Silica-supported Molybdenum Catalysts Research Articles

Silica and boron nitride supported molybdenum and vanadium oxide catalysts for propane oxidation

A range of silica supported molybdenum catalysts and ranges of vanadium catalysts supported on silica and boron nitride have been prepared, characterised and tested for propane oxidation. The silica supported molybdenum catalysts demonstrated appreciable activity for propane total oxidation to carbon oxides, with trace acrolein in some cases. The silica supported vanadium catalysts were also active for propane oxidation, and although the major oxidation product was CO 2, higher selectivity to acrolein was observed when compared to the silica supported molybdenum catalysts. The use of boron nitride as support significantly increased the selectivity towards acrolein. The boron nitride support has a low surface area (SA) (7 m 2 g −1) and it is also hydrophobic. Temperature programmed reduction showed that the vanadium species supported on boron nitride were chemically different from those supported on silica and it is envisaged that this in combination with the characteristics of the boron nitride support are important for the improved acrolein selectivity.

Structures of molybdenum silica catalysts reduced by ethanol, and their relations to catalytic oxidation reactions: I. Structure changes of Mo/SiO 2 with gradual reduction by ethanol

Structural changes of silica supported molybdenum catalysts accompanied with reduction by ethanol were investigated with EXAFS and UV–VIS spectroscopies. Catalysts made from four different precursors were investigated, and it was demonstrated that the dependence on preparation method was small. Catalysts prepared by the reaction between surface hydroxide groups and organometallic complexes are called `fixed catalysts' and some researchers assert that fixed Mo/SiO 2 have uniform and specific molecular-like active sites, which remains controversial. It was found that what had been proposed as characteristics of the molecular-like active sites of the fixed catalysts were merely prejudiced interpretation of a few aspects in the structural changes common to Mo/SiO 2 in general. At least two distinctive states existed in the course of reduction. One has IVCT (intervalence charge transfer) absorption band and corresponds to shallow reductions. The other has 2.6 Å Mo–Mo bond and average oxidation state of it was estimated to be +5. Structure similar to the latter was found also on alumina supported Mo catalysts and can be considered thermodynamically stable.

Mechanisms of Thiophene Hydrodesulfurization on Model Molybdenum Catalysts

Hydrodesulfurization (HDS) activities and selectivities were measured for thiophene, tetrahydrothiophene (THT), and 1-buta-nethiol on silica-supported molybdenum catalysts at a pressure of 1 atm and temperatures ranging from 530 to 795 K. The model catalysts, which were previously characterized, feature isolated molybdenum atoms in the +2, +4, and +6 oxidation states and molybdenum dimers with each molybdenum atom in the +4 oxidation state. Silica-supported MoS2was used for reference. Activities for thiophene and THT HDS correlate with oxidation state. Mo(II) is most active among dispersed catalysts. 1-Butanethiol activities were much larger than thiophene or THT activities and were roughly equal on all dispersed catalysts. Apparent activation energies of 43.4 and 48.5 kJ/mol were determined for thiophene HDS on Mo(II) and MoS2/SiO2, respectively. Apparent activation energies of 132 and 174 kJ/mol were determined for THT HDS on Mo(II) and MoS2/SiO2, respectively. Dihydrothiophene, THT and 1-butanethiol were formed in thiophene HDS over Mo(II) and MoS2/SiO2. The major products of thiophene and THT HDS were 1-butene, 2-butene, andn-butene. Butadiene,i-butane,i-butene, methane, ethane, ethene, propane, and propene were formed in small amounts. Butadiene is thought to be the initial product of thiophene and THT desulfurization and undergoes subsequent hydrogenation and isomerization to yield the observed products. A common mechanism for HDS of thiophene and THT with 2,5-DHT as an intermediate is discussed.

Effect of Catalyst Oxidation State and Structure on Thiophene Hydrodesulfurization over Model Molybdenum Catalysts

Activity and product selectivity have been measured for thiophene hydrodesulfurization (HDS) over model silica-supported molybdenum catalysts at a pressure of 1 atm and at temperatures ranging from 398 to 673 K. The model catalysts feature isolated molybdenum atoms in the +2, +4, and +6 oxidation states and molybdenum dimers with each molybdenum atom in the +4 oxidation state. Silica-supported MoS2, prepared by deposition and decomposition of (NH4)2MoS4, was used for reference. There is a correlation between thiophene HDS activity and molybdenum oxidation state, with Mo(II) most active. Thiophene HDS activity does not show a significant structure dependence for isolated Mo(IV) versus dimeric Mo(IV) catalysts. Activation energies of 51.5 and 49.9 kJ/mol were determined for thiophene HDS over Mo(II) and MoS2/SiO2catalysts, respectively. Butane and butenes are the major products of thiophene HDS with little butadiene detected. Activity and selectivity trends suggest the HDS reaction is initiated by ηlbinding of thiophene on the supported metal catalysts.

Characterization of silica-supported molybdenum catalysts by ESR, XPS and thiophene hydrodesulfurization

Two Mo/SiO2 samples have been prepared by impregnation of silica gel with aqueous solution of molybdophosphoric heteropoly acid or ammonium heptamolybdate. The catalysts were studied by ESR and IR spectroscopy, X-ray diffraction, and XPS. Their activity was measured in the reaction of thiophene hydrodesulfurization. The silica-supported P−Mo heteropoly acid, with a preserved Keggin structure of the heteropoly anion and sulfided by H2S, exhibited two times higher steady conversion in comparison with ammonium heptamolybdate supported on silica at pH=11 and calcined and sulfided at 500°C.

Silica-Supported MoO3 Catalyst Prepared by Alkoxide Method

Abstract Highly dispersed silica-supported molybdenum catalysts were prepared by an alkoxide method, using ethylsilicate and ammonium paramolybdate dissolved in ethylene glycol. By comparing temperature-programmed sulfiding(TPS) spectra of the catalysts with those of alumina or silica-supported catalysts formed by an impregnation method, the structures and the dispersion states of molybdenum oxide in the catalysts prepared by the alkoxide method were discussed.

Infrared investigation of CO adsorption on thermally reduced silica-supported molybdenum catalysts

Molybdenum – silica catalysts thermally reduced in hydrogen have been studied by i.r. spectroscopy using CO as the probe molecule. 12CO and 13CO adsorption at different temperatures (beam temperature and 77 K) were studied as functions of (a) preparation (grafting and impregnation), (b) molybdenum loading, (c) time of reduction treatment, (d) CO pressure and (e) time of CO contact. Infrared spectra show that similar surface species, in differing proportions, are present in all samples. Monocarbonyl species are formed on Mon+ cations probably with 3 ⩽n⩽ 5. Two types of metal carbonyl species are also formed: a —OC—Mo(CO)5 species chemisorbed on Mon+ cations and a ‘physisorbed’ Mo(CO)6 species weakly held on the silica support. Reduction to metal is favoured when the Mo dispersion decreases and when the time of reduction increases.

Catalytic properties of silica-supported molybdenum catalysts in methanol oxidation: The influence of molybdenum dispersion

Two types of Mo SiO 2 catalysts prepared by classical impregnation or by the grafting method have been tested in the methanol oxidation reaction, which is known to be structure sensitive. It is shown that there is a dependence of the formation of formaldehyde and methyl formate, the principal products, on the molybdenum content for grafted catalysts with a higher selectivity for methyl formate. In contrast, the dependence is not clear for the impregnated catalysts, and this appears to be due to the lack of reproducibility in their preparation, and to their Mo dispersion being lower than that of the grafted Mo SiO 2 catalysts. The study of the formation of methyl formate using different reactive mixtures and kinetic calculations led us to propose a mechanism which involves the formation of formaldehyde from methanol on Mo sites, followed by its migration on silica, where it further reacts with methoxy groups to form methyl formate via a hemiacetal intermediate.

EPR investigation of the coordination sphere of molybdenum(5+) ions on thermally reduced silica-supported molybdenum catalysts prepared by the grafting method

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTEPR investigation of the coordination sphere of molybdenum(5+) ions on thermally reduced silica-supported molybdenum catalysts prepared by the grafting methodCatherine. Louis and Michel. CheCite this: J. Phys. Chem. 1987, 91, 11, 2875–2883Publication Date (Print):May 1, 1987Publication History Published online1 May 2002Published inissue 1 May 1987https://doi.org/10.1021/j100295a043RIGHTS & PERMISSIONSArticle Views479Altmetric-Citations85LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Get e-Alerts

Synthesis of acetonitrile from syngas and ammonia over silica-supported molybdenum catalysts modified by silver.

Direct synthesis of acetonitrile from syngas and ammonia over silica-supported molybdenum catalysts was studied. The catalysts were prepared by impregnation of silica followed by calcination and reduction. The reduction treatment in the preparation, however, had little influence on the activity of Mo/SiO2, because the reduction of Mo species on silica proceeds under the reaction atmosphere. Ag had an excellent effect on the catalyst to improve its activity and selectivity to acetonitrile. The optimum reaction temperature was between 450°C and 500°C. The weight change of Mo/SiO2 and Mo-Ag/SiO2 during reduction treatment suggested that MoO2 is the supported Mo species, regardless of whether Ag is added as a promoter or not. From the X-ray diffraction measurements, it was speculated that MoO2 is the active species for the formation of acetonitrile and that Ag has an effect of making MoO2 amorphous or more dispersed. The deactivation of the catalysts may be caused by both crystal growth of MoO2 and surface fouling.

EPR studies on the formation of atomic oxygen(1-) (O-) ions on reduced silica-supported molybdenum catalysts prepared by the grafting method

EPR studies on the formation of atomic oxygen(1-) (O-) ions on reduced silica-supported molybdenum catalysts prepared by the grafting method

Role of atomic oxygen(1-) ions in charge-transfer reactions at the surface of silica-supported molybdenum catalysts prepared by the grafting method

Role of atomic oxygen(1-) ions in charge-transfer reactions at the surface of silica-supported molybdenum catalysts prepared by the grafting method

ALCOHOL SYNTHESIS FROM CO2/H2 ON SILICA-SUPPORTED MOLYBDENUM CATALYSTS

Abstract In the hydrogenation of CO2 over alkali-promoted Mo/SiO2 catalysts, C1–C5 alcohols were selectively obtained at 16 kg/cm2 and 250 °C.

Studies on catalytically active surface compounds: IV. Preparation of [formula omitted] catalysts from MoCl 5 studied by ESR and UV-vis spectroscopy

The preparation steps of silica-supported molybdenum catalysts have been followed by ESR and uv-vis spectroscopy. It is shown that the reaction of MoCl 5 with silanol groups of the support leads to dimeric (the main part) and monomeric surface Mo species. On adsorbing H 2O a surface “solution layer” is formed allowing a fast tumbling of Mo 5+ ions. Simultaneously, the concentration of isolated Mo 5+ ions is increased; however, it does not exceed about 10% of the total Mo concentration. The final hydrolyzing process which includes drying at 378 K in air produces partly oxidized catalysts containing polymolybdate complexes. Although the interaction between the molybdenum species and the silica surface has been described as being relatively weak, no indication has been found that adsorption of water vapor causes any structural changes within the polymeric Mo complexes or its bonding to the surface. Adsorption of wet HCl causes ligand exchange which is accompanied by destruction of the polymeric molybdenum complexes.

Electron paramagnetic resonance study of the reactivity toward carbon monoxide and oxygen of O– ions adsorbed on silica-supported molybdenum catalysts

Using the e.p.r. technique, the reaction at 77 K of an O– radical, formed by N2O adsorption on reduced MoO3/SiO2 samples, with CO or 13CO was followed, and it was possible to detect a CO–2 radical (g1= 2.0029, g2= 2.0016, g3= 1.9974, A1= 199 G, A2= 240 G, A3= 207 G) which was stable at room temperature. This species reacted with O2 at 77 K to give rise to a radical of the type (O2COO)–(g1= 2.0486, g2= 2.0078, G3= 2.0026, A1C= OG, A2C= 4.8 G, A3C= 6 G, A3OI= 140 G, A3OII= 40 G). At 150 K, it disappeared irreversibly, generating an O–2 radical (g1= 2.0175, g2= 2.0097, g3= 2.0040, A3OI= 80 G, A3OII= 70 G) similar to that formed by direct adsorption of oxygen on reduced MoO3/SiO2.