Decomposition of Cellulose in Hot-Compressed Water: Detailed Analysis of the Products and Effect of Operating Conditions

Understanding the reaction pathways of cellulose hydrolysis in hot-compressed water (HCW) is crucial for the optimization of fermentable sugar and chemical production. Advanced analytical strategies are required to better assess the wide range of products from cellulose conversion in HCW. In this work, cellulose conversion in HCW was conducted in an autoclave with sampling upon the reaction time under isothermal conditions (180, 220, and 260 °C from 0 to 120 min). Total water-soluble carbohydrates were quantified (phenol/sulfuric acid method). These products were first characterized by size-exclusion chromatography coupled to evaporative light scattering detection and mass spectrometry (SEC–ELSD–MS). SEC is useful for screening the molecular weight distribution of soluble products. Then, the chemical structure of water solubles has been attributed by hydrophilic interaction liquid chromatography coupled to a linear trap quadrupole Orbitrap mass spectrometer (HILIC–LTQ–Orbitrap–MS). This method notably provides evidence of the formation of a cellobiose conformer under some HCW conditions. A specific high-performance anion-exchange chromatography with pulsed amperometric detection (HPAEC–PAD) method has been developed. This method allows for a selective separation of 5-hydroxymethylfurfural (5-HMF), glucose, fructose, mannose, and oligomers up to cellopentaose. Carboxylic acids were quantified by high-performance liquid chromatography with ultraviolet detection (HPLC–UV). Solid residues obtained after HCW conversion were characterized by X-ray diffraction (XRD) and permanent gas by micro-gas chromatography. The global reaction mechanism of cellulose liquefaction in HCW is rationalized on the basis of these complementary methods. Cellulose conversion first proceeds with heterogeneous hydrolysis (fiber surface) to produce soluble oligomers in competition with pyrolysis (inner fibers with limited mass transfer of water), producing levoglucosan (promoted at a higher temperature). Soluble oligomers produce glucose and isomers by homogeneous hydrolysis (liquid phase). C6 sugars can then undergo further conversion to produce notably 5-HMF and erythrose.