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挥发性有机物(volatile organic compounds,VOCs)是大气中普遍存在的一类化合物,是形成近地面臭氧和二次有机气溶胶(secondary organic aerosol,SOA)的重要前体物[1]. VOCs可以通过人为源和自然源直接排放,也可以通过大气化学反应二次转化形成[2]. 从全球尺度上看,自然源排放(主要是植被排放)VOCs占主导地位,据估算,其排放通量是人为源的十倍以上[3]. 自然源排放的VOCs以烯烃为主,包括异戊二烯、单萜烯、倍半萜烯和一些氧化物[4]. 与人为源排放的VOCs(主要为芳香族化合物)相比,烯烃结构中含有的不饱和双键使其有更高反应活性,更易发生大气氧化反应并生成复杂的二次有机产物[5]. 由于不饱和碳-碳双键的存在,烯烃可以和O3迅速发生臭氧化反应,生成初级臭氧化产物,而其他烃类如烷烃、多环芳烃、醛类、酮类、醇类一般不与O3反应[6]. 因此,在没有竞争关系的情况下,臭氧分解可能是烯烃最重要去除方式. 鉴于烯烃在大气化学中的重要作用,其臭氧分解反应受到研究人员的广泛关注.
目前,烯烃气相臭氧化反应初始阶段的反应机理已被普遍接受,首先臭氧分子与烯烃的碳碳双键发生1,3-偶极环加成反应生成初级臭氧化物(primary ozonide,POZ),处于激发态的POZ 极不稳定,随即裂解为两对羰基化合物和Criegee中间体(Criegee Intermediate, CI)[7],如图1所示. 该反应是一个自发的放热反应过程,产生的大量热量使CI活化,因此会通过单分子分解、碰撞稳定化和双分子反应等途径释放热量以达到能量上的跃迁[8].
CI是烯烃臭氧化反应最重要的反应产物,其重要性体现在多个方面. 首先CI可以通过分子内的1,4-氢位移生成一个中间产物乙烯基氢过氧化物(vinyl hydroperoxide, VHP),并分解生成羟基自由基(·OH)和乙烯氧自由基(vinyloxyl radical)[9]. 其次CI本身是一个重要的强氧化剂,可以将SO2氧化成H2SO4,而气态H2SO4是二元和三元成核理论的重要组成,可以导致新粒子生成(new particle formation,NPF)事件的发生[10]. 此外,碰撞稳定化的CI(stabilized Criegee Intermediate,SCI)可以和大气中多数气态组分发生双分子反应,生成一系列的二次产物. SCI可以和水反应生成有机酸,后者对于大气酸碱平衡的影响十分重要[11];可以与NO2反应生成硝酸自由基(·NO3),从而影响夜间的大气氧化能力[12];也可以与过氧自由基、氢过氧自由基[13]、有机酸[14]和有机胺[15]反应,生成低聚物,最终导致SOA的生成.
由于中间体短暂的大气寿命(CI约10−9 s,SCI为100—500 ms)[16],在实验室模拟研究中准确测定其动力学参数是十分困难的. 研究需要考虑多种反应过程,例如SCI的直接测定、双分子反应产物识别以及NPF事件发生和颗粒物生长的持续观测,对应的反应时间有毫秒到分钟的跨越. 因此,本实验设计了一个可以通过移动管长来控制反应时间的快速流动反应管.
研究SCI的双分子反应一般要经过两个过程,首先烯烃臭氧化分解生成SCI,其次是SCI与其他物种发生双分子反应. 因为烯烃臭氧化反应有多个中间产物生成,如果两个反应在同一个容器中进行,会互相造成干扰,影响最终的实验结果. 本实验设计了多个鞘层的反应管,有多个反应腔,从而让两个反应独立进行.
此外,SCI活跃的化学性质,可以和大气中绝大多数物种发生双分子反应,因而保证反应体系的纯净是非常必要的. 传统的反应容器“烟雾箱”(chamber)的是由聚四氟乙烯制作而成,虽然材质相对惰性不参与反应过程,但其有较严重的壁效应,可以吸收和释放反应体系中的物种,并且是可逆的[17]. 在反应体系浓度较高的情况下(质量浓度数量级mg·m−3)壁效应是可以忽略的,因为释放的污染物量级小到不会对实验造成干扰[18]. 而SCI的大气平均数浓度较低(约105 cm−3)[19],任何杂质的加入都会影响实验的正常进行. 因此,使用光滑无孔、相对惰性的石英作为反应管的主体材料,既解决了壁效应问题,也可以方便定期清理. 同时,石英良好的光透性对在不同光照条件下的反应研究提供了可能.
快速流动反应管的设计优点之一是在固定的受控条件下(如混合浓度、温度、湿度、压力、光照和反应时间等),在连续流动中产生具有相同物理化学性质的反应产物,另一个优点是反应可以在较短的时间内进行,这样给中间产物的测定和反应过程初期的观测提供了可能. 本文对满足这些标准的流动反应管进行了设计和标定.
用于烯烃臭氧化反应的快速流动管的设计与动力学研究
Design and kinetic study of fast flow tube for alkene ozonation reaction
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摘要: Criegee中间体是烯烃气相臭氧化反应过程中最重要的中间产物,可以发生单分子分解生成羟基自由基,也可以与大气中的污染成分发生双分子反应,化学性质十分活跃. 本文设计了大气压力快速流动反应管用于烯烃臭氧化反应及后续二次反应的研究. 该流动管具有良好的温度和流量控制功能;可以同时进行两个独立反应(如烯烃+O3和SCI+SO2);可以通过调节长度控制反应时间;易拆卸清洗,最大限度减少可能影响反应的痕量污染物的存在. 通过3个流体力学无量纲参数雷诺数、斯托克斯数和理查德森数对流动管进行表征,并利用高分辨率化学电离飞行时间质谱对烯烃臭氧化反应及后续双分子反应进行动力学研究. 经验证,新设计的流动管适用于烯烃臭氧化体系的研究,并且能够提供精确的、直接的动力学信息.Abstract: Criegee intermediate is the most important intermediate product in the process of alkene ozonolysis, which can undergo unimolecular decomposition to form hydroxyl radicals or bimolecular reaction with atmospheric pollutants, and has very active chemical properties. In this paper, atmospheric pressure fast flow reaction tube was designed for the study of alkene ozonation reaction and subsequent secondary reactions. The flow tube has good temperature and flow control; two independent reactions (e.g., alkene + O3 and SCI + SO2) can be carried out independently; allows control of the reaction time by adjusting the length; and is easily disassembled for cleaning, minimizing the presence of trace contaminants that may affect the reaction. The flow tube was characterized by three hydrodynamic dimensionless parameters, Reynolds number, Stokes number and Richardson number. Kinetic study of the alkene ozonation reaction and subsequent bimolecular reaction using High Resolution Time-of-Flight Chemical Ionization Mass Spectrometer. The newly designed flow tube was proven to be suitable for the study of alkene ozonation system and to provide accurate and direct kinetic information.
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Key words:
- flow tube /
- alkene ozonation reaction /
- reaction rate /
- yield.
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表 1 不同管长条件下TME与O3反应的动力学参数
Table 1. Kinetic parameters of the reaction between TME and O3 under different tube length
反应管长/cm
Tube length斜率
Slope计算反应时间/s
Calculated reaction
time, tcal计算反应速率/
(cm3·molecule−1·s−1)
Calculated reaction rate, k文献反应速率[6] /
(cm3·molecule−1·s−1)
Literature reaction rate, k有效反应时间/s
Effective reaction
time, teff20 2.51×10−14 6.3 (3.98±0.78)×10−15 (1.13±0.93)×10−15 4.21 30 8.76×10−15 9.4 (0.93±0.14)×10−15 7.75 50 1.57×10−14 15.7 (1.00±0.11)×10−15 13.89 70 2.26×10−14 22.0 (1.03±0.12)×10−15 20.00 90 3.07×10−14 28.3 (1.09±0.11)×10−15 27.17 表 2 不同臭氧浓度条件下TME与O3反应的动力学参数
Table 2. Kinetic parameters of the reaction between TME and O3 under different ozone concentration
O3浓度(V/V)
O3 concentrationO3数浓度/(molecule·cm−3)
O3 number concentration斜率
Slope计算反应速率/(cm3·molecule−1·s−1)
Calculated reaction rate, k文献反应速率[6] /(cm3·molecule−1·s−1)
Literature reaction rate, k1.0×10−7 2.46×1012 0.013 (5.28±0.98)×10−15 (1.13±0.93)×10−15 2.0×10−7 4.92×1012 0.016 (3.25±0.41)×10−15 3.0×10−7 7.38×1012 0.021 (2.85±0.35)×10−15 4.0×10−7 9.84×1012 0.022 (2.24±0.29)×10−15 5.0×10−7 1.23×1013 0.032 (2.60±0.30)×10−15 -
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