Unusual separation property of propylene/propane mixtures through polymer/silver complex membranes containing mixed salts

Previously we have found that the facilitated olefin transport through polymer/silver complex membranes is sensitive to the type of the counteranion of salt; the separation performances including the permeance and selectivity of propylene/propane are in the order of AgBF₄ > AgCF ₃SO₃ ≫ AgNO₃. Here the polymer/silver salt complex membranes consisting of two mixed salts dissolved in poly(2-ethyl-2-oxazoline) (POZ) were prepared and tested for the separation of propylene/propane mixtures. Upon mixing AgNO₃ with AgBF₄ or AgCF₃SO₃, the selectivity of propylene/propane unusually increased in certain composition ranges. The permeation behavior was interpreted in terms of both the activity of silver ion as olefin carrier and the structural change of polymer electrolyte membranes using FT-IR spectroscopy and wide angle X-ray scattering (WAXS). As a result, the anomalous enhancement of selectivity was attributable mainly to the structural compactness of membranes, resulting from the increased transient cross-links of polymer chains by coordination of small sized AgNO₃.