A cheap and effective way of destroying chlorofluorocarbons (CFCs) without
the use of an incinerator has been found by Japanese chemists. Using the
technique, they have decomposed the compounds into substances which do not
harm the ozone layer and which are easy to dispose of.
Of the 680 000 tonnes of CFCs produced worldwide last year, 230 000
tonnes went into refrigerators. All these chemicals need to be destroyed
before they can escape into the environment and destroy the ozone layer.
In 1987, many countries signed the Montreal Protocol, pledging to stop
making CFCs by the end of the century; now the date for stopping production
has been brought forward to 1995. But no one knows how to deal with the
CFCs that are still being made. CFCs are difficult to dispose of because
they are resilient to almost all forms of chemical attack.
However, Kiyonori Shinoda and his colleagues at the Toyama National
College of Technology at Hongo have found that CFCs can be broken down at
temperatures as low as 200 °C (Chemistry Letters, 1992, p 795). They
focused their attention on two compounds: CFC-112 (CCl2F.CCl2F) and the
widely used CFC-113 (CCl2F.CClF2).
Shinoda and his colleagues passed a mixture of ethanol and CFC over
a catalyst of activated charcoal, which turned the CFC into carbon dioxide
(CO2), carbon monoxide (CO), ethyl chloride (CH3CH2Cl)
and hydrogen fluoride (HF). These can all be dealt with separately, and
none poses a threat to the ozone layer above our heads.
The Japanese chemists prepared the catalyst by soaking pellets of the
activated charcoal in a solution of iron (III) chloride (FeCl3) and heating
them until they were dry. They then packed the pellets into a stainless
steel cylinder 40 centimetres long and 2 centimetres in diameter, and heated
this inside an electric oven. The researchers mixed the CFC with four times
as much ethanol by volume, evaporated the solution, and passed the vapour
through the column of catalyst.
Shinoda found that CFC-112 and ethanol started to react at a temperature
as low as 150 °C, but that it took a temperature of 265 °C before
99 per cent of the CFC was decomposed. Under such conditions, the carbon
atoms of the CFCs ended up as carbon monoxide and carbon dioxide, the chlorine
atoms as ethyl chloride, and the fluorine atoms as hydrogen fluoride. At
lower temperatures, some CFC-112 passed through the catalyst unchanged while
some was converted to the other CFC-type materials.
On the other hand, CFC-113 was more resistant to decomposition than
CFC-112. Even at 290 °C, 3 per cent of the CFC-113 passed through the
reactor unaffected, and 8 per cent of the effluent gas was a related CFC
compound, CHFCl. CClF2. However, Shinoda believes that it will be possible
to improve the activated charcoal catalyst and the reactor in order to give
complete decomposition and no unwanted byproducts.
The key to the Japanese work is the charcoal catalyst. Without it, or
without the iron (III) chloride, the process does not work. Shinoda and
his colleagues found that while the pellets became slightly less active
after the first few hours of use, their condition stabilised after about
five hours. For example, the chemists found that CFC-112 underwent 80 per
cent destruction at 180 °C to begin with. This fell to 60 per cent after
5 hours, but was still 60 per cent after 10 hours. Shinoda believes that
the catalyst can be improved to ensure a long working life.