Heterogeneous catalysts participate in chemical reactions in a different phase than the reactants, often using precious or transition metals. They increase reaction rates by altering the transition state and are easily recoverable, making them useful in continuous processes like the petroleum industry’s “moving bed” process.
The term “phase” refers to the state of matter; there are solid, liquid and gaseous phases. A heterogeneous catalyst participates in a chemical reaction that is not part of the same phase as matter as the actual reactants. For example, liquids can undergo a reaction in the presence of a solid catalyst. While the catalyst speeds up the process of a reaction, it is not itself consumed by the reactants. Precious metals or other transition metals are often used as heterogeneous catalysts and can be finely divided for increased surface exposure on a substrate or support.
It is the heterogeneous catalyst rather than the homogeneous variety that is most widely used industrially. The exposed catalytic surface provides sites for weak surface binding of geometrically aligned reactants. This behavior is important, such as in the hydrogenation of carbon-carbon double bonds. Consider an example such as ethylene, which has the chemical structure H2C=CH2. When an ethylene molecule approaches some catalytic surface, it adsorbs or attaches itself to a lower left hydrogen atom and a lower right hydrogen atom – the upper left and right hydrogen atoms remain free.
A single hydrogen molecule, H2 or H‒H, can then add across the double bond, being replaced by a single bond, forming “saturated” ethane, or H3C‒CH3. However, there are two ways hydrogen atoms can add across the double bond. Both can add the double bond from below, or one can add below, while the other adds above the double bond on the other side. Adding both hydrogen atoms to one side of a double bond is called a “cis addition”, while adding one to one side and the other to the other is called a “trans addition”. No doubt, the term “trans-fat” – used to describe an unsaturated fat that has been hydrogenated by trans-addition – will be familiar to the reader.
Catalysts pick up the rate of reactions for which they are used because the reaction pathway is altered by their presence. This changes the transition state and reduces the activation energy required to carry out the reaction. An advantage of such a reaction conducted with a heterogeneous catalyst is the ease of recoverability of the catalyst. Heterogeneous catalysts are particularly suited to continuous process chemical reactions, where material is supplied, reacted, removed, and replaced, continuously. An example of such a heterogeneous catalyst process from the petroleum industry is the use of pelletized catalyst material in the so-called “moving bed” process.
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