Polyphosphazene membrane separations - Review

The further growth of the success of membrane based operations in unusual areas depends on the development of new membranes with tunable properties and/or capable to withstand severe thermal chemical and mechanical environments. Poly(organophosphazene)s (POPs) may give an important contribution for the preparation of such new membranes. In this paper the applications of POP materials in ultrafiltration (UF), nanofiltration (NF), pervaporation (PV), vapor permeation (VP) and gas separation (GS) are reviewed, and some perspectives for future developments are outlined. Interesting results obtained with POP UF membranes indicate that they might be applied in the treatment of organic solvents or aggressive streams, or also for the construction of membrane contactors. However, the versatile and tunable properties of POPs can be fully exploited in membranes whose transport and separation mechanisms rely on differences in solubility and mobility of the feed species to be separated (i.e., NF, PV, VP, and GS). POP based NF membranes have been used for water potabilization and the separation of organic dies from i-PrOH. The research work carried out in industrial and academic laboratories, sponsored also by the US Department of Energy, has probably passed the turning point for the production of commercial PV and VP POP membranes for the separation of organics and/or water from liquid and gaseous streams. Good results were obtained in the separation of acidic species (SO3, H2S, CO2) from permanent gases with dense POP membranes. In perspective, the availability of new hybrid POP-inorganic materials makes it possible to bridge the gap between polymer and ceramic membranes, whose appealing capabilities are still to be explored. The outstanding versatile properties of POP can also be used for the fixation of catalytic centres on suitable membranes for the preparation of catalytic membrane reactors.