Supported Molybdenum Catalysts Research Articles

Characterization and evaluation of ZrO 2 supported hydrotreating catalysts

A series of zirconia supported molybdenum catalysts were prepared and characterized by BETSA, XRD, TPR, FTIR, XPS and oxygen chemisorption. Thiophene, cyclohexene and tetrahydrofuran were taken as model compounds for evaluating their hydrodesulfurization (HDS), hydrogenation (HYD) and hydrodeoxygenation (HDO) activities, respectively. The XRD results indicate that Mo is present as a monolayer up to 6 wt.% loading, whereas MoO 3 crystalline growth is observed beyond this loading. O 2-uptake and catalytic activities also increase up to 6 wt.% Mo-loading but above this loading both start decreasing. There are good correlations between O 2 uptake and all catalytic activities. TPR and FTIR results indicate that at lower loading, MoO 3 is present as tetrahedra form and at moderate loading, both tetrahedral and octahedral forms are found. XPS results reveal that the electron transfer is taking place from support to molybdenum.

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.

Preparation of highly dispersed molybdenum on alumina by thermal decomposition of Mo(CO) 6

Supported molybdenum catalysts are uniquely suitable for a number of reactions, notably the metathesis reaction between hydrocarbons with terminal double bonds. Heterogeneous catalysis offers advantages over homogenous routes with respect to catalyst recycling and separation. A way to produce extremely highly dispersed Mo-metal particles on completely dehydroxylated surfaces from Mo(CO) 6 is described. The metal dispersion is found to depend on: metal loading – higher loading leads to lower dispersion and larger particles; initial dispersion of the precursor – a dry process by subliming the Mo(CO) 6 onto the support was found to give a lower metal dispersion than a wet impregnation process, starting from a pentane solution of Mo(CO) 6. A method for the determination of Mo-dispersion on highly activated alumina surfaces by CO chemisorption has been established.

Hydrodesulfurization of dibenzothiophenes over molybdenum catalyst supported on TiO 2–Al 2O 3

Composite types of TiO 2–Al 2O 3 supports, which are γ-aluminas coated by titania, have been prepared by chemical vapor deposition (CVD), using TiCl 4 as a precursor. Then supported molybdenum catalysts have been prepared by an impregnation method. As supports, we employed γ-alumina, anatase types of titania, and composite types of TiO 2–Al 2O 3 with different loadings of TiO 2. We studied the conversion of Mo from oxidic to sulfidic state through sulfurization by X-ray photoelectron spectroscopy (XPS). The obtained spectra unambiguously revealed the higher reducibility from oxidic to sulfidic molybdenum species on the TiO 2 and TiO 2–Al 2O 3 supports compared to that on the Al 2O 3 support. Higher TiO 2 loadings of the TiO 2–Al 2O 3 composite support led to higher reducibility for molybdenum species. Furthermore, the catalytic behavior of supported molybdenum catalysts has been investigated for hydrodesulfurization (HDS) of dibenzothiophene (DBT) and methyl-substituted DBT derivatives. The conversion over the TiO 2–Al 2O 3 supported Mo catalysts, in particular for the 4,6-dimethyl-DBT, is much higher than that obtained over Al 2O 3 supported Mo catalyst. The ratio of the corresponding cyclohexylbenzene (CHB)/biphenyl (BP) derivatives is increased over the Mo/TiO 2–Al 2O 3. This indicates that the prehydrogenation of an aromatic ring plays an important role in the HDS of DBT derivatives over TiO 2–Al 2O 3 supported catalysts.

Formation of monolayer of molybdena over alumina by unconventional slurry impregnation or solvent assisted spreading method

Alumina supported molybdenum catalysts with variable loading and support particle size were prepared by the unconventional equilibrium adsorption in slurry MoO 3/water. The method can be named either slurry impregnation method, SIM (when compared with impregnation in ammonium molybdate solution) or solvent assisted spreading, SAS (when compared with thermal spreading of MoO 3). The maximum saturated adsorption loading achieved with alumina of surface area 172 m 2 g −1 and particle size 0.16–0.32 mm was 14.4 wt% MoO 3. No other crystalline phase beside γ-Al 2O 3 was found in XRD spectra of the catalysts. The XPS surface ratio Mo Al in oxide form and the thiophene hydrodesulphurization activity in sulphide form increased linearly with the overall composition up to saturated adsorption loading. The hydrodesulphurization activity of the SIM SAS catalysts of corresponding loading was comparable or better than the activity of conventional commercial MoO 3 Al 2O 3 catalyst. All this indicated that the monolayer type catalysts are obtained by SIM SAS method. The electron probe micro analysis of the SIM SAS catalyst prepared from extrudates (1.5 mm) confirmed uniform cross sectional distribution of molybdena. This proved that the SIM SAS method can be used not only for small support particles but also for usual extrudates or tablets. The relation and advantage of the SIM SAS method as compared with other methods of preparation of monolayer molybdena catalysts were discussed.

The Oxidative Dehydrogenation of Cyclohexane and Cyclohexene over Unsupported and Supported Molybdena Catalysts Prepared by Metal Oxide Vapor Deposition

A novel molybdena catalyst has been prepared by metal oxide vapor synthesis (MOVS) and used to promote the vapor phase oxidative dehydrogenation of cyclohexane and cyclohexene. At temperatures in excess of 553 K, the conversion of cyclohexane over γ-alumina supported and unsupported MOVS catalysts and a commercial MoO3/Al2O3sample yielded cyclohexene, benzene, carbon monoxide, and carbon dioxide as the only detected products. The unsupported MOVS sample behaves as an active heterogeneous catalyst but the specific activities are increased by a factor of up to 8 upon supporting the precursor on alumina. The supported MOVS system also exhibits appreciable higher activity than the commercial catalyst and this is attributed to the unique dual dioxo-molybdenum sites present in the active MOVS catalyst. The oxidative dehydrogenation of cyclohexane is viewed as occurring in a stepwise fashion with cyclohexene appearing in the product mixture and the generation of carbon oxides occurring principally from the direct combustion of cyclohexane. Oxygen consumption, specific activities, and molar selectivities for the conversion of both reactants in the overall temperature range 488 K≤T≤673 K over the three molybdena systems are provided. Cyclohexene conversion to benzene exhibits zero and first-order behavior with regard to the hydrocarbon and oxygen concentrations, respectively, and apparent activation energies are recorded for the supported (89 kJ mol−1) and unsupported (126 kJ mol−1) MOVS and commercial (103 kJ mol−1) catalysts. The nature of the reactive oxygen and catalytic sites are considered and the reactivity and stoichiometry of the deep oxidation of both cyclic hydrocarbons are presented.

Metal-Catalyzed Hydrocarbon Conversion Reactions

Palladium-catalyzed acetylene trimerization and the metathesis of olefins catalyzed by molybdenum and its oxides are investigated in ultrahigh vacuum and at high pressures. Benzene is formed on Pd(111) by the reaction between adsorbed acetylene and a surface C4 metallocycle. The resulting benzene evolves in temperature-programmed desorption in two distinct states. The low-temperature state is proposed, following investigations of the desorption kinetics of benzene from Pd(111), to be due to tilted benzene formed on a sterically crowded surface, and the high-temperature state, to be due to flat-lying benzene. The low steady-state benzene formation rate found at high pressures (∼1 atm) is suggested to be due to blocking of the surface by the formation of vinylidene species. Olefin metathesis is found to proceed in two different regimes: one below ∼650 K, which mimics supported molybdena catalysts and where MoO3 is the best catalyst, and another region above this temperature, where the reaction proceeds with a high activation energy (∼60 kcal/mol) and the most effective catalyst is MoO2. The latter kinetics resemble those found for metallic molybdenum, where the reaction is proposed to proceed by a mechanism through which alkenes dissociate and recombine on the surface. This reaction is found to proceed in the presence of a thick carbonaceous layer. The addition of hydrogen is found to increase the rate of both metathesis and cyclotrimerization even though neither of these reactions involves hydrogen directly. This is proposed to be due to the titration of carbonaceous species from the surface. Extension of these ideas to ethylene hydrogenation, a reaction that does involve hydrogen, is successful in rationalizing the kinetics behavior found in that case.

Alcohol synthesis from syngas over supported molybdenum catalysts

Light alcohols as well as hydrocarbons can be obtained from the selective conversion of syngas over supported molybdenum catalysts. It is shown in this study that the activity and the selectivity are very much dependent on the support, the molybdenum content and the alkaline promoter. Indeed the methanol selectivity improves when molybdenum is supported on zinc oxide and promoted with potassium. Moreover, we found that the rate of methanol formation is in good correlation with unreduced molybdenum species. A dual-site mechanism is proposed for the methanol synthesis on such catalysts involving hydrogen activation (and dissociative carbon monoxide adsorption for hydrocarbons formation) and non-dissociative carbon monoxide adsorption, respectively, on reduced and unreduced molybdenum species.

A study of supported molybdenum catalysts for the reduction of SO2 with CH4: Effect of sulphidation method

AbstractAn experimental investigation of the effect of sulphidation method of supported molybdenum catalysts used in the reduction of SO2 with Ch4 was carried out. Two sulphidation methods, one using H2S and the other using a mixture of SO2/CH4, are compared. The methods are evalauted with respect to molybdenum retention, extent of molybdenum sulphidation, and catalytic activity. Mo/Al2O3 catalysts sulphided with H2S are superior to those sulphided under SO2 and CH4 reaction conditions. Although complete sulphidation is never achieved in either of the two cases, the higher activity found with H2S‐sulphided catalysts is attributed to a higher degree of molybdenum sulphidation resulting in a higher MoS2 content. The 15% Mo/Al2O3 catalyst sulphided with H2S at 600°C showed the highest activity and had the highest MoS2 content.

Characterization and Thiophene Hydrodesulfurization Activity of NaX Zeolite Supported Molybdena Catalysts

Abstract A series of NaX zeolite supported molybdena catalysts containing 1—15 wt% MoO3 has been prepared by impregnation method. They have been characterized by X-ray diffraction (XRD), infrared spectroscopy (IR), temperature-programmed reduction (TPR), scanning electron microscopy (SEM), surface area, pore volume, Mo dispersion, and Mo extraction measurements. Zeolite crystallinity was found to be affected greatly only at 15 wt% MoO3 loading. XRD and SEM studies showed absence of any crystalline MoO3 phase. However, presence of bulk MoO3, particularly at higher loadings, was indicated by TPR and Mo extraction results. Tetrahedral surface molybdenum species are formed through strong interaction between molybdena and zeolite lattice through oxygen atoms. A direct relationship between Mo dispersion on the oxidic catalysts and their thiophene conversion activity was observed. In addition to Mo dispersion, acidity of the catalysts was also found to play an important role at high reaction temperature.

Preparation of alumina supported molybdena catalysts by new slurry impregnation method: MoO3/Al2O3 sulfide hydrodesulfurization catalyst

A new slurry impregnation method (SIM) of the preparation of a catalyst or catalyst precursor MoO3/Al2O3 is described. Aqueous slurry of powdered MoO3 is mixed with alumina extradates and the mixture is refluxed. A low solubility of MoO3 is sufficient for transport of MoO3 from powder form via solution to the surface of the support. The catalysts were tested by hydrodesulfurization of thiophene at 1.6 MPa and 280–400°C. Their activity was similar to the activity of industrial and laboratory MoO3/Al2O3 samples prepared by conventional impregnation with solution of ammonium heptamolybdate. The advantage of the SIM method is that calcination, producing nitrogeneous waste gases, is not required and that all deposited molybdenum species are adsorbed and not precipitated.

Preparation of molybdenum-based catalysts by new slurry impregnation method: active carbon supported molybdenum sulfide hydrodesulfurization catalyst

Active carbon supported molybdena catalysts (15% MoO 3 ) were prepared by new slurry impregnation method ( SIM ) using carbon of various origin and particle size. The support was refluxed with the slurry of powdered MoO 3 in water. A low solubility of MoO 3 was sufficient for transport of MoO 3 from the powder via the solution to the surface of the support. Calcination was left out because no other chemicals beside MoO 3 and water were used in the impregnation. The catalysts were presulfided and tested in hydrodesulfurization of thiophene at 1.6 MPa and 300–400°C. Their activity was four times higher than that of a commercial molybdena on alumina catalyst and about the same as the activity of a carbon supported catalyst prepared by the conventional impregnation method (CIM) using ammonium heptamolybdate. The advantage of the new SIM over CIM method is: (i) all molybdena species are deposited by adsorption; precipitation during drying is negligible, (ii) oxidative degradation of the carbon support cannot occur during preparation because calcination is left out; ammonium sulfide is not formed during presulfidation of the uncalcined catalysts.

The Partial Oxidation of Methane on MoO3/SiO2 Catalysts: Influence of the Molybdenum Content and Type of Oxidant

The selective oxidation of methane to formaldehyde at atmospheric pressure was studied over a series of silica supported molybdena catalysts. Two silica supports of substantially different surface areas (80 and 200 m2/g) and molybdenum contents ranging from 0 to 3.5 Mo/nm2 were used for this purpose. The catalytic performance of the two catalyst series was compared for a wide range of experimental conditions, including the influence of the oxidant in the feed (O2 or N2O). The main products of the oxidation reaction were formaldehyde, CO and CO2. Small amounts of C2H6 and CH3OH were also identified. For typical reaction conditions, i.e., reaction temperatures 863 K, W/F 4.5 g h/mol and CH4/O2 = 10 M both methane conversion and formaldehyde selectivity were higher with O2 as oxidant. Furthermore, formaldehyde selectivity reached a maximum for molybdenum concentrations ca. 1 Mo/nm2 with both oxidants. The molybdenum exposure on the surface of fresh and spent catalysts was monitored by X-ray photoelectron spectroscopy.

Electron paramagnetic resonance spectroscopy of catalytic surfaces

A review is presented of the application of electron paramagnetic resonance (EPR) spectroscopy to study surfaces of catalytic interest. Spectra of Mo(V) in supported molybdena catalysts illustrate how EPR can assist in the characterization of such materials. The spectra of an adsorbed stable radical, the superoxide ion, are used to show how surface sites on catalysts can be probed by EPR. Unstable radicals which are conceivable intermediates in catalytic reaction mechanisms can be identified via spin trapping experiments, either chemical or matrix isolation. The mobility of adsorbed radicals on surfaces can be studied by EPR. Finally, recent progress in observing EPR spectra from well-defined single-crystal surfaces is discussed.

ChemInform Abstract: Use of ESCA Valence Bands to Infer Structural Information About the Molybdenum Phase in Supported Molybdenum 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.

Effect of cobalt oxide on surface structure of alumina supported molybdena catalysts studied by in situ Raman spectroscopy

A set of Co promoted 10% Mo/Al2O3 samples have been characterized by means of Raman spectroscopy under ambient as well as in situ dehydrated conditions. Under ambient conditions, the degree of the polymerization of surface molybdenum oxide species decreases with increasing Co loading. Under dehydrated conditions, the polymeric molybdenum oxide species is absent with the addition of only 0.2% Co. At low Co loadings (⩽2%), before the formation of CoMoO4 compound, the spectral features are very similar under ambient conditions. Dehydration causes the upward shift of the Mo=O symmetric stretching mode. A broad band around 920–930 cm−1 was thus observed. This band has been suggested to be associated with the Co-Mo interaction species. In contrast to crystalline CoMoO4, this species shows a reversibility on H2 reduction-O2 reoxidation treatments. From the results obtained, it is proposed that cobalt oxide interacts with the most polymerized molybdenum oxide species to form Co-Mo interaction species and/or crystalline CoMoO4; therefore, the amount of the surface molybdenum oxide species decreases with a change in the molecular structure as a function of the Co concentration.

Use of ESCA valence bands to infer structural information about the molybdenum phase in supported molybdenum catalysts

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTUse of ESCA valence bands to infer structural information about the molybdenum phase in supported molybdenum catalystsJoseph N. Fiedor, Andrew Proctor, Marwan Houalla, Peter M. A. Sherwood, Francis M. Mulcahy, and David M. HerculesCite this: J. Phys. Chem. 1992, 96, 26, 10967–10970Publication Date (Print):December 1, 1992Publication History Published online1 May 2002Published inissue 1 December 1992https://doi.org/10.1021/j100205a067RIGHTS & PERMISSIONSArticle Views105Altmetric-Citations13LEARN 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 (540 KB) Get e-Alerts

Influence of pore structure and chemical properties of supported molybdenum catalysts on their performance in upgrading heavy coal liquids

In the hydroprocessing of solvent-refined coals, both the pore structure and the chemical properties of the catalysts affect the conversion of the heavy materials. Increasing median pore diameter (MPD) of unimodal Ni-Mo/Al 2 O 3 catalysts in the relatively small pore region (up to 150 A) enhanced the conversion of both asphaltene and preasphaltene, but further increasing the MPD up to 730 A mainly promoted preasphaltene conversion. In the runs of the isolated fractions, however, conversions increased with MPD up to 290 A for asphaltene and up to 730 A for preasphaltene

Electron Paramagnetic Resonance Study on Supported Molybdenum Catalysts. Part 1: Interaction of Molybdenum Oxide with Silica and Signal Anlsotropy

AbstractThe effects of reduction, evacuation and adsorbate adsorption on Mo/SiO2 catalysts prepared by impregnation were studied by means of the electron paramagnetic resonance (EPR) technique. Pentavalent molybdenum species in the overlayer (Mo05+, g⟂ = 1.941, g| = 1.895) and at the interface (Moi5+, g⟂ = 1.952, g| = 1.866) between MoO3 and SiO2 were distinguished in the EPR spectra; they were observed from samples reduced at T < 573 K and T > 573 K respectively. The difference in their nature reflected the extent of their interaction with the support. Mo05+ existed in the MoO3 overlayer of the catalyst and interacted only weakly with the support. Moi5+ from the Mo ions connected directly to SiO2 was affected strongly by the support. The anisotropic factor of the measured Moi5+ signals increased with the extent of support interaction.Superoxide ions (O2, g1 = 2.016, g2 = 2.011, g3 = 2.006) were found upon oxygen adsorption on samples evacuated at temperatures above 573 K. The observed O2 may exist at the interface between MoO3 and the support.

Nitrogen containing species as intermediates in the oxidation of ammonia over silica supported molybdena catalysts

The behaviour of ammonia and oxygen over silica supported molybdena catalysts has been studied by means of thermal analysis techniques, such as thermo-gravimetry and heat-flow calorimetry. The composition of the reactants and products was determined by means of mass spectrometric analysis. Nitrogen- and hydrogen-containing species were observed to be present on the catalyst surface after reduction of the catalysts by ammonia and subsequent flushing in helium. Additional evidence for the presence of these species was provided by experiments in which labelled molecules were used as well as by means of X-ray photo-electron spectroscopy. The nitrogen- and hydrogen-containing species could be removed from the surface by O 2. or NO. The reaction products were found to be N 2 and H 2O. In the light of these results, a working model is proposed that describes the adsorption of ammonia on; and the oxidative production of nitrogen and water from ammonia over, silica-supported molybdena catalysts.