吡喃木糖热裂解机理的密度泛函理论研究

A density functional theory study on the mechanism of xylopyranose pyrolysis

采用密度泛函理论理论方法 M062X/6-311++G(d,p),对吡喃木糖的热解反应机理进行了理论计算分析.针对吡喃木糖热解可能发生的化学反应共设计了九条可能的热解路径,并对各路径中的反应物、中间体和过渡态的几何结构进行了能量梯度全优化,并在梯度全优化的基础上计算了各热解反应路径的热力学和动力学参数.文中以两大类方式来设计反应路径:1)木糖首先经过过渡态TS1发生开环反应生成链状中间体2,该步的反应能垒为188.7 kJ/mol,对于中间体2共设计了五种可能的热解反应路径;2)考虑双键同时断裂的情况,木糖先发生脱水反应,接着按C-C和C-O键同时断裂的情况发生开环反应,针于这种情况共设计了四条可能的热解路径.计算结果表明,吡喃木糖热解的主要反应产物有乙醇、乙醛、糠醛、丙酮、酸类、CO_2和CO等小分子化合物.

The pyrolysis mechanism of xylopyranose is investigated by using density functional theory methods at M062X/6-311＋＋G（d,p） level. Nine possible reaction paths were designed, and all the reactants, transition states, intermediates and products were optmized, and the standard thermodynamic and kinetic parameters in each reaction pathway were calculated. In the pyrolysis, two categories of pyrolytic reaction pathways were proposed. In the first category, xylopyranose is firstly converted to acyclic carbonyl isomer 2 via transition state TS1 with energy barrier of 188.7 kJ /mol, and then the isomer 2 further decomposes to form all sorts of small molecules through five possible pyrolytic reaction pathways. In the second category, xylopyranose first undergoes dehydration, and then four possible pyrolytic reaction pathways were proposed through simultaneous cracking of C-O bond and C-C bond. The calculation results show that the major products are low molecular products such as ethanol, acetaldehyde, furfural, acetone , acids, CO2 and CO.