Encapsulation using the sol-gel method

70molmol with hydroquinone being the only product observed. With the encapsulated [Cu (N02)4Pc ] greater selectivity is observed than for the homogeneous complex (100% compared to 87.5% for hydroquinone), while the encapsulated [Cu(Cli4Pc)] shows almost identical selectivity to that obtained by the 

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. 

Although each method has it own merits, in summary the ionic immobilization might be viewed as the method of choice if the system allows for its application, due to the ease of preparation and regeneration, particularly when thinking of continuous reactors with structured packings. 

An exciting developing area is that of the use of resins as supports as these offer some additional properties to oxide supports, such as sweUing and an almost unlimited variety of stable surface groups. 

Regarding characterization, the very concept of single-site catalysts is the precise synthesis on or within insoluble supports of single, uniform catalytic sites, with the same control that has long been the hallmark of homogeneous catalysts. Single-site 

---

See also in sourсe #XX -- [ ]

---