【作者】 黄桃；

【导师】 孙世刚； 陈声培；

【作者基本信息】 厦门大学 ， 物理化学， 2007， 博士

【摘要】 乙醛酸是一种重要的精细化工产品,分子中同时含有醛基和羧基,可与多种化合物发生缩合反应,广泛应用于合成香料、医药、农药、化妆品、油漆、皮革、造纸等工业。乙醛酸可由多种方法合成,其中电合成法用电子作为氧化剂或还原剂,减少污染,降低能源和原材料消耗,具有其它方法无法比拟的优越性。一般采用两种电合成法:草酸电还原法和乙二醛电氧化法。电合成乙醛酸得到广泛关注,具有原料价廉易得、流程简单、副产物少、产品容易分离、产品质量高、反应条件温和,以及无“三废”污染等特点。本文针对乙醛酸电合成中的催化剂和反应过程与机理两个关键科学问题,开展系统深入的研究,取得以下重要结果:1.发展离子色谱电导检测技术,成功地应用于乙醛酸电合成过程中电解液所含物种的定性与定量检测。电合成乙醛酸的电解液中可能存在乙醇酸、乙醛酸、乙二醛和草酸等物种,由于它们的分子结构极为相似,给分离和检测带来困难。运用我们发展的离子色谱电导检测技术获得的结果有:（ⅰ）在草酸电还原合成乙醛酸反应中,电解液主要含乙醇酸、乙醛酸和草酸。选用4.8 mM NaHCO₃+6.0mM Na₂CO₃为洗脱液,虽可实现对草酸定量检测,但此时乙醇酸与乙醛酸的谱峰部分重叠;通过改变洗脱液的浓度,即以0.40 mM NaHCO₃+0.50 mM Na₂CO₃为洗脱液,可改善乙醇酸和乙醛酸两者的分离度,实现同时对乙醇酸和乙醛酸的检测。（ⅱ）在乙二醛电氧化制备乙醛酸中,电解液主要物种有乙醛酸、草酸和乙二醛。以4.8 mM NaHCO₃+6.0 mM Na₂CO₃为洗脱液,可以对乙醛酸和草酸同时进行检测与分析。而乙二醛为中性分子,在离子色谱电导检测中是无法直接进行检测的。但本文巧妙选择2.0 mM NaOH+0.05 mM Na₂CO₃碱性溶液为洗脱液,结果意外发现乙二醛在色谱柱中经碱催化发生坎尼查罗反应生成乙醇酸,从而实现通过离子色谱电导检测技术对乙二醛的检测与分析。2.运用电化学原位红外反射光谱,在分子水平上研究乙醛酸电合成的反应过程与机理。关于草酸电还原和乙二醛电氧化反应过程的深入认识,可为反应条件的选择、催化剂的研制等提供依据,因此具有重要的指导意义。在草酸电还原制备乙醛酸和乙二醛电氧化合成乙醛酸反应过程进行中,红外光谱可原位检测各种中间体、产物和跟踪反应历程等,使电催化的研究深入到分子水平,为反应机理研究提供直接的实验依据。（ⅰ）运用多步电位阶跃、单次电位改变和时间分辨傅立叶变换红外反射光谱,研究了草酸在本体铅电极和nano-Pb/GC电催化剂还原过程中,草酸、乙醛酸和乙醇酸各主要官能团振动吸收谱峰的产生与变化。发现草酸在nano-Pb/GC电极上电还原的还原电位与其在本体Pb电极上相比正移。另外时间分辨红外光谱结果显示:在nano-Pb/GC电极上检测到乙醛酸生成的时间比在本体Pb电极上短。红外光谱研究结果表明nano-Pb/GC具有比本体Pb电极更好的电催化活性。（ⅱ）多步电位阶跃红外反射光谱研究了乙二醛在Pd/GC、Pb/GC和Pb-Pd/GC电极上电氧化过程红外吸收谱峰的变化。研究结果指出起始氧化电位顺序为:Pb-Pd/GC（0.95V）＜Pd/GC（1.00V）＜Pb/GC（1.15V）,说明Pb-Pd/GC催化剂比单金属催化剂（Pd/GC或Pb/GC）具有更好的催化活性。3.采用电沉积法制备了多种Pb基金属电催化剂。电极不仅是实施电子转移的场所,而且作为催化剂参与电化学反应。因此寻找对体系电催化性能较好的电催化剂是至关重要的课题。（ⅰ）针对草酸电还原合成乙醛酸中的电极材料进行研究,采用循环伏安法、计时电位法和电位阶跃法制备了Pb/GC、Bi-Pb/GC、Pt-Pb/GC和Pd-Pb/GC电催化剂,运用场发射扫描电镜对电催化剂形貌进行表征,研究了各个电催化剂对草酸电还原的催化活性。研究结果表明:计时电位法制备的纳米Pb/GC比本体Pb的催化活性高;Pt-Pb/GC对草酸电还原不具有催化活性;而Pd-Pb/GC比Pb/GC对乙醛酸的选择性略好。（ⅱ）对于乙二醛选择性电氧化制备乙醛酸,采用循环伏安法制备了Pb/GC、Pd/GC和不同比例的Pb-Pd/GC电催化剂,研究各个电催化剂对乙二醛电氧化的催化活性。这些电催化剂对乙醛酸的选择性均很高,在89～95%;而在乙二醛转化率方面,Pb-Pd/GC比Pb/GC和Pd/GC高。即二元Pb-Pd电催化剂比单金属Pd或Pb的催化活性好。4.设计电化学流动微反应器,并采用MEMS技术制备了不同尺度的微电极阵列,开展了前期的研究。20世纪90年代中期微反应器技术兴起以来,微结构反应器已被应用于液相反应、气-液反应、光化学与电化学、气相反应等中。已经有利用微反应器进行药物和精细化学品合成的产业化实例。微反应器本身的特点和优点,以及所取得的研究成果均显示出微反应器在精细化工领域的巨大应用价值。本文针对乙醛酸电合成的特点,设计并利用MEMS技术研制梳齿状微电极阵列和相应的微型流动反应器,开展了初步的研究工作。更多还原

【Abstract】 Glyoxylic acid, HOOC-CHO, is an important fine chemical. The molecule contains aldehyde and carboxyl that can react with many compounds by reduction reaction. It can be widely used in the synthesis of perfumery, medicine, pesticide, cosmetic, paint, leather, papermaking and so on. There are many ways to synthesis glyoxylic acid. Among them the electrosynthesis method which employs electron as oxidant or reductant, can significantly reduce pollution. It can also decrease energy and raw material consumption. So this kind of synthesis method shows huge advantages. There are mainly two kinds of electrosythesis methods: electroreduction oxalic acid and selective electrooxidation glyoxal. The electrosynthesis of glyoxylic acid with non-expensive raw material, simple process, little byproduct, easy separation products, high quality, mild reaction conditions, and no "three wastes" pollution characteristics, has attracted wide attention. This work was mainly aimed at the systemic and in-depth studies of electrocatalysts and reaction process and mechanism, two key scientific issues in the electrosynthesis of glyoxylic acid. The following important results have been obtained.1. Based on ion chromatography technology, we have developed a new method that combines ion chromatography with a conductivity detector to separate and determine the substances of glycolic acid, glyoxylic acid, glyoxal and oxalic acid. The method was applied for the first time in quantitative determination of substances involved in the electrosynthesis of glyoxylic acid. In the synthesis of glyoxylic acid, the main species existing in the electrolyte is glycolic acid, glyoxylic acid, glyoxal and oxalic acid. Since the structures of these four substances are similar, quanlitative and quantitative analysis are often difficult to achieve simultaneously. The existing methods and techniques for the analysis of these four substances are not satisfactory. Each method may have one or more disadvantages. With the newly developed method, we have achieved: （ i） In the process of electroreduction oxalic acid to glyoxylic acid, the main species existing in the electrolyte are glycolic acid （byproduct）, glyoxylic acid （main product） and oxalic acid （reactant）. When 4.8 mM NaHCO₃ + 6.0 mM Na₂CO₃ was the eluent, the peak of oxalic acid was well separated from the others in addition to glycolic acid together with glyoxylic acid. The standard calibration equation can be obtained from the detection of the standard solutions under the same experimental conditions. So, quantitative determination of oxalic acid has been achieved. The overlap of peaks for glycolic acid and glyoxylic acid arises difficulties for qualitative and quantitative analysis of them. Fortunately, changing the concentration of the eluent, that is, using 0.40 mM NaHCO₃ + 0.50 mM Na₂CO₃ as the eluent, glycolic acid and glyoxylic acid can be completely separated and simultaneously detected; （ii） In the process of electrooxidation of glyoxal, the electrolyte mainly contains glyoxylic acid （main product）, oxalic acid （byproduct） and glyoxal （reactant）. Selecting 4.8 mM NaHCO₃ + 6.0 mM Na₂CO₃ as the eluent, glyoxylic acid and oxalic acid can be simultaneously detected. For glyoxal, it is a neutral molecule, not existing in the form of an ion. According to the detecting principle of ion chromatography with conductivity detector, the substance of glyoxal could not be directly detected. However, glyoxal is an active molecule that can be easily converted to glycolic acid by catalyst of strong base such as sodium hydroxide. So we skillfully select 2.0 mM NaOH + 0.05 mM Na₂CO₃ alkaline solution as the eluent. We discover that there is a strong peak in the ion chromatogram. Its retention time just coincided with that of glycolic acid. With this discovery, the quantitative determination of glyoxal has been done.2. Study of the reaction process and mechanism of electrosynthesis glyoxylic acid at molecular level using electrochemical in situ FTIR spectroscopy. There are few reports about the reaction mechanism study for electroreduction oxalic acid and electrooxidation glyoxal. Electrochemical in situ FTIR spectroscopy was applied for the investigation of electroreduction oxalic acid and electrocatalytic oxidation of glyoxal into glyoxylic acid at molecular level. It is significant to understand the reaction process and provide direct experimental evidence for reaction mechanism of electroreduction oxalic acid and electrooxidation glyoxal. （i） MSFTIRs, SPAFTIRs and TRFTIRs were used to study the electroreduction of oxalic acid on bulk and nano-Pb/GC electrodes. The results of MSFTIRs and SPAFTIRs demonstrate that the redox potential for the electroreduction oxalic acid on nano-Pb/GC is more positive than that on bulk Pb electrode. And the results of TRFTIRs illustrate that the time to detection of glyoxylic acid production on nano-Pb/GC is shorter than on bulk Pb electrode. So, it can be concluded that nano-Pb/GC exhibits higher electrocatalytic activity than bulk Pb electrode. （ii） MSFTIR spectroscopy was applied to study the process of electrooxidation glyoxal on Pd/GC, Pb/GC and Pb-Pd/GC electrodes. The results demonstrated an order of the initial oxidation potential for glyoxal is Pb-Pd/GC （0.95V） < Pd/GC （1.00V） < Pb/GC （1.15V）. It illustrates Pb-Pd/GC binary electrocatalyst has better catalytic activity than mono-metal electrocatalyst （Pd/GC or Pb/GC）.3. Preparation of various Pb-based metal electrocatalysts by electrochemical method. As we all know, electrode is a media for electronic transfer, and also a catalyst reacting with species involved in reaction, is important for electrochemical reaction. So it is essential to search and prepare electrocatalysts with high catalytic performance. We have done: （i） In the reaction of electroreduction oxalic acid, cyclic voltammetry, chronopotentiometry and potential step were used to prepare Pb/GC, Bi-Pb/GC, Pt-Pb/GC and Pd-Pb/GC electrocatalysts. The field emission scanning electron microscopy was applied to character the morphologies of each electrocatalyst. The catalytic activities for electroreduction oxalic acid of every electrocatalysts were studied by chronopotentiometry and ion chromatography. The catalytic activity of nano-Pb/GC prepared by chronopotentiometry is higher than that of bulk Pb electrode. Pt-Pb/GC has not catalytic activity for electroreduction oxalic acid. The selectivity towards glyoxylic acid for Pd-Pb/GC is slightly better than that for Pb/GC electrode. （ii） For the selective electrooxidation glyoxal, cyclic voltammetry was used to prepare Pb/GC, Pd/GC and Pb-Pd/GC electrocatalysts of different composition. The catalytic activities of various electrocatalysts for electrooxidation glyoxal were studied. These electrocatalysts for the selectivity towards glyoxylic acid are very high in 89～95%. But about the conversion of glyoxal, Pb-Pd/GC is better than Pb/GC or Pd/GC. So it can be concluded that the catalytic activity of the binary metal electrocatalyst Pb-Pd/GC is better than mono-metal catalysts （Pd/GC or Pb/GC）.4. Design of a flow electrochemical microreactor, and fabricate an array of microelectrode with different scale by MEMS technology, and carry out preliminary relevant study. Since the rise of microreactor technology in mid-1990s, it has been successfully applied to many liquid reactions, gas-liquid reactions, photochemistry, electrochemistry, gas reactions and so on. Microreactor has been industrially used for drugs and fine chemicals synthesis. It shows huge value in the field of fine chemicals because of its own characteristics and advantages. According to the characteristics of glyoxylc acid synthesis, we use MEMS technology to prepare comb-microelectrode array and fabricate the corresponding flow microreactor. A relevant preliminary study has been carried out.更多还原