Recycle and Recovery of Rhodium Complexes with Water-Soluble and Amphiphilic Phosphines in Ionic Liquids for Hydroformylation of 1-Hexene

Jointly published by React.Kinet.Catal.Lett. Akadémiai Kiadó, Budapest Vol. 90, No. 1, 53−60 (2007) and Springer, Dordrecht 10.1007/s11144-007-4975-x

RKCL4975

RECYCLE AND RECOVERY OF RHODIUM COMPLEXES WITH WATER-SOLUBLE AND AMPHIPHILIC PHOSPHINES IN IONIC LIQUIDS FOR HYDROFORMYLATION OF 1-HEXENE

Qingrong Peng, Changxi Deng, Yong Yang, Maohua Dai and Youzhu Yuan* State Key Laboratory of Physical Chemistry of Solid Surfaces, and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China

Received April 10, 2006, in revised form June 29, 2006, accepted July 7, 2006

Abstract A biphasic catalysis system composed of ionic liquid and rhodium complexes with water-soluble or amphiphilic phosphine ligands bearing water-soluble groups of sodium sulfonate have been employed for hydroformylation of 1-hexene. The experimental results show that the activity is almost independent of the hydrotropicity of the phosphine ligands in BMI⋅BF4. In this system, the extraction of phosphine species by the organics from the IL phase was quite low but larger than that of rhodium species and showed rather good stability of catalytic activity. A slight decrease in the aldehyde n/i ratio during the catalyst reuse could be recovered, in part, by replenishing certain amount of ligand into the used catalyst system.

Keywords: Hydroformylation, 1-hexene, ionic liquid, rhodium, biphasic catalysis

INTRODUCTION

Ionic liquids (ILs) have attracted attention as green and desirable solvents. They are categorized as fused salts at room temperature and their applications to ______* Corresponding author. Fax: +86-592-2183047; E-mail: [email protected]

54 QINGRONG PENG et al.: HYDROFORMYLATION organic synthesis as alternative reaction media were extensively studied in recent years. In particular, based on their highly charged nature, the IL-phases are ideal for biphasic reactions in organic synthesis and catalysis. A number of papers have described the improvements obtained by substituting water or organic solvent with ILs as solvent for immobilization of organometallic catalysts, resulting in improved catalytic performance and easy catalyst-product separation [1]. An example of the method is the rhodium-catalyzed hydroformylation of olefins using the [Rh(acac)(CO)2]/PPh3 system in ILs such as BMI⋅PF6 and BMI⋅BF4 (BMI = 1-n-butyl-3-methylimidazolium) [2]. The catalyst system showed good performance in the hydroformylation of 1-pentene, although part of the active rhodium catalyst was extracted into the organic phase. Polar ligands such as TPPTS [TPPTS = trisodium salt of tri-(m-sulfophenyl)-phosphine, P(m-C6H4SO3Na)3] and TPPMS [TPPMS = mono-sulfonated triphenylphosphine, P(C6H5)2(m-C6H4SO3Na)] were used to modify the uncharged [Rh(acac)(CO)2] in ILs for olefin hydroformylation. Favre et al. found that the problem of Rh leaching could be minimized by the modification of phosphorus ligands with cationic (guanidinium or pyridinium) or anionic (sulfonate) groups [3]. An easy phase separation has been reported to be viable between the reactant-containing organic phase and the water-soluble phosphine-rhodium complexes-containing IL phase after the hydroformylation [4-8]. It is well documented that the rhodium catalyst ligated with the water-soluble TPPTS ligand exhibits low activity in the hydroformylation of higher olefins because of mass transfer limitations resulting from their low water solubility [9]. Although less well studied than the traditional biphasic approach, recently the amphiphilic concept has attained some importance and a number of amphiphilic phosphines have been synthesized and evaluated in catalysis [10]. Previously, we have reported that the catalytic performances of rhodium complexes with three amphiphilic phosphine ligands, bis-(3-sodium sulfonatophenyl)-(4-tert-butylphenyl)-phosphine (1), phenyl-(3-sodium sulfonatophenyl)-(4-tert-butyl-phenyl)-phosphine (2), and bis-(4-tert-butylphenyl)- (3-sodium sulfonatophenyl) phosphine (3), in hydroformylation of 1-hexene, 1-octene and 1-dodecene show significant enhancements in the reaction rate and higher selectivities towards the normal aldehydes in comparison with those obtained with TPPTS- and TPPDS-rhodium complexes under identical conditions [11]. However, only the catalyst generated from 1 could be recovered many times without significant losses in the catalytic performance. The catalyst based on 3 lost its original activity and selectivity and that based on 2 showed a quick drop in the catalytic activity and a stable selectivity during the recycling, indicating that the amphiphilic phosphines 2 and 3 could not retain rhodium quantitatively in the aqueous phase. The problems of catalyst separation, recycling and mass transfer limitation encountered in aqueous-organic biphasic catalysis systems are inevitably involved systems consisting of IL and organic reactants. But the studies on the QINGRONG PENG et al.: HYDROFORMYLATION 55 metal leaching and the recyclability of the rhodium complexes associated with the IL media for hydroformylation are relatively rare [8]. Herein we report the hydroformylation performance of rhodium complexes of water-soluble phosphine TPPTS and amphiphilic phosphines 1, 2 and 3 dissolved in 1-n-butyl-3-methylimidazolium with different counterions. The objective of the study is to explore catalyst-reactant phase separation and catalyst recycling associated with the IL-organic biphasic system for the hydroformylation of 1-hexene.

SO3Na SO3Na SO3Na P P P 2 2

1 3 2

EXPERIMENTAL

Ligands and rhodium complexes

All ligand syntheses were carried out with standard Schlenk techniques under argon atmosphere and reported elsewhere [11]. The ILs, such as BMI⋅BF4, BMI⋅PF6 and BMI⋅n-C12H25OSO3 were synthesized using the procedures reported in the literature [12-14]. The water content of BMI⋅BF4, BMI⋅PF6 and BMI⋅n-C12H25OSO3 was less than 1 wt.%.

Table 1

Evaluation of 1-hexene hydroformylation in BMI⋅BF4 medium

Ligand Recycling Conv. n/i TOF Content in organic phase (ppb)

number −1 (%) (h ) Rh phosphorus

TPPTS 0 45.9 3.6 92 10.2 48.5 1 62.2 3.2 124 9.8 65.5 2 61.5 2.6 123 4.0 31.6 1 0 64.5 3.9 129 65.7 337.3 1 65.0 3.5 130 49.5 215.7 2 66.5 3.0 133 23.0 105.4

Reaction conditions: [Rh(acac)(CO)2], 1−hexene/Rh = 800 (molar ratio), L/Rh = 5, CO/H2 (molar ratio) = 1, P (CO/H2) = 1.5 MPa, T = 373 K, 1−hexene = 2 mL (0.0161 mmol), IL = 4 mL, reaction time = 4 h, agitation speed = 800 rpm 56 QINGRONG PENG et al.: HYDROFORMYLATION

The structures of phosphines, corresponding rhodium complexes and ILs were investigated by FT-IR, 1H- and 31P-NMR spectroscopy (if any). FT-IR spectra were measured in a Nicolet 740 FTIR spectrometer with a resolution of 4 cm-1. NMR spectra was recorded on a Varian FT Unity+ 500 spectrometer. 31 P-NMR spectra were recorded at 202 MHz at room temperature in CDCl3 for organic-soluble compounds and in D2O for water-soluble ones. The chemical 1 shift was referenced to 85% H3PO4. H-NMR spectra were referenced to SiMe4. The contents of phosphorus and rhodium in the organic phase were determined by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) on an Agilent HP4500 after each reaction. The sample was treated by evaporation to remove organics and then with 5 mL of aqua regia at 363 K for 5-10 min, followed by heating at 363 K for 5-10 min. The resulting solution was diluted with water to 25 mL for measurement.

Table 2 Effect of different ligands in different ionic liquids

Solvent Ligand Conversion (%) n/i TOF (h−1)

BMI⋅BF4 PPh3 90.1 2.3 180 TPPTS 45.9 3.6 92 1 64.5 3.9 129 2 84.4 3.4 169 3 58.5 2.9 117

BMI⋅PF6 PPh3 57.9 3.6 116 TPPTS 3.7 2.7 7 TPPDS 9.4 2.4 19 1 60.2 3.4 120 2 87.3 3.8 174 3 84.8 3.0 170

BMI⋅n-C12H25OSO3 PPh3 82.6 2.5 165 TPPTS 86.6 3.5 173 TPPDS 81.5 2.7 164 1 74.3 3.8 168 2 83.7 3.1 148 3 81.6 2.6 167

Reaction conditions are the same as in Table 1

QINGRONG PENG et al.: HYDROFORMYLATION 57

Hydroformylation

The typical olefin hydroformylation was carried out in a stainless steel autoclave of 60 mL with a magnetic stirrer. After the rhodium complex precursor, ligand, IL and 1-hexene were placed in the autoclave, the reactor was pressurized three times with 1.0 MPa of CO/H2 (1/1, molar ratio). Then the autoclave was pressurized with the same gas mixture at the desired pressure, and heated to the reaction temperature. After the reaction, the reactor was cooled to room temperature and decompressed. Finally the liquids and the catalysts were separated by decantation. The organic phase was analyzed with a gas chromatograph equipped with FID and a capillary column (SE-30, 30 m × 0.32 mm × 0.25 μm).

RESULTS AND DISCUSSION

Phase separation and leaching of phosphine and rhodium species

The Rh-precursor [Rh(acac)(CO)2] was readily dissolved in the ILs such as BMI⋅BF4 and BMI⋅PF6. The water-soluble ligands showed low solubility in those ILs at room temperature. After the reaction, however, a red to brown homogeneous solution containing the IL, Rh-complex with water-soluble phosphine could be obtained, which stayed in the lower phase (IL phase) immiscible with the upper phase of organic reactants. The phase separation was easy and there was no formation of emulsions. To investigate the losses of phosphorus and rhodium from the IL phase, we have measured the contents of phosphorus and rhodium in the organic phase by the ICP-MS after each reaction. Table 1 shows the hydroformylation performance of 1-hexene with Rh-complexes of TPPTS and 1 and the leaching of rhodium and phosphorus from the IL phase into the organic phase. The reaction activities were similar to each other, but the concentrations of rhodium and phosphorus in the product phase using TPPTS-Rh-BMI⋅BF4 system were almost one magnitude lower than those using amphiphilic 1-Rh-BMI⋅BF4 under identical conditions. This might be due to the hydrophobic/hydrophilic property of the ligands. The extraction of Rh and phosphorus species by the reactant from the IL phase was quite low. Using ligand 1 and TPPTS, the ratio of extraction phosphorus/Rh was close to the original L/Rh = 5 (in cycles 0-2). A higher n/i ratio in the case of 1-Rh-BMI⋅BF4 revealed that the larger cone angle of 1 over TPPTS was effective in the IL biphasic catalysis system. The loss of catalytic activity was negligible but a slight decrease in n/i ratio was observed during the catalyst recycling with both complexes.

58 QINGRONG PENG et al.: HYDROFORMYLATION

Variation of ligands and ILs

Table 2 lists the hydroformylation of 1-hexene in the ILs using [Rh(acac)(CO)2] as catalyst precursor and phosphines 1, 2, 3, TPPTS and TPPDS as ligands, respectively. All the reactions were carried out under the same conditions with L/Rh = 5 (mol/mol) and olefin/Rh = 800 (mol/mol). The properties of ILs and ligands showed considerable influence on the catalyst performance. When using BMI⋅n-C12H25OSO3 as the reaction medium, no matter whether the Rh complexes of organic-soluble or aqueous or amphiphilic ligands were used, one phase was formed after the reaction and there was no significant difference in the reaction rate of 1-hexene hydroformylation. This was not the case when using BMI⋅BF4 or BMI⋅PF6 as the reaction media. With PPh3-Rh complex, the reaction rate was higher than that with the Rh complexes of aqueous and amphiphilic ligands, while no phase separation was available after the reaction. Higher reaction rates were obtained, forming a biphasic catalysis system after the reaction, with the Rh-complexes of amphiphilic 1, 2 and 3 both in BMI⋅PF6 and BMI⋅BF4. The reaction rate with the Rh complexes of TPPTS or TPPDS in BMI⋅PF6, however, was considerably lower compared with that in BMI⋅BF4. The results indicated that the Rh complexes of TPPTS and TPPDS ligands with hydrotropic property might not be suitable for the hydroformylation of 1-hexene in the media of hydrophobic ionic liquid BMI⋅PF6.

Recycling use of water-soluble/amphiphilic phosphine rhodium complexes in BIM⋅BF4

The recycling use of the catalysts based on TPPTS, 1, 2, and 3 was examined by performing a series of consecutive runs in BMI⋅BF4. The results are shown in Figs 1A-D. We previously found that the catalysts based on phosphines 2 and 3 gradually lost their original activity and selectivity from the first to sixth run in biphasic system [11], thus the high activity observed with this phosphine was mainly due to the presence of catalyst in the organic phase. The catalysts generated from the phosphines TPPTS, 1, 2 and 3 in the BMI⋅BF4 system were * relatively stable in activity. A small decrease in 1-hexene conversion could be due to the loss of catalyst during decantation and no addition of any fresh catalyst to keep an identical amount of catalysts used. There existed a continuing decrease in aldehyde n/i ratio when using the amphiphilic phosphines 1, 2 and 3 as ligands during the recycling, which is probably connected with the leaching of phosphorus species into the organic phase as shown in Table 1 or/and the degradation of phosphorus species in the course of reaction. Another possible explanation might be the oxidation of phosphorus species due to air invasion during the decantation in the process of recycling runs. It is known that the ratio of L/Rh below the saturation level (for instance, QINGRONG PENG et al.: HYDROFORMYLATION 59 below the cmc of amphiphilic ligand in the case of aqueous biphasic system) is crucial both for the reaction rate and n/i value, owing to the electronic, steric and micellar effects [11,15]. Thus we replenished the ligands of 1, 2 and 3 with a calculated amount of L/Rh = 2 (molar ratio) after the sixth run. The results showed that the n/i ratios were increased almost to those with the fresh catalysts. The results implied that although the rhodium leaching contributed to the drop in catalytic performance, other catalyst deactivation processes like the formation of rhodium carbonyl clusters and the loss and degradation of ligands, if any, might be the likely reasons for the loss of activity and n/i value.

Fig. 1. The recylcing of 1−hexene hydroformylation using rhodium complexes with 1, 2 and 3 as ligands in ionic liquids: (A) TPPTS; (B) ligand 1; (C) ligand 2; (D) ligand 3. ⊗: TOF; : n/i. Reaction conditions are the same as in Table 1

CONCLUSION

The extraction of phosphine species bearing water-soluble groups of sodium sulfonate by the organic reactant from the BMI⋅BF4 phase was quite low but larger than that of rhodium species in the biphasic hydroformylation process of higher olefins. Significant influences due to the hydrophobic/hydrophilic property of ILs and ligands on the catalytic activity of rhodium complex were 60 QINGRONG PENG et al.: HYDROFORMYLATION observed. Higher reaction rates and moderate aldehyde n/i ratios were obtained with rhodium complexes of water-soluble and amphiphilc phosphines in BMI⋅BF4 as compared with that in BMI⋅PF6. The catalyst system exhibited rather good stability in the catalytic activity but a gradual decrease of the aldehyde n/i ratio in the course of catalyst recycling. The activity was almost independent of the water solubility of the phosphine ligands. The decline in n/i ratio could be recovered almost to the original level by replenishing the catalyst phase with a small amount of the corresponding fresh ligand.

Acknowledgements. This work was supported by the NSFC (Nos: 20021002, 20473065 and 20433030), the MOST (G2000048008), the Research Fund for the Doctoral Program of Higher Education (No: 20050384011), the Key Project of Chinese Ministry of Education (NO: 106099) and the Key Project of Fujian Province (No: 2005HZ01-3).

REFERENCES

1. P. Wasserscheid, T. Welton (Eds.): Ionic Liquids in Synthesis. Wiley-VCH 2003. 2. Y. Chauvin, L. Mussmann, H. Olivier: Angew. Chem. Int. Ed., 34, 2698 (1995). 3. F. Favre, H. Olivier-Bourbigou, D. Commereue, L. Saussine: Chem. Commun., 1360 (2001). 4. P. Wasserscheid, H. Waffenschmidt, P. Machnitzki, K. W. Kottsieper, O. Stelzer: Chem. Commun., 451 (2001). 5. P.J.B. Raymond, M.S. Silvana, C.J.K. Paul, P.W.N.M. van Leeuwen: Chem. Commun., 3044 (2002). 6. P.J.B. Raymond, M.S. Silvana, C.J.K. Paul, P.W.N.M. van Leeuwen: Dalton. Trans., 1590 (2004). 7. C.C. Brasse, U. Englert, A. Salzer, H. Waffenschmidt, P. Wasserscheid: Organometallics, 19, 3818 (2000). 8. C.P. Mehnert, R.A. Cook, N.C. Dispenziere, E.J. Mozeleski: Polyhedron, 23, 2679 (2004). 9. B. Cornils, W.A. Herrmann: Aqueous-Phase Organometallic Catalysis, Wiley-VCH, Weinheim, 1998. 10. B.E. Hanson: Coord. Chem. Rev., 185, 795 (1999). 11. Q.R. Peng, Y. Yang, Y.Z. Yuan: Catal. Lett., 88, 219 (2003). 12. A.Z.Z. Paulo, E.L.D. Jeane, E. Sandra: Polyhedron, 7, 1217 (1996). 13. K.S. Brendan, G.P. Dennis: Electrochem. Commun., 3, 712 (2001). 14. R.P. Swatloski, J.D. Holbrey, R.D. Rogers: Green Chem., 5, 361 (2003). 15. Q.R. Peng, X.L. Liao, Y.Z. Yuan: Catal. Commun., 5 (2004) 447.