Influence of structural properties of catalysts at various stages of selective oxidation: from catalyst preparation to catalytic reactors

Solid oxides selective in mild oxidation exhibit a definite crystal morphology. Each crystal face displays a surface crystal field which rules the transformation of the reactant to a given product. The catalytic metastability of the surface which has to be maintained in the operating conditions prevailing in reactors depends on the reactivity of the bulk, because of oxygen diffusion process. Application to VPO, the catalyst for oxidation of n-butane to maleic anhydride, gives the opportunity to examine these properties in the series preparation–activation–catalysis–ageing, in relation to the type of catalytic reactor. This revised version was published online in August 2006 with corrections to the Cover Date.     

Continuously Conducted Selective Hydrogenation of Benzene to Cyclohexene in a Four‐Phase System

Currently, the production of nylon 6 and nylon 6.6 is based on the complete hydrogenation of benzene to cyclohexane over nickel or platinum catalysts and its subsequent oxidation at very low conversions to a mixture of cyclohexanol/cyclohexanone. This mixture is further reacted to ?‐caprolactam or adipic acid. Alternatively, the reaction can be carried out by selective hydrogenation of benzene to cyclohexene and subsequent hydration with acidic catalysts to cyclohexanol. A laboratory method for the selective hydrogenation of benzene in a continuous stirred‐tank reactor (CSTR) using a gas/liquid/liquid/solid system is presented.     

Influence of the preparation conditions on the properties of gold catalysts for the oxidation of glucose

In this work, two deposition–precipitation methods for the preparation of gold catalysts for glucose oxidation were investigated. Thus far, gold colloids immobilized on carbon have been used for catalytic glucose oxidation, but the long-term stability of these systems was not sufficient. To improve the long-term stability we used the deposition–precipitation methods using NaOH (DP NaOH) or urea (DP urea) as precipitation agents as they were described by Haruta and Dekkers, respectively, using alumina as a support material. With these methods, it was possible to prepare highly active and selective catalysts which showed an excellent long-term stability. DP urea was found to be the preferred method, because in contrast to DP NaOH, no losses of gold occurred during the preparation, and it was possible to adjust various gold contents up to 10 wt% Au.     

基于Fe2V4O13的MHF反应器的构建及其特性研究

近年来,在难降解有机废水的处理技术中,Fenton催化氧化法因具有设备简单、操作容易、条件温和、反应速度快和氧化彻底等特点受到了环境工作者的广泛关注。为了解决悬浮态超细粉体Fenton-like催化剂降解污染物的高效率与其分离、连续重复使用的矛盾,将膜分离和多相Fenton-like催化氧化技术进行耦合,通过液相法制备了一种新的多元多相Fenton-like催化剂Fe2V4O13,构建了一种基于这种催化剂的陶瓷膜MHF反应器。研究结果表明：基于Fe2V4O13的MHF反应器为近似全混流反应器,污染较小。该反应器在连续运行过程中膜通量和降解效率都较稳定,处理量远高于普通Fenton-like反应器。     

Oxidation of unsymmetrical dimethylhydrazine over heterogeneous catalysts Solution of environmental problems of production, storage and disposal of highly toxic rocket fuels

The catalytic oxidation of unsymmetrical dimethylhydrazine (UDMH) by air has been studied in a vibro-fluidized catalyst bed laboratory kinetic setup over catalysts Cu_xMg_1−xCr₂O₄/γ-Al₂O₃, 32.9%Ir/γ-Al₂O₃ and β-Si₃N₄ in a temperature range 150–400 °C. The catalyst Cu_xMg_1−xCr₂O₄/γ-Al₂O₃ was found to be optimal regarding high yields of CO₂ and low yields of NO_x. A probable mechanism of UDMH heterogeneous catalytic oxidation is proposed. Catalyst Cu_xMg_1−xCr₂O₄/γ-Al₂O₃ has been further used in the pilot plant specially designed for the destruction of UDMH. Results of testing the main fluidized bed catalytic reactor for UDMH oxidation and the reactor for selective catalytic reduction of NO_x with NH₃ are presented. These results prove that the developed UDMH destruction technology is highly efficient and environmentally safe.     

Low-temperature catalytic combustion of methanol and its decomposed derivatives over supported gold catalysts

Gold can be compared favorably with Pd and Pt in the catalytic combustion of CH₃OH, HCHO and HCOOH when it is deposited on some reducible metal oxides (-Fe₂O₃, TiO₂, etc.). While the supported gold catalysts are less active in H₂ oxidation, they exhibit much higher activities in CO oxidation. For Au/TiO₂, the effect of catalyst preparation was further investigated. Since the activity for CO oxidation of the gold catalysts is not depressed but enhanced by moisture, they are practically applicable to CO removal from air at room temperature. Gold supported on manganese oxide is especially effective in the selective CO removal from hydrogen, indicating its potential applicability to polymer electrolyte fuel cells using the reformed gas of methanol.     

乙苯脱氢-氢选择性氧化催化剂失活机理的研究

考察和分析了Pt/Al2O3体系的乙苯脱氢-氢选择性氧化催化剂的失活原因。研究结果表明,该催化剂的失活主要是由于长期在高温和水蒸气条件下运行,造成Pt晶粒团聚导致的;另外,来自前段脱氢催化剂的铁元素,由于非选择性氧化活性较高也会在一定程度上使催化剂中毒失活;而表面积炭对催化性能的影响不大。     

正丁烷选择氧化中吸附氧与晶格氧的作用

引　言丁烷选择氧化制顺酐是以廉价低碳烷烃作为原料的直接选择氧化反应 ,近年来已引起广泛关注并进行了较多的研究[1] .普遍认为 ,这是一个以复合氧化物VPO作为催化剂 ,晶格氧参与催化循环、按氧化还原机理进行的反应[2 ] .为了提高这一反应的选择性 ,杜邦公司按照序贯氧化 还原原理重新组织催化循环的思想 ,率先提出了双反应器循环流化床 (CFB)的专利 ,并进行了晶格氧正丁烷选择氧化制顺酐的技术开发 ,建立了大规模工业示范装置 ,显示了改善性能的潜力[3] .鉴于在空间上和在时间域内使催化循环各步骤分离进行这两类非定态操作在实质上…     

Advanced TEMKIN Reactor: Testing of Industrial Eggshell Catalysts on the Laboratory Scale

In a variety of reactions in the chemical industry, eggshell catalysts with a thin active layer are applied; they are often crushed for laboratory testing. The destruction of the shell can be avoided by a special reactor design. The presented advanced TEMKIN reactor is a further development of the reactor system for testing eggshell catalysts on the laboratory scale published by Temkin and Kul'kova in 1969. It is suitable for kinetic studies and for the detailed investigation of deactivation processes, as shown on the example of selective hydrogenation of acetylene.     

In situ-electron spin resonance: a useful tool for the investigation of vanadium phosphate catalysts (VPO) under working conditions

Differently prepared (VO)₂P₂O₇ phases and an amorphous V³⁺PO catalyst were investigated under conditions of the selective n-butane oxidation and the toluene ammoxidation, respectively, using a self-constructed in situ-ESR flow reactor in the X-band. By examining the temperature dependence and the line shape of the ESR signals exchange integrals as well as the 2nd and the 4th moment were obtained. These parameters characterize the spin—spin exchange behaviour and, thus, structural and electronic disorder of the catalysts. Increasing structural disorder was found to improve the catalytic performance in the n-butane oxidation. For both catalytic processes a significant reversible alteration of the ESR line shape was observed under working conditions which is discussed in terms of a perturbation of exchange interactions between neighbouring vanadyl centres near the surface the oxidation state of which is assumed to fluctuate between +4 and +5 during the catalytic reaction.     

Redox solid catalysts for the selective oxidation of cyclohexane in air

The selective oxidation of cyclohexane to cyclohexanol, cyclohexanone and adipic acid using molecular oxygen as the oxidant and at moderate temperatures (403 K) has been investigated over four different cobalt‐containing aluminophosphate (AlPO) molecular sieves. There is a correlation between catalytic activity and the fraction of (framework) Co(II) ions that is first oxidised to Co(III) in air. CoAlPO‐36 (pore aperture 6.5 x 7.5 Å) exhibits significant activity for the oxidation of cyclohexane in contrast to CoAlPO‐18, which, although it has the highest fraction of oxidisable cobalt, does not show any activity chiefly because of its smaller pores.     

Activity and product distribution of alumina supported platinum and palladium catalysts in the gas-phase oxidative decomposition of chlorinated hydrocarbons

The complete catalytic oxidation of 1,2-dichloroethane (DCE) and trichloroethylene (TCE) over alumina supported noble metal catalysts (Pt and Pd) was evaluated. Experiments were performed at conditions of lean hydrocarbon concentration (around 1000 ppm) in air, between 250°C and 550°C in a conventional fixed bed reactor. The catalysts were prepared in a range of metal contents from 0.1 to 1 wt%. Palladium catalysts resulted to be more active than platinum catalysts in the oxidation of both chlorinated volatile organic compounds. DCE was completely destructed at 375°C, whereas TCE required 550°C. HCl was the only chlorine-containing product in the oxidation of DCE in the range of 250–400°C. Tetrachloroethylene was observed as an intermediate in the oxidation of TCE, being formed to a significant extent between 400°C and 525°C. CO was also detected in the oxidation of both DCE and TCE over Pd catalysts, though at temperatures of complete destruction, CO₂ was the only carbon-containing product. The Pt catalysts were selective to CO₂ at the studied conditions.     

Miniaturization of screening devices for the combinatorial development of heterogeneous catalysts

The present work is focused on the determination of the advantages, bottlenecks and challenges of miniaturized screening systems which are essential to the success of combinatorial high-throughput methodologies in heterogeneous catalysis. Two different reactor configurations with different degrees of miniaturization were developed for the parallel and fast screening of heterogeneously catalyzed gas phase reactions: a monolithic reactor system acting as a multichannel reactor and a microreaction system based on microfabrication techniques. In both cases, a scanning mass spectrometry technique was successfully applied for quantitative product analysis within 60 s per catalyst. Due to its flexibility and high spatial resolution, this three dimensional scanning MS can be used with different and highly parallel reactor arrays. Many experiments were carried out to study the efficiency and reliability of the different screening systems, with the oxidation of methane, the oxidation of CO, and the oxidative dehydrogenation of i-butane as model reactions. Moreover, chip modules in silicon–glass technology having a number of parallel microchannels were developed, each of them containing a different catalyst. Using this approach, “catalysis-on-a-chip” proved in methane oxidation was possible. Finally, a multibatch reactor consisting of a number of parallel mini autoclaves was developed and tested in the liquid-phase hydrogenation of citral in order to overcome the lack of parallel and fast screening procedures for heterogeneously catalyzed gas–liquid reactions widely spread in the chemical industry.     

Structured catalysts for partial oxidations

The objective of the present work is centred on the analysis of the behaviour of fixed-bed reactors packed with structured catalysts when a partial oxidation takes place. The fluid flow within an element, formed by the intersection of two adjacent corrugated plates, was mathematically modelled and numerical simulations were performed in order to study possible optimising of the catalyst structure. A simple model, involving the continuous stirred tank approach, was used to simulate the performance of the catalyst in an industrial reactor. The profiles of temperature, concentration, velocity and pressure along the reactor were obtained through the global mass balance, partial molar balances, conservation of moment, energy balance and mechanical energy balance. These equations were applied to an exothermic reactional system, the selective oxidation of methanol to formaldehyde, in order to evaluate the possibility of application of these catalysts in industry. Various parameters specific to the catalyst and the reactional system were tested in order to achieve a better understanding of the behaviour of these structured packing. The comparison with the heterogeneous model predictions for a random packing and with industrial values pointed out that the choice of parameters is fundamental to the performance of the catalyst. The adjustments to the parameters allows for significant improvements in some of the more troublesome aspects of the reactional system. Lowering the hot-spot, reducing the progress of the secondary reaction and reducing the pressure drop are the main enhancements that these structured catalysts can offer.     

Development and Application of Oxygen Permeable Membrane in Selective Oxidation of Light Alkanes

In this paper, oxygen permeable membrane used in membrane reactor for selective oxidation of alkanes will be discussed in detail. The recent developments for the membrane materials will be presented, and the strategy for the selection of the membrane materials will be outlined. The main applications of oxygen permeable membrane in selective oxidation of light alkanes will be summarized, which includes partial oxidation of methane (POM) to syngas and partial oxidation of heptane (POH) to produce H₂, oxidative coupling of methane (OCM) to C₂, oxidative dehydrogenation of ethane (ODE) to ethylene and oxidative dehydrogenation of propane (ODP) to propylene. Achievements for the membrane material developments and selective oxidation of light alkanes in membrane reactor in our group are highlighted.     

Towards an efficient process for small-scale, decentralized conversion of methane to synthesis gas: combined reactor engineering and catalyst synthesis

Anthropogenic methane emissions not only pose an increasing global problem but can also be seen as an untapped renewable resource for hydrogen or liquid fuel production. Utilization of this resource requires the development of efficient processes for small-scale, decentralized methane conversion. We see catalytic partial oxidation of methane to synthesis gas as an ideal candidate for such a process. In the present study, we investigate the effect of heat-integration, flow rate and catalyst stability on syngas yields over alumina-supported Pt and Rh catalysts in a reverse-flow reactor configuration. We furthermore demonstrate that novel high-temperature stable nanocomposite catalysts can dramatically lower the noble metal requirement for this reaction, improve catalyst stability and further increase synthesis gas yields. Finally, we demonstrate that the combination of the reverse-flow reactor with these nanocomposite catalysts exploits the efficiency of this reaction route very effectively while maintaining a simple and compact reactor design.     

Cobalt Fischer-Tropsch synthesis: Deactivation by oxidation?

Cobalt catalysts as used in the Fischer-Tropsch synthesis (FTS) are relatively expensive (as compared to iron) and need to have a high metal dispersion and long life to be able to offer a good balance between cost and performance. The oxidation of nano-sized metallic cobalt to cobalt oxide during Fischer-Tropsch synthesis has long been postulated as a major deactivation mechanism. However, to date there is no consistent picture. This paper presents an extensive overview of the literature on this topic of deactivation by means of oxidation for unsupported as well as silica-, alumina- and titania-supported cobalt catalysts. Furthermore, it presents results on the deactivation of an industrial Co/Al₂O₃ catalyst as obtained by pseudo in situ X-ray diffraction, magnetic measurements and X-ray absorption near-edge spectroscopy. These analyses were performed to study the oxidation state of spent industrial Co/Al₂O₃ catalyst samples withdrawn from a slurry reactor operating under realistic FTS conditions, and it was concluded that oxidation can be ruled out as a major deactivation mechanism. Finally, these data together with all relevant literature were used to create a common view on the oxidation behaviour of metallic cobalt during FTS. The apparent discrepancies in literature on the oxidation behaviour of cobalt are most likely due to the lack of direct characterisation of the cobalt oxidation state and due to the comparison of catalysts with varying cobalt crystallites sizes, compared at different reactor partial pressures of hydrogen and water (P_H2O/P_H2). It was shown that the oxidation of cobalt can be prevented by selecting the correct combination of the reactor partial pressures of hydrogen and water (P_H2O/P_H2) and the cobalt crystallite size.     

Niobium as promoting agent for selective oxidation reactions

Literature reports a significant number of studies that describe the various niobium-based catalysts for many uses; selective oxidation reactions constitute a significant part of those works. This contribution makes a brief revision of literature that investigates the promoting effect of niobium on different catalytic formulations used for selective oxidation reactions rather than in the use of niobium as main component.