Competitive Adsorption of Methyl Bromide and Water on Metal Catecholates: Insights from Density Functional Theory

Density functional theory was used to study the interaction of methyl bromide (MeBr) and water with a large number metal catecholates. Differences in the binding mechanism of MeBr and water result in differences in the adsorption selectivity of the alkaline earth, early transition, and late transition metals. The binding of water is primarily driven by electrostatic attraction between the water oxygen atom and the metal, which means the alkaline earth and early transition metals heavily favor water over MeBr. For the more electronegative late transition metals, MeBr donates a significant amount of electron density to the metal, which dominates over the electrostatic binding effect. These metals favor MeBr over water, based on single molecule adsorption calculations. However, calculations of simultaneous adsorption of water and MeBr indicate that MeBr adsorption on late transition metals such as Pt and Au is detrimentally affected by the presence of water, while MeBr adsorption on Ca is more resistant to the presence of humidity. Therefore, despite lower single-component selectivity for MeBr, Ca and other alkaline earth metals might offer advantages for MeBr adsorption applications in humid environments. Also, in the case of four metals (Sc, Y, Hf, and Ta), MeBr is predicted to dissociate and bind separately to the metal as a Br atom and a methyl group, resulting in a very favorable binding energy (>275 kJ/mol).