Boosting the hydroformylation activity of polyoxometalate-anchored Rh single atom catalysts in toluene–water media

Abstract

In this work, water-soluble single atom Rh substituted polyoxometalate (POM) catalysts were constructed. The further ³¹P NMR, FT-IR, ICP-analysis, ESI-MS and HAADF-STEM characterization showed that Keggin-type or Dawson-type Rh-substituted POMs were constructed by a reaction of mono-lacunary POMs with RhCl₃. The Rh substituted POM catalysts were employed in the olefin hydroformylation under toluene–water biphasic conditions. The TOF values of the catalyst can reach as high as 6400 h⁻¹ at 110 °C in propene hydroformylation, along with a superior chemoselectivity towards butyraldehyde/isobutyraldehyde (>95%). The water-soluble single atom Rh substituted POM catalyst showed a great advantage that enables the recovery of the catalytic aqueous phase from the organic product phase (toluene) to be facilitated. The inorganic POM catalysts in the aqueous phase were highly stable without the need for a water-soluble phosphine ligand and can be recycled at least five times without any discernible loss of catalytic activity. The H/D exchange experiments, D NMR and primary isotope effect suggested that water could participate in the hydroformylation reaction and played a crucial role in assisting hydrogen dissociation, enhancing the reaction activity.