Activation Of Oxygen Molecules Research Articles

Pt/MnOx–CeO2 catalysts for the complete oxidation of formaldehyde at ambient temperature

MnOx–CeO2 mixed oxides with a Mn/(Mn+Ce) molar ratios of 0–1 were prepared by a modified coprecipitation method and investigated for the complete oxidation of formaldehyde. The MnOx–CeO2 with Mn/(Mn+Ce) molar ratio of 0.5 exhibited the highest catalytic activity among the MnOx–CeO2 mixed oxides. Structure analysis by X-ray powder diffraction and temperature-programmed reduction of hydrogen revealed that the formation of MnOx–CeO2 solid solution greatly improved the low-temperature reducibility, resulting in a higher catalytic activity for the oxidation of formaldehyde. Promoting effect of Pt on the MnOx–CeO2 mixed oxide indicated that both the Pt precursors and the reduction temperature greatly affected the catalytic performance. Pt/MnOx–CeO2 catalyst prepared from chlorine-free precursor showed extremely high activity and stability after pretreatment with hydrogen at 473K. 100% conversion of formaldehyde was achieved at ambient temperature and no deactivation was observed for 120h time-on-stream. The promoting effect of Pt was ascribed to enhance the effective activation of oxygen molecule on the MnOx–CeO2 support.

Production of hydrogen peroxide by the reaction of hydroxylamine and molecular oxygen over activated carbons

Abstract In situ production of hydrogen peroxide was achieved by the reaction of hydroxylamine and molecular oxygen over reusable activated carbon catalysts at ambient temperature with environmentally benign by-products (nitrogen and water). The yield of hydrogen peroxide was about 46% with the concentration of 0.39 wt% (114 mmol/L). Temperature-programmed desorption analysis and Fourier transformation infrared spectroscopy revealed that the oxygen-containing species, especially quinone/semiquinone-like structures, on the surface of activated carbon played a crucial role in the generation of H2O2, probably through the effective activation of oxygen molecule.

Prooxidant action of xanthurenic acid and quinoline compounds: Role of transition metals in the generation of reactive oxygen species and enhanced formation of 8-hydroxy-2′-deoxyguanosine in DNA†

Xanthurenic acid, a product of tryptophan-NAD pathway, and quinoline compounds produced reactive oxygen species as a complex with iron. Aconitase, the most sensitive enzyme to oxidative stress was inactivated effectively by xanthurenic acid and to a lesser extent by 8-quinolinol in the presence of ferrous sulfate. The inactivation of aconitase was iron-dependent, and was prevented by TEMPOL, a scavenger of reactive oxygen species, suggesting that reduced iron bound to xanthurenic acid or 8-quinolinol can activate oxygen molecule to form superoxide radical. However, kynurenic acid and quinaldic acid without 8-hydroxyl group did not produce reactive oxygen species. Of the quinoline compounds tested, xanthurenic acid and 8-quinolinol with 8-hydroxyl group stimulated the autooxidation of ferrous ion, but kynurenic acid and quinaldic acid did not affect the oxidation of ferrous ion. Hydroxyl group at 8-positions of quinoline compounds was essential for the binding of iron causing the generation of reactive oxygen species. 8-Quinolinol effectively enhanced the ascorbate/copper-mediated formation of 8-hydroxy-2'-deoxyguanosine in DNA, suggesting the quinolinol/copper-dependent stimulation hydroxyl radical formation. Xanthurenic acid and 8-quinolinol as the metal-chelate complexes can show various cytotoxic effects by generating reactive oxygen species through the ferrous or cuprous ion-dependent activation of oxygen molecule.

Oxidation of cyclohexene with molecular oxygen catalyzed by cobalt porphyrin complexes immobilized on montmorillonite

Composite materials were prepared by intercalating cationic porphyrinato cobalt complexes with the substituents of the quaternary ammonium salt of heterocyclic amine into a montmorillonite interlayer. Using these clay interlayer-fixed porphyrinato cobalt complexes as catalyst, the epoxidation of cyclohexene by oxygen molecules was examined. The prepared intercalation compounds have mesopores with an average diameter of about 12 nm, and their specific surface area increased in proportion to the amount of the intercalated porphyrinato cobalt complexes. It was proven that the porphyrinato cobalt complex was intercalated between montmorillonite layers functioned as a pillaring agent. A pillared clay catalyst, which was prepared by intercalating a [meso-tetrakis(1-ethyl-3-pyridinio)porphyrinato] cobalt complex into the montmorillonite interlayer, showed the highest catalytic activity forming 1,2-epoxycyclohexane preferentially in the presence of isobutyraldehyde. It is suggested that montmorillonite contributes to the stabilization of the porphyrinato cobalt complex, and also plays a role in accelerating oxidation by activating oxygen molecules by way of constructing a reaction field that is regulated three-dimensionally through electrostatic interaction with guest molecules.

Mechanistic Studies on the O2-mediated Oxidation of Olefins in the Presence of (Schiff-base)Mn(III) Catalyst and NaBH4

Molecular oxygen would be the most desirable oxidant in organic synthesis due to its economically and environmentally favorable properties. A lot of efforts has long been made to utilize the oxygen molecule as the oxidizing reagent under mild reaction conditions. Probably, the most successful strategies have been the employment of the transition metals as the catalyst for the activation of oxygen molecule. Recently, we have reported the oxidation of olefins to the corresponding alcohols using oxygen molecule as the oxidant, where (schiff-base)Mn(III) complexes were used as the catalysts in the presence of sodium borohydride (eq. 1). During the study of this reaction, we were tempted to investigate reaction mechanism. Described here are the results of our experimental observations and subsequent mechanistic conclusions.

Complete oxidation of formaldehyde over Ag/MnOx–CeO2 catalysts

Complete oxidization of formaldehyde into harmless water and carbon dioxide was obtained over Ag/MnOx–CeO2 catalysts at a temperature as low as 373 K. Structural analysis by X-ray powder diffraction (XRD), X-ray photoelectron spectra (XPS) and temperature-programmed reduction (TPR) measurements revealed that the formation of MnOx–CeO2 solid solution and the subsequent addition of silver species resulted in significant improvement in the effective activation of oxygen molecules in the reaction stream. Accordingly, a consecutive oxygen transfer mechanism starting from the oxygen reservoir of CeO2 to active Ag2O sites through MnOx was proposed. The oxygen species released from the decomposition of Ag2O participated in the oxidation of formaldehyde, and the reoxidation of Ag to Ag2O was achieved through the oxygen species from MnO2. Simultaneously, the transformation of the produced Mn2O3 to MnO2 was realized by the oxygen species from the oxygen reservoir of CeO2, and the Ce2O3 thus formed can be reoxidized into CeO2 by the oxygen in the feed stream. Such a consecutive oxygen pathway was suggested to play an important role in the complete oxidation of formaldehyde.

Quantum Chemical Modeling of Reaction Mechanism for 2-Oxoglutarate Dependent Enzymes: Effect of Substitution of Iron by Nickel and Cobalt

Enzymatic hydroxylation reactions carried out by 2-oxoglutarate (2OG) dependent iron-containing oxygenases were recently implicated in oxygen sensing. In addition to oxygen depletion, two metals, cobalt and nickel, are capable of inducing hypoxic stress in cells by inhibiting oxygenase activity. Two possible scenarios have been proposed for the explanation of the hypoxic effects of cobalt and nickel: oxidation of enzyme-bound iron following cobalt or nickel exposure, and substitution of iron by cobalt or nickel. Here, by using density functional theory calculations, we modeled the reaction route from the reaction components to the high-spin metal-oxide intermediate in the activation of oxygen molecule by 2OG-dependent enzymes for three metal ions Fe(II), Ni(II), and Co(II) in the active site. An initial molecular model was constructed based on the crystal structure of iron-containing asparaginyl hydroxylase (FIH-1). Nickel- and cobalt-containing enzymes were modeled by a consequent replacement of the iron in the active center. The energy profiles connecting stationary points on the potential surfaces were computed by using the intrinsic reaction coordinate (IRC) technique from the located transition states. The results of calculations show that the substitution of iron by nickel or cobalt modifies the reaction energy profile; however, qualitatively, the reaction mechanism remains essentially the same. Thus, we would postulate that if the iron ion in the active site were substitutable by nickel and/or cobalt ions enzyme activity would be considerably altered due to high activation barriers.

Activation of O 2 and CH 4 on yttrium-stabilized zirconia for the partial oxidation of methane to synthesis gas

The isotopic exchange reaction on ZrO 2 and yttrium-stabilized ZrO 2 (YSZ) during catalytic partial oxidation of methane to synthesis gas (CPOM) was studied with transient pulse experiments. The results reveal, surprisingly, that CPOM over both oxides proceeds via a Mars–van Krevelen mechanism. Despite the presence of adsorbed oxygen species, as confirmed by isotopic exchange experiments under reaction conditions, methane is selectively oxidized by lattice oxygen ions on the surfaces of YSZ and ZrO 2. At 900 °C, about 8 and 14% of lattice oxygen in the outermost surface layer of ZrO 2 and YSZ, respectively, can be extracted by methane. Extraction of lattice oxygen results in the formation of surface oxygen vacancies. However, the routes for replenishing oxygen differ for the two oxides. For ZrO 2, the extracted lattice oxygen ions are replenished by direct activation of molecular oxygen at the site of the surface vacancy. The presence of a high concentration of surface oxygen vacancies on YSZ, generated by doping of ZrO 2 with Y 2O 3, permits fast activation of oxygen molecules and fast lattice diffusion of oxygen. The two effects together lead to a rapid replenishment of the surface lattice oxygen extracted by methane. The proposed mechanism explains both the comparatively high activity of YSZ in CPOM and the observation that, in contrast to ZrO 2, lattice oxygen is found exclusively in oxidation products of methane over YSZ during pulse experiments.

Reaction of Cp*RuCl(PPh 3) 2 with dioxygen and formation of a neutral complex Cp*RuCl(O 2)(PPh 3)

Reaction of Cp*RuCl(PPh 3) 2 ( 1) with atmospheric oxygen occurs at room temperature in the presence of acetone or methylene chloride leading to Cp*RuCl(O 2)(PPh 3) ( 2). This complex is remarkably stable in the solid state and it can also release, under not unduly harsh conditions, the activated oxygen molecule, which can oxidize the phosphite L=MeOP[(OCHMe) 2CH 2] to the corresponding phosphate, along with formation of the mono- and disubstituted Cp*RuCl(PPh 3)(L) ( 5), Cp*RuCl(L) 2 ( 6) and [Cp*RuCl(PPh 3)(L) 2]Cl ( 7), complexes. Structural information of phosphite derivatives 5 and 6 has been obtained by X-ray diffraction.

A comparison of nanoscale and large-size BaCl 2-modified Er 2O 3 catalysts for the selective oxidation of ethane to ethylene

The nanoscale Er 2O 3 and 30 mol% BaCl 2/Er 2O 3 catalysts prepared by a modified sol–gel method have been compared with their large-size counterparts for the oxidative dehydrogenation of ethane (ODE) to ethylene. The nanoscale catalysts showed better catalytic performance than the corresponding large-size ones, especially at lower reaction temperatures and shorter contact times. X-ray diffraction results revealed that the extent of cubic Er 2O 3 lattice expansion in nanoscale BaCl 2/Er 2O 3 was larger than that in the large-size one and that more defects were formed in the nanoscale catalyst. We detected further enlargement of the cubic Er 2O 3 lattice in the nanoscale 30 mol% BaCl 2/Er 2O 3 catalyst during the first 40 h of on-stream reaction. The results of X-ray photoelectron spectroscopic investigation demonstrated that after 40 h surface chlorine composition on nanoscale 30 mol% BaCl 2/Er 2O 3 increased slightly, while that on the large-size catalyst decreased. O 2 temperature-programmed desorption studies illustrated that, compared to the large-size catalysts, there were more oxygen species desorbed from the nanoscale ones. In situ Raman studies indicated that the reduction in particle size was beneficial for the activation of oxygen molecules. Taking into consideration the catalytic activity and Raman results, we suggest that dioxygen species such as O 2 δ− (0< δ<1), O 2 −, O 2 n− (1< n<2), and O 2 2− are responsible for the selective oxidation of C 2H 6 to C 2H 4.

Important role of proton in activation of oxygen molecules in heme-containing oxygenases

DFT (density functional theory) calculations have revealed that the position of the proton of the hydroperoxide adduct of an Fe(III)–porphyrin compound influences greatly the electron density of the oxygen atoms of the peroxide ion, suggesting that the position of the proton in the hydroperoxo–iron(III) compound plays an important role in controlling the electronic interaction between the hydroperoxide adduct and the porphyrin system (OOH–π interaction).

Raman spectroscopic and EPR investigations of oxygen species on SrCl 2-promoted Ln 2O 3 (Ln=Sm and Nd) catalysts for ethane-selective oxidation to ethene

The SrCl 2-promoted Ln 2O 3 (Ln=Sm and Nd) catalysts have been investigated for the oxidative dehydrogenation of ethane (ODE) to ethene. With the doping of SrCl 2 into Ln 2O 3, the C 2H 4 selectivity and C 2H 6 conversion were enhanced considerably. We also found that the addition of SrCl 2 to Ln 2O 3 could markedly reduce the deep oxidation of C 2H 4. The 40 mol% SrCl 2/Ln 2O 3 catalysts were stable for 60 h of on-stream ODE reaction. Under the reaction conditions of temperature=640°C and space velocity=6000 ml h −1 g −1, 40 mol% SrCl 2/Sm 2O 3 showed 80.3% C 2H 6 conversion, 70.9% C 2H 4 selectivity, and 56.9% C 2H 4 yield while 40 mol% SrCl 2/Nd 2O 3 gave 63.8% C 2H 6 conversion, 74.3% C 2H 4 selectivity, and 47.4% C 2H 4 yield. X-ray photoelectron spectroscopic and chemical analysis of chloride indicated that the Cl − anions were evenly distributed in the 40 mol% SrCl 2/Ln 2O 3 catalysts. We observed that Cl leaching was insignificant. The results of temperature-programmed desorption of oxygen and temperature-programmed reduction studies demonstrated that the addition of SrCl 2 to Ln 2O 3 enhanced the activation of oxygen molecules. We believe that such improvement is closely associated with the defects formed during the exchanges of ions between the SrCl 2 and Ln 2O 3 phases. X-ray powder diffraction results revealed that the Ln 2O 3 lattices were enlarged, whereas the SrCl 2 lattices contracted in the 40 mol% SrCl 2/Ln 2O 3 catalysts. In situ Raman results indicated that there were dioxygen adspecies such as O 2 2−, O 2 n− (1< n<2), O 2 −, and O 2 δ− (0< δ<1) on the 40 mol% SrCl 2/Ln 2O 3 catalysts. Electron paramagnetic resonance (EPR) results indicated that there were dioxygen O 2 − and mono-oxygen O − adspecies present on the SrCl 2-doped catalysts. Based on the results of in situ Raman and EPR studies as well as the catalytic activity data, we suggest that the O 2 2−, O 2 n− , O 2 −, and O 2 δ− adspecies favor the selective oxidation of C 2H 6 to C 2H 4, whereas the O − adspecies is responsible for the deep oxidation of C 2H 6.

DNA promotes the activation of oxygen molecules by binuclear cobalt(II) compounds

We have observed that binuclear cobalt(II) compounds with H(HPTP), Co 2(HPTP)(CH 3COO)(ClO 4) 2 and Co 2(HPTP)Cl(ClO 4) 2, react with oxygen molecules when DNA is present in the solution, cleaving the DNA (H(HPTP) denotes N, N, N′, N′-tetrakis(2-pyridylmethyl)-1,3-diaminopropane-2- ol).

Dehydrogenation of Propane over Chromia-Pillared Zirconium Phosphate Catalysts

Two series of porous chromia-pillared {alpha}-zirconium phosphate materials, prepared using two different methods of colloidization of the initial phase and with variable chromium contents (CrZrP-Xa and CrZrP-Xb) have been tested in the oxidative and nonoxidative dehydrogenation of propane in a flow reactor at atmospheric pressure. All catalysts are highly selective to propene under nonoxidative conditions at 823 K. In both series of catalysts, the initial activity increases with the chromium content, but generally CrZrP-Xb catalysts are more active than those of series CrZrP-Xa, which is in good agreement with their higher chromium contents and greater dispersions. In all cases, deactivation was detected due to coke formed from undesired reactions. When the reactions were carried out under oxidative conditions at 673 K, the activities were enhanced and the observed deactivation was minimum. The activities found vary between 0.47 and 1.31 {micro}mol of propene g{sup {minus}1} s{sup {minus}1} and are maintained after 200 min of reaction. These activity values were also related to the chromium content, being slightly higher for CrZrP-Xb materials. A parallel study to evaluate the influence of acidity in the obtained results has been carried out. The activities found of these catalysts seem to be related to the presencemore » of Cr(III) centers with vacancies in their coordination sphere. These vacancies, in nonoxidative conditions, can activate the reactive molecules originating propene and hydrogen. On the other hand, in an oxidative atmosphere, Cr(III) species can activate oxygen molecules, through an electronic transference process, yielding propene and water.« less

Fe in MFI metallosilicates, characterization and catalytic activity

Fe was introduced in MFI molecular sieves by four means: direct synthesis, postsynthetic modification of boralite, ion exchange with HZSM-5 in solid or liquid state, and by impregnation. Fe species were characterized by TPR and EPR (dispersed Fe 3+). Additional information was obtained from the FTIR spectra of adsorbed acetonitrile- d 3 and from TPD of ammonia, both of which characterized the acidity of the samples employed. The catalytic activity was examined in oxidative dehydrogenation of ethane, in aromatization of ethane and ethene, in CO oxidation by 18O 2 and in 18O 2+ 16O 2 reactions. The results obtained were compared with those over Ga metallosilicates: it was found that Fe in the zeolite framework appeared only in directly synthesized samples. These were much more active in oxidative dehydrogenation of ethane than HGaZSM-5, but much less active in the aromatization reactions. The oxidation of CO with gaseous oxygen began <100°C, contrary to Ga samples over which the onset of reaction needed a much higher temperature. Significant activation of oxygen molecules, leading to the equilibration of gaseous oxygen molecules as well as to the exchange with lattice oxygens, was observed at 400°C over the most active ferrisilicate.

The oxidative coupling of methane overBa/CO 3LaOCl catalysts

The performance of LaOCl at 800°C was promoted by BaCO 3 in OCM reaction. When 10 mol% BaCO 3 was added, there was little change in CH 4 conversion, but C 2 selectivity was increased from 37% to 66%. With the increase in BaCO 3 loading, the Ba/CO 3 LaOCl catalysts decreased in specific surface area. The improvement in C 2 selectivity is partly due to surface area diminution. The addition of BaCO 3 has also caused surface modification of LaOCl. In the absence of BaCO 3, LaOCl in OCM reaction is capable of generating dioxygen species O 2 2−, O 2 n− and O 2 −, which can cause deep oxidation of CH 4, C 2H 6 and C 2H 4. With the addition of BaCO 3, sites for the activation of oxygen molecules are depleted. The nearby Cl − ions have the ability of destabilizing the BaCO 3, causing it to decompose at ca. 780°C, a decomposition temperature about 200°C lower than that of pure BaCO 3. With the deprivation of dioxygen species, the CH 4 oxidative dehydrogenation process is enhanced, while deep oxidation processes are suppressed. Compared with the deep oxidation reaction processes, the number of sites required in the H-abstraction process is a lot fewer. Hence, although the specific surface area of LaOCl was reduced with the addition of BaCO 3, the conversion of CH 4 over the Ba/CO 3 LaOCl catalysts did not drop significantly.

Direct oxidation of benzene by electron transfer in a multi-functionalized chelate resin system

A polystyrene-based chelate resin, functionalized by tridentate iminodiacetate (ida) moieties, can easily immobilize various components by ion exchange or co-ordination. A multi-functionalized chelate resin immobilized with both ultrafine platinum particles and FeII ions has been prepared successfully by co-ordinating FeII ions to ida moieties after supporting ultrafine platinum particles. Electron transfer from the FeII ions to oxygen produced FeIII ions and activated oxygen in the system, while the FeIII ions were reduced again to FeII ions by molecular hydrogen via the platinum particles. The activation of oxygen molecules and/or a C–H bond of benzene are essential to the direct oxidation of benzene. Thus, benzene has been oxidized directly by activated oxygen to produce p-benzoquinone as the main product, indicating that the multi-functionalized chelate resin works like a dioxygenase enzyme. The oxidizing species for benzene was concluded to be the FeII–dioxo complex, the co-ordinating oxygen of which is activated.

Superoxide Dismutase Enhances Tolerance of Freezing Stress in Transgenic Alfalfa (Medicago sativa L.)

Activated oxygen or oxygen free radicals have been implicated in a number of physiological disorders in plants including freezing injury. Superoxide dismutase (SOD) catalyzes the dismutation of superoxide into O2 and H2O2 and thereby reduces the titer of activated oxygen molecules in the cell. To further examine the relationship between oxidative and freezing stresses, the expression of SOD was modified in transgenic alfalfa (Medicago sativa L.). The Mn-SOD cDNA from Nicotiana plumbaginifolia under the control of the cauliflower mosaic virus 35S promoter was introduced into alfalfa using Agrobacterium tumefaciens-mediated transformation. Two plasmid vectors, pMitSOD and pChlSOD, contained a chimeric Mn-SOD construct with a transit peptide for targeting to the mitochondria or one for targeting to the chloroplast, respectively. The putatively transgenic plants were selected for resistance to kanamycin and screened for neomycin phosphotransferase activity and the presence of an additional Mn-SOD isozyme. Detailed analysis of a set of four selected transformants indicated that some had enhanced SOD activity, increased tolerance to the diphenyl ether herbicide, acifluorfen, and increased regrowth after freezing stress. The F1 progeny of one line, RA3-ChlSOD-30, were analyzed by SOD isozyme activity, by polymerase chain reaction for the Mn-SOD gene, and by polymerase chain reaction for the neo gene. RA3-ChlSOD-30 had three sites of insertion of pChlSOD, but only one gave a functional Mn-SOD isozyme; the other two were apparently partial insertions. The progeny with a functional Mn-SOD transgene had more rapid regrowth following freezing stress than those progeny lacking the functional Mn-SOD transgene, suggesting that Mn-SOD serves a protective role by minimizing oxygen free radical production after freezing stress.

Fcγ receptor-mediated biological activities of human leukemic cell lines and their modulation by transforming growth factor-β1 and interleukin 6

Previously we reported that transforming growth factor-β1 (TGF-β1) remarkably enhanced the differentiation of human leukemic cell lines, HL-60 and THP-1, in the presence of 1α,25-dihydroxyvitamin D 3 (VD 3) and also that it induced Fc receptor for immunoglobulin G (FcγR), type IIIB, in the presence of retinoic acid (RA). The present study revealed that TGF-β1 enhanced the FcγRI- and FcγRII-mediated antibody-dependent cellular cytotoxicity (ADCC) of the cells differentiated in the presence of VD 3 and RA. However, production of active oxygen molecules was suppressed by TGF-β1. On the other hand, IL-6 stimulated production of active oxygen molecules and ADCC of the cells treated with VD 3 and tumor necrosis factor-α (TNF-α). Furthermore, the levels of cell surface FcγRI and FcγRII were not clearly correlated with the ADCC. The TGF-β1 VD 3-treated HL-60 cells were able to synthesize mRNAs for TGF-β1 and TNF-α, although TNF-α protein was not detectable. These results suggest that TGF-β1 has a bifunctional role, either stimulatory or inhibitory, in the modulation of macrophage activities through FcγRs and that IL-6 stimulates certain macrophage activities in mature cells.

New Insights into the Reactivity of Mn(II) Coordination Complexes with Dioxygen

Abstract As a clearer picture of the oxygen activation processes by manganese(II) co-ordination complexes begins to emerge, we are more likely to understand the biological significance of the Mn-O2 interaction in various enzymes and to use the activated oxygen molecule for the catalytic oxidation of organic substrates. This Comment describes some of the recent advancements in the area of dioxygen activation by manganese(II) complexes. Details regarding the binding of the O2 moiety to the metal center and the reaction pathway that is taken during the four electron reduction of the O-O double bond are presented. Particular attention is given to new information on Mn(II) Schiff-base complexes as these results provide strong evidence for a more straightforward and simpler oxygen activation process than previously thought. Also described in this article are examples of unique intermetal oxygen atom transfer reactions that the oxo bridged dimers of Mn Schiff-base complexes undergo with electrophilic metal compo...