Electrochemical reduction of p-chloronitrobenzene (p-CNB) at silver cathode in dimethylformamide
Graphical abstract
Introduction
Chloronitrobenzenes (CNBs), as a group of important raw materials, are extensively used for the production of fine chemicals, polymers, pharmaceuticals, dyes and agrochemicals [1,2]. The large-scale production and uses, as well as the improper disposal of these compounds inevitably make them enter into the environment and contaminate the various water bodies and soil [3,4]. On another hand, due to the potential toxicity of CNBs that can cause cancer and other harmful effects on central nervous system, such as producing fatigue, headache, vertigo, vomiting, general weakness, unconsciousness and even coma [5,6], these compounds are typically classified as environmentally undesired and, have been ranked with priority controlled pollutants by many countries [4,7]. Therefore, effective strategies are strongly required to treat and/or recycle these refractory substances prior to direct discharge.
Many attempts for treating CNBs containing wastewater based on the physical, chemical and biological processes have been made over the past decades [[8], [9], [10], [11], [12], [13], [14], [15], [16]]. These methods can generally be divided into adsorption [8,9], advanced oxidation processes such as photocatalytic oxidation [10], ozonation [11,12], Fenton reaction [13] and electrochemical oxidation [14], catalytic reduction [2] and biodegradation [15,16], as well as the hybrid processes between them like the bioelectrochemical system [4]. Whereas excellent removal performances for CNBs are often achieved, the above-mentioned approaches do suffer from drawbacks of recycling, costly reagents and noble metal usage, the risk of secondary pollution, and catalyst stability and deactivation problems. Moreover, due to the presence of strong electron-withdrawing groups of chloro and nitro on the aromatic ring, it makes the oxidation processes energy intensive and costly. In contrast, the chloro- and nitro-substituents favor the reduction reaction thermodynamically, resulting in the enhanced removal performances of CNBs. In our previous work, we showed that CNBs can be effectively reduced by the iron-based bimetals, where catalytic reduction and dechlorination are readily to occur with the ultimate aniline formation [3]. Nevertheless, the issues on the catalyst loss and deactivation cannot be circumvented and still need to be carefully investigated. Fortunately, electrochemical reduction possesses the capacity to solve the perceived drawbacks encountered by catalytic reduction methods, where the metal corrosion and surface passivation problems could be effectively solved by cathodic polarization. In some cases, the electrolyses of organic chlorides exhibited much higher dechlorination rates than in reactions with zero valent metal/catalyst [17].
As opposed to the treatment of CNBs containing wastewater, which to some extent results in the waste of materials, the recycle purpose, if possible, should always be firstly considered and advocated both from environmental and sustainable point of views. As aforementioned, CNBs are basic raw materials in manufacturing many important chemical products. In fact, the selective production of chloroanilines (CANs) or azo-benzene compounds from CNBs is a key step in those synthesis processes. For instance, CANs are very useful chemical intermediates in the production of analgesic, dyes, herbicides, and antipyretic drugs [18]; dichloroazobenzene and its derivatives are widely used as intermediates in the pharmaceutical and dyes industries, in particular, this type of dyes occupies a quarter of the total organic dyes [19]. The common method for production of CANs and azo compounds is catalytic hydrogenation of CNBs, although the end product is totally different depending on the nature of catalyst. Extensive investigations have been performed in the last decades with the aim of increasing the catalyst selectivity and stability [[20], [21], [22], [23], [24], [25], [26], [27]]. High selectivity of CANs has been achieved over a range of transition metal catalysts, such as Ru, Rh, Ni, Cu, Ag, Au and Pd [[20], [21], [22], [23], [24], [25], [26], [27]]. In recent years, bimetal nanoparticle catalysts representing an active frontier for heterogeneous catalyst development have attracted increasing attention. This is possibly due to the synergistic effect between metals, resulting in the catalytic performance of bimetals superior to their monometallic counterparts [[21], [22], [23]]. In addition to the efforts made to improve the catalytic properties of catalyst, some other strategies, including choosing an appropriate catalyst support (by using its electronic and/or basic property) [20,24] and changing the reaction media (e.g., ionic liquid, supercritical CO2, aqueous-organic solvent, gas phase and solvent-free conditions) [20,[24], [25], [26], [27]], have been developed for achieving effectively selective hydrogenation of CNBs. However, catalytic hydrogenation method does suffer from some limitations: the rigorous working conditions (typically high temperature and pressure), the high cost by using noble metal catalyst (e.g., Pd or Pt-based materials), the potential safety risk of using hydrogen gas as reductant and importantly, the undesired byproducts formation resulting from nonselective hydrogenation. All of these definitely have to be carefully studied in the future works.
Electrochemical reduction using electron as a clean reductant represents a promising and green chemical process, which shows many prominent advantages like environmental friendly, better compatibility, easier controllability, simple and low cost compared to other methods; as a consequence, it has attracted an increasing interest both from environment and synthesis fields. It has been reported in our laboratory that electrochemical reduction can be well applied in the dechlorination and hydrogenation processes [[28], [29], [30], [31], [32], [33], [34], [35]]. However, few studies concerning electrochemical reduction of CNBs have been reported before [19,36], in particular, most of them were mainly investigated around in the last 1960s [37,38]. Recently, Meng and coauthors extended the electrochemical reduction to synthesize 2,2′-dichlorohydrazobenzene from o-CNB, where good outcomes in product yield and selectivity were achieved [19]. The authors attributed the high selectivity to the catalyst property by investigating a series of metal and metal oxides catalysts. It should be noted that the previous works were primarily performed in aqueous conditions [19,37]. Due to the poor solubility of CNBs in water and the resultant mass-transfer limitations, it would restrict the electrochemical transformation of CNBs in large-scale. Because of the remarkable characteristics of organic solvent, such as the large potential window, which may facilitate the reduction of CNBs prior to the solvent discharge, and the high solubility for CNBs that can lead to an enhancement of reaction rate by increasing the substrate concentration, the organic solvent appears to be a better reaction media to initiate the electrochemical reduction of CNBs. Indeed as a promising and green method, electrochemical reduction performed in organic solvent provides a possibility to achieve the effective removal of the CNBs pollutants and/or the selective hydrogenation of CNBs [37]. However, the reduction mechanisms, in particular, the effects of proton donor on the electrochemical reduction of CNBs have not been fully addressed. Although some efforts on the electrochemical reduction of NB and its derivatives, including CNBs, have been made over the last decades, due to the variation of basicity of nitro compounds after the introduction of substituents and the change of solvation in different solvent conditions, it appears impossible to transfer the conclusions made, for instance, for NB in aqueous solution or organic solvent to substituted NBs in the same or different solvent conditions, even the electrode material, supporting electrolyte and the temperature are kept the same [37]. Therefore, it is still interesting and important to investigate the feasibility and the behind mechanisms for the electrochemical reduction of CNBs in different solvent conditions.
The electrochemical reduction of nitro compounds normally involves electron transfer and proton transfer two processes, therefore, for better unravelling the reduction of CNBs, we chose DMF as the aprotic solvent and the proton donor was deliberately added, in order to control both electron and proton transfers. Two types of proton donor, acetic acid (HAc) and water, were chosen as the strong and weak representatives, respectively, to investigate their effects on the reduction of CNBs. In recent years, an increasing number of studies have demonstrated that silver cathode exhibits an excellent catalytic activity for triggering hydrogenation reactions especially in organic and aqueous-organic solvents [[28], [29], [30], [31], [32], [33],[39], [40], [41]]. The previous researches from groups of Gennaro and Amatore mainly attributed the catalytic effect of Ag to the adsorption of substrates onto the metal surface [[42], [43], [44], [45], [46], [47]]. Recently, we proposed the catalyzed DET theory to describe the activation-driving force relationships of electroreductive dechlorination at Ag electrode, and indicated that the catalytic property of Ag is essentially ascribed to the lower of intrinsic barrier energy towards reductive cleavage of CCl bonds [32]. In this context, silver is used as a working electrode in this work. p-CNB is chosen as the research target, in order to investigate the possibility of electrochemical reduction of p-CNB through cyclic voltammetry and electrolysis tools and further to indicate the behind mechanisms. We hope to address the above-mentioned issues through this research and provide an effective tool for the reduction of p-CNB and other CNBs to obtain useful compounds.
Section snippets
Chemicals
p-CNB, p-CAN, HAc and aniline (AN) were analytical grade and purchased from Sinopharm Chemical Reagent CO., Lid (China). N,N-dimethylformamide (DMF), acetone, ethanol and methanol were HPLC grade and purchased from Tedia Company Inc. Tetrabutylammonium tetrafluoroborate (ýC16H36BF4N, TBABF4), 4,4′-dichloroazoxybenzene (DOB) and 4,4′-dichlorohydrazobenzene (DAB) were analytical grade and obtained from Sigma-Aldrich. All chemicals were used directly as received without any treatments. Mili-Q
Cyclic voltammetry of p-CNB
Shown in Fig. 1(a) are cyclic voltammograms recorded at 200 mV s−1 for the reduction of p-CNB at GC (black solid line) and Ag (red solid line) electrodes in DMF containing 0.1 M TBABF4. All the oxidation-reduction potentials of each intermediates and products of p-CNB are collected and provided in the Table 1. The voltammetric behavior for the reduction of p-CNB at GC exhibits two prominent reduction peaks, namely −1.075 (peak a) and −1.954 V (peak b), followed by two successive anodic waves
Conclusions
In this work, the direct electrochemical reduction of p-CNB was investigated at Ag and GC cathodes in DMF containing 0.1 M TBABF4, while the latter is recognized as an inert electrode. The voltammetric reduction of p-CNB displays a series of peaks, namely two reduction waves and three anodic peaks in the return cycle. The first reduction peak was attributed to the conversion of p-CNB to p-CPHA via p-CNSB, while the second one corresponded to the further reduction of p-CPHA to p-CAN, and no
Acknowledgements
The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (No. 5177091149, 51408209, 51509021 and Project 51521006) and the Hunan Provincial Key Research and Development Program (No. 2018SK2025).
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