Isolation of Mixed Compositions of Cellulose Nanocrystals, Microcrystalline Cellulose, and Lignin Nanoparticles from Wood Pulps

Abstract

From a circular economy perspective, one-pot strategies for the isolation of cellulose nanomaterials at a high yield and with multifunctional properties are attractive. Here, the effects of lignin content (bleached vs unbleached softwood kraft pulp) and sulfuric acid concentration on the properties of crystalline lignocellulose isolates and their films are explored. Hydrolysis at 58 wt % sulfuric acid resulted in both cellulose nanocrystals (CNCs) and microcrystalline cellulose at a relatively high yield (>55%), whereas hydrolysis at 64 wt % gave CNCs at a lower yield (<20%). CNCs from 58 wt % hydrolysis were more polydisperse and had a higher average aspect ratio (1.5-2×), a lower surface charge (2×), and a higher shear viscosity (100-1000×). Hydrolysis of unbleached pulp additionally yielded spherical nanoparticles (NPs) that were <50 nm in diameter and identified as lignin by nanoscale Fourier transform infrared spectroscopy and IR imaging. Chiral nematic self-organization was observed in films from CNCs isolated at 64 wt % but not from the more heterogeneous CNC qualities produced at 58 wt %. All films degraded to some extent under simulated sunlight trials, but these effects were less pronounced in lignin-NP-containing films, suggesting a protective feature, but the hemicellulose content and CNC crystallinity may be implicated as well. Finally, heterogeneous CNC compositions obtained at a high yield and with improved resource efficiency are suggested for specific nanocellulose uses, for instance, as thickeners or reinforcing fillers, representing a step toward the development of application-tailored CNC grades.

Figure 1

AFM height images of CNCs produced from different wood pulps (UBSK and BSK) and hydrolysis conditions (58 and 64% sulfuric acid). 10 nm z-scale (dark to light) and x–y scale for images (a–d) indicated in (d). Some of the L-NPs in the UBSK samples are indicated with a dashed-line circle.

Figure 2

Representative AFM topography maps 3 × 3 μm (a,d), s-SNOM 4th harmonic optical amplitude image (a.u.) (b,e), and s-SNOM 4th harmonic optical amplitude phase image (rad.) (c,f) of UBSK 58 (upper row) and UBSK 64 (lower row). The s-SNOM images were acquired using the QCL laser tuned to emission at 1515 cm^–1. Image scale bars are 1 μm.

Figure 3

Photographs of CNC suspensions at 0.5 wt % (a) shear viscosity of suspensions at 1.75 wt %; (b) transmittance spectra of films; (c) photographs of film pieces (∼2 cm², 20 μm-thick) on white and black backgrounds (d). The suspensions and films (a,d) are arranged in the same sequence from left to right: BSK 58, BSK 64, UBSK 58, UBSK 64, and COT 64.

Figure 4

Polarized optical microscopy images of CNC films prepared from different pulp types (UBSK and BSK) and sulfuric acid concentrations (58 and 64 wt %). Figures (a,c) are films cast from UBSK 58 and UBSK 64 CNCs, respectively, and (b,d) are the BSK counterparts. The scale bar for images (a–d) is indicated in (d).

Figure 5

Cross-sectional SEM images of CNC films; figures (a,c) are of UBSK 58 and UBSK 64 films, respectively, and (b,d) are the BSK counterparts. The scale bars for images (a–d) are indicated in (d).