Preparation Of Lignin-based Activated Carbon By Phosphoric Acid Activation

Lignin is an important component of vascular plant cell wall,which is abundant and renewable.At present,the high-value utilization of lignin has not been fully developed,which is mainly limited by its complex structure and properties.Promoting the high-value utilization of lignin is of great significance to improve the economy of biorefinery industry and achieve the goal of"carbon peaking and carbon neutrality".Lignin is an ideal precursor for the preparation of activated carbon due to its rich aromatic structure and surface functional groups.In this paper,activated carbon was prepared by phosphoric acid activation with lignin-rich enzymatic hydrolysis residue as the by-product of furfural production from corn cob.The methods to improve the expansion of lignin in the activation process,the role of phosphoric acid in the activation process and the feasibility of using Mannich reaction to prepare lignin-based modified activated carbon were studied.The specific work and conclusions are as follows:（1）It was found that furfural could inhibit the expansion of lignin when heated.Based on this,a preparation method of lignin-based granular activated carbon was proposed.The core of this method was the chemical reaction between furfural and lignin.In the specific experiment,the iodine value was used as the index and the single factor experiment method was used to optimize the five influencing factors of the preparation process of lignin-based granular activated carbon from furfural,and the influence of each factor on the pore structure and adsorption capacity of the product was discussed.Finally,a series of lignin-based activated carbon was successfully prepared,and the specific surface area of the activated carbon product was up to 1478㎡/g.The iodine value was up to 1010mg/g.Methylene blue adsorption capacity up to 329mg/g;The maximum lateral compression strength can reach 220N/cm.By analyzing the chemical structure of the products obtained by the model reaction,it is believed that the reaction mechanism is the protanation of furfural carbonyl group in the action of phosphoric acid and the substitution of hydrogen on guaiacol methoxyl or phenol hydroxyl ortho carbon to form 6-[（furan-2-yl）（hydroxyl）methyl]-2-methoxyl phenol or 3-[（furan-2-yl）（hydroxyl）methyl]-2-methoxyl phenol.After that,the products are further dehydrated to form a widely cross-linked polymer.Through this reaction,the thermal stability of lignin is improved and some resin-like structures are formed within the molecule,enabling the molding process to be realized.（2）The role of phosphoric acid in the activation process and its influence on the structure of lignin were analyzed through the change of gas production rule during pyrolysis.It was found that phosphoric acid reduced the temperature of CO and CO₂ production,reduced the production of CO and CH₄,and increased the production of H₂ and CO₂.It is speculated that phosphoric acid may promote the cracking of phenolic hydroxyl groups and the polycondensation between aromatic rings by promoting the heterolytic cleavage of chemical bonds and protanation of surface groups,and promote the decarboxylation reaction by catalyzing the heterolytic cleavage of chemical bonds and change the cleavage pathway of methoxyl groups.（3）The feasibility of using Mannich reaction to modify lignin and then prepare modified activated carbon was investigated by using two implementation methods.The results show that the modification reaction can be carried out,And nitrogen doping is beneficial to reduce the surface acidity of phosphoric acid activated carbon products.