Selective pertraction of isomers using membranes having fixed cyclodextrin as molecular recognition sites

A liquid/liquid extraction process mediated by membrane contactors containing fixed recognition sites is being proposed for isomer discrimination. Cyclodextrins (CDs) are able to form inclusion complex of different stability with organic molecules depending on their shape and size. In this paper, we described the selective extraction of the geometrical isomers of xylene using a hydrogel consisting in a poly(vinyl alcohol) matrix grafted with α-CD and β-CD as the complexing moieties. The membrane contactors were prepared by the casting method following cross-linking reaction with hexamethylene diisocyanate (HMDI). Pertraction experiments showed that the transfer of xylenes across the CD-containing membranes was facilitated compared to PVA. The better discrimination was observed for membranes based on α-CD, the more efficient being that containing ca 21 wt-% CD. The order of permeation flux p-xylene > m-xylene > o-xylene followed the affinity order inferred from the stability constants α-CD: xylene in aqueous solution found in literature. These results and the fact that the permeate composition was independent of the feed composition seem to indicate that the limiting step of the transfer is the partitioning of solutes in the hydrogel.