Metal-free carbonaceous materials for lignin Valorization: Insights from sono-, photo-, and sono-photochemical systems in batch and flow configurations

The transition toward sustainable chemical manufacturing has fostered growing interest in lignin valorization, focusing on converting this abundant lignocellulosic biomass component into valuable chemicals and materials. However, the complex structure of lignin has traditionally limited its efficient utilization. This critical review investigates the role of metal-free carbonaceous materials, such as activated carbon, graphene, and carbon nanotubes, while deploying sono-, photo-, and sono-photochemical systems for lignin valorization. These materials, characterized by their high surface area, tunable porosity, and chemical stability, offer a promising alternative to metal-based catalysts, addressing both environmental and economic concerns related to toxicity and cost. The enhancement of lignin conversion processes has been explored via sonocatalysis, photocatalysis, and their synergistic combination in sono-photocatalysis, with particular emphasis on mechanistic understanding through isotope labeling experiments, DFT calculations, and comparative analysis of energy consumption and environmental impact. By systematically comparing batch and flow configurations, and delving into the underlying mechanisms, this review identifies current research gaps and assesses the potential of these carbonaceous materials to propel sustainable lignin valorization technologies, thus contributing to green chemistry and promoting a circular bio-economy.

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