The reactivity and selectivity of Fenton system (Fe2+/H2O2) were improved with N-hydroxyphthalimide (NHPI) as cocatalyst. The oxidation process of benzyl alcohol to benzaldehyde has been studied. The reaction catalyzed by this new Fe2+/H2O2/NHPI system can be well performed under room temperature without adding any organic solvent. Besides, this catalyst system is effective for the oxidation of different alcohols.
The Fenton reagent is a simple catalyst system which is composed of Fe2+ and H2O2. In the reaction process with Fenton’s reagent, the hydroxyl radicals are generated rapidly at the presence of ferrous ion. This catalyst system is proved to be successful for the treatment of organic pollutants in industrial wastewater [
The Fenton system has wonderful water solubility and there are abundant ferrous irons on the earth. Thus, it is possible to develop a mild and inexpensive water phase oxidation process using this system. Despite its numerous advantages, one limitation of traditional Fenton’s reagent must be taken into account: its high oxygen reactivity, which often leads to deep oxidation [
The objective of this study is to control the reactivity of Fenton system and enhance its selectivity. Based on a detailed investigation on a series of additives, we have found that the reactivity of Fenton system can be well controlled and the benzyl alcohol oxidation can be performed with good selectivity at room temperature in water.
All the chemicals were obtained with purities higher than 99%. A typical experiment was carried out in a water bath. The alcohol (20 mmol), FeSO4·7H2O (0.5 mmol), and NHPI (0.5 mmol) were added to a glass reactor. 44 mmol H2O2 (30 wt% in water, 4.5 mL) was added with a syringe pump by 0.2 mL/min speed in 30 min. The reaction mixture was vigorously stirred (about 750 rpm). Reaction mixtures were analyzed using GC and GC-MS.
Different reaction conditions (temperature, concentration, Fe salt, and speed of adding H2O2) have been optimized. Limited by the length, these results were listed in the Supplementary data (Figures S1 and S2 and Tables S1-S2) (see Supplementary Material available online at
It has been revealed that some additives (such as KH2PO4 and KHSO4) to Fenton system can lead to a more effective oxidizing process at 70°C [
Effect of additives for the oxidation of benzyl alcohol to benzaldehyde at 70°Ca.
Entry | Catalyst | Amountb (mol%) | Conv. (%) | Sel.c (%) |
---|---|---|---|---|
1 | FeSO4/NaBF4 | 2.5%/1% | 66.6 | 94.5 (89 : 11) |
2 | 2.5%/10% | 71.0 | 94.3 (88 : 12) | |
3 | FeSO4/NaH2PO4 | 2.5%/1% | 74.7 | 96.0 (87 : 13) |
4 | 2.5%/5% | 2.0 | 96.5 (87 : 13) | |
5 | 2.5%/10% | 1.6 | 93.3 (93 : 7) | |
6 | FeSO4/KH2PO4 | 2.5%/1% | 64.1 | 95.9 (90 : 10) |
7 | 2.5%/2.5% | 4.4 | 95.1 (92 : 8) | |
8 | 2.5%/5% | 1.9 | 89.2 (88 : 12) | |
9 | FeSO4/KHPd | 2.5%/1% | 51.3 | 94.7 (82 : 18) |
10 | 2.5%/2.5% | 32.8 | 86.7 (92 : 8) |
Performing reaction at room temperature is beneficial to save energy and develop safe process. The performance of these inorganic buffers was investigated at 25°C. Unfortunately, the reactivity of these catalyst systems is quite poor at room temperature (Table
Effect of additives for the oxidation of benzyl alcohol to benzaldehyde at 25°Ca.
Entry | Catalyst | Amountb (mol%) | Time (h) | Conv. (%) | Sel.c (%) |
---|---|---|---|---|---|
1 | FeSO4/NaH2PO4 | 2.5%/1% | 0.5 | 3.8 | 95.9 (91 : 9) |
2 | 2 | 8.6 | 95.1 (90 : 10) | ||
3 | 4 | 9.5 | 91.2 (90 : 10) | ||
4 | FeSO4/NaH2PO4 | 2.5%/2.5% | 0.5 | 1.1 | 98.2 (92 : 8) |
5 | 2 | 1.9 | 98.2 (93 : 7) | ||
6 | FeSO4/NaH2PO4 | 2.5%/10% | 0.5 | 1.1 | 98.8 (92 : 8) |
7 | 19 | 1.3 | 98.9 (91 : 9) | ||
8 | FeSO4/KBr | 2.5%/10% | 0.5 | 3.4 | 100 (89 : 11) |
9 | FeSO4/NaBF4 | 2.5%/10% | 0.5 | 1.7 | >99.9 (100 : 0) |
10 | FeSO4/KH2PO4 | 2.5%/2.5% | 0.5 | 1.2 | 98.3 (93 : 7) |
11 | FeSO4/N1111Gly | 2.5%/2.5% | 0.5 | 20.6 | 91.7 (96 : 4) |
12 | FeSO4/N2111OAc | 2.5%/2.5% | 0.5 | 25.7 | 98.4 (87 : 13) |
13 | FeSO4/[HMIM][HSO4] | 2.5%/10% | 0.5 | 1.3 | >99.9 (100 : 0) |
Though ionic liquids have shown attractive role in improving the reactivity of oxidation system [
How to improve the selectivity of Fenton system and maintain its reactivity at the same time are a challenge. To solve this problem and find out an excellent additive, the classical free radical mechanism of Fenton system was carefully analyzed. The whole radical reaction process of Fenton system is shown as follows [
Hydroxyl radical was generated in water phase by a series of complex reactions; then hydroxyl radical can abstract a hydrogen atom from alcohols to form alkyl radicals. If large numbers of hydroxyl radicals accumulate without control, this may cause deep and nonselective oxidation [
So far, there is no report about the second way. If a method can be developed to achieve the second way, the selectivity should be improved without losing catalytic activity. There are two keys to successfully develop this method: first, the additive should mainly exist in the organic phase, which can avoid possible influence on the Fenton system induced by the additive; second, the additive can react with hydroxyl radicals to generate other radicals with high oxidation selectivity quickly (Figure
Oxidation of benzyl alcohol with different catalyst systems at 25°Ca.
Entry | Catalyst | Amountb (mol%) | Conv. (%) | Sel.c (%) |
---|---|---|---|---|
1 | FeSO4/NHPI | 2.5%/2.5% | 36.7 | 99.1 (94 : 6) |
2 | 2.5%/1.25% | 36.0 | 98.2 (92 : 8) | |
3 | 2.5%/5.0% | 31.8 | 95.9 (90 : 10) | |
4 | FeSO4/TEMPO | 2.5%/2.5% | 20.5 | 98.9 (96 : 4) |
5 | FeSO4/4-OH-TEMPO | 2.5%/2.5% | 13.9 | 99.6 (99 : 1) |
6 | Fe2(SO4)3/NHPI | 2.5%/2.5% | 32.1 | 95.2 (87 : 13) |
7 | Fe(NO3)3/NHPI | 2.5%/2.5% | 59.9 | 94.2 (82 : 18) |
8 | FeCl3/NHPI | 2.5%/2.5% | 43.3 | 87.6 (84 : 16) |
Proposed mechanism for the oxidation of benzyl alcohol using NHPI/Fenton system.
To investigate the universality of NHPI/FeSO4/H2O2 system, oxidations of different alcohols were performed (Table
Oxidation of several alcoholsa.
Entry | Substrate | Temp. (°C) | Product | Con. (%) | Sel. (%) |
---|---|---|---|---|---|
1 |
|
25 |
|
36.7 | 93.6 |
2 |
|
25 |
|
4.3 | >99.9 |
3 |
|
25 |
|
12.3 | >99.9 |
4 |
|
55 |
|
50.5 | >99.9 |
5 |
|
55 |
|
4.8 | >99.9 |
6 |
|
55 |
|
13.3 | 90.0 |
7 |
|
55 |
|
49.3 | 97.5 |
8 |
|
55 |
|
41.1 | 96.0 |
9 |
|
55 |
|
54.8 | 82.5 |
To improve the selectivity of Fenton reagent (Fe2+/H2O2) in oxidation reaction, the cooperation between Fenton reagent and a series of additives has been investigated. It was found that the reactivity and selectivity of Fenton system were improved with NHPI as cocatalyst. The oxidation of benzyl alcohol catalyzed by this new Fe2+/H2O2/NHPI system can be well performed under room temperature without adding any organic solvent. Besides, this catalyst system is effective for the oxidation of different alcohols. We hope the catalyst systems developed here provide a new way for a green, innoxious, and cheap candidate for the oxidation of alcohols and similar compounds.
The authors declare that they have no conflict of interests regarding the publication of this paper.
This work was supported by the National Natural Science Foundation of China (no. 21176110).