Elena García-Pérez

Computing the Heat of Adsorption using Molecular Simulations: The Effect of Strong Coulombic Interactions

Molecular simulations are an important tool for the study of adsorption of hydrocarbons in nanoporous materials such as zeolites. The heat of adsorption is an important thermodynamic quantity that can be measured both in experiments and molecular simulations, and therefore it is often used to investigate the quality of a force field for a certain guest-host (g - h) system. In molecular simulations, the heat of adsorption in zeolites is often computed using either of the following methods: (1) using the Clausius-Clapeyron equation, which requires the partial derivative of the pressure with respect to temperature at constant loading, (2) using the energy difference between the host with and without a single guest molecule present, and (3) from energy/particle fluctuations in the grand-canonical ensemble. To calculate the heat of adsorption from experiments (besides direct calorimetry), only the first method is usually applicable. Although the computation of the heat of adsorption is straightforward for all-silica zeolites, severe difficulties arise when applying the conventional methods to systems with nonframework cations present. The reason for this is that these nonframework cations have very strong Coulombic interactions with the zeolite. We will present an alternative method based on biased interactions of guest molecules that suffers less from these difficulties. This method requires only a single simulation of the host structure. In addition, we will review some of the other important issues concerning the handling of these strong Coulombic interactions in simulating the adsorption of guest molecules. It turns out that the recently proposed Wolf method ( J. Chem. Phys. 1999, 110 , 8254 ) performs poorly for zeolites as a large cutoff radius is needed for convergence.

Understanding Gas-Induced Structural Deformation of ZIF-8.

ZIF-8 is a zeolitic imidazolate framework with very good thermal and chemical stability that opens up many applications that are not feasible by other metal-organic frameowrks (MOFs) and zeolites. Several works report the adsorption properties of ZIF-8 for strategic gases. However, despite the vast experimental corpus of data reported, there seems yet to be a dearth in the understanding of the gas adsorption properties. In this work we provide insights at a molecular level on the mechanisms governing the ZIF-8 structural deformation during molecular adsorption. We demonstrate that the ZIF-8 structural deformation during the adsorption of different molecules at cryogenic temperature goes beyond the gas-induced rotation of the imidazolate linkers. We combine experimental and simulation studies to demonstrate that this deformation is governed by the polarizability and molecular size and shape of the gases, and that the stepped adsorption behavior is defined by the packing arrangement of the guest inside the host.

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.

A Simulation Study of Alkanes in Linde Type A Zeolites

Monte Carlo simulations were performed to study the adsorption and diffusion of small hydrocarbons in Linde Type A zeolites as a function of their calcium/sodium ratio. The diffusion studies were focused on methane whereas the adsorption simulations were performed from methane up to pentane. The results obtained showed that an increase in the number of cations in the structure (exchange of univalent sodium ions by divalent calcium ions) led to an increase in the adsorption of linear alkanes at low and medium pressure, but caused a decrease in adsorption at the highest pressures. An increase in the amount of cations favours molecular attraction and hence results in lower mobility. At higher cation loading the ions block the windows interconnecting the LTA cages, leading to a further decrease in diffusion. Methane self-diffusion coefficients obtained from our simulations were twice as high for the Linde Type 5A zeolite as for the Linde Type 4A zeolite. These results are consistent with previous experimental studies and provide a molecular picture of the influence of the zeolite type, the amount of cations contained and their location in the structure.

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.

Molecular simulation of adsorption of n-alkanes in Na-MFI zeolites. Determination of empirical expressions

We performed Configurational-Bias Monte Carlo simulations to provide adsorption isotherms, Henry coefficients and heats of adsorption of alkanes in sodium exchanged MFI- and MOR-type zeolites. We derived empirical expressions from the simulation data to describe the adsorption of linear alkanes in sodium exchanged MFI structures. These expressions adequately describe the Henry coefficient and adsorption enthalpy of n -alkanes as a function of sodium density and temperature. In the high coverage regime we provide an expression for saturation capacities of n -alkanes in the zeolite that combined with the obtained for Henry coefficients, gives a direct estimation of the complete adsorption isotherms of pure adsorbents and mixtures.