Supported aqueous-phase catalysis

TABLE 3.7. Hydroformylation performance of SAPC compared with homogeneous andbiphasic systems 

Horvath performed experiments using substrates with different solubilities in water and showed that, under optimal conditions, this solubility did not influence the activity [67]. These experiments clearly support the fact that the reaction takes place at the organic-water interphase. Furthermore, he performed a hydroformylation reaction in a continuous system and even under reaction conditions no leaching of rhodium complex was detected. Water obviously leaches if the SAPC is used in a continuous flow system, which in a practical application should be compensated for by using water-saturated organic solvents. 

A water-soluble chelating diphosphine ligand (9) based on the xanthene backbone was also studied as supported aqueous phase catalysts. It was shown that this ligand performed well as SAPC since it is much more selective than other SAPC systems reported in literature [68]. Recycling experiments showed that these catalysts retained their activity and selectivity for at least ten consecutive runs, whereas under similar conditions the TPPTS based catalyst showed a reduced performance in the fourth run. 

This shows that the strong chelating effect of the bidentate ligand efficiently retains the metal attached to the support. 

Mortreux and co-workers compared the activity of the SAPC catalysts with that of the homogeneous analogue in the hydroformylation of methyl acrylate [69]. They observed an activity for the SAPC that was strongly dependent on the amount of water present in the system. More remarkably, the optimised activity of the SAPC was higher than that of the homogeneous systems. This effect was ascribed to the polar interactions between the substrate and the silica support. This effect was not observed for nonpolar substrates such as propene, which supported the hypothesis. 

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Figure 3.14. Schematic representation of the concept of supported aqueous phase catalysis...

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. 

Arhancet JP, Davis ME, Merola JS, Hanson BE (1989) Hydroformylation by supported aqueous-phase catalysis a new class of heterogeneous catalysts. Nature 339(6224) 454-455... 

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]. 

Another problem in aqueous catalysis is the very low solubility of organic substrates in water. One way to solve this problem is the use of a promoter ligand another solution seems to be supported aqueous phase catalysis. 

Considering the hydrophilic properties of the support, the effectiveness of polysaccharide aerogel microspheres as catalyst support was evidenced in the so-called Supported Aqueous Phase Catalysis [131]. The stability of the catalyst obtained was investigated in terms of textural stability and catalytic activity in the reaction of substitution of an allyl carbonate with morpholine catalyzed by the hydrosoluble Pd (TPPTS)3 complex [132]. 

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]. 

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. 

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. 

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