The Study On The Preparation Of Lignocellulose Gel And Its Application In Adsorption/catalytic Materials

Lignocellulose material is the one of the most abundant renewable resources on the earth.Developing sustainable technology for preparing high-value lignocellulosic product is becoming a key issue for realizing its efficient utilization.The lignocellulose(LC)with different lignin content is dissolved in N-Methylmorpholine-N-oxide monohydrate(NMMO/H₂O)solvent system via glycerol-swelling and mechanical extrusion pretreatment.The lignocellulose hydrogel(LCG)is fabricated by sol-gel method,and the 3-Aminopropyltriethoxysilane(APTES),is used to improve the mechanical property of lignocellulose gel via crosslinking,for realizing its reuse in subsequent applications.Due to the versatile group of lignin and the porosity in lignocellulose gel,the adsorption removal of lignocellulose gel for heavy metal ions(Cu²⁺,Pb²⁺)and organic dye(Methylene blue,MB,Rhodamine B,RB)can be realized.The lignin in lignocellulose gel can be used as reducing agent for the in-situ synthesis of pallidium(or silver)nanoparticle,and the catalytic system of metal catalysts based on lignocellulosic gel was established.The synergism mechanism of in-situ adsorption-catalysis is introduced into achieve the high-efficiency catalytic reduction of nano catalysts.Lignocelluloses(LC)with different lignin content is successfully dissolved in N-Methylmorpholine-N-oxide monohydrate(NMMO/H₂O)solvent system via glycerol-swelling and menchanical extrusion pretreatment.The results show that the glycerol not only swelled lignocellulose but also cleaved the ether bonds for removing lignin slightly.The mechanical extrusion can dissociate the fiber bundles of LC to thinner fibers and,enhance the accessibility and solubility of the LC in NMMO/H₂O solvent system.The crystal structure of the pretreated LC had no significant change during pretreatment,while the diameters of the fiber bundles were reduced from 50-60 to 10-12?m,as investigated by X-ray diffraction and scanning electron microscopy.The pretreated LC is dissolved from exposed cellulose to LC fiber internal,and the lignin and hemicellulose macromolecules are dissolved in the NMMO/H₂O solvent system after cellulose chains are migrating from the fiber into the solvent.The required dissolving time of lignocellulose increased with lignin content.The lignocellulose with 18.4%lignin can be dissolved in NMMO/H₂O within 5h.The lignocellulose hydrogel(LCG)containing lignin,cellulose,hemicellulose is prepared from lignocellulose solution,and the effect of origin lignin on the physical performance of lignocellulose hydrogel is studied.The mechanical properties of lignocellulose hydrogel exhibits a trend of increasing first and the decreasing with the increasing of lignin content.The lignocellulose hydrogel with 6.5%lignin has the best storage modulus(580 k Pa)and compressive modulus(55 k Pa),indicating that the lignin with a certain content plays a positive role in the mechanical property of hydrogel.When lignin content of LCG is less than 6.5%,the lignin disperses well in LCG.The lignin in LCG with 18.4%lignin is easily to aggregate and destroy the LCG network.Moreover,the lignin/cellulose composite hydrogel is prepared based on nanocellulose as framework,dissolved cellulose as filler,and alkaline lignin as separated lignin.The effect of separated lignin addition on the physical properties of lignin/cellulose composite hydrogel is studied.The composite hydrogel with 10 wt%lignin content exhibits the highest mechanical strength,the storage modulus and loss modulus were 131 k Pa and 13 k Pa.The structure of lignocellulose hydrogel(lignin content,10 wt%)has an abundant porous structure(porosity of96.5%)and high surface area(128.2 m²/g).The LCG is strengthened by cross-linking with a silane coupling agent 3-amino-propyltriethoxysilane(APTES).The formation of Si-O-C cross-links among the lignocellulosic fibrils as a result of the addition of APTES is mainly responsible for the strength improvement.The dynamic storage modulus of APTES-reinforced lignocellulose gel(LCG_A)can reach up to1391 k Pa and the compressive modulus up to 96 k Pa,which shows a 3-fold increase in viscoelastic properties and a 2-fold increase in compressive strength compared with unmodified gel.The adsorption ability of LCG containing original lignin and composite gel containing separated lignin is studied.The effects of original lignin and separated lignin on adsorption efficiency of lignocellulose hydrogel are compared.The LCG as an adsorbent has a certain adsorption capacity for metal ions.The higher the lignin content of LCG,the higher the adsorption amount of metal ions to the gel.The LCG2 with a lignin content of 11.6%has the largest adsorption capacity for metal ions,which is 69.3 mg g^-1(Cu²⁺)and 86.1mg g^-1(Pb²⁺),respectively.Moreover,the LCG2 also has the largest adsorption capacity for organic pollutants,which is 192 mg/g(MB)and 201 mg/g(RB).The cellulose/lignin composite hydrogel exhibits excellent adsorption capacity for heavy metals,which can be attributed to the synergistic effects of physical adsorption(porous3D structure)and chemical adsorption(active sites:carboxyl and phenolic groups).The maximum amount of adsorbed Cu²⁺onto composite hydrogel can reach to 541 mg/g.LCG is served as the host matrix for in-situ generated palladium nanoparticles(Pd NPs)and silver nanoparticles(Ag NPs),creating hydrogel-based metal NPs reactor systems(LCG/Pd NPs and LCG/Ag NPs).The resuts show that(1)LCG/Pd NPs exhibits excellent adsorption ability and catalytic performance due to its porous structure of hydrogel and abundant active groups of lignin.The MB is reduced by LCG/Pd NPs catalytic system within 1min,and Cr⁶⁺(2m M,30m L)was reduced to Cr³⁺with 7 min.The simultaneous adsorption-catalysis process concept is very effective for pollutant remedy:the absorption of pollutants on the porous LCG effectively increases the pollutant concentration on the reaction sites,where the Pd NPs are co-located,enhancing the catalytic reactions.(2)LCG serves as host matrix for immobilization of Ag NPs,and the particle size of Ag NPs can be controlled by lignin content of LCG,which becomes smaller with the increasing of lignin content.The average particle size of Ag NPs reduced by LCG1(lignin content of 6.5 wt%),LCG2(lignin content of 11 wt%),and LCG3(lignin content of 18 wt%)are 42nm,36nm,and 22nm.Furthermore,4-nitrophenolate is gradually converted to aminophenol,and is completely reduced by Ag NPs/LCG3 within 21 min.