(368cc) Polymer blending to overcome permeability/selectivity tradeoff in polybenzimidazole for H2/CO2 separation

My research interests focus on polymer science, gas separation technology, and materials engineering. Specifically, I am focused on:

Polybenzimidazole (PBI) has emerged as a leading membrane material for H₂/CO₂ separation due to its rigid and efficient chain packing and strong size-sieving ability. Not surprisingly, it exhibits a low H₂ permeability of 2.0 Barrer at 35 ^oC, limiting its industrial applications. Herein, we report a facile blending approach to dramatically improve H₂/CO₂ separation properties. First, PBI is blended with a highly permeable polyimide to increase H₂ permeability without losing H₂/CO₂ selectivity, such as 6FDA-DAM with H₂ permeability as high as 610 Barrer. At loadings of 10 - 40 wt%, 6FDA-DAM disperses as a macroscopic discontinuous phase in the blends, validated by their scanning electron microscope (SEM) images, two d-spacings from WAXD patterns, and two glass transition temperatures on DSC curves. The dispersed 6FDA-DAM phase significantly increases H₂ permeability. For example, adding 30 wt% 6FDA-DAM in PBI (PBI/6FDA-DAM-30) increases H₂ permeability to 7.0 Barrer at 35 ^oC without losing H₂/CO₂ selectivity, which can be described using the Maxwel model, incorporating 40 wt% 6FDA-DAM leads to stable H₂ permeability of 80 Barrer and H₂/CO₂ selectivity of 12 at 150 ^oC in simulated practical conditions for 100 h, surpassing Robeson’s upper bound. Second, H₂/CO₂ selectivity of the blends can be effectively improved by doping with polyprotic acids such as phosphoric acid (PPA) and methane trisulfonic acid. These acid molecules cross-link PBI chains via hydrogen bonding and proton transfer, leading to a tighter structure for molecular sieving. For example, H₂/CO₂ selectivity increases from 12 to 26 at 35 ^oC in PBI/6FDA-DAM-20 when doped by PPA at a molar ratio of PPA/PBI repeating units of 0.25. Polymer structures will be thoroughly characterized and correlated with pure- and mixed-gas H₂/CO₂ separation properties at various temperatures. This study demonstrates a facile and efficient approach to improving the gas separation properties of polymers.