Enhancement of hydroformylation performance via increasing the phosphine ligand concentration in porous organic polymer catalysts

Article abstract of DOI:10.1016/j.cattod.2017.06.007

The development of highly efficient and stable Rh-based heterogeneous catalysts for hydroformylation of heavier olefins is of both high fundamental and industrial interest, yet there still remains a tremendous challenge. In this contribution, a series of porous organic polymers bearing various concentrations of triphenylphosphine (PPh₃) moieties, a ligand of industrial choice, was synthesized and their performance in the hydroformylation of styrene, 1-ocetene, and 2-ocetene were investigated after metalation with Rh species. Both concentration of PPh₃ moieties and pore structure of the polymers were found to influence the catalytic performance. The polymer-based rhodium catalysts demonstrated an increase in catalytic activity, selectivity, and stability when the concentration of PPh₃ was increased and the porous polymer constructed by the functional PPh₃ monomer (POL-PPh₃) was found to be optimal among all the solid ligands tested. We anticipate these results will form the basis for a constructive perspective in the development of high performance heterogeneous Rh-based hydroformylation catalysts. Moreover, our observations indicate the considerable potential of porous organic polymers (POPs) as a new generation of heterogeneous catalytic platforms that may prove effective when targeting important but highly challenging reactions.

Full text of DOI:10.1016/j.cattod.2017.06.007

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Catalysis Today
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Enhancement of hydroformylation performance via increasing the
phosphine ligand concentration in porous organic polymer catalysts
Qi Sun, Zhifeng Dai, Xiangju Meng, Feng-Shou Xiao⁎
Key Laboratory of Applied Chemistry of Zhejiang Province and Department of Chemistry, Zhejiang University, Hangzhou 310028, PR China
A R T I C L E I N F O
A B S T R A C T
Keyword:
The development of highly efficient and stable Rh-based heterogeneous catalysts for hydroformylation of heavier
olefins is of both high fundamental and industrial interest, yet there still remains a tremendous challenge. In this
contribution, a series of porous organic polymers bearing various concentrations of triphenylphosphine (PPh3)
moieties, a ligand of industrial choice, was synthesized and their performance in the hydroformylation of
styrene, 1-ocetene, and 2-ocetene were investigated after metalation with Rh species. Both concentration of PPh3
moieties and pore structure of the polymers were found to influence the catalytic performance. The polymer-
based rhodium catalysts demonstrated an increase in catalytic activity, selectivity, and stability when the
concentration of PPh3 was increased and the porous polymer constructed by the functional PPh3 monomer (POL-
PPh3) was found to be optimal among all the solid ligands tested. We anticipate these results will form the basis
for a constructive perspective in the development of high performance heterogeneous Rh-based hydro-
formylation catalysts. Moreover, our observations indicate the considerable potential of porous organic polymers
(POPs) as a new generation of heterogeneous catalytic platforms that may prove effective when targeting im-
portant but highly challenging reactions.
Porous organic polymer
Hydroformylation
Rh catalysis
Heterogeneous catalysis
Phosphine ligand
1. Introduction
catalysts. Examples include the use of fluorous biphase system catalysis
in
a non-aqueous environment and temperature-controlled multi-
Hydroformylation constitutes one of the most powerful and valu-
able tools for CeC bond formation and allows for the straightforward
conversion of inexpensive chemical feedstocks, such as olefins and
syngas, into broadly applicable aldehydes, which serve as major
building blocks and versatile intermediates for numerous chemical
products [1–5]. It has been the subject of extensive research in aca-
demia and industry because of increasing interest in developing an ef-
ficient transformation of the heavier olefins [6,7]. In view of the various
metal catalysts involved in such transformations, rhodium-based cata-
lysts typically work under mild conditions and afford a much higher
selectivity in favor of the greater added value aldehyde product [8–12].
Nonetheless, all commercial plants running these reactions use cobalt
catalysts, which require severe conditions and give poorer selectivities,
because costs incurred in the recovery and recycling of the expensive
Rh-based catalyst make this process prohibitive [13,14]. Solving the
product separation problem in an effective and economically robust
way while retaining the performance of the catalysts would represent a
major step forward in hydroformylation.
component solvent system [15–17]. While reaction rate and selectivity
is excellent, leaching of the catalytic components into the product phase
remains a major drawback. To circumvent the concerns of catalytic
components leaching and immiscibility of heavier olefins in the aqu-
eous biphasic Rh-catalyzed hydroformylation, Pickering emulsions
systems were developed [18,19]. Despite these progresses made in the
homogeneous catalysis, anchoring active species to solid materials
holds great promise for an essential and practical approach to facilitate
separation of the catalyst from the product and thereby to improve the
overall efficiency of the hydroformylation process. Materials such as
inorganic oxides (often silica) or polymers have been extensively in-
vestigated [20–29]. For instance, Davis and coworkers reported the
hydroformylation of higher olefins by supported aqueous-phase cata-
lysts (SAPCs) formed by subtly adsorbing a thin aqueous layer con-
taining a water-soluble catalyst onto a hydrophilic porous material
[30]. The increased interfacial area and the solid nature of the support
lead to the greatly improved activity together with the facilitated cat-
alyst recovery. However, the SAPCs are often very sensitive to the
content of water, and thereby its long-term stability is not adequate. It
is known that covalent anchoring can be robust enough to withstand
Several elegant strategies have been proposed for the hydro-
formylation of heavier olefins with the aim of recycling the Rh
⁎
Corresponding author.
E-mail address: fsxiao@zju.edu.cn (F.-S. Xiao).
http://dx.doi.org/10.1016/j.cattod.2017.06.007
Received 29 October 2016; Received in revised form 23 May 2017; Accepted 13 June 2017
0920-5861/©2017ElsevierB.V.Allrightsreserved.
Pleasecitethisarticleas:Sun,Q.,CatalysisToday(2017),http://dx.doi.org/10.1016/j.cattod.2017.06.007

Q. Sun et al.
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the rather harsh conditions of the catalytic reaction, but these catalysts
unfortunately suffer either from inferior selectivity and activity, or low
recyclability, and hence, is currently not commercially feasible [31,32].
Therefore, there is still a need to develop high efficient and recyclable
Rh-based hydroformylation catalysts.
2.2.2. Synthesis of nonporous polymerized PPh3 (poly-PPh3)
As a typical run, 1.0 g of tris(4-vinylphenyl)phosphine was dis-
solved in 10 mL of ethyl acetate, followed by the addition of 25 mg of
AIBN. The mixture was transferred into an autoclave (20 mL) and
maintained at 100 °C for 24 h. The title polymer was obtained in 68%
yield after being washed with CH2Cl2 and dried under vacuum. Note:
The solvent used in the polymerization media plays a critical role in the
formation of the nanoporous structure of the resultant polymer. To
obtain a highly porous polymer, the formation of sufficiently extensive
interconnected networks is necessary. It is well documented that in the
initial stages of polymerization, the growth of polymer chains is favored
by adding a new monomer unit, followed by the formation of crosslinks
by mutual interconnection of polymer chains. As a consequence, the
polymer synthesized using a porogenic solvent like THF, which is
known to be one of the best compatible solvents with styrenic polymers,
could facilitate the growth of polymer chains into an extended config-
uration, thereby facilitating the formation of a highly crosslinked
polymer with a nanoporous structure. In contrast, using ethyl acetate,
which is incompatible with styrenic polymers, the polymer chains are
forced to aggregate into tighter polymer coils even at a very low
polymerization degree and phase separation occurs, thus impeding the
formation of an extensive network and resulting in the nonporous
structure.
Synthesis of porous polymers with different PPh3 moiety con-
centrations (PDVB-mPPh3, m stands for the mole amount of PPh3
moieties in per gram of polymer). A series of porous polymers with
different PPh3 moiety concentrations was prepared from copolymer-
ization of divinylbenzene and tris(4-vinylphenyl)phosphine at different
ratios. As a typical run, 0.068 g of tris(4-vinylphenyl)phosphine and
0.932 g of divinylbenzene were dissolved in 10 mL of THF, followed by
the addition of 25 mg of azobisisobutyronitrile (AIBN). The mixture was
transferred into an autoclave and maintained at 100 °C for 24 h. After
evaporation of the solvent, a white solid product with the PPh3 moiety
concentration in the polymer at 0.2 mmol/g was obtained in nearly
quantitative yield, which was denoted as PDVB-0.2PPh3.
Porous organic polymers (POPs) containing well-defined metal
catalysts are emerging as amenable materials which combine the merits
of homogeneous and traditional heterogeneous catalysts. Similar to
molecular catalysts, POPs inherit the excellent chemical tunability af-
forded by the wide range of functionalized organic building blocks
employed in their synthesis. Like solid catalyst supports, POPs have
excellent thermal and chemical stability, thus enabling them to tolerate
the harsh reaction conditions (e.g. high temperature and high pressure)
usually employed in heterogeneous catalytic transformations [33–43].
Recently, we have reported the synthesis, characterization, and cata-
lytic efficiency of a porous organic polymer constructed by PPh3 moi-
eties (POL-PPh3), featuring high surface area and very high density of
phosphine species with excellent spatial continuity. After metalation
with Rh species, the resultant catalysts exhibit comparable catalytic
performance in relation to the homogenous counterparts as well as
excellent recyclability in hydroformylation of olefins, thus possessing
great potential for practical applications [44,45]. By taking advantage
of the tunability of polymer synthesis, in this work, a series of porous
organic polymers bearing various amounts of PPh3 moieties was syn-
thesized from copolymerization of tris(4-vinylphenyl)phosphine with
divinylbenzene. We then systematically investigated the influence of
metal concentration and ligand excess on activity and selectivity as well
as the stability of the catalysts. In view of these results, we propose an
explanation for the leaching and low activity phenomena previously
observed with heterogeneous hydroformylation catalysts synthesized
by anchoring phosphine ligands on the conventional solid materials.
Moreover, the influence of the porous structure on the catalytic per-
formance was investigated using nonporous polymerized functionalized
triphenylphosphine as a control sample.
2. Materials and methods
2.2.3. Synthesis of xRh/POL-PPh3 (x stands for the Rh weight percent in the
polymer)
2.1. Materials
As a typical run, 0.1 g of POL-PPh3 was swollen in 40 mL of toluene,
followed by the addition of Rh(CO)2(acac) (5.2 mg) or RhH(CO)
(PPh3)3. After being stirred at room temperature under N2 atmosphere
for 24 h, the mixture was filtered, washed with excess toluene, and
dried at 50 °C under vacuum. The light yellow solid obtained was de-
noted as 2.0 wt% Rh/POL-PPh3.
Solvents were purified according to standard laboratory methods.
THF was distilled over LiAlH4, and 4-bromostyrene was distilled over
CaH2. Other commercially available reagents were purchased in high
purity and used without further purification.
2.2. Catalyst preparation
2.2.4. Synthesis of 2.0 wt% Rh/poly-PPh3
As a typical run, 0.1 g of poly-PPh3 was stirred in 40 mL of toluene,
followed by the addition of 5.2 mg of Rh(CO)2(acac). After being stirred
at room temperature under N2 atmosphere for 24 h, the mixture was
filtered, washed with excess toluene, and dried at 50 °C under vacuum.
The light yellow solid obtained was denoted as 2.0 wt% Rh/poly-PPh3.
2.2.1. Synthesis of POL-PPh3
As a typical run, 1.0 g of tris(4-vinylphenyl)phosphine was dis-
solved in 10 mL of tetrahydrofuran (THF), followed by the addition of
25 mg of azobisisobutyronitrile (AIBN). The mixture was transferred
into an autoclave (20 mL) and maintained at 100 °C for 24 h. The title
polymer was obtained in nearly quantitative yield after being washed
with CH2Cl2 and dried under vacuum. Tris(4-vinylphenyl)phosphine
was synthesized from the treatment of PCl3 (33 mmol in 30 mL of THF)
and (4-vinylphenyl)magnesium bromide solution (100 mmol). The re-
action was quenched by the addition of 50 mL of saturated NH4Cl
aqueous solution. The organic phase was extracted with excess ether,
which was dried over MgSO4. After filtering and purifying by silica gel
chromatography (5% EtOAc/petroleum ether), tris(4-vinylphenyl)
phosphine was obtained as white solid. 1H NMR (400 MHz, DMSO-d6,
298 K, TMS): δ 7.48 (d, 6H, J = 7.6 Hz), 7.22 (t, 6H, J = 7.6 Hz),
6.69–6.76 (m, 3H), 5.85 (d, 2H, J = 18 Hz), 5.30 (d, 2H, J = 10.8 Hz)
ppm. 13C NMR (100 MHz, DMSO-d6) δ 115.76, 126.79, 126.86, 133.76,
133.95, 136.41, 136.55, 138.08 ppm 31P NMR (162 MHz): δ −7.94 (s,
1P) ppm.
2.2.5. Synthesis of 2.0 wt% Rh/PDVB-mPPh3
As a typical run, 0.1 g of PDVB-0.2PPh3 was swollen in 40 mL of
toluene, followed by the addition of 5.2 mg of Rh(CO)2(acac). After
being stirred at room temperature under N2 atmosphere for 24 h, the
mixture was filtered, washed with excess toluene, and dried at 50 °C
under vacuum. The light yellow solid obtained was denoted as 2.0 wt%
Rh/PDVB-0.2PPh3.
2.3. Catalytic tests
2.3.1. Hydroformylation of styrene
Rh catalyst (2.5 μmol), styrene (0.52 g), and toluene (10.0 g) were
added to a stainless steel autoclave (100 mL) with a magnetic stir bar.
After the autoclave was sealed and purged with syngas (CO/H2 = 1:1)
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three times, the pressure of syngas was adjusted to the desired value
and the autoclave was put into a preheated oil bath, stirring at 80 °C for
12 h. After the reaction, the catalyst was removed from the system by
centrifugation and analyzed by gas chromatography (Kexiao Co. GC-
1690 equipped with a flame ionization detector and a Supelco γ-DEX
225 capillary column).
Table 1
Textural parameters of porous polymers with various PPh3 concentration.
Polymer
m (mmol/g)a
BET (m2/g)
Pore Volume (cm3/g)
POL-PPh3
2.9
2.4
1.6
0.8
0.4
0.2
1086
1027
876
841
709
1.70
1.88
1.43
1.08
0.69
0.45
PDVB-2.4PPh3
PDVB-1.6PPh3
PDVB-0.8PPh3
PDVB-0.4PPh3
PDVB-0.2PPh3
For recycling, the catalyst was separated by centrifugation, washed
with degassed toluene, and used directly for the next run.
641
2.3.2. Hydroformylation of octene
a
m stands for the mole amount of PPh3 moieties in per gram of polymer.
As a typical run, a desired amount of Rh catalyst [RhH(CO)(PPh3)3
was used as Rh precursor], 1-octene (3.0 g) or 2-octene (1.5 g), and
toluene (6.0 g) were added into a stainless steel autoclave (30 mL).
After sealing and purging with syngas (CO/H2 = 1:1) for 3 times, the
pressure of syngas was adjusted to the desired value and the autoclave
was heated to 90 °C (2 °C/min), stirring at 90 °C for 4 h. During the
reaction, the syngas was filled from a reservoir to maintain the pres-
sure. After the reaction, the catalyst was taken out from the system by
centrifugation and analyzed by gas chromatography (Agilent 6890 gas
chromatography equipped with a flame ionization detector and a SE-54
capillary column).
surface area as high as 1032 m2/g (Fig. 1A). In addition, the porosity
can also be clearly discerned in the TEM image (Fig. 1B). The X-ray
photoelectron spectroscopy (XPS) spectrum of 2.0 wt% Rh/POL-PPh3
reveals the binding energies of Rh3d5/2 and Rh3d3/2 at 309.1 and
313.9 eV, respectively, which are lower than those of Rh(CO)2(acac)
(309.9 and 314.6 eV). Meanwhile, the P2d binding energy of 2.0 wt%
Rh/POL-PPh3 (131.2 eV) exhibits a higher value than that of the pris-
tine POL-PPh3 (130.4 eV). These results suggest that strong interactions
exist between the Rh and PPh3 moieties in POL-PPh3 (Fig. 2) [46].
It is well established that the ratio of the phosphine to Rh species
plays an important role in the activity and selectivity of hydro-
formylation catalysis [47,48]. Given the excellent spatial continuity of
POL-PPh3, it enabled us to fine-tune the phosphine/Rh ratio to achieve
optimal tradeoff in the resultant solid catalysts: they are highly active
yet retain selectivity. Thus, we turn our attention to screen the effect of
the phosphine/Rh ratio to the reactions by adjusting the Rh loading
amount in POL-PPh3. Considering the increasing interest in branched
aromatic aldehydes in the production of pharmaceutical intermediates
and fine chemicals, the hydroformylation of styrene as a model was
chosen [49]. Initial hydroformylation experiments were performed at
1.0 MPa syngas (CO/H2 = 1:1) and using 0.05 mol% of Rh catalyst in
toluene. As shown in Fig. 3A, activity increased in tandem with the
increment of the Rh loading from 0.5 to 2.0 wt% (the ratio of PPh3 in
POL-PPh3 to Rh from 40 to 10), affording conversion from 61.9% to
higher than 99.5%. Similar catalytic activities were observed when the
Rh loading in the range of 2.0 wt% to 4.0 wt% (PPh3 moieties to Rh
ratio at the range of 10–5), but further increasing the loading amount
from 4.0 wt% to 8.0 wt%, a dramatically decreased conversion from
higher than 99.5% to 65.6% was observed. It is interesting to find that
the selectivity of 2-phenyl propionaldehyde remained almost the same
at the Rh loading from 0.5 wt% to 2.0 wt% (80.1–78.7%), but further
increasing the loading amount, the desired branched aldehyde se-
lectivity experienced a sharp decrease from 78.7% to 58.6%, accom-
panied with the formation of by-product ethyl benzene. These results
indicate the high concentration of free ligands around the metal com-
plexes is necessary to obtain high activity and selectivity, which is in
accordance with corresponding homogeneous catalysis; using excess
PPh3 ligand is often required.
2.4. Characterization
Nitrogen sorption isotherms at the temperature of liquid nitrogen
were measured using Micromeritics ASAP 2020 M and Tristar system.
The samples were outgassed for 10 h at 100 °C before the measure-
ments. ICP analysis was measured with a Perkin-Elmer plasma 40
emission spectrometer. 1H NMR spectra were recorded on a Bruker
Avance-400 (400 MHz) spectrometer. Chemical shifts are expressed in
ppm downfield from TMS at δ = 0 ppm, and J values are given in Hz.
XPS spectra were performed on a Thermo ESCALAB 250 with Al Kα
irradiation at θ = 90° for X-ray sources, and the binding energies were
calibrated using the C1 s peak at 284.9 eV.
3. Results and discussions
Porous polymers bearing various concentrations of PPh3 moieties
were synthesized at 100 °C by solvothermal polymerization of tris(4-
vinylphenyl)phosphine with/without introduction of divinylbenzene in
THF in the presence of azobisisobutyronitrile (AIBN). It is worth men-
tioning that this process affords these POPs in nearly quantitative yield.
To evaluate the pore character of these POPs, N2 sorption isotherms
were investigated at 77 K and all of them exhibit high surface areas in
the range of 641–1083 m2/g (Fig. 1A and Table 1).
Treatment of the resultant polymers with Rh(CO)2(acac) in toluene
afforded the catalysts for investigating their catalytic performance in
the hydroformylation. As a representative sample, POL-PPh3 metalated
with Rh species is illustrated thoroughly. The porous structure of POL-
PPh3 has been preserved after the metalation, as indicated by the N2
sorption isotherms. For example, 2.0 wt% Rh/POL-PPh3 gives the BET
To further examine the effect of the concentration of surrounding
Fig. 1. (A) N2 sorption isotherms and (B) TEM image
of 2.0 wt% Rh/POL-PPh3.
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Fig. 2. P2p XPS spectra of (A) POL-PPh3 and (B)
2.0 wt% Rh/POL-PPh3 samples; Rh3d XPS spectra of
(C) Rh(CO)2(acac) and (D) 2.0 wt% Rh/POL-PPh3
samples.
ligands on catalytic performance, we synthesized a series of polymers
with different PPh3 concentrations by copolymerization of divi-
nylbenzene with tris(4-vinylphenyl)phosphine in different ratios under
solvothermal conditions and compared their activities after loading Rh
(CO)2(acac) (Rh/PDVB-mPPh3, where m stands for the concentration of
PPh3 ligands in the polymers). For further study 2.0 wt% was the
loading amount of choice, given that this afforded the best compromise
between activity and selectivity among the Rh metalated POL-PPh3
catalysts with various loading amount we tested. As shown in Fig. 3B,
the conversion and selectivity steadily increased along with the con-
centration of PPh3 in the polymers. Impressively, the best result in
terms of both conversion of styrene and selectivity of branched alde-
hyde was obtained when employing POL-PPh3 as modification ligands,
which is even comparable with those of homogeneous catalyst Rh
(CO)2(acac)/PPh3 with PPh3/Rh ratio at 10 (Table 2) and competitive
with representative reported homogeneous/heterogeneous catalysts,
even for those using more complicated modification ligands (Table S1).
Worthy of note, it is much superior to those of nonporous Rh/poly-
PPh3, thereby underscoring the benefit of the utilization of porous
homo-polymer as modification ligands. In this context, to identify the
effects of insertion of divinylbenzene on the catalytic performance of
Table 2
Catalytic data in the hydroformylation of styrene over various catalysts.a
Entry
Catalyst
Conv. (%)
Aldehyde select. (%)
b/l (A/B)b
1
Rh/POL-PPh3
Rh/PPh3
Rh/poly-PPh3
> 99.5
> 99.5
32.4
> 99.5
> 99.5
95.5
78.7/21.3
81.3/18.7
50.2/49.8
2c
3c
a
Reaction conditions: styrene (5.0 mmol), toluene (10.0 g), 80 °C, 12 h, CO/H2 = 1:1
(1.0 MPa), Rh catalyst (2.5 μmol), and S/C = 2000.
b
The ratio of branched/linear aldehyde.
The mole ratio of PPh3 to Rh is 10.
c
POL-PPh3, the same PPh3/Rh ratio was kept by adjusting the Rh loading
weight of Rh/POL-PPh3 and Rh/PDVB-1.6PPh3 at 2.0 wt% and 1.1 wt
%, respectively. Notably, the 2.0 wt% Rh/POL-PPh3 clearly out-
performed 1.1 wt% Rh/POL-PPh3, affording the conversion of styrene
Fig. 3. (A) Dependences of catalytic data in the hy-
droformylation of styrene over Rh/POL-PPh3 with
various Rh loadings. (B) Dependence of catalytic
performance in the hydroformylation of styrene over
Rh(CO)2(acac) supported on various polymers with
different PPh3 concentrations. Reaction conditions:
styrene (5.0 mmol), toluene (10.0 g), 80 °C, 12 h,
CO/H2 = 1:1 (1.0 MPa), Rh catalyst (2.5 μmol), and
S/C = 2000.
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Fig. 4. Recycling tests of (A) 2.0 wt% Rh/POL-PPh3
and (B) 2.0 wt% Rh/PDVB-0.2PPh3 in the hydro-
formylation of styrene. Reaction conditions: styrene
(5.0 mmol), toluene (10 g), 80 °C, CO/H2 = 1:1
(1.0 MPa), Rh catalysts (2.5 μmol), 12 h (the reaction
time was determined by the kinetic tests, see also
Fig. S1).
Table 3
Catalytic performance in the hydroformylation of octene over different catalysts.a
Entry
Catalyst
Conv. (%)
Paraffin (%)
Iso-olefins (%)
Aldehydes (%)
n/isob
1
RhH(CO)(PPh3)3
Rh/PPh3
Rh/POL-PPh3
Rh/PPh3
99.4
99.4
99.4
90.4
89.7
3.8
0.7
1.5
4.8
3.3
7.4
0.9
6.4
9.5
8.9
88.8
98.4
92.1
85.7
87.8
0.46
2.01
0.87
–
2c
3
4c,d
5d
Rh/POL-PPh3
–
a
Reaction conditions: 1-octene (3.0 g, 26.7 mmol), toluene (6.0 g), 90 °C, 4 h, CO/H2 = 1:1 (2.0 MPa), Rh catalyst (4.45 μmol), and S/C = 6000.
The ratio of linear/branched aldehyde.
The mole ratio of PPh3 to Rh is 10.
b
c
d
2-octene (1.5 g, 13.4 mmol) was used as a substrate, S/C = 3000.
at > 99.5% vs 74.2% together with branched aldehyde selectivity at
of Rh/POL-PPh3 was further confirmed by its stable outcomes in six
successive runs (Table S2), thus validating its great potential in prac-
tical applicability.
78.7% vs 63.4%, respectively. Given that the only difference in the two
catalysts is the spatial continuity of the PPh3 ligand, therefore it is
reasonable to propose that the high local concentration of the POL-PPh3
is a very significant factor contributing to the excellent catalytic per-
formance.
4. Conclusion
Recyclability and long-term stability of the catalyst are crucial
performance metrics for cost-effective industrial processes. To gauge
the importance of surrounding free ligands on recycling behavior of the
catalysts, 2.0 wt% Rh/POL-PPh3 and 2.0 wt% Rh/PDVB-0.2PPh3 were
taken as examples to study their recyclability in the hydroformylation
of styrene. Remarkably, 2.0 wt% Rh/POL-PPh3 could maintain the
catalytic performance in terms of activity and selectivity even up to the
sixth cycle (Fig. 4A). In sharp contrast, successive decrements were
observed after each cycle with respect to 2.0 wt% Rh/PDVB-0.2PPh3
(Fig. 4B). More importantly, Rh(CO)2(acac)/POL-PPh3 catalyst is
readily recyclable and almost no leaching of Rh species occur (Rh loss
of 0.15% after 5 cycles), while 2.0 wt% Rh/PDVB-0.2PPh3 exhibited the
Rh loss of 8.9% after 5 cycles. These phenomena can be explained that
during the catalytic cycles, it is accompanied by the breaking and re-
formation of the bonds between metal and ligand. As a result, the
catalytic species may break away from the support and become dis-
solved, and this “leaching” process leads to loss of activity of the cat-
alyst when it is recovered by filtration and recycled. In addition, the
existence of free ligands is supposed to reconstitute the damaged cat-
alyst, thus preventing them from decomposing and maintaining the
catalytic performance, further highlighting the advantage of utilization
of POL-PPh3 as modification ligands [50–52].
Given increasing interest in the production of nonanol as an alter-
native plasticizer alcohol [14], we thereby evaluated the catalytic
performance of Rh/POL-PPh3 and corresponding homogeneous cata-
lysts in the hydroformylation of 1-octene and 2-octene. As presented in
Table 3, Rh/POL-PPh3 can efficiently convert 1-octene and 2-octene to
aldehyde products, which is rival to that of homogeneous catalyst Rh/
PPh3 in terms of both activity and selectivity, verifying that the per-
formance of the triphenylphosphine can be fully retained when con-
structed into a porous polymer. Furthermore, the excellent recyclability
In summary, we have disclosed that catalyst performance and sta-
bility was found to depend strongly on the spatial continuity and con-
centration of phosphine ligands as well as the ratio of ligand to the
amount of rhodium in the porous organic polymer catalysts. The ex-
istence of excess free phosphine ligands with excellent spatial con-
tinuity is necessary to obtain high efficiency and inhibit the Rh
leaching. In addition, the benefit of the porous structure in solid ma-
terials for catalysis is also highlighted by the fact that the Rh/POL-PPh3
clearly outperforms that of the nonporous counterparts (Rh/poly-PPh3).
These findings open a new avenue for synthesizing effective Rh-based
catalysts for hydroformylation. Considering the great variety of mod-
ification ligands and diverse strategy of polymer synthesis, there is
undoubtedly much room left for further boosting the catalytic perfor-
mance, by constructing high-performance phosphine ligands such as
xantphos and rationally manipulating the pore structures of the poly-
mers.
Acknowledgments
The authors acknowledge National Natural Science Foundation of
China (, 21333009 and 21422306), National High-Tech Research, and
Development program of China (2013AA065301) for financial support
of this work.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the
online version, at http://dx.doi.org/10.1016/j.cattod.2017.06.007.
5

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CatalysisTodayxxx(xxxx)xxx–xxx
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