Separation of gas mixtures at the industrial scale in US accounts for 10-15% of national energy consumption . Membranes can effectively reduce the enormous amount of energy required for gas separations, compared to conventional phase-change separation processes, such as cryogenic distillation, because membrane separation is a pressure-driven process and does not require intensive energy of liquefying gases to achieve the separation. Currently, polymers dominate gas separation membrane markets due to their appealing manufacturability and separation performance. This fact notwithstanding, polymeric membranes suffer from the productivity-selectivity tradeoff, i. e., the Robeson upper bound . Mixed Matrix Membranes (MMMs) can overcome such challenges, producing membranes with performance well-above the upper bound, since the porosity created by integration of inorganic fillers leads to the deviation of MMMs from the solution-diffusion mechanism which the upper bound is grounded on. However, the interphase between the polymer phase and inorganic dispersant phase in MMMs tends to form non-selective micro-voids or delaminated structures, due to the nature of inhomogeneity and lack of interactive functionalities of the two phases. In this case, gas molecules circumvent filling nanocrystals, leading to an improved flux but undesirably low selectivity. When further increasing the filler loading, membranes can exhibit defects or cracks, indicative of a percolation threshold of dispersant loading in the membrane.
This work aims to tackle the aforementioned challenges and develop ultra-permeable MMMs via functionalizing polymer chains and nanocrystals in the membrane matrix. Metal Organic Frameworks (MOFs), a new family of porous nanocrystals, possess precisely controlled pore sizes close to the molecular dimensions of gas molecules, which enables specific size-sieving amongst similarly-sized gases and synergistic enhancements of performance from both phases. MOFs are tailored with hydrophilic components, such as amino (-NH2) groups, intensively interacting with specially designed pending moieties in the polyimide to foster a firm interphase adhesion. Through insightful molecular design and microstructural engineering, such hybrid membranes mitigate the interfacial delamination and percolation limitation challenges in MMMs effectively. More importantly, the synthetic membranes demonstrate gas separation performance (such as CO2/CH4 and H2/CH4 gas pairs) surpassing the current upper bound considerably.
- S. Sholl and R.P. Lively, Nature 532 (2016), 435-438
- M. Robeson, J. Membr. Sci. 320 (2008) 390â400