Opening the Gate:Framework Flexibility in ZIF-8 Explored by Experiments and Simulations

ZIF-8 is a zeolitic imidazole-based metal-organic framework with large cavities interconnected by narrow windows. Because the small size of the windows, it allows in principle for molecular sieving of gases such as H-2 and CH4. However, the unexpected adsorption of large molecules on ZIF-8 suggests the existence of structural flexibility. ZIF-8 flexibility is explored in this work combining different experimental techniques with molecular simulation. We show that the ZIF-8 structure is modified by gas adsorption uptake in the same way as it is at a very high pressure (i.e., 14 700 bar) due to a swing effect in the imidazolate linkers, giving access to the porosity. Tuning the flexibility, and so the opening of the small windows, has a further impact on the design of advanced molecular sieving membrane materials for gas separation, adjusting the access of fluids to the porous network.</p

Experimental and Simulation Evidence of a Corkscrew Motion for Benzene in the Metal–Organic Framework MIL-47

Some bacteria move with a corkscrew motion, meeting less resistance from surrounding water. A spiral movement is also considered during RNA polymerase translocation and it has been observed for water and proteins. We have found that benzene molecules confined in the metal–organic framework MIL-47 move in a corkscrew fashion. This spectacular diffusion effect could be put into evidence by combining experimental and computation tools; the two experimental techniques, quasielastic neutron scattering and ²H NMR, cover a wide range of time scales

Comparative Guest, Thermal, and Mechanical Breathing of the Porous Metal Organic Framework MIL-53(Cr): A Computational Exploration Supported by Experiments

The breathing of the flexible metal organic framework MIL-53­(Cr) has been widely explored by both experimental and modeling approaches upon the inclusion of different guest molecules within its porosity. This spectacular phenomenon has been only partially tackled by force field based simulations mainly due to the complexity of deriving a set of accurate potential parameters able to capture the associated structural transition implying a unit cell volume change up to 40%. Here, a new parametrization of a flexible force field for the MIL-53­(Cr) framework is realized from an iterative procedure starting with the experimental structural data collected in the presence of CO₂ as the guest molecule. Hybrid osmotic Monte Carlo simulations based on this refined force field are then successfully conducted to reproduce for the first time the complex shape of the CO₂ adsorption isotherm in the whole range of pressures. The structural behavior of the MIL-53­(Cr) under a wide range of applied temperature and pressure is then followed by molecular dynamics simulations. It is established that these two stimuli also induce a similar reversible structural transition toward a contracted phase, with the presence of a hysteresis. Each of these predictions is confirmed by experimental evidence issued from either the literature for the impact of the temperature or from our own high pressure neutron diffraction measurements. This permanent experimental/modeling interplay allows a full validation of the derived flexible force field, a prerequisite for further understanding the microscopic key features that govern the spectacular breathing of such a material