【作者】 章晶晶；

【导师】 易清风；

【作者基本信息】 湖南科技大学 ， 应用化学， 2007， 硕士

【摘要】 己二酸是一种重要的化学品,它不仅是工业生产尼龙的一种重要的中间产物,而且还可用作塑胶及食品添加剂。目前的生产方法大都采用环己醇和环己酮的硝酸氧化法,由于该过程伴有大量的氮氧化物等副产物的产生,因而造成了严重的环境问题,因此开发绿色、洁净的氧化方法合成己二酸,一直吸引着许多化学工作者的注意,而有机电合成反应中的电子就是最清洁的“反应试剂”。本文以镍为阳极,铜为阴极,将环己醇电解氧化为己二酸。同时结合气相色谱和高效液相色谱分析,首次研究了在碱性溶液中,环己醇电解氧化为环己酮、己二酸等产物的动力学过程,探索了环己醇在电化学氧化过程中的反应机理及中间产物的分布规律,从而为己二酸的绿色合成提供重要的指导依据。众所周知,镍电极是一种用途广泛的电极材料,对某些小分子醇类物质的电化学氧化具有高度的电催化活性。然而,环己醇在镍电极上电化学氧化行为却很少有文献报道,本文采用多种电化学方法首次研究了环己醇在镍电极的电化学氧化行为;另外,金属钛作为一种基底材料,由于其具有良好的抗腐蚀性能以及适中的价格,越来越受到人们的关注,本文首次以水热法合成出钛基镍电极并进一步研究了环己醇在该电极上电化学氧化行为。本文研究的主要结论有:1.采用循环伏安、动电流扫描及恒电位阶跃等电化学方法,研究了1.0mol/LNaOH溶液中NOMN电极上环己醇的电化学氧化机理。结果表明,镍电极在阳极扫描过程中产生的Ni（OH）₂发生进一步氧化,所产生的NiOOH能够有效的氧化环己醇。而NiOOH和环己醇发生催化氧化后生成的β-Ni（OH）₂又能在阳极上发生氧化而重新生成NiOOH,因此当溶液中存在环己醇时,在循环伏安图上出现一个新的氧化峰。该氧化峰的峰电流密度大小主要取决于扫描速度和环己醇的浓度。另外,当溶液中添加环己醇后,会大大降低它在动电流扫描中阳极峰电位而大大增加它在电位阶跃中的电流密度。研究结果表明,环己醇在Ni/NiOOH电极上的氧化是一个催化氧化过程,遵循催化反应机理,即Ni²⁺在阳极上首先氧化为Ni³⁺,后者作为氧化剂将环己醇催化氧化,本身被还原为Ni²⁺。2.首次以水合肼作为还原剂,采用水热法制备了一种新型钛基镍电极（Ni/Ti）,SEM图像和EDS能谱图显示,在钛基表面沉积了形状为球形的小颗粒金属镍。利用线性扫描、循环伏安、电位阶跃、电化学交流阻抗等电化学测试方法,研究了在不同环己醇浓度以及不同电位下,Ni/Ti电极在1MNaOH溶液中对环己醇电化学氧化的催化活性进行了研究。结果表明,采用水热法将大量的镍金属小颗粒高度分散在钛基表面而制得的Ni/Ti电极可以降低环己醇氧化的电化学极化阻抗,从而对环己醇氧化表现出优异的催化性能。同时将结果与NOMN电极进行了比较,结果表明,在稳态极化测试中,当溶液含有0.2M环己醇时,在Ni/Ti电极上环己醇发生氧化的起始电位为72mV,低于在NOMN电极上环己醇氧化的起始电位（98mV）;并且电位在410mV附近处,Ni/Ti电极上环己醇氧化的电流密度为9.9 mA/cm²,几乎是NOMN电极上电流密度（1.6 mA/cm²）的6倍。在电化学交流阻抗测试中,在NOMN电极上环己醇氧化反应电荷传质阻力在400mV和450mV电位下分别为2350Ω和2243Ω,而在Ni/Ti电极上仅为290Ω和153Ω,前者几乎是后者的8倍和14倍,这些实验数据都进一步说明与NOMN电极相比Ni/Ti电极对环己醇氧化表现出更为优异的催化性能。实验进一步说明,阳极扫描过程中产生的Ni（OH）₂发生进一步氧化,所产生的NiOOH能将环己醇有效氧化,并且吸附在Ni/Ti电极表面的环己醇与Ni³⁺的反应是该过程的速度决定步骤。3.以镍为阳极,铜为阴极环己醇在1MNaOH溶液中电解氧化结果表明,在电流密度,6 mA/cm²:反应温度,60℃:反应时间,16 F/mol;NaOH浓度,1.0 mol/L条件下,电解产物己二酸的产率最大为42.67%;电解动力学实验表明,该反应具有典型连串反应的特征,并且环己醇电解氧化到环己酮是快反应,环己酮进一步电解氧化到己二酸是慢反应,是该反应的控制步骤。更多还原

【Abstract】 Adipic acid is an important chemical material;it has been widely used not only as an important intermediate in the manufacture of nylon but also as a plasticizer and a food additive.It is produced industrially with the oxidation of cyclohexanol and cyclohexanone using concentrated nitric acid as an oxidant.This process poses environmental constrains. The nitrous oxide emission measurably contributes to global warming,and it will do harm to our environment.Therefore,researchers are interested in the development of simple and environmentally friendly methods of its synthesis.It is well known that electron is the clearest chemical reagent because it will not bring up any pollution.In this paper synthesis of adipic acid by electrochemical oxidation of cyclohexanol was performed,which used copper as a cathodic electrode and nickel as an anodic electrode.Optimization of the electrolytic experimental condition was determined by GC and HPLC for the first time. Then,the concentration distribution of the products was investigated.It is well known,nickel is economically feasible with good electrocatalytic activity for small organic molecule oxidation.However,as far as our knowledge goes,the reaction mechanism for electrooxidation of cyclohexanol has never been appeared in the literature. We investigated the electrocatalytic behavior of NOMN electrode towards cyclohexanol electrooxidation by various electrochemical methods for the first time.In addition,as a substrate,titanium has been receiving increasing attention in recent years for its good corrosion-resistance and reasonable cost.In this paper,we firstly fabricated the supported Ni/Ti electrode by hydrothermal process and investigated its electrocatalytic activity towards cyclohexanol oxidation.The main experiment results obtained are as follows:1.Electrocatalytic oxidation of cyclohexanol was investigated with cyclic voltammograms,linear galvanic voltammograms and chronoamperometric responses on a nickel oxyhydroxide modified nickel（NOMN） electrode prepared by cycling the potential of a nickel electrode in the potential range of 0.1 V to 0.6 V（vs SCE） in alkaline solutions. It was found that cyclohexanol was oxidized by NiOOH generated with further electrochemical oxidation of nickel hydroxide during the anodic potential sweep.One of the products of the reaction between cyclohexanol and NiOOH was Ni（OH）₂ which was subsequently oxidized to NiOOH on the anode.This resulted in the appearance of a new anodic peak in cyclic voltammograms compared with the absence of cyclohexanol and this an（?）dic peak strongly depends upon potential scan rates and cyclohexanol concentrations. In addition,the presence of cyclohexanol in NaOH solutions also lead to the decrease of an（?）dic potentials in linear galvanic voltarnmetric responses and the increase of current densities in chronoamperometric curves.Results showed that the oxidation of cyclohexanol on the NOMN electrode follows the catalytic reaction mechanism.2.Titanium-supported nickel electrode（Ni/Ti） is firstly fabricated by hydrothermal process using hydrazine hydrate as a reduction agent.SEM and EDS show that the surface of Ti substrate was partly covered by nickel particles which were present as small balls with the almost uniform size.Its electrocatalytic activity towards cyclohexanol oxidation has been investigated by cyclic voltammetry（CV）,chronoamperometry（CA）,quasi-steady stale polarization and electrochemical impedance spectroscopy（EIS）.Effects of various parameters such as potential scan rate and cyclohexanol concentration on the electrooxidation of cyclohexanol are investigated.Results show that Ni/Ti electrode acts as an efficient catalyst for the electrooxidation of cyclohexanol in basic media and the results were applied to compare with the NOMN electrode.The pseudo-steady state polarization curves show that the onset potential of cyclohexanol oxidation reaction on Ni/Ti electrode is lower than that on the NOMN electrode,which was found to be around 72mV,compared to the onset potential of approximate 98mV for the NOMN electrode in the presence of 0.2M cyclohexanol.Also at the potential of around 410mV the current density of cyclohexanol oxidation on Ni/Ti electrode is 9.9mA/cm²,which is more than six times higher than NOMN electrode（1.6 mA/cm²）.In addition,the EIS data show that the charge transfer resistances at 400mV and 450mV for the NOMN electrode are 2350 and 2243Ωrespectively,which are over 8 and 14 times larger than those for the Ni/Ti electrode（290Ωand 153Ωrespectively）.These results confirm that the electrocatalytic activity of Ni/Ti electrode towards cyclohexanol oxidation is higher than a nickel oxyhydroxide modified nickel electrode（NOMN）.It is further confirmed that during the anodic potential sweep the electrooxidaton of cyclohexanol follows the formation of NiOOH on the electrode surface and is then catalysed by NiOOH.The rate-determining step for cyclohexanol oxidation is the reaction between the high oxidation state nickel(Ni³⁺) with the cyclohexanol adsorbed on the surface of the Ni/Ti.3.Systhesis of adipic acid by cyclohexanol electrolysis in 1M NaOH solution was carried out using copper as a cathodic electrode and nickel as an anodic electrode.The results of the electrolysis of cyclohexanol show that the optimization of experimental conditions are as follows:current density = 6mA/cm²;reaction temperature=60℃;reaction time = 16F/mol;C_NaOH=1.0 mol/L,the maximum yield of adipic acid under the condition is 42.67%.And study of the kinetics of cyclohexanol oxidation shows that the oxidation of cyclohexanol undergoes two steps:the first one is the formation of cyclohexanone and the second step is the formation of adipic acid from cyclohexanone which is the rate-determining step.It is further revealed that the formation of cyclohexanone is a fast step and formation of adipic acid is a slower one.更多还原