Cora Lind

Silver in Hybrid Membranes Facilitates Separation of Ethene and Ethane

Silver in Hybrid Membranes Facilitates Separation of Ethene and Ethane Researchers at the Osaka National Research Institute in Japan have achieved good selectivity of ethene against ethane in separation processes using inorganic– organic hybrid membranes containing silver(I) ions. The organic poly(N-vinylpyrrolidone) (PVP) part increased the flexibility of the inorganic network and immobilized the silver ions, which can function as olefin carriers. The membranes showed higher selectivity at higher temperatures, as thermal energy enhances the decomplexation rate of the silver olefin complexes. Separation of paraffins and olefins is usually carried out by cryogenic distillation. Among the alternatives that were investigated to replace this energy-intensive process, the use of facilitated support membranes containing Ag+ ions seemed to be promising. While supported liquid membranes and ion-exchange membranes require saturation of the feed gas with solvent, silver polymer membranes can be used to separate paraffins and olefins without water. The limitation of these membranes to low temperatures has now been overcome by replacing the pure polymer membrane with an organic–inorganic hybrid membrane as reported in the March issue of the Journal of the American Ceramic Society. The hybrid membranes were prepared via a sol-gel route using tetraethoxysilane, propyl triethoxysilane, water, and HNO3. After several hours of stirring, PVP and later AgBF4 were added. Membranes were produced by dip-coating on a porous aluminum tube. Two membranes containing ~10 wt% PVP, one with and one without silver, as well as a membrane containing ~20 wt% PVP and silver ions, were prepared. Fourier transform infrared spectrophotometry measurements suggested that the silver ions were bound to the amide group of the PVP segments. The silica matrix gave the membranes enhanced thermal stability; the PVP made the inorganic network more flexible and nonporous. While the membrane without silver ions showed little selectivity for ethene, both of the silver-containing membranes gave PC2H4 /PC2H6 ratios that increased with increasing temperature. At 423 K, values of 1.9 and 2.3 were observed for the membranes containing 10 wt% and 20 wt% PVP, respectively. The researchers said that selectivity of the membranes could be improved by, “(1) raising the temperature to increase the decomplexation rate of C2H4 from Ag+ and (2) increasing the PVP content.” The researchers attribute the latter effect to the ability of the PVP to increase the flexibility of the inorganic framework and the immobilization of Ag+ in the polymer segments. CORA LIND