【作者】 龙秀娟；

【导师】 吴文标；

【作者基本信息】 西南大学 ， 食品科学， 2008， 硕士

【摘要】 本研究是以无机碳为原料，采用电化学技术，考察了常压下，选用具有较高氢超电势的金属和石墨电极，在不同水溶剂体系及非水溶剂体系中，以甲酸盐和草酸盐为目标生成物质对CO₂进行电化学还原的情况，并对产物进行了分析研究。综合考察了各反应体系中反应电压、支持电解盐、反应温度、时间等因素的影响。实验验证了高温下甲酸盐向草酸盐合成的可行性，并总结了最佳反应条件，同时，对草酸盐进一步电化学还原生成食品类物质重要中间体乙醛酸的各种影响因素进行了考察，得出了理想的反应条件。本文第一章在查阅大量参考文献的基础上，对无机碳电化学还原研究背景进行了介绍，总结了当前以CO₂电化学还原生成甲酸及草酸的研究现状。第二章重点考察了水溶剂体系中，多种电极上CO₂的电化学还原生成甲酸的行为。结果表明，在H型电解槽中，采用三电极体系，以铂电极为对电极，甘汞电极为参比电极，当工作电极为铅、石墨电极时有甲酸生成。单因素优化实验结果表明，当槽电压控制在2．9～3．8V之间的时候均有甲酸生成，选择槽电压为3．2V，电极电压为-0．63V（vs．SCE）为理想工作电压：Na₂CO₃、NaHCO₃、K₂CO₃、KHCO₃四种支持电解质对CO₂电化学还原反应有较好的法拉第效率，在四种溶剂体系中均检测到甲酸生成；温度对本实验中甲酸生成影响不大，0℃、25℃、40℃温度甲酸生成量相差不大；在反应时间的影响考察中，在1h反应时间后已有甲酸生成，3小时达到最大生成量。本章还进一步考察了甲酸盐在高温条件下生成草酸盐的情况，结果表明，反应温度控制在400～420℃时，反应时间控制在30～50min内具有较高的草酸钠产率。第三章重点考察了在水溶剂体系和非水溶剂体系中电化学还原CO₂生成草酸的情况。在水溶剂体系中，选择单室型电解槽，加入1g碳粉，以0．1M NaOH为支持电解质，考察了碳-碳、铂-碳、碳-铅、铂-铅，碳-铜、铂-铜等电极体系下草酸的生成情况，结果表明石墨电极对草酸生成具有较好的催化性能，铅电极同样可以促使草酸生成，但是效果不如石墨理想。Cu电极作工作电极时，电解4h后未能检测出草酸生成。以石墨电极做阴阳电极，4h电解后有0．54×10^-4 mol草酸盐生成。单因素优化实验结果表明，当槽电压控制在23．8V，在0．1M NaOH电解液中具有最大的草酸产量0．63×10^-4mol；在阴阳离子对草酸产率的影响考察中发现，碳酸氢根离子对草酸盐生成有促进作用，在碳酸氢钾和碳酸氢钠电解液中均有草酸根检出。在两种铵离子中都未能有草酸根检出，说明相对于钾离子和钠离子，其不利于草酸盐类生成。在氢氧化钠溶液中，加入少量碳粉后有相对高比例的草酸盐生成，在本电解体系中，其产量超过了已有报道的四甲基氯化铵中草酸盐的产量，达到了0．46×10^-4mol；温度对本实验的影响不大，在0℃、25℃、40℃时，草酸盐的生成量相差不大。而在非水溶剂中，主要考察了DMSO中不同支持电解质下草酸的生成情况，在Pb电极上，当电极电位在-0．9V～-1．5V（vs．SCE）之间时，可获得相对较好的草酸产量，其中在-1．2V（vs．SCE）具有最大的草酸产量达0．18×10^-4 mol；支持电解质对草酸产率影响的考察中发现，卤族原子对草酸产量有一定影响，其中从产量上看，Cl^->Br^->l^-，表明Cl^-较利于草酸的生成。以四乙基氯化铵为支持电解质草酸产量明显高于四甲基氯化铵，达到了0．20×10^-4mol。第四章考察了电极材料，电流密度和阳离子交换膜设置对草酸电化学还原生成乙醛酸的影响情况，结果表明，相较于Al、C电极Pb电极较适合做阴电极，而C电极耐腐蚀性较差，高纯度的铅电极做阳电极具有较好的耐腐蚀性；电流密度对草酸转化率的影响较大，本章结果显示在100 mA·cm^-2电流密度下，电解2h可以得到较高的乙醛酸产率；阳离子交换膜对草酸还原亦影响重大，无隔膜装置会影响产物产率。本节通过以铅电极做阴阳电极，100 mA·cm^-2电流密度，室温下电解2h草酸最高得到了42．35％的乙醛酸生成转化率。更多还原

【Abstract】 The target of this study was to synthesize formate and oxalate by electrochemical reduction of inorganic carbon in different water solvent and non-water solvent systems using high hydrogen over-potential metal or graphite electrodes. The effects of voltage, electrolyte, temperature and time on the electrochemical reduction of inorganic carbon were studied. Experimental results showed the feasibility of producing oxalate from formate under high temperature, and the optimal conditions for making oxalate from formate were established by experimentation. Further study was undertaken to make glyoxylate from oxalate, and the best conditions of achieving this were found.The first chapter introduced the background of electrochemical reduction techniques and present research status of transforming CO₂ into formic acid or oxalic acid based on literature review.The second chapter mainly studied on electrochemical reduction of CO₂ to produce formic acid in the aqueous solvent system by using various electrodes. Experimental results showed that the formate was found in electrolyte choosing platinum as the anode electrode, calomel as the parallel electrode, plumbum or graphite as the cathode electrode. The experimental results showed that, when the voltages were controlled in the range of 2.9 - 3.8 V, formate was produced and the best reaction chamber voltage was 3.2 V [the electrode voltage was -0.63 V （vs.SCE）]. Na₂CO₃, NaHCO₃, K₂CO₃ or KHCO₃ electrolyte showed good Faraday efficiency of the electrochemical reduction of CO₂ to produce formate, respectively. Temperature did not affect the production of formic acid a lot. Different reaction times resulted in different amounts of formic acid formed. Formic acid was generated when the electrochemical reduction of CO₂ was carried out for one hour and maximum yield was achieved after three hours. Experimental results showed that the maximum rate of transforming formate into oxalate was obtained at 400 - 420℃for 30 - 50 min.The third chapter mainly studied on the production of oxalate in aqueous or non- aqueous solvent system by the electrochemical reduction of CO₂. Among C-C, Pt-C, C-Pb, Pt-Pb, C-Cu, Pt-Cu electrode systems, graphite electrode showed the best capability of producing oxalate and it was able to produce 0.54×10^-4mol oxalate by electrolysis for 4 h. No oxalate was detected when copper was used as the electrode. The experimental results also showed that, when the voltage was controlled at 23.8 V, it was able to produce 0.63×10^-4mol oxalate in 0.1 M NaOH solution with 1 g carbon powder added. It was found that bicarbonate ion facilitated the production of oxalate, but oxalate could not be found in ammonium solution. In sodium hydroxide solution with some carbon powder added, the amount of oxalate produced was higher compared with other electrolytes (for example, TEABF₄, in which about 0.46×10^-4mol oxalate can be produced). Temperature did not affect the production of oxalate a lot since it was nearly the same at 0℃, 25℃or 40℃. The production of oxalate in non-aqueous solvent system, for example, in DMSO, by electrochemical reduction of CO₂ using different electrolytes was studied. Experimental results showed that when the electrode voltage was between -0.9 - -1.5 V （vs.SCE） oxalate could be produced by using Pb electrode. There was the maximal oxalate output (0.18×10^-4 mol) at -1.2 V （vs.SCE）. The finding also told that Cl^- is more conducive to the production of oxalate than Br^- and （?）, for example, TEACl is better than TEABF₄, in which 0.20×10^-4mol oxalate was produced.The fourth chapter studied on the effects of electrode, current density and cationic exchange membrane set on the efficiency of transforming oxalate into glyoxylate by electrochemical reduction. Experimental results showed that in comparison with aluminium and graphite electrodes, the anti-eroding property of plumbum electrode is better as a cathode.Current density affected the conversion rate a lot. The results also showed that the maximum amount of glyoxylate was achieved by electrolysis at 100 mA·cm^-2 for 2h. Cation exchange membrane set could also make an impact on glyoxylate output. The optimal conditions for transforming oxalate into glyoxylate are as follows: electrolysis at 100 mA·cm^-2 for 2h using plumbum electrode. By this way, 42.35 % oxalate could be transformed into glyoxylate.更多还原