Fabrication And Characterization Of Cellulose Nanocrystal As Well As Cellulose Nanofibril And Their Aerogels

Cellulose is the most abundant natural biopolymer in the world.Nano-cellulose not only has the inherent characteristics of natural cellulose,but also exhibits several unique properties such as high crystallinity index and large surface area.Apart from its renewability and biocompatibility,it offers the opportunity of functionization.Cellulose aerogels,as the third generation of new materials,it can be used not only as a carrier of active matter but also as a template material.Due to their abundant network structure,cellulose aerogels may represent a promising adsorption material.Therefore,it is of great significance to optimize the preparation process of nano-cellulose and its aerogel,which could further improve the use of lignocellulose and improve its performance.In this paper,cellulose nanocrystals?CNCs?and cellulose nanofibers?CNFs?were prepared from microcrystalline cellulose?MCC?and pulp by chemical hydrolysis and mechanical process.Modern analysis techniques were used to characterize their morphology containing physical and chemical properties.The physical forming mechanism of inorganic salt solution and mechanism of amine modification of nano-cellulose aerogel were described deeply.Lignin-containing cellulose nanofibers were isolated by means of grinding from four typical biomass materials:switchgrass,yellow poplar,hybrid poplar and pine.The chemical structures,water retention capability,morphology and thermal degradation properties of the four isolated nanofibers were characterized as well.The energy consumption needed to produce LCNFs were quantified.What's more,the mechanism of lignin filling nano-cellulose aerogel to improve hydrophobicity and mechanical compressive strength was revealed on the basis of experimental analysis.The main conclusions of this paper are as follows:?1?Ammonium persulfate oxidation degradation and sulfuric acid hydrolysis methods are simple and easy to operate.The reaction was severe,and the ratio of solid liquid,reaction time and reaction temperature had a great influence on the yield,morphology of the prepared nanofibers.Enzyme treatment combined with grinding can make the hydrolysis of cellulose under mild conditions.?2?The presence of residual lignin in nanofibers can improve efficiency and reduce energy consumption in processing nanofibers.Mean diameters of nanofibers extracted from switchgrass,yellow poplar,hybrid poplar and pine were 27.9,25.4,24.6 and 21.5 nm respectively.Fibrils were observed after organosolv fractionation and large fibers were reduced after grinding.Residual lignin was found to serve as a gluing agent.Among the four types of LCNFs,pine nanofibers showed the highest WRV.The lignin content reduced the energy required to prepare the LCNFs.Moreover,the more lignin there was,the greater the sample's thermal stability.In conclusion,residual lignin reduces energy consumption and promotes production yield in the production of natural cellulose-based nanofibers.?3?Calcium chloride solution was used to accelerate physical aggregation in the preparation of nano-cellulose aerogels.The addition of calcium chloride results in alteration of charge distribution and makes nano-cellulose particles more likely to be close to each other,which may account for the acceleration of nano-cellulose hydrogel formation.The higher the concentration of CaCl₂ solution and the nanocellulose suspension,the easier to form the nanocellulose wet gel.?4?Nano-cellulose aerogels were prepared via a spontaneous gelation fashion using calcium chloride solution,followed by tert-butyl alcohol solvent displacement and freeze-drying.Addition of calcium chloride?0.25%?accelerated the physical gelation process.The application of tert-butyl alcohol as a solvent contributed to preservation of gel network.The obtained nano-cellulose aerogels had a shrinkage rate of 5.89%.The specific surface area and average pore size was 164.9666 m² g^-1 and 10.01 nm,respectively.With the increase of cellulose contents,nano-cellulose aerogels showed a higher specific surface area and adsorption capacity.?5?Nanocrystalline cellulose aerogels were prepared through supercritical CO₂ drying.The induction of calcium chloride solution maintained the shape of gel and facilitated physical gelation process.The supercritical CO₂ drying aided in preservation of the original gel network and structure.The resultant spherical nanocrystalline cellulose arogels were characterized with respect to shrinkage,specific surface area,thermal degradation and micro-structure.They exhibited a nano-porous network structure composed of mesopores,with a high specific surface area of up to 353m²/g and an average pore size of 8.86nm.They showed a weak shrinkage of4.03%.?6?Amine-modified nanocrystalline cellulose aerogel was successfully fabricated by freeze-drying and supercritical CO₂ drying.The amine group had been successfully introduced based on the formation of C-O-Si bonds between CNC and N-?2-aminoethyl??3-aminopropyl?methyldimethoxysilane?AEAPMDS?.The modification process has been well detected resulting an optimum reaction condition under 100?of 16h at the solid-to-liquid ratio of 1:10.The resultant nanocrystalline cellulose aerogel exhibited a nano-porous network structure composed of mesopores via supercritical CO₂ drying and displayed a honeycomb structure comprised of squares,polygons and circles through freeze drying,possessing a high specific surface area of up to 262m²/g and 120.4 m²/g in turn.This functioned AEAPMDS-CNC aerogel gives a good possibility to capture CO₂ via covalent bonding,its CO₂ adsorption capacity was 1.17 mmol·g^-1,which is 515%more than that of unmodified spherical CNC aerogels.?7?CNF/Nano-Lignin composite aerogel is a super-light mesoporous material displaying a three-dimensional network structure,good thermal stability and low density.The optimum amount of added lignin is 13.5 wt%of the dry weight of the cellulose and the best heating temperature is 160?.At this time,the mechanical compression strength of the resulting aerogel is 0.913 MPa,which is 340%more than that of pure CNF aerogel.?8?The CNF/Nano-Lignin composite aerogel is a three-dimensional porous structure made of nano-filament.The presence of dark globular lignin particles around nano-cellulose does not affect the formation of internal structure of CNF aerogel.After the vacuum oven heating treatment softening and hardening,lignin covers the joint of the fiber and fiber surface.Cellulose branches are coated with lignin nanoparticles,and the"branches"are relatively robust,providing a strong framework for lignin nanoparticles.The pores of the fibers are inter linked by lignin nanoparticles,and lignin acts as a binder.