通过TEMPO增强脱氢和OH吸附促进中性电解质中5-羟甲基糠醛的电催化氧化

Boosting 5-hydroxymethylfurfural electrooxidation in neutral electrolytes via TEMPO-enhanced dehydrogenation and OH adsorption

生物质衍生物的电催化转化为可持续能源的增值利用提供了一条绿色高效的途径.例如,由5-羟甲基糠醛(HMF)氧化可得到生物基聚酯前驱体2,5-呋喃二甲酸(FDCA),对于缓解化石资源带来的能源危机和环境问题具有重要意义.目前,HMF的电催化氧化通常在强碱性(pH>13.5)溶液中进行,但是容易产生难以分离的腐殖质,影响FDCA的工业利用.另外,强碱性介质还存在腐蚀安全隐患、设备维护成本高等问题.中性反应条件有助于改善上述问题,但是缺乏亲电氧物种(例如OH-),HMF和催化剂的活化困难,难以得到高附加值产物FDCA.目前,针对中性介质中HMF的电催化氧化研究少有报道,急需明晰中性介质中HMF的催化机理,并开发中性条件下具有高活性的催化剂体系.本文利用均相催化剂2,2,6,6-四甲基哌啶-1-氧基(TEMPO)和Co_(3)O_(4)电极,对HMF进行中性条件下电催化氧化,1.55 VRHE条件下反应1.5 h,以接近100%转化率获得产物FDCA,产率大于99%.而在无TEMPO的条件下,HMF在Co_(3)O_(4)电极上转化率小于2%.原位红外等实验测试及理论计算结果表明,低电位(1.10 VRHE)下形成的TEMPO+阳离子的活化能显著减低HMF,通过脱氢作用选择性生成中间产物2,5-二甲酰基呋喃(DFF),对调控HMF的氧化路径、促进中性介质电催化氧化起到关键作用.原位X射线光电子能谱和电化学阻抗等结果发现,TEMPO存在时,1.35 V后,在含有高价态Co^(3+)/^(4+)的Co_(3)O_(4)电极上出现更多的水以及OH吸附物种,并导致HMF的氧化电流密度以及FDCA的转化率显著增加.理论计算结果表明,TEMPO自由基与水分子存在较强的氢键作用,有利于促进水活化提供OH.然而,低电位(1.25 V)以Co_(3)O_(4)为电极,或高电位(1.55 V)以泡沫镍为电极,即使添加TEMPO,电极上没有出现相应的OH吸附行为和电化学活性提升现象.因此,活性的Co_(3)O_(4)电极对于协同水的解离形成OH具有重要作用,二者共同作用下,水活化解离形成Co^(3+)/^(4+)-(OH)_(ads)活性中心.在中性溶液中,亲电氧物种OHads提供氧源,促进DFF等甲酰基中间体进一步氧化转化为FDCA.综上,本文加深了对中性介质中HMF的电催化氧化机制的理解,对设计类似电催化剂体系有借鉴作用.

5-Hydroxymethylfurfural(HMF)electrooxidation in neutral conditions is a promising strategy to suppress the formation of humins and corrosive effects on electrochemical devices.However,scarce studies have been reported in neutral media due to the deficiency in electrophilic oxygen(eg OH-)required for the activation of HMF.Herein,2,2,6,6-tetramethyl-piperidine-1-oxyl(TEMPO)/Co_(3)O_(4)was utilized to co-catalyze HMF in neutral media,successfully achieving 2,5-furandicarboxylic acid(FDCA)with yield>99%.It was found that TEMPO could promote HMF dehydrogenation to 2,5-diformylfuran(DFF)and simultaneously activated water via hydrogen-bonding interactions.As a result,the formation of OH*in neutral electrolytes was favored,which was absorbed by electrogenerated active Co species to facilitate subsequent conversion of formyl-group-involved intermediates to FDCA.This work provides a current understanding of the catalytic mechanism for HMFOR in neutral media and guides the design of highly efficient electrocatalysts for biomass upgrading.