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 in zeolites, despite the potential relevance for various applications, among them contaminant removal and drug delivery. The present study aims to fill this gap by comparing adsorption energies and, for selected cases, equilibrium structures of adsorption complexes of emerging organic contaminants 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 emerging organic contaminants in zeolites with the MOR and FAU topologies showed that the absolute values vary widely, whereas the qualitative trend across the set of zeolite-contaminant combinations are not strongly dependent on the choice of functional. For a few cluster models, DFT adsorption energies were compared to reference values obtained with the random phase approximation. When combining these results with observations of previous benchmarking studies, the rev-vdW-DF2 and PBE-D3 functionals emerge as approaches that should be (relatively) free from a systematic tendency to deliver too negative adsorption energies.