The high percentage of branched units with different structures leads to the complex pyrolysis chemistry and product distribution of hemicellulose. In this study, four main branched monosaccharides, i.e., arabinose (Ara), galactose (Gal), galacturonic acid (GacA), and glucuronic acid (GlcA) were employed as typical xylan-based hemicellulose model compounds. The pyrolysis behaviors and mechanisms of these monosaccharides were deeply explored by the combination of pyrolysis experiments and density functional theory (DFT) calculations. Particularly, the roles of structural differences of these monosaccharides were carefully analyzed. Due to the uronic acid group, GacA and GlcA were easier to decompose, having two weightloss peaks with close intensity, and the release of CO had greater intensity than Ara and Gal pyrolysis at low temperatures. GlcA with the equatorial hydroxyl group can undergo a special CC bond breaking reaction to depart the C6 uronic acid group to release HCOOH. The fast pyrolysis of these monosaccharides obtained similar product species but distinct relative contents. The structure of an uncertain hydromethyl-furnaose product was identified as 5-(hydroxymethyl)furan-3(2)-one (5-HMFO) by combined DFT calculations and experimental results. The findings of this work are beneficial to gain a deep understanding of the pyrolysis chemistry of hemicellulose, especially the role of different branches.

中文翻译：

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木聚糖热解的实验和计算研究：主要支链单糖的热解机理

不同结构的支化单元的高比例导致半纤维素复杂的热解化学和产物分布。在本研究中，四种主要的支链单糖，即阿拉伯糖（Ara）、半乳糖（Gal）、半乳糖醛酸（GacA）和葡萄糖醛酸（GlcA）被用作典型的基于木聚糖的半纤维素模型化合物。通过热解实验和密度泛函理论（DFT）计算相结合，深入探讨了这些单糖的热解行为和机制。特别是，仔细分析了这些单糖结构差异的作用。由于糖醛酸基团的存在，GacA和GlcA更容易分解，有两个强度接近的失重峰，且低温下热解CO的释放强度比Ara和Gal更强。带有平羟基的GlcA可以发生特殊的CC键断裂反应，使C6糖醛酸基团脱离，释放出HCOOH。这些单糖的快速热解获得相似的产物种类但相对含量不同。通过结合DFT计算和实验结果，将不确定的羟甲基呋喃糖产物的结构鉴定为5-(羟甲基)呋喃-3(2)-酮(5-HMFO)。这项工作的发现有利于深入了解半纤维素的热解化学，特别是不同分支的作用。