Hydrogen bond network reconstruction of lignite for efficient moisture removal via deep-eutectic-solvent-assisted hydrothermal treatment

Abstract

The hydrogen bond formed between water molecules and oxygen functional groups was an important factor for the moisture content of lignite. Deep eutectic solvent (DES, synthesized at a molar ratio of 1: 2 by ChCl and ZnCl2) with abundant hydrogen bond acceptors and donors was employed to facilitate the removal of moisture in lignite under hydrothermal treatment (HT) condition. At the same upgrading temperature, the dehydration efficiency of lignite upgraded by DES-assisted HT (DES-HT) was higher than HT method. The dehydration efficiency of DES-HT method was increased from 63.99 % (HT) to 68.03 % as the moisture content of upgraded lignite decreased from 8.02 % (HT) to 7.12 % (DES-HT) at 300 °C. Meanwhile, dehydration efficiency of molecular water was increased from 61.34 % (HT) to 66.06 % since the molecular water content of upgraded lignite decreased from 1.79 % (HT) to 1.57 % (DES-HT). In addition, the contents of free water and capillary water in lignite which was upgraded by DES-HT were also lower than those in lignite upgraded by HT. Characterization of upgraded coal via elemental analysis and X-ray photoelectron spectroscopy revealed that DES could weaken the stability of oxygen functional groups and thus promote the decomposition of hydrophilic functional groups such as C-OH/COC and OC-OH. Density functional theory calculations identified that DES could reconstruct the hydrogen bond network with moisture and weaken the intermolecular interaction strength between oxygen functional groups and water molecules during HT process. As a result, both total moisture content and molecular water content of lignite could be removed more effectively by DES-HT.