Water-soluble biphasic hydroformylation catalyst

The prototype industrial process based on this concept is the Ruhrchemie-Rhone Poulenc process for the hydroformylation of propylene to butanal94,219,220 (see Section 7.3.1). Because of the use of appropriately modified water-soluble ligands, the catalyst resides and operates in the aqueous phase. The particular features of this process are the positive energy balance and easy catalyst recovery, namely, the simply circulation of the aqueous catalyst solution. New types of water-soluble Ir and Rh complexes with tris(hydroxymethyl)phosphine222 were described, and the biphasic hydroformylation of 1-hexene was accomplished in ionic liquids.223 A cationic sugar-substituted Rh complex displays high regioselectivity to branched aldehydes.224... 

Likewise, a thermoregulated phase transfer process within the aqueous/organic two-phase system has been reported by Jin and co-workers (cf. Section 3.1.1.1) [290]. A water-soluble supramolecular Rh catalyst based on functionalized /1-cyclodextrin was also described [291]. In a two-phase system this catalyst may function as a carrier for the transfer of both the starting material and the product between the different phases. As an alternative to polar media for biphasic hydroformylation, Chauvin et al., used ionic liquids based on imidazolium salts which are well known for dimerization reactions (cf. Sections 2.3.1.4 and 3.1.1.2.2) [270, 271, 292]. For introduction into technical processes the currently availability and price of ionic liquids could be a drawback, especially for bulk chemicals such as 0x0 products. 

In another report a semi-empirical rate equation has been proposed combining mechanisms of homogeneous and interfacial parameters for biphasic hydroformylation of 1-dodecene with water-soluble Rh-complex catalyst [36]. 

Hydroformylation of olefins using different types of water soluble catalysts has been reported by several investigators (3,1,7). While several other water soluble ligands and catalysts have been studied, Rh-TPPTS is the most suitable and commercially proven catalyst system for biphasic hydroformylation. Since, catalysis in biphasic media is also likely to... 

The hydroformylation of 1-octene using a water-soluble Rh-TPPTS catalyst in a biphasic medium was studied in the absence of any additive or co-solvent by Bhanage [24]. The experiments were carried out under conditions such that the aqueous... 

Hydroformylation of ethylene using a water-soluble Rh-TPPTS catalyst has been investigated [27] using a toluene-water solvent system at 353 K. The effect ofTPPTS concentration on rate shows a maximum at a P/ Rh ratio of 8 1. The rate of reaction first increases with catalyst concentration, and above a certain value it remains constant. The effect of aqueous-phase hold-up shows a maximum in the rate at = 0.4. The apparent reaction orders for the partial pressures of hydrogen and ethylene were found to be one and zero respectively. A strong inhibition in the rate with an increase in Pqq was observed. An interesting example of tandem synthesis of methacrolein in an aqueous biphasic system has been reported by Deshpande et al. [28], in which hydroformylation of ethylene and aldol condensahon reactions occur in two immiscible liquid phases with a high yield of the product Use of a two-phase system prevents contact of the hydroformylation and aldol catalysts, the interaction of which leads to deactivation. 

Examples of applying biphasic system to catalyzed reactions, such as phase-transfer catalysis, show the efficiency over stoichiometric reactions. In a typical catalytic biphasic system, one phase contains the catalyst while the substrate is in the second phase. In some systems, the catalyst and substrates are in the same phase while the product produced is transferred to the second phase. In a typical reaction, the two phases are mixed during the reaction and after completion, the catalyst remains in one phase ready for recycling while the product can be isolated from the second phase. The most common solvent combination consists of an organic solvent combined with another immiscible solvent, which, in most applications, is water. However, there are few examples of suitable water-soluble and stable catalysts, therefore various applications are limited to some extent [4]. Immiscible solvents other than water are recently becoming more applicable in biphasic catalysis due to the better solubility and stability of various catalysts in such solvents. For example, ionic liquids and fluorous solvents have many successful applications in liquid-liquid biphasic syntheses such as Heck reactions and hydroformylations using ionic liquid media, or Baeyer-Villiger reactions... 

An example of a large scale application of the aqueous biphasic concept is the Ruhrchemie/Rhone-Poulenc process for the hydroformylation of propylene to n-butanal (Eqn. (15)), which employs a water-soluble rhodium(I) complex of trisulphonated triphenylphosphine (tppts) as the catalyst (Cornils and Wiebus, 1996). 

A series of water-soluble polyether-substituted triphenyl phosphines (PETPPs) la-c has been successfully employed by Jin et al. [11] in the thermoregulated hydroformylation of 1-dodecene in the biphasic water/toluene system. The catalysts exhibit very good catalytic properties with conversions up to 93% and about 85% selectivity for aldehyde formation. The catalyst derived from rhodium(III) chloride and ligand Ic could be reused in four consecutive cycles without significant loss of activity or chemoselectivity. The n-selectivity of the product aldehydes was not determined. 

For instance, catalysis in liquid/liquid two phases is generally referred to as biphasic catalysis and has widened the practical scope of homogeneous catalysis the catalyst is present in one liquid phase, while reactants and products are present in the other liquid phase. Thus, the catalyst can be separated by simple phase separation. Celanese is operating a 300 000 t/a plant for propylene hydroformylation using a water-soluble rhodium phosphine complex in a biphasic mode of operation at the Ruhrchemie site in Oberhausen [142],... 

With a water-soluble hydroformylation catalyst the overwhelming majority of the reactions take place in an aqueous/organic biphasic mixture for the simple reason of most olefins being insoluble in water. Research in aqueous organometallic hydroformylation is therefore directed to several aims ... 

Veiy recently it was disclosed, that the water-soluble dinuclear complex obtained in the reaction of [ RhCl(COD) 2] and 11-mercaptoundecanoic acid catalyzed the aqueous/organic biphasic hydroformylation of styrene and various arene-substituted styrenes with good activity and useful selectivity to the branched aldehydes (Scheme 4.6) [82], Below pH 4 the acid form of the complex [ Rh(p-S(CH2)ioC02H)(COD) 2] precipitated virtually quantitatively but could be redissolved in water on addition of base. Importantly, higher olefins could also be hydroformylated by this catalyst (for 1-octene TOP = 17.5 h at 55 °C, 35 bar syngas, n/i = 1.0). 

In the hydroformylation of alkenes, the major differences between the [RhH(CO)(PPh3)3], and [RhH(CO)(TPPTS)3] catalysts are the lower activity and higher selectivity of the water-soluble complex in aqueous/organic biphasic systems. Lower activity is not unexpected, since alkenes have limited solubility in water (see 4.1.1.1, Table 3). On the other hand, the higher selectivity towards formation of the linear product deserves more scrutiny. 

As mentioned earlier, in the Ruhrchemie-Rhone Poulenc process for propene hydroformylation the pH of the aqueous phase is kept between 5 and 6. This seems to be an optimum in order to avoid acid- and base-catalyzed side reactions of aldehydes and degradation of TPPTS. Nevertheless, it has been observed in this [93] and in many other cases [38,94-96,104,128,131] that the [RhH(CO)(P)3] (P = water-soluble phosphine) catalysts work more actively at higher pH. This is unusual for a reaction in which (seemingly) no charged species are involved. For example, in 1-octene hydroformylation with [ RhCl(COD) 2] + TPPTS catalyst in a biphasic medium the rates increased by two- to five-fold when the pH was changed from 7 to 10 [93,96]. In the same detailed kinetic studies [93,96] it was also established that the rate of 1-octene hydroformylation was a significantly different function of reaction parameters such as catalyst concentration, CO and hydrogen pressure at pH 7 than at pH 10. 

Similar to the above case, hydroformylation of 1-hexene using a water-soluble rhodium catalyst [RhH(CO)(TPPMS)3] gave lower yields when a-cyclodextrin was added to the biphasic reaction system [14]. Again, the reason was suspected in the interaction between the cyclodextrin and the rhodium catalyst. 

The OATS concept was tested on the catalytic hydroformylation of 1-octene, a hydrophobic substrate. This reaction was selected because it has previously been shown to be inactive for traditional aqueous biphasic systems (18). The catalyst used was a Rh/TPPTS complex, an industrial water soluble catalyst (22). The application of the OATS concept increased catalytic efficiency by a factor of 65 (TOP increased from 5 h for biphasic to 325 h for monophasic). 

Hydroformylation of Mid Range Olefins - Rhodium/tppts catalysts exhibit low catalytic activity in the hydroformylation of mid range olefins (C5-Cg) in a two phase system due to the much lower solubility of such olefins in water. In the Rh/tppts catalysed biphasic hydroformylation of 1-hexene, for example the conversion is only 11-22% with a n/i ratio of aldehydes of 98/2.353,373 The rate of 1-hexene hydroformylation catalysed by Rh/tppts increased by a factor 2.3 when subjected to ultrasound (35 kHz) and high stirring rates.360,361... 

Kalck et /.96,359,362 studied the biphasic hydroformylation of 1-hexene using water soluble dinuclear species such as Rh2(m-S-lBu)2(CO)2(tppts)2 and C0/H20 where H20 acts both as a solvent and hydrogen source according to the water gas shift reaction (Equation 2). The turnover frequency (TOF) obtained was 40 h 1 and the n/i ratio of the aldehyde 96/4.96 Using RhH(CO)(tppts)3 catalysts lower rates were obtained under the same conditions.96... 

Water soluble Rh/tppts and Rh/tppms complexes dissolved in nonaqueous media such as the ionic liquids, l-ethyl-3-methylimidazolium or l-n-butyl-3-methylimidazolium salt have also been used as catalysts in the hydroformylation of 1-pentene employing a two phase system.15,16 The yields obtained were 16-33% (TOF=59-103 h 1) without any leaching of the rhodium from the ionic liquid to the aldehydes/feedstock phase. Rh/PPh3 catalysts exhibited higher rates (TOF=333 h 1) for the same biphasic reaction albeit with leaching of rhodium due to the uncharged nature of the catalytic system.15... 

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