苯为前驱物形成二噁英的反应机理

Reaction mechanism of dioxin formation with benzene as precursor

苯作为垃圾中普遍存在的成分是形成多氯二苯并对二噁英/呋喃(PCDD/Fs)的重要前驱物之一.应用密度泛函理论(DFT)在B3LYP/6-311+G(d,p)水平上研究了苯生成PCDD/Fs的两阶段气相反应机理,获得相关基元反应的势垒与反应热.采用隧道效应校正的变分过渡态理论(VTST)对300~1300K的各基元反应进行速率常数计算.结果表明:苯的氯化过程在合成PCDD/Fs各基元反应中势垒最高,是整个过程的控速步骤;氯酚更有可能被HO·进攻氯苯发生亲核反应后经分子内脱氢形成;邻位Cl能提高芳香烃H的抽取势垒,降低分子反应活性;苯氧自由基经碳碳或碳氧耦合二聚化过程形成不同PCDD/Fs存在竞争机制.相关计算结果可以用于在总二噁英产量基础上评估由苯产生的PCDD/Fs贡献率.

Benzene was one of the important precursors for the formation of polychlorinated dibenzo-p-dioxins （PCDD/Fs） which widespreadly present in waste. The two stage gas-phase formation mechanisms of PCDD/Fs by benzene were studied by density functional theory （DFT） at the B3LYP/6-311＋G （d, p） level, the corresponding potential barriers and reaction heats were calculated. The variational transition state theory （VTST） was used to calculate the rate constants of the elementary reactions in the 300~1300K temperature range. The chlorination process of benzene was the determining step of the whole synthesis process of PCDD/Fs with the highest barrier; Chlorophenol was more likely formed via the intramolecular dehydrogenation of chlorobenzene which attacked by HO· radicals; Ortho Cl can raise the extraction barrier of aromatic hydrocarbon H and reduce the molecular reactivity; competition mechanism existed in the formation of PCDD/Fs which was formed by phenoxy radicals via coupling of carbon-carbon or carbon-oxygen. The calculation results could be applied to evaluate the contribution of PCDD/Fs formed by benzene on the basis of total dioxin production.