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.     

Novel sol–gel‐derived Ag/SiO2–Al2O3 catalysts for highly selective oxidation of methanol to formaldehyde

Novel Ag/SiO₂–Al₂O₃ catalysts with low silver content prepared by the sol–gel method exhibit excellent catalytic properties in the catalytic oxidation of methanol to formaldehyde. The silver content was as low as 2% and the yield of formaldehyde was achieved as 90.3%, which is 16% higher than that of pumice‐supported silver and even 5–6% higher than that of a commercial electrolytic silver catalyst. XRD, XPS and SEM results reveal that all silver was present as Ag⁺ before catalytic reaction and was partially reduced to the metallic state after the reaction. It was also found that silver was aggregated on the surface after its reduction. This revised version was published online in July 2006 with corrections to the Cover Date.     

Dynamic states of V‐oxide species: reducibility and performance for methane oxidation on V2O5/SiO2 catalysts as a function of coverage

Temperature‐programmed in situ Raman spectroscopy is used to understand the effect of surface vanadia coverage on the structure, reducibility and performance for the oxidation of methane on V₂O₅/SiO₂ catalysts. The vanadia coverage on silica has no effect on its structure below its dispersion‐limit loading (“monolayer” coverage); however, the interactions among surface vanadia species under reducing conditions become increasingly important. This interaction appears to operate through the sharing of oxygen sites facilitating the reduction, but it does not alter the total reducibility. The probability for this interaction to take place increases with vanadium oxide surface coverage. It is therefore expected that under reaction conditions the catalyst with higher vanadia coverage would have a greater capacity to release oxygen. This would increase the activity per vanadium site. This revised version was published online in July 2006 with corrections to the Cover Date.     

Synthesis of 3-fluorobenzaldehyde by gas-phase selective oxidation on Fe–Mo oxide/boralite catalysts

The gas-phase selective synthesis of 3-fluorobenzaldehyde from 3-fluorotoluene over bulk iron molybdate and Fe–Mo oxide in a host boralite sample is described. The latter samples, prepared by adding Fe and Mo by chemical vapor deposition to the boralite, show high selectivity in oxidation, making yields of 3-fluorobenzaldehyde of over 40% possible. The pretreatment of the zeolite to eliminate extra-framework boron improves behavior, and secondary post-addition of molybdenum by CVD to increase the Mo/Fe ratio in the catalyst has a similar enhancing effect. The behavior of Fe–Mo/boralite samples proves significantly better, in terms of both specific activity (per mass of active phase) and selectivity, than bulk Fe₂(MoO₄)₃, but at high conversion lower selectivities are found probably due to the presence of limitations in the backdiffusion of 3-fluorobenzaldehyde. This revised version was published online in August 2006 with corrections to the Cover Date.     

Comparative studies on direct conversion of methane to methanol/formaldehyde over La–Co–O and ZrO2 supported molybdenum oxide catalysts

The selective oxidation of methane with molecular oxygen over MoO_x/La–Co–O and MoO_x/ZrO₂ catalysts to methanol/formaldehyde has been investigated in a specially designed high-pressure continuous-flow reactor. The properties of the catalysts, such as crystal phase, structure, reducibility, ion oxidation state, surface composition and the specific surface area have been characterized with the use of XRD, LRS, TPR, XPS and BET methods. MoO_x/La–Co–O catalysts showed high selectivity to methanol formation while MoO_x/ZrO₂ revealed the property for the formation of formaldehyde in the selective oxidation of methane. 7 wt MoO_x/La–Co–O catalyst gave 6.7 methanol yield (ca. 60 methanol selectivity) at 420°C and 4.2 MPa. On the other hand, the maximal yield of formaldehyde ca. 4 (47.8 formaldehyde selectivity) was obtained over 12wt MoO_x/ZrO₂ catalyst at 400 °C and 5.0MPa. 7MoO_x/La–Co–O catalyst showed higher modified H₂-consumption than 12MoO_x/ZrO₂ catalyst. The reducibility and the O^–/O^2– ratio of the catalysts may play important roles on the catalytic performance. The proper reducibility and the O^–/O^2– ratio enhanced the production of methanol in selective oxidation of methane. [MoO₄]^2– species in MoO_x/ZrO₂ catalysts enable selective oxidation of methane to formaldehyde.     

Surface structure and metathesis activity of photoreduced allyl‐based Mo/SiO2 catalysts

XPS and metathesis activity studies were performed on oxidic allylbased Mo/SiO₂ catalysts (0.5 wt% Mo) and the oxidic catalyst following photoreduction to various extents. XPS results and average oxidation state measurements from the amount of CO₂ formed during photoreduction, indicated that controlled photoreduction of the oxidic catalyst, in CO atmosphere, produces a mixture of Mo⁶⁺ and Mo⁴⁺ oxidation states with Mo⁴⁺ in abundances ranging from 0 to 100%. Propene metathesis activity studies were performed on the oxidic and photoreduced allylbased Mo/SiO₂ catalysts. The results indicated that metathesis activity appears on photoreduction and increases linearly with increasing abundance of Mo⁴⁺ present on the catalyst. The linear relationship between the % propene conversion and the % Mo⁴⁺ present on the catalyst is consistent with the proposed uniformity of the Mo species and with the hypothesis that the active site (or active site precursor) is associated with Mo⁴⁺.     

Partial oxidation of methane to syngas over Co/MgO catalysts. Is it low temperature?

Co/MgO catalysts with high Co-loading (>28 wt%) are able to initiate the reaction of methane with oxygen at temperatures around 500 °C. High conversions of methane ( 70%) and very high selectivities for hydrogen and carbon monoxide ( 90%) are obtained at very high reactant gas space velocities (10⁵–10⁶ h^–1). The temperature of the catalyst at the conditions of partial oxidation of methane to form syngas was found to be extremely high (1200–1300 °C); it is about 600–850 °C higher than that previously reported by others. At these temperatures, high temperature homogeneous reactions may prevail. It is suggested that combustion of methane to carbon dioxide occurs on the catalyst with major heat release and that methane and water, respectively methane and carbon dioxide are reformed thermally in an endothermic reaction leading to syngas.     

Low‐temperature selective oxidation of hydrogen sulfide into elemental sulfur on a NiS2/SiC catalyst

Nickel sulfide supported on SiC exhibits a very high activity and selectivity for the direct oxidation of H₂S into elemental sulfur at low reaction temperature (60°C). The presence of water on the catalyst surface could explain the absence of deactivation even at high sulfur loading of the surface. The chemical inertness of the SiC support allowed any detrimental reactions between the active phase and the support itself to be avoided.     

Influence of H2, CO and CO2 co-feeding on the catalytic activity of Rh/Ti–SiO2 during the partial oxidation of methane

The influence of the addition of 1, 2 or 5 vol.% of CO, H₂ or CO₂ to the feed during the partial oxidation of methane (POM) was studied over a Rh/Ti–SiO₂ catalyst. The addition of H₂ or CO decreases the conversion and syngas selectivity. This decrease of performance seems to be related to a higher reduction of the catalyst due to the co-feeding of H₂ or CO. The addition of CO₂ also appears unfavourable to the production of hydrogen but increases the CO yield. A combination of the dry reforming and the reverse water–gas shift reactions is suggested to explain the observed modifications in the product yields.     

Ruthenium supported on new TiO2–ZrO2 systems as catalysts for the partial oxidation of methane

The partial oxidation of methane is studied at 673–873 K over new Ru-based catalysts supported on TiO₂–ZrO₂ with different TiO₂ content. Supports were prepared by a sol–gel method, and RuCl₃ and RuNO(NO₃)₃ were used as ruthenium precursors to prepare the catalysts (1–2 wt% Ru). The effect of the reaction temperature on the catalytic behavior is analyzed, along with the support composition and the Ru precursor used.     

Regeneration behaviors of Fe/Si‐2 and Fe–Mn/Si‐2 catalysts for C2H6 dehydrogenation with CO2 to C2H4

The catalytic performance of Fe/Si‐2 and Fe–Mn/Si‐2 catalysts for conversion of C₂H₆ with CO₂ to C₂H₄ was examined in a continuous‐flow and fixed‐bed reactor. The results show that the Fe–Mn/Si‐2 catalyst exhibits much better reaction activity and selectivity to C₂H₄ than those of the Fe/Si‐2 catalyst. Furthermore, the coking–decoking behaviors of these catalysts were studied through TG. The catalytic performances of the catalysts after regeneration for conversion of C₂H₆ or dilute C₂H₆ in FCC off‐gas with CO₂ to C₂H₄ were also examined. The results show that both activity and selectivity of the Fe–Mn/Si‐2 catalyst after regeneration reached the same level as those of the fresh catalyst, whereas it is difficult for the Fe/Si‐2 catalyst to refresh its reaction behavior after regeneration. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of hydrogen on methane conversion to hydrocarbons in “one‐step” reaction under non‐oxidative conditions at low temperature over Pd–Co/SiO2 catalysts prepared by the sol/gel method

Methane conversion to higher hydrocarbons in a “one‐step” process under non‐oxidative conditions at low temperature was here first introduced and investigated over Co–Pd/SiO₂ catalysts at 250°C as a function of hydrogen concentration in helium and of catalyst composition. A maximum in the production of C₂₊ hydrocarbons including aromatics (benzene and toluene) was observed at 1.3 vol% H₂/He mixture in which one pulse of methane was introduced. Additional hydrogenation with the same H₂/He mixture at 400°C was efficient to remove the larger hydrocarbon fragments already existing on the surface. On pure Pd/SiO₂ the one‐step process is not so efficient as on cobalt‐rich samples, but in the latter case the hydrocarbon removal is the most efficient during high‐temperature hydrogenation. It was found that methane conversion in the one‐step process is at least 2.5 times greater than that measured in the “two‐step” process and, in some cases, 80% of the methane introduced is converted to larger hydrocarbons. The results are discussed in terms of the hydrogen coverage ensuring the optimum hydrogen content in the surface CH_x species leading to chain growth. This revised version was published online in July 2006 with corrections to the Cover Date.     

Comparative study on hydrogen-assisted “one-step” methane conversion over Pd–Co/SiO2 and Pt–Co/NaY catalysts

In our laboratory, methane conversion to higher hydrocarbons in “one-step” process under non-oxidative condition at low temperature was first introduced and investigated over Pd–Co/SiO₂ prepared by sol/gel method [Guczi et al., Catal. Lett. 54 (1998) 33] and over Pt–Co/NaY [Guczi et al., Stud. Surf. Sci. Catal. 119 (1998) 295] bimetallic catalysts. It was found that methane conversion in one-step process is at least 2.5 times higher than that measured in “two-step” process on the same catalysts. In the present work, the two-step and one-step processes are compared. It has been established that in one-step process when methane dissociation occurs in the presence of hydrogen containing helium, not only the production of higher hydrocarbons increases but also the selectivity is shifted towards larger molecules. Palladium–cobalt system proved to be more efficient than the corresponding platinum–cobalt catalysts.     

Oxidation and reduction effects of propane–oxygen on Pd–chlorine/alumina catalysts

Pd–chloride precursor salt was used to prepare Pd/Al₂O₃ catalysts. TPSR measurements showed three distinct reactions for the oxidation of propane on palladium surface under excess of hydrocarbon: complete oxidation, steam reforming and propane hydrogenolysis. Propane oxidation on palladium catalysts was related to the Pd²⁺ sites observed on Pd/Al₂O₃ through infrared of adsorbed carbon monoxide. In fresh catalysts reduced by H₂, the IR spectra showed the linear and bridge adsorbed CO species on the Pd⁰ surface. After propane reaction, a new band at 2130 cm^-1 related to CO adsorption on Pd²⁺ species was noted. Carbon monoxide species adsorbed on Pd⁰ were also observed in all samples after reaction. Our results suggest surface ratios of Pd⁰/PdO during the propane oxidation. On the other hand, time on stream conversions of the complete oxidation of propane were affected by either the water generated during the reaction or added as a reactant at 10 vol%. The water generated by the reaction helped to eliminate chlorine residues in the form of oxychloride species leading to an increasing of the activity. However, the presence of water into the reaction mixture caused a strong decreasing of the activity. The inhibition mechanism of propane oxidation in the presence of water consisted in the dissociative adsorption of water on palladium sites with the possible formation of palladium hydroxide (Pd–OH) at the surface, diminishing the number of active surface sites. Dynamic fluctuations into the reaction conditions supported the idea that a pseudo‐equilibrium adsorption–desorption of water was reached. After water removal or increasing in the reaction temperature the equilibrium was shifted to the direction of OH–Pd decomposition. This behavior suggests that the inhibitory effect of water is a reversible phenomenon, being a function of the amount of water and the reaction temperature. This revised version was published online in July 2006 with corrections to the Cover Date.     

Preparation,characterization, and kinetic evaluation of dendrimer-derived bimetallic Pt–Ru/SiO2 catalysts

A series of silica-supported Pt, Ru, and Pt–Ru catalysts has been synthesized using dendrimer–metal nanocomposite (DMN) precursors prepared by both co- and sequential complexation with metal salts. The catalysts have been characterized by several techniques, including electron microscopy, temperature-programmed titration of adsorbed oxygen, and X-ray diffraction. Liquid-phase selective hydrogenation of 3,4-epoxy-1-butene (EpB) was used as a probe reaction to evaluate their catalytic performance. The bimetallic catalyst prepared by the co-complexation method exhibits a superior catalytic activity compared to the sequential one, and is much more active than a conventional catalyst prepared by incipient wetness. The activity enhancement is attributed to a bifunctional performance of the PtRu alloy sites created, based on a strong correlation between turnover frequencies, and both the alloy compositions and metal surface site distributions. In addition, the co-complexation catalyst is selective toward crotonaldehyde, suggesting that this reaction pathway is favored on the PtRu sites.     

Partial oxidation of methane to synthesis gas over α-Al2O3-supported bimetallic Pt–Co catalysts

The partial oxidation of methane to synthesis gas was studied at atmospheric pressure and in the temperature range of 550–800°C over -Al₂O₃-supported bimetallic Pt–Co, and monometallic Pt and Co catalysts, respectively. Both methane conversion and CO selectivity over a bimetallic Pt_0.5Co₁ catalyst were higher than those over monometallic Pt_0.5 and Co₁ catalysts. Furthermore, the addition of platinum in Pt–Co bimetallic catalysts effectively improved their resistance to carbon deposition with no coking occurring on Pt_0.5Co₁ during 80 h reaction. The FTIR study of CO adsorption observed only linearly bonded CO on bimetallic Pt–Co catalysts. TPR and XPS showed enhanced formation of a cobalt surface phase (CSP) in bimetallic Pt–Co catalysts. The origins of the good coking resistivity of bimetallic Pt–Co catalysts were discussed.     

Synthesis of menthols from citral on Ni/SiO2–Al2O3 catalysts

The one-pot synthesis of menthols from citral was studied on Ni/SiO₂–AlO₃ catalysts containing 3.6%, 8.8% and 11.4% Ni. The yield of menthols increased with the amount of Ni up to 94% on Ni(11.4%)/SiO₂–AlO₃, reflecting the diminution of byproducts formation via acid-catalyzed reactions. The sample deactivation was studied by performing two consecutive catalytic tests. Results showed that Ni(11.4%)/SiO₂–AlO₃ was a stable, active, and highly selective catalyst because it contained the appropriate density and strength of bifunctional acid/Ni⁰ active sites to efficiently promote the hydrogenation/isomerization pathway involved in the reaction network while avoiding coke formation.     

Gas‐phase pinacol conversion on AlPO4, γ‐Al2O3 and SiO2 catalysts

Pinacol (2,3‐dimethyl‐2,3‐butanediol) conversion over AlPO₄ (Al/P = 1) and γ‐Al₂O₃ catalysts proceeded in two parallel reaction pathways with formation of 2,3‐dimethyl‐1,3‐butadiene (by 1,2‐elimination) and 3,3‐dimethyl‐2‐butanone (by rearrangement), the latter being the main reaction product. The activity was in accordance with the surface acidity data as measured versus cyclohexene skeletal isomerization reaction. Thus, AlPO₄ showed the highest activity (almost total conversion at 523 K). The 1,2‐elimination/rearrangement ratio depended on the type of catalyst used and diene formation increased with reaction temperature. Moreover, pinacolone was a reaction intermediate for diene production. This revised version was published online in July 2006 with corrections to the Cover Date.