Abstract
The interactions between metal–organic frameworks (MOFs) and adsorbates have been increasingly predicted and studied by computer simulations, particularly by Grand-Canonical Monte Carlo (GCMC), as this method enables comparing the results with experimental data and also provides a degree of molecular level detail that is difficult to obtain in experiments. The assignment of atomic point charges to each atom of the framework is essential for modelling Coulombic interactions between the MOF and the adsorbate. Such interactions are important in adsorption of polar gases like water or carbon dioxide, both of which are central in carbon capture processes. The aim of this work is to systematically investigate the effect of varying atomic point charges on adsorption isotherm predictions, identify the underlying trends, and based on this knowledge to improve existing models in order to increase the accuracy of gas adsorption prediction in MOFs. Adsorption isotherms for CO2 and water in several MOFs were generated with GCMC, using the same computational parameters for each material except framework point charge sets that were obtained through a wide range of computational approaches. We carried out this work for 6 widely studied MOFs; IRMOF-1, MIL-47, UiO-66, CuBTC, Co-MOF-74 and SIFSIX-2-Cu-I. We included both MOFs with and without open metal sites (OMS), specifically to investigate whether this property affects the predicted adsorption behaviour. Our results show that point charges obtained from quantum mechanical calculations on fully periodic structures are generally more consistent and reliable than those obtained from either cluster-based QM calculations or semi-empirical approaches. Furthermore, adsorption in MOFs that contain OMS is much more sensitive to the point charge values, with particularly large variability being observed for water adsorption in such MOFs. This suggests that particular care must be taken when simulating adsorption of polar molecules in MOFs with open metal sites to ensure that accurate results are obtained.
Highlights
Metal–organic frameworks (MOFs) are porous crystalline materials consisting of coordination bonds between transition-metal cations and organic ligands
We studied isoreticular MOFs (IRMOFs)-1 (Li et al 1999), MIL-47 (Barthelet et al 2002), UiO-66 (Cavka et al 2008), Cu(dpa)2SiF6-i (Nugent et al 2013), CuBTC (Chui et al 1999) and Co-MOF-74 ( known as CPO-27-Co or Co2(dobdc)) (Dietzel et al 2005), with all framework structures obtained from the Cambridge Structural Database.These MOFs were chosen with the aim of covering the most well-known and comprehensively studied “families” of MOF structures, as well as ensuring a large degree of topological diversity
IRMOF-1 (Fig. 1) belongs to a class of MOFs called the isoreticular MOFs (IRMOFs), which are characterised by their cubic topology
Summary
Metal–organic frameworks (MOFs) are porous crystalline materials consisting of coordination bonds between transition-metal cations and organic ligands. Adsorption (2020) 26:663–685 widely investigated as promising adsorbents for carbon dioxide capture, normally by designing new materials with high affinity for C O2 (Kenarsari et al 2013; Torrisi et al 2010). Some MOFs contain open metal sites (OMS), known as coordinatively unsaturated sites (CUS), which are unsaturated metal centers developed upon solvent removal during the activation step. These sites can form strong coordination bonds with adsorbates such as CO2, water or unsaturated hydrocarbons by electron donation from π orbitals of the adsorbate to the metal (Zhang et al 2015). Water adsorbs very strongly at the CUS, and the exact mechanism of competitive adsorption at these sites is still not fully understood
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