K. A. Dubkov

Iron complexes in zeolites as a new model of methane monooxygenase

Iron complexes in the ZSM-5 zeolite matrix (α-centers) are shown to perform single-turnover cycles of methane oxidation to methanol at room temperature when nitrous oxide is used as a source of oxygen. The origin of carbon and oxygen in the product methanol was traced using13C and18O isotopes. Probable structure of α-sites as well as mechanistic features of the reaction allow to consider this system as a first successful model of 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.

NON-CATALYTIC LIQUID PHASE OXIDATION OF ALKENES WITH NITROUS OXIDE. 1. OXIDATION OF CYCLOHEXENE TO CYCLOHEXANONE

A very efficient way of alkenes oxidation to carbonyl compounds is discovered. It is based on remarkable ability of nitrous oxide to interact directly with the double C=C bonds of liquid alkene and to transfer its oxygen, without catalyst aid, to unsaturated carbon atom with nearly 100% selectivity. This oxidation method can be applied to a wide range of organic compounds including aliphatic, cyclic, heterocyclic alkenes and their numerous derivatives.

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.

Biomimetic oxidation on Fe complexes in zeolites

One-step hydroxylation of aromatic nucleus with nitrous oxide (N 2 O) is among recently discovered organic reactions. A high efficiency of FeZSM-5 zeolites in this reaction relates to a pronounced biomimetic-type activity of iron complexes stabilized in ZSM-5 matrix. N 2 O decomposition on these complexes produces particular atomic oxygen form (α-oxygen), whose chemistry is similar to that performed by the active oxygen of enzyme monooxygenases. Room temperature oxidation reactions of α-oxygen as well as the data on the kinetic isotope effect and Moessbauer spectroscopy show FeZSM-5 zeolite to be a successful biomimetic model.

Mechanism of Coke Influence on the Catalytic Activity of FeZSM-5 in the Reaction of Benzene Oxidation into Phenol

The influence of coke formation in the reaction of benzene oxidation by nitrous oxide into phenol on the catalytic activity and concentration of iron-containing active sites (α-sites), which are stabilized in the microporous structure of FeZSM-5 zeolite, is studied. The deactivation by coke is explained by the poisoning of α-sites, whose concentration decreases linearly with an increase in the coke content, rather than by the blocking of zeolite pores. The activity per one α-site remains unchanged. This fact indicates the absence of diffusion limitations associated with coke formation. The toxicity of coke for the α-sites is determined. The coke amount equivalent to 100–120 benzene molecules is shown to result in the deactivation of one active site.

Stoichiometry of oxidation reactions involving α-oxygen on FeZSM-5 zeolite

The stoichiometry of the low-temperature reaction between surface α-oxygen formed by decomposing N2O over Fe-containing ZSM-5 zeolite and methane, hydrogen (deuterium), and carbon monoxide is studied. Methane and hydrogen react with α-oxygen in stoichiometric ratios of 1 : 1.8 and 1 : 1.6, respectively. The observed stoichiometry is due to the mechanisms of the corresponding reactions. According to a mechanism proposed for the interaction of α-oxygen with methane and hydrogen, this reaction is accompanied by the dissociation of CH4and H2molecules. For hydrogen, such a mechanism is supported by IR spectroscopic studies of resulting surface compounds, namely, of new hydroxyl groups that were formed on the zeolite surface in the course of the reaction. α-Oxygen reacts with CO in the ratio of 1 : 1 to form CO2in amounts equal to those of α-oxygen on the surface.

Identification of Active Oxygen Species over Fe Complexes in Zeolites

In the framework of identification problem of active oxygen species involved in the oxidation catalysis a possible role of M=O, O2- and O- species is analyzed. Particular attention is paid to a recently discovered oxygen species Oα formed upon N2O decomposition over Fe complexes in zeolites. High reactivity of α-oxygen coupled with the high concentration allow to reliably identify its participation in the oxidation of methane to methanol and benzene to phenol on the FeZSM-5 surface.

EFFECT OF ZSM-11 CRYSTALLINITY ON ITS CATALYTIC PERFORMANCE IN BENZENE TO PHENOL OXIDATION WITH NITROUS OXIDE

Evolution of ZSM-11 zeolite crystal and pore structure has been studied at 150°C depending on the synthesis time. The amorphous gel was shown to be catalytically inert over the whole induction period, which precedes the crystallization process. The concentration of active sites and the catalytic activity in the oxidation of benzene to phenol is related to the zeolite structure formation and the increasing degree of crystallinity.

High-temperature carboxidation of cyclopentene with nitrous oxide

It is demonstrated by the example of cyclopentene that the noncatalytic oxidation of alkenes with nitrous oxide to carbonyl compounds (carboxidation), which is known to occur in the liquid phase at 150–250°C, can also take place in the gas phase at higher temperatures (300–475°C). Like liquid-phase carboxidation, the gas-phase reaction likely proceeds via a dipolar 1,3-addition mechanism. However, 4-pentenal is formed along with cyclopentanone in the gas phase. The 4-pentenal selectivity increases from 2.5 to 23% as the reaction temperature is raised. High-temperature cyclopentene carboxidation can be carried out in a paraffin melt (bp ∼ 400°C). Filling the reactor with paraffin accelerates the reaction and reduces its activation energy.