Facile one-pot fabrication of cellulose nanocrystals and enzymatic synthesis of its esterified derivative in mixed ionic liquids

Abstract

As an important cellulose derivative, esterified cellulose nanocrystals (E-CNCs) could be applied in biomedical and chemical industries. A facile one-pot enzymatic method was proposed for the preparation of esterified cellulose nanocrystals with methyl laurate by using native cellulose as raw material in a binary ionic liquid (IL) system. 1-Butyl-3-methylimidazolium hydrogen sulfate ([BMIm]HSO₄), a hydrophilic IL, acted as catalyst and solvent to produce cellulose nanocrystals (CNCs). 1-Butyl-3-methyl-imidazolium tetrafluoroborate ([BMIm][BF₄]), which is hydrophobic, was used as a solvent for improving the lipase performance. The key reaction conditions, including mass ratio of [BMIm]HSO₄/[BMIm][BF₄], molar ratio of methyl laurate/anhydroglucose unit (AGU) in cellulose, mass ratio of lipase/cellulose, and reaction time, showed significant influences on the enzymatic transesterification of CNCs. The ester products were confirmed by FT-IR and ¹³C NMR analysis. The crystalline structure of E-CNCs was revealed by XRD. Compared with native cellulose, the E-CNCs showed improved crystallinity. TEM data showed the width of rod-shaped E-CNCs in the range of 10–30 nm. Compared to native cellulose, the thermal stability of E-CNCs was decreased due to the higher surface areas exposed to heat. This is the first successful preparation of E-CNCs with long-chain fatty acids from cheap native cellulose in a one-pot method.