This article reviewed recent development of chemical depolymerization of lignins. There were five types of treatment discussed, including base-catalyzed, acid-catalyzed, metallic catalyzed, ionic liquids-assisted, and supercritical fluids-assisted lignin depolymerizations. The methods employed in this research were described, and the important results were marked. Generally, base-catalyzed and acid-catalyzed methods were straightforward, but the selectivity was low. The severe reaction conditions (high pressure, high temperature, and extreme pH) resulted in requirement of specially designed reactors, which led to high costs of facility and handling. Ionic liquids, and supercritical fluids-assisted lignin depolymerizations had high selectivity, but the high costs of ionic liquids recycling and supercritical fluid facility limited their applications on commercial scale biomass treatment. Metallic catalyzed depolymerization had great advantages because of its high selectivity to certain monomeric compounds and much milder reaction condition than base-catalyzed or acid-catalyzed depolymerizations. It would be a great contribution to lignin conversion if appropriate catalysts were synthesized. 1. Introduction Lignin is a natural resource which exists in woody materials, agricultural residues, and other plant materials (so-called lignocellulosic materials). Lignocellulosic materials consist of 10–30% lignin by weight and 40% by energy [1]. However, it has mainly been used as an energy source in combustion processes, and less than 5% lignin has been used for other purposes nowadays [2]. Because of its high energy content and polymer structure, lignin is considered as a potential renewable resource of chemicals and fuels especially in condition of escalating petroleum price and renewable energy demand. Lignin depolymerization is very promising process which can generate value added products from lignin raw materials. The primary purpose of lignin depolymerization is to convert the complex lignin compound into small molecules for fuels and basic chemicals or oligomers for further application. Considerable amount of research has been done to convert lignin into renewable fuels and chemicals using pyrolysis and gasification methods [3–8]. Pyrolysis refers to the thermal treatment of the biomass or lignin in the absence of oxygen, with or without any catalyst usually at the temperature between 300 and 600°C [3]. The cleavage of OH functional group linked to aliphatic side chain, the breaking of alkyl side chain, aryl ether, and linkage between aromatic rings occur
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