under mild stirring
conditions. The resultant Ce–Sm nitrate solution was added to the dispersed
solution of SiO support. Subsequently, aq. NH solution was added dropwise to the
2
3
above mixture solution until the pH of the solution reached *8.5. The resulting
slurry was filtered off, washed with double-distilled water, and oven-dried at 120 °C
for 12 h. The resulting cake was calcined at 500 °C for 5 h at heating rate of
-
1
5
°C min
following the same procedure under identical conditions and calcined at 500 °C for
h.
in air atmosphere. For comparison, pure CeO was also prepared
2
5
Catalyst characterization
Powder XRD patterns were recorded on a Rigaku diffractometer using Cu Ka
˚
radiation (1.540 A), operated at 40 kV and 40 mA. Diffractograms were recorded in
the 2h range of 10–80° with 2h step size of 0.02° and step time of 2.4 s. XRD phases
present in the samples were identified with the help of Powder Diffraction Files
(
PDFs) from the International Centre for Diffraction Data (ICDD). The average
crystallite size of ceria in various samples was calculated using the Scherrer
equation, and the lattice parameter was estimated by standard cubic indexation
method using the intensity of the most prominent peak (111).
The BET surface area of various samples was determined by N adsorption–
2
desorption measurements using a Micromeritics Gemini 2360 instrument. Prior to
analysis, the samples were oven-dried at 150 °C for 12 h and flushed with argon gas
for 2 h to remove any surface-adsorbed residue. Surface area was calculated by
1
23

Selective allylic oxidation of cyclohexene over a novel…
utilizing the desorption data. A Horiba Jobin–Yvon HR800 Raman spectrometer
equipped with a liquid-nitrogen-cooled charge-coupled device (CCD) detector and
confocal microscope was used to obtain Raman spectra of various samples. The
?
emission line at 638 nm from an Ar laser (Spectra Physics) was focused on the
sample under a microscope with diameter of the analyzed spot being *1 lm, under
ambient conditions. The acquisition time was adjusted according to the Raman
scattering intensity.
TEM/high-resolution electron microscopy (HREM) studies were carried out
using a Tecnai G2 TEM microscope equipped with a slow-scan CCD camera at
accelerating voltage of 200 kV. Carbon-coated Cu grid was used for preparation of
samples for TEM analysis. Preparation of samples for TEM-HREM analysis
involved sonication in ethanol for 2–5 min followed by deposition of a drop on a
copper grid. The specimen was examined under vacuum at room temperature. XPS
measurements were performed using a PHI 5400 instrument with an Al K
(
a
-
7
1486.6 eV) X-ray source at pressure below 10 Torr [36]. Sample preparation for
XPS analysis involved mounting of a few milligrams of sample on carbon tape
supported by silica plate, which was placed in the vacuum chamber of a Thermo
K-Alpha XPS instrument before starting analysis. Thermo Avantage software was
used for XPS analysis of the prepared samples. The binding energies of Ce, Sm, and
O were charge-corrected with respect to the adventitious carbon (C 1s) peak at
284.6 eV. We used a flood gun to remove static charge that developed on the sample
surface during XPS analysis.
NH -temperature programmed desorption experiments were carried out on a
3
Auto-Chem 2910 instrument (Micromeritics). Approximately 30 mg sample was
placed in a quartz tube and degassed up to 300 °C under He flow. Then, anhydrous
NH gas was passed over the sample surface for 30 min, followed by flushing with
3
helium gas to remove physisorbed gas. Then, the amount of chemisorbed NH was
3
-
1
estimated in He gas flowing at rate of 20 mL min from 50 to 800 °C at heating
-
1
rate of 10 °C min
.
Activity measurements
Liquid-phase oxidation of cyclohexene was carried out using TBHP and acetonitrile
as oxidant and solvent, respectively. In a typical experiment, 0.1 g catalyst, 4 mmol
cyclohexene, and 5 ml solvent were charged into a 25-ml three-necked round-
bottomed flask equipped with a condenser. Then, the required amount of TBHP was
slowly added to the reaction mixture under vigorous stirring (*600 rpm). The
mixture was then heated to the desired temperature in an oil bath. After reaction
completion, the liquid products and the catalyst were separated by centrifugation.
The derived products were further diluted with acetonitrile solvent and analyzed by
gas chromatography (GC) using a BP-20 (wax) capillary column and flame
ionization detector. The products were confirmed by GC–mass spectroscopy (MS)
using a DB-5 capillary column and mass detector. Reaction products were also
confirmed by injecting corresponding authentic compounds into the GC. The
obtained cyclohexene conversion and product selectivity values are within the
experimental error of ± 2 percent.
123

B. G. Rao et al.
Results and discussion
Catalyst characterization
Figure 1 shows the XRD profiles of the CeO , CeO /SiO , and CeO –Sm O /SiO
2
2
2
2
2
2
3
catalysts. All samples exhibited fluorite-structured ceria, as evidenced by the
observation of characteristic diffraction peaks of (111), (200), (220), (311), (420),
and (422) in Fig. 1 [37]. This indicates that addition of the dopant (Sm) and support
(
Sm O for the CeO –Sm O /SiO sample can be attributed to several factors,
SiO ) did not alter the structure of CeO . The absence of XRD peaks pertaining to
2 2
2
3
2
2
3
2
3
?
including incorporation of Sm ions into the ceria lattice, amorphous nature of
Sm O , or high dispersion of Sm O species [38, 39]. Also, no XRD peaks
corresponding to the SiO support were observed in the case of the CeO /SiO and
2
3
2 3
2
2
2
CeO –Sm O /SiO samples, indicating amorphous nature of SiO . Close inspection
2
2
3
2
2
of Fig. 1 reveals that the XRD profiles of the CeO /SiO and CeO –Sm O /SiO
2
2
2
2
2
3
samples were quite different compared with that of pristine CeO in terms of peak
2
position and intensity. Changes in the ceria crystallite size and lattice parameter
after addition of Sm and SiO are a probable reason for this observation. As
2
presented in Table 1, the CeO /SiO (8.1 ± 1 nm) and CeO –Sm O /SiO
2
2
2
2
2
3
(
5.2 ± 1 nm) samples showed smaller crystallite size compared with pristine
CeO (8.8 ± 1 nm). This observation indicates a favorable role of the dopant and
2
support in controlling crystal growth of ceria during high-temperature calcination.
Nanoparticles are likely to undergo aggregation at elevated temperatures because of
their exceptional surface energy. Incorporation of a dopant could reduce the surface
energy of the ceria nanoparticles due to formation of strong metal–metal
interactions, hence controlling the particle size, as clearly evidenced in the present
work and in good agreement with literature [40]. The lattice parameter was found to
be 0.540, 0.542, and 0.544 nm for the CeO , CeO /SiO , and CeO –Sm O /SiO
2
2
2
2
2
3
2
samples, respectively.
Fig. 1 Powder XRD patterns of
CeO -Sm O /SiO
2
2
2
3
CeO
Sm
2
, CeO
/SiO
2
/SiO , and CeO
catalysts
2
2
–
#
2
O
3
2
#
CeO2
#
#
#
#
#
CeO /SiO2
2
CeO2
20
30
40
50
60
70
80
o
2
Theta ( )
1
23

Selective allylic oxidation of cyclohexene over a novel…
Table 1 BET surface area (S), average crystallite size (D), lattice parameter (LP), pore volume (V), and
catalysts
pore size (P) of CeO , CeO
2
2
/SiO , and CeO
2
2
2
–Sm O
3
/SiO
2
2 -1 a
S (m g )
b
b
3
V (cm g
-1 c
)
c
Sample
D (nm)
LP (nm)
P (nm)
CeO
CeO
CeO
a
2
2
2
40 ± 2
120 ± 2
193 ± 2
8.8 ± 1
8.1 ± 1
5.2 ± 1
0.540 ± 0.05
0.542 ± 0.05
0.544 ± 0.05
0.114 ± 0.01
0.462 ± 0.02
0.383 ± 0.01
9.84 ± 1
7.53 ± 1
5.40 ± 1
/SiO
2
2
–Sm
3 2
O /SiO
From BET analysis
b
From XRD analysis
c
From Barrett–Joyner–Halenda (BJH) analysis
Figure 2 shows the N adsorption–desorption isotherms of the CeO , CeO /SiO ,
2
2
2
2
and CeO –Sm O /SiO catalysts; the corresponding BET surface area, pore
2
2
3
2
diameter, and pore volume values are presented in Table 1. As shown in Fig. 2,
the pure CeO and CeO –Sm O /SiO samples exhibited a type IV isotherm with
H1 hysteresis loop, attributed to formation of mesopores. Unlike the pure CeO and
2
2
2
3
2
2
CeO –Sm O /SiO samples, the CeO –SiO sample showed a H2 hysteresis loop,
2
2
3
2
2
2
indicating formation of pores with narrow necks and wide bodies. This is probably
due to segregation of silica particles inside pores of ceria (CeO /SiO ), resulting in
the H2 isotherm and wide pore size distribution. The BET surface area of pristine
2
2
2
2
CeO was found to be *40 m /g, increasing significantly to *120 m /g after
2
addition of SiO , indicating the favorable role of the support in enhancing the BET
surface area of CeO . Interestingly, simultaneous addition of Sm and SiO to CeO
2
2
2
2
2
remarkably enhanced the specific surface area from *40 to *193 m /g (Table 1),
indicating a synergetic effect of Sm and SiO in improving the textural properties of
the CeO . The pore size distribution profiles of CeO , CeO /SiO , and CeO –
2
2
2
2
2
2
Fig. 2 N
desorption isotherms of CeO
CeO /SiO , and CeO –Sm
SiO catalysts
2
adsorption–
2
,
CeO -Sm O /SiO
2
2
3
2
2
2
2
2 3
O /
2
CeO /SiO
2
2
CeO2
.4
0
0.6
0.8
1.0
O
Relative pressure (P/P )
123

B. G. Rao et al.
Sm O /SiO catalysts are shown in Fig. S1. A unimodal pore size distribution was
2
3
2
observed for the CeO and CeO /SiO catalysts, whereas a bimodal distribution was
2
2
2
noted for the CeO –Sm O /SiO catalyst. The obtained pore volume and pore size
2
2
3
2
3
-1
values for pure CeO were *0.114 cm g and *9.84 nm, respectively (Table 1).
2
Also, the pore size and pore volume values of CeO /SiO and CeO –Sm O /SiO
3
2
2
2
2
3
2
-1
samples were *7.53 nm and *0.462 cm g
,
and *5.40 nm and
3 -1
0.383 cm g , respectively.
*
Figure 3 shows the Raman spectra of the CeO , CeO /SiO , and CeO –Sm O /
2
2
2
2
2 3
-
1
SiO samples. A major Raman band centered at *465 cm was observed for all
2
catalysts, attributed to the F2g Raman-active mode of fluorite-structured CeO in
space group Fm3m [41, 42]. No Raman bands were found for Sm O , in line with
2
2
3
the XRD results (Fig. 1). Thus, the XRD and Raman results confirm incorporation
of Sm into the ceria lattice, forming a homogeneous ceria solid solution. A
noticeable shift of the F2g peak towards lower wavenumber as well as peak
broadening were observed for the CeO /SiO and CeO –Sm O /SiO samples
2
2
2
2
3
2
compared with pure CeO . Variation in the Ce–O frequencies is one of the key
2
reasons for this observation [43]. In addition to the F2g band, two more Raman
bands were noticed for the CeO –Sm O /SiO sample: The appearance of a band at
2
2
3
2
-
560 cm is attributed to presence of oxygen vacancies, whereas the band at
1
*
*
-
1
606 cm
corresponds to SmO8 defect complex [44, 45]. Since no oxygen
3
?
vacancy band was observed for the CeO /SiO catalyst, the dopant Sm appears to
2
2
play a unique role in generating oxygen vacancies in the cerium oxide lattice.
Formation of oxygen vacancies in the CeO –Sm O /SiO sample can be
attributed to the charge compensation mechanism, due to the difference in ionic
2
2
3
2
3
?
4?
˚
˚
radius between the Sm (1.08 A) and Ce (0.97 A) cations. Figure 4 shows TEM
and HREM images of the CeO –Sm O /SiO sample. Note that the CeO –Sm O /
SiO sample contained sphere-shaped particles and the particle size was found to lie
2
2
3
2
2
2 3
2
in the range of 6–8 nm. The particle size distribution histogram for the CeO2–
Sm O /SiO sample estimated from TEM analysis is shown inset to Fig. 4a. Lattice
2
3
2
d-spacing of *0.31 nm was also estimated, corresponding to the distance between
adjacent (111) crystal planes of fluorite-structured CeO [46].
2
Fig. 3 Raman spectra of CeO
CeO /SiO , and CeO –Sm
SiO catalysts
2
,
/
F2g
CeO -Sm O /SiO
2
2
2
2
O
3
2
2
3
2
2
Ovacancy
SmO8
CeO /SiO
2
2
CeO2
600
3
50
400
450
500
550
650
-
1
Raman shift (cm )
1
23

Selective allylic oxidation of cyclohexene over a novel…
50
40
30
20
10
0
2
-4
4-6
6-8
8-10
Partcle size (nm)
2 2 3 2
Fig. 4 TEM and HREM images of a–c CeO –Sm O /SiO catalyst
Figure 5 shows the Ce 3d XP spectra of the pure CeO , CeO /SiO , and CeO –
2
2
2
2
Sm O /SiO samples. It is well known from literature reports [47, 48] that Ce 3d XP
2
3
2
spectra are complex, due to coexistence of multiple oxidation states as well as
mixing of O 2p and Ce 4f levels in the primary photoemission process. Peaks
labeled u correspond to Ce 3d3/2 spin–orbit states, while those labeled v correspond
0
0
3?
to Ce 3d5/2 contributions. As shown in Fig. 5, peaks labeled u and v belong to Ce
1
1
0
00
00
with electronic configuration 3d 4f whereas other bands labeled u, u , u¢¢¢, v, v ,
1
0
0
4?
and v¢¢¢ belong to the 3d 4f electronic state corresponding to Ce . The presence
4
?
3?
of all these peaks reveals coexistence of Ce and Ce species over the catalyst
surface, indicating the redox nature of the as-prepared catalysts [49–51]. The
binding energy of all the peaks observed for the CeO , CeO /SiO , and CeO –
2
2
2
2
Sm O /SiO samples are presented in Table S1. The Ce 3d spectrum of the CeO –
2
3
2
2
Sm O /SiO sample is notably different from those of the CeO /SiO and CeO
2
2
3
2
2
2
2
samples (Fig. 5). Much higher binding energy was noted for the CeO –Sm O /SiO
2 2 3
catalyst compared with the CeO /SiO and CeO samples, which is probably due to
2
2
2
Fig. 5 Ce 3d XP spectra of
///
CeO -Sm O /SiO
2
Ce 3d
v
2
2
3
CeO
Sm
2
, CeO
/SiO
2
/SiO , and CeO
catalysts
2
2
–
v
/
u
/
///
v
//
u
2
O
3
2
v
u
//
u
CeO /SiO
2
2
CeO2
880
890
900
910
920
Binding energy (eV)
123

B. G. Rao et al.
the fact that the dopant and support could influence the chemical environment of
CeO . Strong synergetic interaction of ceria with the dopant and support could be
2
the reason for this influence on the chemical environment of CeO2 [52–54].
Furthermore, we estimated the relative percentage of cerium species using the area
3
ratio of Ce /Ce ? Ce ; the obtained values are presented in Fig. S2, revealing
?
3?
4?
an order of the samples of CeO –Sm O /SiO (23.1 %) [ CeO /SiO (20.3 %)
2
2
3
2
2
2
?
3
[ CeO (18.0 %). This trend clearly demonstrates that the fraction of Ce was
2
remarkably enhanced by simultaneous introduction of the dopant and support into
ceria.
The Sm 3d5/2 XP spectrum of the CeO –Sm O /SiO sample shows a peak on the
2
2
2
3
higher binding energy side (*1109.2 eV), which reveals presence of Sm in 3?
oxidation state (Fig. S3). Another peak was found at lower binding energy
(
in Sm O [55]. The deconvoluted O 1s spectra of all samples are presented in
*1082.3 eV), which is due to the charge transfer effect of the unpaired 4f electrons
2
3
Fig. S4. The appearance of a peak at *529.2 eV (O ) can be attributed to ceria
I
lattice oxygen. On the other hand, the presence of a peak at *530–531 eV (O ) is
II
due to adsorbed oxygen species of hydroxyl groups. Another peak at
*
species. The Si 2p core-level XP spectra of the CeO /SiO and CeO –Sm O /SiO
531.8–533.2 eV (O ) can be ascribed to adsorbed molecular water and carbonate
III
2
2
2
2
3
2
4
?
catalysts show a peak at *102.8 eV, which is attributed to Si species (Fig. S5)
56, 57].
NH -TPD analysis was undertaken to estimate the acidic properties of the
[
3
catalysts. As shown in Fig. 6, various desorption peaks were observed for the CeO2/
SiO and CeO –Sm O /SiO catalysts. This is due to variation in the activation
energy of NH molecules desorbed from different acidic sites present on the catalyst
2
2
2
3
2
3
surface [25, 58]. The NH desorption profiles could be divided to above and below
3
desorption temperature of 400 °C, corresponding to the low-temperature (LT) and
Fig. 6 NH
3
-TPD profiles of
, CeO –Sm , and
/SiO catalysts
CeO -Sm O /SiO
4
01
2
2
3
2
CeO
CeO
2
/SiO
–Sm
2
2
2 3
O
2
2
O
3
2
6
46
744
3
97
CeO /SiO
2
2
4
54
7
05
400
500
600
700
800
Temeprature (K)
1
23

Selective allylic oxidation of cyclohexene over a novel…
high-temperature (HT) region, respectively. The LT region peaks indicate
desorption of NH3 molecules from weak acidic sites present on the catalyst
surface, whereas the HT region peaks correspond to presence of strong acidic sites.
The CeO –Sm O /SiO catalyst exhibited three peaks, with two peak maxima at
2
2
3
2
higher temperatures (373 and 471 °C), while only one peak was found for the Ce/
SiO catalyst at higher temperature (432 °C). This observation indicates that the
2
CeO –Sm O /SiO catalyst contains a large amount of strong acid sites. It is
therefore clear that simultaneous addition of Sm and SiO results in improved acidic
2
2
3
2
2
strength of CeO , which could play a key role in oxidation of cyclohexene, as
2
discussed below.
Catalytic activity results
Based on the activity results obtained in this work and from literature reports
[
59, 60], plausible reaction pathways can be proposed for oxidation of cyclohexene.
As shown in Scheme 1, oxidation of cyclohexene using TBHP proceeds via two
pathways, namely allylic and epoxidation. Initially, cyclohexene reacts with TBHP
to give cyclohexene oxide (Cy-Oxide) and 3-(tert-butylperoxy) cyclohex-1-ene (Cy-
TBHP) through epoxidation and allylic pathways, respectively. 2-Cyclohexen-1-
hydroperoxide (Cy-HP) could also be obtained from Cy-TBHP with elimination of
tertiary butyl alcohol. However, 2-cyclohexene-1-hydroperoxide is quite unsta-
ble and immediately reacts with cyclohexene to yield cyclohexene oxide (Cy-
Oxide) via the epoxidation route (step 1). In parallel, 2-cyclohexen-1-hydroperoxide
(CY-HP) can decompose upon heating to yield 2-cyclohexen-1-ol (Cy-Ol, step 2) as
well as cyclohexen-1-one (Cy-One, step 3a) with release of water, which is an acid-
TBHP
O
Epoxidationpath
Cyclohexene
Cy-Oxide
Allylic path TBHP
Step 1
O
Step 2
-TBA
H O
O
2
O
O
H
O
Cy-HP
Cy-One
O
Cy-TBHP
Decomposition Step3a
OH
TBHP
Step 3b
Cy-Ol
Cy-One
Scheme 1 Plausible reaction pathways for oxidation of cyclohexene
123

B. G. Rao et al.
catalyzed reaction [18, 61]. Cyclohexen-1-one (Cy-One) can also be formed via
oxidation of 2-cyclohexen-1-ol (Cy-Ol, step 3b) with water as byproduct.
The results achieved for oxidation of cyclohexene over the CeO , CeO /SiO ,
2
2
2
and CeO –Sm O /SiO catalysts are shown in Fig. 7. Catalyst screening was
2
2
2
3
conducted at 70 °C using TBHP and acetonitrile as oxidant and solvent,
respectively, for 4 h. It is obvious from Fig. 7 that CeO alone showed reasonable
conversion of cyclohexene (*35 %), indicating the oxidizing ability of pure CeO2
for this reaction. High selectivity for allylic products (*78 %) but poor selectivity
2
for cyclohexene oxide (12 %) and Cy-TBHP (*10 %) was found with CeO . This
2
observation indicates that the allylic oxidation pathway is favored when using the
CeO catalyst. Interestingly, considerable enhancement in cyclohexene conversion
2
(
CeO /SiO catalyst. On the other hand, the conversion of cyclohexene was
*50 %) and selectivity for allylic products (*81 %) was observed when using the
2
2
significantly increased to *89 % when using the CeO –Sm O /SiO catalyst, with
2
2
3
2
high combined selectivity (*90 %) for Cy-Ol and Cy-One products. These results
indicate a cooperative effect of Sm and SiO in enhancing the catalytic strength of
CeO for oxidation of cyclohexene. As shown in Scheme 1, the allylic oxidation
2
2
pathway of cyclohexene could be enhanced by the dehydration steps of 2-cyclo-
hexen-1-hydroperoxide (step 3a) and 2-cyclohexen-1-ol (step 3b). The dehydration
is a typical acid-catalyzed reaction. Based on the NH -TPD results (Fig. 6), the
3
CeO –Sm O /SiO catalyst contained a large number of strong acidic sites, hence
2
2
3
2
exhibiting higher catalytic efficiency for allylic oxidation of cyclohexene. In
addition, the high BET surface area of the CeO –Sm O /SiO catalyst (Table 1) is
2
2
3
2
expected to play a crucial role in the cyclohexene oxidation reaction, because a
high-surface-area catalyst can provide a greater number of active surface sites to
promote oxidation of cyclohexene molecules, resulting in high conversion of
cyclohexene.
(
A)
50
90
80
70
60
50
40
30
(B)
45
40
35
30
25
20
15
10
5
Conv. of cyclohexene
Sel. of Cy-Oxide
Sel. of Cy-Ol
Sel. of Cy-One
Sel. of Cy-TBHP
CeO2
CeO2/SiO2 CeO2-Sm2O3/SiO2
CeO2
CeO2/SiO2 CeO2-Sm2O3/SiO2
Catalysts
Catalysts
2 2 2 2 2 3 2
Fig. 7 Selective oxidation of cyclohexene over CeO , CeO /SiO , and CeO –Sm O /SiO catalysts:
a conversion of cyclohexene and b selectivity for cyclohexene oxide (Cy-Oxide), 2-cyclohexen-1-ol (Cy-
Ol), 2-cyclohexen-1-one (Cy-One), and 3-(tert-butylperoxy) cyclohex-1-ene (Cy-TBHP)
1
23

Selective allylic oxidation of cyclohexene over a novel…
To optimize the reaction conditions, the effect of various reaction parameters was
investigated for oxidation of cyclohexene using CeO –Sm O /SiO catalyst. The
2
2
3
2
effect of reaction temperature on the cyclohexene conversion and product selectivity
was investigated by varying the temperature from 40 to 70 °C for the reaction over
CeO –Sm O /SiO ; the results are presented in Fig. 8. The conversion of
2
2
3
2
cyclohexene and the selectivity for allylic products gradually increased with
increase of the reaction temperature. When the reaction was performed at room
temperature, very low conversion of cyclohexene was found (*10 %) for time of
4 h, revealing the necessity for higher temperature to activate cyclohexene
molecules to participate in the oxidation reaction. When the reaction was conducted
at 40 °C, the cyclohexene conversion was about *28 %, which increased to
*89 % when the temperature was increased from 40 to 70 °C. This result confirms
the crucial role of reaction temperature in the oxidation of cyclohexene. Also, the
selectivity for allylic products, i.e., Cy-Ol and Cy-One, was found to increase from
*
*
80 to *90 %, while the selectivity for Cy-Oxide decreased from *12 to
5.5 %, with increase of the temperature from 40 to 70 °C.
Figure 9 shows the effect of reaction time (from 1 to 4 h) on the cyclohexene
conversion and product selectivity over Ce–Sm/SiO catalyst at 70 °C. Reasonable
2
conversion of cyclohexene (*28 %) was observed at 1 h of reaction time. Also,
high selectivity for allylic products (*76 %) but poor selectivity for Cy-Oxide
(
Ce–Sm/SiO catalyst in selective oxidation of cyclohexene even for shorter reaction
*15 %) was observed at 1 h of reaction time, indicating a favorable role of the
2
time. The conversion of cyclohexene was observed to be *28, *50, *70, and
*
89 % for reaction time of 1, 2, 3, and 4 h, respectively. The selectivity for Cy-
Oxide and Cy-TBHP products decreased considerably with increase of the reaction
time. On the other hand, high selectivity for allylic products was found for all
reaction times (*76, *81, *84, and *90 % at 1, 2, 3, and 4 h, respectively).
60
50
40
30
20
10
0
90
80
70
60
50
40
30
20
(A)
(
B)
Conv. of cyclohexene
Sel. of Cy-Oxide
Sel. of Cy-Ol
Sel. of Cy-One
Sel. of Cy-TBHP
40
50
60
70
40
50
60
70
o
o
Temperature ( c)
Temperature ( c)
Fig. 8 Effect of reaction temperature on selective oxidation of cyclohexene over CeO
2
–Sm
2
O
/SiO
3 2
catalyst: a conversion of cyclohexene and b selectivity for cyclohexene oxide (Cy-Oxide), 2-cyclohexen-
-ol (Cy-Ol), 2-cyclohexen-1-one (Cy-One), and 3-(tert-butylperoxy) cyclohex-1-ene (Cy-TBHP)
1
123

B. G. Rao et al.
6
5
0
0
9
8
7
6
5
4
3
2
0
0
0
0
0
0
0
0
(A)
(B)
Conv. of cyclohexene
40
Sel. of Cy-Oxide
Sel. of Cy-Ol
Sel. of Cy-One
Sel. of Cy-TBHP
3
2
1
0
0
0
0
1
2
3
4
1
2
3
4
Reaction time (h)
Reaction time (h)
Fig. 9 Effect of reaction time on selective oxidation of cyclohexene over CeO
2
–Sm
2
O
3
/SiO
2
catalyst:
a conversion of cyclohexene and b selectivity for cyclohexene oxide (Cy-Oxide), 2-cyclohexen-1-ol (Cy-
Ol), 2-cyclohexen-1-one (Cy-One), and 3-(tert-butylperoxy) cyclohex-1-ene (Cy-TBHP)
Figure 10 shows the effect of the molar ratio of cyclohexene/TBHP on oxidation
of cyclohexene over Ce–Sm/SiO catalyst under optimized reaction conditions.
2
Very low cyclohexene conversion (*24 %) was observed at 1:0.25 molar ratio of
cyclohexene/TBHP, which is due to insufficient accessibility of TBHP molecules to
interact with cyclohexene molecules. The conversion of cyclohexene was consid-
erably increased with increase of the cyclohexene/TBHP molar ratio. The achieved
cyclohexene conversion was *24, *50, *75, and *89 % for cyclohexene/TBHP
molar ratio of 1:0.25, 1:0.50, 1:0.75, and 1:1, respectively. At molar ratio of
cyclohexene/TBHP of 1:0.25, a considerable amount of epoxidation product (Cy-
Oxide; *28 %) was obtained, which decreased to 5.5 % with increase of the molar
ratio of cyclohexene/TBHP to 1:1. In parallel, the selectivity for allylic products
50
40
30
20
10
0
9
8
7
6
5
4
3
2
0
0
0
0
0
0
0
0
(A)
(B)
Conv. of cyclohexene
Sel. of Cy-Oxide
Sel. of Cy-Ol
Sel. of Cy-One
Sel. of Cy-TBHP
1:0.25
1:0.50
1:0.75
1:1
1
:0.25
1:0.50
1:0.75
1:1
Cyclohexene/TBHP
Cyclohexene/TBHP
Fig. 10 Effect of mole ratio of cyclohexene/TBHP on selective oxidation of cyclohexene over CeO
Sm /SiO catalyst: a conversion of cyclohexene and b selectivity for cyclohexene oxide (Cy-Oxide),
-cyclohexen-1-ol (Cy-Ol), 2-cyclohexen-1-one (Cy-One), and 3-(tert-butylperoxy) cyclohex-1-ene (Cy-
TBHP)
2
–
2
O
3
2
2
1
23

Selective allylic oxidation of cyclohexene over a novel…
was found to increase from *56, *68, *78 to *90 % with increase of the molar
ratio of cyclohexene/TBHP from 1:0.25, 1:0.50, 1:0.75 to 1:1, respectively.
The effect of different solvents on the cyclohexene conversion and product
selectivity was investigated using the Ce–Sm/SiO catalyst; the results are shown in
2
Fig. 11, revealing that both the cyclohexene conversion and product selectivity were
highly dependent on the nature of the solvent. High conversion of cyclohexene
(
*89 %) with superior selectivity (*90 %) for allylic products was found when
using acetonitrile as solvent. On the other hand, *35, *45, and *61 % conversion
of cyclohexene was noted when using dimethylformamide (DMF), ethanol, and
chloroform, respectively, as solvent under similar reaction conditions. Interestingly,
high selectivity for oxidation of cyclohexene to allylic products, namely 2-cyclo-
hexene-1-ol (40 %) and 2-cyclohexene-1-one (50 %), was achieved when using
acetonitrile compared with other solvents under identical conditions. The selectivity
for allylic products with different solvents was as follows: acetonitrile (90 %)
[
that acetonitrile could be the best solvent to attain high conversion with superior
DMF (70 %) [ EtOH (67 %) [ CHCl (59.3 %). It can therefore be concluded
3
selectivity for allylic products.
To understand the significance of the CeO –Sm O /SiO catalyst, a comparison
2
2
3
2
against recently reported catalysts for oxidation of cyclohexene is presented in
Table 2. Various supported noble-metal catalysts, such as PdO/SiO , Au/SiO , Ru/
CeO , and Ru/H-Mont, have been investigated, showing cyclohexene conversion of
2
2
2
7
1, 67.6, 38.1, and 22 %, respectively. Compared with these noble-metal-based
catalysts, much higher cyclohexene conversion (89 %) was observed when using
the CeO –Sm O /SiO catalyst. On the other hand, very low cyclohexene
2
2
3
2
conversion was reported when using Co/CeO2 (36.9 %) or Cu-MOF (33 %)
catalysts with TBHP as oxidant (entries 5 and 6). Also, the VO /SiO and VO /TiO
2
2
2
2
catalysts showed low catalytic performance in terms of both cyclohexene
conversion and selectivity for allylic products (entries 7 and 8) [62, 63]. Overall,
Sel. of Cy-Oxide
Sel. of Cy-Ol
Sel. of Cy-One
Sel. of Cy-TBHP
9
8
7
6
5
4
3
0
0
0
0
0
0
0
(A)
(B)
50
40
30
20
10
0
Conv. of cyclohexene
DMF
EtOH
CHCl3
CH3CN
DMF
EtOH
CHCl3
CH3CN
Solvents
Solvents
Fig. 11 Effect of solvent on selective oxidation of cyclohexene over CeO
2
–Sm
2
O
3
/SiO
2
catalyst:
a conversion of cyclohexene and b selectivity for cyclohexene oxide (Cy- Oxide), 2-cyclohexen-1-ol (Cy-
Ol), 2-cyclohexen-1-one (Cy-One), and 3-(tert-butylperoxy) cyclohex-1-ene (Cy-TBHP)
123

B. G. Rao et al.
Table 2 Comparison of activity of various catalysts for oxidation of cyclohexene
Entry
Catalytic system
Oxidant
Cyclohexene
conversion
Selectivity for
allylic products
Ref.
1
2
3
4
5
6
7
8
9
2.14 % PdO/SiO
2
TBHP
71
81
22
1 % Au/SiO
Ru/CeO
5 % Ru/H-Mont
Co/CeO
Cu-MOF
2
H
2
O
2
67.6
38.1
22
80.2
90.1
100
87.5
96
63
2
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
15
24
2
36.9
33
15
62
5 % VO
2
–SiO
–TiO
/SiO
2
21
17
63
5 % VO
2
2
13
79
63
CeO
2
–Sm
2
O
3
2
90
89
This work
excellent cyclohexene conversion and high selectivity for allylic products were
noted when using the CeO –Sm O /SiO catalyst. Further studies are essential to
2
2
3
2
exploit the present catalyst in place of noble-metal catalysts for not only selective
oxidation of cyclohexene but also other industrially important oxidation reactions.
Conclusions
Highly promising nanostructured CeO /SiO and CeO –Sm O /SiO catalysts were
2
2
2
2
3
2
synthesized by a facile deposition precipitation method and explored for selective
liquid-phase oxidation of cyclohexene using TBHP as oxidant. The catalytic activity
order of the employed catalysts for allylic oxidation of cyclohexene was found to be
CeO \ CeO /SiO \ CeO –Sm O /SiO . All the catalysts showed high selectivity
2
2
2
2
2
3
2
for allylic products, indicating that the ceria-based nanooxide catalysts favor the
allylic oxidation pathway. The reaction temperature, reaction time, cyclohexene/
TBHP molar ratio, and nature of solvent showed a notable influence on the
cyclohexene conversion and selectivity for allylic products. The strong acidic nature
of the CeO –Sm O /SiO catalyst associated with large BET surface area facilitated
2
2
3
2
its outstanding performance for allylic oxidation of cyclohexene. This noble-metal-
free catalyst is expected to find numerous applications in other oxidation reactions
of commercial significance. Further work is under active progress aimed at large-
scale application of this catalyst.
Supplementary data
?
Pore size distribution profiles, Ce concentration, and XP spectra of the catalysts.
3
Acknowledgements B.G.R and P.R.G.N. thank the Council of Scientific and Industrial Research (CSIR),
New Delhi for research fellowships. B.M.R. thanks the DAE, Mumbai for the award of a Raja Ramanna
Fellowship.
1
23

Selective allylic oxidation of cyclohexene over a novel…
Compliance with ethical standards
Conflict of interest The authors declare that they have no conflicts of interest.
References
1
2
3
4
5
6
7
. J. Haber, M. Witkeo, Catal. Lett. 9, 297 (1991)
. S.V. Vyver, Y.R. Leshkov, Catal. Sci. Tech. 3, 1465 (2013)
. F.P. Silva, M.J. Jacinto, R. Landers, L.M. Rossi, Catal. Lett. 141, 432 (2011)
. Y. Cao, H. Yu, F. Peng, H. Wang, ACS Catal. 4, 1617 (2014)
. X. Cai, H. Wang, Q. Zhanga, J. Tonga, Z. Lei, J. Mol. Catal. A 383–384, 217 (2014)
. J. Tong, W. Wang, L. Su, Q. Li, F. Liu, W. Ma, Z. Lei, L. Bo, Catal. Sci. Technol. 7, 222 (2017)
. B.G. Rao, P. Sudarsanam, P.R.G. Nallappareddy, M.Y. Reddy, T.V. Rao, B.M. Reddy, Catal.
Commun. 10, 57 (2017)
8
. Q.X. Luo, M. Ji, S.E. Park, C. Hao, Y.Q. Li, RSC Adv. 6, 33048 (2016)
. G. Zou, D. Jing, W. Zhong, F. Zhao, L. Mao, Q. Xu, RSC Adv. 6, 3729 (2016)
9
1
1
1
1
1
1
0. D.P. Das, K.M. Parida, Catal. Lett. 128, 111 (2009)
1. M. Dusi, T. Mallat, A. Baiker, Catal. Rev. Sci. Eng. 42, 213 (2000)
2. D. Yang, T. Jiang, T. Wu, P. Zhang, H. Han, B. Han, Catal. Sci. Technol. 6, 193 (2016)
3. E.M. Serwicka, J. Połtowicz, K. Bahranowski, Z. Olejniczak, W. Jones, Appl. Catal. A 275, 9 (2004)
4. I.Y. Skobelev, A.B. Sorokin, K.A. Kovalenko, V.P. Fedin, J. Catal. 298, 61 (2013)
5. M. Ghiacia, B. Aghabararia, A.M. Botelho do Regob, A.M. Ferrariab, S. Habibollahic, Appl. Catal. A
3
93, 225 (2011)
6. D. Lahcene, A. Choukchou-Braham, C. Kappenstein, L. Pirault-Roy, J. Sol-Gel Sci. Technol. 64, 637
2012)
1
(
1
1
1
2
2
2
2
7. E. Mikolajska, V. Calvino-Casilda, M.A. Banares, Appl. Catal. A 421–422, 164 (2012)
8. S. El-Korso, S. Bedrane, A. Choukchou-Braham, R. Bachir, RSC Adv. 5, 63382 (2015)
9. Y. Cao, H. Yu, H. Wang, F. Feng, Catal. Commun. 88, 99 (2017)
0. R. Zhang, R. Tang, J. Mater. Sci. 51, 5802 (2016)
1. A. Mouret, L. Leclercq, A. Muhlbauer, V. Nardello-Rataj, Green Chem. 16, 269 (2014)
2. G. Saidulu, P. Bukya, V. Vakati, K.S.R. Rao, D.R. Burri, Catal. Sci. Technol. 3, 409 (2013)
3. D.S. Ovoshchnikov, B.G. Donoeva, B.E. Williamson, V.B. Golovko, Catal. Sci. Technol. 4, 752
(2014)
24. A. Dali, I. Rekkab-Hammoumraoui, A. Choukchou-Braham, R. Bachir, RSC Adv. 5, 29167 (2015)
25. J. Li, Z. Zhang, W. Gao, S. Zhang, Y. Ma, Y. Qu, ACS Appl. Mater. Interfaces 8, 22988 (2016)
26. L. Katta, P. Sudarsanam, B. Mallesham, B.M. Reddy, Catal. Sci. Technol. 2, 995 (2012)
27. M. Wang, F. Wang, J. Ma, M. Li, Z. Zhang, Y. Wang, X. Zhang, J. Xu, Chem. Commun. 50, 292
(2014)
28. X. Liu, J. Ding, X. Lin, R. Gao, Z. Li, W.L. Dai, Appl. Catal. A 503, 117 (2015)
29. J. Liu, X.P. Wu, S. Zou, S. Dai, L. Xiao, X.Q. Gong, J. Fan, J. Phys. Chem. C 118, 24950 (2014)
30. P. Sudarsanam, B. Hillary, M.H. Amin, S.B.A. Hamid, S.K. Bhargava, Appl. Catal. B 185, 213
(2016)
3
3
1. A. Trovarelli, Catal. Rev. 39, 439 (1996)
2. S. Mart ´ı nez-M e´ ndez, Y. Henr ´ı quez, O. Dom ´ı nguez, L. Dornelas, H. Krentzien, J. Mol. Catal. A 252,
226 (2006)
3
3
3. T.X.T. Sayle, F. Caddeo, X. Zhang, T. Sakthivel, S. Das, S. Seal, S. Ptasinska, D.C. Sayle, Chem.
Mater. 28, 7287 (2016)
4. G.B. Della Mea, L.P. Matte, A.S. Thill, F.O. Lobato, E.V. Benvenutti, L.T. Arenas, A. J u¨ rgensen, R.
Hergenr o¨ der, F. Poletto, F. Bernardi, Appl. Surf. Sci. 422, 1102 (2017)
5. P. Sudarsanam, K. Kuntaiah, B.M. Reddy, New J. Chem. 38, 5991 (2014)
6. M.H. Amin, P. Sudarsanam, S.B.A. Hamid, S.K. Bhargava, Appl. Catal. A 492, 160 (2015)
7. T.H. Vuong, J. Radnik, J. Rabeah, U. Bentrup, M. Schneider, H. Atia, U. Armbruster, W. Gr u¨ nert, A.
Br u¨ ckner, ACS Catal. 7, 1693 (2017)
3
3
3
123

B. G. Rao et al.
3
8. A.G.M. Silva, T.S. Rodrigues, A. Dias, H.V. Fajardo, R.F. Gon c¸ alves, M. Godinho, P.A. Robles-
Dutenhefner, Catal. Sci. Technol. 4, 814 (2014)
9. M. Guo, J. Lu, Y. Wu, Y. Wang, M. Luo, Langmuir 27, 3872 (2011)
3
4
4
4
0. B.G. Rao, D. Jampaiah, P. Venkataswamy, B.M. Reddy, Chem. Sel. 1, 6681 (2016)
1. Z. Zhang, Y. Wang, J. Lu, C. Zhang, M. Wang, M. Li, X. Liu, F. Wang, ACS Catal. 6, 8248 (2016)
2. P. Sudarsanam, B. Hillary, B. Mallesham, B.G. Rao, M.H. Amin, A. Nafady, A.M. Alsalme, B.M.
Reddy, S.K. Bhargava, Langmuir 32, 2208 (2016)
4
4
4
3. D.W. Wheeler, I. Khan, Vib. Spectrosc. 70, 200 (2014)
4. L. Li, F. Chen, J.-Q. Lu, M.-F. Luo, J. Phys. Chem. A 115, 7972 (2014)
5. T. Taniguchi, T. Watanabe, N. Sugiyama, A.K. Subramani, H. Wagata, N. Matsushita, M. Yoshi-
mura, J. Phys. Chem. C 113, 19789 (2009)
46. Y. Lin, Z. Wu, J. Wen, K.R. Poeppelmeier, L.D. Marks, Nano Lett. 14, 191 (2014)
47. H. Zhang, F. Gu, Q. Liu, J. Gao, L. Jia, T. Zhu, Y. Chen, Z. Zhong, F. Su, RSC Adv. 4, 14879 (2014)
48. T. Andana, M. Piumetti, S. Bensaid, N. Russo, D. Fino, R. Pirone, Appl. Catal. B 197, 125 (2016)
49. Q. Xie, Y. Zhao, H. Guo, A. Lu, X. Zhang, L. Wang, M.S. Chen, D.L. Peng, ACS Appl. Mater.
Interfaces 6, 421 (2014)
5
5
5
5
5
5
5
5
5
5
6
6
6
6
0. X. Gong, B. Liua, B. Kang, G. Xu, Q. Wang, C. Jia, J. Zhang, Mol. Catal. 436, 90 (2017)
1. X. Lin, S. Zhao, L. Fu, Y. Luo, R. Zhu, Z. Liu, Mol. Catal. 437, 18 (2017)
2. D. Devaiah, T. Tsuzuki, T. Boningari, P.G. Smirniotis, B.M. Reddy, RSC Adv. 5, 30275 (2015)
3. B.G. Rao, P. Sudarsanam, B. Mallesham, B.M. Reddy, RSC Adv. 6, 95252 (2016)
4. H. Zhang, F. Gu, Q. Liu, J. Gao, L. Jia, T. Zhu, Y. Chen, Z. Zhong, F. Su, RSC Adv. 4, 14879 (2014)
5. P. Sudarsanam, A. Rangaswamy, B.M. Reddy, RSC Adv. 4, 46378 (2014)
6. S. Zhu, X. Gao, Y. Zhu, W. Fan, J. Wang, Y. Li, Catal. Sci. Technol. 5, 1169 (2015)
7. B.G. Rao, P. Sudarsanam, A. Rangaswamy, B.M. Reddy, Catal. Lett. 145, 1436 (2015)
8. M.L. Testa, V.L. Parola, L.F. Liotta, A.M. Venezia, J. Mol. Catal. A 367, 69 (2013)
9. Y. Chang, Y. Lv, F. Lu, F. Zha, Z. Lei, J. Mol. Catal. A 320, 56 (2010)
0. P. Bujak, P. Bartczak, J. Polanski, J. Catal. 295, 15 (2012)
1. Y.N. Wei, H. Li, F. Yue, Q. Xu, J.D. Wang, Y. Zhang, RSC Adv. 6, 10710 (2016)
2. D. Jiang, T. Mallat, D.M. Meier, A. Urakawa, A. Baiker, J. Catal. 270, 26 (2010)
3. S. El-Korso, I. Rekkab, A. Choukchou-Braham, S. Bedrane, L. Pirault-Roy, C. Kappenstein, Bull.
Mater. Sci. 35, 1187 (2012)
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