Recent developments in selective catalytic conversion of lignin into aromatics and their derivatives

Lignin (15-30 wt%) is a key component of lignocellulosic biomass, along with cellulose (30-50 wt%) and hemicellulose (20-35 wt%), which is largely obtained as a waste product from pulp/paper and biorefinery industries. Due to its aromatic polymer structure, lignin is an obvious non-fossil source for aromatics and their derivatives. Efficient lignin valorization can assist in achieving two essential energy goals, i.e., boosting the lignocellulosic biorefinery and reducing the utilization of fossil fuels for chemicals synthesis. Hence, developing efficient processes and technologies for transforming this waste lignin into value-added chemicals is of paramount interest in the current biorefinery research. Catalysis plays a crucial role in lignin valorization via efficient cleavage of C-O and C-C bonds, which is greatly dependent on the catalysts' properties and the reaction conditions. In this mini review, we provided a concise account of the most important catalytic routes recently developed for the efficient cleavage of C-O and C-C linkages in lignin systems to produce desired aromatics and their derivatives. Strategies to control unwanted cracking of lignin and char formation especially during catalytic hydroprocessing of lignin are critically discussed. Special attention has been dedicated to understand the role of acid and redox properties of heterogeneous solid catalysts in the selective depolymerization/upgrading of lignin systems.