Ch. Vladov

Effect of phosphorus concentration and method of preparation on the structure of the oxide form of phosphorus-nickel-tungsten/alumina hydrotreating catalysts

Two series of phosphorus containing NiW/Al2O3 catalysts were prepared by different preparation methods and varying phosphorus content from 0 to 7.6 wt.% P2O5. The influence of the phosphorus concentration and the preparation procedure on the structure and the dispersion of the compounds formed in the oxide form of the catalysts was studied. Diffuse reflectance spectroscopy (DRS), infrared spectroscopy (IR) and X-ray photoelectron spectroscopy (XPS) were used for the characterization of the catalysts. It was demonstrated that introduction of phosphorus in NiW/Al2O3 catalysts impedes the formation of NiAl2O4 and increases the amount of Ni2+(Oh) ions in the oxide form of the samples. This effect of phosphorus is better expressed when phosphorus is first introduced to alumina, followed by co-impregnation with nickel and tungsten. Surface AlPO4 is formed in the phosphorus containing samples irrespective of the preparation procedure. Phosphorus is distributed as a monolayer up to concentrations of 1.3 P at/nm2 in both series of catalysts. It was also found that the increased phosphorus content in the samples leads to an increased degree of polymerization of the tungsten species via WOW bonds. The presence of phosphorus changes the dispersion of the active components. Evaluation of the average particle size of tungsten species shows that it increases from 10Afor the phosphorus free sample up to 20–25Afor the samples with high phosphorus content. The relationship between the structure of the phosphorus promoted NiW/Al2O3 catalysts and their hydrodesulfurization activity is discussed.

Effects of carrier gases in the toluene disproportionation on HZSM-5 zeolite

Abstract Toluene disproportionation on HZSM-5 was studied at 573–598 K, 0.1-1 MPa total pressure, 1–2 kPa toluene partial pressure and a WHSV = 0.87 h−1. The degree of conversion depends on the type of the carrier gas under elevated pressures (>0.1 MPa) following the order Ar > N2 > He > H2. In hydrogen the toluene conversion was transiently accelerated following the use of oxygen as a carrier gas, where the major part of the strongly increased activity was due to the formation of methane. The effects are discussed on the basis of the hypothesis that the zeolite Lewis acid sites can be shielded by the carrier gas atoms or molecules and that the hydrogen transfer properties of carbonaceous deposits can be promoted by oxygen.

Catalytic activity of NiPS3 in the reaction of thiophene hydrodesulfurization

Abstract NiPS 3 exhibits a high catalytic activity in thiophene hydrodesulfurization, as well as a hydrogenation and isomerization activity. The promoting effect of phosphorus on nickel-molybdenum hydrodesul-furization catalysts can be explained by the formation of highly dispersed NiPS 3 under the reaction conditions. The mechanism of thiophene hydrodesulfurization over NiPS 3 is discussed in relation to the pronounced ability of that substance to form intercalation compounds. A key step of the mechanism is the activation of hydrogen by intercalation in the van der Waals gap.

MECHANISM OF THE OXIDATIVE HYDROLYSIS OF IRON(II) SULPHATE

The kinetics of the oxidative hydrolysis of iron(II) sulphate at temperature 30°C and pH 4.5 in the concentration range 40–300 gl−1 were studied. The change in the kinetic equations in the presence of inhibitor and catalysts gives additional information about the mechanism of the process. In the presence of manganese(II) ions we observed two parallel mechanisms of oxidation (homogeneous and heterogeneous), in agreement with the obtained kinetic data. The change in the α-FeOOH precipitate structure with time, detected by Mössbauer spectroscopy, allows the suggesting of a mechanism of heterogeneous oxidation related to the crystal growth.

Thermal oscillations during the catalytic hydrogenation of nitrobenzene

Abstract Three commercial Kieselguhr-supported copper, nickel and coppernickel catalysts were investigated under severe conditions in the hydrogenation of nitrobenzene to aniline. Self-sustained oscillations of the catalyst bed temperature in the 438 – 593 K range were observed after a partial deactivation of the studied samples. The oscillation mode was changed upon external heating. The experimental results show that the oscillations are due to mass transfer retardation effects and the nature of the chemical processes. An explanation of the observed phenomenon is proposed, according to which a hot zone is formed in the front layer of the catalyst bed. The zone cycles up and down along the catalyst bed.

ESR study of the surface of industrial Co−Mo/Al2O3 catalyst

An industrial, Co−Mo/Al2O3 hydrodesulfurization catalyst has been studied by means of ESR spectroscopy. It has been established that an oxidation-reduction cycle of reactions occur involving Mo ions in the active complex formation.AbstractМетодом ЭПР спектроскопии проведено исследование промышленных Co−Mo/Al2O3 катализаторов для реакции гидродесульфурирования. Установлено, что окислительно-восстановительный цикл протекает с участием иохов молибдена, образующих активный комплекс.

Structure and activity of a CoMo/Al2O3 catalyst upon modification by gamma irradiation

The activity of a commercial CoMo/Al2O3 catalyst was examined in the reaction of thiophene hydrodesulfurization. The catalytic measurements were performed in a gradientless flow circulating system at atmospheric pressure. It was found that preliminary irradiation of the catalyst by gamma rays gives rise to a considerable increase in catalytic activity and selectivity. The enhancement of activity was retained even after regeneration of the catalyst. As a result of the irradiation some of the tetrahedrally coordinated Co2+ ions were transformed into species with another environment. In addition, upon irradiation some of the Mo6+ ions were reduced to Mo5+ ions and it is suggested that defects are formed on the catalyst surface. Changes in composition and structure of the catalyst samples, which were preserved even after regeneration, are associated with the changes in the catalytic properties.

Kinetics of the isomerization of butenes over an industrial Co-Mo/Al2O3) Catalyst.: I: Determination of the relative rate constants by the method of wei and prater

Abstract The kinetics of the isomerization of butenes over an industrial CoO MoO3/Al2O3 catalyst for hydrodesulphurization of Diesel fractions has been studied by the flow circulation method. The study was carried out under steady-state conditions at atmospheric pressure and in the temperature range 325–425°C. The selectivity of the catalyst for cis-2-butene is practically the same for both the reduced and the sulphided form, but the activity of the reduced form is greater. The selectivity decreases with increasing temperature, with contact time and with 1-butene concentration in the feed, for both the sulphided and the reduced form. Evidence was obtained that suggests that surface reactions are the rate-determining steps under these reaction conditions. The values of the relative rate constants for the three experimental temperatures of 325, 375 and 425°C were calculated by the Wei-Prater method. These values and those obtained for the activation energies show that cis-2-butene is a kinetic control product, as it is formed faster than trans-2-butene which is a thermodynamic control product.

Preparation of fibrous CoMo/Al2O3 hydrotreating catalysts by the sol-gel method

Fibrous CoMo−Al oxide hydrotreating catalysts have been prepared by the sol-gel method. It is shown that their catalytic properties are similar to the properties of commercial catalysts prepared by traditional methods.

Kinetics of isomerization of butenes over an industrial CoOMoO3/Al2O3 catalyst.: II. Kinetic model construction

Abstract The kinetics of isomerization of butenes over an industrial CoO MoO 3 /Al 2 O 3 catalyst have been studied by the flow-circulation method under steady state conditions, at atmospheric pressure and in the temperature interval 325–425°C. As a result of the study a kinetic model is proposed for the reduced form of the catalyst. The kinetic model is of the Langmuir-Hinshelwood type and corresponds to a single site mechanism. The adsorption and desorption of the three isomers, 1-butene, cis-2-butene and trans-2-butene are fast steps at equilibrium under these experimental conditions. The rate-limiting step is a surface reaction. The kinetic constants of the model provide a possibility to evaluate the rate constants of the surface interactions. The model adequacy for the description of the dependence of selectivity and conversion on temperature, contact time and feed composition is extensively illustrated.