Rocío Bueno-Pérez

Effect of air humidity on the removal of carbon tetrachloride from air using Cu–BTC metal–organic framework

We have used interatomic potential-based simulations to study the removal of carbon tetrachloride from air at 298 K, using Cu-BTC metal organic framework. We have developed new sets of Lennard-Jones parameters that accurately describe the vapour-liquid equilibrium curves of carbon tetrachloride and the main components from air (oxygen, nitrogen, and argon). Using these parameters we performed Monte Carlo simulations for the following systems: (a) single component adsorption of carbon tetrachloride, oxygen, nitrogen, and argon molecules, (b) binary Ar/CCl(4), O(2)/CCl(4), and N(2)/CCl(4) mixtures with bulk gas compositions 99 : 1 and 99.9 : 0.1, (c) ternary O(2)/N(2)/Ar mixtures with both, equimolar and 21 : 78 : 1 bulk gas composition, (d) quaternary mixture formed by 0.1% of CCl(4) pollutant, 20.979% O(2), 77.922% N(2), and 0.999% Ar, and (e) five-component mixtures corresponding to 0.1% of CCl(4) pollutant in air with relative humidity ranging from 0 to 100%. The carbon tetrachloride adsorption selectivity and the self-diffusivity and preferential sitting of the different molecules in the structure are studied for all the systems.

Atomic charges for modeling metal–organic frameworks: Why and how

Abstract Atomic partial charges are parameters of key importance in the simulation of Metal–Organic Frameworks (MOFs), since Coulombic interactions decrease with the distance more slowly than van der Waals interactions. But despite its relevance, there is no method to unambiguously assign charges to each atom, since atomic charges are not quantum observables. There are several methods that allow the calculation of atomic charges, most of them starting from the electronic wavefunction or the electronic density or the system, as obtained with quantum mechanics calculations. In this work, we describe the most common methods employed to calculate atomic charges in MOFs. In order to show the influence that even small variations of structure have on atomic charges, we present the results that we obtained for DMOF-1. We also discuss the effect that small variations of atomic charges have on the predicted structural properties of IRMOF-1.

A Simulation Study of Hydrogen in Metal–Organic Frameworks

Molecular simulations have been used to evaluate the effect exerted by metal centres on the adsorption and diffusion of hydrogen in metal–organic frameworks. Simulations were carried out for the MIL-53 (Cr and Al) structures and the isostructural vanadium analogue MIL-47 at room temperature. To validate the models and force fields used in this work, the adsorption isotherms, energies and entropies, and self-diffusivities in Cu–BTC and IRMOF-1 metal–organic frameworks were computed. Using the validated force fields and models, a detailed analysis of the preferential adsorption sites is reported, allowing the energetic contribution in the low-coverage regime (Henry constants and adsorption energies and entropies) to be determined as a function of loading (adsorption isotherms). The influence of each energetic contribution to the charged and uncharged models of hydrogen has also been analyzed.

Controlling Thermal Expansion: A Metal–Organic Frameworks Route

Controlling thermal expansion is an important, not yet resolved, and challenging problem in materials research. A conceptual design is introduced here, for the first time, for the use of metal–organic frameworks (MOFs) as platforms for controlling thermal expansion devices that can operate in the negative, zero, and positive expansion regimes. A detailed computer simulation study, based on molecular dynamics, is presented to support the targeted application. MOF-5 has been selected as model material, along with three molecules of similar size and known differences in terms of the nature of host–guest interactions. It has been shown that adsorbate molecules can control, in a colligative way, the thermal expansion of the solid, so that changing the adsorbate molecules induces the solid to display positive, zero, or negative thermal expansion. We analyze in depth the distortion mechanisms, beyond the ligand metal junction, to cover the ligand distortions, and the energetic and entropic effect on the thermo-structural behavior. We provide an unprecedented atomistic insight on the effect of adsorbates on the thermal expansion of MOFs as a basic tool toward controlling the thermal expansion.

Separation of Amyl Alcohol Isomers in ZIF‐77

The separation of pentanol isomer mixtures is shown to be very efficient using the nanoporous adsorbent zeolitic imidazolate framework ZIF-77. Through molecular simulations, we demonstrate that this material achieves a complete separation of linear from monobranched-and these from dibranched-isomers. Remarkably, the adsorption and diffusion behaviors follow the same decreasing trend, produced by the channel size of ZIF-77 and the guest shape. This separation based on molecular branching applies to alkanes and alcohols and promises to encompass numerous other functional groups.

Cadmium–BINOL Metal–Organic Framework for the Separation of Alcohol Isomers

Abstract The large‐scale isolation of specific isomers of amyl alcohols for applications in the chemical, pharmaceutical, and biochemical industries represents a challenging task due to the physicochemical similarities of these structural isomers. The homochiral metal–organic framework cadmium–BINOL (BINOL=1,1′‐bi‐2‐naphthol) is suitable for the separation of pentanol isomers, combining adsorption selectivities above 5 with adsorption capacities of around 4.5 mol kg−1. Additionally, a slight ability for separation of racemic mixtures of 2‐pentanol is also detected. This behavior is explained based on matching shapes, strength of host–guest interactions, and on the network of hydrogen bonds. The last of these explains both the relative success and shortfalls of prediction methods at high loadings (ideal adsorbed solution theory) or at low coverage (separation factors), which are therefore useful here at a qualitative level, but not accurate in quantitative terms. Finally, the high selectivity of cadmium–BINOL for 1‐pentanol over its isomers offers prospects for practical applications and some room for optimizing conditions.

Influence of Flexibility on the Separation of Chiral Isomers in STW-Type Zeolite

Molecular simulation, through the computation of adsorption isotherms, is a useful predictive tool for the selective capacity of nanoporous materials. Generally, adsorbents are modelled as rigid frameworks, as opposed to allowing for vibrations of the lattice, and this approximation is assumed to have negligible impact on adsorption. In this work, this approach was tested in an especially challenging system by computing the adsorption of the chiral molecules 2-pentanol, 2-methylbutanol and 3-methyl-2-butanol in the all-silica and germanosilicate chiral zeolites STW and studying their lattice vibrations upon adsorption. The analysis of single- and multicomponent adsorption isotherms showed the suitability of STW-type zeolites as molecular sieves for chiral separation processes, which pose a challenging task in the chemical and pharmaceutical industries. Moreover, new experimental adsorption data validate the force field employed. The results reveal that the lattice vibrations of the all-silica framework are sorbate-independent, while those of germanosilicate STW show host-guest coupling modulated by uptake and sorbate type that disrupts the chiral recognition sites. This study indicates that the effects of intrinsic flexibility on the selective capacity of nanoporous materials may range from low to high impact, and some of them could not have been foreseen even after examination of the structural dynamics of an empty framework.

The Si–Ge substitutional series in the chiral STW zeolite structure type

The whole compositional range (Gef = Ge/(Ge + Si) = 0 to 1) of zeolite STW has been synthesized and studied by a comprehensive combined experimental–theoretical approach. The yield of the zeolite goes through a maximum and then drops at the GeO2 side of the series, following the inverse of the calculated free energy curve. The unit cell generally expands, roughly linearly, as the Gef increases, but a notable resilience to expansion is observed at the high silica side. This can be attributed to the more rigid character of SiO2 and the ability of Ge units to deform. Density functional theory calculations provide a new assignment of the previously controversial 19F MAS NMR resonances for occluded fluoride, which is based not only on the number of Ge atoms in the double-4-ring units but also on the way they are associated (namely, no Ge, isolated Ge, Ge pairs or closed Ge clusters). While we found an overall good agreement between the experimental and theoretical trends in preferential occupation of different crystallographic sites by Ge, the theoretical models show more sharp and abrupt tendencies, likely due both to limitations of the approach and to kinetic factors that allow metastable configurations to actually exist.