Prediction of Adsorption Isotherms and Selectivities: Comparison between Classical Density Functional Theory Based on the Perturbed-Chain Statistical Associating Fluid Theory Equation of State and Ideal Adsorbed Solution Theory

Abstract

This study gives an assessment of the predictive capability of classical density functional theory (DFT) for adsorption processes of pure substances and mixtures of spherical and nonspherical molecular species. A Helmholtz energy functional based on the perturbed-chain statistical associating fluid theory (PC-SAFT) is applied to calculate isotherms and selectivities of multicomponent adsorption. In order to unambiguously assess the accuracy of the DFT model, we conduct molecular simulations. Monte Carlo (MC) simulations are performed in the grand canonical ensemble using the transition matrix. Two types of systems are studied: a model system, where fluid−fluid and solid−fluid interactions are defined as (single-site) Lennard-Jones interactions, and a more realistic methane−n-butane mixture in a graphite-like pore. Differences between a slit-shaped and a cylindrical pore geometry are examined for the model system. Adsorption isotherms and selectivities obtained from DFT calculations and MC simulations are found in very good agreement, particularly at high pressures. Capillary condensation observed along adsorption isotherms containing n-butane was accurately predicted, both, in equilibrium pressure and in density-increase. Comparisons with results from the ideal adsorbed solution theory are presented, confirming powerful predictions of the DFT approach.

Nanoporous Gold as a VOC Sensor, Based on Nanoscale Electrical Phenomena and Convolutional Neural Networks

“…The adsorption of VOCs proceeds initially at random sites; subsequently, monolayers and multilayers of adsorbed VOCs form. This process can be described by Brauner-Emmett-Teller (BET) theory [24]. While this process is identical to the one that occurs on a flat gold surface, the number of adsorbed molecules is significantly higher due to the high surface area of NPG.…”

Van der Waals density functional study of hydrocarbon adsorption and separation in metal–organic frameworks without open metal sites

“…Experimentally, the separation of a mixture of two gases is characterized by the ideal adsorbed solution theory (IAST) selectivities as a function of total gas pressure, which are deduced from adsorption isotherms of the individual components. To simulate IAST selectivities, one would typically need to carry out Grand Canonical Monte Carlo simulations [73,74], which are beyond the scope of this work. Here, we attempt to infer qualitative trends on the separation of small hydrocarbons from our static calculations of binding energies.…”

Adsorption of Light Gases in Covalent Organic Frameworks: Comparison of Classical Density Functional Theory and Grand Canonical Monte Carlo Simulations

“…cDFT accurately predicts the adsorption behaviour when compared to GCMC simulations including layering transitions 32 and capillary condensation. 33 The adsorption behaviour of real unordered porous materials, however, is often not well represented by the homogeneous slit pore model with one given pore size. This is because of the ambiguous pore structures with often unknown porosity, chemical composition and pore size distributions.…”