Catalyst recycling using nanofiltration

A. Catalyst Recycling using Nanofiltration in a Continuous-Flow... 

Dijkstra, H.P, van Klink, G.P.M, van Koten, G. (2002) The Use of Ultra- and Nanofiltration Techniques in Homogeneous Catalyst Recycling. Acceleration Chemistry Research, 35, 798-810. 

While a number of dendritic catalysts have been described, catalyst recyclization in most cases is an unsolved problem. Diaminopropyl-type dendrimers bearing Pd-phosphine complexes have been retained by ultra- or nanofiltration membranes, and the constructs have been used as catalysts for allylic substitution in a continuously operating chemzyme membrane reactor (CMR) (Brinkmann, 1999). Retention rates were found to be higher than 99.9%, resulting in a sixfold increase in the total turnover number (TTN) for the Pd catalyst. 

G. Van Koten, The use of ultra- and nanofiltration techniques in homogeneous catalyst recycling, Acc. Chem. Res. 35... 

Fig. 11. Ionic liquid-transition metal catalyst recycle schemes for coupling reactions (a) conventional product isolation via solvent extraction, (b) Organic Solvent Nanofiltration (OSN) used with a biphasic IL/ organic system, and (c) Organic Solvent Nanofiltration (OSN) used with a single phase IL-organic solvent system [Wong et al., 2006].

Converting homogeneous catalytic reactions to heterogeneous versions will benefit the pharmaceutical and fine chemical industries because heterogeneous catalysts are physically separated from reactants and products and can therefore be easily recycled. Dendrimers have been employed as a recoverable catalyst platform using specially designed techniques, such as nanofiltration, precipitation, and two-phase catalysis [25]. 

One of the main applications of dendrimers is in catalysis allowing easy recycling of the homogeneous catalyst by means of nanofiltration. Carbosilane dendrimers functionalized with diphenylphosphine groups at the periphery have been synthesized and characterized. Palladium complexes of these dendrimers have been used as catalysts in the allylic alkylation reaction. These dendrimeric catalysts can be used in a continuous process using a membrane reactor.509... 

The expected contribution of catalysis in this area will derive both from the availability, at low processing costs, of new monomers obtained from biomasses and from the development of an optimized combination of biotechnology processes with classical and new biocatalytic processes. Research priorities for catalysis in the area of polymers from renewable materials for packaging, furniture, domestic water purification and recycling include the need to develop novel catalysts, e.g., for functionalization of polymeric and dendrimeric materials, with side-chain photoactive molecular switches (to be used as smart materials), or the development of multifunctional materials, combining, for example, nanofiltration with catalytic reactivity. 

Dendritic catalysts can be recycled by using techniques similar to those applied with their monomeric analogues, such as precipitation, two-phase catalysis, and immobilization on insoluble supports. Furthermore, the large size and the globular structure of the dendrimer can be utilized to facilitate catalyst-product separation by means of nanofiltration. Nanofiltration can be performed batch wise or in a continuous-flow membrane reactor (CFMR). The latter offers significant advantages the conditions such as reactant concentrations and reactant residence time can be controlled accurately. These advantages are especially important in reactions in which the product can react further with the catalytically active center to form side products. 

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). 

P. T. Witte, S. Roy Chowdhury, J. E. ten Elshof, D. Sloboda-Rozner, R. Neumann, P. L. Alsters, Highly efficient recycling of a sandwich" type polyoxometalate oxidation catalyst using solvent resistant nanofiltration, Chem. Commun. (2005) 1206. 

Catalyst separation from reaction mixtures has been efficiently carried out by using solvent-resistant nanofiltration membranes [75]. Following an alternative approach to solving this problem a quaternary ammonium salt has been immobilized on a soluble poly(ethylene glycol) polymer support. The supported catalyst thus obtained, soluble in solvents commonly used in PTC such as dichloromethane and acetonitrile, was used in a series of standard reactions under PTC conditions with comparable results to those obtained with traditional PTC catalysts [76]. Moreover, it compares favorably to other quaternary salts immobilized on insoluble polystyrene supports [77]. The catalyst can be easily recovered by precipitation with ethereal solvent and filtration and shows no appreciable loss of activity when recycled three times. 

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