Hydrothermal depolymerization of organosolv lignin using Ni-Re catalysts in TiO₂ support via sequential impregnation synthesis

Abstract

This study examines the hydrothermal depolymerisation of organosolv lignin over Ni − Re catalysts supported on TiO₂ and prepared by sequential impregnation method. The effect of different Ni/Re ratios and Ni loadings on the structure of the catalyst and reaction behaviour were investigated. The XRD patterns indicated no change in structure for TiO₂ upon impregnation, and relatively low or absent peaks corresponding to the metals indicated good metal dispersion, particularly at lower loadings. SEM-EDS was used to verify the distribution and presence of Ni and Re on the support. Pressure-time profiles showed that the composition of the catalyst affected the stability of the reaction and the depolymerization behaviour. The results indicated that Ni–Re/ TiO₂ catalysts could be successfully prepared by the sequential impregnation method and XRD and SEM-EDS results confirmed that the TiO₂ support structure was stable after impregnation with the metals and Ni and Re were present and were reasonably well dispersed, particularly at lower loadings of the metals. Pressure–time profiles suggested that the composition of the catalysts affects depolymerization behaviour and reaction stability, with Ni causing more controlled behaviour than the non-catalysed reaction and Ni-Re synergy being more apparent at lower reaction volume or lignin concentration. The pressure behaviour was found to be the best for Ni₄Re₁/ TiO₂ catalyst ratio among the tested ones. However, the GC-MS analysis revealed that the products of lignin depolymerization detected ranged from 5.11 mg/g lignin for Ni₄Re₁/ TiO₂ to 8.38 mg/g lignin for Re₅/ TiO₂, suggesting that the higher the Re content, the more depolymerized the lignin was under the conditions of this study.