有机电化学合成的研究进展

有机电化学合成已成为一种实用、环境友好的合成方法, 广泛应用于氧化、还原以及氧化还原中性反应. 通过精准调控电流或者电位可在温和反应条件下实现一些具有挑战性的化学转化. 然而, 有机电化学反应也存在电极钝化、反应类型受限以及反应活性和选择性不易调控等难题. 日益紧张的能源和环境问题使得电化学合成近年来备受关注. 该综述的主要对象为有机溶液体系中的电化学合成, 从直接电解和间接电解两方面阐述近年来为解决这些难题所取得的进展. 在直接电解方面主要是通过合理的有机电化学反应设计、改变电解模式及设备或者将电化学技术与其它的化学合成技术相融合, 解决电极钝化、反应类型受限等问题. 在间接电解方面主要是利用有机分子或者过渡金属作为分子电催化剂去调控电极和底物之间的电子转移以及反应选择性, 实现一些选择性可控的化学转化.     

《第十五届全国有机电化学与电化学工业学术会议》专辑序言

有机电化学是有机化学和电化学技术相结合的一门科学. 与传统的有机合成方法相比,有机电化学合成无需使用有毒或危险的氧化剂或还原剂、具有反应选择性高、反应条件温和等优点,因而在药物、香料、染料和化纤等精细化学品的合成中得到应用. 尤其是近年来随着环境污染的加剧,有机电合成作为一种绿色的化学合成技术而受到了化学工业界的广泛重视. 从本质上讲,有机电化学以电子为试剂,通过电子的得失实现物质的还原和氧化,即从工艺本身消除了污染的形成,是名符其实的“绿色可持续化学”.     

电化学合成纳米材料和小分子材料在电解制氢领域的应用

氢气是一种清洁、高效、可再生的新型能源,并且是未来碳中和能源供应中最具潜力的化石燃料替代品。因此,可持续氢能源制造具有极大的吸引力与迫切的需求,尤其是通过清洁、环保、零排放的电解水方法。然而,目前的电解水反应受到其缓慢的动力学以及低成本/能源效率的制约。在这些方面,电化学合成通过制造先进的电催化剂和提供更高效/增值的共电解替代品,为提高水电解的效率和效益提供了广阔的前景。它是一种环保、简单的通过电解或其他电化学操作,对从分子到纳米尺度的材料进行制造的方法。本文首先介绍了电化学合成的基本概念、设计方法以及常用方法。然后,总结了电化学合成技术在电解水领域的应用及进展。我们专注于电化学合成的纳米结构电催化剂以实现更高效的电解水制氢,以及小分子的电化学氧化以取代电解水制氢中的析氧共反应,实现更高效、 增值的共电解制氢。我们系统地讨论了电化学合成条件与产物的关系,以启发未来的探索。最后,本文讨论了电化学合成在先进电解水以及其他能量转换和储存应用方面的挑战和前景。     

羧酸盐尿素体系中三价铬电沉积机理

采用循环伏安法、线性扫描法及交流阻抗技术研究了羧酸盐尿素体系Cr³⁺电沉积机理. 结果表明, 该体系中Cr³⁺电沉积有前置反应存在, Cr³⁺放电分两步进行, 分别得1个电子和2个电子, 均不可逆; 且第一步放电步骤是速率控制步骤. 由机理推导的电极反应动力学方程, 计算了电化学反应级数、Tafel斜率、表观传递系数的理论值, 其与实验值相吻合.     

阴极冷却固定床电合成乙醛酸的连续过程模拟

联用反应动力学、电荷衡算和物料衡算建立阴极冷却固定床草酸电合成乙醛酸反应器模型方程.针对反应器连续化操作过程,用正交配置法数值求解该模型方程.结果表明,在连续化操作的反应初期,反应液中的乙醛酸能够较快地达到预定浓度,但继续增加反应器的长度,反应的效果不明显.该模型还模拟了分段温控和部分回流对反应的影响.结果表明,分段温控和部分回流均可较好地提高乙醛酸在阴极液中的浓度.计算结果与试验操作基本相符.该结果对阴极冷却固定床草酸电合成乙醛酸的连续化操作工艺和反应器的优化具有重要的参考意义.     

电化学条件下α-C-H键的官能团化的研究进展

近年来,有机电化学合成已经成为有机合成化学中构造碳碳及碳杂键的重要方法.本文主要综述了目前电化学合成的主要研究方向,着重介绍了作者课题组在电化学合成中利用碘自由基促进苯乙酮α-C-H键的官能团化所取得的研究进展,为绿色有机合成的发展提供了新的思路和方向.     

铜离子在合金硫化镍矿浸出过程的化学动力学

建立了在常压氧化硫酸浸出合金硫化镍矿过程中通氧反应结束后铜离子浸出镍的动力学方程,反应的表观活化能为129.88 kJ.K-1.mol-1.根据该动力学方程,可以进一步对硫酸氧化浸出合金硫化镍矿过程中镍的浸出速率进行推导.     

利用K-L方程估算旋转圆盘电极体系反应动力学电流的误差来源分析

旋转圆盘电极（RDE）体系主要用于低溶解度反应物的电极过程动力学研究. 在利用RDE技术研究不可逆电极反应动力学时,人们常利用Koutecky-Levich方程排除传质的影响,从总电流估算反应的动力学电流. 由于K-L方程是建立在系统满足稳态扩散模型的基础上,实际运用时如果体系偏离稳态扩散,就有可能对估算的动力学参数造成很大误差. 本文以氧气在多晶铂电极上的还原反应为例系统地估算了不同氧气浓度与电极转速下的误差,结果表明低氧气浓度与低圆盘转速的情况不满足稳态扩散条件,若此时仍根据K-L方程利用外推法进行分析,误差可达30%. 因此作者建议,在RDE体系中利用K-L方程估算动力学参数时,最好忽略低浓度与低转速下的数据,直接使用较高浓度与较高转速下的数据进行计算与分析.     

La2O3/BaCO3上甲烷氧化偶联反应动力学与历程

以8％La_2O_3/BaCO_3为催化剂,用无梯度反应器,在720～800℃温度范围内,通过改变甲烷与氧分压进行了一系列的甲烷氧化偶联反应动力学测量,确定了该反应的反应历程。乙烷和大部分CO_x(主要是CO_2)是甲烷氧化的一次产物。乙烯是乙烷串行反应的产物。用幂式动力学方程拟合动力学试验数据,估算了各步反应速度方程的动力学参数。还用脉冲反应技术与过渡应答技术证明了参与活化甲烷的是催化剂表面上的可逆吸附氧。     

铝酸钠溶液中析出一水软铝石过程的宏观动力学

对铝酸钠溶液中析出一水软铝石过程的宏观动力学进行了研究.在考虑主要影响因素的前提下,提出了动力学模型,并通过线性回归计算出方程的参数,给出了相应的动力学方程.研究结果表明:分解反应对A l2O3过饱和度和游离NaOH浓度的反应级数分别为2.24和-2.24,前者与大多数研究者的结论一致,而后者则说明溶液中游离NaOH是对反应影响较大的动力学制约因素;反应的表观活化能为62.248 kJ/mol.     

重铬酸钾电催化合成反应过程阳极稳定性及析气效应研究

针对重铬酸钾传统生产技术存在的高污染、高消耗等问题,研究了电催化合成重铬酸钾绿色新技术。实验研究了自制钌/铱/钛氧化物复合阳极的电催化性能,表征了电极表面形貌。结果表明,该阳极在铬酸钾溶液中具有较低的析氧电位和稳定的高催化活性。实验测得不同反应条件下的阴极析氢速率,建立了析氢速率和析氢量随阳极液铬酸钾初始浓度、反应温度及反应时间变化的数学模型,指出可用宏观测得的阴极析氢量定量表征重铬酸钾电催化反应进程,并讨论了析氢速率和析氢量的影响因素。     

Flow Injection Amperometric System Coupled with a Well-Plate for Fast Screening of Total Antioxidant Capacity

A flow injection (FI) system with an amperometric detection in combination with a 96-well-plate has been developed for high throughput screening of total antioxidant capacity. It is based on the reaction between dichromate and antioxidants Chromium reducing antioxidant capacity (CHROMAC) assay. Antioxidant reduces dichromate leading to a decrease of dichromate concentration, in which reacts further with triiodide reagent. The slow reaction between antioxidant and dichromate takes place in parallel in a 96-well microplate, with the reaction time sufficiently long to reach steady state conditions. Microliter volumes of the remaining dichromate solution were manually injected into the FI amperometric system. Reaction with iodide in acidic medium produces triiodide which undergoes electrochemical reduction on a glassy carbon working electrode at 200?mV versus a Ag/AgCl reference electrode. The decrease of electrical current was directly proportional to the antioxidant capacity. The optimum conditions have been achieved as follow: 160?mg?L^?1 of potassium dichromate, pH of dichromate solution of 2.82, 0.005?mol?L^?1 of potassium iodide, 0.20?mol?L^?1 of hydrochloric acid, and 3.0?mL?min^?1 total flow rate. The developed method was applied for the determination of total antioxidant capacity of tea and herbal infusions and fruit juice samples. The results obtained indicated that the developed method had good correlation with the batch CHROMAC spectrophotometric method and 1,1-diphenyl-2-picrylhydrazyl scavenging assay. The proposed method provides fast and high throughput analysis, convenient operation, and low consumption of chemical reagents and samples.     

Comparison of three electrochemical end-point detection methods to assay potassium dichromate by coulometric titration

Coulometric titrations with three electrochemical end-point detection methods were performed to assay potassium dichromate as a standard for oxidation–reduction titration. The assay as an oxidizing agent was carried out with ferrous ions produced by electrolytically reducing ferric ions. Three end-point detection methods were employed and compared with each other: constant potential amperometry, potentiometry, and constant voltage biamperometry (a dead-stop method). The last one was found to provide high accuracy in the coulometric titration of potassium dichromate. Solution form samples were also measured to confirm the possible existence of chromium(III) in potassium dichromate by both coulometric titration and ion-exchange chromatography with inductively coupled plasma time-of-flight mass spectrometry.     

Differential thermal analysis and temperature profile analysis of pyrotechnic delay systems: Slow burning mixtures of boron and potassium dichromate

The slow burning pyrotechnic reaction between boron and potassium dichromate has been studied by differential thermal analysis and temperature profile analysis. The latter technique in which the combustion process is studied directly, shows the presence of two reaction stages and validates the results from differential thermal analysis obtained under non-ignition conditions. In the first stage potassium dichromate is reduced by boron to form potassium chromate. Differential thermal analysis shows that the reaction takes place below the melting temperature of potassium dichromate and becomes much more rapid above this temperature. The second stage which occurs in the region of 1000 K involves the reaction of potassium chromate with more boron. The first stage acts as a trigger for the second and it is this latter stage which propagates the combustion.     

Differential thermal analysis and temperature profile analysis of pyrotechnic delay systems: mixtures of tungsten and potassium dichromate

The self-propagating combustion reaction between tungsten and potassium dichromate has been studied by differential thermal analysis and temperature profile analysis. Two reaction stages have been distinguished in the combustion where the rate of temperature rise is ? 10⁵ K min^?1: in the first potassium dichromate is reduced by tungsten to form potassium chromate and in the second stage the potassium chromate reacts with more tungsten to form potassium tungstate and chromic oxide.     

Synthesis of Nicotinic Acid by Potassium Dichromate Oxidation

IntroductionNicotinic acid,whose IUPAC name is 3-pyridine formic acid,also known as Vitamine B5,ismainly used as an additive in food,drink andforage industry[1— 3 ] .In medical practice,it is usedto cure pellagra and other relative vitaminedeficiency illness. It is also an intermediate for thesynthesis of important medicines and chemicalmaterials[1,4] . In China,the demand of nicotinicacid is very big. However,there are manyproblems in the current preparation methods ofnicotinic acid,such…     

Kinetic evidence of a common mechanism in the oxidations of diethyl sulfide by dichromates and halochromates of heterocyclic bases

The oxidations of diethyl sulfide by potassium dichromate, pyridinium dichromate, quinolinium dichromate, imidazolium dichromate, nicotinium dichromate, isonicotinium dichromate, pyridinium fluorochromate, quinolinium fluorochromate, imidazolium fluorochromate, pyridinium chlorochromate, quinolinium chlorochromate, and pyridinium bromochromate follow identical kinetic orders—first‐order each with respect to the chromium(VI) reagents, sulfide and hydrogen ion, and moderately inhibited by manganese(II) ion. The energy of activation varies linearly with the logarithm of frequency factor and so does the enthalpy of activation with the entropy of activation. Also, the activation free energies do not differ significantly. The dichromates and halochromates of heterocyclic bases oxidize diethyl sulfide via a common mechanism. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 35: 1–8, 2003     

Apparent Kinetics for Electrosynthesis Process of Chromic Acid

A new green technique for producing chromic acid via an electrosynthesis method was studied.The kinetic experiments were carried out on the direct electrosynthesis reaction of chromic acid from sodium dichromate in a self-made electrosynthesis reactor with a multiple-unit metal oxides combination anode,a stainless steel cathode,and a reinforcing combination Nafion?324 cation exchange membrane.The apparent kinetic data were experimentally measured at different reaction time under different reaction condition...     

Volumetric studies in oxidation-reduction reactions: Reduction with ferrous ethylenediamine sulphate indirect determinations

Ferrous ethylenediamine sulphate has been. used as a reducing agent in acid medium for the indirect volumetric estimations of potassium chlorate, potassium bromate, potassium metaperiodate, potassium, dichromate, potassium ferricyanide, potassium permanganate, potassium persulphate, hydrogen peroxide and ceric sulphate. The excess of ferrous ethylenediamine sulphate added to each of the substances was titrated with standard potassium permanganate and also with. standard potassium dichromate solution. In case of potassium bromate or potassium metaperiodate the end-point was not sharp in potassium dichromate titrations; while accurate volumetric observations were made with standard potassium permanganate solution.     

Catalytic kinetic spectrophotometry for the determination of trace amount of oxalic acid in biological samples with oxalic acid-rhodamine B-potassium dichromate system

A new catalytic kinetic spectrophotometric method was proposed for determining trace oxalic acid based on the catalytic effect of oxalate on the oxidation of potassium dichromate with rhodamin B in 0.10 M of sulfuric acid. Good linearity is obtained over the concentration range 0.40-6.0 microg/mL of oxalic acid. After the reactions of the catalytic and non-catalytic systems were terminated by using 2.00 mL of 4 M sodium hydroxide solution, they can be stable for 3 h at room temperature. The apparent activation energy of the catalytic reaction is 12.44 kJ/mol. The effect of 50 coexisting substances was observed. The method was used to determine trace oxalic acid in tea, spinach and urine samples with satisfactory results.