Effect of silica content on production of p-Xylene from dimethylfuran/ethylene over mesoporous SiO2-Al2O3 catalysts

Article abstract of DOI:10.1166/jnn.2017.13340

Mesoporous SiO₂-Al₂O₃ (SA) catalysts with different SiO₂ contents were prepared, for the selective production of p-xylene from dimethylfuran/ethylene through the combination of cycloaddition and dehydrative aromatization reactions, by a co-precipitation method. For comparison, commercial SiO₂-Al₂O₃ and ZSM-5 zeolites (Si/Al₂ = 30, Si/Al₂ = 80) were also employed as catalysts in the same reaction. The pore size of the catalysts played an important role in determining the catalytic performance in the production of p-xylene. Among the catalysts tested, the order of the yield and production rate of p-xylene was as follows: mesoporous SA > commercial SiO₂-Al₂O₃ > commercial ZSM-5. In the mesoporous SA catalysts in particular, p-xylene yields showed a volcano-shaped trend with respect to the catalyst's SiO₂ content. The SA-60 catalyst, with SiO₂ = 52.3, showed the highest yield (75%) and production rate (57.7 mmol/g-cat · h) because of a catalyst structure with moderate pore size, which prevented side reactions.

Full text of DOI:10.1166/jnn.2017.13340

Article
Journal of
Nanoscience and Nanotechnology
Vol. 17, 2695–2699, 2017
Copyright © 2017 American Scientific Publishers
All rights reserved
Printed in the United States of America
www.aspbs.com/jnn
Effect of Silica Content on Production of p-Xylene from
Dimethylfuran/Ethylene Over Mesoporous
SiO –Al O Catalysts
2
2
3
1ꢀ2
1ꢀ2
1
1
Jin Sil Lee , Sang Yun Kim , Tae-Wan Kim , Soon-Young Jeong ,
1ꢀ∗
2
Chul-Ung Kim , and Kwan-Young Lee
1Research Center for Carbon Resources Conversion, Korea Research Institute of Chemical Technology,
P.O. Box 107, Sinseongno 19, Yusong, Daejeon 305-600, Republic of Korea
2
Department of Chemical and Biological Engineering, Korea University, 5-ga, Anam-Dong, Sunbukku,
Seoul, 136-713, Republic of Korea
Mesoporous SiO –Al O (SA) catalysts with different SiO contents were prepared, for the selec-
2
2
3
2
tive production of p-xylene from dimethylfuran/ethylene through the combination of cycloaddition
and dehydrative aromatization reactions, by a co-precipitation method. For comparison, commercial
SiO –Al O and ZSM-5 zeolites (Si/Al = 30, Si/Al = 80) were also employed as catalysts in the
2
2
3
2
2
same reaction. The pore size of the catalysts played an important role in determining the catalytic
performance in the production of p-xylene. Among the catalysts tested, the order of the yield and
IP: 79.133.106.107 On: Fri, 11 Jan 2019 00:34:51
production rate of p-xylene was as follows: mesoporous SA > commercial SiO –Al O > commercial
Copyright: American Scientific Publishers
2
2
3
ZSM-5. In the mesoporous SA catalysts in particular, p-xylene yields showed a volcano-shaped
Delivered by Ingenta
trend with respect to the catalyst’s SiO content. The SA-60 catalyst, with SiO = 52ꢀ3, showed the
2
2
highest yield (75%) and production rate (57.7 mmol/g-cat · h) because of a catalyst structure with
moderate pore size, which prevented side reactions.
Keywords: p-Xylene, SiO –Al O Catalyst, Cycloaddition, Dimethylfuran, Ethylene.
2
2
3
1
. INTRODUCTION
to p-xylene (Scheme 1 of Ref. [8]).8 The production
of p-xylene through this method is favored by the use
of an solid acid as a catalyst. Until now, various cata-
The development of processes that convert biomass into
chemicals is essential for reducing dependence on non-
renewable resources such as fossil fuel. There is grow-
ing need to develop new routes to chemical intermediates
that utilize alternative feedstocks. p-Xylene (PX) is an
important large-volume commodity chemical in the petro-
chemical industry. In this regard, many researchers have
been searching for alternative routes to the production of
p-xylene from renewable feedstocks. One of the methods
is based on the combination of cycloaddition and dehy-
drative reactions using biomass-derived 2,5-dimethylfuran
1
ꢀ2
lysts such as WOx–ZrO , niobic acid, Y zeolites, H-beta
2
zeolites, and silica-alumina aerogels have been investi-
gated. Interestingly, an amorphous silica-alumina aerogel
has demonstrated activity for p-xylene production compa-
rable to that of H-beta zeolites, which have been reported
as the most active catalysts for this reaction, under identi-
1
–6
6
cal reaction conditions. From this catalysts screening, the
enhanced activity of materials such as amorphous SiO2–
Al O or large-pore zeolites can be attributed to their struc-
2
3
tural properties, which can reduce catalytic deactivation by
means of coke formation and pore resistance.
(
DMF) and ethylene as starting materials and the poten-
6
–11
tial route for this reaction was suggested.
The reac-
In this work, a series of mesoporous SiO –Al O (SA)
2
2
3
tion occurs by symmetry-allowed ꢁ4+2ꢂ Diels-Alder cyclo
addition of ethylene and subsequent aromatization by
acid-catalyzed dehydration in multiple elementary steps
catalysts with different SiO contents were prepared by a
2
co-precipitation method. These materials can serve as effi-
cient acid solid catalysts with mesopore structures. Thus,
mesoporous SA catalysts prepared by this method have
high surface areas and large pore volumes. Moreover, the
∗
Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2017, Vol. 17, No. 4
1533-4880/2017/17/2695/005
doi:10.1166/jnn.2017.13340
2695

Effect of Silica Content on Production of p-Xylene
Lee et al.
composition of these materials can be changed to control
their acidity.
The final Si/Al ratios of the catalysts used were deter-
mined by inductively coupled plasma-atomic emission
spectrometry (ICP-AES, Spectro Ciros Vision).
In particular, the effect of SiO content on the physic-
2
ochemical properties of the SA catalysts and their cat-
alytic performance for the selective production of p-xylene
from dimethylfuran/ethylene through the combination of
cycloaddition and dehydrative aromatization reactions
were investigated, and a comparison with the catalytic
2.3. Test of Catalyst Activity
The reactions to produce p-xylene from 2,5-dimethylfuran
and ethylene were conducted in a 250 mL autoclave
6–8
equipped with an impeller. The reactor was loaded with
ability of commercial SiO –Al O and ZSM-5 zeolites
2
2
3
0.2 g of catalyst for 12.5 mL of 2,5-dimethylfuran and
37.5 mL of n-heptane, and constantly maintained at a tem-
perature of 523 K using a PID controller. A leak test of
the reaction vessel using nitrogen was firstly performed.
Then, the reaction vessel was purged three times with
ethylene gas (C H ꢄ and stirred at 500 rpm while heating
(
Si/Al = 30, Si/Al = 80) for this reaction was also
2
2
carried out.
2
. EXPERIMENTAL DETAILS
2
4
2
.1. Catalyst Preparation
to the reaction conditions. Once the reaction temperature
was achieved, the vessel was pressurized to 50 bar with
ethylene gas, which was continuously added during the
reaction using a C H MFC (20 mL/min). The liquid sam-
A series of SA catalysts with different SiO contents, such
2
as SA-20 (SiO = 83ꢃ6 wt%), SA-40 (SiO = 67ꢃ5 wt%),
2
2
SA-60 (SiO = 52ꢃ3 wt%), SA-70 (SiO = 40ꢃ9 wt%),
2
2
2
4
and SA-80 (SiO = 28ꢃ6 wt%), were prepared by a
2
ple was collected immediately from a sampling port. The
products were identified and quantified, comparing their
retention times with those of pure standards, using a gas
chromatograph (GC, Acme-6100, Young Lin Instrument
Co.) equipped with a flame ionization detector (FID) and
a capillary HP-INNOWAX column (0.32 mm id×0.5 ꢇm
thickness×60 m length). The conversion of DMF, as well
as the selectivities and yields of the products were calcu-
1
2ꢀ13
co-precipitation method.
Alumina precursor (12 g;
AlCl · 6H O, Fluka) was dissolved in 600 mL of distil-
3
2
8ꢀ9
lated water, and the pH of the solution was adjusted to 8
by adding ammonia solution. After maintaining the alu-
mina precursor solution at 328 K for 1 h, an appropriate
amount of silica precursor (Si(OC H ꢄ , Sigma-Aldrich)
2
5 4
was slowly added. The amount of H O in the mother
2
liquor was adjusted such that the H O/Si(OC H ꢄ ratio
2
2
5
4
6–10
lated as described in previous papers.
was 18. The precipitates were dried at 333 K under vac-
IP: 79.133.106.107 On: Fri, 11 Jan 2019 00:34:51
uum for 4 h and then dried at 393 KC oo pv ye rr ing i hg ht :t .A Am l el rt ihc ea n Scientific Publishers
supports were calcined in air at 773 K for 3 h. FD o er li cv oe mr e-d by3I n. g Re En t Sa ULTS AND DISCUSSION
parison, commercial SiO –Al O (SIRAL 1, SIRAL 10 of
3.1. Characterization of Catalysts
2
2
3
SASOL product) and commercial ZSM-5 zeolites (CBV
024E (Si/Al = 30), CBV 8014 (Si/Al = 80) of Zeolyst
Figure 1 shows the powder XRD patterns (a) and NH3-
TPD profiles of SA catalysts (b), and SEM images of
ZSM-5 (c), SA-60 catalysts of the prepared SA catalysts.
The XRD data prove that all of the SA catalysts are amor-
phous with very weak and broad XRD peaks indicating
domains of pure silica and alumina. Each catalyst shows
3
2
2
product) were purchased.
2
.2. Characterization of Catalysts
To confirm the structure of the prepared SA catalysts,
powder X-ray diffraction (XRD) patterns were recorded
on a Rigaku Multiplex instrument operating at 30 kV
and 40 mA (1.6 kW) using Cu-Kꢅ radiation (ꢆ =
ꢀ
a broad maximum peak, centered at 2ꢈ = 22 , attributable
to silica. The intensity of this peak decreased with the
decrease in Si content, and the intensities were in the fol-
lowing order: SA-20 > SA-40 > SA-60 > SA-70 > SA-80.
Information about the physical properties of the cata-
lysts can be found in Table I and Figure 2, which show
the BET surface area (SBETꢄ and pore size distribution, as
well as the nitrogen adsorption isotherms of the SA cata-
lysts, the commercial SiO –Al O (SIRAL 1, SIRAL 10),
0
ꢃ154 nm). Scanning electron micrograph (SEM) images
were obtained with a Hitachi S-4800 microscope oper-
ating at 2 kV. Nitrogen adsorption–desorption isotherms
were obtained at 77 K using a volumetric Micromeritics
Tristar II instrument. To remove water and organic com-
ponents on zeolites, samples were degassed at 573 K for
2
2
3
3
h before measurement. The specific surface area was cal-
and the commercial ZSM-5 zeolites (Si/Al = 30, Si/Al =
2
2
culated using the Brunauer-Emmett-Teller (BET) equation
80)) at 77 K. The Si/Al ratios of the catalysts used were
2
with adsorption points in the range of P/P = 0ꢃ05–0.2.
determined using ICP-AES based on elemental analysis.
As shown in Table I, we found that a decrease in the
SiO content of the SA and commercial SiO –Al O cat-
0
The surface acidity of the catalysts was measured by
loading 50 mg of the catalyst in a glass flow-through cell,
and carrying out the temperature-programmed desorption
2
2
2
3
alysts led to a decrease in their BET surface areas and an
increase in their average pore size. The same trend was
also obtained in the ZSM-5 catalyst when the SiO /Al O
of ammonia (NH -TPD) using a BEL-CAT TPD analyzer
3
with a TCD detector. Before NH adsorption, samples
3
2
2
3
were held at 773 K under flowing helium (50 mL/min) for
mol ratio was increased. Interestingly, the average pore
sizes of the SA catalysts were found to be larger than
1
h to remove adsorbed moisture.
2696
J. Nanosci. Nanotechnol. 17, 2695–2699, 2017

Lee et al.
Effect of Silica Content on Production of p-Xylene
Figure 1. Powder XRD patterns (a) and NH -TPD profiles (b) of SA catalysts, and SEM images (c) of ZSM-5, SA-60 catalysts.
3
that of commercial SiO –Al O and commercial ZSM-5
contents (SA-60, 80), whereas well-developed mesopores
2
2
3
zeolites due to the presence of mesopore structures in the
former. For all SA catalysts, the BET surface areas were in
were found in catalysts with high Si contents (SA-20,
30, 40). On the other hand, commercial ZSM-5 zeolites
(Si/Al = 30, Si/Al = 80) exhibited well-developed uni-
2
the range of 178–379 m /g, whereas the pore volumes and
2
2
3
13
average pore sizes were in the range of 0.66–0.81 cm /g
form micropore structures.
and 15.6–8.9 nm, respectively.
The number of acid sites in each catalyst was quanti-
Figure 2(a) shows the typical nitrogen physisorption
isotherms of the SA catalysts, while Figures 2(b and c)
show the pore size distribution of all catalysts (Fig. 2(b):
SA catalysts and Fig. 2(c): commercial SiO –Al O and
fied from the NH -TPD profile on the basis of the TPD
peak area obtained after injection of a known amount
3
of NH . The surface acid strength peak temperature and
3
the number of acid sites are considered proportional to
the TPD peak area. On the basis of these relationships,
Table II and Figure 1(b) show the analysis performed on
2
2
3
ZSM-5 catalysts such as Siral 1, Siral 10, ZSM-5 (30),
and ZSM-5 (80)). As shown in Figure 2(a), the type of
physisorption isotherm for each SA catalyst depends on its
Si content. SA catalysts with high Si contents (SA-20, 40)
the NH -TPD data to obtain more information about sur-
3
face acidities. As shown in Figure 1(b), in the case of
IP: 79.133.106.107 On: Fri, 11 Jan 2019 00:34:51
showed typical type V isotherms and H -type hysteresis,
the SA catalysts, a broad TPD peak was always found
over a wide range of temperatures (273-1, 173 K). The
total acid sites in the SA catalysts (Table I) increased with
the increase in their Si content, and was in the follow-
3
Copyright: American Scientific Publishers
whereas SA catalysts with low Si contents (SA-60, 70, 80)
Delivered by Ingenta
6
showed typical type IV isotherms and H -type hysteresis.
2
Regarding pore size distributions in SA catalysts, very
broad distributions were obtained for catalysts with low Si
contents (SA-60, 80), whereas well-developed mesopores
were found in catalysts with high Si contents (SA-20,
ing order: SA-20 (1.321 mmol-NH /g-catalyst) > SA-40 >
3
SA-60 > SA-70 > SA-80 (0.502 mmol-NH /g-catalyst).
3
The commercial SiO –Al O (SIRAL 1, SIRAL 10) cata-
2
2
3
3
0, 40). On the other hand, commercial ZSM-5 zeolites
lysts exhibited a large amount of acidity (over 1.0 mmol-
(
Si/Al = 30, Si/Al = 80) exhibited well-developed uni-
NH /g-catalyst), comparable to that found in SA-40 and
SA-60 catalysts, whereas a small amount of acidity (under
2
2
3
1
3
form micropore structures.
Regarding pore size distributions in SA catalysts, very
broad distributions were obtained for catalysts with low Si
1.0 mmol-NH /g-catalyst) was measured in commercial
3
zeolites (Si/Al = 30, Si/Al = 80).
2
2
Table I. Textural properties and acidities of SA, commercial SiO –Al O , and commercial ZSM-5 catalysts.
2
2
3
N Sorption results
NH TPD results
2
3
Pore
Average
Weak acid
(mmol-NH3/
g-catalyst)
Strong acid
(mmol-NH3/
g-catalyst)
Total acid site
(mmol-NH3/
g-catalyst)
SiO2ꢉ
(wt%)
S
volume
pore size
BET
a
2
b
3
c
c
Samples
(m /g)
(cm /g)
(nm)
SIRAL 1
SIRAL 10
SA-20
SA-40
SA-60
SA-70
SA-80
HZSM-5
HZSM-5
99
90
280
378
178
216
272
321
379
308
426
0.51
0.63
0.66
0.75
0.83
0.77
0.81
0.07
0.14
7ꢃ2
6ꢃ4
31ꢃ8
15ꢃ6
12ꢃ2
9ꢃ6
8ꢃ6
5ꢃ3
5ꢃ6
0.566
0.512
0.768
0.608
0.556
0.313
0.307
0.457
0.238
0.475
0.670
0.558
0.550
0.470
0.304
0.195
0.417
0.235
1.031
1.182
1.326
1.158
1.026
0.617
0.502
0.874
0.473
83ꢃ6
67ꢃ5
52ꢃ3
40ꢃ9
28ꢃ6
Si/Al = 30
2
Si/Al = 80
2
Notes. aDetermined by ICP-AES measurement; bCalculated by BET(Brunauer-Emmett-Teller) equation; cBJH (Barret-Joyner-Hallender) desorption pore volume.
J. Nanosci. Nanotechnol. 17, 2695–2699, 2017
2697

Effect of Silica Content on Production of p-Xylene
Lee et al.
Figure 2. (a) Nitrogen adsorption isotherms and (b) pore size distributions corresponding to the adsorption branch of SA, (c) commercial SiO –Al O ,
2
2
3
and commercial ZSM-5 catalysts.
3
.2. Effect of Different Solid Acid Catalysts on
Catalytic Performance
DMF/ethylene cycloadduct, and the secondary addition of
DMF or ethylene.
6
–9
Table II, Figures 3(a and b) show the catalytic activities
of several solid acid catalysts. The production of p-xylene
from DMF and ethylene (50 bar) using heptane as solvent
was measured at 523 K for 24 h.
Among the catalysts tested, the yield and production
rate of p-xylene both decreased in the following order:
mesoporous SA > commercial SiO –Al O > commercial
ZSM-5 zeolites. Thus, a higher p-xylene yield (50–70%)
and p-xylene production rate (29.8–57.5 mmol/g-cat h)
were obtained when SA catalysts were used.
2
2
3
Several catalysts, including our prepared SA (SA-20,
4
0, 60, 70, 80), commercial SiO –Al O (SIRAL 1,
2 2 3
IP: 79.133.106.107 On: Fri, 1 1O nJ at nh e2 o0 t 1h 9e r0 h0 a: n3 d4 ,: 5c 1o mmercial ZSM-5 showed signifi-
SIRAL 10), and commercial ZSM-5 zeolites (Si/Al = 30,
2
Copyright: American Scientific Publishers
Si/Al = 80), were used
cantly low activity toward p-xylene production, indicating
that the presence of micropore size structures cannot
effectively catalyze cycloadduct dehydration. These results
imply that pore size plays an important role in determin-
ing the p-xylene yield and production rate, which can be
attributed to the ability of coke precursors to easily diffuse
2
Delivered by Ingenta
As shown in Table II, p-xylene was the main product
formed through Diels–Alder cycloaddition of ethylene and
subsequent aromatization by acid-catalyzed dehydration in
multiple elementary steps.
The formation of several side products, such as HDO
6
ꢀ13
(
formed from the hydrolysis of DMF) and other unknown
into the well-developed mesopores.
compounds, was also observed. As reported previously,
the unknown compounds were obtained as by-products
of the intramolecular aldol reaction of HBD, the alkyla-
tion of p-xylene with ethylene, the isomerization of the
Among the SA catalysts with different SiO contents, it
2
can be seen from Table II and Figure 3(b) that both DMF
conversion and p-xylene yield increase with the increase
in the Si content of the catalyst up to a certain point,
Table II. Catalytic activities such as the DMF conversion, PX yield, carbon selectivity and p-xylene production rate for the p-xylene production
from DMF with ethylene (50 bar) in heptane as a solvent at 523 K, 24 h with various catalysts.
Carbon selectivity (mol C%)
p-Xylene production
rate (mmol/g-cat h)
DMF conv.
(%)
PX yield
(%)
Unknown
products
Samples
PX
MX
HDO
SA-20
SA-40
SA-60
SA-70
29ꢃ8
47ꢃ4
57ꢃ7
56ꢃ2
46ꢃ7
1ꢃ4
10ꢃ9
6ꢃ4
8ꢃ5
85ꢃ5
95ꢃ0
96ꢃ8
96ꢃ6
93ꢃ4
32ꢃ4
52ꢃ0
39ꢃ4
36ꢃ1
56ꢃ6
72ꢃ4
75ꢃ0
72ꢃ6
69ꢃ3
8ꢃ5
30ꢃ0
24ꢃ1
11ꢃ0
66ꢃ1
76ꢃ2
79ꢃ1
75ꢃ1
74ꢃ2
26ꢃ2
57ꢃ8
61ꢃ0
30ꢃ5
1ꢃ5
2ꢃ4
1ꢃ0
1ꢃ4
0ꢃ7
0
0ꢃ3
0ꢃ3
0
12ꢃ6
11ꢃ0
6ꢃ4
7ꢃ7
10ꢃ7
4ꢃ4
16ꢃ8
5ꢃ2
2ꢃ6
19ꢃ8
10ꢃ4
13ꢃ5
15ꢃ8
14ꢃ4
69ꢃ4
25ꢃ1
33ꢃ5
66ꢃ9
SA-80
SIRAL 1
SIRAL 10
HZSM-5(30)
HZSM-5(280)
Notes. PX: p-xylene; MX: m-xylene; HDO: 2,5-hexanedione; unknown products: carbon loss.
2698
J. Nanosci. Nanotechnol. 17, 2695–2699, 2017

Lee et al.
Effect of Silica Content on Production of p-Xylene
Figure 3. Comparison of catalysts (SA, SIRALs, and ZSM-5s catalysts (a)), the effect of BET surface area (b) and the effect of catalysts reuse
SA-50, ZSM-5) (c) for the p-xylene production from DMF with ethylene (50 bar) in heptane as a solvent at 523 K, 24 h.
(
after which they decrease with a further increase in Si
content. The highest DMF conversion and p-xylene yield
were obtained for the SA-60 catalyst (96.8% and 75%,
respectively).
production rates for p-xylene than commercial SiO2–
Al O and ZSM-5 zeolites. This can be explained by
2
3
the easy diffusion of coke precursors into well-developed
mesopores, indicating that the pore size of catalysts is an
important factor in determining catalytic performance in
the production of p-xylene. Among the catalysts tested, the
SA-60 catalyst, with moderate mesopore size, showed the
highest catalytic performance (DMF conversion of 96.8%;
PX yield of 75.0%) and the highest p-xylene production
rate (57.7 mmol/g-cat ·h).
Although it has been reported in previous studies5–10 that
the p-xylene yields in the Diels–Alder reaction of DMF
and ethylene were affected by the different acid strengths
generated when using different Si/Al ratios of catalysts,
our results evidence that the pore size of the catalyst plays
a more significant role than catalyst acidity in enhancing
p-xylene production. This indicates that proper mesopore
sizes in SA catalysts can lead to higher catalytic activity
for the production of p-xylene from DMF and ethylene.
For catalyst reuse run, two different methods were
Acknowledgment: This work was supported by the
R&D Convergence Program of MSIP (Ministry of Sci-
ence, ICT and Future Planning) and ISTK (Korea Research
IP: 79.133.106.107 On: Fri ,C 1o 1u n Jc ai ln f2o 0r 1 I9n d0 u0 s: t3r i4a :l 5 1S cience and Technology) of the
applied such as solvent washing and calcination method
Copyright: American S Rc iee p nu t bi f li icc Po uf bK l ios rhe ea r (s Grant 13-6-KIER).
Fig. 3(c)). In the first method, the spent catalyst was
then recovered through centrifugation and washing with
(
Delivered by Ingenta
2
-propanol before drying in an oven. Without further calci-
References and Notes
nation, this catalyst was reused in the reaction for the same
reaction time as the previous run with the fresh catalyst.
In a calcination method, spent catalyst was washed, dried,
and calcined under flowing air 923 K for before they were
reused in the reaction. As shown in Figure 3(c), SA-60 cat-
alyst recovered essentially all the catalytic activity with the
same surface area. This results suggest that mesoporosity
of SA-60 offers higher resistance to deactivation by car-
bon deposition than the of microporous materials such as
zeolite, which thus requires less frequent regeneration in
a continuous process. After reaction, we confirmed porous
structure without any special change after reaction. Also,
there is no activity change.
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. CONCLUSION
1
A series of mesoporous SA catalysts, commercial
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2
2
3
ferent SiO2 contents were employed for the produc-
tion of p-xylene via two consecutive reactions: initial
Diels–Alder cycloaddition of ethylene and DMF followed
by subsequent dehydration. Our prepared SA catalysts
with moderate mesopore sizes showed higher yields and
(2012).
1
3. J. Lee, S. Hwang, J. G. Seo, S.-B. Lee, J. C. Jung, and I. K. Song,
Journal of Industrial and Engineering Chemistry 16, 790 (2010).
4. H.-S. Jang, K. Bae, M. Shin, S. M. Kim, C.-U. Kim, and Y.-W. Suh,
Fuel 134, 439 (2014).
1
Received: 24 December 2015. Accepted: 5 May 2016.
J. Nanosci. Nanotechnol. 17, 2695–2699, 2017
2699