Iron Molybdate Catalysts Research Articles

Promotion effects of alkali metals on iron molybdate catalysts for CO2 catalytic hydrogenation

CO2 hydrogenation is an attractive way to store and utilize carbon dioxide generated by industrial processes, as well as to produce valuable chemicals from renewable and abundant resources. Iron catalysts are commonly used for the hydrogenation of carbon oxides to hydrocarbons. Iron-molybdenum catalysts have found numerous applications in catalysis, but have been never evaluated in the CO2 hydrogenation. In this work, the structural properties of iron-molybdenum catalysts without and with a promoting alkali metal (Li, Na, K, Rb, or Cs) were characterized using X-ray diffraction, hydrogen temperature-programmed reduction, CO2 temperature-programmed desorption, in-situ 57Fe Mossbauer spectroscopy and operando X-ray adsorption spectroscopy. Their catalytic performance was evaluated in the CO2 hydrogenation. During the reaction conditions, the catalysts undergo the formation of an iron (II) molybdate structure, accompanied by a partial reduction of molybdenum and carbidization of iron. The rate of CO2 conversion and product selectivity strongly depend on the promoting alkali metals, and electronegativity was identified as an important factor affecting the catalytic performance. Higher CO2 conversion rates were observed with the promoters having higher electronegativity, while low electronegativity of alkali metals favors higher light olefin selectivity.

Methanol oxidation over iron molybdate catalysts. Main and side reactions kinetics

The kinetics of methanol to formaldehyde oxidation over iron molybdates catalysts has been studied. Data were obtained in integral reactor conditions and the rates of the products formation were computed by differentiation of experimental data yield versus contact time. Catalytic tests were performed over stoichiometric (Mo/Fe=1.5 at. ratio) and Mo rich (Mo/Fe=3 at. ratio) catalysts prepared by coprecipitation and sol-gel like techniques. The Mo/Fe atomic ratio of the catalysts seems to have a null effect on the methanol to formaldehyde kinetics what is coherent with the attribution of the active phase to the Fe2(MoO4)3, the main phase of the tested catalysts. A direct correlation between the CO formation and the Mo/Fe atomic ratio was not inferred mainly due to the different degrees of reduction of the tested catalysts (Fe2+/Fe3+), but the Mo/Fe ratio plays an important role on the consecutive oxidation of formaldehyde to CO thus controlling catalyst selectivity.

Activity, Selectivity and Initial Degradation of Iron Molybdate in the Oxidative Dehydrogenation of Ethanol

AbstractIron molybdate catalysts are applied in the industrial FormOx process to produce formaldehyde by oxidative dehydrogenation (ODH) of methanol. Only few studies are available about the applicability of iron molybdate catalysts for the ODH of ethanol to produce acetaldehyde. Herein, an iron molybdate synthesized by co‐precipitation (p) and an iron molybdate prepared by a ball‐milling solid‐state synthesis (bm) are applied as ethanol ODH catalysts. Both materials show attractive acetaldehyde selectivites of >90 % (280 °C: p‐Fe2(MoO4)3: YAcH=90.3 %; bm‐Fe2(MoO4)3: YAcH=60.4 %) and a stable performance. The bulk composition and crystal structure could be confirmed by various characterization techniques and is maintained during ethanol ODH. XPS reveals an enrichment of Mo on the catalyst surface which is slightly decreasing after the catalytic tests. This observation could be a first sign of long‐term deactivation like known from methanol ODH. Comparing the performance of both materials, p‐Fe2(MoO4)3 shows higher activity and aldehyde selectivity. We propose the higher Mo/Fe surface ratio and the lower acidity to be the reasons for these differences.

Iron molybdate catalysts synthesised via dicarboxylate decomposition for the partial oxidation of methanol to formaldehyde

Iron molybdate catalysts were prepared using a sol gel route with malonic acid and oxalic acid and their performance for the selective oxidation of methanol to formaldehyde was evaluated.

Acrolein production by oxidative coupling of alcohols over zinc and cobalt aluminate spinels enlightened by adsorption calorimetry study

This work reports on the production of acrolein by oxidative coupling of biosourced alcohols (methanol and ethanol). This reaction is performed in a two-step process using iron molybdate catalyst to produce formaldehyde and acetaldehyde and zinc and cobalt aluminate spinels, with or without vacancies, to perform the cross-aldolization of aldehydes. In this study, adsorption microcalorimetry of NH3 and SO2 was used to determine the acid-base properties of the catalysts. The catalytic activity was monitored and correlated to catalysts acid-base properties. Moreover, to enlighten the behavior of reactants over the surface and the mechanism of the reaction, an extensive adsorption microcalorimetric study using acetaldehyde and formaldehyde as probe molecules was performed to determine molar enthalpies, molar entropies, thermokinetic parameters but also energy spectra. This broad calorimetric study provided a better understanding of the reaction and allowed to confirm the preferential adsorption of formaldehyde compared to acetaldehyde thus justifying the absence of crotonaldehyde in the products of the catalytic reaction.

A Review and Experimental Revisit of Alternative Catalysts for Selective Oxidation of Methanol to Formaldehyde

The selective oxidation of methanol to formaldehyde is a growing million-dollar industry, and has been commercial for close to a century. The Formox process, which is the largest production process today, utilizes an iron molybdate catalyst, which is highly selective, but has a short lifetime of 6 months due to volatilization of the active molybdenum oxide. Improvements of the process’s lifetime is, thus, desirable. This paper provides an overview of the efforts reported in the scientific literature to find alternative catalysts for the Formox process and critically assess these alternatives for their industrial potential. The catalysts can be grouped into three main categories: Mo containing, V containing, and those not containing Mo or V. Furthermore, selected interesting catalysts were synthesized, tested for their performance in the title reaction, and the results critically compared with previously published results. Lastly, an outlook on the progress for finding new catalytic materials is provided as well as suggestions for the future focus of Formox catalyst research.

Highly Stable Apatite Supported Molybdenum Oxide Catalysts for Selective Oxidation of Methanol to Formaldehyde: Structure, Activity and Stability

AbstractMolybdenum oxide (5 to 20 wt.%) supported on calcium or strontium hydroxyapatite were investigated as catalysts for selective oxidation of methanol to formaldehyde. These catalysts were both active and selective, with a maximum yield achieved at 95 % conversion and 96 % selectivity. The main byproducts were CO (3.2 %) and dimethyl ether (DME, 0.7 %). The catalytic performance of the catalysts was measured for up to 600 h at 350 °C. Compared to an industrial iron molybdate catalyst, the hydroxyapatite based catalysts deactivated slower. The active species were found to be a surface layer of MoOx on the hydroxyapatite support, while excess molybdenum formed crystalline (Ca/Sr)MoO4, acting as a reservoir replenishing surface MoOx lost by volatilization with methanol. The excess molybdenum in the form of (Ca/Sr)MoO4 was found to volatilize significantly slower than the excess MoO3 in iron molybdate catalysts. The combination of high activity and selectivity with low rate of Mo volatilization makes this class of catalysts interesting for industrial production of formaldehyde.

Methanol to Formaldehyde: An Overview of Surface Studies and Performance of an Iron Molybdate Catalyst

Formaldehyde is a primary chemical in the manufacturing of various consumer products. It is synthesized via partial oxidation of methanol using a mixed oxide iron molybdate catalyst (Fe2(MoO4)3–MoO3). This is one of the standard energy-efficient processes. The mixed oxide iron molybdate catalyst is an attractive commercial catalyst for converting methanol to formaldehyde. However, a detailed phase analysis of each oxide phase and a complete understanding of the catalyst formulation and deactivation studies is required. It is crucial to correctly formulate each oxide phase and influence the synthesis methods precisely. A better tradeoff between support and catalyst and oxygen revival on the catalyst surface is vital to enhance the catalyst’s selectivity, stability, and lifetime. This review presents recent advances on iron molybdate’s catalytic behaviour for formaldehyde production—a deep recognition of the catalyst and its critical role in the processes are highlighted. Finally, the conclusion and prospects are presented at the end.

Hydrodeoxygenation (HDO) of oleochemical waste oil into paraffins using iron molybdate (Fe-Mo-O) catalyst

The hydrodeoxygenation (HDO) of 1-dodecanol (C12 fatty alcohol) in oleochemical waste oil was investigated in a continuous fixed-bed reactor using Fe-Mo-O catalyst between 200-300°C, 10-20 bars, and at fixed GHSV of 5121 hr -1. The characterization of the product components was analyzed by GC-MS and further quantified by GC-FID to evaluate the effect of temperature and pressure on the HDO of 1-dodecanol to paraffin such as dodecane and lower carbon number of hydrocarbons. The reaction temperature is the most critical operating parameter that affects the performance of the HDO reaction. Conversion of 1-dodecanol increased up to 98.5% with increasing reaction temperature, while at 250°C, the dodecane selectivity was the highest. Two distinct HDO pathways were distinguished: dehydration-hydrogenation (Path 1) and dehydrogenation-decarbonylation/decarboxylation (Path 2). The high temperature and low pressure of the HDO promoted Path 2 route to produce paraffin with a lower carbon number from the reactant. The selectivity of dodecane was low, probably due to the cracking process that occurred at 300°C. The conversion of 1-dodecanol decreased with the increasing pressure, while dodecane’s production rate follows the reverse trend of the conversion. High pressure of HDO promoted Path 1 route due to the higher selectivity of dodecane. In conclusion, the optimal temperature and pressure for HDO of oleochemical waste oil over Fe-Mo-O catalyst are 250°C and 20 bars, which gave the highest conversion towards dodecane and C12 paraffin.

Modified iron-molybdate catalysts with various metal oxides by a mechanochemical method: enhanced formaldehyde yield in methanol partial oxidation

A mechanochemical method was employed to prepare modified iron molybdate catalysts with various metal salts as precursors. The physicochemical properties of the iron molybdate catalysts were characterized, and their performances in catalyzing the reaction from methanol to formaldehyde (HCHO) were evaluated. Iron molybdate catalysts doped with Co(NO3)2 · 6H2Oand Al(NO3)3 · 9H2O resulted in high HCHO yields. Compared with a commercial catalyst, the HCHO yields in the reaction with the modified catalyst at an optimal Co/Mo molar ratio reached 97.37%. According to chemical state analysis, the formation of CoO and the efficient decrease in the MoO3 sublimation rate could be important factors enhancing the HCHO yield in reactions catalyzed with iron molybdate doped with different Co/Mo mole ratios.

Facial synthesis of ferric molybdate (Fe2(MoO4)3) nanoparticle and its efficiency for biodiesel synthesis via oleic acid esterification

Catalytic esterification of fatty acids, such as oleic acid, is one of the most important chemical pathways for producing biofuels, and attention is now focused on looking for heterogeneous catalysts to avoid the defects of homogeneous catalysts. With this approach, an efficient Lewis acidic catalyst, Fe 2 (MoO 4 ) 3 , was successfully prepared with definite crystalline morphology to catalyze the esterification reaction of oleic acid together with ethanol. Under optimum conditions (70 °C, 9 alcohol: 1 oleic molar ratio and 3% catalyst amount), the catalytic conversion of oleic acid to biodiesel was 90.5% and 92.5 in case of 5% catalyst, indicating that the combined catalyst has good catalytic activity. The kinetic study of ethanol–oleic acid esterification was conducted, as the kinetic study follows first-order kinetics with the calculated activation energy E a = 25.21 kJ mol − 1 and pre-exponential factor A 0 = 2449.16 min − 1 . Thermodynamic showed a positive value of Δ H, Δ S, and Δ G (endothermic reaction). • The iron–molybdate catalysts Fe 2 (MoO 4 ) 3 was prepared using a sol–gel process. • The esterification reaction of oleic acid and ethanol using Fe 2 (MoO 4 ) 3 . • The catalyst showed high catalytic activity at 70 C with O/E molar ratio is 1:9. • The catalyst exhibited high efficiency and reusability up to six times.

Sustainable acrolein production from bio-alcohols on spinel catalysts: Influence of magnesium substitution by various transition metals (Fe, Zn, Co, Cu, Mn)

Acrolein is a widely used intermediate of synthesis for value-added compounds in a number of domains of application. This work reports on the sustainable synthesis of acrolein by oxidative coupling of bio-alcohols, which constitutes a very promising alternative to fossil fuel-based production. The synthesis is performed in two sequential reactors, using an iron molybdate catalyst for oxidation and then a magnesium aluminate spinel where magnesium is partly or totally substituted by transition metals (Fe, Zn, Co, Cu, Mn) as a catalyst for cross-aldolization. The acid-base properties of the latter catalysts were determined using SO2 and NH3 adsorption microcalorimetry. Adsorption microcalorimetry was also used to study the adsorption properties of the reactants, with formaldehyde, acetaldehyde and propionaldehyde as probe molecules, and was complemented by a FT-IR investigation of reactant adsorption in order to better understand the mechanisms of adsorption and reaction. Acrolein production was found to be correlated to the ionic radius of the transition metals used in the catalysts, indicating that electronic effects are likely a factor influencing the acrolein production.

Effects of iron precursors on the structure and catalytic performance of iron molybdate prepared by mechanochemical route for methanol to formaldehyde

Abstract Mechanochemical synthesis has been applied for many novel material preparations and gained more and more attention due to green and high-efficiency recently. In order to explore the influences of iron precursors on structure and performance of iron molybdate catalyst prepared by mechanochemical route, three typical and cheap iron precursors have been used in preparation of iron molybdate catalyst. Many characterization methods have been employed to obtain the physical and chemical properties of iron molybdate catalyst. Results indicate that iron precursors have the significant impact on the phase composition, crystal morphology and catalytic performance in the conversion of methanol to formaldehyde. It is hard to regulate the phase composition by changing Mo/Fe mole ratios for Fe2(SO4)3 as iron precursor. In addition, as for Fe2(SO4)3, the formaldehyde yield is lower than that from iron molybdate catalyst prepared with Fe(NO3)3·9H2O due to the reduction in Fe2(MoO4)3 phase as active phase. Based on mechanochemical and coprecipitation method, the solvent water could be a key factor for the formation of MoO3 and Fe2(MoO4) for FeCl3·6H2O and Fe2(SO4)3 as precursors. Iron molybdate catalyst prepared with Fe(NO3)3·9H2O by mechanochemical route, shows the best methanol conversion and formaldehyde yield in this reaction.

Alkali Earth Metal Molybdates as Catalysts for the Selective Oxidation of Methanol to Formaldehyde—Selectivity, Activity, and Stability

Alkali earth metal molybdates (MMoO4, M = Mg, Ca, Sr, and Ba) were investigated as catalysts for the selective oxidation of methanol to formaldehyde in the search for more stable alternatives to the current industrial iron molybdate catalyst. The catalysts were prepared by either sol-gel synthesis or co-precipitation with both stoichiometric ratio (Mo:M = 1.0) and 10 mol% to 20 mol% excess Mo (Mo:M = 1.1 to 1.2). The catalysts were characterized by X-ray diffraction (XRD), nitrogen physisorption, Raman spectroscopy, temperature programmed desorption of CO2 (CO2-TPD), and inductively coupled plasma (ICP). The catalytic performance of the catalysts was measured in a lab-scale, packed bed reactor setup by continuous operation for up to 100 h on stream at 400 °C. Initial selectivities towards formaldehyde of above 97% were achieved for all samples with excess molybdenum oxide at MeOH conversions between 5% and 75%. Dimethyl ether (DME) and dimethoxymethane (DMM) were the main byproducts, but CO (0.1%–2.1%) and CO2 (0.1%–0.4%) were also detected. It was found that excess molybdenum oxide evaporated from all the catalysts under operating conditions within 10 to 100 h on stream. No molybdenum evaporation past the point of stoichiometry was detected.

Structural dynamics of an iron molybdate catalyst under redox cycling conditions studied with in situ multi edge XAS and XRD.

The structural dynamics and phase transformations of an iron molybdate catalyst with excess molybdenum trioxide (Mo/Fe = 2.0) were studied during redox cycling of the catalyst using in situ multi-edge X-ray absorption spectroscopy (XAS) at the Mo K-edge (transmission mode) and Fe K-edge (fluorescence mode) in combination with X-ray diffraction (XRD). X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analysis showed that heating under reducing conditions with methanol up to 400 °C produced MoO2 and FeMoO4. Linear combination fitting (LCF) analysis showed that iron was reduced completely, while molybdenum remained partly oxidized (60% as Mo(vi)). Complementary in situ XRD also supported the phase transformation due to reduction of Fe2(MoO4)3 and MoO3 to FeMoO4 and MoO2. Subsequent heating under oxidative conditions from 200 to 500 °C transformed the catalyst into its initial state via Fe2O3 and extra MoO3 as intermediate phases. This underlines the segregation and iron enrichment during redox cycling. MoO3 volatilization, observed under industrial reaction conditions of a methanol and oxygen containing atmosphere, causes this segregation to be permanent. Complete regeneration could only be achieved at 500 °C, which is significantly higher than industrial reaction temperatures. Overall, multi edge in situ XAS along with complementary XRD was found to be an ideal tool for tracing the different amorphous and crystalline phases present during redox cycling of the catalyst.

Stability of Iron-Molybdate Catalysts for Selective Oxidation of Methanol to Formaldehyde: Influence of Preparation Method

Iron molybdate/molybdenum oxide catalysts with varying content of Mo (Mo/Fe = 1.6 and 2.0) were synthesized by a mild hydrothermal method and structurally characterized by XRD, XPS, Raman spectroscopy, SEM–EDX, BET and ICP-OES. The stability of the prepared catalysts in selective oxidation of methanol to formaldehyde was investigated by catalytic activity measurements for up to 100 h on stream in a laboratory fixed-bed reactor (5% MeOH, 10% O2 in N2, temp. = 380–407 °C). Excess MoO3 present in the catalyst volatilized under reaction conditions, which lead to an initial loss of activity. Interestingly, the structure of the excess MoO3 significantly affected the stability of the catalyst. By using low temperature hydrothermal synthesis, catalysts with the thermodynamically metastable hexagonal h-MoO3 phase was synthesized, which yielded relatively large crystals (2–10 µm), with correspondingly low surface area to volume ratio. The rate of volatilization of MoO3 from these crystals was comparatively low, which stabilized the catalysts. It was furthermore shown that heat-treatment of a spent catalyst, subject to significant depletion of MoO3, reactivated the catalyst, likely due to migration of Mo from the bulk of the iron molybdate crystals to the surface region. Fe2(MoO4)3/MoO3 catalysts for selective oxidation of methanol were synthesized by hydrothermal synthesis forming large hexagonal-MoO3 crystals. Significantly lower rate of catalyst deactivation due to volatilization of MoO3 under reaction conditions was observed for the large h-MoO3 compared to smaller crystals of thermodynamically stable α-MoO3.

Operando XAS/XRD and Raman Spectroscopic Study of Structural Changes of the Iron Molybdate Catalyst during Selective Oxidation of Methanol

AbstractThe structural changes of an iron molybdate/molybdenum oxide (Mo/Fe=2.0) catalyst for the selective oxidation of methanol to formaldehyde were studied using combined operando X‐ray absorption spectroscopy (XAS) and X‐ray diffraction (XRD) as well as operando Raman spectroscopy. Under operating conditions, the Mo K‐edge XANES spectra showed a transition from a mixture of α‐MoO3 and Fe2(MoO4)3 towards only Fe2(MoO4)3. XRD and Raman spectroscopy also showed disappearance of the α‐MoO3 phase with time on stream. The results evidenced that the α‐MoO3 component evaporated completely, while the Fe2(MoO4)3 component remained stable. This was linked to a decrease in catalytic activity. Further studies unraveled that the rate of α‐MoO3 evaporation increased with increasing MeOH concentration, decreasing O2 concentration and increasing temperature. The simultaneous measurements of catalytic activity and spectroscopy allowed to derive a structure‐activity relationship showing that α‐MoO3 evaporation needs to be prevented to optimize MoO3‐based catalysts for selective oxidation of methanol.

Difference in adsorption properties of Fe(III), Mo(VI) oxides and Fe(III) molybdate as a cause of high selectivity of methanol oxidation on iron molybdate catalyst

Difference in adsorption properties of Fe(III), Mo(VI) oxides and Fe(III) molybdate as a cause of high selectivity of methanol oxidation on iron molybdate catalyst

Modeling of molybdenum transport and pressure drop increase in fixed bed reactors used for selective oxidation of methanol to formaldehyde using iron molybdate catalysts

A dynamic model was developed for a single reactor tube, in which methanol oxidation to formaldehyde over an iron molybdate/molybdenum oxide catalyst takes place simultaneously with transport of MoO3 from the catalyst through the reactor. A previously developed dynamic 1D mathematical model for a single ring-shaped cylindrical catalyst pellet, in which volatilization of MoO3 takes place, was implemented in the reactor model. Known axial profiles in a pilot scale reactor with respect to MeOH and H2O concentration and temperature were used as input to the model. MeOH forms volatile Mo-species with solid MoO3 in the catalyst pellets, which diffuses to the bulk gas phase and is transported through the reactor, leading to MoO3 depleted pellets. Volatilization of MoO3 from the pellets occur at the inlet of the reactor. As MeOH is oxidized down the reactor, the volatile Mo-species decomposes via the reverse reaction that formed them. Deposition of MoO3 downstream in the reactor decreases the void space between the catalyst pellets leading to increased pressure drop. The hydraulic diameter of the catalyst pellets and the porosity of the deposited MoO3 were fitted to experimental data obtained in a pilot plant unit containing a single reactor tube. Furthermore, the model was used to simulate a tube under industrial conditions for up to two years (feed composition 8.4% MeOH, 4% H2O, 10% O2 in N2, bed length = 100 cm and a temperature of 190–346 °C). Finally, two cases where catalyst pellets with no excess MoO3 or shaped as filled cylinders are used in the initial 21 cm of the catalyst bed were simulated. The simulations show that this significantly decreases the rate at which the pressure drop increases. This model is a first step towards a useful tool to predict MoO3 transport, pressure drop increase and estimation of process life time at varying reaction conditions.

Green and rapid synthesis of iron molybdate catalyst by mechanochemistry and their catalytic performance for the oxidation of methanol to formaldehyde

Iron molybdenum catalyst was successfully prepared by mechanochemistry method under ball grinding mill. The physico-chemical properties of iron molybdenum catalyst synthesized with different mechanochemical experiment conditions including the number ratio of large balls to small balls, the quality ratio of agate balls to raw materials and ball milling time, have been investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman, N2 physical absorption, X-ray photoelectron spectroscopy (XPS), etc. The catalytic performance of iron molybdenum samples in the oxidation of methanol to formaldehyde also has been evaluated in a self-built fixed-bed micro-reactor. Results demonstrate that iron molybdenum catalyst prepared by small balls shows the highest formaldehyde yield (95.2%) than the rest samples. With the increasing quality ratio of balls to materials, the phase composition of MoO3/Fe2(MoO4)3 from iron molybdenum catalyst firstly goes down and then goes up, which has close correlation with the formaldehyde selectivity. The catalyst samples prepared under ball grinding time of 30–120 min, show the typical iron molybdenum catalyst diffraction peaks, and the further extension of ball grinding time has no obvious influence on the crystal structure of iron molybdenum catalyst. The samples prepared with ball grinding time of 120 min, has the highest formaldehyde selectivity (97.56%), which could be due to the increasing content of Fe2(MoO4)3 and the improved uniformity of particles with the increasing ball grinding time.