Which of the following catalysts is generally used in the hydrogenation of…

Concept — Hydrogenation is an addition reaction in which molecular hydrogen (H2) adds across the carbon–carbon double bonds of an unsaturated compound and converts it into the corresponding saturated compound. Molecular hydrogen is almost unreactive by itself, so the reaction is run over a finely divided transition-metal surface: the metal chemisorbs H2 and splits it into reactive hydrogen atoms, and at the same time holds the double bond on that surface so the hydrogen atoms can be delivered to it. A metal is useful here only when it binds hydrogen and the double bond with intermediate strength — firmly enough to activate them, loosely enough to let the saturated product leave.

Application — The fatty-acid chains of a vegetable oil carry several C=C double bonds, and that is why the oil is a liquid at room temperature. Hardening the oil (vanaspati manufacture) works as follows:

1. Hydrogen gas is bubbled through the hot oil, at roughly 150–200 °C and a few atmospheres of pressure, in the presence of a finely divided metal.

2. On the metal surface H2 is chemisorbed and dissociates into hydrogen atoms, which is the step that makes the otherwise sluggish hydrogen reactive.

3. A C=C double bond of a fatty-acid chain is held on the same surface, and the two hydrogen atoms add across it, so that double bond becomes a single bond.

4. As the double bonds are saturated the chains straighten and pack closely, so the liquid oil sets into a semi-solid fat; the metal is then filtered out of the product and reused.

The metal that industry uses for this step, in finely divided form, is nickel — it activates hydrogen at a workable temperature, is cheap, and is easily filtered off. Nickel is therefore the catalyst generally used in the hydrogenation of vegetable oils.

Cross-check — the other metals of this set are associated with different catalytic processes:

• Copper — its familiar catalytic role is dehydrogenation, for example ethanol to ethanal at about 573 K, that is, the removal of hydrogen rather than its addition.

• Iron — promoted iron is the catalyst of the Haber–Bosch synthesis of ammonia from N2 and H2.

• Manganese — the well-known catalyst from this element is manganese dioxide, MnO2, which speeds up the decomposition of potassium chlorate and of hydrogen peroxide.