SAP catalyst

Supported aqueous phase (SAP) catalysts (16) employ an aqueous film of TPPTS or similar ligand, deposited on a soHd support, eg, controlled pore glass. Whereas these supported catalysts overcome some of the principal limitations experienced using heterogeneous catalysts, including rhodium leaching and rapid catalyst deactivation, SAP catalysts have not found commercial appHcation as of this writing. 

SLP) systems where a high boiling organic solvent was used as the liquid film. Later, supported aqueous phase (SAP) catalysts where intensively studied [71]. 

To our knowledge, none of the developed SLP and SAP catalysts made their way into a technical process. Obviously, the possibility of using a supported liquid catalyst in a continuous liquid phase reaction is generally very restricted. The reason is that a very low solubility of the liquid in the feedstock/product mixture is enough to remove the catalyst from the surface over time (due to the very small amounts of liquid on the support). Even worse, the immobilised liquid film can be removed from the support physically by the mechanical forces of the continuous flow even in the case of complete immiscibility. 

In future research, it may be interesting to combine supported liquid phase (SLP) catalysts or supported aqueous phase (SAP) catalysts [61,62] with SCCO2 as a mobile phase. 

SAP catalysts have also been applied in the Wacker oxidation584 of higher olefins where the separation of products from the catalyst is cumbersome. Palladium(II) and copper(II) salts immobilized on controlled pore glass CPG-240 in the presence of water catalysed the oxidation of 1-heptene to 2-heptanone in conversions up to 24%.585 Significant isomerization to 2-heptene and 3-heptene (isomerization/oxidation=2/3) was also observed. However, an advantage of SAP-Wacker oxidation catalysts over classical systems is that the Cu(II) is confined to the support and therefore not corrosive whereas aqueous Cu2+ is very corrosive to steel. 

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]. 

Heterogenization of the catalyst Pd/tppts or Pd/tppms was also performed by deposition on silica (SAP catalyst) [86-88]. The activity of the catalyst was found to be dependent on the water content of the support however, the SAP catalyst was found to be drastically more active using PhCN as the co-solvent than its homologous biphasic system. 

Supported liquid-phase catalysts (SLPC) Supported organic-phase (SOP) catalysts Supported aqueous-phase (SAP) catalysts... 

Although SAP catalysts and their non-aqueous analogues have been reported only for less than a decade, it is clear that the reaction chemistries that can be accomplished in this configuration continues to burgeon. This method of immobilization has proven effective and as the realm of water-soluble organometallic catalysts expands, so can the field of SAPCs and its variants. Scientifically and in special, Delmas and his collegues still work on this topic [33]. It is hoped that economic proof will follow. 

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]. 

Purwanto and Delmas [10] proposed the addition of co-solvent (ethanol) to enhance the solubility of 1-octene in the aqueous phase so that the overall reaction rate was increased, and their kinetic study led to a rate model similar to that in homogeneous liquid systems consistently from the point of view of bulk reaction mechanism. Chaudhari et al. [11] reported the improvement of the hydroformylation rate by addition of a small amount of PPhj to the biphasic system to enrich the effective catalyst species at the liquid-liquid interface. Kalck et al. [12] tested two more approaches to improve the mass transfer rate of biphasic hydroformylation of 1-octene and 1-decene with catalyst precursor [Rh2(/i-S Bu)2(CO)2(TPPTS)3j use the phase-transfer agent /i-cyclodextrin to transport the substrate into the aqueous phase to react there (see Section 2.2.3.2.2), and the supported aqueous-phase (SAP) catalyst to increase the reaction area due to the high specific surface area of porous silica (see Section 2.6). The improved conversion and TOF gave informative suggestions for the reaction mechanisms. 

Different inorganic materials have been used as supports in SAPC glass beads of controlled pore size [6,14—17, 24, 39—42,44,45, 53] porous [11,15,18,19, 21, 23, 28-32, 35, 36, 38, 39, 43, 48, 49] and nonporous [28, 33, 48] silica nanoparticles synthetic phosphate [27] carbon [39], and alumina [15,39]. It was shown that glass beads, siKca, and synthetic phosphate gave the best performance. All these supports have a high specific surface with an average diameter of the pores, in the case of porous supports, between 60 and 345 A. The use of chitosan as a natural polymeric support of SAP catalysts for the synthesis of fine chemicals has been reported recently [54]. 

Several methods have been used for preparing the SAP catalysts. According to the preparation procedure, the methods can be classified into two groups (a) indirect methods, when the support is first impregnated with the hydrosoluble catalytic complex, then dried and rehydrated before use [6,14,15,17, 41, 42, 49] (b) direct methods, when the support, catalytic complex, and water are mixed at the same time in the reaction system [15, 21, 29]. In general, the best conversions in the hydroformylation of alkenes by SAPC have been obtained by indirect preparation. However, the direct methods being of much easier implementation than the indirect ones, they are most widely used. 

Using the techniques developed to synthesize the organometallic-based SAP catalysts, several enzyme-based SAP catalyst, using porous glass beads and the enzymes polyphenol oxidase and horseradish peroxidase have been studied [53]. These SAP catalysts were active in the reaction of phenol with O2 or H2O2, respectively. Thus, porous enzyme-based SAP catalysts can be synthesized. 

There are four major classes of heterogenized homogeneous catalysts, and these are listed in Table 6.6 along with subcategories of each class. The most important and recent is the supported aqueous phase (SAP) catalyst. 

---