## 2 Lignocellulosic Biomass and Cellulose  
Lignocellulosic biomass, commonly known as plant biomass, is the most abundant source of sustainable carbon and the most promising raw material for the environmentally benign production of various green products, including biochemicals, bioethanol and biofuels [2]. LB is mainly that portion of a crop which is either not suitable for harvesting or has no other end use. It completely falls under the concept of circular bioeconomy and can thereby be effectively utilized for the production of value-added products to achieve sustainable development goals [4]. The lignocellulosic materials that can be recycled include waste products from agriculture (straw, bark, stalks, leaves and bagasse), forestry (hardwood and softwood), energy crops, weeds, municipal waste, food and other sources [19]. Primarily, the structural components of LB consist of cellulose (35-50%), hemicellulose (20-35%) and lignin (15-30%), which is a polyphenic aromatic polymer [20; 21; 22]. The primary composition of LB varies among the types of biomass, geographical area, climate, time of harvesting etc. [9]. All three components of biomass can be effectively utilized for the production of various value-added materials [1; 11; 23]. However, the major bottleneck in utilization of LB components for the production of different valuable products is its recalcitrant nature. The three polymers in LB (cellulose, lignin and hemicellulose) are compactly packed and interconnected with one another in a heteromatrix to variable degrees and compositions. The intrinsic packaging of these polymers, cellulose crystallinity and degree of polymerization, presence of acetyl group, hydroxyl group and syringyl-to-guaiacyl ratio in lignin are some factors that contribute to overall LB recalcitrance [2; 24]. Biomass recalcitrance prevents mechanical/microbial/enzymatic action on the biomass and limits its conversion into useful materials [25; 26; 27]. Hence, reducing the biomass recalcitrance is a precondition for the efficient conversion of LB which is ideally accomplished through biomass pretreatment. Pretreatment aids in extracting/separating the desirable polymers from the complex network of three main polymers and is therefore one of the major steps in biomass conversion. After the efficient fractionation of LB, cellulose, hemicellulose and lignin polymers can be utilized for the production of various products, including biofuels, hydrogels, vanillin, organic acids and nanomaterials [1; 2; 4; 28]. Cellulose is the most abundantly present polymer across the globe, with an estimated natural production between 10\({}^{10}\) and 10\({}^{11}\) tonnes annually, out of which only about 6 \(\times\) 10\({}^{9}\) tonnes are utilized by various industrial sectors, including the paper, textile, chemical and material industries [29]. Cellulose [(C\({}_{6}\)H\({}_{10}\)O\({}_{5}\))\({}_{n}\)] is made up of repeating \(\beta\)-d-glucopyranoside molecules linked together by \(\beta\)-(1,4)-glycosidic bonds, and constitutes the highly hard crystalline portion of the plant cell wall. The OH groups in the C\({}_{4}\) and C\({}_{1}\) carbon atoms of the glucose units form glycosidic bonds that covalently bind to form several hundred to thousand repeating units, which are linear polysaccharides [30; 31]. Every glucose unit in cellulose is rotated 180 degrees in relation to its neighbors. Hydrogen bonds can be formed between the three free hydroxyl groups (OH) at the C-2, C-3 and C-6 atoms, which hold a crucial role in the crystalline packing (Figure 1). The solubility and other physical attributes of cellulose are controlled by intramolecular and intermolecular hydrogen bonding [3]. Irrespective of the type of LB, a polymeric chain of \(\beta\)-(1-4)-linkage and D-glucose commonly aggregates via Van der Waals interactions and inter/intra molecular hydrogen bonds in nanosized, thread-like assemblies, termed as microfibrils.