The multiple benefits of glycerol conversion to acrolein and acrylic acid catalyzed by vanadium oxides supported on micro-mesoporous MFI zeolites

Article abstract of DOI:10.1016/j.cattod.2016.08.005

The ZSM-5 zeolite (MFI structure, Si/Al?=?40) was treated using NaOH and either oxalic acid or HCl to obtain hierarchical materials with different characteristics, followed by impregnation with vanadium oxides (V₂O₅) to generate redox-active sites. The impact of the multiple treatments on the efficiency and stability of the catalysts in the conversion of glycerol to acrolein and acrylic acid was investigated and correlated with catalyst porosity, acidity, and chemical composition. The treated and impregnated V₂O₅ catalysts were subjected to XRD, ²⁷Al NMR, nitrogen physisorption, TPD-NH₃, TG, and UV–Vis analyses, in order to associate the properties of the catalysts with their activities. The studies showed that the catalytic performance of the materials depended on the acidic and textural properties of the zeolites, which influenced both the dispersion of V₂O₅ and its interaction with the acid sites of the supporting zeolites. All the catalysts provided conversion values exceeding 65%, even after 6?h on glycerol stream. The distribution of products strongly reflected the effects of pore formation, acid treatment with oxalic acid or HCl, and the presence of vanadium oxide. The effects of these modifications resulted in higher selectivity to acrolein and acrylic acid, a reduced rate of coke accumulation in the zeolite, and a longer catalyst lifetime.

Full text of DOI:10.1016/j.cattod.2016.08.005

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The multiple benefits of glycerol conversion to acrolein and acrylic
acid catalyzed by vanadium oxides supported on micro-mesoporous
MFI zeolites
Luiz G. Possato, Thiago F. Chaves, Wellington H. Cassinelli, Sandra H. Pulcinelli,
∗
Celso V. Santilli, Leandro Martins
Instituto de Química, UNESP − Univ. Estadual Paulista, Rua Prof. Francisco Degni 55, 14800-060, Araraquara, SP, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
The ZSM-5 zeolite (MFI structure, Si/Al = 40) was treated using NaOH and either oxalic acid or HCl to
obtain hierarchical materials with different characteristics, followed by impregnation with vanadium
oxides (V2O5) to generate redox-active sites. The impact of the multiple treatments on the efficiency and
stability of the catalysts in the conversion of glycerol to acrolein and acrylic acid was investigated and
correlated with catalyst porosity, acidity, and chemical composition. The treated and impregnated V2O5
Received 12 April 2016
Received in revised form 20 July 2016
Accepted 2 August 2016
Available online xxx
2
7
catalysts were subjected to XRD, Al NMR, nitrogen physisorption, TPD-NH3, TG, and UV–Vis analyses,
in order to associate the properties of the catalysts with their activities. The studies showed that the
catalytic performance of the materials depended on the acidic and textural properties of the zeolites,
which influenced both the dispersion of V2O5 and its interaction with the acid sites of the supporting
zeolites. All the catalysts provided conversion values exceeding 65%, even after 6 h on glycerol stream.
The distribution of products strongly reflected the effects of pore formation, acid treatment with oxalic
acid or HCl, and the presence of vanadium oxide. The effects of these modifications resulted in higher
selectivity to acrolein and acrylic acid, a reduced rate of coke accumulation in the zeolite, and a longer
catalyst lifetime.
Keywords:
Glycerol oxidehydration
V2O5/MFI catalyst
Acrolein
Acrylic acid
Micro-mesoporous zeolites
©
2016 Elsevier B.V. All rights reserved.
1
. Introduction
a high conversion and selectivity to glycols has been obtained by
glycerol hydrogenolysis using catalysts based on metallic Ni, Ru,
and Cu [4–7].
The growth of greenhouse gas emissions and the limited
reserves of easily extracted fossil fuels have led researchers and
industry to pursue new alternatives to replace, even partially, the
use of fossil fuels. As a result, the use of compounds derived from
biomass, such as biogas, ethanol from sugarcane, and biodiesel has
increased in recent years. In the case of biodiesel, production has
increased and consequently there has been an increase in the con-
comitant formation of coproduced glycerol [1–3].
Glycerol is a focus of green catalytic processes, because
this molecule offers interesting chemical versatility that can be
exploited for the formation of compounds that are currently
provided by the petrochemical industry. An example of glycerol val-
orization is the synthesis of 1,2-propanediol and 1,3-propanediol,
which are used as antifreeze fluids in automobiles. In industry, both
compounds are obtained from the hydration of propene. However,
Another important conversion of glycerol into petrochemical-
type compounds is the formation of acrolein and acrylic acid,
which are used in the manufacture of resins. Acrolein can be
obtained by gas phase glycerol dehydration on acid catalysts such
as heteropolyacids [8], impregnated phosphate groups on metal
oxides [9], sulfated zirconia [10], Nb O5 [11], mixed oxides [12],
2
zeolites [13–15], functionalized mesoporous silica [16,17], and
vanadium-silicates [18]. The glycerol can also be dehydrated in
liquid phase [19,20]. Acrylic acid is obtained from the oxidation
of acrolein. The one-step conversion of glycerol into acrylic acid
using bifunctional catalysts with acid and redox sites occurs accord-
ing to Scheme 1 [21–26]. An interesting aspect of coupling the
two reactions is the mutually supporting endothermic dehydra-
0
tion of glycerol (ꢀH = 3.04 kcal/mol) and exothermic oxidation of
0
acrolein (ꢀH = − 61.02 kcal/mol).
For glycerol dehydration using zeolites, maximum performance
of the catalyst is achieved by combining Brønsted acid sites of
medium strength (strong acid sites lead to severe coke formation,
∗ Corresponding author.
E-mail address: leandro@iq.unesp.br (L. Martins).
http://dx.doi.org/10.1016/j.cattod.2016.08.005
0
920-5861/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: L.G. Possato, et al., The multiple benefits of glycerol conversion to acrolein and acrylic acid catalyzed
by vanadium oxides supported on micro-mesoporous MFI zeolites, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.08.005

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2
OH
O
ously with glycerol dehydration to acrolein. Byproducts include
-
H O
2
- H O
2
+
½ O
2
HO
OH
O
OH
O
acetaldehyde, acetol, and acetic acid, as well as very harmful and
deactivating coke molecules. After catalytic experiments with bare
zeolites, the polymerization of bulky molecules on the surfaces
of the catalysts led to coke formation and a characteristic black
appearance. However, in the previous work it was found that when
OH
glycerol
3-hydroxypropanal
acid sites
acrolein
acrylic acid
redox sites
Scheme 1. Glycerol oxidehydration (dehydration combined with oxidation) on
a V O5/zeolite catalyst was used, the coke was continuously oxi-
2
bifunctional acid and redox active sites.
dized due to the presence of well-dispersed vanadium oxides on
the zeolite surface, which maintained the catalytic sites active for
longer periods.
while weak acid sites are less capable of converting glycerol) [27],
porosity (which enhances the diffusion of glycerol and acrolein),
and high specific area (which increases access to catalytic sites).
For instance, members of the lamellar MWW zeolite family, which
includes microporous MCM-22, pillared MCM-36, and delaminated
ITQ-2, offer advantageous characteristics for glycerol dehydration
The aim of the present work was to explore further the multi-
ple benefits of porous V O5/MFI catalysts in the one-step glycerol
2
conversion to acrylic acid. The work focused on zeolite supports
prepared by sequential processes of desilication (in NaOH solu-
tion) and dealumination (in HCl or oxalic acid solutions) in order
to tailor the pores and the quality of acid sites derived from either
aluminum in tetrahedral coordination in the zeolite or from EFA.
Improved transformation of glycerol was achieved on the micro-
[
22]. Following pillarization and delamination of the MWW struc-
ture, the strengths of acid sites decrease, but the increases in
mesopores and specific area raise the overall performance of the
catalyst [28].
mesoporous V O5/MFI zeolites, due to higher catalytic conversion,
2
Despite the attraction of lamellar zeolites for use in glyc-
erol dehydration, the laborious multiple steps and the expense
associated with catalyst preparation are notable disadvantages.
Alternatively, the desilication of commercially available zeolites by
treatment with sodium hydroxide solution seems to be more prac-
tical [29,30]. The alkaline process is simple, with hydroxyl groups
attacking and removing silicon atoms from the zeolite structure,
creating randomly distributed pores in the zeolite crystals. The
diameter and volume of the pores can be tuned by adjusting the
concentration of the alkaline solution and by varying the exposure
time of the zeolite (usually a few minutes) and the desilication
temperature (which normally ranges from room temperature to
a few tens of degrees Celsius) [31–37]. The broad distribution
of mesopore families results in catalytic performance in glycerol
dehydration similar to that of the MWW zeolites.
A disadvantage of the desilication method is that during the zeo-
lite treatment process, aluminum atoms are removed as well as
silicon atoms. Silicon species are mostly found in the alkaline liq-
uid phase, but aluminum tends to form insoluble oligomeric species
that can precipitate on the catalyst surface as extra-framework
aluminum atoms (EFA). Consequently, the mesopores created are
obstructed due to an alkali-induced alumination of the external sur-
faces of the crystals, and the nature of the acid sites of the zeolite
shifts from Brønsted to Lewis acid sites. This catalytic acid behav-
ior must be considered in the design of catalysts by desilication,
because EFA sites are selective in converting glycerol into undesir-
able byproducts. However, the EFA can be removed from the zeolite
by acid leaching; as a result, the selectivity to acrolein is enhanced
and the diffusion of chemicals through the pores is increased due
to the removal of aluminum species.
improved selectivity to acrolein and acrylic acid, extended catalyst
stability, and decreased coke formation.
2. Experimental
2.1. Preparation of zeolite supports
Zeolite of MFI structure (Si/Al mole ratio of 40) was kindly
provided by Zeolyst (USA). The sample was submitted to alka-
◦
line treatment at 60 C for 1 h using an aqueous solution of NaOH
(0.6 mol/L). Detailed information concerning the desilication proce-
dure is provided elsewhere [30,39]. The desilicated zeolite was then
submitted to two different acid treatments using aqueous solu-
tions of hydrochloric or oxalic acids. The acidic treatments were
performed under reflux using 0.1 mol/L acid solution. The MFI sup-
ports were denoted A (parent and untreated MFI zeolite), B (after
alkaline treatment), C (after treatment using H C O ), and D (after
2
2
4
treatment using HCl). After the sequential alkaline and acidic treat-
ments, all the supports (A, B, C, and D) were ion exchanged three
times with NH NO solution, at room temperature. The exchanged
4
3
NH4+ cations were thermally decomposed by heating the samples
for 3 h in a conventional muffle furnace, in air atmosphere, from
◦
◦
◦
25 C to 500 C at a heating rate of 10 C/min.
2.2. Preparation of the catalysts
V-MFI catalysts were obtained by incipient wetness impregna-
tion of the supports using an aqueous solution of vanadyl sulfate
(0.05 mol/L). Approximately 50 mL slurry of vanadyl sulfate solu-
◦
tion and zeolite was stirred for 1 h at 25 C. The water was allowed
◦
In the second step of glycerol conversion to acrylic acid
to evaporate under vacuum at 40 C, and the samples were dried
◦
(Scheme 1), redox active sites are required. Vanadium oxides are
overnight at 100 C. Finally, the catalysts were subjected to a ther-
mal treatment for 2 h in an air atmosphere, with heating from 25 C
to 500 C at a rate of 5 C/min. The contents of V O in all the sam-
◦
strong candidates for this purpose because they possess a very
important redox characteristic, namely the capacity to adopt mul-
tiple oxidation states. On these catalysts, acrolein is oxidized by
◦
◦
2
5
ples was 10 wt.%.
removing a surface oxygen atom from V O5, giving rise to acrylic
2
acid and an oxygen vacancy in V O5-x. In a subsequent step, the
2.3. Characterization of samples
2
catalytic site is oxidized and reestablished by feeding an excess
1
of molecular O2 in the stream (V O5-x + / O → V O5). This redox
X-ray diffractograms of the supports and catalysts were
obtained at the XPD beamline of the Brazilian Synchrotron
Light Laboratory (LNLS), using a Huber 4 + 2 circle diffractometer
equipped with an Eulerian cradle (model 513) placed approx-
imately 13 m from the double-bounce Si(111) monochromator
(␭ = 1.377494 Å) [40]. The data were collected in high-resolution
mode, employing a Si(111) analyzer crystal and a Mythen detec-
tor. Structural parameters were determined by the Rietveld profile
method, using GSAS-EXPGUI software [41,42]. The scale factors,
2
2
2
2
5
+
4+
mechanism and the changes in V /V oxidation states during the
catalytic reaction are known as the Mars-Van Krevelen mechanism
38].
In a recent publication, we described additional useful features
[
of the V O5/zeolite catalytic system [20]. Besides the advan-
2
tages mentioned above, vanadium oxides supported on zeolites
were much less susceptible to deactivation, compared to the bare
zeolites. Several parallel and unknown reactions occur simultane-
Please cite this article in press as: L.G. Possato, et al., The multiple benefits of glycerol conversion to acrolein and acrylic acid catalyzed
by vanadium oxides supported on micro-mesoporous MFI zeolites, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.08.005

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zero shifts, and backgrounds of the peak profiles, together with
the lattice parameters, were refined by fitting with a sixth order
Chebyshev polynomial. Pseudo-Voigt functions were employed for
the peak profile refinements. Other parameters were not refined.
The textural properties of the supports and impregnated cata-
lysts were determined by means of nitrogen adsorption-desorption
retention times of all the compounds were compared with those of
authentic standards. The glycerol conversions (Xglycerol) and prod-
uct selectivities (S) were calculated using the following equations:
nGlinput − n
output
ni
Gl
input
Xglycerol(%) =
× 100 S(%) =
nGlinput − nGloutput
◦
nGl
isotherms obtained at −196 C with a Micromeritics ASAP 2010
Zi
instrument, using a relative pressure (P/P ) interval of 0.001-0.998.
×Z × 100
0
Gl
The samples were previously decontaminated by degassing for 12 h
◦
−5
at 100 C under a vacuum of 1 × 10 Pa. The t-plot method was
used to distinguish between the micro- and mesopore contribu-
tions. Finally, the mesopore distribution was estimated using the
BJH method [24].
where:
input
and nGloutput are the input and output flows of glycerol;
nGI
ni is the flow of the products i (mol/min);
ZGl = 3 (the number of carbon atoms in the glycerol molecule);
Zi = number of carbon atoms in the products.
The deactivation (D) of the catalysts used in the glycerol dehy-
dration was calculated using:
The 27Al NMR spectra were acquired using a Varian INOVA 500
spectrometer equipped with a 7 mm probe and operated at 4.5 kHz.
The experiments were performed using a spinning rate of 78.2 MHz,
acquisition time of 15.4 ms, pulse width of 2.4 ␮s, and recycle delay
of 0.1 s. The Al chemical shifts were referenced to an aqueous
solution of Al(NO ) (1 mol/L). Each spectrum was the result of 256
2
7
(Xglycerol)at timezero − (Xglycerol)after 6h
D(%) =
× 100
3
3
(Xglycerol)at timezero
scans.
Thermogravimetric analyses of the used catalysts (after the glyc-
erol reaction) were performed with a TA Instruments SDT Q600
The glycerol oxidehydration reaction was conducted following
the same protocol as glycerol dehydration, but with the reaction
temperature shifted to 350 C and with 20% of oxygen fed into the
stream (6 mL/min of O2 and 24 mL/min of N2). The O2/glycerol mole
ratio of 4.5 ensured that an excess of oxygen was used.
◦
◦
thermobalance. The samples were heated from 30 to 900 C, at
◦
1
0 C/min, under a 100 mL/min flow of synthetic air.
Scanning electron microscopy (SEM) analyses of the samples
were performed using an Inspect S50 microscope (FEI) operated at
1
1
0 kV, with a secondary electron detector and working distance of
0 mm. After 1 h in an ultrasonic bath, a suspension consisting of the
sample and acetone was deposited on an aluminum stub specimen
mount. The Si/Al mole ratio was determined by energy dispersive
X-ray spectroscopy (EDS), using the same microscope. The spectra
were measured in five different regions, and the composition was
determined by averaging the results obtained for each sample.
The acid sites of the calcined catalysts were determined by
temperature programmed desorption of ammonia (TPD-NH ). Pre-
3
◦
viously, a 200 mg portion of each sample was degassed at 300 C
for 1 h under a 60 mL/min flow of helium. The temperature was
◦
then decreased to 100 C and the sample was exposed for 1 h to a
flow of 1% ammonia in helium (60 mL/min). After surface satura-
◦
tion, the sample was submitted to helium treatment at 100 C for
1
h to remove physisorbed ammonia. The TPD-NH analysis was
3
◦
◦
◦
performed from 100 C to 600 C (at 10 C/min), under a 60 mL/min
flow of helium. The desorbed ammonia was monitored and quan-
tified with a Pfeiffer vacuum mass spectrometer connected to the
outlet stream of the tubular reactor.
2.4. Catalytic tests
The bare zeolite and V-MFI catalysts were tested for glyc-
erol dehydration and oxidehydration, respectively, at atmospheric
pressure. Previously, 100 mg portions of the samples were heated
◦
at 300 C for 15 min, under a 30 mL/min flow of nitrogen. The glyc-
erol dehydration reaction was performed in a fixed-bed reactor
◦
at 300 C for 6 h, using a solution of 10 wt.% glycerol in water. A
flow of 0.05 mL/min of this liquid solution was introduced to a
heated line using a syringe pump (KD Scientific), and the glycerol
was transported to the reactor using a 30 mL/min flow of nitrogen.
The unconverted glycerol and the reaction products were con-
◦
densed continuously in a gas-liquid separator kept at 1 C. The
liquid was periodically collected, weighed, and injected into a gas
chromatograph (Model GC-2014, Shimadzu) equipped with a cap-
illary column (Rtx-1, 30 m, 0.32 mm, 1 ␮m) and a FID detector. For
each injection, a known mass of n-butanol was used as internal
standard. Four drops of standardized solution was diluted in iso-
propanol (1 mL) to prevent excess water from the chromatographic
column. The GC analyses were performed in triplicate, and the
Fig. 1. Rietveld plots of samples A (parent MFI), B (NaOH), C (H2C2O4), and D (HCl),
using ␭ = 1.3775 Å: experimental (black line), calculated (red line), and difference
plot (blue line). (For interpretation of the references to colour in this figure legend,
the reader is referred to the web version of this article.)
Please cite this article in press as: L.G. Possato, et al., The multiple benefits of glycerol conversion to acrolein and acrylic acid catalyzed
by vanadium oxides supported on micro-mesoporous MFI zeolites, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.08.005

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3
. Results and discussion
atoms in the structure tended to distort. The same effect led to
reduced crystallinity of samples B, C, and D.
3
.1. Characterization of the zeolite supports
The relative intensities of the peaks in the NMR spectra were
used for approximate calculation of the amounts of EFA (Table 1).
Comparison of the results for Si/Alglobal and the percentage of EFA
showed that the oxalic acid treatment provided greater selectivity
in removal of EFA without severely affecting the aluminum frame-
work. The micropore volume only changed slightly, from 0.23 to
The XRD patterns of the MFI zeolites before treatment (sam-
ple A in Fig. 1), after the alkaline treatment (sample B), and after
the acidic treatments (samples C and D) revealed the presence of
the main reflections related to the MFI structure: (011), (200), and
3
(
051). The samples submitted to the treatments showed decreases
0.21 cm /g, while the mesopore volume showed a more signifi-
3
in peak intensity, consistent with the reduction of long-range order
caused by the extraction of silicon and aluminum atoms from the
MFI structure [35]. The oxalic acid and HCl treatments also affected
the crystallinity of the samples, as clearly shown in the diffraction
patterns of catalysts C and D, respectively.
The results of structural analysis by Rietveld refinement indi-
cated unit cell expansion after the alkaline treatment [43] (Table 1).
The removal of silicon (which has a mean Si-O bond length of
cant change, from 0.41 to 1.25 cm /g (Table 1). The reactivity of
HCl was evidenced by a more pronounced reduction of the micro-
3
pore volume to 0.18 cm /g. Dealumination of zeolites with oxalic
acid occurs because oxalic acid acts as a hydrolyzing agent by both
solubilizing and removing aluminum oxohydroxide species, and
forming aluminum oxalate complexes [47]. These complexes have
a maximum size of around 0.64 nm, which is compatible with the
pore dimensions of zeolites and enables mobility of the complexes
within the pores [44].
0
.162 nm) and the consequent increase in the concentration of
aluminum in the structure (mean Al-O bond length of 0.175 nm)
The N adsorption-desorption isotherms of the MFI zeolites are
2
3
3
caused unit cell expansion from 5236.9 nm to 5258.1 nm [44,45].
The unit cell expansion was additionally influenced by the pres-
ence of aluminum atoms that remained in oligomeric form inside
the pores of the zeolites. The subsequent acid treatments caused
partial leaching of the EFA, and the structural analyses of these sam-
ples indicated changes in unit cell volume. The decreased unit cell
volumes of the acid treated samples (C and D) revealed that the
treatments were effective. The global chemical analyses of the sup-
ports showed growth in the aluminum fraction from sample A to
sample B (Si/Al ratios of 40 and 18, respectively), and growth in the
silicon fraction from sample B to samples C and D.
Samples C and D were (Table 1) dissimilar in terms of both unit
cell volume and aluminum fraction, due to the differences in the
strengths of the acids used (the pKa values of hydrochloric acid
and oxalic acid are −6.3 and 1.2, respectively) and the mechanism of
EFA leaching. The action of HCl involved solubilization of oligomeric
aluminum oxide species, while oxalic acid acted as both an acid and
a chelating agent.
shown in Fig. 3a. Sample A exhibited a Type I isotherm typical of a
pure MFI microporous structure [48]. After the alkaline treatment,
a hysteresis loop appeared and the capacity of this sample to adsorb
N2 increased, especially at relative pressure (P/P ) above 0.8, due
0
to the creation of intra-crystalline mesopores. Additionally, the dis-
ruption of the micropore structure of the zeolite in alkaline solution
led to a decrease of the adsorption plateau at low P/P characteris-
0
tic of nitrogen adsorption limited by the micropore size. Analyses
of the BJH pore size distribution (Fig. 3b) showed the creation of
mesoporosity in the supports submitted to alkaline and acidic treat-
ments. The pore size distributions broadened after use of the acid
treatments to clean the zeolite surface, giving rise to additional
porosity.
The acid sites of the samples were characterized by
temperature-programmed desorption, using ammonia as
a
basic probe molecule. The TPD-NH3 curves obtained for the bare
and impregnated zeolites are shown in Fig. 4. The bare zeolite
showed well-resolved NH3 desorption peaks, one in the low
The formation and leaching of EFA was followed using 27Al MAS-
NMR (Fig. 2). The chemical shift peak centered at 54 ppm was due
to tetrahedral aluminum atoms in the zeolite structure, while the
signal between 0 and −20 ppm was due to the octahedrally coordi-
nated aluminum of EFA [46]. As mentioned before, the desilication
process was not only responsible for the extraction of silicon atoms
from the structure, but also for aluminum removal, as is clearly
shown in the detail of the decomposed lines for desilicated sample
B. In contrast, the parent zeolite contained only a minor amount
of EFA. A rigorous comparison of the NMR spectra showed that
the peak at 54 ppm became slightly broadened after the chemical
processes, because the tetrahedral arrangement of the aluminum
temperature (LT) region at around 200 C, and one in the high
◦
◦
temperature (HT) region at around 420 C (Fig. 4a). The reference
sample (␥-Al O ) presented a low amount of desorbed NH and a
2
3
3
wide desorption temperature range, characteristic of the presence
of Lewis acid sites. The amounts of NH desorbed from the samples
3
are given in Table 1. The NaOH treatment resulted in increased
NH desorption, due to the decrease in the Si/Al ratio (Table 1), in
3
27
agreement with the results of the Al NMR and chemical analyses.
Treatment of zeolite sample B with oxalic acid led to a decrease in
the quantity of acidic sites, due to the removal of EFA, as found in
2
7
the Al NMR analyses. The treatment with HCl was less effective
in removing the EFA, so the acidity was greater than for the sample
treated with oxalic acid [49].
Table 1
Chemical and textural properties of the bare and impregnated zeolite samples.
Samples
Vmicro
3
Vmeso
(cm /g)
␮mol of
NH3/g
Si/Alglobala
% EFAb
Unit cell
volume (nm )
3
3
(
cm /g)
A
0.33
0.23
0.21
0.18
0
0.26
0.18
0.21
0.20
0
388
464
406
529
110
579
545
427
581
40
18
24
26
12
29
5
11
n/a
n/a
n/a
n/a
n/a
5236.9
5258.1
5250.0
5236.6
n/a
n/a
n/a
n/a
n/a
B − NaOH
C − H2C2O4
D − HCl
Reference Al2O3
V-A
V-B
V-C
V-D
0.41
1.25
1.30
0.89
0
0.21
0.24
0.24
c
n/a
n/a
n/a
n/a
n/a
a
Silicon to aluminum mole ratio determined by EDX (global chemical composition).
%
b
c
EFA determined by deconvolution of the 27Al NMR spectrum (contribution of tetrahedral aluminum species in the zeolite framework).
Not applicable.
Please cite this article in press as: L.G. Possato, et al., The multiple benefits of glycerol conversion to acrolein and acrylic acid catalyzed
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5
5
4 ppm
a
b
Sample B
D
C
B
-
1 ppm
-13 ppm
A
8
0
60 40 20
0
-20
80 60 40 20
0
-20
Chemical Shift (ppm)
Chemical Shift (ppm)
Fig. 2. (a) 27Al NMR spectra of the parent and treated zeolites, and (b) deconvolution of the sample B spectrum.
2
.0
A
B
C
D
a
b
1.5
5
00
1
0
0
.0
.5
.0
0
.0 0.2 0.4 0.6 0.8 1.0
1
10
100
Relative pressure P/P0)
(
Pore diameter (nm)
Fig. 3. (a) Nitrogen adsorption-desorption isotherms (filled and empty points correspond to nitrogen adsorption and desorption, respectively), and (b) BJH mesopore size
distributions from the desorption branches.
Fig. 4. Temperature-programmed desorption of ammonia (TPD-NH3) curves for (a) the bare zeolites, and (b) the impregnated zeolites.
The TPD-NH curve profiles of the vanadium oxide-impregnated
3.2. Characterization of the vanadium oxide-impregnated zeolites
3
samples (Fig. 4b) were significantly different, compared to those
of the bare zeolites (Fig. 4a), with the LT and HT peaks shifted to
lower temperatures. The LT peak increased in intensity, while the
HT peak ended at around 450 C, instead of at the value of ca. 550 C
observed for the bare zeolite. This could be explained by coverage of
the strongest acid sites by vanadium oxide species. The quantities of
Quantitative phase analyses of the diffraction patterns for the
impregnated VA, VB, VC, and VD catalysts (Fig. S1) revealed small
◦
◦
quantities of crystalline V O5 species. In terms of zeolite crys-
2
tallinity, the XRD patterns were similar to those for the bare
zeolites. In the treatment of the XRD data, the crystallography infor-
mation file (CIF) from the International Zeolite Association (IZA)
was adopted for the MFI zeolite, while the 29140 standard from the
desorbed NH were higher for all the samples containing vanadium
3
oxide, reflecting a predominance of weak Lewis acid sites [50,51].
ICSD database was used for V O5. From comparison of the samples,
2
a crystalline V O5 mass fraction of 9.0% was obtained for the parent
2
Please cite this article in press as: L.G. Possato, et al., The multiple benefits of glycerol conversion to acrolein and acrylic acid catalyzed
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the previous finding of greater dispersion of V O5 on the porous
zeolites.
2
VD
VC
VB
VA
373 nm
3
3
.3. Catalytic activity
335 nm
.3.1. Step 1 − glycerol dehydration
The glycerol dehydration reaction occurs in two sequen-
tial reactions. In the first reaction step, glycerol dehydrates on
acid sites, giving rise to a water molecule and a very reactive
3
-hydroxypropene intermediate that rapidly isomerizes via a keto-
enol rearrangement to 3-hydroxypropanal. The 3-hydroxypropanal
molecule then dehydrates, releasing water and acrolein. This reac-
tion sequence occurs at protonic Brønsted acid sites [12,30,54]. In
contrast to the performance of the acid sites of the zeolites, a plot
obtained using a reference alumina (which mainly contained Lewis
acid sites) as catalyst reflected low levels of activity and stability in
the reaction (Fig. 6, Table 2).
2
00
300
400
500
Wavelenght (nm)
Fig. 5. UV–Vis diffuse reflectance spectra of samples containing vanadium oxide.
The catalytic results obtained for samples A, B, C, and D are
summarized in Fig. 6. Regardless of the sample or the reaction
time, the main product of glycerol dehydration was acrolein. Glyc-
erol conversions of 88% and 87% were achieved using sample A
and desilicated sample B, respectively. The beneficial effect of the
micro-mesoporosity of sample B was shown by a decrease in cata-
lyst deactivation from 19% to 7%.
catalyst, which was very close to the 10% mass percentage used in
preparation of the samples, while V O5 mass fractions between 4%
2
and 6.5% were obtained for the desilicated and acid treated samples.
This effect of dispersion on the support, with production of poorly
crystallized V O5 particles, could be attributed to the creation of
2
mesopores and the capacity to cover the zeolite surface during the
wet impregnation process. Comparison of the mesopore volumes
of the bare and impregnated zeolites (Table 1) confirmed the dis-
persion of V O5 in the mesopores. The important influence of V O5
The acid treated samples (C and D) both showed glycerol con-
version exceeding 90% during the first hour of reaction. However,
there was more pronounced deactivation of the sample treated
with hydrochloric acid (sample D), with deactivation values of 5.2%
and 18.4% for samples C and D, respectively. These findings indi-
cated that both acids caused leaching of EFA (Table 1), and that
HCl was less selective than oxalic acid, because it also removed
part of the structural aluminum, damaging the zeolite structure and
consequently affecting the activity of the catalyst in the reaction.
As mentioned before, all the samples were more selec-
tive to acrolein, compared to other reaction products such as
acetaldehyde, 3-hydroxypropanal, and acetol (Fig. 7). The prod-
uct distributions obtained for sample A after 1 and 6 h (Fig. S2)
were indicative of decreased selectivity to acetaldehyde (2 carbon
atoms) and increased selectivity to 3-hydroxypropanal (3 carbon
atoms). This suggested that decomposition of the primary molecule
occurred at the beginning of the reaction, and that this decomposi-
tion was associated with coke formation and COx release [30]. The
decomposition was related to contact of the glycerol molecule and
products with strong acid sites on the catalyst surface that became
progressively covered with coke, resulting in deactivation of the
strong acid sites. This was also reflected in the lower selectivity to
2
2
dispersion on catalytic activity is discussed below.
The diffuse reflectance spectroscopy measurements in the
UV–Vis region (Fig. 5) showed distinct results for the impregnated
samples. The shapes of the spectra in this region were influenced
by the local structures of the vanadium atoms. For example, the
spectral range 333–500 nm provides information about the trans-
5+
fer of low energy electrons between O and V for vanadium atoms
in octahedral coordination [52,53]. On the other hand, the range
2
85–333 nm is related to vanadium in tetrahedral coordination.
The green dashed rectangle highlighted in Fig. 5 indicates a region
influenced by the vanadium oxide environment. Bands at 335 nm
5+
and 373 nm were due to electron charge transfer of V with ter-
minal or bridging oxygen anions, respectively. In other words, the
highlighted region was associated with terminal and supported
vanadium oxides. The main peak occurred at 384 nm, although
shifts to higher energy (or lower wavelength) could occur, depend-
ing on the degree of vanadium oxide coordination by water. Hence,
the systematic increase in intensity of this band complemented
Glycerol dehydration
Glycerol oxidehydration
1
00
1
00
8
0
8
6
4
2
0
0
0
0
0
60
40
20
0
VA - 3 mL/h
A
B
C
D
VA
VB
VC
VD
Al O
2
3
0
1
2
3
4
5
6
0
1
2
3
4
5
6
Time (h)
Time (h)
Fig. 6. Glycerol dehydration and oxidehydration of samples without (left plot) and with vanadium oxide (right plot). The filled and empty symbols correspond to flows of
3
.0 mL/h and 6.0 mL/h of glycerol solution, respectively.
Please cite this article in press as: L.G. Possato, et al., The multiple benefits of glycerol conversion to acrolein and acrylic acid catalyzed
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Table 2
Reactant flow, conversion at time zero, catalyst deactivation, and %wt. of coke after 6 h on glycerol stream: (1) glycerol dehydration, and (2) glycerol oxidehydration.
Reaction
Catalyst
10 wt.% glycerol
solution flow (mL/h)
Conversion at
time zero (%)
Catalyst
deactivation (%)
% wt. of
coke
(
1)
A
B
3
90
89
19.4
7.0
12.6
15.5
C
D
100
100
65
100
89
5.2
18.4
68.5
17.8
14.1
55.5
12.8 (4.7)
12.2
Al2O3
V-A
V-B
V-C
V-D
a
(
2)
6
42.4 (0.0)
22.4 (0.0)
20.9 (0.0)
20.9 (0.0)
93
100
10.4
14.4
a
In brackets are indicated the results of catalyst deactivation under a flow of 3.0 mL/h of glycerol solution.
pockets created by desilication with NaOH and acid leaching, main-
taining the micropores available for glycerol [24].
A
B
C
D
60
50
40
30
20
10
3
.3.2. Steps 1 and 2 − glycerol oxidehydration
As mentioned before, the two steps of glycerol oxidehydration
are (i) the dehydration of glycerol to acrolein on acid sites [2,30],
and (ii) the conversion of acrolein to acrylic acid on redox sites [55].
Therefore, in the impregnated catalysts, the catalytic sites respon-
sible for glycerol dehydration were the Brønsted acid sites, while
V O5 (better represented as V O5-x) was responsible for acrylic
2
2
acid formation. The V O5 impregnation affected the acid sites of
2
the catalysts, increasing the contribution of the Lewis acid sites
and covering the strong Brønsted acid sites, as observed from the
0
0
VA
VB
VC
VD
6
5
4
3
2
1
TPD-NH analyses. As reported previously, the amount of impreg-
3
nated V O5 significantly affected the proportions of acid and redox
2
0
0
0
0
0
0
sites, leading to substantial alterations in catalyst activity [26]. The
main difference between the two groups of samples concerned cat-
alyst stability. No deactivation at all was observed for the samples
impregnated with vanadium oxide. In order to be able to observe
the deactivation and compare samples B, C, and D, the flow of glyc-
erol fed to the reactor was increased from 3 to 6 mL/h (see VA results
in Fig. 6, open and closed symbols). This change enabled detection
of 42.4% deactivation of the VA catalyst. Despite the deactivation
under this new experimental condition, the presence of vanadium
oxide reduced the loss of catalytic activity, compared to the bare
zeolites, demonstrating the multifunctional characteristics of these
catalysts in promoting conversion, with selectivity to acrylic acid
and stability of catalytic activity over longer periods.
The desilicated and impregnated sample (VB) showed decreased
deactivation in the dehydration reaction, compared to the VA cat-
alyst, similar to the behavior observed for the bare zeolites. The
coke was deposited in the formed mesopores, avoiding obstruction
of the micropores. In addition, the removal of EFA by the action of
oxalic acid, followed by impregnation (catalyst VC), resulted in a
further small reduction in deactivation, which decreased to 20.9%.
An interesting result was obtained for the sample treated with
Fig. 7. Selectivity towards condensable products and carbon balance in dehydration
◦
(
top) and oxidehydration (bottom) of the catalysts during 1 h of reaction at 300 C
◦
and 350 C, respectively.
acetic acid after 6 h of reaction (Fig. S2), since acetic acid is a prod-
uct formed from acetaldehyde oxidation. Furthermore, and most
importantly, there was increased selectivity to acrolein.
hydrochloric acid and impregnated with V O5 (sample VD). This
2
zeolite and the reference alumina showed the highest levels of
deactivation in the dehydration of glycerol. However, in the oxide-
hydration reaction, the VD catalyst presented deactivation very
close to that of the VC catalyst, because the action of vanadium
oxide helped to reduce coke formation by oxidizing smaller coke
molecules before oligomerization. Furthermore, the mesoporosity
helped in dispersing vanadium oxide on the surface of the zeolite,
as confirmed by the UV–vis analysis that showed an increase in the
signal at 335–373 nm.
The distribution of the products was also affected by the treat-
ments applied to the samples (Fig. 7 and S3). Use of sample VA
resulted in 5% of acrylic acid after the first hour on stream, with
selectivity to acrylic acid increasing to 25% after 6 h. These sam-
ples contained an average of 12 wt.% coke, so the shift in the acrylic
Different behavior was observed for samples B, C, and D, with
greater amounts of carbonaceous deposits detected, compared
to the parent zeolite, despite lower deactivation. The thermo-
gravimetric analyses of the spent catalysts (Fig. S3) enabled
establishment of relationships between the mesopore volumes and
the amounts of carbonaceous compounds deposited on the cata-
lysts, based on the weight loss. In the case of the purely microporous
sample A, the coke was mainly deposited within straight and sinu-
soidal micropores, or in the entrances of pores, blocking the access
of new glycerol molecules. The effect of coke on catalytic perfor-
mance was more noticeable, even for low coke contents. In porous
zeolites, the coke is preferentially located in the intra-crystalline
Please cite this article in press as: L.G. Possato, et al., The multiple benefits of glycerol conversion to acrolein and acrylic acid catalyzed
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Fig. 8. TG (a) and DTA (b) curves of the catalysts impregnated with vanadium oxide after 6 h of reaction.
acid yield could be explained by the coverage by coke of the more
deactivating sites. It should be noted that the acrylic acid selectivity
showed a dependence on the glycerol flow. A flow of 3.0 mL/h was
not only beneficial to catalyst stability, but also enabled selectiv-
ity to acrylic acid of 17% to be achieved in the first hour. Conditions
providing greater selectivity could therefore be obtained by adjust-
ing the glycerol contact time. The initial acrylic acid selectivity of
the VB catalyst (14%) was higher than obtained for the VA sample.
Other products of glycerol oxidehydration are acetic acid and
acetaldehyde, which both arise from the bifunctional nature of the
catalyst. The acid sites crack the initial three carbon atoms of the
glycerol molecule, resulting in acetaldehyde and one molecule of
COx. The presence of O2 then leads to the oxidation of acetalde-
hyde to acetic acid. The selectivities to these two products were
lower than 10% in the first hour of reaction, and decreased after
position of carbonaceous species on the catalyst surface. The high
porosity of this sample (Table 1) was likely to have contributed
to the oxidation of coke precursors at low temperatures. For com-
parison, the thermogram of the most coked sample A is present in
Fig. 8a . The largest DTA signal (Fig. 8b) represents the two species
of coke formed.
As mentioned, despite the Lewis acid sites are formed in large
amounts after V O5 impregnation, condition that could damage
2
the catalytic activity as shown for ␥-Al O , the maintenance of
2
3
Brønsted acid sites with the coverage of the strong Brønsted acid
sites by the impregnation was an important aspect for maintaining
catalytic activity for a prolonged time.
4. Conclusions
6
h due to deactivation of the sites. This behavior was the oppo-
site of that observed for the acrylic acid selectivity, suggesting that
the same sites were involved. The carbon balance between the glyc-
erol feed and the outflowing condensable liquid products increased
The alkaline treatment of ZSM-5 zeolite with NaOH was effec-
tive for obtaining a micro-mesoporous zeolite by partial disruption
of the crystalline framework due to silicon removal. A portion of the
framework aluminum atoms in tetrahedral coordination was also
removed, but due to their low solubility, these atoms remained
on the zeolite as EFA (extra-framework aluminum). The removal
of EFA was achieved by subsequent acid treatment using either
hydrochloric acid or oxalic acid. The latter was more effective,
because although hydrochloric acid acted to dissolve oligomeric
aluminum oxide species, it also attacked the crystalline zeolite
structure, while oxalic acid additionally acted as a chelating agent.
The micro-mesoporous zeolites were subsequently impregnated
with the glycerol flow and in the presence of V O5. At glycerol solu-
2
tion flow rates of 3.0 mL/h (for the bare zeolites) and 6.0 mL/h (for
the impregnated samples), the average carbon balances were 30%
and 55%, respectively.
The activity results showed that the mesopores in the catalysts
assisted the mass transport of reactant and products, and there-
fore promoted the catalytic activity. In addition, the formation of
the extra porosity assisted in accommodation of the much less
volatile polymeric coke molecules, which remained deposited in
the intra-crystalline pockets within the zeolite mesopores, hence
maintaining access to the micropores. Two types of coke are formed
in glycerol reactions: (1) polyaromatic compounds formed in rad-
ical reactions, and (2) polyglycols formed by the condensation
of neighboring glycerol molecules [26]. Previous characterization
studies [20–22] have shown that both types are present in bare
zeolites, while the polyglycol type is suppressed following impreg-
with vanadyl sulfate solution and calcined to form supported V O5.
2
Spectroscopic measurements in the UV–Vis region indicated that
V O5 was well dispersed on the porous zeolites, due to the high
2
pore volumes. The dispersion of V O5 increased the quantity of acid
2
sites on the zeolites. The beneficial effects of micro-mesoporosity
and V O5 dispersion could be seen in the performance of the cat-
2
alysts during glycerol conversion. The selectivities to acrolein and
acrylic acid were enhanced, and the smaller amounts of coke led to
reduced deactivation.
nation with V O5.
2
The results of the thermogravimetric analyses of the spent cat-
alysts are shown in Fig. 8. The DTA curves of the catalysts with
vanadium oxide showed two exothermic peaks (Fig. 8b). The first
◦
peak, at lower temperature (∼350 C), was associated with the re-
4+
5+
oxidation (exothermic event) of vanadium species (V to V ) in
Acknowledgements
the case of the spent V O5/ZSM-5 catalysts [20]. The second peak, at
2
◦
about 500 C, was due to polyaromatics (known as stubborn coke)
This work was supported by the Brazilian agencies CNPq
(grants 473456/2012-5 and 401679/2013-6) and FAPESP (grants
2013/10204-2, 2013/50023-7 and 2014/11952-5). The authors also
thank the Brazilian Synchrotron Light Laboratory (LNLS) in Camp-
inas for use of the XPD beamline (proposal XPD-17839).
[
8,56,57]. In the first temperature region, some samples showed a
4+
slight increase in mass (Fig. 8a), associated with the oxidation of V
5+
to V (highlighted in Fig. 8b) [26]. The VD sample showed a small
◦
peak near 200 C, which could have been related to the decom-
Please cite this article in press as: L.G. Possato, et al., The multiple benefits of glycerol conversion to acrolein and acrylic acid catalyzed
by vanadium oxides supported on micro-mesoporous MFI zeolites, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.08.005

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Appendix A. Supplementary data
[26] L.G. Possato, W.H. Cassinelli, T. Garetto, S.H. Pulcinelli, C.V. Santilli, L. Martins,
Appl. Catal. A: Gen. 492 (2015) 243–251.
[
27] L.H. Vieira, K.T. Carvalho, E.A. Urquieta-González, S.H. Pulcinelli, C.V. Santilli,
L. Martins, J. Mol. Catal. A: Chem. (2015), http://dx.doi.org/10.1016/j.molcata.
2015.12.019 (in press).
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.cattod.2016.08.
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Please cite this article in press as: L.G. Possato, et al., The multiple benefits of glycerol conversion to acrolein and acrylic acid catalyzed
by vanadium oxides supported on micro-mesoporous MFI zeolites, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.08.005