Hollow porous organic nanospheres for anchoring Pd(PPh3)4 through a co-hyper-crosslinking mediated self-assembly strategy

Article abstract of DOI:10.1039/d0nj00385a

In this work, we present a facile method for anchoring a transition metal catalyst (Pd(PPh₃)₄) into hollow porous organic nanospheres (H-PONs) based on a co-hyper-crosslinking mediated self-assembly strategy by using polylactide-b-polystyrene (PLA-b-PS) and Pd(PPh₃)₄ as precursors. The intrinsic catalytic activity of Pd(PPh₃)₄ could be well retained even if immobilized into H-PONs through a one-step Scholl reaction. Owing to the hierarchically porous structure, high surface area and well-dispersed active sites, the resulting H-PONs-Pd(PPh₃)₄ exhibited excellent catalytic activity, enhanced stability, and good recyclability for the Suzuki-Miyaura coupling in aqueous media. We hope that such a facile synthesis approach can provide a broad platform to support various transition metal catalysts anchored into hollow porous nanomaterials for the heterogeneous catalysis field.

Full text of DOI:10.1039/d0nj00385a

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Hollow porous organic nanospheres for anchoring
Pd(PPh3)4 through a co-hyper-crosslinking
mediated self-assembly strategy†
Cite this:DOI: 10.1039/d0nj00385a
Ying Liu, Li Zhang, Shengguang Gao, Buyin Shi, Haitao Yu and Kun Huang
*
In this work, we present a facile method for anchoring a transition metal catalyst (Pd(PPh3)4) into hollow
porous organic nanospheres (H-PONs) based on a co-hyper-crosslinking mediated self-assembly strategy
by using polylactide-b-polystyrene (PLA-b-PS) and Pd(PPh3)4 as precursors. The intrinsic catalytic activity
of Pd(PPh3)4 could be well retained even if immobilized into H-PONs through a one-step Scholl reaction.
Owing to the hierarchically porous structure, high surface area and well-dispersed active sites, the
resulting H-PONs-Pd(PPh3)4 exhibited excellent catalytic activity, enhanced stability, and good recyclability
for the Suzuki–Miyaura coupling in aqueous media. We hope that such a facile synthesis approach can
provide a broad platform to support various transition metal catalysts anchored into hollow porous nano-
materials for the heterogeneous catalysis field.
Received 21st January 2020,
Accepted 14th March 2020
DOI: 10.1039/d0nj00385a
rsc.li/njc
Tan’s group developed a method by crosslinking triphenylphos-
phine and benzene to produce functionalized KAPs(Ph-PPh3),23
Introduction
As one of the most widely used homogeneous transition metal and then binding Pd with PPh3 groups in the framework to form
catalysts, tetrakis(triphenyl phosphine) palladium (Pd(PPh3)4) KAPs(Ph-PPh3)-Pd. However, this step-by-step method is also
plays an important role in the catalysis of C–C and C–B cross- often accompanied by the deterioration of catalytic reactivity,
coupling reactions. However, homogeneous transition metal selectivity and durability, possibly because of the uncertain
catalysts often suffer from their air sensitivity and poor coordinated chemical microenvironment and partially post-
recyclability.1 Heterogenization of transition metal complexes synthetic metalation with catalytic species. Therefore, developing
may provide an efficient way to address these disadvantages a facile route to carry out the heterogenization of transition metal
because it combines the high catalytic activity of homogeneous complexes onto POPs is still a scientific challenge.11,24,25
catalysis with the inherent advantages of heterogeneous
catalysis.2–6
Besides these, morphology engineering of the supports also
plays a crucial role in optimizing the performance in hetero-
To date, there have been a lot of methods reported to attach geneous catalysis. POP supports with a hollow structure often
catalytic sites onto stable supports (such as zeolites, graphene, showed better catalytic performance than their non-hollow
metal–organic frameworks, etc.)7–10 for converting homogeneous counterparts due to their increased surface areas and reduced
transition metal catalysts into reusable heterogeneous versions. diffusion barriers for substrates.26–28 Recently, our group has
Among the various supports, as a kind of new porous nano- developed a novel method to synthesize hollow porous organic
material, porous organic polymers (POPs)11–16 have received nanospheres (H-PONs) as a kind of support for loading various
much attention due to their high surface area, good chemical catalysts.29 The convenience of functionalization and structure
stability, easy functionalization, and so on.17–19 The common tuning gives them potential applications in heterogeneous
strategy for anchoring transition metal catalysts onto POPs is to catalysis.30–32
synthesize various chelated ligand-functionalized POPs firstly,
Taking these into consideration, herein, we develop a facile
and then capturing the transition metal into the desired sup- strategy for anchoring transition metal catalysts into hollow porous
ports by forming new coordination bonds.20–22 For example, organic nanospheres through a one-step co-hyper-crosslinking
reaction between the tetrakis(triphenyl phosphine) palladium and
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of
polylactide-b-polystyrene diblock copolymer. Owing to the presence
of abundant micro/mesoporous structures, the obtained catalyst
shows not only higher catalytic efficiency, but also better stability
Chemistry and Molecular Engineering, East China Normal University, 500 N,
Dongchuan Road, Shanghai, 200241, P. R. China.
E-mail: khuang@chem.ecnu.edu.cn
than the reported heterogeneous catalysts as well as their homo-
geneous forms. The recyclability and reusability were evaluated by
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0nj00385a
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carrying out the reaction for about ten cycles successfully. In
addition, the excellent stability also allows the catalytic reaction
of Suzuki–Miyaura couplings to be carried out in the presence
of water.
Results and discussion
The detailed synthetic procedure of Pd(PPh3)4-functionalized
hollow porous organic nanospheres (H-PONs-Pd(PPh3)4) was
illustrated as shown in Scheme 1. First, the diblock copolymer
precursor PLA158-b-PS233 was synthesized by reversible addition–
fragmentation chain transfer (RAFT) polymerization of the styrene
(S) monomer with the initiator PLA-TC according to our previous
method.29 1H NMR spectroscopy analyses (Fig. S1, ESI†) and gel
permeation chromatography (GPC) (Fig. S2, ESI†) were respectively
used to analyze the polymeric structure and molecular weight
distribution. Next, a one-step Scholl reaction was carried out to
prepare H-PONs-Pd(PPh3)4 based on the co-hyper-crosslinking
mediated self-assembly strategy developed by our group by using
PLA-b-PS and Pd(PPh3)4 as precursors.33 During the crosslinking
process, the PLA block plays the role of a chemically degradable
section in producing the hollow cavity in the framework, and the PS
block with the Pd(PPh3)4 monomer can be hyper-crosslinked
together to obtain the microporous shell (intrasphere cross-
linking). However, the cross-linking of PS chains on the periphery
of nanospheres (intersphere cross-linking) will interconnect the
nanosphere network units in various directions, leading to the
formation of nanosphere aggregation.
The morphology of Pd(PPh3)4-functionalized hollow porous
polymers with different ratios of Pd(PPh3)4 was firstly investi-
gated and characterized by transmission electron microscopy
(TEM). The results showed that the amount of Pd(PPh3)4 during
the reaction plays an important role in the formation of
H-PONs-Pd(PPh3)4. As shown in Fig. 1B, the well-defined hollow
porous organic nanospheres could be clearly observed for the
H-PONs-Pd(PPh3)4 when the molar ratio of the Pd(PPh3)4 to
benzene ring on the diblock copolymer is 1/15. The average
pore size was measured to be 36 Æ 2 nm and the shell thickness
is 7 Æ 2 nm. The hollow nanosphere morphology disappeared
when the ratio of Pd(PPh3)4 increased to 1/10 and 1/5 (Fig. 1C
and D). However, once the ratio of Pd(PPh3)4 decreased down to
1/20 (Fig. 1A), ill-defined hollow nanosphere morphology can
Fig. 1 TEM images of the Pd(PPh3)4-functionalized hollow porous poly-
mers with different molar ratios of Pd(PPh3)4 to the benzene ring on the
diblock copolymer (A) 1/20; (B) 1/15; (C) 1/10; (D) 1/5.
be formed. The possible mechanism is that the different ratio
of Pd(PPh3)4 in the reaction will greatly influence the self-
assembly of PLA-b-PS and further result in the disappearance
or formation of ill-defined hollow nanosphere morphology. On
the basis of the above results, the catalyst with the molar ratio
of Pd(PPh3)4 to the benzene ring on the diblock copolymer of
1/15 was chosen for further detailed studies (hereafter denoted
as H-PONs-Pd(PPh3)4 (1/15)). Elemental analysis conducted
by ICP-AES (inductively coupled plasma atomic emission
spectroscopy) indicates 1.8 wt% of Pd loading in H-PONs-
Pd(PPh3)4 (1/15).
The Fourier transform infrared (FT-IR) spectra of PLA-b-PS,
H-PONs-Pd(PPh3)4 (1/15) and PPh3 showed that the peak at
1760 cmÀ1 of PLA characteristic carbonyl stretching disappeared,
indicating the complete removal of the PLA block during the
reaction in the presence of FeCl3 as a Lewis acid.32 (Fig. 2A) The
peak at 1116 cmÀ1 can be P–Ar stretching evidence of the existence
of the PPh3 group.15 The peak at 1448 cmÀ1 can be attributed to the
C–P stretching vibration. In comparison with the standard FT-IR
spectrum of PPh3, (Fig. 2A) the peak at 1434 cmÀ1 intensity
significantly decreased, indicating the interaction of PPh3 and
Pd atoms.34–36 The existence of P–Ar was also evidenced by the
1091 cmÀ1 P–Ar stretching in the FT-IR spectrum of PPh3. In
addition, the observed stretching vibration of Pd–P at 407 cmÀ1
can also confirm the interaction of PPh3 and Pd atoms.36 The
13C CP/MAS NMR spectrum (Fig. 2B) of H-PONs-Pd(PPh3)4
(1/15) exhibits two resonance peaks at 142 and 127 ppm, which
are related to the substituted and non-substituted aromatic carbon,
respectively.23 And the chemical shift at 30 ppm in 31P CP/MAS
NMR (Fig. 2C) as well as the new peak at 1116 cmÀ1 in the FT-IR
spectrum suggested that (Pd(PPh3)4) was partially oxidized to form
Scheme 1 Synthetic procedure of H-PONs-Pd(PPh3)4 from PLA-b-PS
and Pd(PPh3)4 precursors.
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Table 1 Pore structure parameters of the samples
a
b
c
d
SBET
Smicro
Smicro
Stot
Entry Samples
(m2
g
À1) (m2
g
À1) (m3
g
À1) (m3 gÀ1
)
1
2
3
4
H-PONs
H-PONs-Pd(PPh3)4 (1/15) 478.7
H-PONs-PPh3-Pd
Ph-PPh3-Pd
804.6
134.5
200.6
269.8
232
0.07
0.09
0.15
0.13
1.28
1.96
2.44
1.76
587.5
547.1
a
b
BET specific surface area from N2 adsorption. Microporous surface
c
d
area calculated from t-plots. DFT micropore volume. Total pore
volume (p/p0 = 0.997).
layer will produce the hollow cavity, which results in the formation
of the mesopores.29 According to the pore size distribution curve
obtained by DFT (Fig. 3B), the maximum size of the micropore and
mesopore is 1.2 nm and 5.3 nm, respectively. Beyond that,
the Brunauer–Emmett–Teller (BET) surface area and microporous
surface area of H-PONs-Pd(PPh3)4 (1/15) reach 478.7 m2 gÀ1 and
200.6 m2 gÀ1, respectively (Table 1). Based on their high surface
area and excellent hierarchical porosity, the obtained H-PONs-
Pd(PPh3)4 (1/15) can be expected to be a superior heterogeneous
catalyst.
To further prove the importance of the hollow structure in
H-PONs-Pd(PPh3)4 (1/15) for catalytic performance, different
types of PPh3-Pd catalysts for comparison were prepared
through a similar hyper-crosslinking strategy as shown in
Scheme 2. Route A: H-PONs-PPh3 was firstly prepared with
PLA-b-PS and PPh3 as precursors, and then mixed with PdCl2
to achieve the metalation of the H-PONs-PPh3 to acquire the
final H-PONs-PPh3-Pd catalyst. Route B: the Ph-PPh3-Pd catalyst
was synthesized similar to H-PONs-Pd(PPh3)4 (1/15) except
for using a single PS block to replace the PLA-b-PS diblock
copolymer as the precursor. The TEM images of H-PONs-PPh3-Pd
and Ph-PPh3-Pd catalysts are shown in Fig. S4 and S5 (ESI†),
which indicate that the hollow porous organic nanosphere
structure still exists in H-PONs-PPh3-Pd. However, a disordered
structure was observed for the Ph-PPh3-Pd catalyst, which further
proved that the PLA block plays a very important role in the
formation of the hollow porous nanosphere structure. Similarly,
both H-PONs-PPh3-Pd and Ph-PPh3-Pd catalysts show a high
surface area and a hierarchically (micro-/meso-pores) porous
structure (Table 1 and Fig. S6, S7, ESI†). Compared with the H-
PONs-Pd(PPh3)4 (1/15) catalyst, H-PONs-PPh3-Pd and Ph-PPh3-Pd
Fig. 2 (A) FT-IR spectra of PLA-b-PS, H-PONs-Pd(PPh3)4 (1/15) and PPh3;
(B) 13C CP/MAS NMR of H-PONs-Pd(PPh3)4 (1/15); (C) 31P CP/MAS NMR of
H-PONs-Pd(PPh3)4 (1/15); (D) XPS of Pd 3d region for H-PONs-Pd(PPh3)4
(1/15).
–PQO bonds in the H-PONs structure.16 The X-ray photoelectron
spectroscopy (XPS) spectra (Fig. 2D) reveal that two different
oxidation states of Pd (Pd0 and Pd2+) in H-PONs-Pd(PPh3)4 (1/15)
coexist due to the partial oxidation of Pd(PPh3)4.23 As shown in
Fig. 2D, Pd 3d5/2 at 337.5 eV and Pd 3d3/2 at 342.7 eV are respectively
attributed to Pd0 species and Pd2+ species. The ratio of Pd0/Pd2+ in
H-PONs-Pd(PPh3)4 (1/15) is 0.90. Compared to the single valence
catalyst, such a mixed valence catalyst is beneficial to carrying out
versatile transformation.37–39 In addition, the P 2d binding energy
(132.2 eV) in H-PONs-Pd(PPh3)4 (1/15) showed an obviously higher
value than that of the reported PPh3-based polymer (130.9 eV)
(Fig. S3, ESI†), which can be ascribed to the strong electron
donation of the triphenylphosphine ligands to Pd.15
To analyze the porous properties of the samples, nitrogen
adsorption–desorption measurement was conducted. As shown
in Fig. 3A, the isotherms show the combination of the typical
type I and type IV curves. And a distinct H4 hysteresis loop at a
high-pressure region indicates the presence of the mesoporous
structure.29 The formation of the micropores could be derived
from the hyper-crosslinking of the PS with Pd(PPh3)4, which
was confirmed by an absorption uptake at a relatively low
pressure. In contrast, the selective removal of the PLA core
Fig. 3 (A) Nitrogen adsorption–desorption isotherms of H-PONs-Pd(PPh3)4
(1/15); (B) pore size distribution calculated by the density functional theory (DFT)
method of H-PONs-Pd(PPh3)4 (1/15).
Scheme 2 Synthetic procedure of H-PONs-PPh3-Pd and Ph-PPh3-Pd.
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Table 2 Suzuki–Miyaura coupling reaction between phenyl chloride and
phenylboronic acid catalyzed by different solid catalystsa
capacity of the hollow porous organic nanosphere structure than
that of non-hollow Ph-PPh3-Pd, which can be more beneficial to
the diffusion of reactants and products and lead to a better
catalytic activity.
In addition, compared with the triphenylphosphine-based
palladium catalysts reported in the literature,21,43,44 the synthe-
sized H-PONs-Pd(PPh3)4 (1/15) showed a great catalytic activity
in the Suzuki–Miyaura coupling reaction (Table S1, ESI†). All
the above experimental results indicated that the morphologies
and highly dispersed Pd(PPh3)4 in the supports play significant
roles in heterogeneous catalysis.
The generality, recyclability and stability of catalysts played
important roles in the practical industrial applications. To
expand the scope of substrates, the Suzuki–Miyaura couplings
of various aryl halides and phenylboronic acid were chosen to
further evaluate the generality of the obtained H-PONs-
Pd(PPh3)4 (1/15).21 As shown in Table 3, the biphenyl in 99%
Entry Catalyst
Pd (wt%) Time (h) Yieldb (%) TOF (hÀ1
)
1
2
Pd(PPh3)4
H-PONs-Pd(PPh3)4 1.8
(1/15)
9.2
2
2
30
99
125
413
3
4
H-PONs-PPh3-Pd
Ph-PPh3-Pd
0.8
0.6
2
2
89
80
370
333
a
Reaction conditions: 0.5 mmol aryl halides, 0.75 mmol phenylboronic
acid, 1.5 mmol anhydrous K2CO3, 2 ml EtOH/H2O, 0.12 mol% Pd, 80 1C,
N2 atmosphere. GC yield.
b
catalysts have a relatively low palladium content under 1 wt%
(0.8 wt% and 0.6 wt%, respectively) through the ICP test.
In order to evaluate the catalytic activity and recyclability of
the obtained H-PONs-Pd(PPh3)4 (1/15), the Suzuki–Miyaura
coupling reaction between phenyl chloride and phenylboronic
acid was chosen as the model reaction.40–42 As shown in Table 2,
the homogeneous catalyst Pd(PPh3)4 (entry 1) just gave a 31%
yield within 2 h. However, the synthesized H-PONs-Pd(PPh3)4
(1/15) catalyst (entry 2) gave an over 99% yield within 2 h. The
heterogeneous catalyst showed a higher catalytic activity than its
homogeneous counterparts. As comparison, the heterogeneous
H-PONs-PPh3-Pd and Ph-PPh3-Pd catalysts under the same reac-
tion conditions (Table 2, entry 3) gave yields of 89% and 80%,
respectively, which are also lower than that of H-PONs-Pd(PPh3)4
(1/15). The kinetic profile curve directly revealed the catalytic
superiority of H-PONs-Pd(PPh3)4 (1/15) over H-PONs-PPh3-Pd,
Ph-PPh3-Pd and pure Pd(PPh3)4 catalysts (Fig. 4A). This pheno-
menon can be attributed to the special hollow porous organic
nanosphere framework structure, high surface area and highly
dispersed Pd(PPh3)4 within the shell of the support. To further
conceptualize the advantage of the hollow porous organic nano-
sphere structure, a simple substrate enrichment test was con-
ducted in the solvent of EtOH/H2O (3/2) similar to the solvent of
the Suzuki–Miyaura coupling reaction. Phenyl chloride and
catalyst H-PONs-Pd(PPh3)4 (1/15) and Ph-PPh3-Pd substrates
were mixed and stirred in a 20 ml vessel, and the sample was
taken every five minutes for the UV test. The UV curves (Fig. S8A,
ESI†) show an obvious enrichment of phenyl chloride in hydro-
phobic H-PONs-Pd(PPh3)4 (1/15), and moreover, the curves
shown in Fig. S8B (ESI†) also showed the greater enrichment
Table 3 Suzuki–Miyaura couplings catalyzed by H-PONs-Pd(PPh3)4 (1/15)a
Time Yieldb TOF
Entry Substrate1
1
Substrate2
(h)
(%)
(hÀ1
)
0.5
99
1650
413
2
3
2
4
99
90
188
4
5
4
4
85
78
177
163
6
0.5
6
99
89
81
91
97
95
91
97
1650
124
113
190
404
396
253
404
7
8
6
9
4
10
11
12
2
2
3
13
2
a
Fig. 4 (A) Kinetic profiles of Suzuki–Miyaura couplings of phenyl chloride
and phenylboronic acid for H-PONs-Pd(PPh3)4.(1/15), H-PONs-PPh3-Pd,
Ph-PPh3-Pd and Pd(PPh3)4. (B) Recycling test of H-PONs-Pd(PPh3)4 (1/15).
Reaction conditions: 0.5 mmol aryl halides, 0.75 mmol arylboronic
acids, 1.5 mmol anhydrous K2CO3, 2 ml EtOH/H2O, 0.12 mol% Pd,
b
80 1C, N2 atmosphere. GC yield.
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yield was produced by the solid catalyst with 0.12 mol% Pd in compared with the corresponding disordered Ph-PPh3-Pd as well as
2 h at 80 1C. As for various types of aryl chlorides bearing the homogeneous molecular catalyst Pd(PPh3)4 under the same
an electron-withdrawing group, such as –COCH3 and –NO2, conditions. In addition, this work provides a new thinking and
H-PONs-Pd(PPh3)4 (1/15) gave good yields within 2–4 h (Table 3, practical method to produce a highly efficient heterogeneous catalyst
entries 9–11). Furthermore, H-PONs-Pd(PPh3)4 (1/15) also has through a simple synthetic process.
excellent catalytic activity for activated aryl chlorides with
electron-donating groups, such as –CH3 and –OCH3 (Table 3,
entries 3–8). And the results of Table 3, entries 3–5, show that
Conflicts of interest
the catalyst can manage the aryl chlorides with substituent
groups at different positions. Notably, under similar condi-
There are no conflicts to declare.
tions, the conversion ratio of phenyl bromide can reach 99%
within 0.5 h (Table 3, entry 1). In addition, we also tested the Acknowledgements
catalytic activities of chlorobenzene with various arylboronic
This work was financially supported by the National Natural
acids bearing the electron-donating group –CH3 and the
Science Foundation of China (21574042, 51273066) and the
electron-withdrawing group-CF3 (Table 2, entries 12 and 13).
Fundamental Research Funds for the Central Universities
Very high yields of the selected arylboronic acid with different
(40500-20104-222047).
substituents indicated the large universality of the H-PONs-
Pd(PPh3)4 (1/15) catalyst. Moreover, the catalyst can be easily
recovered by simple filtration, and no obvious loss of the
catalytic activity was observed after 9 repeated cycles for phenyl
Notes and references
chloride. (Fig. 4B) The TEM image of H-PONs-Pd(PPh3)4 (1/15)
after 9 runs revealed that the hollow nanosphere framework
morphology of the support was well maintained (Fig. S9, ESI†).
To further examine whether the crosslinked Pd(PPh3)4 will split
away from the nanospheres, a ‘‘hot filtration’’ test was
conducted.28 The reaction of phenyl chloride was stopped at
20 min, then the solid was filtered out from the hot solution
and the mother filtrate was allowed to react respectively for
another 120 min under similar conditions. No significant
change in conversion was observed through the GC analyses
(Fig. S10, ESI†). The ICP analyses show nearly no Pd content in
the mother filtrate (0.001 mg LÀ1), which is in accordance with
the ‘‘hot filtration’’ test. The above-mentioned results further
confirmed that the obtained H-PONs-Pd(PPh3)4 (1/15) has
excellent catalytic activity, recyclability and stability. This can
be owed to the high surface area and unique hollow micro-
porous nanosphere frameworks of the support, which can
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For more details please see the ESI.†
Conclusions
In summary, we have reported an efficient strategy to immobilize
and stabilize the homogeneous catalyst Pd(PPh3)4 on the shell of 16 Y. Xu, T. Wang, Z. He, M. Zhou, W. Yu, B. Shi and K. Huang,
hollow porous organic nanospheres by a one-step hyper-crosslinking Macromolecules, 2017, 50, 9626–9635.
reaction. The shells with micropores and hollow mesopores ensured 17 J. Chun, S. Kang, N. Park, E. J. Park, X. Jin, K. D. Kim,
the high dispersion of Pd active sites and the diffusion of organic
reactants and products. The H-PONs-Pd(PPh3)4 (1/15) introduced
here is easy to synthesize and it also has good stability in air and
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