Synergistically improved PIM-1 membrane gas separation performance by PAF-1 incorporation and UV irradiation

Abstract

Super-glassy polymer membranes have suffered from the trade-off relationship between permeability and selectivity for gas separation applications, despite the fact that membrane technology exhibits remarkable energy efficiency advantages over other separation methods. Polymers of intrinsic microporosity such as PIM-1 offer high fractional free volume (FFV) and intermediate gas selectivity, with permeability several orders of magnitude higher than conventional glassy polymers. The methods of producing mixed matrix membranes (MMM) by incorporating nanoparticles into a polymer matrix, or crosslinking, have been widely studied to improve membrane selectivity. While crosslinking and nanoparticle incorporation often increase selectivity or permeability, respectively, this is typically at the expense of the other, limiting transport properties to the Robeson upper bound. Porous aromatic frameworks such as PAF-1 have been shown to significantly increase the permeability of PIM membranes. Here, this nanoparticle additive is coupled with post UV irradiation treatment resulting in a membrane with both significantly improved membrane selectivity (i.e., 16-fold improvement for H₂/CH₄ selectivity, from 5.4 to 90) and high permeability (i.e., P(H₂) = 4800 Barrer). Characterisation of the dual-enhanced membrane revealed that the synergetic performance is caused by a combination of the selective skin layer formed upon UV photo-oxidation with the additional permeable gas transport channels introduced to the bulk matrix by PAF-1. As a result of this dual-approach to membrane enhancement, the PIM-1 MMM exhibited better gas separation performance, surpassing the 2015 upper bounds for H₂/N₂ and H₂/CH₄ as well as 2008 upper bounds for H₂/CO₂ and CO₂/CH₄. Aging studies confirmed that PAF-1 addition, UV irradiation, and both modifications slowed physical aging rate compared to the pure PIM-1 membrane. The performance of this membrane was also investigated at a range of thicknesses, revealing its potential as a candidate for other membrane forms at scale.