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

Storage and separation of gases is 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. A particular category of these materials are the so-called breathing MOFs, in which a reversible structural transformation (variation of pore volume, pore breathing) can be induced by temperature, pressure or interaction with adsorbate molecules. Molecular modeling is an effective way to rapidly obtain microscopic information on the mechanism of adsorption in these materials (in many cases, not accessible using traditional experimental methods) and to shorten the time necessary to implement research results in an industrial process.In the this work we will 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. MOFs with pore limiting diffusion (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). 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. The initial screening allowed us to select the most promising structures for more detailed investigation. We show that 16 materials exhibit very promising selectivity (defined as adsorbed amount of CO2¬ to amount of CH4) in range from 50 to 440. Furthermore, adsorption of methane tends to be reduced with increasing humidity. Reference[1]S. Li, Y. G. Chung, and R. Q. Snurr, “High-Throughput Screening of Metal–Organic Frameworks for CO 2 Capture in the Presence of Water,” Langmuir, vol. 32, no. 40, pp. 10368–10376, 2016.[2]R. Krishna, “Screening metal–organic frameworks for mixture separations in fixed-bed adsorbers using a combined selectivity/capacity metric,” RSC Adv., vol. 7, no. 57, pp. 35724–35737, 2017.[3]Y. G. Chung et al., “Computation-Ready, Experimental Metal–Organic Frameworks: A Tool To Enable High-Throughput Screening of Nanoporous Crystals,” Chem. Mater., vol. 26, no. 21, pp. 6185–6192, Nov. 2014.

Unlike macroscopic objects, any system of nanometric size shows characteristics that strongly depend on its size and geometric form.Here we focus on the influence of confinement on properties of fluids, when the confining volumes are of nanometric size.Using Grand Canonical Monte Carlo simulations we show that when a fluid is confined in nano-volume, delimited by non-attracting pore walls,its density is heterogeneous close to the pore wall, and, on average over the whole pore volume, it is smaller than the density of bulk fluid. Thisfluid heterogeneity, resulting from the nano-confinement, progressively weakens when the pore size increases, and totally disappears inside thepores larger than 5 nm. On the other side, in the limit of very small pores, the fluid density approaches the ideal gas value. This effect should bedistinguished from the well know heterogeneity of density of fluids adsorbed in nanopores, driven by the difference between the strength offluid-fluid and fluid-pore wall interactions, that varies with the distance from the pore wall.We show that the density distribution inside the non-attracting reservoir strongly depends the distance from the pore walls, and the pore size andshape. Such a behavior, although non-intuitive in macroscopic sample, has a simple physical explanation. The energy of a given gas moleculestrongly depends on the number of its nearest neighbors. In the nanopores this number decreases when the molecule is closer to the pore wall.Less intuitive is the observation that at higher pressures (p > 100 bar) the density of the gas close to the wall may be higher than in the middle ofthe pore, and even higher than the one of the bulk gas at the same thermodynamic conditions.

Hydrogen is considered to be the preferred successor to gasoline due to its clean combustion.However, efficient and save storage of hydrogen remains the bottle-neck and one of the main challenges in hydrogen-basedtechnologies. In the past we have justified [1] the necessary conditions (materials physical properties) the hydrogenadsorbents MUST simultaneously fulfilled to be used in mobile applications: it must exhibit the specific surface at least ofthe order of 5000–7000 m2/g, have the average binding energy above 10 kJ/mol, and the density above 0.6 kg/m3. This setof parameters put so strict constrains on possible solutions that such systems are neither yet known nor seem easy tosynthesize.Here we explored the possibility to prepare boron-substituted nanoporous carbons, cheap, light and save material that wassupposed [2,3] to reversibly store hydrogen by physisorption at room temperatures and moderated pressure (< 120 bar). Weshowed that electric arc discharge between graphite electrodes may be optimized to produce graphitized structures with avariety of graphene fragment sizes, forms, and interconnections between them. It also allows to introduce boron heteroatominto graphite-like structure. The as-synthetized material shows the hydrogen binding energy twice as high as unsubstitutedcarbons, but requires post-treatment (activation) as the its surface is low (~200 m2/g).

Here we explored the possibility to prepare boron-substituted nanoporous carbons, cheap, light and save material that wassupposed [2,3] to reversibly store hydrogen by physisorption at room temperatures and moderated pressure (< 120 bar). Weshowed that electric arc discharge between graphite electrodes may be optimized to produce graphitized structures with avariety of graphene fragment sizes, forms, and interconnections between them. It also allows to introduce boron heteroatominto graphite-like structure. The as-synthetized material shows the hydrogen binding energy twice as high as unsubstitutedcarbons, but requires post-treatment (activation) as the its surface is low (~200 m2/g).