| The preparation of high-performance,high value-added chemicals and bio-based materials based on the main chemical components of biomass meets the development requirements of biomass refining.At the same time,it is of great significance to alleviate the resource and environmental problems caused by the excessive utilization of fossil resources.As a typical representative of bio-based nanomaterials,nanocellulose has increasingly become a researchhotspot due to its advantages of biomass materials and superior performance of nanomaterials.However,the traditional preparation methods of nanocellulose have some problems,such as long-time consumption,high energy consumption,environmental pollution of waste liquid and low yield,which limited the development of nanocellulose.Taking a variety of agricultural and forestry biomass as the research object,this paper puts forward a new research idea of extracting nanocellulose under acidic ionic liquid catalyzed alcohol solvent system combined with ultrasonic treatment.The effects of different solvent systems on the structure and chemical components of lignocellulose were investigated,and the reaction mechanism of pretreatment of lignocellulose under acidic ionic liquid catalyzed alcohol solvent system was analyzed.On this basis,the effects of pretreatment temperature,raw material types and pretreatment methods on the yield and crystal structure of nanocellulose were explored.Besides,the method of preparing nanocellulose based on lignin in-situ regeneration was further developed and the role of nanocellulose as enhancer in polyvinyl alcohol biological composite film was evaluated.This paper enriches the transformation ways and methods of waste agricultural and forestry biomass resources to high value-added nanocellulose.The main contents and conclusions of this paper are as follows:(1)A comparative analysis of the pretreatment efficiency of acidic ionic liquid catalyzed alcohol solvent system on poplar was investigated.The effects of carbon chain length,the amount of hydroxyl groups,and hydroxyl group position in alcohols on the yield of the celluloserich materials,the yield of lignin,lignin extraction rate,the yield of nanocellulose,relatively crystallinity and thermal stability of nanocellulose were explored.The results show that the acidic ionic liquid catalyzed alcohol solvent system can effectively break the "biomass recalcitrance" of the wood raw material,which can effectively take approximately 55.4%to 87.1%of lignin,and recover from 3.85%to 10.79%lignin.The longer the carbon chain length of alcohol,the higher the lignin extraction rate and lignin yield.Diol solvent is more suitable for the pretreatment of poplar in the process of extracting nanocellulose due to its higher lignin extraction rate and cellulose-rich materials yield.The lignin extraction rate of 1,4-butylene glycol solvent system was the highest,and the stability of nanocellulose suspension was the best.(2)Poplar was pretreated with acidic ionic liquid-1,4-butylene glycol solvent system to explore the effects of pretreatment temperature on the yield,morphology,particle size,crystal structure and thermal stability of nanocellulose.The results show that the pretreatment temperature had no significant effect on the chemical structure and crystal morphology of cellulose,but the pretreatment temperature affected the lignin extraction rate and cellulose-rich material yield.When the pretreatment temperature was 200?,the lignin extraction rate was the highest(87.1%),the relative crystallinity of the corresponding nanocellulose was the highest(63.05%),and the nanocellulose film had the maximum tensile strength(93.18 MPa)and Young's modulus(5244.9 MPa).When the pretreatment temperature increased from 180? to 220?,the yield of fiber decreased from 46.17%to 31.05%.When the pretreatment temperature is 220?,the excessive pretreatment temperature has a negative impact on the crystalline region of cellulose,resulting in a certain decrease in the relative crystallinity,mechanical properties and thermal stability of nanocellulose.(3)Corn stalk,corn flesh and corncob were pretreated with acidic ionic liquid-1,4-butylene glycol solvent system and traditional bleaching-alkali treatment process respectively and then nanocellulose with different properties was prepared by low intensity ultrasound(400W)to investigated the effects of raw material types and pretreatment methods on the yield,crystal structure and thermal stability of nano cellulose.The results show that the microstructure and chemical composition of raw materials directly affect the cellulose-rich materials yield,lignin content,nanocellulose yield,crystallinity,morphological characteristics and thermal stability of nanocellulose.Among the nanocellulose prepared by acidic ionic liquid-1,4-butylene glycol solvent system combined with low intensity ultrasound(400W),the lignin content and yield of corn stalk nanocellulose are the highest,but the dispersion and optical transparency are the worst.Corncob nanocellulose and corn flesh nanocellulose have better UV blocking ability and optical transparency.Corn flesh nanocellulose has the highest tensile strength and Young's modulus.Compared with the nanocellulose prepared by traditional bleaching-alkali treatment process combined with ultrasound,the lignin containing nanocellulose prepared by acidic ionic liquid catalyzed alcohol solvent system combined with ultrasound has higher yield,better thermal stability and stronger UV shielding ability.(4)Coir with high lignin content was pretreated with acidic ionic liquid-1.2-propylene glycol solvent system.Coir nanocellulose was prepared by lignin in situ regeneration combined with ultrasonic treatment to explore the effect of lignin content on the thermal stability and hydrophobic properties of nanocellulose.The results show that the method of lignin in situ regeneration combined with ultrasonic treatment can significantly improve the yield of nanocellulose(50.28%),which is much higher than that of traditional chlorine containing pulping method(6%).Compared with the traditional pulping method and acidic ionic liquid-1,2propylene glycol solvent system pulping method,the coir nanocellulose based on lignin in situ regeneration has better thermal stability.Besides,the hydrophobic properties of nanocellulose were significantly improved,but the high lignin content reduced the surface structure stability of nanocellulose.(5)In order to evaluate the increasing ability of nanocellulose prepared by acidic ionic liquid catalyzed alcohol solvent system combined with ultrasonication to polymers,corn flesh nanocellulose prepared by 1,4-butylene glycol solvent system in Chapter 3 was selected as a representative,and corn flesh nanocellulose/polyvinyl alcohol biological composite films were prepared by casting method.The effects of nanocellulose addition on the optical,mechanical and hydrophobic properties of the composite films were studied.The results showed that with the increase of nano cellulose content,the tensile strength and Young's modulus of the composite film increased significantly,and the elongation at break decreased significantly.The residual lignin in nanocellulose provides the UV shielding ability of the composite film.When the addition amount of nanocellulose is 16%and 32%,the transmittance of the composite film in the UV range decreases significantly.The residual lignin in nano-cellulose can improve the hydrophobicity of the composite film.When the amount of nanocellulose is 32%,the hydrophobic instantaneous contact angle of the composite film is increased from 67.6° to 74.6°compared with pure polyvinyl alcohol film. |
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