Effect of Fe/Fe2O3 loading on the catalytic activity of sulfonated single-walled carbon nanohorns for the esterification of palmitic acid

Article abstract of DOI:10.1039/c4gc00262h

The effect of dispersion of Fe/Fe₂O₃ nanoparticles in sulfonated single-walled carbon nanohorns (SO₃H/SWCNHs) on their catalytic activity for the esterification of palmitic acid was investigated. A gas-injected arc-in-water (GI-AIW) method was employed to initially synthesize SWCNHs dispersed with iron nanoparticles (Fe-SWCNHs). The Fe-loading amount in the Fe-SWCNHs was varied by changing the number of Fe wires inserted in an anode. The results showed that Fe-loading amount proportionally increased from 6 to 13 wt% with an increase in the number of Fe wires. The surfaces of the Fe-SWCNHs were functionalized with acid functional groups by two sequential steps: impregnation of sulphuric acid and calcination in air. From the characterization results, their acid site concentrations were estimated to be 5.6-8.5 mmol g^-1, suggesting that the catalyst was a solid superacid catalyst. XRD analyses indicated that most of the Fe was transformed to α-Fe₂O₃. The catalytic activity of the SO₃H/Fe-SWCNHs for the esterification of palmitic acid was evaluated to investigate the influence of the Fe-loading on their catalytic activity. The results showed that the yield of methyl palmitate was significantly enhanced by an increase in the Fe-loading amount. It was discovered that the catalytic activity and the magnetic susceptibility of SO₃H/Fe-SWCNHs can be preserved during repeated use, if the Fe-loading amount is large enough.

Full text of DOI:10.1039/c4gc00262h

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Effect of Fe/Fe O loading on the catalytic activity
2
3
of sulfonated single-walled carbon nanohorns for
Cite this: DOI: 10.1039/c4gc00262h
the esterification of palmitic acid
a,b
a
c
Chantamanee Poonjarernsilp,* Noriaki Sano, Nut Sawangpanich,
c
a
Tawatchai Charinpanitkul and Hajime Tamon
2 3 3
The effect of dispersion of Fe/Fe O nanoparticles in sulfonated single-walled carbon nanohorns (SO H/
SWCNHs) on their catalytic activity for the esterification of palmitic acid was investigated. A gas-injected
arc-in-water (GI-AIW) method was employed to initially synthesize SWCNHs dispersed with iron nanopar-
ticles (Fe-SWCNHs). The Fe-loading amount in the Fe-SWCNHs was varied by changing the number of Fe
wires inserted in an anode. The results showed that Fe-loading amount proportionally increased from
6
to 13 wt% with an increase in the number of Fe wires. The surfaces of the Fe-SWCNHs were functiona-
lized with acid functional groups by two sequential steps: impregnation of sulphuric acid and calcination
in air. From the characterization results, their acid site concentrations were estimated to be 5.6–8.5 mmol
g− , suggesting that the catalyst was a solid superacid catalyst. XRD analyses indicated that most of the Fe
1
2 3 3
was transformed to α-Fe O . The catalytic activity of the SO H/Fe-SWCNHs for the esterification of pal-
Received 15th February 2014,
Accepted 4th August 2014
mitic acid was evaluated to investigate the influence of the Fe-loading on their catalytic activity. The
results showed that the yield of methyl palmitate was significantly enhanced by an increase in the Fe-
DOI: 10.1039/c4gc00262h
loading amount. It was discovered that the catalytic activity and the magnetic susceptibility of SO
3
H/Fe-
www.rsc.org/greenchem
SWCNHs can be preserved during repeated use, if the Fe-loading amount is large enough.
to a food crisis. Therefore, the majority of researchers have
focused on non-edible oil or waste cooking oil as a low cost
The increasing demand for fossil fuels has given rise to feedstock for biodiesel production.
Introduction
9
–11
environmental concerns, such as the emission of large
When using non-edible oil for biodiesel production, the
amounts of CO , causing global warming. To help with this amount of FFA in the oil is a concern, because these kinds of
2
issue, several alternatives to fossil fuels have been proposed, oils have high concentrations of FFA; for example, Jatropha
1
–3
12
such as solar energy, biomass, and biodiesel. Biodiesel is a seed oil consists of 48% oleic acid and 34% linoleic acid.
renewable and environmental friendly alternative to pet- Such high amounts of FFA can cause side reactions with alka-
4
roleum-based diesel. It is a mono alkyl ester of fatty acid pro- line catalysts – so called saponification. Conversely, acid cata-
5
duced from vegetable oil or animal fat, which contains lysts possess the advantage of insensitivity to FFA for
1
3,14
saturated hydrocarbons (triglycerides) and free fatty acid saponification.
They can be used with low cost oils that
contain high amounts of FFA. However, the conventionally-
The transesterification of triglycerides and the esterifica- used liquid acid catalysts, such as sulphuric acid, are corrosive,
6
(FFA).
7
,8
tion of FFA are the best schemes for producing high quality so special care is necessary to preserve the reaction systems. In
biodiesel. All fatty acid sources such as animal fats or plant addition, such liquid catalysts have drawbacks with regards to
lipids can be used in biodiesel production. However, the pro- their recovery.
duction of biodiesel from human nutrition sources could lead
Solid acid catalysts, including sulfonated carbon-based
1
4–16
materials,
are considered to be potential candidates to
replace liquid acid catalysts due to their characteristics, such
as high corrosion resistance and the ability to be recovered
and reused. Ideally, a solid acid catalyst should have the
a
Department of Chemical Engineering, Graduate School of Engineering, Kyoto
University, Nishikyo-ku, Kyoto, 615-8510, Japan
17
b
Department of Chemical Engineering, Faculty of Engineering, Rajamangala
characteristics of interconnected pores with a large surface
University of Technology Krungthep, Sathorn, Bangkok, 10120, Thailand.
E-mail: chantamanee.w@rmutk.ac.th; Fax: +66 2287 9734; Tel: +66 2287 9734
1
8
area, a high concentration of strong acid sites, and a hydro-
c
19
Faculty of Engineering, Chulalongkorn University, Bangkok, 10330, Thailand
phobic surface to prevent deactivation by water.
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Various types of solid acid catalysts such as ion exchange
2
0
21
22
resins, sulphate oxide, mixed oxides and derivatives and
1
5,16,23
carbon-based acid solid catalysts
have been studied.
Recently, novel sulfamic-acid-functionalized iron (iron/iron-
2
3
oxide core shell) nanoparticles have been reported. The pres-
ence of the iron/iron-oxide core shell nanoparticles enhanced
the catalytic activity on ring opening of epoxidized soybean
1
7,24,25
oils and the recovery of the catalysts by magnetic force.
It has also been reported that iron-oxide nanoparticles can
adsorb FFA from vegetable oil, which can promote the adsorp-
2
4
tion of FFA on the catalyst surface. Thus, solid acid catalysts
hybridized with iron oxide nanoparticles should achieve a
high performance and act as ideal solid catalysts for the esteri-
fication of FFA. However, the effect of iron oxide on the cataly-
tic activity of solid acid catalysts has not yet been studied so
2
3,24,26
far.
In this work, we propose to use single-walled carbon nano-
horns (SWCNHs). SWCNHs are a type of carbon nanomaterial,
and have suitable properties to meet the requirements for an
ideal solid acid catalyst. SWCNHs have high resistance against
chemicals, suitable pore sizes, thermal stability and a hydro-
2
7
phobic surface in nature. Moreover, it is possible to syn-
thesize SWCNHs hybridized with metal nanoparticles by a
Fig. 1 TEM images of Fe-SWCNHs synthesized with various numbers of
2
8,29
one-step arc discharge technique.
Here, the synthesis con- Fe wires, and the mean diameters of Fe nanoparticles dispersed in the
ditions of SWCNHs dispersed with Fe nanoparticles (Fe- Fe-SWCNHs. (a) 1 wire, (b) 2 wires, (c) 3 wires, (d) 4 wires. (e) TEM image
3
of SO H/Fe-SWCNHs prepared using Fe-SWCNHs synthesized with 4
SWCNHs) were investigated first, focusing on the amount of
Fe-loading on the products to provide a suitable structure.
Next, the catalysts were prepared using the Fe-SWCNHs by an
impregnation method with a subsequent calcination step, and
Fe-wires. (f) Particle size distribution of Fe nanoparticles dispersed in
the Fe-SWCNHs.
their catalytic activity was evaluated by testing with the esterifi- the most of the metallic nanoparticles dispersed in SWCNHs
cation of palmitic acid. Furthermore, the effect of Fe-loading that are synthesized by this method should be migrated in
2
9
in catalyst preparation on the catalytic activity was studied.
SWCNHs. Such a structure may help to avoid the merging of
the metallic nanoparticles when they are calcined.
Although the particle sizes tend to be larger with an
increase in the Fe-wire number in the GI-AIW method, it can
be taken that this change is not significant, so that the high
dispersion of Fe nanoparticles with diameters of 1–11 nm can
be preserved in the present conditions (Fig. 1(f)). It should be
noted that the number density of the Fe nanoparticles in the
Results and discussion
Effect of Fe-wire number in the synthesis process on the
morphologies of Fe-SWCNHs: dispersion of Fe nanoparticles
and Fe-loading weight
As explained in the Experimental section, the Fe-loading Fe-SWCNHs seemed to be higher when the Fe-wire number
amount in Fe-SWCNHs can be varied by changing the number was increased. This tendency is understandable, because the
of Fe wires inserted in a hollow graphite anode used in the increase in Fe wire number should lead to an increase in the
3
0
gas-injected arc-in-water (GI-AIW) method. The transmission concentration of Fe vapor in the arc plasma zone where Fe
electron microscope (TEM) images of Fe-SWCNHs synthesized nanoparticles are formed. It should be noted that the particle
with various numbers of wires are shown in Fig. 1(a–d). These sizes of the Fe nanoparticles did not significantly change,
figures suggest that the Fe nanoparticles, indicated by the dark because the growth rate and the resident time of the Fe vapor
spots, can form simultaneously with the SWCNHs by this in the reaction field in the quenching zone inside the cathode
3
1
method, and that they are highly dispersed in the SWCNHs. is limited. Instead, the number of Fe particles could increase
Analysis by X-ray diffraction (XRD) confirmed that they are with a higher Fe vapor concentration there. It is important to
non-oxidized Fe, as further explained later.
The arc plasma at the anode tip can increase the plasma nanoparticles can be preserved during the calcination step, to
recognize from looking at Fig. 1(e) that the dispersion of Fe
3
1
temperature to approximately 5000 K, and the vapors of prepare the sulfonated acid catalysts (SO
3
H/Fe-SWCNHs).
carbon and Fe emitted from the anode can be quenched
We measured the Fe-loading amount in SWCNHs, and the
during their transfer to cold water. SWCNHs and Fe nanoparti- results are plotted in Fig. 2 as a function of the number of Fe wires
cles are then simultaneously formed from these vapors by a used in the synthesis process. It can be seen that the Fe-loading
self-assembly mechanism. A previous article as reported that amount increases proportionally to the number of Fe wires.
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symbolizes Fe
2
O
3
can apparently be seen in the XRD pattern of
SO H/Fe-SWCNHs. Compared with this peak, the other peaks
3
2 3
of α-Fe O at 35°, 40°, 54° could not be easily distinguished
from the baseline noise due to the small size of the crystals. In
addition to the peak of Fe O , one must pay attention to the
2
3
peak at 45°, representing ferromagnetic Fe. Although the peak
at 45° is less prominent here than in Fe-SWCNHs, we consider
that this XRD suggests that ferromagnetic Fe still remains in
the SO H/Fe-SWCNHs. It should be noted that SO H/Fe-
3 3
SWCNHs can be captured by a permanent magnet, as observed
by using a neodymium magnet of surface magnetic flux
150 mT. The XRD results suggesting the inclusion of ferro-
magnetic Fe are consistent with this magnetic experiment.
The peak around 26° should come from the carbonaceous
Fig. 2 Effect of the number of Fe wires inserted in the anode hole in
the GI-AIW method, on the Fe-loading amount, in percent, in the Fe-
SWCMHs.
parts of the SO
a lower angle, and the appearance of new peaks at 24° and
0°, may be caused by doping of S in structures consisting of
C, Fe and O. Detailed analysis of the structures indicated by
3
H/Fe-SWCNHs. The slight shift of this peak to
3
Transformation of Fe-SWCNHs to SO H/Fe-SWCNHs
3
The solid acid catalysts were prepared from the Fe-SWCNHs by these peaks is not in the scope of this work, but it should be
an impregnation method as detailed in the Experimental done in future.
section. Since the acid sites on these catalysts are considered
Here, it should be noted that the most of the metallic nano-
3
2,33
to be –SO
3
H,
the catalysts prepared here are referred to as particles dispersed in the SWCNHs produced by GI-AIW
2
9
SO H/Fe-SWCNHs.
method are migrated to their carbonaceous part. From this
3
The crystal structure of the Fe nanoparticles dispersed in structural feature, it can be considered that the carbonaceous
the Fe-SWCNHs was characterized by XRD. The diffraction pat- part of the SO H/Fe-SWCNHs should be partially oxidized to
3
terns of the Fe-SWCNHs are shown in Fig. 3, in comparison become porous, so that O2 can diffuse through the carbon-
with pure Fe powders and α-Fe O . The position of the diffrac- aceous part to reach the Fe nanoparticles in Fe-SWCNHs.
2 3
tion peak of Fe-SWCNHs synthesized with four Fe wires According to this pore opening effect, Fe nanoparticles
matches well with the diffraction peak of standard Fe at 45°. migrated in the SWCNHs could be transformed to α-Fe O .
2
3
The other Fe-SWCNHs synthesized with one, two and three Fe
It was found that this morphological change with the
wires also exhibit similar XRD patterns, as shown in Fig. 3. opening of pores on the carbonaceous part was affected by the
These results confirm that the dark particles seen in the TEM Fe-loading amount. Table 1 shows the BET specific surface
images (Fig. 1) are Fe nanoparticles.
The XRD pattern of SO H/Fe-SWCNHs showed several Fe-wire numbers. Because the loading weight of the high-
3
areas of SO H/Fe-SWCNHs, SS,catal, synthesized with various
3
peaks, although the intensities of the peaks should be fairly density Fe became larger when the Fe-wire number was larger,
low as their crystal sizes are so small. The peak at 33° that the specific surface area of the SO H/Fe-SWCNHs became
3
lower accordingly. Here, it should be emphasized that the
specific surface area of the carbonaceous parts in the SO H/Fe-
3
SWCNHs, Ss,c, also decreases with the Fe-loading amount. The
value of Ss,c was calculated by eqn (1) with an assumption that
Table 1 Acid site concentrations, BET surface areas, and average micro
pore diameters of SO
numbers of Fe wires
3
H/Fe-SWCNHs synthesized by inserting various
Acid site
a
b
c
c
concentration
S
s,catal
S
s,c
d
m
+
−1
(m g−1) (m g−1) (nm)
2
2
Catalyst
(H -mmol g
)
3
SO H-SWCNH
0.15
—
749
0.41
(
without Fe)
SO
SO
SO
SO
3
H/Fe-CNH (1 wire) 5.6
3
H/Fe-CNH (2 wire) 8.5
3
H/Fe-CNH (3 wire) 7.5
3
H/Fe-CNH (4 wire) 8.1
68
49
18
9
79
60
23
11
0.68
0.70
0.75
0.79
a
b
Calculated by titration experiments. Determined based on the unit
c
Fig. 3 XRD patterns of Fe powder, Fe-SWCNH, SO
thesized with 4 wires of Fe and α-Fe powder.
3
H/Fe-SWCNH syn- mass of the SO
3
H/Fe-CNH. Determined based on the unit mass of the
2
O
3
carbon part.
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2 3
the exposed surface area of Fe O would be negligible in com-
parison with the carbonaceous part.
Ss;c ¼ ScatalWcatal=ðWcatal ꢀ WFe O Þ
ð1Þ
2
3
where Wcatal, and WFe O are the masses of SO
3
H/Fe-SWCNHs
2
3
and Fe
measured by the removal of the carbonaceous part via
thorough oxidation. SO H-SWCNH without Fe was also pre-
2 3 3
O , respectively, in SO H/Fe-SWCNHs. WFe O was
2 3
3
pared for comparison. Note that the acid functional group
cannot be impregnated well on the SWCNH surface without Fe
dispersion, as shown in Table 1.
It is reported that pores can be opened on pure-carbon
Fig. 4 Methyl palmitate yields obtained by the esterification of palmitic
3
4
SWCNHs by calcination in air. In the case of the SO H/Fe- acid with SO H/Fe-SWCNHs synthesized with various Fe-wire numbers.
3
3
SWCNHs, Fe could catalyze the oxidation of the carbonaceous (The reaction conditions: methanol : palmitic acid mass ratio of 33 : 1,
amount of acid sites at 1.1 mmol reaction temperature of 64 °C, total
reaction time of 60 min.)
parts, so the inclusion of a larger Fe-loading amount could
lead to a widening of the pores. The average micropore dia-
meter as determined by the t-plot method, d
m
, is also shown
in Table 1. It can be seen that dm increases with the Fe-loading adsorbed on the Fe O surface at the SO H/Fe-SWCNHs. Then,
2
3
3
amount. The relatively wide pores generated with large Fe- the adsorbed palmitic acid could diffuse to the acid sites
loading amounts could contribute to the stable re-usability of (SO H-) anchored on the carbonaceous part. It can be con-
3
the SO H/Fe-SWCNHs, as explained in the subsequent section. sidered that the enrichment of palmitic acid on Fe O would
3
2 3
As shown in Table 1, the resulting SO H/Fe-SWCNHs can be so great that the amount of access of the palmitic acid to
3
possess high acid site concentrations of 5.6–8.5 mmol g−
1
,
the acid sites via the Fe O surface could become significantly
2 3
which are comparable with those of the sulfonated solid super- large. We consider that such access via the Fe O could be
2
3
3
5
acid catalysts previously reported. Therefore, the SO H/Fe- comparable to direct access.
3
SWCNHs prepared in this work can be counted as superacid
The yield of methyl palmitate at 60 min reaction time is
catalysts. It should be noted that no acid sites were detected plotted against the Fe-loading weight percentage in Fig. 5 to
on Fe-SWCNHs without the impregnation process. The average clarify the relationship between the Fe-loading amount and
acid site concentration obtained from all SO H-Fe/SWCNHs the catalytic activity of the SO H/Fe-SWCNHs. It can be clearly
3
3
−1
specimens is 7.4 mmol g . The relative error in obtaining the observed that the yield at 60 min is nearly proportional to the
acid site concentration, estimated by repeated analysis, was at Fe-loading weight percentage, in which the yield reaches 92%
a level of 20%. According to the possible error, the acid site at the highest Fe-loading of 13 wt%.
density measured in the present work is considered to be inde-
pendent of the wire number used in the synthesis process.
The catalytic performance of a conventional sulphuric acid
catalyst for the esterification of palmitic acid under the same
conditions can be referred to in our previous work. The
methyl palmitate yield with the conventional catalyst was 20%
in 60 min. In comparison, the yield obtained by the SO H/Fe-
3
SWCNHs catalyst synthesized in the present work was almost
36
Effect of Fe-loading amount on catalytic activities for
esterification
In this work, the effect of Fe-loading amount on the catalytic
activity for the esterification of palmitic acid was investigated
using SO H/Fe-SWCNHs synthesized with various Fe-wire
3
numbers. In order to ensure fair comparison, the amounts of
SO H/Fe-SWCNHs which were supplied into the reactor were
3
adjusted, so that the amount of acid sites used in each batch
was supposedly the same. The change in the methyl palmitate
3
yield with SO H/Fe-SWCNHs of different Fe-loading amounts
is shown in Fig. 4 as a function of reaction time. It can be
observed that the yield increases with an increase in the Fe-
wire numbers used for the synthesis process. This means that
an increase in Fe-loading amount can enhance the catalytic
activities in the esterification of palmitic acid.
The reason for this enhancement effect of Fe-loading can
be discussed by considering the role of the Fe/Fe O nanopar-
2
3
ticles dispersed in SO
3 2 3
H/Fe-SWCNHs. It is known that Fe O
has a high affinity with carboxylic acids, including fatty acids
like palmitic acid. Therefore, palmitic acid can be first yield in 60 min of reaction time.
Fig. 5 Effect of Fe-loading amount on the maximum methyl palmitate
2
4
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five times as high as the yield obtained with the conventional
one.
Reusability of catalysts in esterification
3
The stability of the catalytic activity of the SO H/Fe-SWCNHs
during reuse was investigated. The details of this procedure
are explained in the Experimental section. The results are
3
depicted in Fig. 6 and 7. As presented in Fig. 6, SO H/Fe-
SWCNHs synthesized with an Fe-wire number of 4 exhibits
excellent reusability for esterification. The yield slightly
decreased by 5% in three reaction cycles. In the cases of the
3
SO H/Fe-SWCNHs synthesized with Fe-wire numbers of 1, 2
and 3, the yields decreased by 10–30%. This catalyst de-
activation may be due to pore plugging by the adsorbed palmi-
tic acid molecules, which limits their access to the acid sites.
As mentioned previously, Fe-loading could widen the pores.
Thus, the plugging would not occur when the Fe-loading
amount as large as in the case of an Fe-wire number of 4.
Magnetic properties for re-use of SO H/Fe-SWCNHs
3
Fig. 7 AC magnetic susceptibility (χAC) of the as-prepared Fe-SWCNHs
The stability of the magnetic properties of the SO H/Fe-
SWCNHs during reuse for the esterification reaction was inves-
3
(4 wire) and SO
3
H/Fe-SWCNHs obtained from synthesis processes with
different Fe wire numbers, as function of the frequency of the AC mag-
tigated, as shown in Fig. 7. In this figure, the results obtained netic field (15 mG) applied on the specimen. Re[χAC] and Im[χAC] denote
from SO H/Fe-SWCNHs synthesized with various Fe-wire the real and imaginary parts of the complex number of χAC, respectively.
3
(
3
a) and (b) show the AC magnetic susceptibility of the SO H/Fe-SWCNHs
numbers are shown, and a monotonous trend in the magnetic
properties in accordance with the Fe-wire number is not seen.
before use for the esterification experiment, and (c) and (d) show the
magnetic susceptibility after use for the esterification experiment. Rxn in
(
Fig. 7(a,b) shows the AC magnetic susceptibility of the SO H/
3
c) and (d) denotes the number of repetitions for the esterification
Fe-SWCNHs before they were used for the esterification experi- experiment.
ment, and Fig. 7(c,d) shows the AC magnetic susceptibility
after reuse 1–3 times. The influence of the reuse of SO H/Fe-
3
susceptibility is not decreased remarkably by reuse of the
SO H/Fe-SWCNHs in catalyzing the esterification reaction. In
other words, the magnetic properties of Fe/Fe O dispersing in
SWCNHs on their magnetic properties can be evaluated by a
comparison between (a,b) and (c,d). It can be seen that the
influence of reuse on the AC magnetic susceptibility of the
3
2
3
the SO H/Fe-SWCNHs did not leach out during the esterifica-
3
SO H/Fe-SWCNHs is not significantly changed for any Fe-wire
3
tion reaction.
numbers in any frequency range, in which the AC magnetic
Conclusions
Fe-SWCNHs were used as starting materials for preparing sul-
fonated SWCNHs by an acid impregnation method. It was
found that the Fe-loading amount was proportional to the
number of Fe-wires in the anode used in the GI-AIW method,
and the Fe-loading amounts reached 6–13 wt% in the present
conditions.
3
It was observed that the SO H/Fe-SWCNHs possessed high
acid site concentrations of 5.6–8.5 mmol g− , and so can be
1
regarded as solid superacid catalysts. Most of the Fe nanoparti-
cles dispersed in the SO
3
H/Fe-SWCNHs were oxidized, causing
a transformation to α-Fe O during a calcination step. Never-
2
3
theless, traces of Fe remained, which allowed the magnetic
recovery of the SO H/Fe-SWCNHs using a permanent magnet.
3
The catalytic activity of the SO H/Fe-SWCNHs for the esteri-
3
fication of palmitic acid with methanol was investigated. The
results clearly show that high amounts of Fe loading provide
high yields of methyl palmitate because the presence of the
Fig. 6 Reusability of SO
numbers of Fe wires for the esterification of palmitic acid with methanol.
The reaction conditions were the same as described in Fig. 4.)
3
H/Fe-SWCNHs synthesized with various
(
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Fe/Fe
2
O
3
can enhance the adsorption of palmitic acid mole- to the electrodes by a welding machine (Shindaiwa, STW200A)
cules on the surface of the Fe O nanoparticles. The enriched at 80 A and 40 V. For continuous arc discharge, the anode was
2
3
palmitic acid molecules could then diffuse to the acid sites. In delivered along the axis of the cathode hole with a speed of
H/Fe-SWCNHs was tested, and 1 mm s− . The N
1
gas was injected into the arc plasma zone
addition, the reusability of SO
it was revealed that the deactivation of the catalytic activity of inside the cathode hole at 10 L min when the arc discharge
the SO H/Fe-SWCNHs is significantly reduced when the Fe- was generated for 1 min. Under these conditions, Fe-SWCNHs
3
2
−
1
3
loading amount is large enough. It is noteworthy that the mag- were synthesized and finally collected from the water surface.
netic susceptibility of the SO H/Fe-SWCNHs can be preserved
when they are used repeatedly.
3
Determination of Fe-loading amount (wt%) in Fe-SWCNHs
From a sustainability point of view, firstly, the catalyst pre- As-grown Fe-SWCNHs were oxidized by ambient air in an
pared here does not use any rare metals which are sometimes open-end quartz tube placed in an electric furnace at 900 °C
3
7
used to prepare catalysts for biofuel synthesis, although a for 30 min. All carbonaceous parts were oxidized, and only
high capacity for strong acid sites is realized. Moreover, our reddish brown powders remained there. XRD analysis was
catalyst can be recovered from the solution simply by using a used to confirm that the reddish brown powders were Fe O ,
2
3
permanent magnet. Only a little energy is required for this and no carbon was detected. The weight of the remaining
recovery system. One may worry that the synthesis of the Fe was measured, and then the Fe-loading amount (wt%)
2 3
O
SWCNHs by the arc discharge method is energy intensive and in the Fe-SWCNHs was calculated.
costly. In relation to this, it is reported that the cost of produc-
Synthesis of SO H/Fe-SWCNHs catalyst by an impregnation
method
3
ing SWCNHs can be reduced by an order of two, by optimiz-
ation of the reaction conditions when an electrode of
3
8
moderate purity is employed. In addition, new routes to syn- As-grown Fe-SWCNHs prepared by the GI-AIW method with
thesize SWCNHs at relatively low temperatures are being various Fe-wire numbers were immersed in concentrated sul-
explored. Thus, we expect that the energy-base yield for pre- phuric acid (95%) for 24 h. After acid impregnation, the Fe-
paring the catalyst proposed here may be significantly SWCNHs were washed by distilled water three times and then
3
9
improved by further optimization.
dried in an oven in ambient air at 110 °C for 12 h. Finally, the
Fe-SWCNHs were calcined in a furnace with an air supply of
0 L min− at 400 °C for 3 h. The resulting sulfonated
1
1
3
Fe-SWCNHs (SO H/Fe-SWCNHs) were obtained and kept in a
vacuum desiccator.
Experimental methods
Synthesis of Fe-SWCNHs by an arc discharge method
Morphological characterization of Fe-SWCNHs and SO H/Fe-
SWCNHs catalyst and size distribution of Fe nanoparticles
3
Single-walled carbon nanohorns dispersed with Fe nanoparti-
2
cles (Fe-SWCNHs) were synthesized by a N gas injected arc-in-
3
0
The morphologies of the as-grown Fe-SWCNHs and SO H/Fe-
3
water (GI-AIW) method. The set-up and dimensions of the
reaction system of this method are illustrated in Fig. 8. The
cathode and anode were high purity graphite rods (99.9995%).
Various numbers (from 1 to 4) of Fe wires (0.5 mm in dia-
meter) were inserted in the anode in a hole drilled along its
axis. Both electrodes were submerged in water at room temp-
erature. To generate arc discharge, a DC current was supplied
SWCNHs were analysed by a transmission electron microscope
(
TEM) (JEOL, JEM-1010). The size distributions of the Fe nano-
particles dispersed in Fe-SWCNHs were determined by
measuring the diameters of 500 particles of Fe nanoparticles
in TEM images.
X-ray diffraction analysis
X-ray diffraction patterns were recorded on an X-ray diffracto-
meter (Ringaku, Ultima IV 285 DX) with Cu Kα radiation from
2 3
40 kV and 20 mA. Pure Fe and Fe O powders were also ana-
lyzed to compare their peak positions with those of the
catalyst.
Magnetic properties analysis
3
Analysis of the magnetization in the Fe-SWCNHs and SO H-Fe-
SWCNHs was carried out by measuring the AC magnetic sus-
ceptibility at room temperature with a maximum magnetic flux
of 15 mG (Magqu Co., XacQuan-II).
3
Acid site concentration of the SO H-Fe-SWCNH catalysts
The acid site concentrations of the catalysts were determined
by acid base titration using standard NaCl solution as an ion-
exchange agent. 5.0 ml of 2.0 M NaCl ion-exchanged with
Fig. 8 Set-up of the apparatus for the synthesis of Fe-SWCNHs, and
dimensions of the cathode, anode and Fe wires.
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2
2
5 mg of the catalyst specimen under agitation (150 rpm) for
4 h was titrated with 0.01 M NaOH solution. Phenolphthalein
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Catalytic activity analysis by esterification of palmitic acid
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The catalytic activity of the SO H/Fe-SWCNHs were evaluated
by the liquid phase esterification of palmitic acid with metha-
nol, as follows. A 100 ml three-necked flask equipped with a
water condenser was used as a reactor. This flask was sub-
merged in an oil bath of which the temperature was controlled
by a stirring heater. The solution in the flask was also stirred
by a magnetic bar. Powdery palmitic acid (1.28 g) was placed
into this reactor, until it melted. When the palmitic acid
became a clear solution at 64 °C, 5.27 g of warm methanol
(
99.8%) was added into the reactor. After this, the catalysts
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were added into the reactor with the same amounts of acid
sites at 1.1 mmol. The reaction temperature was controlled at
6
6
4 °C and the reactants were well mixed under an agitation of
00 rpm for 60 min. The methyl palmitate produced from the 10 P. Sinthupinyo, H. Habaki and R. Egashira, J. Chem. Eng.
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mole of methyl palmitate formed
theoretically expected mole of methyl palmitate
1
1
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Reusability of SO
3
H-Fe-SWCNH catalyst
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The stability of the catalytic activity of SO H/Fe-SWCNHs
during reuse for the esterification of palmitic acid was investi-
gated. To reuse the SO H/Fe-SWCNHs remaining in the reac-
3
tion solution, the SO H/Fe-SWCNHs were separated from the
solution using a permanent magnet with a surface magnetic
flux of 150 mT. The catalysts were washed by methanol three
times and dried in air. The catalysts were then used for a new
batch of esterification to start the next reaction cycle. The re-
usability was evaluated by the determination of the methyl pal-
mitate yield at 60 min. In addition, the AC magnetic
susceptibility of the reused catalysts was measured after use
for every cycle.
3
1
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