苯甲醛在光催化反应中氧化还原选择性的理论研究

doi:10.7503/cjcu20150719

摘要/Abstract

摘要：

采用密度泛函理论方法在M06-2X/6-311G^*水平上模拟了不同反应条件下, TiO₂对苯甲醛的光催化还原和氧化的反应. 计算结果表明, 苯甲醛的光催化还原和氧化反应均可在常温下发生; 在缺氧但有乙醇存在的条件下, 乙醇分子可与氧化性物质发生反应, 生成醇自由基, 苯甲醛主要发生光催化还原反应生成苯甲醇; 在有氧气但无乙醇存在条件下, 还原性的光生电子被氧气捕获, 避免了苯甲醛被还原, 主要发生光催化氧化反应生成苯甲酸.

关键词: 苯甲醛, 光催化, 选择性氧化还原机理, 密度泛函理论

Abstract:

The photoelectron and photohole could be generated on the surface of TiO₂ under the UV irradiation. Some reactive species could be produced indirectly. The photoelectron could be trapped by oxygen leading to yield the superoxide anion radicals, while the photohole can react with the solvent molecules to generate the hydroxyl radical and alcohol radical. The substrate may be reduced by the photoelectron directly, or by alcohol radicals. And it may be oxidized by the photohole directly, or by the reactive species of hydroxyl radicals and superoxide anion radicals. The M06-2X/6-311G^* method was employed to investigate the selective redox mechanism of benzaldehyde in solution, which was reduced or oxidized by the reactive species generated during the photo-catalyzed process in different reaction conditions. According to the computation results, the photo-redox reaction of benzaldehyde would be happened in room temperature. In oxygen-free ethanol solvent, the ethanol molecules could react with the oxidizing species to yield the alcohol radicals, while benzaldehyde could be mainly reduced to benzyl alcohol. In oxygen-rich without ethanol condition, the reductive photoelectron is trapped by oxygen to prohibit the reduction of benzaldehyde, so benzaldehyde is mainly oxidized to benzoic acid.

Key words: Benzaldehyde, Photocatalysis, Mechanism of selective redox reaction, Density functional theory

中图分类号:

O644.14

TrendMD:

黄晓, 甘汉麟, 彭亮, 顾凤龙. 苯甲醛在光催化反应中氧化还原选择性的理论研究. 高等学校化学学报, 2016, 37(2): 297.

HUANG Xiao, GAN Hanlin, PENG Liang, GU Fenglong. Theoretical Study on the Selective Redox Mechanism of Benzaldehyde in Photo-catalyzed Reaction^†. Chem. J. Chinese Universities, 2016, 37(2): 297.

图/表 12

Scheme 1 Possible reaction mechanism of benzaldehyde being reduced to benzyl alcohol photo-catalyzed by TiO2

Fig.1 Gibbs free energy profile for the reactions between ·OH and ethanol replete with optimized structures Bond distances are in nm.

Fig.2 Gibbs free energy profile for the reactions between C2H5O· or ·CH2CH2OH and ethanol replete with optimized structures Bond distances are in nm.

Fig.3 Gibbs free energy profile for the process from BHA to ·CO(Ph) replete with optimized structures Bond distances are in nm.

Scheme 2 Possible reaction mechanism of BHA being reduced to BA in ethanol The solid lines represent the dominant pathways.

Fig.4 Gibbs free energy profile for the process from BHA to IM2 replete with optimized structures Bond distances are in nm.

Fig.5 Gibbs free energy profile for the process from IM2 to BA replete with optimized structures Bond distances are in nm.

Scheme 3 Possible reaction mechanism of BHA being oxidized to BAC in water The solid lines represent the dominant pathways.

Fig.6 Gibbs free energy profile for the process from BHA to IM4 replete with optimized structures Bond distances are in nm.

Fig.7 Gibbs free energy profile for the process from BHA to IM5 replete with optimized structures Bond distances are in nm.

Table 1 Relative Gibbs free energies for the processes from IM4 and IM5 to BAC

Species	G/a.u.	ΔG/a.u.	ΔG/(kJ·mol^-1)	Species	G/a.u.	ΔG/a.u.	ΔG/(kJ·mol^-1)
IM4+^·OH	-496.905943	0	0	IM5+^·OH	-420.519329	0	0
BAC+H₂O	-497.083805	-0.177862	-466.98	BAC	-420.680582	-0.161253	-423.37

Table 2 Relative Gibbs free energies for the processes from O2 and BHA to O2·- and IM1 in different solvents

Species	G/a.u.	ΔG/a.u.	ΔG/(kJ·mol^-1)	Species	G/a.u.	ΔG/a.u.	ΔG/(kJ·mol^-1)
O₂(in ethanol)	-150.321004	0	0	BHA(in ethanol)	-345.433966	0	0
$O 2 · -$ (in ethanol)	-150.439021	-0.118017	-309.85	IM1(in ethanol)	-345.522669	-0.088703	-232.89
O₂(in water)	-150.320706	0	0	BHA(in water)	-345.429490	0	0
$O 2 · -$ (in water)	-150.447154	-0.126448	-331.99	IM1(in water)	-345.522900	-0.093410	-245.25

Table 2 Relative Gibbs free energies for the processes from O2 and BHA to O2·- and IM1 in different solvents

Species	G/a.u.	ΔG/a.u.	ΔG/(kJ·mol^-1)	Species	G/a.u.	ΔG/a.u.	ΔG/(kJ·mol^-1)
O₂(in ethanol)	-150.321004	0	0	BHA(in ethanol)	-345.433966	0	0
$O 2 · -$ (in ethanol)	-150.439021	-0.118017	-309.85	IM1(in ethanol)	-345.522669	-0.088703	-232.89
O₂(in water)	-150.320706	0	0	BHA(in water)	-345.429490	0	0
$O 2 · -$ (in water)	-150.447154	-0.126448	-331.99	IM1(in water)	-345.522900	-0.093410	-245.25

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