Effect of the electronic state on low-rank coals with Ca2+ ion exchange

Ca2+-exchanged coal prepared from calcium hydroxide is a good catalyst for steam gasification of low-rank coal with a fixed-bed quartz reactor. However, it is difficult to clarify the effect of Ca2+ ion exchange on low-rank coal by experiment. To investigate the effect of Ca2+ ions on low-rank coals, this study determined the electronic states between Ca2+-exchanged and raw coal models and compared them by B3LYP/6-31G∗ and HF/6-311G∗ calculations. The experiments revealed that the Ca2+ ion exchange method significantly affects gasification of low-rank coal. It is considered that the molecular structure of low-rank coal presents significant changes in the electronic state before and after Ca exchange, even with a simple molecular structure. Therefore, a molecular model was created in which a COOH/OH H+ ion was exchanged with one Ca2+ ion, and quantum chemical calculations were subsequently performed. The results revealed that the –COOCa1/2 IR peak calculated from B3LYP/6-31G∗ appeared at approximately the same position as the experimental IR peak in low-rank coal after Ca exchange. The Ca structure was more stable following interaction with the O in the benzene OH group than with that in the OH group of the tetralin ring containing Ca. Furthermore, the increased negative charge of the benzene ring following Ca interaction was presumed to improve the reactivity between the low-rank coal and steam. In addition, weak bonds in the Ca2+-exchanged coal molecules were investigated by calculating the difference between the values of the Löwdin and Mulliken bond orders before and after Ca2+ exchange. The results indicated that the binding in the molecule was weakened by Ca coordination. From these facts, it is presumed that H+/Ca2+ ion exchange promotes the decomposition of low-rank coal. [Display omitted] •Electronic states of Ca2+-exchanged and raw coal models determined theoretically.•Ca2+ ion exchange significantly affects gasification of low-rank coal.•Ca2+ ion exchange alters electronic state of low-rank coal upon gasification.