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

Abstract

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.

RASPA: molecular simulation software for adsorption and diffusion in flexible nanoporous materials

“…All expressions in Equation (10) are contained in the generalised Hessian matrix, which is the central quantity used in Baker's minimisation scheme. The elastic constants at 0 K can therefore be computed with very high accuracy.…”

“…(The classification was done with respect to porous materials, for pure fluids the meaning of 'adsorbates' reduces to 'molecules'.) The advantage is that the different components of the total energy are available and the interactions can be examined (also the energies in the Ewald Fourier part are split [10]). Cations are considered as part of the framework (they are included in the total mass of the framework).…”

MIL-91(Ti), a small pore metal–organic framework which fulfils several criteria: an upscaled green synthesis, excellent water stability, high CO2 selectivity and fast CO2 transport

“…The adsorption enthalpy at low coverage (∆ℎ) for each gas was calculated through configurational-bias Monte Carlo simulations performed in the NVT ensemble using the revised Widom's test particle insertion method. [66] Additionally, in order to gain insight into the configurational distribution of the adsorbed species in MIL-91(Ti), some additional data were calculated at different pressure including the radial distribution functions (RDF) between the guests and the host.…”

Evaluation of various water models for simulation of adsorption in hydrophobic zeolites

“…Recently, researchers have become more aware of the special nature of charge interactions in nanoporous materials. Straightforward truncated, pairwise Coulombic calculations including methods such as the Wolf-method turn out to be non-transferable to zeolites because of the non-uniform local density (dense framework with open voids) [36]. This work provides a simulation study on water adsorption in MFI-type zeolites using classical force fields to identify the possible limitations of available methods and force field parameters when they are applied to this special and highly hydrophobic system.…”