Sol-gel entrapped catalysts

Sol-gel-entrapped catalysts provide a generic method for the encapsulation of a wide variety of catalysts. Comprehensive reviews are available [59-61]. The essence ofthe concept is captured in Figure 5.6. 

The R D director of SiliCyle, Dr Francois Beland, has been pivotal in guiding the development of the SiliaCat series of sol-gel entrapped catalysts. And I would like to recall here also his and Hugo St-Laurent s visit to Palermo in October 2007. 

Along with these recent technologies, sol-gel entrapped catalysts made of organically modified silicates (ORMOSIL) doped with one or... 

The high catalyst loading typical of sol gel entrapped catalysts ensures a desirably high substrate/catalyst (S/C) ratio as the major part of the heterogeneous catalyst weight originates from the silicate matrix. For example, in a preparative-scale reaction of the alcohol raol-(2-naphthyl)-ethanol only 250 mg of sol-gel CaLB immobilizate could be used per 10 g of substrate. For comparison, all this makes the process based on sol-gel immobilized lipase very competitive with the commercial BASF process using lipase immobilized on Amberlite to produce the amine at a scale of 1000 tons per year. 

Several sol-gel entrapped catalysts are likely to soon find commercial applications. A variety of transition metal catalysts physically entrapped in silica matrices as ion pairs generated from the metal halides and quaternary ammonium or phosphonium salts developed in the mid-1990s by Avnir and Blum resulted in truly heterogeneous, stable and... 

Human life, furthermore, certainly benefits from a less polluted world, and here, again, sol-gel entrapped catalysts are, literally, able to have transferred to within their large inner porosity the whole chemistry of fine chemicals production. Think for instance of an innocuous easily handled orange powder called SiliaCat TEMPO (Chapter 5) that added to a mixture of alcohols at 0 °C with a modest excess of aqueous, cold bleach rapidly converts them into all those fragrances, vitamins, hormones and drugs made of carbonyl compounds. 

The alcohol substrate and at low partial pressure are dissolved in the homogeneous scCOj phase which, upon contact with the powdered sol-gel entrapped catalyst, spills the reactants into the pores where the oxidative dehydrogenation takes place. It then extracts the products. Furthermore, the RuO " immobilization at an imidazolium-modified surface which acts as a potent solubilizing agent accelerates substrate/product dissolution and thus the reaction rate. As evident by the smooth oxidation of the less reactive primary aliphatic alcohol, 1-octanol, a versatile active catalyst is obtained that can be used for the fast conversion of different substrates. Moreover, scCO assists in water removal (thus preventing aldehyde hydration and overoxidation) through its well-known ability of drying solid matrices. 

Gelman F., Blum J., Schumann H., Avnir D. One-pot reactions with sol-gel entrapped catalysts, acids and bases. J. Sol-gel Sci. Technol. 2003 26(1-3) 43-46 Gill L, Ballesteros A. Encapsulation of biologicals within sUicate, siloxane, and hybrid sol-gel polymers An efficient and generic approach. J. Am. Chem. Soc. 1998 120(34) 8587-8598 Gun G., Tsionsky M., Lev O. Voltametric studies of composite ceramic carbon working electrodes. Anal. Chim. Acta 1994 294(3) 261-270... 

Hydroformylation of alkynes by sol-gel catalysts (e.g.. Scheme 24-12) has been extensively studied in several laboratories. Examples are (Cauzzi, 1997 Lindner, 2000 de Campos, 2003 van Leeuwen, 2003). In many cases the sol-gel entrapped catalysts lead to a very high ratio of lineanbranched aldehydes. Notable examples of catalytic carbony-lation are the manufacture of acetic acid from methanol in the presence of entrapped... 

Other notable addition reactions catalyzed by sol-gel entrapped catalysts are the (enantioselective) hydrocyanation of benzaldehyde (Scheme 24-17) (Shvo, 1994) hydrocar-boxylation of amines by a sol-gel bound ruthenium catalyst (Scheme 24-18) (Krocher, 1996), the hydrosilylation of olefins by an immobilized rhodium catalyst (Scheme 24-19) (Capka, 1992b), and the hydration ofalkynes by sol-gel entrapped palladium or platinum compounds (Scheme 24-20) (Blum, 2000 Israelsohn, 2002). 

Whereas die activity of homogeneous catalysts depends often on the nature of the medium the sol-gel entrapped catalysts operate in general in a much wider range of solvents. It is particularly noteworthy that water, which is favored for both its economical and environmental features, can be used for catalytic transformations ofhydrophilic substrates by sol-gel entrapped catalysts. It has been demonstrated, e.g., that the enantioselective... 

Finally, the first commercialized sol-gel-entrapped catalyst is due to Reetz [55]. A number of lipases entrapped in hydrophobic sol-gel matrices can be found that are marketed by Fluka high activity and stability characterize these entrapped enzymes. 

Under certain conditions, one key advantage of sol-gel entrapped catalysts identified by Blum et al. in the early studies in the 1990s is the leach-proof nature of many physically entrapped catalysts [6]. Then, the same sol-gel-entrapped catalyst can be employed in incompatible solvents such as in the case of encapsulated Ru-BINAP and other water-insoluble chiral transition metal catalysts utilized for the enantioselective hydrogenation of itaconic acid in water instead of in organic solvent [32]. 

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