Applied Catalysis A, General 551 (2018) 98–105
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Highly efficient porous organic copolymer supported Rh catalysts for heterogeneous hydroformylation of butenes ⁎
Yuqing Wanga,c, Li Yana, , Cunyao Lia, Miao Jianga, Wenlong Wanga, Yunjie Dinga,b,
T
⁎
a
Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China c University of Chinese Academy of Sciences, 19 Yuquan Road, Shijingshan District, Beijing 100049, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Hydroformylation of butene Porous organic copolymers Synergetic effect
Porous organic copolymer (denoted as CPOL-BP&P) was afforded through the copolymerization of vinyl biphephos and tris(4-vinylphenyl)phosphine monomers under solvothermal conditions, and followed with impregnation method provided a highly efficient Rh/CPOL-BP&P catalyst with high activity (TOF = 11,200 h−1) and regioselectivity (the ratio of linear to branched aldehydes, l:b = 62.2) for heterogeneous hydroformylation of 1-butene. High regioselectivity was also obtained in the hydroformylation of butene mixture (2-butene: l:b = 55.8, isomeric mixture of butenes: l:b = 56.0). The Rh/CPOL-BP&P catalysts were thoroughly characterized by means of nitrogen sorption isotherms, in situ FT-IR, XPS, solid-state 31P MAS NMR, HAADF-STEM, SEM and TEM. The formation of unique coordination bonds with Rh species in the polymer skeleton was determined. Heterogeneous hydroformylation of butenes was effectively realized due to a synergetic effect between PPh3 moiety and biphephos moiety.
1. Introduction Olefin hydroformylation is a very prominent catalytic approach to aldehydes and alcohols in chemical industry [1–3]. As a 100% atomeconomic reaction, nowadays, the production capacity is beyond 12 million tons annually [4,5]. On account of the wide availability of linear aldehydes and alcohols in production of detergents, surfactants and plasticizers, which is attributed to the better physical performance than branch analogues, the linear aldehydes and alcohols become preferred in industry community. Therefore, besides activity, stability and chemoselectivity of the catalysts, the regioselectivity expressed by the linear/branched ratio is also a key parameter to evaluate the reaction effect [6]. With regard to industrial hydroformylation, it is very attractive to selectively produce the linear butyraldehyde whose global consumption occupies more than 50% of all aldehydes by weight [7]. Linear butyraldehyde is a raw material to produce the standard plasticizer bis(2-ethylhexyl) phthalate (DEHP) which is mainly applied in the manufacture of polyvinyl chloride (PVC). However, DEHP has suffered from the potential harm of leakage which leads to pollution and toxicity. To overcome this inherent shortcoming, a much cheaper and more easily available butene mixture has been chosen as a new starting material for new plasticizers, with lower risk of leakage and less toxicity
[8]. Nevertheless, the regioselectivity issue remains to be a big problem when butene mixture is used as the substrate of hydroformylation reaction (Scheme 1). When it comes to the catalysts, the ligand-modified Rh catalysts have been successfully employed in industrial hydroformylation, which provided good catalytic activity and selectivity [9]. However, the separation and recovery as well as recycling of conventional homogeneous catalysts have not been satisfactorily resolved [10]. Many research efforts have been devoted to combining the advantage of catalyst recovery obtained with heterogeneous systems and the much more desirable activity and selectivity associated with homogeneous catalysts [11]. The attractive approaches can be mainly divided into two categories, viz. biphasic catalysis and immobilization [10,12]. As for biphasic catalysis, aqueous biphasic [13], fluorous biphasic [14] and the use of ionic liquids [[14],15] and supercritical fluids [16] have been explored to resolve the separation problems. However, these approaches have the shortcoming that some of the reaction media must be removed from the bulk before catalyst separation process, otherwise, the loss of catalyst would be increased. The supports for the immobilization method could be soluble or insoluble in various solvents [17]. Although the separation problem could be easily circumvented, the traditional heterogenisation technologies often cause a dramatic
⁎ Corresponding authors at: Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China. E-mail addresses:
[email protected] (L. Yan),
[email protected] (Y. Ding).
https://doi.org/10.1016/j.apcata.2017.12.013 Received 14 October 2017; Received in revised form 7 December 2017; Accepted 18 December 2017 Available online 19 December 2017 0926-860X/ © 2017 Elsevier B.V. All rights reserved.
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Scheme 1. Hydroformylation of isomeric mixture of butenes to linear and branched pentanals.
2.2. Methods
drop of catalytic activity and selectivity due to the limitation of mass transfer and the lower concentration of organic ligands around the metal centre [11]. Recently, porous organic polymers (POPs) has emerged as a powerful alternative support material in heterogeneous catalysis and drawn great attention of scientists owing to their excellent properties such as hierarchical porosity, high surface area, large pore volume, low skeleton density and the capability to tune compositions and properties at molecular level [11,18–20]. The POL-PPh3 and CPOL-BP&P polymer have been synthesized in our laboratory [21,22], and CPOL-BP&P supported Rh catalysts displayed excellent activity and selectivity in the hydroformylation of propene and higher olefins. However, the role of two different types of P species in the Rh/CPOL-BP&P catalysts in hydroformylation reaction is ambiguous. Hence, much more works are required to study how the PPh3 and biphephos moieties influence the catalytic performance. Herein, the heterogeneous porous organic copolymer supported Rh catalyst has been extended to the hydroformylation of C4 alkenes. At the same time, the most attractive C4 feedstock like raffinate II contained 1-butene (40%), 2-butene (30%), and saturated C4 (30%) are extremely worthy to be studied. Therefore, in addition to 1-butene as a main research substrate, the hydroformylation of a mixture of 1-butene and 2-butene has also been investigated in this work. Moreover, in order to explore the role of two different types of P species in the framework of Rh/CPOL-BP&P, as a control catalyst, a similar analogue of Rh/CPOL-BP&Ph was obtained by the copolymerization of vinyl biphephos and tri(4-vinylphenyl)benzene. It was found that both of biphephos ligands with the acquired π-acceptor properties and the adequate available PPh3 ligands in Rh/CPOL-BP&P catalyst, cooperatively contributed to the outstanding activity and regioselectivity of Rh/ CPOL-BP&P catalysts in the hydroformylation of 1-butene. The characterizations of the in situ FT-IR, Nitrogen sorption isotherms, XPS, solid-state 31P MAS NMR, HAADF-STEM, SEM, TEM techniques also provided very beneficial evidences to help us understand the synergetic effect between the vinyl-PPh3 and vinyl-biphephos moieties.
2.2.1. Preparations of the catalysts POL-PPh3, CPOL-BP&P (Scheme 3) and polymers with different mass ratio of biphephos/PPh3 (CPOL-0.5BP&10P, CPOL-2BP&10P and CPOL-3BP&10P) were synthesized according to literatures [21,22]. 2.2.1.1. Preparation of CPOL-BP&Ph. As a typical run, in a glove box, in a THF (10 mL) solution of 1,3,5-tri(4-vinylphenyl)benzene (1.0 g) and vinyl biphephos (0.1 g), AIBN (25 mg) was added. After being stirred for 10 min at room temperature, the mixture was transferred into an autoclave and maintained at 100 °C in a muffle furnace for the solvothermal polymerization process without stirring for 24 h. After the reaction, a grey polymer was dried for 5 h under vacuum at 65 °C and obtained in almost quantitative yield. 2.2.1.2. Preparation of POL-PhPh3. As a typical run, in a glove box, in a THF (10 mL) solution of 1,3,5-tri(4-vinylphenyl)benzene (1.0 g), AIBN (25 mg) was added. After being stirred for 10 min at room temperature, the mixture was transferred into an autoclave and maintained at 100 °C in a muffle furnace for the solvothermal polymerization process without stirring for 24 h. After the reaction, the title grey polymer was dried for 5 h under vacuum at 65 °C and obtained in almost quantitative yield. 2.2.1.3. Preparation of 0.125 wt% Rh/POPs. As a typical run, in a THF (20 mL) solution of Rh(acac)(CO)2 (3.1 mg), 1.0 g of CPOL-BP&P was added. After stirring for 24 h under argon at room temperature, the resulting product was filtered, washed several times with excess of THF and dried for 5 h at 65 °C under vacuum. The white solid denoted as Rh/ CPOL-BP&P was obtained. Same procedure was followed to synthesize other Rh/CPOL-BP&Ph, Rh/POL-PPh3 and Rh/POL-PhPh3 catalysts by impregnation of metal precursor Rh(acac)(CO)2 onto the polymers in THF solvent. The exact Rh contents of the polymer catalysts were measured with inductively coupled plasma optical emission spectrometry (ICP-OES) method. 2.2.2. Hydroformylation of C4 olefins The continuous flow fixed-bed reactor applied for C4 alkenes hydroformylation is depicted in Scheme 4. The C4 olefins were taken out of a cylinder with a liquid-phase tube and fed into the reactor via a pump. Furthermore, a precision electronic balance was used for measuring the mass changes of substrates. Syngas flows (CO/H2 = 1:1) was adjusted by a mass-flow controllers before a filter was filled and after a one-way valve was equipped. After the back pressure, which was equipped to maintain the desired reaction pressure and outlet gas flow, the gas stream was online monitoring by an Agilent 3000A Micro gas chromatograph. The liquid phase products were obtained by a condenser and absorption tank and analyzed by an Agilent 6890A gas chromatography equipped with a FID.
2. Experimental 2.1. Materials Tetrahydrofuran (THF) and toluene were distilled from sodium/ benzophenone under argon. Azobisisobutyronitrile (AIBN, AR) was purchased and used without further purification. 1-butene (> 99.9%), 2-butene (trans and cis-2-butene, 1:1), their mixture (1-butene: 60%, trans-2-butene: 20% and cis-2-butene: 20%), Syngas (H2/ CO = 1:1, > 99.9%) were provided by Zhonghao Guangming Chemical Industry Corporation. Rh(acac)(CO)2 was obtained from Aladdin Company. The 1,3,5-tri(4-vinylphenyl)benzene was synthesized according to published procedures [23]. Tris(4-vinylphenyl)phosphine and vinyl-functionalized biphephos ligands were synthesized according to our methods reported previously [18,24]. The monomers of A–C for polymerization are listed (Scheme 2).
2.2.3. Characterizations of catalysts Nitrogen sorption isotherms were carried out with Quantachrome Autosorb-1 system at the temperature of liquid nitrogen. The samples were outgassed for 12 h at 120 °C prior to the measurements. The specific surface areas were calculated from the adsorption data using Brunauer–Emmett–Teller (BET) methods. The pore size distribution 99
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Scheme 2. The structures of vinyl-functionalized organic ligands of (A) vinyl biphephos. (B) tris(4-vinylphenyl)phosphine (3vPPh3).(C) 1,3,5-tri(4-vinylphenyl)-benzene (3v-PhPh3).
Scheme 3. Schematic representation for the synthesis of porous organic polymers POL-PPh3, CPOLBP&P and CPOL-BP&Ph via solvothermal polymerization.
Scheme 4. Schematic flow diagram of the continuous hydroformylation reactor equipment for C4 olefins.
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the best catalytic effect was obtained by the Rh/CPOL-BP&P catalyst. It can be, therefore, assumed that the excellent catalytic performance was probably contributed by the synergetic effect of biphephos and PPh3 moieties with Rh species in the Rh/CPOL-BP&P catalyst. The biphephos moieties with a properly high steric hindrance may lead to a superior regioselectivity, while the high concentration of PPh3 moieties probably not only act as co-coordinated ligands with metal complexes, but also serve as a structure-directing agent of pentanal. Next, the performance of Rh/CPOL-BP&P catalysts with different mass ratios of vinyl biphephos/3vPPh3 were also tested. As shown in Fig. S1 (supporting information), activity increased in tandem with the increment of the mass ratio of vinyl biphephos/3vPPh3 from 0.05 to 0.3, affording the TOF value of pentanal from 8250 to 14,130 h−1. Moreover, simultaneously, the l: b ratio also increased from 52.9 (Rh/ CPOL-0.5BP&10P) to 62.2 (Rh/CPOL-2BP&10P) and 63.7 (Rh/CPOL3BP&P), respectively. These results could be ascribed to the variation of electronic and steric environment in the polymer skeleton with the increased concentration of biphephos units. Given tedious preparation process and expensive nature of vinyl biphephos, Rh/CPOL-1BP&10P catalyst, denoted as Rh/CPOL-BP&P was chosen to conduct the following investigations.
curves were determined from the adsorption branches using the nonlocal density functional theory (NLDFT) method. Pore volume was estimated at the relative pressure P/P0 of 0.995. Transmission electron microscopy (TEM) images were obtained on a JEM-2100 at an accelerating voltage of 200 kV. Scanning electron microscopy (SEM) images was taken on a JSM-7800 F operating at an accelerating voltage of 0.01–30 kV. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) was performed on a JEM-ARM200F operating at 200 kV. XPS spectra were performed on a Thermo Scientific instrument and the binding energies were calibrated using the C1s peak at 284.6 eV. Solid-state 31P MAS NMR spectra were collected on a VARIAN Infinity plus spectrometer equipped with a 2.5 mm probe. 31P MAS NMR spectra were obtained at a frequency of 161.8 MHz, a delay of 3.0 s relative to 85% H3PO4 and a magic angle spinning rate of 10 kHz. Solid-state 13C MAS NMR spectra were recorded under a magic angle spinning rate of 6 kHz. In situ FT-IR spectra were acquired on a Thermo Scientific iS50 instrument ranging from 4000 to 750 cm−1. Each spectrum was obtained by averaging 32 scans taken with 4 cm−1 resolution. The reactor was evacuated and flushed with nitrogen several times. The sample was purified in a flow of N2 at 353 K for 60 min, the background spectrum was recorded. Then a syngas (CO/H2 = 1:1) was passed for 30 min and the spectra were acquired after purging the chamber under N2 at atmospheric pressure for 60 min. The Rh content was determined by using an ICP-OES 7300DV apparatus.
3.2. Characterization of catalysts The pore structures and surface areas of different catalysts were explored by nitrogen adsorption analysis. As shown in Fig. 1A, nitrogen sorption isotherm shows the curve of type-I plus type-IV, suggesting the existence of hierarchical porosity in the four POPs catalysts. As for Rh/ CPOL-BP&P and Rh/CPOL-BP&Ph catalysts, their hierarchical pore structures were further confirmed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images (Fig. 2A, B, C and D). These results showed that Rh/CPOL-BP&P and Rh/POL-PPh3 displayed similar pore size distribution, and the same tendency was found in another group of Rh/CPOL-BP&Ph and Rh/POL-PhPh3 catalysts, suggesting that 3v-PPh3 and 3v-PhPh3 monomers act as very different co-monomers which even influenced the pore structures of Rh/CPOL-BP&P and Rh/CPOL-BP&Ph catalysts. In addition, Rh/CPOLBP&P catalyst possesses the highest BET surface areas (1252 m2/g) and the largest pore volume (2.85 cm3/g) among the four catalysts as shown in Table 2, which is beneficial to the diffusion of the reactants and dispersion of active sites. The substrate of 1-butene should approach the active sites on the Rh/CPOL-BP&P catalyst more sufficiently and easily than Rh/POL-PPh3 and Rh/CPOL-BP&Ph catalysts. Thus is favorable for the acceleration of the reaction rate. Moreover, the ratio of linear/ branched pentanal was about 50 for the homogeneous Rh-biphephos system catalyzed hydroformylation of 1-butene [26], while the corresponding value of the homogeneous Rh-PPh3 catalyzed hydroformylation of 1-butene was 5.8 [27]. Compared with the above homogeneous results, the regioselectivity of heterogeneous Rh/CPOL-BP&P catalyst for hydroformylation of 1-butene was higher (l:b = 62.2). These results indicate that the hierarchical pore structure of CPOL-BP&P support may benefit for the synergetic effects between biphephos moieties and PPh3 moieties, which is related to the formation of unique Rh-P coordination bonds in the polymer skeleton, and is responsible for high activity and linear/branched ratio [28]. Fig. 3 shows the HAADF-STEM measurement results of the Rh/ CPOL-BP&P and Rh/CPOL-BP&Ph catalysts. It seemed that isolated single Rh atoms were uniformly dispersed in Rh/CPOL-BP&P catalyst. Moreover, the Rh species also dispersed in single sites in Rh/POL-PPh3 catalyst [21]. However, a small quantity of Rh clusters was observed in Rh/CPOL-BP&Ph catalyst, suggesting that abundant and excessive phosphorous ligands in the polymer skeleton of catalysts are favorable for the formation of single-site metal species. The in-situ Fourier transforming infrared spectra (FT-IR) of Rh/POLPPh3, Rh/CPOL-BP&P and Rh/CPOL-BP&Ph catalysts taken in synthesis
3. Results and discussion 3.1. Variation of catalysts The hydroformylation of C4 olefins was performed in a fixed-bed reactor due to the heterogeneous nature of our catalysts [25]. At first, different kinds of catalysts were investigated in the hydroformylation reaction of 1-butene, and their catalytic performances are summarized in Table 1. The turnover frequency (TOF), regioselectivity and chemoselectivity for pentanal of the reaction with the catalyst of Rh/CPOLBP&P after 24 h were 11,200 h−1, 62.2 and 94.2%, respectively. These values were much higher than those with Rh/POL-PPh3 (TOF = 490 h−1, l:b = 5.1 and Sel. = 81.4%) or Rh/CPOL-BP&Ph catalyst (TOF = 5754 h−1, l:b = 44.2 and Sel. = 75.9%). It is worthy to notice only one kind of P species exist in Rh/POL-PPh3 and Rh/CPOLBP&Ph, respectively. In fact, the self-polymerization of vinyl-biphephos ligands is very difficult, which is probably due to steric hindrance of this big molecule. Thus, 3v-PhPh3, a similar monomer of 3v-PPh3, was introduced as a co-monomer to crosslink the diphosphite ligand. In addition, in order to detect whether the 3v-PhPh3 component in Rh/ CPOL-BP&Ph catalyst is a key factor on the catalytic performance, POLPhPh3 polymer supported Rh catalyst was prepared. Notably, the catalytic performance was very poor (TOF = 36 h−1, l:b = 4.9 and Sel. = 13.7%), suggesting that the role of 3v-PhPh3 was inconsiderable for the Rh/CPOL-BP&Ph catalyst. These comparative results confirmed Table 1 Hydroformylation of 1-butene catalyzed by four POPs supported Rh catalysts. Catalysts
Rh/POL-PPh3 Rh/CPOL-BP&P Rh/CPOL-BP&Ph Rh/POL-PhPh3
Conv (%)
1.53 26.0 15.1 0.64
TOF (h−1)
490 11,200 5754 36
l:b
5.1 62.2 44.2 4.9
Product selectivity (%) pentanal
2-butene
butane
81.4 94.2 75.9 13.7
2.3 3.6 19.3 –
16.3 2.2 4.8 86.3
Reaction conditions: 0.10 g of catalyst, TOS = 24 h, P = 2 MPa (CO: H2 = 1:1), T = 80 °C, Rh loading at 0.125 wt%, GHSV = 8000 h−1, mass flow of 1-butene = 3.2 g/h.
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Fig. 1. (A) N2 adsorption and desorption isotherms and (B) pore sizes distributions of various POPs catalysts.
gas are shown in Fig. 4. At the same time, trigonal bipyramidal hydridorhodium complexes which are active intermediates in homogeneous hydroformylation were formed on the three different catalysts. As shown in Fig. 4, four absorption bands of Rh/POL-PPh3 catalyst at 2049, 2017, 1967 and 1945 cm−1 could be ascribed to HRh(CO)2(PPh3PS)2 species (PS is the abbreviation of polymer skeleton) [29,30]. The absorption bands at 2049 and 1967 cm−1 are ascribed to ee-HRh (CO)2(PPh3-PS)2 species, while the bands at 2017 and 1945 cm-1 are assigned to ea-HRh(CO)2(PPh3-PS)2 species. Moreover, four carbonyl peaks of the Rh/CPOL-BP&P catalyst appeared at 2049, 2017, 1976 and 1945 cm−1. These features are consistent to those observed for Rh complexes formed with phosphine-phosphite ligands dissolved in organic solvents [31,32]. The bands at 2049 and 1976 cm−1 could be attributed to ee-HRh(CO)2(BP&P-PS) species while the bands at
Table 2 Textural parameters of different kinds of catalysts. Catalysts
Ligands
BET surface area (m2 g−1)
Pore volume (cm3 g−1)
Rh/CPOL-BP&P Rh/CPOLBP&Ph Rh/POL-PPh3 Rh/POL-PhPh3
Biphephos&PPh3 Biphephos&PhPh3
1252 1116
2.85 1.66
PPh3 PhPh3
756 983
1.37 1.41
2017,1945 cm−1 are assigned to ea-HRh(CO)2(BP&P-PS) species. Besides, the band at 2068 cm−1 could be attributed to the active species of HRh(CO)(PPh3)3 [33]. When 3v-PPh3 monomer are replaced by 3vFig. 2. SEM images of (A) 0.125 wt% Rh/CPOLBP&P and (B) 0.125 wt% Rh/CPOL-BP&Ph catalysts; TEM images of (C) 0.125 wt% Rh/CPOL-BP&P and (D) 0.125 wt% Rh/CPOL-BP&Ph catalysts.
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Fig. 3. HAADF-STEM images of (A) 0.125 wt% Rh/ CPOL-BP&P catalyst; (B) 0.125 wt% Rh/CPOLBP&Ph catalyst.
with peak value arising at 132.7 eV (biphephos units) and 131.4 eV (PPh3 units) in the Rh/CPOL-BP&P catalyst and 132.5 eV and 131.3 eV in the CPOL-BP&P support, respectively. This minor positive shift of the peak value in the Rh/CPOL-BP&P catalyst indicated that the interactions existed between Rh species and two types of P species in the polymer framework even when the rhodium loading was low. Interestingly, the percentages of biphephos and PPh3 data (Table. S1, supporting information) indicate that Rh species tend to coordinate preferentially with the P sites from biphephos compared to PPh3 units in the Rh/CPOL-BP&P catalyst. A more pronounced shift of 0.3 eV was observed with the P2p (biphephos) signal between Rh/CPOL-BP&Ph and the support of CPOL-BP&Ph, which indicated that more biphephos species in the polymer skeleton coordinated with Rh species compared with Rh/CPOL-BP&P catalyst (Fig 5C,D). However, the loading of rhodium was too low to be detected by XPS. Thus, to get further insight into the interactions between Rh species and CPOL-BP&P support, the XPS spectrum of 2 wt% Rh/CPOL-BP&P was provided in the Fig S2 (supporting information), which also suggests the coordination of Rh species with two kinds of P species in the CPOL-BP&P support. In the solid-state31P MAS NMR spectrum of Rh/CPOL-BP&P catalyst (Fig. 6A), the peak at −5.8 ppm was consistent with the monomer of tris(4-vinylphenyl) phosphine. An additional peak at 27.3 ppm could be attributed to both of the P species of PPh3 coordinated with Rh species and the oxidation state of phosphorous (P=O). Meanwhile, the peak of 146.3 ppm in the support of CPOL-BP&P could be assigned to the corresponding monomer of biphephos [24], while the catalyst of Rh/ CPOL-BP&P showed a relative low-field peak at 145.3 ppm, indicating the coordination of biphephos with Rh species [41]. The lower peak at 144.8 ppm (Fig.6B) of Rh/CPOL-BP&Ph catalyst suggested that more biphephos in the polymer skeleton coordinated with Rh species. The above peak information indicated that the P species of PPh3 as well as biphephos in the polymer skeleton coordinated with Rh species in the catalyst Rh/CPOL-BP&P. It is unexpected that a new peak at -5.0 ppm observed in the catalyst of Rh/CPOL-BP&Ph, and could probably be attributed to the product of partial decomposition of the biphephos in polymer skeleton [41].
Fig. 4. In-situ FT-IR spectra of the terminal metal carbonyl region recorded over the catalysts (1) Rh/CPOL-BP&Ph; (2) Rh/CPOL-BP&P; (3) Rh/POL-PPh3 (80 °C, absorbed with syngas).
PhPh3 in the Rh/CPOL-BP&Ph catalysts, four peaks at 2073, 2031, 2017 and 1997 cm−1 could be assigned to HRh(CO)2(biphephos-PS) species [34–38]. The absorption bands at 2073, 2017 cm−1 represent the pentacoordinate ee-HRh(CO)2(biphephos-PS) species while the bands at 2031, 1997 cm−1 represent the ea-HRh(CO)2(biphephos-PS) species. The four absorption bands of Rh/CPOL-BP&Ph catalysts are among the highest wavenumbers compared with other catalysts, and the IR band at 1976 cm−1of Rh/CPOL-BP&P is 9 cm-1 higher than the corresponding band at 1967 cm-1 of Rh/POL-PPh3 catalyst, which indicated that the electronic properties of the ligands in the polymer skeleton over different catalyst are distinguishing. In general, the biphephos ligand with high π-acceptor property takes a shift of the νCO bands toward higher wavenumbers [35,39]. Thus, the Rh/CPOL-BP&Ph catalyst shows four highest absorption bands. In addition, the relative intensity of the absorption bands is relevant with the ratio of ee to ea isomers [40]. Thus, it is reasonable that the ratio of ee to ea isomers of Rh/CPOL-BP&P is higher than that of Rh/POL-PPh3. Hence, the π-acceptor characteristic of biphephos and the available adequate PPh3 ligands in Rh/CPOLBP&P catalysts cooperatively facilitated the formation of more active penta-coordinated hydrido-species, particularly for ee isomer, which is probably responsible for the high activity and regioselectivity of Rh/ CPOL-BP&P catalyst in the hydroformylation of 1-butene. The X-ray photoelectron spectra (XPS) of Rh/CPOL-BP&P and Rh/ CPOL-BP&Ph catalysts are shown in Fig.5. They showed doublet signals
3.3. Hydroformylation results 3.3.1. Variation of substrates Encouraged by the high activity of 1-butene hydroformylation over Rh/CPOL-BP&P catalyst, 2-butene and the mixed C4 alkenes were also investigated as substrates for hydroformylation reaction (Table 3). High regioselectivity were obtained for all the three kinds of substrates. At present, the regioselectivity for the productivity of pentanal is only approximate 19 (l:b) when using technical C4-feedstocks like raffinate 103
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Fig. 5. (A) P2p XPS spectrum of CPOL-BP&P, (B) P2p XPS spectrum of 0.125 wt% Rh/CPOL-BP&P, (C) P2p XPS spectrum of CPOL-BP&Ph, (D) P2p XPS spectrum of 0.125 wt% Rh/CPOL-BP&Ph.
II [6]. Furthermore, the high TOF for the hydroformylation of mixed C4 alkenes was also acquired. However, the activity for the hydroformylation of 2-butene remained modest. Thus, we could conclude that the Rh/CPOL-BP&P catalyst has very excellent C4 alkenes (1-butene and isomeric mixture of butenes) tolerance in the way of tuning regioselectivity and activity, with the prospect of the industrialization.
Table 3 Hydroformylation of different C4 alkenes over Rh/CPOL-BP&P catalyst.a TOF (h−1)
Substrate
1-butene 2-butene Mixture of C4 olefinsb
3.3.2. Stability test of Rh/CPOL-BP&P and Rh/CPOL-BP&Ph catalysts As a control experiment, the stability of Rh/CPOL-BP&P and Rh/ CPOL-BP&Ph catalysts in the hydroformylation of 1-butene were tested at 80 °C and 2 MPa. As shown in Fig. 7, the TOF of pentanal of the Rh/ CPOL-BP&P catalyst, maintained at about 5400 h−1 during the period of 100–300 hours, which was obviously higher than the TOF value of 2500 h-1 of the Rh/CPOL-BP&Ph catalyst maintained at 100–300 hours. Moreover, the initial activity in the 100 h of the Rh/CPOL-BP&P catalyst was also higher than the Rh/CPOL-BP&Ph catalyst. As for the ratio of l/b, the value was higher than 61 and almost invariable during the period of 100–300 hours regard to the catalyst of Rh/CPOL-BP&P. However, the l/b ratio obviously decreased from 45 to 18 during the period of 100–300 hours for the catalyst of Rh/CPOL-BP&Ph. The Rh
9020 301 3674
l:b
58.6 55.8 56.0
Product selectivity (%) Pentanal
2-butene
Butane
93.6 20.3 92.6
4.0 51.4c –
2.4 28.3 7.4
a Reaction conditions: 0.10 g of catalyst, TOS = 12 h, P = 2 MPa (CO:H2 = 1:1), T = 80 °C, Rh loading at 0.125 wt%, GHSV = 8000 h−1, mass flow of substrates = 3.2 g/ h. b The composition of mixture of butenes is 60% of 1-butene, 20% of trans-2-butene, 20% of cis-2-butene: c The selectivity for the isomerization of 2-butene to 1-butene.
concentration of the fresh and spent 0.125 wt% Rh/CPOL-BP&P catalyst over 300 h time on steam were 0.1288 wt% and 0.1202 wt%, while the corresponding values for the 0.125 wt% Rh/CPOL-BP&Ph catalyst were 0.1338 wt% and 0.0387 wt%, respectively. The excellent catalytic activity as well as the high stability of the Rh/CPOL-BP&P catalyst could Fig. 6. 31P MAS NMR spectra of two kinds of catalysts: (A) 0.125 wt% Rh/ CPOL-BP&P, (B) 0.125 wt% Rh/CPOL-BP&Ph. Asterisks denote spinning side bands.
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Fig. 7. Stability of the Rh/CPOL-BP&P and Rh/CPOL-BP&Ph catalysts for hydroformylation of 1-butene.
probably ascribed to the synergetic coordination effect of phosphorous both in biphephos and PPh3 moieties while the instability of selectivity of l/b ratio could be attributed to the lack of PPh3 moieties on the Rh/ CPOL-BP&Ph catalyst.
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4. Conclusion To conclude, the heterogeneous porous organic copolymer supported Rh catalysts have been extended to the hydroformylation of C4 olefins conducting in a continuous fixed-bed reactor. For Rh/CPOLBP&P catalyst, excellent activity and high regioselectivity as well as long-term stability in hydroformylation of 1-butene were obtained. At the same time, the high regioselectivity was also obtained in the hydroformylation of butene mixture (2-butene and isomeric mixture of butenes). In-situ FT-IR spectra and XPS spectra as well as the solid-state 31 P MAS NMR indicated that there existed a critical synergetic effect between the different P species of biphephos and PPh3 ligands in Rh/ CPOL-BP&P catalyst, which is responsible for the excellent activity and selectivity as well as the stability. Specifically, biphephos species with the high steric hindrance lead to a superior regioselectivity and the adequate PPh3 species not only act as co-coordinated ligands with Rh species, but also serve as a structure-directing agent to improve the selectivity. Acknowledgements We greatly appreciate the financial support by the Strategic Priority Research Program of Chinese Academy of Sciences Grant No. XDB17000000. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcata.2017.12.013 References [1] R. Franke, D. Selent, A. Börner, Chem. Rev. 112 (2012) 5675–5732. [2] S. Kusumoto, T. Tatsuki, K. Nozaki, Angew. Chem. Int. Ed. 54 (2015) 8458–8461. [3] P.W.N.M. van leeuwen, C. Claver, Rhodium Catalyzed Hydroformylation, Kluwer Academic Publisher, Dordrecht, 2000. [4] M. Haumann, A. Riisager, Chem. Rev. 108 (2008) 1474–1497. [5] A. Kämper, P. Kucmierczyk, T. Seidensticker, A.J. Vorholt, R. Franke, A. Behr, Catal. Sci. Technol. 6 (2016) 8072–8079. [6] M. Beller, Top. Organomet. Chem. 18 (2006) 1–33. [7] C. Chen, P. Li, Z. Hu, H. Wang, H. Zhu, X. Hu, Y. Wang, H. Lv, X. Zhang, Org. Chem. Front. 1 (2014) 947–951. [8] K.-D. Wiese, G. Protzmann, J. Koch, W. Bueschken, German Patent DE19957522 A1
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