Direct hydroxylation of benzene to phenol on Cu-V bimetal modified HMS catalysts

Article abstract of DOI:10.1016/j.catcom.2013.10.005

A series of mesoporous Cu_x-V-HMS catalysts with different copper content were prepared by the co-synthesis method. The addition of copper improves the benzene adsorption ability of the catalyst and the redox property of vanadium species, which

Full text of DOI:10.1016/j.catcom.2013.10.005

Catalysis Communications 43 (2014) 179–183
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Direct hydroxylation of benzene to phenol on Cu–V bimetal modified
HMS catalysts
Liya Hu, Bin Yue ⁎, Xueying Chen, Heyong He ⁎
Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200433, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 July 2013
Received in revised form 12 September 2013
Accepted 7 October 2013
Available online 16 October 2013
A series of mesoporous Cu –V-HMS catalysts with different copper content were prepared by the co-
synthesis method. The addition of copper improves the benzene adsorption ability of the catalyst and
the redox property of vanadium species, which facilitate the benzene hydroxylation reaction. Among
the catalysts studied, Cu0.90–V-HMS exhibited the best catalytic activity with a phenol yield of 29.0%
compared with 20.7% over V-HMS catalyst.
x
©
2013 Elsevier B.V. All rights reserved.
Keywords:
Benzene
Copper
HMS
Phenol
Vanadium
1
. Introduction
copper-based catalysts [5,16,17]. Among various kinds of metal species,
vanadium species, due to their high redox ability and remarkable
stability, showed the excellent catalytic activity in the hydroxylation
of benzene [18,19]. It was reported that the addition of second metal
species may create new redox and acidic properties which are beneficial
to the reaction. Synthesis and catalytic application of such systems have
attracted much attention [15,20].
Phenol is an important intermediate used in the synthesis of phenol
resins, bisphenol-A, caprolactam, etc. Industrially, phenol is produced
mainly through the three-step cumene process, which suffers from a
low one-pass yield of phenol, a low atomic efficiency and an equimolar
amount of acetone byproduct [1,2]. To meet the economic and
environmental demands, the one-step hydroxylation of benzene to
phenol is the most attractive alternative method. Many endeavors
In previous research, we have found that V-HMS catalysts exhibited
good performance in the catalytic hydroxylation of benzene with H O
2 2
have been made using green oxidants, such as O
past few decades [3–5]. However, the conversion of benzene with O
needs a relatively high reaction temperature in order to activate the
oxidant, leading to low phenol yield [6]. Using N O as the oxidant can
provide good selectivity to phenol [7], but the high cost to produce
O as the feedstock limits the application of this technology. Instead,
as an excellent oxidant is widely used [8].
Mesoporous materials as the promising supports have attracted
2
, N
2
O or H
2
O
2
in the
[21]. To further improve catalytic activity of V-containing catalyst,
different amounts of copper were incorporated into the V-HMS catalysts
by co-synthesis method. A higher phenol yield was successfully
achieved over Cu –V-HMS catalysts than copper-free V-HMS catalyst.
x
The role of copper species in the catalytic reaction was also investigated.
2
2
N
H O
2 2
2
2. Experimental
considerable attention due to their structural characteristics, such as
uniform pore size distribution, high surface area, and large pore volume.
Among those mesoporous materials, hexagonal mesoporous silica
2.1. Catalyst preparation
The purchased chemicals were all analytical reagents and were used
(
HMS) with wormlike mesoporosity and large wall thickness has
found promising applications as a support for heterogeneous catalysts
9]. Their silicate framework can be modified by many transition metals
x
without further purification. The Cu –V-HMS catalysts were prepared by
the co-synthesis method, where x represented the Cu/V molar ratio
calculated from the composition of the starting mixture. A typical
procedure proceeded as follows: 1.25 g of dodecylamine (DDA),
10.60g of ethanol, and 8.00g of water were stirred at room temperature
for 1 h. Then 5.34 g of tetraethyl orthosilicate (TEOS) and an aqueous
[
such as V, Ti, Cu, Co, Ti–V, and Fe–V [10–15], resulting in various
catalytic activities.
Studies on the direct oxidation of benzene to phenol with various
metal-based catalysts were reported, such as iron-, vanadium-, and
solution containing 10 mL of H
2
O, 0.23 g of VOSO
4
2
⋅5H O, and different
amount of Cu(NO ⋅3H O (0, 0.1, 0.2, and 0.3 g) were added into the
)
3 2
2
mixture, corresponding to V-HMS, Cu0.45–V-HMS, Cu0.90–V-HMS, and
Cu1.35–V-HMS, respectively. After stirring for another 24 h, the resultant
⁎
Corresponding authors. Tel.: +86 21 65643916.
E-mail addresses: yuebin@fudan.edu.cn (B. Yue), heyonghe@fudan.edu.cn (H. He).
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.10.005

180
L. Hu et al. / Catalysis Communications 43 (2014) 179–183
slurry was filtered, washed with deionized water, and dried overnight at
0 °C. The surfactant was removed by calcination in air at 550 °C for 5 h.
Another Cu0.90-HMS sample was synthesized with the similar procedure
without adding VOSO ⋅5H O.
6
4
2
2
.2. Catalytic reaction
The catalytic activities were evaluated at 70 °C by the reaction of
benzene and H using 80 wt.% acetic acid as a solvent. A typical
2 2
O
reaction was carried out as follows: 0.10 g of catalyst, 2 mL of benzene
and 24 mL of 80 wt.% acetic acid solvent were added to a 150 mL of
three-necked round bottom flask connected with a reflux condenser.
2 2
After stirring at 70 °C for 30min, 7mL of 30wt.% H O was added to the
flask dropwise within 30 min. Then the reaction mixture was stirred for
another 3 h. After reaction the catalyst was separated by centrifugation.
The content of liquid products was analyzed by gas chromatography
(GC) with a flame ionization detector and a HP-5 capillary column. The
conversion of benzene, the selectivity and the yield of phenol were
calculated as the molar ratio of the converted benzene to its initial one,
the formed phenol to the converted benzene, and the formed phenol to
the initial benzene, respectively.
Fig. 1. Low-angle XRD patterns of (a) V-HMS, (b) Cu0.45–V-HMS, (c) Cu0.90–V-HMS, and
d) Cu1.35–V-HMS.
(
the type IV isotherm according to the IUPAC classification, which is the
characteristic of mesoporous materials [22]. Capillary condensation of
nitrogen occurs in the relative pressure P/P range of 0.2–0.3 for all
0
2
.3. Benzene and phenol adsorption test
the samples, suggesting typical mesoporous structure with uniform
pore diameters [9]. Also, each sample displays an obvious hysteresis
loop and the loop becomes flat for the Cu-containing samples, indicating
A calculated amount of benzene (1.1, 2.2, 3.3 and 5.5 mmol) or
phenol (1, 2, 3, 4 and 5 mmol) and 80 wt.% acetic acid were mixed to
obtain a series of 10 mL aqueous solution. Then 20 mg of catalyst was
added into each solution and the mixture was stirred for 44h (benzene)
or 25 h (phenol) at room temperature. The initial and equilibrium
concentrations of benzene or phenol were analyzed by GC.
2
.4. Catalyst characterization
Elemental analysis was performed on a Thermo Elemental IRIS
Intrepid ICP atomic emission spectroscopy. The XRD patterns were
recorded on a Bruker D8 Advances X-ray diffractometer using Cu-K
radiation with a voltage of 40 kV and a current of 40 mA. N adsorption-
α
2
desorption isotherms were obtained at −196 °C using a Micromeritics
Tristar 3000 apparatus. The TEM images were obtained from a JEOL
JEM2011 microscope operated at 200 kV. In H –TPR experiments
2
a sample loaded in a quartz reactor was firstly degassed at room
temperature under He flow and then heated to 800 °C at a rate of
−
1
1
2 2
0°C min under 5% H /Ar flow. The H consumption was monitored
by a thermal conductivity detector on a Micromeritics Chemisorb 2720
apparatus. The UV–vis DRS spectra were measured with a JASCO UV550
UV–vis absorption spectrometer.
3
. Results and discussions
3
.1. Characterization
x
The low-angle powder XRD patterns of the Cu –V-HMS samples are
presented in Fig. 1. A single diffraction peak at 2–3° can be observed for
all the samples, which is a characteristic peak of HMS. Although higher
order Bragg reflections of the hexagonal structure are not resolved, HMS
still has short-range hexagonal symmetry formed by weak, nonionic
interactions between neutral template and inorganic precursors [9].
However, with the increase of copper content, the intensity of the
diffraction peak decreases dramatically, which results from the disorder
of the mesoporous structure. In addition, high-angle XRD patterns of all
samples (not shown) exhibit no diffraction peak of any crystalline phase
of copper or vanadium species, illustrating high dispersion of the copper
and vanadium species.
2 x
The N adsorption-desorption results for the Cu –V-HMS samples
are shown in Fig. 2 and related textural and physicochemical properties
are summarized in Table 1. The isotherms (Fig. 2(A)) are very similar to
Fig. 2. N
2
adsorption–desorption isotherms (A) and pore size distributions (B) of (a) V-HMS,
(b) Cu0.45–V-HMS, (c) Cu0.90–V-HMS, and (d) Cu1.35–V-HMS.

L. Hu et al. / Catalysis Communications 43 (2014) 179–183
181
Table 1
x
Physicochemical properties of Cu –V-HMS samples.
Samples
Cu/V molar ratio in the starting gel
Cu/V molar ratio in the producta
S
BET (m2 g−1
)
Dpore (nm)
V
pore (cm3 g−1
)
V-HMS
0
0
1060
951
833
682
3.8
4.0
4.0
3.8
0.61
0.51
0.46
0.48
Cu0.45–V-HMS
Cu0.90–V-HMS
Cu1.35–V-HMS
0.45
0.90
1.35
0.44
0.82
1.00
a
Analyzed by ICP-AES.
a decrease in total pore volume after the addition of copper species [23].
With the increase of copper loading, the BET surface area decreases
between copper and vanadium species. With further increasing copper
content, the reduction peak at 450°C shifts to 330°C for Cu0.18–V-HMS,
revealing stronger interaction between copper and vanadium species.
In addition, Cu0.90–V-HMS exhibits two reduction peaks at 300 °C
and 340 °C, while these two peaks are well resolved and the peak at
340 °C shifts to 380 °C for Cu1.35–V-HMS. With increasing copper
content, the interaction between copper and vanadium species becomes
stronger, corresponding to the peak shifting from 340 °C to 380 °C. On
the basis of the above discussions, it can be concluded that a strong
interaction exists between copper and vanadium species. Due to this
strong interaction, the reduction of vanadium species becomes easier.
The TEM images of V-HMS and Cu0.90–V-HMS (Fig. S1) exhibit the
typical wormlike mesoporous structure. In addition, both samples
show no evident bulk copper or vanadium species on the HMS surface,
which reveals that the copper and vanadium species are well dispersed.
UV–vis DRS is a useful technique to study the coordination
environment and oxidation states of vanadium species. All the
samples show a band at 220 nm (Fig. 4) assigned to the silica
materials [19]. The peak at 250 nm in Cu0.90–HMS catalyst could be
2
−1
gradually from 1060 to 682 m
g
and the mesoporous volume
. Also, the pore size distribution
declines from 0.61 to 0.48 cm3
g
−1
curves derived from the desorption branch show a narrow peak
Fig. 2(B)) around 3.8–4.0 nm for all samples (Table 1). With the
(
increase of copper loading, although the peak position does not change
appreciably, the peak intensity decreases obviously. The changes in the
textural properties may be attributed to the partial blocking of
mesopores after the incorporation of high content metal species into
the HMS framework [24].
H –TPR curves are shown in Fig. 3. V-HMS exhibits a single reduction
2
peak at 516 °C, which is attributed to the reduction of dispersed
tetrahedral vanadium species [25]. However, the reduction peak at
5
16 °C disappears for the samples of Cu0.45–V-HMS, Cu0.90–V-HMS and
Cu1.35–V-HMS. The main peak at about 220 °C in Cu –V-HMS can be
x
assigned to the reduction of copper species [23]. With the increase of
copper content, the peak at 220 °C shifts towards lower temperature
region, indicating the facilitation of copper reduction. Dong et al. [26]
reported that irrespective of vanadium loading, increasing copper
loading always resulted in the peak shifting to lower temperature. The
δ+
related to the charge transfer of isolated mononuclear Cu species
[28]. In V-HMS, the band at 250 nm can be ascribed to the low-
energy charge transfer of tetrahedral oxygen ligand to central V5
ion in the framework [19,29] and the band in the region of
350–400 nm is associated with the charge transfer of polymerized
+
x
reduction peak at about 300 °C in Cu –V-HMS samples is assigned to
the reduction of dispersed CuO and V–O–Cu species [26], which may
explain the disappearance of the peak of vanadium species at 516 °C
for all Cu-containing samples.
VO
4
species dispersed on the surface [30]. However, Cu0.90–V-HMS
To further prove our speculation, we prepared Cu0.09–V-HMS,
Cu0.18–V-HMS and vanadium-free Cu0.90-HMS samples with the similar
only shows a weak band at 250 nm corresponding to the tetrahedral
5
+
δ+
V
species or isolated mononuclear Cu
existence of the copper species may result in elimination of polymerized
VO species, which is probably caused by the interaction between
species. Therefore, the
preparation method and investigated their reducibility by H
2
–TPR
(Fig. 3(b), (c), and (g)). Cu0.90–HMS shows two reduction peaks at
4
2
13°C and 232°C. The peaks at 213°C can be assigned to the reduction
copper and vanadium species.
of highly dispersed copper species with small particle size, while one
at 232 °C is attributed to the reduction of large CuO particles [27].
Cu0.09–V-HMS shows a new peak at 450 °C probably resulted from the
shift of the peak at 516 °C in V-HMS, implying the weak interaction
Fig. 5 shows the relationship between initial and equilibrium
concentrations of phenol and benzene on V-HMS, Cu0.90–V-HMS and
Cu0.90-HMS samples in the adsorption test. Phenol was reported to be
hardly adsorbed on any catalyst surface regardless of its hydrophilicity
[31]. The equilibrium concentration of phenol shown in Fig. 5(A) is
Fig. 3.
2
H –TPR profiles of (a) V-HMS, (b) Cu0.09–V-HMS, (c) Cu0.18–V-HMS,
(
d) Cu0.45–V-HMS, (e) Cu0.90–V-HMS, (f) Cu1.35–V-HMS, and (g) Cu0.90-HMS.
Fig. 4. UV–vis spectra of (a) V-HMS, (b) Cu0.90–V-HMS, and (c) Cu0.90-HMS.

182
L. Hu et al. / Catalysis Communications 43 (2014) 179–183
Table 2
x
Catalytic performance for benzene hydroxylation to phenol over Cu –V-HMS samples.
Samples
Benzene
Phenol
Phenol
Conversion (%)
Selectivity (%)
Yield (%)
V-HMS
21.7
25.8
31.5
27.2
3.0
95.1
93.3
92.2
93.2
62.4
20.7
24.1
29.0
25.3
1.9
Cu0.45–V-HMS
Cu0.90–V-HMS
Cu1.35–V-HMS
Cu0.90–V-HMS
Reaction condition: 0.10g of catalyst, 2mL of benzene, 24mL of 80wt.% acetic acid, 7mL of
0 wt.% hydrogen peroxide, reaction at 70 °C for 3 h.
3
and keeps almost constant with increasing copper content. However,
Cu0.90-HMS exhibits poor catalytic activity, indicating that sole copper
species is not active in the reaction and the high catalytic activity of
Cu
high phenol yield of 29.0% is achieved over Cu0.90–V-HMS compared
to 20.7% over V-HMS. The excellent activity of Cu –V-HMS catalysts
x
–V-HMS catalysts is mainly attributed to the vanadium species. A
x
may be associated with the strong benzene adsorption and better
redox ability enhanced by the interaction between copper and
vanadium species. Reasonably, the more the benzene adsorbed, the
higher the possibility for the adsorbed benzene to contact with the
active centers, facilitating the hydroxylation of benzene. Also, in the
previous work [15] we found a possible mechanism for benzene
hydroxylation over V-HMS catalysts, in which the vanadium peroxo
species was generated by H O decomposition and further reacted
2 2
with benzene to form phenol. Considering that Cu-HMS shows poor
catalytic activity, vanadium other than copper is the main active species
in Cu
important role in the activation of H
benzene adsorption ability and the appropriate H
Cu –V-HMS being an excellent catalyst. The decrease of catalytic activity
for Cu1.35–V-HMS is attributed to the decrease of redox ability as shown
in the H –TPR results. The slight decrease of phenol selectivity after
x
–V-HMS. Thus, the redox ability of vanadium species plays an
. In combination, the good
activation lead to
2 2
O
2 2
O
x
2
introducing copper may be due to the further oxidation of phenol
facilitated by the copper species.
3.3. Catalyst reusability
Fig. 5. The relationship between initial and equilibrium concentrations of phenol (A)
and benzene (B) on V-HMS, Cu0.90–V-HMS and Cu0.90-HMS.
Cu0.90–V-HMS was used to study the reusability of catalyst. After
reaction, Cu0.90–V-HMS was separated by centrifugation, washed
several times with ethanol and dried at 60 °C. The recovered catalyst
was used in the reaction with a fresh reaction mixture. After five cycles,
the selectivity to phenol remains nearly unchanged and the yield of
phenol only changes from 29.0% to 26.7% (Fig. S2). These results
strongly indicate that the Cu0.90–V-HMS catalyst is a very active and
stable catalyst for the benzene hydroxylation reaction.
higher than the initial concentration on all samples, which should be
explained by the higher adsorption of solvent compared with the solute
phenol. The slope corresponding to V-HMS is bigger than other samples,
suggesting the stronger solvent adsorption on V-HMS. On the other
hand, the adsorption of benzene on three samples shown in Fig. 5(B)
exhibits a lower equilibrium concentration of benzene than the initial
one, indicating the stronger benzene adsorption than the solvent. The
slope of Cu0.9–V-HMS is smaller than that of V-HMS, corresponding to
a stronger benzene adsorption on Cu0.9–V-HMS than that on V-HMS.
According to the above discussion, the addition of copper species
to V-HMS catalyst enhances the adsorption of benzene with much
less effect on phenol adsorption. A similar adsorption phenomenon
4. Conclusions
The Cu
prepared by the co-synthesis method. Their catalytic activities for the
direct hydroxylation of benzene to phenol using H as an oxidant
x
–V-HMS catalysts with different copper content were
2 2
O
were enhanced greatly compared with V-HMS catalyst. The addition
of copper species facilitated the benzene adsorption ability of the
catalysts and the redox property of vanadium species, leading to better
catalytic performance. In addition, the Cu –V-HMS catalysts showed
x
excellent reusability.
2
was observed for TiO embedded mesoporous silica, wherein less
polar substrates such as benzene and toluene showed higher affinity
with the catalyst surface than polar substrates such as phenol and
hydroquinone [32].
3
.2. Catalytic activities
Acknowledgments
The catalytic activities of Cu
The benzene conversion increases from 21.7% over V-HMS to 31.5% over
Cu0.90–V-HMS and decreases to 27.2% over Cu1.35–V-HMS. The phenol
x
selectivity over Cu –V-HMS decreases slightly compared with V-HMS
x
–V-HMS catalysts are shown in Table 2.
This work was supported by the Ministry of Science and Technology
(2009CB623506), the National Natural Science Foundation (21173050,
21371035), and the Shanghai Science and Technology Committee
(10XD1400300).

L. Hu et al. / Catalysis Communications 43 (2014) 179–183
183
Appendix A. Supplementary data
[14] M. Setnička, P. Čičmanec, R. Bulánek, A. Zukal, J. Pastva, Catal. Today 204 (2013)
32–139.
1
[
[
15] S.J. Feng, B. Yue, Y. Wang, L. Ye, H.Y. He, Acta Phys. Chim. Sin. 27 (2011) 2881–2886.
16] K. Lemke, H. Ehrich, U. Lohse, H. Berndt, K. Jähnisch, Appl. Catal. A Gen. 243 (2003)
41–51.
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2013.10.005.
[
[
17] K.M. Parida, D. Rath, Appl. Catal. A Gen. 321 (2007) 101–108.
18] A.N. Kharat, S. Moosavikia, B.T. Jahromi, A. Badiei, J. Mol. Catal. A Chem. 348 (2011)
1
4–19.
References
[19] S. Shylesh, A.P. Singh, J. Catal. 228 (2004) 333–346.
20] R.J. Kalbasi, A.R. Massah, F. Zamani, A.D. Bain, B. Berno, J. Porous. Mater. 18 (2011)
[
[
1] L. Balducci, D. Bianchi, R. Bortolo, R. D'Aloisio, M. Ricci, R. Tassinari, R. Ungarelli,
Angew. Chem. Int. Ed. 42 (2003) 4937–4940.
2] S. Niwa, M. Eswaramoorthy, J. Nair, A. Raj, N. Itoh, H. Shoji, T. Namba, F. Mizukami,
Science 295 (2002) 105–107.
475–482.
[21] S.J. Feng, S.P. Pei, B. Yue, L. Ye, L.P. Qian, H.Y. He, Catal. Lett. 131 (2009) 458–462.
[22] R. Schmidt, E.W. Hansen, M. Stöcker, D. Akporiaye, O.H. Ellestad, J. Am. Chem. Soc.
117 (1995) 4049–4056.
[
[
[
3] A. Kubacka, Z.L. Wang, B. Sulikowski, V.C. Corberán, J. Catal. 250 (2007) 184–189.
4] A. Ribera, I.W.C.E. Arends, S.D. Vries, J. Pérez-Ramírez, R.A. Sheldon, J. Catal. 195
[23] P. Carniti, A. Gervasini, V.H. Modica, N. Ravasio, Appl. Catal. B Environ. 28 (2000)
175–185.
(
2000) 287–297.
[24] T. Tsoncheva, M. Linden, S. Areva, C.H. Minchev, Catal. Commun. 7 (2006) 357–361.
[25] V. Fornés, C. López, H.H. López, A. Martínez, Appl. Catal. A Gen. 249 (2003) 345–354.
[26] L.H. Dong, L.L. Zhang, C.Z. Sun, W.J. Yu, J. Zhu, L.J. Liu, B. Liu, Y.H. Hu, F. Gao, L. Dong,
Y. Chen, Catalysis 1 (2011) 468–480.
[
[
5] N.K. Renuka, J. Mol. Catal. A Chem. 316 (2010) 126–130.
6] T. Miyahara, H. Kanzaki, R. Hamada, S. Kuroiwa, S. Nishiyama, S. Tsuruya, J. Mol.
Catal. A Chem. 176 (2001) 141–150.
[
[
[
7] G.I. Panov, A.K. Uriarte, M.A. Rodkin, V.I. Sobolev, Catal. Today 41 (1998) 365–385.
8] T. Jiang, W.T. Wang, B.X. Han, New J. Chem. 37 (2013) 1654–1664.
9] P.T. Tanev, T.J. Pinnavaia, Science 267 (1995) 865–867.
[27] B. Zhang, Y.L. Zhu, G.Q. Ding, H.Y. Zheng, Y.W. Li, Appl. Catal. A Gen. 443–444 (2012)
191–201.
[28] C.M. Chanquía, A.L. Cánepa, J. Bazán-Aguirre, K. Sapag, E. Rodríguez-Castellón,
P. Reyes, E.R. Herrero, S.G. Casuscelli, G.A. Eimer, Microporous Mesoporous
Mater. 151 (2012) 2–12.
[29] J.S. Reddy, P. Liu, A. Sayari, Appl. Catal. A Gen. 148 (1996) 7–21.
[30] C. Hess, J.D. Hoefelmeyer, T.D. Tilley, J. Phys. Chem. B 108 (2004) 9703–9709.
[31] G. Zhang, J. Yi, J.M. Shim, J.W. Lee, W.Y. Choi, Appl. Catal. B Environ. 102 (2011)
132–139.
[
10] M. Setnička, R. Bulánek, L. Čapek, P. Čičmanec, J. Mol. Catal. A Chem. 344 (2011)
–10.
11] P.E. Boahene, K. Soni, A.K. Dalai, J. Adjaye, Appl. Catal. B Environ. 101 (2011)
94–305.
12] K. Bachari, O. Cherifi, Catal. Commun. 7 (2006) 926–930.
13] S.S. Bhoware, S. Shylesh, K.R. Kamble, A.P. Singh, J. Mol. Catal. A Chem. 255 (2006)
1
[
2
[
[
123–130.
[32] Y. Shiraishi, Y. Sugano, D. Inoue, T. Hirai, J. Catal. 264 (2009) 175–182.