Abstract：
Ultrasonic irradiation holds potential for the selective oxidation of non-volatile organic substrates in the aqueous phase by harnessing hydroxyl radicals as chemical initiators. Here, a mechanistic description of hydroxyl radical-initiated glyoxal oxidation is constructed by gleaning insights from photolysis and radiation chemistry to explain the yields and kinetic trends for oxidation products. The mechanistic description and kinetic measurements reported herein reveal that increasing the formation rate of hydroxyl radicals by changing the ultrasound frequency increases both the rates of glyoxal consumption and the selectivity towards C2 acid products over those from C-C cleavage. Glyoxal consumption also occurs more rapidly and with greater selectivity towards C2 acids under acidic conditions, which favor the protonation of carboxylate intermediates into their less reactive acidic forms. Leveraging such pH and frequency effects is crucial to mitigating product degradation by secondary reactions with hydroxyl radicals and oxidation products (specifically hydrogen peroxide and superoxide). These findings demonstrate the potential of ultrasound as a driver for the selective oxidation of aldehyde functions to carboxylic acids, offering a sustainable route for valorizing biomass-derived platform molecules.
摘要：
通过利用羟基自由基作为化学引发剂，超声波辐射具有选择性氧化水相中非挥发性有机底物的潜力。这里,通过从光解和辐射化学中收集见解来构建羟基自由基引发的乙二醛氧化的机理描述，以解释氧化产物的产率和动力学趋势。本文报道的机理描述和动力学测量表明,与来自C-C裂解的那些产物相比,通过改变超声频率增加羟基自由基的形成速率增加乙二醛消耗速率和对C2酸产物的选择性两者。在酸性条件下，乙二醛的消耗也发生得更快，对C2酸的选择性更高。这有利于羧酸酯中间体质子化为其反应性较低的酸性形式。利用这样的pH和频率效应对于通过与羟基自由基和氧化产物(特别是过氧化氢和超氧化物)的副反应来减轻产物降解是至关重要的。这些发现证明了超声作为醛官能团选择性氧化为羧酸的驱动器的潜力，为生物质衍生的平台分子提供了一条可持续的途径。
