Showing papers on "Disproportionation published in 2007"

Reduction behavior of iron oxides in hydrogen and carbon monoxide atmospheres

Abstract: The reduction of various iron oxides in hydrogen and carbon monoxide atmospheres has been investigated by temperature programmed reduction (TPR H2 and TPR CO ), thermo-gravimetric and differential temperature analysis (TG-DTA-MS), and conventional and “ in situ ” XRD methods Five different compounds of iron oxides were characterized: hematite α-Fe 2 O 3 , goethite α-FeOOH, ferrihydrite Fe 5 HO 8 ·4H 2 O, magnetite Fe 3 O 4 and wustite FeO In the case of iron oxide-hydroxides, goethite and ferrihydrite, the reduction process takes place after accompanying dehydration below 300 °C Instead of the commonly accepted two-stage reduction of hematite, 3 α-Fe 2 O 3 → 2 Fe 3 O 4 → 6 Fe, three-stage mechanism 3Fe 2 O 3 → 2Fe 3 O 4 → 6FeO → 6Fe is postulated especially when temperature of reduction overlaps 570 °C Up to this temperature the postulated mechanism may also involve disproportionation reaction, 3Fe 2+ ⇌ 2Fe 3+ + Fe, occurring at both the atomic scale on two-dimensional interface border Fe 3 O 4 /Fe or stoichiometrically equivalent and thermally induced, above 250 °C, phase transformation—wustite disproportionation to magnetite and metallic iron, 4FeO ⇌ Fe 3 O 4 + Fe Above 570 °C, the appearance of wustite phase, as an intermediate of hematite reduction in hydrogen, was experimentally confirmed by “ in situ ” XRD method In the case of FeO–H 2 system, instead of one-step simple reduction FeO → Fe, a much more complex two-step pathway FeO → Fe 3 O 4 → Fe up to 570 °C or even the entire sequence of three-step process FeO → Fe 3 O 4 → FeO → Fe up to 880 °C should be reconsidered as a result of the accompanying FeO disproportionation wustite ⇌ magnetite + iron manifesting its role above 150 °C and occurring independently on the kind of atmosphere—inert argon or reductive hydrogen or carbon monoxide The disproportionation reaction of FeO does not consume hydrogen and occurs above 200 °C much easier than FeO reduction in hydrogen above 350 °C The main reason seems to result from different mechanistic pathways of disproportionation and reduction reactions The disproportionation reaction wustite ⇌ magnetite + iron makes simple wustite reduction FeO → Fe a much more complicated process In the case of thermodynamically forced FeO disproportionation, the oxygen sub-lattice, a closely packed cubic network, does not change during wustite → magnetite transformation, but the formation of metallic iron phase requires temperature activated diffusion of iron atoms into the region of inter-phase FeO/Fe 3 O 4 Depending on TPR H2 conditions (heating rate, velocity and hydrogen concentration), the complete reduction of hematite into metallic iron phase can be accomplished at a relatively low temperature, below 380 °C Although the reduction behavior is analogical for all examined iron oxides, it is strongly influenced by their size, crystallinity and the conditions of reduction

Controlled Synthesis and Chemical Conversions of FeO Nanoparticles

Abstract: Transition metal oxide nanoparticles of type MO, where M is Mn, Co, Ni, or Fe, have attracted tremendous interest recently because of their potential as electrode materials for rechargeable solid-state batteries, as efficient catalysts for fuel-cell reactions, and as nanoscale magnetic models for understanding nanomagnetism. W&stite (FeO) is one form of the common iron oxides, a group that also includes hematite (aFe2O3), maghemite (g-Fe2O3), and magnetite (Fe3O4). It has a rock-salt structure with Fe and O forming nonstoichiometric FexO (x= 0.83–0.96) and Fe vacancies in an ordered distribution. The structure is not chemically stable and is prone to decomposition into a-Fe and inverse spinel Fe3O4 through a two-step disproportionation process or to oxidation to form Fe3O4, g-Fe2O3, and/or a-Fe2O3. [4] This chemical reactivity makes FeO nanoparticles difficult to make and those prepared from the high-temperature solution-phase decomposition of iron salt have not been fully characterized. Herein we report a facile organic-phase synthesis of monodisperse FeO nanoparticles through high-temperature reductive decomposition of iron(III) acetylacetonate ([Fe(acac)3]) with oleic acid (OA) and oleylamine (OAm) both as surfactants and solvents. The sizes of the particles are tuned from 14 to 100 nm by controlling the heating conditions and the shapes of the particles are controlled to be either spherical or truncated octahedral depending on the volume ratio of OA and OAm used in the reaction. Thermal annealing under an argon atmosphere converted these FeO nanoparticles into composite Fe Fe3O4 nanoparticles, while controlled oxidation of the FeO nanoparticles resulted in the formation of Fe3O4, g-Fe2O3, or a-Fe2O3 nanoparticles. These monodisperse FeO nanoparticles have great potential for catalysis and gas-sensor applications. The chemical conversions of the paramagnetic FeO nanoparticles may also be considered as an alternative, yet better, approach to the synthesis of various magnetic iron oxide or iron nanoparticles with sizes that are difficult to achieve from previous organic-phase syntheses. The nanoparticles were grown from the reaction mixture ([Fe(acac)3] in a mixture of OA and OAm) by controlled heating at 220 8C and 300 8C. In the presence of an excess amount of OAm, spherical nanoparticles were formed, whereas in the presence of equivalent amounts of OA and OAm, truncated octahedral nanoparticles were obtained. The size of both kinds of nanoparticles was tuned by simply controlling the period of heating at 220 8C and 300 8C. For example, 14-nm spherical nanoparticles were synthesized by treating [Fe(acac)3] with OA (8 mL) and OAm (12 mL) at 220 8C and 300 8C, each for 30 min. Extended heating at 300 8C for 1 h gave 22-nm nanoparticles. Heating of a reaction mixture of [Fe(acac)3], OA (10 mL), and OAm (10 mL) at 220 8C and 300 8C, each for 30 min, led to the formation of 32nm truncated octahedral nanoparticles, while heating of the mixture at 220 8C for 1 h and at 300 8C for 30 min gave 53-nm nanoparticles and heating at 220 8C for 30 min and at 300 8C for 1 h yielded 100-nm truncated octahedral nanoparticles. Figure 1 shows transmission electron microscopy (TEM) images of representative FeO nanoparticles. The truncated octahedral shape of the particles can be better seen in the scanning electron microscopy (SEM) image of Figure 1d. These sizeand shape-controlled syntheses suggest that 1) the use of OA and OAm both as solvents and surfactants facilitates the formation and stabilization of FeO nanoparticles; 2) the presence of an excess of OAm facilitates

Role of Cu0 in Controlled/“Living” Radical Polymerization

Abstract: Several propositions have been made about the mechanism in which Cu0 mediates controlled radical polymerization that include (1) exclusive activation of an alkyl halide initiator by exceptionally active Cu0 to generate a propagating radical and a CuI species, (2) instantaneous disproportionation of CuI into Cu0 and CuII in “catalytic” solvents such as DMSO, and (3) deactivation of the radical by CuII to establish an equilibrium between active and dormant polymer chains. It was further postulated that the activation and deactivation processes in this technique, entitled single-electron-transfer living radical polymerization (SET-LRP), occur via outer-sphere electron transfer (OSET) to produce alkyl halide radical anion intermediates. We report herein on our own investigation of the aforementioned mechanism using Cu complexes of tris[2-(dimethylamino)ethyl]amine (Me6TREN). Model studies were employed to quantify disproportionation of CuI/Me6TREN in DMSO, DMF, and MeCN, where comproportionation of Cu0 with C...

A density functional theory computational study of the role of ligand on the stability of CuI and CuII species associated with ATRP and SET‐LRP

Abstract: Atom transfer radical polymerization (ATRP) and single electron-transfer living radical polymerization (SET-LRP) both utilize copper complexes of various oxidation states with N-ligands to perform their respective activation and deactivation steps. Herein, we utilize DFT (B3YLP) methods to determine the preferred ligand-binding geometries for Cu/N-ligand complexes related to ATRP and SET-LRP. We find that those ligands capable of achieving tetrahedral complexes with CuI and trigonal bipyramidal with axial halide complexes with [CuIIX]+ have higher energies of stabilization. We were able to correlate calculated preferential stabilization of [CuIIX]+ with those ligands that perform best in SET-LRP. A crude calculation of energy of disproportionation revealed that the same preferential binding of [CuIIX]+ results in increased propensity for disproportionation. Finally, by examining the relative energies of the basic steps of ATRP and SET-LRP, we were able to rationalize the transition from the ATRP mechanism to the SET-LRP mechanism as we transition from typical nonpolar ATRP solvents to polar SET-LRP solvents. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4950–4964, 2007

A theoretical study of the inner-sphere disproportionation reaction mechanism of the pentavalent actinyl ions.

Abstract: The inner-sphere mechanisms of the disproportionation reactions of U(V), Np(V), and Pu(V) ions have been studied using a quantum mechanical approach. The U(V) disproportionation proceeds via the formation of a dimer (a cation−cation complex) followed by two successive protonations at the axial oxygens of the donor uranyl ion. Bond lengths and spin multiplicities indicate that electron transfer occurs after the first protonation. A solvent water molecule then breaks the complex into solvated U(OH)22+ and UO22+ ions. Pu(V) behaves similarly, but Np(V) appears not to follow this path. The observations from quantum modeling are consistent with existing experimental data on actinyl(V) disproportionation reactions.

Kinetic simulation of single electron transfer–living radical polymerization of methyl acrylate at 25 °C

Abstract: A mechanistic comparison of the ATRP and SET-LRP is presented. Subsequently, simulation of kinetic experiments demonstrated that, in the heterolytic outer-sphere single-electron transfer process responsible for the SET-LRP, the activation of the initiator and of the propagating dormant species is faster than of the homolytic inner-sphere electron-transfer process responsible for ATRP. In addition, simulation experiments suggested that in both polymerizations the rate of deactivation is similar. In SET-LRP, the Cu(II)X2/L deactivator is created by the disproportionation of Cu(I)X/L inactive species, while in ATRP its concentration is mediated by the bimolecular termination. The combination of higher rate of activation with the creation of deactivator via disproportionation provides, via SET-LRP, an ultrafast synthesis of polymers with very narrow molecular weight distribution at room temperature. SET-LRP is mediated by a catalytic amount of Cu(0), and under suitable conditions, bimolecular termination is virtually absent. Kinetic and simulation experiments have also demonstrated that the amount of water available in commercial solvents and monomers is sufficient to induce the disproportionation of Cu(I)X/L into Cu(0) and Cu(II)X2/L and, subsequently, to change the polymerization mechanism from ATRP to SET-LRP. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1835–1847, 2007.

Dioxygen activation at monovalent nickel.

Abstract: The monovalent oxidation state of nickel has received a growing amount of attention in recent years, in part due to its suggested catalytic role in a number of metalloprotein-mediated transformations. In coordination chemistry, nickel(I) is suitable for reductive activation of dioxygen, provided ligands are used that stabilize this less common oxidation state against disproportionation reactions. Two distinct molecular systems have been explored, which have provided access to new nickel–dioxygen structure types, namely, monomeric side-on and end-on superoxo and trans-μ-1,2-peroxo–dinickel complexes. The geometric and electronic structures of the complexes have been established by advanced spectroscopic methods, including resonance Raman and X-ray absorption spectroscopies, and augmented by density functional theory analyses.

Synthesis, Crystal Structure, Characterization, and Catalytic Properties of TNU-9

Abstract: The synthesis, crystal structure, characterization, and catalytic properties of the novel medium-pore zeolite TNU-9 (framework type TUN), one of the most crystallographically complex zeolites known to date, are described. TNU-9 was found to crystallize under hydrothermal conditions at the expense of a lamellar precursor over a very narrow range of SiO(2)/Al(2)O(3) and NaOH/SiO(2) ratios and in the presence of 1,4-bis(N-methylpyrrolidinium)butane and Na+ ions as structure-directing agents. A combination of molecular modeling and Rietveld refinement using synchrotron powder diffraction data confirms the proposed topology of as-made TNU-9 and suggests three or possibly four different sites for the organic within the complex pore structure. The proton form (H-TNU-9) of this new medium-pore zeolite exhibits exceptionally high hydrothermal stability, as well as very strong acidity. When compared to H-ZSM-5, H-MCM-22, H-mordenite, and H-Beta, H-TNU-9 displays unique shape selectivities for the acid-catalyzed reactions of monoaromatic hydrocarbons such as the disproportionation of toluene and the isomerization and disproportionation of m-xylene. In particular, for the isomerization of m-xylene, the ratio of isomerization to disproportionation increases steadily to values in excess of 50 with prolonged time on stream and a high p/o xylene ratio is observed in the products, achieving a value of ca. 6 after only a short time on stream. These results are rationalized on the basis of the unique pore topology of TNU-9.

The role of excess nitroxide in the SG1 (N-tert-butyl-N-[1-diethylphosphono-(2,2-dimethylpropyl)] nitroxide)-mediated polymerization of methyl methacrylate

Abstract: Methyl methacrylate (MMA) polymerizations have been conducted in the presence of large excesses of N-tert-butyl-N-[1-diethylphosphono-(2,2-dimethylpropyl) nitroxide] (SG1) at 110°C. It is demonstrated that such a protocol does not improve control/livingness in the nitroxide mediated polymerization (NMP) of this monomer, instead substantial levels of disproportionation between the nitroxide and propagating radical (PMMA . ) results. The extent of the disproportionation reaction increased with the SG1 concentration, eventually becoming the sole end forming event. Significant disproportionation between SG1 and PMMA . was also observed at substantially lower temperatures (45°C) in the presence of large excesses of SG1.

Synthesis and Characterization of an Iron(II) η2-Hydrazine Complex

Abstract: The iron phosphine complex cis-[Fe(DMeOPrPE)2(η2-N2H4)][BPh4]2 {DMeOPrPE = 1,2-bis[bis(methoxypropyl)phosphino]ethane} was synthesized and structurally characterized. The structure exhibits the first η2 coordination of hydrazine to iron, which may be relevant to intermediates trapped during nitrogenase turnover. The reaction of I with acid results in the formation of ammonia via a disproportionation reaction.

Probing Variable Axial Ligation in Nickel Superoxide Dismutase Utilizing Metallopeptide-Based Models: Insight into the Superoxide Disproportionation Mechanism

Abstract: Nickel superoxide dismutase (NiSOD) is a bacterial metalloenzyme that possesses a mononuclear Ni-center and catalyzes the disproportionation of O2•- by cycling between NiII and NiIII oxidation states. Herein we present evidence from several SOD active metallopeptide maquettes ([Ni(SODM2H(1)X)]; SODM2H(1)X = H2N-XCDLPCG-COOH; X = H, D, or A) that the Ni-center of NiSOD most likely remains five-coordinate during SOD catalysis using thin-film voltammetry. N3− and CN− titration studies suggest that O2•- disproportionation by [Ni(SODM2H(1)X)] proceeds via an outersphere mechanism. Computationally derived values for the nuclear reorganization energy of the [NiII(SODM2)]/[NiIII(SODM2)] self-exchange reaction combined with the experimentally determined value for ko (∼450 s-1) suggest that axial ligation enhances the O2•- disproportionation reaction in [Ni(SODM2)] (and NiSOD by analogy) by optimizing the NiII/NiIII redox couple such that it is close to the midpoint of the O2•- reduction and oxidation couples.

Kinetics and Mechanism of Oxidation Reactions of Porphyrin-Iron(IV)-Oxo Intermediates

Abstract: The kinetics of the reactions of three porphyrin-iron(IV)-oxo derivatives with alkenes and benzylic alcohols were measured. The iron-oxo systems studied were 5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin-iron(IV)-oxo (2a), 5,10,15,20-tetrakis(2,6-difluorophenyl)porphyrin-iron(IV)-oxo (2b), and 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin-iron(IV)-oxo (2c). Species 2 were stable for hours at room temperature as dilute solutions in acetonitrile and reacted hundreds to thousands of times faster in the presence of high concentrations of substrates. Typical second-order rate constants determined from pseudo-first-order kinetic studies are 1-2 x 10(-2) M(-1) s(-1) for reactions with styrene and 3 x 10(-2) M(-1) s(-1) for reactions with benzyl alcohol. The reactivity order for the iron-oxo species was 2a > 2b > 2c, which is inverted from that expected on the basis of the electron demand of the porphyrin macrocycles, and the oxidation reaction was suppressed when excess porphyrin-iron(III) complex was added to reaction mixtures. These observations indicate that the reactions involve disproportionation of the iron(IV)-oxo species 2 to give an iron(III) species and a more highly oxidized iron species, presumed to be an iron(IV)-oxo porphyrin radical cation, that is the true oxidant in the reactions. Analyses of the kinetics of oxidations of a series of para-substituted benzylic alcohols with Hammett sigma+ -substituent constants and with a dual-parameter method developed by Jiang (Jiang, X. K. Acc. Chem. Res. 1997, 30, 283) indicated that considerable positive charge developed on the benzylic carbons in the oxidation reactions, as expected for electrophilic oxidants, and also that substantial radical character developed on the benzyl carbon in the transition states.

Neptunium(V) disproportionation and cation–cation interactions in TBP/kerosene solvent

Abstract: In 30% TBP/OK Np(V) is unstable and disproportionates to Np(IV) and Np(VI). Np(V) readily coordinates to Np(IV) in solution to form a "cation-cation" complex by bonding through an axial oxo group on Np(V). The rate of disproportionation in 30% TBP/OK is > 500 times that in aqueous solution.

Morphology control of Fe2O3 nanocrystals and their application in catalysis

Abstract: We synthesized four iron oxide catalysts with different morphologies and tested their activities in CO disproportionation. The four iron oxides were mesoporous oxide, two different sized iron oxide nano spheres and silica supported iron oxide. Hypothetical equivalent average particle size (EAPS), which was calculated from the surface area and unit cell parameter of the particles, was used to evaluate the catalytic activities of the iron oxides. A size effect (EAPS effect) was observed in these iron oxides. The CO disproportionation test results showed that silica supported iron oxide was the most active due to it having the smallest EAPS.

Reaction mechanism of 4,6-dimethyldibenzothiophene desulfurization over sulfided NiMoP/Al2O3-zeolite catalysts

Abstract: The desulfurization of 4,6-dimethyldibenzothiophene (4,6-DMDBT) was carried out over a pure sulfided NiMoP/Al2O3 catalyst and over mechanical mixtures containing sulfided NiMoP/Al2O3 and an acidic component such as HY and Hβ zeolites at 340 °C under 4 MPa of total pressure in a fixed bed reactor. The transformation of 4,6-DMDBT was also carried out over HY and Hβ zeolites at 350 °C under atmospheric pressure. Over the pure sulfided NiMoP/Al2O3 catalyst, 4,6-DMDBT transformed mainly through two pathways: direct desulfurization leading to 3,3′-dimethylbiphenyl (3,3′-DMBPh) and desulfurization after hydrogenation (HYD) leading to 3-(3′-methylcyclohexyl)toluene (3,3′-MCHT). Over pure zeolites, 4,6-DMDBT underwent both isomerization and disproportionation reactions. Hβ was more selective in isomerization than HY. The use of mechanical mixtures (sulfided NiMoP/Al2O3-zeolite) as bifunctional catalysts allowed an increase of the 4,6-DMDBT reactivity which depended on the acidity and the porosity of the zeolite. Over these bifunctional catalysts, two new categories of products were observed: isomerization and disproportionation products resulting from acidic catalysis and products resulting from their desulfurization. The products obtained by isomerization and disproportionation reactions, which presented approximately the same reactivity in hydrodesulfurization, were more reactive than 4,6-DMDBT. The NiMoP/Al2O3 sulfided catalyst associated with HY-16 zeolite (Si/Al ratio = 16) was the most active of all the mechanical mixtures used for the desulfurization of 4,6-DMDBT. Indeed, HY-16 was the most active zeolite both in isomerization and disproportionation reactions.

Catalytic Transformation of 1,3,5-Trimethylbenzene over a USY Zeolite Catalyst

Abstract: Catalytic transformation of 1,3,5-trimethylbenzene (TMB) has been investigated over a USY zeolite catalyst in a novel riser simulator at different operating conditions. The effect of reaction conditions on the variation of isomerization to disproportionation products ratio, distribution of TMB isomers, xylene/tetramethylbenzenes ratio, and values of p-xylene/o-xylene ratios are reported. Comparisons are made between the results of the 1,3,5-TMB and the earlier reported values of 1,2,4-TMB under similar conditions. Diffusion limitations were observed for the 1,3,5-TMB transformation reactions, while very mild transport effects were seen for the 1,2,4-TMB molecule. This is as a consequence of the kinetic diameters of the reacting molecules (8.6 A for 1,3,5-TMB and 7.6 A for 1,2,4-TMB), which are nearly the size of the passageway of the USY zeolite (7.4 A). Surprisingly, 1,3,5-TMB was found to be more reactive than 1,2,4-TMB at temperatures above 450 °C and reaction times more than 5 s. The experimental resu...

Studies in network formation

Abstract: Networks are formed when free-radical polymerization is initiated (thermally or photochemically) by a system consisting of a metal carbonyl or other zero-valent metal derivative and a polyfunctional halide [e.g., poly(vinyl trichloracetate)] in a monomer for which termination is at least partly by combination. These reactions are suitable for investigations of the formation and also the preparation of networks. In ideal cases the ratio of combination to disproportionation in the termination reaction may be estimated. Some factors which may cause deviation from ideal behavior will also be discussed.

Formation and Thermal Stability of Ni+ Cationic Sites in Ni−ZSM-5

Abstract: Ni−ZSM-5 zeolite reduced in CO was characterized by DRIFTS measurements of adsorbed CO, N2, and H2 molecules and EPR spectroscopy. The results indicate that N2 and H2 molecules adsorbed on Ni+ ions are strongly perturbed, resulting in large bathochromic shifts of their vibration frequencies. Ni+ sites in Ni−ZSM-5 are stable up to moderately high temperatures (550−600 K). At higher temperatures, the Ni+ sites are converted to Ni0 and Ni2+ according to the following disproportionation reaction: 2Ni+ → Ni2+ + Ni0.

Radical chemistry of iron carbonyls

Abstract: It is shown that catalytic carbonylation of various compounds using iron carbonyl complexes is based on two types of reactions [redox disproportionation of iron carbonyl induced by Lewis bases and oxidative addition of Br?nsted and Lewis acids to (hydrido)carbonylferrate anions] comprising single-electron initiation steps and subsequent radical chain reactions. The role of iron carbonyl radical anions as catalysts for carbonylation processes with controlled reduction potential of the medium is noted. Characteristic features of the radical chemistry of iron and other transition metal carbonyls is analysed.

Selective toluene disproportionation over ZSM-5 zeolites

Abstract: Selective toluene disproportionation to benzene and xylenes is investigated over HZSM-5 and modified HZSM-5 zeolites. The zeolites are modified with nickel, magnesium, phosphorus and boron, characterised by sorption, acidity measurement and X-ray photoelectron spectroscopy, and their activity towards toluene disproportionation is discussed in the light of their physico-chemical properties. The two types of active sites have been identified; one promoting disproportionation and the other cracking and dealkylation of toluene. It is found that the latter sites are suppressed due to incorporation of the modifiers. The modified zeolites are found to be para-selective which appears to be consistent with the commonly accepted view that modifiers like magnesium, phosphorus and boron reside in the channels and react with Bronsted acid sites, leading to partial blockages and thus favouring the formation of p-xylene. The effect of poisoning the zeolite with 1-methylisoquinoline is also investigated. In the case of poisoned catalyst, p-xylene is produced at levels up to 99%. The total elimination of other isomers from the products is suggested due to the selective poisoning of the non-selective sites (on the external crystal surface) and to partial blocking of the pore openings.