Highly active and durable WO3/Al2O3catalysts for gas-phase dehydration of polyols

Article abstract of DOI:10.1039/d0ra08340b

Gas-phase glycerol dehydration over WO₃/Al₂O₃catalysts was investigated. WO₃loading on γ-Al₂O₃significantly affected the yield of acrolein and the catalyst with 20 wt% WO₃loading showed the highest activity. The WO₃/Al₂O₃catalyst with 20 wt% WO₃loading showed higher activity and durability than the other supported WO₃catalysts and zeolites. The number of Br?nsted acid sites and mesopores of the WO₃/Al₂O₃catalyst did not decrease after the reaction, suggesting that glycerol has continuous access to Br?nsted acid sites inside the mesopores of WO₃/Al₂O₃, thereby sustaining a high rate of formation of acrolein. Dehydration under O₂flow further increased the durability of the WO₃/Al₂O₃catalyst, enabling the sustainable formation of acrolein. In addition, the WO₃/Al₂O₃catalyst with 20 wt% WO₃loading showed high activity for the dehydration of various polyols to afford the corresponding products in high yield.

Full text of DOI:10.1039/d0ra08340b

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Highly active and durable WO3/Al2O3 catalysts for
gas-phase dehydration of polyols†
abd
Cite this: RSC Adv., 2020, 10, 37538
abcd
Takeshi Aihara,a Katsuya Asazuma,a Hiroki Miura
and Tetsuya Shishido
*
Gas-phase glycerol dehydration over WO3/Al2O3 catalysts was investigated. WO3 loading on g-Al2O3
significantly affected the yield of acrolein and the catalyst with 20 wt% WO3 loading showed the highest
activity. The WO3/Al2O3 catalyst with 20 wt% WO3 loading showed higher activity and durability than the
other supported WO3 catalysts and zeolites. The number of Brønsted acid sites and mesopores of the
WO3/Al2O3 catalyst did not decrease after the reaction, suggesting that glycerol has continuous access
to Brønsted acid sites inside the mesopores of WO3/Al2O3, thereby sustaining a high rate of formation of
acrolein. Dehydration under O2 flow further increased the durability of the WO3/Al2O3 catalyst, enabling
the sustainable formation of acrolein. In addition, the WO3/Al2O3 catalyst with 20 wt% WO3 loading
showed high activity for the dehydration of various polyols to afford the corresponding products in high
yield.
Received 9th September 2020
Accepted 2nd October 2020
DOI: 10.1039/d0ra08340b
rsc.li/rsc-advances
chemicals (e.g. diols, aldehydes, ketones and alkenes).41–48 Many
of these reactions, however, usually require a low substrate feed
Introduction
rate and high reaction temperatures to achieve a high conver-
sion rate. Hence, the development of a catalyst that shows high
activity and selectivity to give the target products under mild
conditions is still a challenge.
The conversion of biomass-derived compounds instead of oil-
derived resources to valuable chemicals is essential for the
sustainable development of society.1–5 Glycerol is one of the
most important biomass-derived compounds. It is produced by
the hydrolysis of glycerides such as those in vegetable oils and
animal fats.6–8 A wide range of commodity chemicals can be
produced from glycerol through various reactions such as
dehydration,9–25 hydrogenolysis26,27 and oxidation28–30 over metal
oxides, zeolites, metal phosphides and metal–organic frame-
works. The dehydration of glycerol is a valuable reaction for the
formation of acrolein, which is an important intermediate for
the production of valuable compounds in the chemical and
agricultural industries, such as acrylic acid and DL-methio-
nine.31–34 Since acrolein is produced by the oxidation of
propylene over a Bi/Mo-mixed oxide catalyst,35–40 a method for
the production of acrolein from glycerol instead of oil-derived
resources should be quite valuable.
On the other hand, the deposition of coke on the catalyst
surface contributes to deactivation of the catalyst in dehydra-
tion. The mechanism of coke formation and the relationship
between coke formation and acid strength and/or porosity of
the catalyst have been widely investigated.49–54 Two deactivation
mechanisms due to coke formation have been proposed so far;
the direct deposition of coke at catalytically active acid sites and
coke formation at the entrance of micropores, which hinders
the substrate from accessing the active site inside the pore. A
better understanding of the deactivation mechanism could be
useful for the design of novel dehydration catalysts with high
durability.
In the course of our investigation of the relationship between
the structure of a two-dimensional tungsten oxide monolayer
supported on metal oxides and their performance as acid
catalysts,55,56 we reported that supported WO3 catalysts showed
high activity for selective biomass conversion, such as the
hydrogenolysis of glycerol and tetrahydrofurfuryl alcohol.57–60
These results encouraged us to further explore the application
of selective conversion of biomass-derived chemicals over sup-
ported WO3 catalysts, in particular polyols to give value-added
chemicals.
Furthermore, solid acid-catalyzed dehydration of polyols has
been reported to be a useful method for obtaining value-added
aDepartment of Applied Chemistry for Environment, Graduate School of Urban
Environmental Sciences, Tokyo Metropolitan University, 1-1 Minami-Osawa,
Hachioji, Tokyo 192-0397, Japan
bResearch Center for Hydrogen Energy-based Society, Tokyo Metropolitan University,
1-1 Minami-Osawa, Hachioji, Tokyo 192-0397, Japan
cResearch Center for Gold Chemistry, Tokyo Metropolitan University, 1-1 Minami-
Osawa, Hachioji, Tokyo 192-0397, Japan
dElements Strategy Initiative for Catalysts & Batteries, Kyoto University, Katsura,
In this study, gas-phase glycerol dehydration over WO3/Al2O3
catalysts was investigated. The optimization of WO3 loading on
Al2O3 enabled us to devise highly active and durable catalysts
for glycerol dehydration. To reveal the effect of coke deposition
Nishikyo-ku, Kyoto 615-8520, Japan
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra08340b
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at the catalyst surface on catalytic activity, the reaction under O2 0.01ꢁ. X-ray photoelectron spectroscopy (XPS) analysis of the
ow and a detailed characterization were carried out. Further- catalysts was performed using a JEOL JPS-9010 MX instrument.
more, the application of WO3/Al2O3 catalyst to the dehydration The spectra were measured using Mg Ka radiation. All spectra
of various polyols was investigated.
were calibrated using C 1s (284.5 eV) as a reference. The surface
properties of samples were obtained from N2 isotherms ob-
tained using a BELSORP-mini II (MicrotracBEL, Japan) at 77 K.
The analysed samples were evacuated at 573 K for 3 h prior to
the measurement. The surface area was estimated by the Bru-
nauer–Emmett–Teller (BET) method. Micropore size distribu-
tions were determined by t-plot and micropore (MP) analysis.
The Barrett–Joyner–Halenda (BJH) method was used to deter-
mine mesopore size distributions. FT-IR spectra were recorded
Experimental
Materials
Glycerol was purchased from Nacalai Tesque, Japan. Polyols,
including ethylene glycol, 1,2-propanediol, 1,2-butanediol, 1,3-
butanediol, 1,4-butanediol and 1,2-pentanediol, were
purchased from Wako Pure Chemical Industries and Tokyo
Chemical Industry, Japan. g-Al2O3 (JRC-ALO-8), ZrO2(JRC-ZRO-
3, monoclinic), TiO2(JRC-TIO-4, rutile) and Nb2O5 (JRC-NBO-1,
orthorhombic) were supplied by Japan Reference Catalyst.
(NH4)10W12O41$5H2O and WO3 were purchased from Wako
Pure Chemical Industries, Japan. All zeolites such as H-ZSM-
5(90), H-b(25), H-Y(5.5) and H-MOR(20), namely, JRC-Z5-90H,
JRC-Z-HB25, JRC-Z-HY5.5 and JRC-Z-HM20, where the number
in parentheses is the SiO2/Al2O3 ratio, were supplied by Japan
Reference Catalyst.
by FT/IR-4200 typeA (JASCO, Japan) with a resolution of 4 cmꢀ1
.
A total of 64 scans were averaged for each spectrum. Each
sample (30 mg) was pressed into a self-supporting wafer with
a diameter of 20 mm. Catalysts were pretreated under 20 kPa of
owing O2 at 773 K for 1.5 h and then evacuated. Catalysts were
exposed to pyridine (0.5 kPa) at 303 K for 30 min, and then
evacuated at 423 K for 30 min. NH3 temperature programmed
desorption (NH3-TPD) was measured using a BELCAT II
(MicrotracBEL, Japan). Fiy mg of the sample was loaded in
a quartz reactor and pretreated under a He ow at 773 K for 1 h.
NH3 adsorption was carried out over 30 min with 5% NH3/He at
373 K followed by purging with He for 15 min. The temperature
was linearly increased from 373–873 K (10 K minꢀ1). The outlet
ow was analysed by means of a TCD and Q-Mass (BELMASS,
MicrotracBEL, Japan). Thermogravimetric analyses (TG) were
performed using a DTG-60H (Shimadzu, Japan) The tempera-
ture range was 303–1273 K (10 K minꢀ1).
Catalyst preparation
Metal oxides and zeolites were calcined at 773 K for 3 h in
owing air. A series of WO3-loaded catalysts were prepared by
impregnation of supports with an aqueous solution of
(NH4)10W12O42, dried at 353 K for 6 h, and then calcined at 1123
K for 3 h in owing air.55,56 The surface coverages of Al2O3, ZrO2
and TiO2 with a WO3 monolayer were estimated by using the
cross-sectional area of a WO6 octahedral unit (0.22 nmꢀ2 61 and
)
were almost 100% at 20, 10 and 2.5 wt% WO3 loading,
respectively.60
Results and discussion
Fig. 1 shows the effect of WO3 loading over g-Al2O3 on the gas-
phase dehydration of glycerol at 588 K with WHSV by glycerol of
4.7 hꢀ1. The catalysts with 0 and 100 wt% WO3 loading were g-
Al2O3 and bulk WO3, respectively. Dehydration over WO3/Al2O3
provided acrolein as a main product together with the forma-
tion of hydroxyacetone as a by-product. The selectivity for
acrolein was around 80% regardless of the WO3 loading on g-
Catalytic dehydration of polyols
Catalytic polyol dehydration was performed in a xed-bed
down-ow glass reactor with an inner diameter of 6 mm at an
ambient pressure of N2 and O2. Prior to the reaction, 100 mg of
a catalyst was placed in the catalyst bed, and the catalyst was
treated at 588 K with N2 for 1.5 h. A 10 mol% aqueous solution
of polyols was fed through the top of the reactor at a prescribed
liquid feed rate together with a carrier gas ow of 20 mL minꢀ1
.
The total substrate feed rate was 5.1 mmol hꢀ1 and the
composition of the mixture gases was substrate/H2O/carrier gas
¼ 1/9/10 (molar ratio). The liquid products were collected in an
ice trap (273 K) and a dry ice–methanol trap (195 K) every hour,
and analysed by a FID-GC (GC2014, Shimadzu) with a 30 m
capillary column of Stabilwax (GL Science, Japan). Gaseous
products were analysed by on-line TCD-GC (GC-8A, Shimadzu,
Japan) with a 3 m packed column (Porapak-Q, GL Science,
Japan). For all the catalysts tested, the carbon balance was
>90%, with a few exceptions.
Characterization
X-ray diffraction (XRD) patterns of the catalyst were recorded by
Rigaku SmartLab with Cu Ka radiation. The samples were
Fig. 1 Dehydration of glycerol to acrolein over WO3/Al2O3 catalysts
with various WO3 loadings. C: yield, ,: selectivity. Conditions: cata-
scanned from 2q ¼ 10–70ꢁ at 10ꢁ minꢀ1 and a resolution of lyst (100 mg), WHSV by glycerol (4.7 hꢀ1), T ¼ 588 K.
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Notably, g-Al2O3 and WO3 showed no activity for this reaction,
suggesting that WO3 loaded on g-Al2O3 was the main active site
for the dehydration of glycerol to form acrolein.
Since previous studies on the relationship between Brønsted
acidity and the activity for glycerol dehydration to form acrolein
have revealed that Brønsted acid sites were a main active site for
acrolein formation,10,62 the number of Brønsted acid sites
(Brønsted acidity) on WO3/Al2O3 catalysts with various WO3
loadings was estimated by pyridine adsorbed FT-IR (Fig. 2 and
S2†). Brønsted acidity increased with an increase in WO3
loading, and WO3/Al2O3 catalyst with 20 wt% WO3 loading
exhibited the highest Brønsted acidity. In contrast, the Brønsted
acidity decreased gradually for catalysts with loadings above
20 wt%. Acrolein was formed in the highest yield with the
catalyst with 20 wt% WO3 loading which showed the highest
Brønsted acidity. We reported that a two-dimensional WO3
monolayer was formed on g-Al2O3 when the WO3 loading was
below 20 wt% and Brønsted acid sites were generated at the
boundaries between the WO3 domains.48,49 The close correlation
between the catalytic activity and Brønsted acidity was observed.
Some researchers reported that dehydration mechanism of
glycerol and the formation of acrolein proceeds over Brønsted
acid sites preferentially to over Lewis acid sites.9,10,16,18,62 Simi-
larly, the active sites of this reaction should be the Brønsted acid
sites at the boundaries between WO3 domains on g-Al2O3. Aer
protonation of hydroxyl group at the secondary carbon of glyc-
erol by the Brønsted acid sites at the boundaries between WO3
domains, the dehydration and keto–enol tautomerism takes
place to give 3-hydroxypropionaldehyde. The further dehydra-
tion of 3-hydroxypropionaldehyde can easily proceed because of
the low stability to produce acrolein.
Fig. 2 Brønsted acidity of WO3/Al2O3 catalysts with various WO3
loadings.
Fig. 3 Dehydration of glycerol to acrolein over various acid catalysts.
C: WO3/Al2O3 with 20 wt% WO3 loading, :: WO3/ZrO2 with 10 wt%
WO3 loading, -: WO3/TiO2 with 2.5 wt% WO3 loading, A: Nb2O5, B:
H-ZSM-5(90), O: H-b(25), ,: H-Y(5.5), >: H-MOR(20). Conditions:
catalyst (100 mg), WHSV by glycerol (4.7 hꢀ1), T ¼ 588 K.
Fig. 3 shows the durability of various solid acid catalysts in
the dehydration of glycerol to acrolein. All the tested catalysts
provided acrolein as a main product (Fig. S3†). Although WO3/
ZrO2 with 10 wt% WO3,60 WO3/TiO2 with 2.5 wt% WO3, H-ZSM-
5(90) and H-b(25) showed high acrolein yield at the initial stage
of the reaction, the rapid deactivation of these catalysts was
observed. H-Y(5.5), H-MOR(20) and Nb2O5 showed low initial
activity. On the other hand, WO3/Al2O3 catalyst with 20 wt%
WO3 loading showed high catalytic performance regarding both
activity and durability.
Al2O3. In contrast, the WO3 loading signicantly affected the
yield of acrolein. The yield of acrolein increased with an
increase in WO3 loading; WO3/Al2O3 catalyst with 20 wt% WO3
loading showed the highest acrolein yield. On the other hand,
excess loading of WO3 led to a decrease in the acrolein yield.
Coke deposition during dehydration is a well-known cause of
catalyst deactivation.49–54 Thus, to identify factors that
Table 1 Physical properties of various catalysts before and after the reaction for 5 h
BET surface area/m2 gcat
Before reaction
ꢀ1
Amount of deposited cokea/mg gcat
Aer reactionb
ꢀ1
Catalyst
WO3/Al2O3 (WO3: 20 wt%)
WO3/ZrO2 (WO3: 10 wt%)
WO3/TiO2 (WO3: 2.5 wt%)
H-ZSM-5 (90)
H-b (25)
H-Y (5.5)
131
82
35
138
198
225
151
109
48
14
460
660
749
502
104
69
18
116
103
25
H-MOR (20)
14
a
Estimated by TG analysis. b Reaction under owing N2 aer 5 h.
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Fig. 4 Effect of O2 treatment on acrolein yield over (C) WO3/Al2O3 with 20 wt% WO3 loading and (B) H-ZSM-5(90) catalysts. Conditions:
catalyst (100 mg), WHSV by glycerol (4.7 hꢀ1), T ¼ 588 K. O2 treatment was carried out after 5 h at the temperature at which deposited coke
completely decomposed; 823 K (WO3/Al2O3) and 923 K (H-ZSM-5), respectively.
a larger surface area than the other supported WO3 catalysts.
These results suggest that WO3/Al2O3 catalyst maintained
a large surface area during the reaction showed the highest
acrolein yield and stability in the dehydration of glycerol.
Oxidative treatment is useful for removing the coke depos-
ited at active Brønsted acid sites.63–65 Hence, we examined the
effect of the oxidative treatment of WO3/Al2O3 and H-ZSM-5(90)
aer the catalytic run on the regeneration of catalytic perfor-
mance. Treatment with O2 ow (at 823 and 923 K for WO3/Al2O3
and H-ZSM-5(90), respectively) successfully recovered the initial
catalytic activity for both WO3/Al2O3 with 20 wt% WO3 loading
and H-ZSM-5(90) (Fig. 4). This suggests that the coke deposited
at active Brønsted acid sites can be removed by oxidative
treatment, and WO3/Al2O3 catalyst can be repeatedly used for
the dehydration reaction.
Fig. 5 Dehydration of glycerol to acrolein over WO3/Al2O3 catalysts
with 20 wt% WO3 loading and under flowing N2 (circle) and O2
(square). Conditions: catalyst (100 mg), WHSV by glycerol (4.7 hꢀ1), T ¼
588 K.
Additionally, the dehydration of glycerol over WO3/Al2O3
catalyst with 20 wt% WO3 loading (Fig. 5) and H-ZSM-5
(Fig. S4†) under owing N2 and O2 gas was investigated. The
reaction under O2 ow with both WO3/Al2O3 and H-ZSM-5
catalysts showed constant acrolein yields of around 95% for
5 h, indicating that the reaction under an O2 atmosphere was
effective for protecting the catalysts from deactivation. Note that
no excess oxidised products (such as acrylic acid) were obtained
in this reaction condition.
contribute to deactivation, we evaluated the BET surface area of
various solid acid catalysts before and aer the reaction for 5 h
as well as the amount of deposited coke (Table 1). Although
coke was deposited on all of the catalyst surfaces, no remark-
able relationship was observed between the amount of depos-
ited coke and deactivation rates of catalysts. The BET surface
areas of zeolite catalysts were drastically decreased aer reac-
tion. On the other hand, the decreases in the surface area of
supported WO3 catalysts were much smaller than those of
zeolite catalysts. Particularly, WO3/Al2O3 catalyst maintained
Table 2 shows the effect of the feed gas on the physical
properties of WO3/Al2O3 and H-ZSM-5 catalysts aer the cata-
lytic run. The amount of deposited coke in the reaction under
Table 2 Physical properties of WO3/Al2O3 with 20 wt% WO3 loading and H-ZSM-5(90) catalysts after reaction under flowing N2 and O2
BET surface area/m2 gcat
Acidityb/mmol gcat
Before reaction Aer reactionc Before reaction Aer reactionc
ꢀ1
ꢀ1
Carrier gas Amount of deposited cokea/mg gcat
ꢀ1
Catalyst
WO3/Al2O3 (WO3: 20 wt%) N2
O2
131
221
138
170
109
460
104
69
116
131
296
158
104
297
48
H-ZSM-5 (90)
N2
O2
30
a
Estimated by TG analysis. b Estimated by NH3-TPD. c Reaction aer 5 h.
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Fig. 6 Pore size distribution of the (A) micropore and (B) mesopore regions in WO3/Al2O3 with 20 wt% WO3 loading and (C) micropore and (D)
mesopore regions in H-ZSM-5(90). Fresh catalyst (circle), after the reaction with N2 (triangle) and O2 (square) for 5 h.
O2 was greater than that under N2 over both catalysts. The acidity because coke deposition on the catalyst caused quite low
acidity of catalysts aer the reaction with owing N2 and O2 was IR transmittance. No difference in the acidity of the WO3/Al2O3
also estimated by NH3-TPD (Fig. S6†). Note that pyridine catalyst between before and aer the reaction was observed
adsorbed FT-IR measurement could not be used to estimate the regardless of the reaction atmosphere. In contrast, the area of
Fig. 7 Dehydration of various polyols over WO3/Al2O3 catalysts with 20 wt% WO3 loading. Conditions: catalyst (100 mg), WHSV by substrate
(4.7 hꢀ1), T ¼ 588 K. (A) Ethylene glycol, (B) 1,2-propanediol, (C) 1,2-butanediol, (D) 1,2-pentanediol, (E) 1,3-butanediol and (F) 1,4-butanediol.
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NH3 desorption peak at 673 K was drastically decreased, indi-
cating that the number of strong acid sites of H-ZSM-5 was
decreased by the reaction. (Fig. S6(B)).† The micropore and
mesopore distributions of WO3/Al2O3 and H-ZSM-5 catalysts are
shown in Fig. 6. No micropores were observed in the WO3/Al2O3
catalyst. Furthermore, no signicant change in pore size
distribution aer the reaction under both N2 and O2 was
observed. On the other hand, H-ZSM-5 showed large number of
micropores and mesopores. The catalytic runs drastically
decreased the number of micropores in H-ZSM-5. These results
suggest that coke deposition at the entrance of the H-ZSM-5
pore reduced the ability of glycerol to access the acid sites
inside the micropores. Based on these results, we can conclude
that the high activity and durability of WO3/Al2O3 are respon-
sible for the fact that the mesoporous nature of WO3/Al2O3 was
not prevented by coke formation at the entrance of pores and
Brønsted acid sites.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This study was partially supported by the Program for Elements
Strategy Initiative for Catalysts & Batteries (ESICB) (Grant
JPMXP0112101003). This work was also supported in part by
Grants-in-Aid for Scientic Research (B) (Grant 17H03459) and
Scientic Research on Innovative Areas (Grant 17H06443)
commissioned by MEXT of Japan. The XAFS experiments at
SPring-8 were conducted with the approval (No. 2019A1717) of
the Japan Synchrotron Radiation Research Institute (JASRI).
Notes and references
The dehydration of polyols other than glycerol can also
provide useful chemicals. Therefore, the dehydration of various
polyols over WO3/Al2O3 catalyst with 20 wt% WO3 loading was
investigated (Fig. 7). The reactions of polyols gave the corre-
sponding aldehydes, dienes, ketones and ethers. In particular,
the dehydration of polyols with hydroxyl groups at the 1,2-
position such as glycerol, ethylene glycol, 1,2-propanediol, 1,2-
butanediol and 1,2-pentanediol afforded the aldehydes (e.g.
acrolein, acetaldehyde, propionaldehyde, butyraldehyde and
valeraldehyde) as main products. On the other hand, the reac-
tion of 1,3-butanediol mainly produced 1,3-butadiene through
double dehydration. In contrast, the dehydration of 1,4-buta-
nediol gave tetrahydrofuran as a sole product. These results
indicate that WO3/Al2O3 catalyst with 20 wt% WO3 loading
shows high activity and durability for the dehydration of various
polyols.
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