Bifunctional WO₃-modified Pd/ZrO₂ catalysts for lignin upgrading: Fixed-bed hydrodeoxygenation and acid-assisted C–C bond cleavage

Lignin is an attractive renewable carbon source for producing sustainable jet fuel components such as cycloalkanes. Its utilization, however, requires both efficient hydrodeoxygenation (HDO) of phenolic monomers and cleavage of recalcitrant C–C linkages in oligomeric fragments. In this work, a Pd/ZrO₂ catalyst was prepared by flame spray pyrolysis and evaluated for deep HDO of lignin-derived alkylated phenols in a continuous fixed-bed reactor, affording high yields of oxygen-free cycloalkanes (propylcyclohexane as the dominant product). To introduce Brønsted acidity required for C–C bond scission, Pd/ZrO₂ was subsequently modified by impregnation with H₃PO₄ or WO₃. Among the catalysts studied, 10 wt% WO₃/Pd/ZrO₂ provided the best overall performance in batch hydrotreatment of 2,2′-biphenol, delivering a 47.8 mol% yield of C₆ monomers (cyclohexane and methylcyclopentane). The optimized WO₃-modified catalyst also enabled effective depolymerization and upgrading of dimethoxybiphenyl and a crude lignin oligomer mixture, demonstrating its potential for integrated lignin-to-hydrocarbon upgrading. These results highlight the importance of combining Pd hydrogenation functionality with accessible Brønsted acid sites on a reducible oxide support to couple C–O removal with acid-assisted C–C scission in lignin conversion.