Hydroformylation water-soluble catalysts

Besides the well-known applications of water-soluble phosphines, e.g., in hydroformylation, water-soluble catalysts may have significant advantages for electrochemical processes in which the much lower resistance of aqueous solutions compared with that of organic solutions would improve the energy efficiency of the process. It is known that the electrochemical reduction of carbon dioxide to carbon... 

A new homogeneous process for hydroformylation of olefins using a water-soluble catalyst has been developed (40). The catalyst is based on a rhodium complex and utilizes a water-soluble phosphine such as tri(M-sulfophenyl)phosphine. The use of an aqueous phase simplifies the separation of the catalyst and products (see Oxo process). 

In another interesting area in the study of hydroformylation, Davis developed the concept of supported aqueous phase (SAP) catalysis.175 A thin, aqueous film containing a water-soluble catalyst adheres to silica gel with a high surface area. The reaction occurs at the liquid-liquid interface. Through SAP catalysis, the hydroformylation of very hydrophobic alkenes, such as octene or dicyclopentadiene, is possible with the water-soluble catalyst [HRh(CO)(tppts)3]. 

The Ruhrchemie/Rhone-Poulenc process is performed annually on a 600,000 metric ton scale (18). In this process, propylene is hydroformylated to form butyraldehyde. While the solubility of propylene in water (200 ppm) is sufficient for catalysis, the technique cannot be extended to longer-chain olefins, such as 1-octene (<3 ppm solubility) (20). Since the reaction occurs in the aqueous phase (21), the hydrophobicity of the substrate is a paramount concern. We overcame these limitations via the addition of a polar organic co-solvent coupled with subsequent phase splitting induced by dissolution of gaseous CO2. This creates the opportunity to run homogeneous reactions with extremely hydrophobic substrates in an organic/aqueous mixture with a water-soluble catalyst. After C02-induced phase separation, the catalyst-rich aqueous phase and the product-rich organic phase can be easily decanted and the aqueous catalyst recycled. 

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). 

The catalytic hydroformylation of long-chain and branched olefins remains a challenge as the activity of the water-soluble catalysts decreases rapidly with increasing chain length of the olefin. Extensive work has been undertaken to overcome these limitations (74). 

More recently, Taqui Khan and co-workers (70) introduced the potentially tetradentate ethylenediaminetetraacetate ligand in the ruthenium coordination sphere in order to obtain an efficient water-soluble catalyst precursor. Indeed, starting from the ruthenium(III) aquo EDTA species [Ru(EDTA)(H20)] , carbonylation gives the paramagnetic carbonyl complex [Ru(EDTA)(CO)] which is able to induce the heterolytic activation of dihydrogen (Scheme 3). The hydroformylation of hex-l-ene performed at 50 bar (CO/H2= 1/1) and 130°C in a 80/20 ethanol-water solvent... 

Synthesis of water-soluble phosphines is nowadays one of the most active areas in hydroformylation research. The oxo synthesis in a two-phase system with water-soluble catalysts, the Ruhrchemie/Rhone-Poulenc process (RCH/ RP), will be discussed in Section 2.1.1.4. Water-soluble catalysts in general are treated in Section 3.1.1.1. Since the last exhaustive reviews in 1993 and 1992 on water-soluble complexes [38], some progress has been made in this area, which will be discussed in Section 2.1.1.5.3. 

Industrial hydroformylation is currently performed in two basic variants the homogeneous processes, where the catalyst and substrate are in the same liquid phase (Shell, UCC, BASF, etc.), and the two-phase process with a water-soluble catalyst (RCH/RP). These processes will be discussed in detail in Section 2.1.1.4. Gas-phase hydroformylation with heterogeneous catalysts plays no role today. The immobilization of homogeneous catalysts will be discussed in Section 3.1.1. Special applications such as SLPC (supported /iquid-phase catalysts) [43] and SAPC (supported aqueous-/7hase catalysts) [44] are not considered further here. Heterogeneous oxo catalysts are not within the scope of this book they are discussed further elsewhere [267]. 

Only limited data are available for the kinetics of oxo synthesis with the water-soluble catalyst HRh(CO)(TPPTS)3. The hydroformylation of 1-octene was studied in a two-phase system in presence of ethanol as a co-solvent to enhance the solubility of the olefin in the aqueous phase [115]. A rate expression was developed which was nearly identical to that of the homogeneous system, the exception being a slight correction for low hydrogen partial pressures. The lack of data is obvious and surprising at this time, when the Ruhrchemie/ Rhone-Pou-lenc process has been in operation for more than ten years [116]. Other kinetic studies on rhodium-catalyzed hydroformylation have been published, too. They involve rhodium catalysts such as [Rh(nbd)Cl]2 (nbd = norbomadiene) [117] or [Rh(SBu )(CO)P(OMe)3]2 [118], or phosphites as ligands [119, 120]. 

Membrane technology is a recent development to separate (or concentrate) water-soluble catalysts (mainly hydroformylation catalysts) [147, 149], although a prior art is known [194, 195]. There are proposals for the use of immobilized or re-immobilized aqueous phases for large-scale processes (cf. Ref. [222] and Section 3.1.1.6). Carbon dioxide as a solvent for biphasic hydroformylations has been described by Rathke and Klinger [184], although the use of CO2 for hydroformylation purposes was described earlier [185]. For the use of supercritical CO2 cf. Section 3.1.13 with non-aqueous ionic liquids cf. Section 3.1.1.2.2. Investigations with supercritical water are in an early state (e. g., Ref. [223]). 

The small droplets act as microreactors when they contain the water-soluble catalysts. For hydroformylation reactions with water-soluble Rh/TPPTS in the droplets, the alkene, carbon monoxide and hydrogen approach the micelle surface where the reaction occurs, as is illustrated in Fig. 5.13. After the reaction is completed, phase separation can be achieved by changing the temperature of the reaction mixture. When the mixture is cooled down an aqueous bottom phase, containing most of the surfactant and the water-soluble catalyst separates from the organic upper phase, which contains the hydrophobic products and unconverted reactants. In case of incomplete catalyst recovery the micelle remaining in the product phase can be separated by means of ultrafiltration. 

The concept of biphasic catalysis requires that the catalyst and product phases separate rapidly to achieve a practical approach to the recovery and recycling of the catalyst. It is obvious that simple aqueous/hydrocarbon systems form two phases under nearly all operating conditions and thus provide rapid product-catalyst separation. Ultimately, however, the application of water-soluble catalysts is limited to low-molecular-mass substrates which have appreciable water-solubility. The problem is illustrated by the data in Table 1, which gives the solubility of some simple alkenes in water at room temperature [1], Although hydrocarbon (alkene)-solubility in water increases at higher temperature, most alkenes do not have sufficient solubility to give practical reaction rates in catalytic applications. The addition of salts further decreases the solubility of hydrocarbons in water. Substrate solubility in water is a significant issue and it is no accident that so-far the practiced and proposed commercial applications of water-soluble catalysts for hydroformylation are limited to propene and butene. 

Hydroformylation of higher alkenes with water-soluble catalysts is difficult to achieve (cf. Section 6.1.3.2). For example, the Rh/TPPTS complex, a perfect catalyst used in the two-phase hydroformylation of propene to generate butanol in the RCH/RP process, if applied to the hydroformylation of 1-hexene only gave conversions as low as 16-22% [4]. 

Several modifications of the water-soluble catalysts using co-solvents (cf. Section 4.3 and [14]), micelle forming reagents (Section 4.5 and [15]), super-critical C02-water biphasic system (cf. Section 7.4 and [16]), SAPC (Section 4.7 and [17]), and catalyst binding ligands (interfacial catalysis) [18, 24] have been proposed to overcome the lower rates observed in biphasic catalysis due to poor solubilites of reactants in water. So far endeavors were centered on innovating novel catalyst and development of the existing systems. However, limited information is available on the kinetics of biphasic hydroformylation. 

The mechanism of the oxo reaction has been extensively studied in the past. A comparative study of both commercially applied oxo catalysts HRh(CO)(TPP)3 (TPP = triphenylphosphine) and HRh(CO)(TPPTS)3 was performed by Horvath [2]. The latter, water-soluble catalyst is considered to react according to the dissociative mechanism. However, remarkable differences exist in the catalytic activity and the selectivity of the organic- and water-soluble catalysts. The latter shows much lower specific activity but an increased selectivity to linear products in the hydroformylation of propene. From an Arrhenius plot it is concluded than the dissociation energy of TPPTS from HRh(CO)(TPPTS)3 is about 30 1 kcal/mol... 

As far as is known, the only industrial application of the water-soluble catalyst for the hydroformylation of 5 -functionalized alkenes has been developed by Kura-ray [17]. In this process, 7-octen-l-al is hydroformylated into nonane-1,9-dial, a precursor of nonene-l,9-diol, by using a rhodium catalyst and the monosulfonated tri-phenylphosphine as water-soluble ligand in a 1 1 sulfolane/water system. At the completion of reaction, the aldehydes are extracted from the reaction mixture with a primary alcohol or a mixture of primary alcohol and saturated aliphatic hydrocarbon (cf. Section 6.9). 

In the meantime, many other catalytic systems have been described for the hydroformylation of fatty compounds, especially by Fell s group [49-51], They used water-soluble catalysts, for instance consisting of Rh4(CO)12 and surface-active sul-fobetaine derivatives of tris(2-pyridyl)phosphine [49]. Other ligand systems are the sodium salt of TPPTS in combination with detergents [50] and the lithium salt of triphenylphosphinemonosulfonic acidTPPMS [51]. 

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