tained as well as the thermo-stabilized freeze-dried
lignin was performed using a TA Q-200 DSC in order to
identify the glass transition temperatures (Tg). 3 to 5 mg
of lignin samples were sealed in an aluminum pan sup-
plied by TA instruments. Initially, the sample was heated
to and maintained at 80˚C for 30 minutes in order to re-
move existing moisture and then cooled to 0˚C. The DSC
thermogram was recorded up to 250˚C employing a 20˚C
/min ramp rate. All analysis was carried out in a nitrogen
atmosphere.The micro-structure of the lignin samples
were analyzed using a scanning electron micro-scope
(SEM), FEI Inspect S50 Netherlands. Gold coating was
Figure 1. Schematic of the various steps involved in the synthesis of carbon nanoparticles fr om lignin.
Copyright © 2012 SciRes. WJNSE
performed for all the samples in order to enhance the
SEM images. Synthesized carbon nanoparticles were
characterized using a transmission electron microscope
(TEM), JEOL 2010F FEG TEM/STEM, employing a 200
kV operating voltage. Brunauer-Emmet-Teller (BET)
surface area analysis of the freeze-dried lignin samples
and the synthesized carbon nanoparticles were measured
with a NOVA station-C, Quantachrome through nitrogen
gas sorption at 77.3 K. First, the samples were flow de-
gased at 55˚C for 8 - 16 hours to remove the volatiles.
BET surface areas were taken from a multipoint plot over
a P/Po range of 0.05 - 0.35.
3. Results and Discussion
The freeze-drying process caused the formation of po-
rous lignin samples. However, these may fuse together
and form lumps during the carbonization process, which
results in bulk carbon material rather than well-defined
carbon nanoparticles. In order to retain this porous struc-
ture, lignin samples underwent a thermal stabilization
process employing a low heating rate of 1˚C/min at
250˚C for 2 hours. The effect of thermal stabilization on
their structural coordination, thermal behavior, and mi-
crostructure were investigated by FTIR, DSC, and SEM
analysis respectively. Figure 2 shows the FTIR spectra
of the as-obtained and thermo stabilized lignin with dif-
ferent KOH formulations. From Figure 2, the FTIR
spectra indicates a characteristic peak of the lignin at
1590 cm1 and 1500 cm1, which represents the aromatic
skeletal vibration [22]. Thermal stabilization of the
freeze-dried lignin caused shifting of the peak at 1590
cm1, which also increases with increasing KOH concen-
tration for both the freeze-dried as well as thermo stabi-
lized lignins. During the thermal stabilization, lignin un-
dergoes condensation reactions in the presence of alkali
metals, which also caused the formation of various or-
ganic compounds such as metal formates as well as ace-
tates [22]. These metal carboxylates have a very strong
absorbance in the region of 1695 - 1540 cm1, which
caused the increased peak intensity at 1590 cm–1 [23].
Thermal stabilization also caused the formation of new
peaks at 1385 cm1 and 1315 cm1, which is attributed to
C-O-C stretching, which evidences the formation of ex-
cess ether groups through condensation. This results in
higher cross-linking and caused significant improvement
in the glass transition temperature.
Figure 3 exhibits the DSC thermograms of freeze-
dried lignin before and after thermo stabilization. The
DSC thermogram of freeze-dried lignin derived without
KOH addition shows a Tg of 89˚C, which increases with
increasing KOH concentration. Ucar et al. reported that
the presence of alkali metal in lignin effectively caused the
cross-linking through condensation, which also results
PL+15% KOH
PL+10% KOH
PL+15% KOH
PL+10% KOH
Figure 2. FTIR spectra of freeze-dried lignin (a) as derived
and (b) thermal stabilized.
in the increased Tg in the DSC thermogram.
Thermo stabilization further enhances the condensa-
tion/cross-linking significantly thereby increasing the
glass transition temperature and retaining the glassy state
of lignin, which is confirmed through the DSC thermo-
grams which do not show a significant Tg point. During
thermal stabilization, lower heating rates increase the Tg
of the lignin samples faster than the actual temperature,
thereby avoiding the possibility of fusing and stabilizing
the foamy structure. SEM analysis confirms this phe-
nomenon, which is shown in Figure 4. The freeze-drying
process caused the formation of porous structures, which
is highly influenced by the presence of KOH. The
freeze-dried lignin solution made without KOH addition
formed a caused the formation of solid mass. Further, the
thermal stabilization caused the successful retention of
Copyright © 2012 SciRes. WJNSE
PL+15% KOH
PL+10% KOH
PL+15% KOH
PL+10% KOH
111 ˚C
Figure 3. DSC thermogram of freeze-dried lignin (a) as
porous structure. This resuis consistent with the re-
were carbonized at
derived and (b) thermo stabilized.
ported literature by Kadla et al. [20].
Thermol stabilized lignin samples
0˚C in nitrogen atmosphere for 2 hours. It was ob-
served that the carbonized lignin without KOH results in
the formation of solid mass where as the lignin samples
modified with KOH yielded ultrafine particles. The chal-
lenging issue in fabricating carbon nanoparticles is the
yield, which indicates the efficiency of the conversion
process. The thermal stabilization yield fraction (YTS) is
the ratio of mass of lignin present after thermal stabili-
zation (mTS) to before (mTS) thermo stabilization process.
Similarly, carbonization yield fraction (YC) is the ratio of
mass of carbonized material (mC) to mass of material
present before carbonization process (mTS). Overall yield
is the product of the yields of thermal stabilization (YTS)
and the carbonization (YC).
(I) A s derived (I I) The rmo-s tabilized
50µm 50µm
50µm 50µm
50µm 50µm
(a) (a)
(b) (b)
(d) (d)
Figure 4. SEM micrographs of freeze-dried lignin (I) as
derived and (II) thermo stabilized ((a) PL; (b) PL + 5 wt%
KOH; (c) PL + 10 wt% KOH; (d) PL + 15 wt% KOH).
mm m
 
  
 
Table 1 summarizes the yields during the various
ages involved in the synthesis of carbon nanoparticles.
The specific surface area of the synthesized carbon
noparticles were measured by employing BET surface
area analysis The measured surface area of the carbon
nanoparticles synthesized from lignin with different
KOH concentrations of 0%, 5%, 10%, and 15 % are 0, 43,
47, and 23 m2/g respectively. From this analysis, it is
confirmed that the addition of KOH to lignin up to 10%
Copyright © 2012 SciRes. WJNSE
increases the surface area and higher concentrations of
KOH decreases the surface area. This may be due to the
tendency of KOH to form agglomerates at higher KOH
concentrations (15%).
The results indicate that the addition of KOH reduces
4. Conclusion
les were successfully synthesized us-
Table 1. Yields for thermaed and carbonized
Sample al Yield of
car Overall yield
e overall yield, which may be due to the oxidation be-
havior of KOH in lignin. Synthesized carbon powders
were further characterized by TEM analysis to confirm
the formation of nanoparticles. TEM micrographs of the
carbon particles prepared from lignin source modified
with different KOH concentration are shown in Figure 5.
Carbon nanopartic
ing lignin (Protobind 2400), a industrial co-product, as a
renewable feedstock. The effect of KOH addition on the
solubility of lignin, the freeze-drying process, thermal
lly stabiliz
carbon nanoparticles.
Yield of therm
name stabilization
(YTS%) bonization
(YC%) (YT%)
PL 92.88 1.3 52.847 1.11 49.08
PL + 5
w88.46 2.5 55.072 1.9 48.71
w 75.97 3.1 56.7 2.1 43.07
w66.82 3.5 57.8 2.5 38.61
t% KOH
t% KOH
PL+ 15
t% KOH
500 nm 100 nm
200 nm 200 nm
(a) (b)
(c) (d)
Figure 5. TEM image of carbon nanoparticles synthe
+ 15 wt% KOH.
he Ontario ministry of agri-
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