How Can a Hydrophobic MOF be Water-Unstable? Insight into the Hydration Mechanism of IRMOFs

International audienceWe report an ab initio Molecular Dynamics study of the hydration process in a model IRMOF material. At low water content (one molecule per unit cell), water physisorption is observed on the zinc cation but the free ⇄ bound equilibrium strongly favors the free state. This is consistent with the hydrophobic nature of the host matrix and its type V isotherm observed in a classical Monte Carlo simulation. At higher loading, a water cluster can be formed at the Zn4O site and this is shown to stabilize the water bound state. This structure very rapidly transforms into a linker-displaced state, where water has fully displaced one arm of a linker and which corresponds to the loss of the material's fully-ordered structure. Thus an overall hydrophobic MOF material can also become water unstable, a feature that was not fully understood until now

Melting of zeolitic imidazolate frameworks with different topologies: insight from first-principles molecular dynamics

Metal–organic frameworks are chemically versatile materials, and excellent candidates for many applications from carbon capture to drug delivery, through hydrogen storage. While most studies so far focus on the crystalline MOFs, there has been a recent shift to the study of their disordered states, such as defective structures, glasses, gels, and very recently liquid MOFs. Following the publication of the melting mechanism of zeolitic imidazolate framework ZIF-4, we use here molecular simulation in order to investigate the similarities and differences with two other zeolitic imidazolate frameworks, ZIF-8 and ZIF-zni. We perform first principles molecular dynamics simulations to study the melting phenomena and the nature of the liquids obtained, focusing on structural characterization at the molecular scale, dynamics of the species, and thermodynamics of the solid–liquid transition. We show how the retention of chemical configuration, the changes in the coordination network, and the variation of the porous volume in the liquid phase are influenced by the parent crystalline framework.<br /

Contribution a la conception d'une station experimentale pour l'etude du comportement des murs de soutenement

SIGLECNRS TD Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Bio-methane purification by adsorption technology: Optimization of operating parameters and adsorbent selection based on PSA process

International audienceAdsorption separation technology is a widely used technology for the separation and the purification of gases and liquids in a wide range of fields. Processes such as pressure swing adsorption (PSA) present an interesting tool for biogas upgrading. The performance of PSA process is controlled by many parameters such as number of beds, cycle step durations, pressure ratio, recycle rate and most importantly the nature and the characteristics of the adsorbent. The selection of an appropriate adsorbent for bio-methane production from biogas by pressure swing adsorption remains a key element for enhancing the performance of PSA technology. Reported experimental results on bio-methane, upgrade from biogas using PSA, show that the performance is highly dependent on the different used adsorbents. In order to rationalize the process of adsorbent selection for a given biogas feed composition, we have developed an approach based on numerical simulations, at the reactor level, of a PSA process used to upgrade the biogas in bio-methane with CH4 purity higher than 98%. These simulations were coupled to molecular-based simulation for optimizing the adsorbent that gives the best performance for a given feed composition. We use the optimal adsorbent characteristics for selecting available adsorbents having similar characteristics. In order to validate the selection of the adsorbents, a series of experimental tests ranging from pure component adsorption isotherms to breakthrough adsorption experiments are carried out together with dynamic flow conditions where the selected adsorbents are characterized using CO 2 /CH 4 gas mixtures. The effects of feed flow rate, number of beds, cycle step durations and purge-to-feed ratio are studied. The optimal operating parameters of the process are obtained from simulations. This cycle will be then tested in PSA pilot plants for biogas upgrading purposes

Structure of Metal–Organic Framework Glasses by Ab Initio Molecular Dynamics

While metal–organic frameworks have been mostly studied in their crystalline form, recent advances have been made on their amorphous phases, both in fundamental understanding and in relation to possible applications. In particular, the zeolitic imidazolate (ZIF) glasses, that can be obtained from quenching liquid ZIFs, have shown promise. However, the details of their microscopic structure are very hard to probe experimentally. Here we use ab initio molecular dynamics simulations to investigate the nature of the ZIF glasses obtained from quenching molten ZIFs in silico. Through computational modeling of the melt–quench process on three different ZIF crystals, we aim to understand the effect of topology and chemistry upon the structure of the glass, compared to crystalline precursor and high temperature liquid. It is the first direct computational description of MOF glasses at the quantum chemical level. </div