Improvement the Activity and Selectivity of Fenton System in the Oxidation of Alcohols

The reactivity and selectivity of Fenton system (Fe/H 2 O 2 ) 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 Fe/H 2 O 2 /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.


Introduction
The Fenton reagent is a simple catalyst system which is composed of Fe 2+ and H 2 O 2 .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 [1][2][3][4][5][6][7][8][9][10][11][12].There are numerous reports concerning pretreatment option to improve the biodegradability of complex wastes and recalcitrant from industry wastewater and landfill leachate [2][3][4].The original pollutants are often transformed to CO 2 and H 2 O due to the strong oxidizing property of the Fenton system [4][5][6][7].In recent studies, Fenton system is coupled with other methods, such as membrane filtration and coagulation, to degrade organic compounds to a larger extent [8,9].Besides, the Fenton reagent is also effective in light alcohols and alkane's oxyfunctionalization [10,13], but the application for selective oxidation is rare [11,12].
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 [5,6,[14][15][16].To apply the Fenton system in chemical synthesis, the most crucial problem is how to control its reactivity and enhance its selectivity.The efficiency of Fenton's reagent is affected by various factors, such as pH, the counterion of Fe 2+ , solvent, iron chelation, UV, and microwaves, which control the oxidation reactivity and regeneration ability of Fe 2+ from Fe 3+ [4,[17][18][19][20].In addition, it has been found that the additives play a vital role to change the reactivity of Fenton system [13,[21][22][23].For example, the reaction with KH 2 PO 4 as additive provided high conversion and good selectivity in the oxidation of benzyl alcohol [13].
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.

Experimental
All the chemicals were obtained with purities higher than 99%.A typical experiment was carried out in a water bath.The alcohol (20 mmol), FeSO 4 ⋅7H 2 O (0.5 mmol), and NHPI (0.5 mmol) were added to a glass reactor.44 mmol H 2 O 2 (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.

The Influence of Different Additives. It has been revealed
that some additives (such as KH 2 PO 4 and KHSO 4 ) to Fenton system can lead to a more effective oxidizing process at 70 ∘ C [13].The role of KH 2 PO 4 was to control the pH of the solution which can change the rate to generate hydroxyl radicals.As a result, the selectivity of the reaction was enhanced [13].Encouraged by this idea, we have tried to further improve the reaction by adding different inorganic buffers in Fenton system.The influence of different additives (NaBF 4 , NaH 2 PO 4 , KH 2 PO 4 , and KHP) at 70 ∘ C was studied (Table 1).Our results with KH 2 PO 4 as additive were very close to those of a previous report [13].It seems that the adding of NaBF 4 , NaH 2 PO 4 , or KHP cannot give better results.
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 2, entries 1-10).Though good selectivity can be obtained, the conversion is quite low.Increasing reaction time or changing the amount of additive could not improve the results at all.

NHPI/FeSO
4 Oxidation Process.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

Oil
Water O H Figure 1: Proposed mechanism for the oxidation of benzyl alcohol using NHPI/Fenton system.
of Fenton system was carefully analyzed.The whole radical reaction process of Fenton system is shown as follows [28][29][30]: 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 [31].Therefore, we believed that there should be at least two ways to improve reaction selectivity of Fenton system.One is to control the generation rate of hydroxyl radicals, and the other is to reduce the accumulation of nonselective hydroxyl radicals and convert them to radicals with higher selectivity, such as phthalimide N-oxyl (PINO), which has been widely used in oxidation reaction [32][33][34][35].Adding inorganic buffers can change the pH of reaction mixture and control the generation rate of radicals.This is the first way mentioned above and it has been widely investigated [13,[18][19][20][21][22][23].Selectivity can be enhanced by this way, but the catalytic activity of this system is quite low at room temperature (Table 2).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 1).N-Hydroxyphthalimide (NHPI) is an excellent choice under these two restrictions, and PINO radical can be generated from NHPI, which has been proven to be catalyst with high oxidation selectivity [32][33][34][35].Then, the effect of NHPI as additive was studied.Indeed, the result is satisfying as expected.Both the selectivity and conversion are obviously improved by adding NHPI (Table 3, entries 1∼3).TEMPO (2,2,6,6-tetramethylpiperidinooxy) and 4-OH-TEMPO (4-hydroxy-TEMPO) also gave better conversion and selectivity comparing with those using inorganic buffers.However, the conversion with TEMPO or 4-OH-TEMPO is lower than that with NHPI.The reactivities of different iron salts were evaluated in the oxidation of benzyl alcohol (Table 3).Obviously, FeSO 4 ⋅7H 2 O cooperating with NHPI gives the highest selectivity in comparison with Fe 2 (SO 4 ) 3 , Fe(NO 3 ) 3 ⋅9H 2 O, and FeCl 3 .

Oxidation of Several Alcohols.
To investigate the universality of NHPI/FeSO 4 /H 2 O 2 system, oxidations of different alcohols were performed (Table 4).Most of these oxidations gave good results.Though the result of benzyl alcohol is satisfying at 25 ∘ C, the converting rates of 4-isopropyl benzyl alcohol and -phenylethyl alcohol are a little low at 25 ∘ C, which may be caused by the replacement of electron donating groups.The conversion of -phenylethyl alcohol achieved 50.5% at 55 ∘ C with excellent selectivity.It should be noted that ortho substituent and bulky groups may also reduce the reaction rate (Table 4, entries 5 and 6).4-Methoxybenzyl alcohol and 4-methylbenzyl alcohol were tested at 55 ∘ C because of the high melting points, and the results were fairly satisfactory (Table 4, entries 7∼8).However, the selectivity of the 2-butanol oxidation is a little poor with this catalyst system.

Conclusion
To improve the selectivity of Fenton reagent (Fe 2+ /H 2 O 2 ) 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 Fe 2+ /H 2 O 2 /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.

Table 1 :
Effect of additives for the oxidation of benzyl alcohol to benzaldehyde at 70 ∘ C a .20 mmol of benzyl alcohol (2.16 g), 2.2 equiv. of H 2 O 2 (44 mmol, 4.5 mL, 30 wt% in water), reaction time: 0.5 h.H 2 O 2 was added with a syringe pump by 0.2 mL/min speed.b Amount of catalyst.c Total selectivity to benzaldehyde and benzoic acid.Value in parentheses means the ratio of benzaldehyde/benzoic acid.
a d KHP: potassium hydrogen phthalate.

Table 2 :
Effect of additives for the oxidation of benzyl alcohol to benzaldehyde at 25 ∘ C a .Fe salt, and speed of adding H 2 O 2 ) have been optimized.Limited by the length, these results were listed in the Supplementary data (FiguresS1 and S2and TablesS1-S2) (see Supplementary Material available online at http://dx.doi.org/10.1155/2014/823054).
a 20 mmol of benzyl alcohol, 2.2 equiv. of H 2 O 2 (30 wt% in water).H 2 O 2 was added with a syringe pump by 0.2 mL/min speed.bAmount of catalyst.cTotal selectivity to benzaldehyde and benzoic acid.Value in parentheses means the ratio of benzaldehyde/benzoic acid.Different reaction conditions (temperature, concentration,

Table 3 :
Oxidation of benzyl alcohol with different catalyst systems at 25 ∘ C a .20 mmol of benzyl alcohol, 2.2 equiv. of H 2 O 2 (30 wt% in water), reaction time: 0.5 h.H 2 O 2 was added with a syringe pump by 0.2 mL/min speed.b Amount of catalyst.c Total selectivity to benzaldehyde and benzoic acid.Value in parentheses means the ratio of benzaldehyde/benzoic acid. a

Table 4 :
Oxidation of several alcohols a .