Supramolecular Structural Changes Of Cellulose During Pretreatments And Preparation Of Nanocrystals

Lignocellulosic biomass is the most widespread renewable material in the nature.It is the main resource for biorefinery to produce bio-energy,functional materials and bio-chemicals.The effective utilization of lignocellulosic biomass can help to relieve the shortage of earth resources,environmental pollution and to achieve the sustainable development.Cellulose is the main component of lignocellulosic biomass,also the main resource for biorefinery.This dissertation focused on the supramolecular structures of cellulose and their distinct behaviors during the biomass preatment and the fabrication of cellulose nanocrystals.Alkaline pretreatment on moso bamboo was observed to explore the dual effects on the hydrolysis of bamboo holocellulose.The concentration for completing the mercerization of cellulose Ⅰ to celluloseⅡ is 12 wt%.During the transformation,the crystallinity of cellulose increased while the crystallite sizes were reduced.The hemicellulose was mostly removed during the pretreatment at high NaOH concentration.Meanwhile,the d-spacings for anti-parallel cellulose II were significantly increased(0.7 um),which enhanced the enzymatic hydrolysis with 95%glucose produced.The difference of ionic liquid[C4mim]Cl pretreatment on the microcrystalline cellulose and moso bamboo holocellulose was determined.The high initial crystallinity(70%)of microcrystalline cellulose helped to retain its crystal structure during the pretreatment,except for some recrystallization of cellulose la to cellulose Iβ after 10 h pretreatment,prohibiting the enzymatic hydrolysis on the sample.The hemicellulose of bamboo was removed together with the allomorphic transformation from cellulose I to cellulose II.As well,the crystallinity of holocellulose decreased to 11.24%,which efficiently deconstructed the biomass recalcitrance,and improved the glucose yield.A physical-chemical treatment called ball-milling was utilized for decrystallization of cotton cellulose.The extended ball-milling time destroyed the intact cotton fiber and caused the depolymerization,increasing specific surface area as well as more pores.Moreover,the chemical reaction was observed on oxidation of hydroxyl groups to carbonyl groups.Both the laboratory and synchroton XRD inferred the crystallinity of nearly 10%after 2 h ball-milling.The crystallite size perpendicular to(200)lattice plane decreased to 4 nm,with more fragments appeared.Meanwhile,the calculation of crystallinity by different analytical methods were performed and compared,which provided more measurements to solve the problems of cellulose crystallinity.To explore the structure of crystalline cellulose,which is harder to be hydrolyzed than amorphous part,a new choline chloride/oxalic acid based deep eutectic solvent was proposed to produce cotton cellulose nanocrystals.Different ratios of choline chloride/oxalic acid and treatment temperature have notable effects on the properties of cellulose nanocrystals as well as the self-assembly of freeze-dried samples.Oxalic acid caused the oxidation and etherification of hydroxyl groups on C6 position,which changed the supramolecular structures of cellulose and accelerated the fibriliation during the following mechanical treatment.Cellulose nanocrystals with different allomorphs were successfully prepared to compare the ability of enzymatic adsorption and degradation on allomorphic crystalline cellulose.The cellulose nanocrystals have different mophorlogic structure,which caused the cnzymatic hydrolysis rate in the following order:CNC II>CNC III>CNC I.The rotation of cellulose chains for cellulose Ⅱ enlarged the d-spacing,accelerating the enzymatic degradation.The CNC III has higher ability to absorb cellulase,which mostly appeared on the middle of the crystal and degraded it by cutting the needle-like crystal into shorter particles.