Preparation Of Lignin Aerogels And Investigation Of Their Properties

Lignin is a natural product with abundant resources,low price,and degradable.Due to the large molecular weight of lignin,complex structure,and few surface active groups,the development and utilization of lignin in various fields are restricted.How to make full use of the functional groups of lignin molecules and truly realize their high-value utilization is still a huge challenge at this stage.In this paper,esterification modification is used to modify the chemical groups on the surface of lignin and optimize the molecular structure.A lignin aerogel adsorption material is prepared using it as a raw material,which is used to adsorb heavy metal ions in wastewater and improve the efficient utilization of lignin.In this paper,an efficient aerogel adsorption material was prepared by modifying acetic acid lignin(AL).Take self-made AL as raw material,use maleic anhydride(MA)to graft double bonds and hydrophilic carboxyl groups through esterification reaction,and use the double bonds on modified lignin(ML)to prepare lignin by free radical polymerization.Aerogel(MLA),and the adsorption performance of aerogel to Cu2+was studied.the result shows:(1)The lignin yield was used as the evaluation index,and the conditions for the preparation of AL by acetic acid method were optimized to determine the optimal process conditions:the cooking time was 3 h,and the amount of sulfuric acid was 3%.Using infrared spectroscopy(FT-IR),nuclear magnetic resonance phosphorus spectroscopy(31P-NMR)and gel chromatography(GPC),it was shown that the hydroxyl content of lignin increased from 4.19 mmol/g to 4.3 mmol/g after AL deacetylation,and the hydroxyl content and molecular weight of AL were comparable to those of alkaline lignin(SL).(2)The influence of factors such as ACL/ML ratio,reaction time and temperature on maleic anhydride(MA)modified lignin was explored and analyzed.The optimal process conditions were determined through orthogonal experiments:ACL/MA ratio was 1:5,the reaction time is 13 h,the reaction temperature is 110℃,and the double bond content reaches 2.783 mmol/g.Combining FT-IR,1H nuclearmagnetic resonance spectrum(1H-NMR),31P spectrum shows that MA has been grafted to lignin,GPC and two-dimensional nuclear magnetic(2D-NMR)found that the addition of DMAP makes the β-O-4 of lignin obviously broken After the modification,the molecular weight of the modified lignin is reduced to 1517,the dispersion coefficient is 1.036,and the molecular weight tends to be uniform.Therefore,the modified lignin is beneficial to the preparation of lignin aerogels(MLA).(3)The influence of lignin content,reaction temperature and aging time on the preparation of MLA was studied,and the optimal process conditions were determined as follows:lignin content is 5%,reaction temperature is 60℃,and aging time is 18 h.Through the analysis of scanning electron microscope(SEM)and specific surface area(BET)results,it can be seen that the internal surface of acrylic aerogel(PAA)is smooth,and the pore size is dominated by macropores.The specific surface area of PAA is 1.050 m2/g,because ML is used as cross-linking Compared with PAA,the internal structure of MLA has a smaller pore size and a "filament" material connection inside.The specific surface area of MLA is increased to 2.921 m2/g,and the number of micropores and mesopores in MLA is significantly increased compared with PAA.At the same time,there is a certain macroporous structure,and the increase of micropores and mesopores provides more adsorption sites for the adsorption of heavy metal ions.(4)The Cu2+adsorption performance of MLA was measured by atomic absorption spectrometer,and the effects of different time,pH,initial concentration,adsorbent quality,and temperature on the adsorption performance were studied.The results showed that:MLA reached adsorption equilibrium at 15 h.After PAA was adsorbed for 22 h,the structure disintegrated due to the lack of lignin,and the adsorption capacity decreased significantly.According to adsorption kinetics,thermodynamics and adsorption isotherms,it shows that MLA adsorbs Cu2+ in a monolayer and the surface of the adsorbate is uniform;the process of MLA adsorbing Cu2+ is an exothermic process;the chemical adsorption is dominated,and the adsorption behavior is spontaneous.When the initial concentration of Cu2+ is 50 mg/L and the addition amount of ML A is only 0.3 g/L,the removal rate of Cu2+reaches 91.16%.Therefore,MLA is a good Cu2+adsorption material.