Study Of Construction And Applications Of Lignin-Based Solid Acid And Porous Carbon Materials

Lignin is the second largest renewable resource after cellulose,which accounts for 15～35%in plant fiber.It is a by-product of pulp and paper,biomass refining and other related industries that use plant cellulose as raw materials.Its global annual output is about 70 million tons.Due to the wide sources of biomass raw materials and different production processes,the lignin structure is not completely determined and the molecular weight is polydisperse,the efficient use of lignin is greatly restricted,only less than 5%,and fine high-end use is insufficient.The industrial lignin is well utilized,and most of the lignin is burned to generate heat,and the value of lignin has not been fully exploited.Therefore,the development of low-cost and high-efficiency technology to convert industrial lignin into high-value-added functional materials to replace low-value combustion and use waste to treat waste can not only reduce environmental pollution caused by combustion,but also can realize the high-value utilization of these industrial wastes,and meet the needs of low-carbon society development,and have important economic benefits and environmental protection values.In view of this,this thesis uses cheap and renewable lignin as raw materials and uses different process conditions to prepare lignin-based polyurethane foam,lignin-based solid acid catalysts and lignin-based porous carbon,and apply them to cellulose.Hydrolysis and dye wastewater treatment provide research ideas for the in-depth development and high-value utilization of lignin.Mainly include the following aspects of work:（1）The lignin extracted from the enzymatic hydrolysis residue of corn stalks by alkali-dissolving and acid precipitation method is used as the raw material,and the reusable lignin-based solid acid is prepared by the partial carbonization-sulfonation method,which is used to catalyze the hydrolysis of cellulose to prepare short rod-like cellulose nanofibers and platform compounds（including glucose,fructose,formic acid,levulinic acid,and 5-hydroxymethyl furfural）.The lignin-based solid acid catalyst has a sulfonic acid group content of 0.60 mmol/g and a total acid density of 4.62 mmol/g.Under the conditions of a hydrolysis time of 12 h,a temperature of 160°C,a cellulose content of 1.2 wt%,and a mass ratio of catalyst to cellulose of 1.5,the yield of cellulose reached 83.9%and 8.8%glucose and 5.4%formic acid was obtained at the same time.The lignin-based solid acid catalyst has strong reusability and still exhibits excellent cellulose catalytic degradation activity after being recycled for 5 times.After the dissolving pulp fibers are pretreated by lignin-based solid acid catalyzed hydrolysis,short rod-like cellulose nanofibers can be prepared through homogenization.The resulting short rod-like cellulose nanofibers have a larger crystallinity（82%）and a lower aspect ratio（10～28）.The film of short rod-like cellulose nanofibers prepared by lignin-based solid acid-catalyzed hydrolysis pretreatment and homogenization has good thermal stability.（2）The lignin extracted from the enzymatic hydrolysis residues of corn stalks by alkali solution and acid precipitation method is used as carbon source,and Fe₃O₄ nanoparticles are used as hard template.After high temperature calcination,H₂SO₄ etching and sulfonation,the lignin-based sulfonated porous carbon contains high density acid groups and porous channel structure.After H₂SO₄ etching and sulfonation,the Fe₃O₄ template is almost completely removed.The surface and interior of the lignin-based sulfonated porous carbon have a porous structure with a high content of sulfur（3.26 wt%）,and the sulfonic acid group density of 2.8mmol/g,carboxyl groups density of 7.2 mmol/g,and phenolic hydroxyl groups density of 3.6mmol/g can be obtained.The porous structure of lignin-based sulfonated porous carbon after H₂SO₄ etching can improve the density of acidic groups,which is conducive to the selective adsorption of organic dyes.Under a single dye system,lignin-based sulfonated porous carbon exhibits good dye adsorption selectivity,and has higher adsorption efficiency for cationic dyes such as methylene blue,malachite green and azure B,while for anionic dyes such as methyl blue,Ponceau 2R and Orange II have lower adsorption efficiency.Under the anion and cation binary composite dye system,the lignin-based sulfonated porous carbon can also selectively adsorb cationic dyes.The adsorption capacity of lignin-based sulfonated porous carbon for methylene blue can reach 420.40 mg/g.The pseudo-second-order kinetic model and Langmuir isotherm model are more suitable to describe the adsorption behavior of methylene blue on the surface of lignin-based sulfonated porous carbon.The lignin-based sulfonated porous carbon has good reusability.After 6 consecutive adsorption-regeneration cycles,the removal efficiency of methylene blue is still as high as 92.3%.（3）Using sodium lignosulfonate as a carbon source and a small amount of KOH as an activator,and the mass ratio of KOH to sodium lignosulfonate is 3:10,a lignin-based porous carbon material with high specific surface area and porous channel structure is prepared by high-temperature carbonization activation.Using only a small amount of KOH as the activator,the specific surface area of the lignin-based porous carbon can be as high as 1615.87 m²/g.The lignin-based porous carbon has a good adsorption effect on the three anionic dyes:methylene blue,acid fuchsin and orange II.When the initial concentration of OrangeⅡis 200 mg/L,the adsorption capacity can reach 199.02 mg/g within 10 minutes.The lignin-based porous carbon can quickly remove 99.51%of OrangeⅡ.When the initial concentration of OrangeⅡis 800mg/L,the adsorption capacity is as high as 785.11 mg/g in 120 minutes.The lignin-based porous carbon can still quickly remove 98.13%of OrangeⅡunder the condition of high dye concentration.The pseudo-second-order kinetic model is more suitable to explain the adsorption behavior of lignin-based porous carbon to Orange II.The lignin-based porous carbon can be eluted and regenerated by ethanol,and has better reusability.After 8 adsorption-desorption cycles,the removal efficiency of Orange II can still reach 83.28%.（4）Using sodium lignosulfonate as a carbon source and a small amount of KOH as an activator,and the mass ratio of KOH to sodium lignosulfonate is 1:10～3:10,a serises of lignin-based hierarchical porous carbon materials with adjustable specific surface area are prepared by high temperature calcination.With the increase of the amount of KOH,the porosity and pore size of the lignin-based hierarchical porous carbon increased.The specific surface area of lignin-based hierarchical porous carbon is 1048.78～1823.88 m²/g,and the pore volume is0.465～0.957 cm³/g.With the increase of KOH addition,the adsorption capacity of lignin-based hierarchical porous carbon materials for Azure B and Orange II gradually increases.After 18adsorption-filtration cycles,the cumulative adsorption capacity of Azure B can reach 860.25mg/g,and the removal efficiency of Azure B still reaches 85.2%.Under the low-concentration anion-cation composite dye system,the lignin-based hierarchical porous carbon still has fast and efficient continuous dye decolorization ability.In the high-concentration single dye system,after 80 minutes of adsorption,the adsorption capacity of Azure B on the lignin-based hierarchical porous carbon increased to 1980.63 mg/g,whose concentration was significantly reduced by 2 orders of magnitude and the removal rate was higher than 99.03%.Using ethanol to elute the the lignin-based hierarchical porous carbon after adsorbing the dyes can realize the closed loop of the adsorbent,water,dyes and ethanol which can be recycled,and this method has great potential in the fast and efficient recovery of organic dyes and water purification.Using renewable lignin with rich carbon content as carbon source,lignin-based solid acid catalyst was prepared by simple and efficient carbonization and sulfonation process.The lignin-based solid acid catalyst has high catalytic hydrolysis efficiency for cellulose,and can be reused after recycling without waste liquid generation.This provides a new idea for cellulose hydrolysis and efficient utilization of lignin.Lignin-based porous carbon was prepared by hard template method and low amount potassium hydroxide activation method,which can adsorb dyes quickly and efficiently,and provide a new way for dye wastewater treatment and high value utilization of lignin.