Controlled partial oxidation of methane to methanol/formaldehyde over Mo–V–Cr–Bi–Si oxide catalysts

The non-catalytic and catalytic oxidations of CH₄ over Mo–V–Cr–Bi–Si oxide catalysts were investigated in a tubular reactor and the catalysts were characterized by XRD, XPS and TPR. Contents of Bi in the catalysts influenced the combination of Mo–V–Bi–O species and, consequently, influenced the TPR reduction temperature of the catalysts. The catalysts exhibited more selective production of methanol when the TPR reduction peaks shifted to lower temperature.     

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.     

Cation effects in the oxidative coupling of methane on phosphate catalysts

The conversion of methane and the selectivities to the various products have been measured at 700 and 775 °C on a variety of phosphates of La(III), Zr(IV), V(V), Cr(III), Mn(III), Mn(II), Fe(III), Co(II), Ni(II), Cu(II), Zn(II), Al(III), B(III), Pb(II), Bi(III) and Sm(III) in the presence and absence of carbon tetrachloride. The conversions reach as high as 30 and 49% at 700 and 775 °C, respectively, with methane and oxygen at partial pressures of 200 and 25 Torr, respectively. The highest C₂₊ selectivities (61 and 82%, respectively) were obtained for lead(II) phosphate at 700 and 775 °C, respectively. In general the conversions and C₂₊ selectivities are enhanced on addition of carbon tetrachloride (1.1 Torr) to the feedstream, although there are notable exceptions. Significantly high selectivities to formaldehyde are observed with a number of the catalysts, in particular 32% with boron(III)phosphate.     

Effect of surface species on activity and selectivity of MoO3/SiO2 catalysts in partial oxidation of methane to formaldehyde

The effect of the nature of surface species on the activity and selectivity of MoO₃/SiO₂ catalysts has been investigated for the partial oxidation of methane to formaldehyde. Characterization techniques including BET surface area, ambient and in situ Raman spectroscopy, X-ray photoelectron spectroscopy, and temperature-programmed reduction were used in conjunction with steady-state reaction studies to relate the presence of different surface species to the activity and selectivity of the catalyst. Results of these experiments indicate the presence of a highly dispersed silicomolybdic species with terminal Mo=O sites appearing at lower MoO₃ loadings. As the weight loading increases, these sites are transformed into polymolybdate species, forming more Mo-O-Mo bridging sites at the expense of Mo=O sites. At high weight loadings, crystalline MoO₃ begins to form. The abundance of the Mo=O sites is believed to affect activity and selectivity in the partial oxidation of methane to formaldehyde.     

Low-temperature catalytic combustion of methane over MnO x –CeO2 mixed oxide catalysts: Effect of preparation method

The effect of preparation method on MnO _x –CeO₂ mixed oxide catalysts for methane combustion at low temperature was investigated by means of BET, XRD, XPS, H₂-TPR techniques and methane oxidation reaction. The catalysts were prepared by the conventional coprecipitation, plasma and modified coprecipitation methods, respectively. It was found that the catalyst prepared by modified coprecipitation was the most active, over which methane conversion reached 90% at a temperature as low as 390 °C. The XRD results showed the preparation methods had no effect on the solid solution structure of MnO _x –CeO₂ catalysts. More Mn⁴⁺ and richer lattice oxygen were found on the surface of the modified coprecipitation prepared catalyst with the help of XPS analysis, and its reduction and BET surface area were remarkably promoted. These factors could be responsible for its higher activity for methane combustion at low temperature.     

Selective Oxidation of Methanol to Formaldehyde Using Modified Iron-Molybdate Catalysts

Methanol selective oxidation to formaldehyde over a modified Fe-Mo catalyst with two different stoichiometric (Mo/Fe atomic ratio = 1.5 and 3.0) was studied experimentally in a fixed bed reactor over a wide range of reaction conditions. The physicochemical characterization of the prepared catalysts provides evidence that Fe₂(MoO₄)₃ is in fact the active phase of the catalyst. The experimental results of conversion of methanol and selectivity towards formaldehyde for various residence times were studied. The results showed that as the residence time increases the yield of formaldehyde decreases. Selectivity of formaldehyde decreases with increase in residence time. This result is attributable to subsequent oxidation of formaldehyde to carbon monoxide due to longer residence time.     

Synthesis and characterization of nanocrystalline Mo–V–W–Fe–O mixed oxide catalyst and its performance in selective methanol oxidation

A mixed oxide catalyst containing Mo, V, W and Fe with the composition of 63, 23, 09 and 06 wt% respectively for the selective oxidation of the methanol to formaldehyde is in reported in this paper for the first time. The characterization of the catalyst was done using BET surface analysis, X-ray diffraction (XRD), Infrared spectroscopy (FTIR), Scanning electron microscopy (SEM) and Energy dispersive X-ray (EDX). The mixed oxide after calcination at 673 K in N₂ which was subjected for the thermal activation in N₂flow at 813 K was used for the methanol selective oxidation. The thermal treatment shows enhanced catalytic performance. Thermal activation of the nanocrystalline Mo_0.63V₂₃W_0.09Fe₀.₀₆O _x precursor oxide in nitrogen atmospheres induces partial crystallization of a Mo₅O₁₄-type oxide only in a narrow temperature range up to 813 K. XRD showed that the thermally activated mixed oxide consists of a mixture of a majority of crystalline Mo₅O₁₄-type oxide and of small amounts of crystalline MoO₃-type and MoO₂-type oxides. The structural analysis suggests that the improvement of the catalytic performance of the MoVWFe oxide catalyst in the selective oxidation of methanol is related to the formation of the catalytic active site such as Mo₅O₁₄-type mixed oxide.     

Low‐temperature oxidation of methane to form formaldehyde: role of Fe and Mo on Fe–Mo/SiO2 catalysts, and their synergistic effects

The partial oxidation of methane with molecular oxygen was performed on Fe–Mo/SiO₂ catalysts. Iron was loaded on the Mo/SiO₂ catalyst by chemical vapor deposition of Fe₃(CO)₁₂. The catalyst showed good low‐temperature activities at 723–823 K. Formaldehyde was a major condensable liquid product on the prepared catalyst. There were synergistic effects between iron and molybdenum in Fe–Mo/SiO₂ catalysts for the production of formaldehyde from the methane partial oxidation. The activation energy of Mo/SiO₂ decreased with the addition of iron and approached that of the Fe/SiO₂. The concentration of isolated molybdenum species (the peak at 1148 K in TPR experiments) decreased as the ion concentration increased and had a linear relationship with the selectivity of methane to formaldehyde. The role of Fe and Mo in the Fe–Mo/SiO₂ catalyst was proposed: Fe is the center for the C–H activation to generate reaction intermediates, and Mo is the one for the transformation of intermediates into formaldehyde. Those phenomena were predominant below 775 K. This revised version was published online in July 2006 with corrections to the Cover Date.     

Correlation between the electrochemical properties of colloidal oxides and supported oxides on metallic substrates

Mixed oxide electrodes of Ti(IV) and Cr(III) were prepared by calcining Cr(OH)₃ layers deposited on metallic titanium supports. This treatment produced a mixed oxide film of TiO₂–Cr₂O₃ covered with a layer of pure Cr₂O₃. The electrochemical response (cyclic voltammetry) shows the presence of two or three oxidation peaks depending on the electrode preparation conditions. One peak may be interpreted as the oxidation of Cr(III) to Cr(VI) species and the appearance of other peaks is due to the presence of chromium atoms in oxidation states higher than (III). The results of chemical analyses, electrophoretic mobilities and acid-base potentiometric titrations on calcined Cr(OH)₃ powders shows that the calcination step in air produced the decomposition of the Cr(III) hydroxide to the Cr(VI) oxide. Soluble Cr(VI) compounds were found in equilibrium with the suspended powder oxide which markedly affected the shape of the titrations curves. From the amount of Cr(VI) present in solution it was possible to correct the experimental ₀-pH curves. These corrected data indicated that Cr(VI) soluble species adsorbed at the Cr₂O₃/electrolyte interface.     

Structure and activity of Sn-Mo-O catalysts: partial oxidation of methanol

This work deals with the catalytic behavior of a series of Sn-Mo-O catalysts in the partial oxidation of methanol. The catalysts of different Sn:Mo ratios, were prepared by co-precipitation and they were investigated by means of dynamic experiments, test reactions (methanol–formaldehyde partial oxidation, isopropyl alcohol decomposition) and physico-chemical characterization (XRD, BET, TPD of NH₃, TPR, XPS and EPR).It was observed that interdispersion between MoO₃ and SnO₂ favors a superficial architecture in which the Sn-Mo interaction plays a major role modifying the reactivity of the lattice oxygens and the reducibility of Mo ions and, therefore, the catalytic behavior. The partial oxidation of methanol induces a reordering of the catalyst structural organization leading to a Mo surface enrichment. The absence of chemical shifts for Sn (XPS) suggests that the O–Mo bond is mainly responsible for the methanol reaction.     

Methane Partial Oxidation by Unsupported and Silica Supported Iron Phosphate Catalysts: Influence of Reaction Conditions and Co-Feeding of Water on Activity and Selectivity

The partial oxidation of methane to methanol and formaldehyde by molecular oxygen has been investigated over crystalline and silica supported FePO₄at a pressure of 1 atm and in the temperature range of 723–973 K. The quartz phase of FePO₄, as well as silica supported FePO₄prepared by impregnation (5 wt%), were examined in a continuous flow reactor. Experiments carried out over FePO₄show high selectivity to formaldehyde at low conversion and suggest that formaldehyde is the primary reaction product, but selectivity decreased rapidly as conversion was increased. The highest space-time yield of formaldehyde observed for this catalyst was 59 g/kg_cat-h. Above 5% methane conversion, carbon oxides were the only products. For silica-supported FePO₄, formaldehyde selectivity did not fall off rapidly, exhibiting a formaldehyde selectivity of 12% at about 10% conversion (STY=285 g/kg_cat-h). Quantifiable yields of methanol were observed at very low conversion levels, i.e. below 3% (STY=11 g/kg_cat-h). Addition of steam (up to 0.1 atm partial pressure) into the feed stream increased the selectivity to methanol (∼25 g/kg cat/h with up to 3% selectivity) and formaldehyde (∼487 g/kg cat/h with up to 94% selectivity) for the silica-supported FePO₄catalyst. Steam addition had little effect on catalyst activity. Characterization results indicate the presence of FePO₄, as well as fivefold coordinate Fe³⁺in silica supported catalyst samples, and this species is proposed to be responsible for methane activation. After catalysis in the presence of steam, the fivefold coordinate iron is present, but a significant fraction of the FePO₄has been reduced to Fe₂P₂O₇. Enhanced selectivity in the presence of steam is attributed in part to the ease of the reversible formation of surface hydroxyl groups (P-OH) from pyrophosphate (P-O-P) groups.     

Effect of high temperature treatment on the properties of fe-mo oxide catalysts

In this study the structural and compositional changes that Fe-Mo oxide catalysts undergo during treatment at elevated temperatures were investigated. Commercial and laboratory prepared catalysts were examined. The changes in catalyst properties were determined by low temperature nitrogen adsorption, scanning electron microscopy, X-ray diffraction, X-ray fluorescence and thermogravimetric analysis. The activities of the catalysts for the oxidation of methanol in air to formaldehyde were measured using an integral bed reactor. The results showed that treatment at elevated temperatures resulted in the growth and segregation of Fe₂(Mo0₄)₃ and Mo0₃ crystals resulting in a loss of specific surface area. The changes in specific activity can be explained in terms of molybdenum depletion from a molybdenum rich iron molybdate phase.     

Laser Raman spectroscopy (LRS) and time differential perturbed angular correlation (TDPAC) study of surface species on Mo/SiO2 and Mo,Na/SiO2. Their role in the partial oxidation of methane

Silica-supported molybdenum (1.6 and 5.0 wt%) and molybdenum (5 wt%)-sodium (0.4 wt%) catalysts have been characterized by laser Raman spectroscopy (LRS), time differential perturbed angular correlation (TDPAC), temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS). The presence of different molybdenum species was correlated with activity and selectivity to formaldehyde during the methane partial oxidation reaction. The main species identified on the Mo(5.0 wt%) /SiO₂ surface were MoO₃ and monomeric species with a single Mo=O terminal bond. The pre-impregnation of the silica support with sodium strongly diminishes the Mo=O concentration due to the formation of Na₂Mo₂O₇ species and tetrahedral monomers with a high degree of symmetry. As a result of these modifications, both methane conversion and formaldehyde formation are strongly inhibited. The combination of LRS and TDPAC techniques resulted in a powerful tool for the identification and quantification of the molybdenum species present on the surface of a silica support. This revised version was published online in July 2006 with corrections to the Cover Date.     

The dynamic states of silica-supported metal oxide catalysts during methanol oxidation

The in situ Raman spectra of silica-supported metal oxide catalysts (containing surface metal oxide species of V, Nb, Cr, Mo, W and Re) were measured during methanol oxidation. Stable surface methoxy species were found to form via reaction with the surface Si---OH groups for all the catalysts investigated. The surface Si---OH groups were also titrated by the surface metal oxide species and the surface concentration of Si---OH groups decreases with increasing metal oxide loading. The stable surface M---OCH₃ were only found for the V₂O₅/SiO₂ catalysts. The surface metal oxide species were all influenced by the methanol oxidation reaction. The surface rhenium oxide species were removed from the silica surface and the surface molybdenum oxide species were partially agglomerated to crystalline β-MoO₃ particles by the formation of Re-methoxy and Mo-methoxy complexes. The surface niobium oxide and tungsten oxide species were partially reducing by the net reducing methanol oxidation environment. In situ Raman spectra for the CrO₃/SiO₂ catalysts could not be obtained due to the formation of reduced chromium oxide species during methanol oxidation which gave rise to sample fluorescence. The in situ Raman observations provided a fundamental basis for understanding the selectivity patterns of the silica-supported metal oxide catalysts during methanol oxidation. However, the mechanism by which the silica support ligands activate the redox properties of the surface metal oxide species is not completely understood.     

Preparation, Characterisation and Catalytic Behaviour of a New TeVMoO Crystalline Phase

TeM_xMo_1.7O mixed oxides (M = V and/or Nb; x = 0-1.7) have been prepared by calcination of the corresponding salts at 600 °C in an atmosphere of N₂. A new crystalline phase, with a Te/V/Mo atomic ratio of 1/0.2-1.5/1.7, has been isolated and characterised by XRD and IR spectroscopy. This phase is observed in the TeVMo or TeVNbMo mixed oxide but not in the TeNbMo mixed oxide. The new crystalline phase shows an XRD pattern similar to Sb₄Mo₁₀O₃₁ and probably corresponds to the M1 phase recently proposed by Aouine et al. (Chem. Commun. 1180, 2001) to be present in the active and selective MoVTeNbO catalysts. Although these catalysts present a very low activity in the propane oxidation, they are active and selective in the oxidation of propene to acrolein and/or acrylic acid. However, the product distribution depends on the catalyst composition. Acrolein or acrylic acid can be selectively obtained from propene on Nb-free or Nb-containing TeVMo catalysts, respectively. The presence of both V and Nb, in addition to Mo and Te, appears to be important in the formation of acrylic acid from propene.     

Selective oxidation of methane to formaldehyde on supported molybdate catalysts

A series of silica-supported molybdena catalysts with variable molybdenum content has been prepared and tested in the selective oxidation of methane to formaldehyde. The nature of the supported oxide phases has been studied by Fourier transform IR of chemisorbed NO molecule, X-ray photoelectron spectroscopy, UV-Visible reflectance and X-ray diffraction techniques. The results show that molybdenum oxide is highly dispersed on the silica at low molybdenum concentrations where two-dimensional polymolybdates are developed. Three-dimensional MoO₃ crystals grow in the region of high molybdenum concentrations. The analysis of the combined data suggests that there is a close relationship between methane conversion and formaldehyde selectivity and the presence of highly dispersed polymolybdate structures on the silica surface.     

Crystalline Mo-V–W-mixed Oxide with Orthorhombic and Trigonal Structures as Highly Efficient Oxidation Catalysts of Acrolein to Acrylic Acid

We succeeded in introducing W in Mo₃VO_x with keeping the orthorhombic, trigonal, and amorphous structures. Synthesized crystalline Mo(W)₃VO_x with orthorhombic and trigonal structures, both of which possess heptagonal channels, showed catalytic activity for gas-phase selective acrolein oxidation to acrylic acid superior to amorphous Mo(W)₃VO_x and to tetragonal Mo₃VO_x. The results strongly suggest that the crystalline Mo(W)₃VO_x with orthorhombic and trigonal structures are real active phase of industrial acrolein oxidation catalysts based on Mo, W, and V oxides. Furthermore, we found that the resulting W-containing catalyst showed less-dependency of water partial pressure in the reactant feed on the acrolein conversion.     

Effect of the catalyst supports on the removal of perchloroethylene (PCE) over chromium oxide catalysts

The oxidation of perchloroethylene (PCE) was investigated over chromium oxide catalysts supported on TiO₂, Al₂O₃, SiO₂, SiO₂–Al₂O₃ and activated carbon. The phase of chromium oxide on the catalyst surface is critical for the oxidation of PCE. The catalytic activity of PCE removal enhances as the formation of Cr(VI) species on the catalyst surface increases. The surface area and the type of the catalyst supports were also essential for high performance in the PCE oxidation. In addition, the structure of Cr(VI) on the catalyst surface also plays an important role for the decomposition of PCE. The polymerized Cr(VI) mainly formed by the interaction of metals with the support is the active reaction site for the present reaction system. CrO_x/TiO₂ reveals the strongest PCE removal activity among the catalysts examined in the present study. This revised version was published online in July 2006 with corrections to the Cover Date.     

Catalytic Performance of the Sb–V Mixed Oxide on Sb–V–O/SiO2 Catalysts in Methane Selective Oxidation to Formaldehyde

Sb–V–O/SiO₂ catalysts were prepared and investigated in methane selective oxidation with O₂ as oxidant. Sb–V–O/SiO₂ catalysts are active and selective in methane selective oxidation. The formaldehyde yield obtained on Sb–V–O/SiO₂ catalysts is clearly higher than that for VO_X/SiO₂ and SbO_X/SiO₂ catalysts. A one-pass formaldehyde yield up to 3% was obtained on Sb–V–O/SiO₂ catalysts at 650 °C. XRD and UV Raman studies showed that the phase of Sb–V mixed oxide on Sb–V–O/SiO₂ catalysts transformed with decreasing Sb/V ratio from Sb₂VO₅ to SbVO₄/VSb_1-XO_4-1.5X phase. The Sb–V mixed oxide in Sb₂VO₅ phase is more active and selective than that in SbVO₄/VSb_1-XO_4-1.5X phase.