M. Chen et al.
Molecular Catalysis 498 (2020) 111239
a decreased activity was observed for the recycled catalyst. Hou and
Tanaka [32] reported the acidic resin-supported palladium complexed
catalysts that could be recycled five times with a consistent yield. In
general, the conventional solid catalysts show a relatively low activity or
poor reusability because of the poor chemical modification and low
surface areas. Therefore, there is still a strong desire to develop a het-
erogeneous catalyst combining the advantages of homogeneous and
heterogeneous catalysts for alkene hydroesterification.
measurement was performed on a JEM-2100 transmission electron mi-
croscope with an accelerating voltage of 200 kV. The high-angle annular
dark-field scanning transmission electron microscopy (HAADF-STEM)
measurement was carried out on a JEOL-ARM200 F electron micro-
scope. X-ray photoelectron spectroscopy (XPS) analysis was carried out
on a Thermo ESCALAB 250Xi spectrometer using a 15 kV Al Kα X-ray
source as the radiation source. The 31P solid-state NMR experiments
were carried out on VARIAN’s Infinitypius NMR spectrometer. The in
situ FTIR measurement with CO was carried out using Bruker’s Tensor
27 Fourier transform infrared spectrometer in the range of 4000–400
Porous organic polymers (POPs) [33–36] are emerging as a versatile
platform for the development of heterogeneous active metal highly
dispersed or single-atom catalysts [37–42], due to its large surface area,
hierarchical porosity, high stability, and flexibility in building block
design. Owing to the rich and strong anchoring sites of the ligands,
different transition metals can be introduced into the solid polymers
through the formation of stable metal-ligand complexes via some coor-
dination bonds. Such heterogenized metal catalysts cannot ease the
leaching of active metal species and thus facilitate catalyst and product
separation. In our previous work [43], PPh3@POP was developed by
hydrothermal polymerization of tris-vinyl-functionalized PPh3 ligand in
the presence of copolymerized monomers (Scheme 1). And the
PPh3@POP polymer supported Au [44], Rh [37,45], Ru [46], Zn [47],
Pd [48,49] metal catalysts exhibited excellent catalytic performance in
hydroamination, hydroformylation, hydrogenation, and coupling re-
actions. It is worth mentioning that the PPh3@POP supported palladium
catalyst not only gave a high yield of 99 % of 4-acetyl-biphenyl but also
can be reused 10 times without any activity drop in the coupling re-
actions of aryl chlorides. As an extension of our previous work, the
hybrid catalyst further evaluated in the hydroesterification of olefins. To
our delight, the catalyst was found to be highly efficient, giving activity
and selectivity approaching those of their corresponding homogeneous
counterpart under the same conditions and it can be reused 5 times
without loss of activity.
cmꢀ 1. All spectra were recorded with 32 scans at a resolution of 4 cmꢀ 1
.
Before the test, the sample was evacuated at 150 ◦C for 30 min, and then
cooled to 30 ◦C and purged with helium for 30 min to collect background
spectrum A. After exposure to 2 % CO for 20 min, the He purged for 30
min and recorded a spectrum B. The difference between the spectrum B
and the background spectrum A is the infrared spectrum of the sample
adsorbed CO.
2.4. Hydroesterification of olefins
Hydroesterification of olefins was carried out in an autoclave.
Typically, 50 mg catalyst, 2 ml methanol, 1 mmol styrene and 25 mg p-
toluenesulfonic acid (p-TsOH) were sequentially added to the autoclave.
The reactor was then sealed and purged three times with pure CO, fol-
lowed by adjusting the pressure to 2.0 MPa. The temperature was raised
to 90 ◦C in a water bath for 5 h. Afterward, the reactor was cooled to
room temperature with ice-water, the liquid samples were centrifuged to
isolate the catalyst, and then the solution was analyzed off-line by
Agilent 7890B gas chromatography with a HP-5 capillary column and an
FID detector. The selectivity of the product is calculated by the sum of
the linear and branched ester. Hydroesterification of other olefins were
tested according to the same procedure described above for styrene. The
hot filtration reaction was performed in the presence of the Pd/
PPh3@POP catalyst for 1 h, and then the solid catalyst was filtered off at
the reaction temperature. The resulting filtrate was placed under the
same reaction conditions as the original reaction and analyzed by GC.
The TOF value was calculated according to:
2. Experimental
2.1. Materials
All reagents were of analytical grade and used as purchased without
further purification unless otherwise stated. Tetrahydrofuran (THF) was
distilled from sodium and PCl3 was distilled under argon prior to use.
m ole of esters
m ole of Pd(OAc)2 × reaction tim e
TOF =
2.2. Preparation of catalysts
3. Results and discussion
All operations were carried out under anaerobic anhydrous condi-
tions. PPh3@POP and divinylbenzene (DVB) was prepared according to
our previous report [43]. The procedure for the synthesis of
Pd/PPh3@POP was as follows. 1.0 g PPh3@POP was added to 20 mL
THF solution containing 14.7 mg palladium acetate and stirred at room
temperature for 24 h under N2. The solid sample was centrifugally
separated, washed with excessive THF, and dried at 65 ◦C under vac-
uum. The obtained sample was denoted as xPd/PPh3@POP, where x
represents the Pd content in wt.%. The Pd/Al2O3, Pd/SiO2 and Pd/DVB
catalysts were also prepared according to the above operation.
3.1. Catalyst characterization
The Pd/PPh3@POP catalysts were thoroughly characterized by
Table 1
Characterization data of the Pd/PPh3@POP catalysts and the support.
a
b
Samples
Vtotal
Vmicro
Smesob (m2
SBETc (m2
Pdd
(cm3 gꢀ 1
)
(cm3 gꢀ 1
)
gꢀ 1
)
gꢀ 1
)
(wt. %)
PPh3@POP
0.2Pd/
1.80
1.64
0.10
0.08
855
791
1067
981
–
0.20
PPh3@POP
0.5Pd/
2.3. Characterization of catalysts
1.83
1.55
1.45
1.67
0.06
0.06
0.04
0.06
811
726
600
818
941
905
689
944
–
PPh3@POP
0.7Pd/
0.64
0.56
–
The specific surface area, pore volume, and pore distribution of the
samples were measured by adsorption-desorption of N2 at 77 K on a
Quantachrome Autosorb-1 instrument. The samples were outgassed at
120 ◦C for 24 h before measurements. The X-ray diffraction (XRD) was
performed on an X’per PRO X-ray diffractometer from PANalytical using
PPh3@POP
0.7Pd/
PPh3@POPe
1.0Pd/
PPh3@POP
Cu K
α
radiation in a scanning range of 10◦–90◦ at a speed of 10◦ minꢀ 1
.
a
Determined from the amount of N2 adsorbed at p/p0 = 0.99.
b
c
The tube pressure was 40 kV and tube flow was 40 mA. The thermog-
ravimetric analysis (TG) experiments of the catalysts were carried out on
a STA 499 F3 thermogravimetric analyzer. SEM study was measured
t-plot method.
BET method.
d
e
ICP-OES.
using
a
FEI Quanta 200
F
scanning electron microscope. TEM
Catalyst after 5 cycles.
2