Highly selective sulfonated polyethersulfone (SPES)-based membranes with transition metal counterions for hydrogen recovery and natural gas separation

Two types of SPES polymers with different degrees of sulfonation (DS) were synthesized by adjusting the used amount of chlorosulfonic acid and confirmed via elemental analysis and XPS in this work. TGA degradation curves show a difference in the thermal stability between the PES and SPES polymers due to the graft of sulfonic groups. The SPES-H membranes with different DS values were fabricated using a solution-casting method and their gas separation results exhibit a decrease in the permeability and an increase in the selectivity with increasing the degree of sulfonation because of the effect of electrostatic crosslinking. Novel transition metal counterions (i.e. Ag⁺ and Zn²⁺) were applied to conduct the ion-exchange treatment of SPES-H polymer samples to change the affinity properties of some specific penetrants in the membrane. EDX data confirm the nearly complete replacement of the proton in the SPES-H sample by metal counterions. The SPES membrane containing Zn²⁺ exhibits the best gas separation performance in this study due to a combination of the strong electrostatic crosslinking and the affinity between Zn²⁺ and some specific gas molecules (e.g. O₂ and CO₂). Compared to flat dense PES membranes, the SPES membrane containing Zn²⁺ exhibits impressive permselectivity increments of approximately 140%, 86%, 18% and 57% for He/N₂, H₂/N₂, O₂/N₂ and CO₂/CH₄ gas pairs, respectively. Both CO₂-induced plasticization tests and CO₂/CH₄ mixed gas measurements have been carried out to examine the applicability of these newly developed SPES membranes with different ion forms in industry. Results prove that this type of membrane material is a promising candidate for hydrogen recovery and natural gas separation.