Two-step catalytic hydrogenation of lignin to high-yield monocyclic and polycyclic aromatic hydrocarbons

The valorisation of lignin into high-value aromatic hydrocarbons remains challenging because of the complex structure and oxygen-rich nature of lignin. This study presents a model-compound-guided two-step approach using a MoS 2 catalyst for the extensive depolymerisation and deoxygenation of lignin. In the first step, C–O bond cleavage was demonstrated at 300–325 °C using model compounds; for real lignin liquefaction, the first step was optimised at 400 °C to achieve complete dissolution and fragmentation before the second step at 450 °C for C–C bond cleavage. Remarkably, this process converts most aromatic structures into detectable low-molecular-weight compounds, yielding 77.9 wt% oil, together with 19.3 wt% water and 9.6 wt% gases, so that the mass balance closes at 107.7 wt% on a lignin basis with the excess accounted for by hydrogen uptake. The product distribution features 29.2 wt% monocyclic aromatic hydrocarbons (MAHs) and 33.9 wt% polycyclic aromatic hydrocarbons (PAHs), collectively representing 81% of detectable products. Notably, the oxygen content is reduced to 0.27 wt%, demonstrating effective deoxygenation. Unlike conventional lignin-derived black oils, the product oil obtained herein exhibits superior quality with a transparent appearance. This study establishes an efficient pathway for the conversion of lignin into MAHs and into polycyclic aromatics suitable as chemical feedstocks, addressing key challenges in the valorisation of lignin. The presented strategy provides a framework for the sustainable utilisation of lignin in biorefineries. The first-step black oil containing GC-undetectable C–C-linked oligomers was largely converted into detectable MAHs and PAHs in the second step, as supported by GPC and GC–MS analyses.