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Cite this: Chem. Commun., 2014, 50, 11844 Received 21st May 2014, Accepted 9th August 2014
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Porous organic ligands (POLs) for synthesizing highly efficient heterogeneous catalysts† Qi Sun,ab Miao Jiang,c Zhenju Shen,b Yinying Jin,a Shuxiang Pan,a Liang Wang,a Xiangju Meng,*a Wangzhi Chen,*a Yunjie Ding,*c Jixue Lib and Feng-Shou Xiao*a
DOI: 10.1039/c4cc03884c www.rsc.org/chemcomm
We reported a universal route for synthesizing porous organic ligands (POLs) via solvothermal polymerization. The POLs were obtained quantitatively, showing high surface area, large pore volume, hierarchical porosity, and superior stability. The POL bearing a triphenylphosphine supported rhodium catalyst (Rh/POL-PPh3) exhibits high activity and excellent recyclability in 1-octene hydroformylation.
Organic ligands such as triphenylphosphine (PPh3) are of critical importance in homogeneous catalysis. However, the difficulties of recovery and recycling restrict their more extensive application.1 Heterogenization of metal complexes has been expected to solve this problem,2 where polymers and porous silicas are the most commonly used materials for such supports.3 Mesoporous silicas have robust porous structure and high surface areas, but their chemical nature limits their potential for chemical modification processes. On the other hand, diffusion limitation remains as a drawback for conventional polymer supports, due to their low surface areas. More recently, another approach emerged as a powerful alternative, in which porous organic polymers (POPs) are served as a versatile platform for deployment of highly stable and recyclable heterogeneous catalysts by taking advantage of their permanent porosity and the ability to tune their compositions and properties at the molecular level.4 However, in these materials the concentration of organic ligands is usually low due to the presence of copolymerized monomers. Thus organic ligands on these materials cannot be cooperative with each other efficiently, which is required for the formation and stabilization of the highly active species in the heterogeneous catalysts.5 The development of novel catalytic systems that combine the advantages of homogeneous and heterogeneous catalysis is therefore still a major aim of modern chemistry.
a
Department of Chemistry, Zhejiang University, Hangzhou 310028, P.R. China. E-mail: [email protected]; Fax: +86 571-8827-3698 b Electron Microscopy Centre, Zhejiang University, Hangzhou 310027, P.R. China c Dalian Institute of Chemical Physics, Dalian 116023, P.R. China † Electronic supplementary information (ESI) available: Synthesis details, characterization, and activities. See DOI: 10.1039/c4cc03884c
11844 | Chem. Commun., 2014, 50, 11844--11847
Scheme 1 Structures of vinyl-functionalized organic ligands of (A) triphenylphosphine (PPh3), (B) 2,20 -bipyridyl (bpy), and (C) salen.
Table 1 Textural parameters of POLs synthesized from various vinylfunctionalized organic ligands
POLs
Ligands
BET surface area (m2 g1)
Pore volume (cm3 g1)
POL-PPh3 POL-bpy POL-salen
PPh3 bpy Salen
1086 525 490
1.70 0.86 0.38
In this work, we report a synthetic methodology of porous organic ligands (POLs) bearing phosphorous, nitrogen, and oxygen donors. The POLs were obtained via solvothermal polymerization of corresponding vinyl-functionalized organic ligands (Scheme 1 and Table 1). As a typical example, a porous organic ligand of PPh3 (POL-PPh3) was synthesized from solvothermal polymerization of vinyl-functionalized PPh3 (3V-PPh3) in the presence of AIBN (azobisisobutyronitrile) at 100 1C in an autoclave (Schemes S1–S3, ESI†). Notably, the achieved POLs play the roles as both supports and ligands simultaneously. After treating with metal precursors, the obtained materials such as heterogeneous catalysts (M/POLs) exhibited excellent performance in hydroformation of 1-octene. In addition, due this feature of the heterogeneous catalysts, they can be easily adopted in a fixed-bed reactor. For a typical example of POLs, POL-PPh3 was selected for subsequent studies in detail. Fig. 1A shows the 13C MAS NMR spectrum of POL-PPh3, exhibiting an additional peak at 41 ppm assigned to the polymerized vinyl groups (Fig. S1, ESI†), except
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Fig. 1 (A) 13C MAS NMR spectrum, (B) 31P MAS NMR spectrum, (C) N2 sorption isotherms, (D) SEM image, (E) TEM image, and (F) TG curve of POL-PPh3.
for the peaks associated with the vinyl-functionalized PPh3 monomer. The 31P MAS NMR spectrum (Fig. 1B) shows that POL-PPh3 gives a single peak at 5.8 ppm, which is in good agreement with that of the monomer, indicating that P species are stable during the polymerization. Fig. 1C shows N2 sorption isotherms of the POL-PPh3, which give the curve of type-I plus type-IV. A steep increase in the curve at relative pressure less than 0.01 is due to the filling of micropores, while a hysteresis loop at a relative pressure of 0.7–0.95 is mainly from the contribution of sample mesoporosity. Correspondingly, the pore sizes are distributed at 0.7, 1.5, and 3–70 nm (Fig. S2, ESI†). These results suggest that POL-PPh3 has hierarchical porosity, which is also confirmed by the scanning electron microscopy (SEM) image and the transmission electron microscopy (TEM) image of POL-PPh3 (Fig. 1D and E). The BET surface area and pore volume of the POL-PPh3 are as high as 1086 m2 g1 and 1.70 cm3 g1, which are favorable for the dispersion of catalytically active sites.6 Furthermore, TG analysis shows that POL-PPh3 has superior thermal stability, giving the decomposition temperature starting at 440 1C (Fig. 1F), which is even higher than that of Nafion NR50 (330 1C), one of the most stable resins. Even if POL-PPh3 is treated in boiling water for 240 h, the pore structures are almost unchanged (Fig. S3 and S4 and Table S1, ESI†), indicating its superior hydrothermal stability. This is remarkable since most of the MOF catalysts are normally sensitive to water and relatively high temperature.7 It is particularly emphasized that POL-PPh3 has very high concentration of PPh3 species at 2.94 mmol g1 because of
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polymerization by vinyl-functionalized PPh3 itself. In comparison, the concentration of PPh3 species in the conventional heterogeneous catalysts reported previously is below 0.62 mmol g1.8 The high concentration of organic ligands is very favorable for the formation of a suitable coordination environment for ligands and metal species especially for multiple coordination.5 Very importantly, POL-PPh3 can be prepared in a relatively large scale synthesis. Under the same conditions, the solvothermal polymerization of 50 g of vinyl-functionalized PPh3 in a 200 mL autoclave gave ca. 50 g of POL-PPh3 with almost quantitative yield. In contrast, many porous polymers such as Covalent Organic Frameworks (COFs) have been obtained in relatively low yields.9 It is worth noting that the POLs are a kind of unique porous materials. Compared with Metal–Organic Frameworks (MOFs), they have very high thermal, hydrothermal, and chemical stabilities;7 compared with conventional immobilization of organic ligands in polymers, they have very high surface area and large pore volume;3c compared with copolymerization of monomers with organic ligands,10 they have very high concentration of organic ligands and high thermal stabilities; compared with sole micropores in porous materials,8 the hierarchical porosity in the POLs is very favorable for diffusion of reactants and products in organic transformation. Because organic ligands play so important roles in organic transformation,1 the improvement of organic ligands should be a breakthrough for organic synthesis. Therefore, the novel POLs with these unique advantages should be widely applied. After loading various metals, POLs still show very high surface areas. For example, POL-PPh3 supported Rh(CO)2(acac), denoted as Rh(CO)2(acac)/POL-PPh3, shows the BET surface area and pore volume of 1032 m2 g1 and 1.69 cm3 g1 (Fig. S5 and Table S1, ESI†), respectively. The HAADF-STEM image and EDX mappings show that the dispersion of Rh species in the sample are very high and uniform (Fig. S6, ESI†). More interestingly, the XPS spectra of P2d show that Rh(CO)2(acac)/POL-PPh3 gives relatively high binding energy (131.2 eV), compared with that of POL-PPh3 (130.4 eV, Fig. S7A and B, ESI†). In contrast, the binding energies of Rh3d5/2 and Rh3d3/2 in Rh(CO)2(acac)/ POL-PPh3 shift to 309.1 and 313.9 eV from 309.9 and 314.6 eV of Rh(CO)2(acac), respectively (Fig. S7C and D, ESI†), due to coordination of POL-PPh3 with Rh species.11 It is well known that hydroformylation is an extremely important industrial process, and right now more than 8 million tons per year of aldehydes and alcohols are produced worldwide annually through this process.12 Particularly, hydroformylation of higher olefins to yield aldehydes or alcohols is of great interest because their wide applications in production of detergents, plasticizers, solvents, and surfactants. Rhodium homogeneous catalysts typically work under mild conditions, giving good activity and selectivity to the desired aldehydes. Nonetheless, all commercial plants carrying out these reactions use cobalt catalysts, which require much harsher reaction conditions and give poorer selectivities, because the rhodium homogeneous catalysts decompose when attempting to distill the high boiling point product from the reaction systems.13 Therefore, the development
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Table 2 Catalytic data for 1-octene hydroformation over various catalystsa
Run
Catalyst
Conv. (%)
Select.c (%)
Yield (%)
1 2 3 4 5b
RhH(CO)(PPh3)3 Rh(CO)2(acac) RhH(CO)(PPh3)3/POL-PPh3 Rh(CO)2(acac)/POL-PPh3 RhH(CO)(PPh3)3/POL-PPh3
99.4 98.5 99.4 99.0 99.4
88.8 55.9 92.1 89.0 90.2
88.3 55.1 91.5 88.1 89.7
(0.46) (0.70) (0.87) (1.35) (0.87)
a Reaction conditions: CO/H2 = 1 : 1 (2.0 MPa), 1-octene (3.0 g), S/C at 6000, toluene (6.0 g), 90 1C for 4 h, Rh loading at 2.0 wt%. b Recycled 6 times. c Catalytic selectivity in parentheses was the molar ratio of linear to branched aldehyde in the products.
of Rh-based heterogeneous catalysts that retain the high activities and selectivities of their homogeneous analogues are urgently needed in commercial plants. Table 2 presents catalytic data for hydroformation of long-chain 1-octene over various catalysts. Clearly, the heterogeneous catalysts of RhH(CO)(PPh3)3/POL-PPh3 and Rh(CO)2(acac)/POL-PPh3 not only can be easily separated from the reaction system by filtration, but also exhibit much higher selectivity for the corresponding aldehydes than those of homogeneous catalysts of RhH(CO)(PPh3)3 and Rh(CO)2(acac) under similar conversions (Table 2 and Table S2, ESI†). This feature should be related to the high concentration of PPh3 in the Rh/POL-PPh3 catalysts, in good agreement with those of corresponding homogeneous catalysts.13c The heterogeneous catalyst also showed excellent stability, and it could be reused at least 6 times, with negligible loss of its activity (Table S3, ESI†). The excellent catalytic performance over the Rh/POL-PPh3 catalyst might be related to the high concentration of PPh3 species and the enrichment of reactants in the catalyst (Fig. S8, ESI†).14 Similarly, POL-bpy and POL-salen materials are carefully characterized, as shown in Fig. S9–S14 (ESI†). They also show high surface areas, large pore volume, hierarchical porosity, high concentration of organic ligands, and can be served as efficient catalyst supports. e.g., after POL-bpy supported with CuBr2, the obtained CuBr2/POL-bpy catalyst exhibits comparable catalytic performance with the corresponding homogeneous
Table 3 Aerobic oxidation of primary alcohols to aldehydes over homogeneous Cu/bpy and heterogeneous Cu/POL-bpy catalystsa
Yield (%) Entry
R
Time (h)
CuBr2/bpyb
CuBr2/POL-bpyc
1 2 3 4d
H CH3 CH3O H
6 5 5 6
96.4 98.6 99.2 —
94.2 96.8 97.1 92.2
a
Reaction conditions: alcohol (1.0 mmol), catalyst (5.0 mol%), TEMPO (5.0 mol%), KOH (10 mol%), CH3CN (2.0 mL), H2O (1.0 mL), 1 atm of air, and RT. b 2,2 0 -Bipyridyl (5 mol%), CuBr (5.0 mol%). c Cu loading at 5.0 wt% and 5.0 mol% was used. d Recycled 5 times.
11846 | Chem. Commun., 2014, 50, 11844--11847
analogue (CuBr2/bpy) in oxidations of primary alcohols to aldehydes using O2 as an oxidant (Table 3 and Table S5, ESI†).15 In addition, this heterogeneous catalyst can be easily recycled at least 5 times without an obvious loss in the activities. In summary, we report a facile and universal route for synthesizing various porous organic ligands. After supporting with metal species, the obtained heterogeneous catalysts exhibited not only high activities and selectivities but also excellent recyclabilities. Furthermore, except for applications of these POLs for organic transformation, they could be used as various advanced materials. For example, POL-bpy supported Eu3+ is an outstanding fluorescent material (Fig. S15, ESI†). This work is supported by NSFC (U1162201, 21333009 and 21273197), National High-Tech Research and Development program of China (2013AA065301), and Fundamental Research Funds for the Central Universities (2013XZZX001).
Notes and references ´n and S. Cabrera, Chem. Soc. Rev., 2013, 42, 774; 1 (a) J. Alema (b) J. Magano and J. R. Dunetz, Chem. Rev., 2011, 111, 2177; (c) T. P. Yoon and E. N. Jacobsen, Science, 2003, 299, 1691. 2 (a) B. C. Gates, Catalytic Chemistry, Wiley, New York, 1992; (b) J. M. Fraile, J. I. Garcı´a and J. A. Mayoral, Chem. Rev., 2009, ¨nemann and W. R. Thiel, Angew. 109, 360; (c) S. Shylesh, V. Schu Chem., Int. Ed., 2010, 49, 3428; (d) K. T. Wan and M. E. Davis, Nature, 1994, 370, 449. 3 (a) C. E. Song and S. Lee, Chem. Rev., 2002, 102, 3495; (b) S. Minakata and M. Komatsu, Chem. Rev., 2009, 109, 711; (c) C. A. McNamara, M. J. Dixon and M. Bradley, Chem. Rev., 2002, 102, 3275; (d) M. Benaglia, A. Puglisi and F. Cozzi, Chem. Rev., 2003, 103, 3041. 4 (a) Y. Zhang and S. N. Riduan, Chem. Soc. Rev., 2012, 41, 2083; (b) P. Kaur, J. T. Hupp and S. T. Nguyen, ACS Catal., 2011, 1, 819; (c) X. Zhu, C. Tian, S. M. Mahurin, S.-H. Chai, C. Wang, S. Brown, G. M. Veith, H. Luo, H. Liu and S. Dai, J. Am. Chem. Soc., 2012, 134, 10478; (d) X. Du, Y. Sun, B. Tan, Q. Teng, X. Yao, C. Su and W. Wang, Chem. Commun., 2010, 46, 970; (e) Z. Xie, C. Wang, K. E. deKrafft and W. Lin, J. Am. Chem. Soc., 2011, 133, 2056; ( f ) S. Fischer, J. Schmidt, P. Strauch and A. Thomas, Angew. Chem., Int. Ed., 2013, 52, 12174; (g) T. Yasukawa, H. Miyamura and S. Kobayashi, J. Am. Chem. Soc., 2012, 134, 16963. 5 (a) J. W. J. Knapen, A. W. van der Made, J. C. de Wilde, P. W. N. M. van Leeuwen, P. Wijkens, D. M. Grove and G. van Koten, Nature, 1994, 327, 659; (b) A. Dahan and M. Portnoy, J. Am. Chem. Soc., 2007, 129, 5860. 6 (a) D. E. De Vos, M. Dams, B. F. Sels and P. A. Jacobs, Chem. Rev., 2002, 102, 3615; (b) Y. Yue, Z.-A. Qiao, P. F. Fulvio, A. J. Binder, C. Tian, J. Chen, K. Nelson, X. Zhu and S. Dai, J. Am. Chem. Soc., 2013, 135, 9572; (c) P. Kuhn, A. Forget, D. Su, A. Thomas and M. Antonietti, J. Am. Chem. Soc., 2008, 130, 13333. 7 C. Serre, Angew. Chem., Int. Ed., 2012, 51, 6048. 8 B. Li, Z. Guan, W. Wang, X. Yang, J. Hu, B. Tan and T. Li, Adv. Mater., 2012, 24, 3390. 9 (a) S.-Y. Ding, J. Gao, Q. Wang, Y. Zhang, W.-G. Song, C.-Y. Su and W. Wang, J. Am. Chem. Soc., 2011, 49, 19816; (b) H. M. El-Kaderi, ´s, A. P. Co ˆte, R. E. Taylor, M. O’Keeffe J. R. Hunt, J. L. Mendoza-Corte and O. M. Yaghi, Science, 2007, 316, 268; (c) T. Ben, H. Ren, S. Ma, D. Cao, J. Lan, X. Jing, W. Wang, J. Xu, F. Deng, J. M. Simmons, S. Qiu and G. Zhu, Angew. Chem., Int. Ed., 2009, 48, 9457; (d) A. I. Cooper, Adv. Mater., 2009, 21, 1291. 10 X. Feng, D. Bao, K. Li and M. Bao, J. Mol. Catal. A: Chem., 2010, 323, 16. 11 K. Mukhopadhyay, A. B. Mandale and R. V. Chaudhari, Chem. Mater., 2003, 15, 1766. ¨rner, Chem. Rev., 2012, 112, 5675; 12 (a) R. Franke, D. Selent and A. Bo (b) J. Pospech, I. Fleischer, R. Franke, S. Buchholz and M. Beller, Angew. Chem., Int. Ed., 2013, 52, 2852; (c) O. Diebolt, H. Tricas, Z. Freixa and P. W. N. M. van Leeuwen, ACS Catal., 2013, 2, 128; ¨rner, Chem. Rev., 2012, 112, 5675; (d) R. Franke, D. Selent and A. Bo (e) J. Potier, S. Menuel, M.-H. Chambrier, L. Burylo, J.-F. Blach,
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14 (a) C. U. Pittman, Jr. and R. M. Hanes, J. Am. Chem. Soc., 1976, 5402; ˜es, E. N. Dos Santos and (b) H. J. V. Barros, C. C. Guimara E. V. Gusevskaya, Catal. Commun., 2007, 8, 747; (c) F. Liu, L. Wang, Q. Sun, L. Zhu, X. Meng and F.-S. Xiao, J. Am. Chem. Soc., 2012, 134, 16948; (d) Z. Chen, Z. Guan, M. Li, Q. Yang and C. Li, Angew. Chem., Int. Ed., 2011, 50, 4913; (e) H. Zhang, X. Pan, X. Han, X. Liu, X. Wang, W. Shen and X. Bao, Chem. Sci., 2013, 4, 1075. 15 (a) P. Gamez, I. W. C. E. Arends, J. Reedijk and R. A. Sheldon, Chem. Commun., 2003, 2414; (b) J. M. Hoover and S. S. Stahl, J. Am. Chem. Soc., 2011, 133, 16901.
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P. Woisel, E. Monflier and F. Hapiot, ACS Catal., 2013, 3, 1618; ( f ) B. C. E. Makhubela, A. Jardine and G. S. Smith, Green Chem., 2012, 14, 338; ( g) G. Parrinello and J. K. Stille, J. Am. Chem. Soc., 1987, 107, 7122; (h) S. C. Bourque, F. Maltais, W.-J. Xiao, O. Tardif, H. Alper, P. Arya and L. E. Manzer, J. Am. Chem. Soc., 1999, 121, 3035; (i) R. Abu-Reziq, H. Alper, D. Wang and M. L. Post, J. Am. Chem. Soc., 2006, 128, 5279. 13 (a) D. J. Cole-Hamilton, Science, 2003, 299, 1702; (b) B. Cornils and W. A. Herrmann, J. Catal., 2003, 216, 23; (c) C. U. Pittman Jr. and R. M. Hanes, J. Am. Chem. Soc., 1976, 98, 5402.
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Chem. Commun., 2014, 50, 11844--11847 | 11847
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Cite this: Chem. Commun., 2014, 50, 11844 Received 21st May 2014, Accepted 9th August 2014
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Porous organic ligands (POLs) for synthesizing highly efficient heterogeneous catalysts† Qi Sun,ab Miao Jiang,c Zhenju Shen,b Yinying Jin,a Shuxiang Pan,a Liang Wang,a Xiangju Meng,*a Wangzhi Chen,*a Yunjie Ding,*c Jixue Lib and Feng-Shou Xiao*a
DOI: 10.1039/c4cc03884c www.rsc.org/chemcomm
We reported a universal route for synthesizing porous organic ligands (POLs) via solvothermal polymerization. The POLs were obtained quantitatively, showing high surface area, large pore volume, hierarchical porosity, and superior stability. The POL bearing a triphenylphosphine supported rhodium catalyst (Rh/POL-PPh3) exhibits high activity and excellent recyclability in 1-octene hydroformylation.
Organic ligands such as triphenylphosphine (PPh3) are of critical importance in homogeneous catalysis. However, the difficulties of recovery and recycling restrict their more extensive application.1 Heterogenization of metal complexes has been expected to solve this problem,2 where polymers and porous silicas are the most commonly used materials for such supports.3 Mesoporous silicas have robust porous structure and high surface areas, but their chemical nature limits their potential for chemical modification processes. On the other hand, diffusion limitation remains as a drawback for conventional polymer supports, due to their low surface areas. More recently, another approach emerged as a powerful alternative, in which porous organic polymers (POPs) are served as a versatile platform for deployment of highly stable and recyclable heterogeneous catalysts by taking advantage of their permanent porosity and the ability to tune their compositions and properties at the molecular level.4 However, in these materials the concentration of organic ligands is usually low due to the presence of copolymerized monomers. Thus organic ligands on these materials cannot be cooperative with each other efficiently, which is required for the formation and stabilization of the highly active species in the heterogeneous catalysts.5 The development of novel catalytic systems that combine the advantages of homogeneous and heterogeneous catalysis is therefore still a major aim of modern chemistry.
a
Department of Chemistry, Zhejiang University, Hangzhou 310028, P.R. China. E-mail: [email protected]; Fax: +86 571-8827-3698 b Electron Microscopy Centre, Zhejiang University, Hangzhou 310027, P.R. China c Dalian Institute of Chemical Physics, Dalian 116023, P.R. China † Electronic supplementary information (ESI) available: Synthesis details, characterization, and activities. See DOI: 10.1039/c4cc03884c
11844 | Chem. Commun., 2014, 50, 11844--11847
Scheme 1 Structures of vinyl-functionalized organic ligands of (A) triphenylphosphine (PPh3), (B) 2,20 -bipyridyl (bpy), and (C) salen.
Table 1 Textural parameters of POLs synthesized from various vinylfunctionalized organic ligands
POLs
Ligands
BET surface area (m2 g1)
Pore volume (cm3 g1)
POL-PPh3 POL-bpy POL-salen
PPh3 bpy Salen
1086 525 490
1.70 0.86 0.38
In this work, we report a synthetic methodology of porous organic ligands (POLs) bearing phosphorous, nitrogen, and oxygen donors. The POLs were obtained via solvothermal polymerization of corresponding vinyl-functionalized organic ligands (Scheme 1 and Table 1). As a typical example, a porous organic ligand of PPh3 (POL-PPh3) was synthesized from solvothermal polymerization of vinyl-functionalized PPh3 (3V-PPh3) in the presence of AIBN (azobisisobutyronitrile) at 100 1C in an autoclave (Schemes S1–S3, ESI†). Notably, the achieved POLs play the roles as both supports and ligands simultaneously. After treating with metal precursors, the obtained materials such as heterogeneous catalysts (M/POLs) exhibited excellent performance in hydroformation of 1-octene. In addition, due this feature of the heterogeneous catalysts, they can be easily adopted in a fixed-bed reactor. For a typical example of POLs, POL-PPh3 was selected for subsequent studies in detail. Fig. 1A shows the 13C MAS NMR spectrum of POL-PPh3, exhibiting an additional peak at 41 ppm assigned to the polymerized vinyl groups (Fig. S1, ESI†), except
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Fig. 1 (A) 13C MAS NMR spectrum, (B) 31P MAS NMR spectrum, (C) N2 sorption isotherms, (D) SEM image, (E) TEM image, and (F) TG curve of POL-PPh3.
for the peaks associated with the vinyl-functionalized PPh3 monomer. The 31P MAS NMR spectrum (Fig. 1B) shows that POL-PPh3 gives a single peak at 5.8 ppm, which is in good agreement with that of the monomer, indicating that P species are stable during the polymerization. Fig. 1C shows N2 sorption isotherms of the POL-PPh3, which give the curve of type-I plus type-IV. A steep increase in the curve at relative pressure less than 0.01 is due to the filling of micropores, while a hysteresis loop at a relative pressure of 0.7–0.95 is mainly from the contribution of sample mesoporosity. Correspondingly, the pore sizes are distributed at 0.7, 1.5, and 3–70 nm (Fig. S2, ESI†). These results suggest that POL-PPh3 has hierarchical porosity, which is also confirmed by the scanning electron microscopy (SEM) image and the transmission electron microscopy (TEM) image of POL-PPh3 (Fig. 1D and E). The BET surface area and pore volume of the POL-PPh3 are as high as 1086 m2 g1 and 1.70 cm3 g1, which are favorable for the dispersion of catalytically active sites.6 Furthermore, TG analysis shows that POL-PPh3 has superior thermal stability, giving the decomposition temperature starting at 440 1C (Fig. 1F), which is even higher than that of Nafion NR50 (330 1C), one of the most stable resins. Even if POL-PPh3 is treated in boiling water for 240 h, the pore structures are almost unchanged (Fig. S3 and S4 and Table S1, ESI†), indicating its superior hydrothermal stability. This is remarkable since most of the MOF catalysts are normally sensitive to water and relatively high temperature.7 It is particularly emphasized that POL-PPh3 has very high concentration of PPh3 species at 2.94 mmol g1 because of
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polymerization by vinyl-functionalized PPh3 itself. In comparison, the concentration of PPh3 species in the conventional heterogeneous catalysts reported previously is below 0.62 mmol g1.8 The high concentration of organic ligands is very favorable for the formation of a suitable coordination environment for ligands and metal species especially for multiple coordination.5 Very importantly, POL-PPh3 can be prepared in a relatively large scale synthesis. Under the same conditions, the solvothermal polymerization of 50 g of vinyl-functionalized PPh3 in a 200 mL autoclave gave ca. 50 g of POL-PPh3 with almost quantitative yield. In contrast, many porous polymers such as Covalent Organic Frameworks (COFs) have been obtained in relatively low yields.9 It is worth noting that the POLs are a kind of unique porous materials. Compared with Metal–Organic Frameworks (MOFs), they have very high thermal, hydrothermal, and chemical stabilities;7 compared with conventional immobilization of organic ligands in polymers, they have very high surface area and large pore volume;3c compared with copolymerization of monomers with organic ligands,10 they have very high concentration of organic ligands and high thermal stabilities; compared with sole micropores in porous materials,8 the hierarchical porosity in the POLs is very favorable for diffusion of reactants and products in organic transformation. Because organic ligands play so important roles in organic transformation,1 the improvement of organic ligands should be a breakthrough for organic synthesis. Therefore, the novel POLs with these unique advantages should be widely applied. After loading various metals, POLs still show very high surface areas. For example, POL-PPh3 supported Rh(CO)2(acac), denoted as Rh(CO)2(acac)/POL-PPh3, shows the BET surface area and pore volume of 1032 m2 g1 and 1.69 cm3 g1 (Fig. S5 and Table S1, ESI†), respectively. The HAADF-STEM image and EDX mappings show that the dispersion of Rh species in the sample are very high and uniform (Fig. S6, ESI†). More interestingly, the XPS spectra of P2d show that Rh(CO)2(acac)/POL-PPh3 gives relatively high binding energy (131.2 eV), compared with that of POL-PPh3 (130.4 eV, Fig. S7A and B, ESI†). In contrast, the binding energies of Rh3d5/2 and Rh3d3/2 in Rh(CO)2(acac)/ POL-PPh3 shift to 309.1 and 313.9 eV from 309.9 and 314.6 eV of Rh(CO)2(acac), respectively (Fig. S7C and D, ESI†), due to coordination of POL-PPh3 with Rh species.11 It is well known that hydroformylation is an extremely important industrial process, and right now more than 8 million tons per year of aldehydes and alcohols are produced worldwide annually through this process.12 Particularly, hydroformylation of higher olefins to yield aldehydes or alcohols is of great interest because their wide applications in production of detergents, plasticizers, solvents, and surfactants. Rhodium homogeneous catalysts typically work under mild conditions, giving good activity and selectivity to the desired aldehydes. Nonetheless, all commercial plants carrying out these reactions use cobalt catalysts, which require much harsher reaction conditions and give poorer selectivities, because the rhodium homogeneous catalysts decompose when attempting to distill the high boiling point product from the reaction systems.13 Therefore, the development
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Table 2 Catalytic data for 1-octene hydroformation over various catalystsa
Run
Catalyst
Conv. (%)
Select.c (%)
Yield (%)
1 2 3 4 5b
RhH(CO)(PPh3)3 Rh(CO)2(acac) RhH(CO)(PPh3)3/POL-PPh3 Rh(CO)2(acac)/POL-PPh3 RhH(CO)(PPh3)3/POL-PPh3
99.4 98.5 99.4 99.0 99.4
88.8 55.9 92.1 89.0 90.2
88.3 55.1 91.5 88.1 89.7
(0.46) (0.70) (0.87) (1.35) (0.87)
a Reaction conditions: CO/H2 = 1 : 1 (2.0 MPa), 1-octene (3.0 g), S/C at 6000, toluene (6.0 g), 90 1C for 4 h, Rh loading at 2.0 wt%. b Recycled 6 times. c Catalytic selectivity in parentheses was the molar ratio of linear to branched aldehyde in the products.
of Rh-based heterogeneous catalysts that retain the high activities and selectivities of their homogeneous analogues are urgently needed in commercial plants. Table 2 presents catalytic data for hydroformation of long-chain 1-octene over various catalysts. Clearly, the heterogeneous catalysts of RhH(CO)(PPh3)3/POL-PPh3 and Rh(CO)2(acac)/POL-PPh3 not only can be easily separated from the reaction system by filtration, but also exhibit much higher selectivity for the corresponding aldehydes than those of homogeneous catalysts of RhH(CO)(PPh3)3 and Rh(CO)2(acac) under similar conversions (Table 2 and Table S2, ESI†). This feature should be related to the high concentration of PPh3 in the Rh/POL-PPh3 catalysts, in good agreement with those of corresponding homogeneous catalysts.13c The heterogeneous catalyst also showed excellent stability, and it could be reused at least 6 times, with negligible loss of its activity (Table S3, ESI†). The excellent catalytic performance over the Rh/POL-PPh3 catalyst might be related to the high concentration of PPh3 species and the enrichment of reactants in the catalyst (Fig. S8, ESI†).14 Similarly, POL-bpy and POL-salen materials are carefully characterized, as shown in Fig. S9–S14 (ESI†). They also show high surface areas, large pore volume, hierarchical porosity, high concentration of organic ligands, and can be served as efficient catalyst supports. e.g., after POL-bpy supported with CuBr2, the obtained CuBr2/POL-bpy catalyst exhibits comparable catalytic performance with the corresponding homogeneous
Table 3 Aerobic oxidation of primary alcohols to aldehydes over homogeneous Cu/bpy and heterogeneous Cu/POL-bpy catalystsa
Yield (%) Entry
R
Time (h)
CuBr2/bpyb
CuBr2/POL-bpyc
1 2 3 4d
H CH3 CH3O H
6 5 5 6
96.4 98.6 99.2 —
94.2 96.8 97.1 92.2
a
Reaction conditions: alcohol (1.0 mmol), catalyst (5.0 mol%), TEMPO (5.0 mol%), KOH (10 mol%), CH3CN (2.0 mL), H2O (1.0 mL), 1 atm of air, and RT. b 2,2 0 -Bipyridyl (5 mol%), CuBr (5.0 mol%). c Cu loading at 5.0 wt% and 5.0 mol% was used. d Recycled 5 times.
11846 | Chem. Commun., 2014, 50, 11844--11847
analogue (CuBr2/bpy) in oxidations of primary alcohols to aldehydes using O2 as an oxidant (Table 3 and Table S5, ESI†).15 In addition, this heterogeneous catalyst can be easily recycled at least 5 times without an obvious loss in the activities. In summary, we report a facile and universal route for synthesizing various porous organic ligands. After supporting with metal species, the obtained heterogeneous catalysts exhibited not only high activities and selectivities but also excellent recyclabilities. Furthermore, except for applications of these POLs for organic transformation, they could be used as various advanced materials. For example, POL-bpy supported Eu3+ is an outstanding fluorescent material (Fig. S15, ESI†). This work is supported by NSFC (U1162201, 21333009 and 21273197), National High-Tech Research and Development program of China (2013AA065301), and Fundamental Research Funds for the Central Universities (2013XZZX001).
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