Car-Parrinello molecular dynamics study on the interaction between lignite and water molecules

Water-desorption from lignite surface is an important issue in energy usage, however, corresponding researches are inadequate. Herein, in order to comprehensive understand the process of water-desorption from lignite surface, the interaction between water molecules and carboxyl group of lignite were investigated systematically from 298.15 K to 440 K, by using Car–Parrinello molecular dynamics (CPMD) method. The result indicated that in the key carboxyl group, the hydrogen site (H51) showed stronger adsorption capacity for water molecules than the oxygen site (O23) of the carboxyl group. Based on the statistical result, the water molecules adsorbed at H51 and O23 were considered as removed after 360 K. Vibration analysis was performed to acquire stretching frequencies of carboxyl group before and after water adsorption. The NBO analysis was used to study the reorientation of water molecules in a 0.14 ps trajectory when they replace with each other. The NBO orbitals obtained show that when water molecule is replacing, the two lone pair orbitals of O31 and O32 interact with the σ antibonding orbital of O24 H51 bond at the same time. It enhances the formation of new H-bond and facilitates the fracture of old H-bond. This work can not only reveal the mechanism of water desorption from lignite, but also lay valuable foundations for efficient utilization of lignite resource.