吡喃木糖和O-乙酰基吡喃木糖热解形成羟基乙醛的机理研究

以半纤维素主要成分木聚糖的两种单体--吡喃木糖和O-乙酰基吡喃木糖为模型化合物,运用密度泛函理论(DFT),采用B3LYP方法和6-31+G(d,p)基组进行计算,研究了吡喃木糖热解形成HAA的6条可能的反应路径和O-乙酰基吡喃木糖热解形成HAA的3条可能的反应路径。由此确定了吡喃木糖热解形成HAA的最优路径为:吡喃木糖首先开环得到链式木糖,然后C3羟基和C2氢脱水,随后经重排和逆醇醛缩合反应生成包含C4/C5的HAA;该路径的决速步骤为脱水反应,能垒为253.3 kJ/mol。O-乙酰基吡喃木糖热解形成HAA的最优路径为:O-乙酰基吡喃木糖首先支链断裂脱出乙酸(AA),开环后的链式中间体经氢转移反应得到包含C4/C5的HAA;该路径的决速步骤为最后的氢转移反应,能垒为317.6 kJ/mol。

Mechanism for the formation of hydroxyacetaldehyde by the pyrolysis of xylopyranose and O-acetyl-xylopyranose

Xylopyranose and O-acetyl-xylopyranose, the two monomers of xylan, were employed as the model compounds to study the mechanism for the formation of hydroxyacetaldehyde (HAA) from xylan by pyrolysis. Six possible pathways from xylopyranose and three from O-acetyl-xylopyranose were proposed by employing the density functional theory (DFT) at B3LYP/6-31+G(d,p) level; the energetically favored pathways for HAA formation were revealed. Xylopyranose may undergo ring-opening, dehydration, rearrangement and retro-aldol reactions sequentially, to form HAA that contains C4/C5; the rate-determining step is the dehydration reaction, with an energy barrier of 253.3 kJ/mol. From O-acetyl-xylopyranose, the side chain is cleaved in the first place, forming acetic acid (AA) and a cyclic intermediate; the ring-opening and H-shift reactions happen afterwards from the cyclic intermediate to generate HAA containing C4/C5; the rate-determining step is the H-shif reaction, with an energy barrier of 317.6 kJ/mol.