Most reactions happen in a solvent, a liquid that dissolves the reactants so their molecules can meet. Making a medicine often uses more solvent, by mass, than everything else put together.
Many solvents burn easily or harm the people who breathe them, and once used they become waste. A greener solvent is safe to handle and easy to use again. Water is the safest of all: it does not burn, it is not poisonous and it costs almost nothing. Most wall paint today is water-based. Its colour and binder are spread through water, which dries off into the air.
Carbon dioxide as a solvent
The supercritical carbon dioxide that decaffeinates coffee is a greener solvent: it is cheap, it does not burn, and it turns back into a gas when the pressure is let off, so the plant collects it and uses it again. The same fluid takes the flavour out of hops for brewing beer.
Catalysts that come back
A catalyst speeds up a reaction and comes out unchanged, ready for the next batch, so one load of catalyst can make many tonnes of product. A chemical that is used up in the reaction has to be bought for every batch and ends up as waste.
Ibuprofen shows the difference. The first factory route, from the Boots company in the 1960s, took six steps. Its first step used aluminium chloride, which was used up and left as waste. In 1992 a new route from the BHC company, a joint venture of Boots and Hoechst Celanese, took over at a factory in Bishop, Texas. It takes three steps, each with a catalyst: hydrogen fluoride, then nickel, then palladium. The hydrogen fluoride is collected and used again, over and over.
| Measure | Boots, 1960s | BHC, 1992 |
|---|---|---|
| steps | 6 | 3 |
| atom economy | about 40% | about 77% |
| step 1 uses | aluminium chloride, used up | hydrogen fluoride, used again |
In the new route the only by-product is ethanoic acid, and the factory collects that and sells it too, which takes the share of atoms put to use to about 99%. The route won a United States Presidential Green Chemistry Challenge Award in 1997.
Enzymes
Enzymes, nature's catalysts, work in water at mild temperatures, and each one speeds up just one reaction, so they make few by-products. Fixed onto tiny beads and packed into a tube, an enzyme can work for months as a sugar syrup flows past. Much of the sweet syrup in soft drinks in the United States is made this way: an enzyme turns part of the glucose from maize starch into fructose.