Modification of cobalt-loaded zeolite catalysts for the selective fractionation of kraft lignin into guaiacyl phenolic monomers: Effect of oxophilic metal ions

Lignin, an abundant byproduct of lignocellulosic biorefineries, exhibits significant potential as a renewable feedstock for the production of value-added aromatic chemicals and high-energy-density biofuels due to its complex, highly aromatic polymeric structure. In this study, we investigated the effect of a Co-loaded HZSM-5 (50) (HZ) catalyst on oxidative valorization of kraft lignin under mild conditions of air. The bio-oil yield was increased with the increasing loading of Co, and was obtained to be 45.3 wt.% at 140°C/ 30 min. The desilication of zeolite exhibited a promising impact on the depolymerization of lignin, and the bio-oil was significantly enhanced to 51.8 wt.% with 10Co/HZ-n catalyst. Herein. The effect of metal promoters (Zn, Fe, and Nb) were also comprehensively determined by their successive loading on 10Co/HZ-n catalyst. The maximum bio-oil yield (67.3 wt.%) was evaluated with 10Co-3Nb/HZ-n catalyst at 120°C/ 30 min, reaction conditions. Additionally, the reusability performance of the 10Co-3Nb/HZ-n catalyst was examined for up to four catalytic cycles. The elemental analysis of bio-oils showed a significant improvement in the quality in terms of high calorific value (36.8 MJ/kg) for the 10Co-3Nb/HZ-n. Additionally, vanillin (68.2%) was observed as the main selective catalytic product, and its maximum selectivity was also determined for the optimum 10Co-3Nb/HZ-n bio-oil. The study emphasises the role of high metal valent ions in the selective fractionation of lignin into valuable phenolic compounds.