solid separation system for highly effective recycling of homogeneous catalyst based on a phosphine-functionalized polyether guanidinium ionic liquid

Molecular Catalysis 475 (2019) 110503

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A novel homogeneous catalysis–liquid/solid separation system for highly effective recycling of homogeneous catalyst based on a phosphinefunctionalized polyether guanidinium ionic liquid

T

⁎

Xin Jina, , Jianying Fenga, Shumei Lib, Hongbing Songa, Cong Yua, Kun Zhaoa, Fangfang Konga a State Key Laboratory Base for Eco-Chemical Engineering, College of Chemical Engineering, Qingdao University of Science and Technology, 53 Zhengzhou Street, Qingdao, 266042, China b College of Physical Education, Qingdao University of Science and Technology, 53 Zhengzhou Street, Qingdao, 266042, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Catalyst recycling Homogeneous catalysis Hydroformylation Ionic liquids Phosphine ligands

Herein we report a novel homogeneous catalysis–liquid/solid separation catalytic system for highly effective recycling of homogeneous catalyst based on a phosphine-functionalized polyether guanidinium ionic liquid (PPolyGIL) with a room temperature liquid/solid phase transition characteristic. This novel catalytic system has perfectly integrated the advantages of both the homogeneous and biphasic catalysis to realize the combination of the high activity, easy catalyst recycling and long service-life of the catalyst without a significant loss of the activity, selectivity and Rh for more than 30 cycles in the Rh-catalysed hydroformylation of higher olefins. The long service-life of the catalyst could be attributed to the efficient and practical strategy of the catalyst separation.

1. Introduction Over the last decades, homogeneous catalysts have been studied extensively because of their excellent activity and selectivity under mild conditions, but there is a considerable difficulty in the separation and recycling of the expensive catalysts [1–3]. Although various innovations have been developed in this area to deal with this problem [4–8], including catalyst immobilization on inorganic supports or organic polymers [9–15], aqueous-organic biphasic catalysis [16], supported aqueous-phase catalysis (SAPC) [17], fluorous biphasic catalysis [18–20], catalysis in supercritical fluids [21–24], thermoregulated phase transfer (or separable) catalysis [25–28], temperature-dependent multi-component solvent (TMS) system [29–39], release and catch catalysis [40–42], and tunable and switchable solvent systems [43–47], etc., it still remains a tremendous challenge to combine the advantages of homogeneous and heterogeneous catalysis to achieve a higher activity, more effective separation and recycling of catalyst and longer life time of catalyst. In recent years, ionic liquids (ILs) have attracted increasing attention as a green carrier for recycling transition metal catalysts in homogeneous catalysis [48–62], in which the highly efficient immobilization of catalysts in ILs is the key. In order to reduce the loss of noble metal catalysts, ionic tags, such as imidazolium, pyridinium and

⁎

guanidinium, could be attached to the skeleton of ligands to enhance the affinity between catalysts and ILs [63–88]. However, this strategy could be limited by the complicated process in the modification of ligands. In our opinion, it is simpler and more economic to design and synthesize “task-specific ionic liquids” [89–94] tailored to commercially available ligands with the wide application potential based on the designability of ILs. More recently, we reported a halogen-free polyether-functionalized guanidinium IL (PolyGIL, 1 in Scheme 1) with a room temperature liquid/solid phase transition characteristic (melting point: Tm =23.8 °C), and tested it for the Rh-catalyzed biphasic hydroformylation of higher olefins using a TPPTS ligand [95]. However, this biphasic IL system exhibited only a low catalytic activity with TOFs being less than 200 h−1 due to the widespread mass transfer limitation in the biphasic IL catalytic system. With the aim of establishing the highly effective and long-life catalytic system, and in conjunction with our previous work [95], we have synthesized a phosphine-functionalized polyether guanidinium ionic liquid (P-PolyGIL, 2 in Scheme 1) [96] with a room temperature liquid/ solid phase transition characteristic, and designed a novel homogeneous catalysis–liquid/solid separation catalytic system for highly effective recycling of Rh catalyst in the hydroformylation of higher olefins. In this system, the catalytic reaction is performed in a

Corresponding author. E-mail address: [email protected] (X. Jin).

https://doi.org/10.1016/j.mcat.2019.110503 Received 1 June 2019; Received in revised form 3 July 2019; Accepted 4 July 2019 2468-8231/ © 2019 Elsevier B.V. All rights reserved.

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Scheme 1. A phosphine-functionalized polyether guanidinium ionic liquid (P-PolyGIL), and our previous and current strategies for recycling Rh catalyst in hydroformylation of higher olefins.

homogeneous solution phase with Rh catalyst, P-PolyGIL, PolyGIL and organic solvent I (e.g. toluene), and after the reaction the Rh catalyst, PPolyGIL and PolyGIL can be easily separated by a simple precipitation from solvent I and product by adding a non-polar solvent II (e.g. alkanes). The advantages of homogeneous and biphasic catalysis herein were combined perfectly with the high activity, much simplified catalyst separation and long life time of catalyst.

HRMS was recorded on a Q-ToF Ultima Global mass spectrometry using the electrospray technique. ICP-AES data were measured on an IRIS Istrepid II XSP apparatus for determining the Rh leaching. Differential scanning calorimetry (DSC) measurement was performed on a Mettler Toledo DSC1 instrument under N2. The thermal decomposition temperature was measured with a Mettler Toledo TGA/ DSC1 instrument under N2.

2. Experimental section

2.2. Synthesis and characterization of P-PolyGIL 2

2.1. Materials and methods

A 100 mL Schlenk flask was charged with 0.5 g (0.78 mmol) TPPTS·4H2O, 2.1 g (2.3 mmol) 1 and 25 mL of degassed acetonitrile under an argon atmosphere. The mixture was stirred at 25 °C for 72 h and filtered. Then the filtrate was evaporated under reduced pressure to give a yellow viscous liquid in 90% yield. The yellow viscous liquid slowly solidified below 10 °C. 1H NMR (500.0 MHz, D2O): δ = 7.87 (d, 3H, P-Ph-H), 7.82 (d, 3H, P-Ph-H), 7.59 (t, 3H, P-Ph-H), 7.52 (t, 3H, PPh-H), 3.84–3.44 (m, 200H, OCH2CH2), 3.39 (s, 9H, OCH3), 2.95 (s, 36H, 6×N(CH3)2); 13C NMR (125.7 MHz, CDCl3): δ = 162.07, 146.54, 136.03, 134.00, 130.39, 127.85, 126.27, 71.31, 69.93, 69.63, 68.97, 58.40, 44.13, 39.31; 31P NMR (202.4 MHz, D2O): δ = −5.11; HRMS (Q–ToF MS, ES+): m/z = 746.4994, calcd. for C34H72O14N3 ([CH3(OCH2CH2)14TMG]+): 746.5009; m/z = 790.5256, calcd. for C36H76O15N3 ([CH3(OCH2CH2)15TMG]+): 790.5271; m/z = 834.5518,

The 1-alkenes and TPPTS were purchased from Acros Company. The RhCl3·3H2O was purchased from ABCR Company. All other reagents were obtained commercially, except as noted. The PolyGIL 1 was synthesized according to literature methods [95]. All reactions were carried out under an argon atmosphere using the standard Schlenk techniques. The solvents and reagents were rigorously deoxygenated and dehydrated prior to use. The hydroformylation reactions were performed in a stainless steel autoclave. The conversion and selectivity were determined by GC using an OV101 capillary column and the FID detector. GC/MS data were recorded on an Agilent 6890/5973 GC–MS apparatus with a DB-35MS capillary column. NMR spectra were recorded on Bruker 500 MB instrument. 2

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calcd. for C38H80O16N3 ([CH3(OCH2CH2)16TMG]+): 834.5533; m/ z = 878.5779, calcd. for C40H84O17N3 ([CH3(OCH2CH2)17TMG]+): 878.5795; m/z = 922.6041, calcd. for C42H88O18N3 ([CH3(OCH2CH2)18TMG]+): 922.6057; HRMS (Q–ToF MS, ES−): m/ z = 166.3132 (z = 3), calcd. for C18H12O9PS3: 166.3133; m/ z = 249.9730 (z = 2), calcd. for C18H13O9PS3: 249.9735.

through a convenient ion exchange reaction between the PolyGIL (1, in Scheme 1) with TPPTS (Scheme 1) without complicated synthetic steps. TPPTS is known to be the simplest sulfonated phosphine ligand, which is commercially available and widely used in catalytic reactions. The differential scanning calorimetry (DSC) analysis showed that the melting point temperature of 2 is 19.3 °C, which indicated that the P-PolyGIL, similar to the PolyGIL, displays a room temperature liquid/ solid phase transition property. We attributed this result to the high rotational freedom and flexibility of the ether chain, which led to a decrease in the lattice energy of TPPTS, in turn to a drop in the melting point [97]. In further experiment, the solubility of 2 and 1 in different solvents was determined. In contrast to the TPPTS ligand and some classic imidazolium ILs, which are normally insoluble in less polar organic solvents (e.g. toluene, alkanes), both 2 and 1 are readily soluble in aromatic solvents (e.g. toluene) and completely immiscible with alkanes. Therefore, a homogeneous system can be made by mixing 1, 2 and toluene, after which both 1 and 2 can be precipitated from toluene by adding an alkane through a room temperature liquid/solid phase transition to allow a liquid/solid biphasic separation. In order to make sure the catalytic system to be “homogeneous”, we investigated the phase behavior of the 1/2/toluene/1-alkenes (as substrates for the hydroformylation) systems by the cloud titration method [29]. The cloud titration experiments indicated that the mixtures of 1, 2, toluene and 1-alkenes with an appropriate ratio at a certain temperature could become a homogeneous reaction systems. Therefore, a homogeneous catalysis–liquid/solid separation catalytic system based on 2 and 1 was established and tested using the Rh-catalysed hydroformylation of higher olefins as the model reactions (Fig. 1). Typically, RhCl3·3H2O, 2 and 1 were mixed with a certain volume of toluene, and the mixture was maintained at the hydroformylation temperature and pressure for 24 h to pre-form the precursor (Rh-2) of the active Rh catalyst in situ. The system with Rh-2, 1 and toluene was still homogeneous after being cooled to room temperature (25 °C). After the 1-alkene was added at room temperature, the system became biphasic, where the upper layer was the mixture of 1-alkene and toluene and the lower layer was the mixture of Rh-2 and 1. When the temperature was increased to 85 °C, the reaction system became homogeneous again (Fig. 1, A) and then the hydroformylation was carried out with 5.0 MPa of syngas. After the reaction, the mixture was cooled to room temperature (25 °C), which remained homogeneous (Fig. 1, B). Upon adding n-heptane whose volume was 0.5–2 times of that of toluene, a phase separation occurred where the upper layer was the mixture of toluene, n-heptane and the

2.3. Evaluation of the initial catalyst activity and selectivity of Rh-2 catalyst for the hydroformylation of C8-C14 1-alkenes under the homogeneous catalysis–liquid/solid separation system (the results were shown in Table 1) An autoclave was charged with RhCl3·3H2O (1.0 mg, 3.88 × 10-3 mmol), 2 (0.12 g, 3.88 × 10-2 mmol), 1.0 g 1 and 2.5–4.5 mL toluene (2.5 mL for 1-octene, 3.0 mL for 1-decene, 4.0 mL for 1-dodecene and 4.5 mL for 1-tetradecene) under an argon atmosphere. The catalyst precursor solution was stirred at 85 °C for 24 h under 5.0 MPa of syngas (CO/H2 = 1:1). Subsequently, C8-C14 1-alkenes (3.88 mmol) and 0.1 mL internal standards (cyclohexane for 1-octene, n-octane for 1decene, n-decane for 1-dodecene and n-dodecane for 1-tetradecene) were added. The reactor was pressurized with syngas to 5.0 MPa, followed by heating the reaction system to 85 °C while stirring. After 0.5 h, the autoclave was rapidly cooled in an ice bath. Upon releasing the gases, n-heptane (4–7 mL) was added to extract the product, and the upper toluene/heptane layer was removed for GC analysis. 2.4. Cycling of Rh-2 catalyst in the hydroformylation of 1-octene under the homogeneous catalysis–liquid/solid separation system (the results were shown in Fig. 2) An autoclave was charged with RhCl3·3H2O (1.0 mg, 3.88 × 10-3 mmol), 2 (0.24 g, 7.76 × 10-2 mmol), 1.0 g 1 and 2.5 mL toluene under an argon atmosphere. The catalyst precursor solution was stirred at 85 °C for 24 h under 5.0 MPa of syngas (CO/H2 = 1:1). Subsequently, 0.6 mL 1-octene (3.88 mmol) and 0.1 mL cyclohexane as an internal standard were added. The reactor was pressurized with syngas to 5.0 MPa, followed by heating the reaction system to 85 °C while stirring. After 2 h, the autoclave was rapidly cooled in an ice bath. Upon releasing the gases, 4 mL n-heptane was added to extract the product. The upper toluene/heptane layer was removed for GC analysis, and the fresh 1-octene and toluene were added to the solidified 1 and Rh-2 in the lower layer for next recycle. 3. Results and discussion We first synthesized the P-PolyGIL (2, in Scheme 1) in CH3CN

Table 1 The initial activity and selectivity of Rh-2 for hydroformylation of C8-C14 1-alkenes in the homogeneous catalysis–liquid/solid separation system a. Entry

Catalytic system

Reaction medium

Ligand

Alkene

t (h)

Conversion.

1 2 3 4 5 6 7 8 9

Biphasic Biphasic Biphasic Biphasic Homogeneous Homogeneous Homogeneous Homogeneous Homogeneous

1 1 1 1 1/PhMe 1/PhMe 1/PhMe 1/PhMe 1/PhMe

TPPTS TPPTS TPPTS TPPTS TPPTS 2 2 2 2

1-Octene 1-Decene 1-Dodecene 1-Tetradecene 1-Octene 1-Octene 1-Decene 1-Dodecene 1-Tetradecene

5 5 5 5 2 0.5 0.5 0.5 0.5

36 15 7 6 79 61 64 41 43

b

(%)

Sald c (%)

n/i

97 92 75 92 95 91 95 93 91

2.3 2.4 2.4 2.4 2.3 2.6 2.5 2.5 2.5

d

TOF e (h−1) 70 28 10 11 375 1110 1216 763 783

a: RhCl3·3H2O 1.0 mg, ligand/Rh = 10, 1-alkenes/Rh = 1000, p (H2/CO = 1:1) =5.0 MPa, T =85 °C, the experimental results were obtained after 24 h of active catalyst preformation; entries 1-4: biphasic system (Rh-TPPTS/1/1-alkenes), with 1 g of 1, calculated from the data reported by Jin [95]; entry 5: homogeneous system (Rh-TPPTS/1/1-alkenes/PhMe), with 1 g of 1 and 2.5 mL of PhMe; entries 6-9: homogeneous system (Rh-2/1/1-alkenes/PhMe), with 1 g of 1 and 2.5–4.5 mL of PhMe (2.5 mL for 1-octene, 3.0 mL for 1-decene, 4.0 mL for 1-dodecene and 4.5 mL for 1-tetradecene), the volume of toluene was determined by a cloud titration method [29]; b: conversion of 1-alkenes was determined on GC with the cyclohexane (for 1-octene), n-octane (for 1-decene), n-decane (for 1-dodecene) and ndodecane (for 1-tetradecene) as internal standards; c: the combined selectivity to normal aldehyde and 2-methyl aldehyde; d: ratio of normal aldehyde to 2-methyl aldehyde; e: turnover frequency (TOF): mol (aldehyde) per mol (rhodium) per hour; for Rh-2, the initial TOFs were calculated within the first 0.5 h of the reaction, at which time the conversion of 1-alkenes reached to.40–65%. 3

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Fig. 1. A pattern diagram of the separation, recovery and recycling of Rh catalyst under the homogeneous catalysis–liquid/solid separation system for the hydroformylation of higher olefins.

In Table 1, the initial activity and selectivity of Rh-2 were evaluated for the hydroformylation of C8-C14 1-alkenes in the homogeneous catalysis–liquid/solid separation system. For comparison, the corresponding data [95] in Rh-TPPTS/1/1-alkenes biphasic hydroformylation system were also included in Table 1 (entries 1–4, Table 1). As listed in Table 1, the activity of Rh-TPPTS in the biphasic system was very low with TOF values of only 10–70 h−1, which indicated that the 1-alkenes must overcome the resistance in the mass transport and diffusion to arrive at the catalytically active site due to their poor solubility in 1. In order to improve the mass transport in the biphasic system, the toluene was introduced into the biphasic system to transform it into a homogeneous system. As a result, a higher conversion (79%) was observed for 1-octene (entry 5, Table 1), and the TOF value (375 h-1) was 5 time the one in the biphasic system (entry 1 vs. entry 5, Table 1), indicating that the hydroformylation might proceed under a homogeneous conditions. It is clear that Rh-TPPTS is insoluble in the toluene due to the strong polarity of the TPPTS ligand. Thus, it can be speculated that in the 1/toluene system, the ion exchange between TPPTS and 1 may have occurred to generate the new ligand 2 in situ, which is more soluble in toluene and allows the hydroformylation to proceed under a homogeneous condition. Therefore, here the polyether guanidinium cation ([Me(EO)16TMG]+) serves as a phase transfer agent. To verify this speculation, the homogeneous 1/toluene system using Rh-2 as catalyst (Table 1, entries 6–9) was evaluated. As expected, about 40%–65% of C8–C14 1-alkenes were converted within 0.5 h, and the TOF values reached to 700–1300 h-1, which were almost 15–80 time of the TOF values in the corresponding biphasic system

Fig. 2. Recycling of Rh-2 catalyst under the homogeneous catalysis–liquid/ solid separation system for hydroformylation of 1-octene, reaction conditions: RhCl3·3H2O = 1.0 mg, 2/Rh = 20, 1-octene/Rh = 1000, 1 g of 1, 2.5 mL of PhMe, p(H2/CO = 1:1) =5.0 MPa, T =85 °C, t =2 h.

aldehyde products and the bottom layer included Rh-2 and 1. Below 10 °C, both 1 and Rh-2 at the bottom were solidified and then the upper layer including products could be readily decanted and thus separated (Fig. 1, C). The solid bottom layer (Fig. 1, D) could be carried to the next catalytic cycle as the catalyst phase. The volatile toluene and nheptane can be readily removed from the aldehydes by distillation.

4

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affinity between the P-PolyGIL and PolyGIL was improved to promote the immobilization of Rh-2 in 1 and lower the Rh leaching, thanks to the structural similarity of the P-PolyGIL to PolyGIL, such as SO3− anion and polyether guanidinium cation moieties. Finally, the polyether guanidinium cations [Me(EO)16TMG]+ might form a large steric hindrance around the catalytically active Rh species to prevent the formation of the low-active rhodium cluster species [74,98], thus stabilizing the Rh catalyst. It should be emphasized that there is a certain similarity between our catalytic system and the temperature-dependent multi-component solvent (TMS) systems that are widely reported for the efficient separation of homogeneous catalysts [29–39], because the catalytic reactions in both systems are all carried out under a homogeneous condition and both systems adopt a two-phase separation way to recover the catalysts. Therefore, we compare our system with two typical TMSsystems [31,33] in Table 2. By comparison, it can be found that our system is far superior to the TMS systems in the catalytic activity (TOF), catalyst service-life and Rh leaching. For example, in the first TMSsystem (entry 1, Table 2), the incomplete ion exchange between TPPTS and [Me(EO)16NEt3][OMs] may result in a lower TOF value (298 h−1), which is similar to our homogeneous system with Rh-TPPTS (entry 1 in Table 2 vs. entry 5 in Table 1). In addition, the major limitation of the TMS-systems is that the ratio between two or more solvents must be strictly regulated. However, with the increase of the number of catalyst cycling, the partial leaching of one solvent to another may cause a change in the proportion of solvents, which may lead to the failure of the two-phase separation (a main reason for the lower TTON value) and excessive Rh loss. In contrast, the highlight of our work is to adopt a more practical strategy of adding another solvent after the reaction to divide the catalytic system into two phases, which has a loose requirement for the ratio of several solvents, and more convenient operation, thus easily achieving a long-term cycling of the catalyst with a higher TTON value and lower Rh leaching. A patent [99] related to this article had been filed in 2013 and authorized in 2014.

Fig. 3. Photographs of the homogeneous catalysis–liquid/solid separation system for the hydroformylation of 1-octene, (a) corresponding to “A” in Fig. 1; (b) corresponding to “B” in Fig. 1; (c) corresponding to “C” in Fig. 1, top layer: toluene/n-heptane/aldehydes phase, bottom layer: solidified Rh-2/1 phase.

(entry 1 vs. entry 6, entry 2 vs. entry 7, entry 3 vs. entry 8, entry 4 vs. entry 9, Table 1). However, the TOF value of the homogeneous system with Rh-TPPTS is only one third of that of the homogeneous system with Rh-2 (entry 5 vs. entry 6, Table 1), which may be attributed to the incomplete ion exchange between TPPTS and 1 in the toluene. For all the tested 1-alkenes, the chemoselectivities for aldehyde were all higher than 90% under the homogeneous system with isomeric alkenes (2-alkenes and 3-alkenes) as the main by-products, and the regioselectivities (n/i ratio of ˜2.5) were moderate, similar to that in the biphasic system. In the subsequent experiments, the service-life of Rh-2 catalyst in the homogeneous catalysis–liquid/solid separation system was examined by the recycling experiments using the hydroformylation of 1-octene as a model reaction (Fig. 2). It was found that Rh-2 exhibited a long life time in this catalytic system. For 30 continuous cycles, the conversion of 1octene could be kept above 90% without a significant deterioration in the aldehydes selectivity (83%–96%) and n/i ratio (2.3–2.7). The total turnover number (TTON) could reach to 28,214 after 34 runs. The ICP analysis indicated that the Rh leaching in a single cycle was less than 0.1%, and no IL loss was observed. The photos of the homogeneous catalysis–liquid/solid separation system for the hydroformylation of 1octene are shown in Fig. 3. The high efficiency and long service-life of Rh catalyst could be attributed to the structure and property of both P-PolyGIL and PolyGIL, as well as the efficient and practical catalyst recycling strategy. First, the homogeneous reaction system ensured the high activity and good selectivity of the hydroformylation reaction. Secondly, the “liquid/solid separation” simplified the separation operating of Rh catalyst and retarded the oxidation of the ligand by limiting the exposure of the catalyst to air to achieve the highly efficient recycling of the catalyst so that the separation process could be carried out in air. Thirdly, the

4. Conclusion In summary, we developed a new homogeneous catalysis–liquid/ solid separation system for separating and cycling the Rh catalyst in the hydroformylation of higher olefins based on a phosphine-functionalized polyether guanidinium ionic liquid (P-PolyGIL) with a room temperature liquid/solid phase transition characteristic. This novel catalytic system has perfectly combined the advantages of both the homogeneous and biphasic catalytic systems to achieve the integration of the high activity, facile catalyst recycling and long life time of catalyst without a significant loss of the activity, selectivity and Rh for more than 30 runs. This catalytic system exhibits a better ability than the TMS-system in the separation, recovery and recycling of catalyst, thanks to its efficient and practical catalyst separation strategy. Despite the fact that the new system introduces solvents (toluene and heptane) that would then have to be removed, the use of solvents greatly enhances the TOF value of the hydroformylation reaction and realizes the efficient separation and recycling of catalyst, which effectively

Table 2 A comparison of our system with the temperature-dependent multi-component solvent (TMS) systems for the Rh-catalyzed hydroformylation of 1-alkenes. Entry 1 2 3 4

a b c c

System TMS-System TMS-System Our system Our system

Reaction medium [Me(EO)16NEt3][OMs]/PhMe/n-heptane propylene carbonate/dodecane/1,4-dioxane 1/PhMe 1/PhMe

Ligand

Substrate

TPPTS P(OPh)3 2 2

1-Dodecene 1-Octene 1-Octene 1-Dodecene

TOF (h−1) 298 404 1110 763

n/i

TTON d

n.r. 10.5 2.6 2.5

11950 n.r. 28,214 —

e

Rh Loss (%)

Ref.

0.45–0.82 2.8 0.06-0.1 —

[31] [33] — —

a: RhCl3·3H2O as the catalyst precursor, TPPTS/Rh = 8, p (1:1 H2/CO) =5.0 MPa, T =110 °C, t =5 h; b: HRh(CO)(PPh3)3 as the catalyst precursor, P(OPh)3/ Rh = 12, p (1:1 H2/CO) =1.5 MPa, T =90 °C, t =2 h; c: RhCl3·3H2O as the catalyst precursor, 2/Rh = 10, p (1:1 H2/CO) =5.0 MPa, T =85 °C, t =0.5 h; d: n.r.: not reported; e: Calculated based on data from the references in the last column of the Table 2. 5

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improves the atom economy and reduces the consumption of energy. Therefore, we believe the added separation processes of solvents are acceptable. We can anticipate that the homogeneous catalysis–liquid/ solid separation system could be applied in other homogeneous catalytic reactions.

[30]

[31]

Acknowledgments

[32]

We gratefully thank the support of this investigation by the National Natural Science Foundation of China (21576144) and the Natural Science Foundation of Shandong Province (ZR2014BM009).

[33]

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