Table 4.

Classification of lignin depolymerization

Depolymerization	Subtype	Objective
Thermal	Pyrolysis and hydrothermal liquefaction	Controlled thermal degradation to give bio-crude
	Gasification	Controlled degradation of aromatics to give combustible gas
	Combustion	Oxidative energy generation
Chemical	Base/acid catalysis	Ether bond/side chain cleavage under milder condition
	Oxidation/Reduction	Bond cleavage coupled with further conversion to minimize re-polymerization
	Capping depolymerization	Minimize re-polymerization by temporary protection
	Ionic liquid/Supercritical fluid, etc	Solubilizing lignin to improve selectivity/mass & energy transfer
Metallic Catalytic		Selective bond cleavage and coupling with redox reactions
Biological	Enzymatic, growing cell	Oxidative depolymerization or catabolizing lignin