Supported aqueous-phase catalysts SAPC

Renewed interest in this method came recently from its adaptation to the immobilization of water/ organic solvent biphasic catalysts, resulting in the so-called supported aqueous phase catalysts (SAPCs).117 The molecular catalyst is immobilized via water, which is hydrogen bonded to the surface silanol groups reactants and products are in the organic phase (Figure 11)... 

In a somewhat different approach, supported-aqueous-phase-catalysts (SAPC, see Chapter 5, Section 5.2.5 of this book) have been combined with supercritical CO2 in catalytic hydrogenation [55], Ruthenium was supported on silica and combined with the ligand TPPTS in water, after which a scC02/H2 phase was applied together with the substrate. Better levels of conversion were obtained using scC02 than the equivalent system with toluene for the hydrogenation of cinnamaldehyde. 

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]. 

Supported aqueous phase catalyst (SAPC) involves generating a thin water film on a high surface area support surrounded by a bulk organic solvent.20 SAPC suffers from mass transfer problems despite the sharp increase in interfacial surface area and leaching problems of the water-soluble catalyst. The SAPC method has been effectively applied to cyclohexene oxidations using a simple water-soluble ammonium molybdate catalyst with the aid of supported surfactant molecules.21... 

Initially, water was used as the hydrophilic liquid and these catalysts are therefore denoted as supported aqueous-phase catalysts (SAPCs) [7-10], Subsequently, we expanded this concept to other hydrophilic liquids such as ethylene glycol and glycerol [11], Reactions of liquid-phase, hydrophobic organic reactants take place at the film-organic interface. SAP catalysis differs significantly from SLP catalysis in that the latter is used for gas-phase reactants whereas the former is specifically designed for liquid-phase substrates. Additionally, with SLP catalysis, the reaction proceeds homogeneously in the supported film while in SAP catalysis it occurs at... 

Supported aqueous-phase catalysts (SAPC) can be seen as a special case of adsorption, whereby a water-soluble catalyst dissolved in a very polar solvent is adsorbed on a hydrophilic support forming a water film on the inner surface of the support [30,31]. In the case of supported liquid-phase catalysis (SLPC),the water film on the inner surface is replaced by a solvent of low vapor pressure (e.g.jphthalic acid esters) [2]. The reaction itself takes place in the supportedUq-uid or at the interface of the supported liquid film, or in the gas phase or organic phase when dealing with SLPC or SAPC, respectively. The use of SLPC catalysts is generally restricted to the synthesis of low-boiling compounds. 

Reduction of unsaturated acid 17 in a two-phase Ac0Et/H20 system using the ruthenium complex associated with ligand 6 allowed the preparation of naproxen (18) in 81% ee, ee values in the range 78-83% being obtained over several recycles of the catalytic solution [Eq. (5)] [22]. The analogous supported aqueous-phase catalyst (SAPC) was also prepared and used in this reduction, an enantioselectivity... 

Due to interfacial reactions, one of the main advantages of SAPC upon biphasic catalysis is that the solubility of the olefins in the catalytic aqueous-phase does not limit the performance of the supported aqueous phase catalysts (SAPCs) the turnover frequencies (TOFs) are roughly independent of olefin carbon number [17]. This has been shown to be true also for carbon numbers as high as 17 [15]. 

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... 

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