Production of nanocellulose from supramolecularly delignified wood

Abstract

This work focused on the practical and theoretical aspects of pulp delignification and overall nanocellulose production. Nanocellulose as an ecological composite material has gained interest worldwide and multiple global institutes have researched its use in products as a standalone component or as a filler material. From biomedical applications to aerospace engineering, this nanoscale cellulose-polymer has a variety of possible uses. But as nanocellulose is cumbersome to manufacture due to large energy costs, large scale production of the material has not yet seen the light of day.

Cellulose is a large linear polysaccharide in the cellular walls of wooden plants, as well as numerous other natural sources such as bacterium and algae. This polymer is tightly bound into cellular structures by e.g., van der Waals forces, hydrogen bonds and ionic interactions, which makes separation of pure cellulose difficult. This problem has been answered in the past by ecologically hazardous processes which utilize harsh chemicals, which are widely used in pulp manufacturing. These production processes have been made as eco-friendly as possible through recycling efforts, but nonetheless, the process itself remains dangerous.

Using eutectic solvents, which can be manufactured using only environmentally friendly chemicals, pulp purification may be made easy and energy efficient. Eutectic solvents have a lower melting point than one or more of its constituents, and subsequently form two or more distinct phases when cooled. The molar ratio of the components is behind this phenomenon, with and optimal ratio producing a liquid solution. A deep eutectic point (DES) is considered when the melting point differs vastly from the individual constituents.

DES have been trialed in biomass valorization efforts as the solvent is able to efficiently fractionate biomass, separating lignin, and hemicellulose from the cellulose polysaccharide. This effectiveness rivals processes which use hot temperatures and toxic chemicals with a fraction of the energy expenditures, whilst being more ecological. The separated cellulose may be subsequently processed into desired products, such as nanocellulose.

The experimental section of this work focuses on producing nanocellulose from delignified pulp produced with eutectic solutions, and the subsequent valorization of produced nanocellulose. The raw pulp was delignified in varying temperatures and durations to examine the effectiveness of the eutectic solvent treatment. The constituents of the solvents were methanesulfonic acid (MSA) and thymol, which have been proven by yet unpublished work to produce effective delignification. The delignified pulp batches were subjected to ultrasonication in identical conditions to produce nanocellulose suspensions with water. The acquired suspensions were subjected to testing and manufactured into composites with polyvinyl alcohol (PVA), to explore the possibilities and characteristics of the acquired nanocellulose variants. The testing series included viscosity analysis with a hybrid rheometer (HR), ultraviolet-visible (UV-vis) spectroscopy, tensile strength analysis, gravimetric yield analysis, and reviewing the characteristics of membranes produced from said nanocellulose batches. The testing series also included optical analysis with varying electron microscopy imaging methods.

Initial results indicated efficient delignification with higher temperatures and longer reaction durations when inspecting the imaging results. Gravimetric yield analysis indicated that the largest yield of nanocellulose may be produced in only certain reaction conditions, and that higher temperatures and durations do not necessarily produce the highest yield. The purest cellulose was however produced in higher temperatures and durations, but no direct correlation with nanocellulose production and lesser amounts of lignin were observed.