Fast pyrolysis is a promising technology for the production of renewable fuels and chemicals from lignocellulosic biomass. The product distribution (bio-oil, char) and the composition of bio-oil are significantly influenced by the presence of naturally occurring alkali and alkaline-earth metals (AAEMs). In this paper, we investigate, at the molecular level, the influence of Na(I), K(I), Ca(II), and Mg(II) ions on glycosidic bond breaking reactions using density functional theory. Glycosidic bond breaking reactions are categorized as direct C–O breaking mechanisms, namely, transglycosylation, glycosylation, and ring contraction and the two-step pathways, which include the mannose pathway, dehydration, and ring opening. Our calculations show that in the absence of metal, transglycosylation and dehydration pathways (activation barriers ∼55 kcal.mol^–1) are kinetically most facile. The linkage type (α- or β-1,4) has an insignificant effect on kinetics of glycosidic bond cleavage. Mg(II) ions have a pronounced effect on lowering the activation barriers of glycosylation, ring contraction, and the mannose pathway, requiring activation enthalpies of 32–52 kcal.mol^–1. Conversely, Mg(II) and Ca(II) ions inhibit the dehydration pathway. Na(I) and K(I) ions do not significantly influence the activation barriers of glycosidic bond cleavage reactions, as the reduction is only about 5–10 kcal.mol^–1. Thus, AAEM ions exhibit different catalytic effects on glycosidic bond breaking reactions.

中文翻译：

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碱和碱土金属对生物质热解过程中糖苷键断裂的影响：以纤维二糖为模型化合物的DFT研究

快速热解是一种从木质纤维素生物质生产可再生燃料和化学品的有前途的技术。天然存在的碱金属和碱土金属（AAEM）的存在会显着影响产品分布（生物油，炭）和生物油的成分。在本文中，我们使用密度泛函理论在分子水平上研究了Na（I），K（I），Ca（II）和Mg（II）离子对糖苷键断裂反应的影响。糖苷键断裂反应被归类为直接的C–O断裂机理，即转糖基化，糖基化和环收缩以及两步途径，其中包括甘露糖途径，脱水和开环。我们的计算表明，在没有金属的情况下，糖基化和脱水途径（激活屏障〜55 kcal.mol^–1）在动力学上最容易。连接类型（α-或β-1,4）对糖苷键裂解的动力学影响不大。Mg（II）离子对降低糖基化，环收缩和甘露糖途径的激活屏障具有显著作用，要求激活焓为32–52 kcal.mol ^–1。相反，Mg（II）和Ca（II）离子抑制脱水途径。Na（I）和K（I）离子不会显着影响糖苷键裂解反应的激活屏障，因为还原仅约5–10 kcal.mol ^–1。因此，AAEM离子对糖苷键断裂反应表现出不同的催化作用。