ISSN   1004-0595

CN  62-1224/O4

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抗氧剂协同作用机理的加速反应分子动力学研究

Accelerated Reaction Molecular Dynamics Study of the Synergistic Mechanism of Antioxidants

  • 摘要: 受阻酚类抗氧剂与二苯胺类抗氧剂复配使用往往表现出优异的协同抗氧化作用,但其详细的化学反应路线尚不明确. 本文中利用1种结合元动力学(Metadynamics)与反应力场(ReaxFF)的加速反应分子动力学方法,模拟了季戊四醇酯基础油添加2种典型抗氧剂(2,6-二叔丁基对甲酚(2,6-Di-tert-butyl-4-methylphenol, BHT)、4,4'-二辛基二苯胺(4,4'-Di-iso-octyldiphenylamin, ODA))在不同温度工况下(350和500 K)的热氧化分解过程. 结果表明:酯基数量的增多、烷烃链长度的增加以及仲氢和叔氢数量的增大会导致多元醇酯基础油氧化安定性减弱;烷基过氧自由基以及烷氧自由基是加速基础油烷烃链链式反应的关键;BHT、ODA通过有效捕捉这2种自由基进而提高体系氧化安定性. 2种抗氧剂优异的协同作用来源于低温下BHT苯酚脱出的氢原子有效提高了ODA以及N-羟基二苯胺的再生效率. 详细反应路径表明,ODA存在多个反应活性位点是其抗氧性能优于BHT的重要原因之一.

     

    Abstract:
    The combined use of diphenylamine antioxidants and hindered phenol antioxidants often exhibits excellent synergistic effects. However, current studies on the relevant mechanisms are primarily focused on non-in situ experimental tests. Therefore, the detailed reaction network and reaction mechanism remain unclear. Chemical reaction molecular dynamics (Reactive Force Field, ReaxFF) has gained widespread application for its advantage of unbiasedly revealing complex reaction processes at the atomic scale without requiring any prior knowledge. However, due to the typical industrial application temperature range of diphenylamine antioxidants and hindered phenol antioxidants, which is approximately 233 K to 493 K, the ReaxFF method, limited to high-temperature chemical simulations (>2000 K), is not applicable. Collective variable-driven hyperdynamics (CVHD) is an accelerated reaction molecular dynamics method combining metadynamics and ReaxFF. It increases the probability of chemical reactions by introducing bias potentials in the non-transition state region. Thus, the simulation of chemical reactions at lower temperatures can be realized effectively.
    Under industrial operating temperatures, a study was conducted using CVHD to explore the quantitative structure-property relationship between the molecular structure and oxidation stability of neopentyl polyol ester lubricants. Additionally, the synergistic antioxidative mechanisms of hindered phenolic antioxidants and diphenylamine antioxidants in pentaerythritol ester-based oils were analyzed. The results indicated that the oxidation stability of neopentyl polyol ester lubricants decreased with an increase in the number of ester groups and the length of the alkane chain. However, a higher degree of alkane branching contributes to the addition of more active secondary and tertiary hydrogens, thereby reducing the oxidative stability of synthetic ester lubricants. And under practical operating temperatures (350 and 500 K), the CVHD method reproduced the oxidation process of pentaerythritol ester lubricating oil and the antioxidative mechanisms of different antioxidants. The results indicated that the effective capture of alkyl peroxyl radicals and alkoxyl radicals was crucial for the antioxidative mechanism. Moreover, the key products and reaction pathways aligned well with experimental conclusions. From an atomic perspective, the synergistic mechanism between two types of antioxidants was revealed: benefiting from intermolecular hydrogen transfer, the active hydrogen atom released from the phenol group of 2,6-Di-tert-butyl-4-methylphenol(BHT) at low temperatures promoted the regeneration of diphenylamine (R15) and N-hydroxydiphenylamine (R16), thereby enhancing the antioxidative performance. Notably, the reaction pathway of R16 had not been reported previously. Compared to the easily decomposable BHT at 500 K, 4,4'-Di-iso-octyldiphenylamin(ODA) still exhibited good antioxidative performance. Moreover, the identification of a new potential reaction pathway (R13) suggested that ODA’s superior antioxidative capability over BHT was attributed, in part, to the presence of multiple active reaction sites for capturing peroxyl radicals.
    The above study on the synergistic antioxidative mechanisms of the two main antioxidants effectively demonstrated the accuracy and universality of the CVHD method. This method was applicable to various areas such as thermal decomposition, thermal oxidation, and tribological chemical reactions, especially for chemical reaction simulations at lower temperatures. The research provided a clear chemical reaction network and mechanism at the atomic scale for the antioxidation of synthetic ester-based lubricants, offering crucial guidance for formulating synthetic ester lubricants with excellent oxidation stability.

     

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