We all know that microwaves can be used to heat your dinner more quickly than a normal oven. Chemists want to speed up chemical reactions in a similar way. By using microwaves instead of heating a reaction, is it possible to increase the rate of a reaction?
This question is important because if it is, the chemicals we need every day for clothes, packaging, toiletries and many other things can be made more cheaply and greenly. In the mid 1980s chemists began experimenting with microwaves and found that they could cause reactions to go much faster --- But how?
When food is placed in a normal oven, it heats from the outside in. It can take take quite a long time for the heat to penetrate inside thick food, which is why it takes a long time to cook a Sunday roast. Microwaves heat food in a very different way, and this depends on what materials they encounter. Some materials allow microwaves to travel straight through them, like light travels through a window. Other materials, like metal, do not allow the either light or microwaves through. Lots of materials are in between, and act like sunglasses do with light --- they let some through, but the amount is reduced.
The microwave ovens used in homes are very powerful, and when food absorbs this energy, it causes the food to heat up. Because the microwaves can travel through the food (which only absorbs some), the very centre of the food gets heated at the same time at the outside, and the time needed to cook something is much shorter. The microwaves which pass all the way through the food are reflected back through it until all the microwave power is used in heating your dinner. This means that heating the food all the way through using a microwave is much quicker than using a normal oven, which only heats the outside.
When water gets hot enough (100 degrees Celcius, known as its boiling point) it usually turns into steam. However, under certain circumstances, it is possible to heat a liquid above its boiling point. Boiling is where a liquid turns into a gas, and this is why when you boil a pan of water, you can see bubbles forming in it. The bubbles are steam, or water in a gaseous state.
Bubbles tend to form on what are called nucleation sites, or nucleation points. These are usually tiny scratches or imperfections on the walls of whatever is holding the liquid. This is why when you have a fizzy drink, the bubbles form a trail which comes from a single point --- this is a nucleation centre. If there are no nucleation sites, then bubbles find it difficult to form, and boiling can not happen. This means that it is possible to heat things above their boiling point, and this is known as superheating. If a bubble does form when the liquid is superheated, it expands very rapidly, and can cause all the liquid to boil out of the container at once. This has injured some people and great care should be taken when heating liquids in the microwave.
Microwaves are good at causing superheating for two reasons. Firstly, people often use glass or ceramic containers when heating things in microwave ovens. These have smooth sides, and are less likely to have nucleation centres than metal pans. The second reason is that microwaves can heat inside a liquid, and this means the centre of the liquid can be hotter than the outside. When this happens, the liquid in contact with the walls may be below the boiling point, and the middle hotter. As the hot part is not in contact with the nucleation centres at the walls, superheating occurs. Chemical reactions generally go faster at high temperatures, but the temperature of a reaction in a liquid is sometimes limited by the boiling point of the solvent which the reactants are dissolved in. Reactions occurring in superheated solvents are not limited in this way, and can be much faster, sometimes reducing the time for a reaction from 15 hours normally to 15 minutes in a microwave.
A chemical reaction can be thought of as a process which converts one or more chemicals into different substances. These chemicals can be in any one of three 'phases', which are gas, liquid or solid. Those put in at the start of the reaction are the reactants, and those made by the reaction are known as the products. If all of the reactants and products are in the same phase, the reaction is called a homogenous reaction. If there is more than one phase involved, the reaction is known as a heterogeneous reaction.
Reactions can be sped up by using a catalyst, as well as heating them in microwaves. A catalyst is a material which speeds up a reaction, but does not get used up. If you think that a normal reaction starts with reactants and finishes with products, then a reaction with a catalyst starts with reactants and a catalyst, and finishes with products and a catalyst. Catalysts work by providing a different way for the materials to react and allows the reaction to go faster. In a similar way to naming reactions, if the catalyst is in the same phase as the reactants and products, it is a homogeneous reaction. If the catalyst is a different phase to the reactants and products, the reaction is heterogeneous. In practise, heterogeneous catalysts are usually solids, and react with liquids or gases. Using solid catalysts makes it easy to remove the products from the catalyst after the reaction, leaving the product pure and letting the catalyst be recycled.
Studying catalysis and how catalysts affect reactions is important because it allows people to perform reactions which may be too slow or hard to achieve normally. This makes it possible to use less energy to make the things we need, quickly, cheaply, and more cleanly. It is estimated catalysts are used in the production of 60% of all commercially made chemicals. Combining the advantages of catalysis with the advantages of microwave heating could lead to even greater benefits. Experiments have shown that microwaves can heat heterogeneous catalytic reactions at least as effectively as conventional methods, and in some cases much better. To understand why, first we must know more a bit more about catalysts and how they are affected by microwaves.
Heterogeneous catalysts work by providing a surface which the reactants stick to (adsorb). This weakens any chemical bonds they have, and brings the reactants close together, making them more likely to react. This surface needs to attract the reactants, if they adsorb too strongly, will not release the products, and the catalyst is said to be 'poisoned'. The best materials for catalysis are usually a type of metal called a transition metal. These are often expensive, so to create the greatest surface area, and make the metal go as far as possible, it is made into tiny particles. These are immobilised on a cheaper material to keep stop them clumping together, and catalysts of this form are called 'supported metal catalysts'.
As mentioned earlier, different materials interact with microwaves in different ways. Some materials are 'transparent' and let the microwaves pass through without heating. Other materials are 'lossy' and absorb some of the microwave power so that they heat up. Also, how materials act can depend on other factors, such as their shape, size or temperature. Large, flat pieces of metal reflect microwaves like a mirror. Metal powders absorb microwaves very strongly and heat rapidly. Supported metal catalysts have small metal particles which are very lossy, and absorb microwaves. As reactions are normally faster at higher temperatures, and occur on the metal surface of a catalyst, heating only the metal using a microwave should save energy. This idea of selectively heating the active metal prompted some scientists to investigate what happens when catalysts are heated in this way.
What they found out is quite complicated and surprising. Some catalysts appear to work the same when heated by microwaves, and some appear better. Some of the catalysts that work better in a microwave then keep their higher activity if they are heated normally after the microwave treatment!
What makes different catalysts behave in different manners is unclear, and some people believe that the strange properties seen when microwaving catalysts can be explained because the microwaves are causing the metal particles to become very hot, whilst the support remains relatively cool. It is possible that the differences reported are because it is hard to measure temperature in a microwave. It is even harder to measure the temperature of the metal particles separately from the support, so nobody really knows what temperatures the catalysts reach, or if different parts are different temperatures.
Another group of people think there may be a special effect that the microwaves have on the reacting chemicals. This is known as a 'non-thermal' or 'microwave effect'. Exactly what form this would take is not known, but any such effect would be a very exciting discovery.
To work out what is happening will require more time and people to look at catalytic reactions in a microwave. With a little bit of research, we may be able to save energy when making things, and possibly create new materials with exciting possibilities for the future.
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