摘要
Silver nanoparticles prepared by the direct reduction of AgNO3 in aqueous solution were compacted into coins and used as the cathode for the electrocatalytic carboxylation of 1-phenethyl bromide with CO2. The influences of the working electrode, charge, current density and temperature were investigated. Under optimized conditions, 98% yield of 2-phenylpropionic acid was obtained. The reaction was performed under very mild conditions and no added catalyst was required in the electrolyte. Yields that varied from moderate to excellent were also achieved with other benzyl bromides. This electrode has good stability and reusability, and the yield and selectivity of 2-phenylpropionic acid could be maintained during reuse for 10 times.
CO_2作为主要的温室气体,CO_2固定利用引起了广泛的关注,同时它还是一种丰富无毒的C1资源,将其作为原料合成高附加值的化学品,不仅可以缓解温室效应,而且还可以缓解能源危机,具有重要的经济和战略意义.在CO_2的资源化利用中,制备2-苯基丙酸意义重大.2-苯基丙酸是一种重要的医药中间体,可用于合成布洛芬、酮洛芬等用途广泛的药剂.因此,其制备方法引起了人们的广泛关注.在典型的合成2-苯基丙酸均相催化体系中,经常使用Co,Ni和Pd等过渡金属催化剂,虽然得到的目标产物产率较高,但催化剂成本高,且很难循环使用,从而限制了其实际使用.电催化法为2-苯基丙酸的合成提供了一条新的途径.本课题组利用手性钴配合物作为催化剂电羧化不对称合成了手性2-苯基丙酸,其产率和ee值分别为37%和83%.此外,我们还制备了Co负载的纳米Ag电极,以其为工作电极不对称羧化1-溴乙基苯与CO_2反应,得到目标产物2-苯基丙酸的产率为58%,ee值为73%.在前期工作的基础上,本文利用无负载的纳米银电极(Ag NPs)为工作电极,电催化1-溴乙基苯与CO_2羧化制备2-苯基丙酸.银纳米电极是利用水合肼还原Ag NO3溶液经抽滤、干燥、压片而成.为了研究Ag NPs催化CO_2与1-溴乙基苯反应,在一室型电解池中,以Ag NPs为阴极,镁电极为牺牲阳极,以CH_3CN-TEAI(0.1 mol/L)溶液为电解质溶液,底物浓度为0.1 mol/L,饱和CO_2的氛围下进行恒电流电解,经后处理,可得目标产物2-苯基丙酸.为了提高2-苯基丙酸的产率,我们探讨了工作电极、电解电量、电流密度以及反应温度等条件对反应的影响,从而得到优化条件为反应温度0℃、电解电量2.5F/mol、电流密度5 m A/cm2,此时2-苯基丙酸的产率可达98%.在优化条件下,我们还研究了一系列苯基卤代物,如溴化苄、溴苯、α-溴苯乙酸、2-溴代萘、二苯基溴甲烷和1-氯乙基苯的电羧化反应.反应后可以得到相应的羧酸,并取得较好的收率(67%-88%).结果表明,纳米银电极对催化该类反应具有很好催化活性和普适性.本文所采用的条件都比较温和,无需高温或高压.在最优条件下,所制纳米银电极可重复使用至少10次,且保持催化活性不变.经过X射线衍射和扫描电镜表征发现,重复使用后纳米银电极的组成和微结构都保持不变.因此,该纳米银电极具有制备方法简单、催化活性高,稳定性好等特点,具备一定的应用前景.
基金
supported by the National Natural Science Foundation of China(21203066,21373090,21473060)~~
