Isolation Of Cellulose From Sisal Waste And Preparation Of Cellulose Nanofibrils

Sisal waste is an agricultural waste of sisal after fiber processing,which is rich in cellulose.It is of great significance to study the resource utilization of sisal waste for the development of sustainable and modern agriculture.With the knowledge of the chemical composition and microstructure of sisal waste,lignin was identified as the limiting factor for resource utilization.In this thesis,UV-catalyzed hydrogen peroxide pretreatment(UV/H₂O₂)combined with hot alkali extraction were developed to consecutively remove lignin and hemicellulose and recover cellulose from sisal waste.Then,cellulose nanofibrils(CNF)were separated from sisal waste cellulose by TEMPO oxidation-ultrasonic two-step method.The effects of preparation conditions on the structure and size of CNF were studied,and the disintegration mechanism of CNF was speculated.Main contents and results are as follows:(1)Analysis of the chemical composition and structure of sisal waste.Raw sisal waste was pre-processed by crushing,enzyme inactivation and dewaxing.The treated sisal waste was mainly composed of lignocellulosic components.The content of cellulose was 56%and the average length and width were 1 mm and 5?m,respectively.The content of hemicellulose and lignin was 7%and 13%,respectively.Hemicellulose is a heteropolysaccharide with xylan as the main chain and arabinose and glucuronic acid as the side chain.Lignin is a guaiacyl-syringyl-p-hydroxyphenyl(GSH)compound,which contains a high proportion of ferulic acid and p-coumaric acid.Based on the above analysis,a structural model of lignocellulose of sisal waste was proposed.(2)Isolation and characterization of cellulose isolated from sisal waste.Cellulose was isolated from sisal waste by UV/H₂O₂ pretreatment and hot alkali extraction.The removal rates of lignin and hemicellulose reached 77.9%and 33.4%respectively under the UV/H₂O₂ pretreatment with 0.1 m L/g H₂O₂ for 6 h.The pretreated sisal waste was extracted with 2 mol/L Na OH at 80?for 90 min,resulting in cellulose with yield of63.3%and purity of 88.5%,respectively.SEM,FT-IR,XRD and(CP/MAS)¹³C NMR analysis showed that cellulose extracted from sisal waste was a microfiber bundle with a diameter of 7?m and a length of 20?m.The cellulose belonged to a typical cellulose?structure,and the crystallinity index(Cr I)was 51.4%.The possible mechanism of cellulose isolation from sisal waste is as follows:On the one hand,saponification occurs between lignin and hemicellulose under alkaline conditions;on the other hand,H₂O₂ is decomposed by UV catalysis to produce a large amount of OH·,which attacks the ester bond between lignin and hemicellulose.Both reactions promote the degradation and dissolution of lignin and hemicellulose.And the residual hemicellulose is further separated from the non-fibrous layer by hot alkali,leading to an efficient extraction of cellulose from sisal waste.(3)Method and mechanism of cellulose nanofibrils(CNF)preparation.CNF was disassembled from sisal waste cellulose by a combined chemical-mechanical disassembly method of TEMPO oxidation and powerful ultrasound.The optimal TEMPO oxidation condition was Na Cl O₂10 mmol for 6 h under neutral p H.The oxidized cellulose was further disassembled and dispersed in water by high intensity ultrasound at 1200 W for 30 min.The yield of cellulose nanofibers was 78%and the content of carboxyl groups was 1.25 mmol.SEM and AFM showed that the obtained CNF had a network structure with a diameter of about 50 nm and a length of more than2?m.According to the changes of the microstructure of CNF under different preparation conditions,the mechanism of TEMPO oxidation-ultrasonic method for preparing CNF is that TEMPO system attacks the amorphous and surface of crystalline regions of cellulose.The cavitation effect of ultrasonic has longitudinal cutting effect on the crystal surface of oxidized cellulose.