Haile et al. Bioresour. Bioprocess.
(2021) 8:35
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up of β-1,4-linked xylose residues. They consist of α-arabinofuranose and α-glucuronic acids pendant branches that contribute to cross-linking of cellulose microfibrils and lignin through ferulic acid residues. Xylan can be extracted from several locations in the pulp mill. During kraft pulping, xylan is partly dissolved in the cooking liquor but part of it will be redeposited onto the fibers in the later parts of the cook. The dis- solved xylan can be isolated from black liquor for its utili- zation in the production of different biomaterials. Among the different methods used for xylan extraction, those utilizing alkali are most commonly and widely used. Xylan is constituted by the black liquor along with frac- tions lignin fractions. The extraction of xylan from black liquor requires two-step precipitation. First, black liquor acidification is conducted and this is followed by precipi- tation by ethanol which ultimately provides the fractions of xylan (Stoklosa 2014). The xylan hemicellulose separa- tion is done using potassium hydroxide (KOH) for hard- wood and sodium hydroxide for softwood in aqueous media. The mechanism of separation of the hemicellulose is achieved through hydrolysis which involves the break- age of ester linkages between xylan and other alkaline black liquor components. Xylan can be biorefined into lactic acid (Fig. 10). First, xylan is converted to low molecular weight sugars by hydrolysis using enzymes or chemically using acid hydrolysis. Then the resulting low molecular weight sug- ars can be converted to lactic acid by fermentation or alkali oxidation. Biomass-derived lactic acid is an impor- tant renewable chemical building block for synthesiz- ing bioplastics (Fernández-Rodríguez et al. 2019; Chen
2012). Polylactic acid (polylactide) used as a precursor for plastics can be obtained by polymerization of a lactic acid refinery of xylan. Polylactic acid (polylactide) [PLA] is biodegradable as well as recyclable polyester made from renewable feed- stock. PLA is synthesized by the condensation polym- erization of lactic acid or ring-opening polymerization of the corresponding lactide. As one of the highly biode- gradable and crystalline polymers the lactic acid polymer (PLA) has a high melting point and outstanding mechan- ical characteristics (Gordobil et al. 2015; Singhvi et al. 2019). It can be used for bioplastic manufacturing used for different engineering applications. Biomaterials of versatile need can be obtained from PLA using advanced manufacturing techniques such as electrospinning, espe- cially for medical products. The major challenges in con- version and utilization of xylan are of two ways. One is the difficulty in obtaining fully purified xylan with the grade required for polymer production and the other is the lower melting pint of the PLA produced which limits its application for high-performance materials. Different researches are ongoing especially in maximizing extrac- tion effectiveness of xylan and usage of modified polym- erization and spinning technologies for manufacturing higher grade PLA products. Beneficiation of cellulose for high value-added materials Production of cellulose nanocrystals [CNC] from cellulosic residues Nanocellulose is a unique and promising natural com- pound derived from ordinary biomass. It is currently the most environmentally friendly compound that is
Fig. 10 Xylan to PLA route
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