Influence of coal treatments on the Ni loading mechanism of Ni-loaded lignite char catalysts

Abstract

Many kinds of lignite coals have been used as catalyst supporters for preparing the Ni-loaded lignite char catalyst. However, these coals have different properties; especially ash content. The ash in coal affects the mechanism of Ni-loading on the position of the functional group structures in coal. In catalyst preparation, it is interesting that the difference in coal properties might directly influence the mechanism of Ni loading. To prove this point, the coal sample needed to be treated before the catalyst preparation. MM coal (original coal) was treated with acid (HClMM and AceMM coals), alkali (NaMM coal) and alkali followed by acid treatments (NaHClMM and NaAceMM coals). Then, Ni was loaded on the five treated coals by the ion-exchange technique. The Ni-loaded lignite coals were pyrolyzed at 650 °C under a N₂ atmosphere to prepare the Ni-loaded lignite char catalysts. The Ni-loading mechanisms were studied via FTIR, XRD, AAS and SEM-EDS analyses. The results showed that the different treatments affected the ash content and the functional groups in the coals. The decreases in the ash contents of HClMM, AceMM, NaHClMM and NaAceMM coals indicated that the exchangeable metallic species were removed by transforming metal-carboxylates into carboxyl groups. The transformations of metal-carboxylates were confirmed by the increased Δϑ(COO⁻) value. For acid treatment, the ion exchange of Ni was controlled by carboxyl groups, while in alkali treatment it occurred through hydroxyl and metal-carboxyl groups. In alkali followed by acid treatment, Ni ions were exchanged with hydroxyl and carboxyl groups. The Ni ion forms, Ni(NH₃)₆²⁺ and/or Ni(H₂O)₆²⁺, appeared on the modified coals. Through pyrolysis, the Ni ion was reduced to Ni metal that was observed in the XRD patterns of the catalysts. The Ni contents of the catalysts were in the range of 16.51–20.07 wt%. The thermal behaviours of the catalysts were presented via TGA-DTG.