Hydrogenation of Alkenes with Metal Catalysts
Hydrogenation of Alkenes with Metal Catalysts: Introduction
Addition of hydrogen to alkenes is made possible only by using catalysts, with heterogeneous catalysis used more often than the homogeneous process. Finely dispersed transition metals with a large surface area, such as palladium, platinum and nickel, are most frequently used in heterogeneous catalysis. Very often, the metal is bound to inert support material to facilitate handling. The most important metal catalysts are finely dispersed palladium-on-charcoal Pd/C, platinum oxide PtO2 (in situ reduction) and Raney nickel in the form of porous nickel with an extremly large surface. This catalyst is prepared by treatment of a nickel-aluminum alloy with concentrated sodium hydroxide. Both educts, hydrogen and olefin, are adsorbed (coordinated to) on the surface of the metal catalyst but move relatively freely. During the adsorption, the σ bond of hydrogen and the π bond of the olefin are weakened. Transfer of the hydrogen atom to carbon takes place by insertion into the coordinatively bound C-metal bond of the olefin.
Addition has been shown to be syn-stereospecific with the hydrogen entering from the less-hindered side of the molecule. Overall, the reduction is exotherm. For homogeneous catalysis, complexes of rhodium, cobalt and ruthenium, for example the Wilkinson catalyst RhCl(Ph3P), are being employed.
- Hardening of fats