Electronic State of Low-Rank Coals with Exchanged Sodium Cations

Our previous experimental study showed that Na⁺-exchanged coal prepared from low-cost natural soda ash is an excellent catalyst for steam gasification of low-rank coals using fixed-bed quartz reactors. However, it is difficult to experimentally clarify the effect of Na ion exchange on low-rank coal. In order to investigate the influence of Na⁺ ions on low-rank coal, this study determined the electronic state between the Na⁺-exchanged coal model and raw coal model and compared them using RHF/6-311G* and B3LYP/6-31G*. The experiments revealed that Na ion exchange has a significant effect on low-rank coal gasification. The model structure of low-rank coal is considered to change significantly in terms of the electronic state before and after Na exchange even with a simple main molecular structure. Molecular models where H of COOH/OH was ion-exchanged with one, two, and three Na ions were developed, and quantum chemical calculations were performed. The results showed that when the number of Na⁺-exchanged sites is increased, the electron state on the coal molecule becomes more negatively charged in the case of the Na⁺-exchange coal model. It is presumed that this contributes to enhancing the reactivity of low-rank coal and water vapor. In addition, weak bonds in the Na⁺-exchanged coal molecule were examined by calculating the difference in the value of the Mulliken and Löwdin bond orders before and after Na⁺ exchange. The results showed that the increase in the number of exchanged Na⁺ in the low-rank coal molecule model increased the number of weak bonds in the molecule. It is presumed that this contributes to enhancing the decomposition of low-rank coal.