摘要
对氨基苯酚(p-Aminophenol，简称PAP)是一种重要的有机精细化工原料和医药中间体，主要应用于医药、染料和显影剂等的生产。目前国内外生产PAP的方法有四种：①对硝基苯酚铁粉还原法；②对硝基苯酚催化加氢还原法；③硝基苯催化加氢还原法；④硝基苯直接电解还原法。硝基苯电解还原法的优点是产品质量好(纯度高、含铁量低)，经简单处理即可达到制药要求，而且整个工艺过程基本上无污染，所以成为目前国内外研究开发的热点。本文以硝基苯为原料，在硫酸体系中，研究了不同电极材料上直接电还原合成对氨基苯酚的电极过程，并对其合成工艺进行了探讨。
论文的主要研究工作为：运用化学反应工程理论，结合硝基苯的电还原机理，设计出用于硝基苯直接电还原合成对氨基苯酚实验的板框式电解槽，并利用脉冲响应法测定了该板框式电解槽的停留时间分布数据，研究了流型随流量变化的规律；通过动态循环伏安法研究了硝基苯在不同电极上、不同条件下的循环伏安特性，初步评选出了用于硝基苯电还原的电极材料；结合循环伏安的测定结果，在H型隔膜电解槽中采用三电极体系测定了硝基苯在不同电极上、不同条件下的稳态极化曲线，得到了各主要因素的影响规律及硝基苯在各个电极上电还原的交换电流密度数值；在基础研究结论指导下，采用板框式电解槽，在高温、强酸、贫氧系统中，进行了一系列的电解实验，分别考察了电极材料、电流密度、通电量、硝基苯浓度、硫酸浓度等因素对收率及电流效率的影响，最终选择以蒙乃尔合金电极作为阴极、铅合金为阳极、Dupont Nofion 417阳离子交换膜为隔膜，得到由硝基苯直接电还原制对氨基苯酚的最佳电解工艺条件：反应温度85℃，电流密度500A·m~(-2)，硫酸浓度20％(wt．)，硝基苯的表观浓度6.75％(wt．)，通电量为理论通电量的0.95倍。在此条件下，连续电解600小时，对氨基苯酚的收率和电流效率都稳定在70％以上，这也证明了所设计的电解槽与反应是相匹配的，选择的电极材料和隔膜是比较稳定的。同时在对蒙乃尔阴极的活性考察中发现，蒙乃尔合金在连续使用600小时以后，其活性及选择性均下降。
为了揭示蒙乃尔合金阴极在连续使用数百个小时后电催化活性剂选择性下降的原因，采用X射线衍射仪(XRD)、电子扫描电镜(SEM)和电子能谱分析仪(EDS)对电解实验所用的蒙乃尔阴极材料在电解实验前后的表面状态进行了表征，结果表明：所使用的蒙乃尔合金阴极材料在电解实验前后均属于Cu-Ni合金，使用600hr后，由于表面层元素组成和几何排布发生了变化，即合金中表面自由焓较小的Cu元素向合金表面“富集”，形
成一种大理石结构的微晶，表面粗糙、不均一，这是造成其对硝基苯电还原的催化活性及
选择性随着使用时间延长而逐渐下降的主要原因，同时也为此类电极的修饰改进提供了方
向。
P-aminophenol , abbreviated as PAP, is an important organic fine chemical and medicine intermediate. It mainly applies to the production of medicines, dyes developing agents and so on. At present the methods of PAP production mainly include:(l) P-nitrophenol powdered iron reduction method;(2) p-nitrophenol catalytic hydrogenation method;(3)nitrobenzene catalytic hydrogenation method;(4) nitrobenzene direct electroreduction method in domestic and abroad. The nitrobenzene electroreduction method has the merits of high product quality (high purity, low iron content), agreement with the drugs manufacture requirements after simple treatment,moreover having no pollutes basically in the technological process, therefore it becomes the present hotspot of R&D in the domestic and abroad. This dissertation studied the electrode process of direct electroreduction nitrobenzene to /j-aminophenol on different electrodes in sulfuric acid systems. The synthetic technology has also been discussed in this dissertation.
The main research work of this dissertation is as follows: Based on the chemical reaction engineering theory and the nitrobenzene electroreduction mechanism, the plate and frame electrolyzer was designed and applied to the process of electroreduction nitrobenzene to p-aminophenol. This dissertation measured the residence time distribution data of the electrolyzer by means of the pulse response method and studied the rule of flow pattern along with the current capacity change. Under different conditions ,this dissertation studied the cyclic voltammetry properties of nitrobenzene on different electrodes by means of the dynamic cyclic voltammetry method and the electrode materials applied to the NB electroreduction have been selected primarily.Baseed on the results of cyclic voltammetry tests, this dissertation used p-electrode systems to measure the steady-state- polarization curves of nitrobenzene on different electrodes in H-type diaphragm cell, and obtained each primary factor s influence rule and the excha
nge current density value of each electrode.And then on the basis of the conclusions of fundamental researchs upword, the effects of influence factors on the the yield of p-aminophenol and the current efficiency,such as electrode material, current density, quantity of electricity circulated, nitrobenzene concentration . sulpuric acid concentration and so on, have been studied in detail in the systems of hign temperature , strong acidity and oxygen-poor
through a series of electrolysis experiments in this paper.After all using Pb as anode Monel metal as cathode Dupont Nafion 417 cation-exchange membrane as membrane, this dissertation obtained the optimum technological condition: reaction temperature about 85C, current density 500A m-2, sulphuric acid concentration 20%(wt.) , nitrobenzene apparent concentration 6.75% (wt.), quantity of electric charge 95% of theory.Under this condition, the average yield of p-aminophenol and the current efficiency could reach above 70% during the 600 hours continual electrolysis ,which indicated that the electrolyzer structure matched the reaction well and the electrode & diagram materials were stable. Meanwhile during the course of investigation on the stability of Monel metal, its activeness and selectivity droped after 600 hours.
In order to disclosure the reason why the electrocatalysis activeness and selectivity droped after being used several hundred hours continuously, the x-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscope (EDS) have been used to characterize the surface condition variations of Monel metal. The results indicated: Monel metal belongs to the Cu-Ni alloy around the electrolysis experiment.And after being used for 600hr, the surface layer element composition and geometry distribution of the alloy changed, namely in the alloy the Cu element /which superficial free enthalpy was smaller, "concentrated" to the alloy surface,and formed one kind of marble structure microcrystallite.Thus the electrode surface becamed rough a
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