Tuning the thermal reactivity of low-rank coal: The structural role of phenolic hydroxyl groups from molecular aggregation to pore collapse

Phenolic hydroxyl groups significantly influence the efficiency of low-rank coal direct liquefaction, yet their specific role in thermal reactivity is unclear. This study deciphers the fundamental mechanism by which phenolic hydroxyl groups govern the thermal reactivity of Naomaohu long flame coal (NL), a low-rank coal, identifying them as a key regulator of its chemical and physical structure. Through hydrothermal treatment (240–320 °C) of demineralized NL, the phenolic hydroxyl concentration in it achieved a selective reduction from 2.94 to 0.08 mmol/g, while the carboxyl group remained stable at 0.21–0.25 mmol/g. A decrease in these groups was found to promote aromatic condensation and a rise in C C/C H bonds, which in turn triggered pore structure collapse, surface smoothing, and specific surface area decreasing. This structural coarsening was directly linked to a raised energy barrier for thermal reaction, with the activation energy being increased from 28.26 to 43.11 kJ/mol. At the electronic level, it was demonstrated that the reaction energy barrier is lowered by phenolic hydroxyls through a homogenization of the molecular electrostatic potential and a weakening of the critical C O bonds. Thus, the role of phenolic hydroxyls is conclusively resolved, and their concentration is established as a primary factor controlling coal reactivity, which provides immediate implications for the strategic upgrading of low-rank coal.