Thermal stability of (0.15–0.35 wt%) rhodium on low-loaded ceria-supported rhodium catalysts

The thermal stability of rhodium on ceric oxides submitted to high-temperature reduction (773–1173 K) and aging in air at 1173 K was studied by high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy and benzene hydrogenation. Catalysts were prepared by anionic exchange of rhodium chloro complexes on both high (144 m² g^-1) and low specific surface area ceria (6 m² g^-1). In reducing conditions, both types of catalysts stabilized 5 nm metal rhodium particles. Calcination in air at 1173 K pointed out the interest of rhodium exchanged over a low surface precalcined ceria. In that case, all the metal remained at the surface of CeO₂ after calcination whereas 50% of the initial supported metal was buried into sintered ceria for the catalyst prepared from the high-surface CeO₂. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

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.     

Initial carbon–carbon bond formation during synthesis gas conversion to higher alcohols on K–Cu–Mg5CeOx catalysts

Isotopic tracer studies of alcohol synthesis pathways using ¹³CO/H₂/¹²CH₃OH mixtures have shown that ethanol is formed predominantly by direct reactions of ¹³CO on Cu_0.5Mg₅CeO_x promoted with K at short residence times, without significant involvement of the ¹²CH₃OH present in the feed. The observed decrease in ¹³C content in ethanol with increasing residence time is caused by reverse aldol reactions of higher alcohols, which contain lower ¹³C contents because of the significant involvement of ¹²CH₃OH in their formation, and by reactions of methyl formate with ¹²CH₃OH-derived species. The effects of residence time on the ¹³C distribution within reaction products and on the rates of formation of ethanol, methyl formate, and methyl acetate are consistent with the intermediate role of methyl formate and methyl acetate in ethanol formation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Efficient and selective conversion of hexose to 5-hydroxymethylfurfural with tin–zirconium-containing heterogeneous catalysts

Efficient and selective production of 5-hydroxymethylfurfural (HMF) from the hexose is achieved in the presence of heterogeneous Sn-based catalyst. The mixed SnO₂–ZrO₂ is prepared from zirconium n-propoxide and different metal Sn precursors using Sol–gel method. The sulfated SnO₂–ZrO₂ (SO₄^2 −/SnO₂–ZrO₂) is obtained by the impregnation method with H₂SO₄ solution. All catalytic materials are detected with XRD, TG, SEM, TEM and BET techniques in order to reveal the physical properties and structures of these materials. When these materials were used in the dehydration of fructose, it was found that the suitable ratio of Sn/Zr is 0.5, and the catalytic activity of SO₄^2 −/SnO₂–ZrO₂ is higher than that of SnO₂–ZrO₂ where more than 75.0% yield of HMF was obtained for 2.5 h at 120 °C. The effects of reaction temperature and reaction time were also investigated. Moreover, the recycling experiment of catalyst shows that the catalytic activity can be almost kept unchanged after being used five times.     

磷酸铝AlPO4-5分子筛的研究进展

介绍了AlPO4-5磷酸铝分子筛合成进展.综述AlPO4-5作为载体在催化加氢反应方面的研究进展,在择形催化,催化氧化,脂化反应以及其吸附性能在分离领域的应用.对今后发展的方向和应用前景进行了可行性展望.     

Adsorption and reaction of CO on vanadium oxide–Pd(111) “inverse” model catalysts: an HREELS study

The room temperature adsorption and reaction of CO on Pd(111) surfaces decorated with submonolayer coverages of vanadium oxide – so-called inverse model catalysts – have been studied by high-resolution electron energy loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS). The HREELS surface phonon spectra of the V oxide phases have been measured and used to monitor the changes in the oxide as a result of the interaction with CO. The intramolecular C–O stretching frequency of CO adsorbed on the V-oxide/Pd(111) surfaces displays two vibrational loss components as a function of CO coverage as it has been observed on the clean Pd(111) surface. The relative intensities of the two vibrational features as a function of V oxide coverage however suggest that the balance of CO adsorption sites is modified as compared to clean Pd(111) by the presence of the V oxide–Pd phase boundary. Preferential population of high coordination adsorption sites by CO in the vicinity of the oxide–metal interface is proposed. The analysis of the V oxide phonon spectra indicates that adsorbed CO partially reduces the V oxide at the boundaries of the oxide islands to the Pd metal. The reduction of V oxide by CO is dependent on the oxygen content of the V oxide phase. The reduction of V oxide is confirmed by the XPS V 2p core level shifts.     

AlPO4-5分子筛大晶体的合成

研究了阳离子表面活性剂十六烷基三甲基溴化铵(CTAB)对水热晶化法合成AlPO4-5大晶体的影响.实验过程中以拟薄水铝石为铝源,正磷酸为磷源,三正丙胺为模板剂,在晶化原液中添加一定量的氢氟酸、异丙醇以及CTAB.通过改变CTAB的添加量,在晶化温度180℃,晶化时间24.0 h的条件下,合成出了大而完美的六角棱柱形的AlPO4-5晶体,晶体尺寸可达890 μm×120 μm.实验结果表明,晶体的大小及完美程度与表面活性剂中烷基三甲基铵阳离子与晶化液中的负电荷基团[AlF6]3-和[PF6]1-的吸附作用有关.     

Preparation and characterisation of titania-supported vanadium–phosphorus oxide catalysts

A series of vanadium–phosphorus oxides (mainly with V P ) supported on pigmentary anatase (10 m² g^-1) has been prepared using aqueous NH₄VO₃ and (NH₄)H₂PO₄ solutions, with loadings up to 11.3 wt%, equivalent to about 12.7 monolayers. Characterisation by X-ray diffraction, laser Raman spectroscopy, X-ray photoelectron spectroscopy and temperature-programmed reduction suggests that the main phase present at loadings below about 10 wt% is an amorphous V–P oxide which exists chiefly as blocks of disordered material. The presence of small amounts of crystalline -VOPO₄ and of V₂O₅ is indicated at the highest loadings, especially when and V P ratios are used. The two materials having the lowest loadings are active for methanol oxidation at 473–533 K, and show high selectivity to formaldehyde. This revised version was published online in June 2006 with corrections to the Cover Date.     

Synthesis,structure and behavior of new polycaprolactam copolymers based on poly (ethylene oxide)–poly (propylene oxide)–poly (ethylene oxide) macroactivators derived from Pluronic block copolymers

Novel series of poly (CL–co–Pluronic) polymers were successfully synthesized in situ by ring-opening polymerization (ROP) of ε-caprolactam (ε-CL). The copolymerization was activated by new type macroactivators (MAs) based on hydroxyl-terminated poly (ethylene oxide)-poly (propylene oxide)-poly (ethylene oxide) [PEO-PPO-PEO] triblock copolymers, known under the trade name Pluronic^®. Toluene-2,4-diisocyanate (TDI) was used to obtain the isocyanate-terminated Pluronic prepolymers. The corresponding MAs were synthesized in situ with an N-carbamoyllactam structure. As an initiator of the copolymerization processes was used sodium lactamate (NaCL). To confirm the influence over the copolymerization process, microstructure, composition and molecular weight of the polymeric products two new types MAs based on Pluronic (P123 and F68) have been varied from 2 to 10 wt.% (vs. the monomer ε-CL). The structure of the both Pluronic based macroactivators (MAs) and accordingly obtained two series poly (CL-co-Pluronic) polymers were confirmed by ¹H NMR and FT-IR analyses. Additionally, the structure, molecular weight, physicomechanical behavior, thermal stability and morphology of the new synthesized poly (CL–co–Pluronic) polymers have been investigated by Differential Scanning Calorimetry (DSC), Wide-Angle X-ray Diffraction (WAXD), Thermogravimetric Analysis (TGA) and Scanning Electron Microscopy (SEM) analysis.     

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.     

Hydration of ethene over W–P mixed metal oxide catalysts

W–P mixed metal oxide catalysts are active and selective for the gas-phase hydration of ethene to ethanol. The activity and selectivity of this catalytic reaction depend on the W/P atomic ratio. However, ethene conversion slightly decreases at higher W/(W + P) atomic ratio. The selectivity for ethanol increases with the W/P atomic ratio and reaches the highest value (92%) at W_0.81P_0.19O_x. The W_0.81P_0.19O_x catalyst is less active than the conventional H₃PO₄/SiO₂ catalyst, but the activity is maintained for a long time without the supply of any catalyst components. The reaction temperature does not affect substantially the rate of ethene hydration over the W_0.81P_0.19O_x catalyst. The H₂O/ethene molar ratio of 0.4 is the most appropriate for both reaction rate and selectivity. The active species of W–P mixed metal oxide are amorphous. But there is Keggin structure of W–P oxide species (PW₁₂O₄₀ ³⁻) in the presence of steam. And the species are the active sites for the hydration of ethene, confirmed by in situ Raman spectroscopy. This revised version was published online in July 2006 with corrections to the Cover Date.     

Alumina-supported manganese- and manganese–palladium oxide catalysts for VOCs combustion

Alumina-supported manganese- and palladium–manganese oxide catalysts were prepared and tested in the combustion of formaldehyde. Total combustion of formaldehyde/methanol was achieved at 220 °C over a 18.2% Mn/Al₂O₃ catalyst. This temperature decreased either to 90 or 80 upon adding 0.1% or 0.4% Pd, respectively, to the base 18.2% Mn/Al₂O₃ catalyst. The combined use of X-ray diffraction, temperature-programmed reduction and photoelectron spectroscopy (XPS) techniques revealed that a Mn⁴⁺/Mn³⁺ oxide(s) and PdO_x species are present on the surface of the fresh catalysts and remain along the catalytic reaction.     

Steam reforming of methanol over copper–manganese spinel oxide catalysts

The steam reforming of methanol was studied over a series of copper–manganese spinel oxide catalysts prepared with the urea–nitrate combustion method. All catalysts showed high activity towards H₂ production with high selectivity. Synthesis parameters affected catalyst properties and, among the catalysts tested, the one prepared with 75% excess of urea and an atomic ratio Cu/(Cu + Mn) = 0.30 showed the highest activity. The results show that formation of the spinel Cu_xMn_3 − xO₄ phase in the oxidized catalysts is responsible for the high activity. Cu–Mn catalysts were found to be superior to CuO–CeO₂ catalysts prepared with the same technique.     

Metal–support interactions on rhodium model catalysts

Metal–support interactions on supported rhodium catalysts were studied by using specially prepared Rh/TiO₂/Mo model systems. For their characterization and the analysis of modifications due to various heat treatments several surface analytical methods were applied: low-energy ion scattering, X-ray photoemission spectroscopy and thermal desorption spectroscopy. Heating in ultrahigh vacuum to 670 K leads to Rh agglomeration followed (above 720 K) by encapsulation including the formation of reduced titanium oxide species. These morphological and chemisorption changes are reversible upon reoxidation and low-temperature reduction and thus exhibit the characteristic features of strong metal–support interactions. For the effective mechanism a reaction is suggested that involves oxygen chemisorption on the Rh clusters and partial reduction of the surrounding support oxide.     

Kinetics studies of nano-structured cobalt–manganese oxide catalysts in Fischer–Tropsch synthesis

The nano-structured cobalt/manganese oxide catalyst was prepared by thermal decomposition of [Co(NH₃)₄CO₃]MnO₄ precursor, and was tested for the Fischer–Tropsch reaction (hydrocarbon forming) in a fixed-bed micro-reactor. Experimental conditions were varied as follow: reaction pressure 1–10 bar, H₂/CO feed ratio of 1–2 and space velocity of 3600 h^?1 at the temperature range of 463.15–523.15 K. On the basis of carbide and/or enolic mechanisms and Langmuir–Hinshelwood–Hougen–Watson (LHHW) type rate equations, 30 kinetic expressions for CO consumption were tested and interaction between adsorption HCO and dissociated adsorption hydrogen as the controlling step gave the most plausible kinetic model. The kinetic parameters were estimated with non-linear regression method and the activation energy was 80.63 kJ/mol for optimal kinetic model. Kinetic results indicated that in Fischer–Tropsch synthesis (FTS) rate expression, the rate constant (k) has been increased by decreasing the catalyst particle size. The catalyst characterization was carried out using different methods including powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and Brunauer–Emmett–Teller (BET) surface area measurements.     

Oxidative coupling of methane at 600 °C on CaNiK oxide catalysts

We are herewith reporting new data which modifies, explains and extends our earlier work on this subject. Previous observations of low CH₄ conversion and high selectivity to C₂ hydrocarbons were erroneous due to the formation of bulk CaCO₃ in the catalyst. The carbonate was detected by powder X-ray diffraction and was shown to accumulate during the reaction and decompose during regeneration. Catalytic runs which incorporated an internal standard revealed a deficit in the C balance consistent with carbonate formation. Actual CH₄ conversions were 20% with 15% selectivity to hydrocarbons. The effect of steam in promoting coupling over combustion was affirmed.     

Effects of particle size on catalytic conversion of ethanol to propylene over H-ZSM-5 catalysts—Smaller is better

In this study, H-ZSM-5 catalysts with different particle sizes were prepared by adding different amount of water to the starting gel before crystallization. These H-ZSM-5 catalysts were characterized by scanning electron microscopy, N₂ adsorption/desorption measurements, X-ray fluorescence, X-ray diffraction, and temperature programmed desorption of ammonia. It was found that the olefin (ethylene, propylene and butene) yield decreased with the increase in the particle size of the catalysts. The paraffin (ethane, propane and butane) yield trended to increase with the increase of particle size. The C₅ + products yield increased considerably with the increase of particle size. The results implied that the formation of the C₅ + products and hydrogen was inhibited over the smaller particle size catalyst, furthermore the formation of paraffin decreased. So the propylene yield could be increased over smaller H-ZSM-5 catalyst.