Abstract
Selective pyrolysis of lignin into aromatics remains a major challenge due to its high oxygen content and structural complexity. Solid acid catalysts, like zeolites, are well known as effective catalysts for pyrolysis processes. However, the role of acid site density and strength in controlling aromatic yields in lignin pyrolysis remains insufficiently understood. Here, three zeolite framework topologies, namely FAU, MFI, and BEA, with distinct acidity profiles were systematically evaluated for lignin pyrolysis to disentangle zeolite framework topology and acidity effects. Significant variations in bio-oil and non-oxygenated aromatics (benzene, toluene, xylene (BTXs), and napthalenes) yields demonstrated that framework topology-dependent acid site strength plays a more dominant role than acid site density (bulk/external) in enhancing aromatics yield. Thermogravimetric analysis revealed that primary lignin depolymerization is not responsible for the product distribution variations observed. Instead, secondary reactions of oxygenated lignin fragments with acid sites dictate the overall deoxygenation performance. H-ZSM-5 exhibited the highest aromatics yields (100–132 mg g-1lignin), followed by H-Beta (35–52 mg g-1 lignin) and H-Y (8–22 mg g-1 lignin) zeolites. These values correspond to 4 to 16 % of theoretical maximum BTXs ring utilization efficiency (RUE), a benchmark that we propose to evaluate catalyst efficiency in lignin pyrolysis, defined as the ratio of experimentally obtained BTXs to the theoretical maximum based on phenylpropanoid units (C9) in lignin. Regeneration studies revealed gradual catalyst deactivation due to the conversion of strong acid sites (SAS) to intermediate acid sites (IAS) resulting in lower BTXs RUE per cycle. Overall, this work highlights the role of zeolite framework topology-driven confinement effects in controlling the acid site strength and thereby, governing lignin pyrolysis performance.