体系对醇氧化活性和选择性的改善外文翻译资料

 2022-05-01 21:42:05

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

Guoqiang Yang, Qiuxing Lin, Xingbang Hu, Youting Wu, and Zhibing Zhang

School of Chemistry and Chemical Engineering and National Engineering Research Center for Organic Pollution Control and Resource, Nanjing University, Nanjing 210093, China

Published 24 March 2014

Academic Editor: David W. Mazyck

Copyright copy; 2014 Guoqiang Yang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

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.

1. Introduction

The Fenton reagent is a simple catalyst system which is composed of Fe2 and H2O2. In the reaction process with Fentonrsquo;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 . There are numerous reports concerning pretreatment option to improve the biodegradability of complex wastes and recalcitrant from industry wastewater and landfill leachate. The original pollutants are often transformed to CO2 and H2O due to the strong oxidizing property of the Fenton system. In recent studies, Fenton system is coupled with other methods, such as membrane filtration and coagulation, to degrade organic compounds to a larger extent. Besides, the Fenton reagent is also effective in light alcohols and alkanersquo;s oxyfunctionalization, but the application for selective oxidation is rare.

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 Fentonrsquo;s reagent must be taken into account: its high oxygen reactivity, which often leads to deep oxidation. 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 Fentonrsquo;s reagent is affected by various factors, such as pH, the counterion of Fe2 , solvent, iron chelation, UV, and microwaves, which control the oxidation reactivity and regeneration ability of Fe2 from Fe3 . In addition, it has been found that the additives play a vital role to change the reactivity of Fenton system. For example, the reaction with KH2PO4 as additive provided high conversion and good selectivity in the oxidation of benzyl alcohol.

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.

2. Experimental

All the chemicals were obtained with purities higher than 99%. A typical experiment was carried out in a water bath. The alcohol (20thinsp;mmol), FeSO4·7H2O (0.5thinsp;mmol), and NHPI (0.5thinsp;mmol) were added to a glass reactor. 44thinsp;mmol H2O2 (30thinsp;wt% in water, 4.5thinsp;mL) was added with a syringe pump by 0.2thinsp;mL/min speed in 30thinsp;min. The reaction mixture was vigorously stirred (about 750thinsp;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.

3. Results and Discussion

3.1. The Influence of Different Additives

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. The role of KH2PO4 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. 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 (NaBF4, NaH2PO4, KH2PO4, and KHP) at 70°C was studied. Our results with KH2PO4 as additive were very close to those of a previous report. It seems that the adding of NaBF4, NaH2PO4, 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. 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. Though ionic liquids have shown attractive role in improving the reactivity of oxidation system, it se

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2.实验摘要

以N-羟基邻苯二甲酰亚胺（NHPI）为助催化剂，Fenton体系（Fe2 / H2O2）的反应活性和选择性得到提高。已经研究了苯甲醇到苯甲醛的氧化过程。这种新的Fe2 /H2O2/NHPI体系催化的反应可以在室温下良好地进行，无需添加任何有机溶剂。此外，该催化剂体系对于不同醇的氧化是有效的。

  1. 介绍

Fenton试剂是一种由Fe2 和H2O2组成的简单催化剂体系。在使用芬顿试剂的反应过程中，在亚铁离子存在下迅速生成羟基自由基。该催化剂体系在工业废水中处理有机污染物方面被证明是成功的。有许多关于提高工业废水和垃圾渗滤液中复杂废物和顽固废物生物降解性的预处理方法的报道。由于Fenton体系的强氧化性，原来的污染物经常转化为CO2和H2O。在最近的研究中，Fenton系统与膜过滤和凝结等其他方法结合使用，可以更大程度地降解有机化合物。此外，芬顿试剂在轻醇和烷烃的氧化官能化方面也是有效的，但是选择性氧化的应用很少见。

Fenton体系具有极好的水溶性，地球上有丰富的亚铁。因此，可以开发使用该系统的温和且廉价的水相氧化工艺。尽管有许多优点，但必须考虑到传统芬顿试剂的一个局限性：其高氧反应性常常导致深度氧化。为了在化学合成中应用Fenton体系，最关键的问题是如何控制其反应性并提高其选择性。芬顿试剂的效率受pH值，Fe2 的抗衡离子、溶剂、铁螯合、紫外线和微波等因素的影响，控制Fe3 对Fe2 的氧化反应和再生能力。此外，已经发现添加剂对改变Fenton体系的反应性起着至关重要的作用。例如，与KH2PO4作为添加剂的反应提供了高转化率和良好的苯甲醇氧化选择性。

本研究的目的是控制Fenton体系的反应性并提高其选择性。在对一系列添加剂进行详细研究的基础上，我们发现Fenton体系的反应活性可以得到很好的控制，并且苯甲醇氧化可以在室温下在水中具有良好的选择性。

所有这些化学品的纯度均高于99％。典型的实验是在水浴中进行的。将醇（20mmol），FeSO4·7H 2 O（0.5mmol）和NHPI（0.5mmol）加入到玻璃反应器中。 在30分钟内用注射泵以0.2mL/分钟的速度加入44mmol H2O2（30wt％在水中，4.5mL）。剧烈搅拌反应混合物（约750rpm）。使用GC和GC-MS分析反应混合物。

不同的反应条件（温度、浓度、Fe盐和加入H2O2的速度）已经被优化。

3.结果与讨论

3.1不同添加剂的影响

已经揭示了芬顿体系中的一些添加剂（如KH2PO4和KHSO4）可以导致在70℃下更有效的氧化过程。 KH2PO4的作用是控制可以改变产生羟基自由基速率的溶液的pH值。结果，反应的选择性增强。受到这一想法的鼓舞，我们试图通过在Fenton体系中添加不同的无机缓冲剂来进一步改善反应。研究了不同添加剂（NaBF4，NaH2PO4，KH2PO4和KHP）在70℃下的影响。我们用KH2PO4作为添加剂的结果与之前的报告非常接近。似乎添加NaBF4，NaH2PO4或KHP不能产生更好的结果。

在室温下进行反应有利于节能和开发安全过程。在25℃下研究了这些无机缓冲液的性能。不幸的是，这些催化剂体系在室温下的反应性很差。虽然可以获得很好的选择性，但转换率很低。增加反应时间或改变添加剂的量不能改善结果。尽管离子液体在改善氧化体系的反应性方面显示出有吸引力的作用，但似乎该化合物不能与Fenton体系良好配合。[HMIM][HSO4]（己基甲基咪唑硫酸氢盐）作为添加剂的转化在25℃下仅为1.3％。添加N1111Gly（甘氨酸四甲基铵）或N2111OAc（乙酸三甲基乙基铵）可加速反应，但选择性也不太好。

3.2 NHPI/FeSO4氧化工艺

通过一系列复杂的反应在水相中产生羟基自由基;那么羟基自由基可以从醇中提取氢原子形成烷基自由基。如果大量的羟基自由基在没有控制的情况下积累，这可能会导致深度和非选择性氧化。因此，我们认为应该至少有两种方法来提高Fenton体系的反应选择性。一个是控制羟基自由基的生成速率，另一个是减少非选择性羟基自由基的积累并将其转化为具有较高选择性的自由基，如已广泛用于氧化反应的邻苯二甲酰亚胺N-氧基（PINO）。加入无机缓冲剂可改变反应混合物的pH值并控制自由基的生成速率。这是上面提到的第一种方式，它已被广泛研究。通过这种方式可以提高选择性，但是该系统在室温下的催化活性很低。

到目前为止，还没有关于第二种方式的报告。如果可以开发一种方法来实现第二种方法，则应该改进选择性而不失去催化活性。成功开发这种方法有两个关键：一是添加剂主要存在于有机相中，可以避免添加剂对Fenton体系的影响；其次，添加剂可以与羟基自由基反应，快速生成具有高氧化选择性的其他自由基。在这两个限制条件下，N-羟基邻苯二甲酰亚胺（NHPI）是一个很好的选择，并且可以从NHPI产生PINO自由基，NHPI已被证明是具有高氧化选择性的催化剂。然后，研究了NHPI作为添加剂的效果。事实上，结果如预期般令人满意。通过添加NHPI，选择性和转化率都得到明显改善。TEMPO（2,2,6,6-四甲基哌啶氧基）和4-OH-TEMPO（4-羟基-TEMPO）与使用无机缓冲剂相比也具有更好的转化率和选择性。然而，TEMPO或4-OH-TEMPO的转化率低于NHPI。在苯甲醇的氧化中评价不同铁盐的反应性。显然，FeSO4·7H2O与NHPI配合使用，与Fe2(SO4)3，Fe(NO3)3·9H2O和FeCl3相比，具有最高的选择性。

3.3 几种醇的氧化

为了研究NHPI/FeSO4/H2O2体系的普遍性，进行了不同醇的氧化。大部分这些氧化都有很好的效果。尽管在25℃下苯甲醇的结果令人满意，但在25℃下4-异丙基苄醇和alpha;-苯乙醇的转化率稍低，这可能是由于供电子基团的置换引起的。 alpha;-苯乙醇的转化率在55℃达到50.5％，具有极好的选择性。应该指出，邻位取代基和庞大的基团也可能降低反应速率。由于熔点高，在55℃下测试4-甲氧基苄醇和4-甲基苄醇，结果相当令人满意。但是，该催化剂体系对2-丁醇氧化的选择性稍差。

4.结论

为了提高Fenton试剂（Fe2 / H2O2）在氧化反应中的选择性，研究了Fenton试剂与一系列添加剂的协同作用。发现用NHPI作为助催化剂可提高Fenton体系的反应活性和选择性。该新型Fe2 /H2O2/NHPI体系催化苯甲醇的氧化反应可在室温下良好进行，无需添加任何有机溶剂。此外，该催化剂体系对于不同醇的氧化是有效的。我们希望在这里开发的催化剂体系为醇和类似化合物的氧化提供了一种绿色，无毒和廉价的候选物的新方法。

参考文献

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