Preparation of Cellulose Nanocrystals by Synergistic Action of Ionic Liquid and Recyclable Solid Acid under Mild Conditions

Abstract

Cellulose nanocrystals (CNCs) are nanoscale particles made from cellulose. They have many unique properties such as being lightweight, stiff, and renewable, making them promising for a variety of applications in a wide range of industries, including materials science, energy storage, and biomedicine. In this paper, a two-stage (swelling-SA-catalyzed) method including IL pretreatment and solid acid hydrolysis process was developed to extract CNCs with high purity and good thermal stability from microcrystalline cellulose (MCC). In the first stage, the swelling of MCC in ionic liquid was studied with the assistance of ultrasonication, and it was found that the amorphous regions became more disordered while the crystalline areas were selectively retained under the conditions of 30 min of reaction time, 45 °C of temperature, 2% of ionic liquid water content and 1:4 mass ratio of cellulose to ionic liquid. CNCs were extracted using solid acid hydrolysis, with a 45 wt% solid acid to cellulose ratio and a 5.0 h hydrolysis process at 45 °C. The morphology, crystallinity, surface characteristics and thermo stability of the sample were characterized by atomic force microscopy (AFM), X-ray diffraction (XRD) and thermogravimetric analysis (TGA), respectively. Results demonstrated the highly thermostable CNCs were successful extracted with rodlike shape of 300 ± 100 nm in length and 20 ± 10 nm in width. Solid acid recovery and reuse were also studied, revealing a promising candidate that can reduce the environmental impact associated with chemical products.

Keywords: cellulose nanocrystals; hydrolysis; ionic liquids; recycling; solid acid; swelling.

Figure 1

Effects of different species of ionic liquids on (A) swelling degree, (B) crystallinity degree, and (C) swelling selectivity of MCC. (D) Chemical structures of the three ionic liquids.

Figure 2

Effect of swelling time on sizes of MCC in [AMIm][Cl]. ((A) 0 min; (B) 15 min; (C) 30 min; (D) 45 min; (E) 60 min; (F) 75 min).

Figure 2

Effect of swelling time on sizes of MCC in [AMIm][Cl]. ((A) 0 min; (B) 15 min; (C) 30 min; (D) 45 min; (E) 60 min; (F) 75 min).

Figure 3

Effects of swelling parameters on the properties of the MCC in [AMIm][Cl]. (a) Effect of water content. (b) Effect of time. (c) Effect of mass ratio of cellulose to ionic liquid.

Figure 4

AFM images of CNCs extracted via swelling−SA−catalyzed hydrolysis process.

Figure 5

X-ray spectra of (1) the original MCC, (2) the CNCs extracted with swelling−SA−catalyzed hydrolysis method at 45 °C for 5 h, and (3) the extracted CNCs hydrolyzed in 65.0 wt% H₂SO₄ at 45 °C for 5 h.

Figure 6

FTIR spectra of (a) the original MCC; (b) the CNCs extracted with swelling−SA−catalyzed hydrolysis method for 5 h at 45 °C; (c) the extracted CNCs hydrolyzed in 65.0 wt% H₂SO₄ for 5 h at 45 °C.

Figure 7

Size distribution of CNCs with different methods. a: the extracted CNCs hydrolyzed in 65.0 wt% H₂SO₄ at 45 °C for 5 h; b: the CNCs extracted with swelling−SA−catalyzed hydrolysis method at 45 °C for 5 h.

Figure 8

TGA analysis of a: the original MCC, b: the CNCs extracted with swelling−SA−catalyzed hydrolysis method at 45 °C for 5 h, and c: the extracted CNCs hydrolyzed in 65.0 wt% H₂SO₄ at 45 °C for 5 h.

Figure 9

Length of the CNCs after each reuse cycle.