Studying the adsorption of emerging organic contaminants in zeolites with dispersion-corrected density functional theory calculations: From numbers to recommendations
It has been established that adsorption energies obtained from dispersion-corrected density functional theory (DFT) calculations show a considerable dependence on the choice of exchange-correlation functional and dispersion correction. A number of investigations have employed different approaches to compute adsorption energies of small molecules like methane, ethane, or carbon dioxide in different types of zeolites (all-silica, protonated, cation-exchanged), using reference values from high-level calculations and/or experiments. Such comparative studies are lacking for the adsorption of larger functional organic molecules such as pharmaceuticals or personal care products, despite the potential relevance for applications, e.g., in contaminant removal or drug delivery. The present study aims to fill this gap by comparing adsorption energies and, for selected cases, equilibrium structures of emerging organic contaminants adsorbed in all-silica zeolites, employing a total of 13 dispersion-corrected DFT approaches. Methods using a pairwise (D3) dispersion correction as well as non-local van der Waals density functionals were included. A comparison of adsorption energies obtained for a variety of contaminants in MOR- and FAU-type zeolites showed that absolute values vary widely, whereas qualitative trends across the set of zeolite-guest combinations are not strongly dependent on the choice of functional. For selected cluster models, DFT adsorption energies were compared to reference values obtained with coupled cluster (DLPNO-CCSD(T)) calculations. Although all DFT approaches delivered systematically more negative adsorption energies than the coupled cluster reference, the rev-vdW-DF2 functional emerged as functional for which this tendency is least pronounced.