Recycling crosslinked polystyrene-supported catalyst

Amphiphilic resin supported ruthenium(II) complexes similar to those displayed in structure 1 were employed as recyclable catalysts for dimethylformamide production from supercritical C02 itself [96]. Tertiary phosphines were attached to crosslinked polystyrene-poly(ethyleneglycol) graft copolymers (PS-PEG resin) with amino groups to form an immobilized chelating phosphine. In this case recycling was not particularly effective as catalytic activity declined with each subsequent cycle, probably due to oxidation of the phosphines and metal leaching. 

There are reports of numerous examples of dendritic transition metal catalysts incorporating various dendritic backbones functionalized at various locations. Dendritic effects in catalysis include increased or decreased activity, selectivity, and stability. It is clear from the contributions of many research groups that dendrimers are suitable supports for recyclable transition metal catalysts. Separation and/or recycle of the catalysts are possible with these functionalized dendrimers for example, separation results from precipitation of the dendrimer from the product liquid two-phase catalysis allows separation and recycle of the catalyst when the products and catalyst are concentrated in two immiscible liquid phases and immobilization of the dendrimer in an insoluble support (such as crosslinked polystyrene or silica) allows use of a fixed-bed reactor holding the catalyst and excluding it from the product stream. Furthermore, the large size and the globular structure of the dendrimers enable efficient separation by nanofiltration techniques. Nanofiltration can be performed either batch wise or in a continuous-flow membrane reactor (CFMR). 

Polystyrene-supported iodobenzene has been used instead of iodobenzene as a catalyst for the a-tosyloxylation of ketones and alcohols with mCPBA and p-toluenesulfonic acid [111]. In particular, two kinds of polymer-supported Phi, the standard linear poly(4-iodostyrene) and the macroporous crosslinked poly(4-iodostyrene), have been used as recyclable catalysts, which could be recovered from the reaction mixture by simple filtration in 90-100% yield. Recovered poly(4-iodostyrene) could be reused for the same reaction to provide the corresponding a-tosyloxyketone in good yields [111]. Recyclable ionic-Uquid-supported iodoarenes have also been used as catalysts in the a-tosyloxylation of ketones with /nCPBA and p-toluenesulfonic acid [112]. 

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. 

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