Electrosynthesis Of Glyoxylic Acid: Catalyst Preparation And In Situ FTIR Spectroscopic Studies

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.