In situ preparation of palladium nanoclusters in cerium metal-organic frameworks Ce-MOF-808, Ce-UiO-66 and Ce-BTC as nanoreactors for room temperature Suzuki cross-coupling reaction

Ultrasmall thiolated palladium nanoclusters (Pdn(l-Cys)m) were immobilized inside the cavities of three kinds of cerium-organic frameworks (Ce-MOFs), with different structures and coordinate environments Ce-MOF-808, Ce-UiO-66 and Ce-BTC as sacrificial templates, donated as core-shell catalysts (Pdn@Ce-MOFs). The Pdn(l-Cys)m clusters have an ultrasmall size with narrow distribution (1 ± 0.2 nm), amounting to a cluster size of ca. 40 Pd atoms. For comparison, the palladium nanoclusters were loaded over the three Ce-MOFs with the well-known impregnation method, donated as supported material catalysts (Pdn/Ce-MOFs). Pdn@Ce-MOFs and Pdn/Ce-MOFs catalysts were utilized for the Suzuki cross-coupling reaction in an environmentally friendly solvent system (ethanol/water) under mild reaction conditions. Pdn@Ce-MOFs catalysts showed a remarkably high catalytic activity, due to the unique atomic packing structure and electronic properties of the palladium nanoclusters and the porosity and high specific surface area of the Ce-MOFs that have a great influence on the Suzuki cross-coupling reaction. Pdn@Ce-MOF-808 showed the highest turnover frequency (132,530.1 h−1) observed in the microwave-assisted Suzuki cross-coupling reaction with a very low Pdn loading (0.007 mol%, 0.0224 μmol Pd). Moreover, the Pdn@Ce-MOF-808 catalyst was readily recovered and recycled in at least five cycles without significant leaching or loss of catalytic activity. The particle size of the palladium nanoclusters inside the Ce-MOFs cavities is still around 1 ± 0.2 nm as confirmed by high resolution-transmission electron microscopy (HR-TEM) analysis. The crystallinity of the Pdn@Ce-MOFs and Pdn/Ce-MOFs catalysts was characterized by X-ray diffraction (XRD). The charge of unloaded palladium clusters (Pdn(l-Cys)m) and loaded palladium clusters in Ce-MOFs (Pdn@Ce-MOFs) and on Ce-MOFs (Pdn/Ce-MOFs) was investigated by X-ray photoelectron spectroscopy (XPS). Liquid nitrogen sorption isotherms at _196 °C for the prepared catalysts were measured to investigate their texture properties. [Display omitted] •In situ preparation of palladium nanoclusters in Ce-MOFs as core–shell catalysts.•Pd nanoclusters inside the Ce-MOFs cavities acting as nanoreactors.•Performance of the core–shell catalysts is better than the supported material catalysts.•The MOF's pores prevent the growing up of the palladium nanoclusters.•Achieving the highest turnover frequency (132,530.1 h−1) in Suzuki reaction till now.