Moderated Molybdenum–Zeolite Interaction and Tunable Lewis–Brønsted Acid Sites’ Synergy for Efficient Kraft Lignin Depolymerization in Solvent-Donated Hydrogen

Abstract

Despite lignin’s massive potential for biobased clean fuel production, developing effective catalysts for selectively cleaving its interunit bonds to harness this potential continues to be challenging. In this study, an HZSM-5-supported MoO₃ catalyst was developed for a one-pot, hydrothermal kraft lignin depolymerization to maximize liquid fuel yield. A record-high 85.1% yield of petroleum ether-soluble product (a potential gasoline substitute), an unprecedented 45.3% yield of aromatic monomer, and a meager 4.9% char yield (corresponding to a 95.1% lignin conversion) were realized at 300 °C and 24 h in a catalytic transfer hydrogenolysis. The resulting liquid fuel afforded a calorific value of 35.97 MJ/kg, significantly improving over the 25.45 MJ/kg in the original kraft lignin. Catalyst characterization results showed that an optimal acidity characteristic and a suitable Mo-zeolite interaction were pivotal to the efficient cleavage of the C–O and C–C bonds in the kraft lignin. The analyses of the spent catalyst revealed that the formation of inactive Mo⁴⁺ species and carbonaceous deposits on the catalyst’s surface was responsible for the initial catalyst deactivation, but recalcination reinstated the catalyst activity. This study extends the frontiers of knowledge in rational catalyst development for lignin valorization applications.