Enzymatic Oxidation of Lignin: Challenges and Barriers Toward Practical Applications

Lignocellulosic biomass represents perhaps the most abundant renewable resource with a potential to replace fossil‐based feedstock for sustainable energy, chemical and materials production. Among the three major lignocellulosic biomass components (i. e. cellulose, hemicellulose and lignin), lignin is a macromolecule with an aromatic skeleton with a variety of functional groups (e. g. hydroxyl, methoxy, carbonyl, double bond) and carries a higher energy density. The unique structure makes lignin an intriguing substrate for energy, chemicals and materials productions. However, the high molecular weight and complex macromolecular structure have made lignin a challenging substrate to be transformed by many conversion methods. Microbial enzyme degradation and modification of lignin have been subjected to a significant amount research in the last a few decades. Yet so far little success has been demonstrated to merit the use of enzymatic technology for lignin transformation at a commercial scale. This paper provides an updated review of the development of lignin degrading/modifying enzymes with an emphasis on identifying the key barriers and challenges toward practical applications of microbial enzymes for lignin valorization with a hope to generate new insights and direction that can overcome these challenges.