Chen Hongwen, Iyer Jayendran, Liu Yue, Krebs Simon, Deng Fuli, Jentys Andreas, Searles Debra J, Haider M Ali, Khare Rachit, Lercher Johannes A
Department of Chemistry and Catalysis Research Center, Technical University of Munich, Garching 85748, Germany.
Renewable Energy and Chemicals Laboratory, Department of Chemical Engineering, Indian Institute of Technology Delhi, New Delhi 110016, India.
J Am Chem Soc. 2024 May 22;146(20):13949-13961. doi: 10.1021/jacs.4c01911. Epub 2024 May 13.
Aqueous-phase electrocatalytic hydrogenation of benzaldehyde on Cu leads not only to benzyl alcohol (the carbonyl hydrogenation product), but Cu also catalyzes carbon-carbon coupling to hydrobenzoin. In the absence of an organic substrate, H evolution proceeds via the Volmer-Tafel mechanism on Cu/C, with the Tafel step being rate-determining. In the presence of benzaldehyde, the catalyst surface is primarily covered with the organic substrate, while H* coverage is low. Mechanistically, the first H addition to the carbonyl O of an adsorbed benzaldehyde molecule leads to a surface-bound hydroxy intermediate. The hydroxy intermediate then undergoes a second and rate-determining H addition to its α-C to form benzyl alcohol. The H additions occur predominantly via the proton-coupled electron transfer mechanism. In a parallel reaction, the radical α-C of the hydroxy intermediate attacks the electrophilic carbonyl C of a physisorbed benzaldehyde molecule to form the C-C bond, which is rate-determining. The C-C coupling is accompanied by the protonation of the formed alkoxy radical intermediate, coupled with electron transfer from the surface of Cu, to form hydrobenzoin.
苯甲醛在铜上的水相电催化氢化不仅生成苄醇（羰基氢化产物），而且铜还催化碳-碳偶联生成氢化苯偶姻。在没有有机底物的情况下，析氢在铜/碳上通过Volmer-Tafel机理进行，其中Tafel步骤是速率决定步骤。在苯甲醛存在的情况下，催化剂表面主要被有机底物覆盖，而H*覆盖率较低。从机理上讲，吸附的苯甲醛分子的羰基O上的第一次氢加成导致表面结合的羟基中间体。然后，羟基中间体在其α-C上进行第二次也是速率决定的氢加成，形成苄醇。氢加成主要通过质子耦合电子转移机理发生。在平行反应中，羟基中间体的自由基α-C攻击物理吸附的苯甲醛分子的亲电羰基C以形成C-C键，这是速率决定步骤。C-C偶联伴随着形成的烷氧基自由基中间体的质子化，并与来自铜表面的电子转移耦合，形成氢化苯偶姻。
