Department of Chemical and Biomolecular Engineering , University of California at Berkeley , Berkeley , California 94720 , United States.
Department of Chemical Engineering , University of California at Davis , One Shields Avenue , Davis , California 95616 , United States.
J Am Chem Soc. 2019 Mar 6;141(9):4010-4015. doi: 10.1021/jacs.8b13013. Epub 2019 Feb 22.
Although essentially molecular noble metal species provide active sites and highly tunable platforms for the design of supported catalysts, the susceptibility of the metals to reduction and aggregation and the consequent loss of catalytic activity and selectivity limit opportunities for their application. Here, we demonstrate a new construct to stabilize supported molecular noble-metal catalysts, taking advantage of sterically bulky ligands on the metal that serve as surrogate supports and isolate the active sites under conditions involving steady-state catalytic turnover in a reducing environment. The result is demonstrated with an iridium pair-site catalyst incorporating P-bridging calix[4]arene ligands dispersed on siliceous supports, chosen as prototypes because they offer weakly interacting surfaces on which metal aggregation is prone to occur. This catalyst was used for the hydrogenation of ethylene in a flow reactor. Atomic-resolution imaging of the Ir centers and spectra of the catalyst before and after use show that the metals resisted aggregation and deactivation, remaining atomically dispersed and accessible for catalysis. This strategy thus allows the stabilization of the catalysts even when they are weakly anchored to supports.
虽然本质上分子贵金属物种为设计负载型催化剂提供了活性位点和高度可调的平台，但金属对还原和聚集的敏感性以及由此导致的催化活性和选择性丧失限制了它们的应用机会。在这里，我们展示了一种稳定负载型分子贵金属催化剂的新结构，利用金属上的空间位阻配体作为替代载体，并在稳定的催化转化条件下隔离活性位点，其中涉及还原环境中的催化周转。这一结果以含有 P 桥联杯[4]芳烃配体的铱双位催化剂为例得到证明，这些配体分散在硅载体上，被选为原型，因为它们提供了相互作用较弱的表面，金属容易在这些表面上聚集。该催化剂用于乙烯在流动反应器中的加氢反应。铱中心的原子分辨率成像和使用前后催化剂的光谱表明，金属能够抵抗聚集和失活，保持原子分散状态并可用于催化。因此，即使催化剂与载体的结合较弱，这种策略也可以稳定催化剂。
