Catalytic lignin valorisation by depolymerisation, hydrogenation, demethylation and hydrodeoxygenation: Mechanism, chemical reaction kinetics and transport phenomena

The lignin-to-chemicals valorisation has increased the interest of the scientific community in exploring the effective lignin depolymerisation. Lignin is the most abundant natural resource of aromatic components with a high potential to be converted into the various chemicals, thus increasing the level of an integrated biorefinery. This review focuses on lignin depolymerisation mechanism, chemical reaction kinetics and transport phenomena studies, recently introduced in the field of lignin chemistry to understand the reactivity of lignin macromolecule in two- or three-phase systems with a heterogeneous catalyst, liquid solvent and gaseous source. Lignin depolymerisation involves several parallel and sequential reactions of aryl-ether bond cleavage leading to a complex mixture of numerous depolymerised components. Theoretical and mathematical approaches for understanding and predicting catalytic parameters with the kinetic modelling are also discussed. The modelling approaches and kinetic data from various works in the literature has been thoroughly systematically reviewed, processed and consistently presented and benchmarked in graphical and tabular form. The aspects of modelling heat and mass transfer during lignin depolymerisation and upgrade has also been reported.