Molecular Dynamics Study of the Role of Water in the Carbon Dioxide Intercalation in Chloride Ions Bearing Hydrotalcite

Molecular dynamics simulation was used to study the role of water in the intercalation of CO₂ with a model Mg–Al–Cl-hydrotalcite mineral at ambient pressure and temperature. The ClayFF force field was used along with a model Mg–Al–Cl-hydrotalcite containing different amounts of water (H₂O) and carbon dioxide (CO₂) molecules in its interlayer spacing. It was observed that high CO₂ content, say 3.85 mmol g^–1, could be achieved at low water concentrations or even without the presence of water. However, high water concentrations (e.g., 2 H₂O molecules/hydrotalcite unit cell, the maximum allowed water concentration observed experimentally) could also yield similar CO₂ content, but in this case, the presence of water led to a significant interlayer spacing expansion (from 23.0 Å (no water) to 28.5 Å). The expansion was likely due to the change in the orientation distribution of the CO₂ molecules. Analyzing the orientation of CO₂ molecules revealed that they preferred to orientate parallel to the mineral surface at low water concentrations. However, as water concentration increased, CO₂ molecules exhibited a wider range of orientations with a significant fraction of them orienting more or less perpendicular to the mineral surface, especially at high CO₂ contents. The observed change in the orientation of CO₂ was attributed to the dipole interaction between H₂O and CO₂ molecules and the reduced interaction between CO₂ and the hydroxyl groups on hydrotalcite. Also, it was observed that water molecules formed extensive hydrogen bond networks. All of the above findings seem to explain the contradicting results reported in the literature that water is needed under certain conditions to increase the amount of CO₂ captured by hydrotalcites. Here, we showed that high amounts of CO₂ can be intercalated with the presence of water