Entrapped sol-gel complexes

When only the enzyme was entrapped in the sol-gel, the rhodium complexes poisoned the enzyme, resulting in low yields, whereas when both enzyme and complex were trapped in separate sol-gel mixtures, good yields were achieved. In one particular case, this procedure resulted in an increase in yield of 1-pentyl undecano-ate from 1 -undecenoic acid and 1 -pentanol by a factor of 6.5, indicating successful site isolation of the mutually interfering catalysts. 

The limitations of the method come from the stability of the complexes under sol gel conditions however, even biocatalysts can be suitable for entrapment in sol-gel materials.87... 

It should be pointed out that the addition of substances, which could improve the biocompatibility of sol-gel processing and the functional characteristics of the silica matrix, is practiced rather widely. Polyethylene glycol) is one of such additives [110— 113]. Enzyme stabilization was favored by formation of polyelectrolyte complexes with polymers. For example, an increase in the lactate oxidase and glycolate oxidase activity and lifetime took place when they were combined with poly(N-vinylimida-zole) and poly(ethyleneimine), respectively, prior to their immobilization [87,114]. To improve the functional efficiency of entrapped horseradish peroxidase, a graft copolymer of polyvinylimidazole and polyvinylpyridine was added [115,116]. As shown in Refs. [117,118], the denaturation of calcium-binding proteins, cod III parvalbumin and oncomodulin, in the course of sol-gel processing could be decreased by complexation with calcium cations. 

McEvoy A.K., McDonagh C.M., MacCraith B.D., Development of a fiber opic dissolved oxygen sensor based on quenching of a ruthenium complex entrapped in a porous sol-gel film, SPIEProc. 2508, 190-198, (1995). 

Table 42.4 Enantioselective hydrogenation of itaconic acid using sol-gel-entrapped Rh complexes [51].

Sensors based on the above reaction scheme have been developed for Al3+, Zn2+, Cu2+, Ca2+, Pb2+, Hg2"1", K+, Li+, etc. A polycation, protamine sensor has also been developed using 2/7/-dichlorofluorescein octadecyl ester (DCFOE) doped in polymer membranes. However, most of these sensors are pH dependent due to the pH dependence of the cation complexation reactions. The cation ion indicators can be immobilized on any solid support, such as silica, cellulose, ion-exchange resin, porous glass, sol-gel, or entrapped in polymer membranes. 

The sol-gel entrapment of the metal complexes [Ru(p-cymene)(BINAP)Cl]Cl and the rhodium complexes formed in situ from the reaction of [Rh(COD)Cl]2 with DlOP and BPPM has been reported by Avnir and coworkers [198]. The metal complexes were entrapped by two different methods the first involved addition of tetramethoxysilane to a THF solution of the metal complex and triethylamine, while the second method was a two-step process in which aqueous NH4OH was added to a solution of HCl, tetramethoxysilane and methanol at pH 1.96 followed by a THF solution of the appropriate metal complex. The gel obtained by each method was then dried, crushed, washed with boiling CH2CI2, sonicated in the same solvent and dried in vacuo at room temperature until constant weight was achieved. Hydrogenation of itaconic acid by these entrapped catalysts afforded near-quantitative yields of methylsuccinic acid with up to 78% e.e. In addition, the catalysts were found to be leach-proof in ethanol and other polar solvents, and could be recycled. 

These were prepared by tethering Rh and Pt complexes to silica-supported metal catalysts (metal = Pd, Ni, Ru, Au). The catalysts are very active in the hydrogenation of benzene derivatives to the corresponding substituted cyclohexanes under mild conditions. The activities are higher than those of the separate homogeneous complexes, complexes just tethered to silica, or the silica-supported heterogeneous catalysts. When the sol-gel-entrapped [Rh2Co2(CO)12] complex was heat-treated at 100°C, immobilized metallic nanoparticles were formed.425 The catalyst thus prepared efficiently catalyzed substituted benzene derivatives. 

Sol-gel preparation methods offer the possibility of entrapment of the metallic complex. This method has been used for bimetallic Rh-Pd177 and Rh-Co178 systems, with silica employed as the support material. The metallic nanoparticles in these catalysts are very efficient. However, their... 

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