Vladimir I. Sobolev

Generation of active oxygen species on solid surfaces. Opportunity for novel oxidation technologies over zeolites

Generation of surface oxygen species and their role in partial oxidation reactions catalyzed by metal oxides are discussed. Main attention is paid to a new concept related to a recent discovery of remarkable ability of Fe complexes stabilized in a ZSM-5 matrix to generate a new form of surface oxygen (α-oxygen) from N2O. At room temperature, α-oxygen exhibits a high reactivity typical for the active oxygen of monooxygenases, and mimics its unique ability to perform selective oxidation of hydrocarbons. This opens new opportunity for creating novel technologies based on biomimetic strategy. A process of direct oxidation of benzene to phenol, recently demonstrated by Solutia on a pilot plant scale, is evidence of great potential of this approach.

Oxidative hydroxylation using dinitrogen monoxide: a possible route for organic synthesis over zeolites

Abstract The use of zeolites as catalysts for selective oxidation reactions has been considered. Particular attention has been paid to recent studies of a gas-phase oxidative hydroxylation of aromatics by dinitrogen monoxide. The best results were obtained for a simple reaction, the oxidation of benzene to phenol over Fe-containing ZSM-5 zeolites. It proceeds with nearly 100% selectivity at 25–30% benzene conversion. Further progress in this promising direction, the hydroxylation of aromatics, can provide new efficient technologies in organic synthesis. Peculiarities of dinitrogen monoxide as an oxidant and the reaction mechanism are discussed.

Kinetic isotope effects and mechanism of biomimetic oxidation of methane and benzene on FeZSM-5 zeolite

Abstract Earlier, iron complexes stabilized in a ZSM-5 zeolite matrix have been shown to produce a new form of surface oxygen (called α-oxygen) upon decomposition of N2O. α-Oxygen exhibits a very high reactivity typical for oxygen of monooxygenases (MO) and mimics its unique ability in selective oxidation of hydrocarbons at room temperature. Kinetic isotope effect (KIE) measurements reported here reveal additional similarities between MO and the model. Depending on the temperature, the value of KIE for oxidation of methane with α-oxygen ranges from 1.9 to 5.5. For the oxidation of benzene the value of KIE is 1.0. This indicates that both biological and chemical oxidation of methane involves a rate limiting CH bond cleavage, whereas the reaction with benzene is probably limited by the formation of an epoxy-type intermediate. The assumed structure of the active sites as well as some features of the oxidation mechanism allow one to consider FeZSM-5-N2O system as a new and successful model for methane monooxygenase.

Selective oxidation of methane to methanol on a FeZSM-5 surface

Abstract Methane is selectively oxidized to methanol at room temperature over the interaction with α-form of the surface oxygen produced on FeZSM-5 zeolite by N2O decomposition.

On the role of Brønsted acidity in the oxidation of benzene to phenol by nitrous oxide

Abstract Oxidation of benzene to phenol with nitrous oxide being an untypical reaction for zeolite catalysis arouses a great interest among researchers. Several hypotheses concerning reaction mechanism are discussed. Some of them assume a key role of the Bronsted acid centers (BAC), though few experimental support for this idea is available. The goal of this work is to study the Bronsted acidity of FeZSM-5 zeolite catalyst (0.13 wt.% Fe 2 O 3 ) to reveal a relationship (if any) between the BAC concentration and the catalytic activity in benzene oxidation. Various pretreatments of the sample including a high-temperature calcination up to 900°C were used to vary the BAC concentration, which was measured by infrared spectrometry. No evidence of the Bronsted acidity importance for catalyzing the reaction was found. What is more, a formal inverse correlation was discovered. Thus, a manifold decrease in BAC concentration was accompanied by a remarkable increase in the oxidation rate, with a small acid recreation under the reaction conditions used. The obtained results support an earlier suggested redox mechanism of the reaction involving a special oxygen form produced at N 2 O decomposition on Fe-containing active sites.

“Double‐Peak” Catalytic Activity of Nanosized Gold Supported on Titania in Gas‐Phase Selective Oxidation of Ethanol

Recent years have seen a growing amount of fundamental research dealing with selective oxidation of alcohols and polyols using molecular oxygen (air) as a cheap and clean oxidant in the presence of solid catalysts. In this respect, supported gold nanoparticles have attracted great attention owing to their unique catalytic properties under mild conditions. Moreover, gold catalysts are becoming increasingly important for the conversion of biomass-derived alcohols, such as ethanol and glycerol, into other useful chemicals. Bioethanol, in particular, is an example of a promising renewable feedstock to obtain corresponding products of oxidation and concurrent reactions; acetaldehyde, 1,1-diethoxyethane, ethyl acetate and acetic acid, which are normally formed one by one with increase of temperature. As a result, more or less complex mixtures of these products or a predominant individual product can be obtained depending on the reaction conditions and the nature of the catalyst (gold particle size, support, preparation procedure). Notably, exactly the same spectrum of products, for example, aldehydes, acetals, esters and carboxylic acids, is typically produced in the presence of other supported metals (e.g. , Pt, Pd, Ru) or metal oxide catalysts (e.g. , V2O5, NbMoVOx), although generally under harsher conditions. Therefore, it seems quite possible that all reactions indicated above have similar mechanistic aspects. However, no generally accepted mechanism has, to date, been formulated for these reactions (see, however, Refs. [8–11]), as it has been for gold-catalyzed reactions of CO, O2, H2, and other small molecules. [12–13] During our monitoring of a gas-phase oxidation of ethanol, a large variety of solid catalysts, such as supported noble metals, metal oxides, and multicomponent systems have been tested. Among them, about thirty different supported gold catalysts were examined. Herein we report an unusual catalytic behavior of Au/TiO2, which was the only tested catalyst to give rise to a second low-temperature peak of activity. The most active gold catalysts are known to be those that contain small particles of gold (<10 nm in diameter), especially on reducible supports such as titania or ceria. 14] Taking this into account, several samples of 2 wt % Au/TiO2 were prepared according to a direct ion-exchange procedure 15] using ammonia as a washing agent to give an average gold particles size close to 2 nm (Figure 1; see also the Supporting Information). The catalytic performance of Au/TiO2 (d = 1.9 1 nm) sample

Room-temperature oxidation of hydrocarbons over FeZSM-5 zeolite

Summary Interaction of a variety of organic molecules with α-oxygen formed by N2O decomposition over FeZSM-5 zeolites leads to products of selective hydroxylation. O-insertion occurs rapidly at room temperatures and affects both aromatic and aliphatic C-H bonds.

Generation of Reactive Oxygen Species on Au/TiO2 after Treatment with Hydrogen: Testing the Link to Ethanol Low‐Temperature Oxidation

The gold catalyzed aerobic oxidation of alcohols is currently of great interest for the following reasons: 1) biomass-derived alcohols are a promising, renewable organic feedstock; 2) they use cheap, green oxidants, such as molecular oxygen (air) ; and 3) gold catalysts show extraordinarily high activities under mild conditions. The mechanistic aspects of gold catalyzed oxidation reactions of CO, H2, and other small molecules have been extensively studied, but a generally accepted oxidation mechanism for alcohols under similar conditions has not yet been formulated (however, see references [7–10]). Notably, the efficient low-temperature (50–120 8C) gold catalyzed oxidation of alcohols is known to proceed only in the liquid phase during the course of a long-term batch reaction with high oxygen pressure or intensive O2 flow. [1–4] On the other hand, we recently found that metallic gold nanoparticles supported on TiO2 gave rise to “double peak” catalytic activity in the gasphase oxidation of ethanol to acetaldehyde. The temperature, at which the first peak of activity occurred, 120 8C, was unusually low for the gas-phase reaction of primary alcohols. In contrast, gold supported on Al2O3 and SiO2 showed more usual behaviors, which were analogous to the second peak activity of Au/TiO2 at temperatures above 200 8C. [11] To account for these results, we proposed that specific active oxygen species form on the Au/TiO2 surface under mild reaction conditions and suggested hydrogen as a probable cofactor in their generation. Indeed, H2 can be produced concurrently as a result of ethanol anaerobic dehydrogenation and can thus participate in catalytic activity. In the present work, our efforts were focused on the role of hydrogen in the catalytic activity of gold supported on TiO2, Al2O3, and SiO2 matrixes to provide insights into the different profiles of ethanol oxidation. For this purpose, the O2 isotope exchange technique was applied in order to estimate the relative activity of the surface oxygen species. Oxygen isotope exchange (OIE) is a very sensitive direct method for evaluating the reactivity of surface oxygen atoms, which is a subject of primary interest in heterogeneous oxidation catalysis. Usually, as in the case of metal oxides, OIE is observed at 220–700 8C, although some metal oxides induce OIE at low temperatures after calcination and cooling in a vacuum. A highly reactive oxygen species, so called aoxygen, is generated on the Fe-ZSM5 zeolite surface after treatment with N2O. These species readily oxidize organics (benzene, methane, etc.) as well as perform OIE at room temperature. A special but relevant case, although photoinduced, is the OIE over TiO2, which also occurs at room temperature due to the involvement of reactive oxygen species, probably O anion radicals. Notably, a correlation was found between the TiO2 activity for the photoinduced OIE and the oxidation of isobutane, methanol, and ethanol over TiO2 upon ultraviolet (UV) irradiation. Generally, in a heterophase system comprising molecular dioxygen (O2) in the gas phase and oxygen on the surface of a solid (O), two isotope exchange reactions take place: 1) the hetero-exchange reaction [Equation (1)] , which is conveniently monitored by a fraction of the O isotope in the gas phase (a), and 2) the homo-exchange reaction [Equation (2)] , which is monitored by the parameter y = *C34 C34 (the difference between the equilibrium (*C34) and the current (C34) values for the fraction of asymmetric isotopic OO molecules). Obviously, the low-temperature activity of a catalyst in an oxygen homoexchange can indicate the involvement of very active surface oxygen species by analogy with the cases of a-oxygen and illuminated TiO2. [17]

Effect of transition metal oxide additives on the activity of an Ag/SiO2 catalyst in carbon monoxide oxidation

The catalysts of silver supported on mesoporous silica modified with Co3O4, CeO2, and ZrO2 were prepared by an impregnation method; characterized by X-ray diffraction analysis, temperature-programmed reduction, and low-temperature nitrogen adsorption; and studied in a model reaction of CO oxidation. It was found that the Ag/SiO2 system exhibited high activity in the reaction of CO oxidation, and the addition of transition metal oxides led to reduction of the temperature of 50% CO conversion by 40°C. The modification of Ag/SiO2 with cerium dioxide was found most effective because of the interaction of silver particles and CeO2 on the surface of silica gel.

Selective oxidation of alcohols over Si3N4-supported silver catalysts

Silver-containing catalysts supported on ceramic silicon nitrides and modified with Zr or Al are considered. The catalytic activities of the catalysts in the dehydrogenation and oxidative dehydrogenation of ethanol into acetaldehyde are compared. The introduction of oxygen into the reaction mixture decreases the temperature of 100% ethanol conversion to 270°C and increases the acetaldehyde selectivity to 95% for all of the catalysts. According to temperature-programmed reduction data, the simultaneous presence of Fe and Ag on the catalyst surface enhances the activity of the supported catalysts. It is hypothesized that the Ag/Fe interface plays the key role in the formation of active sites on the catalyst surface.