Skeletal Copper Catalysts

Glycine Salt Over 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. 

Chitwood (2) found that copper compounds exhibited only a short period of maximum catalytic activity for the dehydrogenation of ethanolamine to glycine salt. In this study, the catalytic activity of a skeletal copper catalyst was tested in repeated use. The catalyst used was prepared by selectively leaching CuAl2 particles in a 6.1 M NaOH solution at 293 K for 24 hours. Figure 1 shows the profiles of hydrogen evolved versus reaction time. 

The effect of reactant concentrations on reaction rate was studied using unpromoted skeletal copper catalysts initially leached at 278 K and then... 

In order to investigate the relationship between the surface area of skeletal copper and activity, the same sample of catalyst was tested in four successive runs. Rate constants was compared with that of another sample prepared in the same way but pretreated in 6.2 M NaOH at 473 K before use. Figure 4 shows that the first order rate constants, calculated so as to take into account the mass of catalyst relative to the volume of solution, decreased in the first three cycles but then stabilised. The surface areas, measured on small samples taken after reaction, mirrored this pattern. The rate constant, and the surface area, for the pretreated catalyst was similar to those obtained in cycles 3 and 4. It is apparent that activity and surface area are closely related for the unpromoted skeletal copper catalyst and that the pretreatment in NaOH at 473 K is approximately equivalent to three repeated reactions in terms of stabilising activity and surface area. 

CuCrO.Ol and CuCrO.l) there was no obvious deactivation, but due to their lower initial activities they had no advantage compared with the unpromoted skeletal copper catalyst. For the low chromia content skeletal copper catalyst (CuCr0.002), and the unpromoted skeletal copper catalyst, the deactivation in the first cycle was significant. However, CuCr0.002 had both a higher initial activity and a higher, stable residual activity than the unpromoted skeletal copper catalyst. 

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.

The oxidative dehydrogenation of ethanolamine over skeletal copper catalysts at temperatures, pressures and catalyst concentrations that are used in industrial processes has been shown to be independent of the agitation rate and catalyst particle size over a range of conditions. A small content of chromia (ca. 0.7 wt %) provided some improvement to catalyst activity and whereas larger amounts provided stability at the expense of activity. 

The structure of skeletal catalysts is so fine that electron microscopes are required for sufficient resolution. The use of a focussed ion beam (FIB) miller has enabled a skeletal copper catalyst to be sliced open under vacuum and the internal structure to be imaged directly [61], Slicing the catalyst enabled viewing beyond the obscuring oxide layer on the surface. A uniform, three-dimensional structure of fine copper ligaments was observed [61], which differed from the leading inferred structure at the time of parallel curved rods [54],... 

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. 

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. 

Table 4 shows the surface properties of skeletal copper catalysts produced by leaching a 50wt% Cu alloy in aqueous sodium hydroxide solution at 293 K. It shows that the surface area decreases with increasing particle size of the alloy. Table 5 shows the effect of temperature of extraction on the surface area and pore structures of completely leached 1000-1180 fim particles of the 50wt% Cu alloy. The results show... 

A major industrial process that uses skeletal copper catalysts is the liquid-phase hydrolysis of nitriles to... 

This process is conducted in the liquid phase using fixed beds of skeletal copper catalysts and temperatures from 300 to 400 K. Higher temperatures lead to catalyst fouling by polymerization of the product acrylamide. This deactivation can be reversed by washing the catalyst with caustic soda solution. This regeneration is a positive advantage of skeletal copper over other forms of copper catalysts used in this industrial process. 

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. 

A D Tomsett, Pore Development in skeletal Copper Catalysts, PhD Thesis, University of New South Wales, Sydney, Australia, 1987... 

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