仿生无金属氢化物供体催化剂用于 CO 还原。
Biomimetic Metal-Free Hydride Donor Catalysts for CO Reduction.
机构信息
Department of Chemistry, University of Illinois at Chicago, 845 W. Taylor Street, Chicago, Illinois 60607, United States.
Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 Cass Avenue, Lemont, Illinois 60439, United States.
出版信息
Acc Chem Res. 2022 Mar 15;55(6):844-856. doi: 10.1021/acs.accounts.1c00708. Epub 2022 Feb 24.
The catalytic reduction of carbon dioxide to fuels and value-added chemicals is of significance for the development of carbon recycling technologies. One of the main challenges associated with catalytic CO reduction is product selectivity: the formation of carbon monoxide, molecular hydrogen, formate, methanol, and other products occurs with similar thermodynamic driving forces, making it difficult to selectively reduce CO to the target product. Significant scientific effort has been aimed at the development of catalysts that can suppress the undesired hydrogen evolution reaction and direct the reaction toward the selective formation of the desired products, which are easy to handle and store. Inspired by natural photosynthesis, where the CO reduction is achieved using NADPH cofactors in the Calvin cycle, we explore biomimetic metal-free hydride donors as catalysts for the selective reduction of CO to formate. Here, we outline our recent findings on the thermodynamic and kinetic parameters that control the hydride transfer from metal-free hydrides to CO. By experimentally measuring and theoretically calculating the thermodynamic hydricities of a range of metal-free hydride donors, we derive structural and electronic factors that affect their hydride-donating abilities. Two dominant factors that contribute to the stronger hydride donors are identified to be (i) the stabilization of the positive charge formed upon HT via aromatization or by the presence of electron-donating groups and (ii) the destabilization of hydride donors through the anomeric effect or in the presence of significant structural constrains in the hydride molecule. Hydride donors with appropriate thermodynamic hydricities were reacted with CO, and the formation of the formate ion (the first reduction step in CO reduction to methanol) was confirmed experimentally, providing an important proof of principle that organocatalytic CO reduction is feasible. The kinetics of hydride transfer to CO were found to be slow, and the sluggish kinetics were assigned in part to the large self-exchange reorganization energy associated with the organic hydrides in the DMSO solvent. Finally, we outline our approaches to the closure of the catalytic cycle via the electrochemical and photochemical regeneration of the hydride (R-H) from the conjugate hydride acceptors (R). We illustrate how proton-coupled electron transfer can be efficiently utilized not only to lower the electrochemical potential at which the hydride regeneration takes place but also to suppress the unwanted dimerization that neutral radical intermediates tend to undergo. Overall, this account provides a summary of important milestones achieved in organocatalytic CO reduction and provides insights into the future research directions needed for the discovery of inexpensive catalysts for carbon recycling.
二氧化碳催化还原为燃料和高附加值化学品对于发展碳循环技术具有重要意义。催化 CO 还原的主要挑战之一是产物选择性:一氧化碳、分子氢、甲酸盐、甲醇和其他产物的形成具有相似的热力学驱动力,因此很难选择性地将 CO 还原为目标产物。科学界已经做出了巨大的努力,开发可以抑制不希望发生的氢析出反应并使反应定向生成所需产物的催化剂,这些产物易于处理和储存。受自然光合作用的启发,在光合作用中,卡尔文循环中的 NADPH 辅助因子用于还原 CO,我们探索仿生无金属氢供体作为 CO 选择性还原为甲酸盐的催化剂。在这里,我们概述了我们最近在控制无金属氢化物向 CO 转移的热力学和动力学参数方面的发现。通过实验测量和理论计算一系列无金属氢供体的热力学氢化物,我们得出了影响其供氢能力的结构和电子因素。确定了两个对较强氢供体有贡献的主要因素是(i)通过芳构化或存在供电子基团稳定 HT 形成的正电荷,以及(ii)通过变旋作用或在氢化物分子中存在显著结构限制使氢供体失稳。具有适当热力学氢化物的氢供体与 CO 反应,实验证实了甲酸盐离子(CO 还原为甲醇的第一步)的形成,这为有机催化 CO 还原是可行的提供了重要的原理证明。发现氢化物向 CO 的转移动力学较慢,部分原因是 DMSO 溶剂中有机氢化物的自交换重组能较大。最后,我们概述了通过电化学和光化学从共轭氢受体(R)再生氢化物(R-H)来闭合催化循环的方法。我们说明了质子耦合电子转移不仅可以有效地降低发生氢化物再生的电化学势,还可以抑制中性自由基中间体倾向于经历的不需要的二聚化。总的来说,本报告总结了在有机催化 CO 还原方面取得的重要里程碑,并提供了对发现用于碳循环的廉价催化剂所需的未来研究方向的见解。