Selective oxidation of methane with air over silica catalysts

Partial oxidation of methane by oxygen to form formaldehyde, carbon oxides, and C₂ products (ethane and ethene) has been studied over silica catalyst supports (fumed Cabosil and Grace 636 silica gel) in the 630–780 °C temperature range under ambient pressure. The silica catalysts exhibit high space time yields (at low conversions) for methane partial oxidation to formaldehyde, and the C₂ hydrocarbons were found to be parallel products with formaldehyde. Short residence times enhanced both the C₂ hydrocarbons and formaldehyde selectivities over the carbon oxides even within the differential reactor regime at 780 °C. This suggests that the formaldehyde did not originate from methyl radicals, but rather from methoxy complexes formed upon the direct chemisorption of methane at the silica surface at high temperature. Very high formaldehyde space time yields (e.g., 812 g/kg cat h at the gas hourly space velocity = 560 000 (NTP)/kg cat h) could be obtained over the silica gel catalyst at 780 °C with a methane/air mixture of 1.5/1. These yields greatly surpass those reported for silicas earlier, as well as those over many other catalysts. Low CO₂ yields were observed under these reaction conditions, and the selectivities to formaldehyde and C₂ hydrocarbons were 28.0 and 38.8%, respectively, at a methane conversion of 0.7%. A reaction mechanism was proposed for the methane activation over the silica surface based on the present studies, which can explain the product distribution patterns (specifically the parallel formation of formaldehyde and C₂ hydrocarbons).     

Catalytic behavior of YBa2Cu3O7-x in the partial oxidation of methanol to formaldehyde

The partial oxidation of methanol to formaldehyde was studied over YBa₂Cu₃O_7-x catalyst in a flow reactor. The structural change of YBa₂Cu₃O_7-x before and after the reaction was measured by means of XRD and iodometric titration method. The catalytic characteristics of YBa₂Cu₃O_7-x for the partial oxidation of methanol to formaldehyde was due to copper ions. It was found that Cu⁺² was responsible for the higher selectivity for formaldehyde.     

On the Synergy Effect in MoO3–Fe2(MoO4)3 Catalysts for Methanol Oxidation to Formaldehyde

Methanol oxidation to formaldehyde was studied over a series of Fe–Mo–O catalysts with various Mo/Fe atomic ratio and the end compositions Fe₂O₃ and MoO₃. The activity data show that the specific activity passes through a maximum with increase of the Mo content and is the highest for Fe₂(MoO₄)₃. The selectivity to formaldehyde, on the other hand, increases with the Mo content in the catalyst. A synergy effect is observed in that a catalyst with the Mo/Fe ratio 2.2 is almost as active as Fe₂(MoO₄)₃ and as selective as MoO₃. Imaging of a MoO₃/Fe₂(MoO₄)₃ catalyst by SEM and TEM shows that the two phases form separate crystals, and HRTEM reveals the presence of an amorphous overlayer on the Fe₂(MoO₄)₃ crystals. EDS line-scan analysis in STEM mode demonstrates that the Mo/Fe ratio in the amorphous layer is ~2.1 in the fresh catalyst and ~1.7 in the aged catalyst. The enrichment of Mo at the catalyst surface is confirmed by XPS data. Raman spectra give evidence for the Mo in the amorphous material being in octahedral coordination, which is in contrast to the crystalline Fe₂(MoO₄)₃ bulk structure where Mo has tetrahedral coordination. X-ray diffraction (XRD) analysis gives no support for the formation of a defective molybdate bulk structure. The results presented give strong support for the Mo rich amorphous structure being observed on the Fe₂(MoO₄)₃ crystal surfaces being the active phase for methanol oxidation to formaldehyde.     

Production of hydrogen over bimetallic Pt–Ni/δ-Al2O3: II. Indirect partial oxidation of LPG

Indirect partial oxidation (IPOX) of a 75:25 propane:n-butane mixture, which was used as a model for LPG, was studied over the bimetallic 0.2 wt%Pt–15 wt%Ni/δ-Al₂O₃ catalyst in 623–743 K temperature range. The effects of steam to carbon ratio (S/C), carbon to oxygen ratio(C/O₂) and residence time (W/F (g cat-h/mol LPG)) on the hydrogen production activity, selectivity and product distribution were studied in detail. The results are compared with the results obtained in the IPOX of pure propane. An Increasing Temperature Program (ITP) was applied during all experiments and the results showed that the presence of n-butane in the feed enhances hydrogen production activity and selectivity. Considering the well established distribution network of LPG and the superior performance of the bimetallic Pt–Ni catalyst in the IPOX of LPG, Pt–Ni system seems a very promising catalyst alternative to be used in commercial fuel processors.     

Continuous gas–liquid aerobic oxidation of toluene catalyzed by [T(p-Cl)PPFe]2O in a series of three stirred tank reactors

The liquid-phase catalytic aerobic oxidation of toluene by [T(p-Cl)PPFe]₂O was studied in a series of three stirred tank reactors. The effects of operation mode (including semi-batch and continuous operation), reaction temperature, catalyst concentration, average residence time, and air flow rate on the oxidation process were examined. The experimental results showed that continuous oxidation had no advantage over the total yield and selectivity of benzaldehyde and benzyl alcohol in comparison with semi-batch oxidation. And the reaction temperature was the most significant factor influencing on continuous oxidation of toluene. It is also found that adopting sequentially decreased temperature in the three series reactors could improve the yield and selectivity of benzaldehyde and benzyl alcohol in this process. Under which at the higher conversion of toluene, the total yield to benzaldehyde and benzyl alcohol increased 17.05% or 43.62% respectively in comparison with adopting sequentially increased or same temperature in the three series reactors.     

Kinetics of formaldehyde oxidation on a vanadia-titania catalyst

The heterogeneous catalytic oxidation of formaldehyde in the gas phase may be considered as an alternative to the multistep liquid-phase synthesis of formic acid. Monolayer vanadia-titania catalysts are active and selective in the oxidation of formaldehyde to formic acid. Detailed investigation of kinetics of formaldehyde oxidation over a monolayer vanadia-titania catalyst was carried out. It was established that byproduct form via a consecutive-parallel reaction network. CO₂ results from formaldehyde oxidation via parallel pathway and from formic acid overoxidation via consecutive pathway; CO forms from formic acid via consecutive pathway. It was shown that oxygen and water accelerate formic acid formation and that water retards CO formation. Based on experimental data, a kinetic model of formaldehyde oxidation was developed. The kinetic model was used in the mathematical simulation of the formaldehyde oxidation process and in the determination of dynamic and design parameters of the reactor. Formic acid production by the gasphase oxidation of formaldehyde is unique and does not have any analogue. As opposed to conventional technologies, it is energy-saving, environmentally friendly, and technologically simple. An enlarged-scale pilot plant using this technology is under construction.     

Partial oxidation of methane to synthesis gas on a Rh/TiO2 catalyst in a fast flow porous membrane reactor

The partial oxidation of methane to synthesis gas has been studied over a 3% Rh/TiO₂ catalyst in a fixed bed and a novel membrane reactor under autothermal conditions using O₂ as oxidant. The membrane reactor allows the partial oxidation reaction to be performed without premixing the reactants reducing the risk of explosion even at low methane/oxygen ratios. The membrane reactor can operate autothermally and at millisecond residence time. Methane conversions of up to 65% with CO and H₂ selectivities of 90 and 82% respectively have been achieved. The low methane oxygen ratio and the high flow rates are the key factors to attain autothermal behavior. The most sensitive factor to attain high conversion and selectivities appears to be short contact time but high temperature. A kinetic model was used to interpret the experimental results. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

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.     

Pulse-MS study of the partial oxidation of methane over Ni/La2O3 catalyst

The CH₄ direct oxidation reaction was studied at 600°C by the pulse-MS transient method over the Ni/La₂O₃ catalyst. Over the freshly prepared catalyst (which contains NiO), the CO selectivity and CH₄ conversion increased and attained constant values as the number of CH₄/O₂ pulses increased. Over the reduced catalyst (containing Ni), as the number of CH₄/O₂ pulses increased, the CO selectivity and CH₄ conversion decreased before they reached the same constant values as over the fresh catalyst. The CO selectivity increased as the residence time of the reactants shortened, implying that CO was directly generated without the preformation of CO₂. The activation energies of CH₄ dehydrogenation in the presence and absence of oxygen have been calculated using the bond-order conservation Morse-potential approach. The results indicate (1) the direct dehydrogenation steps are more likely to occur; (2) the transient oxygen species adsorbed on-top of the metal atoms promote dehydrogenation; (3) the oxygen species adsorbed on bridge or hollow sites do not promote dehydrogenation.     

Direct conversion of methane in formaldehyde at very short residence time

To obtain new kinetic data, a quartz annular flow microreactor was chosen for the experimental investigation of the gas phase partial methane oxidation to formaldehyde under high temperature (1173–1273 K) and short residence time (20–60 ms). High formaldehyde selectivity (up to 80%) is achieved, but the yield remains low. For a single pass operation, the best HCHO experimental yield is 2.4%. The kinetic simulation using GRI-Mech 3.0 shows an overall good agreement with the experimental data, though there does exist some discrepancies for some conditions. The general pathways of the reaction show that HCHO is formed mainly from direct oxidation of methyl radical CH₃ by O₂, and partly from dehydrogenation or oxidation of methoxy radical CH₃O.     

Kinetics of 2,2,6,6‐Tetramethylpiperidine Oxidation over Sn2+ on Ion Exchange Resin

The oxidation of 2,2,6,6‐tetramethylpiperidine (TEMP) over Sn²⁺ on ion exchange resin is carried out in a batch reactor. The influence of the reactant concentrations, reaction temperature and catalyst amount is investigated. Within the temperature range studied, 303–323 K, the reaction follows the second order kinetic equation ‐r_TEMP = kC_TEMPC_H2O2, with reaction rate constant k = 2.2 × 10⁷exp (‐51200/RT) L/mol h. We propose a reaction mechanism different from that for catalyzation by tungstate. The catalyst likely forms complexes with hydroxyl radicals, keeping their concentration steady. At the same time, the catalyst also decreases the decomposition of hydrogen peroxide caused by high temperatures and the reactant 2,2,6,6‐tetramethylpiperidine itself.     

Production of hydrogen by short contact time partial oxidation and oxidative steam reforming of propane

Partial oxidation (POX) and oxidative steam reforming (OSR) of propane have been studied over Rh-impregnated alumina foams in the short contact time regime. The experiments were performed over a wide temperature range (300–1000 °C) at close to atmospheric pressure. It was found that a furnace temperature of 700 °C is optimal for the production of hydrogen for both reactions (POX and OSR) in this system. Variations in the total flow rate revealed an effect of residence time on the product distribution during both POX and OSR. The production of hydrogen was hardly affected by the residence time, but an influence of the residence time on the selectivity to all other products was observed. Hydrocarbon byproducts were increasingly formed at shorter residence times while formation of partial and complete oxidation products increased with longer residence times. The Rh foam catalyst also showed promising stability under strong oxidation conditions.     

Electrochemical effect of gold nanoparticles on Pt/α-Fe2O3/C for use in methanol oxidation in alkaline solution

A catalyst containing gold nanoparticles with Pt/α-Fe₂O₃/C was prepared by a co-precipitation method and its catalytic activity for the oxidation of methanol, formaldehyde, and formic acid in alkaline solutions was evaluated by an electrochemical method and high-performance liquid chromatography (HPLC). The addition of gold nanoparticles improved catalytic activity only for the oxidation of methanol and formaldehyde, and not for the oxidation of formic acid. HPLC analysis was performed for methanol oxidation to detect the oxidative products. In HPLC analysis, only formate anion could be detected in the electrolyte solution and the ratio of formate anion obtained to the total passed charge in Pt/nano-Au/α-Fe₂O₃/C was less than that in Pt/C, indicating that formic acid is not the final product of methanol oxidation. These results show that gold nanoparticles promoted methanol oxidation up to CO₂.     

Removal of formaldehyde from aqueous solutions via oxygen reduction using a reticulated vitreous carbon cathode cell

The removal of formaldehyde from waste streams to <0.3 ppm has been demonstrated using a cell with a reticulated vitreous carbon cathode; the formaldehyde is oxidized by hydrogen peroxide, formed at the cathode by reduction of oxygen. In most electrolytes studied (e.g. NaOH, NaCl and Na₂SO₄), the formaldehyde is oxidised only to formic acid. On the other hand, the addition of a low concentration of an iron salt (i.e. 0.5 mm), catalyses the complete oxidation to carbon dioxide. The removal of formaldehyde can be achieved in media of low ionic strength (< 10 mm) although the use of iron salts necessitates the adjustment of pH to 3 to maintain the catalyst in solution.     

Improving selectivity during methane partial oxidation by use of a membrane reactor

A reactant-swept catalytic membrane reactor for partial oxidation of methane to formaldehyde has been modeled. Kinetic parameters were taken from the literature for a V₂O₅/Sio₂ methane partial oxidation catalyst, and membrane parameters characteristic of commercially available materials were used. The models show that the selectivity for formaldehyde can be significantly improved by using a membrane reactor.     

Regeneration studies of redox catalysts

Cited advantages of circulating fluidized bed reactors (CFB) include higher selectivity and conversion together with the ability to optimize the process conditions of each vessel independently—temperature, gas partial pressure and residence time. DuPont commercialized a CFB process to produce maleic anhydride in which a vanadium pyrophosphate (VPO) was cycled between a fast bed riser and an air fed regenerator. Together with VPO, we examined two other redox catalyst systems—MoVSb (acrylic acid from propane) and FeMoO (methanol to formaldehyde).The lattice oxygen capacity of the FeMoO catalyst was about five times higher than either the VPO or MoVSb with little adsorbed carbon but a significant quantity of chemisorbed water. Above 350 °C, carbon deposition was detected and increased with increasing temperature. Carbon deposition decreased with increasing temperature for the MoVSb system and its lattice oxygen capacity was slightly higher than for VPO. The carbon deposition pattern for VPO was the opposite of the MoVSb and increased with temperature. Based on a hydrogen and carbon mass balance during the catalyst re-oxidation treatment, the molecular composition of the adsorbed species were C₄H₆ and C₃H₃—like for the VPO and MoVSb, respectively.Based on the high lattice oxygen capacity, the formaldehyde reaction appears to be ideally suited for development in a CFB. Whereas the lattice oxygen contribution of the MoVSb is equivalent to VPO, less oxygen is required to produce acrylic acid (compared to maleic anhydride) so the incentive of developing a CFB process should be greater than for butane oxidation to maleic anhydride.     

Selective oxidation of propane to oxygenates over mesoporous SBA-15-supported potassium catalysts

As a novel catalyst system for the selective oxidation of low alkanes, mesoporous SBA-15-supported potassium catalysts were firstly employed for the selective oxidation of propane to oxygenates by using molecular oxygen as oxidant. It was found, compared with bare mesoporous SBA-15, that the selectivities to the oxygenates including formaldehyde, acetaldehyde, acrolein and acetone were remarkably enhanced over K _x /SBA-15(K:Si = x:100, mol) catalysts, and the main products were acrolein and acetone. At 500 °C, the yield of the oxygenates can reach 464% over K_3.0/SBA-15, which is the highest value over SBA-15–supported potassium catalysts. The catalysts were characterized by XRD and BET techniques. The results demonstrated that the catalytic performance was strongly dependent on the potassium content of the catalysts. Furthermore, the highly dispersed potassium on the catalyst surface was shown to be important to orientate the reaction toward the production of oxygenates. The obtained results showed that mesoporous structure, uniform pore sizes and appropriate pore surface area were favorable for the selective oxidation of propane. The samples with moderate amount of potassium promoted the selectivity to the oxygenates.     

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.     

Mass transfer and influence of the local catalyst activity on the conversion in a riser reactor

Gas–solids contacting in risers has been studied based on measurements of mass transfer controlled CO oxidation over a Pt/γ-alumina catalyst, and on experimental results published by Ouyang et al. (1995) for the kinetically controlled ozone decomposition. In the present experiments, the catalyst activity was varied by mixing the active catalyst particles with similar, but inert γ-alumina particles (in ratios from 150 to 2500 m³_inert/m³_cat), whereas Ouyang and co-workers varied the operating temperature ( de 300 à to 500 K). Mass transfer controlled CO oxidation occurs at temperatures above 750 K. A negative square root dependency has been observed for the relationship between the Sherwood number and the solid hold-up. Increasing the gas velocity always improves the gas–solids contacting. The local catalyst activity appears to be an important parameter. As an important conclusion of the present work, it can be stated that at a high local activity, the conversion rate per unit volume of catalyst decreases significantly due to local depletion of reactant.