Wei Li thesis defense on July, 26th 2021

15 July 2021
This thesis was completed in our laboratory, under the supervision of Dr. Yoshiharu NISHIYAMA. The subject is: "Preparation of cellulose oligomers by acid hydrolysis." Click on the title for more information.

Abstract:

” The study was motivated by the necessity to produce cellulose oligomers with controlled molecular weight and molecular weight distribution by a simple, scalable and aqueous-solution based method, with the purpose of understanding the mechanisms in the swelling, dissolution, hydrolysis and regeneration processes and establishing characterization methods of molecular weight distribution of cellulose oligomers. The swelling behavior of cellulose in sulfuric acid was followed at -20 ˚C using X-ray diffraction. At low temperature, a cellulose-sulfuric acid crystalline complex was found as the first example of cellulose-acid crystallosolvate. This crystalline complex had an orthorhombic unit cell (a = 18.24 Å, b = 25.98 Å, c = 25.05 Å) that contained 4 cellulose chains, 48 sulfuric acid molecules and 144 water molecules. This complexation was acid-concentration dependent and occurred only above 62 wt%, indicating that the lack of water molecules drove the complexation to form a stable hydration state of sulfuric acid. To characterize the molecular weight of cellulose oligomers, derivatization methods of cellulose and the different characterization methods were compared. Compared to nitration, carbanilation was more suitable for size exclusion chromatography (SEC) since light scattering is more sensitive to cellulose carbanilates than to cellulose nitrates. For determining the degree of substitution (DS) of cellulose carbanilate, a liquid-state 1H nuclear magnetic resonance (NMR) based method using N-H proton intensity has smaller error from side products compared to other methods such as elemental analysis. Four different methods were compared to characterize the degree of polymerization (DP) and DP distribution of cellulose oligomers, namely, liquid-state 1H NMR and solid-state 13C cross polarization magic angle spinning (CP/MAS) NMR spectroscopies, mass spectroscopy (MS) and SEC. SEC was found to be suitable for relatively higher molecular weight oligomer fractions, while the other three spectroscopic methods were suitable for lower molecular weight fractions. The effects of various hydrolysis and regeneration conditions, and acid species on the DP and DP distribution of cellulose were investigated. Different conditions provided oligomers with different average DP values (5<DPw<37), DP distributions and yields. For instance, phosphoric acid (H3PO4) high-temperature hydrolysis significantly shortened the preparation time: the cellulose oligomers obtained from the hydrolysis at 23 ˚C for 42 days exhibited almost the same average DP and yield as the cellulose oligomers from a 3-day the hydrolysis at 40 ˚C. The resulting DP distribution from this oligomer production method was tentatively explained by a model based on random chain scission and a fractionation cutoff that depended on the solubility of cellulose oligomers in a given solvent. This thesis serves as a solid basis for a better control of molecular weight distribution of cellulose oligomers from an acid hydrolysis pathway. The knowledge on the characterization of cellulose oligomers will also be useful for further investigation and utilization of cellulose oligomers in both industries and academic research communities.”

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