The complete conversion of bleached kraft pulp into dissolving pulp and two xylo-oligosaccharides through a deep eutectic solvent-assisted biorefinery

The preparation methods for refining kraft pulp into dissolving pulp are generally limited by an aggressive chemical input and overlook the valorization of the xylan component. This work demonstrated an integrated process involving deep eutectic solvent (DES) pretreatment followed by mechanical refining and enzymatic hydrolysis (DME) for the preparation of the dissolving pulp and co-production of two xylo-oligosaccharides (XOSs). The technical feasibility of the DME process was assessed with typical bleached bamboo and poplar kraft pulp. It was shown that the employed aqueous alkaline DES consisting of choline hydroxide and urea (40% concentration) could initially purify cellulose while selectively extracting alkali-soluble XOSs by the unique bonding behavior of the DES. The resulting dissolving pulp showed considerably high cellulose contents of 94.3% and 91.1% for the bleached bamboo pulp and bleached poplar pulp, respectively. The overall cascade process significantly improved the cellulose reactivity and quality (alpha-cellulose > 90%, S10/S18 < 8%), which could meet the high criterion for commercial dissolving pulp while extracting 69.4% of the content of high-purity XOSs. When 100 g bleached pulp was subjected to this technology route, a high yield of over 80 g cellulose was obtained for both the substrates while simultaneously producing the alkaline and water-soluble XOSs (approximately 10 g). It was proposed that the strong hydrogen-bonding-accepting ability of the ChOH-urea DES system could purify cellulose through XOS selective solubilization while swelling cellulose to ease the subsequent mechanical fibrillation. Subsequently, the mechanically treated cellulose with enhanced accessibility to xylanase provided an additional technique for further cellulose purification through a facile enzymatic hydrolysis. Therefore, this proposed integrated technique could refine bleached kraft pulp into dissolving pulp with enhanced cellulose reactivity while co-producing high-purity XOS product streams.