Microwave Chemistryby Gavin Whittaker
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If anyone gets a headache listening to Ajay Bose talk about his work, pain relief is close at hand. Chances are, Bose himself will whip up a batch of aspirin. Nothing unusual about that - after all, he is a chemist. What's odd is how he goes about it. Forget the beakers, flasks and tubes of the classic chemistry lab, or the wasted minutes waiting for the reaction to run its course. Bose simply shoves all the ingredients together in a microwave oven. Ninety seconds later and "ping", the aspirin is ready.
Bose, like me, is one of a group of chemists who have abandoned Bunsen burners and electric hotplates in favour of microwave ovens. Microwaves are revolutionising the way we do our chemical cookery, even though precisely how they work is sometimes something of a mystery. Just the same, the effects are spectacular. Reactions run in record time, and without the need for toxic or flammable solvents. What's more, reactions that normally lead to an unholy mixture of chemicals can sometimes be tweaked to favour the desired product In Edinburgh we are looking to use microwaves to make new superconducting materials. And Bose, in his labs at the Stevens Institute of Technology in New Jersey, is finding faster ways to make not only aspirin but also several antibiotics, and anticancer compounds such as taxol.
So how do microwaves work their magic? Is there some special "microwave effect", or do they simply heat the initial mix of chemicals? The debate has been running since 1988, when Robert Gedye at Laurentian University in Ontario, Canada, reported that certain reactions worked more than a thousand times as fast as normal when the usual sources of heat are replaced by microwaves (see "The chemist's quick cookbook", New Scientist, 12 November 1988, p 56).
In 1992, things started looking bleak for the microwave effect when Mike Mingos of Imperial College, London, and postgraduate student David Baghurst showed how microwaves heat solvents above their normal boiling points. Most chemists now think that this superheating is why reactions run faster. Water, for example, hits 105°C instead of 100°C before boiling, and acetonitrile, another popular solvent, gets to 120°C, an amazing 38°C higher than its usual boiling point. This is because microwaves heat all the solvent in a flask directly (see "Let's do the twist"), allowing it to reach a higher than usual temperature before bubbles can form and it boils. A Bunsen burner, by contrast, heats the edges of a flask where bubbles form much more readily, so the heat is transferred much more slowly to the interior.
However they work, microwaves give chemists a unique method for choosing which chemicals emerge from their reactions. Take, for example, the reaction that adds a sulphonic acid chemical group to naphthalene - an organic molecule that resembles two benzene molecules fused edge to edge. This group can attach itself in either of two positions, and in 1993, Didier Stuerga of the University of Bourgogne in Dijon, France, discovered that microwaves allowed him to control which one predominated. Low microwave power, like normal heating methods, gave an equal mixture of the two products, 1- and 2-naphthalenesulphonic acid. High microwave power heated the reaction faster and gave almost 100 per cent of the latter chemical. Stuerga hopes to use microwaves in similar ways in other reactions to give only the desired product
Another effect unique to microwaves is that they selectively heat some materials faster than others. Working for my D. Phil in 1993, I was able to exploit this effect to develop a speedy synthesis for metal chalcogenides - compounds of metals with sulphur or selenium. These materials could have applications for storing energy in batteries, and as semiconductors, but they can take a long time to make by conventional means.
The normal way of making metal sulphides is simply to mix sulphur and the metal, both in powder form, and heat them in a sealed tube. The trouble is that the sulphur vaporises as it warms up, and if the temperature gets too high or rises too quickly, the pressure of the sulphur vapour will blow the tube to bits. To avoid this, chemists heat the mixture slowly and cautiously, even though this means it may take a week or more for the ingredients to combine and form the metal sulphide. However, in my experiments with microwaves I found I could heat the mixture very quickly, without fear of an explosion. This is because microwaves heat only the metal not the sulphur. Instead of taking days, the reaction is over in just 15 minutes. In 1994, Andrew Barron at Harvard University used this method to make copper- and indium-based materials that hold promise for making solar cells, that are usually produced by more costly techniques.
Meanwhile, some chemists still insist that microwaves are more than just a different way of applying heat. They believe that the radiation somehow interferes with atoms and molecules, and are looking for a specific microwave effect. Ruth Wroe, who works at EA Technology in Chester, thinks she has found one. Using a furnace partially powered by microwaves, Wroe has been testing the effect of microwaves on the production of ceramics. In essence, ceramics are giant networks of atoms or ions, and the usual way of a making these materials is by heating tiny grains of a mixture of ingredients to high temperatures Where the grains are pressed together, they fuse by exchanging atoms or ions to form a unified network - a process known as sintering. Wroe decided to use microwaves to heat the material up very quickly. "If you can get the energy in faster, you can process quicker", she says
Wroe compared what happened when she sintered zirconia using normal heating and microwave heating, using the same temperature in both cases. Zirconia shrinks as the grains fuse together, and Wroe was delighted to find that when microwaves were used, the material shrank faster. "When microwave power was removed," she says, and heating was done by normal methods, "we saw the shrinkage rate revert to normal."
She thinks that microwaves lower the energy barriers which the ions must overcome as they move through zirconia. The microwave electric field helps "pick up" ions and shunt them between grains, bonding them together. "Most people would still say there isn't an athermal effect," says Wroe, "but we've proved it." This microwave effect is already being put to use. Last year EA Technology completed its first commercial microwave-enhanced furnace for processing ceramics, capable of processing between 15 and 20 tonnes of material per day. Wroe says it reduces energy consumption by 60 per cent and processing time by 70 per cent.
Here in Edinburgh, my colleagues and I are hoping to harness the same microwave effect to make high temperature superconducting materials, or materials which have the property known as giant magnetoresistance, which could be useful in building read-write heads for disc drives or in magnetic switches. These materials are made of ions, which microwaves should be able to shunt around and so lead to better ways of synthesising them. This might also lead to, say, improved superconducting properties at higher temperatures.
To tackle these materials, we are building a microwave reactor that will allow us to focus microwave energy on a piece of material without having to put it in an oven. Such a "remote heating" technique will make it possible to heat materials to very high temperatures and in special high-pressure containers - we are hoping to work at more than 1000 B0C and at 100 times normal atmospheric pressure. Combined with microwaves, these conditions may be vigorous enough to transform the materials and their properties.
Meanwhile, Bose has found that even low-power microwaves can have some surprising effects. "Enzymes seem to behave slightly differently if you heat them gently with microwaves," he says. It's as though the enzymes become energised. Last year, Bose took the enzyme cellulase, which breaks down cellulose compounds, and gave it a gentle dose of low-power microwaves. Afterwards, he found that the treated cellulase broke down microcellulose twice as fast as the untreated enzyme. Bose has seen the same mysterious effect with other enzymes, and the only explanation he can think of is that the microwaves somehow change the molecules' shape.
However, according to Bose, the debate about how microwaves work and whether there is a microwave effect is beside the point. "The fact is that microwaves do some amazing things," he says. The majority of his work aims to use microwaves to speed up the production of pharmaceuticals, and he is achieving some impressive results. Wyeth-Ayerst Research in New York asked Bose to see if he could speed up the synthesis of a new drug it was developing. " they had a reaction which took four days and gave 50 per cent yield," he says. "Using microwaves, I got the reaction to run at 90 per cent yield in just four hours." Bose has many similar success stories.
The cleanness of microwave chemistry is another attraction - microwaves do away with the need for a solvent. Waste solvents are a major problem for the chemicals industry: they can be toxic and flammable, and disposing of them is often expensive. "The best solvent is no solvent," says Jack Hamelin at the University of Rennes in France. But getting reactions to work without any solvent is tricky.
Microwaves are helping Hamelin to tackle this problem by building on an approach developed by André Loupy of the University of Paris-Sud. Like Loupy, he soaks up all the chemicals involved in the reaction onto a sponge-like support material, such as alumina. When Hamelin heats the sponge and its contents with microwaves, the adsorbed chemicals react with one another. It can all be over in as little as one minute, giving a more efficient, cleaner result. "Microwaves, in the absence of solvents, give high yields and better purity," says Hamelin.
Rajender Varma of Sam Houston State University, Texas, who is also working on solvent-free reactions, agrees with Hamelin. "In most of our reactions, products are obtained which do not need further purification," he says. And with no solvent to worry about, the process is much cleaner. "Solvent conservation has an enormous impact on reduction of waste discharged," says Varma.
Hamelin sees his work as "a springboard to clean, economical, and safe industrial processing". He has already used microwaves to make several insecticides, which are now being tested by Du Pont and Rhône-Poulenc, and has made a range of anticancer compounds which are being tested at the National Cancer Institute near Washington DC.
Rudy Abramovitch of Clemson University in South Carolina is using microwaves to tackle the chemicals industry's past environmental sins. The soil at old factory sites is often contaminated with chemicals known as PCBs (polychlorinated biphenyls) and PAHs (polycyclic aromatic hydrocarbons) from spills and leaks. Abramovitch is using microwaves to break down these PCBs and PAHs into less toxic compounds. The work is still at the lab stage, but its prospects look promising. "We started off decomposing individual PCBs in soil samples, then moved on to commercial PCB mixtures known as aroclors," says Abramovitch. The levels of contamination were "well above those found in the environment", he says.
The technique requires small amounts of catalysts to be added to the soil being treated. These substances react with the PCBs and PAHs, and cause them to break down when the microwaves are fired at the soil.
The technique removes virtually all traces of PCBs and PAHs. If the land is to be reused for anything other than growing fruit and vegetables, "then it is more or less 100 per cent decontaminated", says Abramovitch.
So far, only small soil samples have been treated. Now Abramovitch is planning to scale things up and commercialise the work: pilot-scale studies in the field "are going to be a piece of cake", he says.
Microwaves have clearly come a long way already from the domestic oven in the kitchen. But the pioneering researchers still have some work to do to get their techniques widely accepted "This may be because most scientists view the use of microwave energy as cooking," he says. A few examples of the power of microwaves should change their minds. Microwave chemistry may not be cooking, but it's certainly hot stuff.
Let's do the twist
Microwaves can be thought of as high-frequency electric fields. They cause liquids and solids to heat up in two distinct ways - conduction and dielectric polarisation.
The conduction effect causes metals and other materials that conduct electricity to heat up - and also explains why cutlery throws off sparks in a microwave oven. When microwaves hit a conductor or semiconductor, electrons or ions in the material experience an extremely high voltage. This pulls them in one direction through the material, creating a current, which heats it up. Sparks fly when the voltage in the metal becomes so high that is causes an electric discharge, like lightning.
Dielectric polarisation, on the other hand, causes molecules to twist backwards and forwards in the microwave's rapidly changing electric field. Polar molecules such as water try to orient themselves in the direction of the field. But before they can line up, the field switches to the opposite direction and the molecules try to swivel round and orient this way instead. So energy from the microwaves causes what looks like random jiggling of the molecules into thermal energy, heating up the material.