Selective photocatalytic conversion of lignin via metal-doped anodic TiO2 nanotubes

Abstract

Lignin, an underutilized lignocellulosic biomass often considered waste, is a rich source of fuel and aromatic chemicals. Utilizing lignin for sunlight-induced photocatalytic reactions to produce high-value bulk and fine chemicals is regarded as a bio-based economically viable strategy. Herein, we report a facile preparation strategy for a one-dimensional metal-doped nanostructured TiO₂ photocatalyst aimed at the selective conversion of lignin into valuable chemicals. Specifically, we successfully synthesized and applied metal-doped anodized TiO₂ nanotubes (NTs) to efficiently convert lignin into valuable chemical feedstock, with a focus on cleaving the β–O–4 linkage. Various metal dopants (Cr, Fe, Co, and Cu) were incorporated into TiO₂ NTs to enhance charge separation, improve charge transfer efficiency, and increase the availability of active sites responsible for lignin bond cleavage. Optical and photoelectrochemical (PEC) analyses revealed slight band edge modulation, which facilitates the generation of active species essential for lignin conversion. Selective conversion to vanillin, a high-value chemical, was achieved, with Ti-O_x sites and the 1D nanotubular structure playing crucial roles in the process. Additionally, the reusability of the doped TiO₂ systems and the distinct effects of different metal dopants on the photocatalytic process were thoroughly investigated. Overall, this study presents metal-doped TiO₂ NTs as a promising approach for the efficient and selective conversion of lignin into valuable chemical feedstock, contributing to sustainability and bio-based resource utilization.