Silver on alumina catalyst

Ethylene oxide is produced in large, multitubular reactors cooled by pressurized boiling Hquids, eg, kerosene and water. Up to 100 metric tons of catalyst may be used in a plant. Typical feed stream contains about 30% ethylene, 7—9% oxygen, 5—7% carbon dioxide the balance is diluent plus 2—5 ppmw of a halogenated moderator. Typical reactor temperatures are in the range 230—300°C. Most producers use newer versions of the Shell cesium-promoted silver on alumina catalyst developed in the mid-1970s. 

Ethylene can be oxidized to EO over a silver-on-alumina catalyst in 1-in-diameter tubes approximately 20 ft long. A modem EO plant produces 200 tons/day, with a typical reactor consisting of 1000 tubes with an EO selectivity of 80% with a 4 1 C2H4 02 ratio at approximately 50% conversion of Oz. EO formation is mildly exothermic, while the competing complete combustion reaction... 

In 1986, a process to produce 1 by the continuous, vapor phase oxidation of 1,3-butadiene over a silver on alumina catalyst was discovered by Monnier and Muehlbauer of the Kodak Corporate Research Laboratories (10). The process was further developed and commercialized by Eastman Chemical Company at its Longview, Texas plant (11). Following this discovery of an economical process for 1, the production of 2,5-DHF was once again of commercial interest. 

Figure 7.14. NOx to N2 conversion over silver on alumina catalyst depending on reactor arrangements.

Figure 7.15. NOx to N2 activity with octane as a reducing agent over silver on alumina catalyst and a platinum oxidation catalyst depending on the distance between the catalysts (K. Eranen, L.-E.Lindfors, F. Klingstedt, D.Yu.Murzin, Continuous reduction of NOx with octane over a silver/alumina catalyst in oxygen-rich exhaust gases combined heterogeneous and surface mediated homogeneous reactions, Journal of Catalysis, 219 (2003) 25).

Fig. 8.6 NOx to N2 activity with octane as a reducing agent over a single bed and four layer silver on alumina catalyst. (From L-E. Lindfors, K. Eranen, F. Klingstedt, D.Yu. Murzin, Silver/alumina catalyst for selective catalytic reduction of NO, to N2 by hydrocarbons in diesel powered vehicles. Top. Catal. 28 (2004) 185. Copyright 2004 Springer).

Ethylene oxide synthesis is one of the largest-volume industrial processes with a produchon rate of some plants of several 100 0001 a (see original citahons in [4]). In 1995, the world capacity for ethylene oxide was approximately 11 200 0001 a. As industrial catalyst silver on alumina is employed. In addihon to large produc-... 

The preparation of precious metal supported catalysts by the HTAD process is illustrated by the synthesis of a wide range of silver on alumina materials, and Pt-, Pt-Ir, Ir-alumina catalysts. It is interesting to note that the aerosol synthesis of alumina without any metal loading results in a material showing only broad reflections by XRD. When the alumina sample was calcined to 900°C, only reflections for a-alumina were evident. The low temperature required for calcination to the alpha-phase along with TEM results suggest that this material was formed as nano-phase, a-alumina. Furthermore, the use of this material for hexane conversions at 450°C indicated that it has an exceptionally low surface acidity as evidenced by the lack of any detectable cracking or isomerization. 

Al-Saleh et al. [Chem. Eng. J., 37 (1988) 35] performed a kinetic study of ethylene oxidation over a silver supported on alumina catalyst in a Betty reactor. At temperatures between 513-553 K and a pressure of 21.5 atm, the observed reaction rates (calculated using the CSTR material balance) were independent of the impeller rotation speed in the range 350-1000 rpm (revolutions per minute). A summary of the data is ... 

Ag on alumina is an effiicient catalyst for deNOx removal but the drawback is die simultaneous formation of CO, requiring an oxidation catalyst behind a bed of silver on alumina. The activity depends on the distance between the catalysts, e.g. residence time between Ag/alumina and oxidation catalyst. When the Pt-oxidation catalyst is placed immediately behind the Ag/alumina bed, a significant drop in the NO to N2 activity is observed in comparison with the single Ag/alumina bed. As expected the oxidation catalyst removes completely the produced CO. However, when the distance between the two catalysts is extended, the conversion of NO to N2 improves to levels close to those recorded over the single Ag/alumina bed (Figure 7.15). 

Selective oxidation of primary OH groups in carbohydrate derivatives has been achieved using A -oxoammonium salts generated from (2,2,6,6-tetramethyl-piperidin-l-yl)oxy (TEMPO) and its derivatives as catalysts. The stoichiometric oxidants employed include sodium hypochlorite [48-50], sodium hypobromite [51, 52], and ammonium peroxodisulfate (using silver on alumina as a co-catalyst) [53, 54]. A representative protocol is shown in Scheme 12. 

Hernandez Carucci et of. [29] used a plug flow model for epoxidation of ethylene on silver/R-alumina catalysts. The silver/R-alumina catalysts were wash coated in stainless steel microchannels. Due to low conversions (<0.30% for kinetic experiments), the concentration and temperature gradients were small. The effect of internal diffiision could be neglected due to pure silver catalyst. The model was based on the competitive adsorption of ethylene and molecular oxygen over the silver surface. It was compared with experimental data in the microchaimels and had a very high degree of agreement (97%). 

Catalyst Selectivity. Selectivity is the property of a catalyst that determines what fraction of a reactant will be converted to a particular product under specified conditions. A catalyst designer must find ways to obtain optimum selectivity from any particular catalyst. For example, in the oxidation of ethylene to ethylene oxide over metallic silver supported on alumina, ethylene is converted both to ethylene oxide and to carbon dioxide and water. In addition, some of the ethylene oxide formed is lost to complete oxidation to carbon dioxide and water. The selectivity to ethylene oxide in this example is defined as the molar fraction of the ethylene converted to ethylene oxide as opposed to carbon dioxide. 

Ethylene oxide (qv) was once produced by the chlorohydrin process, but this process was slowly abandoned starting in 1937 when Union Carbide Corp. developed and commercialized the silver-catalyzed air oxidation of ethylene process patented in 1931 (67). Union Carbide Corp. is stiU. the world s largest ethylene oxide producer, but most other manufacturers Hcense either the Shell or Scientific Design process. Shell has the dominant patent position in ethylene oxide catalysts, which is the result of the development of highly effective methods of silver deposition on alumina (29), and the discovery of the importance of estabUshing precise parts per million levels of the higher alkaU metal elements on the catalyst surface (68). The most recent patents describe the addition of trace amounts of rhenium and various Group (VI) elements (69). 

Rearrangement of fluorine with concomitant ring opening takes place in fluorinated epoxides Hexafluoroacetone can be prepared easily from perfluo-ropropylene oxide by isomerization with a fluorinated catalyst like alumina pre treated with hydrogen fluoride [26, 27, 28] In ring-opening reactions of epoxides, the distribution of products, ketone versus acyl fluoride, depends on the catalyst [29] (equation 7) When cesium, potassium, or silver fluoride are used as catalysts, dimenc products also are formed [29]... 

Eranen, K., Klingstedt, F., Arve, K. et al. (2004) On the mechanism of the selective catalytic reduction of NO with higher hydrocarbons over a silver/alumina catalyst, J. Catal. 227, 328. 

Promoters may influence selectivity by poisoning undesired reactions or by increasing the rates of desired intermediate reactions so as to increase the yield of the desired product. If they act in the first sense, they are sometimes referred to as inhibitors. An example of this type of action involves the addition of halogen compounds to the catalyst used for oxidizing ethylene to ethylene oxide (silver supported on alumina). The halogens prevent complete oxidation of the ethylene to carbon dioxide and water, thus permitting the use of this catalyst for industrial purposes. 

The catalyst consists of silver supported on alumina and, while it is reasonably specific, appreciable amounts of C02 and H20 are also formed. Over the range of interest, the yield of ethylene oxide is relatively constant so that for present purposes, we may regard the reaction stoichiometry as... 

Compressed oxygen, and fresh and recycled ethylene, are heated, mixed, and then passed through a reactor with fixed beds of catalyst— silver oxide deposited on alumina pellets. In recent years the catalyst has been improved by the addition of promoters and inhibitors. (Promoters—in this case compounds of alkali or alkaline rare earth metals—enhance the activity of the catalyst inhibitors—in this case chlorine compounds—chloroethane, or vinyl chloride, reduce its rate of activity decline.)... 

In support of the conclusion based on silver, series of 0.2, 0.5, 1.0, 2.0, and 5.0 % w/w of platinum, iridium, and Pt-Ir bimetallic catalysts were prepared on alumina by the HTAD process. XRD analysis of these materials showed no reflections for the metals or their oxides. These data suggest that compositions of this type may be generally useful for the preparation of metal supported oxidation catalysts where dispersion and dispersion maintenance is important. That the metal component is accessible for catalysis was demonstrated by the observation that they were all facile dehydrogenation catalysts for methylcyclohexane, without hydrogenolysis. It is speculated that the aerosol technique may permit the direct, general synthesis of bimetallic, alloy catalysts not otherwise possible to synthesize. This is due to the fact that the precursors are ideal solutions and the synthesis time is around 3 seconds in the heated zone. 

The search for a new epoxidation method that would be appropriate for organic synthesis should also, preferably, opt for a catalytic process. Industry has shown the way. It resorts to catalysis for epoxidations of olefins into key intermediates, such as ethylene oxide and propylene oxide. The former is prepared from ethylene and dioxygen with silver oxide supported on alumina as the catalyst, at 270°C (15-16). The latter is prepared from propylene and an alkyl hydroperoxide, with homogeneous catalysis by molybdenum comp e ts( 17) or better (with respect both to conversion and to selectivity) with an heterogeneous Ti(IV) catalyst (18), Mixtures of ethylene and propylene can be epoxidized too (19) by ten-butylhydroperoxide (20) (hereafter referred to as TBHP). 

The same catalyst compositions used in the more important methane steam reforming [Eq. (3.1), forward reaction], may be used in methanation, too.222 All Group VIE metals, and molybdenum and silver exhibit methanation activity. Ruthenium is the most active but not very selective since it is a good Fischer-Tropsch catalyst as well. The most widely used metal is nickel usually supported on alumina or in the form of alloys272,276,277 operating in the temperature range of 300-400°C. 

Supported versions of such silver-catalyzed, three-component couplings have been recently reported. Silver oxide on multiwall carbon nanotubes or on alumina,102 as well as silver-doped zeolites,103 proved to be efficient and reusable catalysts for such coupling reactions. In the former, water was used as solvent, while in the latter, no solvent was required, making it a truly green process. 

Silver-alumina type catalysts are by far the most widely used, especially since they are the main catalytic source in the epoxidation of ethylene. Therefore, they are readily available and already have undergone extensive studies. Many systems have sought to utilize the presence of NO (another harmful environmental species) in gas feeds. In this case, the NO species would be reduced to N2, causing oxidation of the hydrocarbon with the support of the catalyst. Studies have helped to elucidate the active species on the catalyst surface at varying temperatures and species leading to the desired products (31). Results from a recent study point to the active silver species being a [Ag O Al] bound intermediate that leads to N2 formation (32). If the silver is present in nanoparticle form, it is simply believed to be a spectator. Other work showed mixed results on the benefit of silver-based alumina systems for the oxidation of methane and higher hydrocarbons. The effect is dependent on the type of reactor system prepared (33,34). 

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