Subcritical Water as Reaction Environment: Fundamentals of Hydrothermal Biomass Transformation

Abstract

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.

Influence of High-Pressure Steam Pretreatment on the Structure of Rice Husk and Enzymatic Saccharification in a Two-Step System

“…These results were in accordance with the work of Zakaria et al (2015a), who reported a similar decrease in the pH from 4.10 to 3.39 when the temperature increased from 170 to 210 °C during the hydrothermal pretreatment of oil palm frond fibre (OPFF). The decrease in pH was attributed to the accumulation of acetic acid, which resulted from the cleavage of acetyl groups located in the hemicellulose matrix, and resulted in hemicellulose degradation (Möller et al 2011;Xiao et al 2013;Ho et al 2014).…”

Hydrothermal Depolymerization of Lignin: Understanding the Structural Evolution

“…Hydrothermal depolymerization is also an efficient process for biomass utilization (Demirbas 2010;Moeller et al 2011;. For example, Akhtar et al (2010) reported that empty palm fruit bunches could be liquefied efficiently in alkaline hot compressed water, where more than 60.1% of liquid products were phenolic; however, aromatic bio-char was generated.…”

Biomimetic Conversion of Glucose to Organic Acid Facilitated by Metalloporphyrin under Mild Conditions

“…On the contrary, the selectivity of the unidentified products decreased markedly before 1.5 h, and then increased remarkably with prolonged time. After 1.5 h, the decomposition of the C-C bond cleavage products to gas products or humins could easily take place (Möller et al 2011;Yin and Tan 2012). As shown in Fig.…”