Hydrogenation catalysts

Key point: Alkenes are hydrogenated on supported metal particles by a process that involves H₂ dis sociation and migration of H. to an adsorbed ethene molecule. Skeletal nickels can be used to reduce alkanals to alkanols. A milestone in heterogeneous catalysis was Paul Sabatier’s observation in 1890 that nickel catalyses the hydrogenation of alkenes. He was in fact attempting to synthesize Ni (C₂H₄ )₄ in response to Mond, Langer, and Quinke’s synthesis of Ni (CO)₄ (Section 22.18). However, when he passed ethene over heated nickel he detected ethane. His curiosity was sparked, so he included hydrogen with the ethene, whereupon he observed a good yield of ethane. The hydrogenation of alkenes on supported metal particles is thought to proceed in a manner very similar to that in metal complexes. As pictured in Fig. 26.20, H₂, which is dissociatively chemisorbed on the surface, is thought to migrate to an adsorbed ethene molecule, giving first a surface alkyl and then the saturated hydrocarbon. When ethene is hydrogenated with D2 over platinum, the simple mechanism depicted in Fig. 26.20 indicates that CH₂DCH₂ D should be the product. In fact, a complete range of C₂Hn D6 n ethane isotopologues is observed. It is for this reason that the central step is written as reversible; the rate of the reverse reaction must be greater than the rate at which the ethane molecule is formed and desorbed in the final step. One of the most important classes of heterogeneous hydrogenation catalysts are the ‘skeletal nickels’, which are used for various processes such as conversion of alkanals (aldehydes) to alkanols (alcohols), as in

and reduction of alkylchloronitroanilines to the corresponding amines. Skeletal nickels and similar catalytically active metal alloys are produced by preparing a metal alloy such as NiAl at high temperature and then selectively dissolving most of the aluminium by treatment with sodium hydroxide. Other metals, such as molybdenum and chromium, can be added to the original alloy and may act as promoters that affect the catalyst’s reactivity and selectivity for certain reactions. The resulting spongy or porous metals are rich in nickel (>90 per cent) and their high surface areas lead to very high catalytic activities. One further application of these catalysts is to the conversion of naturally occurring polyun saturated fats, which are liquids, to solid polyhydrogenated fats, as in margarine.

Figure 26.20 Schematic diagram of the stages involved in the hydrogenation of ethene by deuterium on a metal surface.

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مواضيع ذات صلة

Sulfur dioxide oxidation

Ammonia synthesis

Surface migration

Chemisorption and desorption

Surface metal sites

Surface acidic and basic sites

Surface acidic and basic sites

Surface area and porosity

Methanol carbonylation: ethanoic acid synthesis

Palladium-catalysed C-C bond-forming reactions

Asymmetric oxidations

Wacker oxidation of alkenes