X-rays illuminate catalytic potential of zero-valent palladium MOF

Society’s transition from fossil-fuel technologies requires the development of new catalysts to facilitate efficient and sustainable chemical reactions with minimal environmental impact. Currently, precious metals like palladium and platinum dominate these catalysts, particularly for multi-electron transformations, such as those involved in energy conversion processes. Efforts are underway to miniaturize catalyst particle sizes, reducing the amount of precious metal within materials. However, designing and optimizing new catalysts requires precise understanding and control of active site structures. Surface-supported nanoparticle catalysts pose a challenge in preparation and characterization, often existing as dynamic ensembles of different sizes and structures [1]. 

Metal-organic frameworks (MOFs) typically utilize rigid, positively charged inorganic nodes tethered by negatively charged organic linkers. While these frameworks control the heterogeneity of small nanoparticle catalysts within defined voids [2], they compromise the porosity of the parent MOF host (Figure 1). Embedded nanoparticles are less stabilized compared to metal atoms within inorganic nodes. A more elegant approach involves employing zero-valent metal clusters, essentially well-defined (sub)nanoparticles, as structural nodes within new frameworks.
 

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Fig. 1: Conventional MOFs, featuring cationic inorganic nodes linked by anionic organic linkers, have been used to control the heterogeneity of nanoparticle catalysts (grey circles) confined within.

For decades, research on MOFs has heavily relied on single-crystal X-ray crystallography for structural characterization. The ability to grow single crystals measuring >1-10 mm in all dimensions has often been crucial for project success. However, this poses a significant challenge for frameworks constructed from covalent bonds, which are less reversible and thus harder to crystallize compared to electrostatic interactions in conventional MOFs. Consequently, frameworks built from charge-neutral building blocks tend to form statistical, disordered polymers with nanoscopic crystallite domains.

To simplify the framework synthesis, the desired zero-valent clusters were pre-assembled as molecular entities (Pd₃ in Figure 2a), using well-established organometallic chemistry. Subsequently, the supporting isocyanide ligands of the molecular Pd₃ cluster were replaced in solution by a chemically similar, but di-topic linker, resulting in the formation of nanocrystalline Pd₃-MOF as a red powder. Despite efforts, obtaining single crystals of Pd₃-MOF suitable for single-crystal X-ray diffraction remained challenging. 

Although crystals large enough for single-crystal X-ray diffraction experiments were not attainable, Bragg peaks were evident in powder X-ray diffractograms. However, their broadness indicated very small crystalline domains. The advent of dedicated electron diffractometers has accelerated the field of electron crystallography [3]. Electrons interact more strongly with matter than X-rays, enabling diffraction from crystals ranging from 10 to 100 nm, thereby significantly expanding the range of materials that can be characterized. Consequently, the crystal structure of Pd₃-MOF was determined by merging datasets from five individual crystals to achieve a resolution of 1 Å (Figure 2a). The material exhibits a layered framework with modest internal porosity, capable of adsorbing up to 1 mol of CO₂ per {Pd}₃. 

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Fig. 2: a) Single-crystal X-ray diffraction structure of Pd₃ and 3D electron diffraction structure of Pd₃-MOF prepared by a simple ligand exchange strategy. b) XANES (left) and R-space Fourier Transform-EXAFS (right) at the K-edge of palladium, confirming the integrity of the {Pd(0)}₃ clusters within Pd₃-MOF.

The primary goal of this research was to develop a MOF containing well-defined, nanoparticle-like zero-valent metal clusters, aiming to deepen understanding of how catalyst performance is influenced by active site structures. Although electron crystallography revealed the atomic structure of Pd₃-MOF, its value is limited without a more sophisticated understanding of the electronic structures of the palladium centres.

To address this, X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine-structure spectroscopy (EXAFS) experiments were performed at beamline ID12, providing crucial evidence of the preserved integrity of the zero-valent {Pd(0)}₃ clusters within Pd₃-MOF (Figure 2b). While less relevant to these diamagnetic {Pd(0)}₃ clusters, element-specific X-ray magnetic circular dichroism (XMCD), also available at ID12, can offer unparalleled insights into the electronic structures of clusters with more intriguing magnetic properties.

The {Pd(0)}₃ clusters supported by the framework in Pd₃-MOF exhibit remarkable resilience, enduring exposure to solvents and air for hours to days. This robustness stands in stark contrast to the Pd₃ organometallic precursor cluster, which decomposes within seconds upon exposure to air. Additionally, the {Pd (0)}₃ clusters within Pd₃-MOF display catalytic activity typical of palladium nanoparticles, as evidenced by the successful hydrogenation of styrene, a classical test reaction for detecting the presence of palladium.

In conclusion, this novel modular synthesis strategy, coupled with the availability of dedicated electron diffractometers, could unlock a new class of metal-organic frameworks, featuring nanoparticle-like metal clusters as structural nodes. The prospects of drawing from mature fields such as organometallic and molecular cluster chemistry to design frameworks with bespoke catalytic active sites and predictable activities is exciting. Additionally, X-ray absorption spectroscopy and X-ray magnetic circular dichroism, available at ID12, will offer crucial insights into the element-specific electronic structures of these active sites.

Principal publication and authors
A zero-valent palladium cluster-organic framework, X. Liu (a), J.N. McPherson (a), C.E. Andersen (a), M.S.B. Jørgensen (a), R.W. Larsen (a), N.J. Yutronkie (b), F. Wilhelm (b), A. Rogalev (b), M. Giménez-Marqués (c), G. Mínguez Espallargas (c), C.R. Göb (d), K.S. Pedersen (a), Nat. Commun. 15, 1177 (2024); https://doi.org/10.1038/s41467-024-45363-3
(a) Technical University of Denmark, Kgs Lyngby (Denmark)
(b) ESRF
(c) Instituto de Ciencia Molecular, Universidad de Valencia, Valencia (Spain)
(d) Rigaku Europe SE, Neu-Isenburg (Germany)

References
[1] T.W. Hayton et al., Inorg. Chem. 62, 13165 (2023).
[2] K. Kollmansberger et al., Chem. Soc. Rev. 51, 9933 (2022).
[3] K.-N. Truong et al., Symmetry 15, 1555 (2023).
 

About the beamline: ID12

Beamline ID12 is dedicated to polarisation-dependent X-ray spectroscopy in the tender and hard X-ray range (2 -15 keV). The main research activities are focused on the studies of the electronic and magnetic properties of a wide range of systems, from the bulk permanent magnet to paramagnetic monolayers on surfaces in an element- and orbital-selective manner. A large variety of dichroic experiments sensitive either to magnetism, chirality or both can be performed under multiple extreme conditions of magnetic field (up to 17 Tesla), temperature (up to 800 K, down to 2 K) and pressure (up to 60 GPa). The exceptional stability of the optics, combined with a highly efficient detection system, enables reliable measurement of dichroic signals with an unprecedented signal-to-noise ratio.