Supported aqueous-phase catalysis systems

The development of supported aqueous-phase catalysis (SAPC) opened the way to hydroformylating hydrophobic alkenes such as oleyl alcohol, octene, etc. (cf. Section 4.7 [17]). SAPC involves dissolving an aqueous-phase HRh(CO)(TPPTS)3 complex in a thin layer of water adhering to a silica surface. Such a catalyst shows a significantly high activity for hydroformylation. For classical liquid-liquid systems, the rate of hydroformylation decreases in the order 1-hexene > 1-octene > 1-decene however, with SAP catalysts, these alkenes react at virtually the same rate and the solubility of the alkene in the aqueous phase is no longer the ratedetermining factor [26]. 

Although the first aim of the use of a water-soluble palladium catalyst in allylic alkylation in a two-phase system was the recycling of the catalyst, this methodology finds quite interesting applications in the deprotection of peptides as well as in the selective alkylation of uracils and thiouracils. More recently, the effective use of supported aqueous-phase catalysis as well as asymmetric alkylation in water in the presence of surfactants or amphiphilic resin-supported phosphines open new applications and developments for the future. 

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. 

Of all the catalyst systems studied, Rh-TPPTS is the most suitable and commercially proven catalyst system for biphasic hydroformylation. Several modifications of the water-soluble catalysts using co-solvents [15], surfactants and micelle-forming reagents [16], a supercritical C02-water biphasic system [17], supported aqueous-phase catalysis [18], and catalyst-binding ligands (interfadal catalysis) [19] have been proposed to overcome the lower rates observed in biphasic catalysis due to poor solubilities of reactants in water (see Sections 2.2.3.2 and 2.3.3.3). So far, endeavors have been centered on innovating novel catalyst systems from the viewpoint of efficient catalyst recycle and rate enhancement, but limited information is available on the kinetics of biphasic hydroformylation. 

The concept of supported Hquid catalysis is not restricted to liquid salts. In order to apply the concept of uniform surface properties and efficient catalyst immobilization, several authors investigated the SLP concept during the 1970s and 1980s [5-11]. However, later studies revealed that the evaporation of the loaded liquid cannot be avoided completely during operation. This is especially a problem when using water as the Hquid phase [12-17]. In these supported aqueous phase (SAP) systems, the thin film of water evaporated quickly under reaction conditions, making the concept appHcable only for slurry-phase reactions with hydrophobic reaction mixtures. 

A particular advantage of SAP catalysis is that reaction with hydro-phobic substrates occurs at the aqueous/organic interface allowing water insoluble higher olefins to be hydroformylated using the RhH(CO)(tppts)3 system. In the biphasic hydroformylation using RhH(CO)(tppts)3 the reaction rates decrease in the order 1-hexene > 1-octene > 1-decene. In contrast, using the same catalyst in a silica supported aqueous phase similar activities and selectivities were obtained in the hydroformylation of these olefins. The low n/i ratio observed in... 

Soluble polymer-bound catalysts can be expected to receive continued attention as they offer specific advantages. By comparison to aqueous two-phase catalysis, a range of substrates much broader with respect to their solubility can be employed. By comparison to heterogenization on solid supports, the selectivity and activity of homogeneous complexes can be retained better. However, it must also be noted that to date no system has been unambiguously proven to meet the stability and recovery efficiency required for industrial applications. 

---