Effect of coordination number on the formation of silver nanoparticles in polymer/silver salt complex membranes

Polymer/silver salt complex membranes consisting of AgBF4 dissolved in poly(2-ethyl-2-oxazoline) (POZ) exhibited higher separation performance for propylene/propane mixtures but gradual decrease of separation properties, primarily due to the reduction of silver ions to silver nanoparticles. In this study, the effect of salt concentration on the formation of silver nanoparticles in POZ/AgBF4 complex membranes was investigated. Separation test showed that 1/0.5 POZ/AgBF4 complex membrane exhibited improved long-term stability on the membrane separation performance for propylene/propane mixtures compared to 1/1 POZ/AgBF4 complex membrane. The stability improvement would be attributed to the suppression of the reduction of silver ions to silver metal nanoparticles, resulting from the higher coordination number of silver ions to carbonyl oxygens of POZ in 1/0.5 POZ/AgBF4 membrane than 1/1 membrane. Transmission electron microscopy (TEM) and UV–visible spectra results confirmed that the rate of silver metal nanoparticle formation in the 1/0.5 POZ/AgBF4 complex membrane was significantly retarded compared to that of 1/1 POZ/AgBF4 complex membrane. It is therefore concluded that the coordination number of silver ions for carbonyl oxygens is of pivotal importance in controlling the reduction reaction of silver ions to silver metals and consequently improving the membrane separation performance for propylene/propane mixtures.