Vanadium phosphorus oxide modified by silver doping: A highly effective catalyst for allylic oxidation of cycloolefins

Article abstract of DOI:10.1016/j.catcom.2011.08.018

Allylic oxidation of cycloolefins was carried out over Ag-VPO catalyst using tert-butyl hydroperoxide (TBHP) as oxidant in argon atmosphere, under our reaction conditions, attack of the activated C-H bonds was preferred instead of the epoxidation of C=C bond and corresponding α, β-enones were obtained as the main products. It is a high yielding, safe and eco-friendly method.

Full text of DOI:10.1016/j.catcom.2011.08.018

Catalysis Communications 15 (2011) 103–107
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Vanadium phosphorus oxide modified by silver doping: A highly effective catalyst for
allylic oxidation of cycloolefins
a,
b
a
a,
a,c
Junhua Liu ⁎, Fang Wang , Zhenmao Ma , Jun Lin ⁎, Zhenggui Gu
a
College of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210097, China
College of Sciences, Nanjing University of Technology, Nanjing 210009, China
Research Institute for Chemical Resources Exploitation of Jiangsu Yanjiang, Nanjing 210097, China
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 June 2011
Received in revised form 21 July 2011
Accepted 12 August 2011
Available online 28 August 2011
Allylic oxidation of cycloolefins was carried out over Ag-VPO catalyst using tert-butyl hydroperoxide (TBHP)
as oxidant in argon atmosphere, under our reaction conditions, attack of the activated C–H bonds was pre-
ferred instead of the epoxidation of C=C bond and corresponding α, β-enones were obtained as the main
products. It is a high yielding, safe and eco-friendly method.
©
2011 Elsevier B.V. All rights reserved.
Keywords:
Allylic oxidation
Cycloolefin
Silver
Vanadium phosphorus oxides
1
. Introduction
benzaldehyde over silver doped VPO (Ag-VPO) catalyst in an environ-
mentally benign oxidation protocol [12]; herein we report an effec-
tive allylic oxidation of cycloolefins to enones under the similar
conditions.
Allylic oxidation into α, β-unsaturated ketones is an organic reac-
tion of significant interest to organic chemists with applications in
areas ranging from agricultural products to pharmaceuticals [1]. Due
to the ease in separation, recovery and recycling for continuous pro-
cessing, heterogeneous catalytic oxidation is a more acceptable path-
way compared to homogeneous counterparts, so there has been an
increasing interest in the development of heterogeneous catalysts
for this reaction. Recent trends in this area have fallen into two
broad categories: (i) development of new transition metal substitut-
ed/constituted redox molecular sieves [2–4], and (ii) immobilization
of known homogeneous oxidation catalysts on the surface or in the
cavities of suitable hosts such as zeolites and structurally tailored
sol–gel materials [5–7]. However, the research in this area still is
not enough and it is necessary to develop a better solid catalyst to
achieve both high activity and selectivity for enones.
2. Experimental
2.1. Materials
TBHP (70% in water) and other reagents were all obtained from
commercial sources without any further purification.
2.2. Catalyst preparation
Silver doped vanadium phosphorus oxides catalysts were pre-
pared by refluxing an appropriate quantity of V
amount of AgNO (0.467 g) in a mixture of isobutanol (30 mL) and
benzyl alcohol (15 mL) for 12 h (120 °C), followed by addition of
85% H PO (1.8 mL), the coprecipitated powder were refluxed for fur-
2 5
O (2.5 g) and
3
Vanadium phosphorus oxides catalysts (VPO) are known to cata-
lyze the mild oxidation of n-butane to maleic anhydride (MA), very
good and promising results have been obtained on bulk VPO or sup-
ported VPO catalysts so far [8,9], also, catalytic performance may be
improved by adding specific doping agents to the VPO compositions
3
4
ther 6 h to give a light green precipitate (110 °C), the precipitate was
filtered off, dried at 110 °C overnight and then calcined in air at 400 °C
for 4 h. VPO sample was prepared under the same procedure without
[10,11]. However, there are few reports on the oxidations of other hy-
3
the addition of AgNO .
drocarbons, such as alcohols, alkenes, using metal-doped VPO cata-
lysts. We have previously presented an oxidation of styrene to
2.3. Catalyst characterization
The morphologies of obtained VPO and Ag-VPO particles were ex-
amined by transmission electron microscope (TEM) (JEM-1200EX),
scanning electron microscope (SEM) (JSM-5600 LV). The crystals of
⁎
Corresponding authors. Tel.: +86 25 83598233.
E-mail addresses: liujh2010@yahoo.cn (J. Liu), linjun72@126.com (J. Lin).
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.08.018

104
J. Liu et al. / Catalysis Communications 15 (2011) 103–107
’
the catalysts were studied by X-ray diffraction (XRD) (X Pert PRO
PANalytical). Surface composition was determined by X-ray photo-
electron spectroscopy (XPS), using an ESCALAB 210 XPS system
with a Mg Kα source. The fourier transformed-infrared spectroscopy
(
FT-IR) spectra of samples were obtained on a Bruker IFS 120HR FT-
IR spectrometer. Specific surface area was measured by N adsorp-
2
tion–desorption isothermals at 77 k over an ASAP2010 instrument.
Elemental analysis was determined by X-ray fluorescence spectrome-
try (XRF) (Magix PW 2403 XRF).
2
.4. Catalyst test
Catalytic reactions were carried out according to the following
procedure. A mixture of catalyst, substrate and acetonitrile was stir-
red under argon atmosphere in a 25 ml round-bottom flask equipped
with a condenser at room temperature for 30 min, after the addition
of tert-butyl hydroperoxide, the reaction was started by immersing
the flask in the oil bath kept at the reaction temperature, then carried
out with vigorous stirring under argon atmosphere for a certain time.
After filtration and extraction with solvent, the filtrate was concen-
trated by rotary evaporator then was analyzed by gas chromatogra-
phy (GC 201) with a SE-54 capillary column, the nature of the
products were also determined by GC-MS (VARIAN CP-3800, with a
Chrompack DB-1 column).
Fig. 2. FT-IR spectra of VPO and Ag-VPO catalysts.
−
1
pyrophosphate phases are: 742–765 cm
P–O–P bond), 1085, 1025 cm
(symmetric stretching of
−
1
(P–O stretching), in the cases where
silver has been introduced during the phosphatization step, it was ob-
served that the P–O stretching was modified. The ν(P–O stretching) at
−
1
1085 cm
has a stronger intensity in the impregnated catalyst than
unpromoted ones, which is probably due to the formation of silver
vanadyl pyrophosphate. Examination of the entire infrared spectrum
suggests that the peaks of characteristic bands of VPO are weaker than
Ag-VPO catalyst, meanwhile, the V=O absorption band of the catalyst
containing silver is significantly shifted to lower wave number (from
3
. Results and discussion
3
.1. Catalyst characterization
−
1
−1
BET surface areas of the bulk VPO and Ag-VPO catalyst are 14.6
986 cm
to 972 cm ), while the P–O–P absorption bands shifted to
2
−1
−1
and 16.2 m /g, respectively.
higher wave number (from 742 cm
to 763 cm ). The introduction
Fig. 1 shows the XRD patterns of bulk VPO and Ag-VPO catalyst. β-
of metal ions into the crystal lattice brought about a shift of V=O
wave number to lower frequencies [14], while the higher shift in P–
O–P wave number indicates the promoter atoms affect the layer link-
ages [15], so these above results strongly suggest that promoter ele-
ment silver is located in the crystal lattice of vanadyl pyrophosphate.
Fig. 3a, b shows SEM images of VPO and Ag-VPO solids respective-
ly, parent VPO and Ag-doped VPO solids are different slightly in the
shape of crystallites, the addition of silver increase the sizes of crystal-
lites. The particles of VPO are composed of lamellar crystallites; while
the Ag-VPO is composed of the fastener-like crystallites and sizes of
particles become larger. Fig. 3c shows the TEM image of the Ag-VPO
catalyst, one can see that a uniform compound Ag-VPO is formed
and no dissociative Ag particles are found on the surface, which
makes it possible to recycle the catalyst.
VOPO
these solids, and (VO)
in these solids. Compared with the VPO parent, the peaks correspond-
ing to β-VOPO phase were decreased and two new phases were
formed after adding the silver, the X-ray lines corresponding to Ag
VO )(PO ) (JCPDS: 81–2149) and Ag(V 10) (JCPDS: 81–2364)
phases have been observed in Ag-VPO solid, and the X-ray lines due
to the β-VOPO phase decreased after adding silver.
Fig. 2 shows the FT-IR spectra of bulk VPO and Ag-VPO catalyst.
4
phase (JCPDS: 27–948) was the main crystalline phase in
2 2 7
P O
phase (JCPDS: 41–698) was also observed
4
2
(
2
4
2 2
P O
4
−
1
4+
There appears a strong band at 970 cm
[ν V =O], in all cases the
−
1
5+
band at 947 cm
13], the peak intensity at 970 cm
than that of bulk VPO, which means that part of V species are reduced
pertaining to V species is visible but not obvious
−
1
[
after adding silver is stronger
5+
4+
to
V
.
Other characteristic bands corresponding to vanadyl
Fig. 4a shows high-resolution XPS spectra of the V 2p3/2 photo-
electron peaks for VPO and Ag-VPO catalysts. Evaluation of the oxida-
tion states of vanadium requires peak deconvolution procedure, the
measured V 2p3/2 peak could be well fitted with a single Gaussian
5+
4+
with a maximum at 518.4 eV (V ), and the position of the V com-
ponent obtained independently lies at 516.7 eV. Because the curve fit-
ting and the deconvolution are performed in the same way for the Ag-
VPO and VPO catalysts under study, this technique allows to monitor
5
+
4+
the changes in the relative ratio of V
and V
[16], as shown in
Fig. 3, the oxidation states V5 /V
+
4+
ratios of the VPO parent and
Ag-VPO catalyst were 97/3 and 88/12, respectively. Both XRD and
XPS results showed that the addition of silver decreased the average
5+
surface oxidation state of vanadium, part of V species were reduced
to V4 ones. The high resolution XPS spectrum of the Ag 3 d region
was shown in Fig. 4b, the Ag 3 d5/2 region shows two distinct peaks
components, a large and dominating centred at 368.0 eV (92.5%)
and a smaller one at higher energy of 364.7 eV (7.5%). About the bind-
ing energies for Ag 3 d5/2 peaks, some researchers considered that the
peaks at 367.5 and 368.3 eV were commonly assigned to the ionic and
metallic silver, respectively [17]; the others argued that it is difficult
to assign the binding energies for Ag 3 d5/2 peaks in the case of the
+
Fig. 1. XRD patterns of VPO and Ag-VPO catalysts. Symbols: (VO)2P2O7 (●); β-VOPO4
▲); Ag2(VO2)(PO4) (○); Ag(V2P2O10)(*).
(

J. Liu et al. / Catalysis Communications 15 (2011) 103–107
105
Fig. 4. XPS spectra of (a) V2p3/2 region of VPO and Ag-VPO catalysts and (b) Ag3d re-
gion of Ag-VPO catalyst.
the presence of interaction [17]. The binding energy of 364.7 eV is
to beassigned to the silver vanadyl pyrophosphate.
3.2. Catalytic activity
Initially, the allylic oxidation of cyclohexene was examined as a
model reaction under different conditions; the results are shown in
Table 1. The Ag-VPO exhibited the highest catalytic activity and selectiv-
ity for cyclohex-2-enone in acetonitrile under an argon atmosphere,
cyclohex-2-enol and cyclohex-2-ene-1, 4-dione were primary bypro-
ducts. Obviously, the yield of cyclohex-2- enone increased gradually
with increasing polarity of solvents, polarity of solvent was a crucial fac-
tor to the activity and selectivity of catalyst. Based on the experimental
data, acetonitrile (ε: 37.5, entry 4)Ndichloroethane (ε: 10.4, entry 3)N
dichloromethane (ε: 3.4, entry 2)Ntoluene (ε: 2.4, entry 1), the polarity
of solvent was determined by dielectric constant ε. No promising results
were achieved except acetonitrile among these solvents (Table 1, en-
tries 1–4), excellent reaction performance were obtained using acetoni-
trile as a solvent, which could be assigned to the presence of acidic
Fig. 3. SEM images of (a) VPO (b) Ag-VPO catalysts and (c) TEM image of Ag-VPO
catalyst.
samples consisting of silver dispersed in carrier matrices because
their XPS signals could change due to different support interactions,
they gave example of the binding energies for Ag(0) 3 d5/2 as high
as ca. 370 eV, which were higher than 368.2 eV reported in the refer-
ence [18]. In our experiments, TEM showed that no dissociative Ag
particles are found on the surface. In view of literature data, we spec-
ulate the XPS signal at 368.0 eV could be ascribed to ionic silver, in
this case the binding energy is slightly higher than the standard
ionic silver, so the silver species responsible for the main peak are lo-
cated in more electropositive surroundings, which is further testify
protons of CH
and TBHP), as well as the strong interaction of the CH
3
CN molecules (in favor of dissolutions of cyclohexene
CN molecules
3
with the active sites [19]. In scheme, acetonitrile activates TBHP by
-
forming a perhydroxyl anion (tBuOO ), which nucleophilically attacks
the nitrile to generate
a peroxycarboximidic acid intermediate
(Scheme 1, I), which is a good oxygen transfer agent [20,21]. The allylic
oxidation of cyclohexene gave 82% conversion and 62% cyclohex-2-
enone selectivity using the VPO as the catalyst (Table 1, entry 5),

106
J. Liu et al. / Catalysis Communications 15 (2011) 103–107
Table 1
Allylic oxidation of cyclohexene using different solvents .
a
Entry
Solvent
Conversion
Product selectivity (%)b
A
b
(%)
B
C
Others
1
2
3
4
5
6
7
8
9
Toluene
13.7
29.2
49.3
97.2
82.0
N.R.
46.1
62.4
7.0
28.0
48.7
67.8
1.0
25.0
–
69.3
49.0
28.6
88.6
62.0
–
2.0
-
1.2
6.2
10.2
–
0.7
2.3
2.4
4.2
2.8
–
Dichloromethane
Dichloroethane
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
c
d
e
f
72.8
13.8
48.1
26.6
75.2
51.0
-
9.7
-
0.6
1.3
0.9
g
h
a
Reaction conditions: Ag-VPO catalyst 0.1 g (Ag: 0.5 mol%), cyclohexene 10 mmol, solvent 5 mL, TBHP 3 mL (70% in water), 82 °C, Ar, 6 h.
Determined by GC and GC-MS.
VPO parent 0.1 g.
Blank experiment, without catalyst.
N.R.: Reaction did not occur.
Reaction time was 3 h.
b
c
d
e
f
g
h
Air atmosphere.
Hydrogen peroxide 2.2 mL (50% in water) as oxidant.
which showed the addition of silver improved the catalytic activity and
Next, we carried out oxidation of various cycloolefins under opti-
mum conditions, the results are listed in Table 2.
product selectivity. The existence of Ag-VPO catalyst was crucial be-
cause no product was found without catalyst (Table 1, entry 6). The
products during the reaction period were analyzed, mainly cyclohex-
As shown in Table 2, different cycloolefins showed high reaction
activity for the allylic oxidations to corresponding enones. Both cyclo-
hexene and cyclopentene gave more than 86% yields (Table 2, entries
1–2), glutaraldehyde was main byproduct when cyclopentene was
oxidized. Remarkably, cyclohexene with side chains at allylic posi-
tions, such as 3-methylcyclohexene, 3-phenylcyclohexene, showed
slightly higher selectivity (93%–94%) than cyclohexene (Table 2, en-
tries 4–5). Cyclohexene contains a range of extra electron-donating
as well as electron-withdrawing groups have also been tolerated in
the reaction, proceeded readily to afford the corresponding products
in a certain time (Table 2, entries 6–7), the results showed that elec-
2
7
-enol was obtained when the reaction time was 3 h (Table 1, entry
), and the cyclohex-2-enol was further oxidized to cyclohex-2-enone
with the extended time. The reaction was much slower involving aque-
ous tert-butyl hydroperoxide in air atmosphere, only gave 62.4% cyclo-
hexene conversion and 46.9% enone yield (Table 1, entry 8), also, no
good yield could be observed when hydrogen peroxide was used as ox-
idant (Table 1, entry 9). When the t-butylhydroperoxide was used as
terminal oxidant, and stirred with Ag-VPO catalyst in acetonitrile
under argon atmosphere, it shows a very pronounced efficiency for al-
lylic oxidation of cyclohexene to enones (97.2% cyclohexene conversion
and 88.6% enone selectivity). Although there have been many reports
on the development of heterogeneous catalysts for this reaction, the
enone yield of the cyclohexene oxidation over Ag-VPO catalyst is the
highest among these metal-contained catalysts to the best of our
knowledge.
3
tron-donating groups such as CH O– was beneficial to allylic oxida-
tion, and some electron-withdrawing groups such as –CHO reduced
the reaction rates. Because the Ag-VPO catalyst was prepared by in-
situ synthesis and formed a uniform compound, so the catalyst was
easily separated from the reaction mixture, ICP analysis of filtrate
confirmed that no leaching of Ag and V species occurred. It was
shown that the recovered Ag-VPO could be reused without any loss
of its catalytic activity (Table 2, entry 3); meanwhile, the catalyst
was filtered off after ca. 50% conversion at the reaction temperature,
further treatment of the filtrate under similar reaction conditions
did not afford to any products.
From above experimental results, the addition of silver improved
the conversion and selectivity of allylic oxidation using VPO as a cat-
alyst where it reduced part of V5 species to V , which suggested
the silver located in the crystal lattice of vanadyl pyrophosphate
was essential to improve the catalytic performance and maintain
+
4+
the balance of V5 /V . As shown in our previous report, an appro-
+
4+
priate ratio of V5 /V4+ species is ideal for maximum effectiveness
+
[
12]. This also explains why the reaction works better in argon atmo-
5+
sphere, as the presence of oxygen (air) hinders the reduction of V
species back to V4 , which may destroy the balance of V /V , so
the oxidation of cyclohexene at air atmosphere gave lower catalytic
result (Table 1, entry 8), similar reports can be seen in other litera-
tures [22,23]. Based on the mechanism reported by the pioneers
and our experimental details, the allylic oxidation of cyclohexene
pathway over Ag-VPO catalyst was proposed (Scheme 1, cyclohexene
as an example). In this scheme, when the cyclohexene was oxidized
+
5+ 4+
Scheme 1. Probable mechanism for the allylic oxidation of the cyclohexene.

J. Liu et al. / Catalysis Communications 15 (2011) 103–107
107
Table 2
a
Allylic oxidation of various cycloolefins over Ag-VPO catalyst .
Entry
1
Substrate
Product
Time (h)
6
Conversionb (%)
98
Selectivityb (%)
91
2
6
97
96
97
89
89
93
3c
4
7
7
6
5
6
97
98
94
90
7
9
87
91
a
Reaction conditions: Substrate (10 mmol), Ag-VPO (0.1 g, Ag: 0.5 mol%), acetonitrile (5 mL), TBHP (3 mL), 82 °C, Ar.
Determined by GC and GC-MS.
Recycled experiment.
b
c
5
+
species was reduced to V4+, then
[3] A. Sakthivel, S.E. Dapurkar, P. Selvam, Applied Catalysis A: General 246 (2003)
to cyclohex-2-enone, part of V
the V
4
+
5+
283–293.
species was oxidized to V
at the presence of TBHP to
[
[
[
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close this circle, so the balance of V5+/V
4+
was maintained. During
4
+
5+
the circle, dispersed V
species in combination with V
species
4+
may be responsible for the oxidation wherein the V act as a dynam-
ic oxidizing center [24]. The redox mechanism is also evident from
the color changes of the reaction mixture, where the initial color of
the reaction mixture was brown (mainly V ) that slowly changed
to greenish yellow and then to green (V ), finally, it further changed
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5
+
to brown (mainly V ).
[
(
2001) 275–286.
4
. Conclusions
[
[
[
[
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In conclusion, the Ag-VPO was demonstrated to be an efficient
heterogeneous catalyst for the allylic oxidation of cycloolefins to
enones. The present catalytic system can offer significant benefits in
achieving the simple and clean synthesis of enones.
[
Acknowledgment
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(
[
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This work was supported by the Natural Science Foundation of
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(
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