Isolation Of Lignin Containing Nanocellulose By Acidic Deep Eutectic Solvent And Organic Acid Pretreatment

With the increasing concern of environmental pollution and the awareness of environmental protection,the traditional nanocellulose isolation techniques,e.g.,H₂SO₄ hydrolysis,have been unable to fully meet the needs of modern society,and the economic and environmental concerns associated with these conventional isolation techniques pose major obstacles for commercialization.In this paper,the thermomechanical pulp(TMP,the raw material for newspaper)was used as the raw material to produce high value-added lignin containing nanocellulose isolation by using green,economical,and recoverable deep eutectic solvents and organic acid.This study provides a new way to make full use of lignocellulose resources to prepare nanocellulose with high yield and helps to alleviate a series of issues caused by the decline in the current quantity of TMP demand.Therefore,the research on lignin containing nanocellulose is of considerable theoretical significance and practical value.The main conclusions of this research are as follows.(1)This work demonstrates that the TMP can be converted into high value-added lignin-containing nanocellulose(LNC)by using a lactic acid-choline chloride(Ch Cl)based acidic DES pretreatment with a high production yield.After the microfluidization of pretreated samples for twice,the yields of LNC were between 70?85%with the lignin content about 30%at designed parameters.The LNC shows excellent thermal stability but poor dispersibility in water.Increasing the pretreatment intensity can improve the efficiency of esterification reaction between lactic acid and lignocellulose,which is conducive to the isolation of LNC.Experiments have demonstrated the reusability of the DES,but the introduced impurities such as sugars and lignin,should be removed as much as possible,and the lactic acid should also be fed in time.otherwise,the efficiency of DES pretreatment will decrease dramatically.(2)In this chapter,oxalic acid based DES,which can introduce carboxyl group on the surface of cellulose,was selected to improve the dispersibility of LNC in water.The hydrolysis efficiency increased with increasing pretreatment temperature and oxalic acid(OA)concentration,with the highest LNCs yield of 57%being achieved at 1:1 OA:Ch Cl ratio and 90°C for 6 h.Under such reaction conditions,ribbon-like(width?7.1 nm,thickness?3.7 nm)LNCs with 0.30 mmol/g surface carboxylic acid groups,60%cellulose I?crystallinity,high thermal stability(T_max=358°C),and32.6%lignin content were produced.Spherical and slightly elongated lignin nanoparticles of 20?50nm in diameter can be observed in the LNC suspension and can co-assemble with the nanocellulose into continuous sub-micron fibers by freeze-drying.Uniform distribution of these lignin nanoparticles along the assembled fibers could be observed by the auto-fluorescence of lignin using a confocal microscope.(3)Based on the aforementioned research,a novel tertiary acid DES containing p-Tolunesulfonic acid was designed for the isolation of lignin containing nanocellulose.Lignin-containing cellulose nanocrystals(LCNC),and high yield can be successfully obtained from TMP under optimal conditions(pretreated at 80°C for 3 h followed by high-speed blending for 30 min).The isolated LCNCs showed a cellulose I crystal structure and crystallinity of 57.4%with a thickness of 3.3 nm and a width of 5.9 nm.Compared to conventional binary DES(DES₂)containing only choline chloride and oxalic acid,a ternary DES(DES₃)presents obvious advantages,including 70%more production yield(66%for DES₃ vs.39%for DES₂),significantly increased lignin content(47.8%for DES₃ vs.28.4%for DES₂),higher surface carboxyl contents(0.43 mmol/g for DES₃ vs.0.29 mmol/g for DES₂),and slightly higher thermal stability(T_max of 358°C for DES₃ vs.T_max of 344°C for DES₂).(4)The TMP can also be converted into LCNC using recyclable molten oxalic acid dihydrate with a high yield.LCNC with high yield(70.2%)and good thermal stability(T_max=340°C)was successfully prepared from TMP by using molten oxalic acid dihydrate hydrolysis at 110°C for 30min after microfluidization.The obtained LCNC also showed a ribbon-like structure and relatively uniform in size(length?200?400 nm,width?7 nm,height?3.9 nm).The surface carboxyl content of LCNC prepared in this research was 0.58 mmol/g,even higher than that prepared by the DES pretreatment method mentioned above.What's more,the oxalic acid dihydrate exhibited good recyclability and isolation efficacy,and the morphology of the obtained LCNC showed no change even after the oxalic acid was reused four times.