Showing papers in "Topics in Catalysis in 2003"

Brief Overview of the Partial Oxidation of Methane to Synthesis Gas

Abstract: A review of the main developments in the partial oxidation of methane to synthesis gas since the first paper in 1929 to the present day is given. The reaction is discussed from the view of the thermodynamics; the main catalysts studied for the reaction are summarised, and the reaction mechanism is discussed. The review is not comprehensive, but it is designed to provide a general background to the most important developments in the field.

Deactivation of Copper Metal Catalysts for Methanol Decomposition, Methanol Steam Reforming and Methanol Synthesis

Abstract: Laboratory and industrial results are reviewed to elucidate the general features of the deactivation of supported copper metal catalysts in various reactions involving methanol as reactant or product. Most catalyst types are based on Cu/ZnO formulations that contain stabilisers and promoters such as alumina, alkaline earth oxides and other oxides. These additional materials have several roles, including the inhibition of sintering and absorption of catalyst poisons. All copper catalysts are susceptible to thermal sintering via a surface migration process, and this is markedly accelerated by the presence of even traces of chloride. Care must be taken, therefore, to eliminate halides from copper catalysts during manufacture, and from reactants during use. Operating temperatures must be restricted, usually to below 300°C. In methanol synthesis involving modern promoted Cu/ZnO/Al2 O3 catalysts neither poisoning nor coking is normally a significant source of deactivation; thermal sintering is the main cause of deactivation. In contrast, catalyst poisoning and coking have been observed in methanol decomposition and methanol steam reforming reactions.

Multifunctionality of Active Centers in (Amm)oxidation Catalysts: From Bi–Mo–Ox to Mo–V–Nb–(Te, Sb)–Ox

Abstract: Catalytic centers in selective (allylic) oxidation and ammoxidation catalysts are multimetallic and multifunctional. In the historically important bismuth molybdates, used for propylene (amm)oxidation, they are composed of (Bi3+)(Mo6+)2 complexes in which the Bi3+ site is associated with the α-H abstraction and the (Mo6+)2 site with the propylene chemisorption and O or NH insertion. An updated reaction mechanism is presented. In the Mo–V–Nb–Te–Ox systems, three crystalline phases (orthorhombic Mo7.5V1.5NbTeO29, pseudohexagonal Mo6Te2VO20, and monoclinic TeMo5O16) were identified, with the orthorhombic phase being the most important one for propane (amm)oxidation. Its active centers contain all necessary key catalytic elements (2V5+/Mo6+, 1V4+/Mo5+, 2Mo6+/Mo5+, 2Te4+) for this reaction wherein a V5+ surface site (V5+ = O ↔ 4+V•–O•) is associated with paraffin activation, a Te4+ site with α-H abstraction once the olefin has formed, and a (Mo6+)2 site with the NH insertion. Four Nb5+ centers, each surrounded by five molybdenum octahedra, stabilize and structurally isolate the catalytically active centers from each other (site isolation), thereby leading to high selectivity of the desired acrylonitrile product. A detailed reaction mechanism of propane ammoxidation to acrylonitrile is proposed. Combinatorial methodology identified the nominal composition Mo0.6V0.187Te0.14Nb0.085Ox for maximum acrylonitrile yield from propane, 61.8% (86% conversion, 72% selectivity at 420 °C). We propose that this system, composed of 60% Mo7.5V1.5NbTeO29, 40% Mo6Te2VO20, and trace TeMo5O16, functions with a combination of compositional pinning of the optimum orthorhombic Mo7.5V1.5±xNb1±yTe1±zO29±δ phase and symbiotic mop-up of olefin intermediates through phase cooperation. Under mild reaction conditions, a single optimum orthorhombic composition might suffice as the catalyst; under demanding conditions this symbiosis is additionally required. Improvements in catalyst performance could be attained by further optimization of the elemental distributions at the active catalytic center of Mo7.5V1.5NbTeO29, by promoter/modifier substitutions, and incorporation of compatible cocatalytic phases (preferably epitaxially matched). High-throughput methods will greatly accelerate the rational catalyst design processes.

New Supported Pd and Pt Alloy Catalysts for Steam Reforming and Dehydrogenation of Methanol

Abstract: The catalytic performances of supported Group 810 metal (Co, Ni, Ru, Pd, Ir and Pt) catalysts for steam reforming of methanol, CH3OH + H2O → CO2 + 3H2, and dehydrogenation of methanol to methyl formate, 2CH3 OH → HCOOCH3 + 2H2, are markedly affected by the kinds of supports as well as the metals used. The selectivity for steam reforming and the formation of methyl formate was markedly improved when Pd or Pt were supported on ZnO, In2O3 and Ga2O3. The combined results of temperature-programmed reduction, XRD, XPS and AES revealed that Pd-Zn, Pd-In, Pd-Ga, Pt-Zn, Pt-In and Pt-Ga alloys were formed upon reduction. Over the catalysts having an alloy phase, the reactions proceeded selectively, whereas over the catalysts having a metallic phase, methanol was decomposed to carbon monoxide and hydrogen predominantly. It was shown that the reactivity of formaldehyde intermediate over the Pd and Pt alloys was markedly different from that over metallic Pd and Pt. Over Pd and Pt alloys, aldehyde species were stabilized and transformed into carbon dioxide and hydrogen or methyl formate by nucleophilic addition of water or methanol, respectively. By contrast, over metallic Pd and Pt, aldehyde species were rapidly decarbonylated to carbon monoxide and hydrogen.

Fischer–Tropsch on Iron with H2/CO and H2/CO2 as Synthesis Gases: The Episodes of Formation of the Fischer–Tropsch Regime and Construction of the Catalyst

Abstract: Fischer–Tropsch synthesis experiments have been performed with reduced precipitated Fe-Al-Cu-K2O catalysts, using a H2/CO and a H2/CO2 synthesis gas. Samples of the reaction products and of the catalyst were taken at distinct run lengths. The product samples were analyzed in detail and from their composition the kinetic data of elemental reaction steps (growth, branching, desorption as olefin or paraffin) calculated, applying the extended model of “nontrivial surface polymerization”. The catalyst samples were characterized by BET, XRD, Mossbauer spectroscopy, XPS and TPD in hydrogen and thus specifically phase changes of e.g. alpha iron, iron oxides, and iron carbides observed. It has been measured how the product composition changed with time, up to the steady state of synthesis. Several episodes with their own kinetic regimes were identified. These were then correlated with compositional and structural changes of the catalyst. This is addressed as “construction of the true iron Fischer–Tropsch catalyst”.

Hybrid Inorganic–Organic Solids: An Emerging Class of Nanoporous Catalysts

Abstract: Nanoporous hybrid materials, both metal-organic coordination polymers and hybrid metal oxides, have recently developed into an important new class of solid-state compounds. Potential applications in the field of heterogeneous catalysis include acid, shape-selective, chiral, ship-in-a-bottle, and shape-recognition-driven reactions. Here, we summarize the synthesis, structural features, chemical functionality, and catalytic properties of this unique family of materials.

On the issue of the active site and the role of ZnO in Cu/ZnO methanol synthesis catalysts

Abstract: The problem concerning the active site and the role of ZnO in Cu/ZnO-based methanol synthesis catalysts can be consistently explained based on the literature results by distinguishing CO2 and CO hydrogenations. Although only metallic copper has some activities for methanol synthesis by the hydrogenation of CO2, Cu-Zn alloying in Cu particles is responsible for the major promotional role of ZnO in industrial Cu/ZnO-based catalysts. The morphology effect reported in the literature will probably appear for the system of highly dispersed Cu particles supported on ZnO. As for the hydrogenation of CO, Cu+ species or Cu-O-Zn sites are the active sites for methanol synthesis. The spillover effect of the Cu-ZnO system is not significant compared to the effect of ZnO on the creation of the Cu-O-Zn site.

Nanoparticles in Catalysis

Abstract: In this article, we report studies of two new forms of highly active supported catalysts. First, those derived from supported carbonylate clusters—nanocatalysts and second, those produced from the heterogenization of known chiral homogeneous systems.

Structural Characterization of the Orthorhombic Phase M1 in MoVNbTeO Propane Ammoxidation Catalyst

Abstract: The structure of the orthorhombic phase in the MoVNbTeO propane ammoxidation catalyst system has been characterized and refined using a combination of TEM, synchrotron X-ray powder diffraction (S-XPD), and neutron powder diffraction (NPD). This phase, designated as M1 by Ushikubo et al. [1], crystallizes in the orthorhombic space group Pba2 (No. 32) with a = 21.134(2) A, b = 26.658(2) A, and c = 4.0146(3) A. The formula unit is Mo7.5V1.5NbTeO29. Bond valence sum calculations indicate the presence of d 1 metal sites neighbored by d 0 metal sites. The d 1 sites are occupied by a distribution of Mo5+ and V4+, whereas the d 0 sites are occupied by a distribution of Mo6+ and V5+. Out-of-center distortions in d 0 octahedra are consistent with the second-order Jahn–Teller effect and lattice effects. We argue that the V5+–O–V4+/Mo5+ moieties adjacent to Te4+ and Mo6+ sites in the [001] terminal plane provide a spatially isolated active site at which the selective ammoxidation of propane occurs.

Fundamentals of Methanol Synthesis and Decomposition

Abstract: Fundamental studies of methanol synthesis and decomposition (mainly over Cu-based catalysts) have been carried out. Various kinetic approaches, i.e. TPD study after various chemical treatments of catalyst, non-steady-state transformation of strongly adsorbed species, tracer technique, and steady-state kinetics, have been used. The macroscopic mechanism and detailed reaction scheme of methanol synthesis, as well as the kinetic description of the process have been established and proven. Methanol synthesis over Cu-based catalysts was found to occur by CO2 hydrogenation only, which was coupled with the water-gas shift reaction. Methanol decomposition and steam reforming over Cu-based catalysts have been studied. It was shown that methanol decomposed into a mixture of CO and H2 via methyl formate as an intermediate. Methanol transformation into the mixture of CO2 and H2 occurred by interaction of methanol and water. The reaction proceeded as the reverse methanol synthesis reaction, accompanied by the reverse water-gas shift reaction.

Control of Metal Dispersion and Structure by Changes in the Solid-State Chemistry of Supported Cobalt Fischer–Tropsch Catalysts

Abstract: Controlling preparation variables in supported cobalt Fischer–Tropsch catalysts has a dramatic effect on the dispersion and distribution of cobalt, and determines how active and selective the resulting catalyst will be. We detail specific examples of catalyst synthesis strategies for modifying interactions between the support and the cobalt precursor, promoting reduction, stabilizing catalysts to high-temperature treatments, minimizing deleterious support metal interactions, and controlling the distribution of cobalt on large support particles. It is important to optimize the support and precursor interaction strength, so that it is strong enough to obtain good dispersion but not too strong to prevent low temperature reduction. We show examples in which formation of surface complexes and epitaxial matching of precursor and support structures improves dispersion dramatically. Reduction promoters can help in those cases where support–precursor interactions are too strong. We show how substitutions of silicon into a titania lattice stabilizes surface area and retards formation at high oxidation temperatures of cobalt ternary oxides that reduce only at very high temperatures—an important consideration if oxidative coke removal is necessary. In addition, surface treatment of TiO2 with an irreducible oxide like ZrO2 can inhibit deleterious support interactions that can block surface cobalt sites. Selectivity can also be dramatically altered by catalyst synthesis. We illustrate a case of large (2 mm) SiO2 particles onto which cobalt can be added either uniformly or in discrete eggshells, with the eggshell catalysts having substantially higher C5+ selectivity. These approaches can lead to optimal Fischer–Tropsch catalysts with high activity and C5+ selectivity, good physical integrity, and a long life.

Major and minor reactions in Fischer-Tropsch synthesis on cobalt catalysts

Abstract: Minor reactions, accompanying the major reactions for building straight-chains of aliphatic hydrocarbons from the reactants CO and H2 on the surface of cobalt catalysts, can contribute substantially to the understanding of the regime of Fischer–Tropsch synthesis. This goal affords precise mass balances, precise determination of product composition and consistent kinetic schemes for obtaining the right kinetic coefficients. The concept of self-organization of the Fischer–Tropsch regime is established from time dependence of activity, selectivity and catalyst structure. A process of thermodynamically controlled restructuring/segregation of the cobalt surface is addressed and understood as activating the catalyst and specifically, disproportionating on-plane sites into sites of lower coordination (on-top sites) and higher coordination (in-hole sites). These different sites appear to collaborate in the Fischer–Tropsch regime, with steps of coordination chemistry (comparable to those of transition metal complexes) on on-top sites and dissociation (specifically of CO) on in-hole sites and further in principle suppressed reactions on on-plane sites. This concept is developed and illustrated here with the results of several investigations such as tracing of activity and selectivity during the initial episodes of synthesis, experiments with added (14C-labeled) olefins and variation of synthesis parameters to see their specific influences. As minor reactions of coordination chemistry on on-top sites, reversible CH2 cleavage from alkyl chains, CO insertion and ethene insertion are visualized. On on-plane sites CO methanation, olefin hydrogenation and olefin double bond shift are noticed, but much inhibited. As compared to Fischer–Tropsch on iron catalysts, the common Fischer–Tropsch principle appears to be the inhibition of chain desorption to allow for growth reactions of the adsorbed chains. Minor reactions and detailed kinetics on iron and cobalt catalysts differ basically.

Steam reforming of methanol over Cu/CeO2 catalysts studied in comparison with Cu/ZnO and Cu/Zn(Al)O catalysts

Abstract: A series of Ce1-xCuxO2-δ mixed oxides were synthesized using a co-precipitation method and tested as catalysts for the steam reforming of methanol. XRD patterns of the Ce1-xCuxO2-δ mixed oxides indicated that Cu2+ ions were dissolved in CeO2 lattices to form a solid solution by calcination at 773K when x < 0.2. A TPR (temperature-programmed reduction) investigation showed that the CeO2 promotes the reduction of the Cu2+ species. Two reduction peaks were observed in the TPR profiles, which suggested that there were two different Cu2+ species in the Ce1-xCuxO2-δ mixed oxides. The TPR peak at low temperature is attributed to the bulk Cu2+ species which dissolved into the CeO2 lattices, and the peak at high temperature is due to the CuO species dispersed on the surface of CeO2. The Ce1-xCuxO2-δ mixed oxides were reduced to form Cu/CeO2 catalysts for steam reforming of methanol, and were compared with Cu/ZnO, Cu/Zn(Al)O and Cu/AL2O3 catalysts. All the Cu-containing catalysts tested in this study showed high selectivities to CO2 (over 97%) and H2. A 3.8wt% Cu/CeO2 catalyst showed a conversion of 53.9% for the steam reforming of methanol at 513K (W/F = 4.9 g h mol-1), which was higher than that over Cu/ZnO (37.9%), Cu/Zn(Al)O (32.3%) and Cu/AL2O3 (11.2%) with the same Cu loading under the same reaction conditions. It is likely that the high activity of the Cu/CeO2 catalysts may be due to the highly dispersed Cu metal particles and the strong metalsupport interaction between the Cu metal and CeO2 support. Slow deactivations were observed over the 3.8wt% Cu/CeO2 catalyst at 493 and 513K. The activity of the deactivated catalysts can be regenerated by calcination in air at 773K followed by reduction in H2 at 673K, which indicated that a carbonaceous deposit on the catalyst surface caused the catalyst deactivation. Using the TPO (temperature-programmed oxidation) method, the amounts of coke on the 3.8wt% Cu/CeO2 catalyst were 0.8wt% at 493K and 1.7wt% at 513K after 24h on stream.

Cu–ZnO and Cu–ZnO/Al2O3 Catalysts for the Reverse Water-Gas Shift Reaction. The Effect of the Cu/Zn Ratio on Precursor Characteristics and on the Activity of the Derived Catalysts

Abstract: Comparison is made between Cu–ZnO and alumina-supported Cu–ZnO as catalysts for the reverse water-gas shift (RWGS) reaction For both types of catalyst the Cu/Zn ratio has been varied between Cu-rich and Zn-rich compositions By applying X-ray diffractometry, X-ray line broadening, optical reflectance spectroscopy and other techniques the effects on the structural and physical properties of the hydroxycarbonate precursors, the calcined products and the ultimately derived catalysts are determined The presence of alumina decreases the crystallite size of the CuO and ZnO particles produced on calcination and at high Cu/Zn ratios increases the dispersion of copper in the final catalyst The activities of the catalysts for the RWGS reaction at 513K are compared and the most active are shown to be those which are Cu rich (Cu/Zn > 3) and contain alumina as support The activities of all the catalysts can be rationalized by referring the activity to unit surface area of copper metal

Propane Oxidation on MoVTeNbO Mixed Oxide Catalysts: Study of the Phase Composition of Active and Selective Catalysts

Abstract: Several phases reported as minor or major phases in the active MoVTeNbO catalysts have been prepared and investigated for the oxidation of propane into acrylic acid. Activity and selectivity of pure phases and mixtures of phases obtained either directly from synthesis or by co-grinding have been compared. The results obtained confirmed that the orthorhombic M1 phase is the most active and selective phase and is responsible for the major part of the efficiency of the best catalysts. However, they also clearly demonstrated that a synergism due to a cooperation between phases occurs, similar to that previously proposed between the M1 [(Te2O)M20O56] and M2 [(TeO)M3O9] phases for the ammoxidation of propane. The origin of this phase cooperation is discussed.

Calcination of Co-Based Fischer–Tropsch Synthesis Catalysts

Abstract: The calcination of Co-based slurry-phase Fischer–Tropsch synthesis catalysts was investigated. Fischer–Tropsch synthesis is part of the gas-to-liquids (GTL) process that produces gas oil and naphtha from natural gas. For the GTL process, the preparation of highly active Co-based catalysts is of utmost importance. This paper shows that the conditions during the calcination of impregnated cobalt precursors have a significant influence on the performance of the final catalyst. The options of calcination in rotary kilns, furnaces and fluidized-bed reactors were considered. It was found that the catalyst performance is strongly dependent on the heating rate and the air-space velocity during the preferred option of fluidized bed calcination. The postulation that Co3O4 is not the preferred oxide phase of the calcined intermediate catalyst is supported by a temperature-programmed reduction (TPR) study.

Identification and Characterization of Active Sites and Their Catalytic Processes—the Cu/ZnO Methanol Catalyst

Abstract: In this paper, we review a series of hybrid QM/MM studies performed on the methanol synthesis catalyst. This work has required development of a new embedded cluster approach suitable for polar surfaces of ionic oxides, which has been implemented in the computer code ChemShell. Five themes have been pursued: simulation of polar oxide surfaces of ZnO; their hydrogenation; adsorption of reactants, intermediates and products on active sites; characterization of active sites by their spectroscopic signature; investigation of metal-support interactions and modeling of the formation and oxidation of supported nanoclusters.

Trends in Fischer–Tropsch Reactor Technology—Opportunities for Structured Reactors

Abstract: Reactor design for Fischer–Tropsch synthesis has always been a nonoptimal compromise. Using the systematic approach for multiphase reactor selection by Krishna and Sie, the requirements for three different strategy levels, namely catalyst design, injection and dispersion strategies, and hydrodynamic flow regime, are analyzed. The presently commercially used reactor types are discussed on the basis of this analysis. Two novel reactor types, gas lift reactor and monolithic reactor, are introduced as alternative solutions. Especially, the monolithic reactor appears to fit the reactor needs of the Fischer–Tropsch synthesis very well.

Naturally chiral surfaces

Abstract: Chiral surfaces are of growing importance as a result of their potential for use in enantioselective chemical processes. By far, the most widely used and commonly studied chiral surfaces are those prepared by templating of an achiral surface with chiral organic ligands. It is possible, however, to prepare naturally chiral surfaces by a number of means. This paper describes the various types of chiral surfaces. In addition, data are presented to suggest that naturally chiral surfaces of metals can be prepared by a process of imprinting in which chiral adsorbates induce reconstructions that create chiral kinks on metal surfaces.

Reflections on routes to enantioselective solid catalysts

Abstract: The intellectual and practical challenges of preparing enantioselective solids are discussed and two routes to the possible assembly of chiral inorganic structures highlighted. These synthesis methods are the imprinting of amorphous structures and the crystallization of chiral, crystalline, and microporous materials. For both types of solids, there is the potential to create chirality within the structure that may allow for long-term, robust use in catalytic reactions without significant loss of enantioselectivity.

Mechanisms of Iron Activation on Fe-Containing Zeolites and the Charge of α-Oxygen

Abstract: According to previous Mossbauer data [1] α-sites formation at the activation of Fe-containing zeolites is accompanied by irreversible “self-reduction” of the iron, proceeding without participation of an external reducing agent. Reduced Fe2+ ions are inert to O2 but are reversibly oxidized to Fe3+ by N2O, generating the α-oxygen species, Oα, which provide selective oxidation of hydrocarbons.

On the Enhanced Selectivity of HZSM-5 Modified by Chemical Liquid Deposition

Abstract: The external surfaces and pore mouth regions of HZSM-5 samples with different crystal sizes were modified by chemical liquid deposition (CLD) with tetraethoxysilane (TEOS), which led to the passivation of unselective acid sites. The modification was found to be more effective for zeolite samples of larger crystal size. The diffusivity of o-xylene was substantially reduced after silylation, while the diffusivity of toluene hardly changed. Dealumination of the external surface of the zeolite crystals enhanced the silylation effects; this was related primarily to the removal of acid sites associated to extra-framework alumina. For the modified catalysts, a significant increase in the selectivity to p-xylene in the disproportionation of toluene was achieved.

A study of the functionalities of the phases in Mo-V-Nb-Te oxides for propane ammoxidation

Abstract: Catalysts belonging to the Mo-V-Nb-Te-O system have been prepared with both a slurry method and hydrothermal synthesis and were tested for propane and propylene ammoxidation to acrylonitrile. All samples were characterized with BET, XRD, ICP and XPS. The catalysts were found to consist of three phases, to which activity and selectivity correlations were made. The results indicate that both an orthorhombic phase and a hexagonal phase are needed to have an active and selective catalyst. The orthorhombic phase is the most active for propane conversion although less selective than the hexagonal phase for the conversion of formed propylene to acrylonitrile.

Carbon Dioxide Reforming of Methane Over Nanocomposite Ni/ZrO2 Catalysts

Abstract: A systematic study of the “size effect” of zirconia nanocrystals on nickel-catalyzed reforming of methane with CO2 shows that extremely stable Ni/ZrO2 catalysts are obtainable by hydrogen reduction of impregnated nickel nitrate on zirconia particles with sizes less than 25 nm. The same preparation method with larger particles of zirconia results in catalyst samples that deactivate rapidly in the reforming reaction. Comprehensive characterization with XRD, TPR/TPD, and TEM shows that the stable Ni/ZrO2 catalysts are better described as nanocomposites of size comparable to Ni metal (9-15 nm) and zirconia (7-25 nm) nanoparticles. The high percentage of the Ni-zirconia boundary or perimeter in the nanocomposite catalysts is believed to be crucial for the extremely stable catalytic activity.

In Situ Surface Analysis in Selective Oxidation Catalysis: n-Butane Conversion Over VPP

Abstract: In situ analysis of the surface of a working selective oxidation catalyst is an essential yet rarely conducted experiment in attempts to derive structure–function relationships. The case study of n-butane oxidation over vanadyl pyrophosphate (VPP) is used to develop a general working hypothesis and to illustrate that the molecular properties of the substrate sets boundary conditions on the surface chemical properties of the catalyst. Experiments using in situ X-ray photoelectron spectroscopy (XPS) and in situ low-energy X-ray absorption spectroscopy are used to derive compositional, electronic, and geometric structural information of the surface of the working VPP. These data allow the conclusion that a surface phase different from VPP must be present covering at least part of the active material. The recent data together with literature observations are used to derive a scenario explaining the function of VPP as a unique catalytic system.

Propane Selective Oxidation Over Monophasic Mo–V–Te–O Catalysts Prepared by Hydrothermal Synthesis

Abstract: Two distinct phases, orthorhombic and hexagonal, of Mo–V–Te–O mixed oxide catalysts were prepared separately by the hydrothermal synthetic method and solid-state reaction, and these catalysts were tested for propane selective oxidation to acrylic acid. The hydrothermally synthesized orthorhombic phase of the Mo–V–Te–O catalyst showed high activity and selectivity for the oxidation of propane into acrylic acid. This catalyst also showed extremely high catalytic performance in the propene oxidation, producing acrylic acid in a high yield. The hexagonal Mo–V–Te–O catalyst was formed via the solid-state reaction between the orthorhombic Mo–V–Te–O and α-TeVO4. This phase showed poor activity to both propane and propene oxidations, although the hexagonal phase was constructed with the octahedra of Mo and V similar to the orthorhombic phase. Reaction kinetics study over the catalyst with orthorhombic structure revealed that propane oxidation was of first order with respect to propane and nearly zero order with respect to oxygen, suggesting that the rate-determining step of the reaction is C–H bond breaking of propane to form propene. Structural effects on the catalytic oxidation performance were discussed.

Low-Temperature Fischer–Tropsch Synthesis on Cobalt Catalysts—Effects of CO2

Abstract: Effects of CO2 on low-temperature Fischer–Tropsch synthesis were investigated with four different cobalt catalysts in an experimental study. CO2 was found to behave as an inert gas component with three catalysts, however, a negative effect on Fischer–Tropsch reaction rate and catalyst deactivation was observed in one case (Co-La-Ru-SiO2). CO2 effects in a large-scale FTS slurry reactor were simulated by means of a mathematical reactor model using the kinetic information gained in the experiments. The reactor volume required for achieving a desired CO conversion must be higher if the syngas contains CO2, more strongly in cases where the catalyst exhibits a deactivation behavior in the presence of CO2. These model calculations can contribute to process optimization with respect to CO2 removal before synthesis.

Design, Synthesis, and Catalytic Properties of Silica-Supported, Pt-Promoted Iron Fischer–Tropsch Catalysts

Abstract: Silica-supported iron catalysts (Fe/SiO2, FePt/SiO2, and FePtK/SiO2) were prepared using a novel nonaqueous (acetone) evaporative deposition technique. This preparation leads to relatively well-dispersed iron phases at modest (10%) metal loadings. Moreover, catalytic activities of these catalysts for Fischer–Tropsch synthesis are high and comparable to industrially relevant precipitated iron catalysts. Catalyst activities were tested following a nonregular L18 orthogonal array that enabled the number of 150-h activity tests to be reduced from 54 to 18; this statistical design was augmented with five additional runs to provide replication. Primary independent variables affecting catalysts' activity were promoter type, pretreatment gas composition (H2, H2/CO, or CO), pretreatment temperature (250, 280, or 320 °C), and reaction temperature (250 or 265 °C); iron carbide level measured from Mossbauer spectroscopy was correlated with activity in a separate analysis. Activity was found to increase in the order Fe/SiO2, FePt/SiO2, and FePtK/SiO2. For a given catalyst composition, activity increases to a maximum with increasing pretreatment temperature and increasing time. Catalyst activity was also positively correlated with increasing chi-carbide content for Fe/SiO2 and FePt/SiO2 catalysts but not for FePtK/SiO2. While pretreatment atmosphere greatly influences initial activity–time behavior, activity is less dependent on pretreatment after about 150 h of reaction. Steady-state methane and C2+ hydrocarbon selectivities (CO2-free basis) for the FePtK/SiO2 catalyst at 250–265 °C, 10 atm, and H2/CO = 1 are 7–9 and 91–93%, respectively, while its hydrocarbon productivity at 250 °C (normalized to 15 atm, H2/CO = 0.7) of 0.27 g HC/gcat/h is comparable to those reported for unsupported precipitated iron catalysts of high activity and selectivity. These results indicate that preparation of an active, selective, stable, attrition-resistant supported iron catalyst for Fischer–Tropsch synthesis is feasible. Promise for additional improvements in catalyst performance through application of advanced preparation methods and optimization of catalyst chemical and physical properties is also indicated.

Is True Ethane Oxydehydrogenation Feasible

Abstract: Oxydehydrogenation of ethane as a route to ethylene has the attractive feature of removing the thermodynamic equilibrium conversion limitation of the simple dehydrogenation. For example, in the dehydrogenation of ethane to ethylene, the maximum conversion possible at 1000 °C is 51%, while essentially complete (100%) conversion is possible even at ambient conditions. The best catalysts discovered to date are those from Union Carbide's work (in the late 1970s and early 1980s), which operate at 300-400 °C. These reducible Mo–V–Nb oxide catalysts are thought to react via a surface ethoxide intermediate on a Mo or V site that can then undergo a β-elimination process to form ethylene. On the other hand, the surface ethoxide can be oxidized further to form surface acetate, which leads to acetic acid on hydrolysis with water. Aside from these low-temperature reducible catalysts, many catalysts containing reducible metal oxides and non-reducible metals are known to convert ethane to ethylene at 500-800 °C. It is proposed that these catalysts are essentially dehydrogenation catalysts where the H2 formed from straight dehydrogenation or during a surface intermediate stage after H-abstraction is converted to H2O, thereby shifting the dehydrogenation equilibrium. Therefore, the big question, the challenge, and the opportunity remains as to whether true oxydehydrogenation is possible at relative low-to-moderate temperatures? This challenge/opportunity will be discussed in the backdrop of some of the recent advances in alkane activation.

A study on the properties and mechanisms for NOx storage over Pt/BaAl2O4-Al2O3 catalyst

Abstract: The NOx storage catalyst Pt/BaAl2O4-Al2O3 was prepared by a coprecipitation--impregnation method. For fresh sample, the barium mainly exists as the BaAl2O4 phase except for some BaCO3 phase. The BaAl2O4 phase is the primary NOx storage phase of the sample. EXAFS and TPD were used for investigating the mechanism of NOx storage. It is found that two kinds of Pt sites are likely to operate. Site 1 is responsible for NO chemisorption and site 2 for oxidizing NO to nitrates and nitrites. When NO adsorbs on the sample below 200 °C, it mainly chemisorbs in the form of molecular states. Such adsorption results in an increase of the coordination magnitude of Pt-O, and a decrease of that of Pt-Pt and Pt-Cl. The coordination distance of Pt-Pt, Pt-Cl and Pt-O also increases. When the adsorption occurs above 200 °C, NO can be easily oxidized by O2, and stored as nitrites or nitrates at the basic BaAl2O4. Site 2 is regenerated quickly. A high adsorption temperature is favorable for nitrate formation.