The effect of the position of cross-linkers on the structure and microenvironment of PPh3moiety in porous organic polymers

Article abstract of DOI:10.1039/d0ta12316a

Three trivinyl functionalization triphenylphosphine (3vPPh₃)-based porous organic ligands (3vPPh₃-POLs) with cross-linkers in different positions were obtained through solvothermal polymerization. By simply changing the position of the cross-linkers (vinyl groups) attached to the PPh₃monomer, the resulting porous organic polymer (POP) materials acquired diverse hierarchical porous structures, and the microenvironment of POPs was sequently regulated. Among the three 3vPPh₃-POLs, the BET surface areas ranged from 168 to 1583 m²g^?1, while the proportion of micropores changed from 0.0% to 52.0%. Benefiting from the unique structure, Rh ions could be coordinated and dispersed as a single site inm-3vPPh₃-POL to form HRh(CO)₂(PPh₃-POL)₂species, which endowed the Rh/m-3vPPh₃-POL catalyst with an activity similar to that in the homogeneous system, anl/bratio (the ratio of the linear aldehyde to the branched aldehyde) approximately as high as 10, and stability for a duration of more than 500 h in the hydroformylation of 1-octene.

Full text of DOI:10.1039/d0ta12316a

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Materials Chemistry A
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PAPER
The effect of the position of cross-linkers on the
structure and microenvironment of PPh3 moiety in
porous organic polymers†
Cite this: J. Mater. Chem. A, 2021, 9,
9165
Guangjun Ji,‡ab Cunyao Li,‡a Dong Xiao,c Guoqing Wang,ab Zhao Sun,ab Miao Jiang,a
c
a
ac
*
*
*
Guangjin Hou,
Li Yan and Yunjie Ding
Three trivinyl functionalization triphenylphosphine (3vPPh3)-based porous organic ligands (3vPPh3-POLs)
with cross-linkers in different positions were obtained through solvothermal polymerization. By simply
changing the position of the cross-linkers (vinyl groups) attached to the PPh3 monomer, the resulting
porous organic polymer (POP) materials acquired diverse hierarchical porous structures, and the
microenvironment of POPs was sequently regulated. Among the three 3vPPh3-POLs, the BET surface
areas ranged from 168 to 1583 m2 ꢀ1, while the proportion of micropores changed from 0.0% to 52.0%.
g
Benefiting from the unique structure, Rh ions could be coordinated and dispersed as a single site in m-
3vPPh3-POL to form HRh(CO)2(PPh3-POL)2 species, which endowed the Rh/m-3vPPh3-POL catalyst
with an activity similar to that in the homogeneous system, an l/b ratio (the ratio of the linear aldehyde
to the branched aldehyde) approximately as high as 10, and stability for a duration of more than 500 h in
the hydroformylation of 1-octene.
Received 20th December 2020
Accepted 11th March 2021
DOI: 10.1039/d0ta12316a
rsc.li/materials-a
illustrated that the C^C groups could cooperate with N atoms
from pyridine to coordinate with Pd(II), and served as both cross-
1 Introduction
linkers and coordination sites. The pyridine-based CMP could be
used as a heterogeneous ligand for the Pd(II)-catalyzed oxidative
Heck reaction with high linear selectivity (the ratio of linear
products to branched products was more than 100 : 1).22 We also
introduced vinyl groups to organic ligand monomers and
prepared a new kind of POPs through solvothermal polymeriza-
tion, denoted as porous organic ligands (POLs). POLs possess
high surface area, large pore volume, hierarchical porosity,
favorable stability, and highly exposed functional sites.23
Compared with other fabricating methods, the polymerization of
vinyl groups in functional monomers is believed to be the most
effective and relatively cheaper way to get functional POPs.
Recently, a series of POLs has been constructed and used in many
elds.24–28 It is worth mentioning that we have achieved the
industrial production and application of POLs as a bifunctional
material to prepare a heterogeneous catalyst for the hydro-
formylation of ethylene. However, the effects of the position of
cross-linkers and the intrinsic structure of monomers on the
properties of POLs are very far from being understood.
As a new class of functional materials, porous organic polymers
(POPs) have got the full attention of researchers due to their
extensive application in gas sorption and storage,1 energy storage,2
proton conduction3 and catalysis.4–7 The POP family consists of
hyper-cross-linked polymers (HCPs),8 polymers of intrinsic
microporosity (PIMs),9 conjugated microporous polymers
(CMPs)10 and covalent organic frameworks (COFs).11 Various cross-
linking methods have been employed to fabricate these elegant
materials, for example, COFs can be cross-linked by boroxine,12
boronate ester,13 imine,14 triazine,15 etc. HCPs can be fabricated by
external crosslinking,16 post-crosslinking17 or direct poly-
condensation.18 PIMs are prepared by the formation of dibenzo-
dioxin19 and formation of imide linkages.20 CMPs are prepared by
the polymerization of monomers with conjugate structures.21
A series of CMPs was prepared through the Sonogashira cross-
linking reaction of 1,3,5-triethynyl-benzene (C^C) with bipyr-
idine in our previous study. The characterization result
aDalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics,
Chinese Academy of Sciences, Dalian, 116023, P. R. China. E-mail: yanli@dicp.ac.
cn; dyj@dicp.ac.cn
bUniversity of Chinese Academy of Sciences, Beijing 100039, P. R. China
cState Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese
Academy of Sciences, Dalian, 116023, P. R. China. E-mail: ghou@dicp.ac.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ta12316a
Based on our previous works, we believe that the properties
of POPs could be optimized by regulating the skeletons; herein,
we provide a new strategy to regulate the skeletons of POPs by
simply changing the position of the vinyl groups attached to the
functional monomer. Tris(vinphenyl)phosphine (3vPPh3) was
employed as a template to demonstrate the feasibility of this
regulation strategy. Three 3vPPh3-POLs were prepared accord-
ing to this strategy, and each of them displayed different
‡ These authors contributed equally to this work.
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Synthesis of p-3vPPh3, m-3vPPh3 and o-3vPPh3
Synthesis of p-3vPPh3. First, 60 mmol 4-bromostyrene was
dissolved in 60 mL THF, added dropwise to 63 mmol magne-
sium chips in 1 hour, then reacted for 4 hours to get solution
A. Next, 20 mmol PCl3 was dissolved in 20 mL THF, added
dropwise to solution A at 253 K in 1 hour, then reacted for 2
hours. Aer the reaction, 50 mL of saturated NH4Cl aqueous
was added. The organic phase was extracted with excessive
amount of ethyl acetate, which was dried with MgSO4. Aer
ltering and purifying by silica gel chromatography (EtOAc/
Petroleum ether ¼ 1/20), p-3vPPh3 (white solid) was ob-
tained. 1H NMR (400 MHz, chloroform-d) d 7.41–7.32 (m, 6H),
7.27 (t, J ¼ 7.8 Hz, 6H), 6.70 (dd, J ¼ 17.6, 10.9 Hz, 3H), 5.77 (d, J
¼ 17.6 Hz, 3H), 5.27 (d, J ¼ 10.9 Hz, 3H). 13C NMR (101 MHz,
chloroform-d) d 138.03, 136.62 (d, J ¼ 10.6 Hz), 136.39, 133.91
(d, J ¼ 19.6 Hz), 126.35 (d, J ¼ 7.1 Hz), 114.74. 31P NMR (162
MHz, chloroform-d) d ꢀ6.76.
Synthesis of m-3vPPh3. Under the same conditions as p-
3vPPh3. On replacing 4-bromostyrene with 3-bromostyrene, m-
3vPPh3 (pale yellow oil) was obtained. 1H NMR (400 MHz,
chloroform-d) d 7.46–7.34 (m, 6H), 7.28 (td, J ¼ 8.3, 7.9, 1.4 Hz,
3H), 7.18 (d, J ¼ 7.2 Hz, 3H), 6.63 (dd, J ¼ 17.6, 10.9 Hz, 3H), 5.66
(d, J ¼ 18.2 Hz, 3H), 5.20 (d, J ¼ 11.5 Hz, 3H). 13C NMR (101
MHz, chloroform-d) d 137.76 (d, J ¼ 7.7 Hz), 137.29 (d, J ¼ 11.2
Hz), 136.54, 133.07 (d, J ¼ 16.5 Hz), 131.96 (d, J ¼ 23.0 Hz),
128.80 (d, J ¼ 6.3 Hz), 126.58, 114.50. 31P NMR (162 MHz,
chloroform-d) d ꢀ5.10.
Scheme 1 The preparation method of the three 3vPPh3 monomers
and their corresponding POLs; the three 3vPPh3 monomers were
constituted by the same organic block and vinyl group, while pos-
sessing different structures.
Synthesis of o-3vPPh3. Under the same conditions as p-3vPPh3.
On replacing 4-bromostyrene with 2-bromostyrene, o-3vPPh3
structures. Also, Rh ions were introduced into these 3vPPh3-
POLs (denoted as Rh/3vPPh3-POLs) to further validate the
transformation of the microenvironment of the PPh3 moiety
among these 3vPPh3-POLs, as Rh–P complexes are sensitive in
hydroformylation reaction. Scheme 1 shows the regulation
strategy and structures of the three 3vPPh3 monomers and the
corresponding 3vPPh3-POLs. The characterization of Rh/
3vPPh3-POLs by MAS NMR, EXAFS and in situ FTIR illustrated
that the microenvironment of the PPh3 moiety was different
when the three 3vPPh3-POLs were coordinated with Rh ions.
1
(white solid) was obtained. H NMR (400 MHz, chloroform-d)
d 7.59 (dd, J ¼ 7.3, 4.5 Hz, 3H), 7.46–7.20 (m, 6H), 7.12 (t, J ¼
7.4 Hz, 3H), 6.74 (dd, J ¼ 7.6, 4.4 Hz, 3H), 5.61 (d, J ¼ 17.2 Hz,
3H), 5.18 (d, J ¼ 10.9 Hz, 3H). 13C NMR (101 MHz, chloroform-d)
d 142.41 (d, J ¼ 23.0 Hz), 135.41 (d, J ¼ 24.6 Hz), 134.07, 133.69
(d, J ¼ 12.3 Hz), 129.04, 127.93, 125.47 (d, J ¼ 4.5 Hz), 115.83. 31
NMR (162 MHz, chloroform-d) d ꢀ29.65.
P
Synthesis of p-3vPPh3-POL, m-3vPPh3-POL and o-3vPPh3-
POL
Synthesis of p-3vPPh3-POL. First, 1.0 g p-3vPPh3 and 25.0 mg
AIBN were dissolved in 10 mL THF. Aer stirring for 30 minutes
at room temperature, the mixture was transferred into an
autoclave, then maintained at 373 K for 24 h. Aer evaporation
of THF under vacuum, the polymer p-3vPPh3-POL was obtained
as a white solid.
Synthesis of m-3vPPh3-POL. First, 1.0 g m-3vPPh3 and 25.0 mg
AIBN were dissolved in 10 mL THF. Aer stirring for 30 minutes
at room temperature, the mixture was transferred into an
autoclave, then maintained at 373 K for 24 h. Aer evaporation
of THF under vacuum, the polymer m-3vPPh3-POL was obtained
as a white solid.
Synthesis of o-3vPPh3-POL. First, 1.0 g o-3vPPh3 and 25.0 mg
AIBN were dissolved in 10 mL THF. Aer string for 30 minutes
at room temperature, the mixture was transferred into an
autoclave, then maintained at 373 K for 24 h. Aer evaporation
of THF under vacuum, the polymer o-3vPPh3-POL was obtained
as a pale-yellow solid.
2 Experimental
2.1 Materials
All solvents were puried according to standard laboratory
methods. Azobisisobutyronitrile (AIBN) (99%), tetrahydrofuran
(THF) (99%) and PCl3 (97%) were purchased from Tianjin Kemiou
Chemical Reagent Co., Ltd. Rh(CO)2(acac) (99%), 1-hexene (97%),
1-octene (99%), 1-heptene (98%) and 1-dodecene (97%) were ob-
tained from J&K Scientic Ltd. Sodium (98%), benzophenone
(98%), 4-bromostyrene (98%), 3-bromostyrene (98%) and 2-bro-
mostyrene (98%) were obtained from Aladdin. THF was distilled
over sodium/benzophenone under Ar atmosphere, and PCl3 was
puried by distillation under Ar atmosphere before use.
2.2 Methods
Unless otherwise noted, all manipulations were carried out
under Ar atmosphere in a glove-box or using standard Schlenk
techniques.
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Synthesis of Rh/p-3vPPh3-POL, Rh/m-3vPPh3-POL and Rh/o-
Journal of Materials Chemistry A
used as the carrier gas and the column front pressure was kept
at 4 psi. The injection port was held at 523 K, 4 psi while the
split ratio was kept at 50 : 1. As for the ame ionization detector
(FID), the temperature was held at 493 K and the ow rate of H2
was kept at 30 mL minꢀ1 while the ow rate of air was held at
300 mL minꢀ1. n-Butanol was used as an internal standard and
the GC was also calibrated using quantitative analysis of
substrates and products.
3vPPh3-POL
Synthesis of Rh/p-3vPPh3-POL. One gram p-3vPPh3-POL and
6.3 mg Rh(acac)(CO)2 were added to 30 mL THF. Aer stirring
for 24 h at room temperature, the resulted product was sepa-
rated by using a centrifuge. The crude product was washed with
THF (3 ꢁ 5 mL). Aer evaporation of THF under vacuum, the
slight yellow catalyst Rh/p-3vPPh3-POL was obtained.
Synthesis of Rh/m-3vPPh3-POL. One gram m-3vPPh3-POL and
6.3 mg Rh(acac)(CO)2 were added to 30 mL THF. Aer stirring
for 24 h at room temperature, the resulted product was sepa-
rated by using a centrifuge. The crude product was washed with
THF (3 ꢁ 5 mL). Aer evaporation of THF under vacuum, the
slight yellow catalyst Rh/m-3vPPh3-POL was obtained.
Synthesis of Rh/o-3vPPh3-POL. One gram o-3vPPh3-POL and
6.3 mg Rh(acac)(CO)2 were added to 30 mL THF. Aer stirring
for 24 h at room temperature, the resulting product was sepa-
rated by using a centrifuge. The crude product was washed with
THF (3 ꢁ 5 mL). Aer evaporation of THF under vacuum, the
slight yellow catalyst Rh/o-3vPPh3-POL was obtained.
3 Results and discussion
3.1 Effects of the position of cross-linkers on the structure
evolution
Firstly, as shown in the upper line of Scheme 1, three kinds of
tris(vinphenyl)phosphine (3vPPh3) with cross-linkers (vinyl
groups) in different positions were designed and successfully
synthesized by precisely controlling the position of the cross-
linkers in functional monomers, and their NMR spectra are
displayed in Fig. S1.† Then, the three 3vPPh3-POLs were
prepared through solvothermal polymerization with yields of
100%. To gure out the differences in the structure among the
3vPPh3-POLs, nitrogen sorption isotherms, thermogravimetric
analysis (TGA) and scanning electron microscopy (SEM) were
employed to characterize the 3vPPh3-POLs. Table 1 shows that
p-3vPPh3-POL, m-3vPPh3-POL and o-3vPPh3-POL possess
2.3 Catalytic performance tests
As a typical run for autoclave reaction, 5.0 g of toluene, 1.0 g of
1-octene, and 0.060 g of Rh/m-3vPPh3-POL catalyst were added
into an autoclave. Aer sealing and purging with syngas
(CO : H2 ¼ 1 : 1) 6 times, the autoclave was heated to 383 K
within 30 min, followed by adjusting the pressure of syngas to
1.0 MPa and stirring at 383 K for 12 h. During the reaction, the
pressure in the reactor was held by the injection of syngas
(CO : H2 ¼ 1 : 1) via a pressure regulator. Aer cooling the
autoclave in an ice bath and releasing the pressure, a sample
was taken from the autoclave by ltration or centrifugation and
analyzed.
different BET surface areas, from 1583 m2 gꢀ1 to 168 m2 gꢀ1
,
and pore volumes, from 2.70 cm3 gꢀ1 to 0.15 cm3 gꢀ1. As shown
in Fig. 1a, both m-3vPPh3-POL and p-3vPPh3-POL exhibit the
curve of type I plus type-IV, while o-3vPPh3-POL does not,
indicating that m-3vPPh3-POL and p-3vPPh3-POL possess
intrinsic hierarchical porosity, which is further conrmed by
the SEM images (Fig. 1d).
As calculated by non-local density functional theory
(NLDFT), the pore sizes of m-3vPPh3-POL and p-3vPPh3-POL are
distributed from 0.59 nm to 2.84 nm (Fig. 1b), while the
proportion of micropores in m-3vPPh3-POL is more than double
that in p-3vPPh3-POL (Table 1). Compared with m-3vPPh3-POL
and p-3vPPh3-POL, o-3vPPh3-POL possesses lower surface area
and pore volume, and is mesoporous only. The TGA curves of all
three 3vPPh3-POLs also show different decomposition temper-
atures. The decomposition temperatures of p-3vPPh3-POL and
m-3vPPh3-POL are higher than 700 K while that of o-3vPPh3-POL
is 650 K (Fig. 1c), which indicates that p-3vPPh3-POL and m-
3vPPh3-POL possess better thermal stability. As shown in
Fig. 1d–f, the SEM images exhibit the amorphous morphology
of 3vPPh3-POLs. Moreover, the SEM images of p-3vPPh3-POL
In the recycling experiment, the cooled autoclave was opened
in a glove box; a lter was used to separate the catalyst, and the
catalyst could be directly used in the next run.
We also performed the xed-bed hydroformylation of long-
chain olens to test the stability of Rh/m-3vPPh3-POL cata-
lysts. The length of the xed-bed reactor was 40 cm, and the
inner diameter was 9 mm. As a typical run, 12 mL of quartz sand
was lled at the bottom of the reactor, followed by lling in
a little silica wool (<1 mL) and 0.30 g of Rh/m-3vPPh3-POL
catalyst (about 2.0 mL), then a little silica wool (<1 mL) and
another 12 mL of quartz sand was lled at the top of the reactor.
Thus, the catalyst was put just in the middle of the reactor. The
reactor was heated by a tubular furnace and the temperature
was monitored by a thermocouple which was just put in the
thermocouple hole beside the sample hole. The effluent passed
through a condenser to capture the liquid products. A simple
owchart of the hydroformylation equipment is listed in
Scheme S1.†
Table 1 Textural parameters of p-3vPPh3-POL, m-3vPPh3-POL and
o-3vPPh3-POL
BET surface
area (m2 gꢀ1
Pore volume
Micropore
(%)
The products were analyzed off-line by an Agilent 7890B gas
chromatograph with an HP-5 column (30 m length, 0.3 2 mm
diameter). The column temperature was initially kept at 313 K
for 5 min and then raised to 373 K at 5 K minꢀ1, maintained at
373 K for 2 min then raised to 448 K at 10 K minꢀ1. Argon was
Samples
)
(cm3 gꢀ1
)
p-3vPPh3-POL
m-3vPPh3-POL
o-3vPPh3-POL
1583
841
168
2.70
1.68
0.15
23.5
52.0
0.0
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ranging from 129 to 146 ppm are assigned to the aromatic
carbons of 3vPPh3-POLs, and the signal at 40 ppm is attributed
to the polymerized vinyl groups. There is a very small peak at
112 ppm in the 13C MAS NMR spectrum of p-3vPPh3-POL, which
could be assigned to unpolymerized vinyl groups, indicating
that p-3vPPh3-POL is highly polymerized.33 Similarly, the small
peak at 113 ppm in the 13C MAS NMR spectra of m-3vPPh3-POL
declares its high degree of polymerization. However, the great
differences in the 13C CP and one pulse MAS NMR between m-
3vPPh3-POL and p-3vPPh3-POL declare the discrepancy in their
structures, which comes from the monomer gap, to be precise,
the difference in the position of the vinyl groups.
To clearly clarify whether the frameworks of p-3vPPh3-POL
and m-3vPPh3-POL are changed under external environment,
the kinetic performances of the polymer frameworks were
characterized by 13C{1H} R-type RF irradiation and two-
dimensional separated local eld (2D R-SLF) experiments.
These experiments measure the residual 13C–1H dipolar
coupling which is then used as an indicator to monitor the
motions of the 3vPPh3-POL frameworks upon post treatments
and reactions. The 13C–1H dipolar coupling of CH groups from
both aromatic rings and polymerized vinyl groups of m-3vPPh3-
POL and p-3vPPh3-POL was measured (as shown in Fig. 3). The
results show that CH groups in both m-3vPPh3-POL and p-
3vPPh3-POL have a 13C–1H dipolar coupling strength of 22 kHz,
indicating that the frameworks of these materials are rather
rigid, since 13C–1H dipolar coupling for CH and CH2 groups in
Fig. 1 (a) Nitrogen sorption isotherms of different 3vPPh3-POLs
materials. (b) Pore size distribution of different 3vPPh3-POL materials.
(c) TGA and DTG curves of different 3vPPh3-POLs materials. (d) The
SEM image of p-3vPPh3-POL. (e) The SEM image of m-3vPPh3-POL. (f)
The SEM image of o-3vPPh3-POL.
and m-3vPPh3-POL reveal a highly developed pore structure,
while that of o-3vPPh3-POL shows a poor pore structure.
To further conrm the transformation of the framework in
3vPPh3-POLs, m-3vPPh3-POL and p-3vPPh3-POL were charac-
terized by the 13C cross-polarization (CP) and one pulse magic
angle spinning (MAS) NMR, shown in Fig. 2. The strong signals
Fig. 3 (a) 13C{1H} 2D R-SLF spectra of p-3vPPh3-POL and Rh/p-
3vPPh3-POL in different stages. (b) The F1 dimension slices of 13C peak
at 132 ppm spectra of (a). (c) 13C{1H} 2D R-SLF spectra of m-3vPPh3-
Fig. 2 13C MAS NMR spectrum of m-3vPPh3-POL and p-3vPPh3-POL. POL and Rh/m-3vPPh3-POL in different stages. (d) The F1 dimension
CP mode was employed for testing the framework structure of slices of 13C peak at 132 ppm spectra of (c). The residual dipole
samples, one pulse mode provided more refined analysis of the coupling is calculated from the splitting based on the scaling factor of
framework structure.
the recoupling sequence used.
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Journal of Materials Chemistry A
a rigid framework retains a strength of a one-bond distance and provide abundant coordinating sites to coordinate with Rh ions
is about 22 kHz, but less than 22 kHz when the dipolar coupling and promote Rh ions to uniformly distributed as a single site. It
is averaged by molecular motions.32 Also, the 13C–1H dipolar is worth mentioning that the hierarchical porosity of m-3vPPh3-
coupling strength of the CH groups of these POLs is retained POL is preserved aer the reaction, demonstrating the superior
well aer Rh doping, catalyst activation and reaction, suggest- structure stability of 3vPPh3-POLs. Also, the abundant uncov-
ing their strong structural stability upon post treatment and ered P atoms in 3vPPh3-POLs are ready to coordinate with Rh
reaction.
ions and x them as a single site; therefore, the microenviron-
ment of the PPh3 moiety in 3vPPh3-POLs plays an important
role in their properties and especially in their catalysis
performance.
3.2 Effects of the position of cross-linkers on
microenvironment of 3vPPh3 moiety
The 31P MAS NMR spectrum of m-3vPPh3-POL exhibits two
signals (Fig. 4a), one is a strong signal at ꢀ3 ppm, which is
attributed to P atoms in the m-3vPPh3-POL framework, the other
is a small peak at 25 ppm, which could be assigned to the slight
oxidation of phosphorus (P]O).26 A signal at 30 ppm is raised
in Rh/m-3vPPh3-POL, and increased aer syngas treatment. In
the used Rh/m-3vPPh3-POL, the signal intensity at 30 ppm
remains unchanged. By spectrum tting, the peaks between 29
to 34 ppm are assigned to P multiply coordinated with Rh,34,35
conrming that there are multiple coordination bonds between
Rh and P in Rh/m-3vPPh3-POL, which leads to the formation of
Rh–P complexes and is benecial for catalysis. Compared with
m-3vPPh3-POL, p-3vPPh3-POL exhibits a little change in its 31P
MAS NMR spectra (Fig. 4b). The P in p-3vPPh3-POL shows a peak
at ꢀ6 ppm and a peak of P]O at 23 ppm. Moreover, a peak at
28 ppm appears in the fresh Rh/p-3vPPh3-POL, it is shied to
30 ppm and increased aer syngas treatment, and then main-
tains its strength but is shied back to 28 ppm aer the reac-
tion,36 indicating that Rh is also multiply coordinated with P in
p-3vPPh3-POL, but the Rh–P complexes are different from those
in m-3vPPh3-POL. Though the 31P MAS NMR spectrums conrm
that Rh is multiply coordinated with P in both 3vPPh3-POLs, P
atoms in p-3vPPh3-POL are more electronegative than those in
In 3vPPh3-POLs, the functional sites are mainly presented by P
in the PPh3 moiety. As an embodiment of the functional sites of
3vPPh3-POLs, P atoms clearly reect the transformation of the
microenvironment of the PPh3 moiety in 3vPPh3-POLs, by their
capacity of coordinating with Rh ions.23,31 Therefore, Rh ions
were introduced to coordinate with P atoms, which could
intuitively reveal the changes in the PPh3 moiety.
As conrmed by our previous works, the high surface area
and large pore volume of the POLs are in favor of exposing more
functional P sites, which promote the dispersion and anchoring
of metals.29,30 Rh/m-3vPPh3-POL was selected as a representative
sample to detect these functional sites. The pore structure and
dispersion of Rh ions in Rh/m-3vPPh3-POL were characterized
through SEM, TEM and HADDF-STEM. The SEM images of Rh/
m-3vPPh3-POL and used Rh/m-3vPPh3-POL (Fig. S2a and b†)
reveal that the hierarchical porosity of m-3vPPh3-POL is main-
tained well aer immobilizing Rh ions and even aer the
reaction. The TEM images of Rh/m-3vPPh3-POL and used Rh/m-
3vPPh3-POL (Fig. S2c and d†) display that Rh ions are highly
dispersed in m-3vPPh3-POL. The HADDF-STEM images and
HADDF-EDS element maps of Rh/m-3vPPh3-POL and used Rh/
m-3vPPh3-POL (Fig. S2e and f†) further conrm that P atoms are
uniformly dispersed in the 3vPPh3-POL framework, which
Fig. 4 (a) Changes in the 31P MAS NMR spectra of m-3vPPh3-POL and Rh/m-3vPPh3-POL in different stages. (b) Changes in the 31P MAS NMR
spectra of p-3vPPh3-POL and Rh/p-3vPPh3-POL in different stages.
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Fig. 5 (a) The in situ FT-IR spectra of Rh/3vPPh3-POLs; the samples were purged with nitrogen at 383 K, and the background spectra were
collected. Then, the samples were allowed to absorb syngas (CO/H2 ¼ 1 : 1) for 1 h and purged with nitrogen, and collected every 5 min until the
spectrogram was unchanged. (b) The Rh K-edge k3-weighted Fourier transform spectra of EXAFS; the activated samples were treated with
syngas. (c) The preparation method, difference in coordination state and the Rh–P complexes among Rh/3vPPh3-POLs.
m-3vPPh3-POL. In a word, the microenvironment of the PPh3 assigned to the corresponding species and displayed in Table 2.
moiety is regulated by changing the position of the cross-linkers There are four peaks that could be assigned to the HRh(CO)2(-
(vinyl groups) attached to the PPh3 monomer, which inuences PPh3-POL)2 species.39 An equilibrium of two trigonal-
the interaction between Rh ions and P atoms.
bipyramidal isomers exists in the HRh(CO)2(PPh3-POL)2
To discover the interaction between 3vPPh3-POLs and Rh compound and each of the isomers has two peaks. The ee-
ions, we compared the change in P 2p binding energy of 3vPPh3- HRh(CO)2(PPh3-POL)2 species at 1961 cmꢀ1 and 2050 cmꢀ1
POLs with the use of XPS (Fig. S3†). Aer modifying with contains two equatorial phosphine ligands while the ea-
0.25 wt% Rh ions, the P 2p binding energy of all the three HRh(CO)2(PPh3-POL)2 species at 1950 cmꢀ1 and 2002 cmꢀ1
3vPPh3-POLs is increased by 0.1 eV, respectively, suggesting that contains an equatorial and an apical phosphine ligand. The
there is interaction between 3vPPh3-POLs and Rh ions, which is HRh(CO)(PPh3-POL)3 species has one characteristic peak at
derived from the coordination between Rh ions and P atoms in 2064 cmꢀ1. Aer the treatment with syngas, all of the Rh/
3vPPh3-POLs.
3vPPh3-POLs possess the ea-HRh(CO)2(PPh3-POL)2 species, ee-
As is known, Rh ions are capable of coordinating with P HRh(CO)2(PPh3-POL)2 species and HRh(CO)(PPh3-POL)3
atoms and forming different kinds of Rh–P complexes, which
may coexist in equilibrium in a homogeneous system.37,38 To
further disclose the differences in the Rh–P complexes among
the three 3vPPh3-POLs, characterization of the Rh–P complex
was carried out by means of in situ FT-IR, as shown in Fig. 5a.
The samples were in situ treated with syngas (CO : H2 ¼ 1 : 1) for
1 h in a cell at 383 K, then N2 was introduced to replace syngas.
The wavenumbers of the observed signals are indicated and
Table 2 Assignments of the IR bands observed after syngas absorp-
tion processes over Rh/PPh3-POLs
Rh–P species
Wave numbers (cmꢀ1
)
Ref.
ee-HRh(CO)2(PPh3POL)2
ea-HRh(CO)2(PPh3POL)2
HRh(CO)(PPh3POL)3
1961, 2050
1950, 2002
2064
37 and 43
37 and 43
44
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Table 3 The curve-fitting analysis of the EXAFS traces
s2
R-Factor
(%)
ꢀ
R (A)
ꢀ3 ꢀ2
Sample
Coordination
CN
(ꢁ10 A )
Rh/p-3vPPh3-POL
Rh–P
Rh–O
Rh–P
Rh–C
Rh–P
Rh–O
Rh–P
Rh–C
Rh–P
Rh–O
Rh–P
Rh–C
2.6
1.9
2.7
1.3
2.4
2.0
2.2
1.9
2.9
1.7
2.8
1.3
2.34
2.08
2.24
1.72
2.34
2.08
2.31
1.72
2.33
2.08
2.31
1.77
6.6
3.5
4.0
9.0
5.9
4.2
3.8
3.1
6.7
4.5
3.8
1.3
0.65
1.81
0.77
1.73
0.83
1.45
Rh/p-3vPPh3-POL-actived
Rh/m-3vPPh3-POL
Rh/m-3vPPh3-POL-actived
Rh/o-3vPPh3-POL
Rh/o-3vPPh3-POL-actived
species. However, compared with p-3vPPh3-POL and o-3vPPh3- Rh/3vPPh3-POLs possesses a different composition of Rh–P
POL, m-3vPPh3-POL presents an apparent tendency to form the complexes. It is more likely to generate the HRh(CO)2(PPh3-
HRh(CO)2(PPh3-POL)2 species rather than the HRh(CO)(PPh3- POL)2 species rather than the HRh(CO)(PPh3-POL)3 species on
POL)3 species, which means that Rh in m-3vPPh3-POL has Rh/m-3vPPh3-POL-actived, while relatively more HRh(CO)(PPh3-
a stronger affinity with CO.40–42 The in situ FT-IR spectra of the POL)3 species are generated on Rh/p-3vPPh3-POL-actived and
three Rh/3vPPh3-POLs show great difference, illustrating that Rh/o-3vPPh3-POL-actived. The difference in coordination state
the microenvironment of the PPh3 moiety in the three 3vPPh3- and the Rh–P complexes among Rh/3vPPh3-POLs are illustrated
POLs is diverse, which endows Rh/3vPPh3-POLs with different in Fig. 5c.
ratios of Rh–P complexes and consequently distinctive catalytic
The results of MAS NMR, in situ FTIR and EXAFS spectra
performance.
prove that the microenvironments of the PPh3 moiety in the
To get a more specic understanding about the coordination three 3vPPh3-POLs are various, which is inuenced by the
state of Rh with P species in 3vPPh3-POLs, EXAFS character- position of the cross-linkers attached to the PPh3 monomer,
ization was employed. As shown in Fig. 5b, no Rh–Rh bonds can and the subsequent Rh/3vPPh3-POL catalysts may display
be detected in any of the tested samples, indicating that Rh ions dissimilar behaviors in catalytic reactions. Thus, the strategy of
are dispersed exclusively as a single site by P species in 3vPPh3- regulating the structure and property of POPs by simply
POLs. According to the tting result (Table 3), in the fresh Rh/ changing the position of the cross-linkers attached to the
3vPPh3-POL catalysts, Rh atom is coordinated with O atoms and monomer has been successfully established.
P atoms, in which the Rh–P bonds are derived from Rh coor-
dinated with P species in 3vPPh3-POLs and Rh–O bonds are
derived from acetylacetone (acac) of Rh(acac)(CO)2.26 Aer
3.3 Performance tests of 3vPPh3-POLs in hydroformylation
syngas treatment, the coordination state of Rh atoms changes
a lot. In the Rh/p-3vPPh3-POL-actived sample, each Rh atom is
coordinated with 1.3 C atoms and 2.7 P atoms, while in the Rh/
o-3vPPh3-POL-actived sample, each Rh atom is coordinated with
1.3 C atom and 2.8 P atoms. Nevertheless, each Rh atom is
coordinated with 1.9 C atoms and 2.2 P atoms in the Rh/m-
3vPPh3-POL-actived sample, which is signicantly different
from those in the Rh/p-3vPPh3-POL-actived and Rh/o-3vPPh3-
POL-actived samples. The proportion of HRh(CO)2(PPh3-POL)2
and HRh(CO)(PPh3-POL)3 species in different Rh/3vPPh3-POLs
are tted and listed in Table 4, which illustrates that each of the
It is commonly recognized that the hydroformylation reaction is
sensitive to the Rh–P complexes; as a consequence, the hydro-
formylation reaction was selected as a probe reaction to assess
the difference in 3vPPh3-POLs in catalytic performance.
Initially, Rh(CO)2(acac) was employed in the hydro-
formylation of 1-octene in a homogeneous system, shown in
Fig. 6a (detailed data are listed in Table S1†). The conversion of
1-octene is 99% while the aldehyde selectivity is only 8.5%, and
the l/b ratio is 0.6; therefore, 1-octene is mainly converted to iso-
alkenes and alkane by Rh(CO)2(acac). Then, Rh(CO)2(acac) was
coordinated with the three 3vPPh3 monomers in the
Table 4 The proportion of HRh(CO)2(PPh3-POL)2 and HRh(CO)(PPh3-POL)3 species in different activated Rh/3vPPh3-POLs
Active species proportion (%)
Samples
HRh(CO)2(PPh3-POL)2
HRh(CO)(PPh3-POL)3
Rh/p-3vPPh3-POL-actived
Rh/m-3vPPh3-POL-actived
Rh/o-3vPPh3-POL-actived
30
85
25
70
15
75
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Fig. 6 (a) Test of different Rh catalysts in hydroformylation of 1-octene. None is Rh ions; p-3vPPh3 is Rh-p-3vPPh3; m-3vPPh3 is Rh-m-3vPPh3;
o-3vPPh3 is Rh-o-3vPPh3; p-POL is Rh/p-3vPPh3-POL; m-POL is Rh/m-3vPPh3-POL; o-POL is o-3vPPh3-POL. (b) Rh/m-3vPPh3-POL catalytic
performance test in long-chain olefin hydroformylation reactions. (c) Rh/m-3vPPh3-POL recycle performance in 1-octene hydroformylation.
Reaction conditions: substrate (1.0 g), toluene (5.0 g), Rh catalysts (0.060 g, Rh wt% ¼ 0.25%), T ¼ 383 K, P ¼ 1.0 MPa, t ¼ 12 h. Conversion is
indicated by the circles, selectivity is indicated by the bars. (d) Stability tests of Rh/m-3vPPh3-POL in 1-octene hydroformylation reaction in
a fixed-bed reactor. Reaction conditions: substrate (0.8 mL hꢀ1), syngas (1000.0 mL hꢀ1), catalyst (0.30 g, Rh wt% ¼ 0.25%), T ¼ 383 K, P ¼
1.0 MPa. The products were collected in a cooling tank and analysed every 12 h.
hydroformylation of 1-octene in a homogeneous system. With performance in the heterogeneous system. Rh/p-3vPPh3-POL
the introduction of p-3vPPh3, the aldehyde selectivity and l/ displays an aldehyde selectivity of 65.5% and an l/b ratio of 3.0.
b ratio are increased to 89.0% and 2.9 on Rh-p-3vPPh3, respec- However, only 47.1% aldehyde selectivity and an l/b ratio of 2.3
tively. Comparatively, Rh-m-3vPPh3 displays a higher aldehyde are attained by Rh/o-3vPPh3-POL, which may be caused by its
selectivity (93.1%) and l/b ratio (4.0). Also, an aldehyde selec- lower BET surface area and poor porosity. To our delight, the
tivity of 92.8% and an l/b ratio of 4.9 are obtained by using Rh-o- highest aldehyde selectivity (88.6%) and l/b ratio (ca. 10) are
3vPPh3.
obtained on the Rh/m-3vPPh3-POL catalyst. Obviously, the
Aer that, 3vPPh3-POLs were engaged to support and coor- excellent performance of Rh/m-3vPPh3-POL could be assigned
dinate with Rh(CO)2(acac), named Rh/3vPPh3-POLs, and to its unique structure and microenvironment of the PPh3
applied in the heterogeneous hydroformylation of 1-octene. moiety in m-3vPPh3-POL.
Compared with the corresponding homogeneous Rh-3vPPh3
Next, Rh/m-3vPPh3-POL was employed to optimize the reac-
catalysts, Rh/3vPPh3-POLs show considerably different catalytic tion conditions. As shown in Table S2,† with the syngas
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Table 5 Rh concentration in fresh and used catalysts
samples through polymerization. The resulting rigid 3vPPh3-
POLs possessed different BET surface areas, pore volumes and
pore structures. Moreover, the microenvironment of the PPh3
moiety, the negativity and the hindrance of P in the three
3vPPh3-POLs were different, which led to their different abilities
to coordinate with Rh to catalyze some hydroformylation reac-
tions. Remarkable improvement in chemo-selectivity and
regioselectivity in the hydroformylation reaction were achieved
on Rh/m-3vPPh3-POL, and over 500 h performance tests further
conrmed their prominent stability and industrial application
potential. Given the efficiency and feasibility of this regulation
strategy, we expect that the strategy of regulating the position of
cross-linkers will be instructive in designing functional POP
materials.
Rh concentration
Sample
(%)
Rh/m-3vPPh3-POL (fresh sample)
Rh/m-3vPPh3-POL (used sample)
0.2656
0.2615
pressure increasing, aldehyde selectivity is improved while l/
b ratio is repressed. Table S3† exhibits that enhanced temper-
ature could increase the conversion of 1-octene, but decrease
the aldehyde selectivity. The optimized syngas pressure was
xed at 1.0 MPa while the reaction temperature was xed at 383
K. Under optimal conditions, the turnover frequency (TOF) is as
high as 2221.6 hꢀ1 (Table S4†), representing the superior
activity of Rh/m-3vPPh3-POL.
Conflicts of interest
To extend the scope of Rh/m-3vPPh3-POLs in the hydro-
formylation reaction, we also employed 1-hexene, 1-heptene
and 1-octene as substrates in an autoclave reactor, and the
result is shown in Fig. 6b. For the hydroformylation of 1-hexene,
1-heptene and 1-octene, the Rh/m-3vPPh3-POL catalysts display
high selectivity of aldehydes (above 88.6%), high l/b ratio (above
10) and low selectivity of alkanes (lesser than 2.8%). The main
by-products are isomeric alkenes, which can be converted to the
corresponding initial substrates.
Recyclability is an important index for catalysts. As shown in
Fig. 6c, in recycling tests, Rh/m-3vPPh3-POL can be reused at
least 8 times with high selectivity, activity and l/b ratio. The used
Rh/m-3vPPh3-POL was analyzed by ICP-OES, which showed that
the Rh ions are durably xed by m-3vPPh3-POL (Table 5),
probably owing to the high density of P atoms and the multiple
Rh–P coordination bonds.
To exploit the stability and feasibility of Rh/m-3vPPh3-POL
for industrial applications, we also performed the hydro-
formylation of 1-octene in a xed-bed reactor. The simple
owchart is shown in Scheme S1,† and the reaction temperature
and pressure were xed at 383 K and 1.0 MPa, respectively, the
LHSV of 1-octene was xed at 0.4 hꢀ1, and the GHSV of syngas
(CO : H2 ¼ 1 : 1) was xed at 500 hꢀ1. As shown in Fig. 6d, the
selectivity of aldehydes and linear selectivity (the proportion of
linear aldehyde in all aldehydes) are more than 85% and 90%,
respectively, during the time-on-stream over 500 h. During the
stability test, the selectivity of isomerized alkenes is about 10%
to 12%, and the alkane selectivity is below 3%. The TOF of
aldehyde on Rh/m-3vPPh3-POL is above 800 hꢀ1 during the test.
The xed-bed reactor test further demonstrated the excellent
stability of Rh/m-3vPPh3-POL, implying its great potential for
industrial application.
There are no conicts to declare.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (no. 91845101, 22002152), the Trans-
formational Technologies for Clean Energy and Demonstra-
tion”, Strategic Priority Research Program of the Chinese
Academy of Sciences, grant no. XDA 21020900, the Strategic
Priority Research Program of the Chinese Academy of Sciences,
grant no. XDB 17000000, and DICP & QIBEBT (grant no. DICP &
QIBEBT UN201704). +shanghai. Rh K-edge X-ray absorption ne
structure (EXAFS and XANES) spectra were obtained at the
BL14W1 beamline of Shanghai Synchrotron Radiation Facility
(SSRF), Shanghai Institute of Applied Physics (SINAP), Chinese
Academy of Sciences. Technical support and helpful discussion
by Prof. Zheng Jiang are gratefully acknowledged.
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