Water Clusters in Lignite and Desorption Energy Calculationby Density Functional Theory

摘要

The interaction of water and hydrophilic sites with hydroxyl, carboxyl, and multiple oxygen-containing functional groups (OFGs) in lignite molecules was studied by density functional theory. The adsorption of water molecules on the lignite surface initially resulted in the formation of hydrogen bond-driven stable rings by three to four water molecules, followed by the formation of three-dimensional water clusters like a ″patchwork″. Aqueous layer thickness obtained from the water cluster size was 0.4–0.6 nm, which was consistent with the experimental data. Thus, pore-filling water beyond this range was less affected by the OFGs on the surface. Calculation of the adsorption energy predicts that the water clusters were primarily formed in the hydrophilic sites with three OFGs (site 1, including a carbonyl group, an alcoholic hydroxyl group and an etheroxy group in tetrahydropyran), then in COOH, and in O–H. For isolated hydroxyl groups, the interaction between the hydroxyl group and water molecules was weaker than that between the water molecules. When the water cluster waslocated at the hydrophilic sites with two or more OFGs, the adsorptionenergy of lignite–water interaction was higher than that ofwater–water interaction. Investigating the thermodynamics ofthe adsorption process at a molecular scale will help in understandingboth drying and resorption process of dried lignite during industrialproduction.