An antimony(V) substituted Keggin heteropolyacid, H4PSbMo11O40: Why is its catalytic activity in oxidation reactions so different from that of H4PVMo11O40?

Application of matrix-assisted laser desorption/ionization in the studies of phosphotungstic acid

RATIONALE

Phosphorotungstic acid (PTA) has many applications, especially in the field of catalysis, due to its structural properties. However, the structure of PTA is studied mainly using theoretical methods. Matrix-assisted laser desorption/ionization (MALDI) has the potential to be an effective method for the experimental study of heteropolyacids. Limitations of MALDI are the high molecular weight of the particles and the complex distribution of isotopic peak intensities. Both problems can be solved by automatically identifying observed signals by generating hypothetical molecular formulas and estimating their isotopic distributions.

METHODS

Phosphotungstic acid was studied under conditions of laser desorption/ionization in the absence and in the presence of the matrix. Three types of matrices were used: 2,5-dihydroxybenzoic acid in water, α-cyano-4-hydroxycinnamic acid in acetonitrile, and sinapic acid (SA) in tetrahydrofuran. Part of the peaks in the resulting mass spectra was identified using in-house developed software that implements the automated isotopic distribution brute force.

RESULTS

The most informative mass spectra were obtained using SA as the matrix, which enabled the detection of particles containing PTA dimers for the first time. The compositions of particles incorporating PTA dimers were determined in an automated manner and can be written as [H3PW12O40]2·2H2O (m/z = 5791.2 Da) and [H3PW12O40]2·4H2O (m/z = 5836.5 Da). Other observed species included (WO3)n·PO3 -, HPO2·(WO3)n, and WO2·(WO3)n clusters, with the latter containing W in mixed oxidation states.

CONCLUSIONS

The combined use of MALDI and an automated identification procedure provided valuable experimental data on the structure and fragmentation of phosphotungstic acid. To the best of our knowledge, this study was the first to report on particles containing phosphotungstic acid dimers.

Elucidating the role of H2O in promoting the formation of methacrylic acid during the oxidation of methacrolein over heteropolyacid compounds

The involvement of water in the selective oxidation of MAL to MAA over a pure Keggin-type H₃PMoO₁₂O₄₀ catalyst was investigated using an in situ DRIFTS reactor coupled with a mass spectrometer for the first time to elucidate the reaction pathway associated with water. Comparing the spectra and activity data using D₂O instead of H₂O during transient switching experiments has allowed us to evaluate the possible active sites where D₂O is activated. It has been found that, during the cycling switches of D₂O in and out of the MAL + O₂ gas feed at 320 °C, the formation of MAA-OD product is increased and decreased when D₂O is added and removed, respectively. This suggests that the deuterium from D₂O is involved in the production of gas phase MAA-OD. In addition, the in situ DRIFTS-MS results obtained from the isotopic switches between D₂O and H₂O reveal changes in the characteristic infrared bands of the Keggin unit between 1200 and 600 cm^-1. It is found that the isotopic exchange possibly occurs on the bridging oxygen of Mo-O-Mo unit, where water is activated for the formation of MAA. Based on the in situ DRIFTS-MS analysis from the transient switching experiments, the reaction mechanism associated with the effect of water on the selective oxidation of MAL to MAA over Keggin-type H₃PMoO₁₂O₄₀ catalyst is proposed.

Polyoxometalate catalysts with co-substituted VO2+ and transition metals and their catalytic performance for the oxidation of isobutane

The catalytic activity for the oxidation of isobutane is significantly improved by the synergistic effect between VO²⁺ and Cu²⁺ co-doped into the secondary structure of HPCs.

Synthetic chemistry with nitrous oxide

Nitrous oxide (N₂O, ‘laughing gas’) is a very inert molecule. Still, it can be used as a reagent in synthetic organic and inorganic chemistry, serving as O-atom donor, as N-atom donor, or as a oxidant in metal-catalyzed reactions.

N2O reduction at a dissymmetric {Cu2S}-containing mixed-valent center

Through a bio-inspired approach of the active site of the metalloenzyme nitrous oxide reductase, we isolated and characterized a dinuclear mixed-valent dicopper complex capable of N₂O reduction at room temperature.