Abstract
In recent years, cellulose nanocrystals (CNCs) have emerged as a leading biomass-based nanomaterial owing to their unique functional properties and sustainable resourcing. Sulfated cellulose nanocrystals (sCNCs), produced by sulfuric acid-assisted hydrolysis of cellulose, is currently the predominant form of this class of nanomaterial; its utilization leads the way in terms of CNC commercialization activities and industrial applications. The functional properties, including high crystallinity, colloidal stability, and uniform nanoscale dimensions, can also be attained through carboxylated cellulose nanocrystals (cCNCs). Herein, we review recent progress in methods and feedstock materials for producing cCNCs, describe their functional properties, and discuss the initial successes in their applications. Comparisons are made to sCNCs to highlight some of the inherent advantages that cCNCs may possess in similar applications.
Highlights
Functionalization of CarboxylatedInterest in biomass-based nanomaterials has grown in the last decade owing to their unique functional properties and sustainable resourcing
Sulfuric acid hydrolysis is the predominant method for producing cellulose nanocrystals (CNCs), there are many other methods reported in the literature that allow for the preparation and isolation of CNCs at various levels of technology readiness level (TRL)
The values displayed for the degree of oxidation (DO) or carboxyl content of carboxylated cellulose nanocrystals (cCNCs) correspond to values prior to the amidation reactions; Carboxyl content; EDC: N-(3-dimethylaminopropyl)-N0 -ethylcarbodiimide hydrochloride; NHS: N-hydroxysuccimide; MES: 2-(N-morpholino)ethanesulfonic acid buffer; DMTMM: 4-(4,6-dimethoxy-1,3,5-tr,azin-2-yl)-4-methylmorpholinium chloride; CDI: 1,10 -carbonyldiimidazole
Summary
Interest in biomass-based nanomaterials has grown in the last decade owing to their unique functional properties and sustainable resourcing. Nanomaterials 2021, 11, 1641 the industry leader, Celluforce, which has a production capacity of 1 ton per day for sCNCs. sulfuric acid hydrolysis is the predominant method for producing CNCs, there are many other methods reported in the literature that allow for the preparation and isolation of CNCs at various levels of TRL. Efforts in developing alternative CNCs’ production methods to sulfuric acid hydrolysis have been driven by the various challenges associated with using sCNCs in downstream applications, including low thermal stability [26] and nanomaterial aggregation [27]. The sulfuric acid hydrolysis method is a harsh reaction due to the corrosiveness of the acid employed and favors the use of pure cellulosic feedstocks (forestry pulp materials, for example) to produce sCNCs. When biomass waste streams (such as raw lignocellulosic biomass containing cellulose, lignin, pectin, and other biopolymers) are used in the same process, it often leads to a low-yielding, impure CNC product that requires additional purification steps [20]. This review is focused on the direct synthesis of cCNCs from biomass feedstocks and excludes reactions performed on nanocelluloses and other types of cellulose derivatives
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