Functionalized ionic liquids for lignite dissolution and treatment

BACKGROUND As a low-rank coal, lignite (frequently referred to as 'brown coal') has a high latent moisture content and low energy density and thus has low value as a solid fuel, often being burned at power stations close to where it is mined. For this reason, an effective pretreatment is essential to the better utilization of low-rank coal for energy and materials. Ionic liquids (ILs) are alternatives to conventional volatile organic solvents for pretreating coal due to their tailorable structures and physical properties, as well as their ability to partially dissolve/swell coal components. This study aimed to rationally design ionic liquids having a high hydrogen-bond basicity, based on the acetate anion, paired with ether-functionalized cations (especially the less-expensive piperidinium) for lignite dissolution and treatment. RESULTS Lignite treated by these functionalized ionic liquids at 105 degrees C led to the dissolution of 15.7-24.5 wt% lignite with a concomitant increase in coal aromaticity and rank, a reduction in coal decomposition residues, and the production of smaller and more uniform particles. The ether-functionalized N-(2-ethoxyethyl)-N-methylpiperidinium acetate is a particularly attractive alternative to conventional imidazolium ILs due to its lower cost, high hydrogen-bond basicity, and a strong capability for improving the coal aromaticity. CONCLUSION Our results have demonstrated that pretreatment with functionalized ionic liquids can improve the coal aromaticity and rank, decrease coal decomposition residues, and yield smaller and more uniform particles, which enables a better utilization of low-rank coal for fuel and materials. (c) 2021 Society of Chemical Industry (SCI).

Automated summary

The study developed ionic liquids with high hydrogen-bond basicity to pretreat lignite, resulting in partial dissolution of lignite, increased coal aromaticity and rank, reduced coal decomposition residues, and the production of smaller, more uniform particles. The ether-functionalized N-(2-ethoxyethyl)-N-methylpiperidinium acetate showed promise as an alternative to conventional imidazolium ILs due to its lower cost and ability to improve coal aromaticity. Overall, pretreatment with functionalized ionic liquids can enhance the properties of low-rank coal for better utilization in fuel and materials.