Polymer supported metal catalysts catalyst recycling

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. 

Polymer-supported catalysts incorporating organometaUic complexes also behave in much the same way as their soluble analogues (28). Extensive research has been done in attempts to develop supported rhodium complex catalysts for olefin hydroformylation and methanol carbonylation, but the effort has not been commercially successful. The difficulty is that the polymer-supported catalysts are not sufftciendy stable the valuable metal is continuously leached into the product stream (28). Consequendy, the soHd catalysts fail to eliminate the problems of corrosion and catalyst recovery and recycle that are characteristic of solution catalysis. 

In the early 1970 s, Bayer et al. reported the first use of soluble polymers as supports for the homogeneous catalysts. [52] They used non-crosslinked linear polystyrene (Mw ca. 100 000), which was chloromethylated and converted by treatment with potassium diphenylphosphide into soluble polydiphenyl(styrylmethyl)phosphines. Soluble macromolecular metal complexes were prepared by addition of various metal precursors e.g. [Rh(PPh3)Cl] and [RhH(CO)(PPh3)3]. The first complex was used in the hydrogenation reaction of 1-pentene at 22°C and 1 atm. H2. After 24 h (50% conversion in 3 h) the reaction solution was filtered through a polyamide membrane [53] and the catalysts could be retained quantitatively in the membrane filtration cell. [54] The catalyst was recycled 5 times. Using the second complex, a hydroformylation reaction of 1-pentene was carried out. After 72 h the reaction mixture was filtered through a polyamide membrane and recycled twice. 

In the present study, we describe the methods of preparing the silica hybrids of linear and branched fiinctional polysiloxanes which could be used as a support for metal complex catalysts. The way in which the catalyst operates when it is attached to the polysiloxane moiety of the hybrid suspended in a polysiloxane solvent should be similar to the way it operates when in solution. Thus, its high catalytic activity is expected. On the other hand, it is easily separated from the reaction products and may be recycled or used in the continuous process. A high catalytic activity and specificity may be achieved if a polymer with a highly branched structure is used for the immobilization of catalysts [1-3]. Considerable amounts of catalytic groups may be placed in the external part of the branched macromolecule. 

The need for a process for the direct conversion of benzene to phenol was mentioned in the foregoing. A polymer-supported vanadyl chelate has been used (at 1 mol%) to obtain phenol (100% selectivity at 30% conversion) by treatment with 30% hydrogen peroxide.209 There was no leaching of the metal ion. The catalyst could be recycled ten times before it started breaking up. Further work with an inorganic support might allow the development of a commercial process. 

The desire to convert benzene directly to phenol with 30% hydrogen peroxide was mentioned in Chap. 4. A polymer-supported salicylimine vanadyl complex (1 mol%) was used to catalyze this reaction. Phenol was obtained in 100% yield at 30% conversion.217 There was no leaching of the metal. The catalyst was recycled ten times after which it started to break up. Oxidation of ligands is often a problem with oxidation catalysts. Inorganic supports not subject to such oxidation need to be tried to extend the life of such catalytic agents. 

Polymer supported Palladium(II) complex catalyst was synthesised using chloromethylated styrene-divinyl benzene copolymer as a support by sequential attachment of glycine and a metal salt solution. It was characterized using various methods such as IR, UV-Vis, ESR, DTA-TGA, SEM and surface area measurement. Swelling studies, moisture content and bulk density have also been investigated. The catalytic activity of the catalyst was tested for the oxidation of toluene. The effect on it of various parameters has been seen. The recycling efficiency has also been studied. 

Polymer supported Pd(II)-glycine complex was found to be stable upto 150°C on the basis of DTA-TG analyses. A change in morphology of the catalyst is indicative of anchoring the ligand as well as the metal ions on the surface of polymer. The catalytic activity of the catalyst was tested for oxidation of toluene under various conditions. The recycling efficiency... 

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