Supported Aqueous-Phase Catalysts

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

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. 

In another interesting area in the study of hydroformylation, Davis developed the concept of supported aqueous phase (SAP) catalysis.175 A thin, aqueous film containing a water-soluble catalyst adheres to silica gel with a high surface area. The reaction occurs at the liquid-liquid interface. Through SAP catalysis, the hydroformylation of very hydrophobic alkenes, such as octene or dicyclopentadiene, is possible with the water-soluble catalyst [HRh(CO)(tppts)3]. 

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)... 

The supported aqueous phase methodology was applied to the system Pd(OAc)2/5 TPPTS, a catalytic precursor for the Trost-Tsuji reaction. The characterization of the solid by 31P MAS NMR confirms the presence of Pd°(TPPTS)3 as the main surface species. The catalytic properties of the solid were tested for the allylic substitution of E-cinnamylethylcarbonate by different nucleophiles such as ethyl acetoacetate, dimethyl malonate, morpholine, phenol, and 2-mercapto-pyridine. The absence of palladium leaching was demonstrated, and having solved the problem of water leaching from the solid to the organic phase, the SAP-Pd catalyst was successfully recycled several times without loss in its activity. It was used in a continuous flow experiment which... 

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. 

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

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. 

Supported aqueous phase (Chapter 3, Section 3.6, Chapter 5, Section 5.2.5) and supported ionic liquid phase catalysts, Chapter 7, Section 7.3) are probably not suitable for use with higher alkenes because the liquid feed slowly dissolves some of the water or ionic liquid changing the nature of the catalyst and leading to catalyst leaching. 

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. 

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. 

Co2(CO)6(tppts)2 is a brown-colored solid which is moderately stable in air, but is best handled and stored under an inert gas atmosphere. The compound is very soluble in water and insoluble in organic solvents like toluene or hexane. It exhibits in the 31P NMR (109.3 MHz, D20, 5°Q a singlet at 5 68.8 ppm. The IR displays a strong carbonyl stretching vibration at 1954 cm-1. The significant SO-absorptions are detectable at 1224 (sh, vst), 1200 (vst), 1039 (vst), and 623 (vst) cm-1. The compound has been used for carbonylation of phenyl ethyl bromide21 and on supported aqueous phase catalysts for the hydroformylation of olefins.22... 

An interesting new concept of catalyst immobilization is the use of supported aqueous phase catalysts. Here, the catalyst is immobilized in a thin water layer adhered within the pores of a high-surface-area porous support. A new Rh catalyst of this class with ligand 11 is stable, recyclable, and highly selective in the hydroformylation of higher alkenes to linear aldehydes.236... 

An attempt at recycling the rhodium catalyst was made my Delmas [46]. An immobilized rhodium catalyst in a supported aqueous phase could convert limo-nene to 95% of the desired product. The selectivity for the linear product was 70%. A kinetic study showed that the reaction rate was first order. 

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... 

Benaissa M, Jauregui-Haza UJ, Nikov I, Wilhelm AM, Delmas H (2003) Hydroformylation of linalool in a supported aqueous phase catalyst by immobilized rhodium complex kinetic study. Catal Today 79-80 419-125. doi 10.1016/s0920-5861(03)00074-9... 

Figure 5 Rendering of the silica-water-substrate phase in the supported aqueous phase catalysts...

Supported aqueous-phase catalysts can also be used to advantage. These supported catalysts have a thin aqueous film adhering to silica gel that contains the water-soluble complex (131). These catalysts are particularly useful for the hydroformylation of substrates such as oleyl alcohol (132). Since these catalytic reactions occur at the phase boundary, characteristics such as the water content can cause changes both in the reactivity and in the linear branched chain ratio of the product aldehyde. 

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]. 

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]. 

Those inunobilization procedures generally yielding supported solid-phase catalysts (SSPCs) have already been described in the preceding sections this section deals with catalytically active species (e. g., Wilkinson s or Vaska s complex) that are dissolved in liquids (therefore the catalyst is exactly the same as in homogeneous catalysis) or are even liquids themselves supported on porous solids. The principal structure of such an SLPC and the principal difference from an SAPC (Supported Aqueous-Phase Catalyst) is shown in Figure 2. 

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