The change of hydrogen bonding network during adsorption of multi-water molecules in lignite: Quantitative analysis based on AIM and DFT

During the adsorption process of multi-water molecules, two types of hydrogen bonds are gradually formed in lignite. One type of hydrogen bonds exists between H₂O molecules and oxygen-containing functional groups, wheras the other exists between different H₂O molecules. As reported by previous studies, strength distributions of the above two types of hydrogen bonding networks are heterogeneous, however, the change rules of the hydrogen bonding network as well as its heterogeneity from the adsorption of single water molecule to multi-water molecules remain to be investigated. In this work, we quantitatively analyzed the change of hydrogen bonding networks during the adsorption of multi-water molecules in lignite. Analysis of Atoms in Molecules (AIM) and density functional theory (DFT) calculation were performed to calculate the hydrogen bond energy. Using the standard deviation of hydrogen bond energy as an indicator, we found that the heterogeneities of hydrogen bonding networks in lignite are enhanced with the proceeding of H₂O molecule adsorption, which can be mainly ascribed to the adsorption of multi-water molecules near carboxyl group. Specifically, from the adsorption of three to four H₂O molecules, the standard deviation of hydrogen bond energy between carboxyl group and H₂O molecules increases about 1.5 times (from 14 kJ/mol to 35 kJ/mol), whereas that of hydrogen bond energy between different H₂O molecules decreases from 16 kJ/mol to 9 kJ/mol. Based on the above findings, gradient dewatering strategy is proposed for the removal of adsorbed H₂O molecules in lignite.