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PRINCIPAL PUBLICATION AND AUTHORS

Zeolites as equilibrium-shifting agents in shuttle catalysis, J. Dallenes (a), J. Wuyts (a), N. Van Velthoven (a), A. Krajnc (b), G. Mali (b), O. Usoltsev (c), A. Bugaev (d), D. De Vos (a), Nat. Catal. 6, 495-505 (2023); https:/doi.org/10.1038/s41929-023-00967-8 (a) Centre for Membrane Separations, Adsorption, Catalysis, and Spectroscopy for Sustainable Solutions (cMACS), KU Leuven, Leuven (Belgium) (b) Department of Inorganic Chemistry and Technology, National Institute of Chemistry, Ljubljana (Slovenia) (c) The Smart Materials Research Institute, Southern Federal University, Rostov-on-Don (Russia) (d) Paul Scherrer Institute, PSI, Villigen (Switzerland)

REFERENCES

[1] B.N. Bhawal et al., ACS Catal. 6, 7528-7535 (2016). [2] X. Fang et al., Science 351, 832-836 (2016). [3] J.C. Reisenbauer et al., Org. Process. Res. Dev. 26, 1165-1173 (2022).

in the selective dimerisation of ethylene, the reaction co- product from propionitrile.

To mechanistically depict the zeolite s role at the molecular level, in-situ X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy experiments were performed at the Pd K-edge at beamline BM23. In the absence of zeolite, the XANES spectra of transfer hydrocyanation reactions coincide with the Pd(0) reference and the EXAFS spectra show dominant Pd-P contributions, presumably originating from the catalyst complex Pd(0)(Xphos)2 (i.e., the resting state of the homogeneous transfer catalyst). In the presence of zeolite, the Pd edge position slightly shifts to higher energies, indicating a more oxidised state of Pd (Figure 91a). Moreover, the signals in the EXAFS region are shifted towards Pd-O contributions, and the magnitude of this shift is proportional to the amount of zeolite that is added to the reaction mixture (Figure 91b). By comparing these spectra with reference Pd(OAc)2-loaded zeolite materials, it was possible to assign these Pd-O contributions to mononuclear square planar Pd(II)-zeolite species derived from the complexation between Pd(OAc)2 and the zeolite s

acid sites. Control X-ray absorption spectroscopy (XAS) experiments showed that the dimerisation of ethylene to butenes coincides with the transformation of the Pd(II)- zeolite species to subnanometric Pd(0)-carbide clusters, clearly indicating that zeolite-bound Pd(II) is the active dimerisation catalyst.

In conclusion, this work shows how the equilibrium of complex, reversible catalytic reactions can be shifted by using zeolites as shape-selective tandem reaction catalysts. In-situ XAS studies revealed the molecular structures of the catalytic entities that operate in a synergistic fashion, allowing for precise control of the tandem reaction system and the reaction equilibrium.

Fig. 91: a) XANES spectra of Pd(OAc)-ZSM-5 (purple), Pd(0)(Xphos)2 (brown) and recovered Pd-containing zeolite after reaction (red). b) In-situ EXAFS spectra of reactions with no Xphos and with zeolite (purple), 2.5 mol% Xphos ligand and with zeolite (blue), 5 mol% Xphos and with zeolite (green) and 5 mol% Xphos ligand and without zeolite (brown). The arrows indicate increased shift towards Pd(II)-O.