Gas phase glycerol oxidative dehydration over bifunctional V/H-zeolite catalysts with different zeolite topologies

Article abstract of DOI:10.1016/j.cattod.2016.08.011

The oxidative dehydration of glycerol to acrylic acid can be performed in one single step using bifunctional catalysts containing both acid and redox sites. In this sense, zeolites containing transition metals are promising catalysts. Zeolites of different topologies: FAU, FER, MEL, MFI, MOR, MWW and OFF were synthesized and further impregnated with 5% vanadium. The catalysts were characterized and evaluated in the gas phase glycerol oxidative dehydration. All catalysts showed high conversions of glycerol (100-78%). The acrolein selectivity decreases with the total density of acid sites in these zeolites. The selectivity to acrylic acid is related to the ability of each topology of stabilizing the redox pair V⁵⁺/V⁴⁺. The best performances were observed for zeolite catalysts with MWW, BEA and MFI topologies.

Full text of DOI:10.1016/j.cattod.2016.08.011

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Gas phase glycerol oxidative dehydration over bifunctional
V/H-zeolite catalysts with different zeolite topologies
Thamyris Q. Silvaa, Maurício B. dos Santosa, Alex A.C. Santiagoa, Danilo O. Santanaa,
Fernanda T. Cruza, Heloysa M.C. Andradea,b, Artur J.S. Mascarenhasa,b,∗
a Laboratório de Catálise e Materiais, Departamento de Química Geral e Inorgânica, Instituto de Química, Universidade Federal da Bahia, R. Barão do
Jeremoabo, s/n, Ondina, 40170-280, Salvador-Bahia, Brazil
b Instituto Nacional de Ciência e Tecnologia em Energia e Ambiente (INCT-E&A), Universidade Federal da Bahia, R. Barão do Jeremoabo, s/n, Ondina,
40170-280, Salvador, Bahia, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
The oxidative dehydration of glycerol to acrylic acid can be performed in one single step using bifunctional
catalysts containing both acid and redox sites. In this sense, zeolites containing transition metals are
promising catalysts. Zeolites of different topologies: FAU, FER, MEL, MFI, MOR, MWW and OFF were
synthesized and further impregnated with 5% vanadium. The catalysts were characterized and evaluated
in the gas phase glycerol oxidative dehydration. All catalysts showed high conversions of glycerol (100-
78%). The acrolein selectivity decreases with the total density of acid sites in these zeolites. The selectivity
to acrylic acid is related to the ability of each topology of stabilizing the redox pair V5+/V4+. The best
performances were observed for zeolite catalysts with MWW, BEA and MFI topologies.
© 2016 Elsevier B.V. All rights reserved.
Received 10 May 2016
Received in revised form 11 July 2016
Accepted 2 August 2016
Available online xxx
Keywords:
Glycerol oxidative dehydration
Acrylic acid
V/H-zeolite catalysts
1. Introduction
available for further catalytic cycles of reaction and improving cat-
alyst lifetime [9].
The conversion of glycerol into valuable products has been
investigated as a strategy to attain a sustainable biodiesel pro-
duction chain [1–3]. Gas-phase glycerol dehydration to acrolein
is one of the most promising route, because acrolein is an impor-
tant intermediary in the production of acrylic acid, superabsorbent
polymers, pharmaceuticals and plasticizers. Many acid catalysts
were investigated in the literature, such as heteropolyacids, mixed
oxides, phosphates and zeolites, with satisfactory glycerol conver-
sion and acrolein selectivity [4,5]. However, rapid deactivation by
coke formation is still the main limitation in their use for industrial
purposes and a lot of effort has been payed to solve this problem.
Among the investigated strategies, one could mention the use of
moving bed reactors [6], alternating cycles of reaction and coke
burning [7] or oxygen co-feeding [8]. This last option seems to be
the most adequate for practical purposes, since oxidative condi-
tions favors desorption of the oxygenate oligomer compounds that
would act as coke precursors, maintaining the surface acid sites
The acrolein, produced in a first bed containing an acid cata-
lyst (usually a zeolite or a heteropolycompound), can be further
oxidized in a consecutive bed containing a redox catalyst, such as V-
W-Nb mixed oxides [10,11], resulting in high yields of acrylic acid.
On the other hand, the use of catalysts containing both acid sites
and redox species has been envisaged as a way to produce acrylic
acid in a one-step oxy-dehydration process [1]. In most cases, it
is observed an acrolein yield higher than to acrylic acid [1,12,13].
Recently, catalysts based on phosphoric acid modified W-V-Nb cat-
alysts have shown acrylic acid yields around 60% in a short time run
[14]. There is no mention to long time stability.
Redox molecular sieves, such as zeolites containing transition
metal oxides, are promising alternative catalysts for this purpose,
but there are few studies in the literature. Pestana et al. [1] have
evaluated the zeolite-␤ (BEA) containing 5 wt.% or 10% of vana-
dium, prepared by impregnation or physical mixture, in the glycerol
oxidative dehydration at 275 ◦C, observing a selectivity to acrylic
acid of 25% and 12%, respectively, at a glycerol conversion of ca.
75%. Acrolein and acetol were the main products formed. Other
oxygenates, such as acetaldehyde and acetic acid were observed as
minor byproducts, but a significant amount of unidentified prod-
ucts with high boiling points were also formed during the reaction.
∗
Corresponding author at: Laboratório de Catálise e Materiais, Departamento de
Química Geral e Inorgânica, Instituto de Química, Universidade Federal da Bahia, R.
Barão do Jeremoabo, s/n, Ondina, 40170-280, Salvador-Bahia, Brazil.
E-mail addresses: ajsmascarenhas@yahoo.com.br, artur@ufba.br
(A.J.S. Mascarenhas).
http://dx.doi.org/10.1016/j.cattod.2016.08.011
0920-5861/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: T.Q. Silva, et al., Gas phase glycerol oxidative dehydration over bifunctional V/H-zeolite catalysts with
different zeolite topologies, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.08.011

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2.2. Characterization of the catalysts
The X-ray diffraction patterns (XRD) were collected on a Shi-
madzu XRD-6000, operating with CuK␣ radiation at a voltage of
40 kV, current of 30 mA, and graphite monochromator, in the region
of 1.4◦–80◦ 2␪ at scan rate of 2◦ min−1
.
Elemental analyses were performed by energy dispersion X-ray
spectrometry in an equipment Shimadzu EDX 720, operating with
rhodium radiation source at 15 kV (Na-Sc) or 50 kV (U-Ti) and a
10 mm collimating slit.
Textural analyses were carried out by nitrogen physisorption
on a Micromeritics ASAP 2020, at −196 ◦C. The samples were pre-
treated at 350 ◦C for 3 h under vacuum (2 ␮m Hg) before collecting
the isotherms. BET, t-plot and BJH models were used to obtain
textural properties.
Temperature programmed desorption of ammonia profiles
(NH3-TPD) were collected in a Micromeritics 2720 Chemsorb. Ini-
tially the samples were pretreated in a helium flow at 300 ◦C for 1 h.
NH3 adsorption (9.9% mol/mol NH3/He) was carried out at ambient
temperature with a flow rate of 25 mL min−1 for 1 h. After removal
of physisorbed ammonia at 150 ◦C for 1 h, the samples were heated
from room temperature up to 800 ◦C, at 10◦ min−1, under helium
flow (25 mL min−1). The desorbed ammonia was monitored using
a thermal conductivity detector (TCD). All NH3-TPD profiles were
deconvoluted and the amounts of weak, moderate and strong acid
sites were calculated.
The temperature programmed reduction profiles using H2 (H2-
TPR) were conducted in a Micromeritics 2720 Chemsorb. The
samples (50 mg) were pretreated in an air flow at 300 ◦C for 1 h.
After cooling down to room temperature, a flow of 30 mL min−1
of 10 mol% H2/N2 is admitted to the reactor, and the sample
is heated from room temperature to 1000 ◦C at a heating rate
␤ = 10 ◦C min−1. H2 consumption was monitored using a thermal
conductivity detector.
Fig. 1. X-ray diffraction patterns of V/H-zeolites with different topologies. (Symbols
indicate the following phases: , erionite zeolite and ♣, layered silicate RUB-18).
Based on XPS measurements, the authors correlated the catalytic
performances to the vanadium dispersion in the zeolite pores.
In a recent publication, Possato et al. [12] have studied the use of
V2O5/H-ZSM-5 (MFI topology) prepared by using vanadyl sulphate
(VOSO4) or ammonium metavanadate (NH4VO3) as vanadium pre-
cursors. At 350 ◦C, glycerol conversions up to 97% and selectivity to
acrylic acid of 17% were attained for the catalyst prepared by wet
impregnation with VOSO4 [12]. According to the authors, the pres-
ence of vanadium improved the catalyst lifetime, because catalyzes
both the oxidation of acrolein to acrylic acid and the oxidation
of coke precursors. The formation of acrylic acid depends on the
dispersion of vanadium oxide, which facilitates the redox cycle
Diffuse reflectance spectra (DRS) in the UV–vis region were
collected in a Thermo-scientific Evolution 600 spectrometer oper-
ating with a Harrick Praying MantisTM accessory in the range of
200–800 cm−1
.
Thermogravimetry (TG/DTG) of spent catalysts were conducted
in a Shimadzu TGA-50 in a temperature range of 10–1000 ◦C at a
heating rate of 10 ◦C min−1 under air flow (50 mL min−1). The coke
content was calculated from mass loss in the range of 300–1000 ◦C.
EPR spectra of V/H-Zeolites samples were collected in tubular
quartz cuvettes (width of 0.3 mm) at 90 K (liquid nitrogen) with
an EMX plus Bruker spectrometer using 100 kHz field modulation
and 20 G standard modulation-width. Before the analysis, the solids
samples were dried overnight at 100 ◦C.
V
4+/V5+, as suggested by XPS and DTA analyses.
In this work, a series of catalysts were prepared by wet impreg-
nation with 5 wt.% of vanadium using NH4VO3 and evaluated in the
gas phase oxidative dehydration of glycerol, in order to evaluate the
effect of zeolite topology on the nature, dispersion and reducibility
of vanadium species in the selectivity to acrylic acid.
2. Experimental
2.3. Catalytic tests
2.1. Preparation of the catalysts
The catalytic activity was evaluated in a borosilicate glass
vertical fixed-bed reactor, containing 0.1 g of catalyst dispersed
in glass beads, operating at atmospheric pressure and 320 ◦C
for 10 h. A solution of 36 wt.% glycerol in water was fed using
Zeolite ZSM-5 (MFI), Beta (BEA), ferrierite (FER), zeolite Y (FAU),
offretite (OFF) and mordenite (MOR) were synthesized according to
the IZA methods [15]. The zeolite ZSM-11 (MEL) was synthesized
by the method proposed by Gonzales et al. [16]. Zeolite MCM-22
(MWW) was synthesized with molar ratio SiO2/Al2O3 = 30 by the
method proposed by Carric¸ o et al. [7]. The calcined materials were
ion exchanged with a solution with 0.1 mol L−1 NH4NO3 and fur-
ther calcined to obtain the acid form of the respective zeolites.
The thus prepared H-zeolites were wet impregnated by adding the
adequate volume a solution of 0.2 mol L−1 ammonium metavana-
date (NH4VO3) in order to obtain 5% of vanadium. The solvent was
removed in a rotary evaporator at 65 ◦C, 30 rpm, under reduced
pressure. The samples V/H-zeolite were calcined at 500 ◦C, for 3 h,
under air flowing (50 mL min−1).
a
peristaltic pump operating at 2 mL/h and an air flow of
30 mL min−1(W/F = 39.7 mmol g s−1). The reaction products were
condensed and absorbed in 10 mL of 0.1 wt% hydroquinone solu-
tion, used as polymerization inhibitor, and then analyzed by
gas chromatography on a GC-FID Perkin Elmer Clarus 500 oper-
ating with a flame ionization detector using a CPWax column
(15 m × 0.53 mm × 1.2 ␮m). Aliquots of 1 ␮L of the solution were
injected. Both the injection port and detector were kept at 250 ◦C,
while the following temperature program was used in the oven: i)
the initial temperature was 50 ◦C for 1 min; ii) then the column was
heated up to 80 ◦C, using a heating rate of 10 ◦C min−1, and kept
at this temperature for 3 min; and iii) finally heated up to 220 ◦C
Please cite this article in press as: T.Q. Silva, et al., Gas phase glycerol oxidative dehydration over bifunctional V/H-zeolite catalysts with
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3
H-ZSM-5
V/H-ZSM-5
H-MCM-22
H-β
V/H-MCM-22
V/H-
H-Ferrierite
V/H-Ferrierite
H-Y
H-Offretite
H-ZSM-11
H-Mordenite
V/H-Y
V/H-Offretite
V/H-ZSM-11
V/H-Mordenite
100 200 300 400 500 600 700 800 900
100 200 300 400 500 600 700 800 900
Temperature (°C)
Temperature (°C)
(a)
(b)
Fig. 2. NH3-TPD profiles of the H-zeolites of different topologies: (a) before and (b) after impregnation with 5 wt% vanadium.
at a heating rate of 12 ◦C min−1, and kept at this temperature for
V/H-ZSM-5
V/H-MCM-22
V/H-β
1 min. The analysis were performed in triplicates. Pure glycerol and
analytical grade reagents (possible products) were used to prepare
analytical curves in the concentration range expected during the
experiments, using 1-butanol as internal standard.
The glycerol conversion, product selectivities and yields were
quantified according to the following equations:
Conversion(%) = 100 x(nglycerol,in−nglycerol,out)/nglycerol,in
Productselectivity(%) = 100 xnproduct,formed/nglycerol,consumed (2)
Productyield(%) = 100 xnproduct,formed/nglycerol,in (3)
(1)
V/H-Ferrierite
V/H-Y
In which nglycerol,in indicates the inlet amount of matter of
glycerol; nglycerol,out is the outlet amount of matter of glycerol;
V/H-Offretite
nproduct,formed is the amount of matter of a given product in the efflu-
ents, and nglycerol,consumed is the amount of glycerol converted during
the catalytic reaction.
V/H-ZSM-11
V/H-Mordenite
3. Results and discussion
3.1. Catalyst characterization
200
400
600
800
1000
The X-ray diffraction patterns of calcined V/H-zeolites samples
with different topologies are shown in Fig. 1. The X-ray diffraction
patterns confirmed the formation of a crystalline structure of the
desired topologies as compared with the respective IZA standard
[17]. No peaks of vanadium oxides were observed in the powder
diffraction patterns, suggesting that they are very well dispersed in
the pore systems of the zeolites.
Temperature (°C)
Fig. 3. H2-TPR profiles for the catalysts V/H-zeolites of different topologies.
close to the expected one. Differences can be accounted by the
analysis error.
The as-synthesizes ferrierite presented low crystallinity, even
before the impregnation with NH4VO3. The diffractograms of the
samples V/H-offretite (OFF) and V/H-mordenite (MOR) suggest that
these zeolites are slightly contaminated with the erionite zeolite
(ERI) and the layer silicate RUB-18 phases [18], respectively.
The EDX elemental analysis of the V/H-zeolite catalysts are
shown in Table 1. The experimental values of the molar ratio
SiO2/Al2O3 are close to those found for the materials prepared by
the IZA verified synthesis methods [15]. The vanadium content is
The acidity of the samples was studied by NH3-TPD and the
profiles of the parent H-zeolites and the respective samples after
impregnation with vanadium are shown in Fig. 2. Quantification of
acid sites density is shown in Table 2.
The comparison of acid site strength of zeolites with differ-
ent topologies by using only NH3-TPD profiles should be avoided,
because different confinement events must be considered and NH3
heat of protonation/adsorption does not change significantly for
different zeolite topologies [19,20]. Furthermore, 13C MAS NMR of
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Table 1
Elemental analysis and textural properties for the V/H-zeolites of different topologies.
Catalyst
EDX
Textural properties − N2 adsorption
SiO2/Al2O3
Nominal
V (wt.%)
SBET
Smicro
Sexternal
(m2 g−1
Vmicro
Vmeso
a
b
b
b
c
Experimental
(m2 g−1
)
(m2 g−1
)
)
(cm3 g−1
)
(cm3 g−1
)
V/H-ZSM-5
V/H-MCM-22
V/H-␤
V/H-Ferrierite
V/H-ZSM-11
V/H-Mordenite
V/H-Y
30
30
30
15.2
50
30
10
16.5
20
22
25
12
42
14
3
5.1
5.4
4.2
5.2
4.8
4.6
5.4
4.7
301
304
453
187
484
342
418
364
209
214
368
137
362
296
362
335
92
90
85
50
122
46
57
29
0.097
0.099
0.230
0.069
0.159
0.130
0.170
0.156
0.065
0.063
0.041
0.164
0.091
0.092
0.050
0.008
V/H-Offretite
5
a
Surface area determined by BET method.
Micropore surface area, external surface area and micropore volume determined by t-plot method.
Average mesopore volume, determined by BJH method.
b
c
Table 2
Acid site strength distribution and quantification based on NH3-TPD profiles of V/H-zeolite catalysts.
Sample
Type of site
Tm (◦C)
Density of acid sites (mmol g)
Partial
Total
Moderate
Strong
Strong
264
390
477
0.27
0.30
0.13
0.70
V/H-
ZSM-
5
(1.03)a
Weak
Moderate
Strong
224
317
471
0.27
0.38
0.23
0.88
V/H-
MCM-
22
(0.88)a
Moderate
Strong
Strong
273
423
573
0.60
0.83
0.10
1.53
V/H-
␤
(1.59)a
Moderate
Strong
Moderate
Strong
269
429
268
420
0.41
0.28
0.34
0.43
0.69
V/H-
Ferrierite
V/H-
(0.87)a
0.77
(0.92)a
ZSM-
11
Moderate
Strong
Strong
275
449
594
0.52
0.76
0.16
1.44
V/H-
(1.13)a
Mordenite
Moderate
Strong
Strong
253
349
491
0.34
1.21
0.45
2.00
V/H-
Y
(2.18)a
Moderate
Strong
Strong
Strong
Strong
266
358
456
520
561
0.33
0.55
0.50
0.15
0.57
2.06
V/H-
Offretite
(2.10)a
a
Values in parenthesis indicates the total density of acid sites before impregnation of the H-zeolite with vanadium.
adsorbed acetone has detected no differences in acid strength in
high silica zeolites [21]. However, in this work, only a comparison
for the same zeolite topology, before and after impregnation with
5 wt.% of vanadium, has been performed.
Fig. 3 shows the profiles of H2-TPR for the catalysts V/H-zeolite
with different topologies. One could observe a large reduction peak
between 300 and 700 ◦C, due to the reduction of vanadium oxide
species. This peak can be shifted to lower or to higher tempera-
tures depending on the dispersion and location of vanadium species
in the porous systems (channels and/or cavities) of the different
zeolite topologies [21]. The high temperature peak (Tm > 800 ◦C)
corresponds to the non-stoichiometric reduction V+5 → V+3,32 [22].
A tentative assignment of the reduction peaks can be performed
by quantifying the molar ratio H2/V. The nominal values for the
reduction V4+ → V3+ and V5+ →V3+ are 0.5 and 1.0, respectively [23].
For all samples the molar ratio H2/V is between these nominal val-
ues, as shown in Table 3, what suggests that vanadium exists in
both oxidation states, V5+ and V4+ in different proportions, in the
fresh catalysts. For the material V/H-Y the molar ratio H2/V is 0.91,
which indicates that vanadium is better stabilized as V5+ in FAU
topology. The zeolites with topologies MWW and BEA have shown
molar ratios H2/V close to 0.5, but it is unacceptable that all vana-
dium atoms present in these catalysts would be in the V4+ oxidation
All materials have shown high acid sites densities (Table 2). From
the NH3-TPD profiles of Fig. 2a, one could observe that zeolites
H-ZSM-5, H-MCM-22, H-Ferrierite, H-Y and H-ZSM-11 have pre-
sented two events of NH3 desorption, suggesting the presence of at
least two types of sites with different acid strength. Besides, zeolites
H-␤ and H-mordenite showed three events, while four desorption
events are seen for zeolite H-offretite. For these last materials, it is
important to consider the presence of contaminant phases of other
zeolite topologies and/or layered silicates detected by XRD.
The values of acid sites density for the H-zeolite samples corre-
late well with their respective molar ratio SiO2/Al2O3, differences
being accounted by the material crystallinity and the presence of
defects. In general, the addition of vanadium leads to a decrease in
the acidity with respect to the parent H-zeolites (Table 2), but also
creates new acid sites, probably of Lewis type.
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Table 3
H2-TPR quantitative analyses of V/H-zeolite with different topologies, indicating onset and maximum reduction temperature, and molar ratio H2/V.
Sample
T0 (◦C)
TM1 (◦C)
TM2 (◦C)
TM3 (◦C)
H2/V
V/H-ZSM-5
V/H-MCM-22
V/H-␤
V/H-Ferrierite
V/H-Y
V/H-Offretite
V/H-ZSM-11
V/H-Mordenite
430
388
380
269
389
463
352
374
588
540
506
514
582
500
396
467
620
586
555
550
627
575
545
538
–
–
–
–
924
857
–
0.60
0.47
0.54
0.61
0.91
0.38
0.69
0.44
–
amount of V4+ ions (Fig. 5). Vanadium(IV) has an electronic con-
figuration d1, with net electron spin (S = 1/2) and net nuclear spin
(I = 7/2), thus each signal originates 2I + 1 = 2 × 7/2 +1 = 8 lines due
to hyperfine interactions [27].
The analysis of the spectra shown in Fig. 5a–h revealed that,
even without a reduction pretreatment, all samples have presented
V/H-ZSM-11
V/H-ZSM-5
a signal with g = 1.936 and A = 203 G, g = 1.990 and A = 83 G.
ꢀ
ꢀ
⊥
⊥
These tensor values are consistent with the presence of VO2+ ions in
an axially symmetric environment, probably with V4+ in a distorted
octahedral geometry or square pyramidal coordination, typical of
hydrated VO2+ ions in cation exchange sites [28,29].
V/H-β
V/H-mordenite
V/H-offretite
Observing the EPR spectra, one could relate the nor-
malized intensities with the content of V4+ species
in each sample, and the following order is observed:
V/H-␤ > V/H-MCM–22 > V/H-offretite > V/H-ZSM–11 > V/H-
V/H-ferrietite
V/H-Y
mordenite > V/H-ZSM–5 ∼ V/H-ferrierite ∼ V/H-Y,
which
is
consistent to the previously discussed based on TPR analysis.
V/H-MCM-22
200 300 400 500 600 700 800
3.2. Catalytic tests
Wavelength (nm)
Fig. 6 shows the conversion of glycerol and the selectivity to
products as a function of reaction time in the oxidative dehydration
of glycerol.
Fig. 4. Diffuse reflectance UV–vis spectra recorded at room temperature of V/H-
zeolite of different topologies. Spectra were measured using the respective H-zeolite
as reference.
The glycerol conversion for all catalysts was high during the
10 h reaction runs and the main reaction product is acrolein, and
not acrylic acid, regardless of the catalyst employed. Comparing
the results with those previously reported in the absence of oxy-
gen [7,9], deactivation by coke deposition is highly inhibited in the
presence of oxygen and a decrease of ca. 20% in glycerol conversion
was observed for V/H-MCM-22 and V/H-ZSM-11 catalysts, while
V/H-ZSM-5 and V/H-mordenite were more stable during the 10 h
of reaction.
The catalyst V/H-ZSM-11 has presented higher selectiv-
ity to acrolein in the first hours, but V/H-MCM-22 was
more stable in terms of selectivity to acrolein. The follow-
ing order was observed for the selectivity to acrolein after
10 h of reaction: V/H-MCM–22 > V/H-ZSM–11 ∼ V/H-ZSM–5 ∼ V/H-
␤ ∼ V/H-ferrierite > V/H-mordenite > V/H-Y > V/H-offretite.
The selectivity to acrylic acid is very low in the first hour, but
increases with the reaction time, indicating that re-adsortion of
acrolein on vanadium oxides species is limiting the oxidation step.
After 10 h on stream, catalysts V/H-MCM-22 and V/H-␤ reach 20%
of selectivity to acrylic acid. The results obtained with the catalyst
5% V/H-Beta are similar to those reported by Pestana et al. [1] for
the same type of catalyst, even if operating at higher temperature.
Besides, the catalytic performances obtained here with the catalyst
V/H-ZSM-5 is superior to those reported by Possato et al. [12] with
a similar catalyst.
state, because the materials have the characteristic yellow-brown
color of V2O5. As the molar ratio H2/V nominal for V5+ to V4+ reduc-
tion is also 0.5, it would be more reasonable to assume that V5+ in
the V/H-MCM-22 and V/H-␤ materials do not reduce until V3+ and
that the redox cycle V5+/V4+ would be favored in these topologies.
Finally yet importantly, the molar ratio H2/V for V/H-offretite and
V/H-mordenite samples is lower than 0.5, which suggests the for-
mation of non-stoichiometric species of vanadium oxide [23], what
can also occur for other materials.
The nature and chemical environment of the vanadium species
were also studied by diffuse reflectance spectroscopy in the UV/vis
region (Fig. 4). In the spectra of Fig. 4, it is possible to observe bands
at 270 and 340 nm, that can be assigned to ␲(t2) → d(e) and to
␲(t1) → d(e) charge transfer transitions of tetrahedral V5+ [24]. A
band at 380 nm reveal the presence of V5+ ions in oxide clusters
dispersed in the porous systems of the zeolites. Hydrated species,
like octahedral vanadium ions contribute to the absorption in the
300–500 nm region and, finally a wide band from 500 to 800 nm is
typical of d–d transitions of V4+ [25,26]. Except for V/H-Y, all sam-
ples present a contribution above 500 nm, confirming that V4+ ions
are present in these zeolite topologies. These results reinforce the
attribution of the H2-TPR reduction peaks herein proposed.
In order to obtain more information about the nature and
coordination of the V-oxide species present within the zeolite
framework by monitoring the V4+ species, the samples were ana-
lyzed by EPR at 90 K. It is important to remember that V5+ (d0) is
silent in the electron paramagnetic resonance spectroscopy. The
EPR spectra of all samples confirmed the presence of significant
The product distribution in the oxidative dehydration of glycerol
for the V/H-zeolites with different topologies is shown in Table 4 for
the condensates obtained after 2 and 10 h of reaction, respectively.
Even though acrolein was the main product, significant amounts of
acrylic acid, acetic acid and acetaldehyde were also formed. Coke
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8
7
6
5
4
3
2
1
0
V/H-MCM-22
25
20
15
10
5
V/H-ZSM-5
0
-5
-10
-1
-2
2500
-15
3000
3500
4000
4500
2500
3000
3500
4000
4500
H (Gauss)
H (Gauss)
(a)
(b)
40
9
V/H-ferrierite
V/H-β
8
30
20
10
0
7
6
5
4
3
-10
-20
2
1
II
2500
3000
3500
H (Gauss)
(c)
4000
4500
2500
3000
3500
4000
4500
H (Gauss)
(d)
6
30
V/H-offretite
V/H-Y
25
20
15
10
5
5
4
3
2
1
0
0
-5
-10
-15
-20
-1
-2
2500
3000
3500
H (Gauss)
(e)
4000
4500
2500
3000
3500
4000
4500
H (Gauss)
(f)
V/H-mordenite
V/H-ZSM-11
12
10
8
15
10
5
6
4
2
0
0
-5
-2
-4
2500
-10
2500
3000
3500
4000
4500
3000
3500
4000
4500
H (Gauss)
H (Gauss )
(g)
(h)
Fig. 5. EPR spectra recorded at 90 K of calcined V/H-Zeolite with different topologies.
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Table 4
Catalytic performance for the gas-phase oxydehydration of glycerol over V/H-zeolites with different topologies, after 2 h and 10 h.
Catalyst
MFI
MWW
BEA
FER
FAU
OFF
MEL
MOR
Conversion (%)a
99.4 (95.9)
53.8 (46.8)
3.2 (12.0)
95.3 (76.2)
64.1 (42.8)
10.4 (16.0)
93.8 (87.6)
52.8 (40.5)
6.1 (17.7)
90.7 (81.7)
52.1 (37.7)
2.9 (5.5)
91.1 (81.3)
30.8 (23.2)
0.5 (1.1)
97.1 (84.2)
24.2 (12.7)
0.7 (1.8)
99.9 (80.3)
75.6 (40.9)
1.7 (11.6)
98.0 (89.7)
46.8 (32.1)
0.6 (3.2)
Acrolein yield (%)a
Acrylic Acid yield (%)
Molar selectivity (%)
Acrolein
1-hydroxyacetone
Propionaldehyde
Acetaldehyde
Acetic acid
Allyl Alcohol
Propionic acid
Acrylic acid
54.2 (48.7)
0.1 (0.1)
2.1 (2.3)
3.0 (3.8)
1.4 (2.0)
0.0 (0.2)
0.0 (0.0)
3.2 (12.5)
36.0 (30.5)
67.3 (56.6)
0.3 (0.3)
3.7 (3.1)
6.5 (5.5)
1.1 (4.1)
0.6 (0.6)
0.9 (1.4)
10.9 (21.0)
8.7 (7.8)
56.4 (46.3)
0.4 (0.2)
4.2 (1.6)
8.3 (5.6)
5.2 (6.0)
1.6 (0.4)
2.4 (2.2)
6.5 (20.2)
15.0 (17.7)
57.4 (46.4)
0.6 (0.9)
4.4 (4.8)
10.6 (13.5)
5.4 (12.1)
3.5 (4.1)
0.9 (1.1)
2.4 (6.7)
14.2 (10.4)
33.8 (28.6)
0.3 (0.3)
0.5 (0.5)
7.6 (8.0)
1.7 (4.8)
0.3 (0.5)
0.4 (1.1)
0.5 (1.3)
54.9 (55.0)
24.9 (15.1)
0.2 (0.3)
0.3 (0.3)
2.8 (5.8)
6.2 (7.1)
0.2 (0.4)
0.5 (2.3)
0.7 (2.1)
64.2 (66.6)
75.7 (50.9)
0.0 (0.3)
0.9 (0.3)
14.6 (4.0)
5.3 (2.9)
0.0 (0.1)
1.1 (2.0)
1.7 (14.5)
0.8 (25.0)
47.8 (35.8)
0.1 (0.4)
0.4 (0.5)
7.6 15.5)
1.8 (7.3)
0.2 (0.8)
1.8 (0.7)
0.6 (3.6)
40.3 (35.5)
Others
Coke (%)c
7.5
10.3
12.7
5.0
7.2
4.1
6.7
6.8
a
Reaction conditions: 320 ◦C, 36.6% glycerol; glycerol flow = 2 mL/h, catalyst weight = 0.10 g.
Table 5
Comparison of V/H-zeolite catalyst performances in the gas-phase glycerol oxidehydration, after 10 h of run, with data reported in the literature.
Catalyst
WHSV (h-1
)
T (oC)
cglicerol (%)
Sacrolein (%)
Sacrylic acid(%)
Reference
V/H-MCM-22
V/H-b
V/H-ZSM-5
V/H-ZSM-11
V2O5-ZSM-5
5%V-BEA
8.6
8.6
8.6
8.6
3.07
1.51
7.56
320
320
320
320
350
275
275
76.2
87.6
95.9
80.3
97
56.2
46.3
48.7
50.9
15
21
This work
This work
This work
This work
[12]
Zeolitic
20.2
12.5
14.5
17
25
23
75
100
45
13
[1]
[38]
Fe4.0-BEA-50
Non
MoV/SiW/Al2O3
0.31
0.31
0.31
0.31
1.24
1.21
0.11
0.42
0.6
0.6
0.6
1.41
1.93
0.5
300
350
450
450
290
300
300
250
300
300
300
310
265
340
340
340
100
100
100
100
100
90
2.9
35.2
3
0.9
17
20
10
8.2
0.1
3
2
11
3
6.9
9.3
18
12.1
19.8
46.2
9
34
26
25.7
30.5
24
26
28
25
50
56.6
40.9
60
[30]
[30]
[30]
[30]
[31]
[31]
[32]
[33]
[34]
[34]
[34]
[35]
[36]
[37]
[37]
[37]
zeolitic SiW/Al2O3 and Mo3VOx
SiW/Al2O3 and Mo3VOx
SiW/Al2O3 and Mo3VOx (phy.mix.)
V-W-Nb-oxides
V-W-oxides
V-Mo-oxides
Mo-V-W-oxides
V-W-oxides
V-W-oxides
V-Mo-Te-Nb-oxides
V-W-oxides
V-W-Nb-oxides
Cs(VO)0.2(PMo)0.5(PW)0.5
1Cs(VO)0.2PMoW
H0.1Cs2.5(VO)0.2(PMo12O40
90
100
100
100
99
100
100
100
100
100
0.5
0.5
)0.25(PW12O40
)
0.75
contents determined by TG analysis under oxidative conditions
after 10 h of catalytic run were also shown in Table 4.
In order to establish structure-activity relationships for the glyc-
erol oxidative dehydration, acrolein and acrylic acid yields were
plotted as a function of acid sites density and molar ratio V5+/V4+
(determined by H2-TPR), respectively. The results are shown in
Fig. 7a and b.
In general, Fig. 7a suggests that acrolein yield tends to
decrease with the increase of acid sites density. Probably this is
a consequence of deactivation by coke (see Table 4) and/or non-
condensable products (gases), which were not quantified. The best
results were observed for the catalysts whose acid sites density
was near to 0.8 mmol g−1, however among these catalysts it was
noticeable the influence of the zeolite topology, with V/H-ZSM-11
(MEL) > V/H-MCM-22 (MWW) > V/H-ZSM-5 (MFI) ∼ V/H-ferrierite
(FER).
i) iglycerol interacts via C2-hydroxyl group with zeo-
lite Brönsted acid sites, producing by dehydration
3-hydroxipropionaldehyde as an intermediary, which is
further dehydrated to produce acrolein [31];
ii) acrolein re-adsorbs on vanadium species and is oxidized to
acrylic acid, with consequent reduction of V5+ to V4+, which
stabilizes the oxygenated species as acrylate [32];
iii) V4+ ions are re-oxidized to V5+ by consuming molecular oxygen
in the process.
3.3. Camparison with literature
The bifunctional V/H-zeolites catalysts of different zeolite
topologies were compared those found in the literature. Table 5
shows not only glycerol conversion and selectivities to acrolein and
acrylic acid, but also the reaction conditions as temperature and
weight hourly space velocity (WHSV = mass flow/catalyst mass).
For catalysts in the literature, acid function is provided by a zeo-
lite [1,12,33], a heteropolyacids [30] or an acid support, such as
alumina [34] or niobia [8,10,35], while transition metal oxides, as
vanadium, molybdenum or tungsten, or their associations provide
the catalyst redox function [8,10,32,34,36,37]. MoVW catalyst, con-
taining niobium or not in their formulation, are the most promising
On the other hand, for acrylic acid (Fig. 7b), one can observe that
those catalysts presenting a molar ratio V5+/V4+ around 0.5 have
shown higher yields, confirming that the stabilization of the redox
pair V5+/V4+ favors the catalytic cycle and suggests that the reac-
tion occurs according to the Mars-van Krevelen mechanism [30],
as follows:
Please cite this article in press as: T.Q. Silva, et al., Gas phase glycerol oxidative dehydration over bifunctional V/H-zeolite catalysts with
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100
95
90
85
80
75
100
90
2 h
10 h
V/MEL
80
70
60
50
40
30
20
10
0
V/MWW
V/MFI
V/BEA
V/MOR
V/FER
V/H-ZSM-5
70
V/FAU
V/OFF
V/H-MCM-22
β
V/H-β
65
V/H-Ferrierite
V/H-Y
60
V/H-Offretite
V/H-ZSM-11
55
50
V/H-Mordenite
0,6
0,7
0,8
1,4 1,6 1,8 2,0 2,2
1
2
3
4
5
6
7
8
9
10
Acid sites density (mmol g-1)
Time (h)
(a)
(a)
100
90
80
70
60
50
40
30
20
10
0
30
25
20
15
10
5
V/H-Y
V/H-ZSM-5
V/H-MCM-22
V/H-β
V/H-Offretite
V/H-ZSM-11
2 h
V/MWW
V/BEA
10 h
V/H-Mordenite
V/H-Ferrierite
V/MFI
V/MEL
V/FER
V/MOR
V/OFF
V/FAU
0,9
0
0,2
0,3 0,4
0,5
0,6
0,7
0,8
1,0
1
2
3
4
5
6
7
8
9
10
Molar ratio V5+/V4+
Time (h)
(b)
(b)
50
45
40
35
30
25
20
15
10
5
V/H-ZSM-5
V/H-MCM-22
V/H-β
V/H-Ferrierite
V/H-Y
V/H-Offretite
V/H-ZSM-11
V/H-Mordenite
Fig. 7. Structure-activity relationships: (a) dependence of acrolein yield (after 2 h
and 10 h) on the acid sites density; and (b) acrylic acid yield (after 2 h and 10 h) as
a function of the molar ratio V5+/V4+. Reaction conditions: 320 ◦C, 36.6% glycerol;
glycerol flow = 2 mL/h, catalyst weight = 0.10 g.
atures of 320 ◦C, suggesting that the catalytic performance can be
improved by adjusting the hydrodynamic properties of the catalytic
reactor. Additionally, as the oxygen content is crucial to the regen-
eration of redox sites, catalytic performance should be improved
by increasing molar ratio O2/glycerol.
4. Conclusions
0
Bifunctional catalysts V/H-zeolite, prepared by impregnation of
V2O5 on the acid zeolites of different topologies, have shown to
be very active in the gas-phase oxidative dehydration of glycerol,
achieving high glycerol conversions and producing mainly acrolein
and acrylic acid. Among these catalysts, those with MWW and BEA
topologies resulted in better catalytic performances, with selectiv-
ities to acrylic acid of about 20%, but a significant coke deposition
was observed for these more open topologies. Characterization
techniques suggested that acrolein production is favored on cata-
lysts in which acid sites densities are around 0.8 mmol g−1, but the
effect of zeolite topology must be considered. On the other hand,
acrylic acid selectivities are related to the ability of a specific zeo-
lite topology to stabilize the redox pair V5+/V4+, as evidenced by
H2-TPR, DRS UV–vis and EPR, suggesting that a Mars van Krevelen
mechanism is occurring. These catalysts are promising for practi-
1
2
3
4
5
6
7
8
9
10
Time (h)
(c)
Fig. 6. Glycerol conversion (a), acrolein selectivity (b) and acrylic acid selectiv-
ity over V/H-Zeolite catalysts. Reaction conditions: 320 ◦C, 36.6% glycerol; glycerol
flow = 2 mL/h, catalyst weight = 0.10 g.
catalysts, with higher acrylic acid selectivities than those based on
zeolites. However, it is necessary to consider that they operate with
low space velocities, which means that space time (W/F) is high,
facilitating acrolein re-adsorption on vanadium species (step ii).
Bifunctional V/H-MCM-22 (MWW topology) reached maximum
21% of acrylic acid selectivity under WHSV = 8.6 h−1 and temper-
Please cite this article in press as: T.Q. Silva, et al., Gas phase glycerol oxidative dehydration over bifunctional V/H-zeolite catalysts with
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cal uses, but reaction parameters as space velocity and molar ratio
oxygen/glycerol must be optimized.
[15] Verified Syntheses of Zeolitic Materials, in: H. Robson (Ed.), 2nd ed., Elsevier,
Amsterdam, 2001.
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121 (2009) 26–33.
Acknowledgments
[17] M.M.J. Treacy, J.B. Higgins, Collection of Simulated XRD Power Patterns for
Zeolites, 4th ed., Elsevier, Amsterdam, 2001.
[18] F.S.O. Ramos, H.O. Pastore, Micropor. Mesopor. Mater. 177 (2013) 143–150.
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Initiation (PIBIC/UFBA) for the scholarship.
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Please cite this article in press as: T.Q. Silva, et al., Gas phase glycerol oxidative dehydration over bifunctional V/H-zeolite catalysts with
different zeolite topologies, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.08.011