Catalytic Oxidation of Cyclopentene by O2 over Pd(II)-SBA-15 Complexes

Article abstract of DOI:10.1007/s10562-017-2131-4

Abstract: Novel catalysts with Pd(II)-picolinamide complexes anchored into the channels of mesoporous material SBA-15 were prepared for chemical transformation of cyclopentene, and characterized in details. Spectra of ²⁹Si NMR, ¹³C NMR and XPS revealed the organic ligands were grafted into the SBA-15 and Pd(II) complexes formed. Spacial diversity of the complexes, especially distances from –C=O to N on pyridyl cycles, may influence electronic distribution of conjugated system and further the catalytic activity. With the help of the newly synthesized catalytic materials, a new heterogeneous oxidation system was developed for selective catalytic transformation of cyclopentene to cyclopentanone with molecular oxygen as the sole oxidant. Analytic results of the reaction mixtures indicated that all catalysts exhibited high activity, while the cat.1 and cat.2 with 2-pyridinecarbonyl or 3-pyridinecarbonyl on the ligands gave better yields of cyclopentanone. 96.2% conversion of cyclopentene and 76.3% yied of cyclopentanone were achieved over the catalyst cat.2 under the conditions of 0.7?MPa O₂, 323?K and 6?h reaction. In addition, the catalysts were also appealing for easy separation and recyclable property. Graphical Abstract: [Figure not available: see fulltext.] A new heterogeneous reaction system was developed for the catalytic oxidation of cyclopentene to cyclopentanone by molecular oxygen over novel catalysts. The synthesized catalysts are comprised of Pd(II)-picolinamide complexes anchored into the channels of SBA-15. The new system was efficient for the mentioned reaction, and the catalysts were reusable.

Full text of DOI:10.1007/s10562-017-2131-4

Catal Lett
DOI 10.1007/s10562-017-2131-4
Catalytic Oxidation of Cyclopentene by O2 over Pd(II)-SBA-15
Complexes
Lumin Yue1 · Zhenwei Wang1 · Lele Bao1 · Wei Fu1 · Li Xu1 · Jun Li1 · Guanzhong Lu1
Received: 14 March 2017 / Accepted: 24 June 2017
© Springer Science+Business Media, LLC 2017
Abstract Novel catalysts with Pd(II)-picolinamide com-
plexes anchored into the channels of mesoporous mate-
rial SBA-15 were prepared for chemical transformation
of cyclopentene, and characterized in details. Spectra
of 29Si NMR, 13C NMR and XPS revealed the organic
ligands were grafted into the SBA-15 and Pd(II) complexes
formed. Spacial diversity of the complexes, especially dis-
tances from –C=O to N on pyridyl cycles, may influence
electronic distribution of conjugated system and further the
catalytic activity. With the help of the newly synthesized
catalytic materials, a new heterogeneous oxidation system
was developed for selective catalytic transformation of
cyclopentene to cyclopentanone with molecular oxygen as
the sole oxidant. Analytic results of the reaction mixtures
indicated that all catalysts exhibited high activity, while the
cat.1 and cat.2 with 2-pyridinecarbonyl or 3-pyridinecar-
bonyl on the ligands gave better yields of cyclopentanone.
96.2% conversion of cyclopentene and 76.3% yied of cyclo-
pentanone were achieved over the catalyst cat.2 under the
conditions of 0.7 MPa O2, 323 K and 6 h reaction. In addi-
tion, the catalysts were also appealing for easy separation
and recyclable property.
Graphical Abstract
A new heterogeneous reaction system was developed for
the catalytic oxidation of cyclopentene to cyclopentanone
by molecular oxygen over novel catalysts. The synthesized
catalysts are comprised of Pd(II)-picolinamide complexes
anchored into the channels of SBA-15. The new system
was efficient for the mentioned reaction, and the catalysts
were reusable.
Keywords Cyclopentene oxidation · Pd(II)-complexes ·
Heterogeneous catalysis · Cyclopentanone
1 Introduction
Pd(II)-containing catalysts were widely adopted for the oxi-
dation of terminal alkenes to corresponding ketones [1–4].
The homogeneous palladium catalyst was an effective cat-
alyst for Wacker reaction and the reaction has become an
important process in chemical industry. However, it was
difficult to separate the homogeneous palladium catalyst
from reaction mixture, residual catalyst may contaminated
products and reactors.
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-017-2131-4) contains supplementary
material, which is available to authorized users.
Ligand modulation of catalyst anchored to supports
was an effective method to alleviate separation difficul-
ties [5–8]. Moreover, reusability, high activity, and low
metal loading may be achieved by this way. For example,
a reusable heterogeneous catalyst with KIT-6-SH ligand,
brought about a satisfying recyclability (metal leaching
* Jun Li
junliecust0967@sina.com
1
School of Chemical and Environmental Engineering,
Shanghai Institute of Technology, Shanghai 201418,
People’s Republic of China
Vol.:(0123456789)
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L. Yue et al.
~0.49 ppm after 8 cycles) and activity (yield: 93%) in
the Suzuki–Miyaura reaction [7]. Heterogeneous cata-
lysts with several organic functional groups were attract-
ing more and more attentions [9].
2 Materials and Methods
2.1 Chemicals
Metals can be anchored to functionalized support-
ers via coordinating with functional groups. The spatial
position of functional groups may change the electronic
density distribution around active center and further the
catalytic performance. This spatial effect had enormous
potential for controlling the heterogeneous reaction,
which can be used to synthesize specific target prod-
ucts. The activity vs distance behavior may be quali-
tatively different. In the case of acid and thiol groups,
the catalytic activity for bisphenol synthesis reached the
highest when the two groups were as close as possible
[10]. Therefore, how to locate these functional groups
on ligand compounds was a crucial issue in order to
improve the catalytic performance of polyfunctional het-
erogeneous catalysts.
All reagents were purchased from Sinopharm in analyti-
cal grade without further purification. These reagents are
listed as flowing: Tetraethylorthosilicate (TEOS, 99.99%),
P123 (average Mn ~5800), HCl (36.0 ~38.0%), ethanol
(≥99.8%), 2-picolinate acid (99%), 3-pyridinecarboxylic
acid (99.5%), 4-picolinate acid (99%), thionyl chloride
(≥99%), dimethylbenzene (≥99.8%), 3-aminopropyltri-
ethoxysilane (≥99.8%), dichloromethane (≥99.5%), ace-
tone (≥99%), palladium chloride (≥99%), cupric chloride
(99%), cyclopentene (98%), cyclopentanone (99%), KBr
(SP, ≥99%).
2.2 Preparation of Catalysts
2.2.1 Synthesis of SBA‑15 (Step I)
Mesoporous silica SBA-15 with a large quantity of
surface silanol groups can be grafted on by organosilane
[10–13]. Silanized mesoporous silica SBA-15 function-
alized with a variety of functional groups was widely
used as an ideal supporter for immobilizing metal-
complexes or metal nanoparticles [14–16]. Although
metal-complexes coordinated with the functional groups
distributed evenly on the heterogeneous catalyst, the
electron density distribution over the ligand groups
varied with the anchored sites. Catalytic efficiency
increased with enhancing the basicity at the coordination
sites or the electron availability of the chelating ring,
which accelerated the π-bond formation between metal
and olefin [17].
SBA-15 was prepared according to the reported process
[10–16, 18, 19]. In a typical process, P123 (4 g) was dis-
solved in HCl solution (120 ml, 2 M) with continuously
stirring at 40°C for 2 h. TEOS (8.5 g, 0.04 mol) was intro-
duced in with stirring for another 24 h at the same tempera-
ture. The obtained mixture was transferred into a Teflon-
lined autoclave and then hydrothermally kept at 100°C for
48 h. The suspension was filtrated and washed with ethanol
for several times. The filter cake was dried at 100°C for 3 h
and calcined at 550°C for 6 h in a muffle furnace to obtain
SBA-15.
2.2.2 Synthesis of Pyridinecarbonylchloride (Step II)
Inspired by above ideas, we have developed novel
heterogeneous catalysts for the selective oxidation of
cyclopentene in which Pd(II) coordinate with two func-
tional groups. In particular, acylamino and pyridyl were
grafted on SBA-15 via two-step surface modification.
The activity and stability of the heterogeneous catalysts
were investigated in the selective oxidation of cyclo-
pentene by O2. Three developed catalysts all gave high
activity and reusability in the selective oxidation of
cyclopentene to cyclopentanone by molecular oxygen.
To the best of our knowledge, this is the first report
of cyclopentene oxidation over heterogeneous Pd(II)
catalysts with molecular oxygen as the oxidant. The per-
formance of the Pd(II)-complexes is satisfying for more
than 96% conversion of the substrate and 76% selectivity
to the goal product under favorable conditions. The strat-
egy to immobilize metal ions on a solid support may set
a good example for other reactions with transition metal
cations as active ingredients of catalysts.
In a typical process [20], 2-picolinic acid (6.15 g) was dis-
solved in methylbenzene (70 ml). Thionylchloride (11.9 g)
was added into the solution drop wise. The mixture was
refluxed at 100°C for 10–12 h with vigorous stirring under
nitrogen atmosphere. Finally, 2-pyridine acyl chloride was
obtained after removing the solvent by rotary evaporation.
Similarly, 3-pyridine acyl chloride and 4-pyridine acyl
chloride was synthesized from 3-picolinic acid and isonico-
tinoyl chloride, respectively.
2.2.3 Modifying of SBA‑15 by Bifunctional Compounds
(Step III)
SBA-15 was silanized with aminopropyltriethoxysi-
lane according to the reported method [11, 12]. SBA-15
(2.5 g) reacted with aminopropyltriethoxysilane (5 g)
in the mixed solvent of dimethylbenzene (100 ml) and
dichloromethane (20 ml) under nitrogen atmosphere for
24 h with vigorously stirring. The obtained suspension
1 3

Catalytic Oxidation of Cyclopentene by O2 over Pd(II)-SBA-15 Complexes
was filtered, washed with ethanol and dried under vac-
uum at 120 °C for 3 h, marked as SBA-15-R.
radiation (λ = 0.15405 nm). Data were collected over
the 0.5 ≤ 2θ ≤ 6° region. Scanning electron microscopy
(SEM) images were acquired using BRUKERS-3400N.
Transmission electron microscopy (TEM) micrographs
were obtained with a Tecnai G2 20 (FEI Company). Spe-
cific surface area measurement and porosity analysis were
performed using N2 adsorption isotherms (Micromeritics,
ASAP 2010 MICROPORE dry Analyzer). UV–Visible
diffuse reflectance spectra were measured using UV-3600
(Shimadzu) between 200 and 600 nm. FT-IR spectra were
recorded using NICOLET iZ10 with 64 scans between
400 and 4000 cm−1 [18, 19, 21] with KBr as the standard.
Elemental analysis (%C and %N) were analyzed using
Elementar Vario EL III. The loadings of Palladium in
the samples were determined by ICP-AES (Perkinelmer,
Optima 7000 DV). Thermogravimetric analysis was per-
formed using a Netzsch thermoanalyzer STA 449F3 at a
heating rate of 10 °C/min in air.
SBA-15-R was dispersed with 100 ml dimethylben-
zene, followed by the introduction of 2-pyridinecarbonyl
chloride (2 g). The mixture was refluxed for 24 h, cooled
to room temperature, and filtered with the mixed solvent
of 30 ml acetone and 30 ml ethanol for three times. The
obtained filter cake dried at 120 °C for 5 h under vacuum,
marked as SBA-15-R-2Py (support 1). 3-Pyridinecar-
bonyl chloride and isonicotinoyl chloride were handled
in similar processes, and marked with SBA-15-R-3Py
(support 2) and SBA-15-R-4Py (support 3). The amount
of immobilized pyridine was determined by elemental
analysis.
2.2.4 Immobilization of Pd(II) [or Cu(II)] Onto Modified
SBA‑15 (Step IV)
The modified SBA-15 reacted with PdCl2 (or CuCl2) in
acetone for 72 h. The suspension was filtered and then
washed with ethanol and dried at 120°C for 3 h under vac-
uum. In this way, target catalysts SBA-15-R-2Py-Pd (cat.1),
SBA-15-R-3Py-Pd (cat.2), SBA-15-R-4Py-Pd (cat.3), SBA-
15-R-2Py-Cu (sub-cat.1), SBA-15-R-3Py-Cu (sub-cat.2)
and SBA-15-R-4Py-Cu (sub-cat.3) were obtained, the pro-
cesses are presented schematically in Schemes S1, S2 and
S3 (Supporting Materials), whereas the final structures of
the catalysts are graphically shown in Fig. 1. The loadings
of Palladium in the samples was determined by ICP.
2.4 Activity Test of the Catalyst
Cyclopentene oxidation was carried out in a Teflon-lined
stainless steel reactor (100 ml) equipped with a magnetic
stirrer. In a typical reaction, the reactor was flushed with
molecular oxygen, then cyclopentene (4.5 g) was mixed
with solvent (6 g), water (0.3 g), O2 (0.2 MPa), cat.
(0.1 g) and sub-cat. (0.3 g). Pure molecular oxygen was
recharged into the reactor to 0.7 MPa when reaction tem-
perature 50 °C was attained. After reaction, the mixture
of the reactant and products was cooled down to room
temperature and centrifuged to separate the catalyst. The
products were analyzed by HPLC (High Performance
Liquid Chromatography, Agilent 1100).
2.3 Catalyst Characterization
X-ray diffraction patterns (XRD) were carried out
with a Bruker D8 X-ray diffractometer using Cu–K
Fig. 1 Structures of the cata-
lysts
1 3

L. Yue et al.
2.5 Recycling Studies
3 Results and Discussion
Recycling studies were also carried out with cyclopen-
tene oxidation. The reaction was performed under follow-
ing conditions: cyclopentene (4.5 g), ethanol (6 g), water
(0.3 g), cat. (0.1 g, Pd-0.04 mmol), sub-cat. (0.3 g), reac-
tion temperature 50°C, O2 pressure 0.7 MPa and reaction
time 6 h. The catalyst was filtered, washed with ethanol and
dried under vacuum at 100°C for 3 h. Then it was reused in
the next run. The loadings of Pd in the catalyst were deter-
mined using ICP before and after each run.
3.1 Characterization of Pd-Catalysts
Figure 2 shows small-angle XRD patterns of SBA-15 and
other three samples cat.1, cat.2 and cat.3. The main dif-
fraction peaks (100) for the samples cat.1, cat.2 and cat.3
are similar to that of the SBA-15, indicating the long-range
ordered mesoporous structure of the SBA-15 was basically
retained after modification [22, 23].
SEM images (Fig. 3) reveal that all samples keep
wheat-like macrostructures identical to that of SBA-15
[24]. Both the uniformity of the mesopore size and the
existence of mesopore channels of these catalysts are con-
firmed by TEM image (Fig. 3e). The TEM image reveals
the absence of any palladium particles inside the channels,
indicating the functionalization and loading steps have no
negative impact on the ordered mesoporous structure of the
catalysts.
Table 1 gives the results for N2 adsorption–desorption,
including pore diameters (DBJH), BET surface area (SBET
)
and total pore volumes (Vtotal) of these catalysts. The type
IV profile with a sharp hysteresis loop indicates that all
catalysts have the highly ordered mesoporous structure, as
presented in Figure S1 (Supporting Materials). Nitrogen
adsorption–desorption studies demonstrates that modifica-
tion on SBA-15 channel surface brings about a significant
decrease of SBET, DBJH and Vtotal. The mesoporous con-
struction is still maintained at the same time, as is con-
firmed with SEM results mentioned above. The presence
of Pd in functionalized mesoporous SBA-15 catalyst was
quantified by ICP, and the presence of C and N were also
confirmed by Elemental Analysis.
Fig. 2 Small-angle XRD patterns of SBA-15 (a), cat.1 (b), cat.2 (c)
and cat.3 (d)
Fig. 3 SEM images of samples: a SBA-15, b cat.1, c cat.2, d cat.3; e TEM image of cat.2
1 3

Catalytic Oxidation of Cyclopentene by O2 over Pd(II)-SBA-15 Complexes
Table 1 Results of the
N2 adsorption–desorption
measurement and elemental
analysis
Sample
SBET (m2 g−1
)
Vtotal (cm3 g−1
)
DBJT (nm)
C (wt%)
N (wt%)
Pda (wt%)
SBA-15
Support 1
Cat.1
Support 2
Cat.2
Support 3
Cat.3
409.18
309.93
286.19
271.76
266.98
255.95
239.25
0.97
0.55
0.50
0.53
0.49
0.48
0.46
8.16
6.14
6.12
7.00
6.48
6.64
6.64
–
14.3
–
13.6
–
14.6
–
–
3.6
–
3.5
–
3.6
–
–
–
4.82
–
4.76
–
4.72
aThe loadings of Pd in the samples were determined by ICP
C-2
S-2
C-1
S-1
C-3
S-3
-1
1570cm-1
1616cm-1
1713cm
1644cm-1
1411cm-1
1200
1300
1400
1500
1600
1700
1800
Wavenumber / cm-1
Fig. 5 Solid-state diffuse-reflectance UV/visible spectrum of cata-
lysts and PdCl2 (inset)
Fig. 4 FT-IR spectra of the supports 1, 2 and 3 (S‑1, S‑2 and S‑3)
and catalysts 1, 2 and 3 (C‑1, C‑2 and C‑3)
Figure 4 shows the FT-IR spectra for the supports and
catalysts in the range of 1150–1800 cm−1. The bands
centered at 1659, 1644 and 1713 cm−1 for the lines of
support 1, support 2, support 3 are assigned to the vibra-
tions of carbonyls in the supports [14, 18], the band
peaks shifted to 1606, 1603 and 1647 cm−1 respectively
after Pd-ligand complexes were formed, correspond-
ing to the vibrations of carbonyls on the acylaminos for
cat.1, cat.2 and cat.3, indicating O on the acylaminos
may be involved in the coordination of Pd ions and the
ligands. The bands in the range of 1620–1400 cm−1 rep-
resent the vibration absorptions of the bonds C=C, and
C=O in the pyridine rings [14, 24]. The absorption peaks
centered at 1591, 1597 and 1616 cm−1 for the supports
shifted to 1570, 1580 and 1577 cm−1 respectively for the
corresponding catalysts, evidencing N in the the pyri-
dine rings taking part in the formation of the Pd-ligand
complexes. The absorption peaks centered at 1243, 1242,
and 1247 cm−1 for support 1, support 2 and support 3
are ascribed to vibration absorptions of the bonds N–H
on the acylaminos of the ligands [21, 24], which shift to
Fig. 6 Thermogravimetric analysis results of different samples
1235, 1232 and 1237 cm−1 in turn owing to coordinations
of N in the second amides with Pd ions.
Generally speaking, characteristic peaks of the stretch-
ing bands of the functional groups involved in coordination
1 3

L. Yue et al.
move to lower wavelength [6, 14, 24]. The donor atoms are
the nitrogen atoms in pyridine rings and the nitrogen atoms
of amides. C=N peak moving to lower wave number means
the formation of metal–ligand bonds [14].
atoms on the ligands as shown in Schemes. For cat. 2 and
cat. 3, palladium cations also coordinate with two N atoms
on the respective ligands, the structures and performances
of the complexes may impacted by the conjugated π bonds
on the pyridyl rings, especially for the cat. 3.
The larger shift of C=N stretching band means stronger
bonding between Pd(II) and organic ligand. Shifting to
low wave number of C=O peak is probably caused by the
pyridine ring. Molecular structure of N-(3-(triethoxysilyl)
propyl)picolonicamide (TPP) is not planar, and there is an
angle between pyridyl ring and –C–N bond [25]. So there
has something different of the coordination between Pd(II)
and the donors for these three catalysts. Stable Pd(II)-com-
plexs of five-membered cycles are to be formed easily with
cat.1 owing to the favorable locations of the coordinating
These developed catalysts are detected by the solid-state
diffuse UV/visible spectrum, as shown in Fig. 5. Absorb-
ance bands appear around near 265 and 400 nm, while the
untreated precursor material (PdCl2) shows no absorbance
band around similar regions (Fig. 5, inset). It is supposed
that these bands near 265 nm (cat.2, cat.3) and 275 nm
(cat.1) are the characteristic absorption of pyridine deriva-
tives and the band near 400 nm is the characteristic absorp-
tion of coordinated Pd(II) which initially locates nearly
C before and D after the catalytic reaction
A before and B after the catalytic reaction
(1)
(2)
338.1
Pd 3/2, 5/2
338.1
Pd 3/2, 5/2
343.3
343.4
C
A
B
D
348
344
340
336
332
328
348
344
340
336
332
328
binding energy (eV)
binding energy (eV)
E before and F after the catalytic reaction
338.1
(3)
Pd 3/2, 5/2
334.3
E
F
348
344
340
336
332
328
bending energy (eV)
Fig. 7 XPS spectra of cat.1 (1), cat.2 (2) and cat.3 (3) before and after use
1 3

Catalytic Oxidation of Cyclopentene by O2 over Pd(II)-SBA-15 Complexes
at 300 nm in free-state. The corresponding absorption of
Pd(II) shifts to long wavelength which is also an indication
of the formation of metal–ligand bonds.
There are several reports in literature about changes
occurring in the thermal degradation of metal complexation
as well as chemical modified catalysts. For example, the
correlations of thermal degradation properties of a series
of metal complexes has been reported [26]. The decrease
in the observed thermal stability is intimately related to
the specific type of metal complex. Figure 6 shows ther-
mogravimetric analysis results for picolinamide-modified
SBA-15 and their metal complexes. All the samples exhibit
two stage degradation. Supports give the first degradation
temperature range between 380 and 410°C and the second
degradation range between 540 and 590°C. In contrast, cat-
alysts show the first degradation range of 330–370°C and
the second degradation range of 400–430°C. Obviously,
the thermal stability at higher temperature range decreases
after metal coordination although all catalysts keep steady
before 200°C.
Fig. 8 29Si NMR spectra of SBA-15 and the cat.2
X-ray photoelectron spectroscopy (XPS) measurements
were carried out for cat.2 before and after the oxidation
reaction. Pd 3d3/2, 5/2 XPS spectra are shown in Fig. 7(1–3).
For Fig. 7(2)—A, a single chemical state is present with
Pd 3d3/2 and 3d5/2 peaks at binding energies of 343.4 and
338.1 eV, respectively. Pd 3d5/2 338.1 eV for the cat.2 is
similar to the published data with Pd(OAc)2 supported on
PMO [27], indicating the presence of Pd(II) species in the
cat.2. The binding energy of 338.1 eV is lower than that for
PdCl2 (338.5 eV) [28], indicating that there is an electronic
interaction between Pd (II) and the ligands for the cat2. In
Fig. 7(2)—B, besides the peaks 343.4 and 338.1 eV, shoul-
ders appear to the right of the main peaks respectively, evi-
dencing there may be new state of Pd species upon reac-
tion, which is presumably ascribed to Pd particles formed
during the reaction. For cat.1, and cat.3, XPS spectra are
analogous to that of the cat.2, indication the three catalysts
underwent similar processes and displayed like behaviors.
The presence of organic functional groups was fur-
ther confirmed by NMR spectroscopy. 29Si NMR spec-
tra of SBA-15 and the cat.2 are shown in Fig. 8, dis-
tinct resonances can be observed for the siloxane
[Qn =Si–(OSi)n–(OH)4−n, n=2–4; Q2 at −93, Q4 at
−111 ppm and Q3 at −103 ppm] and organosiloxane
[Tm =RSi(OSi)m–(OH)3−m, m=1–3; T3 at −68 ppm and T2
at −60 ppm] species [29, 30]. The appearance of T3 and T2
peaks for cat.2 confirms a strong covalent linkage between
the organic groups and the silica surface.
2
1
3
8
6
9
7
4
5
200
160
120
80
40
0
-40
Fig. 9 13C NMR spectrum of the cat.2
dramatically decreases in height indicating strong chemical
bonding between the Pd and amide group, which in turn
effects the peak of C4 attached to the amide group [31].
3.2 Catalytic Performance of the Pd(II) Complexes
for the Title Reaction
The catalyst activity are tested using cyclopentene oxida-
tion (Scheme 1). The date presented in Table 2 indicates
that solvent, reaction temperature, catalyst loading and O2
pressure are critical for the reaction outcomes.
The obtained materials were also characterized by
means of solid state 13C NMR spectroscopy. The peak
assignments are mentioned in Fig. 9, the strong signals at
13.7, 25.5 and 47.1 ppm are attributed to the methylene car-
bons (C1, C2 and C3) respectively. The signal at 175.3 ppm
It is well known that the nature of solvent plays a very
important role in the catalytic reactions carried out in liquid
1 3

L. Yue et al.
Table 3 Effect of reaction time on cyclopentene oxidation over cat.2
phase. To study the influence of solvent nature, the oxida-
tion of cyclopentene with oxygen using cat.2 as catalyst,
are carried out at 50°C and 0.7 MPa O2 using ethanol,
acetonitrile, dimethyl benzene, and polyethylene glycol as
solvents respectively. Table 2 (entry 1–4) shows the cyclo-
pentene conversion for all the solvents used. The activity
order for these four solvents, acetonitrile>ethanol>poly-
ethylene glycol>dimethylbenzene, are observed. However,
the selectivity for cyclopentone with acetonitrile solvent is
lower than ethanol solvent. Ethanol are selected in the fol-
lowing reactions.
Catalyst
Reaction Conversion/%
time/h
Selectivity/%
Yield/%
Cat.2
Cat.2
Cat.2
Cat.2
Cat.2
2
4
6
8
10
61.45
86.57
96.24
99.29
99.89
67.2
66.9
79.04
71.02
67.89
41.336
57.921
76.10
70.52
67.82
Reaction condition: cyclopentene 4.543 g, ethanol 6 g, water 0.3 g,
50°C, 0.7 MPa O2, Cat. 0.1 g
The effect of O2 pressure on the reaction is investigated
(Table 2, entry 1, entries 14–17). As listed in Table 2,
0.7 MPa is the best O2 pressure for the reaction with high
yield.
achieved for the reaction at 50°C (Table 2, entry 1, entries
10–13).
The effects of temperature on the reaction are examined.
The obvious improvement in the conversion (96.2%) is
The influences of catalyst loading are also investigated.
The best result is obtained with 0.7 mol‰ (the molar ratio
of Pd: cyclopentene) of catalyst (Table 2, entries 1, 5–9).
Finally, with a reliable set of conditions in hand, cat.1
and cat.3 were investigated in the optimal conditions
(50°C, 0.7 MPa O2, ethanol solvent) when cat.2 was used
for the same reaction. The results show the yield of cat.3 is
lower compared to those of the cat.1 and cat.2. The inferior
Scheme 1 Oxidation of cyclopentene to cyclopentone
Table 2 Results of
Entry
Cat.
Cat (mol‰)a
Solvent
T(℃)
PO2 (MPa)
Con. (%)
Yields (%)
cyclopentene oxidation over the
developed catalysts
1
2
3
4
5
6
7
8
Cat.2
Cat.2
Cat.2
Cat.2
Cat.2
Cat.2
Cat.2
Cat.2
Cat.2
Cat.2
Cat.2
Cat.2
Cat.2
Cat.2
Cat.2
Cat.2
Cat.2
Cat.1
Cat.3
Pd0–S
0.7
0.7
0.7
0.7
4
2
1.3
1
0.5
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0.7
Ethanol
50
50
50
50
50
50
50
50
50
30
40
60
70
50
50
50
50
50
50
50
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0.3
0.5
1.0
1.5
0.7
0.7
0.7
96.2
97.8
48.8
61.7
98.8
98.3
97.6
97.5
89.2
49.3
78.3
95.4
90.6
84.1
89.5
99.3
99.8
95.9
95.1
0
76.3
52.3
27.0
45.3
55.7
66.8
69.2
71.4
55.1
20.6
46.2
69.6
58.6
45.7
64.2
71.3
67.2
75.1
65.6
–
Acetonitrile
Dimethyl benzene
Polyethylene glycol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
9
10
11
12
13
14
15
16
17
18
19
20b
Ethanol
Reaction condition: cyclopentene (4.543 g, 0.067 mol), solvent 6 g, water 0.3 g, reaction time 6 h
aMolar ratio of Pd: cyclopentene
bPd0–S denotes a catalyst Pd0-SBA-15 prepared using incipient wetness impregnation with PdCl2 as the
source of palladium and SBA-15 as the support followed by hydrogen reduction
1 3

Catalytic Oxidation of Cyclopentene by O2 over Pd(II)-SBA-15 Complexes
Table 4 Recycle performance
Catalysts
Yielda
Run 1
Pd (wt%)b
Fresh
of the developed catalysts
Run 2
Run 3
Run 4
Fourth run
Cat.1
Cat.2
Cat.3
75.1
76.1
65.6
70.3
71.8
60.7
64.9
71.1
58.9
61.3
69.7
56.8
4.82
4.76
4.72
4.28
4.43
4.25
Reaction condition: Cyclopentene (4.543 g, 0.067 mol), Ethanol 6 g, Water 0.3 g, 50°C, 0.7 MPa O2, Cat.2
0.1 g, Sub-cat.2 0.3 g, reaction time 6 h
aYield of cyclopentanone (%)
bLoadings of Pd in fresh and the used catalysts were determined by ICP analysis
yield with cat.3 may be attributed to the unfavorable spacial
structure of this Pd(II) complex.
relatively strong and the formation rate of the Pd(0) in the
circulation of Pd(II)–Pd(0)–Pd(II) can be faster than the
aggregation of Pd(0).
The diverse performances of the cat.1and cat.2 and cat.3
for the selective oxidation of cyclopentene may be attrib-
uted to the different structures of the three Pd(II) complexes
stemming from three distinct ligands which were initially
formed by 2-pyridinecarbonyl chloride, 3-pyridinecarbonyl
chloride and 4-pyridinecarbonyl chloride respectively. Spa-
cial structures of the complexes may be related to the steric
hindrance or the electron density of Pd(II) in the complex.
For cat.3, conjugated π electrons in the pyridyl rings may
have important impacts on the performance of the catalyst
leading to the lower selectivity in the goal product com-
pared to the other two catalysts. Pd nanoparticles supported
on mesoporous material SBA-15 (denoted as Pd0–S in the
Table 2) did not catalyze the talked reaction efficiently as
shown in the above Table 2.
4 Conclusions
Selective catalytic oxidation of cyclopentene was success-
fully implemented with molecular oxygen as the sole oxi-
dant and Pd(II)-Picolinamide Complexes as catalysts in a
heterogeneous reaction system. The catalysts were effective
for the aforementioned reaction with high yields of the goal
product. The synthesized Pd-complex catalysts can be eas-
ily recovered by filtration. In other words, the catalytic oxi-
dation system is environmentally friendly. Furthermore, a
good yield was obtained even after the catalyst was reused
for many times. More work is necessary to achieve a better
understanding about the effects of the ligand structures on
the final outcomes of the oxidation reaction.
Results of the oxidation reaction are influenced by reac-
tion time. Conversion of the cyclopentene increased with
reaction time, while the highest selectivity to the goal prod-
uct appeared after 6 h reaction leading to the best yield of
cyclopentenone. Too long reaction time resulted in poor
yields of the goal product as shown in Table 3.
Acknowledgements The authors gratefully acknowledge the finan-
cial support of Consortium Plan Project of Shanghai (LM201411), the
support of Shanghai Institute of Technology, and the constant support
and advice from the Applied Catalysis Group.
3.3 Catalyst Reusability
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Reusability is also a virtual feature for supported cata-
lysts because of leaching of metal species during reaction
[6, 16, 32, 33]. The Pd leaching and reusability results of
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respectively after four recycles). Hence, the heterogeneous
catalysts shown in this paper can be used for several times
without major change in the activity. It can be deduced that
the coordinate bonds formed in these three catalysts are
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