Abstract :
[en] Lignocellulosic substrates constitute a promising alternative resource for the sustainable production of energy (biofuels), biobased products and organic compounds. In the past, the extraction and the valorization of cellulose (into fermentescible glucose) constituted the central axis of lignocellulosic biorefinery processes. Degraded hemicelluloses and lignins were however recovered as side-products with no possibilities of high-added value applications.
Within the context of an integrated biorefinery, and for economic reasons, the recovery and the non-energetic transformation of lignins have opened recently new horizons. Lignin is a cross-linked phenolic polymer and is considered as a potential alternative to petrochemical polymers or as a source of antioxidants for cosmetics and food industry, resins, chelating agent...
As the final application of lignin is dependent of both extraction process and type of lignocellulosic sources, the development of fast and efficient physicochemical characterization methods is thus a prerequisite to optimize extraction processing conditions.
In this study, beech wood particles (Fagus sylvatica L.) collected in the region of Gaume (Belgium) were delignified at atmospheric pressure by a mixture of formic acid/acetic acid/water. The effect of cooking time and temperature was evaluated on delignification and on chemical structure of lignins obtained by precipitation from the black liquor after treatments.
To study the delignification, a central composite design and response surface methodology were used for the optimization of two treatments parameters, i.e. time (between 1h30 and 4h30) and temperature (from 87 to 107°C). These two variables were optimized for delignification yield, pulp yield, concentration of degradation products (furfural and hydroxymethylfurfural) and for the amount of residual linkages between lignin constitutive units.
On the basis of our results, higher delignification yields were obtained when cooking time and temperature increased. However, for high cooking times and temperatures, pulp yield decreased because hemicelluloses and cellulose were hydrolyzed and the production of furfural and hydroxymethylfurfural was augmented.
The structural and physico-chemical features of extracted lignins were investigated with different analytical tools, namely infrared spectrometry, thermogravimetric analysis, size-exclusion chromatography, mono-dimensional nuclear magnetic resonance (NMR 1H and NMR 13C) and bidimensional NMR (heteronuclear HSQC experiments).
The characterization of lignins indicated the occurrence of a repolymerization phenomenon when both cooking time and temperature increased. Indeed, HSQC NMR spectra presented oxidized syringyl units. Otherwise, thermogravimetric analysis and HSQC analysis showed the presence of hemicelluloses in lignin samples from soft treatments (1h30, 87°C). Some linkages between lignin and hemicelluloses were not cleaved under these experimental conditions.
