二甲醚自由基+O2反应体系的从头算热力学和动力学分析

Symposium (International) on Combustion Pub Date : 1998-01-01 DOI:10.1016/S0082-0784(98)80406-2

Takahiro Yamada, Joseph W. Bozzelli, Tsan Lay

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The dimethyl-ether radical CH3OC·H2 (ΔHf298o=0.1 kcal/mol) adds to O2 to form a peroxy radical CH3OCH2OO·(ΔHf298o=−33.9 kcal/mol). The peroxy radical can undergo dissociation back to reactants or isomerize via hydrogen shift (Ea,rxn=17.7 kcal/mol) to form a hydroperoxy alkyl radical C·H2OCH2OOH, (ΔHf298o=−26.5 kcal/mol). This alkyl radical can undergo β-scission reaction to formaldehyde (CH2O)+hydroperoxy methyl radical (C·H2OOH), (Ea, rxn=24.7 kcal/mol). The hydroperoxy methyl radical rapidly decomposes to a second CH2O plus OH. 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引用次数: 4

摘要

采用从头算方法分析了CH3OC·H2+O2反应体系的反应途径和动力学，确定了反应物、中间自由基和过渡态化合物的热力学性质。生成焓(ΔHf298o)采用CBS-q//MP2(full)/6-31G(d,p)等速反应法测定。熵(s2980)和热容(Cp(T) 300≤T/K≤1500)是用理论的MP2(full)/6-31G(d,p)水平的几何参数和振动频率确定的。量子Rice-Ramsperger-Kassel (QRRK)分析用于计算与能量相关的速率常数k(E)，并使用主方程来解释碰撞稳定。二甲基醚自由基CH3OC·H2 (ΔHf298o=0.1 kcal/mol)与O2结合形成过氧自由基ch3och200·(ΔHf298o=−33.9 kcal/mol)。过氧自由基可以解离回反应物或通过氢位移异构化(Ea,rxn=17.7 kcal/mol)形成羟基烷基自由基C·H2OCH2OOH (ΔHf298o=−26.5 kcal/mol)。该烷基自由基可与甲醛(CH2O)+羟基甲基自由基(C·H2OOH)发生β-裂解反应，(Ea, rxn=24.7 kcal/mol)。氢氧甲基迅速分解成第二个CH2O + OH。CH3OCH2 +O2到2CH2O+OH的反应势垒低于反应回CH3OC·H2+O2所需的能量，为二甲基醚氧化提供了低能链传播路径。OH+CH3OCH3→CH3OC⋅H2+H2O(1)+) CH3OCH3·H2+O2→2CH2O+OH¯(2)CH3OCH3+O2→2CH2O+H2O的TS计算值与实验值的比较表明，为了与Sehested等人的数据相匹配，需要降低cs -q计算的C·H2OCH2OOH→C·H2OOH+CH2O的Ea, rxn。重要反应的速率常数为(k=A(T/ k) exp(−Ea/RT})， A单位cm3/(mol s)， Ea单位kcal/mol): k1， (2.33×1063)(T/ k)−16.89e−11.89/RT, CH3OC·H2+O2⇒ch3och200·;CH3OC·H2+O2⇒CH2O+CH2O+OH在1atm下的k3， (T/K)−5.46e−8.59/RT。

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Thermodynamic and kinetic analysis using AB initio calculations on dimethyl-ether radical+O2 reaction system

Reaction pathways and kinetics are analyzed on CH₃OC·H₂+O₂ reaction system using ab initio calculations to determine tehrmodynamic properties of reactants, intermediate radicals, and transitionstate (TS) compounds. Enthalpies of formation (ΔH_f298^o) are determined using the CBS-q//MP2(full)/6-31G(d,p) method with isodesmic reactions. Entropies (S₂₉₈^o) and heat capacities (C_p(T) 300≤T/K≤1500) are determined using geometric parameters and vibrational frequencies obtained at the MP2(full)/6-31G(d,p) level of theory. Quantum Rice-Ramsperger-Kassel (QRRK) analysis is used to calculated energy-dependent rate constants, k(E), and the master equations is used to account for collisional stabilization. The dimethyl-ether radical CH₃OC·H₂ (ΔH_f298^o=0.1 kcal/mol) adds to O₂ to form a peroxy radical CH₃OCH₂OO·(ΔH_f298^o=−33.9 kcal/mol). The peroxy radical can undergo dissociation back to reactants or isomerize via hydrogen shift (E_a,rxn=17.7 kcal/mol) to form a hydroperoxy alkyl radical C·H₂OCH₂OOH, (ΔH_f298^o=−26.5 kcal/mol). This alkyl radical can undergo β-scission reaction to formaldehyde (CH₂O)+hydroperoxy methyl radical (C·H₂OOH), (E_{a, rxn}=24.7 kcal/mol). The hydroperoxy methyl radical rapidly decomposes to a second CH₂O plus OH. The reaction barriers for CH₃OCH₂ +O₂ to 2 CH₂O+OH are lower than the energy needed for reaction back to CH₃OC·H₂+O₂, and provide a low-energy chain propagation path for dimethyl-ether oxidation. $\begin{matrix} O H + C H_{3} O C H_{3} \to C H_{3} O C \cdot H_{2} + H_{2} O (1) \\ \underline{+) C H_{3} O C \cdot H_{2} + O_{2} \to 2 C H_{2} O + O H} (2) \\ C H_{3} O C H_{3} + O_{2} \to 2 C H_{2} O + H_{2} O \end{matrix}$ Comparison of calculated falloff with experiment indicates that the CBS-q calculated E_{a, rxn} for the TS of C·H₂OCH₂OOH→C·H₂OOH+CH₂O needs to be lowered in order to match the data of Sehested et al. Rate constants of important reactions are (k=A(T/K) exp(−E_a/RT}), A in cm³/(mol s), E_A in kcal/mol): k₁, (2.33×10⁶³)(T/K)^−16.89e^−11.89/RT for CH₃OC·H₂+O₂⇒CH₃OCH₂OO·; k₃, (T/K)^−5.46e^−8.59/RT for CH₃OC·H₂+O₂⇒CH₂O+CH₂O+OH at 1 atm..gif">

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Symposium (International) on Combustion

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