The separation of related molecules with similar physical/chemical properties is of prime industrial importance and practically entails a substantial energy penalty, typically necessitating the operation of energy-demanding low temperature fractional distillation techniques. Certainly research efforts, in academia and industry alike, are ongoing with the main aim to develop advanced functional porous materials to be adopted as adsorbents for the effective and energy-efficient separation of various important commodities. Of special interest is the subclass of metal-organic frameworks (MOFs) with pore aperture sizes below 5-7 Å, namely ultra-microporous MOFs, which in contrast to conventional zeolites and activated carbons show great prospects for addressing key challenges in separations pertaining to energy and environmental sustainability, specifically materials for carbon capture and separation of olefin/paraffin, acetylene/ethylene, linear/branched alkanes, xenon/krypton, etc. In this tutorial review we discuss the latest developments in ultra-microporous MOF adsorbents and their use as separating agents via thermodynamics and/or kinetics and molecular sieving. Appreciably, we provide insights into the distinct microscopic mechanisms governing the resultant separation performances, and suggest a plausible correlation between the inherent structural features/topology of MOFs and the associated gas/vapour separation performance.

Adsorption-based phenomena are important in gas separations, such as the treatment of greenhouse-gas and toxic-gas pollutants, and in water-adsorption-based heat pumps for solar cooling systems. The ability to tune the pore size, shape and functionality of crystalline porous coordination polymers--or metal-organic frameworks (MOFs)--has made them attractive materials for such adsorption-based applications. The flexibility and guest-molecule-dependent response of MOFs give rise to unexpected and often desirable adsorption phenomena. Common to all isothermal gas adsorption phenomena, however, is increased gas uptake with increased pressure. Here we report adsorption transitions in the isotherms of a MOF (DUT-49) that exhibits a negative gas adsorption; that is, spontaneous desorption of gas (methane and n-butane) occurs during pressure increase in a defined temperature and pressure range. A combination of in situ powder X-ray diffraction, gas adsorption experiments and simulations shows that this adsorption behaviour is controlled by a sudden hysteretic structural deformation and pore contraction of the MOF, which releases guest molecules. These findings may enable technologies using frameworks capable of negative gas adsorption for pressure amplification in micro- and macroscopic system engineering. Negative gas adsorption extends the series of counterintuitive phenomena such as negative thermal expansion and negative refractive indices and may be interpreted as an adsorptive analogue of force-amplifying negative compressibility transitions proposed for metamaterials.

Natural gas must be dehydrated before it can be transported and used, but conventional drying agents such as activated alumina or inorganic molecular sieves require an energy-intensive desiccant-regeneration step. We report a hydrolytically stable fluorinated metal-organic framework, AlFFIVE-1-Ni (KAUST-8), with a periodic array of open metal coordination sites and fluorine moieties within the contracted square-shaped one-dimensional channel. This material selectively removed water vapor from gas streams containing CO, N, CH, and higher hydrocarbons typical of natural gas, as well as selectively removed both HO and CO in N-containing streams. The complete desorption of the adsorbed water molecules contained by the AlFFIVE-1-Ni sorbent requires relatively moderate temperature (~105°C) and about half the energy input for commonly used desiccants.

Molecular simulations have largely contributed to the emergence of Metal Organic Frameworks (MOFs) not only for the resolution of the crystal structures of the most complex and poorly crystallized solids but also to enumerate all the plausible structures constructed by the assembly of a large diversity of inorganic and organic building blocks. Besides this in silico design of novel MOFs which has been only rarely validated so far by the post-synthesis of the desired material, a computational effort has been deployed to modulate the chemical and topological features of existing architectures specifically targeted for societally-relevant applications. Molecular modelling has been also frequently used to guide interpretation of the experimental data by providing a deep understanding of the microscopic adsorption/separation mechanism with the objective to drive the synthesis effort towards tuned materials with the required features for an optimization of their properties. This presentation will highlight the invaluable contribution of the computational approaches from the birth of novel MOFs and their structure elucidations to the characterization and understanding of their properties, throughout recent advances our groups have made in this field. A special emphasizes will be devoted to a series of recent MOFs that show promising adsorption/separation performances for natural gas upgrading, carbon capture and interesting features for mechanical energy storage and proton conduction.

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Negative gas adsorption (NGA) in ordered mesoporous solids is associated with giant contractive structural transitions traversing through metastable states. Here, by systematically downsizing the crystal dimensions of a mesoporous MOF (DUT-49) from several micrometers to less than 200 nm, counterintuitive NGA phenomena are demonstrated to critically depend on the primary crystallite size. Adsorbing probe molecules, such as n-butane or nitrogen, gives insights into size-dependent activation barriers and thermodynamics associated with guest-induced network contraction. Below a critical crystal size, the nitrogen adsorption-induced breathing is completely suppressed as detected using parallelized synchrotron X-ray diffraction–adsorption instrumentation. In contrast, even the smallest particles show NGA in the presence of n-butane, however, associated with a significantly reduced pressure amplification. Consequently, the magnitude of NGA in terms of amount of gas expulsed and pressure amplification can be tuned, potentially paving the way towards innovative concepts for pressure amplification in micro- and macro-system engineering.

A microporous Al trimesate-based Metal Organic Framework (MOF), denoted MIL-96(Al), was selected as a porous hybrid filler for the processing of Mixed Matrix Membranes (MMMs) for CO 2 /N 2 post combustion separation. First, the structural model of MIL-96(Al) initially reported was revisited using a combination of synchrotron-based single crystal X-ray diffraction (XRD), solid state Nuclear Magnetic Resonance (NMR) spectroscopy and Density Functional Theory (DFT) calculations. In a second step, pure MIL-96 (Al) crystals differing by their size and aspect ratio, including anisotropic hexagonal platelets and nanoparticles of about 70 nm in diameter, were prepared. Then, a combination of in situ IR spectroscopy, single gas and CO 2 /N 2 co-adsorption experiments, calorimetry and molecular simulations revealed that MIL-96(Al) nanoparticles show a relatively high CO 2 affinity over N 2 owing to strong interactions between CO 2 molecules and several adsorption sites such as Al 3+ Lewis centers, coordinated water and hydroxyl groups. Finally, the high compatibility between MIL-96(Al) nanoparticles and the 6FDA-DAM polymer allowed the processing of homogeneous and defect-free MMMs with a high MOF loading (up to 25 wt%) that outperform pure polymer membranes for CO 2 /N 2 separation.

Structure/properties relationships for a series of isostructural (MOFs) with thesoctopology were explored for various gas separation applications.