Valorization of industrial lignin to value-added chemicals by chemical depolymerization and biological conversion

Abstract Lignin is a renewable aromatic resource and the most abundant aromatic biopolymer on Earth. As a byproduct of paper pulping, more than 50 million tons of waste lignin is generated annually and used primarily for heat and supplemental energy through simple combustion. Lignin production is also expected to increase as a byproduct of lignocellulosic biofuel industries. Thus, demand on effective lignin valorization technologies is increasing. Although many attempts have been made to develop lignin valorization technologies based on thermochemical or biological processes, efficient degradation of lignin and upgrading to value-added products remain challenging. This paper reviews recent developments and the advantages and/or drawbacks of each thermochemical depolymerization and biological conversion process, and explores the possibilities for sustainable industrial application of waste lignin using an integrated valorization method that combines thermochemical depolymerization and biological conversion. Furthermore, various value-added products such as vanillin, muconic acid, polyhydroxyalkanoates, and lipid representing as comprehensive proof of concepts for great potentiality of lignin are thoroughly discussed.