Cellulose nanocrystals' production in near theoretical yields by 1-butyl-3-methylimidazolium hydrogen sulfate ([Bmim]HSO4) - mediated hydrolysis

We report on near theoretical yield production of cellulose I nanocrystals (CNCs) using a two-step hydrolysis with the mildly acidic ionic liquid (IL) 1-butyl-3-methylimidazolium hydrogen sulfate ([Bmim]HSO4) in aqueous solution from common cellulosic sources. Two successive Taguchi experimental plans were performed to evaluate the impact of selected reaction parameters (T, t, H2O:IL ratio) and their interactions on the CNCs' yield from bleached softwood kraft pulp (SWP), bleached hardwood kraft pulp (HWP) and microcrystalline cellulose (MCC). With these experimental plans, the molar yield for extraction of nanocrystals was optimized to near theoretical levels, reaching 57.7 ± 3.0%, 57.0 ± 2.0%, and 75.6 ± 3.0%, for SWP, HWP and MCC, respectively. The reaction yields corresponded to a relative crystalline region recovery of 84.1 ± 5.3%, 71.7 ± 1.3%, 76.0 ± 2.0% from SWP, HWP and MCC, respectively. The collected nanocrystals exhibited high aspect ratios (36-43), negligible sulfur content (0.02-0.21%) and high solvent dispersibility in comparison to those obtained with the traditional sulfuric acid method. Additionally these near theoretical yields were achieved for mild reaction conditions with the combined severity factor of 2 and 3 for MCC and pulp, respectively. Overall this two-stage IL-mediated preparation of nanocrystals combines the advantages of achieving high product quality, high reaction yields and mild conditions.