低浓度 NaOH 冻融预处理调控纤维素聚集态结构及 Fock 反应性能

Regulating cellulose supramolecular structure and Fock reactivity via low-concentration NaOH freeze-thaw pretreatment

摘要

【目的】为推进纤维素的高值化利用，亟需解决传统高浓度 NaOH 丝光化处理易导致纤维素降解，以及均相反应体系溶剂体系复杂、回收困难等问题。为此，本研究构建低浓度 NaOH 冻融预处理方法，旨在实现纤维素高效润胀、聚合度保持与反应性能提升，同时阐明冻融过程中纤维素结构活化与反应性能提升的作用机制。【方法】本研究以脱脂棉纤维为原料，采用低浓度 NaOH 冻融预处理对纤维素进行活化。通过对预处理前后纤维素的润胀行为、形貌、结晶结构、热稳定性及水分吸附–脱附行为进行表征，分析其结构变化和分子链可及性；并结合 Fock 反应性能测试和乙酰化反应，评价纤维素反应性能的提升效果，进一步阐明 NaOH 冻融预处理的作用机制。【结果】（1）NaOH 冻融预处理可以促进纤维素润胀，使纤维呈现球状或串珠状结构，并有助于保持较高聚合度。（2）NaOH 冻融预处理后，纤维素结晶度明显降低，且晶型转变所需 NaOH 质量分数降低。当 NaOH 质量分数由 0% 升高至 10% 时，纤维素结晶度由 80.21% 降低至 48.55%；在 NaOH 质量分数为 4% 时，纤维素晶型由Ⅰ型转变为Ⅱ型。（3）NaOH 冻融预处理可显著提升纤维素的反应性能。相同 NaOH 质量分数下，4% 和 8% NaOH 冻融处理样品的 Fock 反应性能分别为 77.43% 和 81.78%，较对应常温 NaOH 处理样品分别提高 52.27% 和 50.08%。（4）机制分析表明，低质量分数 NaOH 可削弱纤维素原有氢键网络并促进纤维初步润胀；冻融过程中 NaOH 局部浓缩、冰晶体积膨胀及解冻后水分通道的形成进一步促进碱液渗透和分子链松弛，从而增加反应位点暴露，提高纤维素反应性能。【结论】本研究通过低质量分数 NaOH 润胀与冻融过程协同作用，实现了棉纤维在非溶解条件下的温和结构活化。该方法能够降低纤维素结晶有序性，提高分子链可及性，并在较好保持聚合度的同时显著提升 Fock 反应性能和乙酰化取代度，为纤维素化学改性的温和预处理提供了有效途径。

Abstract

[Objective] To promote the high-value utilization of cellulose, it is necessary to address the cellulose degradation caused by traditional high-concentration NaOH mercerization treatment, as well as the complex solvent systems and difficult solvent recovery associated with homogeneous reaction systems. In this study, a low-concentration NaOH freeze-thaw pretreatment method was developed to achieve efficient cellulose swelling, maintain the degree of polymerization, and improve cellulose reactivity. Meanwhile, the mechanism of cellulose structural activation and reactivity enhancement during the freeze-thaw process was clarified. [Method] Defatted cotton fibers were used as the raw material, and cellulose was activated by low-concentration NaOH freeze-thaw pretreatment. The swelling behavior, morphology, crystalline structure, thermal stability, and water adsorption-desorption behavior of cellulose before and after pretreatment were characterized to analyze structural changes and molecular-chain accessibility. Fock reactivity tests and acetylation reactions were further conducted to evaluate the improvement in cellulose reactivity and clarify the mechanism of NaOH freeze-thaw pretreatment. [Result] (1) NaOH freeze-thaw pretreatment promoted cellulose swelling, resulting in spherical or bead-like swollen morphology, while helping to maintain a relatively high degree of polymerization. (2) After NaOH freeze-thaw pretreatment, the crystallinity of cellulose decreased significantly, and the NaOH concentration required for cellulose crystal transition was reduced. When the NaOH concentration increased from 0% to 10%, the crystallinity decreased from 80.21% to 48.55%; a cellulose crystal transition from cellulose I to cellulose II occurred at a NaOH concentration of 4%. (3) NaOH freeze-thaw pretreatment significantly improved cellulose reactivity. Under the same NaOH concentration, Fock reactivity values of the samples treated with 4% and 8% NaOH freeze-thaw pretreatment were 77.43% and 81.78%, respectively, representing relative increases of 52.27% and 50.08% compared with the corresponding samples treated with NaOH at room temperature. (4) Mechanistic analysis indicated that low-concentration NaOH weakened the original hydrogen-bond network of cellulose and promoted initial fiber swelling. During the freeze-thaw process, local enrichment of NaOH, ice-crystal expansion, and the formation of water channels after thawing further promoted alkali penetration and molecular-chain relaxation, thereby increasing the exposure of reaction sites and improving cellulose reactivity. [Conclusion] Through the synergistic effect of low-concentration NaOH-induced swelling and the freeze-thaw process, mild structural activation of cotton fibers was achieved under non-dissolving conditions. This method reduced the crystalline order of cellulose, enhanced molecular-chain accessibility, and significantly improved Fock reactivity and the degree of substitution after acetylation while maintaining a relatively high degree of polymerization. This study provides an effective approach for the mild pretreatment of cellulose prior to chemical modification.

关键词

冻融预处理 / 纤维素晶型转变 / 低浓度碱处理 / 聚合度保持 / Fock 反应性能 / 纤维素乙酰化

Key words

freeze-thaw pretreatment / cellulose crystal transition / low-concentration alkali treatment / degree of polymerization / Fock reactivity / cellulose acetylation

参考文献

[1] Liao Y, de Beeck B O, Thielemans K, et al. The role of pretreatment in the catalytic valorization of cellulose[J].Molecular Catalysis, 2020, 487: 110883.

[2] Deng Y, Zhu T, Cheng Y, et al. Recent advances in functional cellulose-based materials: classification, properties, and applications[J].Advanced Fiber Materials, 2024, 6(5): 1343-1368.

[3] Zhang Q, Zhu E, Li T, et al. High-value utilization of cellulose: Intriguing and important effects of hydrogen bonding interactions: a mini-review[J].Biomacromolecules, 2024, 25(10): 6296-6318.

[4] 董元锋, 刘若飞, 董翠华, 等. 黏胶级溶解浆反应性能及其改善研究进展[J].中国造纸学报, 2020, 35(4): 71-76.

[5] Dong Y F, Liu R F, Dong C H, et al. Reactivity and improvement of dissolving pulp for processing viscose[J].Transactions of China Pulp and Paper, 2020, 35(4): 71-76.

[6] 梁芳敏, 焦健, 朱北平, 等. 两种机械处理方式对生物质原料反应性能的影响[J].林业工程学报, 2023, 8(2): 117-123.

[7] Liang F M, Jiao J, Zhu B P, et al. Effects of two different mechanical pretreatment methods on reactivity of biomass raw materials[J].Journal of Forestry Engineering, 2023, 8(2): 117-123.

[8] Jiao C, Xiong J. Accessibility and morphology of cellulose fibres treated with sodium hydroxide[J].BioResources, 2014, 9(4): 6504-6513.

[9] Albán Reyes D C, Skoglund N, Svedberg A, et al. The influence of different parameters on the mercerisation of cellulose for viscose production[J].Cellulose, 2016, 23(2): 1061-1072.

[10] Fang Z, Li B, Liu Y, et al. Critical role of degree of polymerization of cellulose in super-strong nanocellulose films[J].Matter, 2020, 2(4): 1000-1014.

[11] Cai J, Zhang L. Rapid dissolution of cellulose in LiOH/urea and NaOH/urea aqueous solutions[J].Macromolecular Bioscience, 2005, 5(6): 539-548.

[12] Kugo Y, Nomura S, Isono T, et al. Elucidating the structural changes of cellulose molecules and dynamics of Na ions during the crystal transition from cellulose I to II in low temperature and low concentration NaOH solution[J].Carbohydrate Polymers, 2024, 332: 121907.

[13] Cao L, Zhu J, Deng B, et al. Efficient swelling and mercerization of bagasse fiber by freeze-thaw-assisted alkali treatment[J].Frontiers in Energy Research, 2022, 10: 851543.

[14] Zhang Y, Deng W, Liu C, et al. Preparation of antibacterial dialdehyde nanocellulose using LiBr·3H₂O non-dissolving pretreatment promoted periodate oxidation[J].ACS Sustainable Chemistry & Engineering, 2023, 11(17): 6641-6651.

[15] 田超, 黄雁, 杨小博, 等. 溶解浆的反应性能及相关研究进展[J].中国造纸, 2015, 34(1): 61-66.

[16] Tian C, Huang Y, Yang X B, et al. The reactivity of dissolving pulp and its research progress: A review[J].China Pulp & Paper, 2015, 34(1): 61-66.

[17] Tingaut P, Zimmermann T, Lopez-Suevos F. Synthesis and characterization of bionanocomposites with tunable properties from poly(lactic acid) and acetylated microfibrillated cellulose[J].Biomacromolecules, 2010, 11(2): 454-464.

[18] Budtova T, Navard P. Cellulose in NaOH-water based solvents: A review[J].Cellulose, 2016, 23(1): 5-55.

[19] Lindman B, Medronho B, Alves L, et al. The relevance of structural features of cellulose and its interactions to dissolution, regeneration, gelation and plasticization phenomena[J].Physical Chemistry Chemical Physics, 2017, 19(35): 23704-23718.

[20] Huang Z, Liu C, Feng X, et al. Effect of regeneration solvent on the characteristics of regenerated cellulose from lithium bromide trihydrate molten salt[J].Cellulose, 2020, 27(16): 9243-9256.

[21] Asmare F W, Liu X, Qiao G, et al. Investigation and application of different extraction techniques for the production of finer bamboo fibres[J].Advances in Bamboo Science, 2024, 7: 100070.

[22] Chen H, Yu Y, Zhong T, et al. Effect of alkali treatment on microstructure and mechanical properties of individual bamboo fibers[J].Cellulose, 2017, 24(1): 333-347.

[23] Banvillet G, Depres G, Belgacem N, et al. Alkaline treatment combined with enzymatic hydrolysis for efficient cellulose nanofibrils production[J].Carbohydrate Polymers, 2021, 255: 117383.

[24] El Oudiani A, Chaabouni Y, Msahli S, et al. Crystal transition from cellulose I to cellulose II in NaOH treated Agave americana L. fibre[J].Carbohydrate Polymers, 2011, 86(3): 1221-1229.

[25] Rincón-Iglesias M, Lizundia E, Costa C M, et al. Tailoring electrical and mechanical properties of all-natural polymer composites for environmentally friendlier electronics[J].ACS Applied Polymer Materials, 2020, 2(4): 1448-1457.

[26] Ling Z, Chen S, Zhang X, et al. Exploring crystalline-structural variations of cellulose during alkaline pretreatment for enhanced enzymatic hydrolysis[J].Bioresource Technology, 2017, 224: 611-617.

[27] Garg M, Apostolopoulou-Kalkavoura V, Linares M, et al. Moisture uptake in nanocellulose: the effects of relative humidity, temperature and degree of crystallinity[J].Cellulose, 2021, 28(14): 9007-9021.

[28] Sayadi S, Brouillette F. Silylation of phosphorylated cellulosic fibers with an aminosilane[J].Carbohydrate Polymers, 2024, 343: 122500.

[29] Zhang J, Luo J, Tong D, et al. The dependence of pyrolysis behavior on the crystal state of cellulose[J].Carbohydrate Polymers, 2010, 79(1): 164-169.

[30] Kim U J, Eom S H, Wada M. Thermal decomposition of native cellulose: Influence on crystallite size[J].Polymer Degradation and Stability, 2010, 95(5): 778-781.

[31] Salmén L, Larsson P A. On the origin of sorption hysteresis in cellulosic materials[J].Carbohydrate Polymers, 2018, 182: 15-20.

[32] Ye X Y, Sun Q, Huang H J, et al. Unveiling the structural characteristics of acetone-insoluble substances in various cellulose diacetate[J].Carbohydrate Polymers, 2025, 360: 123616.

[33] Kono H, Hashimoto H, Shimizu Y. NMR characterization of cellulose acetate: Chemical shift assignments, substituent effects, and chemical shift additivity[J].Carbohydrate Polymers, 2015, 118: 91-100.

[34] Le Moigne N, Navard P. Dissolution mechanisms of wood cellulose fibres in NaOH-water[J].Cellulose, 2010, 17(1): 31-45.

[35] Li J, Liu Z, Feng C, et al. Green, efficient extraction of bamboo hemicellulose using freeze-thaw assisted alkali treatment[J].Bioresource Technology, 2021, 333: 125107.

基金资助

国家自然科学基金项目(32571986)