Subcritical water, that is, water above the boiling and below critical point, is a unique and sustainable reaction medium. Based on its solvent properties, in combination with the often considerable intrinsic water content of natural biomass, it is often considered as a potential solvent for biomass processing. Current knowledge on biomass transformation in subcritical water is, however, still rather scattered without providing a consistent picture. Concentrating on fundamental physical and chemical aspects, this review summarizes the current state of knowledge of hydrothermal biomass conversion in subcritical water. After briefly introducing subcritical water as a reaction medium, its advantages for biomass processing compared to other thermal processes are highlighted. Subsequently, the physical-chemical properties of subcritical water are discussed in the light of their impact on the occurring chemical reactions. The influence of major operational parameters, including temperature, pressure, and reactant concentration on hydrothermal biomass transformation processes are illustrated for selected carbohydrates. Major emphasis is put on the nature of the carbohydrate monomers, since the conversion of the respective polymers is analogous with the additional prior step of hydrolytic depolymerization.

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The hydrothermal liquefaction of cellulose in subcritical water using reaction times from 25 to 110 minutes is studied. Major emphasis is put on the impact of the cellulose crystallinity on the degree of liquefaction, on the yields of the major reaction products, glucose and 5-(hydroxymethyl)furfural (5-HMF), and on the composition of the solid reaction residues. It is demonstrated that the crystallinity index (CI) of the used cellulose starting material plays a crucial role in the reaction mechanisms, the reaction rates and obtained product yields. Amorphous starting materials show a larger extent of liquefaction. Thus, 64% total liquid carbon is obtained for cellulose with a CI of 42 (MCC 42 ), whereas that of MCC 74 (CI 74) is 45%. The major liquefaction products are glucose and 5-HMF. It is shown that the cellulose conversion commences with a fast decomposition of the amorphous material domains, followed by a slower conversion of the crystalline cellulose domains.

The uncatalyzed microwave-assisted hydrothermal conversion of xylose and xylan as hemicellulose model compound to furfural at temperatures of 160 -240 °C and reaction times of 5 -120 min is investigated. With acceptable reaction rates already at 200 °C the conversion of xylose provides furfural with a maximum yield of 49%.The conversion of xylan, however, requires lower temperatures (160 -180 °C) in order to promote the hydrolysis of the polymer, yielding xylose as furfural precursor.

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