Ultrathin Pd Films on PBI-HFA Membranes for Selective Hydrogen Separation

A polymeric gas separation membrane utilizing polybenzimidazole (PBI) based on 4,4'-(hexafluoroisopropylidene)bis(benzoic acid) (HFA) with an effective permeating area of 8.3 cm² and a thickness of 30(± 2) mm was prepared and its gas permeation properties were investigated using H₂, CO₂, CO, and N₂ at various temperatures ranging from 24 to 200 ^oC.  The PBI-HFA membrane not only exhibited excellent H₂ permeability, but it also displayed superior gas separation performance particularly for H₂/N₂ and H₂/CO₂.  The permeation parameters for both permeability and selectivity ( PH₂ and α(H₂/N₂); PH₂ and α(H₂/CO₂)) obtained for the new material were found to surpass those reported by Robeson in 2008.  The gas permeation properties of the prepared membrane were found to be dependent on trans-membrane pressure difference as well as temperature.

A gas-tight palladium (Pd) membrane was fabricated on a PBI-HFA membrane by a vacuum electroless plating technique (VELP).  The thickness and effective permeating area of the Pd/PBI-HFA membrane were estimated in the range of 228–261 nm, and 8.3 cm², respectively.  The Pd membrane prepared using the VELP technique had a more uniform microstructure than those manufactured by a conventional electroless plating method (CELP), with no delamination or agglomerated Pd particles observed.  The gas permeation properties of the prepared Pd membranes were evaluated using H₂, N₂, CO₂, and CO as functions of:35 to 200 °C temperature, 0.4 to 0.8 MPa pressure difference, and membrane preparation methods including CELP, VELP, and VELP following a H₂O₂ surface treatment (H-VELP).  The Pd film fabricated by the H-VELP method showed superior performance, and proved to be impermeable to nitrogen as well as carbon monoxide.  Additionally, the Pd membranes prepared by H-VELP with a 228 nm thickness also demonstrated an excellent gas tightness with an infinite H₂/N₂ permselectivity, and its separation performance was above the Robeson’s 2008 upper bound.