Efficient Modeling of Water Adsorption in MOFs Using Interpolated Transition Matrix Monte Carlo.

With the specter of accelerating climate change, securing access to potable water has become a critical global challenge. Atmospheric water harvesting (AWH) through metal-organic frameworks (MOFs) emerges as one of the promising solutions. The standard numerical methods applied for rapid and efficient screening for optimal sorbents face significant limitations in the case of water adsorption (slow convergence and inability to overcome high energy barriers).

Quasicontinuous Cooperative Adsorption Mechanism in Crystalline Nanoporous Materials.

The hase behavior of confined fluids adsorbed in nanopores differs significantly from their bulk counterparts and depends on the chemical and structural properties of the confining structures. In general, phase transitions in nanoconfined fluids are reflected in stepwise adsorption isotherms with a pronounced hysteresis. Here, we show experimental evidence and an interpretation of the reversible stepwise adsorption isotherm which is observed when methane is adsorbed in the rigid, crystalline metal-organic framework IRMOF-1 (MOF-5).

Hydrogen Storage in Pure and Boron-Substituted Nanoporous Carbons-Numerical and Experimental Perspective.

Nanoporous carbons remain the most promising candidates for effective hydrogen storage by physisorption in currently foreseen hydrogen-based scenarios of the world's energy future. An optimal sorbent meeting the current technological requirement has not been developed yet. Here we first review the storage limitations of currently available nanoporous carbons, then we discuss possible ways to improve their storage performance.

Pore opening and breathing transitions in metal-organic frameworks: Coupling adsorption and deformation.

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. Understanding of the mechanism of these transformations is essential for their potential applications in gas adsorption, separation and storage.

Self-Assembled Two-Dimensional Nanoporous Crystals as Molecular Sieves: Molecular Dynamics Studies of 1,3,5-Tristyrilbenzene-C Superstructures.

Due to their unique geometry complex, self-assembled nanoporous 2D molecular crystals offer a broad landscape of potential applications, ranging from adsorption and catalysis to optoelectronics, substrate processes, and future nanomachine applications. Here we report and discuss the results of extensive all-atom Molecular Dynamics (MD) investigations of self-assembled organic monolayers (SAOM) of interdigitated 1,3,5-tristyrilbenzene (TSB) molecules terminated by alkoxy peripheral chains C containing carbon atoms (TSB3,5-C) deposited onto highly ordered pyrolytic graphite (HOPG). structural and electronic properties of the TSB3,5-C molecules were initially determined using second order Møller-Plesset (MP2) calculations.

Tailoring the separation properties of flexible metal-organic frameworks using mechanical pressure.

Metal-organic frameworks are widely considered for the separation of chemical mixtures due to their adjustable physical and chemical properties. However, while much effort is currently devoted to developing new adsorbents for a given separation, an ideal scenario would involve a single adsorbent for multiple separations. Porous materials exhibiting framework flexibility offer unique opportunities to tune these properties since the pore size and shape can be controlled by the application of external stimuli.

Collective Breathing in an Eightfold Interpenetrated Metal-Organic Framework: From Mechanistic Understanding towards Threshold Sensing Architectures.

Functional materials that respond to chemical or physical stimuli through reversible structural transformations are highly desirable for the integration into devices. Now, a new stable and flexible eightfold interpenetrated three-dimensional (3D) metal-organic framework (MOF) is reported, [Zn(oba)(pip)] (JUK-8) based on 4,4'-oxybis(benzenedicarboxylate) (oba) and 4-pyridyl functionalized benzene-1,3-dicarbohydrazide (pip) linkers, featuring distinct switchability in response to guest molecules (H O and CO ) or temperature. Single-crystal X-ray diffraction (SC-XRD), combined with density functional theory (DFT) and grand canonical Monte Carlo (GCMC) simulations, reveal a unique breathing mechanism involving collective motions of eight mixed-linker diamondoid subnetworks with only minor displacements between them.

Computer modeling of 2D supramolecular nanoporous monolayers self-assembled on graphite.

Nano-porous two-dimensional molecular crystals, self-assembled on atomically flat host surfaces offer a broad range of possible applications, from molecular electronics to future nano-machines. Computer-assisted designing of such complex structures requires numerically intensive modeling methods. Here we present the results of extensive, fully atomistic simulations of self-assembled monolayers of interdigitated molecules of 1,3,5-tristyrilbenzene substituted by C6 alkoxy peripheral chains (TSB3,5-C6), deposited onto highly-ordered pyrolytic graphite.

Boron-neutron Capture on Activated Carbon for Hydrogen Storage.

This work investigates the effects of neutron irradiation on nitrogen and hydrogen adsorption in boron-doped activated carbon. Boron-neutron capture generates an energetic lithium nucleus, helium nucleus, and gamma photons, which can alter the surface and structure of pores in activated carbon. The defects introduced by fission tracks are modeled assuming the slit-shaped pores geometry.

Intermediate states approach for adsorption studies in flexible metal-organic frameworks.

Adsorption studies in flexible metal-organic frameworks are challenging and time-consuming. It is mainly because the mechanism of adsorption, defined by structural framework properties, is constantly modified during the process, as the framework transformation depends on the adsorption uptake. We propose here a new approach to investigate adsorption in such complex systems, in which the simulations of adsorption in a deforming framework are replaced by the analysis of adsorption in intermediate rigid structures.

Benchmarking of GGA density functionals for modeling structures of nanoporous, rigid and flexible MOFs.

The adequate choice of the interaction model is essential to reproduce qualitatively and estimate quantitatively the experimentally observed characteristics of materials or phenomena in computer simulations. Here we present the results of a benchmarking of density-functional theory calculations of rigid and flexible metal-organic frameworks (MOFs). The stability of these systems depends on the dispersion interactions.

Modeling of adsorption of CO in the deformed pores of MIL-53(Al).

Molecular simulations were performed to predict CO adsorption in flexible metal-organic frameworks (MOFs). A generic force field was fitted to our experimental data to describe the non-bonded (electrostatic and van der Waals) interactions between CO molecules and the large pore (lp) and narrow pore (np) forms of the MIL-53(Al) framework. With the new validated force field, it is possible to predict CO uptake and enthalpy of adsorption at various applied external pressures that will modify the structure's pore configuration and allow us to have more control over the adsorption/desorption process.

Screening the Effect of Water Vapour on Gas Adsorption Performance: Application to CO Capture from Flue Gas in Metal-Organic Frameworks.

A simple laboratory-scale protocol that enables the evaluation of the effect of adsorbed water on CO uptake is proposed. 45 metal-organic frameworks (MOFs) were compared against reference zeolites and active carbons. It is possible to classify materials with different trends in CO uptake with varying amounts of pre-adsorbed water, including cases in which an increase in CO uptake is observed for samples with a given amount of pre-adsorbed water.

Modeling of low temperature adsorption of hydrogen in carbon nanopores.

We simulated the low temperature (T = 77 K) hydrogen adsorption in carbon slit-shaped nanopores using consecutively united atom (UA) and all atom (AA) representation of hydrogen molecule. We showed that both approximations give comparable estimation of the amount stored, for the wide range of pore width (0.6-2.

Unique bonding nature of carbon-substituted Be2 dimer inside the carbon (sp(2)) network.

Controlled doping of active carbon materials (viz., graphenes, carbon nanotubes etc.) may lead to the enhancement of their desired properties.

Understanding universal adsorption limits for hydrogen storage in nano porous systems.

Despite of more than 15 years of research, no materials possess the adsorbing properties required for mobile storage. At this time of state-of-the-art technology, the essential question should be asked: why is it so difficult to prepare a material with the desired properties? Here, we discuss the sources of physical limitations of existing materials and indicate the directions for further material research.

Open carbon frameworks - a search for optimal geometry for hydrogen storage.

Properties of a new class of hypothetical high-surface-area porous carbons (open carbon frameworks) have been discussed. The limits of hydrogen adsorption in these carbon porous structures have been analyzed in terms of competition between increasing surface accessible for adsorption and the lowering energy of adsorption. From an analysis of an analytical model and simulations of adsorption the physical limits of hydrogen adsorption have been defined: (i) higher storage capacities in slit-shaped pores can be obtained by fragmentation/truncation of graphene sheets into nano-metric elements which creates surface areas in excess of 2600 m(2)/g, the surface area for infinite graphene sheets; (ii) the positive influence of increasing surface area is compensated by the decreasing energy of adsorption in the carbon scaffolds of nano-metric sizes; (iii) for open carbon frameworks (OCF) built from coronene and benzene molecules with surface areas 6500 m(2) g(-1), we find an impressive excess adsorption of 75-110 g H2/kg C at 77 K, and high storage capacity of 110-150 g H2/kg C at 77 K and 100 bar; (iv) the new OCF, if synthesized and optimized, could lead to required hydrogen storage capacity for mobile applications.

Hypothetical high-surface-area carbons with exceptional hydrogen storage capacities: open carbon frameworks.

A class of high-surface-area carbon hypothetical structures has been investigated that goes beyond the traditional model of parallel graphene sheets hosting layers of physisorbed hydrogen in slit-shaped pores of variable width. The investigation focuses on structures with locally planar units (unbounded or bounded fragments of graphene sheets), and variable ratios of in-plane to edge atoms. Adsorption of molecular hydrogen on these structures was studied by performing grand canonical Monte Carlo simulations with appropriately chosen adsorbent-adsorbate interaction potentials.

Computational investigation of the adsorption of carbon dioxide onto zirconium oxide clusters.

A theoretical investigation of the adsorption of CO₂ onto ZrO₂ is presented. Various cluster models were used to mimic different basic and acidic sites on the surface. The method used was the density functional theory with the generalized gradient approximation and including Grimme's empirical model in order to properly describe the weak interactions that may occur between the adsorbate and the surface.

Molecular simulations of intermediate and long alkanes adsorbed on graphite: tuning of non-bond interactions.

The interplay between the torsional potential energy and the scaling of the 1-4 van der Waals and Coulomb interactions determines the stiffness of flexible molecules. In this paper we demonstrate for the first time that the precise value of the nonbond scaling factor (SF)--often a value assumed without justification--has a significant effect on the critical properties and mechanisms of systems undergoing a phase transition, and that, for accurate simulations, this scaling factor is highly dependent on the system under consideration. In particular, by analyzing the melting of n-alkanes (hexane C6, dodecane C12, tetracosane C24) on graphite, we show that the SF is not constant over varying alkane chain lengths when the structural correlated transformations are concerned.