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 crosslinking 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 αCD: 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.

Stable polymer membranes containing fixed CD recognition sites were successfully tested for selective pertraction of xylene isomers. The facilitated transport by CD was evidenced by an increase of permeability compared to control PVA membrane. α-CD was found more effective to selectively extract the xylene isomers than β-CD. Flux observed for pertraction of single isomers and of the o-/p- binary mixtures was in the same order as the binding constants to α -CD i.e. p-xylene > m-xylene > o-xylene. The fabricated membranes exhibited a p-xylene selectivity for low p-xylene feed mole fraction (<70%) and a o-xylene selectivity for higher p-xylene feed mole fraction. The p-xylene enrichment factor observed at 10% p-xylene in the feed equal to 6, is the higher value ever reported for the separation of xylene isomers using CD containing membranes. These results are consistent with a limiting step of the transport due to a saturation of recognition sites at the feed interface.