A DFT study on the reaction pathway for the oxidation of C6H2(OH)3CH3 to hydroxyl benzoic acid
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摘要: 运用密度泛函（DFT）理论，采用Materials Studio 8.0，用GGA/BP方法研究了C6H2（OH）3CH3氧化成羟基苯甲酸的反应路径。结果表明，甲基上的氢原子被氧化成羟基以及羟基被氧化为醛基及醛基被氧化成羧基均为放热过程。分子C6H2（OH）3CH3中甲基氧化成羧基的主路径为三个氢原子氧化反应路径，其路径为C6H2（OH）3CH3+3O→C6H2（OH）3C（OH）3→C6H2（OH）3COOH+H2O，该路径受限于羟基直接被氧化成羧基过程，需克服130 kJ/mol的反应势垒，反应速率常数对数ln（k）为-22.96 s-1；醛基、羟基优先被氧化成羧基的顺序为：-CHO > -C（OH）3 > -HC（OH）2 > -H2C（OH）；提高反应温度、氧气浓度均有利于羟基苯甲酸的生成，适当的催化剂有利于促进整个反应的进行。Abstract: The reaction pathways for the oxidation of C6H2(OH)3CH3 oxidizing into hydroxyl benzoic acid were investigated by using density functional theory (DFT) method at the GGA/BP levels with Materials Studio 8.0 program. The results illustrated that the reactions for the oxidation of hydrogen on the methyl into hydroxyl, the hydroxyl to aldehyde, and then the aldehyde to carboxylic are all exothermic. As the main path, the oxidation of C6H2(OH)3CH3 to hydroxyl benzoic acid follows:C6H2(OH)3CH3+3O → C6H2(OH)3C(OH)3 → C6H2(OH)3COOH+H2O; as the controlling step, the conversion of hydroxyl to carboxyl exhibits a high energy barrier (130 kJ/mol) and a low reaction rate (ln(k)=-22.96 s-1). The oxidation of hydroxyl and aldehyde to carboxylic acid follows the sequence of -CHO > -C(OH)3 > -HC(OH)2 > -H2C(OH). An increase in the temperature and oxygen concentration is beneficial to the formation of hydroxyl benzoic acid, whereas appropriate catalyst can promote the whole reaction process.
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图 6 甲烷氧化成水和二氧化碳的反应过程示意图
Figure 6 Reaction process for the oxidation of methane to water and carbon dioxide
图 7 甲基上单个氢原子被氧化成羟基过程中键长的变化
Figure 7 Changes of bond length during the oxidation hydrogen in methyl to hydroxyl
图 8 羟基被氧化成醛基过程中键长的变化
Figure 8 Changes of bond length during the further oxidation of hydroxyl to aldehyde
图 9 醛基被氧化成羧基过程中键长的变化
Figure 9 Changes of bond length during the oxidation of aldehyde to carboxyl
图 10 单个氢原子氧化反应的反应势垒
Figure 10 Energy barrier of a single hydrogen atom oxidation reaction
图 11 甲基上两个氢原子先后被氧化成羟基、醛基过程中键长的变化
Figure 11 Changes of bond length during the oxidation of two hydrogen atoms into hydroxyl and aldehyde
图 12 醛基被氧化成羧基过程中键长的变化
Figure 12 Changes of bond length during further oxidation of aldehyde to carboxyl
图 14 甲基上三个氢原子先后被氧化成羟基、羧基过程中键长的变化
Figure 14 Changes of bond length during the oxidation of three hydrogen atoms to hydroxyl and carboxyl
图 15 三个氢原子氧化反应的反应势垒
Figure 15 Energy barrier of three hydrogen atom oxidation reaction
表 1 反应物、中间体、过渡态及生成物的总能量，过渡态频率、反应能量、反应势垒及反应速率常数
Table 1 Total energies of reactants, products, intermediate, and transition states; frequency of transition states; reaction energies, energy barrier, and reaction rate constant
Total energies
E/HaFrequency of transition
states /cm-1Reaction energies
Ee/(kJ·mol-1)Energy barrier
Eb/(kJ·mol-1)Reaction rate constant
ln(k)/s-1IM0 -572.4485192 TS0-1 -572.4476524 -124.77 -452.010 2.276 28.539 IM1 -572.6206809 IM1A -572.6206558 TS1A-2 -572.4952856 -1526.41 -9.791 142.352 -27.970 IM2 -572.5993533 IM2A -646.7025609 TS2A-3 -646.6834768 -647.04 -2.402 50.103 9.245 IM3 -646.7034749 
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表 2 反应物、中间体、过渡态及生成物的总能量、过渡态频率、反应能量、反应势垒及反应速率常数
Table 2 Total energies of reactants, products, intermediate, and transition states; frequency of transition states; reaction energies, energy barrier, and reaction rate constant
Total energies
E/HaFrequency of transition
states /cm-1Reaction energies
Ee/(kJ·mol-1)Energy barrier
Eb/(kJ·mol-1)Reaction rate constant
ln(k) /s-1IM0 -647.8643879 TS0-1 -647.6631287 -1381.09 -33.962 528.406 -183.711 IM1 -647.8773231 TS1-2 -647.8250685 -1574.66 -25.383 137.193 -25.889 IM2 -647.8869909 IM2A -646.7025609 TS2A-3 -646.6834768 -647.04 -2.402 50.103 9.245 IM3 -646.7034749
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表 3 反应物、中间体、过渡态及生成物的总能量、过渡态频率、反应能量、反应势垒及反应速率常数
Table 3 Total energies of reactants, products, intermediate, and transition states, frequency of transition states, reaction energies, energy barrier, and reaction rate constant
Total energies
E/HaFrequency of transition
states /cm-1Reaction energies
Ee/(kJ·mol-1)Energy barrier
Eb/(kJ·mol-1)Reaction rate constant
ln(k) /s-1IM0 -723.1418687 TS0-1 -723.1363946 -104.99 -2.774 14.372 23.659 IM1 -723.1429255 TS1-2 -723.0934644 -1327.50 -62.560 129.859 -22.930 IM2 -723.1667535
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