| Summary: | The adsorption equilibrium of methane (CH<sub>4</sub>) and carbon dioxide (CO<sub>2</sub>) on the metal–organic framework (MOF) UiO-66 is studied via molecular simulation. UiO-66 is a versatile MOF with vast potential for various adsorption processes, such as biogas upgrading, CO<sub>2</sub> capture, and natural gas storage. The molecular simulations employ the grand canonical Monte Carlo (GCMC) method, covering a temperature range of 298–343 K and pressures up to 70 bar for CH<sub>4</sub> and 30 bar for CO<sub>2</sub>. The accuracy of different forcefields in describing the adsorption equilibria is evaluated. Two modelling approaches are explored: (i) lumping each hydrogen atom in the MOF framework to the heavy atom it is bonded to (united atom approximation) and (ii) considering explicit hydrogen atoms. Additionally, the influence of electrical charges on CO<sub>2</sub> adsorption is also evaluated. The findings indicate that the most effective forcefield to describe the adsorption equilibrium is a united atom forcefield based on the TraPPE parametrization. This approach also yields an accurate calculation of the isosteric heat of adsorption. In the case of CO<sub>2</sub>, it is observed that the use of electrical charges enhances the prediction of the heat of adsorption, especially in the low-coverage region.
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