Analysis Of Physicochemical Properties Of Bamboo Lignin From Kraft Pulping And Carbon Dots Synthesis

Lignin,the second most abundant natural renewable organic polymer after cellulose,is an amorphous complex aromatic bio-based macromolecule with a three-dimensional network structure rich in sp² hybridization.The black liquor from bamboo pulp papermaking contains a large amount of lignin resources,and analyzing its physical and chemical properties is critical to increasing its added value.Therefore,the acid precipitation method was firstly used to separate lignin from the black liquor of bamboo pulp papermaking mills in this study,the ash,cellulose,and hemicellulose in bamboo lignin were reduced by washing,and its chemical structure and physicochemical properties were interpreted.Secondly,the structure and performance of bamboo lignin-based carbon dots were optimized by controlling the hydrothermal reaction time,hydrothermal reaction temperature and material-to-liquid ratio.Finally,bamboo lignin was modified using low-temperature pretreatment technology,and the effect of treatment temperature on the fluorescence properties of bamboo lignin-based carbon dots was explored.The main research conclusions are as follows:（1）The washing process reduced the content of ash,cellulose,and hemicellulose in crude lignin,and the purity of bamboo lignin was increased from 82.88%to 96.55%.Bamboo lignin is a typical GSH type lignin,withβ-O-4 bonds connecting the units mainly,and its molecular weight is low（905-967 g/mol on average）.Bamboo lignin contains a large amount of p-coumaric acid.（2）The pyrolysis process of bamboo lignin can be divided into three stages:dehydration and drying,primary pyrolysis,and slow decomposition of residues.The pyrolysis products of bamboo lignin mainly include minor molecular gases such as H₂O,CO₂,CO,and CH₄,as well as light tar components such as aromatics,acids,ketones,aldehydes,alcohols,alkanes,phenols,and ethers.CO₂ is a major product of pyrolysis of bamboo lignin,which release temperature is mainly concentrated at 400-800℃.The combustion process of lignin is divided into four stages,the main combustion stage is the IV stage,and the weight loss rate is about 41-65%.The washing process changed the chemical composition,thermochemical behavior,and ash melting properties of bamboo lignin ash.（3）Changes in pH and solvent types affect the aggregation behavior of bamboo lignin in solution,which alters its fluorescence characteristics.The optimized process for processing BL-CDs from bamboo lignin is 10 g/L with a material-to-liquid ratio,a reaction time of 10 h,and a reaction temperature of 200℃.The average particle size of the synthesized BL-CDs is 3.11 nm,with visible lattice fringes and a lattice spacing of 0.24 nm.The three-dimensional network structure of lignin is destroyed during the synthesis process.The structural units are linked together by new chemical bonds to form macromolecules with conjugate planes;the conjugated area grows in size,and the degree of graphitization increases.（4）The lignin sample undergoes degradation reactions during the low-temperature pretreatment process.All connecting bonds are broken when the temperature rises to 275℃.Most of the G-type lignin and esterified p-coumaric acid were broken when the temperature increased from 275℃to 300℃.Besides,when the temperature increased to 325℃,the esterified p-coumaric acid signal disappeared completely,leaving a small amount of S and G lignin units.The S and G type lignin units are completely pyrolyzed when the temperature rises to 350℃.The low-temperature pretreatment increases the sp² hybridization area of the sample and enhances the diffraction intensity of graphitic carbon.