Showing papers in "Catalysis Today in 1994"

Application of ZrO2 as a catalyst and a catalyst support

Abstract: Examples of the application of ZrO 2 for catalysts and catalyst supports are reviewed. The specificity of the structure and the surface properties including the behavior of surface OH groups are introduced. The catalytic properties of dispersed and promoted ZrO 2 are presented. The novel application to the photocatalytic total decomposition of water is also presented.

Oxidation of alcohols with molecular oxygen on platinum metal catalysts in aqueous solutions

Abstract: The platinum-catalyzed oxidation of alcohols with molecular oxygen in aqueous medium was discovered one and a half centuries ago [ 1,2]. The method has been improved with the general development of heterogeneous catalysis. At the beginning of this century it was found [ 31 that not only platinum, but dispersed palladium was also a good catalyst for the reaction. Platinum on various supports was tested and compared by Heyns [ 41 when he extended the method to the synthesis of L-ascorbic acid. The next milestone was the promotion of platinum metals with heavy metals such as Bi or Pb to overcome the deactivation problems and change the direction (selectivity) of the reaction [ 5-71. The present review will be focused on the application of biand multi-metallic catalysts in the aqueous phase oxidation of alcohols with molecular oxygen. The reactions discussed are the transformation of primary alcohols to aldehydes or carboxylic acids, and secondary alcohols to ketones. Special attention will be given to the various reasons for the loss of catalytic activity and the role of promoters in the suppression of deactivation. A detailed discussion of the earlier results of alcohol oxidation on platinum metal catalysts, especially in the field of carbohydrate chemistry, can be found in previous reviews [ 818 1. The mild reaction conditions and high selectivity in the oxidation of polyols made the method very attractive for carbohydrate chemistry. As an example, the catalytic route for the oxidation of D-glucose to D-gluconic acid based on the application of promoted Pd catalysts in aqueous alkaline medium can compete economically with the generally applied biochemical oxidation [ 19,201. Various chelating agents, detergent builders or food additives may be prepared by this way from relatively cheap starting materials of natural origin.

Carbon dioxide reforming of methane to synthesis gas over supported Ni catalysts

Abstract: Carbon dioxide reforming of methane to synthesis gas has been investigated over supported Ni catalysts in the temperature range of 500–850°C. Addition of CaO (10mol%) promoter to the Ni/γ-Al2O3 resulted in an increase of reaction rate and an improvement of catalyst stability, which may be related to enhanced reducibility of the promoted catalyst. The kinetic studies show that the overall reaction can be described by a Langmuir-Hinshelwood mechanistic scheme, assuming that methane dissociation is the rate determining step. In addition to adsorbed CO and formate species, three types of carbonaceous species, Cα, Cβ and Cγ, were found to exist on the Ni catalyst. While the active Cα, species is suggested to be responsible for CO formation, the less active Cβ and Cγ species are attributed to causing catalyst deactivation.

Novel catalytic decomposition and reduction of NO

Abstract: Nitrogen monoxide (NO) is thermodynamically unstable relative to Nz and O2 at low tempera~res, and therefore its catalytic decomposition is the simplest and cheapest method for the removal of NO from exhaust streams [ l-51. To date, however, no suitable catalyst of consistently high activity has been found, except for a few described later. Most previous attempts to develop practical decomposition catalysts have dealt with noble metals and metal oxides. Some of these materials are active in the reduced state, but oxygen contained in the feed gas or released by the decomposition of NO competes with NO for the adsorption sites and poisons the activity. To remove surface oxygen and regenerate catalytic activity, high reaction temperatures and/or gaseous reductants are required. Thus, catalytic reduction processes using NH,, HC or CO have been applied for removing NO. In the present catalytic reduction processes, selective and non-selective methods are used in industrial boilers and vehide engines, respectively. Many books and reviews [4-lo] have described the details of these processes. Here the disadvantages or problems that each of the present reduction processes suffers are summarized [ 91. (a) In the selective catalytic reduction system on V,OS-TiO,-W03 with ammonia as a reductant there are several disadvantages such as high costs of facilities and inning and leakage of unreacted ~monia. (b) The automobile catalytic converter is the only technology available for meeting the most stringent emission standards. In this technology Pt-Pd-Rh catalysts are preferentially used with several limitations such as using unleaded gasoline and maintaining a specified air/fuel ratio. However, this system cannot meet the requirements of newly developed engines in which the air/fuel ratio has been made lean to an air-rich region, because the exhaust contains a considerable amount of oxygen and the present three-way catalysts do not work under such conditions.

Deactivation of supported nickel catalysts during the reforming of methane by carbon dioxide

Abstract: Several supported nickel catalysts were tested for the methane reforming reaction at 700°C. The initial activity is found to depend essentially on the state of the nickel phase (reduction and dispersion) and little on its environment (support, additive). The product distribution is controlled by the WGS equilibrium. The zero order aging process is mostly due to carbon deposition, though slight Ni sintering also occurs. Among the deposited carbon, only a stable form, possibly arising from the CO disproportionation, would poison the Ni particles. Another form, less stable and arising from methane activation, is rapidly accumulated on the catalyst, but at a low level and limited extent.

Acid-base bifunctional catalysis by ZrO2 and its mixed oxides

Abstract: Examples of acid-base bifunctional catalysis by ZrO2 and its mixed oxides are summarized. The hydrogenation of olefins, carbon monoxide and aromatic carboxylic acids over ZrO2 are also examples of acid-base bifunctional catalysis which occur by heterolytic splitting of a hydrogen molecule (H2) into a proton (H+) and a hydride ion (H−) on the acid-base pair sites. Industrial applications of ZrO2 catalysts are demonstrated, the importance of acid-base bifunctional catalysis by ZrO2 and its mixed oxides being emphasized.

Sulfated zirconia as a hydrocarbon conversion catalyst

Abstract: Platinum containing sulfated zirconia is a more active catalyst for hydrocarbon isomerization and cracking than most, if not all, zeolite cracking catalysts. A survey of the literature data suggests that the preparation technique, initial calcination and final catalyst treatment play important roles in determining catalyst activity. The final treatment probably is the most important and least controlled step in most studies to date. Whether the catalytic site involves Lewis or Bronsted acidity is debated; our data indicate Bronsted acidity as more likely. The role of a supported metal, e.g., Pt, is to provide a hydrogenation role but many attribute a second role for it in generating acidity through hydrogen spillover. For hexadecane conversions, increasing the hydrogen pressure increases the cracking to lower molecular weight hydrocarbons. The presence of small amounts of hydrogen donors are claimed to improve isomerization selectivity.

Selective reduction of NO by hydrocarbons and oxygenated hydrocarbons over metal oxide catalysts

Abstract: A survey of recent studies on metal oxide and related catalysts for the selective reduction of NO by hydrocarbons and oxygenated hydrocarbons over metal oxide catalysts is presented, focussing mainly on the work of the author's group. The contents include single metal oxides, sulfate-promoted metal oxides, binary metal oxides, metal-supported oxides and supported noble metals. Then the catalytic performance of alumina, one of the most active single metal oxides is described. Reaction mechanisms concerning the behavior of these catalysts are also discussed.

Heterogeneous catalytic oxidations in the manufacture of fine chemicals

Abstract: In bulk chemicals manufacture, traditional environmentally unacceptable processes have largely been replaced by cleaner, catalytic alternatives. Indeed, catalytic oxidation is the single most important technology for the conversion of hydrocarbon feedstocks (olefins, alkanes and aromatics) to industrially important oxygenated derivatives [ 11. Both heterogeneous, gas phase oxidations employing solid catalysts and homogeneous liquid phase oxidations are applied (see Table 1) . The latter tend to employ soluble metal salts as catalysts. The fine chemicals industry, in contrast, is the domain of synthetic organic chemists who, generally speaking, clung to the use of stoichiometric quantities of classical inorganic oxidants such as potassium dichromate and potassium permanganate. Because much smaller production volumes are involved there was

Nature of hydrous zirconia and sulfated hydrous zirconia

Abstract: The structure of crystalline hydrous zirconias and their methods of formation are described. A mechanism for the hydrolytic polymerization of Zr IV to obtain the several forms of hydrous oxide is presented. The method by which sulfate ion binds to the surface of zirconia particles is explored. It is concluded that the superacidity of sulfated zirconia arises from the presence of isolated bisulfate groups acting in conjunction with strong adjacent Lewis acid sites. Thus, both Bronsted and Lewis acid sites are present in sulfated oxides.

Selective catalytic reduction of nitrogen monoxide by methane on zeolite catalysts in an oxygen-rich atmosphere

Abstract: Selective reduction of NO by CH 4 on zeolite catalysts has been studied. After an investigation of the catalytic features of various cation-exchanged zeolites, we found that Ga-ZSM-5 and In-ZSM-5 were highly active and selective for NO reduction by CH 4 . We also showed that this reaction was moderately promoted even by H-form ZSM-5, mordenite, and ferrierite. From the catalytic performance of Ga-and In-ZSM-5 in NO-CH 4 -O 2 , NO 2 -CH 4 -O 2 , and NO-O 2 reactions, it is concluded that the selective reduction of NO on these catalysts proceeds in two stages: (1) NO is oxidized to NO 2 on zeolite acid sites, (2) NO 2 and CH 4 react on gallium or indium sites. The highly selective features of Ga and In in the zeolite for the NO 2 -CH 4 reaction seem to be attributed to the coordinatively unsaturated nature of these sites which adsorb both of these reactants on the same site. Effect of water vapor on NO conversion was also investigated. It was found that Ga-ZSM-5 was strongly inhibited by steam, while In-ZSM-5 was fairly active even in the presence of 10% steam.

Study of mixed steam and CO2 reforming of CH4 to syngas on MgO-supported metals

Abstract: The activity of mixed steam and CO2 reforming of CH4 to produce synthesis gas was investigated and compared with those of steam reforming alone and CO2 reforming alone at 600–900°C under atmosphere pressure on MgO-supported noble metals. Mixed reforming shows a far lower CH4 conversion than the value for thermodynamic equilibrium. The activity decreases following the order Ru,Rh > Ir > Pt,Pd. Little deactivation was observed for Ru, Rh and Ir catalysts. An isotope labelled 13CO2 experiment was carried out in situ for mixed reforming on Rh/MgO and the results suggest that CO2 dissociates as CO-M and O-M. The results of the temperature program reaction (TPR) of mixed reforming shows that CH4 adsorbs and dissociates before reaction starts and that CO2 reforming and steam reforming start simultaneously. A possible reaction mechanism is discussed.

Biocatalysis with polyphenol oxidase: a review

Abstract: A comprehensive review of the properties of polyphenol oxidase (EC 1.14.18.1) is given, including information relevant to the application of the enzyme as a biocatalyst, in aqueous and organic media. Literature reporting the use of the enzyme as a biocatalyst, and as a model enzyme in general studies of biocatalysis, is summarised.

Reforming of methane with carbon dioxide to synthesis gas over supported Rh catalysts

Abstract: Reforming of methane with carbon dioxide to synthesis gas (CO/H2) has been investigated over rhodium supported on SiO2, TiO2, γ-Al2O3, MgO, CeO2, and YSZ (ZrO2 (8 mol% Y2O3)) catalysts in the temperature range of 650–750°C at 1 bar total pressure. A strong carrier effect on the initial specific activity, deactivation rate, and carbon accumulation was found to exist. A strong dependence of the specific activity of the methane reforming reaction on rhodium particle size was observed over certain catalysts. Tracing experiments (using 13CH4) coupled with temperature-programmed oxidation (TPO) revealed that the carbon species accumulated on the surface of the Rh/Al2O3 catalyst during reforming reaction at 750°C are primarily derived from the CO2 molecular route. The amount of carbon present on the working catalyst surface which is derived from the CH4 molecular route is found to be very small.

Modelling and simulation of heterogeneous oxidation of methane on a platinum foil

Abstract: The heterogeneous oxidation of methane-air mixtures in a stagnation point flow onto a platinum foil is investigated numerically and results are compared with experiments. The analysis includes a detailed reaction mechanism in the gas phase as well as on the surface. The heterogeneous ignition occurring around 600°C, extinction, and autothermal behaviour are interpreted in terms of elementary steps at the gas-surface interface.

Zirconocene catalysts for olefin polymerization

Abstract: Metallocenes in combination with methylalumoxane as cocatalyst form extremely active catalysts for olefin polymerization and have become a research topic of growing interest in recent years. Meanwhile a great number of symmetric and chiral zirconocenes have been synthesized to give totally different structures of isotactic, syndiotactic, stereoblock, or isoblock polymers. The isotactic sequence length of polypropylene is influenced by the nature of the ligands of the metallocenes. It has become possible to polymerize cyclic olefins like cyclopentene, norbornene, or dimethanooctahydronaphthalene with different chiral zirconocenes without any ring opening reaction. The crystalline polycycloalkenes show extremely high melting points which are between 400 and 600°C. Copolymers of cyclo-alkenes with ethene are amorphous and transparent with a glass transition temperature between 120 and 160° C and they can be used as materials for optical discs and fibers.

Chapter 3.2 X-ray diffraction analysis

Abstract: X-ray diffraction analyses were carried out by nine laboratories. The equipment and experimental conditions of analysis were generally different. However, the results obtained were very similar and coherent. Samples with a high vanadium content showed exclusively the presence of TiO 2 (anatase) and crystallites of V 2 O 5 as long as not pretreated. No TiO 2 (rutile) or other vanadium oxide was observed. V 2 O 5 was also observed in low vanadium content samples in two analyses. No other crystalline phases could be observed, although the presence of very weak lines were detected. These lines were not identified as vanadium oxide, titanium oxide or vanadium-titanium oxide. In the pretreated samples containing vanadium, the presence of V 2 O 5 lowered the temperature of the phase transition of anatase to rutile.

Research on new DeNOx catalysts for automotive engines

Abstract: The performance and durability of Cu-ZSM-5 catalysts for selective reduction of NOx in an oxidizing atmosphere were studied. These catalysts exhibited substantial NOx conversion performance in a leanburn engine exhaust and the simulated gas. However, the temperature dependence, SV performance, and thermal stability must still be improved.

Partial oxidation of alkanes over noble metal coated monoliths

Abstract: The production of olefins and synthesis gas (CO and H2) from CH4, C2H6 C3H8, and n-C4H10 in the presence of air or O2 at atmospheric pressure has been examined over monoliths coated with various metals at residence times between 10−3 and 10−2 s. Experiments are carried out by feeding gases at atmospheric pressure and 25°C into an autothermal monolith reactor operating between 800 and 1200°C. In CH4 oxidation, > 90% selectivity to syngas is achieved over Rh at > 90% CH4 conversion, while Pt forms more H2O, Ir sinters, and Ni volatilizes. In C2H6 oxidation, Pt forms up to 70% selectivity to C2H4, while Rh gives up to 70% selectivity to syngas, and Pd forms solid carbon. Carbon formation is suppressed under conditions where graphite formation is predicted on some metals, but forms readily on others. The C2H6 results can be interpreted quite simply in terms of hydrogen abstraction by adsorbed oxygen to form the alkyl, followed by β-hydrogen elimination to yield the olefin (Pt) or pyrolysis to yield syngas (Rh) or carbon (Pd). When C3H8 and n-C4H10 are used, considerable cracking to ethylene plus alkane is also observed.

Chapter 7.1 X-ray photoelectron spectroscopy of TiO2/V2O5 catalysts

Abstract: A comparison of the XPS data produced by several laboratories working with the same V2O5/TiO2 catalysts samples is performed and reveals large discrepancies. From the results reported by the participating laboratories (influence of pretreatment, of data acquisition conditions and of data treatment procedures), recommendations are proposed in order to improve the reproducibility of XPS studies.

Carbon dioxide reforming of methane over lanthanum-modified catalysts in a fluidized-bed reactor

Abstract: Nickel on lanthanum-modified aluminas were prepared and tested as catalysts for the CO 2 + CH 4 ⇔ 2CO + 2H 2 reaction in a fluidized-bed reactor. Attrition tests show that lanthanum increases the strength of the carrier significantly. TPR and XRD results indicate that nickel enters into positions in the Al-O spinel blocks of the subsurface LaAl 12 O 19 phase. This results in lower reducibility and thus lower activity for the reforming reaction as compared to nickel on unmodified alumina. While the unmodified catalyst deactivated rapidly under the chosen test conditions, the modified catalysts seem to be stable over several hours. However, the stability is sensitive to the pretreatment conditions of the catalysts.

Nature and properties of nickel-containing mixed oxides obtained from hydrotalcite-type anionic clays

Abstract: Anionic clays with a hydrotalcite-type (HT) structure may be useful precursors of multicomponent catalysts, such as, for instance, Ni/Al catalysts for steam reforming and methanation reactions. The structure and the synthesis of HT anionic clays are briefly reviewed to illustrate the specific properties of these phases and the key factors involved in the preparation of pure compounds. By comparing the properties of the oxides obtained by thermal decomposition of Ni/Al and Ni/Cr HT anionic clays at different temperatures, it was found that the HT structure is necessary, but not sufficient, to obtain well dispersed and stable samples; also necessary is the presence of aluminum as the trivalent element. However, for both Ni/Al and Ni/Cr samples the formation of stoichiometric spinel phases, which takes place at different temperatures depending on the composition, has a negative effect, giving rise to remarkable increases in both NiO crystal size and reducibility. On the basis of bulk and surface analyses (before and after NaOH treatment), a model for Ni/Al oxides obtained from HT anionic clays is proposed, which involves the presence of NiO, a Ni-doped alumina phase and a spinel-type phase. This last phase is thought to be responsible for the thermal stability and low reducibility of the NiO particles.

OCM in a fixed-bed reactor: limits and perspectives

Abstract: The oxidative coupling of methane (OCM) over a La 2 O 3 /CaO catalyst was studied in a poly tropic fixed-bed reactor (I.D. = 15 mm, W/F = 0.15 g · s/ml). Reaction conditions for stable operation were determined. (1) A minimum inlet temperature of 580°C was necessary to initiate the reaction. (2) The maximum hot-spot temperature of 1000°C limited the highest oxygen inlet concentration to 20%. The temperature gradients in the bed amounted to 250 K. The influence of the reaction conditions on the C 2+ selectivity was investigated by testing the effects of temperature ( T inlet = 580–860°C), oxygen concentration (C O 2 = 5–20%) and particle diameter ( d p = 250–350 μm, and pellets of h p = 4 mm and d p = 4 mm). The C 2+ selectivity ran through a maximum with increasing temperature and decreased with rising inlet oxygen concentration. Mass-transfer limitations, which occurred when applying pellets, resulted in a drop of C 2+ selectivity. Highest C 2+ yields amounted to 15.5% ( X CH 4 = 31%, S c 2+ = 51%). Distributed feed of oxygen was tested as a means to cope with the high temperature gradients and to increase C 2+ . selectivity. Upon applying this mode of operation, oxygen concentrations up to 30% could be converted. However, no improvement of C 2+ selectivity and yield compared to cofeed operation was achieved.

Role of anions in the surface area stabilisation of zirconia

Abstract: Interest in the use of zirconia as both a catalyst and a support has been growing over the last few years. The use of sulphated zirconia as a superacid catalyst has generated considerable interest because of the need to reformulate gasoline using more environmentally friendly catalysts. The presence of sulphate has been shown to enhance the stability of both the surface area and tetragonal phase of the zirconia. The influence of anions (sulphate and carbonate) on the properties of zirconium hydroxide during calcination was studied. It was concluded that the crystallization of zirconia from zirconium hydroxide occurred via three stages. The overlap of these stages is determined by both the anions present and the drying conditions. In addition sulphate increases zirconium-zirconium separation which makes oxide formation more difficult.

Fixed-bed reactors with periodic flow reversal: experimental results for catalytic combustion

Abstract: The influence of design and operating parameters on the behavior of a fixed-bed reactor with periodic flow reversal has been studied in a laboratory set-up for the case of catalytic total oxidation. The results are in accordance with detailed model simulations published elsewhere. They show that the periodic operation is completely dominated by the regenerative heat exchange and that steady-state kinetics can be used. Like any other autothermal reactor, a fixed-bed reactor with periodic flow reversal has to be operated in the ignited steady state. It was shown that totally and partially ignited steady states may exist under the same operating conditions if several combustible components with different ignition temperature are present in the feed. Hot gas withdrawal from the middle of the packed bed proved to be a suitable method to utilize almost all of the heat of reaction at the highest temperature in the reactor and to prevent high temperature peaks at the respective exit valves. Together with an appropriate design of the fixed bed, composed of inert front and end sections with low effective axial conductivity and an active portion with large axial conductivity, hot gas withdrawal allows for an efficient control of the reactor under the conditions of both high and low feed concentrations.

Catalysis and large-scale conversion of natural gas

Abstract: The efficiency of large-scale conversion of natural gas into chemicals is ca. 80% of the thermodynamic optimum. Syngas manufacture constitutes a major fraction of costs. New approaches to syngas manufacture are summarized and feasibilities of direct conversion routes are discussed. Catalysis may play an increasing role in the power industry. This includes catalytic combustion, fuel cells, chemical recuperation, and energy transport systems.