Soft porous crystals undergo large structural transformations under a variety of physical stimuli. Breathing-like transformations, occurring with a large volume change, have been associated with an existence of bi-stable or multi-stable crystal structures. Here, we provide a detailed, multiscale qualitative and quantitative analysis of the adsorption-induced “breathing” transformations in two metal organic frameworks (MOFs): MIL-53(Al) which is a reference case of our approach, and recently synthetized JUK-8, which does not show any bistability without adsorbate. The proposed approach is based on atomistic simulations and does not require any empirical or adjustable parameters. It allows for a prediction of potential structural transformations in MOF and, in the case of adsorption-induced deformations, a calculation of the adsorption stress. We also show that the quantitative agreement between calculated and experimental results critically depends on the quality of the dispersion energy correction.

The paper presents a brief review of the recent developments in the field of absorption of atomic and molecular species in carbon cellular structures. Such absorbing objects can be distinctly recognized among a large family of carbon porous materials owing to potential and already observed in experiments very high capacity to soak and to keep inside different substances, which at usual conditions outside the porous matrices may often stay only in a gaseous form. High capacity filling is attained owing to single graphene-like walls separating different cells in the whole structures providing their lightweight. This property of cellular structures makes them very promising for numerous technological applications such as hydrogen storage in fuel cells and molecular sieving in membranes made from such structures or for their usage in microelectronics, photovoltaics and production of Li-ion batteries. Independently of the targeted applications gases are good candidates for probing tests of carbon matrices themselves.

Due to their unique geometry complex, self-assembled nanoporous 2D molecular crystals offer a broad landscape ofpotential applications, ranging from adsorption and catalysis to optoelectronics, substrate processes, and future nanomachineapplications. Here we report and discuss the results of extensive all-atom Molecular Dynamics (MD) investigations of self-assembledorganic monolayers (SAOM) of interdigitated 1,3,5-tristyrilbenzene (TSB) molecules terminated by alkoxy peripheral chains Cncontaining n carbon atoms (TSB3,5-Cn) deposited onto highly ordered pyrolytic graphite (HOPG). In vacuo structural andelectronic properties of the TSB3,5-Cn molecules were initially determined using ab initio second order Møller−Plesset (MP2)calculations. The MD simulations were then used to analyze the behavior of the self-assembled superlattices, including relaxed latticegeometry (in good agreement with experimental results) and stability at ambient temperatures. We show that the intermoleculardisordering of the TSB3,5-Cn monolayers arises from competition between decreased rigidity of the alkoxy chains (loss ofintramolecular order) and increased stabilization with increasing chain length (afforded by interdigitation). We show that theinclusion of guest organic molecules (e.g., benzene, pyrene, coronene, hexabenzocoronene) into the nanopores (voids formed byinterdigitated alkoxy chains) of the TSB3,5-Cn superlattices stabilizes the superstructure, and we highlight the importance of alkoxychain mobility and available pore space in the dynamics of the systems and their potential application in selective adsorption.

Nano-porous two-dimensional molecular crystals, self-assembled on atomically flat host surfaces offer abroad range of possible applications, from molecular electronics to future nano-machines. Computerassisteddesigning of such complex structures requires numerically intensive modeling methods. Here wepresent the results of extensive, fully atomistic simulations of self-assembled monolayers of interdigitatedmolecules of 1,3,5-tristyrilbenzene substituted by C6 alkoxy peripheral chains (TSB3,5-C6), depositedonto highly-ordered pyrolytic graphite. Structural and electronic properties of the TSB3,5-C6 moleculeswere determined from ab initio calculations, then used in Molecular Dynamics simulations to analyze themechanism of formation, epitaxy, and stability of the TSB3,5-C6 nanoporous superlattice. We show thatthe monolayer disordering results from the competition between flexibility of the C6 chains and theirstabilization by interdigitation. The inclusion of guest molecules (benzene and pyrene) into superlatticenanopores stabilizes the monolayer. The alkoxy chain mobility and available pore space defines thesystems dynamics, essential for potential application.

We discuss the mechanism of the structural transformations of gas adsorbed in microporous crystalline solids. We show that it is possible to induce structural transformations in a confined system by simply varying the number of molecules adsorbed in the pore. We found that the mechanism of these novel, adsorption-induced structural transformation in nano-pores differs from the capillary condensation. First, the structure of the confined gas is determined by a competition between adsorption sites attractive forces and intermolecular interaction. Second, at low temperature, the transformation is discontinuous because it is defined by limited number of accessible adsorption sites [1,2]. In the case of methane adsorbed in IRMOFs porous structures the character of transformation depends on temperature but also on the IRMOF structure: it changes from strongly discontinuous (especially at low temperatures), to continuous transition. The mechanism of the transformation is also modified by the size of the gas molecules and the strength of interaction. However, even discontinuous transition can produce continuous isotherms. We will show that the continuous isotherm can be the effect of statistical dynamical switching between two phase, characterized by different number of adsorbed molecules. We show the simulated microscopic mechanism and experimental observations support such statistical interpretation. This work was supported by the Polish National Science Centre (NCN, grant no. 2015/17/B/ST8/00099). The calculations have been partially performed at the WCSS computer center of The Wroclaw University of Science and Technology, grant no 33.References[1] Heterogeneous melting of methane confined in nano-pores. Dundar, E.; Boulet, P.; Wexler, C.; et al. J. Chem. Phys. 145, 144704 (2016)[2] Adsorption-Induced Structural Phase Transformation in Nanopores. Kuchta, Bogdan; Dundar, Ege; Formalik, Filip; et al. Angewandte Chimie In. Ed. 56, 16243–16246. (2017)

Introduction/PurposeStorage and separation of gases are one of the most challenging processes when they have to be implemented at industrial scale. Hundreds of potential materials have been tested over the past years, especially for gas storage (hydrogen and methane for fueling applications) and separation of biogas (a mixture of 55 – 75% of methane 25 – 45% of carbon dioxide). Today, MOFs (Metal-Organic Frameworks) [1-2] are considered as the most promising materials for such applications. MOFs are hybrid, nanoporous materials with a very large surface area, in which pore sizes, shapes and adsorption energies can be almost unlimitedly tailored to fit the required adsorption characteristics.MethodsIn this work, we present simulations of methane and carbon dioxide adsorption in a wide range of MOF structures from CoRE-MOF database [3], and at variable content of humidity. Over 1000 structures have been tested using Grand Canonical Monte Carlo method (as implemented in RASPA code) to find the optimal material for the CH4/CO2 separation in conditions of increasing humidity.ResultsMOFs with pore limiting diameter (PLD) over 3.3 Å were chosen for the simulations. Due to the humid conditions, we chose structures from the middle range of hydrophilicity, kH between 5·10-2 mol/kg·Pa (CuBTC) and 5·10-6 mol/kg·Pa (ZIF-8). The initial screening allowed us to select the most promising structures for more detailed investigation. We show that 20 materials exhibit very promising selectivity (defined as the adsorbed amount of CO2­ to the amount of CH4) in the range from 100 to 900 and total uptake above 0.5 mol/kg.ConclusionsThe results provide clear information about different mechanisms of adsorption for selected structures. Furthermore, the adsorption of methane tends to be reduced with increasing humidity. Materials with a growing level of selectivity for CO2 and CH4 with increasing hydration degree are the most promising from the future application point of view.

Low frequency lattice vibrations (phonons) can be used as indicators of a variety of structural transformations. In particular, conformational changes in flexible metal-organic frameworks (MOFs) can be successfully explained by analyzing materials phonon spectrum in frequency range below 200 cm-1 [1,2]. It has been shown that such lattice vibrations are responsible for pressure- and adsorption-induced gate-opening in zeolitic imidazolate frameworks (ZIFs), and pore breathing in materials from MIL-53 family. In this work we present a phonon-based methodology to analyze MOFs deformations that can be detected in adsorption experiments but cannot be explained at the microscopic level without theoretical support based on numerical simulations.As a case study we have chosen to analyze adsorption-induced transformations in ZIF-8. The step-like adsorption isotherm observed in this material was interpreted in the literature as a signature of gate-opening (enlargement of the pore free volume) induced by the increasing number of guest molecules entering the pore. To analyze this phenomenon at the microscopic level, we first calculated and visualized normal modes of the lattice vibrations. For that, density functional theory (DFT) with PBE functional and empirical dispersion correction D3(BJ) was used. Only the modes which fulfill two criteria: (i) their frequency is below 200 cm-1, and (ii) the related lattice deformation leads to increase of the pore free volume, have been further analyzed. These deformed structures, (including one related to gate-opening, pore breathing and one which combines both deformations) were selected to serve as the adsorbent model. Grand Canonical Monte Carlo (GCMC) simulations have been performed to calculate the isotherms of adsorption in selected structures. Surprisingly, in the structure related to gate-opening, the mode, located at ~40 cm-1, and considered in the literature as the one responsible for step-like form of the isotherm did not provided an uptake larger than the undistorted structure. On the other hand, in the structure resulting from simultaneous pore breathing and gate-opening deformation the adsorption is significantly increased. Therefore, we consider that this deformation, induced by pore breathing and gate-opening phonons explains better the experimental observations..1. F. Formalik, et al., Microporous and Mesoporous Materials, (2018),2. M. R. Ryder, et al., Physical Review Letters, 113 (2014), 215502-21550