Supported aqueous-phase catalysis SAPCs

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

Research in this field started in the wake of the reports of SL-PC. Consisting of a catalyst-containing supported liquid layer for CF reactions in the gas phase, the concept was transferred to batch reactions, using a catalyst dissolved in a supported aqueous phase. This was first referred to as supported aqueous-phase catalysis (SAPC) by Davis in an article published in Nature in 1989. Later, the concept was extended, using a variety of names, but the essence has remained the same a supported catalyst-philic phase. 

The development of supported aqueous-phase catalysis (SAPC) [275, 276] is a new and efficient way to facilitate the hydroformylation of longer olefins. Most of the SAP catalysts described in the literature use TPPTS as ligand. Only a few sulfonated diphosphine ligands were examined [277]. A water-soluble chelating diphosphine ligand with a wide natural bite angle, based on a xanthene backbone, was studied as a SAP aqueous catalyst. This ligand showed a much better selectivity than the SAP catalysts known so far [278]. 

Supported aqueous-phase catalysis (SAPCs) offer a second relevant possibility for heterogenization of homogeneous catalysts [132, 133]. Initially, water was used as the hydrophilic liquid and these catalysts are therefore denoted as SAPCs [134]. 

A variant of this technique is supported aqueous-phase catalysis (SAPC), in which the polar catalyst phase is heterogenized on a solid support [61-68], The principle of this technique is shown in Figure 2. The organometallic complex, e.g.,... 

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]. 

Another approach in order to recycle the palladium-catalyst was the use of supported aqueous-phase catalysis (SAPC). Alkylation of (E)-cinnamyl ethyl carbonate by ethyl acetoacetate or morpholine occurred in acetonitrile or benzonitrile using Pd(OAc)2-TPPTS supported on silica. No leaching of the catalyst was observed, allowing proper recycling of the catalyst [22-26], Polyhydroxylated supports such as cellulose and chitosan were also successfully used in this approach [27-29]. 

Another possible way to separate they catalyst from the fatty products was found by Davis [52-54] and further investigated by Fell [55]. This new method is supported aqueous-phase catalysis (SAPC cf. Section 4.7). On a hydrophilic support, e.g., silicon oxide with a high surface area, a thin aqueous film is applied which contains the water-soluble rhodium catalyst, for instance HRh(CO)L3 with sodium TPPTS ligands. Oleyl alcohol and syngas react at the organic/aqueous interface and form the formylstearyl alcohol in a yield of 97%. The catalyst can be separated from the product by simple filtration without loss of activity. 

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. 

The alternative could arise from development of an elegant immobilization method designed specifically to convert liquid-phase reactants the supported aqueous-phase catalysis (SAPC) [6]. SAPC is a special case of the supported Hquid-phase catalysis whose development began according to proposals by Moravec [7] and Rony [8, 9]. SAPC is very promising due to its high capacity in conversion and selectivity, its better stability, and also the very easy recovery of the catalyst from the organic phase. Comprehensive reviews of SAPC are available [1,10-12]. 

I Ls can also be immobilized by impregnation of an inorganic support. This is a direct transposition of the Supported Aqueous-Phase Catalysis (SAPC) concept to ionic liquids (see Section 2.6). Supported Ionic Liquid Phase (SILP) catalysts containing Rh-biphosphine ligands were applied to perform continuous-flow gas-phase hydroformylation of propene in [BMIMJIPFg] or [BMIMJIRSOJ (R=octyl). 

Immobilization of ILs on a solid support is the direct transposition of the well-documented supported aqueous-phase catalysis (SAPC see Section 2.6). For ILs this strategy can be of particular interest if we consider that the IL-phase remains liquid during reaction and is easy to maintain on the support due to its negligible volatility. Furthermore, because of the ease of separation and the possibility of using a fixed-bed reactor, a solid catalyst can be highly industrially advantageous for the production of aldehydes. 

Another possibility is to dissolve the catalyst in water, which is supported on a solid phase such as silica. The catalyst is not directly anchored to the solid phase but is dissolved in a film of water, which in turn is linked to the surface of the solid. This approach is termed supported aqueous phase catalysis (SAPC) and has successfully been applied to allylic carbonates. " " ... 

As an alternative to the heterogenization of homogeneous catalysis, there are some proposals to realize a solid catalyst with an immobilized species in aqueous/organic media. This concept, a continuation of the SLPC as mainly published and highlighted by Scholten et al., consists of a thin film of catalytic material that resides on a high-surface-area support such as controlled-pore glass, silica, zeolites. Thus this concept of supported aqueous phase catalysis (SAPC) contains both a hydrophilic liquid and a hydrophilic organometallic catalytic complex on a solid support as shown in Fig. 12.15. "... 

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