Rhodium catalysts polymer-based

This approach can be illustrated by describing the preparation of the polymer rhodium catalyst II (Sec. 9-lg). The synthesis is based on a nucleophilic substitution reaction of chlor-omethylated polystyrene [Grubbs and Kroll, 1971] ... 

Stereoregular polyacetylenes have been attracting great attention due to the characteristic features based on their hehcal structure and the conjugated main chain [22, 23], The dynamic structure change in the helical polymers is particularly attractive. Stereoregular polyacetylenes with the c/s-transoidal structure have been synthesized by rhodium catalysts from various acetylene derivatives. 

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. 

Michalska, Z. M., Strzelec, K. (2000), Synthesis and physicochemical characterization of polymeric supports and polymer-supported rhodium catalysts based on polyamides having a pyridine moiety. Reactive Functional Polymers, 44,189-99. 

Ionic liquids have already been demonstrated to be effective membrane materials for gas separation when supported within a porous polymer support. However, supported ionic liquid membranes offer another versatile approach by which to perform two-phase catalysis. This technology combines some of the advantages of the ionic liquid as a catalyst solvent with the ruggedness of the ionic liquid-polymer gels. Transition metal complexes based on palladium or rhodium have been incorporated into gas-permeable polymer gels composed of [BMIM][PFg] and poly(vinyli-dene fluoride)-hexafluoropropylene copolymer and have been used to investigate the hydrogenation of propene [21]. 

Most of the more recent studies have concentrated on rhodium. An effective system for a gas-phase reaction was reported by Arai et al. (107). The catalyst support was silica gel, which was desirable for its high surface area properties (293 m3/g). This was covered with a polymer formed from styrene and divinylbenzene, either by emulsion (A) or by solution (B) polymerization. Each of these base materials was then functionalized by the reactions shown in Eq. (49). 

Adsorption is commonly used for catalyst removal/recovery. The process involves treating the polymer solution with suitable materials which adsorb the catalyst residue and are then removed by filtration. Panster et al. [105] proposed a method involving adsorbers made from organosiloxane copolycondensates to recover rhodium and ruthenium catalysts from solutions of HNBR. These authors claimed that the residual rhodium could be reduced to less than 5 ppm, based on the HNBR content which had a hydrogenation conversion of over... 

The nature of the support can have a very profound influence on the catalyst activity. Thus, phosphinated polyvinyl chloride supports are fairly inactive (75), and phosphinated polystyrene catalysts are considerably more active (57), but rather less active particularly when cyclic olefins are the substrates than phosphinated silica supports (76). Silica-supported catalysts may be more active because the rhodium(I) complexes are bound to the outside of the silica surface and are, therefore, more readily available to the reactants than in the polystyrene-based catalysts where the rhodium(I) complex may be deep inside the polymer beads. If this is so, the polystyrene-based catalysts should be more valuable when it is desired to hydrogenate selectively one olefin in a mixture of olefins, whereas the silica-based catalysts should be more valuable when a rapid hydrogenation of a pure substrate is required. 

Development of molecularly imprinted enantioselective hydrogenation catalysts based on immobilised rhodium complexes was reported by Gamez et al. [29]. The imprinted catalysts were prepared by polymerising Rh(I)-(A,A -dimethyl-l,2-diphe-nylethanediamine) with di- and tri-isocyanates, using a chiral alkoxide as the template (9). The imprinted polymer, after removal of the template, was tested for the reduction of ketones to alcohols. An enhanced enantioselectivity was observed in the presence of the imprinted polymeric catalyst, in comparison to the control polymer. 

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