Hydrothermal dissolution of willow in hot compressed water as a model for biomass conversion

Biomass has wide applications as a source of clean energy and as a raw material for different chemical stocks. Dissolution of willow as a model system for biomass conversion has been investigated in the 200–350 °C temperature range. The dissolution process was studied using a batch-type (diamond-anvil cell) and a continuous flow process reactor. A 95% dissolution of willow was achieved. The lignin and hemicellulose in willow were fragmented and dissolved at a temperature as low as 200 °C and a pressure of 10 MPa. Cellulose dissolved in the 280–320 °C temperature range. A dissolution mechanism is proposed, which involves a rapid fragmentation and hydrolysis of lignin, hemicellulose and cellulose to form oligomers and other water–soluble products, such as glucose. The re-condensation behavior of the dissolved oligomers is the main challenge for efficient dissolution. A continuous flow process is more effective and simpler in this regard than is a batch process. The results of this work show that hot, compressed water affords a viable alternative to corrosive chemicals and toxic solvents, thereby facilitating the utilization of biomass as a source of renewable fuel and chemical feedstocks.