Highly permeable chemically modified PIM-1/Matrimid membranes for green hydrogen purification

Polymers of intrinsic microporosity, e.g. PIM-1, are attractive materials for gas separation and energy development, which is ascribed mainly to their superior permeability. The H₂ and CO₂ permeability of PIM-1 is about 1300-4000 Barrer and 3000-8000 Barrer, respectively. However, it has a relatively low H₂/CO₂ selectivity of 0.4-0.8. Different from the previous UV rearrangement approach, for the first time we report here a viable method to tune the intrinsic properties of PIM-1 blend membranes from being CO₂-selective to H₂-selective via blending with Matrimid and subsequent cross-linking the mixed matrix membrane with diamines at room temperature. The ideal H₂/CO₂ selectivity of the membrane after modification by 2 h triethylenetetramine (TETA) improved dramatically from 0.4-0.8 to 9.6, with a H₂ permeability of 395 Barrer. The modified membranes also show exceptional separation performance, surpassing the present upper bound for H ₂/CO₂, H₂/N₂, H₂/CH ₄ and O₂/N₂ separations. Positron annihilation lifetime spectroscopy (PALS) and Field emission scanning electron microscopy (FESEM) revealed that the diamine cross-linking successfully alters the membrane morphology from a dense to a composite structure. The X-ray diffraction (XRD) analysis and sorption data confirmed that the modified membrane has a smaller d-spacing and a decrease in the diffusivity coefficient. Our results also affirmed that the spatial structure, rather than the pK_a value, of the diamine is the prevailing factor which governs the reactivity of diamines towards the PIM-1/Matrimid membrane due to the low concentration of cross-linkable polyimides distributing randomly in the polymer matrix. The fundamentals and knowledge gained throughout this study may facilitate the development of polymeric membranes for green H₂ enrichment processes.