Probing the Evolution of Palladium Species in Pd@MOF Catalysts during the Heck Coupling Reaction: An Operando X-ray Absorption Spectroscopy Study

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 (H 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.

Aerobic Homocoupling of Arylboronic Acids Catalyzed by Regenerable Pd(II)@MIL‐88B‐NH2(Cr)

“…[21] Our group has developed several procedures using Pd nanoparticles supported on MIL-101-NH 2 (Cr) and MIL-88B-NH 2 (Cr). The excellent catalytic activity of the resulting materials has been demonstrated in a wide variety of transformations, including Suzuki-Miyaura cross-couplings, [22] CÀ H activation/ halogenations, [23] selective C=C hydrogenations, [24] Heck couplings, [25] different carbonylation reactions [26] and the aerobic oxidation of benzylic alcohols. [27] The catalytic methods we developed using these heterogeneous Pd catalysts were efficient, and in some cases they even surpassed their homogeneous counterparts.…”

In situ XAS study of the local structure and oxidation state evolution of palladium in a reduced graphene oxide supported Pd(ii) carbene complex during an undirected C–H acetoxylation reaction

“…This implies a process of pre-catalyst activation in the beginning of the reaction which has been realized as an important procedure for the catalyst which will be further discussed in the EXAFS analysis. 29,39 Table 1 Refined distances (d/Å), and mean number of distances (N) and Debye-Waller factor (σ 2 /Å 2 ) in selected scans using 1@rGO and 1 as catalysts. Single scattering at outer shells and multiple scatterings are not shown in the table…”

“…24,25 With improved control of the chemical conditions several successful examples have been reported. [26][27][28][29] These studies have proven that in situ XAS is a powerful tool to study the catalytic species in solution and solution-solid reaction mixtures.…”

In Situ Structural Determination of a Homogeneous Ruthenium Racemization Catalyst and Its Activated Intermediates Using X‐Ray Absorption Spectroscopy

“…To reach this goal a temperature and stirring controlled in situ/operando reactor has been applied in the present study. The in situ/operando reactor was recently successfully used to obtain structural information on in‐house developed heterogeneous catalysts in operando studies …”

“…The in situ/operando reactor was recently successfullyu sed to obtains tructural information on in-house developed heterogeneous catalysts in operando studies. [41,42] The aim of the presentw ork has been to study the activation process of Ru II catalysts 1a-c utilizing synchrotron radiation in an in situ XASsetup using in situ/operando reactor.…”