Cellulose Valorization for the Development of Bio-based Functional Materials via Topochemical Engineering
Introduction
Bio-based natural resources are of significant scientific importance in this era due to their rich existence, renewability, easy accessibility, low cost, etc. Cellulose, the naturally occurring biopolymer abundant in plants and woods has been a potential starting material for valorization as their supply from the forest products industry and agricultural wastes is huge and inexpensive. Cellulose is a polymer substance, with the repeating units of monosaccharide containing secondary and tertiary -OH groups. These -OH groups are highly prone to undergo any modification using chemical reagents or by physical methods. Topochemical engineering is one strategy through which directed assembly of new guest molecules on substrate or disassembly of that host substrate units can be performed to produce multifunctional materials. This book chapter aims to disseminate the knowledge around topochemical engineering of cellulose-based functional materials with the emphasis on valorization techniques, material compositions, and applications, for the readers of sustainable biomaterials chemistry.
We are living in the age of rapid industrialization and modernization of our lives. But these developments are highly dependent on natural resources. There are two types of resources: (1) Fossil deposits and (2) Forests. Fossil resources are nonrenewable hence unsustainable, whereas forest products are highly abundant and renewable. These resources have turned to be potential replacement to non-renewable fossil resources as they are depleting fast. Wood Pulp is a lignocellulosic material containing cellulose, hemicelluloses, and lignin. The annual production of pulp exceeds 160 million tonnes. This pulp feedstock is hugely acquired from forest products industries to get converted into bulk chemicals and other technical materials. Bioresources offer sustainable chemical or material production with environment compatibility and recyclability. Thus, the sustainable development goals fixed by United Nations can be achieved via bio-based chemistry.
The raw material, pulp fibres, from wood and annual plants is usually obtained by mechanical, chemo-mechanical, or chemical treatment and then bleached prior to further modifications. Mechanical and chemo-mechanical treatments lead to higher amounts of lignin and hemicelluloses left unremoved from pulp, whereas pulp treated by chemical reagents yields high-quality cellulose fibres without contaminants. Kraft pulping and sulfite pulping are the techniques majorly used in chemical pulping. These techniques combined with other bleaching methods like Elemental Chlorine Free (ECF) or Total Chlorine Free (TCF) sequences produce cellulose fibres for high-end applications. Pulps treated with Kraft process and ECF bleaching offer variety of applications like paper, tissues, absorbents, and packaging materials. On the other hand, the process in which pulps are processed until pre-hydrolysis step of Kraft process or by sulfite followed by TCF bleaching delivers pulps with high soluble cellulose fibres as this combined process eliminates lignin and hemicelluloses very efficiently. This dissolved cellulose can be chemically modified into cellulose derivates further. Thus, different methods of pulping and bleaching techniques have great impact on the quality of the resultant pulp, which offers variety of pulp grades suitable for different purposes depending on residual amounts of lignin and hemicelluloses present in the material. All pulps possess anionic groups (AGs) that are active sites for interactions with other reagents. The amount of AGs on fibres depends on the macromolecular properties, amount of hemicelluloses of wood raw material, and dissolution/reaction of biopolymers during pulping and bleaching. Cationic chemicals, polyelectrolytes are usually applied to interact with AGs in the production of fibre-based materials (1).
Cellulose is a biopolymer containing repeating units of anhydroglucose. Hence, it is a polysaccharide. These units contain 1° hydroxyl groups at C6 position and 2° hydroxyl groups at C2, C3 positions on its carbon Skelton. These hydroxyl functional groups are chemically reactive; hence they are sites of opportunity to modify cellulose, for example, cellulose nitrates, esters, and ethers. Oxidation is an important chemical reaction that can be carried out on cellulose. During oxidation the primary -OH groups at position C6 converted into -COOH and secondary -OH groups at positions C2, C3 into -CHO and -COOH (2, 3). Thus cellulose can be modified from neutral to anionic or cationic, hydrophilic to hydrophobic character, with increased mechanical strength and chemical reactivity. Chemical functionalizations can be tuned in cellulose to result desired hydrophilicity/hydrophobicity, crystallinity, stereo-regularity, multi-chirality, thermal and mechanical stability, biocompatibility, and sustainability.
For a long time, cellulose has been viewed as just a pulp and paper source material. But due to the new inventions that led to the potential applications of cellulose fibres in thin films, textiles and personal care products, the scenario in pulp market changed very fast. These are the motivations to explore chemical and physical properties of cellulose for value addition and finally the manufacturing of sustainable novel applied materials. This chapter focuses on topochemical engineering approach on cellulose that can be done either by directed assembly or disassembly via the design of intermolecular interactions in a topological space. The challenge is to disturb the intra/intermolecular hydrogen bonding in cellulose fibres to separate them as independent cellulose molecules with right employment of solvents, reagents, process of extrusion, and drying. Post disassembly, these new morphologies are functionalized or assembled with new guest molecules possessing desired properties. These modifications unveil new surfaces and interfaces with targeted functionalities suitable for a variety of technical material applications. The product materials include well-defined objects such as tissues, sponges, textile fibres, film or sheets, and spherical beads having desired specifications.
