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

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journal contribution
posted on 12.04.2018 by Zeinab Naderi Khorshidi, Mohammad Khalkhali, Hao Zhang, Phillip Choi
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.