Biphasic Hydroformylation Using Ionic Liquids - ACS Symposium

Chapter 29

Biphasic Hydroformylation Using Ionic Liquids

Downloaded by UNIV OF OKLAHOMA on October 27, 2014 | http://pubs.acs.org Publication Date: July 25, 2002 | doi: 10.1021/bk-2002-0818.ch029

Peter Wasserscheid and Horst Waffenschmidt Institut für Technische Chemie und Makromolekulare Chemie, University of Technology Aachen, Worringer Weg 1, D-52074 Aachen, Germany

Two different approaches for the biphasic hydroformylation of 1-octene using ionic liquids are presented in detail. a) the Pt-catalyzed hydroformylation in chlorostannate ionic liquids; b) the Rh-catalyzed hydroformylation in hexafluorophosphate systems. For the latter case, the use of special ionic ligands is required to achieve full catalyst immobilization in the ionic liquid. The synthesis and application of successful cationic ligands systems is described.

Why Use Ionic Liquids As Solvents For Biphasic Hydroformylation? Biphasic catalysis is a well-established method for effective catalyst separation and recycling. In the case of Rh-catalyzed hydroformylation reactions this principle is technically realized in the Ruhrchemie-Rhone-Poulenc-process, where water is used as catalyst phase (/, 2, 3). Unfortunately, this process is limited to C2-C5-olefins due to the low water solubility of higher olefins. Nevertheless, the hydroformylation of e.g. 1-octene is of technical interest for the synthesis of linear nonanal. To produce the latter in high selectivity, it is

© 2002 American Chemical Society

In Ionic Liquids; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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374 necessary to achieve a highly regioselective hydroformylation. This can be realized by application of special ligand systems. Obviously, the additional costs for these ligands make it even more economically attractive to develop methods for an efficient catalyst separation and recycling. Recently, a new approach has been adopted for biphasic catalysis, involving the use of catalyst solvents known as ionic liquids, which are simply salt mixtures of low melting points (< 100°C). Ionic liquids form biphasic systems with many organic liquids such as e.g. α-olefines. Moreover, their non-volatile character allows distillative product separation from the catalyst without the formation of azeotrops and without solvent contamination in the product (4, 5, 6). Depending on the coordinative properties of its anion, the role of an ionic liquid in homogeneous catalysis can be regarded as either "innocent" solvent or solvent/cocatalyst. Ionic liquids with tetrafluoroborate or hexafluorophosphate ions, for example, have to be considered as neutral and inert solvents in most hydroformylation reactions (if carbene formation from the imidazolium cation is avoided). In these cases the function of the ionic liquid is solely to provide a polar, weakly coordinating medium for the transition metal catalyst that additionally offers special solubility for feedstock and products. However, ionic liquids formed by the reaction of a halide with a Lewis acid (e.g. chloroaluminate or chlorostannate melts) generally act as well as a cocatalyst. The reason for this is the Lewis acidity or basicity which is always present (at least latent), and which results in strong interactions with the catalyst complex. In many cases, the Lewis-acidity of an ionic liquid is used to convert the neutral catalyst precursor into the corresponding cationic active form. In biphasic hydroformylation reactions both types of ionic liquids have been successfully applied. While in the Pt-catalyzed hydroformylation slightly acidic chlorostannate ionic liquids are known to activate the precursor complex in a Lewis-acid/base reaction (7), hydroformylation with Rh-complexes has been carried out in "neutral" ionic liquids such as e.g. hexafluorophosphate systems (S, 9,10). The following chapters intend to give a summary of recent progress in both areas. For several reasons only those examples will be discussed where 1-octene has been used as the substrate: (a) in order to allow a comparative discussion of the different approaches; (b) since all tested ionic liquids form biphasic reaction systems with 1-octene; (c) due to the technical importance of the 1-octene hydroformylation; 1-nonanal is an important intermediate for the synthesis of plasticizer alcohols; (d) since 1-octene hydroformylation with aqueous catalyst solutions is not successful; 1-octene solubility in water is only in the range of


375

octane hydrogénation

1-octene


hydroformylation Ο

n-nonanal

i-nonanal

Figure 1: Hydroformylation of 1-octene 0.0001 mol% at 25°C (3) while 1-octene solubility in e.g. [BMIM] [PF ] is about 2.5 mol% at 25°C (11). In figure 1, general aspects of the 1-octene hydroformylation are summarized. 6

Hydroformylation Of 1-Octene In Chlorostannate Ionic Liquids Using Pt-Catalysts Platinum(II) complexes with phosphine ligands promoted by S n C l are known as hydroformylation catalysts since 1976 (12). Systems with monodentate phosphines have been investigated in the hydroformylation of terminal and internal olefins (13,14, 15, 76, 77,18,19,20). Pt/Sn catalysts with diphosphines have been successfully applied to the asymmetric hydroformylation (27,22,23,24,25) and to the highly regioselective hydroformylation of internal, functionalized olefins (26). The Platinum-catalysed hydroformylation of ethylene in a tetraethylammoniumchlorostannate molten salt (melting point: 78°C) was described by Parshall as early as 1972 (27). This publication showed for the first time the potential of a chlorostannate ionic liquid as reaction medium for a homogeneous Platinium-catalyst. However, the relatively high melting point of the chlorostannate salt used by Parshall caused some restrictions for reaction parameters and processing conditions. The experiments were carried out under 2


376 extreme conditions (syngas pressure of 400 bars). Finally, in Parshall's publication the activity of the catalyst has not been determined quantitatively. Therefore it is not possible to compare the catalyst's performance in the chlorostannate melt with the results in conventional organic solvents. Recently, room temperature liquid chlorostannate ionic liquids were used to investigate the potential of Pt-complexes in the hydroformylation of 1-octene (7). Those low melting chlorostannate salts have been described by Ling et al. as solvents for electrodeposition of tin and other electrochemical applications (28). Chlorostannate ionic liquids are easily prepared by mixing imidazolium or pyridinium chloride salts with the appropriate amount of S n C l (7). T o form the ionic catalyst solution, e. g. (PPh ) PtCl is added to the chlorostannate ionic liquids. A change in color from yellow to red is observed. Probably, this is attributed to the abstraction of chloride atoms from the Pt-complex by the acidic Sn Cl '-species of the ionic liquid (as shown in figure 2). This assumption can be supported by recording the Lewis-acidity of the chlorostannate ionic liquid before and after the addition of (PPh ) PtCl by S n - N M R (7). The results of this investigation corresponded very well to an acid-base reaction of both chloride atoms of the Platinum-complex with the acidic ionic liquid.


2

3

2

2

2

5

119

3

P R

3'..

tl

Cl

Pt'^ PR ^ ^Cl 3

[cation] Sn Cls 2

2

2

,ci

PR*,

P

Pt' P R ^ ^SnCl 3

[cation] S n C l

R

„ei

3 " .

SnCl " 3

3

| P * 3 ^

P

t

^

D

+ [cation] S n C l

3

3

Figure 2: Activation of (PPh ) PtCl2 in an acidic chlorostannate ionic liquid (proposed mechanism) 3

2

Table 1 shows the results obtained with (PPh ) PtCl in two different chlorostannate ionic liquids in comparison to the experiment in C H C 1 under identical conditions (7). While the reaction is found to be monophasic in C H C 1 , a biphasic reaction takes place with each of the chlorostannate ionic liquids. Moreover, no leaching of the Pt-catalyst is detected, which can be interpreted as another strong hint for the postulated ionic structure of the active Pt-catalyst. The ionic catalyst solution can be recovered after catalysis by simple phase separation. The hydroformylation in C H C 1 shows (in good accordance to the known literature (26)) reasonable activity with very high selectivity to the linear nonanal 3

2

2

2

2

2

2

2


2

377

Table 1: Hydroformylation of 1-octene using different solvents b>

Conversion TOF S(n-nonanalf'S(octane) [%] [%] [V] [%]

Solvent a

[4-MBP] Cl/SnCl [BMIM] Cl/SnCl CH C1

19.7

103

96.0

29.4

22.3

126

95.0

41.7

25.7

140

98.3

9.4

2

b

2

c

2

2

Conditions: 0.02 mmol PtCl (PPh ) , 0.1 mmol PPh , 20 mmol 1-octene, 5 ml solvent, p(CO/H )= 90 bar, T= 120 °C, t= 2h; Entries a,b: X cn= 0.51. a) The selectivity for nnonanal was calculated in the following way: [S(n-nonanal)]= amount of nnonanal/amount of all hydroformylation products, b) The selectivity for octane was calculated in the following way: [S(octane)]= amount of octane/amount of all products.


2

3

2

3

2

Sa

(entry c in table 1). The undesired hydrogénation of 1-octene to octane is moderate in CH C1 . In contrast, in both chlorostannate ionic liquids slightly lower rate, still very high n/iso-selectivity, but higher hydrogénation activity (entries a and b in table 1) is obtained. Interestingly, the undesired hydrogénation activity depends significantly on the cation of the ionic liquid (41.7% hydrogenated product with l-n-butyl-3-methylimidazolium[BMIM] cation versus 29.4% with l-w-butyl-4-methylpyridinium[4-MBP] cation). More detailed studies to identify the best reaction conditions revealed that in the chlorostannate ionic liquids the highest ratio of hydroformylation to hydrogénation is found at high syngas pressure and low temperature. A t 80 °C and 90 bar CO/H -pressure more than 90 % of all product are n-nonanal and isononanal, the ratio between these two hydroformylation products being as high as 98.6:1.4 (n:i = 72.4) (7). In conclusion, these results show that room temperature liquid chlorostannate ionic liquids are versatile solvents for the regioselective hydroformylation of 1-octene. Moreover, the experiments reveal some unique properties of chlorostannate ionic liquids. In contrast to other known ionic liquids, the chlorostannate system combine a certain Lewis-acidity with high compatibility to functional groups. The first led in the hydroformylation of 1octene to the activation of (PPh ) PtCl by a Lewis-acid-base reaction with the acidic ionic liquid medium. The high compatibility to functional groups is demonstrated by the catalytic reaction in presence of C O and hydroformylation products. 2

2

2

3

2

2


378

Hydroformylation Of 1 -Octene With Rh-Catalysts In Ionic Liquids Already in 1995, Chauvin et al. described the biphasic hydroformylation of 1-pentene with [Rh(CO) acac]/triarylphosphine in e.g. [BMIM] [PF ] (8, 29). However, with none of the tested ligands it was possible to combine high activity, complete retention of the catalyst in the ionic liquid and high selectivity for the desired linear hydroformylation product. The use of PPI13 resulted in significant leaching of the Rh-catalyst out of the ionic liquid layer. This could be suppressed by the application of sulfonated triaryl phosphine ligands, but a major decrease in catalytic activity was found with these ligands. A l l ligands used in Chauvin 's work showed poor selectivity to the desired linear hydroformylation product (n/iso-tatio between 2 and 4). Obviously, the Rhcatalyzed, biphasic hydroformylation of higher olefins in ionic liquids requires the use of ligand systems that are specifically designed for this application.


2

6

Cationic phosphine ligands designed for the use in ionic liquids

Cobaltoceniumdiphosphine ligands A first ligand system designed for the use in ionic liquids was described in 2000 by Salzer et al. (9). These authors used successfully cationic ligands with cobaltocenium backbone for the biphasic, Rh-catalyzed hydroformylation of 1octene. l,r-Bis(diphenylphosphino)cobaltocenium hexafluorophosphate (cdpp) proved to be an especially promissing ligand. The compound can be synthesized according to figure 3 by mild oxidation of l,l'-bis(diphenylphosphino)cobaltocene with C2CI6 and anion exchange with [NH4][PF ] in acetone, (for detailed ligand synthesis see (9)). The catalytic systems are prepared in situ by mixing [Rh(CO)2acac] with the ligand in [BMIM] [PF ] at room temperature. [BMIM] [PF ] is prepared by reacting the chloride salt (see literature (30) for detailed synthesis) with H P F after a method described by Fuller and Carlin (31) or purchased from Solvent Innovation GmbH, Cologne (32). The results obtained in the biphasic hydroformylation of 1-octene are presented in table 2. In order to evaluate the properties of the ionic diphosphine ligand with cobaltocenium backbone, the results with ligand cdpp are compared with those obtained with PPI13, two common neutral bidentate ligands and with NaTPPTS as standard anionic ligand (9). 6

6

6

6


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Biphasic Hydroformylation Using Ionic Liquids - ACS Symposium

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