Promoted Skeletal Copper Catalysts

The oxidative dehydrogenation of ethanolamine to sodium glycinate in 6.2 M NaOH was investigated using unpromoted and chromia promoted skeletal copper catalysts at 433 K and 0.9 MPa. The reaction was first order in ethanolamine concentration and was independent of caustic concentration, stirrer speed and particle size. Unpromoted skeletal copper lost surface area and activity with repeated cycles but a small amount of chromia (ca. 0.4 wt%) resulted in enhanced activity and stability. 

Recently, a novel process for the preparation of chromia promoted skeletal copper catalysts was reported by Ma and Wainwright (8), in which Al was selectively leached from CuA12 alloy particles using 6.1 M NaOH solutions containing different concentrations of sodium chromate. The catalysts had very high surface areas and were very stable in highly concentrated NaOH solutions at temperatures up to 400 K (8, 9). They thus have potential for use in the liquid phase dehydrogenation of aminoalcohols to aminocarboxylic acid salts. 

Table 1 Compositions and surface areas of unpromoted and chromia-promoted skeletal copper catalysts ...

Figure 6 First order rate constants for repeated cycles of ethanolamine dehydrogenation over chromia-promoted skeletal copper catalysts under standard conditions.

Promoted skeletal copper was also imaged with the FIB. In particular, both zinc- and chromium-promoted skeletal copper have a structure similar to that of un-promoted skeletal copper, but on a much finer scale [110,111], This observation agrees with the increased measured surface areas for these promoted catalysts. Figure 5.2a shows the fine uniform ligaments in a zinc-promoted skeletal copper catalyst. 

L. Ma, D.L. Trimm and M.S. Wainwright Promoted skeletal copper catalysts for methanol synthesis, in Advances of Alcohols Fuels in the World, - Proceedings of the XII International Symposium on Alcohol Fuels, Beijing, China, Tsinghua University Press, 1998, pp. 1-7. 

Unpromoted and chromia-promoted skeletal copper catalysts were prepared as described in detail previously (10, 11, 14, 15) by leaching a CUAI2 alloy, sieved to 106-211pm, in a large excess (500 mL) of 6.1 M NaOH, either alone or containing Na2Cr04 (0.004 M), for 24 hours at 5°C. 

The addition of other metals to promote skeletal catalysts has been the subject of a number of investigations including the use of V, Cr, Mn, and Cd for hydrogenation of nitro compounds [23], Cd in the hydrogenation of unsaturated esters to unsaturated alcohols [24], and Ni and Zn for the dehydrogenation of cyclo-hcxanol to cyclohexanone. The use of Cr as a promoter is particularly attractive as copper chromite catalysts arc used in a wide range of industrial applications. Lainc and co-workers [25] have made a detailed study of the structure of chromium promoted skeletal copper catalysts. 

Skeletal Cu-Zn catalysts show great potential as alternatives to coprecipitated Cu0-Zn0-Al203 catalysts used commercially for low temperature methanol synthesis and water gas shift (WGS) reactions. They can also be used for other reactions such as steam reforming of methanol, methyl formate production by dehydrogenation of methanol, and hydrogenolysis of alkyl formates to produce alcohols. In all these reactions zinc oxide-promoted skeletal copper catalysts have been found to have high activity and selectivity. 

The activity and stability of skeletal catalysts can be improved with the use of additives, often referred to as promoters. These can be added to the alloy before leaching, or alternatively can be added to the leaching solution [16-19], An example is the use of zinc to promote skeletal copper for the catalytic synthesis of methanol from synthesis gas [20-22], Mary other promoters have been considered, both inorganic and organic in nature. 

Following the development of sponge-metal nickel catalysts by alkali leaching of Ni-Al alloys by Raney, other alloy systems were considered. These include iron [4], cobalt [5], copper [6], platinum [7], ruthenium [8], and palladium [9]. Small amounts of a third metal such as chromium [10], molybdenum [11], or zinc [12] have been added to the binary alloy to promote catalyst activity. The two most common skeletal metal catalysts currently in use are nickel and copper in unpromoted or promoted forms. Skeletal copper is less active and more selective than skeletal nickel in hydrogenation reactions. It also finds use in the selective hydrolysis of nitriles [13]. This chapter is therefore mainly concerned with the preparation, properties and applications of promoted and unpromoted skeletal nickel and skeletal copper catalysts which are produced by the selective leaching of aluminum from binary or ternary alloys. 

It is the reprecipitation of Zn(OH)2 in the porous skeletal copper which provides promotion in methanol synthesis, water gas shift, and other reactions. The highly dispersed reprecipitated Zn(OH)2 decomposes at around 400 K to form ZnO which is an active promoter of copper catalysts. 

This study reports improved stabilities of skeletal Cu catalysts for use in organic synthesis reactions. The promoted skeletal Cu catalysts have been characterised by measuring their resistance to structural rearrangement in caustic solutions, thermal stabilities and activities for the reactions of methanol dehydrogenation and methyl formate hydrogenolysis. Comparisons have been made with an unpromoted skeletal Cu catalyst and a commercial coprecipitated copper chromite catalyst. 

Figure 1 bet (a) and Cu (b) surface areas of the promoted (CuCrl 2) and unpromoted (Cul) skeletal copper catalysts aged in 6.1 M NaOH at 323K for the period up to 170 hours. 

Figure 7 XRD profiles for the promoted (CuCrl) and unpromoted (Cul) skeletal copper catalysts and commercial Cu-0203T catalyst that are pretreated in H2 at the temperatures of 513K and 613K for 1 hour, respectively.

L. Ma and M.S. Wainwright, Development of skeletal copper-chromia catalysts I. Structure and activity promotion of chromia on skeletal copper catalysts for methanol synthesis. Appl. Catal. A General, 187 89-98, 1999. 

Promoter deposition through different mechanisms can account for different catalyst properties. In particular, chromate depositing as chromia does not easily redissolve but, zinc oxide does redissolve once the leach front passes and the pH returns to the bulk level of the lixiviant. Therefore, chromate can provide a more stable catalyst structure against aging, as observed in the skeletal copper system. Of course, promoter involvement in catalyst activity as well as structural promotion must be considered in the selection of promoters. This complexity once again highlights the dependence of the catalytic activity of these materials on the preparation conditions. 

Figure 6 Copper crystallite sizes of the promoted (CuCr3) and unprompted (Cu2) skeletal Cu catalysts corresponding to Figure 5.

Activated skeletal catalysts including nickel, copper, cobalt and molybdenum or chromium-promoted nickel are available commercially. 

Although palladium occupies the dominant position in semi-hydrogenation catalysts, it is by no means the only metal suitable for formulation into a viable catalyst. Mention has already been made of the nickel boride alternatives, with or without copper promotion, for example. Other examples include the skeletal catalyst Raney nickel [69], alumina-supported nickel [70], and aluminum phosphate-supported nickel [71] (Eqs 21 and 22) ... 

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