Molybdenum Oxide Catalysts Research Articles

Molecular modelling study of solid acidity in molybdenum oxide supported on silica-alumina

Factors influencing the solid acidity in molybdenum oxides supported on silica-alumina were studied theoretically by means of self-consistent Hartree-Fock (HF), electron-correlated Møller-Plesset (MP2) and local density functional (LDF) quantum mechanical calculations of acid site models. This work was aimed at elucidating relationships between the chemical composition, surface geometry and the electronic properties of the solid acid catalysts. Local structures of unsupported and supported molybdenum oxide inferred from earlier experimental results were studied using cluster models. Relationships between the structural transformations in the molybdenum oxide tetrahedra and the changes in Brönsted-Lewis acidity were identified. The experimentally observed effect of the catalyst support composition on the acid strength of the supported molybdenum oxide catalyst was interpreted in terms of the calculated charge redistribution and molecular orbital energies. An acid site structure is proposed to explain the effect of Mo loading on the Brönsted acidity. The calculated changes in infrared vibration frequencies agree with the measured ones and support the mechanism proposed for the acidity changes on Mo loading.

Carboxylate-Alumoxanes: Precursors for Heterogeneous Catalysts

ABSTRACTCarboxylate-alumoxanes are organic substituted alumina nano-particles synthesized from boehmite in aqueous solution which are an inexpensive and environmentally benign precursor for the fabrication of nano-, meso-, and macro-scale aluminum based ceramics. The use of carboxylate-alumoxanes as a novel high surface area alumina support for heterogeneous catalysis will be discussed. The ability to perform further chemistry on the organic ligands of the carboxylate-alumoxanes allows for attachment of catalysts. During calcination, the organic ligands are burned out, leaving behind the catalyst in a well-dispersed manner. To demonstrate this concept, the metathesis of C16 olefins using a molybdenum oxide catalyst supported on alumina will be discussed using the carboxylate-alumoxane method.

A study of “superacidic” MoO3/ZrO2 catalysts for methane oxidation

A series of zirconia-supported molybdenum oxide catalysts with different molybdenum loadings prepared using conditions reported to generate "superacidity" have been evaluated for their performance as catalysts for methane oxidation. A marked dependence of Mo content on activity has been observed, with the most active material being that with intermediate molybdenum content. 5 wt MoO3/ ZrO2 compares favourably with Zrcursive Greek chiCe1-cursive Greek chiO2 for methane combustion. The presence of MoO3 is observed to stabilise the tetragonal polymorph of ZrO2 and, as Mo content is increased, dispersed MoO3 crystallites are formed as evidenced by Raman spectroscopy. Temperature-programmed reduction studies evidence differences in the reduction behaviour of the materials as a function of loading. The results indicate that molybdenum oxide supported on monoclinic zirconia gives rise to the most active catalyst. It is tentatively suggested that the formation of a MoO3 monolayer during reaction may be of importance.

Structure–Activity Relationships in the Ammoxidation of Ethylene in the Absence of Molecular Oxygen over γ-Al2O3-Supported Molybdenum Oxide Catalysts

This study focuses on the structure–activity relationships in the reaction of ethylene with ammonia to acetonitrile in the absence of gaseous oxygen over γ-Al2O3-supported molybdenum catalysts. Previous work has proved that this reaction is structure-sensitive and that two mechanisms for the formation of acetonitrile exist. The first mechanism is based on ammoxidation with consumption of lattice oxygen and operates on freshly calcined catalysts. The second mechanism operates without the consumption of lattice oxygen on catalysts submitted to long reaction times, independent of pretreatment, and thus, it is based on oxidative ammonolysis. By applying various physico-chemical techniques, such as XRD, HREM, XPS, and LEIS, different solid state properties of the catalyst were identified. Catalysts were analysed right after pretreatment and at different times on stream. From this a relationship was derived between the solid state property of the catalyst and its catalytic property. On freshly calcined catalysts molybdenum is present as Al2(MoO4)3which is highly dispersed on the γ-Al2O3surface. At a loading of 10 wt% Mo, where the maximum adsorption capacity of alumina is exceeded, only 77% of the γ-Al2O3is covered with Al2(MoO4)3, indicating that adsorption occurs at specific sites. The ammoxidation mechanism is suggested to be active on this Al2(MoO4)3structure. When the catalyst is pretreated with hydrogen, the molybdenum surface species are best described as MoO2-like. A decrease in the dispersion of approximately 50% was found upon hydrogen pretreatment. It was also shown that, after long reaction times (>24 h), a highly dispersed MoO2-like structure was formed, independent of the pretreatment, containing both Mo(IV) and Mo(VI) ions which had reached a structural and chemical equilibrium. It was concluded that the oxidative ammonolysis mechanism is operational on this structure.

The Sulfidation of γ-Alumina and Titania Supported (Cobalt)Molybdenum Oxide Catalysts Monitored by EXAFS

The sulfidation of γ-alumina- and titania-supported(cobalt)molybdenum oxide catalysts has been studied with X-rayabsorption spectroscopy and temperature programmed sulfidation (TPS).The catalysts were stepwise sulfided at temperatures between 298 and673 K and their structure was determined with EXAFS spectroscopy. Onalumina oxygen–sulfur exchange starts at a temperature just aboveroom temperature resulting in the formation of monomeric and dimericmolybdenum sulfide species containing disulfide ligands. Between 448and 523 K the disulfide ligands are reduced with hydrogen and onlydimeric molybdenum sulfide species with a Mo–Mo distance of 2.77 Åremain. Above 523 K these dimers aggregate to larger MoS2particles.In contrast, on titania molybdenum oxide is sulfided to yieldisolated molybdenum sulfide monomers at 523 K, which similarlyaggregate to MoS2particles between 523 and 623 K. Nomolybdenum sulfide dimers are observed as intermediates. Both onalumina and titania all intermediates are anchored to the support viaMo–O linkages with a bond distance of 2.0 Å. The addition of Co toMo/Al2O3accelerates the rate of sulfidation, demonstrated by the TPS patterns. However, on titania a lowering of the sulfidation rate is observed next to the presence of molybdenum sulfide dimers, that are not present in the unpromoted catalyst. These observations can be ascribed to the presence of CoMoO4in the oxidic catalyst.

Isomerization of heptane on molybdenum oxide catalysts with a small amount of nickel

T. Matsuda, H. Sakagami and N. Takahashi, J. Chem. Soc., Faraday Trans., 1997, 93, 2225 DOI: 10.1039/A701855J

Effects of Water Vapor upon Partial Oxidation of Mathane over Highly-dispersed MoO3/SiO2

Abstract Effects of water vapor upon the partial oxidation of methane on silica-supported molybdenum oxide catalyst strongly depend upon the dispersion states of Mo ions, and highly dispersed Mo ions depress the production of CO2 by an addition of water vapor into the feed gas.

An Atomic Force Microscopy Study of the Morphological Evolution of the MoO3(010) Surface during Reduction Reactions

Atomic force microscopy was used to characterize the nanometer-scale structural evolution of the MoO3(010) surface during reaction with hydrogen at 400°C. Two primary surface modifications were identified. First, water vapor, when present in the reactor either as an impurity or as an oxidation product, accelerates the volatilization of MoO3and leads to the formation of surface voids. The presence of these voids increases the density of (100) and (h0l)-type surface sites on the basal planes. Second, oxygen removal leads to the formation of crystallographic shear planes. The intersection of these defects with the (010) surface creates 2 Å high steps along 〈001〉 and their presence indicates that the surface vacancy concentration has reached an upper limit. The mechanisms by which these structural changes might influence the reactivity of molybdenum oxide catalysts are discussed.

Inhibition of Methanol Oxidation by Water Vapor—Effect on Measured Kinetics and Relevance to the Mechanism

The kinetics of methanol oxidation to formaldehyde was studied over an iron molybdenum oxide catalyst in a continuous flow reactor with external recycling at temperatures of 200–300°C. The kinetics of the reaction were well described by a power law rate expression of the formr=kPCH3OHxPO2yPH20z, wherex=0.94±0.06,y=0.10±0.05, andz=−0.45±0.07. The measured activation energy was 98±6 kJ/mol. When product inhibition by water vapor is not taken into account in such a power law kinetic rate expression, the apparent reaction orders in methanol and oxygen,x′andy′, and the activation energyE′are all lower than their true values:x′=(1−δ)x,y′=(1−δ)y, andE′=(1−δ)E, where δ=−z/(1−z). Methanol chemisorbs dissociatively to form methoxy and hydroxyl groups, and the rate-determining step is the decomposition of the methoxy intermediate. Product inhibition occurs through kinetic coupling, whereby water vapor chemisorbs dissociatively to form hydroxyl groups, which serve to reduce the steady state concentration of methoxy groups on the catalyst surface by reacting with them to reform methanol.

Comparison of Silica-Supported MoO3and V2O5Catalysts in the Selective Partial Oxidation of Methane

Silica-supported molybdenum oxide and vanadium oxide catalysts have been prepared with a wide range of surface metal loadings (0.3–3.5 metal atom/nm2). Isolated surface metal oxide species are dominant at surface metal loadings on silica below ca. 1 metal atom/nm2while at higher loadings, the metal oxide species tend to aggregate into crystalline MoO3and V2O5particles. Both catalyst series have been tested in the selective oxidation of methane with molecular oxygen at atmospheric pressure. The reactivity for methane conversion appears to be essentially related to dispersed isolated surface metal oxide species, with the surface vanadium oxide species being more reactive than molybdenum oxide species. The higher reactivity of the surface vanadium oxide sites may be related to the different nature of the interaction of the oxides with oxygen. Silica-supported vanadium oxide catalysts, despite their higher activity, result in lower yields to HCHO than silica-supported molybdenum oxide catalysts.

Surface structures of supported tungsten oxide catalysts under dehydrated conditions

The molecular structures of the WO 3/support (Al 2O 3, TiO 2, Nb 2O 5, ZrO 2, SiO 2, and MgO) catalysts under in situ dehydrated conditions have been investigated by Raman spectroscopy. The series of catalysts was synthesized by the aqueous incipient wetness method. The WO 3/support catalysts, with the exception of the WO 3 SiO 2 and WO 3 MgO catalysts, possess a highly distorted, octahedrally coordinated surface tungsten oxide species with one short WO bond (mono-oxo tungsten oxide species) at high surface coverages. The WO 3 SiO 2 catalysts exhibit strong Raman features of crystalline WO 3 particles due to the relative low density and reactivity of the surface hydroxyl groups. The WO 3 MgO catalysts possess non-stoichiometric compounds, Mg x (WO 4) y and Ca x (WO 4) y , at low tungsten oxide contents and crystalline MgWO 4 and CaWO 4 at high tungsten oxide contents. This result is attributed to the high aqueous solubility of MgO as well as the CaO impurity and the strong acid-base interaction between WO 2− 4 with Mg(OH) 2 and Ca(OH) 2. The current findings for supported tungsten oxide catalysts parallel the previous findings for supported molybdenum oxide catalysts and reflect the similar surface structural chemistry of these two oxides.

Non-uniform surface kinetics with two types of sites: the case of ethanol oxidation on molybdenum oxide

Temkin's theory of rates of catalytic reactions on non-uniform surfaces is extended to the MoO3-catalyzed oxidation of ethanol to acetaldehyde. Two types of sites are assumed to be present, an oxygen atom site that can be modeled with uniform properties and a metal atom site characterized by non-uniform properties both for ethanol chemisorption to an ethoxide intermediate and the conversion of this intermediate to acetaldehyde. The rate-limiting step is the cleavage of a C-H bond in the absorbed ethoxide intermediate. Non-uniform surface kinetics leads to a kinetic rate expression of the form $$v = kP_{C_2 H_5 OH}^{1 - m} P_{O_2 }^{(1 - m)/4} P_{H_2 O}^{ - (1 - m)/2} $$ . Such a rate expression, withm=0.14, is shown to provide a good fit to kinetic data for the selective oxidation of ethanol on a silica supported molybdenum oxide catalyst.

Selectivities in methanol oxidation over silica supported molybdena

Selectivities in methanol oxidation over silica supported molybdenum oxide catalysts were investigated in relation with the phase distribution. The supported catalysts were prepared by impregnation with ammonium heptamolybdate. In addition to crystalline MoO3, Mo containing cluster species of 1–2 nm size were observed by STEM even from a used catalyst with 13% catalyst loading. The percentage of Mo present as crystalline MoO3 increases with the catalyst loading. An ESCA study indicates that part of surface Mo in the supported catalysts is reduced to Mo5+. The dimethyl ether selectivity increases with the catalyst loading and its formation occurs over the crystalline MoO3 phase. The Selectivities to CO and methyl formate are greatly enhanced because of the presence of support, and are relatively independent of the catalyst loading and phase distribution. The dependence and independence of the Selectivities of different byproducts on the loading make the silica supported catalysts with high catalyst loadings less selective for the partial oxidation of methanol to formaldehyde.

Application of a molybdenum oxide catalyst supported on Al-pillared montmorillonite in the propene metathesis reaction

The catalytic behavior of a molybdenum oxide catalyst supported on Al-pillared montmorillonite in the propene metathesis reaction has been investigated. Comparison with two reference catalysts (Mo/SiO2 and Mo/Al2O3) was also studied. Pillared clay catalysts show an important activity loss and activity recovery by air regeneration was obtained.

Genesis and Stability of Silicomolybdic Acid on Silica-Supported Molybdenum Oxide Catalysts: In-Situ Structural-Selectivity Study on Selective Oxidation Reactions

The formation of silicomolybdic acid (SMA, H4SiMo12O40) on silica-supported molybdenum oxide catalysts has been studied by in situ Raman spectroscopy, by TGA measurements, and for the selective oxidation of methane to formaldehyde. The formation of silico-molybdic acid requires exposing the MoO3/SiO2 to air saturated with water for several hours at room temperature. The large amount of water deposited on the silica support allows solubilization of part of the silica support in the presence of solvated heptamolybdate species, which leads to the formation of silicomolybdic acid. Desorption of water via thermal treatments breaks the silicomolybdic acid into dehydrated or partial hydrated species which are stable up to ca. 573 K. Above 573 K, only an isolated and distorted mono-oxe surface molybdenum oxide species is observed by in situ Raman spectroscopy. Consequently, the silicomolybdic acid species on SiO2 should not result in catalytic behavior different from that of conventional MoO3/SiO2 catalysts for reactions taking place above 573 K. Unlike surface molybdenum oxide species, the surface SMA species on SiO2 are stable during methanol oxidation at 503 K and do not transform into crystalline β-MoO3 phase. The selective oxidation of methane to formaldehyde (843-883 K) shows no difference between conventional silica-supported molybdenum oxide and silica-supported silicomolybdic acid catalysts. In situ Raman spectroscopy studies during methanol oxidation at temperatures above 573 K reveal that the surface silicomolybdic acid species are not stable and transform into crystalline β-MoO3.

Catalytic Properties of Supported Molybdenum Oxide Catalysts: In Situ Raman and Methanol Oxidation Studies

The oxidation of methanol was studied over supported molybdenum oxide catalysts as a function of the specific oxide support (TiO2,ZrO2, Nb2O5, and A1203) and molybdenum oxide loading (surface coverage). The surface molybdenum oxide species were selective for the production of formaldehyde, and the oxide support sites yielded dimethyl ether (alumina and niobia) and methyl formate (zirconia) or were relatively inactive (titania). The turnover frequency (TOF) for the selective oxidation of methanol to formaldehyde varied by a factor of 2-4 with surface molybdenum oxide coverage and a factor of approximately 10 with the specific oxide support at monolayer coverage. The molecular structures of the surface molybdenum oxide species (isolated, tetrahedral at low coverages and polymerized, octahedralketrahedral at high coverages) did not affect the reaction selectivity but did appear to influence the slight increase in TOF with surface coverage. The order of magnitude variation in TOF with the specific oxide support correlated with the reducibility of the support and suggests that the Mo-0-support bond is critical in controlling the TOF. In situ Raman studies during methanol oxidation revealed that the supported molybdenum oxide species were 100% dispersed up to monolayer coverage. The percent reduction of the surface molybdenum oxide species, reflected by the decrease in the Raman intensity of the Mo-0 bond, during methanol oxidation was not a strong function of surface coverage and the specific oxide support. This suggests that the order of magnitude variation in the TOF with the specific oxide support is primarily related to the activity per site of the surface molybdenum oxide species rather than variation in the number of participating sites.

Surface Structures of Supported Molybdenum Oxide Catalysts: Characterization by Raman and Mo L3-Edge XANES

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTSurface Structures of Supported Molybdenum Oxide Catalysts: Characterization by Raman and Mo L3-Edge XANESHangchun Hu, Israel E. Wachs, and Simon R. BareCite this: J. Phys. Chem. 1995, 99, 27, 10897–10910Publication Date (Print):July 1, 1995Publication History Published online1 May 2002Published inissue 1 July 1995https://doi.org/10.1021/j100027a034RIGHTS & PERMISSIONSArticle Views2487Altmetric-Citations300LEARN 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 (2 MB) Get e-Alerts

ChemInform Abstract: Alumina Supported Molybdenum and Tungsten Oxide Catalysts. Surface Area and Pore Size Distribution from Nitrogen Adsorption and Mercury Penetration.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

ChemInform Abstract: Molecular Structures and Reactivity of Supported Molybdenum Oxide Catalysts.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Reactions of propene on supported molybdenum and tungsten oxides

Alumina-supported tungsten and molybdenum oxide catalysts with a wide range of MO 3 loadings were studied in the metathesis of propene. The catalysts were activated by several procedures in order to correlate the oxidation state of Mo or W, as characterized by XPS, and the acidic character, as determined by IR of adsorbed pyridine, with the main metathesis reaction and side olefin reactions. The catalytic activity of the samples under standard conditions was found to be invariant with the pretreatment (air or He). This leads to the conclusion that the activation of catalysts takes place under the propene atmosphere during the first minutes of reaction. XPS studies of fresh and spent catalysts showed that the active sites for metathesis seem to be W(VI) or Mo(VI) species. Both MoO 3 Al 2 O 3 and WO 3 Al 2 O 3 catalysts are active in parallel reactions such as polymerization, isomerization and cracking. The product distribution for the metathesis reaction differed greatly from that expected. Thus, for all catalysts the ethene/butene ratio is lower than 1 and isobutene was the main component of the C 4 fraction, its proportion increasing with the metal oxide content. Studies by IR of adsorbed pyridine showed the Brønsted acid sites generated on MoO 3 or WO 3 in samples with high metal oxide content to be responsible for the lateral acid-catalyzed reactions.