To meet the growing demand for “green products”, many chemistry-based industrial sectors are investing in plant-based products. These have the advantage of being renewable and are therefore adjudged to be more “natural”. Present in wood but also in many industrial waste products, lignin could become a key resource for the chemistry of tomorrow. A good knowledge of the product is required in order to ensure that it is used efficiently. This is the task that the Industrial Biological Chemistry Unit of Gembloux Agro-Bio Tech has set itself. Following two years of research, the researchers have succeeded in establishing a significant correlation between the pre-treatment applied to these plants and the anti-oxidant properties of the lignin extracted from them.

Replacing car fuels by plants…barely a few decades ago this scenario would have been laughable, but today, the dream has become reality. People are no longer surprised by posters boasting the incorporation (at undoubtedly lower prices) of bioethanol or biodiesel in petrol and diesel on sale at the pumps. These “green” fuels, or biofuels, are produced in biorefineries from vegetables as banal as beetroot, wheat, rapeseed oil etc.; or from sugar cane or corn etc. in other countries.

“Can we describe these vegetables as banal”? Perhaps not, because these vegetables also form the staple diet of billions of people. Regardless of how well-intentioned the use of these products may be (in particular, that of reducing pollution by combustion engines), their use for energy purposes poses a problem of competition with the use of the same plants as food. This is why current research efforts are concentrated on second-generation biofuels. These do not involve use of the vegetables themselves, but the residues of the production of vegetables or their transformation: straw, leaves, stems, etc. This second-generation of biofuels also uses forest-based residues (for example, from saw-mill chips or the paper industry), but also biodegradable waste from parks and gardens, or fast-growing vegetables planted specifically for this purpose (known as short-rotation).

Apart from having the characteristic of being the most abundant source of renewable carbon on the planet, these raw materials are lignocellulose-based. This means that they are composed both of cellulose (40% to 60 % depending on the plant variety), hemicellulose (20% to 40 %, a polymer of sugar) and, finally, lignin (10% to 25 %). Thanks to different types of pre-treatment (physical, thermal or chemical); modern industries succeed in easily decomposing the lignocellulosic matter in order to gain access to these three components. The latter are not only used for the manufacture of biofuels or paper, they are also used - this is particularly so in the case of lignin - in the manufacture of new products (materials, cosmetics, pharmaceutical products etc.) or interesting molecules for the chemical industry. “Once the fermentable sugars have been extracted from the raw materials, lignin remains”, explains Aurore Richel, head of the Biological and industrial Chemistry Unit of Gembloux Agro-Bio Tech (University of Liege). “This aromatic polymer has the particularity of having a very complex chemical structure. Thanks to work done in the past ten years, we now know that lignin from Wood has a more or less identical structure regardless of the species in question. We also know, thanks to sophisticated tools such as spectrometry or chromatography, that the structure of herbaceous plants also displays certain constants. In fact the same applies to all categories of large plants”.