Silica heterogeneous polymerization catalysts from

Scheme 71 Preparation of heterogeneous neodymium-based diene polymerization catalysts on modified silica (adopted from [306])...

The advantages that heterogeneous catalysts have is that they are easily separable from the product, and can be recycled. A number of studies have been conducted in which ligands have been attached or bound to polymeric material to provide an immobilized ligand, and these include polyacrylate and silica [27], polyurea [28], polythiourea [29], polyether [30, 31] and dendritic [32] systems. Upon metal coordination, the immobilized catalysts have retained most of the activity and selectivity, but they now provide the advantage of simple separation and recycling. For exam-... 

From this brief discussion it is clear that, even if the CO-reduced catalyst is simpler than the industrial catalyst, the high degree of heterogeneity of the Cr(II) species present on the surface of the amorphous silica makes the comprehension of the polymerization mechanism on the Phillips catalyst a complex and difficult task that still requires work (5). The heterogeneity of the Cr(II) structure is reflected in a... 

In a similar vein, a series of papers published between 2002 and 2008 contains spectacular claims of highly enantioselective asymmetric additions of water to styrenes, unsaturated carboxylic acids, or simple terminal alkenes [34-Al]. The catalysts used are of the heterogeneous type and based on chiral biopolymers such as wool, gelatin, or chitosan as solid supports (sometimes in combination with silica or ion-exchange resins) that are doped with transition metal salts. This series of papers contains spectacular claims, insufficient experimental data, and erroneous chemical structures for the biopolymers used. As earlier work from the same group of authors on asymmetric catalysis on bio-polymeric supports is irreproducible [42], one is well advised to await independent confirmation of those results. 

Several heterogeneous catalysts have been shown to effect related multicomponent couplings. These include cross-linked polymeric ionic liquid material-supported copper (Cu-CPSIL), silica-dispersed CuO (CuO/Si02), and imidazolium-loaded Merrifield resin-supported copper (Cu-PSIL), all of which can be used in water at room temperature to arrive at 1,4-disubstituted-1,2,3-triazoles from alkyl halides, NaNs, and terminal alkynes. Each can be filtered and reused several times with minimal loss of efficacy. Multistep flow synthesis, specifically including generation of underused vinyl azides and their subsequent click conversions to vinyl triazoles, has also been reported. ... 

In a heterogeneous SPS polymerization at a typical reaction temperature (40-90 °C) with a silica-supported catalyst, the reaction mixture undergoes a series of physical changes as SPS particles precipitate out from the liquid phase. Initially, the total solid content (TSC) in the reaction mixture is very small and it is a clear liquid (Fig. 8.5a). 

---