Showing papers on "Disproportionation published in 1993"

Bacterial disproportionation of elemental sulfur coupled to chemical reduction of iron or manganese.

Abstract: A new chemolithotrophic bacterial metabolism was discovered in anaerobic marine enrichment cultures. Cultures in defined medium with elemental sulfur (S0) and amorphous ferric hydroxide (FeOOH) as sole substrates showed intense formation of sulfate. Furthermore, precipitation of ferrous sulfide and pyrite was observed. The transformations were accompanied by growth of slightly curved, rod-shaped bacteria. The quantification of the products revealed that S0 was microbially disproportionated to sulfate and sulfide, as follows: 4S0 + 4H2O → SO42- + 3H2S + 2H+. Subsequent chemical reactions between the formed sulfide and the added FeOOH led to the observed precipitation of iron sulfides. Sulfate and iron sulfides were also produced when FeOOH was replaced by FeCO3. Further enrichment with manganese oxide, MnO2, instead of FeOOH yielded stable cultures which formed sulfate during concomitant reduction of MnO2 to Mn2+. Growth of small rod-shaped bacteria was observed. When incubated without MnO2, the culture did not grow but produced small amounts of SO42- and H2S at a ratio of 1:3, indicating again a disproportionation of S0. The observed microbial disproportionation of S0 only proceeds significantly in the presence of sulfide-scavenging agents such as iron and manganese compounds. The population density of bacteria capable of S0 disproportionation in the presence of FeOOH or MnO2 was high, > 104 cm-3 in coastal sediments. The metabolism offers an explanation for recent observations of anaerobic sulfide oxidation to sulfate in anoxic sediments.

Pillared clays: characterization of acidity and catalytic properties and comparison with some zeolites

Abstract: The pore structure and the acidic properties of pillared clays based on montmorillonite and hectorite intercalated with aluminium-, zirconium- and chromium-hydroxy oligomers are compared. Furthermore, the catalytic activity of these catalysts was analyzed in the case of toluene disproportionation. Except for the zirconium-pillared clay the other catalysts show BET surfaces between 250 m2/g and 300 m2/g up to a temperature of 400 °C. At higher temperatures the structure collapses because of dehydroxylation of the clay sheets. All pillared clay catalysts show a bimodal pore-size distribution with macro-pores between the primary clay crystals. The acidity is characterized by means of IR-spectros-copy with pyridine as sensor molecule. Chromium-pillared clays have stronger Lewis-acid sites than the other pillared clays. The number of Lewis-sites correlates with the number of pillars, whereas the BrOnsted-acid sites are mainly located in the clay sheets. The metal concentration in the dispersion of oligomer-solution and clay-mineral is a preparation parameter, which influences the pillar density. An Al-montmorillonite has higher Lewis- and BrOnsted-acidity than a comparable Al-hectorite. Among the tested catalysts pillared clays based on chromium are most active for the choosen model reaction because of their large macroporosity and the existence of strong Lewis-acid sites. In comparison to a H-ZSM-5 and a H-Y-zeolite the catalytic activity of the optimal Cr-pillared clay is between the activities of these catalysts. Furthermore coking mainly takes place on Bronsted-acid sites especially during the first minutes of reaction. The effective stationary kinetics of toluene disproportionation catalyzed by the optimal Cr-pillared clay can be described by a pseudo-first order expression for the reaction velocity.

Unexpected disproportionation mechanism for proton-transfer reactions between 17-electron metal hydride cation radicals and neutral 18-electron metal hydrides

Abstract: The hydrides CpRu(PR 3 ) 2 H ((PR 3 ) 2 =(PPh 3 ) 2 , 1; dppm, 2; dppe, 3; and dppp, 4) undergo one-electron oxidations at -0.1 to -0.3 V vs Cp 2 Fe/Cp 2 Fe + . The oxidations of 1 and 4 are nearly chemically reversible in acetonitrile; in THF, the oxidation of 3 also exhibits partial chemical reversibility. Chemical oxidation of 1-4 in acetonitrile yields 1:1 mixtures of CpRu(PR 3 ) 2 (NCMe) + and the dihydride or dihydrogen complexes CpRu(PR 3 ) 2 H 2 + , indicative of proton transfer from the hydride cation radicals to the neutral hydrides

A functional model of manganese catalase. Mass spectrometric and visible spectral evidence for {MnIV(O)}2 and MnIIMnIV(O) intermediates

Abstract: A dinuclear manganese(II) complex [Mn2(L)(PhCO2)2(NCS)][L = 2,6-bis{N-[2-(dimethylamino)ethyl]iminomethyl}-4-methylphenolate(1–)] decomposes H2O2 catalytically indimethylformamidesolution; two oxomanganese(IV) species {MnIV(O)}2 and MnIIMnIV(O) are detected for the first time as intermediates in the H2O2 disproportionation reaction based on mass spectrometric and visible spectral studies.

Activated zeolite beta and its use for hydrocarbon conversion

Abstract: The present invention relates to a zeolite beta catalyst characterized by critical limits of weak and strong acid species and exceptionally high catalytic activity. The catalyst is activated at a temperature effective to substantially reduce the concentration of strong acid species, i.e., hydronium cations, without substantially reducing the concentration of weak acid species, i.e., hydroxoaluminum cations, preferably following a calcining step wherein a synthesized zeolite beta catalyst containing a templating agent is calcined at a temperature in the range of from about 200° to 1000° C. in order to remove a substantial portion of the catalyst templating agent and an ion-exchanging step wherein the calcined catalyst is ion-exchanged with a salt solution containing at least one hydrogen forming cation selected from NH4 + and quaternary ammonium. Conversion processes utilizing the catalyst of the invention, including isomerization of paraffins and alkylaromatics and disproportionation of aromatics, also are disclosed.

Catalytic studies with dealuminated Y zeolite. Part 1.—Catalyst characterisation and the disproportionation of ethylbenzene

Abstract: A series of highly crystalline, hydrothermally dealuminated Y zeolites has been prepared and further modified by extracting non-framework aluminium with Na2H2EDTA. The zeolites were comprehensively characterised using chemical analysis, X-ray diffraction, mid-infrared spectroscopy, 27Al magic-angle-spinning nuclear magnetic resonance (27Al MAS NMR), analysis of N2 adsorption isotherms and temperature-programmed desorption of NH3 to determined acid site concentrations. It is shown that aluminium from the framework resides in the micropores and freshly developed mesopores in a number of forms. Aluminium extraction with Na2H2EDTA increases both the pore volumes and the concentration of Bronsted acid sites, but leaves an average of five charge-balancing Al atoms per unit cell.Catalytic studies of ethylbenzene disportionation at 473 K show that Bronsted acid sites are the seat of an activity which increases following Na2H2EDTA extraction. There is evidence, however, for a synergistic interaction between Bronsted acid sites and extrastructural hydroxo-aluminium species in unextracted catalysts to yield sites of higher activity than in extracted catalysts. The reaction is believed to proceed by a dealkylation/alkylation mechanism which accounts for minor yields of ethene, which is held responsible for coke formation and catalyst deactivation.

Coupling and disproportionation reactions of alkyl iodides on a single-crystal copper surface : alkyl radicals versus metal alkyls

Abstract: Previous studies of straight-chain alkyl iodides (C 2 -C 5 ) adsorbed on a Cu(111) surface at submonolayer coverages have shown that these molecules dissociate at ∼120 K to generate adsorbed alkyl groups and iodine atoms. The surface-bound alkyls (with an alkyl-metal bond energy of ∼30 kcal/mol) undergo β-hydride elimination above -200 K. Here, the thermal chemistry of these alkyl iodides at high surface coverage (up to saturation of the monolayer) is investigated by using temperature-programmed reaction/desorption to identify the surface reaction products and reaction kinetics

Ethylbenzene dealkylation and realkylation over pore size regulated MFI zeolite

Abstract: The dealkylation and realkylation of ethylbenzene over silica-deposited MFI zeolite have been studied. The relationship between para selectivity enhancement and extent of silica deposition is established. The effects of reaction conditions like variation in reaction temperature and weight hourly space velocity on the high para selectivity feature of modified zeolite are examined. A study of the kinetics of the reaction in the temperature range 523--573 K is carried out. The estimated activation energy is similar to that reported for toluene disproportionation over MFI zeolite.

Bromous acid/cerium(4+): reaction and HBrO2 disproportionation measured in sulfuric acid solution at different acidities

Abstract: The rate of the reaction of bromous acid with Ce 4+ , which is responsible for the slowing down of the autocatalytic oxidation of Ce 3+ by bromate in the Belousov-Zhabotinsky reaction, is measured directly by spectroscopic methods in sulfuric acid medium at different acidities. BrO 2 radicals are identified as primary reaction products. The rate constant decreases from 40 000 M -1 s -1 in 0.3 M H 2 SO 4 to 1000 M -1 s -1 in 3 M H 2 SO 4

Magnetic field effects on the rates of chlorine molecule(1-) and bromine molecule(1-) reactions: the dependence of the rates of radical disproportionation and oxidation of manganese(II) complexes on field intensity

Abstract: Magnetokinetic effects on the rates of disproportionation of Cl 2 - and Br 2 - and on their reactions with Mn(OH 2 ) 6 2+ and Mn(EDTAH) - have been investigated as a function of the magnetic field intensity, B, between 0 and 6.5 T. The dependence of the rate constants on B has been rationalized in terms of the spin evolution in paired reactants. The isotropic hyperfine coupling-induced multiplet transitions and the Δg mechanism make significant contributions to such effects.

Synthesis, characterization, thermal stability, acidity and catalytic properties of large-pore MAPO-46

Abstract: Large-pore MAPO-46 with three-dimensional pore structure and hexagonal symmetry has been synthesized. MAPO-46 has AFS topology. The effect of the organic template (di-n-propylamine) concentration in the gel on the crystallinity and phase purity of MAPO-46 was studied. The N2, H2O and n-hexane sorption capacity of MAPO-46 is higher than MAPO-36 and AIPO4-5, which is consistent with the presence of 12- and 8-ring pore openings and a large structural void volume in the crystal structure. Morphological investigations, surface and thermal analysis of the material have been carried out. Effects of thermal treatment on the structural stability and N2 sorption capacity at 78 K of MAPO-46 have also been investigated. MAPO-46 exhibits a lower thermal stability than MAPO-36 and AIPO4-5. In situ IR spectroscopic investigations of chemisorbed pyridine on MAPO-46 revealed the presence of Bronsted and Lewis acid sites. The site-energy distribution on MAPO-46 was investigated by the temperature-programmed desorption (TPD) of pyridine and the stepwise-programmed desorption (SPD) of the base from 323 to 673 K. MAPO-46 possesses a stronger acidity than MAPO-5, SAPO-5 and AIPO4-5. MAPO-46 possesses a significant catalytic activity in the cracking of aliphatic hydrocarbons, conversion of o-xylene, disproportionation of toluene and also conversion of ethanol to aromatics at 673 K. In ethanol, o-xylene, toluene and cumene conversion reactions, MAPO-46 exhibited higher catalytic activity than MAPO-5. The acidity and catalytic activity results both indicated that MAPO-46 has a lower acidity and catalytic activity than MAPO-36. The deactivation of MAPO-46 in the cumene cracking reaction was found to be slower than that of MAPO-36 and MAPO-5 in the same reaction.

An Infrared Study of an Active no Decomposition Catalyst

Abstract: NO was adsorbed on Cu-ZSM-5 at temperatures between 173 K δ+ (1900 cm −1 ), NO δ− (1810 cm −1 ) and dimeric or dinitrosyl (NO) 2 (1824 cm −1 and 1730 cm −1 ). Disproportionation of NO at low temperature produced N 2 O and NO 2 . Bands stemming from adsorption of these species were also observed. It was found that an equilibrium exists between the NO δ− and (NO) 2 species bound to reduced Cu sites. The results suggest that the same (NO) 2 species which is a possible intermediate in NO disproportionation at low temperature (≤ 298 K) can be a transient state in NO decomposition at high temperature (> 600 K).

Kinetics of toluene disproportionation over unmodified and modified ZSM-5 zeolites

Abstract: The kinetics of toluene disproportionation has been studied over both unmodified and Si-Mg modified ZSM-5 catalysts using an integral reactor and taking into account the reversibility of the reaction. It has been found that xylene dealkylation is the major secondary reaction, whereas toluene dealkylation can be considered negligible. For the unmodified ZSM-5, heterogeneous models based as the alkyl-transfer mechanism allowed the experimental results to be fit better than first and second pseudohomogeneous models, the best concordance being obtained when toluene adsorption is assumed as the rate-limiting step. The corresponding kinetic equations has been further applied to the modified zeolite (SiMg/ZSM-5), leading to the development of a kinetic model which includes the effect of the toluene and p-xylene intracrystalline diffusion rate and the presence of nonselective acid sites on the external zeolite surface. This model describes adequately the selective formation of p-xylene over modified ZSM-5, reproducing the experimental product distribution with an average relative error of 2.8%

Energetics of acetic acid enol in aqueous solution. A new method for thermodynamic estimation

Abstract: A new disproportionation calculation allows the estimation of the free energy of formation of the enol of acetic acid as 65 ± 2 kcal/mol. The value of pKE derived from this free energy, pKE = 21 ± 2, is in satisfactory agreement with information from the literature about rates of exchange. Analysis of the data on rates of exchange of the C-H protons of acetic acid using Marcus theory allows an independent estimate of the enol content. Exchange in acid and in base lead to internally consistent estimates, pKE = 19.3 ± 2.2, which are within the combined uncertainties of the values from the thermodynamic estimate.

Reactions of benzyl alcohol and dibenzyl ether over zeolites

Abstract: The reaction of benzyl alcohol and dibenzyl ether on several zeolites was studied in the temperature range 300–500 °C. The major products were benzaldehyde and toluene due to disproportionation, anthracene due to intermolecular alkylation followed by cyclodehydration, benzyltoluenes due to alkylation followed by hydrogenolysis, and stilbene. The alkylation course was strongly influenced by shape selectivity with the large pore zeolites mainly yielding anthracene and the medium pore zeolites benzyltoluene. The formation of stilbene was a new observation which has a bearing on the mechanism of the methanol-to-hydrocarbon reactions.

Study on aqueous polymerizations of vinyl monomers initiated by metal oxidant–chelating agent redox initiators

Abstract: In this study, a series of chelating type reductants containing redox pairs were tested as the initiator for aqueous polymerizations. The redox pairs consist of Ce(IV) or several first-row transition metals coupled with chelating agents of amino acids, dibasic acids, or diamine. The initial rates and limiting conversions of acrylamide polymerization initiated by those redox pairs were determined. The reductive reactivity of the chelating agents with Ce(IV) and the oxidative half-wave potential of Ce(III)-chelating agent/Ce(IV)-chelating agent were measured to evaluate the feasibility of these redox pairs as initiators. After the evaluation, the redox pairs other than Ce(IV)-amino acid type chelating agent were precluded to be promising initiators for aqueous polymerizations. Those Ce(IV)-amino acid type chelating agent redox pairs which could form at least two five- or six-membered rings were found to be potential initiators. The Ce(IV)-NTA pair was the most promising one. The mechanism of initiation of the redox pairs was proposed and further confirmed by the 13C- and 1H-NMR spectra of NTA-terminated polyacrylamide. The complex formation constants (K) and disproportionation constants (kd) of the Ce(IV)-amino acid type chelating agent redox initiators for acrylamide polymerization were evaluated. The factors governing the parameters of chelated complexes and the performance of polymerizations were discussed. These redox pairs were also used as the initiators for aqueous polymerizations of acrylic acid and acrylonitrile. © 1993 John Wiley & Sons, Inc.