Cleaner Preparation Of Functional Nanocellulose By Organic Acids

Preparation and applications of high value-added cellulose nanocrystals(Cellulose Nanocrystals,CNC)and cellulose nanofibrils(Cellulose Nanofibrils,CNFs)from renewable cellulosic raw materials has been one of the hottest topics in the research of nanotechnology and nanomaterials.Nanocellulose with its excellent properties,such as nanoscale size,excellent elastic modulus and Young’s modulus,low thermal expansion coefficient,good biocompatibility and its naturally occurring renewability and degradability,makes it a promising natural polymer material,which has been successfully applied in biomedical materials,food packaging materials,photoelectric materials and other fields.The preparation of nanocellulose is the first and the most important step before applying nanocelluloses for different purposes,such as preparing new materials.Prepared nanocellulose from various fiber raw materials in an efficient and cleaner way while maintaining its original natural performance or adding simultaneously new functionality is very important for the development of nanocellulose.Currently,there are several pathways to prepare nanocellulose,such as inorganic acid hydrolysis,enzyme hydrolysis,oxidation method and mechanical method,etc.However,there are many disadvantages include high cost,large-scale product specifications,severe cellulose degradation,equipment corrosion and environment pollution.Therefore,there is strong need to develop the preparation of nanocellulose efficiently and environmentally friendly.The object of this work was to develop a cleaner method to prepare nanocellulose through organic acid(citric acid,oxalic acid,formic acid and tartaric acid)hydrolysis.Compared with strong mineral acids used in conventional method,the organic acids in this work have low toxicity,moderate acidity and little environment pollution.Besides,the organic acids can be recovered through rotary evaporation,crystallization or rectification methods,and the recovery rate is more than 90%.In addition,citric acid,oxalic acid,and tartaric acid are poly-carboxylic acids.During the hydrolysis reaction,carboxyl groups can be introduced on the surface of nanocellulose through esterification reaction.The introduction of carboxylic groups increased the amounts of charged groups on nanocellulose,which correspondingly improved the dispersibility of the obtained nanocellulose and facilitating further functionalization.The aid of ultrasound can greatly improve the yield of nanocellulose,which is beneficial for the development of its future industrial production.In this article,firstly,citric acid hydrolysis combined with ultrasound-assisted was used to prepare nanocellulose.The yield,size,carboxyl content,dispersibility and thermal stability of the obtained nanocellulose were analyzed.It was found that with the increase of acid concentration and hydrolysis time,the yield of CNC gradually increased up to 32.2% at 80% acid concentration,100?C,4 h hydrolysis time,and the corresponding yield of CNF gradually decreased.The content of carboxyl group also increased along reaction conditions.The highest carboxyl group content of CNC and CNF were 0.65 mmol/g and 0.30 mmol/g,respectively,both of which had good dispersibility.The diameter of CNC was between 20-30 nm and the length was 250 nm to 450 nm;while the diameter of CNF was 30-60 nm and the length was 500-1000 nm.Both diameter and length of the CNC and CNF gradually became smaller as the reaction conditions stronger.Without considering the international definitions of CNC and CNF,but based on the size,the effective yield of CNC was higher than 90%.Under the conditions of 80% acid concentration,100℃ and reaction time of 4 h,the yield of the CNC without ultrasonic was 10.6%,which is much lower than the yield of the CNC under ultrasound of 32.2%,indicating that ultrasound played an important role on the yield of CNC.Secondly,oxalic acid hydrolysis combined with ultrasonication was used to prepare nanocellulose.The yield,size,carboxyl content,dispersibility and thermal stability of the obtained nanocellulose were analyzed.The yield of CNC gradually increased with the acid concentration and reaction time,reached up to 59.72% at 80% acid concentration and 2 h hydrolysis time.While the diameter and length decreased when the reaction condition became stronger,CNC had a diameter of 20-40 nm and a length of 400-500 nm.The highest carboxyl content of CNCs was 0.37 mmol/g,and that of CNF was 0.23 mmol/g,both of which gradually increased with the strengthening of reaction conditions.Finally,tartaric acid and formic acid hydrolysis to prepare nanocellulose were further studied.The yield of CNC increased with hydrolysis time when tartaric acid was used,and the highest yield was 37.09%.Due to the low solubility of tartaric acid,only 50% concentration of tartaric acid solution can be prepared.The diameter and length became smaller as the reaction time increased.The diameter was 45-50 nm while the length was 500-600 nm.The yield of CNF produced by tartaric acid decreased as reaction time increased which was as our expect.The diameter of the CNF ranges from 60 nm to 65 nm,while the length between 900 nm and 1450 nm.The carboxyl content of the CNC and CNF gradually increased with the deepening of the reaction conditions,the highest contents of carboxyl groups on CNC and CNF were both about 0.30 mmol/g.There was no CNC found when 80% formic acid was used,but the yield of CNF was over 90%.In summary,the work presented in this thesis mainly explored a novel method of using different organic acids to prepare nanocellulose,and simultaneously obtained surface-modified CNC and CNF in higher yield successfully.The yield,size,carboxyl content,dispersibility and thermal stability of CNC and CNF under various conditions were mainly investigated,and the optimal preparation conditions and processes were discovered.Under all conditions,more than 90% organic acids can be successfully recovered,which greatly reduced the production costs and defects to the environmental.The carboxylic CNC and CNF presented in this work offer commercial success and lower toxic risks,which means they have the potential to find applications as environmentally friendly,sustainable,and new bio-based nanomaterials in hightech fields,such as biomaterials.