Robust and efficient amide-based nonheme manganese(III) hydrocarbon oxidation catalysts: substrate and solvent effects on involvement and partition of multiple active oxidants

Two new mononuclear nonheme manganese(III) complexes of tetradentate ligands containing two deprotonated amide moieties, [Mn(bpc)Cl(H₂O)] ( 1 ) and [Mn(Me₂bpb)Cl(H₂O)] ? CH₃OH ( 2 ), were prepared and characterized. Complex 2 has also been characterized by X‐ray crystallography. Magnetic measurements revealed that the complexes are high spin (S=5/2) Mn^III species with typical magnetic moments of 4.76 and 4.95 μ_B, respectively. These nonheme Mn^III complexes efficiently catalyzed olefin epoxidation and alcohol oxidation upon treatment with MCPBA under mild experimental conditions. Olefin epoxidation by these catalysts is proposed to involve the multiple active oxidants Mn^V?O, Mn^IV?O, and Mn^III? OO(O)CR. Evidence for this approach was derived from reactivity and Hammett studies, KIE (k_H/k_D) values, H₂¹⁸O‐exchange experiments, and the use of peroxyphenylacetic acid as a mechanistic probe. In addition, it has been proposed that the participation of Mn^V?O, Mn^IV?O, and Mn^III? OOR could be controlled by changing the substrate concentration, and that partitioning between heterolysis and homolysis of the O? O bond of a Mn‐acylperoxo intermediate (Mn? OOC(O)R) might be significantly affected by the nature of solvent, and that the O? O bond of the Mn? OOC(O)R might proceed predominantly by heterolytic cleavage in protic solvent. Therefore, a discrete Mn^V?O intermediate appeared to be the dominant reactive species in protic solvents. Furthermore, we have observed close similarities between these nonheme Mn^III complex systems and Mn(saloph) catalysts previously reported, suggesting that this simultaneous operation of the three active oxidants might prevail in all the manganese‐catalyzed olefin epoxidations, including Mn(salen), Mn(nonheme), and even Mn(porphyrin) complexes. This mechanism provides the greatest congruity with related oxidation reactions by using certain Mn complexes as catalysts.     

Cerium(IV)‐Driven Water Oxidation Catalyzed by a Manganese(V)–Nitrido Complex

The study of manganese complexes as water‐oxidation catalysts (WOCs) is of great interest because they can serve as models for the oxygen‐evolving complex of photosystem II. In most of the reported Mn‐based WOCs, manganese exists in the oxidation states III or IV, and the catalysts generally give low turnovers, especially with one‐electron oxidants such as Ce^IV. Now, a different class of Mn‐based catalysts, namely manganese(V)–nitrido complexes, were explored. The complex [Mn^V(N)(CN)₄]²⁻ turned out to be an active homogeneous WOC using (NH₄)₂[Ce(NO₃)₆] as the terminal oxidant, with a turnover number of higher than 180 and a maximum turnover frequency of 6 min⁻¹. The study suggests that active WOCs may be constructed based on the Mn^V(N) platform.     

A natural manganese ore as a heterogeneous catalyst to effectively activate peroxymonosulfate to oxidize organic pollutants

Heterogeneous transition metal catalysts are indispensable in improving environmental pollution. However, their fabrication is often costly and cumbersome, and they can easily pollute the environment. This study proposed using a natural Gabonese ore (GBO) containing Mn_xO_y and Fe_xO_y as catalysts to degrade orange II (OII) via peroxymonosulfate (PMS) activation. The GBO + PMS system exhibited extraordinarily high stability and catalytic activity towards OII elimination (92.2%, 0.0453 min^?1). The reactive oxygen species (ROS) generated in the system were identified using radical scavenging tests and electron spin-resonance (ESR) analysis. Singlet oxygen (¹O₂) represented the dominant reactive species for OII degradation, while the system presented a lower reaction energy barrier and was effective in a broad pH range (2–10). This work also proposed the activation mechanism for the GBO + PMS system and OII degradation pathways. This study revealed a new approach for exploring inexpensive, eco-friendly, efficient, and stable heterogeneous transition metal catalysts.     

Accelerated ZnMoO4 photocatalytic degradation of pirimicarb under UV light mediated by peroxymonosulfate

This paper reports the optimized synthesis of zinc molybdates by the hydrothermal method and the combination of ZnMoO₄ and peroxymonosulfate (PMS) under UV irradiation for the degradation of pirimicarb. The as‐prepared ZnMoO₄ photocatalyst was characterized using X‐ray diffraction, scanning electron microscopy, X‐ray photoelectron spectroscopy and UV–visible diffuse reflectance spectroscopy. The effects of operational parameters in the ZnMoO₄/PMS/UV system were evaluated and the results indicated the highest performance is achieved with pH = 9.0, 1 mM PMS and 1 g l^?1 ZnMoO₄. The degradation efficiency of pirimicarb was 98% after 3 h in the photocatalytic process. A photodegradation mechanism is proposed based on scavenger and electron spin resonance studies to decide the main active species and by using chromatography–mass spectrometry to identify the major intermediates. Pirimicarb degradation is found to be mainly driven by holes and ?O₂^? radicals, with the contribution of ?OH and SO₄^?? radicals enhancing the process in the tested catalytic system. The mechanism is proposed involving two routes, dealkylation and decarbamoylation. Lastly, the zinc molybdate photocatalyst is shown to be stable, reusable and efficient in the removal of pirimicarb from real water samples in the presence of PMS, demonstrating potential application in the treatment of contaminated and/or environmental water.     

Influences of steric bulkiness in hydrotris(pyrazolyl)borate ligands on ethylene polymerization reaction

Manganese(II) complex catalysts with hydrotris(pyrazolyl)borate ligands have been examined on their catalytic performance in ethylene polymerization and ethylene/1‐hexene copolymerization. The activities of [Mn(L6)(Cl)(NCMe)] ( 1 ) and [Mn(L10)(Cl)] ( 2 ) activated by Al(i‐Bu)₃/[Ph₃C][B(C₆F₅)₄] for ethylene polymerization go up to 326 and 11 kg mol (cat^?1) h^?1, respectively, (L6^? = hydrotris(3‐phenyl‐5‐methyl‐1‐pyrazolyl)borate anion, L10^? = hydrotris(3‐adamantyl‐5‐isopropyl‐1‐pyrazolyl)borate anion). In particular, for ethylene/1‐hexene copolymerization, complex 1 gives high‐molecular‐weight poly(ethylene‐co‐1‐hexene)s with the highest M_w of 439,000 in manganese olefin polymerization catalyst systems. Moreover, the 1‐hexene incorporation by complex 1 seems more efficient than that by [Mn(L3)(Cl)] ( 4 ) (L3^? = hydrotris(3‐tertiary butyl‐5‐isopropyl‐1‐pyrazolyl)borate anion). In this work, we demonstrated that the coordination geometry and coordination number are also important factors for ethylene polymerization reaction as well as steric hindrances and ligand frameworks in our manganese(II) catalysts. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5720–5727, 2009     

Reverse Micelle Synthesis of Colloidal Nickel–Manganese Layered Double Hydroxide Nanosheets and Their Pseudocapacitive Properties

Colloidal nanosheets of nickel–manganese layered double hydroxides (LDHs) have been synthesized in high yields through a facile reverse micelle method with xylene as an oil phase and oleylamine as a surfactant. Electron microscopy studies of the product revealed the formation of colloidal nanoplatelets with sizes of 50–150 nm, and X‐ray diffraction, energy dispersive X‐ray spectroscopy, and X‐ray photoelectron spectroscopy studies showed that the Ni–Mn LDH nanosheets had a hydrotalcite‐like structure with a formula of [Ni₃Mn(OH)₈](Cl^?) ? n H₂O. We found that the presence of both Ni and Mn precursors was required for the growth of Ni‐Mn LDH nanosheets. As pseudocapacitors, the Ni–Mn LDH nanosheets exhibited much higher specific capacitance than unitary nickel hydroxides and manganese oxides.     

Promotion of Mn Doped Co/CNTs Catalysts for CO Hydrogenation to Light Olefins

With various contents, Mn was introduced into carbon nanotubes (CNTs) supported cobalt catalysts and the obtained Mn‐Co/CNTs catalysts were investigated for CO hydrogenation to light alkenes and characterized by N₂ adsorption, X‐ray diffraction (XRD), X‐ray photoelectron spectra (XPS), H₂ temperature programmed reduction (TPR), CO temperature programmed desorption (TPD) and transmission electron microscope (TEM). The results indicate that the addition of a small amount of Mn (0.3 wt%) to CNTs‐supported Co catalyst significantly increased the selectivity of C₂–C₄ olefins and decreased the selectivity of CH₄. However, with further addition of Mn to the cobalt catalysts, the CH₄ selectivity decreased obviously along with the increase of the C₅₊ selectivity. Compared with the unpromoted catalysts, the Mn‐promoted cobalt catalysts increased the C₂^?–C₄^?/C₂⁰–C₄⁰ molar ratio.     

Solvothermal Synthesis and Crystal Structures of Two Manganese Complexes [Mn(II)(acac−)2(4,4′‐bipy)]n (bipy=4,4′‐bipyridine) and [Mn(III)(acac−)3]·4CO(NH2)2

Two special manganese complexes [Mn(II)(acac^?)₂(4,4′‐bipy)]_n (bipy=4,4′‐bipyridine) (complex 1 ) and [Mn(III)(acac^?)₃]·4CO(NH₂)₂ (acacH=acetylacetone) (complex 2 ) were synthesized in the same strategy by solvothermal method. Single crystal X‐ray diffraction revealed the complex 1 consists of one‐dimensional infinite coordination chain, with the manganese centers bridged by 4,4′‐bipy. And free carbamides of complex 2 connect with each other through the hydrogen bonds to form a 14‐membered carbamide ring and a zig‐zag plane. Both enantiomers of Mn(III)(acac^?)₃ exist in the structure, forming a racemate. Furthermore, these enantiomers and those zig‐zag planes are linked with hydrogen bonds to form an unique spatial network.     

HPW/PDMAEMA‐b‐PMAA/ZIF‐8 Ternary Lamellar Composite and the Photocatalytic Degradation of Methylene Blue

PDMAEMA‐b‐PMAA block copolymers were prepared by the sequential RAFT polymerization of DMAEMA and tBMA, followed by hydrolysis. Phosphotungstic acid (HPW) was anchored to the PDMAEMA blocks through electrostatic interactions and the as‐obtained HPW/PDMAEMA‐b‐PMAA was added to the synthesis of ZIF‐8. During the formation of ZIF‐8, the PMAA blocks coordinated to the Zn²⁺ ions through their carboxy groups, along with the HPW groups that were anchored to the PDMAEMA blocks. In this way, the block copolymer could consolidate the interactions between HPW and ZIF‐8 and prevent the leakage of HPW. Finally, the HPW/PDMAEMA‐b‐PMAA/ZIF‐8 ternary lamellar composite was obtained and the structure of the HPW/PDMAEMA‐b‐PMAA/ZIF‐8 hybrid material was characterized by using powder X‐ray diffraction (PXRD), X‐ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). As a photocatalyst, the HPW/PDMAEMA‐b‐PMAA/ZIF‐8 ternary lamellar composite showed excellent photoactivity for the degradation of methylene blue (MB). The rate of degradation of MB was 0.0240 min^?1, which was 7.5‐times higher than that of commercially available P25 (0.0032 min^?1). In the presence of H₂O₂, the kinetic degradation parameters of the composite reached 0.0634 min^?1, which was about 19.8‐times higher than that of P25.     

Thermal decomposition of magnesian kutnahorite

This work investigates the thermal decomposition of magnesian kutnahorite, which belongs to the dolomite group.The DTA curve measured in static air using a small amount of sample (5.0 mg) is quite different from those published previously. This difference might be due to the effect of a self-generated CO₂ atmosphere.In a CO₂ flow of 100 ml min^?1, magnesian kutnahorite decomposes in four steps. Mg-kutnahorite → CaCO₃ + Mg₂MnO₄ + Mn₃O₄ + MgO → CaCO₃ + CaMnO₃ + MgO → CaCO₃ + CaMnO₃ + Ca₂MnO₄ + MgO → CaMnO₃ + Ca₂MnO₄ + MgO + CaO.However, in a mixed gas flow of CO₂ at 95 ml min^?1 and CO at 5 ml min^?1, it decomposes, like dolomite, in two steps. Mg-kutnahorite → CaCO₃ + (Mg,Mn)$\text{O}\text{-}$ → (Ca, Mn)O + (Mg,Mn)$\text{O}\text{-}$.It has been found that the equilibrium redistribution of Mn between (Ca, Mn)$\text{O}\text{-}$ and (Mg, Mn)$\text{O}\text{-}$ is achieved at the second decomposition step. This is supported by theoretical considerations.Consequently, when the O₂ partial pressure in the atmosphere is low enough to keep Mn in a bivalent state, the Mn bearing dolomite group mineral decomposes in a similar manner to dolomite itself.