Advancing sustainable lignin valorisation: utilizing Z-scheme photocatalysts for efficient hydrogenolysis of lignin's β-O-4, α-O-4, and 4-O-5 linkages under ambient conditions

Abstract

Lignin, a crucial component of lignocellulosic biomass, holds immense promise for advancing biorefineries and producing sustainable energy alternatives. This study engineered a CN/rGO/BMO (2 : 1) heterojunction photocatalyst with varying Pd NP loading to selectively target the hydrogenolysis of challenging C–O ether bonds in lignin model compounds. Specific focus was on β-O-4, α-O-4, and the formidable 4-O-5 linkages. The 4-O-5 linkage, due to its high dissociation energy, poses a significant challenge. Electrochemical and spectral analyses unveiled improvements in charge separation, leading to delayed recombination of charge carriers in the heterojunction photocatalyst. Decorating the heterojunction with Pd NPs exhibited an elevated work function and a low Fermi energy level, facilitating the accommodation of photogenerated electrons and enabling efficient H₂ dissociation. Such enhancement facilitated the cleavage of all three linkages, including the resistant 4-O-5 bonds under ambient conditions. The photocatalyst exhibited effective cleavage, yielding aromatic (toluene, phenol, ethylbenzene) and aliphatic (cyclohexane, ethylcyclohexane) products. Selectivity modulation was achieved by adjusting the time, hydrogen pressure, and Pd loading. Furthermore, this photocatalytic approach successfully transformed simulated lignin bio-oil containing all three linkages into valuable monomers. The alcoholic solvents efficiently harnessed photogenerated holes and prevented electron–hole recombination. The photocatalyst also demonstrated the capability to produce monomers from the native lignin extracted from teak wood sawdust. Emphasizing the focus on the reaction pathway and mechanism, scavenging studies and analyses were conducted, including UPS and VBXPS, to establish the Z-scheme charge transfer mechanism within the heterojunction. These findings provide a sustainable and efficient pathway for lignin valorisation in biorefineries, significantly contributing to the advancement of green fuels and aligning with the principles of green chemistry.