SAPC biphasic catalysis

Horvath recognized that SAPC solved the problem posed by the solubility of lypophilic substrates in aqueous biphasic catalysis with water-soluble homogeneous catalysts. He compared biphasic aqueous-organic catalysis with SAPC, in order to clarify whether in SAPC the catalyst remained dissolved in the... 

SAPC can perform a broad spectrum of reactions such as hydroformylation, hydrogenation and oxidation, for the synthesis of bulk and fine chemicals, pharmaceuticals and their intermediates. Rhodium complexes are the most extensively used, but complexes of ruthenium, platinum, palladium, cobalt, molybdenum and copper have also been employed [63-65]. Owing to interfacial reactions, one of the main advantages of SAPC upon biphasic catalysis is that the solubility of the reactant in the catalytic aqueous-phase does not limit the performance of the supported aqueous phase catalysts. 

Fig. 3 Positioning of aqueous biphase catalysis different approaches according to the variation of the application phase of catalysts. FBS, fluorous biphase system (cf. Section 7.2 [24] PEG, polyethylene glycol cf. Sections 4.6.3 and 6.1.3.2 SAPC, and SLPC, cf. Section 4.7).

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 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. 

Many typical parameters usually investigated in biphasic catalysis, such as temperature, pressure, the excess of water-soluble ligand, and the nature of the reactants and the catalyst, have roughly similar effects on SAPC. 

As seen before, a major difference with respect to biphasic catalysis is the low dependence on substrate solubility in the catalytic aqueous phase as the SAPC reaction occurs at the interface. SAPC is strongly dependent on the water content of the solid support. Two types of water content effects have been reported usually SAPC is efficient over a very restricted hydration range where activity exhibits a clear peak, while only recently a large plateau was observed in a higher hydrahon range. 

Davis and Hanson developed a new concept of immobilizing homogeneous catalysts denoted as supported aqueous phase catalysts (SAPC) [15]. They reasoned that in aqueous biphasic catalysis the reaction mainly takes place at the interface. In order to increase this interface they used a high-surface-area hydrophilic support (figure 5). These materials have a thin film of water adhered to the surface, in which the water-soluble catalyst is dissolved. The reaction, performed in an organic solvent such as toluene, occurs at the water-organic interface. The supported catalyst has a... 

The activity is low (TOP < 100 h ) in most cases for both rhodium- [42, 45] and cobalt-based [39, 41] catalysts. Ligand modification is a powerful tool to influence the selectivity in SAPC. Using a Xantphos derivative as ligand results in a high Hnear to branched selectivity of >30 [46] as it does in biphasic catalysis [47]. The weakness of SAPC is that it is too sensitive to the content of water in substrates, and therefore its stabihty in long-term operation is not satisfactory. 

Substitution of the water-biphasic procedure by supported aqueous phase catalysis (SAPC, see below) [39,10,26b,64] ... 

The transitions of supported liquid-phase catalysts (SLPC) and supported aqueous-phase catalysts (SAPC) are dealt with in Section 3.1.1.3, while special aspects of clusters and colloids are discussed in Sections 3.1.1.4 and 3.1.1.5 and those of aqueous-phase, re-immobilized catalysts in Section 3.1.1.6. The combination of heterogeneous catalysis with aqueous (biphasic) techniques is also under investigation, e. g., [209]. 

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