Catalytic Depolymerization Of Lignin By Solid Base For Preparation Of Aromatic Compounds

Lignin is the second largest biomass energy in the world other than cellulose.Due to the abundant aromatic units in its polymer structure,it can be depolymerized to produce small molecule compounds like aromatic compounds,which is the important component for utilization of lignin efficiently and comprehensively.However,the utilization of lignin is still low currently,and most of lignins in the form of papermaking waste are discharged into the river or they are burned directly,which also results in environment pollution and resources waste seriously.Therefore,it is one of the important ways to achieve the high value and resource utilization of lignin by converting it into high value-added chemicals via chemical methods.Catalyst is the key factor of depolymerizing lignin among chemical methods.To depolymerize lignin's inside complex chemical bonds so as to achieve its efficiently conversion,the design and development of multifunctional catalysts are important methods.In this paper,composite metal oxide solid bases prepared by calcining hydrotalcite precursor with adjustable metal components were used as catalyst to catalyze depolymerization of lignin.Firstly,MgAl,Ni MgAl,La MgAl,Mg Fe and Ni Mg Fe hydrotalcites were prepared by hydrothermal method.The crystal structure and apparent morphology of the samples were characterized by XRD,SEM and FT-IR.The grain size of MgAl and Mg Fe hydrotalcite were controlled by temperature adjustment,concentration adjustment,introduction of hydroxy compounds and segmented crystallization.Then,the catalytic performances of the composite metal oxide solid bases were evaluated in hydrothermal reactor and the liquid phase products were analyzed by GC-MS.The results of the study are as follows:(1)The optimum parameters for the preparation of MgAl hydrotalcite obtained through crystal structure characterization: solution p H=12,temperature 180°C,crystallization time 24 h.(2)The solution p H is a key factor for the preparation of Mg Fe and Ni Mg Fe hydrotalcites.When the p H was 10,nano-layered Mg Fe and Ni Mg Fe hydrotalcites with hexagonal crystal structure could be prepared.Metal hydroxides were generated under other p H conditions.(3)Compared with other grain size control methods,the hydrotalcite grain size could be controlled efficiently by segmented crystallization method to produce nano-scale hydrotalcite.Under optimal conditions,the grain size of MgAl hydrotalcite and Mg Fe hydrotalcits were controlled as 45-65 nm and 40-55 nm respectively.(4)The optimum calcination temperature of hydrotalcite for preparation of composite metal oxide solid base was 600°C.(5)MgAlOx,NiMgAlOx,LaMgAlOx,MgFeOx and Ni MgFeOx composite metal oxide solid bases were used to catalyze lignin,and the products mainly included phenols,esters and carboxylic acids compounds.Compared with other catalysts,the NiMgAlOx-20% showed most excellent catalytic performance,and the depolymerization conversion of lignin was up to 95%,the liquid yield was 73.34%,the percentage of phenolic compounds in the products amounted to 83.19%.