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Abstract
The mechanism of the Heck C-C coupling reaction catalyzed by Pd@MOFs has been investigated using operando X-ray absorption spectroscopy (XAS) and powder X-ray diffraction (PXRD) combined with transmission electron microscopy (TEM) analysis and nuclear magnetic resonance (1H NMR) kinetic studies. A custom-made reaction cell was used, allowing operando PXRD and XAS data collection using high-energy synchrotron radiation. By analyzing the XAS data in combination with ex situ studies, the evolution of the palladium species is followed from the as-synthesized to its deactivated form. An adaptive reaction mechanism is proposed. Mononuclear Pd(II) complexes are found to be the dominant active species at the beginning of the reaction, which then gradually transform into Pd nanoclusters with 13-20 Pd atoms on average in later catalytic turnovers. Consumption of available reagent and substrate leads to coordination of Cl- ions to their surfaces, which causes the poisoning of the active sites. By understanding the deactivation process, it was possible to tune the reaction conditions and prolong the lifetime of the catalyst.
Highlights
Sustainable catalysis by transition metals[1] is essential for further advances in the large-scale production of specialty chemicals for pharmaceuticals, advanced materials or agrochemicals
Kinetic profiles of the Heck reaction using Pd(II)@MIL-101-NH2 as precatalyst are presented in Figure 3a, where both conversion and temperature are plotted as a function of time
After an initial short period of rapid conversion at 60 °C, a plateau was reached at ca. 70−80 °C (20−30 min after olefin addition) with a clearly diminished activity (Figure 3a, solid blue line)
Summary
Sustainable catalysis by transition metals[1] is essential for further advances in the large-scale production of specialty chemicals for pharmaceuticals, advanced materials or agrochemicals. In this context, C−C coupling reactions remain indispensable for creating the backbone of organic molecules. Recycle or replace the heavy metals involved in these processes, for economic and environmental reasons, a thorough understanding of reaction mechanisms is required. Heterogeneous mechanisms are notoriously difficult to monitor,[4,5] and adapted operando methods are highly demanded to achieve a deeper understanding.[6]
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