Hydrothermal liquefaction of pinecone kraft lignin into selective phenolic: Optimization of Ni and Mo impregnation on ceria

Abstract

Pine is an abundant forestry waste and is a main cause of forest fires in China's mountain area owing to its flammable nature. Therefore, its utilization for the production of valuable phenolics can contribute a small to fix it. This study examined Ni-impregnated ZrO2, CeO2, and MgO catalysts for hydrothermal liquefaction of pinecone lignin, which was extracted by applying the kraft process. The 5 wt%-Ni/CeO2 catalyst was found to be efficient with a 59.2 wt% bio-oil yield corresponding to 77.6% of total conversion in ethanol. This bio-oil yield was enhanced significantly to 65.7 wt% on applying the 3 wt% Mo along with 5Ni/CeO2. The effect of diverse solvents e.g., water, ethanol, methanol, IPA, and co-solvent mixture was also explored to achieve the greatest liquefaction of this PC kraft lignin. Amongst all, methanol/water (1:1) was highly compatible with the maximum liquefaction of kraft lignin (77.8%) at optimum 280 °C/30 reaction conditions. The GCMS analysis of optimum bio-oil attributes the higher selectivity of vanillin (67.4%) and validates the presence of guaiacyl sub-aromatic units in the lignin. The elemental analysis of cat-M-W bio-oil exhibits a considerable improvement in calorific value (38.4 MJ/kg) compared to that of raw lignin (28.1 MJ/kg). Additionally, the reusability test of the 5Ni–3Mo/CeO2 catalyst attributes the best performance of the catalyst up to two cycles without applying any kind of regeneration. Additionally, XRD and BET analysis of catalysts showed the impregnation of both metals (Ni & Mo) at the CeO2 surface in the oxide form.