The SAPC Concept of Immobilization

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 

Horvath conducted several other interesting experiments with rhodium SAPCs (Table 1) [12]. Clearly, the water-solubility of the olefins does not limit the performance of the SAPCs since the TOFs (turnover frequencies) are essentially independent of olefin carbon number. This has been shown to be true also for carbon numbers as high as 17 [13]. Additionally, Horvath conducted experiments aimed at observing rhodium loss into the organic phase. He concluded that the SAPC does not leach catalytically active rhodium species under hydroformylation conditions. Another critical test for leaching was performed by Horvath. He conducted a 38 h continuous-flow experiment in a trickle-bed reactor and showed no loss of rhodium by elemental analysis. Thus, the combined data from all work shows that 

For long-term stability, the SAPC must remain assembled. To test for this type of stability, it was investigated whether the components can self-assemble. The rhodium complex HRh(CO)(TPPTS)3, TPPTS and water were loaded into a reactor with cyclohexane and 1-heptene. The reactor was pressurized with approx. 70 bar H2 + CO (CO H2, 1 1) and heated with stirring to 100°C. A second experiment was carried out in a manner similar to the one previously described except that CPG-240 was added also. The components of the SAPC self-assemble to form an SAPC and carry out the hydroformylation reaction [13]. Upon termination of the reaction, the solid collected contained HRh(CO)(TPPTS)3 and TPPTS. This test indicates that, under the conditions of the experiment, the individual components of the SAPC are more stable assembled in an SAPC configuration than separated. Therefore, the reverse, i.e., the separation of the solution and complex from the support, is not likely to happen under reaction conditions. 

The water content of HRh(CO)(TPPTS)3-based SAPCs has a great influence on their performance. For example, when 1-heptene is hydroformylated, the TOF increases by two orders of magnitude when the water content of the catalyst increases from approx. 2.9 wt.% to approx. 9 wt.% (Table 2). 

The foregoing discussions show that the SAPC immobilization concept does reveal the desired properties of activity and selectivity with no catalyst leaching. Table 3 provides a summary of the catalytic materials/reactions reported using this immobilization technique. 

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