Electron Acceptor Sites Research Articles

Factors influencing deactivation of zeolites in methanol transformation

Acidic properties, catalytic activity and coking of H-mordenite, H-erionite, H-ZSM-5 as well as H-ZSM-5 containing Fe ions at exchangeable sites and in the form of supported iron oxide were investigated by temperature-programmed desorption of ammonia and transformation of methanol and ethylene. The amount of coke in the zeolite and the gaseous product composition were followed as a function of the reaction time-on-stream. It has been shown that zeolites deactivation by coking is primarily influenced by the zeolite structural type, the reactant character and the presence of electron acceptor sites (Fe cations), while the effect of the amount and acid strength of structural OH groups is of less importance.

Orientation of Electron Transport Activities in the Membrane of Intact Glyoxysomes Isolated from Castor Bean Endosperm

Intact glyoxysomes were isolated from castor bean endosperm on isometric Percoll gradients. The matrix enzyme, malate dehydrogenase, was 80% latent in the intact glyoxysomes. NADH:ferricyanide and NADH:cytochrome c reductase activities were measured in intact and deliberately broken organelles. The latencies of these redox activities were found to be about half the malate dehydrogenase latency. Incubation of intact organelles with trypsin eliminated NADH:cytochrome c reductase activity, but did not affect NADH:ferricyanide reductase activity. NADH oxidase and transhydrogenase activities were negligible in isolated glyoxysomes. Mersalyl and Cibacron blue 3GA were potent inhibitors of NADH:cytochrome c reductase. Quinacrine, Ca(2+) and Mg(2+) stimulated NADH:cytochrome c reductase activity in intact glyoxysomes. The data suggest that some electron donor sites are on the matrix side and some electron acceptor sites are on the cytosolic side of the membrane.

Catalytic hydrogenation of carbon monoxide on ruthenium Y-zeolites: Effect of support on activity and selectivity

The specific activity of ruthenium supported on zeolite Y in the hydrogenation of carbon monoxide is enhanced when the charge-compensating sodium ions are replaced by multivalent cations or protons. This effect is attributed to an increase in electron deficiency of the metal crystallites owing to an electron transfer from the metal to electron-acceptor sites in the support. Hydrogen then competes more successfully with carbon monoxide for available surface sites, resulting in an increased hydrogenation activity. This conclusion is supported by the marked decrease in the selectivity to olefins which is observed. The electronic influence of the support is greater for small metal particles.

Properties of Y-type zeolites with various silicon/aluminium ratios obtained by dealumination with silicon tetrachloride. Distribution of aluminium and hydroxyl groups and interaction with ethanol

Y-type zeolites with Si/Al ratios of 3.4–20, prepared by the dealumination of NaY (Si/Al = 2.5) with SiCl4, have been characterized using X-ray diffraction analysis, sorption-capacity data and by obtaining infrared spectra of the framework, OH groups and adsorbed pyridine. Dealuminated zeolites exhibit good crystallinity and some aluminium released from the lattice remains in extralattice positions in the zeolite cavities or in an amorphous phase to provide strong electron-acceptor sites. The zeolites were obtained directly in the H form; further exchange with NH+4 affected their proton-donor properties only slightly. In addition to weakly acidic or non-acidic hydroxyls (bands at 3740 and 3620 cm–1), strongly acidic framework hydroxyls were present (bands at 3630 and 3560 cm–1) in amounts dependent on the amount and distribution of aluminium. An analysis of substances released during temperature-programmed desorption from zeolites with preadsorbed ethanol revealed the influence of dealumination on the catalytic activity: the maximum of evolved ethylene as a main product was reached on moderately dealuminated zeolites (Si/Al = 3.4–6) with an appreciable number of both strong electron-acceptor and strong proton-donor sites.

PtMo bimetallic catalysts supported on Y-zeolite: I. Characterization of the catalysts

Bimetallic PtMoY catalysts have been prepared by adsorption and decomposition of Mo(CO) 6 in a Y-type zeolite containing 10-Å Pt particles trapped in the zeolite cages. Part of the Mo atoms are deposited onto the Pt particles which produces a weak increase of the particle size measured by CTEM and STEM and a progressive decrease of the amount of H 2 or CO chemisorbed on platinum. At high Mo contents these chemisorptive properties are completely masked. It was checked by XPS, X-ray absorption edge spectroscopy, and ir spectroscopy with CO and NO as probe molecules that the Pt particles are initially electron deficient but the electronic structure becomes similar to that of bulk platinum once the Mo atoms were added. This is probably because the Pt atoms are then shielded from the electron acceptor sites of the support. The Mo atoms bonded to Pt are probably in a low valence state, but there is a fraction of Mo not bonded to Pt, especially in the zeolite outer layers of Mo-rich samples, which is oxidized to Mo IV-Mo VI by oxido-reduction reactions with the zeolite protons.

On the redox properties of certain oxide surfaces

This study gives details of the ESR effects observed when quinhydrone and hydroquinone/ p-benzoquinone mixtures of varying composition ratio are adsorbed on high surface-area samples of silica-alumina, γ-alumina, magnesium oxide, and zinc oxide. The purpose is to test the hypothesis that in considering the phenomena of radical formation on surfaces that posses both electron donor and electron acceptor sites, it is necessary to explicitly take into account an indirect cooperative interaction between the donor and acceptor sites in analyzing the data. The correctness of the hypothesis is demonstrated and the implications thereof as concerns the currently used method to determine redox site density on these types of oxide surfaces is briefly considered. Preliminary data are provided on the donor and acceptor site density on the four materials listed above as determined by the quinhydrone method.

Electron-transfer at alumina surfaces. Part 5.—Oxidation of aromatic amines

The oxidation of primary, secondary and tertiary aromatic amines on oxide surfaces has been studied, using e.s.r. spectroscopy. All the amines give adsorbed radicals whose spectra are not well defined. Addition of a polar molecule displaces the surface species. The nature of the desorbed product depends on the oxide surface involved, e.g., with diphenylamine on alumina the desorbed product is diphenyl nitroxide, whereas from a silica–alumina (∼13 % Al2O3) surface NN′-diphenylbenzidine is obtained. These facts and other evidence indicate that the electron-acceptor sites on silica-aluminas of low alumina content differ substantially from those on alumina. On hydrogen Y zeolite two types of site capable of oxidizing aromatic amines exist, depending on the degree of dehydroxylation of the zeolite surface. One of these sites corresponds to an oxidizing site present in silica-alumina; the other is more closely related to the oxidizing centre found on the alumina surface. Possible mechanisms for the surface reactions are considered.

Oxidation of Aromatic Amines on Oxide Surfaces

The oxidation of primary, secondary and tertiary aromatic amines on oxide surfaces has been studied, using ESR spectroscopy. All the amines give adsorbed radicals whose spectra are not well defined. Addition of a polar molecule displaces the surface species. The nature of the desorbed product depends on the oxide surface involved, e.g., with diphenylamine on alumina the desorbed product is diphenylnitroxide, whereas from a silica-alumina (∼13% Al2O3) surface N,N'-diphenylbenzidine is obtained. These facts and other evidence indicate that the electronacceptor sites on silica-aluminas of low alumina content differ substantially from those on alumina. On hydrogen Y zeolite two types of site capable of oxidizing aromatic amines exist, depending on the degree of dehydroxylation of the zeolite surface. One of these sites corresponds to an oxidizing site present in silica-alumina; the other is more closely related to the oxidizing centre found on the alumina surface.

Étude polarographique de la mobilisation du fer de la ferritine

Polarographic study of the mobilization of ferritin iron Polarographic study allows to propose a model for mobilization of ferritin iron: an equilibrium exists between iron core and small quantities of iron outside the protein. These iron atoms would be lying on electron acceptor sites including SH groups. The number of sites is dependent on iron content of ferritin. Therefore, the iron could be removed by the action of reducing agents such as xanthine oxidase or ascorbic acid, and then chelated by a complexing agent.

Synthetic offretite: II. Decomposition of organic cations and generation of acidity

The organic transformations and associated structural changes during thermal decomposition of a tetramethylammonium (TMA) offretite have been investigated. In the aluminosilicate structure, lattice associated hydroxyl groups, confirmed as protonic in nature by interaction with NH 3, were generated from TMA cations, an unusual proton precursor. Dehydroxylation of acid offretite at approximately 500 °C occurred to give electron acceptor (Lewis) sites which coordinated with NH 3. A complex mixture of coke, trimethylamine, and twenty-one low molecular weight molecules, including methanol, carbon monoxide, methyl ether, C 3-C 4 olefins, and hydrogen can be formed from vacuum fragmentation of the bulky TMA cations within the narrow offretite channels. A mechanistic interpretation, which includes superimposition of many “classical” reaction patterns rationalizes the observed product distributions.

Electron-transfer at alumina surfaces. Part 3.—Reduction of aromatic nitro-compounds

The generation of the anion radicals of various mono-, di- and tri-nitro aromatic compounds by electron transfer at the surface of partially dehydrated catalytic aluminas and silica-aluminas is reported. An increase in the alumina content of the latter results in an increase in reducing power. Maximum reducing activity in alumina and in silica-aluminas occurs with samples dehydrated in the range 600-700°C. A surface defect site containing an excess of oxide ions probably acts as the electron donor on both types of surface; molecular oxygen is not involved in the electron-transfer process. The adsorbed species is stable for long periods and the parent compound can be recovered unchanged from the catalyst surface. The e.s.r. spectrum obtained for all the adsorbed nitro-radicals can be analyzed in terms of a strong anisotropic interaction with one nitrogen atom. Thus, the radical-surface complex behaves essentially as an ion-pair in which the unpaired electron is largely localized on one of the nitro-groups. An increase occurs in the spin density on the nitro-group as the activation temperature is increased up to 700°C. On silica-alumina catalysts of high alumina content, as on pure aluminas, powerful electron-donor and electron-acceptor sites can be stabilized in close proximity and these active centres are to some degree interdependent.

Electric fields at the surface and interface of SiO 2 films on silicon

At solid surfaces and interfaces the potential energy of electrons has a sharp discontinuity with respect to the adjacent bulk phases. This change of potential give rise to parallel opposing layers of charged defects or ions within insulating or semiconducting solids. The concentration of charge and the electric field depend not only on the interfacial potential drop, but also on the potential and concentration of electron donor or acceptor sites in the interfacial region. At the surface of thin silicon oxide films grown on silicon, oxide ions provide a layer of negative charge; an equal positive charge (usually sodium ions) is concentrated in the oxide within about 200 Å of the surface. A similar double layer develops at the interface with silicon, where electrons donated to the silicon provide a negative charge having an adjacent positive charge layer in the oxide. Both positive charge layers exhibit a Poisson-Boltzmann distribution, with a potential drop at the surface always about three times greater than at the interface. Sodium oxide impurities are the dominant electron-donors; these supply the electrons for the negative surface and interfacial layers (as well as for any competing electron-acceptor sites). A high density of acceptor sites can be introduced into silicon dioxide by conditions tending to produce oxygen vacancies, and when the electrons from sodium oxide donors are trapped by such acceptor sites the sodium ions are localized near the trapped electrons. Under these conditions few electrons and sodium ions reach the surface or interface. Since a high surface density of oxide ions requires a high concentration of sodium ions, it is not surprising that the rate of oxidation of silicon increases with sodium content.

Clay Minerals as Electron Acceptors and/or Electron Donors in Organic Reactions

AbstractCertain clay minerals have the ability to catalyze the polymerization of some unsaturated organic Compounds (styrene, hydroxyethyl methacrylate) and yet to inhibit polymer formation from other closely related monomers (e.g. methyl methacrylate). This apparently contradictory behaviour of the clay minerals can be rationalized in terms of electron accepting and electron donating sites in the silicate layers. The electron acceptor sites are aluminium at crystal edges and transition metals in the higher valency state in the silicate layers; the electron donor sites are transition metals in the lower valency state.The catalyzed polymerizations involve the conversion of the organic molecule to a reactive intermediate; thus where the clay mineral accepts an electron from the vinyl monomer a radical-cation is formed, where the organic compound gains an electron it forms a radical-anion. Examples of these reactions are discussed.The inhibition of polymerization processes involves the conversion of reactive organic intermediates, such as free radicals, which have been formed by heat or radical initiators, to non-reactive entities. For example, loss of an electron from the free radical gives a carbonium ion; in some cases this will not undergo polymerization. An example of this type is the thermal polymerization of methyl methacrylate.The color reactions on clay minerals are useful in predicting the electron accepting or electron donating behaviour of the clay minerals because they proceed by similar mechanisms to the polymerization reactions. For example, the benzidine blue reaction is a one electron transfer from the organic molecule to the electron accepting sites in the mineral (aluminium edges, transition metals in higher valency state).Masking of the crystal edge with a polyphosphate destroys the electron accepting properties of the crystal edge; this technique can be used to control the reactivity of the mineral and to distinguish between the crystal edge and transition metal sites as electron-acceptor sites in the clay minerals.