We use infrared (IR) spectroscopy, gel permeation chromatography (GPC), and liquid chromatography-mass spectrometry (LC-MS) in multistage dissolution experiments in various solvents to investigate the solubility and molecular structure of humins formed during fructose dehydration. We demonstrate that the soluble fraction of humins correlates positively with the donor number of solvents resulting in significant dissolution at room temperature in solvents with high donor number. Most of the solubilized humins fragments have relatively low molecular weight (M_w), and the same species are present in different quantities as the residual solid is repeatedly dissolved in the same solvent. In contrast to the common belief of humins consisting of polymers of large M_w, we postulate for the first time that they are spatially and chemically heterogeneous and consist of insoluble macromolecules and small soluble species that are weakly associated within the structure. The solubility profiles of dissolved species in acetonitrile and methanol are different in terms of M_w but possess similar IR spectra, indicative of similar functional groups in dissolved species. Furthermore, we hypothesize that the identified humins fragments form through aldol condensation between 5-hydroxymethylfurfural and its hydrated products and through condensation of furanic species.

中文翻译：

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布朗斯台德酸催化果糖脱水过程中形成的腐殖质的结构分析

我们在各种溶剂中的多阶段溶解实验中使用红外（IR）光谱，凝胶渗透色谱（GPC）和液相色谱-质谱（LC-MS），研究了果糖脱水过程中形成的腐殖质的溶解度和分子结构。我们证明，腐殖质的可溶部分与溶剂的供体数正相关，导致室温下在具有高供体数的溶剂中显着溶解。大多数可溶的腐殖质片段具有相对较低的分子量（M _w），并且相同的物质以不同的量存在，因为残留的固体反复溶解在相同的溶剂中。与由大分子量M _w的聚合物组成的人用腐殖质的普遍看法相反，我们首次假定它们在空间和化学上是异质的，并且由不溶的大分子和在结构内弱关联的小的可溶物质组成。溶解物质在乙腈和甲醇中的溶解度曲线的M _w不同，但具有相似的IR光谱，表明溶解物质中的官能团相似。此外，我们假设，通过5-羟甲基糠醛及其水合产物之间的羟醛缩合以及呋喃类化合物的缩合形成了确定的腐殖质片段。