Toward an Understanding of the Microstructure and Interfacial Properties of PIMs/ZIF-8 Mixed Matrix Membranes

A study integrating advanced experimental and modeling tools was undertaken to characterize the microstructural and interfacial properties of mixed matrix membranes (MMMs) composed of the zeolitic imidazolate framework ZIF-8 nanoparticles (NPs) and two polymers of intrinsic microporosity (PIM-1 and PIM-EA-TB). Analysis probed both the initial ZIF-8/PIM-1 colloidal suspensions and the final hybrid membranes. By combination of dynamic light scattering (DLS) and transmission electron microscopy (TEM) analytical and imaging techniques with small-angle X-ray scattering (SAXS), the colloidal suspensions were shown to consist mainly of two distinct kinds of particles, namely, polymer aggregates of about 200 nm in diameter and densely packed ZIF-8-NP aggregates of a few 100 nm in diameter with a 3 nm thick polymer top-layer. Such aggregates are likely to impart the granular texture of ZIF-8/PIMs MMMs as shown by SEM-XEDS analysis. At the molecular scale, modeling studies showed that the surface coverage of ZIF-8 NPs by both polymers appears not to be optimal with the presence of microvoids at the interfaces that indicates only a moderate compatibility between the polymer and ZIF-8. This study shows that the microstructure of MMMs results from a complex interplay between the ZIF-8/PIM compatibility, solvent, surface chemistry of the ZIF-8 NPs, and the physicochemical properties of the polymers such as molecular structure and rigidity

Cation mobility and the sorption of chloroform in zeolite NaY

International audienceMolecular dynamics simulations at temperatures of 270, 330, and 390 K have been carried out to address the question of cation migration upon chloroform sorption in sodium zeolite Y. The results show that sodium cations located in different sites exhibit different types of mobility. These may be summarized as follows: (1) SII cations migrate toward the center of the supercage upon sorption, due to interactions with the polar sorbate molecules. (2) SI‘ cations hop from the sodalite cage into the supercage to fill vacant SII sites. (3) SI‘ cations migrate to other SI‘ sites within the same sodalite cage. (4) SI cations hop out of the double six-rings into SI‘ sites. In some instances, concerted motion of cations is observed. Furthermore, former SI‘ and SI cations, having crossed to SII sites, may then further migrate within the supercage, as in (1). The cation motion is dependent on the level of sorbate loading, with 10 molecules per unit cell not being enough to induce significant cation displacements, whereas the sorption of 40 molecules per unit cell results in a number of cations being displaced from their original positions. Further rearrangement of the cation positions is observed upon evacuation of the simulation cell, with some cations reverting back to sites normally occupied in bare NaY

Microscopic Model of the Metal-Organic Framework/Polymer Interface: A First Step toward Understanding the Compatibility in Mixed Matrix Membranes

International audienceAn innovative computational methodology integrating density functional theory calculations and force field-based molecular dynamics simulations was developed to provide a first microscopic model of the interactions at the metal-organic framework (MOF) surface/polymer interface. This was applied to the case of the composite formed by the polymer of intrinsic microporosity, PIM-1, and the zeolitic imidazolate framework, ZIF-8, as a model system. We found that the structure of the composite at the interface is the result of both the chemical affinity between PIM-1 and ZIF-8 and the rigidity of the polymer. Specifically, there is a preferential interaction between the -CN groups of PIM-1 and the NH terminal functions of the organic linker at the ZIF-8 surface. Additionally, the resulting conformation of the polymer gives rise to interfacial microvoids at the vicinity of the MOF surface. The porosity, rigidity, and density of the interfacial polymer were analyzed and compared to those for the bulk polymer. It was shown that the polymer still feels the impact of the MOF surface even at long distances above 15-20 Å. Further, both the polydispersity of the polymer and the flexibility of the MOF surface were revealed to only slightly affect the properties of the MOF/interface. This work, which delivers a microscopic picture of the MOF surface/polymer interactions at the interface, would lead, in turn, to the understanding of the compatibility in MOF-based mixed-matrix membrane

Role of MOF surface defects on the microscopic structure of MOF/polymer interfaces: A computational study of the ZIF-8/PIMs systems

International audienceThe influence of defects at the metal-organic framework (MOF) surface on the microscopic structure of a MOF/polymer composite has been studied by a computational methodology that combines density functional theory calculations with force field-based molecular dynamics simulations. This has been applied to composites formed by ZIF-8 and two different polymers of intrinsic microporosity: PIM-1 and PIM-EA-TB. Analysis of the MOF/polymer interactions, surface coverage, polymer conformation/stiffness and a full characterization of the interfacial voids are provided. We found that, although the nature of the MOF/polymer interactions changes in the presence of defects, the coverage and conformation of the polymer, as well as the morphology of the "interfacial microvoids" remain practically unchanged from a microscopic point of view. These results suggest that there is no microscopic evidence that defective MOF surfaces drastically change the geometry of the MOF/polymer interface and the strength of the physisorption-type interactions in play. (C) 2017 Elsevier Inc. All rights reserved

The effect of pore shape on hydrocarbon selectivity on UiO-66(Zr), HKUST-1 and MIL-125(Ti) metal organic frameworks: Insights from molecular simulations and chromatography

International audienceConfigurational Bias Grand Canonical Monte Carlo simulations have been used to show that the alkane isomer adsorption selectivity of porous MOF materials containing two pore types depends on the orientation of organic linkers' phenyl groups. These simulations were performed at low pressure (0.1 kPa) using mixtures of n-hexane and its branched isomers (2,2-dimethylbutane, 2,3-dimethylbutane and 2-methylpentane). Where possible, we compared the results with our gas chromatography results. In typical 1D narrow pore materials, the linear isomer is usually preferentially adsorbed over its branched isomers. In MOF materials exhibiting a 3D pore system with two pore types, a large one interconnected by smaller pores, the selectivity order is the inverse. Here, we show that this depends on the degree of opening of the access windows, which can allow it to be either ''closed'', or to mimic a small pore channel. The consequence of this is the possibility (in the linear/branched mixture case) for the linear alkane to remain linear and thus maximize its interactions with the pore. The linear/aromatic mixture case considers a mixture of benzene and n-hexane, to show that a more favorable packing efficiency pushes the selectivity towards the aromatic molecule, regardless of the degree of the pore opening, although n-hexane can increase its competitiveness for the adsorption sites in materials where it can remain in mostly linear conformations

Multifaceted study of the interactions between CPO-27-Ni and polyurethane and their impact on nitric oxide release performance

S.M.V. would like to thank the EPSRC for funding opportunities under grant agreement EP/K005499/1. S.M.V. and D.N.M. would further like to acknowledge the EPSRC Capital for Great Technologies grant (EP/L017008/1) and the EPSRC Strategic Equipment Resource grant (EP/R023751) for funding and supporting electron microscopy facilities at the University of St Andrews. M.J.D. and S.J.W. would like to acknowledge the ProDIA project that has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement no. 685727.A multifaceted study involving focused ion beam scanning electron microscopy techniques, mechanical analysis, water adsorption measurements, and molecular simulations is employed to rationalize the nitric oxide release performance of polyurethane films containing 5, 10, 20, and 40 wt % of the metal-organic framework (MOF) CPO-27-Ni. The polymer and the MOF are first demonstrated to exhibit excellent compatibility. This is reflected in the even distribution and encapsulation of large wt % MOF loadings throughout the full thickness of the films and by the rather minimal influence of the MOF on the mechanical properties of the polymer at low wt %. The NO release efficiency of the MOF is attenuated by the polymer and found to depend on wt % of MOF loading. The formation of a fully connected network of MOF agglomerates within the films at higher wt % is proposed to contribute to a more complex guest transport in these formulations, resulting in a reduction of NO release efficiency and film ductility. An optimum MOF loading of 10 wt % is identified for maximizing NO release without adversely impacting the polymer properties. Bactericidal efficacy of released NO from the films is demonstrated against Pseudomonas aeruginosa, with a >8 log10 reduction in cell density observed after a contact period of 24 h.Publisher PDFPeer reviewe

Coadsorption of n‑Hexane and Benzene Vapors onto the Chromium Terephthalate-Based Porous Material MIL-101(Cr) An Experimental and Computational Study

International audienceThe adsorption of n-hexane−benzene mixture onto a chromium terephtalate-based porous material (MIL-101(Cr)) has been studied experimentally and theoretically. The adsorption isotherms of the single components show that MIL-101(Cr) has a better affinity for benzene than for n-hexane. This is in good agreement with the enthalpies of adsorption determined at low coverage. Values of −68 kJ*mol−1 and −61 kJ*mol−1 were found for benzene and n-hexane, respectively. These are consistent with the simulated enthalpies of adsorption and also with the benzene/n-hexane selectivities which range between 2 and 3 depending on the equilibrium pressure. The saturation plateau obtained with nhexane is 30% lower than that obtained with the adsorption of benzene onto MIL-101(Cr). In the case of the mixture of n-hexane−benzene, the saturation plateau is located between those obtained after adsorption of the single components. This is an indication that the coadsorption of n-hexane and benzene does not occur at the expense of one component of the mixture. However, the kinetics of adsorption of the mixture shows that benzene is adsorbed preferentially at low coverage. This is consistent with the chromatographic separation of n-hexane−benzene mixture by MIL-101(Cr)