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Energy-efficient approaches to propylene/propane separation such as molecular sieving are of considerable importance for the petrochemical industry. The metal organic framework NbOFFIVE-1-Ni adsorbs propylene but not propane at room temperature and atmospheric pressure, whereas the isostructural SIFSIX-3-Ni does not exclude propane under the same conditions. The static dimensions of the pore openings of both materials are too small to admit either guest, signalling the importance of host dynamics for guest entrance to and transport through the channels. We use ab initio calculations together with crystallographic and adsorption data to show that the dynamics of the two framework-forming units, polyatomic anions and pyrazines, govern both diffusion and separation. The guest diffusion occurs by opening of the flexible window formed by four pyrazines. In NbOFFIVE-1-Ni, (NbOF 5 ) 2- anion reorientation locates propane away from the window, which enhances propylene/propane separation.

The selective capture of carbon dioxide in porous materials has potential for the storage and purification of fuel and flue gases. However, adsorption capacities under dynamic conditions are often insufficient for practical applications, and strategies to enhance CO(2)-host selectivity are required. The unique partially interpenetrated metal-organic framework NOTT-202 represents a new class of dynamic material that undergoes pronounced framework phase transition on desolvation. We report temperature-dependent adsorption/desorption hysteresis in desolvated NOTT-202a that responds selectively to CO(2). The CO(2) isotherm shows three steps in the adsorption profile at 195 K, and stepwise filling of pores generated within the observed partially interpenetrated structure has been modelled by grand canonical Monte Carlo simulations. Adsorption of N(2), CH(4), O(2), Ar and H(2) exhibits reversible isotherms without hysteresis under the same conditions, and this allows capture of gases at high pressure, but selectively leaves CO(2) trapped in the nanopores at low pressure.