Alumina-supported Molybdenum Oxide Research Articles

Catalytic dehydrogenation of isobutane over supported MoOx/K-Al2O3

Potassium-promoted, alumina-supported molybdenum oxide is investigated for non-oxidative isobutane dehydrogenation, with an emphasis on sub-monolayer coverages. After pre-reduction in H2, initial isobutene turnover frequencies are largely independent of Mo loading, but deactivation rate constants increase by >100-fold as loadings increase to monolayer, leading to a >13-fold difference in reaction rates at extended time on stream. Mo oxidation state by in situ X-ray absorption spectroscopy is stable with time on stream, arguing against continued catalyst restructuring as the origin of deactivation. Across the set of loadings, isobutane dehydrogenation and coke formation are correlated with partially reduced Mo4+ and deeply reduced Moδ+ sites, respectively. Given the stable dehydrogenation activity of low-loaded Mo, in contrast to the rapid deactivation of high-loaded Mo, sub-monolayer metal oxides may warrant further investigation as light alkane dehydrogenation even when their bulk counterparts may not.

Multiwavelength Raman Spectroscopic Characterization of Alumina-Supported Molybdenum Oxide Prepared by Vapor Deposition

Molybdenum oxide species on alumina have been prepared via gas phase deposition of bis(ethylbenzene) molybdenum with loadings in the range of 0.05–3.1 Mo atoms/nm2. These materials have been characterized relative to analogous impregnated samples using UV–Visible diffuse reflectance spectroscopy, UV Raman spectroscopy, and visible Raman spectroscopy. The presence and distribution of monomolybdates, two- and three-dimensional polymolybdates, and Al2(MoO4)3 depends on the Mo loading and synthesis technique employed, especially at low loadings. Alternately dosing bis(ethylbenzene) molybdenum and water generates uniform polymolybdates at all loadings. In contrast, pulsing the precursor into the reactor without intervening water leads to monomolybdates at low loadings and polymolybdates at high loadings. The ability to influence the distribution of surface molybdenum oxide species can facilitate the rational design of molybdenum based catalysts.

Synthesis of formaldehyde from dimethyl ether on alumina-supported molybdenum oxide catalyst

The selective oxidation of DME to formaldehyde over alumina-supported MoOx catalyst (prepared by dry impregnation) is studied in this work. The activity and stability of the catalyst were evaluated in a fixed-bed continuous reactor at different temperatures and reactant concentrations.The influence of the main operating conditions, (DME, O2, CO2 and CO feed concentrations; reaction temperature) on reaction rate and product selectivity was experimentally determined. Thus, DME conversion decreases on increasing DME feed concentration and increases on increasing O2 feed concentration. Formaldehyde selectivity remained almost unaffected. A reaction mechanism, based on a Mars-van-Krevelen redox cycle representing DME oxidation to formaldehyde was used as a basis to develop a kinetic model for the reaction. The resulting simplified model suggests power law dependences for the reaction rate of 0.2 for the O2 and 0.5 for the DME.

Aromatic hydrocarbon production by the online catalytic cracking of lignin fast pyrolysis vapors using Mo2N/γ-Al2O3

This study investigates the online catalytic cracking of lignin fast pyrolysis vapors using Mo2N/γ-Al2O3 prepared by nitriding an alumina-supported molybdenum oxide precursor with nitrogen hydrogen mixtures though temperature programming. The activity and selectivity of the catalyst toward aromatic hydrocarbons were determined in the pyrolysis-gas chromatography/mass spectrometry system. Results show that the catalyst has a significant function in the pyrolysis process. In the presence of the catalyst, the primary pyrolysis products from lignin are catalytically converted into aromatic products, benzene and toluene, as well as to an insignificant quantity of dimethylbenzene, ethylbenzene, trimethylbenzene, and naphthalene. The highest aromatic hydrocarbon yield of 17.5% is obtained using Mo2N/γ-Al2O3 (the catalyst-to-lignin weight ratio=4) at 700°C; by contrast, this yield is only 1.4% when no catalyst is used. Furthermore, the highest benzene yield of 70.1% is obtained using Mo2N/γ-Al2O3 (catalyst-to-lignin weight ratio=4) at 850°C. Under this condition, the monocyclic aromatic hydrocarbons together contribute >95% of the total aromatic hydrocarbon yield, whereas the selectivity toward naphthalene is only 2.2%.

Brønsted acid generation of alumina-supported molybdenum oxide calcined at high temperatures: Characterization by acid-catalyzed reactions and spectroscopic methods

The acidic properties of alumina-supported molybdenum oxide (MoO3/Al2O3) calcined at high temperatures, with MoO3 loadings of 5–30wt%, were investigated using acid-catalyzed reactions (benzylation of anisole and isomerization of α-pinene), and Fourier-transformed infrared spectroscopy (FT-IR). The structure of MoO3 on Al2O3 was characterized by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectra (XPS), and X-ray absorption fine structure (XAFS) analysis. The correlation between acidic property and structure is discussed. Brønsted acid sites, where acid-catalyzed reactions take place, are generated on MoO3/Al2O3 by calcination at high temperatures. 11wt% MoO3/Al2O3 calcined at 1073K exhibited the highest activity, and the largest numbers of Brønsted acid sites were generated. XPS and Mo K-edge XAFS revealed that molybdenum oxide monolayer domains with distorted MoO6 units and small MoO3 clusters were formed and molybdenum oxide monolayer domains were stabilized on alumina below 11wt% of MoO3 loading. Brønsted acid sites are probably generated at boundaries between molybdenum oxide monolayer domains and/or small MoO3 clusters. When the MoO3 loading was sufficient to form two-dimensional molybdenum oxide overlayers (>11wt%), some of the Brønsted acid sites on MoO3/Al2O3 was covered with Al2Mo3O12 having no Brønsted acid site, resulting in a lowering of the catalytic activity.

An operando Raman study of structure and reactivity of alumina-supported molybdenum oxide catalysts for the oxidative dehydrogenation of ethane

The structural and catalytic properties of supported MoO 3/Al 2O 3 catalysts with Mo surface densities, n s , in the range 1.1–12.5 Mo/nm 2 were studied for the oxidative dehydrogenation of ethane by in situ Raman spectroscopy with simultaneous catalytic measurements at temperatures of 400–550 °C. Isolated mono-molybdates (MoO 4) and polymolybdates are formed at various proportions (depending on the loading) on the catalyst surface under dehydrated conditions; bulk Al 2(MoO 4) 3 crystals are formed at n s exceeding the monolayer. Under reactive environment (C 2H 6/O 2), the Raman features attributed to Mo O and Mo O Mo modes appear to be perturbed, indicating that surface MoO x species are reduced to lower valence states. The combined information on catalyst structure under reaction conditions and catalytic data shows a concurrence between the trends for activity per Mo (turnover frequency [TOF]) and for the number of Mo O Al bonds per Mo as a function of n s . The catalytic activity is related to the anchoring Mo O support bonds. Structure–activity/selectivity relations are derived and discussed.

Mechanistic Study of Selective Oxidation of Dimethyl Ether to Formaldehyde over Alumina-supported Molybdenum Oxide Catalyst

XPS and IR spectroscopies were used to investigate the surface intermediates of dimethyl ether (DME) oxidation to formaldehyde over MoOx/Al2O3 catalyst. The reaction performances were tested by employing three typical reaction conditions, depending on the O2/DME ratio and the reaction temperature. When there was sufficient oxygen present in the reaction media, a terminal or bridged CH3O* species formed by DME dissociation was highly active and rapidly reacted with lattice oxygen to produce formaldehyde, leading to higher selectivity of HCHO. When oxygen was consumed completely or only DME was present in the reaction media, CH3O species bonded to more than two Mo atoms (μ-OCH3) and CH x (x=1–3) species attached to the Mo atoms were observed and the relative ratio of (μ-OCH3) /Mo–CH x was significantly dependent on the reduction degree of MoO x domains. The (μCH3O) species was related to the formation of CH3OH or CO x , and the Mo–CH x species led to the formation of CH4.

Adding a third dimension to operando spectroscopy: a combined UV-Vis, Raman and XAFS setup to study heterogeneous catalysts under working conditions

The potential of combined operando UV-Vis/Raman/XAFS has been explored by studying the active site and deactivation mechanism of silica- and alumina-supported molybdenum oxide catalysts under propane dehydrogenation conditions.

Oxidative dehydrogenation of propane over Mo/g-Al2O3. effect of precursor

Alumina supported molybdenum oxide was prepared using ammonium heptamolybdate and molybdenum acetate as different precursors. The catalysts were characterized by BET, AAS, SEM-EDAX, XRD, TPR and surface acidity measurements. The characterization results and the catalytic behavior in oxydehydrogenation of propane were similar irrespective of precursor.

Inelastic neutron scattering studies of catalysts

Inelastic neutron scattering is a spectroscopic technique which enables one to observe vibrational and rotational spectra of catalysts and sorbed species. The spectrum is due to energy transfer from the incident neutrons to the scatterer. Since hydrogen is the strongest scatterer the INS technique is especially useful for investigating motions involving hydrogen atoms. I shall review applications of neutron scattering in catalytic studies and describe our INS studies of molybdenum-containing catalysts using the time focused crystal analyzer spectrometer (TFXA) at the ISIS pulsed neutron source at the Rutherford Appleton Laboratory. Particular topics will include chemisorbed hydrogen, the nature and binding of water in alumina-supported molybdenum oxide catalysts, the search for metal-hydrogen and sulfur-hydrogen species in molybdenum disulfide catalysts, and the interaction of benzene and thiophene with the catalyst, species in zeolites, and surface reactions of hydrogen.

Epoxidation of Fatty Acid Methyl Esters with Organic Hydroperoxides and Molybdenum Oxide

AbstractEpoxidation of monounsaturated fatty acid methyl esters with an hydroperoxide and a catalyst was undertaken. Methyl oleate, elaidate, petroselinate, erucate, ricinoleate and 10‐undecenoate were tested with cumyl‐ and tert‐butylhydroperoxide. The heterogeneous alumina supported molybdenum oxide was used as a catalyst. In most cases, the reaction proceeded smoothly, giving only the expected epoxide in high yield. 10‐Undecenoate, in which the double bond is less nucleophilic, exhibited lower selectivity and conversion rate. Methyl esters with a disubstituted double bond gave rise to pure stereoisomeric epoxide, corresponding to a syn‐addition mechanism, as shown by the 1H‐NMR spectra. Thus, the peracid as well as the catalytic hydroperoxide epoxidation work in the same way.

A Raman and ultraviolet diffuse reflectance spectroscopic investigation of alumina-supported molybdenum oxide

Summary A thorough calorimetric investigation of graphite-supported iridium particle chemistry was conducted. A careful determination of the particle size distribution assisted in the interpretation. The stoichiometry of oxygen, carbon monoxide, and hydrogen was determined: CO/Ir, = 0.50, O/h, = 1.0, H/h, = 1.0. The differential heats of adsorption of these molecules on the iridium surface was found to be very similar to those reported for alumina-supported iridium particles, suggesting support effects are minimal. Differential rate data were made available for the first time. It is clear that this basic information regarding adsorption stoichiometries, rates, and heats will be useful in future studies of the surface composition of multimetallic supported catalysts that contain iridium. The calorimeter allowed a study of the interaction between hydrogen and oxygen on an iridium surface. The rate, heat, stoichiometry, and sequence of steps that occur when differential hydrogen reduction of a preoxidized surface is carried out were all studied. It was found that the heat of reaction and the rate of reaction between dosed hydrogen and adsorbed oxygen progressively decreased. The measured integral heat, however, agreed with a simple thermodynamic model. The results of this study suggests that differential calorimeters can be valuable tools for providing information of the type required for detailed kinetic modeling of reactions taking place on catalyst surfaces. They also suggest that this type of calorimetry can provide detailed insight into the mechanism of surface reaction, an area of intense research at the present time.

Effect of pretreatment conditions on hydrocarbon reactions on alumina-supported molybdenum catalysts

Isomerization and cracking of 2-methylpentane were studied on alumina supported molybdenum oxide catalysts. Before all catalytic experiments, the catalyst was reduced in situ under hydrogen flow. The pretreatment conditions appeared to affect strongly the properties of the catalyst. By lowering the heating rate used to reach the reduction temperature (from 150°C/15 min to 50°C/15 min) or by increasing the reduction duration (from 2 h to 6 h), the catalytic properties could be altered from a high level of isomerization activity to a high level of cracking activity. The nature of the two kinds of catalytically active sites is discussed.

Surface analysis of alumina-supported molybdenum oxide (MoO3), nickel oxide (NiO), and NiO-MoO3 by low energy ion scattering spectroscopy

A series of alumina-supported MoO/sub 3/, NiO, and NiO-MoO/sub 3/ catalysts with varying amount of MoO/sub 3/ and/or NiO has been examined by low energy ion scattering spectroscopy. The evolution of the surface structure of a MoO/sub 3//Al/sub 2/O/sub 3/ sample during the preparation and after the addition of the promoter (cobalt or nickel), as well as the effect of the calcination temperature, have been studied. The results, when compared with previous studies by X-ray photoelectron and laser Raman spectroscopy, confirm the description of the supported species as small oxomolybdenum entities, well dispersed and occupying only a small fraction of the support surface. A strong shielding effect of the Mo element by the promoter Co or Ni has been detected; this gives direct evidence of a chemical interaction between the promoter and the oxomolybdenum species. A description of this association as an isopolymolybdate salt of Co or Ni is proposed.