Effect of Nb doping in WO3/ZrO2 catalysts on gas phase dehydration of glycerol to form acrolein

Article abstract of DOI:10.1039/c7ra08154e

Gas-phase dehydration of glycerol to produce acrolein was investigated over tungstated zirconia catalysts with and without niobium doping, in order to correlate the catalytic activity and life-stability with the acid/base properties of the catalysts. The results of NH₃- and CO₂-TPD experiments showed that the tungstated zirconia catalysts contained both acidic and basic sites, and the amounts decreased with the increase in calcination temperature. The most suitable calcination temperature for 20% WO₃/ZrO₂ as the catalyst in the gas phase dehydration of glycerol to form acrolein was 450 °C in consideration of achieving high acrolein yield and long lifetime of the catalyst. Not only the amount of acid but also the amount of base affected the acrolein yield. Doping a small amount of niobium further increased the life-stability of the catalysts, attributed to the decrease in the number of basic sites. The influence of Nb is mostly in neutralization of surface acidity electronically instead of geometric blocking of the basic sites. Moreover, fine tuning the ratio of acid/base sites was found to be important in achieving good catalytic performance in the gas-phase dehydration of glycerol to generate acrolein. The ratio of acid/base sites in the range of 4-5.4 gave the highest glycerol conversion, and acrolein yield and lowest catalyst decay rate. The spent catalysts were easily regenerated by calcination at 450 °C, and the catalytic activities were retained well even after the second time of reuse.

Full text of DOI:10.1039/c7ra08154e

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Effect of Nb doping in WO3/ZrO2 catalysts on gas
phase dehydration of glycerol to form acrolein†
Cite this: RSC Adv., 2017, 7, 41880
*
Ku-Hsiang Sung and Soofin Cheng
Gas-phase dehydration of glycerol to produce acrolein was investigated over tungstated zirconia catalysts
with and without niobium doping, in order to correlate the catalytic activity and life-stability with the acid/
base properties of the catalysts. The results of NH3- and CO2-TPD experiments showed that the tungstated
zirconia catalysts contained both acidic and basic sites, and the amounts decreased with the increase in
calcination temperature. The most suitable calcination temperature for 20% WO3/ZrO2 as the catalyst in
the gas phase dehydration of glycerol to form acrolein was 450 ꢀC in consideration of achieving high
acrolein yield and long lifetime of the catalyst. Not only the amount of acid but also the amount of base
affected the acrolein yield. Doping a small amount of niobium further increased the life-stability of the
catalysts, attributed to the decrease in the number of basic sites. The influence of Nb is mostly in
neutralization of surface acidity electronically instead of geometric blocking of the basic sites. Moreover,
fine tuning the ratio of acid/base sites was found to be important in achieving good catalytic
performance in the gas-phase dehydration of glycerol to generate acrolein. The ratio of acid/base sites
in the range of 4–5.4 gave the highest glycerol conversion, and acrolein yield and lowest catalyst decay
rate. The spent catalysts were easily regenerated by calcination at 450 ꢀC, and the catalytic activities
were retained well even after the second time of reuse.
Received 24th July 2017
Accepted 22nd August 2017
DOI: 10.1039/c7ra08154e
rsc.li/rsc-advances
Dehydration of glycerol to form acrolein has been exercised
since 1930, Schering-Kahlbaum claimed the rst patent of
1. Introduction
acrolein production from glycerol in gas-phase over lithium
phosphate supported on pumice stone.4 The rate of acrolein
decomposition was found to be faster than that of acrolein
formation in the absence of acid catalyst. Since then, sulfuric
acid was used as the catalyst in liquid phase dehydration of
glycerol.5,6 However, corrosion hazard and difficulties in
handling and product separation are the drawbacks. Although
solid acid catalysts such as clay and alumina supported phos-
phoric acid7,8 and heteropolyacids9,10 were effective as the
dehydration catalysts in liquid phase, it is still hazardous to
operate the reaction under supercritical water pressure.
Gas-phase dehydration of glycerol to produce acrolein has
been promoted by solid acids. Dubois et al.11,12 studied the cata-
lytic performances of various solid acids, including zeolites,
Naon and WO3/ZrO2, in gas-phase dehydration of glycerol, and
concluded that solid acids with Hammett acidities (HAs) in the
range of ꢁ10 to ꢁ16 were suitable catalysts. They also reported
for the rst time that WO3/ZrO2 had high activity in catalyzing
gas-phase dehydration of glycerol to produce acrolein. In 2007,
Chai et al.13 examined a large group of solid materials including
zeolites, SAPOs, metal oxides, supported phosphoric and heter-
opoly acids, and sulfated and tungstated zirconia, in catalyzing
gas-phase dehydration of glycerol. They concluded that solids
with HAs in the range of ꢁ3 to ꢁ8.2 were most active catalysts in
generating acrolein. Among the solids, 15 wt% WO3/ZrO2 showed
To decrease the global demand for fossil fuels and reduce CO2
emission, there has been great interest in producing fuels and
chemicals from renewable feedstock. Biodiesel is considered
one of the best choices among several alternative fuels because
of its CO2-neutral production and slight impact on engine
performance.1 Biodiesel is currently synthesized by base-
catalyzed transesterication of triglyceride with short chain
alcohols on a large scale, accompanied by the formation of
10 wt% of glycerol as byproduct.2 Although political regulations
have pushed the production of biofuels in order to fulll the
CO2 reduction objectives xed by the Kyoto climate protocol
and lately the Paris Agreement, it is not yet possible to produce
any of these biofuels as economical competitors to fossil fuels.
One of the possible ways to reduce the production cost of bio-
diesel is using glycerol to produce value-added chemicals.
When considering a commercial target, the catalytic dehydra-
tion of glycerol to acrolein is the most promising route for
valorizing glycerol because acrolein has many applications in
industrial processes, such as the precursors of acrylic acid and
biocides.3
Department of Chemistry, National Taiwan University, Roosevelt Rd. Sec. 4, Taipei
106, Taiwan. E-mail: chem1031@ntu.edu.tw; Fax: +886-2-33668671
† Electronic supplementary information (ESI) available: TPD-NH3 and CO2, TG
proles and catalyst reusability. See DOI: 10.1039/c7ra08154e
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the highest glycerol conversion and acrolein selectivity. Never- was carried out in a Micromeritics TriStar 3000 system. Prior to
theless, deactivation of the catalysts with time-on-stream (TOS) the physisorption, the sample was degassed at 200 ꢀC under
was the main concerns. In 2011, Ulgen et al.14 reported the use of 10ꢁ3 torr for at least 8 h. Acidity and basicity of the catalysts
WO3/TiO2 instead of WO3/ZrO2 as the catalyst in gas-phase were determined by temperature-programmed desorption of
dehydration of glycerol, and they found that co-feeding of NH3 and CO2, respectively, using an AutoChem 2910 system
oxygen could reduce deactivation rate of the catalyst.
with a TCD detector. About 0.25 gram of the sample was heated
Niobium based catalysts have been tested in dehydration of in a 50 mL minꢁ1 of He ow at calcination temperature for 1 h
glycerol to form acrolein.15–20 Chai et al.15 reported that the to remove the adsorbed moisture. Then, 5% ammonia in He or
niobia calcined at 400 C gave the highest glycerol conversion pureꢀCO2 gas (50 mL minꢁ1) was passed through the sample at
ꢀ
and acrolein selectivity. Shiju et al.16 studied silica supported 100 C for 1 h, followed by purging the sample with pure He
niobia catalysts and found that the glycerol conversion and (30 mL minꢁ1) for another hour. The TCD signals of the effluent
acrolein selectivity depended on the niobia loading and calci- were recorded when the sample was heated from 100 to 600 ꢀC
nation temperature. However, in both aforementioned works, with a ramping rate of 10 ꢀC minꢁ1. FTIR spectra were con-
the niobia catalysts showed glycerol conversions decreased ducted using a Nicolet Magna FT-IR 550 spectrometer. The
rapidly with TOS. Lauriol-Garbey et al.19,20 used zirconium and resolution of the recorded spectra is 4 cmꢁ1. 25 mg of the
niobium mixed oxides (ZrNbO) as catalysts for glycerol dehy- sample was pressed into a disk and placed inside the cell. Aer
ꢁ4
ꢀ
dration. The ZrNbO catalysts gave the recorded high conversion heated at 450 C under 10 torr for 1 h and cooled down to
of 82% even aer 177 h on stream with acrolein selectivity room temperature, the sample was exposed to pyridine vapor
remaining 72%. However, these results have not been repeated for 30 min and evacuated to 10ꢁ4 torr for another 30 min to
by other research groups. Lately, the same group reported SiO2- remove pyridine vapor in the gas phase. The IR spectra were
doped WO3/ZrO2 as catalysts in dehydration of glycerol.21 recorded under 10ꢁ4 torr while the temperature was raised to
However, the catalytic lifetime was much shorter than that of 400 ꢀC at 100 ꢀC intervals. Scanning electron microscopy (SEM)
ZrNbO. Recently, niobium species supported on a zirconium photographs were taken using a Hitachi S-4800 Field Emission
doped porous silica were used as the catalysts, but acrolein Scanning Electron Microscope.
selectivity of 45 mol% was obtained aer 2 h TOS.22 In the
present study, we disclosed a novel nding that doping a small
amount of niobium to WO3/ZrO2 catalysts would increase the
life-stability of the catalysts. Moreover, relatively high acrolein
selectivities (ca. 70%) could be retained. Efforts were put in
correlation of the active sites with the decay rate of the catalysts.
2.3 Catalytic reaction
Gas-phase dehydration of glycerol was carried out at 290 ꢀC
under ambient pressure in a vertical xed-bed tubular quartz
reactor. The catalyst (300 mg) in ne powders was mixed with
quartz (300 mg) and then sandwiched in the middle of the
reactor with quartz wool. Prior to the reaction, the catalyst was
pretreated at 290 ꢀC for 1 h under owing dry nitrogen
(60 mL minꢁ1). An aqueous solution of 10 wt% glycerol was fed
2. Experimental methods
2.1 Catalyst synthesis
into the reactor by a syringe pump with a ow rate of 8 mL hꢁ1
.
Tungstated zirconia was prepared by wet impregnation method.
For the WO3/ZrO2 catalyst of 20% weight ratio, a solution con-
taining 0.2656 g (NH4)6H2W12O40$xH2O dissolved in 5 mL
deionized water was added dropwise onto 1.2926 g Zr(OH)4
powders under static condition, and the mixture was le steady
The products were collected hourly by an ice-water trap and
analyzed by a gas-chromatography (GC) equipped with a capil-
lary column (BP-20) and a FID detector. The glycerol conversion
and selectivities of identied products were calculated accord-
ing to the following equations:
for 1 h before the solvent was removed by rotary evaporation.23,24
ꢀ
Aer drying at 100 C overnight, the powders were calcined in Conversion (%) ¼ (initial mol ꢁ final mol) of glycerol/
ꢀ
air at temperatures in the range of 400 to 800 C for 3 h with
initial mol of glycerol ꢂ 100%
(1)
a ramping rate of 1 C minꢁ1. The niobium doped tungstated
ꢀ
Selectivity (%) ¼ (specific product in mol)/
(initial mol ꢁ final mol) of glycerol ꢂ 100% (2)
zirconia catalysts were prepared by the procedures similar to the
foregoing method, except proper amounts of niobium oxalate
were added in the solution of ammonium metatungstate
hydrate for impregnation on Zr(OH)4 powders. Aer drying at
100 ꢀC overnight, the powders were calcined at 450 ꢀC for 3 h in
air. The resultant materials were designated as X-NbWOx/ZrO2-
450, where the WO3/ZrO2 weight ratios were kept at 20%, and X
varied in 1, 3, and 5% is the Nb2O5/(WO3 + ZrO2) weight ratio.
3. Results and discussion
3.1 Effect of calcination on WO3/ZrO2 catalysts
3.1.1 Characterization. XRD patterns of the WO3/ZrO2
catalysts show that calcined at 400 ꢀC to be amorphous (Fig. 1).
As the calcination temperature increases, the peaks of the
tetragonal ZrO2 (2q ¼ 30.3, 34.4, 50.3 and 60.0ꢀ) start to show
2.2 Catalyst characterization
Powder X-ray diffraction (PXRD) patterns were recorded with and grow with temperature. At calcination temperature of
a Panalytical X'Pert diffractometer using Cu Ka radiation in an 800 ꢀC, monoclinic ZrO2 (characteristic peaks at 2q ¼ 28.2 and
operating mode of 45 kV and 40 mA. Nitrogen physisorption 31.5ꢀ) appears. Meanwhile, the monoclinic WO3 phase (2q ¼
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Fig. S1(A) (ESI†) shows that the total acid amounts of the
WO3/ZrO2 catalysts measured by NH3-TPD decrease with calci-
nation temperature, and the desorption peaks appear mainly in
ꢀ
the range of 150–500 C. These results are in consistence with
that reported by Garcıa-Sancho et al.25 that the acidic strengths of
´
the WO3/ZrO2 catalysts were in the range of weak to moderate.
ꢀ
ꢀ
However, it is noticed that the sample calcined at 400 C have
additional desorption peaks at temperatures above 400 C. The
high temperature peak decreases in intensity and shis to
temperatures above 470 ꢀC for the sample calcined at 450 ꢀC.
Fig. S1(B) (ESI†) compares the base amounts of WO3/ZrO2
catalysts calcined at different temperatures measured by CO2-TPD
experiment. Again, the total base amounts of the WO3/ZrO2
catalysts decrease with the calcination temperꢀature. Three peak
maxima can be seen at ca. 190, 380 and 530 C on the sample
calcined at 400 ꢀC. The last high temperature peak is hardly seen
on the sample calcined at 450 ꢀC. In combination of the CO2-TPD
and NH3-TPD results, it can be concluded that most of the surface
hydroxyl groups condensate in the temperature range of 400–
450 ꢀC. Moreover, the high temperature peak observed on the
ꢀ
NH3-TPD prole of WO3/ZrO2 calcined at 450 C should not be
Fig. 1 XRD patterns for WO3/ZrO2 catalysts calcined at: (a) 400, (b)
450, (c) 500, (d) 600, (e) 700 and (f) 800 ꢀC, in comparison to the
JCPDS patterns of tetragonal and monoclinic ZrO2 and monoclinic
WO3.
due to condensation of surface hydroxyl groups. Instead, it
corresponds to some strong acidic sites present on the sample.
Table 2 summarizes the acid and base densities based on the
desorption peak areas of NH3-TPD and CO2-TPD, respectively,
of WO3/ZrO2 calcined at different temperatures. It is noticed
that those calcined at 400–600 ꢀC have acid densities in a close
range of 0.145–0.165 sites per nm2, while those calcined at 700
and 800 ꢀC have slightly lower and higher acid density of 0.130
and 0.183 sites per nm2, respectively. On the other hand, base
densities decrease almost linearly with the increase of calcina-
tion temperature. As the surface area decreases with calcination
temperature, Fig. 2 shows that the acid and base densities as
a function of surface area varied in the reverse trend. Base
density increases with surface area, while acid density decreases
with surface area but merely in a narrow range. These results
infer that the amount of basic sites increases more abruptly
than that of acid sites with the increase of surface area, and the
latter is almost linearly proportional to the surface area.
Table
1 Textural properties of WO3/ZrO2 catalysts calcined at
different temperatures and those doped with Nb
SBET
Pore
Pore volume
Sample
(m2 gꢁ1
)
diametera (nm)
(cm3 gꢁ1
)
WO3/ZrO2-400
WO3/ZrO2-450
WO3/ZrO2-500
WO3/ZrO2-600
WO3/ZrO2-700
WO3/ZrO2-800
1% NbWOx/ZrO2-450
3% NbWOx/ZrO2-450
5% NbWOx/ZrO2-450
254
223
189
138
134
62
233
236
242
3.7
4.4
5.1
5.3
6.3
12
5.0
5.3
5.6
0.241
0.222
0.248
0.221
0.239
0.224
0.278
0.302
0.302
a
Determined by BJH method at peak maximum of pore size
3.1.2 Catalytic performance of WO3/ZrO2 calcined at
different temperatures. Fig. 3 shows the results of gas phase
dehydration of glycerol catalyzed by WO3/ZrO2 calcined at
different temperatures. The glycerol conversions are all nearly
100% at the beginning, and then they decay with time on stream.
Moreover, the decay rate is faster for the catalyst calcined at
higher temperatures (Fig. 3A). On the other hand, the acrolein
selectivities vary in similar trends with time-on-stream over all
WO3/ZrO2 catalysts calcined at different temperatures (Fig. 3B).
The acrolein selectivities increase in the rst two hours and
become steady with time on stream. The low acrolein selectivities
in the rst hour may be due to that the system is not in dynamic
stability. Other researchers also reported lower than 90% calcu-
lated carbon balance in the initial reaction time.14,26 The
conversions and products distribution in 3–4 h and those in 11–
12 h are tabulated in Table 3. Acrolein is the major product, while
hydroxylacetone (HA), acetaldehyde (AD), and allyl alcohol (AA)
are minor products. Moreover, there are ca. 20 mol% of carbon
distribution prole.
23.2, 23.6 and 24.4ꢀ) is also detected, suggesting that the crys-
talline WO3 is formed and dispersed on the ZrO2 support.
Nitrogen sorption isotherms of the calcined WO3/ZrO2
catalysts show type IV isotherms with the hysteresis loops
appeared at p/p0 of 0.5–0.9, implying that these materials own
mesopores. The derived textural properties are given in Table 1.
A decrease in surface area and increase in pore diameter are
observed with increasing the calcination temperature, and both
are originated from sintering of the metal oxide particles at high
temperatures.
NH3- and CO2-TPD experiments were used to measure the
surface concentrations of acidic and basic sites, respectively.
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Table 2 Results of NH3-TPD and CO2-TPD measurements on WO3/ZrO2 catalysts calcined at different temperatures
Acid amount
Acid density
Base amount
Base density
(nmꢁ2
Decay rateb
(%/h)
Catalyst
(1018 gꢁ1
)
(nmꢁ2
)
(1018 gꢁ1
)
)
WO3/ZrO2-400
WO3/ZrO2-450
WO3/ZrO2-500
WO3/ZrO2-600
WO3/ZrO2-700
WO3/ZrO2-800
1% NbWOx/ZrO2-450
3% NbWOx/ZrO2-450
5% NbWOx/ZrO2-450
36.8
0.145
15.1
11.4
8.10
6.81
5.20
1.73
8.46
7.84
7.57
0.0595
0.0510
0.0430
0.0494
0.0388
0.0279
0.0363
0.0332
0.0313
ꢁ0.32c
32.6 (2.9)a
31.2
0.146 (0.013)a
0.165
ꢁ0.39c/ꢁ1.37e
ꢁ0.70d
22.8
17.4
11.3
30.3 (3.3)
33.7 (3.3)
34.8 (5.3)
0.165
0.130
0.183
0.130 (0.014)
0.143 (0.014)
0.144 (0.022)
ꢁ2.00d
ꢁ7.50d
ꢁ9.50d
ꢁ0.37e
ꢁ0.14e
ꢁ0.63e
Values in parenthesis are strong acid sites (>500 ꢀC on NH3-TPD proles). b From the slopes of glycerol conversions vs. TOS based on TOS. c From
a
the slopes of glycerol conversions vs. TOS based on TOS in 3–9 h. d From the slopes of glycerol conversions vs. TOS based on TOS 3–6 h in Fig. 3A.
e
From the slopes of glycerol conversions vs. TOS based on TOS 7–12 h in Fig. 5A.
Dalil's group has conrmed the coke formation at initial
stage of gas phase glycerol dehydration, and the amount of coke
deposited on WO3/TiO2 catalysts was found to increase with
TOS.27–29 Since the selectivities of Unknowns over our catalysts
are quite steady as a function of TOS (3–4 h vs. 11–12 h in Table
3). It is also consistent with the nding by Dalil's group.
In comparison of WO3/ZrO2 calcined at different tempera-
tures, those calcined at 450–700 ꢀC give relatively higher acrolein
selectivities of 68–71% (Fig. 1 and Table 3). Moreover, acrolein
selectivities retain almost unchanged with TOS for catalysts
calcined at 450–600 ꢀC. For the catalyst calcined at lowest
Fig. 2 Correlation of acid/base densities with the surface areas of (A)
WO3/ZrO2 calcined at different temperatures, and (B) WO3/ZrO2-450
doped with various amounts of Nb; ( ) total acid, ( ) weak acid, ( ) base.
temperature, WO3/ZrO2-400, which contains the largest amounts
of both acidic and basic sites, is most stable on TOS. However,
the acrolein selectivity is among the lowest. In contrast, WO3/
ZrO2 calcined at highest temperatures, WO3/ZrO2-700 and -800,
imbalance, which are shown as “Unknowns”. Since all the used
catalysts turn black, it is assumed that the Unknowns are coke
and polymeric materials.
have very fast decay rates and larger amounts of coke formation.
Judging by the stabilities of the catalysts and high acrolein
selectivities, the most suitable calcination temperature for WO3/
Fig. 3 Evolution of (A) glycerol conversion and (B) acrolein selectivity as a function of time on stream at 290 ꢀC for the WO3/ZrO2 catalysts
calcined at different temperatures. 0.3 g catalyst, GHSV ¼ 1117 hꢁ1, 60 mL minꢁ1
N2 flow rate.
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Table 3 Product distributions in glycerol dehydration over WO3/ZrO2 catalysts calcined at different temperaturesa
Product selectivity (%)
Catalysts
Conversion (%)
Acrolein
AD
AA
HA
Unknowns
WO3/ZrO2-400
WO3/ZrO2-450
WO3/ZrO2-500
WO3/ZrO2-600
WO3/ZrO2-700
WO3/ZrO2-800
99 (95)
99 (92)
99 (79)
99 (47)
97 (17)
87
62 (57)
68 (70)
71 (71)
70 (68)
69 (65)
63
6 (4)
6 (4)
6 (5)
4 (3)
2 (1)
1
2 (1)
1 (2)
2 (3)
2 (3)
2 (3)
1
9 (18)
9 (12)
4 (9)
7 (8)
4 (6)
6
21 (20)
16 (12)
17 (12)
17 (18)
23 (25)
29
a
Reaction conditions: 290 ꢀC, 0.3 g catalyst, GHSV ¼ 1117 hꢁ1, 60 mL minꢁ1 N2 ow rate. AD ¼ acetaldehyde; AA ¼ allyl alcohol; HA ¼ 1-
hydroxylacetone. Selectivity for unknowns ¼ (100 ꢁ known products)%. The selectivity data were obtained at TOS ¼ 3–4 h, and those in
parenthesis obtained at TOS ¼ 11–12 h of reaction.
ZrO2 catalyst is 450 ꢀC. Therefore, the catalysts studied hereaer that Nb would neutralize the basic sites present on the zirconia
were calcined at 450 ꢀC.
support.15 However, the change in base amounts is less signi-
cant as the doping amount of Nb is further increased. The
phenomenon can be explained by that the inuence of Nb is
mostly in neutralization of surface acidity electronically instead
of geometrically blocking the basic sites. Moreover, Fig. S3(B)†
shows that the basic sites of strongest strength with the CO2
desorption peak appeared above 400 ꢀC are suppressed
completely, while those of weakest strength with the desorption
peak appeared at 100–330 ꢀC are partially suppressed and those
around 380 ꢀC are hardly affected by Nb-doping.
The acid and base densities estimated from the desorption
peak areas of NH3-TPD and CO2-TPD of Nb-doped WO3/ZrO2-
450 samples are also summarized in Table 2. The acid densities
are almost unchanged (0.143–0.146 nmꢁ2), except the one
doped with 1% Nb2O5 has lower value of 0.130 nmꢁ2, while the
base density decreases from 0.0510 nmꢁ2 to 0.0313–0.0363
nmꢁ2 with Nb doping. Fig. 2B plots the acid/base densities on
Nb-doped WO3/ZrO2-450 as a function of surface area. It clearly
shows that the acid densities are constrained in a narrow range
while the base densities decrease with the increase in surface
areas.
The nature of the acidic sites was examined by taking the FT-IR
spectra of the catalysts aer pyridine adsorption. Fig. 4 compares
the FT-IR spectra of WO3/ZrO2-450 and that doped with 3%
Nb2O5. At the desorption temperature of 100 ꢀC, both samples
show a strong peak at 1445 cmꢁ1 attributing to pyridine adsorbed
on the Lewis acid sites and a weak peak at 1545 cmꢁ1 attributing
to pyridine adsorbed on the Brønsted acid sites. In addition, the
peaks at 1575 and 1610 cmꢁ1 correspond to pyridine adsorbed on
the weak and strong Lewis acid sites, respectively. At the desorp-
tion temperature of 300 ꢀC, the 1545 cmꢁ1 peak disappears
completely while the 1445 and 1610 cmꢁ1 peaks still retain some
intensity for both samples. These results imply that both Nb2O5-
doped and undoped WO3/ZrO2-450 materials contain mainly the
Lewis acidic sites, and the strength of Lewis acidic sites is stronger
than that of Brønsted acid sites. Moreover, 3% NbWO3/ZrO2-450
contains slightly larger amount of strong Lewis acidic sites than
WO3/ZrO2-450, judging by the peak intensities of the FT-IR spectra
taken at desorption temperatures of 200–300 ꢀC.
3.2 WO3/ZrO2 catalysts doping with niobium
3.2.1 Characterization. The WO3/ZrO2 catalysts doping with
1–5% Nb2O5 show similar XRD patterns and nitrogen sorption
isotherms as those of WO3/ZrO2-450. However, the surface area,
pore diameter, and pore volume of the Nb-doped catalysts
increase slightly in comparison to those of pristine WO3/ZrO2-450
(Table 1). These results infer that the incorporation of Nb(V) in
WO3/ZrO2 probably helps the dispersion of WO3 on ZrO2.
Fig. S2 (ESI†) shows the SEM photographs of WO3/ZrO2-450
catalyst and those doped with 1% and 3% Nb2O5. All of them are
aggregates of nanoparticles ca. 10 nm in diameter and hard to
estimate the individual oxide particle size. It is apparent that
the doping of 1–3 wt% Nb2O5 does not cause the drastic change
of the morphology of WO3/ZrO2 oxides, and no particles of other
morphologies can be seen. This result implies that Nb is
probably distributed homogeneously in the oxides.
Fig. S3 (ESI†) shows the NH3- and CO2-TPD proles of
WO3/ZrO2-450 catalysts doped with different amounts of Nb2O5,
in comparison with those of pristine WO3/ZrO2-450. Fig. S3(A)
(ESI†) shows that all the samples have similar NH3 desorption
ꢀ
proles: a broad band covering from 100 to 450 C and a weak
peak appeared above 450 ꢀC. The possibility that the high
temperature peaks are due to the condensation of surface
hydroxyl groups can be excluded because they do not show in the
CO2-TPD proles (Fig. S3(B), ESI†). Hence, the presence of the
small high temperature peaks in NH3-TPD proles implies that
the materials contain a small amount of strong acidic sites,
probably attributed to formation of Nb2O5 clusters. Nevertheless,
the majority of the acidic sites is in the weak to medium strength
region. The intensities of the low temperature peaks decrease
when only 1 wt% Nb2O5 is doped, and contrarily increase when it
is further increased to 3 and 5% (Fig. S3(A), ESI†). However, the
positions of desorption maxima retain at almost the same
ꢀ
temperature around 200 C, implying no signicant change in
acid strength of the catalysts with niobium doping.
The CO2-TPD proles in Fig. S3(B) (ESI†) show that the base
amounts decrease markedly when as low as 1% Nb2O5 is doped
on WO3/ZrO2-450. The decrease in base amount is attributed to
3.2.2 Catalytic performance of Nb-doped WO3/ZrO2. Fig. 5
shows the results of dehydration of glycerol catalyzed by Nb-
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Fig. 4 FT-IR spectra of (A) WO3/ZrO2-450, (B) 3% NbWOx/ZrO2-450 after pyridine adsorption at various evacuation temperatures: (a) 100 ꢀC (b)
200 ꢀC (c) 300 ꢀC (d) 400 ꢀC.
Fig. 5 Evolution of (A) glycerol conversion and (B) acrolein selectivity as a function of time on stream at 290 ꢀC for the WO3/ZrO2-450 and
NbWOx/ZrO2-450 catalysts with different Nb2O5 weight ratios to (WO3 + ZrO2). 0.3 g catalyst, GHSV ¼ 1117 hꢁ1, 60 mL minꢁ1
N2 flow rate.
doped WO3/ZrO2-450, in comparison to that of pristine WO3/ZrO2-
450. The decay in glycerol conversion upon TOS is markedly
slowed down over the Nb-doped catalysts. Table 4 compares the
conversions and product distributions in 3–4 h and those in 23–
24 h. All the catalysts give nearly complete conversion in 3–4 h
TOS, but that drastic decreases to 28% aer 23–24 h TOS is seen
over pristine WO3/ZrO2-450. In contrast, all the Nb-doped cata-
lysts can retain relatively high conversions of 74–81% aer 23–
24 h TOS. The catalytic stabilities vary with the doping amounts of
Nb2O5 in the order of 5% < 1% < 3%. Acrolein is the major
product, and the acrolein selectivity is not changed with Nb-
doping or TOS. It retains around 70% over all the catalysts. HA,
AD, and AA are obtained as the minor products and their selec-
tivities are not affected by Nb-doping, while the Unknowns
decrease slightly with Nb-doping. The optimal catalytic perfor-
mance was obtained over 3% NbWOx/ZrO2-450 with 81% glycerol
conversion and 72% acrolein selectivity at 24 h TOS.
3.3 Reusability of catalyst
The reusability of 3% NbWOx/ZrO2-450 catalyst was examined.
The spent catalyst aer reaction at 290 ꢀC for 24 h was regenerated
by calcination at 450 ꢀC for 3 h to burn the coke disposed on the
surface of the catalyst. Fig. S4 (ESI†) showed the TG prole of the
used 3% NbWOx/ZrO2-450 catalyst obtained in air. The coke
deposition on the used catalyst is about 19 wt%. Moreover, the
prole indicates that 17 wt% is burned away at temperature below
450 ꢀC and only 2 wt% coke is still remained aer calcination at
450 ꢀC. Fig. S5 (ESI†) compares the catalytic performances of the
fresh and regenerated 3% NbWOx/ZrO2-450 catalysts. The results
showed that the glycerol conversion and selectivity to acrolein are
well retained even the catalyst is regenerated and reused for
second time. The similar variation patterns in glycerol conversions
and acrolein selectivities as a function of TOS provide a solid
evidence that the initial low acrolein selectivities are due to that
the system is not in dynamic stability, instead of rapid coke
formation as proposed by Dalil et al.29 These results also infer that
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Table 4 Product distributions in glycerol dehydration over WO3/ZrO2-450 with and without Nb2O5 dopinga
Product selectivity (%)
Catalysts
Conversion (%)
Acrolein
AD
AA
HA
Unknowns
WO3/ZrO2-450
99 (28)
99 (77)
99 (81)
99 (74)
68 (70)
69 (70)
69 (72)
71 (70)
6 (6)
8 (5)
7 (5)
8 (5)
1 (3)
2 (2)
1 (2)
2 (3)
9 (15)
11 (14)
9 (15)
16 (6)
10 (9)
14 (6)
8 (5)
1% NbWOx/ZrO2-450
3% NbWOx/ZrO2-450
5% NbWOx/ZrO2-450
11 (17)
a
Reaction conditions: 290 ꢀC, 0.3 g catalyst, GHSV ¼ 1117 hꢁ1, 60 mL minꢁ1 N2 ow rate. AD ¼ acetaldehyde; AA ¼ allyl alcohol; HA ¼ 1-
hydroxylacetone; selectivity for unknowns ¼ (100 ꢁ known products)%. The selectivity data were data obtained at TOS ¼ 3–4 h, and those in
parenthesis obtained at TOS ¼ 23–24 h of reaction.
2 wt% coke deposition on the 3% NbWOx/ZrO2-450 catalyst does main product.31 Kinage et al.32 studies Na-doped metal oxides as
not signicantly vary the acid/base active sites and has negligible the catalysts for glycerol conversion and proposed that the basic
inuence on the catalyst activity.
sites were responsible for HA formation. Chai et al.13 suggested
that the strong acidic sites of Hammett constant in ꢁ8.2 to ꢁ3.0
were more efficient than those of medium to weak acid sites in
acrolein formation. Besides, Lewis acids such as Al2O3 and
Nb2O5 showed lower acrolein selectivities (ca. 40 mol%) than
the materials containing Brønsted acids, such as supported
phosphoric acid and heteropoly acid (60–70 mol%). On the
other hand, the polyglycerols or acetalization products of glyc-
erol were considered to form and be adsorbed on basic sites.33,34
In our case, the glycerol consumption rate increases almost
linearly with both amounts of acid and base sites, based on the
3.4 Catalytic active sites
Zirconia is considered to be one of the amphoteric oxides and
contains both acidic and basic sites on its surface. Addition of
WO3 to zirconia was found to signicantly decrease the amount
of basic sites and slightly increase the acidic sites.30 When used
as the catalysts in gas-phase dehydration of glycerol, ZrO2 gave
complicated products with acetol (also known as hydrox-
ylacetone, HA) as the largest portion of products (25% selec-
tivity), while WO3/ZrO2 gave acrolein of 70% selectivity as the
Fig. 6 Correlations of (A) specific glycerol consumption rates, (B) acrolein yields, and (C) decay rates (average in 3–12 h TOS) with total acidic/
basic sites on WO3/ZrO2 calcined at 400 to 700 ꢀC (acid -; base ,), or NbWOx/ZrO2 catalysts (acid +; base *).
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catalytic results of WO3/ZrO2 calcined at different temperatures correlation can be drawn, implying that the catalytic performance
(Fig. 6A). Nevertheless, there are optimal amounts of acid and is less affected by the acid and base densities on the surface.
base sites to obtain the maximum acrolein yield (Fig. 6B), which
Fig. 7 correlates the catalytic performances with the ratio of
are 31–33 ꢂ 1018 sites per g catal for acidic sites and 7–12 ꢂ 1018 acid/base sites. It shows the best performances are over Nb-
sites per g catal for basic sites. This is in consistence with the doped WO3/ZrO2-450 catalysts which have acid/base ratios of
30
ˇ ´
observation by Stosic et al. that the selectivity of acrolein is 4.0–5.4. It is also noticed that there appears a narrow range of
largely governed by the balance between acidic and basic active acid/base ratios of 3.25–3.5 giving lowest glycerol consumption
sites. Fig. 6 also shows that when 1–5% Nb2O5 is doped in rate, low acrolein yield, and fast catalyst decay rate. These
WO3/ZrO2-450, the acid amounts retain at relatively high values results demonstrate again that ne tuning the ratio of acid/base
of 30–35 ꢂ 1018 sites per g catal, while the base amounts are sites is important in achieving good catalytic performance in
shrunk in a narrower range of 7.5–8 ꢂ 1018 sites per g catal. gas-phase dehydration of glycerol to generate acrolein.
Both are still in the range of the optimal amounts of acid and
base sites to obtain the optimal acrolein yield.
In our case, most of the acidic sites on WO3/ZrO2-450 with and
without doping Nb2O5 determined by pyridine adsorption are
Lewis acid sites and the Brønsted acid sites are few and relatively
weak. Therefore, Lewis acid sites seem to be the active sites for
glycerol dehydration, and around 70% selectivities of acrolein are
obtained. This nding is different from the literature reports that
Lewis acids showed lower selectivities in the acrolein production
than the materials containing Brønsted acids.13 However, under
the reaction condition where a large amount of water is present,
it cannot be excluded that some Brønsted acidity may be gener-
ated through coordinative interaction of water molecules with
Lewis acid sites on the catalysts. On the other hand, Kinage
et al.32 proposed that the basic sites were responsible for
hydroxylacetone (HA) formation. Indeed, higher HA selectivities
are obtained on the catalysts containing larger amounts of basic
sites, such as WO3/ZrO2-400 and WO3/ZrO2-450 (Table 3).
As the decay rate is concerned, Fig. 6C shows that it decreases
almost linearly with both amounts of acid and base sites, and
those trends are in reverse to those of glycerol consumption rate.
WO3/ZrO2-400 is the most stable catalyst among WO3/ZrO2
calcined at different temperatures (Fig. 3 and Table 3), while
WO3/ZrO2-800 which contains lowest amounts of acid sites and
almost no basic sites gives the lowest glycerol conversion, acrolein
selectivity and faster decay rate. Since the acid sites, which are
active for glycerol dehydration, are also active for coke formation,
the fast decay rate observed over WO3/ZrO2-800 should be due to
coking on the active sites. With the increasing amount of basic
sites on WO3/ZrO2 catalysts, the decay rate slows down. Therefore,
basic sites should contribute the suppression of coke formation.
For Nb-doped WO3/ZrO2-450, the product selectivities in
3–4 h TOS are almost unaffected upon Nb-doping, while the
Unknowns selectivities decrease with decreasing the amount of
basic sites. It has been proposed that too many basic sites would
lead to formation of macromolecules, such as polyglycerols or
acetalization products of glycerol.33,34 Therefore, ne tuning the
amount of basic sites is important. This conclusion is demon-
strated in Fig. 6, in which the glycerol consumption rate, acro-
lein selectivity and catalyst decay rate all vary more drastically
with the base amount than acid amount, and the optimal
acrolein selectivity is obtained in a narrower range of base
amount than that of acid amount.
Fig. 7 Correlations of (A) specific glycerol consumption rates, (B)
acrolein yields, and (C) decay rates (average in 3–12 h TOS) with
acidic/basic ratios on WO3/ZrO2 calcined at 400 to 700 ꢀC (-) or
Fig. S6 (ESI†) intends to correlate glycerol consumption rate,
acrolein yield, and catalyst decay rate as a function of acid and
base densities on the surface in the unit of sites per nm2. No good NbWOx/ZrO2 catalysts (C).
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41888 | RSC Adv., 2017, 7, 41880–41888
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