Molecular simulation for physisorption characteristics of O2 in low-rank coals

Highlights: • The macromolecular model of five low-rank coals were constructed. • The physisorption behavior of oxygen in low-rank coals were simulated. • Oxygen in low-rank coals tends to gather around the aliphatic hydrocarbons. • Aromatic rings, hydroxyl and ether with strong physisorption capacity for oxygen. In this paper, systematic research of the physisorption characteristics of oxygen in low-rank coals had been carried out using the Grand Canonical Monte Carlo (GCMC) and the Density Functional Theory (DFT) methods based on the assumption of no chemisorption. Firstly, the surface molecular structure parameters of five different low-rank coals were determined, the coal molecules and their unit cells structure were constructed; Secondly, Oxygen physisorption behaviour in coal molecular unit cells was simulated based on the GCMC and the Molecular Dynamics (MD) method; Finally, the physisorption parameters for oxygen physisorption at each adsorption site were simulated based on the DFT. The results show that the microporous structure of coal molecules is positively correlated with the total physisorption amount of oxygen and has an effect on the physisorption heat; oxygen is gathered around aliphatic hydrocarbons, the mutual distances of methyl and methylene to oxygen were 3.57 Å and 3.81 Å, respectively; the adsorption capacity of the low-rank coal molecules is effected by aromatic, oxygenated aliphatic hydrocarbons, and the degree of condensation of polycyclic aromatic hydrocarbons, the physisorption energy of the aromatic ring, hydroxyl and ether bonds to oxygen were −3.4790 kcal/mol, −2.9933 kcal/mol and −2.9663 kcal/mol respectively. This research will enable us to better understand the physisorption mechanism of oxygen in low-rank coals.