Separation by Hydrodynamic Volume Size

Better suited for the separation of low molecular weight products from polymeric catalysts is ultrafiltration or nanofiltration [83,84]. As there is no sharp distinction between these two types, only the term ultrafiltration is used for 

Van Koten and coworkers examined the permeabihty of nanofiltration membranes (SelRO-MPF-60 MWCO 400 g mol ) using coloured metalloden-drimers ([G0]-[G3]) of increasing molecular weight and size (=0.6-1.0 nm) in CHjClj [87]. The retention times were measured via UV-Vis spectroscopy ([Gl] fi/2=108 h [G2] fi/2=300 h [G3] fi/2 60 days). In this way the authors showed that dendrimers do not have to be exceptionally large for successful retention. Subsequently they fimctionahsed the support of choice with a catalyt-ically active centre instead of the dye. Nanofiltration membrane-capped immersion vials were developed and used to compartmentahse homogeneous dendritic catalysts. These catalytic systems could be regenerated and stored for months without losing their activity. 

The same principle has been used in so-called continuous membrane reactors [83,85,88-95]. In this case the membrane is used to retain a soluble polymer bound catalytic species. Low molecular weight substrates are transformed continuously in the reactor and ideally pure products can be collected beyond the membrane. This can lead to easy separation and an increase in the total turnover number of the catalyst [85]. However, these systems are very demanding on support and membrane, since for efficient use a retention of more than 99.9% has to be guaranteed. In addition, even the best membranes cannot prevent metal leaching. 

In 1999 Brinkmann and Kragl reported on a dendrimer supported Pd-catal-ysed allylic substitution reaction performed in a continuous membrane reac- 

The required pressure for ultrafiltration can also be generated by centrifugation [ 1 ]. The separation time depends on the rotation speed of the centrifuge. However, due to the low solvent compatibility of the commercially available membrane ampoules this technique can only be employed conditionally. 

In conclusion, membrane techniques still need much improvement. New membranes that are compatible with a variety of organic solvents and organometalHc reagents are required to develop this area of supported catalysts to its full potential [84,89]. 

---