Microencapsulation metal compounds

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... 

Type 3 metal complexes involve the physical interaction of a metal complex, chelates, or metal cluster with an organic polymer or inorganic high molecular weight compound. The preparation of type 3 compounds differs from those of type 1 and type 2, as they are ultimately achieved through the use of adsorption, deposition by evaporation, microencapsulation, and various other methods. 

Chemical stabilization is the alteration of the chemical form of the contaminants to make them resistant to aqueous leaching. Solidification/Stabilization processes are formulated to minimize the solubility of metals by controlling pH and alkalinity. Entrapment or microencapsulation may immobilize anions, which are more difficult to bind in insoluble compounds. Chemical stabilization of organic compounds maybe possible, but the mechanisms involved are poorly understood [22]. 

The reaction involves the amine-catalyzed conversion of an aldehyde into a nitroalkene by reaction with nitromethane followed by a transition-metal-catalyzed Michael addition of p-dicarbonyl compounds in the same reaction vessel. Typically, amine catalysts and nickel complexes are incompatible due to their tendency to chelate and to render each other inactive. However, microencapsulation of poly(ethyleneimine) (PEI) forms catalyst 140, which can successfully be used in tandem with the nickel-based catalyst 141 (Figure 3.6). 

The term microencapsulating implies obtaining nanoparticles of a compound in protecting covers which consist of film-forming polymeric materials [60]. The encapsulated substance forming the nucleus of the microcapsule may be a metal complex or a metal nanoparticle. The polymeric cover separates the nanoparticles from each other and from the environment and serves also as a stabilizer. The cover can be made from natural polymers, for instance proteins... 

A new strategy (at the time) of microencapsulated osmium tetroxide was published by Kobayashi and coworkers in 1998 [38]. The metal is immobilized onto a polymer on the basis of physical envelopment by the polymer and on electron interactions between the n electrons of the benzene rings of the polystyrene-based polymer and a vacant orbital of the Lewis acid. Using cydohexene as a model compound it was shown that this microencapsulated osmium tetroxide (MC OSO4) can be used as a catalyst in the dihydroxylation with NMO as stoichiometric oxidant (Scheme 1.15). 

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