In-situ and ex-situ catalytic pyrolysis of lignin in rotary kilns with metal-modified acidified ZSM-5

• Lignin was pyrolyzed in a self-designed rotary kiln with metal modified acidified ZSM-5. • In-situ catalysis favored vanillin production while ex-situ catalysis promoted guaiacol formation. • Mg@ZSM-5 demonstrated exceptional guaiacol selectivity, achieving 49.77% yield in in-situ catalysis and 55.48% yield in ex-situ catalysis. Biomass serves as a sustainable clean energy source, effectively mitigating greenhouse effects and environmental pollution. Lignin, the one of the main components of biomass, constitutes the most abundant aromatic resource in nature yet faces utilization challenges due to structural complexity and heterogeneity, resulting in low conversion efficiency and poor product selectivity. Catalytic pyrolysis is an effective means of utilization, but systematic studies on in-situ versus ex-situ catalytic modes and catalyst types remain limited. In this study, a series of metal modified acidified ZSM-5 catalyst using dealkaline lignin was studied in a rotary kiln reactor, and the effects of different catalysts and catalytic modes on the distribution of pyrolysis products were analyzed. Ex-situ catalysis favors higher guaiacol selectivity compared to in-situ methods, with H@ZSM-5 selectivity surging from 27.52% to 42.61%, Fe@ZSM-5 rising from 27.18% to 42.46%. Transition metal catalysts exhibited mode-dependent behaviors: Both Ni@ZSM-5 and Fe@ZSM-5 were more selective to vanillin in in-situ catalysis and more selective to guaiacol in ex-situ catalysis. Mg@ZSM-5 consistently preserved methoxy groups across both modes through alkaline suppression of deoxygenation, attaining 49.77% guaiacol in in-situ and 55.48% in ex-situ. These findings provide a reference for lignin valorization and reactor-catalyst synergistic optimization.