Energetics of Zn2+ adsorption in silicate MEL-type nanoporous material

Density-functional-based and ab initio calculations were implemented at different computational levels to estimate the binding energy of Zn2+ ion adsorbed on the available sites of a silicate MEL-type adsorbent. B3LYP and MP2 were used in combination with the 6-31G*, 6-31+G*, LanL2DZ, 6-311+G*, and Def2-TZVP basis sets. The zinc cation was found to preferentially occupy the 6MR sites followed by the cage-like positions. Nevertheles, all of the available sites exhibited negative amounts for the Gibbs free energy and enthalpy of adsorption with the corresponding population-averaged values of –160.84 and –169.53 kcal/mol at the B3LYP/Def2-TZVP level. Overall, the B3LYP/LanL2DZ method illustrated the highest deviation from the others both in trends and absolute values of binding energy. While the absolute binding energy ranged from 131.23 to 230.79 kcal/mol over different sites, the population-averaged binding energies altered from 146.08 to 162.54 kcal/mol depending on the method employed.