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This is the text of an article written by me for New Scientist, published on
28th Feb. 1998. © New scientist (1998) Reproduced with permission.
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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


Controlled Cookery

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.

Energised enzymes

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.

Cleaning up

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.

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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.

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