The diffusion of acid catalysts into cellulose network is important
in the hydrolysis reaction. In solids, the diffusion processes
are much slower than that in gases and liquids; and the hydrolysis
of cellulose powder is step-by-step from the surface of to inner,
resulting in a slow depolymerization rate. Hydrogels are crosslinked
hydrophilic polymers swollen in water or an aqueous environment
with the spaces between the polymer chains filled with
water (Amsden, 1998). The diffusion rates in hydrogel lie between
those in liquids and in solids and depend strongly on the concentration
and degree of swelling of polymers (Masaro and Zhu,
1999). To further confirm the diffusion of Na+ and Cl ions, the
HCl regenerated cellulose hydrogel was washed with water, air
dried at room temperature and then characterized by XRD (see
Fig. S3 in Supplement materials). The peaks for NaCl disappeared
completely and the spectrum was very similar with that of the
air dried hydrogel from H2O regeneration. This suggested that
the Na+ and Cl ions diffused out from the hydrogel during washing.
Thus, the high acid hydrolysis rate can be expected due to the
easy diffusion of ions into the hydrogel.