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

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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

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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: Porous organic polymer Hydroformylation Rh catalysis Heterogeneous catalysis Phosphine ligand

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 polymerbased 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 (POLPPh3) 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.

1. Introduction Hydroformylation constitutes one of the most powerful and valuable 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 academia and industry because of increasing interest in developing an efficient transformation of the heavier olefins [6,7]. In view of the various metal catalysts involved in such transformations, rhodium-based catalysts 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. Several elegant strategies have been proposed for the hydroformylation of heavier olefins with the aim of recycling the Rh

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catalysts. Examples include the use of fluorous biphase system catalysis in a non-aqueous environment and temperature-controlled multicomponent 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 aqueous 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 investigated [20–29]. For instance, Davis and coworkers reported the hydroformylation of higher olefins by supported aqueous-phase catalysts (SAPCs) formed by subtly adsorbing a thin aqueous layer containing 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 catalyst 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

Corresponding author. E-mail address: [email protected] (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/ © 2017 Elsevier B.V. All rights reserved.

Please cite this article as: Sun, Q., Catalysis Today (2017), http://dx.doi.org/10.1016/j.cattod.2017.06.007

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2.2.2. Synthesis of nonporous polymerized PPh3 (poly-PPh3) As a typical run, 1.0 g of tris(4-vinylphenyl)phosphine was dissolved 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 configuration, 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 concentrations (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 copolymerization 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.

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. 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 afforded 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 catalytic efficiency of a porous organic polymer constructed by PPh3 moieties (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 synthesized 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 performance 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) 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 denoted as 2.0 wt% Rh/POL-PPh3.

2.1. Materials 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 dissolved 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 reaction 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) 2

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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. GC1690 equipped with a flame ionization detector and a Supelco γ-DEX 225 capillary column). For recycling, the catalyst was separated by centrifugation, washed with degassed toluene, and used directly for the next run.

Table 1 Textural parameters of porous polymers with various PPh3 concentration.

2.3.2. Hydroformylation of octene 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 pressure. 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).

Polymer

m (mmol/g)a

BET (m2/g)

Pore Volume (cm3/g)

POL-PPh3 PDVB-2.4PPh3 PDVB-1.6PPh3 PDVB-0.8PPh3 PDVB-0.4PPh3 PDVB-0.2PPh3

2.9 2.4 1.6 0.8 0.4 0.2

1086 1027 876 841 709 641

1.70 1.88 1.43 1.08 0.69 0.45

a

m stands for the mole amount of PPh3 moieties in per gram of polymer.

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 pristine 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 hydroformylation 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 selectivity experienced a sharp decrease from 78.7% to 58.6%, accompanied with the formation of by-product ethyl benzene. These results indicate the high concentration of free ligands around the metal complexes is necessary to obtain high activity and selectivity, which is in accordance with corresponding homogeneous catalysis; using excess PPh3 ligand is often required. To further examine the effect of the concentration of surrounding

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 measurements. 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(4vinylphenyl)phosphine with/without introduction of divinylbenzene in THF in the presence of azobisisobutyronitrile (AIBN). It is worth mentioning 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 POLPPh3 has been preserved after the metalation, as indicated by the N2 sorption isotherms. For example, 2.0 wt% Rh/POL-PPh3 gives the BET

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 divinylbenzene 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 concentration of PPh3 in the polymers. Impressively, the best result in terms of both conversion of styrene and selectivity of branched aldehyde 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/polyPPh3, 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 2c 3c

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

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. c The mole ratio of PPh3 to Rh is 10.

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 outperformed 1.1 wt% Rh/POL-PPh3, affording the conversion of styrene Fig. 3. (A) Dependences of catalytic data in the hydroformylation 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 hydroformylation 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 2c 3 4c,d 5d

RhH(CO)(PPh3)3 Rh/PPh3 Rh/POL-PPh3 Rh/PPh3 Rh/POL-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 – –

a b c d

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. 2-octene (1.5 g, 13.4 mmol) was used as a substrate, S/C = 3000.

of Rh/POL-PPh3 was further confirmed by its stable outcomes in six successive runs (Table S2), thus validating its great potential in practical applicability.

at > 99.5% vs 74.2% together with branched aldehyde selectivity at 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 performance. 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 reformation of the bonds between metal and ligand. As a result, the catalytic species may break away from the support and become dissolved, and this “leaching” process leads to loss of activity of the catalyst when it is recovered by filtration and recycled. In addition, the existence of free ligands is supposed to reconstitute the damaged catalyst, 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 alternative plasticizer alcohol [14], we thereby evaluated the catalytic performance of Rh/POL-PPh3 and corresponding homogeneous catalysts 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 performance of the triphenylphosphine can be fully retained when constructed into a porous polymer. Furthermore, the excellent recyclability

4. Conclusion In summary, we have disclosed that catalyst performance and stability was found to depend strongly on the spatial continuity and concentration of phosphine ligands as well as the ratio of ligand to the amount of rhodium in the porous organic polymer catalysts. The existence of excess free phosphine ligands with excellent spatial continuity is necessary to obtain high efficiency and inhibit the Rh leaching. In addition, the benefit of the porous structure in solid materials 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 modification ligands and diverse strategy of polymer synthesis, there is undoubtedly much room left for further boosting the catalytic performance, by constructing high-performance phosphine ligands such as xantphos and rationally manipulating the pore structures of the polymers.

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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