The ability to comprehensively deconstruct cellulose is key when developing new biorefinery systems. This study presents an in-depth overview of using supercritical water treatment as a green alternative to depolymerize cellulose into low-molecular-weight cellulose (LMWC). Using commercial microcrystalline cellulose (MCC) as starting material, experiments conducted on two pilot reactors yielded cellulosic products with a range of degrees of polymerization between 30 and 110, a range difficult to obtain with conventional techniques. The unique ability of nearcritical and supercritical water to hydrolyze cellulose and selectively dissolve the low molar mass fraction allowed recovering LMWC after precipitation without the need for any catalyst or antisolvent. The LMWC yield was maximized after near-complete cellulose hydrolysis, while longer reaction times favored water-soluble cello-oligosaccharides and glucose. Using a quenching reactor at nearcritical reaction conditions LMWC accounted for nearly 20 wt.% of the final mass balance. In contrast, increasing the MCC consistency while using a depressurization reactor at supercritical water conditions created additional shear-induced degradation which further enhanced the hydrolysis mechanism and allowed reaching a higher yield of 55 wt.%. This setup could produce a LMWC output up to 200 g.hr-1 with an energy requirement as low as 66 kJ.g-1. LMWC precipitated as nanocellulose within a few hours after treatment, driven by the general insolubility of cellulose in aqueous media. The precipitation was characterized as a two-step phenomenon where particles grow upon stacking of LMWC chains, followed by flocculation and sedimentation. Further morphological and structural analyses revealed that the properties of the final LMWC material were highly dependent on parameters at the end of the reaction, namely the temperature and concentration. Changing these parameters resulted in materials with different shapes and allomorphy, including ribbon-like cellulose II and lamellar cellulose IVII particles. The material properties of ribbon-like cellulose II produced using the quenching reactor were further investigated. This distinct kind of nanocellulose, a gel-like material, was tested as emulsifier for oil-in-water Pickering emulsions and as stabilizer in aqueous graphite nanoflake suspensions. In a comparative assessment with microfibrillated cellulose (MFC) mechanically produced from MCC, the LMWC exhibited superior dispersion and stabilization ability. These benefits resulted from a favorable aspect ratio, higher surface free area, as well as higher viscosity compared to MFC. This study clarified the prerequisites and operating conditions for the direct chemical-free production of low-molecular-weight nanocelluloses from MCC. This one-step continuous process enables the production of this material at larger scale, thus opening the way to new applications, including as emulsifier and stabilizer in complex multiphasic systems.
|Translated title of the contribution||Low-Molecular-Weight Nanocellulose Produced Using Supercritical Water Treatment|
|Publication status||Published - 2020|
|MoE publication type||G5 Doctoral dissertation (article)|
- low-molecular-weight cellulose
- supercritical water