Potential of hydrothermal treatments in lignocellulose biorefineries

The aqueous fractionation of native lignocellulosic materials with hot, compressed water (also known as hydrothermal processing or autohydrolysis) has been proposed as a fractionation method for biorefineries, as it enables the simultaneous removal of water-soluble extractives and the solubilization of hemicelluloses, yielding a solid phase enriched in lignin and cellulose. When the operation is carried out under favorable conditions, the liquid phase from autohydrolysis contains valuable products derived from hemicelluloses, including oligosaccharides that keep the major structural features of the polymer (including substitution by acetyl groups). Post-hydrolysis of oligosaccharides in media catalyzed by acids or enzymes yields fermentable media containing hemicellulosic sugars; whereas harsh acid hydrolysis conditions result in the production of levulinic acid. Subsequent delignification of autohydrolyzed solids enables the separation of the three major lignocellulose components (hemicelluloses, cellulose, and lignin). As an alternative way for using autohydrolyzed solids, they can be used as substrates for the enzymatic hydrolysis of cellulose, leaving lignin as a solid by-product. The sustainable manufacture of materials, fuels and chemicals from the biorefinery schemes cited above is discussed. Particular attention is paid to hemicellulose-derived products. (C) 2011 Society of Chemical Industry and John Wiley & Sons, Ltd