Terrific timber

Supercritical carbon dioxide has found many uses in the past couple of years, mostly as an alternative to organic solvents. Researchers from CSIRO in Australia have now found another use for it — improving the performance of wood preservatives.

The team, led by Abdul Qader, used supercritical CO2 to carry preservatives through the structure of the wood. The gas is raised to above its critical point — 31.1°C and 72.9 atmospheres — and pumped at the same high pressure, along with the preservatives, into the wood.

'The CO2 acquires a liquid-like ability to dissolve compounds,' explains Qader. 'At the same time, it retains a gas's ability to penetrate fine structures such as the micropores of wood, unrestricted by the high surface tensions associated with liquids.' Unlike conventional preserving techniques, which involve simply dipping wood into a preservative solution, using supercritical CO2 should allow hardwoods to be treated effectively.

Wood treated by this method does not need to be dried or stored to allow vapours to dissipate, as the CO2 returns to a gaseous state as soon as the pressure is reduced.

Meanwhile, research from chemists at Lyon University could soon see wood making its appearance in drugs manufacturing too. It'll still be hard to spot, however; Françoise Quignards and Agnès Choplin are using wood as the basis for catalyst supports.

The researchers are working on supported aqueous phase catalysis, which is often used when a synthesis calls for the combination of two complex organic molecules. It generally uses palladium catalysts, dissolved in water then coated onto porous glass beads, which are recovered after the reaction. This technique has two main problems, however: the reaction mixture is often reluctant to form close contact with the glass beads; and the palladium gradually leaches away from the support.

The researchers found that both problems could be avoided if the catalyst was supported on cellulose, rather than glass. The cellulose keeps a tight hold on the palladium, and the ability of the cellulose to swell when wet improves the contact between active sites and the reaction mixture. Unexpectedly, the activity of the catalyst appears to peak twice — when the catalyst absorbs a quarter of its weight in water, and again when it absorbs two-thirds of its weight. The researchers believe that the low cost, biodegradability and interesting properties of wood-based catalysts could prove valuable.