Iron Molybdate Catalysts Research Articles

Acrolein production from methanol and ethanol mixtures over La- and Ce-doped FeMo catalysts

The acrolein production from methanol and ethanol mixtures over iron-molybdate-based catalysts was studied. The reaction to acrolein can be described by two successive steps: the first consists on the oxidation of both alcohols into their corresponding aldehydes and the second step is the subsequent aldol condensation of the as-formed aldehydes. The iron-molybdate catalysts were modified by doping with La and Ce (1%mol) in order to improve the aldol condensation step of the process. Series of catalysts were thus synthesized with different Mo/Fe ratios (i.e., 1.5, 2.0 and 2.5) and calcined at three different temperatures (i.e, 350 °C, 400 °C and 450 °C). The best catalytic performance was observed for FeMoCe2.0 (400 °C) for which the acrolein yield reached 42% (T = 320 °C, MetOH/EtOH = 1, GHSV = 3900 h−1). Furthermore, all the samples were characterised by TGA-DSC, HT-XRD, XPS, BET, LEIS, XRF, CO2-TPD, Pyridine (FTIR) and NH3 (calorimetry) adsorption. The increase in acrolein yield observed upon La and Ce doping was attributed to acid/base properties modification.

Deactivation behavior of an iron-molybdate catalyst during selective oxidation of methanol to formaldehyde

Molybdenum loss, phase transformations and catalytic performance of an iron molybdate catalyst.

Photo-Fenton-like degradation of Rhodamine B dye from waste water using iron molybdate catalyst under visible light irradiation

Iron molybdate Fe2(MoO4)3 nanopowders with different morphologies have been successfully tailored using two different techniques; co-precipitation and sol gel auto-combustion methods Evidently, X-ray diffraction (XRD) profiles manifested that monoclinic Fe2(MoO4)3 with space group P21/a was acquired at different annealing temperatures from 400 to 600°C for 2h using both routes. FE-SEM micrographs evidenced that the formed particles was exhibited aggregates of plates staked with different directions for the sample annealed at 400°C which it was converted to the brain or an array of fused spheres for the samples annealed at 600°C using co-precipitation strategy. Otherwise, the microstructure of the formed particles was appeared as a homogeneous porous spherical cluster for Fe2(MoO4)3 sample annealed at 400°C which it was showed a honey comb like for the sample annealed at 600°C. Overall, Fe2(MoO4)3 nanopowder prepared by co-precipitation method at 400°C was exhibited the highest photo Fenton catalytic activity with efficiency ∼97% for degradation of RhB dye after 60min. In this regards, the highest catalytic activity was attributed to the strong absorption of Fe2(MoO4)3 in the visible-light region due to high surface area of the distinct morphologies and generation of reactive OH from H2O2 synergistically activated by both Fe3+ and MoO42−.

Preparation of NO-doped β-MoO3 and its methanol oxidation property

The major drawback of the industrial iron molybdate catalysts which is their deactivation problem has driven the study of alternative catalysts for formaldehyde production from methanol. In this paper, NO-doped β-MoO3 was successfully synthesized from the commercial molybdic acid powder (H2MoO4) and characterized by differential thermal analysis (DTA), X-ray Diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Results obtained from XRD and Raman spectroscopy indicated that the synthesized sample has all features of the well-known β-MoO3 except for the presence of a new small peak. The curve-fitting of XPS spectra revealed that nitrogen-containing species may be present in the form of negatively charged nitrogen oxide in the prepared sample. Due to its metastable nature, NO-doped β-MoO3 may be transformed into the thermally stable α-MoO3 at temperature higher than 400 °C as pointed out by DTA study. However, when the reaction temperature was as low as 300 °C, the catalyst was stable for partial methanol oxidation with no significant change in activity during 30 h of catalytic study. Methanol conversion and formaldehyde selectivity were maintained at about 98% and 99%, respectively.

Preparation of iron molybdate catalysts for methanol to formaldehyde oxidation based on ammonium molybdoferrate(II) precursor

It was demonstrated that iron molybdate catalysts for methanol oxidation can be prepared using Fe(II) as a precursor instead of Fe(III). This would allow for reduction of acidity of preparation solutions as well as elimination of Fe(III) oxide impurities which are detrimental for the process selectivity. The system containing Fe(II) and Mo(VI) species in aqueous solution was investigated using UV–Vis spectroscopy. It was demonstrated that three types of chemical reactions occur in the Fe(II)–Mo(VI) system: (i) formation of complexes between Fe(II) and molybdate(VI) ions, (ii) inner sphere oxidation of coordinated Fe(II) by Mo(VI) and (iii) decomposition of the Fe–Mo complexes to form scarcely soluble Fe(III) molybdate, Mo(VI) hydrous trioxide and molybdenum blue. Solid molybdoferrate(II) prepared by interaction of Fe(II) and Mo(VI) in solution was characterized by EDXA, TGA, DTA and XRD and a scheme of its thermal evolution proposed. The iron molybdate catalyst prepared from Fe(II) precursor was tested in methanol-to-formaldehyde oxidation in a continuous flow fixed-bed reactor to show similar activity and selectivity to the conventional catalyst prepared with the use of Fe(III).

Design and stabilisation of a high area iron molybdate surface for the selective oxidation of methanol to formaldehyde.

The performance of Mo-enriched, bulk ferric molybdate, employed commercially for the industrially important reaction of the selective oxidation of methanol to formaldehyde, is limited by a low surface area, typically 5-8 m(2) g(-1). Recent advances in the understanding of the iron molybdate catalyst have focused on the study of MoOx@Fe2O3 (MoOx shell, Fe2O3 core) systems, where only a few overlayers of Mo are present on the surface. This method of preparing MoOx@Fe2O3 catalysts was shown to support an iron molybdate surface of higher surface area than the industrially-favoured bulk phase. In this research, a MoOx@Fe2O3 catalyst of even higher surface area was stabilised by modifying a haematite support containing 5 wt% Al dopant. The addition of Al was an important factor for stabilising the haematite surface area and resulted in an iron molybdate surface area of ∼35 m(2) g(-1), around a 5 fold increase on the bulk catalyst. XPS confirmed Mo surface-enrichment, whilst Mo XANES resolved an amorphous MoOx surface monolayer supported on a sublayer of Fe2(MoO4)3 that became increasingly extensive with initial Mo surface loading. The high surface area MoOx@Fe2O3 catalyst proved amenable to bulk characterisation techniques; contributions from Fe2(MoO4)3 were detectable by Raman, XAFS, ATR-IR and XRD spectroscopies. The temperature-programmed pulsed flow reaction of methanol showed that this novel, high surface area catalyst (3ML-HSA) outperformed the undoped analogue (3ML-ISA), and a peak yield of 94% formaldehyde was obtained at ∼40 °C below that for the bulk Fe2(MoO4)3 phase. This work demonstrates how core-shell, multi-component oxides offer new routes for improving catalytic performance and understanding catalytic activity.

Methanol oxidation on Fe2O3 catalysts and the effects of surface Mo.

The adsorption of methanol on haematite has been investigated using temperature programmed methods, combined with in situ DRIFTS. Model catalysts based on this material have then been made with a shell-core configuration of molybdenum oxide monolayers on top of the haematite core. These are used as models of industrial iron molybdate catalysts, used to selectively oxidise methanol to formaldehyde, one of the major chemical outlets for methanol. Haematite itself is completely ineffective in this respect since it oxidises it to CO2 and the DRIFTS shows that this occurs by oxidation of methoxy to formate at around 200 °C. The decomposition behaviour is affected by the absence or presence of oxygen in the gas phase; oxygen destabilises the methoxy and enhances formate production. In contrast, when a monolayer of molybdena is placed onto the surface by incipient wetness, and it remains there after calcination, the pathway to formate production is blocked and formaldehyde is the main gas phase product in TPD after methanol dosing.

The surface of iron molybdate catalysts used for the selective oxidation of methanol

The oxidation of methanol to formaldehyde is a major chemical process carried out catalytically and iron molybdate is one of the major catalysts for this process. In this paper we explore the nature of the active and selective surfaces of iron molybdate catalysts and show that the effective catalysts comprise molybdenum rich surfaces. We conclude that it is therefore important to maximise the surface area of these active catalysts and to this end we have studied catalysts made using a new physical grinding method with oxalic acid. For super-stoichiometric materials (Fe:Mo=1:2.2) the reaction data show that physical mixing produces effective catalysts, possibly offering an improvement over the conventional co-precipitation method.

Rules for Selective Oxidation Exemplified by Methanol Selective Oxidation on Iron Molybdate Catalysts

A number of simple rules are proposed which dictate for high selectivity to formaldehyde during methanol oxidation on iron molybdate catalysts. The reaction is of the Mars–van Krevelen type, that is, it is surface lattice oxygen that is the active species. However, we also show that the material is quite a flexible one, in that the bulk oxygen anions become mobile enough at moderate temperatures to exchange rapidly with the surface vacancies that are created during the reaction. Further, and essential to high selectivity, is the fact that Mo is preferentially segregated to the surface layer of the catalyst, with no Fe present there. Finally, it can also be shown that the important oxidation state for the cations at the surface is the highest one (6+ for Mo), with lower oxidation states being detrimental to selectivity. It is likely that these basic rules are also important for a range of other catalytic processes/catalysts.

A cubic Fe4Mo4 oxo framework and its reversible four-electron redox chemistry.

The potential of iron molybdates as catalysts in the Formox process stimulates research on aggregated but molecular iron-molybdenum oxo compounds. In this context, [(Me3TACN)Fe](OTf)2 was reacted with (nBu4N)2[MoO4], which led to an oxo cluster, [[(Me3TACN)Fe][μ-(MoO4-κ(3)O,O',O″)]]4 (1, Fe4Mo4) with a distorted cubic structure, where the corners are occupied by (Me3TACN)Fe(2+) and [Mo═O](4+) units in an alternating fashion, being bridged by oxido ligands. The cyclic voltammogram revealed four reversible oxidation waves that are assigned to four consecutive Fe(II) → Fe(III) transfers and motivated attempts to isolate compounds containing the respective cations. Indeed, a salt with a Fe(II)2Fe(III)2Mo(VI)4 constellation, [Fe4Mo4](TCNQ)2 (2), could be isolated after treatment with TCNQ. The Fe(II)Fe(III)3Mo(VI)4 stage could be reached via oxidation with DDQ or 3 equiv of thianthrenium hexafluorophosphate (ThPF6), giving [Fe4Mo4](DDQ)3 (4) or [Fe4Mo4](PF6)3 (5), respectively. The fully oxidized Fe(III)4Mo(VI)4 state was generated through oxidation with 4 equiv of ThPF6, leading to [Fe4Mo4](PF6)4, which showed a unique behavior: upon storage, one of the [Mo═O](4+) corners inverts, so that the terminal oxido ligand is located in the interior of the cage, leading to the formation of [[(Me3TACN)Fe]4[μ-([MoO4]3[MoO4(MeCN-κN)])-κ(3)O,O',O″)](PF6)4 (7). In this form, the compound could no longer be employed to enter the cyclic voltammogram recorded for 1, 3, and 5 from the oxidized side; no discrete redox events were observed. Compounds 1-3 and 7 were characterized structurally and 1, 3, and 7 additionally by SQUID measurements and Mössbauer spectroscopy. The data reveal a high degree of charge delocalization. (16)O/(18)O exchange experiments with labeled water performed with 1 revealed an interesting parallel with the Formox catalyst: water-(18)O exchanges its label with all of the oxido ligands (bridging and terminal). This property relates to the ion mobility being held responsible for the activity of iron molybdate catalysts compared to neat MoO3 or Fe2O3.

Tungstate promoted vanadium phosphate catalysts for the gas phase oxidation of methanol to formaldehyde

Formaldehyde is an important intermediate in the pharmaceutical, plastic and automobile industries which is produced commercially via methanol oxidation. A series of tungstate promoted vanadium phosphate catalysts were prepared and characterised using a combination of XRD, UV-vis spectrometry, XPS and ammonia TPD. The 25 wt% WO3–VPO catalyst exhibited good activity for the selective gas phase oxidation of methanol with >90% conversion at around 250 °C. This is a considerably lower temperature than used in the current industrial process using either silver or iron molybdate catalysts. This increased activity at low temperatures is ascribed to the increase in Brønsted acidity and redox properties of the tungstate doped materials.

Evolution of active catalysts for the selective oxidative dehydrogenation of methanol on Fe2O3 surface doped with Mo oxide

Iron molybdate catalysts are used for the selective oxidation of methanol to formaldehyde. In this paper we have attempted to understand what determines high selectivity in this reaction system by doping haematite with surface layers of Mo by incipient wetness impregnation. This works well and the Mo appears to form finely dispersed layers. Even very low loadings of Mo have a marked effect on improving the selectivity to formaldehyde. Haematite itself is a very poor catalyst with high selectivity to combustion products, whereas, when only 0.25 monolayers of Mo are deposited on the surface, formaldehyde and CO selectivities are greatly enhanced and CO2 production is greatly diminished. However, even with as much as seven monolayers of Mo dosed on to the surface, these materials achieve much less selectivity to formaldehyde at high conversion than do the industrial catalysts. The reason for this is that the Mo forms a 'skin' of ferric molybdate on a core of iron oxide, but does not produce a pure Mo oxide monolayer on the surface, a situation which is essential for very high yields of formaldehyde.

Fe2(MoO4)3/MoO3 nano-structured catalysts for the oxidation of methanol to formaldehyde

Nano-structured iron molybdate materials, comprising Fe2(MoO4)3 nano-particles anchored onto MoO3 nano-rods, have been synthesised and evaluated as catalysts for the selective oxidation of methanol to formaldehyde. The catalysts were benchmarked against a standard iron molybdate catalyst prepared by co-precipitation and were found to have comparable performance under the test reaction conditions. The materials have been characterised using a combination of electron microscopy, powder X-ray diffraction and Raman spectroscopy. The synthesised MoO3 nano-rods were found to have a uniform structure, 8–10μm in length with a rectangular cross-section of 50–100nm×100–200nm. Fe was impregnated onto the MoO3 nano-rods via incipient wetness with an aqueous solution of Fe(NO3)3·9H2O. Calcination of these precursors formed 20–200nm Fe2(MoO4)3 islands on the surface of the nano-rods via a solid-state diffusion mechanism, and the size of the Fe2(MoO4)3 islands could be controlled by varying the Fe loading. The effect of the temperature and duration of calcination were investigated, and it was found that the optimum conditions were 450–500°C for 2h. At 350°C, the temperature was too low for the solid-state reaction between the MoO3 nano-rod and the surface Fe to occur, and no Fe2(MoO4)3 was formed. At higher temperatures, the nano-rod morphology was compromised, and irregular, partially coalesced particles of Fe2(MoO4)3 were generated, which had a lower catalytic performance than the nano-structured Fe2(MoO4)3/MoO3 materials. At 400°C, the solid-state reaction occurred sufficiently slowly for mechanistic aspects of the Fe2(MoO4)3 island formation to be elucidated by transmission electron microscopy.

Iron Molybdate Catalyst Stabilized by Calcium Oxide for Methanol to Formaldehyde Conversion

Regularities of methanol oxidation by atmospheric oxygen on the catalyst Fe2(MoO4)3/MoO3/CaO = = 1 : 0.7 : 0.3 have been studied. It was found that the speed of the process in a model flow reactor obeys the first order equation with the observed rate constant of 1.0 sec1 and the apparent activation energy of 65 kJ/mol. It was established that the selectivity of formaldehyde in the process with once-through conversion over 45 % sharply decreases. The mechanism of alcohols heterogeneous catalytic oxidation, including the stages of organic compounds interaction both with metal cations and oxygen atoms of the catalyst surface was suggested. Вивчено закономірності окиснення метанолу киснем повітря на каталізаторі складу Fe2(MoO4)3/MoO3/CaO = 1 : 0,7 : 0,3. Знайдено, що швидкість процесу в модельному проточному реакторі підпорядковується рівнянню першого порядку зі спостережуваною константою швидкості 1,0 с-1 і уявною енергією активації 65 кДж/моль. Встановлено, що селективність процесу по формальдегіду при ступені перетворення метанолу більше 45 % різко зменшується. Запропоновано механізм гетерогенно-каталітичного окиснення спиртів, який включає стадії взаємодії органічних сполук як з катіонами металу, так і атомами кисню поверхні каталізатору.

Origin of the synergistic interaction between MoO 3 and iron molybdate for the selective oxidation of methanol to formaldehyde

The origin of the enhanced catalytic performance of bulk iron molybdate catalysts with excess crystalline MoO 3 for methanol oxidation to formaldehyde was investigated with MoO 3, Fe 2O 3, Fe 2(MoO 4) 3, MoO 3/Fe 2(MoO 4) 3 and model supported MoO 3/Fe 2O 3 catalysts. Low-energy ion scattering (LEIS) analysis of the outermost surface layer revealed that the molybdate catalysts possess a monolayer of surface MoO x species. Temperature programmed CH 3OH-IR spectroscopy revealed that both intact surface CH 3OH * and surface CH 3O * species are present on the catalysts with both yielding HCHO for the redox molybdate catalysts. The addition of excess crystalline MoO 3 to the crystalline Fe 2(MoO 4) 3 phase significantly increases the overall steady-state catalytic performance toward HCHO formation. The enhanced catalytic performance of bulk iron molybdate catalysts in the presence of excess MoO 3 is related to the formation of a surface MoO x monolayer on the bulk Fe 2(MoO 4) 3 phase. Thus, the catalytic active phase for bulk iron molybdate catalysts is the surface MoO x monolayer on the bulk crystalline Fe 2(MoO 4) 3 phase and the only role of the excess crystalline MoO 3 is to replenish the surface MoO x lost by volatilization during methanol oxidation.

A Combined Multi-Technique In Situ Approach Used to Probe the Stability of Iron Molybdate Catalysts During Redox Cycling

A setup combining a number of techniques (WAXS, XANES and UV–Vis) has been used to probe the stability of an iron molybdate catalyst during redox cycling. The catalyst was first reduced under anaerobic methanol/helium conditions, producing formaldehyde and then regenerated using air. Although in this test-case the catalyst and conditions differ from that of a commercial catalyst bed we demonstrate how such a setup can reveal new information on catalyst materials. In particular we observe the formation of two phases during reduction; one which we propose to be an oxygen deficient ‘pseudo-molybdate phase’, the other a molybdenum carbide-like phase, both produced as oxygen is removed from the catalyst. Standard in situ techniques could detect such transient phases, however, the information from multiple techniques, allows us to more accurately identify the nature of these materials and to carry out appropriate complementary ex situ measurements to aid in the analysis. This and similar setups therefore offer a way to more quickly and accurately observe reaction pathways within a catalyst, which may for example, result in the deactivation of the material by different routes to those observed previously. Additionally, the specific combination of these techniques with on-line mass spectrometry, allows us to monitor the activity of the catalyst surface and here observe that different catalytic mechanisms may occur during different stages of the redox process. Therefore this setup should allow for the observation of many novel variations in a catalyst’s reactivity, leading to the improvement of current and development of new materials.

Selectivity and Activity of Iron Molybdate Catalysts in Oxidation of Methanol

The selectivity and activity of iron molybdate catalysts prepared by different methods are compared with those of a commercial catalyst in the oxidation of methanol to formaldehyde in a continuous tubular bed reactor at 200-350 oC (473-623 oK), 10 atm (1013 kPa), with a methanol-oxygen mixture fixed at 5.5% by volume methanol: air ratio. The iron(III) molybdate catalyst prepared by co-precipitation and filtration had a selectivity towards formaldehyde in methanol oxidation comparable with a commercial catalyst; maximum selectivity (82.3%) was obtained at 573oK when the conversion was 59.7%. Catalysts prepared by reacting iron (III) and molybdate by kneading or precipitation followed by evaporation, omitting a filtration stage, were less active and less selective. The selectivity-activity relationships of these catalysts as a function of temperature were discussed in relation to the method of preparation, surface areas and composition. By combing this catalytic data with data from the patent literature we demonstrate a synergy between iron and molybdenum in regard to methanol oxidation to formaldehyde; the optimum composition corresponded to an iron mole fraction 0.2-0.3. The selectivity to formaldehyde was practically constant up to an iron mole fraction 0.3 and then decreased at higher iron concentrations. The iron component can be regarded as the activity promoter. The iron molybdate catalysts can thus be related to other two-component MoO3-based selective oxidation catalysts, e.g. bismuth and cobalt molybdates. The iron oxide functions as a relatively basic oxide abstracting, in the rate-controlling step, a proton from the methyl of a bound methoxy group of chemisorbed methanol. It was proposed that a crucial feature of the sought after iron(III) molybdate catalyst is the presence of -O-Mo-O-Fe-O-Mo-O- groups as found in the compound Fe2(MoO4)3 and for Fe3+ well dispersed in MoO3 generally. At the higher iron(III) concentrations the loss of selectivity is due to the presence of iron oxide patches or particles which catalyze the total oxidation of methanol, and the loss of activity to blocking of molybdenum sites.

An iron molybdate catalyst for methanol to formaldehyde conversion prepared by a hydrothermal method and its characterization

A one-step, low-temperature hydrothermal method has been successfully employed to prepare iron molybdate catalysts with Mo:Fe ratios ranging from 1.5:1 to 3.0:1. The resulting materials were characterized using a number of techniques including: XRD, Raman, N 2 adsorption, SEM/EDX, DTA, EDXRD and combined XRD/XAS. The catalytic oxidative dehydrogenation of methanol to formaldehyde has been used as a test reaction. For Mo:Fe ∼ 1.5, phase-pure Fe 2(MoO 4) 3 resulted from syntheses performed at temperatures as low as 100 °C in under 4 h. For samples with a Mo:Fe ∼ 3 detailed analysis of XRD, Raman and EXAFS data revealed the formation of a high surface area possessing, mixed phase material consisting of a poorly crystalline Mo 5O 14 and an amorphous Fe 2(MoO 4) 3 type precursor. Both phases proved to be thermally unstable above a calcination temperature of 300 °C, going on to form high surface area mixed Fe 2(MoO 4) 3/MoO 3. Continued heating of this mixed oxide material resulted in sintering and to a decrease in the surface area. When both mildly (200 °C) and then more severely calcined (300 °C), this mixed phase sample showed a higher selectivity for formaldehyde production than a conventionally prepared (via co-precipitation) iron molybdate catalyst.

Optimization of the methanol oxidation over iron–molybdate catalysts

The oxidation of methanol on industrial iron–molybdenum catalysts was investigated with a view to shed more light on the possibilities to optimize the process. Stainless steel pseudo-isothermal reactor, located in a precisely controlled thermostat was used to ensure conditions of the process as close as possible to the industrial. The catalytic activity was evaluated under steady state conditions and methanol conversion above 99.0% on the basis of the shape of the temperature profile in height of the catalyst layer, the situation of the “hot spot” and the temperature of the gas flow at exit of the catalyst layer. The outlet gas mixture was analyzed for CH 2O, CH 3OH, CO, dimethylether (DME), and CO 2 content in the analytical section. The influence of the process parameters on the catalyst activity and selectivity was analyzed and it was concluded that oxygen concentration in the feed mixture has to be equal to or higher than the methanol concentration and water concentration in the range of 2.0–3.0% does favor the selectivity of the process. The increase of methanol concentration in the gas mixture fed in the reactor leads to a dramatic change both of the temperature profile of the catalyst layer and the distribution of the reaction products. A number of laboratory experiments were performed to clarify the influence of the most important parameters of the process (temperature, space velocity and methanol concentration) on the efficiency of the adiabatic layer.

Methanol Oxidation over Iron-Molybdate Catalysts: Optimization of the Process

This research aims to shed more light on the possibilities to optimize the process of selective oxidation of methanol over oxide catalysts using an adiabatic layer by determining border values of basic parameters of the process in the adiabatic layer that ensure a maximum rate of methanol oxidation at high selectivity. Laboratory experiments were performed to clarify the influence of the most important parameters of the process (temperature, space velocity and methanol concentration) on the efficiency of the adiabatic layer. An industrial experiment was carried out to verify the results obtained.