Water catalyst immobilization

Immobilization of catalysts is an important process design feature (see Chapter 9.9). A recent example of catalyst immobilization is the biphasic approach which seems superior to immobilization on solids, as successfully proven in the Ruhrchemie/Rhone Poulenc process for the hydro-formylation of olefins.286 Supported liquid phase catalysis was devised as a method for the immobilization of homogeneous catalysts on solids. When the liquid phase is water, a water-soluble catalyst may be physically bound to the solid. 

Another way to omit the fluorous solvent would be to utilize a catalyst immobilization solvent that is not fluorinated, such as water. We demonstrated the application of a phase change after reaction permits facile recycle of hydrophilic catalysts. This method is called OATS (Organic-Aqueous Tunable Solvent) (15). 

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... 

Rhodium(I) complexes immobilized on silica using 3-(3-silylpropyl)-2,4-pentanedio-nato ligands (38) show good activity in the hydrosilylation of 1-octene with HSi(OEt)3 at 100°C60. The immobilized Rh catalysts are prepared by (i) reaction of (EtO)3Si(CH2)3C(COMe)2Rh(CO)2 with untreated silica (Catalyst A), (ii) reaction of Rh(acac)(CO)2 (acac = acetylacetonato = 2,4-pentanedionato) with silica modified by [(EtO)3Si(CH2)3C(COMe)2] prior to the complexation (Catalyst B), (iii) reaction of [Rh(CO)2Cl]2 with a polycondensate of [(EtO)3Si(CH2)3C(COMe)2] , Si(OEt)4 and water (Catalyst C) and (iv) sol-gel processing of (EtO)3Si(CH2)3C(COMe)2Rh(CO)2 and Si(OEt)4 (Catalyst D). The Catalysts A and B show ca three times better activity than their homogeneous counterparts, while the Catalyst D exhibits only low activity and the Catalyst C is inactive60. 

Catalysts immobilized on resins are quite often used for catalytic reactions. Phase-transfer conditions, guaranteeing separation and recovery of the catalysts by use of water/organic solvent biphasic systems, are another possibility. Unlike homogeneous processes, these kind of catalytic processes take account of product loss and substrate selectivity. Furthermore,... 

Controlled potential electrolysis of 32 inl0% water in CF3CH2OH gave 21 turnovers (O2 per 32) whereas the activity jumps to 33,500 turnovers for the catalyst immobilized on an ITO electrode in aqueous solution. Unfortunately, no attempts to use chemical oxidants in a completely homogeneous system for 32 are reported. 

Some further studies still deal with the Friedel-Crafts acylation in fluorous fluids. These fluids all have very unusual properties such as high density and high stability, low solvent strength and extremely low solubility in water and organic compounds, and, finally, nonflammability. These properties allow their easy handling and reuse. Friedel-Crafts acylation of electron-rich aromatic substrates can be very efficiently performed in a fluorous biphasic system (FBS), which represents a benign technique for phase separation, and catalyst immobilization and recycling. 

The concept of supported Hquid catalysis is not restricted to liquid salts. In order to apply the concept of uniform surface properties and efficient catalyst immobilization, several authors investigated the SLP concept during the 1970s and 1980s [5-11]. However, later studies revealed that the evaporation of the loaded liquid cannot be avoided completely during operation. This is especially a problem when using water as the Hquid phase [12-17]. In these supported aqueous phase (SAP) systems, the thin film of water evaporated quickly under reaction conditions, making the concept appHcable only for slurry-phase reactions with hydrophobic reaction mixtures. 

Recyclability requires rates and yields of reactions to be maintained at a reasonable level after repeated reactions. In particular, reactions containing a transition metal catalyst immobilized into the IL of a biphasic reaction system have proved to be recyclable. Generally, recycling is based on the non-volatile nature of ILs and the solubility differences between ILs, organic compounds and water. Products can be extracted from ILs with a nonpolar solvent or they can be separated by distillation. A water-immiscible IL can be washed with water to get a water-soluble product or side products out of the reaction mixture [Karkkainen, 2007]. 

A water-soluble ruthenium complex, HRu(C0)Cl(TPPMs)3-2H20, was used for hydrogenation of olefins in the biphasic system decalin-water [155], Ruthenium catalyst immobilized on a hydrophilic microporous resin turned out to be more selective for partial hydrogenation of benzene to cyclohexene than the same catalyst immobilized on charcoal. 

Hydroformylation of 1-octene under aqueous two-phase conditions was studied using an n N-heterocyclic carbene rhodium catalyst immobilized to an amphiphilic, water-soluble block copolymer support. The catalyst showed high activity up to 2360h turnover frequency at 100°C and 50bar CO H2 = 1 1 pressure in four consecutive cycles [103]. 

PHOTOPRODUCTION OF HYDROGEN FROM WATER USING IMMOBILIZED BIOLOGICAL AND SYNTHETIC CATALYSTS... 

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