Biphasic Hydroformylation of Higher Olefins

Steven D. Dietz, Claire M. Ohman, Trudy A. Scholten, Steven Gebhard and 

A method has been developed for the continuous removal and reuse of a homogeneous rhodium hydroformylation catalyst. This is done using solvent mixtures that become miscible at reaction temperature and phase separate at lower temperatures. Such behavior is referred to as thermomorphic, and it can be used separate the expensive rhodium catalysts from the aldehydes before they are distilled. In this process, the reaction mixture phase separates into an organic phase that contains the aldehyde product and an aqueous phase that contains the rhodium catalyst. The organic phase is separated and sent to purification, and the aqueous rhodium catalyst phase is simply recycled. 

Currently, worldwide production of aldehydes exceeds 7 million tons/year (1). Higher aldehydes are important intermediates in the synthesis of industrial solvents, biodegradable detergents, surfactants, lubricants, and other plasticizers. The process, called hydroformylation or more familiarly, the Oxo process, refers to the addition of hydrogen and the formyl group, CHO, across a double bond. Two possible isomers can be formed (linear or branched) and the linear isomer is the desired product for these applications. 

One approach that has been successfully used to separate the catalyst from the product aldehyde is to use a biphasic system in which the rhodium catalyst is soluble in water and the product is soluble in an organic phase. This approach is used by Hoechstdlhone-Poulenc to produce more than 600,000 t/year of butyraldehyde (a lower aldehyde) (2). Unfortunately, this process caimot be used to produce higher aldehydes because the water solubihty of the higher olefins that are the feedstock is very low, which dramatically reduces the reaction rate. 

To eliminate the need to recover the product by distillation, researchers are now looking at thermomorphic solvent mixtures. A thermomorphic system is characterized by solvent pairs that reversibly change from being biphasic to monophasic as a function of temperature. Many solvent pairs exhibit varying miscibility as a function of temperature. For example, methanol/cyclohexane and n-butanol/water are immiscible at ambient temperature, but have consolute temperatures (temperatures at which they become miscible) of 125°C and 49°C, respectively (3). 

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Thermoregulated Phase Transfer Catalysis - A conceptual advance in the field of biphasic hydroformylation of higher olefins is the use of rhodium catalysts generated from nonionic tenside phosphines, such as ethoxylated tris(4-... 

Using RuCl(CO)(TPPTS)(BISBIS) the biphasic aqueous hydroformylation of higher olefins in the presence of the cationic surfactant CTAB ensures a TOF > 700 h and regioselectivity >96% for the linear aldehyde Piperazinium cationic surfactants were also successfully applied as catalysis promotion agents in the aq. biphasic hydroformylation of higher olefins. The property of surfactant and ligand can be assumed by the same molecule, e.g. di-sulfonated cetyl(diphenyl)phosphine 42. 1-Dodecene is hydroformylated in water/toluene (3 1) under mild conditions [olefin/Ru = 2500, CO/H2=1, P(CO + H2) = 15 bar, 42/Ru = 10] with TOF = 188 Another approach to... 

Thermoregulated phase-transfer catalysis has been used successfully for the aqueous biphasic hydroformylation of higher olefins [13, 18]. A reasonable explanation for the satisfactory catalytic reactivity is that it results from the thermoregulated properties of Rh/TRL complexes. As shown in Table 1, average turnover frequencies (TOFs) of250 h for 1-dodecene and 470 h for styrene have been achieved. Even the hydroformylation of oleyl alcohol, an extremely hydrophobic internal olefin, exhibits a yield of 72% [24]. 

Indeed, high activity was obtained in the biphasic hydroformylation of higher olefins in the presence of Rh/PETPP complexes as catalysts, which demonstrated that the water solubility of the substrate did not influence the effectiveness of the TRPTC system. Some TRPT catalysts such as PETPPs 17, P-N bidentate (PEDPA... 

The consequence of low alkene solubihty is in that industrially the RCH-RP process can be used only for the hydroformylation of C2-C4 olefins. In all other cases the overah production rate becomes unacceptably low. This is what makes the hydroformylation of higher olefins one of the central problems in aqueous/organic biphasic catalysis. Many solutions to this problem have been suggested (some of them will be discussed below), however, any procedure which increases the mutual solubihty of the organic components and the aqueous ingredients (co-solvents, surfactants) may... 

Hydroformylation of higher olefins provide long chain alcohols which find use mainly as plasticizers. No aqueous/organic biphasic process is operated yet for this reaction, for several reasons. First, solubility of higher olefins is too small to achieve reasonable reaction rates without applying special additives (co-solvents, detergents, etc.) or other means (e.g. 

In conclusion it can be said, that micellar effects offer useful possibilities to tune the reactivity and separation characteristics of aqueous/organic biphasic hydroformylations. Nevertheless, the added sensitivity of the systems to small changes in process variables and the added cost of surfactants and/or specially synthetized ligands have to be justified by high added value products or on grounds of process cost savings. Whether this will happen on industrial scale (perhaps in the hydroformylation of higher olefins) remains to be seen. 

Figure 5 Flow diagram of the Union Carbide Process for hydroformylation of higher olefins catalysed by Rhltppms in a single phase with biphasic catalyst separation.

The aqueous biphasic processes require a minimum solubility of the reactants S in the catalyst phase [196, 205]. Therefore, hydroformylation of higher olefins (approx. > Cg) or functionally substituted olefins is more difficult but offers various advantages, such as the simplification of reaction sequences and reduced expenditure for the catalyst cycle. So far, work on these biphasic processes for the conversion of higher olefins, except for Kuraray s recent devel-... 

According to Horvath, the problems arising from the limited reciprocal solubilities of the water phase and higher olefins should be overcome by application of the SAPC technique [138, 144]. For this and other biphasic - but nonaqueous -processes, see Sections 3.1.1.2 and 3.1.1.3. A new concept concerning inner lipophilic cavities and hydrophilic surfaces of a-cyclodextrins may offer new possibilities for the hydroformylation of higher olefins [142]. Asymmetric hydro-formylations are dealt with in Section 2.9. 

In the hydroformylation of higher olefins, even using the aqueous biphasic method, it has to be assumed that, owing to the reduction in the reaction rate caused by decreasing solubility of the olefins, the actual reaction should be single phase and the separation should be two phase. From today s point of view, this leaves the following routes to a solution ... 

In aqueous two-phase hydroformylation of 1-octene and 1-dodecene the amphiphilic ligands of type 25 (n = 10, 12) have been shown to form Rh catalysts that are superior to Rh/TPPTS systems [129]. The bicyclic ligands 26 were considered to be of interest as substitutes for TPPMS in the new oxo process developed by Union Carbide for the hydroformylation of higher olefins using N-methylpyrroli-done or polyalkylene glycols as solvents [7, 51, 52], Rh(I) complexes [Rh(26)2]+ [96] showed, however, a very poor performance as catalysts in biphasic systems for hydrogenation and hydroformylations in contrast to non-functionalized 1-phospha-norbornadiene [98], This was explained by formation of P,P(0) chelates blocking... 

The most severe dra wback in homogeneous catalysis is the separation of the catalyst from the reaction mixture. The industrial success of the aqueous two-phase hydroformylation ofpropene to n-butanal [1] in Ruhrchemie AG in 1984 represents the considerable progress in this field. However, aqueous/organic biphasic catalysis has its limitations when the water solubility of the starting materials proves too low, as in hydroformylation of higher olefins (see Chapter 1). To solve this issue, a variety of approaches have been attempted. Additions of co-solvents [2] or surfactants [3, 4] to the system or application of tenside ligands [5, 6] and amphiphilic phosphines [7, 8] are ways to increase the reaction rates. Other approaches such as fluorous biphase system (FBS see Chapter 4) [9], supported aqueous phase catalysis (SAPC see Section 2.6) [10], supercritical CO2 (cf. Chapter 6) [11] and ionic liquids (cf Chapter 5) [12] have also been introduced to deal with this problem. 

The rhodium/TPPTS-catalyzed hydroformylation of higher olefins in organic/ aqueous biphasic system in the presence of double long-chain cationic surfactants (37) was studied at 100 °C and 20 bar CO H2 = 1 1 pressure. The reaction rate was comparable with that in homogeneous catalysis system [111]. 

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