纤维素热分解机理的分子反应动力学研究

Pyrolysis Mechanism of Cellulose Based on Molecular Reaction Dynamics Simulation

采用基于反应力场的分子动力学模拟方法研究了纤维素热解的动态演变规律,着重考察了自由基演变历程和热解产物分布.构建原子数为1152的纤维素大分子模型,对其进行500~2800 K的热解模拟,得到产物分布和中间自由基的演变历程.结果表明,低温下纤维素热解以一次反应为主,主要热解产物为焦炭和生物油;高温下发生了二次反应,一次反应生成的生物油经历二次反应裂解为小分子气体或缩聚为焦炭.900 K是一次反应向二次反应的转折温度,且发生二次反应比一次反应所需时间更长.低温下纤维素热解生成的生物油中含氧量较高,高温下生物油中的含氧官能团会进一步裂解生成更小的气体分子片段,部分H在高温下迁移为气相.统计并探究了CO、CO_(2)、H_(2)O和C2H_(4)O_(2)4种主要热解产物的演变规律,反应轨迹研究表明,除CO与CO_(2)之间的相互转化,CO主要通过羰基(CHO)断裂生成,CO_(2)的释放主要是羧基(CHO_(2))的断裂和重整所致.

The molecular dynamics method based on reactive force field(ReaxFF)was employed to study the pyrolysis mechanism of cellulose.A reasonable model of cellulose with 1152 atoms was constructed.The simulation of cellulose pyrolysis was carried out to explore the distribution of products and the evolution of intermediate radicals within the range of 500—2800 K.It was found that the pyrolysis of cellulose at low temperature mainly experienced primary reaction,with char and bio-oil as the main pyrolysis products.Through the secondary reaction,the bio-oil produced by the primary reaction was cracked to gas or condensed to char.900 K was the transition temperature from primary reaction to secondary reaction,and the time required for secondary reaction to occur is longer than that for primary reaction.The bio-oil generated by cellulose pyrolysis had a high oxygen content at low temperature.The oxygen-containing functional groups in the bio-oil would be further cleaved into gaseous molecular fragments and some of the H migrated into gas phase at high temperature.Compared with the primary pyrolysis process,the secondary reaction required much longer time.In addition,the evolution tendencies of the main pyrolysis products(CO,CO_(2),H_(2)O and C2H_(4)O_(2))were given.The reaction trajectory showed that in addition to the mutual conversion between CO and CO_(2),the breakage of carbonyl group(CHO)led to CO generation,while the release of CO_(2) was mainly due to the breakage and reformation of carboxyl group(CHO_(2)).