The Zr-containing polyoxometalates (POMs), including (Et2NH2)8{[α-PW11O39Zr­(μ–OH)­(H2O)]2}·7H2O (1), effectively catalyze the hydrolysis of nerve agent simulants at near-neutral pH. Analogous Zr-containing heterogeneous systems are much-studied and effective nerve-agent hydrolysis catalysts, but due to their heterogeneous nature, it is very challenging to know the exact structure of the catalytic sites during turnover and to clarify at the molecular level the elementary mechanistic processes. Here, under homogeneous conditions, hydrolysis rates of the nerve-agent simulant methyl paraoxon catalyzed by 1 are examined as a function of pH, ionic strength, catalyst, and substrate concentrations. In addition, the specific effect of three commonly used buffers is examined, revealing that acetate functions as a co-catalyst, phosphate inhibits hydrolytic activity, and 2-(N-morpholino)­ethanesulfonic acid (MES) has no effect on the hydrolysis rate. Spectroscopic (31P nuclear magnetic resonance) and computational studies demonstrate how each of these buffers interacts with the catalyst and offer explanations of their impacts on the hydrolysis rates. The impact of the nerve-agent hydrolysis product, methyl phosphonic acid, is also examined, and it is shown to inhibit hydrolysis. These results will aid in the design of future Zr-based hydrolysis catalysts.

The reaction of reduced polyoxometalates (POMs) with dioxygen is centrally important in POM catalysis. We report that this process, as represented by the one-electron-reduced Keggin complexes POMred (α-AlW12O40 6–, α-SiW12O40 5–, and α-PW12O40 4–), is efficiently catalyzed by copper complexes. The Cu-catalyzed pathway is dominant in the presence of as low as ∼0.1 μM of Cu, a copper concentration that is typically lower than the copper ion contamination in aqueous solutions. The reaction kinetics and mechanism have been comprehensively studied in sodium sulfate buffer at pH 2.0. The catalytic pathway includes a reversible reduction of Cu­(II) by POMred, followed by a fast reoxidation of Cu­(I) by O2 to regenerate Cu­(II). The rate constants of the first catalytic steps were determined by three approaches and found to be (1.8 ± 0.3) × 105 and 57 ± 15 M–1 s–1 for SiW12O40 5– and PW12O40 4–, respectively. These reactions are thermodynamically more favorable and therefore proceed significantly more quickly than those for the direct outer-sphere electron transfer to O2. The proposed reaction mechanism quantitatively describes the experimental kinetic curves over a wide range of experimental conditions.

Polyoxometalates (POMs) and in particularly Keggin heteropolytungstates are much studied and commercially important catalysts for dioxygen-based oxidation processes. The rate-limiting step in many POM-catalyzed O-based oxidations is reoxidation of the reduced POM by O. We report here that this reoxidation process, as represented by the one-electron-reduced Keggin complexes POM (α-PWO and α-SiVWO) reacting with O, is efficiently catalyzed by a combination of copper and iron complexes. The reaction kinetics and mechanism have been comprehensively studied in sulfate and phosphate buffer at pH 1.8. The catalytic pathway includes a reversible reaction between Cu(II) and Fe(II), followed by a fast oxidation of POM by Fe(III) and Cu(I) by O to regenerate Fe(II) and Cu(II). The proposed reaction mechanism quantitatively describes the experimental kinetic curves over a wide range of experimental conditions. Since the oxidized forms, α-PWO and α-SiVWO, are far better oxidants of organic substrates than the previously studied POMs, α-SiWO and α-AlWO, this synergistic Fe/Cu cocatalysis of reduced-POM reoxidation could well facilitate significant new O/air-based processes.

Our group reported that the polyoxometalate Na 10 [Co 4 V 2 W 18 O 68 ]•26H 2 O (Co 4 V 2 ) is an active water oxidation catalyst and provided characterization of this system (J. Am. Chem. Soc. 2014, 136 (26), 9268). Two recent publications called into question the stability of Co 4 V 2 , one noting the miss-assignment of a 51 V NMR peak (Inorg. Chem. 2016, 55 (11), 5343) and another providing additional stability studies (ACS Catal., 2017, 7 (1), 7). We report here solution studies that further clarify stability limitations in this system by locating the correct 51 V NMR resonance of Co 4 V 2 and the other V-containing species present. Furthermore, we demonstrate that the observed catalytic activity cannot be explained simply by Co(II) aq , but arises from multiple active WOC species in solution. Key points about investigating such complex equilibrating aqueous catalyst systems are addressed.

In this study, transmission Raman spectroscopy was explored for the direct measurement of the density of packed polyethylene (PE) pellets. A simple and direct transmission Raman measurement of packed, solid granules or pellet samples without pretreatment is greatly advantageous. Initially, the optimal packing thickness of PE pellets for transmission Raman measurement was determined by investigating the reproducibility of triplicate spectra collected by varying the thickness from 2 to 9 cm. Once determined, transmission Raman spectra were collected for 25 different grades of PE pellets and the partial least squares method was used to determine the sample density. The resulting accuracy was 0.00067 g·cm −3 , while that obtained using backscattering measurements was 0.00083 g·cm −3 . To investigate possible inhomogeneity within a pellet, Raman line mapping was performed over the face of a sectioned pellet and spectral variations among the mapped spectra were examined using principal component analysis. In addition, differential scanning calorimetry was performed on three samples prepared separately by cutting a pellet into left, middle, and right sections. Based on both studies, internal pellet inhomogeneity was found to be minute, but was clearly present. The correct sample representation of internally inhomogeneous PE pellets by the transmission Raman measurement eventually improved the accuracy for density determination. Finally sample-to-sample two-dimensional correlation analysis was used to further examine the origin of the improved accuracy.