Highly efficient and selective high-pressure CO2-H2O pre-treatment of agro-industrial residues to produce C5-sugars with high enzymatic hydrolysis yields

Nowadays, agricultural activity generates annually 140 billion metric tonnes of residues worldwide. Among them a dominant part are lignocellulosic residues which can be converted into biofuels and chemicals. The conversion of these residues into carbohydrate-derived fermentable sugars constitutes a great challenge, essentially due to recalcitrance of their structures. Up to now, most of the biomass processing technologies are characterised by low yields and selectivities, which burden the production of fuels and chemicals at competitive costs. In this work, the selective hydrolysis of agro-industrial-based hemicelluloses into C₅-sugars using high-pressure CO₂-H₂O mixture is for the first time proposed. The scrutinised system brings the benefits from the use of auto-generated acidic environment guiding to enhancement of C₅-sugars selectivity and yield. Without an additional chemical catalyst, this novel system outperform water-based technologies allowing to get total sugars yield as high as 84 % in comparison to 67 % with water-only reaction (autohydrolysis) at higher temperature. Besides the hemicellulose conversion, both chemical and physical effects of CO₂ coupled to enzymatic process resulted in a glucan conversion yield of 82 %, which represents an improvement of 26 % over those obtained for water-based technologies. First, the influence of various operating conditions, such as temperature, reaction time, initial CO₂ pressure, ratio biomass/water, on hydrolysis of hemicelluloses into C₅-sugars and final enzymatic hydrolysis yield were investigated to help to define optimal operational conditions. Then, the influence of CO₂ on processed materials morphology was studied by scanning electron microscopy and Fourier transform infrared spectroscopy.