Heteropolyacid catalysts for Diels-Alder cycloaddition of 2,5-dimethylfuran and ethylene to renewable p-xylene

Article abstract of DOI:10.1016/j.cattod.2016.12.032

The Diels-Alder cycloaddition of biomass-derived furans and subsequent dehydration are promising routes for the sustainable production of commodity chemicals such as p-xylene (PX). In this paper, we have investigated the catalytic performances of a range of phosphotungstic acid (HPW) and silicotungstic acid (HSiW) catalysts supported on various oxides, i.e., SiO₂, Al₂O₃, TiO₂ and ZrO₂ and their structure-activity correlation in the conversion of 2,5-dimethylfuran (DMF) and ethylene to PX. The characterization studies of the catalysts using XRD, BET, Raman and ³¹P MAS-NMR spectroscopy reveal that all of the supported heteropolyacid (HPA) catalysts (except HPW/ZrO₂) retain their Keggin structure on the surface of oxide supports. Results from ammonia- and n-propylamine-TPD studies show that all of the supported HPA catalysts possess well-defined Br?nsted acid sites with the total acidity decreasing in the following order: HPA/SiO₂?>?HPA/Al₂O₃?>?HPA/ZrO₂?>?HPA/TiO₂. The conversion of DMF and the initial rate of PX production generally increase with an increase in the total acidity, with HPA/SiO₂ being the most active catalyst. The turnover frequency of PX production for HPA/SiO₂ is also considerably greater than those for the HPAs supported on Al₂O₃, ZrO₂, and TiO₂, which suggests that the higher activity of HPA/SiO₂ is at least partly due to the enhanced strength of Br?nsted acid sites. Both the silica-supported HSiW and HPW catalysts demonstrate remarkably high PX selectivity (82–85%) at high DMF conversion (91–94%) at 250?°C after 6?h reaction. The effects of reaction conditions such as acid loading, reaction temperature, and reaction time have also been investigated with the most active silica-supported HSiW catalysts to optimize the PX yield.

Full text of DOI:10.1016/j.cattod.2016.12.032

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Heteropolyacid catalysts for Diels-Alder cycloaddition of
,5-dimethylfuran and ethylene to renewable p-xylene
2
a,1
a,b,1
, Young-Kwon Park , Dong Jin Suha,d,
c
Yanuar Philip Wijaya , Haryo Pandu Winoto
a,b
a,b,e
a,b,∗
Hyunjoo Lee , Jeong-Myeong Ha
Clean Energy Research Center, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea
Clean Energy and Chemical Engineering, Korea University of Science and Technology, Daejeon 34113, Republic of Korea
School of Environmental Engineering, University of Seoul, Seoul 02504, Republic of Korea
Department of Green Process and System Engineering, Korea University of Science and Technology, Daejeon 34113, Republic of Korea
Green School, Korea University, Seoul 02841, Republic of Korea
, Jungho Jae
a
b
c
d
e
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 19 August 2016
Received in revised form
The Diels-Alder cycloaddition of biomass-derived furans and subsequent dehydration are promising
routes for the sustainable production of commodity chemicals such as p-xylene (PX). In this paper, we
have investigated the catalytic performances of a range of phosphotungstic acid (HPW) and silicotungstic
acid (HSiW) catalysts supported on various oxides, i.e., SiO2, Al2O3, TiO2 and ZrO2 and their structure-
activity correlation in the conversion of 2,5-dimethylfuran (DMF) and ethylene to PX. The characterization
1
9 November 2016
Accepted 16 December 2016
Available online xxx
3
1
studies of the catalysts using XRD, BET, Raman and P MAS-NMR spectroscopy reveal that all of the sup-
ported heteropolyacid (HPA) catalysts (except HPW/ZrO2) retain their Keggin structure on the surface of
oxide supports. Results from ammonia- and n-propylamine-TPD studies show that all of the supported
HPA catalysts possess well-defined Brønsted acid sites with the total acidity decreasing in the following
order: HPA/SiO2 > HPA/Al2O3 > HPA/ZrO2 > HPA/TiO2. The conversion of DMF and the initial rate of PX
production generally increase with an increase in the total acidity, with HPA/SiO2 being the most active
catalyst. The turnover frequency of PX production for HPA/SiO2 is also considerably greater than those
for the HPAs supported on Al2O3, ZrO2, and TiO2, which suggests that the higher activity of HPA/SiO2 is
at least partly due to the enhanced strength of Brønsted acid sites. Both the silica-supported HSiW and
HPW catalysts demonstrate remarkably high PX selectivity (82–85%) at high DMF conversion (91–94%)
Keywords:
Biomass
Diels-Alder
2
,5-Dimethylfuran
p-Xylene
Heteropolyacids
◦
at 250 C after 6 h reaction. The effects of reaction conditions such as acid loading, reaction temperature,
and reaction time have also been investigated with the most active silica-supported HSiW catalysts to
optimize the PX yield.
©
2016 Elsevier B.V. All rights reserved.
1
. Introduction
polyethylene terephthalate (PET), a thermoplastic polymer with
applications in fibers, packaging, and electric devices manufac-
ture [4–6]. Fig. 1 presents a fully sustainable integrated process for
the production of PX from biomass (the major steps are marked
with the bold arrow lines). Glucose, as the major product from
the hydrolysis of cellulose, can be isomerized to fructose using
biological (i.e. enzymes) [7] or thermochemical catalysts such as
Sn-containing beta zeolite [8]. Fructose can then be dehydrated to
5-hydroxymethylfurfural (HMF), which is subsequently converted
to 2,5-dimethylfuran (DMF) via hydrogenolysis [9,10]. Meanwhile,
glucose can also be fermented to produce ethanol [11], which is
then dehydrated to ethylene [12,13]. The reaction between DMF
and ethylene can then occur via tandem Diels-Alder cycloaddition
The potential of utilizing biomass as a renewable resource to
reduce the dependency on petroleum-based resources has driven
research efforts towards the development of sustainable pro-
cesses for the production of aromatic chemicals, such as benzene,
toluene, and xylenes (BTX) [1–3]. The sustainability of BTX aro-
matics is critical because they are used widely in fuels and in the
chemicals industry as the precursors for polymers [3]. In partic-
ular, p-xylene (PX) is a precursor chemical for the synthesis of
∗ Corresponding author at: Clean Energy Research Center, Korea Institute of Sci-
ence and Technology, Seoul 02792, Republic of Korea.
(DA) and dehydration reactions to produce PX.
E-mail addresses: jjae@kist.re.kr, john.jjae@gmail.com (J. Jae).
Co-first authors.
1
http://dx.doi.org/10.1016/j.cattod.2016.12.032
0
920-5861/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: Y.P. Wijaya, et al., Heteropolyacid catalysts for Diels-Alder cycloaddition of 2,5-dimethylfuran and
ethylene to renewable p-xylene, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.12.032

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Fig. 1. A sustainable process for conversion of biomass to renewable p-xylene (PX), a precursor chemical for synthesis of polyethylene terephthalate (PET) and the reaction
network for PX production from DMF and ethylene via a DA-dehydration cascade.
In the recent years, intensive research has been done over a
range of acid catalysts [14–21] because a promising route for the
production of renewable PX from DMF was introduced over the H-Y
zeolite with good selectivity (∼75%). It was reported that the overall
rate of PX formation is controlled by the initial DA reaction step over
H-Y because Brønsted acids cannot catalyze the DA reaction [14].
A DFT study also revealed the rate-limiting step to be the DA reac-
tion when Brønsted acids are used and dehydration when Lewis
acids such as Na-Y are used [15]. In a different study, acidic oxides
acids and solid acid catalysts, such as zeolites and metal oxides
[22,24,25,28]. Keggin-type HPAs have a discrete ionic structure
n−
that consists of heteropolyanions with the formula [XW12O40
]
(X = P5+, Si , Al , Co , etc.) and countercations (H , H O , H5O2+,
4+
3+
2+
+
+
3
etc.), which stabilize the Keggin units [22,28–31]. This network
structure enables the protons to be very active and extremely
mobile, not only on the surface, but also within the interior of the
solid HPA crystallites [22,23,29]. In addition, the Keggin-structure
HPAs are more thermally stable with a simpler synthesis proce-
dure than the HPAs of other structures [24,27]. The supported HPA
has been reported to exhibit higher catalytic activity than silica-
alumina, oxides, and acidic zeolites for relatively low temperature
reactions [23,29,30], such as the synthesis of dimethyl ether (DME)
via dehydration of methanol over oxide-supported HPA [29,30].
Overall, HPAs are regarded as economically and environmentally
attractive catalysts that can be used widely in both homogeneous
and heterogeneous reactions [22,32].
(
such as WOx-ZrO and niobic acid) were reported to have superior
2
activity for the production of aromatics (∼77% PX selectivity) and
higher resistance to coke deposition than zeolite Y due to its strong
Brønsted acidity [16]. More recently, higher PX selectivity (90%)
at almost complete (∼99%) DMF conversion was achieved over H-
BEA zeolite (Si/Al = 12.5) in n-heptane for 24 h reaction [17]. On
the other hand, another research group reported that microporous
materials, such as H-BEA zeolite, are less resistant to coke depo-
sition than mesoporous materials [16,20]. Its microporosity is the
cause of the mass-transfer limitation to the catalytic active sites due
to the significant pore resistance, leading to catalyst deactivation
In this study, we have investigated the catalytic performances
of a range of phosphotungstic acid (HPW) and silicotungstic acid
(HSiW) catalysts supported on various oxides, i.e., SiO , Al O , TiO
2
2
3
2
[
20] and the requirement of frequent regeneration in a continuous
and ZrO2 and their structure-activity correlation in the conversion
of DMF and ethylene to PX. Their catalytic performances were also
compared with those of H-Beta zeolites (H-BEA, Si/Al = 12.5 and 19)
and silica-alumina aerogel (SAA, Si/Al = 1), which were previously
reported as highly active catalysts. The activity correlation of HPAs
supported on different oxides was explored based on the acid site
density and acid strength. The effects of reaction conditions, such
as acid loading, reaction temperature, and reaction time, were also
investigated with the most active silica-supported HSiW catalyst.
Importantly, this study demonstrated the superior catalytic perfor-
mance of silica-supported HSiW towards the presently most active
commercial catalyst for PX production, H-BEA (Si/Al = 12.5).
process [16]. As an alternative, a mesoporous beta zeolite with a
morphology similar to a nanosponge (NSP-BEA) was introduced as
a highly active and stable catalyst (∼80% PX selectivity, ∼99% DMF
conversion, at 24 h reaction with 50 bar ethylene pressure) [20].
The advantage of using a mesoporous catalyst was also reported, in
which approximately 70% PX selectivity at 90% DMF conversion was
achieved over silica-alumina aerogel (SAA) with well-defined large
mesopores of 20–30 nm at a shorter reaction time (6 h) [18]. All
previous reports revealed the good catalytic performances of solid
acid catalysts with strong Brønsted acid sites for PX production
from DMF via DA and dehydration reactions; however, most of the
reactions for producing high PX yield (>80%) were carried out for
a long time (>24 h). A long-time reaction could be tedious, imprac-
tical, and economically unattractive. Herein, we propose another
potential catalyst for achieving a comparably high PX yield and
remarkable PX selectivity within a shorter reaction time.
2. Experimental
2.1. Catalyst preparation
Heteropolyacids (HPAs) with a Keggin structure, such as
All of the supported HPA catalysts were prepared by the incipi-
phosphotungstic acid (H PW12O40
)
and silicotungstic acid
3
ent wetness method with an ethanol solution at room temperature,
as described elsewhere [29]. Bulk phosphotungstic acid (HPW,
Aldrich, >99.9%) and silicotungstic acid (HSiW, Aldrich, >99.9%)
(
H SiW12O40), have received a lot of interest as promising acid
4
catalysts for a range of chemical reactions owing to their purely
strong Brønsted acidity [22–27]. Solid HPAs possess stronger
acidity and a unique structure that differs from ordinary mineral
◦
were used as the reference materials and dried overnight at 100 C.
The incipient wetness volume of the ethanol solution was esti-
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mated based on the ratio of HPA to the oxide supports (10–20 wt%).
After impregnation, the samples were kept at room temperature
for 24 h to reach a uniform distribution of HPA clusters onto the
◦
oxide supports, and then dried in an oven at 100 C for 24 h.
The silica-, alumina-, titania-, and zirconia-supported HPW and
HSiW are referred to as HPW/SiO2 and HSiW/SiO , HPW/Al O
2
2
3
and HSiW/Al O , HPW/TiO and HSiW/TiO , and HPW/ZrO2 and
2
3
2
2
HSiW/ZrO , respectively. SiO2 was used as received for impregna-
2
◦
tion, whereas Al O , TiO , and ZrO were calcined at 400 C in air
2
3
2
2
for 5 h before impregnation.
H-BEA-25 (CP814E, Si/Al atomic ratio = 12.5), H-BEA-38
CP814C, Si/Al atomic ratio = 19), and H-ZSM-5 (CBV3024E, Si/Al
(
atomic ratio = 15) were purchased from Zeolyst. Silica-alumina
aerogel (SAA) with Si/Al atomic ratio = 1 (SAA-57) was prepared
using the method described elsewhere [18]. The zeolites and SAA
◦
were calcined at 550 C in air for 4 h before reaction.
2.2. Characterization
Powder X-ray diffraction (XRD, Shimadzu LabX-XRD 6000) mea-
surements were performed using Cu K␣ radiation (␭ = 1.54 Å) from
◦
5
to 50 2␪. The Brunauer-Emmett-Teller (BET) surface areas
and porosity of catalysts were determined by recording the N2
adsorption-desorption isotherms using a BELSORP-mini II appa-
ratus (BEL Japan, Inc.). Before analysis, the samples of bulk HPW
◦
and HSiW were outgassed at 150 C for 3 h, whereas the samples
◦
of the supported catalysts were outgassed at 200 C for 3 h. The
Raman spectra of bulk and supported HSiW catalysts were recorded
in a Jobin Yvon T64000 with a visible laser (532 nm) excitation.
31
The P MAS NMR spectra of bulk and supported HPW catalysts
were recorded in a Bruker Avance 400 WB spectrometer at a fre-
quency of 8 kHz. The acid properties of the bulk and supported HPA
catalysts were determined by ammonia-temperature programmed
desorption (NH -TPD) measurements using a BELCAT-B catalyst
3
analyzer coupled with a thermal conductivity detector (TCD) and
a mass spectrometer (MS). TPD was performed over the tempera-
◦
ture range, 100–850 C, after the saturation of catalyst surface with
Fig. 2. Raman spectra for bulk and supported HSiW catalysts (top) and support
materials (bottom).
◦
NH3 at 100 C for 25 min. TPD of n-propylamine (Sigma Aldrich,
99%) was also carried out to confirm the formation of Brønsted
>
acid sites on the supported HPA catalysts. Samples (50 mg) were
impregnated with 0.25 cc of propylamine and vacuum dried at 40 C
◦
tion, Raman spectroscopy, 31P MAS NMR, and ammonia- and
propylamine-TPD analyses. The XRD patterns of the bulk HPW
prior to analysis. Subsequently, the TPD measurements were per-
◦
◦
◦
◦
formed over the temperature range, 100–750 C, with gas detection
and HSiW showed intensive reflections at 10.3 , 25.3 , and 34.8
2␪, belonging to the typical Keggin structure of HPAs (see Fig. S1)
33,34]. These XRD peaks were not observed for Al O , TiO , and
through a MS.
[
2
3
2
2
.3. Catalytic experiments
ZrO2 supported HPAs, indicating that HPW and HSiW are highly
dispersed on the supports. Meanwhile, the XRD patterns of HPAs
◦
◦
The reactions were performed in a 160 mL high pressure Parr
supported on SiO2 exhibited definite peaks at 10.3 , 25.3 , and
◦
reactor. A mixture of DMF (3.9 mL) and 1,4-dioxane (30 mL) and
each HPA catalyst (0.05–0.15 g) were charged into the reactor
at first. After sealing the reactor, the reactor was purged three
times with N . The reactor was then pressurized with ethylene
to 20–30 bar and heated to 225–300 C. The reaction mixture was
34.8 2␪. This suggests that heteropolyanions tend to agglomerate
into larger clusters on the silica support, forming X-ray detectable
crystallites. Fig. 2 shows the Raman spectra of bulk and supported
HSiW catalysts. The spectrum of bulk HSiW exhibited intense bands
2
◦
−1
at 998, 974, 882, 555, and 227 cm , which were assigned to the
stirred at 350 rpm with an impeller and the reactions were per-
formed for 1–10 h. At the end of the reaction, the reactor was cooled
by air cooling fan and the liquid content was sampled and analyzed
by GC-FID (Agilent 7890A) and GC–MS (Agilent 5975). The quantifi-
cation of products was undertaken by creating calibration curves
for each product compound using standard chemicals.
symmetric and asymmetric stretching modes of ꢀ(W = O), asym-
metric stretching mode of ꢀ(W
O W), and the bending modes
of ꢀ(O Si O) and ꢀ(W W) [34,35]. All the oxide-supported
O
−
1
HSiW catalysts showed a Raman band at 998 cm for the W = O
stretching band, indicating that the Keggin structure of HSiW was
well preserved on the surface of the oxide supports. However,
−
1
the band intensity at 998 cm was varied with support used and
3
. Results and discussion
HSiW/Al O3 exhibited a relatively weaker intensity of the band as
2
compared to those of other supported HSiW, implying that less
W = O bonds are present on HSiW/Al O . In addition to the Raman
3
.1. Characterization of bulk and supported HPA catalysts
2
3
−
1
peak at 998 cm , HSiW/TiO showed strong bands at 144, 399, 519,
2
−
1
The structural, surface and acid properties of the bulk and
and 639 cm , which were related to the Raman bands of the TiO2
nanoparticles.
supported HPA catalysts were investigated by XRD, N2 physisorp-
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that zirconia disrupts the Keggin structure of HPAs owing to its
basic properties, resulting in strong interaction between HPA and
zirconia surface [31].
Table 1 lists the textural properties (BET surface area and total
pore volume) of the HPAs supported on different oxides. The BET
surface areas and total pore volumes of the neat supports decreased
in the order: SiO > Al O > ZrO > TiO . In general, the addition of
2
2
3
2
2
HPW and HSiW to the supports resulted in a slight decrease in sur-
face area, except for Al O . In the case of Al O , it is possible that
2
3
2
3
HPA molecules are mostly located outside the pore, which leads
to an increase in the surface area of the alumina-supported HPAs.
The surface areas and pore volumes of the HPAs supported on the
oxides decreased in the same order as the neat supports. Assum-
ing a Keggin unit cross section of 144 Å2 for HPA molecules [30],
1
4
5% HPA loading will correspond to a monolayer HPA coverage of
2
−1
5 m g . Because all of the oxide supports used in this study had
2
−1
surface areas above 45 m g , it is expected that all the supported
HPA catalysts would have submonolayer HPA coverage.
The acidic properties of the bulk and supported HPA catalysts
were investigated using NH -TPD. Fig. 4 shows the NH -TPD curves
for the neat supports and HPA catalysts, and Table 1 lists the total
3
3
Fig. 3. 31P NMR spectra of bulk and supported HPW catalysts.
acidity. The bulk HPW and HSiW exhibited three NH desorption
3
◦
◦
peaks at 100–200 C (denoted as ␣-peak), 200–450 C (denoted as
-peak), and 450–600 C (denoted as ␥-peak). The ␣-peak was
Fig. 3 shows the 31P MAS NMR spectra of bulk and supported
HPW catalysts. Hydrated HPA species of the Keggin structure give
a strong resonance in the range of −15 to −18 ppm, depending
◦
␤
assigned to the desorption of hydrogen-bonded NH3 molecules,
while the ␤- and ␥-peaks were attributed to the decomposition
of ionically bonded NH4+ ion coordinated in the different crys-
tal lattices [25]. Regarding the neat supports, Al O , TiO , and
on their hydration degree [36]. HPW supported on SiO , Al O ,
2
2
3
and TiO2 exhibited a peak at −17.8, −16.0 and −17.8 ppm, respec-
tively, indicating that the Keggin structure of HPW remains intact
on these supports. Meanwhile, HPW/ZrO2 showed a broad peak
at −8.6 ppm, which can be assigned to lacunary species formed
2
3
2
ZrO2 showed two NH3 desorption peaks at temperatures of 150
◦
◦
◦
and 430 C, 150 and 270 C, and 150 and 430 C, respectively,
◦
whereas SiO2 showed only a weak and broad peak at 400 C. In
by partial decomposition of HPW on ZrO , with a shoulder peak
2
terms of the total acidity, the Al O3 support showed the high-
2
at −17.8 ppm [36,37]. This result indicates that a great amount of
est total acidity (0.31 mmol/g) followed by TiO2 (0.28 mmol/g),
HPW on ZrO stays as fragment forms of HPW. It was reported
2
Fig. 4. NH3-TPD profiles of Keggin HPA catalysts: bulk HSiW and HPW (a), metal oxide supports (b), oxide-supported HSiW (c), and oxide-supported HPW (d).
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Table 1
Summary of textural and acid properties of catalysts used in this study.
SBETa (m /g)
2
Vporeb (cm /g)
3
Dporec (nm)
Cacidd (mmol/g)
Entry
Catalyst
Si/Al
1
2
3
4
5
6
7
8
9
SiO2
Al2O3
TiO2
ZrO2
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
12.5
1
176
87
58
1.48
1.03
0.69
0.41
0.22
1.62
1.01
0.73
0.34
0.19
2.28
1.01
0.66
0.41
0.49
1.39
0.34
0.23
58.1
21.3
43.6
7.1
37.4
37.4
43.6
76.5
6.3
37.4
43.6
21.3
23.6
6.3
0.02
0.31
0.28
0.17
2.03
1.05
0.82
0.36
0.50
2.30
1.14
1.04
0.40
0.54
89
HPW
6.6
150
92
48
77
5.8
152
103
51
15%-HPW/SiO2
15%-HPW/Al2O3
15%-HPW/TiO2
15%-HPW/ZrO2
HSiW
15%-HSiW/SiO2
15%-HSiW/Al2O3
15%-HSiW/TiO2
15%-HSiW/ZrO2
H-BEA-25
SAA-57
10
11
12
13
14
15
16
17
18
82
[18]
[18]
[18]
[18]
545
411
475
358
9.4
28.1
3.8
1.12
1.16
1.21
1.10
H-BEA-38
H-ZSM-5
19
15
2.4
Notation: aBET surface area, bSingle point total pore volume (P/P0 = 0.98), cAverage pore diameter obtained by BJH method, dNH3-TPD total acid site; n.a. = not applicable.
ZrO2 (0.17 mmol/g), and SiO2 (0.02 mmol/g). In general, supported
HPA catalysts showed higher total acidity than their parent sup-
ports, indicating that the impregnation of HPA into the supports
induced new acid sites. In particular, HPW (1.05 mmol/g) and
HSiW (1.14 mmol/g) supported on SiO2 showed much higher
total acidity compared to SiO2 (0.02 mmol/g). The NH -TPD pro-
3
files of HPW/SiO2 and HSiW/SiO2 showed three NH3 desorption
◦
peaks appearing at 150, 390, and 475 C and at 130, 400, and
4
◦
75 C, respectively, which should be due to the interaction of
NH3 molecules with HPA. In the case of HPAs supported on Al O ,
2
3
TiO2 and ZrO , an additional NH desorption peak appeared at
2
3
◦
3
50–380 C, compared to their parent supports. The total acid-
ity of HPW and HSiW supported on SiO , Al O , TiO , and ZrO
2
2
3
2
2
decreased in the order: SiO > Al O > ZrO > TiO . It appears that
2
2
3
2
2
the weak interaction of silica with the HPAs keeps their Brønsted
acid character and results in an increase in total acidity.
TPD of n-propylamine was also performed to confirm the forma-
tion of Brønsted acid sites on the supported HPA catalysts. Because
propylamine decomposes to propene and ammonia only at the
Brønsted acid sites, the number of molecules that decompose can
be indicative of the Brønsted acidity [38]. It was found that propy-
lamine decomposed completely to ammonia and propene over all
of the supported HSiW catalysts at 350–380 C (see Fig. S2). There-
fore, it is believed that the supported HSiW catalysts used in this
study possess predominantly Brønsted acid sites.
Fig. 5. DMF conversion, PX selectivity, and PX production rate for the DA-reaction
of DMF and C2H4 (20 bar), at 250 C for 1 h over various solid acid catalysts. Acid
loading of all the oxide-supported HPAs = 15 wt%.
◦
the highest PX product rate (77.98 mmol/(g.h)) with the highest PX
selectivity (∼70%).
◦
HSiW/SiO2 also exhibited higher PX production rate than
H-BEA-25 (74.11 mmol/(g.h)), SAA-57 (54.97 mmol/(g.h)), and H-
ZSM-5 (15.51 mmol/(g.h)), and higher selectivity to PX than those
catalysts at similar DMF conversion (42–49%). In order to bet-
ter compare the activity of different acid catalysts, the turnover
frequency (TOF) for the production of PX over each catalyst was cal-
culated, as shown in Table 1. Because Brønsted acidity determines
the overall rate of PX production [16,18], the TOF was calculated
by normalizing the rate of PX production by the number of proton
sites. In the case of HPA/SiO2, the proton site densities were calcu-
lated assuming that all HPA protons (15 wt% loading) are equally
available for the reaction. Similarly, for H-zeolites, the proton site
densities were calculated from their Si/Al atomic ratios, assum-
ing that the Al content is equal to the number of proton sites.
3.2. Catalytic performances of bulk and supported HPA catalysts
At the initial stages, the catalytic performances of the bulk and
supported HPA catalysts were investigated in a short reaction time
1 h) and compared with the previously tested catalysts, such as
(
H-BEA-25, H-BEA-38, H-ZSM-5, and SAA-57. As shown in Fig. 5 and
Table 2, bulk HPW and HSiW were quite active for the conversion
of DMF (41–71%) and selective toward the PX production (46–53%
selectivity), demonstrating their effectiveness for catalyzing the
DA reaction of DMF and ethylene. Regarding the oxide-supported
HPAs, their activity and selectivity to PX was strongly dependent
on the type of support. For both HPW and HSiW, the conversion of
DMF and the selectivity to PX decreased in the following order:
HPA/SiO > HPA/Al O > HPA/TiO > HPA/ZrO . The PX production
−
1
It was found that the TOF values for HPW/SiO2 (407.8 h ) and
−
1
HSiW/SiO2 (371.3 h ) were considerably greater than those for H-
−
1
−1
−1
BEA-25 (60.0 h ), HBEA-38 (72.8 h ), and SAA-57 (47.4 h ). The
higher TOF values over HPA/SiO2 suggest that they have stronger
Brønsted acid sites, which can accelerate the rate of cycloadduct
dehydration. Overall, silica-supported HPA catalysts demonstrated
the superior activity and selectivity toward PX production than did
zeolites and SAA.
2
2
3
2
2
rate also decreased in the same order, indicating that silica bet-
ter promotes the activity of HPAs, followed in order by alumina,
titania, and zirconia, possibly due to its higher surface area, larger
pore volume, and higher acidity (see the comparison in Table 1).
Among the supported HPAs, the silica-supported HSiW exhibited
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Table 2
◦
Summary of the catalytic performance results for DMF conversion to PX over various solid acid catalysts. Reaction condition: P = 20 bar, T = 250 C, and t = 1 h.
TOF (h 1
−
)
Entry
Catalyst
XDMF (%)
SPX (%)
SHD (%)
SOLI (%)
C.B. (%)
PX rate (mmol/(g h))
1
2
3
4
5
6
7
8
9
HPW*
HPW
41.22
68.47
47.17
9.65
5.24
3.87
43.07
71.56
42.36
44.62
6.09
50.52
53.12
57.58
17.25
8.03
0.00
8.10
5.62
0.00
0.00
0.00
0.00
7.16
4.95
3.54
0.00
0.00
5.04
0.00
3.82
0.00
1.38
2.19
2.50
4.89
0.00
0.00
1.63
2.23
0.29
3.83
0.00
0.00
0.93
0.00
8.33
0.00
82.11
75.44
84.10
92.49
95.18
96.42
82.19
68.72
89.60
81.36
95.72
95.35
86.02
86.43
89.58
96.68
169.21
96.97
65.25
3.90
1.21
0.73
177.34
87.06
77.98
55.01
4.73
7161.2
–
15%-HPW/SiO2
15%-HPW/Al2O3
15%-HPW/TiO2
15%-HPW/ZrO2
HSiW*
407.8
24.4
6.90
4.54
6626.4
–
371.3
266.3
22.5
3.78
60.0
47.4
72.8
16.1
7.57
51.89
46.11
69.81
50.14
29.80
6.47
57.60
56.77
57.17
69.15
HSiW
15%-HSiW/SiO2
15%-HSiW/Al2O3
15%-HSiW/TiO2
15%-HSiW/ZrO2
H-BEA-25
SAA-57
H-BEA-38
H-ZSM-5
1
1
1
1
1
1
1
0
1
2
3
4
5
6
4.97
0.79
48.62
36.63
49.15
10.77
74.11
54.97
55.90
15.51
Notation: *mCAT = 0.05 g; XDMF = 2,5-dimethylfuran conversion, SPX = p-xylene selectivity, SHD = 2,5-heanedione selectivity, SOLI = oligomers selectivity; C.B. = carbon balance;
TOF = turnover frequency, calculated as the p-xylene formation rate per surface proton site.
HPAs and the PX selectivity decreases in the following order:
HPA/SiO > HPA/Al O > HPA/TiO > HPA/ZrO . In regard to the sup-
2
2
3
2
2
ported HPW, the highest PX selectivity (73%) was achieved over
5%-HPW/SiO2 at approximately 81% DMF conversion. Regarding
1
the supported HSiW, the highest PX selectivity (85%) was achieved
over 15%-HSiW/SiO2 at approximately 94% DMF conversion. The
zirconia- and titania-supported HPAs consistently exhibited very
low PX yields (<29%) at low DMF conversions (<54%).
The origin of the higher activity of the silica-supported HPAs
for PX production was investigated. According to the literature,
the overall rate of PX formation has a linear dependence on the
density of Brønsted acid sites in the catalysts [19]. This is because
the initial DA reaction of DMF and ethylene is a thermal process,
while the dehydration of the DMF/ethylene cycloadduct cannot
proceed uncatalyzed, thereby making the second dehydration the
rate-limiting step for PX production [15]. Therefore, the correla-
tion between the acid site density estimated from TPD of ammonia
and the initial rate of PX production was examined. The acid site
density of the supported HPA catalysts was modified by subtract-
ing the support acidity from the overall acidity of the supported
HPA catalysts in order to consider only the Brønsted acid sites.
Because impregnated HPW and HSiW species contain predomi-
nantly Brønsted acid sites, the calculated acidity can be a reasonable
approximation of the density of Brønsted acid sites. As shown in
Fig. 7(a), the PX production rate was proportional to the acid site
concentration with bulk HSiW, which possesses the highest acid
concentration (2.30 mmol/g), with the highest rate of PX produc-
tion (177.34 mmol/(g h)). The silica-supported HPAs, which possess
the higher acid site concentrations, exhibited the higher PX produc-
tion rates than other oxide-supported HPAs. These results suggest
that the high activity of the silica-supported HPAs is mainly due to
their high concentration of Brønsted acid sites.
In addition to the acid site concentrations, the acid strength
can affect the catalytic activity. It has been reported that an
interaction between HPA and the oxide supports can largely
influence the acid strength of the HPA catalysts. According
to the initial enthalpies of ammonia adsorption (ꢁHNH3) for
the HPA catalysts measured by NH3 adsorption calorimetry,
the acid strength of the HPA catalysts decreased in the order:
HPW > HSiW > HPW/SiO ≈ HSiW/SiO > HPW/TiO > HPW/ZrO
Fig. 6. DMF conversion, products selectivity, and carbon balance for the DA-reaction
of DMF and C2H4 (20–30 bar), at 250 C for 6 h over oxide-supported HPW cata-
lysts (a) and oxide-supported HSiW catalysts (b). Products are classified as p-xylene
◦
(
(
PX), alkylated p-xylene (AP), 3,6-dimethyl-2-cyclohexenone (CH), 2,5-hexanedione
HD), 3-methyl-2-cyclopentenone (CP), and oligomers (OLI).
3.3. Effect of support on catalytic activity
2
2
2
2
The catalytic performances of all the oxide-supported HPAs
[24,30,39]. The supported HPA catalysts exhibited the weaker
acidity than the bulk HPAs owing to the interaction of HPAs with
the supports, and the acid strength of the supported HPA catalysts
decreased with increasing HPA-support interaction. Although
the NH3 adsorption calorimetry technique does not discriminate
Brønsted and Lewis acid sites, the measured acid strength showed
were investigated further at an extended reaction time (6 h).
Among all the different porous materials, silica appears to be the
most effective catalyst support, either for the HPW or HSiW cata-
lyst, and for both short (1 h) and extended reaction times. Fig. 6
clearly shows the superior performance of the silica-supported
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Fig. 8. DMF conversion, PX selectivity and yield for the DA-reaction of DMF and C2H4
◦
30 bar) at 250 C for 6 h in over HSiW/SiO2 with different acid loadings (5–20 wt%).
(
Fig. 7. Correlations (a) between surface acidity and PX production rate and (b)
between initial heat of NH3 adsorption and TOF.
a good correlation with the rate of acid-catalyzed reaction (e.g.,
methanol dehydration) [30]. Accordingly, the correlation between
the acid strength and the turnover frequency (TOF) of PX formation
was examined, as shown in Fig. 7(b). TOFs for the supported HPA
catalysts were calculated per surface Brønsted site, assuming that
all HPA protons were equally available for the reaction. On the
other hand, in the case of bulk HPAs, the proton site densities
Fig. 9. DMF conversion, PX selectivity and yield for the DA-reaction of DMF and
C2H4 (30 bar) at different temperatures (200–300 C) for 6 h over 15%-HSiW/SiO2.
◦
also increases sharply from 53 to 82% with increasing temperature
◦
from 200 to 250 C. Interestingly, when the reaction temperature
◦
was increased further to 275 C, the DMF conversion and PX selec-
2
were calculated using a Keggin unit cross section of 144 Å and
tivity also increased slightly to 96 and 85%, respectively. Even at
+
−1
and
the catalyst surface areas (i.e., HPW = 0.023 mmol (H ) g
◦
3
00 C, the DMF conversion and PX selectivity were still high (95
+
−1
HSiW = 0.023 mmol (H ) g ) to consider only surface proton site.
Notably, a linear relationship was observed between the TOF and
catalyst acid strength, showing that the higher the acid strength,
the higher the turnover rate of PX formation. Therefore, this result
suggests that the higher activity of HPA/SiO2 for PX production is
at least partly due to the enhanced strength of Brønsted acid sites.
and 83%, respectively), indicating that the 15%-HSiW/SiO has good
2
thermal stability in the reaction and it is not easily deactivated
◦
by coke under high temperature reaction conditions (>250 C). As
summarized in Table S4, the carbon balances for all the reactions at
different temperatures are relatively constant (84–86%), suggesting
that the higher reaction temperature does not necessarily promote
the formation of larger oligomers or solid residues.
3
.4. Effect of the reaction conditions
In addition to the acid loading and reaction temperature, the
effects of the reaction time were also investigated over 15%-
HSiW/SiO2 in comparison to the commercial zeolite, H-BEA-25.
Fig. 10 clearly shows that both catalysts show similar DMF
conversion profiles. However, the PX production rates over 15%-
HSiW/SiO2 were higher than those over H-BEA-25. The highest
PX yield over 15%-HSiW/SiO2 was 82% within 8 h, whereas over
H-BEA-25, it took longer time (12 h) for a lower PX yield (68%).
Importantly, as mentioned previously in the effects of the reaction
temperature, the carbon balances for the different reaction times
with 15%-HSiW/SiO2 were also relatively constant (84–88%), and
no apparent change was observed. Meanwhile, at a longer reaction
time, the carbon balance for the reaction with H-BEA-25 was much
lower (77%) due to the more significant formation of byproducts
(such as alkylated products and oligomers) and the possible for-
mation of a solid residue (coke) on the catalyst, as confirmed by the
As the best-performing catalyst in this study, the silica-
supported HSiW was used to investigate further the effect of
reaction conditions, such as acid loading, reaction temperature,
and reaction time on the DMF conversion and PX selectivity. As
shown in Fig. 8, increasing the HSiW loading from 5 to 15 wt%
resulted in increasing DMF conversion, PX selectivity, and PX yield,
as expected. However, when the acid loading was increased fur-
ther to 20 wt%, the DMF conversion and PX selectivity decreased
slightly. This probably indicates that 15 wt% is the optimal acid
loading for submonolayer coverage in the HSiW/SiO , in which all
the HPA protons are equally available for the reaction.
The reaction temperature, as shown in Fig. 9, also affects the
conversion of DMF to PX. Over the HSiW supported on silica, DMF
conversion increases linearly from 35 to 91% and the PX selectivity
2
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Fig. 10. DMF conversion and PX yield as a function of time for the DA-reaction of
◦
DMF and C2H4 (30 bar) at 250 C over 15%-HSiW/SiO2 in comparison to H-BEA-25.
Table 3
Summary of the catalytic performance results for DMF conversion to PX over the
fresh and spent HSiW/SiO2 catalysts calcined at different temperatures. Reaction
◦
condition: P = 30 bar and T = 250 C.
Catalyst
t (h)
XDMF (%)
YPX (%)
SPX (%)
C.B. (%)
◦
Fresh, calcined at 100
Spent, uncalcined
C
6
6
6
6
6
1
1
1
1
94.1
65.1
10.1
51.7
1.6
46.2
50.6
26.5
0.3
80.2
49.4
3.1
37.8
0.5
29.1
29.8
10.2
–
85.2
76.0
30.7
73.1
33.2
63.0
58.9
38.6
–
87.7
85.4
93.0
86.2
98.9
81.1
78.0
76.8
–
◦
◦
◦
◦
◦
◦
◦
Spent, calcined at 325
Spent, calcined at 400
Spent, calcined at 550
Fresh, calcined at 100
Fresh, calcined at 300
Fresh, calcined at 400
Fresh, calcined at 500
C
C
C
C
C
C
C
Fig. 11. TGA and DTG curves for the fresh and spent HSiW/SiO2 catalysts.
GC/MS spectra for the liquid products from the reactions shown in
Figs. S6 and S7 .
◦
4
00 C (see Fig. S7). Overall, TGA and XRD measurements suggest
◦
that the coke can be completely removed after calcination at 400 C
without the decomposition of Keggin structure, but the catalyst
activity was not fully recovered. In order to clarify the reason behind
3.5. Reusability of HSiW/SiO2 catalysts
The stability and reusability of HSiW/SiO2 catalysts were inves-
◦
the lower activity of the spent catalyst calcined at 400 C, the fresh
tigated. For this purpose, the spent catalyst from the reaction of
DMF and ethylene was reused under the standard reaction condi-
tions. As shown in Table 3, the recycled catalyst showed a lower
DMF conversion than the fresh HSiW/SiO2 (65.1% vs 94.1%) and
a lower selectivity to PX (76% vs 85.2%). Thermogravimetric (TG)
analysis of the spent catalyst revealed the formation of carbona-
ceous deposits on the spent catalyst, comprising of approximately
catalyst was also subjected to calcination at different temperatures
and was tested for the reaction. The activity was found to be much
dependent on the calcination temperature. The activity of the cat-
alyst was highest in the calcination temperature of 100 and 300 C,
showing the DMF conversion of 45 and 50% at 1 h reaction time and
was decreased by half (26.5% DMF conversion) with an increase in
the calcination temperature to 400 C. This result implies that the
calcination should be carried out at temperatures below 300 C for
the regeneration of the spent catalyst to keep the highest activity
of HSiW/SiO2 catalyst; however, the coke could not be removed at
such low temperatures as seen in the TGA results. The decrease in
◦
◦
5
% of the catalyst weight (see Fig. 11). In the meantime, Induc-
◦
tively Coupled Plasma (ICP-OES) analysis of the product solution
indicated that HSiW (tungsten species) leaching is negligible (<1%).
Therefore, the deactivation of the HSiW/SiO2 can be attributed to
the loss of some active sites by deposition of coke.
◦
the activity of the catalyst calcined at 400 C might be due to the
Because the DTG curve of the spent catalyst showed a peak at
partial degradation of Keggin structure of HSiW supported on sil-
ica, affecting its acidity. Thus, the challenge remains to establish
the method for regenerating the coked HSiW/SiO2 catalyst.
◦
4
50 C, the calcination temperature of the spent catalyst for regen-
◦
eration was explored at a temperature ranging from 325 to 550 C.
The catalysis results showed that 400 C was the optimal temper-
◦
ature for calcination, but the initial activity of the catalyst could
not be fully regained, showing only 51.7% conversion and 73.1% PX
selectivity. TGA analyses of the spent catalysts calcined at different
4. Conclusions
temperatures confirmed that the calcination temperature above
In summary, silica-supported HSiW demonstrated remark-
able catalytic performance for PX production from DMF (∼82%
yield) and should be considered as a promising catalyst for
this reaction among all the oxide-supported HPA catalysts.
Importantly, the turnover frequency of PX production for
◦
4
00 C is required to fully remove the coke. The XRD measurements
◦
showed that the calcination temperature of 550 C results in the
decomposition of Keggin structure, forming WO crystallites, while
3
the Keggin structure is intact at the calcination temperatures below
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HSiW/SiO2 was 6–8 times greater than those for HBEA-25
and SAA-57, which were previously reported as highly active
catalysts. Compared to other oxide-supported HPAs, the higher
activity of HPA/SiO2 was mainly due to their higher acid site
density and stronger Brønsted acidity. Results from NH3-
TPD showed that the total acidity of the catalysts decreased
[3] S.K. Green, R.E. Patet, N. Nikbin, C.L. Williams, C.-C. Chang, J. Yu, R.J. Gorte, S.
Caratzoulas, W. Fan, D.G. Vlachos, Appl. Catal. B 180 (2016) 487–496.
[
4] D.I. Collias, A.M. Harris, V. Nagpal, I.W. Cottrell, M.W. Schultheis, Ind.
Biotechnol. 10 (2014) 91–105.
[5] J.J. Pacheco, M.E. Davis, Proc. Natl. Acad. Sci. U. S. A. (2014) 201408345.
[
[
6] Y. Tachibana, S. Kimura, K.-i. Kasuya, Sci. Rep. 5 (2015).
7] A. Illanes, M. Zuniga, S. Contreras, A. Guerrero, Bioprocess. Eng. 7 (1992)
199–204.
in
while the rate of PX production decreased in the order
HPA/SiO > HPA/Al O > HPA/TiO > HPA/ZrO . The turnover
the
order
HPA/SiO > HPA/Al O > HPA/ZrO > HPA/TiO ,
[8] M. Moliner, Y. Román-Leshkov, M.E. Davis, Proc. Natl. Acad. Sci. U. S. A. 107
(2010) 6164–6168.
2
2
3
2
2
[
9] Y. Román-Leshkov, C.J. Barrett, Z.Y. Liu, J.A. Dumesic, Nature 447 (2007)
82–985.
2
2
3
2
2
9
frequency of PX production for bulk and silica-supported HPA cat-
alysts were also significantly greater than those for HPAs supported
[10] J. Jae, W. Zheng, R.F. Lobo, D.G. Vlachos, ChemSusChem 6 (2013) 1158–1162.
[11] Y. Lin, S. Tanaka, Appl. Microbiol. Biotechnol. 69 (2006) 627–642.
[
[
[
12] A. Morschbacker, J. Macromol. Sci. Polym. Rev. 49 (2009) 79–84.
13] M. Zhang, Y. Yu, Ind. Eng. Chem. Res. 52 (2013) 9505–9514.
14] C.L. Williams, C.-C. Chang, P. Do, N. Nikbin, S. Caratzoulas, D.G. Vlachos, R.F.
Lobo, W. Fan, P.J. Dauenhauer, ACS Catal. 2 (2012) 935–939.
15] N. Nikbin, P.T. Do, S. Caratzoulas, R.F. Lobo, P.J. Dauenhauer, D.G. Vlachos, J.
Catal. 297 (2013) 35–43.
16] D. Wang, C.M. Osmundsen, E. Taarning, J.A. Dumesic, ChemCatChem 5 (2013)
2044–2050.
17] C.-C. Chang, S.K. Green, C.L. Williams, P.J. Dauenhauer, W. Fan, Green Chem.
on TiO2 and ZrO , indicating that the weak interaction of silica
2
with the HPAs allow them to keep strong Brønsted acidity, thereby
accelerating the rate of cycloadduct dehydration. Both the silica-
supported HSiW and HPW catalysts were capable of producing
[
[
[
[
remarkably high PX selectivity (82–85%) at high DMF conversion
◦
(
91–94%) at 250 C after 6 h reaction. Overall, the silica-supported
HPAs can be an alternative potential catalyst for the tandem DA
and dehydration reactions of DMF and ethylene to produce PX
with high selectivity. This study would contribute to the design
and development of improved catalysts for the efficient synthesis
of renewable chemicals from biomass, as a part of the sustainable
efforts to reduce the dependency on petroleum-derived chemicals.
16 (2014) 585–588.
18] Y.P. Wijaya, D.J. Suh, J. Jae, Catal. Commun. 70 (2015) 12–16.
[19] R.E. Patet, N. Nikbin, C.L. Williams, S.K. Green, C.-C. Chang, W. Fan, S.
Caratzoulas, P.J. Dauenhauer, D.G. Vlachos, ACS Catal. 5 (2015) 2367–2375.
[
20] T.-W. Kim, S.-Y. Kim, J.-C. Kim, Y. Kim, R. Ryoo, C.-U. Kim, Appl. Catal. B 185
2016) 100–109.
[21] S. Song, G. Wu, W. Dai, N. Guan, L. Li, J. Mol. Catal. A: Chem. 420 (2016)
34–141.
(
1
[
[
[
22] I.V. Kozhevnikov, Chem. Rev. 98 (1998) 171–198.
Acknowledgements
23] Y. Izumi, R. Hasebe, K. Urabe, J. Catal. 84 (1983) 402–409.
24] A.M. Alsalme, P.V. Wiper, Y.Z. Khimyak, E.F. Kozhevnikova, I.V. Kozhevnikov, J.
Catal. 276 (2010) 181–189.
25] J.K. Kim, J.H. Choi, J.H. Song, J. Yi, I.K. Song, Catal. Commun. 27 (2012) 5–8.
26] M. Misono, N. Mizuno, K. Katamura, A. Kasai, Y. Konishi, K. Sakata, T. Okuhara,
Y. Yoneda, Bull. Chem. Soc. Jpn. 55 (1982) 400–406.
This research was supported by the National Research Coun-
cil of Science & Technology (NST) grant by the Korea government
[
[
(
MSIP) (No. CAP-11-04-KIST). This work was also supported by the
New & Renewable Energy Core Technology Program of the Korea
Institute of Energy Technology Evaluation and Planning (KETEP)
granted financial resource from the Ministry of Trade, Industry &
Energy, Republic of Korea (No. 20153030101580).
[27] N. Mizuno, M. Misono, Curr. Opin. Solid State Mater. Sci. 2 (1997) 84–89.
[
28] I. Kozhevnikov, K. Kloetstra, A. Sinnema, H. Zandbergen, H. Van Bekkum, J.
Mol. Catal. A: Chem. 114 (1996) 287–298.
29] R.M. Ladera, J.L.G. Fierro, M. Ojeda, S. Rojas, J. Catal. 312 (2014) 195–203.
[
[30] W. Alharbi, E.F. Kozhevnikova, I.V. Kozhevnikov, ACS Catal. 5 (2015)
186–7193.
7
[
[
[
[
[
[
31] B.B. Bardin, R.J. Davis, Appl. Catal. 200 (2000) 219–231.
32] M. Timofeeva, Appl. Catal. A 256 (2003) 19–35.
Appendix A. Supplementary data
33] J.C. Juan, J. Zhang, M.A. Yarmo, J. Mol. Catal. A: Chem. 267 (2007) 265–271.
34] A.H. Jadhav, H. Kim, I.T. Hwang, Bioresour. Technol. 132 (2013) 342–350.
35] H. Atia, U. Armbruster, A. Martin, J. Catal. 258 (2008) 71–82.
36] S.M. Kumbar, G. Shanbhag, F. Lefebvre, S. Halligudi, J. Mol. Catal. A: Chem. 256
(2006) 324–334.
37] T.H. Kang, J.H. Choi, Y. Bang, J. Yoo, J.H. Song, W. Joe, J.S. Choi, I.K. Song, J. Mol.
Catal. A: Chem. 396 (2015) 282–289.
38] C. Pereira, R. Gorte, Appl. Catal. A 90 (1992) 145–157.
39] W. Alharbi, E. Brown, E.F. Kozhevnikova, I.V. Kozhevnikov, J. Catal. 319 (2014)
174–181.
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.cattod.2016.12.
032.
[
References
[
[
[
[
1] T.R. Carlson, Y.-T. Cheng, J. Jae, G.W. Huber, Energy Environ. Sci. 4 (2011)
45–161.
2] T.R. Carlson, G.A. Tompsett, W.C. Conner, G.W. Huber, Top. Catal. 52 (2009)
41–252.
1
2
Please cite this article in press as: Y.P. Wijaya, et al., Heteropolyacid catalysts for Diels-Alder cycloaddition of 2,5-dimethylfuran and
ethylene to renewable p-xylene, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.12.032