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

Abstract

Abstract 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.