Catalytic behaviour of multiphasic oxide catalysts containing lanthanides (La,Ce, Pr,Sm, Tb) in the selective oxidation of isobutene to methacrolein

The catalytic performances of five lanthanide oxides (La₂O₃, Sm₂O₃, CeO₂, Pr₆O₁₁ and Tb₄O₇) for the selective oxidation of isobutene to methacrolein are evaluated within the framework of the remote control mechanism. Mechanical mixtures of these oxides with typical donor (Sb₂O₄) or acceptor (MoO₃) phases of spill-over oxygen were prepared and tested for their activity in the isobutene-to-methacrolein oxidation at 400°C. Amongst the five lanthanide oxides tested, only CeO₂ and Pr₆O₁₁ were found to display significant cooperation effects for the investigated reaction, with enhanced yields and selectivity for partial oxidation and concomitant decrease of CO₂ production. The fresh and used catalysts were characterized by X-ray diffractometry, and the occurrence of solid state reactions between the partner oxides outside the reaction conditions was investigated in parallel in the temperature range 400–500°C. No new phase was observed in the case of the mixtures with La₂O₃, Sm₂O₃, CeO₂ and Tb₄O₇. Account taken of the absence of any new phase in the CeO₂-MoO₃ system, it can be concluded that CeO₂ is a potential donor of spill-over oxygen. The situation in the MoO₃-Pr₆O₁₁ mixtures is more complex, owing to the generation of various praseodymium molybdates, together with the oxocarbonate Pr₂CO₅. The latter phase was shown to have no intrinsic tendency to produce methacrolein, but it seems that some of the praseodymium molybdates present in the working catalysts may exhibit noticeable catalytic properties.     

Interaction of isobutene and methacrolein with silica-supported uranium and molybdenum oxide catalysts

The adsorption of isobutene (in the temperature range 309–341K) and methacrolein (in the temperature range 296–336 K) on a Mo/(Mo+U)=0.89 catalyst were studied. The equilibrium data were analysed according to Freundlich's model. The high coverages observed at lower temperatures suggest some participation of physical adsorption in the reversible adsorption. Entropy calculations showed that these adsorbed species are best described by a mobile model which assumes bidimensional translation and two rotational degrees of freedom. Isobutene is weakly chemisorbed, producing reversible species which were identified as a π-complex formed by interaction with surface OH groups through π-electrons (infrared bands at 1655, 1615, 1470 and 1375 cm^?1), and only a small fraction of isobutene interacts strongly via lattice oxygen forming intermediate species in the oxidation reaction. However, in the presence of oxygen at 523 K a majority of the surface species are strongly held by the surface (infrared bands at 1775, 1723, 1610, 1460 and 1350 cm^?1 associated to a methacrylic complex and bands at 1590 and 1538 cm^?1 of carbonate-carboxylate species).     

甲基丙烯醛一步法制甲基丙烯酸甲酯催化剂研究进展

介绍了目前生产甲基丙烯酸甲酯(MMA)的几种工业化合成路线,其中甲基丙烯醛直接氧化酯化法因其环境友好而得到更多的关注。列举了甲基丙烯醛一步法制备MMA的几类催化剂,包括其制备方法、催化性能、使用条件等,指出钯系催化剂的研制将成为今后的主要发展方向,在改进催化剂性能的同时更应加强基础理论方面的研究。     

Partial oxidation of butane to syngas using nano-structure Ni/zeolite catalysts

In this study, performance of nano-structure Ni over different zeolite supports in partial oxidation of butane was investigated. First, partial oxidation process was performed without catalyst to evaluation of optimal conditions. For in situ reduction of catalysts, H₂ produced from homogenous reaction was used. Catalytic partial oxidation was carried out using nano-structure nickel catalysts supported by ZSM5, mordenite and Y. Each catalyst was synthesized through reverse microemulsion method. The catalysts were characterized by BET surface area, XRD, SEM and TGA. Highest butane conversion (≈89%) observed in the presence of Ni/Y catalyst. Also Ni/Y shows the highest overall selectivity to CO and H₂ as the most desired partial oxidation products. Results from TGA showed that the minimum quantity of formatted coke was related to Ni/Y, which confirmed the stability of butane conversion versus time for this catalyst.     

Partial oxidation of methane to synthesis gas over Rh-hexaaluminate-based catalysts

The partial oxidation of methane to synthesis gas over catalysts consisting of Rh supported on hexaaluminates (BaAl₁₂O₁₉, CaAl₁₂O₁₉ and SrAl₁₂O₁₉) was investigated at atmospheric pressure and high reactant dilution in order to compare their performances within the kinetic-controlling regime. Comparison with the results obtained over a commercial Rh/-Al₂O₃ system indicates that hexaaluminate catalysts are active and selective in this reaction. Despite of the higher surface area of the support, hexaaluminate-supported catalysts were found less stable, active and selective than an -Al₂O₃-supported catalyst.     

Partial oxidation of CH4 with air to produce pure hydrogen and syngas

The gas–solid reaction between methane and the lattice oxygen of Ni, Co, and Fe-oxides loaded on various support materials produced a synthesis gas (hydrogen and carbon monoxide) at 600–800 °C. Metal oxides were reduced to metals or lower valence oxides, and they were re-oxidized to oxides by introducing air after the reaction. Thus, production of hydrogen or synthesis gas free from nitrogen can be achieved alternatively without using pure oxygen. As a metal oxide, Fe₂O₃ and Rh₂O₃-loaded on Y₂O₃ exhibited the highest H₂ selectivity of 60.1% with a moderate CH₄ conversion of 54% and a high lattice oxygen utilization of 84% at 800 °C.     

Partial oxidation of methane to synthesis gas over Co/Ca/Al2O3 catalysts

A series of calcium-modified alumina-supported cobalt catalysts were prepared with a two-step impregnation method, and the effect of calcium on the catalytic performances of the catalysts for the partial oxidation of methane to syngas (CO and H₂) was investigated at 750 °C. Also, the catalysts were characterized by XRD, TEM, TPR and (in situ) Raman. At 6 wt.% of cobalt loading, the unmodified alumina-supported cobalt catalyst showed a very low activity and a rapid deactivation, while the calcium-modified catalyst presented a good performance for this process with the CH₄ conversion of 88%, CO selectivity of 94% and undetectable carbon deposition during a long-time running. Characterization results showed that the calcium modification can effectively increase the dispersion and reducibility of Co₃O₄, decrease the Co metal particle size, and suppress the reoxidation of cobalt as well as the phase transformation to form CoAl₂O₄ spinel phases under the reaction conditions. These could be related to the excellent catalytic performances of Co/Ca/Al₂O₃ catalysts.     

Partial oxidation of methane over rhodium catalysts for power generation applications

The partial oxidation of methane (POM) to syngas, i.e. H₂ and CO, over supported Rh catalysts was investigated at atmospheric pressure. The influence of support material, Rh loading and the presence of water vapor on the methane conversion efficiency and the product gas composition was studied. The catalysts containing ceria in the support material showed the highest activity and formation of H₂ and CO. By increasing the Rh loading, a decrease of the ignition temperature was obtained. The addition of water vapor to the reactant gas mixture was found to increase the ignition temperature and the formation of hydrogen, which is favorable for combustion applications where the catalytic POM stage is followed by H₂-stabilized homogeneous combustion.     

Partial oxidation kinetics of m-hydroxybenzyl alcohol with noble metal catalysts in supercritical carbon dioxide

Partial oxidation of m-hydroxybenzyl alcohol was studied over several supported noble metal catalysts in a temperature range from 373 to 413 K, up to 2 MPa of oxygen pressure and 20 MPa of carbon dioxide pressure. The major product detected was m-hydroxybenzaldehyde. A charcoal supported palladium catalyst gave the highest yield of the aldehyde. For high temperature above 393 K and high oxygen pressure above 0.5 MPa, total oxidation was observed, which caused the selectivity of m-hydroxybenzaldehyde to decrease. Supercritical carbon dioxide medium seemed to improve heat dissipation of the reaction to allow the partial oxidation of m-hydroxybenzyl alcohol to occur under mild conditions. The partial oxidation of benzyl alcohol over a charcoal supported palladium catalyst was also examined for comparison and the major product formed was benzaldehyde. The conversion of benzyl alcohol and the selectivity to benzaldehyde was higher than those for the case of partial oxidation of m-hydroxybenzyl alcohol.     

Partial oxidation of alkanes to oxygenates in supercritical carbon dioxide

The catalytic partial oxidation of propane in supercritical carbon dioxide has been investigated in a stirred batch reactor. Various metals (oxides) have been used as supported catalysts with respect to their activity and selectivity for the formation of oxygenates. The reactions run with a 1:2.3–2.9:68–108 molar ratio of propane:synthetic air:CO₂ at 453–573 K and 80–100 bar. Using a precipitated 2.4 wt.% Co₃O₄–SiO₂ catalyst at 573 K, a total oxygenate (i.e. acetic acid, acetone, acetaldehyde, methanol) selectivity of 59% and a propene selectivity of 21% were obtained at a propane conversion of 12 mol%. The same catalyst has been used to investigate the influence of the supercritical conditions and initial feed composition on the reaction, varying the density of CO₂ and the concentration of synthetic air, respectively.     

Partial oxidation and combined reforming of methane on Ce-promoted catalysts

Pt/ZrO₂ and Pt/Ce_0.14Zr_0.86O₂ catalysts containing 0.5 and 1.5 wt.% Pt were studied in order to evaluate the effect of the support reducibility and metal dispersion on the catalyst stability for the partial oxidation and the combined partial oxidation and CO₂ reforming of methane. The Pt/Ce_0.14Zr_0.86O₂ catalysts proved to be more active, stable and selective than Pt/ZrO₂ catalysts during the partial oxidation reaction. No increase in deactivation was observed when the CH₄:O₂ feed ratio was increased from 2:1 to 4:1. In addition, no water formation was observed at the high CH₄:O₂ ratios. The activity of the catalyst is dependent upon both the dispersion and the ability of the catalyst to resist carbon deposition.

The addition of CO₂ resulted in a decrease in the methane conversion and a decrease in the H₂/CO ratio for the Ce_0.14Zr_0.86O₂ and ZrO₂ supported catalysts. Small increases in the temperature of the bed have been recorded during the partial oxidation reaction. However, within a few minutes the temperature stabilizes below the furnace temperature providing indirect evidence for the combined combustion and reforming mechanisms previously proposed. The 1.5 wt.% Pt/CeZrO₂ catalyst shows promise for the autothermal reforming reaction based on the stability during transient operation.     

Partial oxidation of light paraffins on supported superacid catalytic membranes

Superacid-supported catalytic membranes were found to be active and very selective in the partial oxidation of light paraffins (C₁–C₂) with H₂O₂ under mild conditions (T_R: 80–110°C; P_R: 1.4 bar) in a three-phase catalytic membrane reactor (3PCMR). Among different catalytic membranes investigated, Nafion-based ones showed the best performance in terms of both activity and selectivity. Addition of Fe²⁺ ions in the liquid phase enhances the reaction rate, however, a volcano-shaped trend between reaction rate and concentration of Fe²⁺ was observed. Reaction temperature drastically affects both reaction rate and product distribution. A reaction pathway based on the electrophilic hydroxylation of the C–H bond on superacid sites and subsequent reaction of the activated paraffin with OH radicals has been proposed.     

Partial oxidation of methane to synthesis gas. Behaviour of different Ni supported catalysts

The behaviour of Ni supported catalysts, obtained using Ni(NO₃)₂ and Ni-acetylacetonate as precursor compounds, is analyzed. It is observed that initial activities and selectivities are similar for both systems, but the stability differs significantly. The systems show different carbon structures and sintering rates, depending on the precursor compound employed.     

Partial oxidation reactions on phosphate-based catalysts

In this work emphasis has been placed on phosphate-based catalysts used for partial oxidation reactions, such as vanadyl pyrophosphate, iron phosphates and hydrogen/hydroxy-phosphates, zirconium hydrogen phosphates as layered compounds used to stabilise/entrap Cr and V oxyhydroxy-macrocations. It is shown that, in partial oxidation reactions, the catalyst surface behave in a rather dynamic and labile way, reconstructing under activation and/or catalytic reaction conditions and adapting itself to the stereochemistry of the reactants. The active sites are shown to have a molecular size, to be isolated and to present several catalytic functions, as hydrocarbon activation, H atom abstraction, lattice oxygen incorporation and electron transfer through the solid material to allow the redox process to occur. The metal cations are the active species and the role of the phosphate tetrahedra is not only to bind the MO₆ octahedra together to constitute a dense or layered compound but also to bring some specific redox and acid–base properties. This revised version was published online in August 2006 with corrections to the Cover Date.     

Partial oxidation of methane to syngas over BaTi1 − xNixO3 catalysts

Performances of BaTi_1 − xNi_xO₃ perovskites, prepared using sol–gel method, as catalysts for partial oxidation of methane to syngas have been studied. The catalysts were characterized by XRD, BET and TEM. The experimental studies showed the calcination temperature and Ni content exhibited a significant influence on catalytic activity. Among catalysts tested, the catalyst BaTi_0.8Ni_0.2O₃ exhibited the best activity and excellent stability.     

Selective oxidation with air on metal catalysts

Oxidation of organic molecules with air on metal catalysts has been known for a long time but there has been a renewed interest in recent years because these catalytic reactions are environmentally safe and could replace stoichiometric oxidations. This paper describes several oxidation reactions conducted either at high temperatures in the gas phase or at moderate temperatures in the liquid phase; in both cases they proceed via a mechanism of oxidative dehydrogenation on the metal surface. Ethylene glycol was converted to glyoxal at 550°C on Ag/SiC catalyst with a 70% yield provided promoters were added to the reaction feed (diethylphosphite or iodine) or deposited on the catalyst (LiPO₄ or H₃PO₄). The promoters improve the conversion and selectivity by modifying the structure and the oxygen concentration on the surface of silver. Oxidation of glyoxal to glyoxylic acid, glucose to gluconic acid and glycerol to various oxygenated derivatives were conducted in water at 60°C in the presence of carbon-supported palladium or platinum catalysts. Bismuth promoter, deposited on the platinum metals by redox reaction, improves the catalyst activity by preventing over-oxidation of the metal surface and favors the oxidation of secondary alcohol functions into keto-derivatives. At higher reaction temperatures, platinum catalysts produce C-C bond rupture with the formation of carboxylic acids with smaller chains. Thus, cyclohexanol was converted into C₆, C₅, and C₄ diacids with a 45% selectivity to adipic acid on Pt/C catalysts at 150°C.     

异丁烯齐聚反应精馏的耦合工艺研究

针对异丁烯齐聚反应和产物提纯过程相分离的工业现状,在自行设计的实验装置上进行了异丁烯齐聚催化反应精馏耦合工艺的研究,分别考察了系统操作状态随时间的变化规律和影响反应精馏过程的主要因素。结果表明:为了得到较高的异丁烯转化率和二聚物选择性,采用全回流操作,进料位置宜设在反应段上方;回流温度过低和塔釜温度过高都对过程不利;添加适当的液相惰性组分,有利于改善填料层的分离效果。在转化率相同的情况下,该反应精馏过程的二聚物选择性大大高于固定床微分反应器和高压釜式反应器的实验结果。     

Partial oxidation of ethanol on Ru/Y2O3 and Pd/Y2O3 catalysts for hydrogen production

The partial oxidation of ethanol was investigated over Ru and Pd catalysts supported onto yttria over a wide range of temperatures (473–1073 K). The product distributions obtained over these catalytic systems were correlated with diffuse reflectance infrared spectroscopy analyses (DRIFTS). Results showed that reaction route depended strongly on the type of metal. The decomposition of ethoxy species to CH₄ and CO or oxidation to CO₂ was promoted by Pd, and the acetaldehyde desorption was predominant over Ru in the low temperature region. Furthermore, the acetate and carbonate formation prevailed over Pd, which explained the lower acetaldehyde selectivity. The presence of CH₄ and CO₂ at high temperature is assigned to the decomposition of acetate species via carbonates over Pd-based catalysts. Ru was more suitable system for H₂ production than Pd by achieving a selectivity of about 59%.     

甲醇氧化重整催化剂的研究

制备并优选了甲醇氧化重整的催化剂 ,实验表明在相同的温度下 ,Pd催化剂、Cr2 O3 ZnO氧化物催化剂和Cu催化剂相比 ,活性、选择性以及氢产率都表现为Cu >Cr Zn >Pd。其中 ,Cu催化剂在低温时、Cr Zn催化剂在高温下表现出良好的活性和选择性 ,而且Cr Zn催化剂的热稳定性较好     

Partial oxidation of n-hexadecane and polyethylene in supercritical water

In this study, we show the results of partial oxidation experiments of n-hexadecane (n-C₁₆) and polyethylene (PE) in supercritical water (SCW). The experiments were carried out at 673 or 693 K of reaction temperature and 5 or 30 min of reaction time using a 6 cm³ of a batch type reactor. Water density ranged from 0.1 to 0.52 g/cm³ (water pressure: 20–40 MPa). The loaded amount of oxygen was set to 0.3 of the ratio of oxygen atom to carbon atom. Some experiments were made using CO instead of oxygen for the partial oxidation of n-C₁₆ and PE to explore the effect of water gas shift reaction. In the results of partial oxidation of n-C₁₆, the yield of CO and some compounds containing oxygen atoms, such as aldehydes and ketones increased with increasing water density. Moreover, 1-alkene/ n-alkane ratio in the products decreased with increasing water density. The 1-alkene/n-alkane ratio was lower than that of pyrolysis in SCW. Also for the case of PE experiments, in dense SCW (0.42 g/cm³), the 1-alkene/n-alkane ratio in partial oxidation was lower than that in SCW pyrolysis. In the case of CO experiments for n-C₁₆ and PE, 1-alkene/n-alkane ratio was a little lower than that of pyrolysis in SCW. These results show that the yield of n-alkane, which is a hydrogenated compound, was higher through water gas shift reaction in SCW and also through partial oxidation in SCW. Therefore, these results suggest the possibility of hydrogenation of hydrocarbon through partial oxidation followed by the water gas shift reaction.