Ethylene hydroformylation in imidazolium-based ionic liquids catalyzed by rhodium–phosphine complexes

Catalysis Today 200 (2013) 54–62

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Ethylene hydroformylation in imidazolium-based ionic liquids catalyzed by rhodium–phosphine complexes Yanyan Diao a , Jing Li a , Ling Wang a , Pu Yang a , Ruiyi Yan a , Li Jiang a,b , Heng Zhang a,c , Suojiang Zhang a,∗ a Beijing Key Laboratory of Ionic Liquids Clean Process, State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China b Graduate University of Chinese Academy of Sciences, Beijing 100049, PR China c School of Chemistry and Chemical Engineering, Qufu Normal University, Qufu 273165, PR China

a r t i c l e

i n f o

Article history: Received 30 March 2012 Received in revised form 26 June 2012 Accepted 29 June 2012 Available online 13 August 2012 Keywords: Hydroformylation Ionic liquids Ethylene Rh catalyst

a b s t r a c t In this research, the catalytic activity of a rhodium-based (Rh) catalyst with imidazolium-based ionic liquids (IBILs) as solvents for ethylene hydroformylation was studied. The structures of IBILs had an important influence on the activity and stability of the Rh catalyst. The IBILs with longer cation side chains, which were the strong steric hindrances around the Rh catalyst, were more unfavorable for the catalytic activity. The turnover frequency (TOF) of the Rh catalyst was 10627 h−1 when [Bmim][BF4 ] was used as solvent. The activity of the Rh complexes in the ionic liquid is better than they do in toluene. We used electrospray ionization mass spectrometry to characterize the catalyst after the reaction and found that [Bmim]+ acts as a ligand of the Rh catalyst to form a new active catalytic site [Rh(CO)(PPh3 )2 (Bmim)(BF4 )]+ through the coordination of the Rh atom with the imidazole-2-C group of [Bmim][BF4 ], and it was essential for the stabilization of the Rh catalyst and prevented the formation of low-active Rh clusters. In addition, the catalyst recycling test showed that the Rh catalyst could be reused with [Bmim][BF4 ] as solvent without obvious loss of catalytic activity. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The hydroformylation reaction (Scheme 1), which was discovered by Roelen in 1938, is a powerful method for functionalizing C C bonds to provide aldehyde and alcohol compounds. It is also an important industrial homogeneous catalytic process [1–3]. Thus, hydroformylation has attracted continuous and increasing academic and industrial interest. The general metal catalysts used in the hydroformylation of common olefins are rhodium (Rh) [4–8], cobalt [9,10] and platinum [11]. The supported metal catalysts have also been used in an attempt to combine the efficiency of a homogeneous catalyst with the practical advantages of a heterogeneous system [12,13]. Moreover, different reaction media have been employed for hydroformylation, such as normal organic solvents [14,15], aqueous media [16], and supercritical CO2 [17]. However, the unsatisfactory catalytic activity, high cost, low thermal stability of catalysts, and the requirement of toxic organic solvents (DMF or toluene), as well as the difficulty of separation are still the disadvantages of the process that need to be overcome. Ionic liquids have variable physicochemical characteristics, negligible vapor pressure, and structures that can be tailored to

∗ Corresponding author. Tel.: +86 10 8262 7080; fax: +86 10 8262 7080. E-mail address: [email protected] (S. Zhang). 0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cattod.2012.06.031

R CH=CH2 + CO + H2

Rh-Cat.

CH 3 R CH2 CH2CHO + R CHCHO

R=H, CH3, CnH 2n+1 Scheme 1. Hydroformylation reaction of olefins to produce aldehydes.

the reaction requirements, which make them possible alternatives to conventional organic solvents. The hydroformylation of olefins catalyzed by rhodium complexes in ionic liquids has been reported [18–22]. The first application of ionic liquids in hydroformylation was reported by Chauvin et al. [23]. They reported the biphasic hydroformylation of 1-pentene with (acetylacetonato)dicarbonylrhodium(I) [Rh(CO)2 acac] as catalyst in 1-butyl-3methylimidazolium hexafluorophosphate ([Bmim][PF6 ]). For the Rh-catalyzed hydroformylation of methyl-3-pentenoate, Wasserscheid [24] showed that the addition of an ionic liquid significantly increased the catalyst’s lifetime and overall productivity. The catalyst was stabilized by the non-volatile ionic liquid during the distillation process. However, no reports have been published on the analysis of the active Rh catalyst in ionic liquid solution, particularly the interaction between the ionic liquid and Rh catalyst. Electrospray ionization mass spectrometry (ESI-MS) is a technique that allows the ions in solution to be transferred into the gas phase, where they can be analyzed and eventually

Y. Diao et al. / Catalysis Today 200 (2013) 54–62

characterized with minimal fragmentation. Thus, the straightforward analytical applications of ESI-MS technique to the characterization of organometallic intermediates have caught the attention of chemists [25]. ESI-MS is also rapidly becoming the preferred technique for mechanistic studies and the high-throughput screening of homogeneous catalysis [26,27]. To the best of our knowledge, ionic liquids are mainly used as solvent in the biphasic hydroformylation of higher olefins to simple the separation process. To extend the previous work, we chose the Rh-catalyzed ethylene hydroformylation to demonstrate the great potential of ionic liquids as solvents for homogeneous catalysis even in cases where the reaction mixture is monophasic. The use of ionic liquids as solvent significantly enhanced the overall productivity and the catalyst’s lifetime. Imidazolium-based ionic liquids (IBILs), particularly the structures of the cations and anions, have an important influence on the activity and stability of the Rh catalyst. With [Bmim][BF4 ] as solvent, the effects of parameters, such as the reaction temperature, pressure, the amount of catalyst used, as well as the ratio of ligand to Rh catalyst, on ethylene hydroformylation were also investigated. The catalyst can be reused several times without additional regeneration and no obvious loss in activity and selectivity. Having recognized the pivotal importance of characterizing organometallic intermediates, we decided to take advantage of the ability of ESI-MS to transfer ionic species to the gas phase smoothly and to investigate the structure of the active Rh catalyst in ionic liquids solution and the interaction between Rh analyst and ionic liquids. We also aim to probe the effect of the ionic liquids on the activity and stability of the Rh catalyst. 2. Experimental 2.1. Reagents All reagents and gases used in this study were either of chemical or analytical grades. The rhodium chloride trihydrate (Sinopharm; 99%) was supplied by the Beijing Cuibolin Non-ferrous Metal Technology Development Center. The propionaldehyde (Sinopharm; >98%), triphenylphosphine (Sinopharm; CP; abbreviated as PPh3 ), 1-methylimidazole (Acros; 99%), 1-propylimidazole (Acros; 99%) and 1-butylimidazole (Acros; 99%) were produced by the Beijing Chemical Plant. The hydrogen (99.99%), carbon monoxide (99.9%), and ethylene (99.99%) were supplied by the Beijing Analytical Instrument Factory. All reagents were used as received. 2.2. Preparation of ionic liquids and catalyst 2.2.1. Preparation of ionic liquids A series of IBILs were prepared using standard methods [28–36]. Briefly, N-methylimidazole was reacted with a little excess of 1bromobutane in a round-bottom flask at room temperature for 5 h to produce [Bmim]Br. The halide-based ionic liquids were purified by recrystallization and dried overnight under vacuum conditions at room temperature. For [Bmim][BF4 ] or [Bmim][PF6 ], [Bmim]Br was ion-exchanged with equal mole of sodium tetrafluoroborate (NaBF4 ) or sodium hexafluorophosphate (NaPF6 ) in aqueous solution. The obtained IBILs were extracted using dichloromethane, and the amount of halide impurities in the ionic liquids were qualitatively determined by adding silver nitrate. Then IBILs were dried under vacuum conditions at 343 K for two to three days before use. 2.2.2. Preparation of catalyst The catalyst HRh(CO)(PPh)3 [Carbonylhydridotris (triphenylphosphine)rhodium(I)] was synthesized as follows (Scheme 2) [37]. The experiment was performed using standard Schlenk techniques under nitrogen atmosphere. A solution of

RhCl 3·H2O

55

PPh3

Rh(PPh3)3Cl

HCHO KOH 80ºC

HRh(CO)(PPh3)3

Scheme 2. Synthesis of the Rh catalyst.

0.053 g (2 mmol) rhodium trichloride 3-hydrate in 4 mL ethanol was added to a vigorously boiling solution of 0.53 g (20 mmol) triphenylphosphine in 20 mL of ethanol. After stirring for 15 s, an aqueous formaldehyde solution (2 mL, 40%, w/v solution) and a solution of 0.16 g potassium hydroxide in 20 mL hot ethanol were added rapidly and successively into the above mixture while stirring. The mixture was heated under reflux for 10 min and then cooled to room temperature. The bright yellow crystalline product was filtered, washed with ethanol and water, then were dried and kept in Schlenk bottle under vacuum conditions before use. 2.3. Characterization of ionic liquids and catalyst ESI-MS experiments were performed in positive ion mode on a Bruker micrOTOF-Q II mass spectrometer, with m/z from 50 to 2500. The infusion flow rate of 180 ␮L h−1 was maintained by a syringe pump. Basic vacuum ESI condition at 4.0 × 10−7 mbar was employed. The fourier transform infrared (FT-IR) spectra in the regions (400–4000 cm−1 ) were recorded via the KBr pellet technique using 0.5 mg of sample and 100 mg of KBr on a Thermo Nicolet 380 spectrometer (Thermo Electron Company). Elemental analysis was performed on a vario EL element analyser. 2.4. Hydroformylation procedure The experimental apparatu used for the both batch and recycling reaction is described in Scheme 3. Batch experiments were conducted in a flat-bottomed, magnetically stirred 150 mL steel autoclave equipped with a 120 mL teflon tube. The catalyst HRh(CO)(PPh3 )3 (0.0270 g, 0.03 mmol), and the ligand triphenylphosphine (0.0774 g, 0.3 mmol; Rh:PPh3 = 1:10) were dissolved in a solvent of propionaldehyde (5 mL) and IBILs (5 mL), and then added into the 120 mL teflon tube in the stainless steel autoclave. The reactor was vacuumed first, then purged with feed gas (C2 H4 :CO:H2 = 1:1:1), and finally, placed under continuous pressure of 2 MPa. After attaining 2 MPa pressure, the reactor was heated to 100 ◦ C using an oil bath and magnetically stirred at 800 rpm. The reaction was performed at a constant pressure of 2 MPa by feeding mixed gas. The total amount of feed gas was measured using a mass flow meter. The reaction was conducted for 2 h and before it was allowed to cool to room temperature, and then, the excess gas was slowly vented. The reaction mixtures were analyzed via GC (Agilent 6890, equipped with a DB-624 capillary column and an FID detector) and GC–MS. The catalytic activities were expressed in terms of ethylene conversion (C, %), propionaldehyde selectivity (S, %), and turnover frequency (TOF, h−1 ), which were determined via GC using n-hexane as internal standard. These parameters are defined as: C=

Q1 − Q2 × 100% Q1

S=

22400 · np × 100% Q1 − Q2

TOF =

np ncat · t

where Q1 is the volume of ethylene into the reactor (mL); Q2 is the volume of ethylene unreacted (mL); np is the mole number of propionaldehyde produced during reaction (mol); TOF is the Turnover frequency (h−1 ); t is the Reaction time (h); ncat is the mole number of catalyst added in the reaction (mol). The mole number of gas was calculated using standard conditions. Recycling experiments were similar to batch experiments. When the reaction was finished after 2 h and cooled to room temperature, the vacuum pump was opened and propionaldehyde product was collected in a cooling batch. The catalyst HRh(CO)(PPh3 )3 ,

56

Y. Diao et al. / Catalysis Today 200 (2013) 54–62

HRh(CO)( PPh3) 3

694.7

Intens. x10 6

+MS, 5.0-12.1s #(4-11) 655.0785

Transmittance (%)

2036.3

0.8

1477.2 1086.6

1921.0 1432.1

742.4

513.6

PPh3

0.6 0.4 0.2

917.1712

279.0943

0.0 500 3500

3000

2500

2000

1500

1000

500

1000

1500

m/z

Fig. 2. ESI-MS analysis of catalyst HRh(CO)(PPh3 )3 .

Wavenumber(cm-1) Fig. 1. FT-IR spectra of PPh3 and HRh(CO)(PPh3 )3 .

ionic liquids, PPh3 and part of the propionaldehyde remained in the autoclave, to which fresh feeding gas was added. Then, a new catalytic reaction was started. The same procedure was repeated for several cycles. The amount of feed gas was measured using a mass flow meter in every reaction, and the catalytic activity of HRh(CO)(PPh3 )3 in every reaction was expressed by the amount of feed gas. 3. Results and discussion 3.1. Characterization of catalyst To confirm the structure of the Rh catalyst, FT-IR spectroscopic studies were conducted. Fig. 1 shows the IR spectra of the catalyst and triphenylphosphine. The spectrum of the catalyst displayed a very strong peak corresponding to the CO stretching frequency centered at about 1921.0 cm−1 , a typical strong peak corresponding to

the Rh-H stretching frequency centered at about 2036.3 cm−1 [37], and nine typical peaks centered at 3449.5, 3053.6, 1583.7, 1477.2, 1432.1, 1086.6, 742.4, 694.7, and 513.6 cm−1 associated with the stretching and bending frequencies of triphenylphosphine. The theoretical contents of different elements of the HRh(CO)(PPh3 )3 catalyst were 71.90% C, 5.05% H, 1.74% O, meanwhile, the experimental results of the element analysis were 71.80% C, 5.12% H, 1.83% O. From the results of the FT-IR and element analysis, the structure of the synthesized catalyst was HRh(CO)(PPh3 )3 . In methanol solution, the structure of HRh(CO)(PPh3 )3 was analyzed via ESI-MS in positive ion mode (Fig. 2). The main peak at 655.0785, is attributed to the active species [Rh(CO)(PPh3 )2 ]+ , and the peak at 917.1712, is attributed to the active species [Rh(CO)(PPh3 )3 ]+ . Fig. 3 shows the comparison of the experimental isotopic distribution of the mixture and the theoretical isotopic distribution as simulated by the Bruker Xmass 6. 1. 2 software for the peak at 655.0785 and 917.1712. The appearance of [Rh(CO)(PPh3 )2 ]+ was mainly caused by the dissociation of [Rh(CO)(PPh3 )3 ]+ in the solution, as shown in Scheme 4.

Scheme 3. Experimental apparatus used for both batch and recycling hydroformylation.

Y. Diao et al. / Catalysis Today 200 (2013) 54–62 Intens. x106

+MS, 7.0-11.0s #(6-10) 655.0785

+MS, 5.0-12.1s #(4-11)

917.1712

Experomental

0.8 0.6

Intens. x105

57

Experomental 918.1751

2

656.0835

0.4

1

0.2

919.1768

657.0870

0.0 x106

C 37 H 30 O P 2 Rh ,655.08

655.0821

3

Theoretical

0.8

0 x105

0.6

920.1787 917.1733

2

0.4

C 55 H 45 O P 3 Rh ,917.17

Theoretical 918.1766

656.0855

1

0.2

919.1800

657.0889

920.1833

0

0.0 655

656

657

658

m/z

917

918

919

920

921

m/z

Fig. 3. Comparison of experimental isotopic distribution and theoretical isotopic distribution simulated by Bruker Xmass 6.1.2 software at the peaks of 655.0785 and 917.1712.

PPh3

[Rh(CO)(PPh3)2]+

[Rh(CO)(PPh3)3]+

Scheme 4. The equilibrium of [Rh(CO)(PPh3 )3 ]+ and [Rh(CO)(PPh3 )2 ]+ in methanol solvent.

3.2. Activities of Rh catalyst with various ionic liquids as solvents on ethylene hydroformylation The activities of the HRh(CO)(PPh3 )3 catalyst in different solvents during ethylene hydroformylation were examined. The corresponding results are presented in Table 1. The catalytic activity of the Rh catalyst in the [Bmim][BF4 ] solution (Table 1, entry 3) was better than that in the traditional toluene and propionaldehyde solutions (Table 1, entries 1and 2). Table 1 showed that the structures of the ionic liquids had a significant influence on the activity of the Rh catalyst during ethylene hydroformylation. Based on the activity of the Rh catalyst with [Bmim][BF4 ], [Hmim][BF4 ] and [Omim][BF4 ] as solvents, the carbon numbers of the chain in the N1 position (the mark of C and N in the imidazole ring of ionic liquids as shown in Scheme 5) of the imidazolium-based cation increased from 4 to 8, the TOF of the catalyst decreased from 10627 h−1 to 1123 h−1 . This result indicates that the lengthening of the Table 1 Effect of various ionic liquids as solvent for ethylene hydroformylation.a Entry

Solvents

1 2 3 4 5 6 7 8 9 10 11 12

Toluene Propionaldehyde [Bmim][BF4 ] [Hmim][BF4 ] [Omim][BF4 ] [Bpim][BF4 ] [Bbim][BF4 ] [Bmim][PF6 ] [Bmim][OTs] [Bmim][OAc] [Bmim][SCN] [Bmim][HSO4 ]

Conversion (%) 98.90 97.64 98.19 95.22 71.41 97.18 97.12 96.92 –b – – –

Selectivity (%) 99.10 77.80 98.14 94.79 92.64 92.32 92.26 92.85 – – – –

TOF (h−1 )

9856 7558 10627 5411 1123 8450 8237 7912 – – – –

a Conditions: 3 mmol L−1 , HRh(CO)(PPh3 )3 , Rh: PPh3 = 1:10, 100 ◦ C, 2.0 MPa, C2 H4 : H2 : CO = 1: 1: 1, 800 rpm, 2 h. b No reaction, [Bmim] = 1-butyl-3-methylimidazolium; [Hmim] = 1[Omim] = 1-octyl-3-methylimidazolium; hexyl-3-methylimidazolium; [Bpim] = 1-butyl-3-propylimidazolium; [Bbim] = 1-butyl-3- butylimidazolium; [OTs] = Tosylate; [OAc] = Acetate.

carbon chain length in the N1 position have adverse effects on catalytic activity. Similarly, when [Bmim][BF4 ], [Bpim][BF4 ] and [Bbim][BF4 ] were used as solvents, the length of carbon chains in the N3 position of the imidazolium cation increased from 1 to 4, the TOF of the catalyst decreased from 10627 h−1 to 8237 h−1 . This result shows that the increased lengths of the chains in the N1 or N3 position could more or less reduce the catalytic activity of the Rh catalyst. The anions of the ionic liquids also had an important influence on the activity of the Rh catalyst. The TOF of 7912 h−1 in the [Bmim][PF6 ] solution was lower than that in the [Bmim][BF4 ] solution. Some reports [38,39] on the hydroformylation of olefins in [Bmim][PF6 ] have shown that the PF6 − group could be dissociated weakly, which is possibly why the TOF of the Rh catalyst in the [Bmim][PF6 ] solution was slightly lower than that in the [Bmim][BF4 ] solution. The Rh catalyst had no activity in the [Bmim][OTs], [Bmim][OAc], [Bmim][SCN] and [Bmim][HSO4 ] solutions. 3.3. Analysis of Rh catalyst after ethylene hydroformylation by ESI-MS After ethylene hydroformylation, the existential state of the Rh catalyst in toluene and ionic liquid solution were all analyzed by ESI-MS. As shown in Fig. 4, there are four typical peaks in toluene centered at 627.1042, 655.0925, 889.1886, and 917.1809, which could be attributed to the active catalytic sites [Rh(PPh3 )2 ]+ , [Rh(CO)(PPh3 )2 ]+ , [Rh(PPh3 )3 ]+ , and [Rh(CO)(PPh3 )3 ]+ respectively. Scheme 6 shows that these species are in equilibriums between

H (2)C

Hm+2Cm (3)N

(4)C H

+

N(1)

CnHn+2

C(5) H

Scheme 5. The structure of imidazolium-based cation and the mark of C and N in the imidazole ring.

58

Y. Diao et al. / Catalysis Today 200 (2013) 54–62

Fig. 4. ESI-MS catalysis of active Rh catalyst structure in toluene solution after ethylene hydroformylation.

HRh(PPh3)2

CO

HRh(CO)(PPh3)2

PPh3

HRh(PPh3)3

PPh3 CO

HRh(CO)(PPh3)3

Scheme 6. The equilibrium of Rh catalyst on ethylene hydroformylation.

them, and they are all active catalysts. As shown in Fig. 5 and Fig. 6, the Rh catalyst also existed as several states in the [Bmim][BF4 ], notably at the peak of 793.1966, which was attributed to the new active catalytic site [Rh(Bmim)(CO)(PPh3 )2 ]+ , and at the peak of 365.2457, 591.3706, 817.5091, and 1043.6361, which could be attributed to the clusters of [(Bmim)2 (BF4 )]+ , [(Bmim)3 (BF4 )2 ]+ , [(Bmim)4 (BF4 )3 ]+ , and [(Bmim)5 (BF4 )4 ]+ respectively. Based on the analysis, [Bmim]+ acts as a ligand of the Rh catalyst to form a new active catalytic site for hydroformylation. In addition, previous studies [20] have reported on the analysis of imidazole-based heterocyclic fragment ionic liquids used in the rhodium catalyzed hydroformylation process via 1 H NMR. The H atoms of imidazole-2 are unstable, and can easily release free H+ . The remaining imidazole ring constitutes a heterocyclic carbine Intens. x104 1.5

+MS, 4.2-9.2s #(4-9) 365.2507

Experomental

Fig. 5. ESI-MS catalysis of active Rh catalyst structure in [Bmim][BF4 ] after ethylene hydroformylation.

with a pair of electrons that can be used as ligands of the Rh catalyst. In the Figs. 7 and 8, in the [Omim][BF4 ], [Omim]+ can also act as a ligand of the Rh catalyst to form a new active catalytic site [Rh(Omim)(CO)(PPh3 )2 ]+ at 849.2590. Another active catalytic site [Rh(CO)(PPh3 )2 ]+ was also observed at 655.0823. The peak at 477.3761 is attributed to the cluster of [(Omim)2 (BF4 )]+ . In the catalytic reaction, the acyl-rhodium complex reacts with di-hydrogen is a rate-limiting step. The cations of the IBILs can be Intens.

Experomental

2000 1500

1.0

794.2002

1000 0.5

364.2536

500

366.2527

0.0 x104 1.50

C 16 H 30 B F 4 N 4 ,365.25

1.25

Theoretical

365.2497

C 45 H 44 N 2 O P 2 Rh ,793.20 793.1978

Theoretical

1500

0.75

794.2012

1000

0.50

0.00 363

795.2032

0 2500 2000

1.00

0.25

+MS, 4.2-9.2s #(4-9)

793.1969

364.2530

500

366.2527

795.2045

0 364

365

366

367

m/z

792

793

794

795

796

797

m/z

Fig. 6. Comparison of experimental isotopic distribution and theoretical isotopic distribution simulated by Bruker Xmass 6.1.2 software at the peaks of 365.2507 and 793.1969.

Y. Diao et al. / Catalysis Today 200 (2013) 54–62

Intens. x105 1.25

h−1 to about 1.0 × 104 h−1 . As Rh concentration further increased from 3.0 mmol L−1 to 3.5 mmol L−1 , the TOF and conversion rate remained almost constant. Within the range of Rh concentrations, the selectivity remained almost constant. Therefore, the suitable Rh concentration was set to 3.0 mmol L−1 .

+MS, 4.2-10.2s #(4-10) 477.3761

1.00 0.75

849.2590

0.50

655.0823 0.25 849.2590

0.00 400

600

800

1000

1200 m/z

Fig. 7. ESI-MS analysis of active Rh catalyst structure in [Omim][BF4 ] after catalytic reaction.

associated with the [Rh-H] species [40,41]. These associations made the [Rh-H] species more active and enhanced their reactions with dihydrogen, thus increasing the catalytic activity. However, the experimental results showed that an increase of the carbon chain length, regardless of whether it is in the N1 or N3 position, would lead to a decline in catalytic activity. This finding indicates that the imidazolium cation around the rhodium center speeds up the reaction of Rh-H. Meanwhile, the steric effect of the imidazolium cation also has an effect on the association and dissociation of the Rh–PPh3 complex. 3.4. Influence of reaction conditions on ethylene hydroformylation in [Bmim][BF4 ] The reaction conditions generally have important influence on the activity of catalyst in the homogeneous reaction. Thus, we investigated the TOF of Rh catalyst on the different reaction conditions (catalyst concentration, PPh3 /Rh ratio, temperature and pressure) using [Bmim][BF4 ] as solvent. 3.4.1. Effect of Rh concentration Fig. 9 shows the effect of a series of Rh concentration on ethylene hydroformylation. As the Rh concentration increased from 1.0 mmol L−1 to 3.0 mmol L−1 , the number of active rhodium centers also increased and the TOF steadily increased from 2000 Intens. x10 5

+MS, 4.2-10.2s #(4-10) 477.3761

1.25

Experomental

3.4.2. Effect of PPh3 /Rh ratio Fig. 10 shows the conversion, selectivity and TOF of a series of PPh3 /Rh ratios on ethylene hydroformylation. The PPh3 /Rh ratio was found to have an important effect on catalyst activity. The lowest catalytic activity was observed with the lowest PPh3 /Rh ratio in the test range. As the PPh3 /Rh ratio increased from 1 to 70, the TOF increased from 2000 h−1 to about 1.0×104 h−1 . In addition, the catalytic activity also increased; whereas the selectivity was remained above 96% in all test PPh3 /Rh ratios. As the PPh3 /Rh ratio continued to increase to 90, the change in TOF was not obvious. Therefore, the PPh3 /Rh ratio of 70 was chosen as the most appropriate ratio in this test range. 3.4.3. Effect of temperature The Effect of temperature on ethylene hydroformylation is shown in Fig. 11. The reaction was very sensitive to temperature. As the temperature increased from 80 ◦ C to 110 ◦ C, the TOF increased remarkably, from below 4000 h−1 to about 1.3 × 104 h−1 . As the temperature further increased from 110 ◦ C up to 130 ◦ C, the TOF decreased to 7000 h−1 , As the temperature increased from 80 ◦ C to 110 ◦ C, the increasing of thermal energy makes the molecules of materials move faster, and increase the collision probability of molecules and catalysts, thus enhance catalytic activity. However, at temperatures over 110 ◦ C, the rhodium complex began to become unstable. Thus the catalytic activity continued to decline as the temperature kept rising. The decline in selectivity began at 100 ◦ C rather than at 110 ◦ C. This phenomenon showed that a great number of side reactions were produced when temperature increased to above 100 ◦ C. Hence, the most appropriate reaction temperature in the test range should be 100 ◦ C. 3.4.4. Effect of pressure Fig. 12 presents the conversion, selectivity and TOF of a series of pressure conditions on ethylene hydroformylation. From 0.5 MPa to 1.5 MPa, the TOF rapidly increased from 1500 h−1 to about 11500 h−1 and then moderately decreased at pressures ranging from 1.5 MPa to 2.5 MPa until the TOF became constant. The conversion Intens.

0.75

3000

850.2627

2000

0.50 478.3783

476.3786

1000 479.3798

0.00 x10 5

C 24 H 46 B F 4 N 4 ,477.38 477.3751

1.25

Theoretical

6000

4000 3000 476.3782

477

478

Theoretical

850.2638

2000

478.3780

479

480

851.2671

1000

479.3813 476

C 49 H 52 N 2 O P 2 Rh ,849.26 849.2604

5000

0.75 0.50

851.2633

0

1.00

0.00

Experomental

5000 4000

0.25

+MS, 4.2-10.2s #(4-10) 849.2590

1.00

0.25

59

m/z

0

852.2705 849

850

851

852

853 m/z

Fig. 8. Comparison of experimental isotopic distribution of and theoretical isotopic distribution simulated by Bruker Xmass 6.1.2 software at the peaks of 477.3761and 849.2590.

Y. Diao et al. / Catalysis Today 200 (2013) 54–62

1.2x104

90

1.0x104

80

8.0x103

-1

100

TOF / h

Conversion & Selectivity /%

60

Conversion Selectivity

70

4.0x103

60 50

6.0x103

2.0x103

1.0 1.5 2.0 2.5 3.0 Rh Concentration /( mmol·L-1 )

3.5

1.0

1.5

2.0

2.5

3.0

Rh Concentration /(mmol· L-1)

3.5

Conditions: PPh3/Rh = 10, 100 °C, 2.0 MPa, C2H4: H2: CO= 1: 1: 1, 2 h. Fig. 9. Effect of Rh concentration on ethylene hydroformylation. 4

2.0x10

95

4

1.5x10 -1

90

TOF / h

Conversion & Selectivity /%

100

3

5.0x10

80 75

4

1.0x10

Conversion Selectivity

85

1

5

10

30 50 PPh3/Rh

70

0.0

90

1

5

10

30 50 PPh3/Rh

70

90

Conditions: 3 mmol·L-1 HRh(CO)(PPh3)3, 100 °C, 2.0 MPa, C2H4: H2: CO= 1: 1: 1, 2 h. Fig. 10. Effect of PPh3 /Rh ratio on ethylene hydroformylation.

3.5. Recycling experiments of catalyst Using ethylene, CO and H2 as substrates, we conducted recycling experiments to examine the recyclability of HRh(CO)(PPh3 )3 in the [Bmim][BF4 ] solution using the similar industrial production conditions of toluene as solvent (100 ◦ C and 2.0 MPa). The catalytic activity of HRh(CO)(PPh3 )3 was expressed by the amount of feed gas. As shown in Fig. 13, no obvious decrease in the gas flow for six repeated runs indicated the high stability of the Rh catalyst. In addition, the ESI-MS analysis results (Fig. 14) proved that the Rh catalyst in the [Bmim][BF4 ] solution did not change after six runs and the active catalytic sites of catalyst were [Rh(CO)(PPh3 )2 ]+ (655.0818) and [Rh(Bmim)(CO)(PPh3 )2 ]+ (793.1871).

100

1.4x10

98

1.2x10

96

1.0x10

4

-1

4 4

TOF / h

Conversion & Selectivity /%

and selectivity both remained above 95% in all conditions of pressure, and the highest conversion rate and selectivity were observed at 1.5 MPa. From 0.5 MPa to 1.5 MPa, the increased feed gas pressure caused the increased solubility of the gas in the liquid phase, which strengthened the mass transfer between the gas-liquid states and thus enhanced the TOF of the catalyst. From 1.5 MPa to 2.5 MPa, the decrease in TOF was caused by the competition of excess CO and PPh3 in coordination with the Rh catalyst. When the pressure was higher than 2.5 MPa, a delicate balance was established between the feed gas concentrations in the liquid phase and the competitive coordination, leading to the observed stable catalytic activity. Therefore, the pressure of 1.5 MPa would be the most suitable choice for the system.

3

8.0x10

94

3

Conversion Selectivity

92

6.0x10

3

4.0x10

90

3

80

90 100 110 Temperature /ºC

120

130

2.0x10

80

90 100 110 120 Temperature /ºC

130

Conditions: 3 mmol·L -1 HRh(CO)(PPh3)3, PPh3/Rh = 10, 2.0 MPa, C2H4: H2: CO= 1: 1: 1, 2 h. Fig. 11. Effect of temperature on ethylene hydroformylation.

Y. Diao et al. / Catalysis Today 200 (2013) 54–62

4

1.2x10

4

1.0x10

95 -1

3

TOF /h

Conversion & Selectivity /%

100

61

Conversion Selectivity

90

8.0x10

3

6.0x10

3

4.0x10

3

2.0x10

85 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Pressure /MPa

Conditions: 3 mmol·L

-1

0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Pressure / MPa

HRh(CO)(PPh3)3, PPh3/Rh = 10, 100 °C, C2H4: H2: CO= 1: 1: 1, 2 h.

Fig. 12. Effect of pressure on ethylene hydroformylation.

interacts as a ligand with Rh catalyst to form a new active catalytic site for hydroformylation, which is essential for the stabilization of the Rh catalyst and prevention of the formation of low active Rh clusters. Thus, a high TOF (10627 h−1 ) could be achieved under mild conditions. In addition, the catalyst recycling test showed that the Rh catalyst could be reused with [Bmim][BF4 ] as solvent without obvious loss of catalytic activity. The organic green process presented here could have potential applications in industry due to its easy product separation and high efficiency.

20000

Q / mL

15000

10000

5000

Acknowledgements

0

1

2

3

4

5

6

Recycle times Conditions: PPh3/Rh = 10, 100 °C, 2.0 MPa, C2H4: H2: CO= 1: 1: 1, 2 h.

This work is financially supported by General Program Youth of National Natural Science Foundation of China (21006106), National Basic Research Program of China (973 Program) (2009CB219901), and Key Program of National Natural Science Foundation of China (21036007).

Fig. 13. Recycling experiments of ethylene hydroformylation in [Bmim]BF4 .

References Intens. x105

+MS, 5.2-8.2s #(5-8)

365.2538

5

655.0818

4 3

793.1871

2

591.3795 1043.6382 1269.75811494.8915

1 0 400

600

800

1000

1200

1400

m/z

Fig. 14. The ESI-MS analysis of active Rh catalyst structure in [Bmim][BF4 ] after recycling reaction.

4. Conclusions IBILs, which are used as solvents on ethylene hydroformylation, have an important influence on the activity and stability of the Rh catalyst. The increased length of the chains in the N1 or N3 position of the imidazole ring reduced the catalytic activity of the catalyst, and the anions of ionic liquids also had significant influence on the activity of the catalyst. In the [Bmim][BF4 ] solution, [Bmim]+

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