Gey Marten Niklas, Schröder Uwe
Institute of Biochemistry, University of Greifswald Felix-Hausdorff-Str. 4 17489 Greifswald Germany
RSC Adv. 2025 Jul 16;15(31):25132-25140. doi: 10.1039/d5ra02859k. eCollection 2025 Jul 15.
One of the major challenges in the oxidation of the carbohydrate-based 5-(hydroxymethyl)-furfural (HMF) to 2,5-furandicarboxylic acid (FDCA) - a reaction of great relevance for the production of biopolymers - is the need for alkaline conditions. Many of the published oxidation systems operate at pH 13-14, at which a self-polymerization of HMF to strongly colored humic substances occurs. To date, this side reaction has only been investigated to a limited extent. This study presents a first investigation of the humic substance formation of alkaline HMF solutions upon storage over a period of more than 200 hours. A comparison of the HMF degradation with that of its benzyl analog 4-(hydroxymethyl)-benzaldehyde (HMB) showed that humic substance formation is caused by the opening of the furan ring, which subsequently induces linking of the individual HMF molecules. Parallel to this, the Cannizzaro reaction proceeds, which, in the observed concentration range of 20-100 mM, converted approx. 20% of the initial HMF. Further analysis of the humic substance formation by UV/Vis spectroscopy revealed that this process can be separated into a "build-up" phase (within the first 24 h) and an "aging" phase (after 24 h), in which the colored humic material is decolorized again due to the presence of dissolved atmospheric O. Based on the solubility at different pH values, the formed humic material was classified as a mixture of humic acids and fulvic acids, while (fully insoluble) humins were not formed. Finally, FTIR spectroscopy was utilized to carry out a structural investigation of the acid-insoluble humic acid fraction.
将基于碳水化合物的5-(羟甲基)-糠醛(HMF)氧化为2,5-呋喃二甲酸(FDCA)是生物聚合物生产中一项极具意义的反应，其中一个主要挑战是需要碱性条件。许多已发表的氧化体系在pH值为13 - 14的条件下运行，在此条件下HMF会自聚合成颜色很深的腐殖质。迄今为止，对这种副反应的研究还很有限。本研究首次对碱性HMF溶液在储存200多小时后的腐殖质形成情况进行了研究。将HMF的降解情况与其苄基类似物4-(羟甲基)-苯甲醛(HMB)的降解情况进行比较，结果表明腐殖质的形成是由呋喃环的开环引起的，随后导致单个HMF分子发生连接。与此同时，坎尼扎罗反应也在进行，在观察到的20 - 100 mM浓度范围内，约20%的初始HMF发生了转化。通过紫外/可见光谱对腐殖质形成的进一步分析表明，这个过程可以分为一个“积累”阶段(在最初的24小时内)和一个“老化”阶段(24小时后)，在“老化”阶段，由于溶解的大气中的O的存在，有色腐殖质会再次脱色。根据在不同pH值下的溶解度，所形成的腐殖质被归类为腐殖酸和富里酸的混合物，而(完全不溶的)胡敏素并未形成。最后，利用傅里叶变换红外光谱对酸不溶性腐殖酸部分进行了结构研究。
