Y. Wang: From Fibers to Sugars: Harnessing Gaseous Hydrogen Chloride for Cellulose Conversion

Content of the thesis: 

According to the so-called fringed-fibrillar model, the short, disordered regions in cellulose microfibrils are more susceptible to acid hydrolysis, which gives rise to the leveling-off degree of polymerization (LODP). In addition, one severe drawback of acid hydrolysis is the production of humins as an unwanted side reaction to glucose conversion. This dissertation utilizes a new pathway for cellulose degradation, namely using HCl gas, where the fiber morphology is minimally affected in the entire process. Purification of the products is relatively unproblematic from a gas-solid mixture, and a gaseous catalyst is more straightforward to recycle than the aqueous counterpart. This thesis was to explore the use of anhydrous HCl gas for producing sugars and nanocellulose from biomass by suppressing humin formation and analyzing the LODP behavior. 

Here, we investigate how various cellulose polymorphs/materials were hydrolyzed by HCI gas with a  30–50 wt.% water content. The 50–70% glucose yields were obtained from cellulose I and II polymorphs, while a >90% monosaccharide conversion was acquired from cellulose IIIII after a mild post-hydrolysis. In addition, alcoholysis has raised attention because it can significantly increase biopolymers' solubility and depolymerization rate, inhibit harmful humin formation, and produce valuable platform chemicals. The cotton-based cellulose fibers were firstly soaked in various alcohols (ethanol, 2-propanol, t-butanol, and ethylene glycol) and exposed to anhydrous HCl gas as an acid catalyst, the impact of these different systems on the LODP was explored. 

Cellulose nanofibrils (CNFs) have attracted significant attention due to their extraordinary mechanical and optical properties. In a foundational effort, we investigated the formation of CNF production from agricultural waste, namely from potato fibers, which are side streams with a global production of millions of tons annually. Remarkably, we obtained short CNFs (~500 nm) simultaneously in a one-step HCl(g) hydrolysis process involving alkali treatment, NaClO2 addition, and gaseous HCl hydrolysis. Subsequent film preparation demonstrated that even short CNFs could form strong transparent films. 

Overall, these findings provide a foundational understanding of the practical implementation of HCl gas hydrolysis with various cellulose polymorphs/materials. The study explores HCl (g) hydrolysis as a pathway to produce nanocellulose and platform chemicals from biomass.

Keywords: Cellulose polymorphs/materials, Gas-solid reactions, Alcoholysis, Leveling-off degree of polymerization, Humins, Monosaccharide generation, Cellulose nanofibrils

Opponent: Prof. Monica Ek, KTH Royal Institute of Technology, Sweden

Supervisor: Prof. Eero Kontturi, Aalto University School of Chemical Engineering

Link to electronic thesis: Thesis available for public display 10 days prior to the defence at Aaltodoc

Link to the remote defense: LINK