L. Petrov

Immobilization of 12-molybdophosphoric and 12-tungstophosphoric acids on metal-substituted hexagonal mesoporous silica

Abstract Hexagonal mesoporous silicate (HMS) materials containing Ti, Zr and Al ions were prepared using the neutral template route. The effect of the cation on the immobilization of 12-molybdophosphoric and 12-tungstophosphoric acids on mesoporous materials was studied. The texture of the samples was studied by the N2 adsorption–desorption isotherms, and their structure by X-ray diffraction (XRD), UV-Vis diffuse reflectance (DRS), infrared (IR) and X-ray photoelectron spectroscopy (XPS), and 1 H and 31 P nuclear magnetic resonance (NMR). Incorporation of different metals into the framework of HMS led to change in the structural and surface properties of mesoporous silica. All metal-containing samples showed a lower surface area compared to that of pure silica. The introduction of Zr and Al cations led to the development of textural mesoporosity in addition to the framework mesoporosity, characteristic of HMS and Ti-containing mesoporous silica (TiHMS) materials. IR and 1 H and 31 P NMR spectroscopic data revealed that the species present on the surface of HMS and TiHMS mesoporous materials after impregnation with solutions of 12-molybdophosphoric and 12-tungstophosphoric acid are non-degraded heteropoly anions with a preserved Keggin unit. Partial degradation of the anion was observed for zirconium-containing silicate samples (ZrHMS)-supported acids, the extent of which was stronger for supported molybdophosphoric acid in comparison with that of tungstophosphoric acid.

Characterization of molybdenum hydrodesulfurization catalysts supported on ZrO2-Al2O3 and ZrO2-SiO2 carriers

Two series of Mo-containing catalysts supported on alumina and silica, respectively, modified with different amounts of zirconium, were prepared by wet impregnation method. X-ray diffraction, temperature-programmed reduction (TPR), X-ray photoelectron and Fourier transform infrared spectroscopic techniques were used for physicochemical characterization of the samples in oxidic, reduced and sulfided state. The effect of zirconium content on the state and dispersion of molybdenum is reported: introduction of a small amount of zirconium into the support leads to a significant increase of the dispersion of supported molybdenum oxide species, especially, on silica. ZrO2-Al2O3 supported Mo catalysts show higher Lewis acidity compared to those supported on ZrO2-SiO2, Zirconium influences the reducibility of molybdenum. The reduction of molybdenum oxide species on ZrO2-Al2O3 supports proceeds easily compared to that supported on ZrO2-SiO2, however increasing the Zr02 content leads to a decreasing of the hydrogen consumption during the reduction for both series catalysts due to a strong interaction between molybdenum species and modified supports. The maximum in HDS activity is observed for Mo/ZrO2-SiO2 catalysts with the lowest Zr content (6.6 wt.%) related to a higher dispersion of molybdenum sulfide species

XPS characterization of zirconium-promoted CoMo hydrodesulfurization catalysts

CoMo hydrodesulfurization catalysts (HDS) supported on mixed ZrO2-Al2O3 and ZrO2-SiO2 oxides have been prepared via a two-step impregnation using the incipient wetness method. The S-BET method, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used for characterization of the samples. Increasing the ZrO2 content decreases the surface areas and the pore volumes of supported CoMo catalysts. The effect of Zr content on the dispersion of Mo and Co species is more pronounced in the case of CoMo/ZrO2-SiO2 samples; a higher dispersion corresponds to a higher ZrO2 content. The sulfidation of molybdenum forms MoS2. A higher degree of sulfidation of molybdenum is observed for CoMo catalysts supported on ZrO2-Al2O3. A comparison between surface properties and catalytic activity behaviors of CoMo catalysts in HDS of thiophene is presented

Kinetic model of nitrobenzene hydrogenation to aniline over industrial copper catalyst considering the effects of mass transfer and deactivation

Abstract A kinetic model was developed of nitrobenzene hydrogenation to aniline over industrial copper catalyst, considering the effects of mass transfer and deactivation. The model describes experimental data with good accuracy. Based on the model, a rapid method for estimating the working time of industrial catalyst for aniline production is described.

Effect of phosphorus on the surface state of alumina-supported nickelmolybdenum catalysts for hydrodesulphurization

Abstract The influence of concentration and method of phosphorus introduction in the alumina-supported nickel—molybdenum catalysts on the surface properties and catalytic activity in thiophene hydrodesulphurization (HDS) have been investigated. As a result of a strong interaction between phosphorus and support aluminium phosphates are formed. Molybdenum compounds, among them iso- and heteropoly compounds, are formed on the surface of Al 2 O 3 irrespective of the presence of phosphorus. The promoting effect of phosphorus is expressed in optimization of the phase composition of the catalyst and a decrease in coke formation. A 1 wt.-% ensures the optimum composition of the polymolybdate surface structures and their homogeneous distribution.

Effect of niobium on the surface properties of Nb2O5-SiO2-supported Mo catalysts

Nb2O5-SiO2 oxides with different Nb content (0.8-4.4 wt.%) were prepared by impregnation of silica with ethanol solution of niobium ethoxide, Nb(OC2H5)(5). Mo catalysts supported on the Nb2O5-SiO2 carriers were prepared by wetness impregnation method of the supports with an aqueous solution of ammonium heptamolybdate, (NH4)(6)MO7O24. The samples were characterized by X-ray photoelectron spectroscopy (XPS) and diffuse reflectance spectroscopy (DRS). The chemical state and dispersion of metal components (Nb and Mo) were studied in the oxides and Mo-containing catalysts. It was shown a good correlation between the amount of Nb and XPS intensity Nb/Si ratios in oxide supports. Generation of new acid sites (Lewis and Bronsted) in Nb2O5-SiO2 samples was easily understood when considering the change in the binding energies of Nb 3d and 0 Is photoelectrons, due to a strong interaction between Nb atoms and silica. It was found that addition of niobium improves the dispersion of molybdenum on silica

Kinetics of ethylene oxidation over a silver catalyst in the presence of dichloroethane

Abstract The kinetics of ethylene oxidation over a silver catalyst in the presence of dichloroethane has been studied by the circulation flow method with a gradientless reactor. Dichloroethane inhibits the total oxidation reaction to a greater extent than the partial oxidation reaction. In this way it promotes the selectivity for ethylene oxide. The promoter was added to the feed gas stream in trace quantities ∮.2 to 28 ppm. The experiments were carried out under steady-state conditions, at atmospheric pressure and in the interval 240 – 292°C. Additional factors that could influence the selectivity, temperature, feed composition and degree of conversion of ethylene, were also investigated. It was established that the influence of the conversion is negligible. A linear model of the influence of the dichloroethane concentration on the selectivity could be applied to most series of the experiments. The selectivity-promoting effect of dichloroethane is stronger at higher temperatures. The selectivity is most sensitive to promoter concentration when the feed gas stream has the composition 50% O2, 25% C2H4 and 25% Ar. A kinetic model consisting of two rate equations - for the partial and for the complete oxidation is proposed, including a dependence on the partial pressure of dichloroethane. The model describes satisfactorily the influence of all factors on the selectivity. The role of dichloroethane is to inhibit the dissociative adsorption of oxygen on the silver surface.

A kinetic model of steady state ethylene epoxidation over a supported silver catalyst

Abstract A kinetic study of selective ethylene oxidation to ethylene oxide over a silver catalyst has been carried out by the circulation flow method. The silver was promoted by a Ca additive and was supported on alumina. The experiments were conducted under steady state conditions in the temperature interval 210 – 292°C and at atmospheric pressure. As a result of this kinetic study two rate equations, for the partial and for the complete oxidation reactions, are proposed. A reaction scheme is put forward, according to which adsorbed molecular oxygen produces ethylene oxide, whereas the atomic oxygen is responsible for the complete oxidation reaction. The empirical kinetic model corresponds to a Rideal-Eley type of mechanism. Selectivity decreased with temperature increase and with decrease of the oxygen content in the feedstock.

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.

Fourier transform infrared spectroscopic study of surface acidity by pyridine adsorption on Mo/ZrO2-SiO2(Al2O3) catalysts

Acidity of the oxidic molybdenum catalysts supported on mixed ZrO2-SiO2 and ZrO2-Al2O3 carriers was investigated by Fourier transform infrared spectroscopy of pyridine adsorption. Deposition of molybdenum on ZrO2-SiO2 and ZrO2-Al2O3 supports leads to formation of surface Brønsted acid sites. The number of the Brønsted and Lewis acid sites in supported-molybdenum catalysts depends on both the ZrO2 content and the type of the support. With increasing ZrO2 content, the Lewis acid sites increase for both series of catalysts. The Brønsted acid sites are higher for Mo/ZrO2-SiO2 samples compared to those for Mo/ZrO2-Al2O3 and also increase with zirconia.