Materials research is key to enable synthetic membranes for large-scale, energy-efficient molecular separations. Materials with rigid, engineered pore structures add an additional degree of freedom to create advanced membranes by providing entropically moderated selectivities. Scalability - the capability to efficiently and economically pack membranes into practical modules - is a critical yet often neglected factor to take into account for membrane materials screening. In this Progress Article, we highlight continuing developments and identify future opportunities in scalable membrane materials based on these rigid features, for both gas and liquid phase applications. These advanced materials open the door to a new generation of membrane processes beyond existing materials and approaches.
Understanding the crystal-size dependence
of both guest adsorption and structural transitions of nanoporous
solids is crucial to the development of these materials. We find that
nano-sized metal–organic framework (MOF) crystals have significantly
different guest adsorption properties compared to the bulk material.
A new methodology is developed to simulate the adsorption and transition
behavior of entire MOF nanoparticles. Our simulations predict that
the transition pressure significantly increases with decreasing particle
size, in agreement with crystal-size-dependent experimental measurements
of the N2-ZIF-8 system. We also propose a simple core–shell
model to examine this effect on length scales that are inaccessible
to simulations and again find good agreement with experiments. This
study is the first to examine particle size effects on structural
transitions in ZIFs and provides a thermodynamic framework for understanding
the underlying mechanism.
Characterization techniques beyond microscopy, scattering and spectroscopy approaches are needed to understand and improve sub-angstrom discrimination between penetrants in carbon molecular sieve (CMS) membranes. Here we use a method based on molecular scale gas diffusion probes to understand relevant membrane properties at the required level of detail. We further use this method to consider hypotheses about the evolution of structure responsible for fundamental properties of CMS materials derived from a high performance CMS precursor polymer, 6FDA:BPDA-DAM. While 6FDA:BPDA-DAM derived CMS membranes display a ~230 % improvement in CO 2 permeability when compared to Matrimid ® derived CMS formed under the same conditions, the diffusional selectivity for these two materials are very similar at 35 and 38.5, respectively. These results indicate a non-trivial connection between CMS precursor material structure and resulting performance. Linking hypotheses about structural changes likely to occur during pyrolysis with the probe data provides insights regarding transformation of the random coil polyimide into ultra-rigid CMS, with exquisite size and shape diffusion selectivity. The results provide a framework for understanding and tuning properties of this special class of materials with important technological advantages in energy-intensive gas separations.
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