Synergistic effects of hydrogen donation and catalysis by deep eutectic solvents on tar upgrading in coal pyrolysis

The efficient utilization of low-rank coal is constrained by the low yield and high heavy-component content of tar from conventional pyrolysis. To overcome this, a novel deep eutectic solvent (DES) composed of NiCl 2 and ethylene glycol (EG) was synthesized as a multifunction reagent for the catalytic pyrolysis of Naomaohu coal. Results demonstrated that the NiCl 2 -EG DES improved both tar yield and quality. The optimal performance was achieved at a 1:4 ratio (NMH-4), where the tar yield increased to 19.26% from 10.77% for raw coal, and the light fraction (boiling point ≤ 360 °C) yield increased from 7.67% to 15.08%. Kinetic analysis via the KAS method revealed a lower average apparent activation energy (224.18 kJ·mol −1 vs. 247.34 kJ·mol −1 for raw coal) and a higher pre-exponential factor, confirming enhanced pyrolysis reactivity. The synergistic mechanism was elucidated by FTIR, XRD and GC-MS characterization: NiCl 2 is reduced to metallic Ni via carbothermal reaction, and the in-situ formed Ni catalyzes the cleavage of C O, C-O, and C-C bonds, while the active hydrogen produced by EG decomposition stabilizes the pyrolysis free radicals, thereby inhibiting the recombination and promoting the formation of light fraction. This synergy also facilitated the deoxygenation of phenols and the ring-opening of polycyclic aromatic hydrocarbons. The hydrogen-donating pathway, verified using 4-ethylphenol, showed that EG-derived active hydrogen preferentially stabilizes 4-hydroxyphenyl radicals, increasing phenol yield. This work demonstrates an effective strategy to integrate catalysis and hydrogen donation by designing a dedicated deep eutectic solvent, offering an effective route for coal pyrolysis upgrading.