Molecular Designed Dispersion Method Research Articles

SBA-15 loaded with iron by various methods as catalyst for DeNOx process

SBA-15 modified with iron by various methods was tested as catalyst of the DeNOx process. Two methods were used for deposition of iron: (i) molecular designed dispersion method to introduce Fe in the form of monomeric cations and (ii) ion-exchange of iron acetate oligocations, [Fe3O(OAc)6·AcOH·2H2O]+, for SBA-15 modified with propylsulfonic groups to introduce aggregated iron oxide species. The obtained samples were characterized with respect to: textural parameters (low-temperature N2 adsorption), structure (XRD, TEM), efficiency of surface species anchoring (DRIFT, FTIR-PAS, TG-DTG and elemental analysis), form and aggregation of deposited iron species (Mossbauer spectroscopy, UV-vis-DRS, H2-TPR, CO-FTIR), distribution of iron oxide species in the samples (TEM-EDX). SBA-15 modified with iron was found to be effective catalysts of selective reduction of NO with ammonia. It was suggested that iron deposited in the form of aggregated FeOx species presents higher catalytic activity in the studied process comparing to monomeric Fe3+ cations.

V2O5/SBA-15 nanocatalysts for the selective synthesis of 2,3,5-trimethyl-1,4-benzoquinone at room temperature

The molecular designed dispersion method yielded monodispersed nanoscale (4–7.5 nm sized) V2O5 dimeric species on SBA-15, which oxidized TMP into TMBQ with H2O2 under green conditions.

Catalytic activity of rhodium grafted on ordered mesoporous silica materials modified with aluminum in N2O decomposition

Abstract Three different ordered mesoporous silica materials, such as MCM-41, Al-containing MCM-41 (with 1.6 wt% of Al incorporated into the silica framework) and SBA-15, were prepared. Furthermore, aluminum (1.8 wt%) was grafted on the silica surface by the molecular designed dispersion method (MDD). In a next step, rhodium (1.4–2.9 wt%), as an active metal for the catalytic reaction of N 2 O decomposition, was introduced by the MDD technique. The following order of the catalytic activity was found under an inert atmosphere: Al(incorporated)-MCM-41 + Rh > MCM-41 + Al(grafted) + Rh > SBA-15 + Rh ≈ MCM-41 + Rh. The influence of textural properties, total acidity and rhodium dispersion on the activity of catalysts was studied.

Effects of copper and vanadium deposition in multi-walled hydrogen trititanate and mixed-phase anatase/trititanate nanotubes

Metal incorporation and doping is one of the routes commonly explored to improve visible light photocatalytic activity of titania and related materials. In this work, we explore the effect of copper and vanadium deposition and incorporation in multi-walled hydrogen trititanate and mixed-phase anatase/trititanate nanotubes. The molecular designed dispersion method is used to introduce the metals in the materials. Electron paramagnetic resonance facilitates a full characterization of the metal sites before and after calcination and allows determining the UV- and visible-light-induced formation of paramagnetic intermediates. Vanadium or copper deposition on the tubes leads to increased formation of F(+) centres under visible or UV illumination. The EPR results are linked to the activity towards rhodamine 6G photobleaching of the different materials.

SBA-15 mesoporous silica modified with rhodium by MDD method and its catalytic role for N2O decomposition reaction

SBA-15 mesoporous silicas modified with rhodium were studied as catalysts for the N2O decomposition reaction. Rhodium was deposited on SBA-15 by the Molecular Designed Dispersion (MDD) method using Rh(acac)3 as a precursor of active phase. The same method was used for the deposition of Cu, Fe, Al and Ti. The SBA-15 support modified with metals were characterized with respect to metal loading (EPMA), structure (XRD), texture (BET), morphology (SEM), Rh dispersion (oxygen chemisorption), surface acidity (pyridine adsorption) and chemical nature of introduced copper and iron species (UV–vis-DRS). The rhodium-containing SBA-15 samples were found to be active catalysts for the N2O decomposition reaction. Deposition of Al on the Rh-loaded catalyst increased its activity. An opposite effect was observed for the samples modified with Cu and Fe.

SBA-15 mesoporous silica modified with metal oxides by MDD method in the role of DeNO x catalysts

SBA-15 mesoporous silica was modified with metal (Al, Ti, Cu, Fe) oxides by the molecular designed dispersion (MDD) method using acetylacetonate complexes of metals as precursors of the catalytically active components. The modified mesoporous silicas were characterized with respect to texture (BET), composition (EPMA), coordination and aggregation of transition metal species (UV–vis-DRS), reducibility of the deposited transition metals (TPRed) and surface acidity (FT-IR). Deposition of aluminium and titanium species on the SBA-15 surface significantly increased its acidity, mainly by generation of strong Lewis acid sites. Copper and iron deposited on the surface of pure SBA-15 were present nearly exclusively in the form of mononuclear cations. Deposition of Fe or Cu on the SBA-15 supports modified with alumina or titania resulted in a formation of significant amounts of oligomeric metal oxide clusters. The SBA-15 based samples have been found to be active and selective catalysts of the DeNOx process. The modification of the silica surface with titanium or aluminium prior to the deposition of iron or copper significantly improved the activity of the SBA-15 based catalysts.

Direct spectroscopic detection of framework-incorporated vanadium in mesoporous silica materials

Framework-incorporated vanadium mesoporous silica materials with different contents in vanadium were obtained by a facile, direct synthesis at room temperature, using VOSO4 x 5H2O as the vanadium precursor. The porous characteristics of the samples and the coordination environment of the vanadia in the structure were studied by a combination of techniques: X-ray diffraction, N2-adsorption/desorption, FT-Raman, FTIR-PAS and UV-Vis-DR, electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) spectroscopy. A structural comparison is made using pulsed EPR and ENDOR spectroscopic techniques between vanadia deposited on the surface of MCM-41 by the Molecular Designed Dispersion method and as-synthesised samples of vanadia incorporated in the mesoporous silica framework using the above-mentioned synthesis method. The EPR study on the non-calcined samples proves the incorporation of a high amount of vanadium in the silica framework by the observation of a strong hyperfine coupling of the unpaired electron with 29Si. It demonstrates the feasibility for EPR to reveal structural information on true incorporation of metal ions in framework positions leading to metal oxides.

Preparation and Characterization of Vanadium Oxide Species Supported on Mesoporous Silica for the Hydroxylation of Benzene

The homogeneous liquid-phase direct catalytic oxidation of benzene to phenol was performed at 60 °C in acetonitrile solvent using various catalysts of VOx supported on aluminum-substituted mesoporous silica (Al-MPS) and hydrogen peroxide as the oxidant. We employed the molecular designed dispersion method to prepare the VOx catalyst supported on Al-MPS. The deposit of monolayer VO(acac)2 complexes (acac: acetylacetonate) could be achieved by hydrogen bonding or a ligand-exchange mechanism and subsequently decomposition of organic acac ligands in an oxygen atmosphere at elevated temperature, yielding the six coordination of VV center. The octahedral vanadium oxide samples possess much mobile ligands (i.e., H2O or solvent) around the catalytic center that could be easily substituted by hydrogen peroxide and yielded the active peroxo intermediate for the hydroxylation of benzene. From the studies of powder X-ray diffraction, N2 adsorption and desorption isotherms, diffuse reflectance UV−vis, and electron pa...

Influence of the MCM-41 morphology on the vanadia deposition by a molecular designed dispersion method

Abstract Spherical, irregular and gyroidal MCM-41 were used for the deposition of VOx by the molecular designed dispersion method (MDD), in liquid phase, using vanadyl acetylacetonate. The molecular designed dispersion method using metal complexes on a highly porous support is a suitable method to prepare high quality catalysts. Deposition of VO(acac)2 was carried out at room temperature and with a decreasing temperature gradient. The porous characteristics and coordination environment of the supported vanadium oxide was studied by a combination of different techniques such as N2 adsorption/desorption, FT-Raman and FTIR-PAS. The results show that vanadia is present at the surface of the pore walls. Different morphologies of the support materials has no influence on the reaction mechanism for the VO(acac)2 deposition. However, the vanadium loading is significantly influenced by the morphologies of the support. The spherical MCM-41 gives the highest loading of vanadium, although it has a smaller surface area, pore volume and pore diameter than irregular and gyroidal MCM-41. Dye adsorption experiments prove that MCM-41 with various morphologies have different accessibilities, which is also related to differences in the vanadium loadings.

Preparation and Characterization of Vanadium Oxide Deposited on Thermally Stable Mesoporous Titania

Vanadium oxide was deposited on mesoporous titania by the molecular designed dispersion method to investigate the potential properties of these catalysts. Mesoporous titania was synthesized following the evaporation-induced self-assembly (EISA) method with a subsequent treatment with ammonia to increase the thermal stability. As a result, the mesoporous titania obtained shows a high surface area (approximately 350 m2/g) and high stability. Vanadium oxide was deposited by the MDD method using a vanadyl acetylacetonate complex that was transformed into VOx after a controlled calcination in air flow at 300 degrees C. The mesostructure and porosity characteristics of titania remain even until the maximum V-loading was reached (0.4 mmol/g), as it was shown by N2 sorption measurements at -196 degrees C. The catalysts were characterized by chemical analysis, Fourier transform infrared-photoacoustic spectroscopy (FTIR-PAS), UV-vis diffuse reflectance (DR), and Fourier transform Raman spectroscopy. Raman spectra showed isolated V species for the different V-containing catalysts. Furthermore, UV-vis-DR revealed a higher contribution of polymeric species as the V loading increases. The VOx/mesoporous titania catalysts were highly active in the selective catalytic reduction of NOx. A high activity in the NO conversion was observed, which increases with increasing metal loading.

Catalytic performance of various mesoporous silicas modified with copper or iron oxides introduced by different ways in the selective reduction of NO by ammonia

Mesoporous silicas (MCM-48, SBA-15, MCF), reflecting various porous structures, were modified with copper and iron oxides by two different methods. For a first series of the samples the molecular designed dispersion (MDD) method using acetylacetonate complexes of copper and iron was applied for the deposition of transition metal oxides on the silica supports. A second series of the catalysts was obtained by the incipient impregnation technique using aqueous solutions of the suitable metal nitrates. The modified materials were characterized with respect to the texture (BET), composition (electron microprobe analysis), coordination of the transition metals (UV–vis–DRS) and surface acidity (NH3-TPD, FTIR). The mesoporous silica supports modified with transition metal oxides were tested as catalysts of the selective reduction of NO with ammonia. The catalytic performance of the studied samples depended on the method used for the deposition of transition metal oxide as well as the kind of mesoporous silica used as a catalytic support. In general, the Cu-containing mesoporous samples effectively operated at lower temperatures than silicas modified with iron. The samples obtained by the MDD method have been found to be more active and selective compared to the analogous samples prepared by the impregnation technique. An introduction of water vapor into the reaction mixture only slightly decreased the NO conversion and selectivity towards N2 over the MCF mesoporous silica modified with copper or iron oxide.

Characterization of Vanadium and Titanium Oxide Supported SBA-15

Supported vanadium and titanium oxide catalysts were prepared by adsorption and subsequent calcination of the vanadyl and titanyl acetylacetonate complexes, respectively, on mesoporous SBA-15 by the molecular designed dispersion (MDD) method. Liquid and gas phase depositions at different temperatures were carried out with vanadyl acetylacetonate, and the different results together with those of titanyl acetylacetonate in the liquid phase deposition were discussed. The bonding mechanism, the influence of the metal interaction with the support material, and differences due to the way of deposition and the temperature were investigated by TGA, chemical analysis, FTIR, and Raman spectroscopy. Elevated dissolving temperatures in the liquid phase led to higher final loadings on the SBA-15 without the formation of clusters, even at high loadings. The decomposition of the anchored vanadium and titanium complexes, their thermal stability, and the conversion to the covalently bound VO(x) and TiO(x) species on SBA-15 were studied and investigated by in situ transmission IR spectroscopy. In general, the titanium complex is more reactive than the vanadium complex toward the surface of SBA-15 and has a higher thermal stability. The MDD method of the VO(acac)2 and TiO(acac)2 enables to create a dispersed surface of supported VO(x) and TiO(x), respectively. The structure configurations of VO(x) and TiO(x) oxide catalysts obtained at different metal loadings were studied by Raman spectroscopy. Pore size distributions, XRD, and N2 sorption confirmed the structural stability of these materials after grafting. VO(x)/SBA-15 and TiO(x)/SBA-15 samples, with different metal loadings, were also catalytically tested for the selective catalytic reduction (SCR) of NO with ammonia.

Modification of MCM-48-, SBA-15-, MCF-, and MSU-type Mesoporous Silicas with Transition Metal Oxides Using the Molecular Designed Dispersion Method

Four various mesoporous silicas (MCM-48, SBA-15, MCF, and MSU) were modified by the molecular designed dispersion method using Fe(acac)3, Cr(acac)3, and Cu(acac)2 complexes. The deposition was performed at the same concentration of the metal acetylacetonate (acac) complex in a toluene solution. All as-synthesized samples were investigated by diffuse reflectance infrared Fourier transform spectroscopy, Fourier transform infrared photoacoustic spectroscopy, and thermogravimetric analysis. The calcined materials were studied with respect to their textural properties (Brunauer-Emmett-Teller adsorption isotherm) and chemical composition (electron microprobe analysis). It allowed elucidation of the mechanism of interaction between the acac complex and the silanol groups. For the MCM-48, SBA-15, and MCF materials, the formation of hydrogen bonding was found for the chromium- and copper-modified samples, whereas the Fe-containing materials showed the ligand exchange mechanism. The strong interaction of the MSU support and the different acetylacetonate complexes, resulting in a loss of at least one acac ligand, was observed. The mesoporous silicas modified with transition metal oxides were studied by UV-vis-DR spectroscopy. The different metal dispersions were found for the samples containing various transition metal oxides.

Structural Investigation of Vanadyl-Acetylacetonate-Containing Precursors of TiOx−VOxMixed Oxides on SBA-15

TiOx−VOx mixed-oxide supported SBA-15 catalysts were prepared by the molecular designed dispersion method (MDD) using acetylacetonate complexes. Two different synthesis route depositions are presented to prepare these mixed-oxide catalysts in a very controlled way. Continuous wave and pulse EPR spectroscopy is employed to determine the location of the VO(acac)2 complexes in the mixed-oxide precursors (before calcination) in comparison to other supports. It is shown that the different synthesis pathways lead to differences in the local structure of the VO2+ complexes and to a distinctive degree of spreading of the metals on the support surface. This is confirmed by additional characterizations of the final catalyst in terms of chemical composition and porosity. Depending on the way of preparation of the mixed oxides, the final catalysts present significant differences in pore structure, showing partially blocked or open mesopores. In the case of the mixed-oxide samples a higher dispersion of the metals is observed compared to the disposition of VO(acac)2 on the pure SBA-15 support. Both hyperfine sublevel correlation (HYSCORE) and IR spectroscopy show that the VO(acac)2 complex, in contrast to the TiO(acac)2 complex, has a great tendency to increase its coordination sphere by coordinating water molecules, but that this tendency is highly reduced when VO(acac)2 is present in a Ti environment.

Synthesis, characterization and hydrodesulphurisation activity of CoMo/γ-Al 2O 3 catalyst prepared through molecular designed dispersion method

CoMo/γ-Al2O3 catalysts for hydrodesulphurisation activity were prepared by making use of the molecular designed dispersion (MDD) method. Molybdenum and cobalt pyrrolidine-N-carbodithioate (Pydtc) complexes were used for the incorporation of metals on the support. The catalysts were characterized by elemental analysis, low temperature oxygen chemisorption, temperature programmed reduction (TPR) and laser Raman spectroscopy. The hydrodesulphurisation activity of all the catalysts were carried out and results were compared with those of the catalysts prepared through the conventional method. Higher molybdenum dispersion, smaller molybdenum clusters, lower reduction temperature of catalyst and better hydrodesulphurisation activity were observed for the catalysts prepared through the MDD method.

Synthesis and characterization of alumina-supported vanadium oxide catalysts prepared by the molecular designed dispersion of VO(acac)2 complexes

Alumina-supported vanadium oxide catalysts have been prepared by the molecular designed dispersion method, using the vanadyl acetylacetonate complex (VO(acac)2). The complex has been adsorbed on the support from solution, followed by thermal conversion into the corresponding supported vanadium oxide catalyst. The formation of the VO(acac)2–support material has been studied by both spectroscopic (FTIR, FT-Raman, ESR, UV–VIS–DRS; DRS: diffuse reflectance spectroscopy) and chemical methods. The present study reveals that VO(acac)2 complexes react preferentially with the hydroxy groups of the alumina, and only to a limited extent with the coordinatively unsaturated Al3+ sites. In addition, the deposition of the VO(acac)2 complex on the alumina surface yields different supported vanadium configurations as a function of the surface loading, due to both ligand exchange and hydrogen bond interactions. The thermal conversion of the adsorbed VO(acac)2 complexes into the supported vanadium oxide catalyst has been studied by TGA, GC-FTIR and in situ IR, which provides additional information on the VO(acac)2–support interaction. Characterization of these supported vanadium oxide catalysts by FTIR, FT-Raman and UV–VIS–DRS reveals the formation of highly dispersed VOx functionalities.

Synthesis of Supported Transition Metal Oxide Catalysts by the Designed Deposition of Acetylacetonate Complexes

The molecular designed dispersion method is used to prepare supported transition metal oxide catalysts (Mo, Cu, V). The corresponding transition metal acetylacetonate complexes were reacted with silica or alumina, and subsequently thermally converted into the supported metal oxide. The deposition of these acetylacetonate complexes depends on the geometry and stability of the complex, the support properties and the synthesis procedure. By thorough control of the reaction parameters, this novel synthesis method enables the creation of uniform and highly dispersed supported transition metal oxides. Therefore, the molecular designed dispersion method plays an important role in the development of a new generation of catalysts. This paper is forwarded to the special issue of the Langmuir journal, devoted to the ISSHAC-3 symposium.

Spectroscopic characterization of an MoOx layer on the surface of silica. An evaluation of the molecular designed dispersion method

Silica-supported molybdenum oxide catalysts have been prepared by liquid and gas phase deposition, followed by calcination of the deposited molybdenyl acetylacetonato complex. Fourier-transform infrared spectroscopy indicated a hydrogen bonding anchorage mechanism for the liquid phase deposition and a two step reaction mechanism for the gas phase deposition. After calcination of the absorbed molybdenum complexes, the supported molybdenum oxides were characterized by combining Fourier-transform infrared and Raman spectroscopy. X-ray diffraction was used to probe the possible clustering towards crystallites. An evaluation of the molecular designed dispersion method has been made by comparing the deposited molybdena structures obtained by the designed dispersion of MoO2(acac)2 with catalysts prepared by the conventional impregnation method using ammonium heptamolybdate. It is concluded that the molecular designed dispersion method results in a better grafting (more Si–O–Mo bonds) and thus a stronger metal oxide–support interaction than the conventional impregnation methods.