Full Papers
themselves. Thus, with oxygen as the only byproduct, ozone
keeps salt generation at a minimum. Nevertheless, a small
amount of salt has been shown to improve the oxidation effi-
ciency by disrupting the hydrogen-bond network of cellulose,
and thereby, making some of the hydroxyl groups better ac-
cessible to periodate. For example, Sirvio et al. performed salt-
Spent periodate from DAC preparation was completely re-
generated by ozone treatment over three consecutive recy-
cling steps. The observed consumption of hydroxyl ions during
ozonation could be an advantage in the recycling process be-
cause it decreased the amount of acid necessary to return to
the acidic conditions desired for cellulose oxidation. Minor
losses of iodate in the recycling process result from the
workup procedure after the oxidation step, which can be over-
come by collection of the DAC washing solutions. Recycled
periodate maintained good oxidation performance throughout
subsequent DAC preparations, and showed no differences
from fresh periodate. The presented recycling process for
periodate offers an environmentally friendly, economically fea-
sible, and sustainable pathway for the production of oxidative-
ly functionalized celluloses and other polysaccharides.
assisted oxidation with an amount of NaCl that corresponded
À
to the concentration of salt which would accumulate from IO
4
[44]
recycling after 17 cycles. This indicates that the presence of
salt could be tolerated to a certain extent, after which per
iodate would be precipitated and recovered as a salt, given
[
6]
a sufficient amount of alkali, or purified by other means.
Another particular benefit of using ozone in alkaline
medium is the removal of organic impurities, such as low-mo-
lecular-weight acids and hydroxyacids from cellulose or poly-
saccharide degradation, by oxidation. These impurities are
completely oxidized, that is, converted into CO2 and water,
under the strongly oxidizing conditions. They would thus not
further contribute to the total organic carbon (TOC) content in
the effluents and also could not undergo any side reactions,
such as condensation to chromophoric substrates, with cellu-
lose or DAC.
Experimental Section
Materials
All chemicals used for optimization of the regeneration process
were obtained as p.a. grade from Sigma–Aldrich (Schnelldorf, Ger-
many) and were used as received. Acetonitrile and phosphoric acid
Regardless of the change in pH, recycling of periodate evi-
dently has to be carried out separately from cellulose oxidation
to prevent side reactions of the polymer with the secondary
oxidation agent, for example, cellulose degradation or further
oxidation of aldehydes to carboxyl groups. Recycling of per
iodate with ozone also allows the preparation of DAC of vari-
ous degrees of oxidation by using higher periodate concentra-
(
85%) were of the highest purity available and obtained from
Merck (Vienna, Austria) and Roth (Graz, Austria), respectively.
Fully bleached sulfite dissolving pulp (beech) from Lenzing AG
(
weight-average molar mass of the pulp was 290 kgmol with
a starting carbonyl content of 24.3 mmolkg . Full characterization
Lenzing, Austria) was used as the cellulosic test substrate. The
À1
À1
[45]
of this pulp was previously reported by our group.
[
43]
tions. In our study, we evaluated the regeneration of per
iodate up to 100 mm solutions of iodate. If desired, even satu-
rated solutions of iodate (e.g., 400 mm) can be recycled by
ozone treatment, providing the pH is kept high enough, if only
in the form of precipitated salt.
Preparation of DAC
Air-dried cellulose sheets (10 g) were torn into small pieces of 1–
2
2
cm and disintegrated in water by using a kitchen blender. After
The proposed ozone-based regeneration method should be
suitable for recycling of spent periodate not only from oxida-
tion of cellulose, but also from oxidation of starch and other
glucans. If the oxidation product was soluble, for example,
fully oxidized DAC or low-molecular-weight carbohydrates, the
precipitation of iodate prior to regeneration could be achieved
removing excess water by filtration with a Büchner funnel, the
water content of the cellulosic pulp was approximately 50%wt.
The wet cellulose was immersed in a solution of sodium periodate
(
0.26 g) to a total volume of 120 mL and was oxidized at 558C to
[43]
a DO of 2%, according to a method reported previously. Ali-
quots were taken every 10 min to monitor both periodate con-
sumption and iodate formation, by measuring their absorbance at
l=290 nm after ion separation on a reversed-phase column. When
all periodate was consumed, the reaction solution was filtered off,
and the pulp was thoroughly washed with deionized water to
remove residual iodate. Pulp samples were frozen without drying
and stored for further analysis by gel permeation chromatography
[9]
by the addition of metal ions or organic solvents.
Conclusions
This study demonstrates efficient periodate recycling by ozone
treatment with a great potential for large-scale applications in
DAC production. Alkaline conditions that promote the forma-
tion of oxygen radicals and a high ozone concentration at
room temperature proved to be most beneficial for high con-
version efficiency. Direct monitoring of reaction kinetics
showed that 100% yields were attained when the concentra-
tion of alkali was at least equal to the initial iodate concentra-
tion to be converted, or higher. Insight into the mechanism of
ozone oxidation, by means of hydroxyl radical scavengers and
DFT calculations of potential energies, assisted better under-
standing of the recycling process and confirmed that hydroxyl
radicals were centrally involved in the oxidation system.
(
GPC).
Recycling of periodate
After cellulose oxidation, the reaction solution containing about
1
0 mm iodate was allowed to cool and the pH was adjusted to 12
with 4.5m NaOH. An amount of about 100 mL was treated with
ozone for 20 min at room temperature. Finally, 12m HCl was used
to acidify the regenerated solution of periodate to pH 4.5, which
was required for the next oxidation cycle (Scheme 1). Recycled
periodate was reused to repeatedly oxidize cellulose to DAC, as de-
scribed above. Control reactions with fresh solutions of periodate
were run in parallel. The starting amount of cellulose was adjusted
ChemSusChem 2016, 9, 825 – 833
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