It’s easy to see that the bleached CCR was a cellulose-rich and
hemicellulose-depleted waste residue (the data are listed in Section
2.2). The hemicelluloses had been removed during furfural production.
After the bleaching process, the lignin in the CCR had also
been mostly removed. As shown in Fig. 1a and b, the bleached CCR
was white powder with particle size of dozens of microns. Small
amount of nanofibers and ordered nano-sized pores (Fig. 1c) could
be detected on the particle surfaces before any further processing.
The porous structure might make bleached CCR a more processable
lignocellulosic material for nanocellulose production. In the
following sections, the nanocellulose obtained from four different
methods (sulfuric acid hydrolysis, FA hydrolysis, TEMPO-mediated
oxidation and PFI refining) was referred as SCNC, FCNC, TCNF and
PCNF, respectively.
3.1. Self-assembled structure
Drying process of nanocellulose suspension is important for its
storage, transportation and generation of various desired structures
(Jiang & Hsieh, 2013). The nanofibers usually aggregate
into bundles during the drying process (Isogai, 2013). The SEM
images of self-assembled structures of nanocellulose are shown
in Fig. 2a–d. During the freeze-drying process, the SCNC (Fig. 2a)
and FCNC (Fig. 2b) aggregated into 2D sheet-like structures. The
SCNC self-aggregated into petal shaped sheets, which were mutually
connected and no visible porosity could be seen on the sheets.
In contrast, the FCNC formed larger sheets with nanometer-sized
pores on the surfaces. Both TCNF (Fig. 2c) and PCNF (Fig. 2d)