Study On The Cleavage Of β-O-4Bonds Of Lignin Model Compounds In Ionic Liquid

Biomass is a potential alternative energy. At present, the hot research focuses on producing platform compounds and liquid alkanes by using cellulose, hemicellulose and sugars worldwide. However, in the network of lignocellulose, lignin generally fills the spaces between cellulose and hemicellulose, so cellulose and hemicellulose microfibrils are encapsulated in a matrix consisted of lignin and will not be eroded in nature. Based on the points above, lignin may also reduce the reactivities of cellulose and hemicellulose, which would have a negative effect to the biorefinery with cellulose and hemicellulose as feedstocks. Effectively depolymerizing lignin is conductive to liberate cellulosic and hemicellulosic fractions from lignocellulose, which is vital to the exploration of renewable energy based on lignocellulose. β-O-4bonds are the major linkages in lignin. Cleaving the (3-0-4bonds is an effective strategy for the degradation of lignin, while preserving the aromatic character of lignin fragments. However, few literatures on cleaving β-0-4bonds have been reported. Recently, ionic liquids have been used as alternative solvents in a variety of research fields. Rogers et al reported that ionic liquids could dissolve cellulose, which solved the problem of dissolving lignocellulose and promoted the development of studying the conversion of biomass with ionic liquids as the media.The objective of this dissertation is to explore an effective approach to depolymerize lignin by cleaving the β-0-4bonds in ionic liquids. Guaiacylglycerol-(3-guaiacyl ether (1-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)-1,3-propanediol, GG) and veratrylglycerol-(3-guaiacyl ether (1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)-1,3-propanediol, VG) were used as lignin model compounds. The main results in this dissertation are summarized as follows:Organic bases, such as1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD),7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD),1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and1,1,3,3-tetramethylguanidine (TMG), could effectively promote the conversion of a phenolic lignin model compound, GG, in1-butyl-2,3-dimethylimidazolium chloride ([BdMIM]Cl). It was found that these organic bases could convert GG into a quinone methide intermediate, which underwent the subsequent reactions. Thus, GG conversion and product yields were highly dependent on the basicity of base. Compared with other organic bases, TBD showed a unique promotion effect to the cleavage of the (3-0-4bond of GG. When no extra nucleophile was added, GG conversion and guaiacol (2-methoxyphenol) yield reached up to94.3%and42.3%, respectively, with a1:1molar ratio of TBD to GG after2h at150℃. The reason that TBD could promote the cleavage of [3-0-4bond of GG was possibly because TBD could function as a nucleophile to attack the quinone methide intermediate by two naked lone electron pairs in its structure.AICl3, CUCl2and FeCl3could effectively catalyze the hydrolytic cleavage of (3-0-4bonds of GG and VG in1-butyl-3-methylimidazolium chloride ([BMIM]Cl), leading to the production of guaiacol. Water was important to the cleavage of β-O-4bonds. Compared with the results for the hydrolysis of GG over HCl, H2SO4and metal sulfates, respectively, it was speculated that AICl3, CuCl2and FeCl3might react with water to produce HCl, and HCl worked as the catalyst for the hydrolytic cleavage of the β-0-4bond. Under optimized conditions, the maximum yields of guaiacol from GG were79.8%,70.4%and68.7%, respectively, in [BMIM]Cl with AlCl3, CuCl2and FeCl3, while the maximum yield of guaiacol from VG was73.7%in [BMIM]Cl with AlCl3.The β-0-4bonds of GG and VG could be effectively hydrolyzed in an acidic ionic liquid,1-H-3-methylimidazolium chloride ([HMIM]Cl), working both as solvent and catalyst. The maximum yield of guaiacol from GG was76.4%with a30:1molar ratio of H2O to GG after1h at150℃, while the maximum yield of guaiacol from VG was78.4%with a30:1molar ratio of H2O to VG after2h at150℃. When an initial concentration of8wt%-32wt%of GG and VG were treated in [HMIM]Cl, more than70%and65%yields of guaiacol could be obtained from GG and VG, respectively. After extracting the products by organic solvent,[HMIM]Cl could be reused without decrease of activity in subsequent experimental runs.It was found that acid-catalyzed hydrolytic cleavage of β-0-4bonds of GG and VG possibly followed two pathways:1. GG and VG underwent acid-catalyzed dehydration first to form EE and VEE, respectively, then the (3-0-4bonds of EE and VEE were hydrolyzed subsequently, leading to the production of guaiacol and Hibbert's ketones;2. GG and VG underwent acid-catalyzed condensation to form GG dimers and VG dimers by their hydroxide groups, respectively, then the (3-0-4bonds of dimers followed subsequent hydrolytic cleavage to produce guaiacol.