Silver on alumina

Hydrogenation of Acetylenes. Complete hydrogenation of acetylenes to the corresponding alkanes, which maybe requited to remove acetylenic species from a mixture, or as a part of a multistep synthesis, may be accompHshed using <5 wt % palladium or platinum on alumina in a nonreactive solvent under very mild conditions, ie, <100°C, <1 MPa (10 atm). Platinum is preferred in those cases where it is desired to avoid isomeri2ation of the intermediate olefin. Silver on alumina also can be used in this appHcation as can unsupported platinum metal. 

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 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-... 

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... 

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. 

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.

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). 

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).

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. 

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).

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