Effect of metal cations on the gas separation performance of sulfonated poly (phenylene oxide) membranes

The polymer polarity and density of sulfonated poly (phenylene oxide) (SPPO) membranes were manipulated by exchanging the proton of the sulfonic groups with mono-, di-, and tri-valent metal cations. The performance of the TFC membranes coated with ion-exchanged SPPO, toward the separation of CO2/CH4 and O2/N2 gas systems, showed to correlate with the location of the metal in the universal periodic table of the chemical elements. Altering the metal cation that replaces the proton of the sulfonic groups has altered: the cross-linking forces, the hindrance effects due to the voluminous metal size, and the SO3-Metal groups polarity due to the change in the metal electronegativity.

1. Exchanging the proton of the sulfonic groups in a SPPO membrane with a monovalent metal cation decreases the membrane density as a result of the insertion of a more voluminous cation. As a result, the permeance of gases in the monovalent metal cation exchanged SPPO membranes has increased. 2. The permeance of O2, N2, and CH4 gases, decreases as the atomic weight of the metal cation increases, i.e. by going down along the first group of the periodic table of the chemical elements. This is the result of increasing hindrance due to an increase in the size of the monovalent cation replacing the proton of the sulfonic groups. However, the permeance of CO2 gas increases in the same direction due to an increase in the interaction of the gas with the membrane as the polarity of the membrane increases, which is evidenced by an increase in the electronegativity difference between the metal and the oxygen atoms in the sulfonate group. 3. Exchanging the proton of the sulfonic groups in SPPO membrane with divalent metal cations increases the density of the membrane as − divalent cations can cross-link two SO3 groups. However, the residual charge increases as the size of the divalent metal cation increases, which results in an increase in CO2 solubility, by going along the second group of the periodic table of the chemical elements. 4. The permeance of all gases shows a maxmum while the permselectivity shows a minimum at Ca2+ by going down the periodic table for divalent metals. This is mainly the result of two conflicting effects: the metal cation size and the cross-linking bond strength. 5. The highest permeance of all gases was obtained with the Al 3+ -form of SPPO with relatively high permselectivity and separation efficiency, although the membrane density was relatively low. It is believed that this is the result of having some salt complex formation of the electrolyte used with the sulfonate group as: AlCl(SO3)2 or AlCl2SO3.