Effect of interlayer hydration on CO2 adsorption in iron-montmorillonite: A DFT study

Abstract

Growing atmospheric CO2 concentrations have intensified the demand for efficient carbon capture and storage (CCS) systems. Montmorillonite (MMT), a smectite clay mineral, is widely studied for its structural flexibility and CO2 intercalation capacity. This study evaluates iron-modified montmorillonite (Fe-MMT) as a CO2 adsorbent using density functional theory (DFT), systematically investigating a hydration gradient of zero to four interlayer water molecules in a fully periodic Fe-MMT model to examine the interplay between interlayer swelling, water content, and CO2 adsorption. The Fe-MMT model was constructed via Fe2+ ion-exchange substitution (verified by charge balance and a high-spin magnetic moment of 3.92 μB), and analyzed using spin-polarized DFT+U (U = 4.0 eV) with the Grimme DFT-D2 dispersion correction. Under dry conditions, CO2 adsorption requires +1.26 eV of interlayer widening energy while releasing only −0.89 eV of binding energy, giving a net endothermic balance of +0.37 eV that is energetically unfavorable. Within the hydration range investigated (n = 0–4), three interlayer water molecules yield the most favorable adsorption condition, with an adsorption energy of −0.37 eV at the ground-state DFT level, eliminating the dry-state widening penalty and enabling favorable CO2 adsorption. PDOS and charge density analyses show that Fe modifies the local interlayer electronic environment, while CO2 stabilization at n = 3 H2O is governed primarily by long-range dispersion and electrostatic interactions through the hydrogen-bonded water network rather than direct Fe–CO2 orbital overlap. These results identify Fe-MMT as a promising CCS candidate, with interlayer hydration as a key parameter for optimizing CO2 capture performance.