(519a) Mechanisms to Enhance Permeation in Hybrid MOF-Polymer CO2 Capture Membranes

Current carbon capture processes like amine absorption are highly energy intensive and, consequently, has delayed our progress towards a carbon neutral society. Membranes are poised to become a competitive technology to replace amine absorption due to their passive operation, low costs, and scalability but currently cannot meet desired performance targets. Hybrid membranes, consisting of inorganic nanomaterials dispersed within a polymer matrix, present new opportunities to meet these targets as they are not confined by traditional transport relationships found in pure polymer systems.  However, the optimization of hybrid membranes requires a fundamental understanding of the dominant transport mechanism in these materials which has not yet been achieved. In this work, we explore the transition of transport pathways between phases in hybrid membrane through where porous crystalline metal-organic frameworks acts as the inorganic nanomaterial.

Specifically, we describe the results of a hybrid system comprising polysulfone and the MOF, UiO-66-NH₂ as a function of MOF loading. The MOF was chosen because of its sub-nanometer pore sizes and promising separation selectivity of carbon dioxide over methane and nitrogen in both low and high pressures conditions. Pure gas permeability, diffusion, and solubility properties were collected for the gases of interest. Due to the porous nature of MOF, the gas permeability is higher in the hybrid system than pure polymer systems as observed in other reported MOF/polymer systems. Surprisingly, and not observed previously, a dramatic jump in permeability occurs in systems which contain more than 30 wt% UiO-66-NH₂. This breakthrough event is attributed to the formation of a percolating pathway of interconnected UiO-66-NH₂ nanomaterials, wherein the inorganic phase becomes the dominant transport pathway. Permselectivity of CO₂/CH₄ and CO₂/N₂ are maintained at all loadings of UiO-66-NH₂ investigated.  Powder x-ray diffraction measurements reveal that the framework crystalline structure of the MOF remains intact during membrane processing. These results show the possibility of creating hybrid membranes, which exhibit completely different transport mechanism found in conventional polymer systems.