Hydrotalcite supported cobalt and tungsten catalysts for valorization of lignin into valuable phenolics

Abstract

Designing of an efficient catalytic system to break down the sub-aromatic units from the lignin complex 3D structure is a challenging task in biorefinery. Herein, we have designed and prepared the cobalt-impregnated hydrotalcite HT500 basic catalysts for the valorization of an industrial lignin. Catalysts were synthesized using base (e.g., NH3, NaOH) and without the use of the base. The 10 wt% Co/HTNaOH catalyst showed excellent liquefaction tendency by yielding maximum bio-oil (72 wt%) together with high selectivity of acetosyringone under mild reaction conditions (70 °C/3h). This may be owing to the efficient breakage of β-O-4 linkages together with the activation of the Ar-Cα-OH bonds. With the incorporation of W (Co-W/HTNaOH), the selectivity towards acetosyringone was enhanced remarkably (65 area%). The 1H, 13C, and 2D-NMR analysis of raw lignin and bio-oil validate the cleavage of Ar-Cα-OH and ether linkages (β-O-4) without disturbing C–C linkages present in sub-aromatic units of lignin e.g., syringyl and guaiacyl. The Co-W/HTNaOH showed good catalytic performance up to two successive cycles upon regeneration. The XRD, BET, TEM, SEM, XPS, and FT-IR characterization of fresh and regenerated catalysts did not show any significant loss in catalytic active sites, indicating good catalyst regeneration.