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.