Polymer supported metal catalysts preparation

The second general method, IMPR, for the preparation of polymer supported metal catalysts is much less popular. In spite of this, microencapsulation of palladium in a polyurea matrix, generated by interfacial polymerization of isocyanate oligomers in the presence of palladium acetate [128], proved to be very effective in the production of the EnCat catalysts (Scheme 3). In this case, the formation of the polymer matrix implies only hydrolysis-condensation processes, and is therefore much more compatible with the presence of a transition metal compound. That is why palladium(II) survives the microencapsulation reaction... 

Recently, Chaudhari compared the activity of dispersed nanosized metal particles prepared by chemical or radiolytic reduction and stabilized by various polymers (PVP, PVA or poly(methylvinyl ether)) with the one of conventional supported metal catalysts in the partial hydrogenation of 2-butyne-l,4-diol. Several transition metals (e.g., Pd, Pt, Rh, Ru, Ni) were prepared according to conventional methods and subsequently investigated [89]. In general, the catalysts prepared by chemical reduction methods were more active than those prepared by radiolysis, and in all cases aqueous colloids showed a higher catalytic activity (up to 40-fold) in comparison with corresponding conventional catalysts. The best results were obtained with cubic Pd nanosized particles obtained by chemical reduction (Table 9.13). 

Aluminum chloride and its derivatives are the most familiar Lewis acids and are routinely employed in many Lewis acid-promoted synthetic transformations. The first polymer-supported metal Lewis acids to be studied were polymers attached by weak chemical or physical interactions to a Lewis acid. In the 1970s Neckers and coworkers reported the use of styrene-divinylbenzene copolymer-supported AlCl,- or BF3 as catalyst in condensations, esterifications, and acetalization of alcohols [11,12]. This type of polymer-supported AICI3 (1) is readily prepared by impregnation of a polystyrene resin with AICI3 in a suitable solvent. Subsequent removal of the solvent leaves a tightly bound complex of the resin and AICI3. The hydrophobic nature of polystyrene protects the moisture-sensitive Lewis acid from hydrolysis, and in this form the Lewis acid is considerably less sensitive to deactivation by hydrolysis. This polymer complex could be used as a mild Lewis acid catalyst for condensation of relatively acid-sensitive dicyclopropylcarbinol to an ether (Eq. 1) [13],... 

Here a compact survey on recent developments in the field of polymer-supported metal species for catalytic epoxidation is given. Progress since 2000 is considered focusing on catalyst preparation, catalytic performance, and catalyst recyclability rather than physicochemical properties of the polymers. 

Many books and reviews cover the preparation, characterization4 and use of homogeneous5 and heterogeneous6 catalysts. Others describe supported metal catalysts,7 other supported catalysts and reagents,8 and their use in preparative chemistry,9 the use of polymers as supports,10 trends in industrial catalysis,11 and the environmentally friendly nature of solid catalysts and reagents.12... 

The final step in the preparation of polymer-supported metal nanoparticles is the generation of the nanoparticles within the polymer, which is usually accomplished by reduction of the polymer-bound metal precursors. Often, techniques similar to the preparation of conventional metal catalysts supported on inorganic solids are employed. 

The number and diversity of transition metal phosphine complexes is vast and a wide range have been used as catalysts for synthetic organic transformations for many years. It therefore comes as little surprise that the preparation of polymer-supported metal phosphine complexes and assessment of their catalytic activity has attracted much attention. Supported phosphine ligands and their metal complexes prepared from 1981 to 2001 can be found in a review published in 2002. Discussed here are examples in the literature from 1996 to the present together with a selected number of those from 1981 to 1996 where particularly notable synthetic methods have been used or where key points should be raised. 

The polyamide obtained by polycondensation of 2,6-diaminopyridine and 2,6-pyridine dicarboxylic acid was the first polymer to assemble itself into a double helix (DNA-type) in solution. The synthesis and physicochemical characterization of some polymer-supported rhodium catalysts based on polyamides containing 2,6- and 2,5-pyridine units were reported by Michalska and Strzelec (2000) these catalysts were used for the hydrosilylation of vinyl compounds such as phenylacetylene. Chevallier et al. (2002) prepared polyamide-esters from 2,6-pyridine dicarboxylic acid and thanolamine derivatives and investigated their polymer sorption behavior towards heavy metal ions. Finally, Scorlanu et al. (2006) also prepared a polymer with improved performance based on polyureas containing 2,6-pyridine moiety and polyparabanic acids, and polymethane-ureas containing 2,6-pyridine rings. 

Design parameters of the anode catalyst for the polymer electrolyte membrane fiiel cells were investigated in the aspect of active metal size and inter-metal distances. Various kinds of catalysts were prepared by using pretreated Ketjenblacks as support materials. The prepared electro-catalysts have the morphology such as the sizes of active metal are in the range from 2.0 to 2.8nm and the inter-metal distances are 5.0 to 14.2nm. The electro-catalysts were evaluated as an electrode of PEMFC. In Fig. 1, it looked as if there was a correlation between inter-metal distances and cell performance, i.e. the larger inter-metal distances are related to the inferior cell performance. 

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