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

Kinetic Monte Carlo Simulation of the Synthesis of Periodic Mesoporous Silicas SBA-2 and STAC-1: Generation of Realistic Atomistic Models

SBA-2 and STAC-1 are two related periodic mesoporous silicas (PMSs) that have regular networks of spherical, interconnected pores; the pores are similar in the two materials but the networks differ in their symmetry. The nature of the interconnected network of pores in these materials gives rise to interesting properties related to their potential use in separation processes. In this work, we extend a kinetic Monte Carlo (kMC) technique, originally derived for MCM-41, a simpler PMS, and apply it to mimic the condensation, aggregation, deformation, and calcination stages of the synthesis of SBA-2 and STAC-1. Our simulated synthesis results suggest that the pores are connected through windows formed during micelle aggregation because of the close packing of the spherical micelles and the presence of water molecules at the silica micelle interface. The simulated materials were validated by comparing properties such as unit cell size, pore size, pore shape, and wall density to results from experimental X-ray diffraction (XRD), transmission electron microscopy (TEM), density measurements, and Si-29 NMR Quantitative agreement between simulated and experimental nitrogen isotherms was achieved demonstrating the realism of the pore models obtained by the kMC simulations. Our results highlight the importance of a realistic, rough pore surface for the prediction of adsorption at low pressures in these materials.</p

Flexibility and swing effect on the adsorption of energy-related gases on ZIF-8: combined experimental and simulation study

ZIF-8, a prototypical zeolitic porous coordination polymer, prepared via the self-assembly of tetrahedral atoms (e.g. Zn and Co) and organic imidazolate linkers, presents large cavities which are interconnected by narrow windows that allow, in principle, molecular sieving. However, ZIF-8 shows flexibility due to the swing of the imidazolate linkers, which results in the adsorption of molecules which are too large to fit through the narrow window. In this work, we assess the impact of this flexibility, previously only observed for nitrogen, and the level of agreement between the experimental and simulated isotherms of different energy-related gases on ZIF-8 (CO2, CH4 and alkanes). We combine experimental gas adsorption with GCMC simulations, using generic and adjusted force fields and DFT calculations with the Grimme dispersion correction. By solely adapting the UFF force field to reduce the Lennard-Jones parameter e, we achieve excellent agreement between the simulated and experimental results not only for ZIF-8 but also for ZIF-20, where the transferability of the adapted force field is successfully tested. Regarding ZIF-8, we show that two different structural configurations are needed to properly describe the adsorption performance of this material, demonstrating that ZIF-8 is undergoing a structural change during gas adsorption. DFT calculations with the Grimme dispersion correction are consistent with the GCMC and experimental observations, illustrating the thermodynamics of the CH4 adsorption sites and confirming the existence of a new adsorption site with a high binding energy within the 4-ring window of ZIF-8.</p

A novel structural form of MIL-53 observed for the scandium analogue and its response to temperature variation and CO2 adsorption

The scandium analogue of the flexible terephthalate MIL-53 yields a novel closed pore structure upon removal of guest molecules which has unusual thermal behaviour and stepwise opening during CO2 adsorption. By contrast, the nitro-functionalised MIL-53(Sc) cannot fully close and the structure possesses permanent porosity for CO2