Continuous flow membrane reactor

Fig. 7.2 a) Schematic represenation of a continuous flow membrane reactor and b) theo-reticai reiative concentrations of (dendritic)... 

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. 

Fig. 6. Allylic alkylation in a continuous flow membrane reactor using dendritic ligand 5c (flow rate 50mLh reactor volume 20 mL, Koch MPF-60 NF membrane, molecular weight cut-off = 400 Da slight increases are due to pump failures) (19b).

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). 

Membrane technology has been performed using either micro-, ultra- or nanofiltration or reverse osmosis in either batch-wise or continuous-flow membrane reactors (CFMR). 

Another important issue evolves if the dendritic catalyst will be used in a continuous-flow membrane reactor. Generally, two forms of leaching have to be considered when dendritic transition metal catalysts are used in such reactors depletion of the dendritic catalyst through the membrane and metal dissociation (possibly after decomposition) from the dendrimer resulting... 

Abstract Enantioselection in a stoichiometric or catalytic reaction is governed by small increments of free enthalpy of activation, and such transformations are thus in principle suited to assessing dendrimer effects which result from the immobilization of molecular catalysts. Chiral dendrimer catalysts, which possess a high level of structural regularity, molecular monodispersity and well-defined catalytic sites, have been generated either by attachment of achiral complexes to chiral dendrimer structures or by immobilization of chiral catalysts to non-chiral dendrimers. As monodispersed macromolecular supports they provide ideal model systems for less regularly structured but commercially more viable supports such as hyperbranched polymers, and have been successfully employed in continuous-flow membrane reactors. The combination of an efficient control over the environment of the active sites of multi-functional catalysts and their immobilization on an insoluble macromolecular support has resulted in the synthesis of catalytic dendronized polymers. In these, the catalysts are attached in a well-defined way to the dendritic sections, thus ensuring a well-defined microenvironment which is similar to that of the soluble molecular species or at least closely related to the dendrimer catalysts themselves. 

Hyperbranchedpolytriallylsilanes functionalized with NCN [C H3X(CH2NMe2)2-2,6] ligands were reported by van Koten and Frey [19]. The soluble supports were used as ligands for the Pd-catalyzed aldol condensation of benzaldehyde and methyl isocyanate. Activities similar to that of parent NCN-Pd complexes were observed (Scheme 2). The hyperbranched polymeric systems showed similar properties to those of analogous dendritic compounds, indicating that structural perfection is not always required. The polymers were purified by means of dialysis, showing a potential application in continuous-flow membrane reactors. 

Ref. 192) The setup of the continuous flow membrane reactor is shown in Figure 4.6. A microporous silica membrane with an average pore size of 0.6 nm was used in the reactor. The characteristics of the reactor are summarized in Table 4.4. First, the active catalyst was generated in situ in a high-pressure reactor from [RhCl(COD)]2 and P(C6H4-/t-SiMe2CH2CH2C8Fiv)3, which were introduced into the system by... 

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