Hydrocarboxylation of Olefins Catalyzed by Polyoxometalate-Anchored Palladium Single-Atom Catalysts

Abstract

Herein, the Keggin-type polyoxometalate (POM)a n c h o r e d p a l l a d i u m s i n g l e -a t o m c a t a l y s t s [ ( n -C 4 H 9 ) 4 N] 6 [SiW 11 O 39 Pd] and [(n-C 4 H 9 ) 4 N] 5 [PW 11 O 39 Pd] were constructed and employed in the olefin hydrocarboxylation reaction under two-phase conditions. Both the conversion of styrene and the yield of the carboxylic acids can reach as high as 95% at 110 °C. The characterization by high-angle annular darkfield scanning transmission electron microscopy (HAADF-STEM), electrospray ionization mass spectrometry (ESI-MS), and in situ Fourier-transform infrared (FT-IR) spectra of CO adsorption indicated the incorporation of the isolated Pd atom into the POM structure. The POM-anchored Pd catalysts can be directly stored under an ambient atmosphere and easily recovered in consecutive catalytic recycling. Tetrabutylammonium bromide (TBAB) played a crucial role in stabilizing/dispersing the Pd catalyst. Moreover, the catalytic systems showed a unique "pseudo-liquid-phase" behavior and the reactants were absorbed into the flexible POM anions. The catalysts exhibited a wide substrate suitability and have been applied under phosphine ligand-free conditions. The catalytic mechanism studies involving D 2 O showed that the hydrocarboxylation reaction proceeds through the "Pd−H" pathway.

Ruthenium Single-Atom Anchored in Polyoxometalate-Ionic Liquids for N-Formylation of Amines with CO2 and H2

“…In addition, the HRTEM and STEM analysis of the reused TOMA-SiWRu is shown in Figure S11. It is obvious that no obvious particle aggregation was found and the highly uniform distribution of Ru, W, and Si in the Ru-POM- IL catalyst is still maintained even if the catalyst was reused for six runs, in line with our previous studies. , …”

“…Toluene and morpholine could diffuse into the framework of bulk POM, further expanding the sponge-like aggregates due to the specific "pseudo liquid phase" behavior of POMs. 62,63 As such, binary ILs behave as an amphiphilic microreactor, BMImOAc was distributed in the polar region (red color), while the Ru-POM-IL was prone to reside in low polar regions (black color, Figure 5d,f), which permits the substrate or solvent molecules to access to the active sites of the POM anions. Next, the effects of organic cations in the Ru-POM-IL catalysts on the reaction were examined (Figure S10a).…”

“…It is obvious that no obvious particle aggregation was found and the highly uniform distribution of Ru, W, and Si in the Ru-POM-IL catalyst is still maintained even if the catalyst was reused for six runs, in line with our previous studies. 20,63 The substrate scope of the Ru SAC is evaluated in the Nformylation reaction (Table 2). Not only aliphatic secondary amines, like dimethylamine, diethylamine, dibutylamine, pyrrolidine, and piperidine (entries 1−5), but also the aliphatic primary amines, such as propylamine, butylamine, hexylamine, and cyclohexylamine (entries 6−9), can give outstanding yields of formamides.…”

“…Interestingly, after centrifugation of the mixture, the volume of the sponge-like IL phase returned to its original state, and the biphasic system was reserved (Figure S9d). Toluene and morpholine could diffuse into the framework of bulk POM, further expanding the sponge-like aggregates due to the specific “pseudo liquid phase” behavior of POMs. , As such, binary ILs behave as an amphiphilic microreactor, BMImOAc was distributed in the polar region (red color), while the Ru-POM-IL was prone to reside in low polar regions (black color, Figure d,f), which permits the substrate or solvent molecules to access to the active sites of the POM anions.…”

Formation, Reactivity, and Catalytic Behavior of a Keggin Polyoxometalate/Bipyridine Hybrid in the Epoxidation of Cyclooctene with H2O2

“…Polyoxometalates (POMs) are discrete metal-oxide clusters composed of early transition-metal ions and oxo ligands . Keggin heteropolyanions (HPAs) of general formula [XM 12 O 40 ] n − (mainly M = Mo or W and X = P, Si, or Ge) are one of the most explored subsets of POMs in catalytic applications, particularly in the oxidation of organic substrates. − Much of the interest in these anions has emerged from their catalytic efficiency in oxidation reactions when combined with H 2 O 2 as an oxidant. Relevant examples involve homogeneous ,,− ,− and heterogeneous oxidations − catalyzed by Keggin HPAs.…”