Reductive or oxidative catalytic lignin depolymerization: An overview of recent advances

Lignin, one of the main biopolymers in lignocellulosic biomass, is a promising feedstock for producing aromatic chemicals and BTX-fuels. Despite high availability, lignin remains under-valorized to chemical formation motivating research towards upgrading through depolymerization. Catalytic approaches are particularly studied since they can influence the selectivity of depolymerization. Next to thermal or acid/base depolymerization, catalytic routes under either oxidative or reductive atmosphere represent a very attractive way to produce aromatics for fine chemistry and energy. This literature overview shows that the depolymerization outcome depends strongly on the reaction conditions. While under oxidative atmosphere highly functionalized high-added value chemicals like vanillin are obtained, alkylphenolics and BTX are mainly observed when conducting the reaction under reductive atmosphere. Additionally, results depend mainly on the nature of the catalyst for the former approach, whereas reaction conditions play an essential role when operating under hydrogen. Understanding and comparing these quite complex methodologies over the last ten years constitutes the focus of this review.