Abstract
Industrial kraft pulping processes provide abundant lignin feedstocks, which are renewable and valuable resources for developing diverse chemicals and functional materials. However, this industrial-grade lignin usually exhibits a strong odor, dark color, heterogeneous chemical structure, and even complex chemical constituents depending on the wood species utilized, the delignification process employed, and the subsequent steps involved in lignin recovery. The high heterogeneity in structure and chemical constituents can result in variations in the quality of downstream products, posing a significant challenge in developing industrial lignin for high-value-added products.
The presented thesis focused on clarifying the heterogeneity in chemical constituents and sublinkage compositions of industrial softwood kraft lignin (ISKL). Additionally, it aimed to elucidate the effects of these heterogeneities on
lignin fractionation, the physicochemical properties of lignin fractions, and the
formation of lignin nanoparticles.
Results on the chemical constituents showed that ISKL consisted of heterogeneous lignin polymers, lignin-carbohydrate complexes, extractives, aromatic monomers, and other products generated during the kraft delignification process. The sublinkage compositions of lignin fractions obtained through a novel solvent fractionation approach indicated that the lignin can be classified as follows: 1) One fraction exhibited a higher proportion of sublinkages with reduced side chain (stilbenes and enol ether) while containing a lower amount of native sublinkages. It also presented a lower molar mass, a higher phenolic hydroxyl groups (PhOH) content than aliphatic hydroxyl groups (AlkOH), a higher carbon content, and stronger solubility in a low polarity solvent. 2) One fraction showed a relatively stable proportion of native sublinkages and stilbenes, while the proportion of resinol and benzyl ethers increased with their increased molar mass. This fraction also maintained a relatively constant content of PhOH and AlkOH, and elemental composition. 3) One fraction primarily consisted of native sublinkages, a certain amount of carbohydrates, and a smaller percentage of sublinkages with reduced side chains. It also presented a higher molar mass and an elevated content of AlkOH compared to PhOH. These structural characteristics collectively contributed to its reduced carbon content and improved solubility in solvents with a moderate
dispersion and a stronger hydrogen bonding capacity. 4) A low-yield fraction exhibited a moderate molar mass, a high content of native sublinkages, carbohydrates, and fewer sublinkages with reduced side chains. The variation in the sublinkages composition of different fractions results in them presenting
different thermal degradation behaviors and variable abilities to form lignin
nanoparticles. Finally, the heterogeneous chemical constituents, especially, fatty
acids play a major role in the formation of hollow lignin nanoparticles rather
than lignin itself.
The presented thesis focused on clarifying the heterogeneity in chemical constituents and sublinkage compositions of industrial softwood kraft lignin (ISKL). Additionally, it aimed to elucidate the effects of these heterogeneities on
lignin fractionation, the physicochemical properties of lignin fractions, and the
formation of lignin nanoparticles.
Results on the chemical constituents showed that ISKL consisted of heterogeneous lignin polymers, lignin-carbohydrate complexes, extractives, aromatic monomers, and other products generated during the kraft delignification process. The sublinkage compositions of lignin fractions obtained through a novel solvent fractionation approach indicated that the lignin can be classified as follows: 1) One fraction exhibited a higher proportion of sublinkages with reduced side chain (stilbenes and enol ether) while containing a lower amount of native sublinkages. It also presented a lower molar mass, a higher phenolic hydroxyl groups (PhOH) content than aliphatic hydroxyl groups (AlkOH), a higher carbon content, and stronger solubility in a low polarity solvent. 2) One fraction showed a relatively stable proportion of native sublinkages and stilbenes, while the proportion of resinol and benzyl ethers increased with their increased molar mass. This fraction also maintained a relatively constant content of PhOH and AlkOH, and elemental composition. 3) One fraction primarily consisted of native sublinkages, a certain amount of carbohydrates, and a smaller percentage of sublinkages with reduced side chains. It also presented a higher molar mass and an elevated content of AlkOH compared to PhOH. These structural characteristics collectively contributed to its reduced carbon content and improved solubility in solvents with a moderate
dispersion and a stronger hydrogen bonding capacity. 4) A low-yield fraction exhibited a moderate molar mass, a high content of native sublinkages, carbohydrates, and fewer sublinkages with reduced side chains. The variation in the sublinkages composition of different fractions results in them presenting
different thermal degradation behaviors and variable abilities to form lignin
nanoparticles. Finally, the heterogeneous chemical constituents, especially, fatty
acids play a major role in the formation of hollow lignin nanoparticles rather
than lignin itself.
| Original language | English |
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| Supervisors/Advisors |
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| Place of Publication | Åbo |
| Publisher | |
| Print ISBNs | 978-952-12-4452-0 |
| Electronic ISBNs | 978-952-12-4453-7 |
| Publication status | Published - 2024 |
| MoE publication type | G5 Doctoral dissertation (article) |