P. Gómez-Álvarez

Insights into the microscopic behaviour of nanoconfined water: Host structure and thermal effects

The hydration of nanoporous materials is relevant to many fundamental and industrial applications. In this context, zeolites are usually used, but metal–organic frameworks constitute an emerging class of useful materials. The singular properties of water ascribed to the molecular association lead to a variety of behaviors in the confining systems that are not well understood. There is little of both experimental and computational information available, and most of which are limited to room temperature. This work addresses the adsorption and structural properties of water in a series of porous materials for a fixed topology, in particular LTA, and at various temperatures in the range of 298–573 K. The targeted structures were the all-silica zeolite ITQ-29, the aluminosilicate form with charge-balancing sodium and calcium cations LTA-5A, and the Zn-based zeolitic imidazolate framework. The adsorption process was computed using Monte Carlo simulations in the grand canonical ensemble and comprehensively rationalized at the molecular level on the basis of energetic factors and radial distribution functions. The structure of confined water for the different hydration levels was characterized in detail by using a specific criterion of hydrogen bond formation. The influence of both host characteristics and temperature on the microscopic behaviour of adsorbed water was assessed. Overall, this work proves that water–water hydrogen bonding is enhanced by the hydrophobic character of the pore walls and the large confining spaces. The increase in temperature induces progressive destruction of hydrogen bonds, but the majority of water molecules remain associated even when saturation is not reached. Concerning the thermodynamics of water adsorption, it is mainly affected by the peculiarities of the pore structure.

Hydrogen bonding of water confined in zeolites and their zeolitic imidazolate framework counterparts

Hydrogen bonds play a crucial role in the macroscopic behaviour of associated liquids. In particular, the singular properties of water are generally ascribed to the association at molecular level, and a significant number of studies have been focused on gaining understanding of this subject. However, a consistent description of this phenomenon when confining water in nanoporous materials is still lacking. This work is aimed at elucidating the effect of confinement in various structures on the hydrogen bonding of water using molecular simulation techniques. In particular, we considered pure silica hydrophobic zeolites and in their respective zeolitic imidazolate framework counterparts, which have larger pores and stronger affinity to water due to their chemical composition. Adsorption of water in these structures was computed via Monte Carlo simulations in the Grand-Canonical ensemble using previously validated force fields. A geometric criterion of hydrogen bonding formation was applied over generated configurations and allowed the computation of the structure of confined water. Our results at high hydration indicate considerable changes in relation to bulk water, which were found to be quite sensitive to the confining structure.

Aqueous Solutions of Ionic Liquids: Microscopic Assembly.

Aqueous solutions of ionic liquids are of special interest, due to the distinctive properties of ionic liquids, in particular, their amphiphilic character. A better understanding of the structure-property relationships of such systems is hence desirable. One of the crucial molecular-level interactions that influences the macroscopic behavior is hydrogen bonding. In this work, we conduct molecular dynamics simulations to investigate the effects of ionic liquids on the hydrogen-bond network of water in dilute aqueous solutions of ionic liquids with various combinations of cations and anions. Calculations are performed for imidazolium-based cations with alkyl chains of different lengths and for a variety of anions, namely, [Br](-), [NO3](-), [SCN](-) [BF4](-), [PF6](-), and [Tf2N](-). The structure of water and the water-ionic liquid interactions involved in the formation of a heterogeneous network are analyzed by using radial distribution functions and hydrogen-bond statistics. To this end, we employ the geometric criterion of the hydrogen-bond definition and it is shown that the structure of water is sensitive to the amount of ionic liquid and to the anion type. In particular, [SCN](-) and [Tf2N](-) were found to be the most hydrophilic and hydrophobic anions, respectively. Conversely, the cation chain length did not influence the results.

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.

Highly selective zeolite topologies for flue gas separation

The separation of carbon dioxide from flue gas is essential for the reduction of greenhouse gas emissions. In adsorptive methods, the challenge lies in the choice of suitable porous materials. Among all zeolite topologies, a number of adsorbents with pore dimensions in the range of the guest molecules were identified to allow an excellent separation by diffusion, and MRE and AFO zeolite topologies appear to be the best candidates based on equilibrium adsorption. Also, it was found that the behavior of this gas mixture in DFT and APD zeolites differed from the normal behavior.

Improving Olefin Purification Using Metal Organic Frameworks with Open Metal Sites

The separation and purification of light hydrocarbons is challenging in the industry. Recently, a ZJNU-30 metal-organic framework (MOF) has been found to have the potential for adsorption-based separation of olefins and diolefins with four carbon atoms [H. M. Liu et al. Chem.-Eur. J. 2016, 22, 14988-14997]. Our study corroborates this finding but reveals Fe-MOF-74 as a more efficient candidate for the separation because of the open metal sites. We performed adsorption-based separation, transient breakthrough curves, and density functional theory calculations. This combination of techniques provides an extensive understanding of the studied system. Using this MOF, we propose a separation scheme to obtain a high-purity product.