Pd0-PyPPh2@porous organic polymer: Efficient heterogeneous nanoparticle catalyst for dehydrogenation of 3-methyl-2-cyclohexen-1-one without extra oxidants and hydrogen acceptors

Article abstract of DOI:10.1016/j.mcat.2018.07.007

In this contribution, we have developed an efficient and recyclable porous organic polymer (POP) supported Pd nanoparticle catalyst (Pd°-PyPPh₂@POP) for dehydrogenation of 3-methyl-2-cyclohexen-1-one. This heterogeneous catalytic system represents a totally clean process without using any extra oxidant and hydrogen acceptors. The SEM-EDS mapping images of the Pd°-PyPPh₂@POP catalyst reveal the highly uniformly dispersed character of C, Pd, P and N elements. The coordination bonds between Pd nanoparticle and exposed P atom as well as N atom on the surface of PyPPh₂@POP polymer are confirmed by means of solid-state ³¹P NMR and XPS. Importantly, both P atom and pyridyl ring on the PyPPh₂@POP polymer are themselves used as solid base over the Pd°-PyPPh₂@POP catalyst, leading to a catalytic conversion of 88.2% even without the employment of inorganic base additives (K₂CO₃). Our results have provided a strategy for designing highly active bifunctional POP supported nanoparticle catalysts.

Full text of DOI:10.1016/j.mcat.2018.07.007

MolecularCatalysis456(2018)49–56
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Pd0-PyPPh2@porous organic polymer: Efficient heterogeneous nanoparticle
catalyst for dehydrogenation of 3-methyl-2-cyclohexen-1-one without extra
oxidants and hydrogen acceptors
⁎
Xingkun Chena,b, Wenlong Wangb, Hejun Zhub,⁎, Wenshao Yanga, Yunjie Dinga,b,c,
a
Hangzhou Institute of Advanced studies, Zhejiang Normal Universtity, 1108 Gengwen Road, Hangzhou 311231, PR China
b
Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences 457 Zhongshan Road, Dalian 116023, PR China
The State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences 457 Zhongshan Road, Dalian 116023, PR China
c
A R T I C L E I N F O
A B S T R A C T
Keywords:
Porous organic polymer
Palladium
Nanoparticle
Dehydrogenation
Phenol
In this contribution, we have developed an efficient and recyclable porous organic polymer (POP) supported Pd
nanoparticle catalyst (Pd°-PyPPh2@POP) for dehydrogenation of 3-methyl-2-cyclohexen-1-one. This hetero-
geneous catalytic system represents a totally clean process without using any extra oxidant and hydrogen ac-
ceptors. The SEM-EDS mapping images of the Pd°-PyPPh2@POP catalyst reveal the highly uniformly dispersed
character of C, Pd, P and N elements. The coordination bonds between Pd nanoparticle and exposed P atom as
well as N atom on the surface of PyPPh2@POP polymer are confirmed by means of solid-state 31P NMR and XPS.
Importantly, both P atom and pyridyl ring on the PyPPh2@POP polymer are themselves used as solid base over
the Pd°-PyPPh2@POP catalyst, leading to a catalytic conversion of 88.2% even without the employment of
inorganic base additives (K2CO3). Our results have provided a strategy for designing highly active bifunctional
POP supported nanoparticle catalysts.
1. Introduction
insoluble supports, such as polymers, silica method in commercial ex-
ploitation [19–24]. Thus, Liu et al. report AC (activated carbon) sup-
Phenols and its derivatives are important precursors and core
structures of useful chemical molecules in the field of pharmaceuticals,
agrochemicals and polymers et al [1,2]. The introduction of functional
groups in meta-position of phenolic hydroxyl group still remains a key
challenge in the preparation of these molecules due to the strong
electronic directing effect of −OH group which cause ortho- and para-
substituent easier [3–6]. Thus, intensive efforts have been devoted to
achieve a facile route to substituted phenols. Recently, palladium-cat-
alyzed aerobic dehydrogenation of cyclohexenes is emerging as a ver-
satile strategy to achieve this goal. Stahl and co-workers develop an
efficient Pd/Py-based homogeneous catalytic system to dehydrogenate
cyclohexanone to phenols by using molecular oxygen (Scheme 1a)
[7–15]. However, oxidative dehydrogenations for the synthesis of
phenols are frequently hindered by over-oxidation due to the inherent
nature of susceptible to oxidation of phenols [16]. In addition, the
homogeneous nature of this system limits the catalyst recycling process
[17,18]. To solve the catalyst recycling problem and realize the sus-
tainable green catalysis, immobilization of metal complex onto
ported palladium catalyst (Pd°@AC) for dehydrogenation of cyclohex-
anone to phenol in the context of oxidant- and hydrogen-acceptor-free
conditions (Scheme 1b) [25]. Nonetheless, in such a catalytic system,
large amount of additional K2CO3 need to be added to achieve high
activities, which will cause metal contamination of the products and
pollutions to the environment [26]. Porous organic polymers (POP) are
one of the promising supports because of their large surface area, ad-
vanced porous structure, low skeleton density, flexible synthetic
strategy, ready functionality and high stability [27–35]. Intense and
continuous efforts have led to the rapid development of various POP
with useful structures and properties, but there is still a persistent re-
quirement for the design and synthesis of POP containing specific
functional groups to improve their performance in heterogeneous cat-
alysis. The incorporation of coordination groups is regarded as an ef-
ficient method to stabilize nanoparticles (NPS) [36–45]. Many mono-
dentate and bidentate nitrogen-containing POP supported nanoparticles
are employed for heterogeneous catalysis but supported nanoparticles
catalyst by using POP containing both of nitrogen and phosphine
⁎
Corresponding authors at: Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences 457 Zhongshan Road,
Dalian 116023, PR China.
E-mail addresses: zhuhj@dicp.ac.cn (H. Zhu), dyj@dicp.ac.cn (Y. Ding).
https://doi.org/10.1016/j.mcat.2018.07.007
Received 2 April 2018; Received in revised form 3 July 2018; Accepted 5 July 2018
2468-8231/©2018ElsevierB.V.Allrightsreserved.

X. Chen et al.
MolecularCatalysis456(2018)49–56
nanocatalyst exhibits relatively high activities even without additional
K2CO3. From the viewpoint of efficiency and green chemical process,
the heterogeneous POP supported nanocatalyst combines excellent re-
activity and eliminated pollutions as well as easy product separation.
2. Experimental
2.1. Chemicals
All reagents are of analytical grade and used as purchased without
further purification unless otherwise stated. Tetrahydrofuran (THF) is
distilled from sodium and pyridine is distilled from calcium hydride
under argon.
2.2. Catalyst preparation
All operations are carried out under argon conditions. PyPPh2@POP
is synthesized according to our previously report (Scheme 2). The
porous organic polymer support with smaller surface area which is
denoted as PyPPh2@POPs is synthesized by changing the amount of
AIBN and solvent during the solvothermal producing process. The
procedure for preparing Pd°-PyPPh2@POP is as follows. In a typical
run, 0.16 g PyPPh2@POP is added into a solution of 0.0143 g PdCl2 in
6 ml acetonitrile. After stirring 2 h at room temperature, the mixture is
dried under reduced pressure at 65 °C to give a brown solid. The sample
is denoted as PdCl2-PyPPh2@POP. And then it is reduced in a stream of
H
2 (100 ml/min) at the temperature of 300 °C (heating rate: 9 °C /min)
for 5 h. In addition, the Pd°-PyPPh2@POP catalyst could also be pre-
pared by using Pd(OAc)2. The porous organic polymer supported cat-
alyst with smaller surface area is denoted as Pd°-PyPPh2@POPs.
Pd°@AC and Pd°@Al2O3 catalysts are prepared by the same procedure.
Scheme 1. Dehydrogenation of 3-methyl-2-cyclohexen-1-one or 3-methyl-cy-
clohexanone to 3-methyl phenol.
2.3. Characterization
coordination sites has not yet been reported [46,47].
Nitrogen sorption isotherms are measured using an AutoSorb-1 in-
strument. The Pd°-PyPPh2@POP catalyst is outgassed for 24 h at 120 °C
before measurements. SEM and SEM-EDS mapping study is performed
on a JSM-7800 F scanning electron microscope. The thermal stability of
Recently, we report a novel two-fold cross-linked porous organic
polymer PyPPh2@POP, which is synthesized through polymerization of
vinyl-functionalized diphenyl-2-pyridylphosphine (2- PyPPh2 ligand)
containing both P and N [48]. Herein, we report PyPPh2@POP sup-
ported palladium nanoparticle catalyst (Pd°-PyPPh2@POP, Scheme 2)
for oxidant- and hydrogen acceptor-free dehydrogenation of 3-methyl-
2-cyclohexen-1-one to 3-methyl phenol. This novel heterogeneous
Pd°-PyPPh2@POP is detected in N2 flow (20 ml min−1
) using a
Diamond TG/DTA instrument, with a heating rate of 10 ℃ min−1 up to
1000 °C. Solid-state 31P NMR experiments are carried out on an Infinity
plus 400 spectrometer operating at 161 MHz. 13C MAS NMR spectra are
recorded under a magic angle spinning rate of 6 KHz. XPS spectra are
carried out on a ESCALAB 250Xi Alα radiation. TEM study is performed
on a JEM-2100 transmission electron microscopy.
2.4. Catalytic performance testing
In a typical run, 25 ml flask is charged with 0.016 g Pd°-PyPPh2@
POP catalyst, 0.90 g 3-methyl-2-cyclohexen-1-one and then the reactor
is sealed. After replaced 5 times with argon, the contents are heated to
140 °C and stirred for 12 h. Then the reactor is cooled to room tem-
perature with ice-water, the liquid samples are centrifuged to isolate
the catalyst. And then the solution is analysed offline by Agilent 7890 A
gas chromatography with a HP-5 capillary column and FID detector
using isopropanol as an internal standard. Pd°@AC and Pd°@Al2O3 are
used in the reaction according to the same procedure described above
for the Pd°-PyPPh2@POP catalyst.
2.5. Hot filtration test
The reaction is carried out in the presence of the Pd°-PyPPh2@POP
catalyst in 1 h and at that point the solid insoluble catalyst is filtered off
at the reaction temperature. The resulting liquid filtration is placed
under the same conditions as the original reaction for another 1.5 h and
also analysed by GC.
Scheme 2. Preparation of Pd°-PyPPh2@POP catalyst.
50

X. Chen et al.
MolecularCatalysis456(2018)49–56
Fig. 1. (A) N2 sorption isotherms. (B) Pore width distribution curves calculated by NLDFT method. a) PyPPh2@POP, b) Pd°-PyPPh2@POP (reduced at 150 °C) and c)
Pd°-PyPPh2@POP (reduced at 300 °C). (C) 13C NMR of PyPPh2@POP. (D) 31 P NMR of a) PyPPh2@POP and b) Pd°-PyPPh2@POP.
2.6. Catalyst reusing test
21.0 ppm appears, which is attributed to P atoms coordinated to Pd
nanoparticles. It indicates that an interaction between palladium ac-
tives and the PyPPh2@POP support is present on the heterogeneous
Pd°-PyPPh2@POP catalyst. This interaction is very favourable for sta-
bilizing palladium nanoparticles during the dehydrogenation reaction
process.
After being charged with Pd°-PyPPh2@POP catalyst, 3-methyl-2-
cyclohexen-1-one and DMA, the reactor is sealed. After replaced 5 times
with argon, the contents are heated to 140 °C and stirred for 5 h. Then
the reactor is cooled to room temperature with ice-water, the liquid
samples are centrifuged to isolate the catalyst. And then the solution is
analysed offline by Agilent 7890 A gas chromatography with a HP-5
capillary column and FID detector using isopropanol as an internal
standard. The isolated catalyst is used in the reaction according to the
same procedure described above for next run.
Thermogravimetry (TG) analysis demonstrates that the weight loss
of Pd°-PyPPh2@POP starts over 330 °C, which indicates that the Pd°-
PyPPh2@POP catalyst is thermally stable (Fig. S1). Representative
scanning electron microscope image (SEM, Fig. 2A) of Pd°-PyPPh2@
POP further confirms the rough surfaces and the existence of hier-
archical porosities. Transmission electron micrograph (TEM, Fig. 2B)
shows that Pd nanoparticles are uniformly distributed in polymer
support and the average size of Pd nanoparticles increases from 1.81 nm
to 3.40 nm (Fig. S3-8). TEM images of the used Pd°-PyPPh2@POP cat-
alyst reveal the stablization of palladium nanoparticles during the re-
action process (Fig.5). Element distribution in the Pd°-PyPPh2@POP
catalyst is determined by the scanning electron microscopy-energy-
disperse X-ray spectroscopy (SEM-EDS mapping).The image reveals the
highly dispersed character of all functional elements (C, Pd, P and N),
which are well integrated in the catalyst (Fig. 3). This kind of in-
tegration in the polymer creates a very favourable coordinative con-
dition for stabilizing palladium nanoparticle, which plays a significant
role in enhancing the activity of the catalyst.
X-ray photoelectron spectroscopy analysis (XPS) spectra of Pd°-
PyPPh2@POP also reveal the valence state of functional elements
(Fig. 4). The C1 s peaks (Fig. 4B) distribute near 284.6, 285.3 and
287.5 eV are assigned to CeC, C–P and C–N bonds, respectively. The
two peaks at 132.7 and 131.4 eV are assigned to 2p1/2 and 2p3/2 for P
species, respectively (Fig. 4C). The N1 s regions of Pd°-PyPPh2@POP
are deconvoluted into two peaks (399.0 and 400.7 eV), indicating the
presence of two nitrogen species (Fig. 4D). This confirms that a co-
ordination interaction of Pd and the pyridine sites is present on the
PyPPh2@POP catalyst [45]. In addition, two peaks at 340.9 eV and
335.8 eV for Pd°-PyPPh2@POP catalyst are assigned to 3d5/2 and 3d3/2
for Pd° species (Fig.4E), while the other two peaks at 343.0 and
3. Results and discussion
The Pd°-PyPPh2@POP catalyst is thoroughly characterized by var-
ious characterization techniques. N2 adsorption and desorption iso-
therms of Pd°-PyPPh2@POP and PyPPh2@POP are shown in Fig. 1A.
The obvious hysteresis loops suggest the coexistence of micro- and
mesopores in the framework. The pore size distribution curves calcu-
lated by nonlocal density functional theory method (NLDFT) are dis-
played in Fig. 1B, which further confirm the hierarchical structures of
these materials, and the hierarchical porous structure facilitates the
mass transfer process. The BET surface area and pore volume of the
catalyst (reduced at 300 °C) are 311 m2 g−1 and 0.49 cm3 g−1, re-
spectively. The relatively high surface area and large pore volume of
the Pd°-PyPPh2@POP catalyst play a significant role in enhancing the
reaction rates.
13C magic angle spinning (MAS) spectra are used to characterize the
PyPPh2@POP polymer (Fig. 1C). The signal at around 32.2 ppm is at-
tributed to the CH2 group directly linked to the aromatic ring, and the
peaks at around 20 ppm are ascribed to the “−CH2CH2-” linker. It
implies that the structure of the 2-vinyl-functional diphenyl-2-pyr-
idylphosphine monomer is not destroyed during the polymerization
process. Solid state 31P NMR spectrum of PyPPh2@POP (Fig. 1D) shows
just one signal at -4.6 ppm, which is at the same position of the ligand
monomer. Remarkably, for Pd°-PyPPh2@POP catalyst, a new peak at
51

X. Chen et al.
MolecularCatalysis456(2018)49–56
Fig. 2. (A) SEM image of Pd°-PyPPh2@POP (reduced at 150 °C), (B) TEM image of Pd°-PyPPh2@POP (reduced at 300 °C).
337.8 eV for PdCl2-PyPPh2@POP catalyst are assigned to 3d5/2 and 3d3/
reactant molecular. Stahl et al. claims that the generated hydrogen
during the reaction process can be removed by oxygen and thus ac-
celerating the reaction rate. Therefore, control experiments are per-
formed using O2 instead of Ar. The results show that the use of O2 as gas
atmosphere leads to an improvement in the selectivity of 3-methyl
phenol (from 48.3% to 75.9%, Table 1, Entry 5), however, a drastically
decrease in the conversion from 98.3% to 1.9% is also observed. After
screening the gas atmosphere conditions, we find that inert gases (Ar
and CO2) are favorable for achieving high activities (Table Entries 3 and
6).
for Pd2+ species (Fig. 4F), respectively. This confirms that the Pd(II)
2
species are completely reduced into Pd(0) in Pd° -PyPPh2@POP cata-
lyst. The surface compositions of the Pd°-PyPPh2@POP catalyst which
are detected by XPS are shown in Table S1 Entry1. By comparison of the
XPS results of the relative percentage of Pd on the surface in Pd°-
PyPPh2@POP catalyst with the theoretical values, the Pd percentage
decreases from 0.61 mol% (Table S1 Entry 2) to 0.4 mol% (Table S1
Entry 1), which implies that some palladium atoms are buried inside
the tunnel of PyPPh2@POP support.
In order to explore prospects for the proposed oxidant- and hy-
drogen-acceptor-free palladium nanoparticles catalyzed dehydrogena-
tion methods, our study is initiated through investigating 3-methyl-2-
cyclohexen-1-one over Pd°-PyPPh2@POP catalyst under various con-
ditions. In the beginning, the dehydrogenation reaction proceeds slowly
to afford phenol with conversion of only 14.8% (Table1, Entry 1). A
satisfactory activity is got after 10 mol% of K2CO3 is added during the
reaction process. The effects of solvents are also evaluated, and it is
interesting to find that the solvent is unfavorable for the reaction
(Table 1, Entry 4). This may be due to the fact that the solvent could be
coordinated with Pd center and thus suppress the coordination with the
Table 2 presents the reaction performance for the dehydrogenation
of 3-methyl-2-cyclohexen-1-one over various catalysts. To improve the
efficiency of this Pd°-PyPPh2@POP catalyst for dehydrogenation, var-
ious Pd loading and reducing temperature are screened in detail. The
increasing of Pd loading from 0.022 mmol g−1 to 0.5 mmol g−1 led to
the improvement of conversion of 3-methyl-2-cyclohexen-1-one from
0% to 98.5% (Table 2, Entries 1–5). Additionally, the catalytic activity
increases drastically when the reducing temperature increases from 150
to 300 °C (Table 2, Entries 2 and 3). This result might be explained by
the changes of the particle size of palladium (Fig. S3-S8). The average
size of Pd NPS increases from 1.81 nm to 3.40 nm with Pd loading
Fig. 3. SEM-EDS mapping of Pd°-PyPPh2@POP catalyst. (A) C with red color, (B) P with yellow color, (C) N with green color and (D) Pd with cyan color (For
interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
52

X. Chen et al.
MolecularCatalysis456(2018)49–56
Fig. 4. (A) XPS spectra of Pd°-PyPPh2@POP, XPS spectra of (B) C 1 s, (C) P 2p, (D) N 1 s and (E) Pd in the Pd°-PyPPh2@POP catalyst. (F) XPS spectra of Pd in the
PdCl2-PyPPh2@POP catalyst.
ranging from 0.125 mmol g−1 to 0.75 mmol g−1. And higher reducing
temperature leads to the increasing of Pd NPs, according to Fig. S8. In a
control experiment, we carry out the reaction without K2CO3. Pd°-
PyPPh2@POP which is reduced at 150 °C gives a conversion of only
17.4% (Table 2 Entry 6) when K2CO3 is not added in this reaction
system. The Pd°-PyPPh2@POPs catalyst with surface area of no more
than 200 m2 g−1 exhibits much lower activity (Table 2 Entry 7).
However, it is observed that the reaction works better when Pd°-
PyPPh2@POP is reduced at 300 °C (a good conversion of 88.2% is ob-
tained). Furthermore, when traditional inorganic material supported
palladium catalyst are used as catalyst instead of Pd°-PyPPh2@POP, the
conversion is less than 30% (Table 2, Entries 8, 9, 11 and 12). And the
previously reported Pd°@AC catalyst gave a relatively high conversion
of 26.9% when it was reduced at 300 °C. The relatively good perfor-
mance of Pd°-PyPPh2@POP in dehydrogenation reaction could be at-
tributed to the following reasons: 1) The palladium nanoparticles are
uniformly distributed in Pd°-PyPPh2@POP according to TEM images
because of that the P and N functional atoms on the surface of polymer
can effectively stabilize the Pd nanoparticles through the coordination
effect with Pd-P and Pd-N bonds, and thus prevent the aggregation of
Pd nanoparticles. Moreover, palladium nanoparticles in the interior
cavities can be restricted to a proper size by the confinement effect of
the pores in POP. 2) Both P atom [49] and pyridyl ring in PyPPh2@POP
can act as a solid base which play an important role in accelerating
reaction rates, the N atom of the pyridyl ring works as an acceptor of
the H atom, thus favouring the dehydrogenation process. 3) The rela-
tively high surface area and large pore volume of Pd°-PyPPh2@POP
make the substrate easy to access the active Pd species. In addition, the
optimized Pd°-PyPPh2@POP catalyst (Table 2. Entry 10) also proves to
be effective in dehydrogenation of several other ketone and quinone to
the corresponding phenol derivatives (Table 3). Varying the substituent
on the ketone and quinone do have some effect on outcome of the re-
action. To our delight, both ketone and no substituted quinone are
dehydrogenated with good activities. However, relatively lower activity
is obtained in the case of quinone with bluky substitutes. This may be
attributed to the steric hindrance of bulky substitutes which restricts
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MolecularCatalysis456(2018)49–56
Table 1
a
Reaction condition optimization
.
Entry
Conditions
Conv.
(%)
Select.(%)
TOF
(h−1
)
3-methyl cresol
3-methyl-cyclohexanone
1
2
140 °C/Ar/12h
14.8
10.1
98.3
58.7
1.9
100.0
100.0
0
68.1
88.6
49.9
94.5
97.8
100.0
51.2
50.2
48.3
27.0
75.9
49.5
49.1
–
48.8
48.4
50.8
47.1
47.9
46.9
48.8
49.8
51.7
73.0
24.1
50.5
50.9
–
51.2
51.6
49.2
52.9
52.1
53.1
12.3
8.4
81.9
48.9
1.5
83.3
83.3
0
56.8
73.8
499
315
163
55.6
5% K2CO3/140 °C/Ar/12h
10% K2CO3/140 °C/Ar/12h
10% K2CO3/140 °C/Ar/12h/DMF
10% K2CO3/140 °C/ O2/12h
10% K2CO3/140 °C/ CO2/12h
10% K2CO3/140 °C/H2+Ar(70%)/12h
10% K2CO3/140 °C/Ar/12h
10% K2CO3/140 °C/Ar/12h
10% K2CO3/140 °C/Ar/12h
10% K2CO3/140 °C/ Ar /1h
10% K2CO3/140 °C/ Ar /3h
10% K2CO3/140 °C/ Ar /6h
10% K2CO3/140 °C/ Ar /18h
3
4b
5
6
7
8c
9d
10e
11
12
13
14
a
Basic Reaction conditions: 3-methyl-2-cyclohexen-1-one (8 mmol), 0.1 mol% Pd°-PyPPh2@POP catalyst. Pd loading was 0.25 mmol/g, 1 atm of initial pressure of
gas atmosphere. Conversion and selectivity were based on gas chromatography (GC) with Isooctyl alcohol as an internal standard.
b
Solvent (2 ml).
0% catalyst.
0.025% catalyst.
0.05% catalyst.
c
d
e
the hydrogen atom transferring process.
not changed, although the polymer has swelled in the organic solvent
(Fig. S9). In addition, TEM analysis of the recovered catalyst after the
third cycle shows that there is hardly any change in palladium nano-
particle size of the Pd°-PyPPh2@POP catalyst (Fig. 5). The valence state
of Pd element in Pd°-PyPPh2@POP after catalytic cycles is investigated
by XPS. Remarkably, The binding energy of Pd(0) (341.9 and 336.8 eV)
on the used Pd°-PyPPh2@POP catalyst is higher than the fresh catalyst
Pd(0) (340.9 and 335.8 eV) probably because of the formation of H–Pd
species [50] which are considered as one of the active species of the
The reusability of the catalyst is also investigated.
Dimethylacetamide (DMA) is used as solvent to facilitate the recycling
process though it will lead to a decrease of catalytic activity. After
completion of the reaction (5 h), the catalyst is separated by cen-
trifugation and then reused for next round of reaction. In the 3rd run, a
conversion of 49.8% is obtained with the reused catalyst, which par-
allels to the result of the first recycle of 43.7%. SEM images reveal that
the porous structure of the Pd°-PyPPh2@POP catalyst after 3 cycles is
Table 2
Dehydrogenation of 3-methyl-2-cyclohexen-1-one over various catalystsa.
Entry
Catalyst
Conv.(%)
Select.(%)
3-methyl cresol
TOF
3-methyl-cyclohexanone
(h−1
)
1
0.125-Pd0-PyPPh2@POP
0.25-Pd0-PyPPh2@POP
0.25-Pd0-PyPPh2@POP
0.5-Pd0-PyPPh2@POP
0.75-Pd0-PyPPh2@POP
0.5-Pd0-PyPPh2@POP
0.5-Pd0-PyPPh2@POPs
0.5-Pd0@AC
13.4
59.3
83.1
98.5
95.8
17.4
6.3
13.5
12.5
88.2
26.9
25.2
43.7
46.5
49.8
44.4
49.1
49.6
47.5
47.7
49.1
47.7
47.3
49.0
50.4
49.2
48.9
45.0
45.9
46.4
55.6
50.9
50.4
52.5
52.3
50.9
52.3
52.7
51.0
49.6
50.8
51.1
55.0
54.1
53.6
134
593
831
985
958
14.5
5.3
11.3
10.4
73.5
22.4
21.0
10.9
11.6
12.5
2
3b
4
5
6c
7c,d
8c
9c
0.5-Pd0@Al2O3
10b,c
11b,c
12b,c
13b,c,e
14b,c,e
0.5-Pd0-PyPPh2@POP
0.5-Pd0@AC
0.5-Pd0@Al2O3
0.5-Pd0-PyPPh2@POP(1st)
0.5-Pd0PyPPh2@POP(2nd)
0.5-Pd0-PyPPh2@POP(3rd)
15b,c,
e
a
Basic Reaction conditions: 3-methyl-2-cyclohexen-1-one (8 mmol), 1 atm of initial pressure of Ar. The catalyst denoted as x-Pdy@POP, where x represents Pd
loading was x mmol/g and y represents the value state of Pd. Pd catalyst (0.1 mol% Pd) which was reduced at 150 °C, K2CO3 (10 mol%), 140 °C, 1 h, 1 atom of Ar gas
atmosphere. Conversion and selectivity were based on gas chromatography (GC) with Isooctyl alcohol as an internal standard.
b
The catalysts were reduced at 300 °C.
c
The reaction was carried out without K2CO3, 12 h.
d
The surface area of Pd°-PyPPh2@POPs catalyst is no more than 200 m2 g−1 which is lower than that of Pd°-PyPPh2@POP catalyst.
e
3-methyl-2-cyclohexen-1-one (4 mmol), 1 atm of initial pressure of Ar, Pd catalyst (0.8 mol% Pd), 140 °C, 5 h, 2 ml DMA.
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MolecularCatalysis456(2018)49–56
confirming that leaching is negligible. To ensure the heterogeneous
nature of the catalytic process, the hot filtration test is carried out when
the insoluble Pd°-PyPPh2@POP catalyst is removed from the hot reac-
tion mixture by filtration, the reaction stops. It indicates that the cat-
alyst is heterogeneous in nature.
Table 3
Dehydrogenation of ketone and quinone over Pd°-PyPPh2@POPa.
Entry
1
substrate
Phenol
Conv. (%)
99.5
Select(%)
50.8
TOF (h−1
)
20.7
31P NMR and XPS experiments results reveal that there is a strong
interaction between Pd and PyPPh2@POP support. The reaction me-
chanism of Pd°-PyPPh2@POP catalyst for dehydrogenation of 3-methyl-
2-cyclohexen-1-one is proposed, as shown in Scheme 3, which is similar
to that of Liu’s report [25]. The dehydrogenation reaction starts with a
keto-enol tautomerism equilibrium process in the presence of solid
pyridyl ring which acts as a solid base. The Pd-H2-PyPPh2@POP species
(A) are formed when the Pd°-PyPPh2@POP catalyst is treated with H2.
Then the β-position of 3-methyl-2-cyclohexen-1-one is dehydrogenated
to give the intermediate (B) together with the Pd-H2-PyPPh2@POP
species (A). The intermediate (B) is subsequently changed into the
important intermediate (C), and H2 is released due to that the N atom of
the pyridyl ring plays a key role in accelerating H atoms transfer [48].
After that, the intermediate (C) is dissociated to generate the Pd°-
PyPPh2@POP catalyst and the intermediate (D). Considering that the 3-
methyl-cyclohexanone is detected in the reaction mixture, a hydro-
genation process is also presented in Scheme 3, which is similar to the
dehydrogenation process except that it proceeds in the opposite direc-
tion.
2
3
99.7
100
99.2
99.9
20.8
20.8
4
5
100
99.9
99.9
20.8
18.8
90.2
6
7
38.3
21.6
99.9
99.9
8.0
4. Conclusions
0.45
In conclusion, we have developed an efficient and reusable porous
organic polymer supported palladium nanoparticle (Pd°-PyPPh2@POP)
catalyst featuring dual coordination sites (P and N species) for the de-
hydrogenation of 3-methyl-2-cyclohexen-1-one without any extra oxi-
dants and hydrogen acceptors. The present study reveals that the use of
Pd°-PyPPh2@POP instead of traditional inorganic material supported
palladium catalysts (Pd°@AC and Pd°@Al2O3) significantly increases
the reaction rate. Moreover, because of P atom and pridyl ring in
PyPPh2@POP which acted as solid base, the Pd°-PyPPh2@POP catalyst
still exhibits high activity without adding any inorganic bases (such as
K2CO3).
a
Basic Reaction conditions: ketone and quinone 4 mmol, 2 ml DMA, 0.032 g
Pd°-PyPPh2@POP (Pd loading 0.5 mmol g−1, reduced at 300 °C), time 12 h,
1 atm of initial pressure of Ar. Conversion was based on gas chromatography
(GC).
reaction (Fig. S2) [25]. Pd leaching of the heterogeneous Pd°-PyPPh2@
POP catalyst in reaction solution is detected by ICP-OES. It is found that
the amount of Pd in the first reaction solution is too low to be detected,
Fig. 5. TEM image of 0.5-Pd°-PyPPh2@POP catalyst which was reduced at 300 °C after the third circle.
55

X. Chen et al.
MolecularCatalysis456(2018)49–56
Scheme 3. Proposed mechanistic pathway of dehydrogenation of 3-methyl-2-cyclohexen-1-one on the Pd°-PyPPh2@POP catalyst.
Acknowledgements
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thank Mr. Jike Jiang and Miss. Huimin Gong for technical assistance.
This work is supported by the National Natural Science Foundation of
China (21273227) and the Strategic Priority Research Program of the
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