纤维素热解形成左旋葡聚糖机理的理论研究

A theoretical study on the mechanism of levoglucosan formation in cellulose pyrolysis

采用密度泛函理论UB3LYP/6-31G(d)方法,对模型化合物纤维二糖热解反应机理进行了量子化学理论计算研究。设计了三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,计算了不同温度下热解反应的标准热力学和动力学参数。计算结果表明,糖苷键均裂而形成两个自由基中间体IM1a和IM1b,吸收热量为321.26 kJ/mol,中间体IM1a经过渡态TS1a进一步形成左旋葡聚糖P1,反应势垒为202.72 kJ/mol;与分步反应相比,纤维二糖经过渡态TS2协同反应直接形成左旋葡聚糖P1和吡喃葡萄糖P2的反应势垒低于分步反应的总势垒,其反应势垒为377.54 kJ/mol;H+的加入有利于糖苷键的断裂,断裂形成的中间体IM3很难进一步反应形成左旋葡聚糖。

To understand the mechanism of levoglucosan formation in cellulose pyrolysis,the pyrolysis of cellobiose as a model compound was investigated theoretically by using UB3LYP/6-31G（d） methods.Three kinds of pyrolysis reaction paths were designed;the equilibrium geometries of the reactants,intermediate,transition states,and products were optimized.The standard thermodynamic and kinetic parameters of pyrolysis reaction were calculated at different temperatures.The results showed that the free radicals IM1a and IM1b can be formed by homolysis of glycosidic bond and the reaction is endothermic with an energy of 321.26 kJ/mol.Free radical IM1a may react further via transition state TS1a and lead to the formation of levoglucosan,with an energy barrier of 202.72 kJ/mol.Compared to consecutive reactions,concerted reaction of the formation of levoglucosan P1 and glucopyranose P2 via the transition state TS2 in pyrolysis of cellobiose has a lower energy barrier of 377.54 kJ/mol.Addition of H＋ would be in favor of breakage of glycosidic bond;the intermediate IM3 formed can hardly transform to levoglucosan,which is in accord with the related analysis of experimental results.