Topic 9 · page 10 of 15

Catalysts in industry

Industry uses catalysts to make processes more sustainable in two ways. A lower activation energy means less energy is needed, and a better route can mean fewer steps, less waste and a higher atom economy.

11 min read9.7

Read first: Catalysts and reaction profiles

A sustainable process uses fewer resources, makes less waste and does less harm to the environment, while still being worth running. The specification names two ways a catalyst helps: less energy and higher atom economy.

Less energy 9.7

A catalyst lowers the activation energy, so a reaction can run fast enough at a lower temperature, and sometimes at a lower pressure, than it would without one.

The chain of reasoning is the one to write in an exam:

The catalyst provides a route with a lower activation energy.
So a lower temperature is enough to make the reaction fast.
So less fuel is burned to heat the reactor, which means lower costs and less carbon dioxide.

For example, the iron catalyst in the Haber process makes ammonia fast enough at about 450 °C and 200 atm. Without it the reaction would be far too slow at that temperature, and the only way to speed it up, a higher temperature, would cut the yield (page 14).

Higher atom economy 9.7

A catalyst can allow a different route, with fewer steps and fewer unwanted by-products. That raises the atom economy, which you met in Topic 1.

A route with a high atom economy turns more of the reactant atoms into the product you want, so there is less waste to separate, dispose of or treat.

Worked exampleTwo routes to ethanol

Both routes make ethanol, C₂H₅OH. Compare their atom economies. ( $M_\text{r}$ : C₂H₄ = 28.0, H₂O = 18.0, C₂H₅OH = 46.0, C₆H₁₂O₆ = 180.0, CO₂ = 44.0)

Hydration of ethene, with a phosphoric acid catalyst:

\ce{C2H4(g) + H2O(g) -> C2H5OH(g)}

Only one product, so the sum of the products is 46.0.
Atom economy = 46.0 ÷ 46.0 × 100 = 100%

Fermentation of glucose:

\ce{C6H12O6(aq) -> 2C2H5OH(aq) + 2CO2(g)}

Desired product: 2 × 46.0 = 92.0. All products: 92.0 + 2 × 44.0 = 180.0.
Atom economy = 92.0 ÷ 180.0 × 100 = 51.1%

Industry also develops catalysts to cut the number of steps in making a drug. A well-known example is ibuprofen: the older route had about 40% atom economy, and a newer catalytic route has about 77%. These are rounded figures from textbooks.

Check yourself

Give two ways in which a catalyst can make an industrial process more sustainable.
Explain why a catalyst allows a reaction to be run at a lower temperature.
Calculate the atom economy for ethanol made by hydrating ethene, and for ethanol made by fermenting glucose (see the worked example), and say which is better.
Explain why a process with a higher atom economy is more sustainable, even if its percentage yield is the same.
A company finds a catalyst that lets a reaction run at 200 °C instead of 400 °C. Give two benefits and one possible drawback.

Answers cover these until you have tried the questions

Less energy: a lower activation energy means a lower temperature is enough. Higher atom economy: a route with fewer steps and fewer by-products, so less waste.
The catalyst provides an alternative route with a lower activation energy, so a larger proportion of molecules have enough energy even at a lower temperature, and the rate is fast enough without heating as much.
Hydration: 46.0 ÷ 46.0 × 100 = 100%. Fermentation: 92.0 ÷ 180.0 × 100 = 51.1%. Hydration is better, since all the atoms of the reactants end up in the product.
A higher atom economy means a smaller proportion of the reactant atoms is wasted as by-products. With the same percentage yield, less waste has to be separated, disposed of or treated, and less raw material is used per tonne of product.
Benefits: less fuel is burned to heat the reactor, so lower energy costs and lower CO₂ emissions; milder conditions are also safer. Drawback (any one): the catalyst may be expensive, may be poisoned by impurities, or may need to be separated from the product.