M. N. Mikhailov

The structure of active sites in a molybdenum/zeolite catalyst for methane dehydroaromatization: a DFT study

Mononuclear active sites in molybdenum-zeolite catalysts for methane dehydroaromatization were studied within the framework of the density functional theory (DFT). The optimized structures of mononuclear sites in molybdenum oxide, molybdenum oxycarbide, and molybdenum carbide were obtained. A mechanism of carbidization i.e., formation of molybdenum oxycarbide and molybdenum carbide from molybdenum oxide was proposed. Carbidization using three (radical, cationic, and anionic) structures of mononuclear active sites was studied. It was found that the anionic structure provides the best conditions for methane activation on the mononuclear active site.

One-dimensional heterogeneous model of a Fischer-Tropsch synthesis reactor with a fixed catalyst bed in the isothermal granules approximation

A one-dimensional heterogeneous model has been developed for a cat6alytic fixed-bed Fischer-Tropsch (FT) synthesis reactor in the isothermal granules approximation. The FT process has been simulated for a laboratory-scale reactor. The effects of the linear gas velocity and of the inner diameter of the reactor on the thermal stability of the process are considered. The size of the reactor is limited by the possibility of a “thermal explosion” occurring in the frontal layer of the catalyst. Raising the linear gas velocity enhances heat transfer, thereby reducing the overheating of the catalyst bed. The synthesis of solid hydrocarbons can be conducted in reactors no larger than 18 mm in diameter. According to calculations, the maximum temperature drop in a 3-, 4-, and 6-m-long reactor is 4.7, 4.2, and 3.6°C, respectively. The corresponding CO conversion is 35.0, 34.4, and 33.9%, respectively. For producing liquid hydrocarbons in a high-performance reactor, it is necessary to decrease its inner diameter to 12 mm. In this case, the maximum temperature drop at a reactor length of 3, 4, and 6 m is 9.6, 8.7, and 7.6°C, and the CO conversion is 78.0, 77.4, and 76.7%, respectively. The mathematical model devised here provides means to estimate the necessary design parameters of the reactor and the appropriate FT synthesis conditions for producing liquid or solid hydrocarbons.

On the possibility of the detachment of hydrogen as a result of electron capture by a Broensted center on zeolites

The structure and reactivity of the radical anion center in ZSM-5 zeolite were studied by the density functional theory method. It was shown that the interaction of the hydrogen zeolite form with adsorbed olefins and aromatic hydrocarbons could be accompanied by electron transfer from hydrocarbon molecules to the Broensted acid center with the formation of a radical anion fragment. The radical anion fragment formed is unstable, which contributes to the probability of the exothermic process (ΔE = −21 kcal/mol) of atomic hydrogen detachment with the activation energy not exceeding 10–12 kcal/mol. The atomic hydrogen split off can initiate hydrocarbon transformations, and such a radical anion center can play the role of a catalytic activity carrier on acid zeolites.

Investigation of the molybdenum state in the Mo(4%)/HZSM-5 catalyst for methane aromatization

The Mo(4%)/HZSM-5 catalyst for methane aromatization prepared by the impregnation of ammonium heptamolybdate followed by calcination at 700°C was studied. The formation of molybdenum ions in the oxidation state 5+ during catalysis and the presence of graphitized carbonaceous sediments on the surface of the catalyst were confirmed by diffuse reflectance infrared Fourier transform spectroscopy (DRIFT), X-ray photoelectron spectroscopy (XPS), and ESR spectroscopy.

Framework composition and activity of platinum-containing high-silica zeolites in n-hexane isomerization

The conversion of n-hexane on Pt-containing dealuminated mordenites and ZSM-5 zeolites was studied. The framework composition and the concentrations of extraframework aluminum compounds in mordenites were determined by X-ray diffraction analysis. It was demonstrated that the aluminum content of the framework affected the activity in n-hexane isomerization. It was found that a Pt-mordenite catalyst containing a considerable amount of extraframework aluminum compounds exhibited maximum activity. A quantum-chemical study of the interaction of platinum with Brønsted and Lewis acid sites was performed. It was hypothesized that oxidized surface platinum nanoparticles were the active sites of Pt-containing high-silica zeolites. These nanoparticles were formed by the interaction of platinum clusters with proton sites or extraframework aluminum compounds. An alternative mechanism was proposed for the conversion of alkanes to exclude the direct participation of acid sites.

The role of hole defects in the formation of active sites in the catalyst for methane dehydroaromatization

The formation of active sites in the molybdenum-zeolite catalyst for methane dehydroaromatization was studied by the density functional theory method. The interaction of MoO2(OH)2 particle with the Brønsted site, anionic site, and electron hole of the zeolite was studied. The mechanism governing the formation of mononuclear active sites was proposed. It was shown that the formation of the MoO2 mononuclear active site with participation of electron hole of the zeolite is thermodynamically possible and is accompanied by electron density transfer from zeolite oxygen atom to molybdenum atom.

The Role Played by Ga-Pt Nanoparticles in the Aromatization of Lower Alkanes on ZSM-5 Zeolites

The activity and selectivity of Pt-Ga/ZSM-5 zeolites in the aromatization of ethane was studied. The temperature intervals of the stability of gallium oxides on the surface of zeolites were determined by X-ray diffraction. The formation of GaPt clusters was observed after high-temperature treatment of a mixture of platinum-containing zeolites with gallium oxides. Density functional theory was used to study the state of gallium clusters in zeolites by quantum-chemical methods. The interaction of gallium with the H-form of ZSM-5 zeolites was shown to result in the formation of cationic centers containing reduced single-charged gallium cations. The introduction of platinum stabilized alloyed Ga-Pt particles localized in zeolite channels. These particles increase catalyst activity in the aromatization of lower alkanes.

Optimizing the parameters of the steam-carbon dioxide conversion of methane by Gibbs energy minimization

The thermodynamic equilibrium for the steam-carbon dioxide conversion of methane was studied by Gibbs energy minimization. The degree of coke formation, the content of methane and carbon dioxide in the synthesis gas, and the synthesis gas H2/CO ratio were plotted as functions of the molar ratios of CO2/CH4 and H2O/CH4 in the initial mixture at different temperatures and pressures. The regions of the optimum CH4/CO2/H2O molar ratios for steam-carbon dioxide conversion were discovered, with no coke formation taking place in these regions. The optimized CH4/CO2/H2O molar fractions characterized by the minimum content of methane and carbon dioxide in the synthesis gas were found for each region.

Phase composition of Mg—Al mixed oxides, their activity and selectivity in the ethanol condensation reaction

Layered hydroxides with a molar ratio of metals Mg: Al: M = 3: 1: 1 (M = Fe, Ce, Zr, Cr) were prepared and served as a basis to obtain the mixed oxides MgAlOx, MgAlCrOx, MgAlCeOx, MgAlZrOx, and MgAlCrOx. Powder X-ray diffraction was used to study the phase composition of the oxides. It was suggested that the catalyst active surface is related to the presence of spineltype X-ray amorphous compounds. Ammonia adsorption was used to determine the total acidity, and deuterated acetonitrile adsorption was applied to estimate the strength of acid sites. The catalytic properties of complex oxides were studied in the ethanol condensation reaction. An attempt was made to correlate the catalyst activity and selectivity and the distribution of acid and base sites on the catalyst surface.

Nature of active sites in hybrid metal-zeolite catalysts for the Fischer-Tropsch synthesis

The temperature-programmed reduction, powder X-ray diffraction, and oxygen titration were applied to study reducibility, phase composition, and structure of the active surface of the hybrid metal-zeolite catalysts Mβ@Co/Al2O3 (M = Pd, Co, and Fe). The results of physicochemical studies were compared with the data on activity and selectivity of the catalysts in the synthesis of liquid hydrocarbons from the synthesis gas. The nature of transition metal and the method of its introduction into the catalyst influence the composition of synthetic liquid hydrocarbons. A comparison of the Feβ@Co/Al2O3 catalyst prepared by the ion-exchange method that exhibits the highest activity in the synthesis of liquid hydrocarbons with a similar catalyst Fe/(Hβ@Co/Al2O3) prepared by the impregnation method indicated a distinct advantage of the ion exchange procedure. A mechanism of isomerization and cracking of primary linear alkanes on the Mnδ+ clusters in the Mβ@Co/Al2O3 systems was proposed. The mechanism explains the main features governing the group and fractional composition of the obtained synthetic hydrocarbons.