Perfluorosulfonic membranes like Nafion are much more permeable to ammonia than to hydrogen or nitrogen. This permeability is a strong, complex function of temperature. At room temperature, this membrane has a selectivity about three thousand and an ammonia flux about five times greater than that through polyvinylammonium thiocyanate, the other known highly ammonia-selective membrane. This flux depends on the membrane's counterion in the following order: H+ > Na+ > Ag+ NH+4 > Zn2+ > Li+ > Cu2+ > K+. As the temperature rises, the ammonia flux and selectivity both decrease, much as they do in the thiocyanate-based membrane. Flux and selctivity pass through a minimum around 150°C. At higher temperatures, the ammonia flux rises and the membrane remains up to sixty times more permeable to ammonia than to nitrogen. The flux now depends on the membrane's counterion in a different order: Ag+ > H+ > Na+ > Li+ > Cu2+ > NH+4. The variation of flux with ammonia pressure implies that the transport mechanism is different at lower and at higher temperatures.
Bibliographical noteFunding Information:
This work was largely supported by the National Science Foundation (CTS 89-12634). Other support came from Hoechst-Celanese and from General Mills.
- gas separations
- ion exchange membranes