S. Damyanova

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

Effect of mixed titania-alumina supports on the phase composition of NiMo/TiO2Al2O3 catalysts

NiMo catalysts supported on mixed TiO2Al2O3 oxides containing between 0 and 22 wt.−% TiO2 have been prepared and characterized by temperature-programmed reduction (TPR), infrared (IR) and diffuse reflectance (DR) spectroscopy, BET and ammonia adsorption measurements. The samples have been tested in hydrodesulphurization (HDS) of thiophene. The spectral results show that the proportion of weakly bonded MoO42− groups is higher for NiMo/TiO2Al2O3 catalysts compared to catalysts contained only molybdenum. The increase in TiO2 content in the mixed supports leads to a decrease of inactive NiAl2O4 spinel. An increase of Ni2+ ions in octahedral surroundings, included in a structure analogous to NiTiO3 is observed. Acidity shows the nickel-molybdenum surface interaction to be a result of the formation of a precursor of NiMoO4-like structure. The TPR results demonstrate that addition of titania into the support facilitates the Mo6+ reduction to Mo5+ and Mo4+. The influence of nickel on the molybdenum reducibility is revealed by a decrease of the temperature of reduction of the molybdenum species. The HDS activity of NiMo/TiO2Al2O3 catalysts reaches a maximum at 17 wt.−% TiO2 due to formation of an active phase with an optimal composition of the oxidic precursors.

Acid-redox properties of titania-supported 12-molybdophosphates for methanol oxidation

Physicochemical and catalytic properties of titania-supported 12-molybdophosphates have been investigated. The samples were characterized by the S(BET) method, Fourier-transform infrared (FTIR) and X-ray photoelectron (XPS) spectroscopies, temperature-programmed desorption (TPD) of ammonia and temperature-programmed reduction (TPR). The effect of temperature pretreatment on the catalytic behaviour of the samples in methanol oxidation was investigated. A significant change of selectivity to the main reaction products: dimethyl ether (DME) and formaldehyde (HCHO), as a function of the acid properties of the catalysts was observed. The highest selectivity to DME at 523 K suggested that the undecomposed molybdophosphoric acid (HPMo) on titania behaves as an acid catalyst. Pretreatment of the samples at higher temperatures (up to 723 K) and replacement of the protons in HPMo by cations (Co and Ni) lead to development of the redox properties (HCHO formation) of the catalyst, due to the suppression of the Bronsted acidity. The IR and XPS results provided clear evidence for the preservation of Keggin unit up to 623 K pretreatment after test reaction.

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

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.

CO2 reforming of CH4 over Rh-containing catalysts

Abstract Two series Rh catalysts were prepared depending on the kind of the support: (i) Rh supported on zeolite and (ii) on oxide carriers. Rh/NaY zeolite catalysts were prepared by ion-exchange from an aqueous solution of [Rh(NH 3 ) 5 ]Cl 3 . Rh catalysts supported on γ-Al 2 O 3 , Nb 2 O 5 and TiO 2 were prepared by incipient wetness impregnation method of the carrier with an aqueous solution of RhCl 3 ·2H 2 O. The catalysts were characterized by X-ray diffraction (XRD) spectroscopy, nuclear magnetic resonance (NMR), temperature-programmed desorption of hydrogen (TPD-H 2 ). Test reaction for Rh catalysts was CO 2 reforming of methane at different reaction temperatures. TPD data showed that the dispersion of metal particles depends on the pretreatment activation procedure; highest dispersion was observed for samples previously calcined and then activated in H 2 . Catalysts activated directly in H 2 , steam or submitted directly to the reaction conditions showed large metal particles. The effect of the protons on the destabilization of the zeolite framework during reaction was revealed by the higher Si/Al ratios, due to a dealumination of the zeolite. Neutralization of protons with solution of NaOH leaded to a stabilization of the dispersion and zeolite structure. Correlation between the dispersion and specific activity in CO 2 reforming was found for zeolite-supported Rh catalysts. When the activities are compared by turnover frequencies, oxide-supported Rh catalysts are significantly more active compared to zeolite-supported ones due to a higher degree of participation of the reverse water–gas shift reaction (WGSR). The difference in activity and thermal stability was related to the nature of the support.

Surface properties of titania-supported 12-molybdophosphoric acid hydrodesulphurization catalysts

Abstract Titania-supported 12-molybdophosphoric acid and its Co(Ni) salts were studied by X-ray photoelectron spectroscopy and Fourier transform IR spectroscopy in the mid-infrared range and of adsorbed nitric oxide and pyridine. Hydrodesulphurization of thiophene was used as test reaction of the samples. The results provide additional support for the preservation of phosphoromolybdate anion on titania-support after its supporting. The highest activity of nickel promoted catalysts may be caused by an increase in dispersion of molybdenum and its lighter reduction.

CO2 reforming of CH4 over Ru/zeolite catalysts modified with Ti

Abstract Ru/NaY and Ru/USY catalysts were prepared by ion exchange from an aqueous solution of [Ru(NH 3 ) 6 ]Cl 3 . The effect of titanium on the structure of Ru/zeolite introduced by ion exchange or impregnation method from aqueous solution of (NH 4 ) 2 TiO(C 2 O 4 ) 2 ·H 2 O was investigated. The samples were characterized by different techniques, using X-ray diffraction (XRD), UV-Vis diffuse reflectance spectroscopy (DRS), temperature-programmed desorption of hydrogen (TPD-H 2 ), Fourier transform infrared spectroscopy (FTIR) of CO adsorption and transmission electron microscopy (TEM). Introduction of titanium into Ru/NaY zeolite by ion exchange caused a significant structural collapse of the zeolite framework. Contrary to that, TEM data showed that Ti incorporated into Ru/USY by ion exchange was well distributed within the zeolite cavities. The crystallinity of the USY zeolite decreased significantly after introduction of Ti by impregnation method and a separate phase of titania was formed. The effect of Ti content on the catalytic behavior of zeolite-supported Ru catalysts in the reaction of CO 2 reforming of CH 4 was elucidated. The higher specific activity of Ti-containing catalysts related to CO formation was caused by the both factors, the acceleration of CH x O decomposition (precursor for CO formation) on metal-support interface and participation of the reverse of water gas shift reaction.

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