Selective conversion of lignin to ethylbenzene

Abstract

Lignin, an abundant renewable aromatic resource, has a complex structure composed of various C₉ propyl phenol units with cross-linked C–C, ester and ether linkages. Herein, we report a two-step process for the selective production of ethylbenzene from corncob lignin valorization. This process starts with hydrodeoxygenation of lignin to C₈ ethylcyclohexane (42%), C₉–C₁₇ cyclic alkanes (bio-jet fuel range, 38%), and some C₃–C₇ alkanes (gasoline range, 20%) over Ni/Silicalite-1 (Ni/S-1) at 300 °C and 6 MPa H₂ using a non-polar solvent. After separation by distillation, the subsequent dehydrogenation of ethylcyclohexane over Pt–Sn/Al₂O₃ leads to the formation of ethylbenzene with 99.3% yield at 500 °C and 0.5 MPa H₂ in a continuous flow reactor. It is demonstrated that the selective production of C₈ ethylcyclohexane from lignin specifically proceeds by thermal cracking and hydrogenolysis on Ni nanoparticles (NPs). The thermal cracking of lignin eliminates the C_β–C_γ bond in the side 3-C chain of the depolymerized C₉ radicals, as formed by the homolytic cleavage of β-O-4 linkages in lignin via a quinone methide reaction pathway. Additionally, Ni NPs facilitate the decarboxylation of the carbonyl groups formed from the end hydroxyl and ester groups of C₉ lignin units. The established strategy enables the selective production of value-added ethylbenzene from lignin valorization and the simultaneous formation of a qualified bicyclic alkane bio-jet fuel with high thermal stability.