O-乙酰基-吡喃木糖热解反应机理的理论研究

为了从微观上理解半纤维素热解过程及其主要产物的形成演变机理,采用密度泛函理论方法B3LYP/6-31G++(d,p),对O-乙酰基-吡喃木糖的热解反应机理进行了量子化学理论研究。在热解过程中,O-乙酰基-吡喃木糖中的O-乙酰基首先脱出,形成乙酸和中间体IM₁,该步反应能垒为269.4 kJ/mol。IM₁进一步发生开环反应形成IM₂,开环反应能垒较低,为181.8 kJ/mol。对中间体IM₂设计了四种可能的热解反应途径,对各种反应的反应物、产物、中间体和过渡态的结构进行了能量梯度全优化,计算了各热解反应途径的热力学和动力学参数。计算结果表明,反应路径(4)和反应路径(2)是O-乙酰基-吡喃木糖热解的主要反应通道,乙酸、乙醛、乙醇醛、丙酮、CO、CO₂、CH₄等小分子产物是热解的主要产物。这与相关实验结果分析是一致的。

Theoretical studies on pyrolysis mechanism of O-acetyl-xylopyranose

In order to understand the pyrolysis mechanism of hemicellulose and to identify the formation pathways of key products during pyrolysis, the pyrolysis processes of O-acetyl-xylopyranose are investigated by using density functional theory methods at B3LYP/6-31G++(d,p) level. In the pyrolysis, O-acetyl-xylopyranose firstly decomposes to form acetic acid and IM₁ with an energy barrier of 269.4 kJ/mol, and then IM₁ is converted to acyclic carbonyl isomer IM₂ with a low energy barrier of 181.8 kJ/mol. IM₂ further decomposes to form all sorts of small molecules through four possible pyrolytic reaction pathways. The equilibrium geometries of the reactants, transition states, intermediate and products were fully optimized, and the standard thermodynamic and kinetic parameters of every reaction pathway were calculated. The calculation results show that reaction pathways (2) and (4) are the major reaction channels in pyrolysis of O-acetyl-xylopyranose and the major products are low molecular products such as acetic acid, acetaldehyde, glycolaldehyde, acetone, CO, CO₂ and CH₄, which is according with related analysis of experimental results.