Surfactant-assisted synthesis of Mo-V mixed oxide catalysts for upgraded one-step conversion of glycerol to acrylic acid

Article abstract of DOI:10.1039/c8ra01443d

The catalytic properties of Mo-V mixed oxides hydrothermally synthetized in the presence of ionic surfactants (SDS and CTAB) were investigated in the gas-phase oxidative dehydration of glycerol. The presence of surfactants promoted a change in morphology of MoV₂O₈ phase, directing to formation of rod-shaped crystals, and, consequently, an increase in macroporosity of materials, generated by intercrystallite spaces, when compared to a reference sample. Rod-like morphology stabilized the MoV₂O₈ mixed oxide phase during glycerol conversion, avoiding migration of vanadium from crystalline to amorphous phase, like observed in the reference sample, favoring the dynamic of reduction/reoxidation of vanadium and, consequently contributing to an increase in efficiency and stability of the catalyst. Both SDS and CTAB catalysts presented higher productivity of acrylic acid and good catalytic stability, with no coke formation and considerable decrease in CO_X evolution during 6 h of reaction. SDS presented the best catalytic results with 100% of conversion, 57% of acrylic acid selectivity and 36% of CO_X selectivity.

Full text of DOI:10.1039/c8ra01443d

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Surfactant-assisted synthesis of Mo–V mixed oxide
catalysts for upgraded one-step conversion of
glycerol to acrylic acid†
Cite this: RSC Adv., 2018, 8, 11975
Letıcia F. Rasteiro,
Luiz H. Vieira,
Celso V. Santilli
and Leandro Martins
*
´
The catalytic properties of Mo–V mixed oxides hydrothermally synthetized in the presence of ionic
surfactants (SDS and CTAB) were investigated in the gas-phase oxidative dehydration of glycerol. The
2 8
presence of surfactants promoted a change in morphology of MoV O phase, directing to formation of
rod-shaped crystals, and, consequently, an increase in macroporosity of materials, generated by
intercrystallite spaces, when compared to a reference sample. Rod-like morphology stabilized the
2 8
MoV O mixed oxide phase during glycerol conversion, avoiding migration of vanadium from crystalline
to amorphous phase, like observed in the reference sample, favoring the dynamic of reduction/
reoxidation of vanadium and, consequently contributing to an increase in efficiency and stability of the
catalyst. Both SDS and CTAB catalysts presented higher productivity of acrylic acid and good catalytic
Received 14th February 2018
Accepted 21st March 2018
stability, with no coke formation and considerable decrease in CO
presented the best catalytic results with 100% of conversion, 57% of acrylic acid selectivity and 36% of
CO selectivity.
X
evolution during 6 h of reaction. SDS
DOI: 10.1039/c8ra01443d
rsc.li/rsc-advances
X
one catalyst with bifunctional properties is getting more atten-
Introduction
15
tion due to several advantages. Mixed oxides based on
, are prominent catalysts
the coproduced glycerol, the search for new uses for it becomes to perform the one-step conversion of glycerol due to the
Due to the increase in biodiesel production and, consequently, molybdenum and vanadium, Mo V O
x y z
1,2
indispensable. Many studies have been reported for glycerol moderate acidity and well-known oxidative potential of vana-
valorisation in the production of different substances using dium in these structures, conducted by the Mars–van-Krevelen
3
–5
6–8
8,16
various processes like dehydration, oxidative dehydration,
mechanism.
However, the main disadvantages of these
9
10
11
acetalization, acetylation and hydrogenolysis. A high value- catalysts are the textural properties, like low specic areas and
8
,17–19
added product that can be obtained from glycerol oxidative porosity,
and the instability of the mixed oxide phases in
20
dehydration and that has attracted attention because of the the reaction conditions. The solid-state reaction and/or
increase in its market demand is acrylic acid. Nowadays, spreading of mixed oxide phases when applied in catalytic
acrylic acid is industrially produced from a two-step oxidation of systems under high temperatures and oxidant atmospheres can
12
20
propylene, a non-renewable route, since propylene is derived lead to a restructuration of phases to form single component
12,13
from petroleum.
oxides and/or migration of active species to amorphous phase.
In the last years, several studies demonstrated the viability of
The use of surfactants in the synthesis of catalysts is
the catalytic conversion of glycerol to acrylic acid by means of a method to create porous structures in the material and/or to
21,22
a two-stage reaction depicted in Scheme 1. In the rst stage, alter surface characteristics.
The addition of surfactants in
glycerol is dehydrated to 3-hydroxypropanal on acid sites, fol- the synthesis of Mo can bring new textural properties and
V O
x y z
lowed by subsequent dehydration and formation of acrolein. In lead to a stabilization of the mixed oxide phases in these cata-
the second stage, the oxidation of acrolein to acrylic acid occurs lysts, not yet studied.
14
on metal–oxygen redox sites. Therefore, for the selective
synthesis of acrylic acid from glycerol, the catalysts need to
12
possess acid and oxidant sites. This route of glycerol valori-
zation can be performed with one or two catalysts, but the use of
Instituto de Qu ´ı mica, Unesp – Universidade Estadual Paulista, Rua Prof. Francisco
Degni 55, CEP 14800-900 Araraquara, SP, Brazil. E-mail: leandro@iq.unesp.br
Scheme 1 Two-step glycerol oxydehydration: dehydration to acrolein
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c8ra01443d
followed by oxidation of acrolein to acrylic acid.
1
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 11975–11982 | 11975

View Article Online
RSC Advances
Paper
In a previous work, we studied the effect of synthesis and in the samples were identied with Crystallographica Search
calcination atmospheres in the formation of Mo–V mixed oxides Match soware and quantied by the Rietveld renement
crystalline phases prepared by a hydrothermal method and method using TOPAS® 4.2 soware. The occupancy and
evaluated their performance in the one-pot glycerol oxy- temperature factors of all atoms were xed, unit cell parame-
6
dehydration to acrylic acid. We identied the MoV
2
O
8
and ters, scale factor and atomic positions (except for special posi-
0.35
Mo4.65V O14 as the most active phases in the reaction but with tions: 0, 1/4, 1/3, 1/2 etc.) were rened. Sample displacement,
low stability during reaction. Herein, we describe the zero error and intensity corrections (Lorentz and polarization
surfactant-assisted synthesis and structural characterization of factors were xed in 26.4, characteristic value for graphite
x y z
Mo V O catalysts and explore how the presence of SDS and monochromators) were rened. The background was tted
CTAB surfactants in the synthesis mixture inuences the using a sixth-order Chebyshev polynomial function. Funda-
composition of phases, porosity, crystal morphology, and mental parameters prole tting (FPPF) were used for the peak
catalytic activity of these materials in the one-step oxidative prole renements.
dehydration of glycerol.
Specic BET area of the samples were determined by
nitrogen sorption measurements performed at liquid nitrogen
ꢀ
temperature (ꢁ196 C), with relative pressure interval between
Experimental
Hydrothermal synthesis of the Mo–V mixed oxides
0
.001 and 0.998, using a Micromeritics ASAP 2010 system. The
samples were pre-treated under vacuum (ꢂ10 ꢃ 10 Pa) for
ꢁ
6
ꢀ
23
1
2 h at 200 C.
The synthesis of the mixed oxides was performed based on
a hydrothermal method described in our previous work. In
a typical synthesis, vanadyl sulfate (VOSO
6
Density of the samples was evaluate by helium pycnometry
using a Micromeritics AccuPyc 1330 and then transferred to
a Micromeritics GeoPyc 1360 to evaluate the total porosity of the
samples.
4
) and ammonium
paramolybdate ((NH Mo 24$4H O) were dissolved separately
4
)
6
7
O
2
in distilled water. Both mixtures were maintained under stirring
ꢀ
Hg intrusion porosimetry was used to evaluate the textural
characteristics of the materials in the macro- and mesopore
ranges. The measurements were carried out in a Micromeritics
Autopore III equipment, operating for the determination of
pores with a minimum diameter of 3 nm. The pore diameter
was calculated by means of the Washburn equation (r ¼
for 10 min at 25 C and aer the complete dissolution, they were
ꢀ
mixed and stirred for another 10 min at 25 C, resulting in
a solution with a pH of 2.8 and a Mo/(Mo + V) mole ratio of
6,8
0
.60. The mole proportion of the reagents was 1 VOSO : 0.2
4
4 6 7 2 2
(NH ) Mo O24$4H O : 300 H O. Aerwards, sodium dodecyl
sulfate (SDS) or cetyltrimethylammonium bromide (CTAB) were
added to the solution and maintained under stirring for 40 min
ꢁ
2g cos q/P), using surface tension and contact angle values of
0.489 N m and 135 , respectively.
ꢁ
1
ꢀ
ꢀ
at 25 C. The mole ratio surfactant/(Mo + V) was 0.10. Later, the
Particle size and shape of the catalysts were observed by SEM
images on a FEI Magellan 400L microscope. Samples were
previously deposited on an aluminum sample holder and
subsequently sputtered with gold for 30 s.
solution was transferred to a 50 mL Teon vessel contained in
a stainless steel autoclave reactor, occupying approximately
70% of the vessel capacity. The reactor was hermetically closed
and then purged with pure O in successive cycles and nally
2
Thermogravimetric analysis (TGA) of the spent catalysts was
was pressurized with O
2
to 6 bar. The reactor was placed in
ꢁ1
ꢀ
conducted under a ow rate of 100 mL min of O
, using an
2
a glycerol bath heated at 160 C and kept for 48 h under static
conditions. The precipitate was separated by centrifugation and
SDT Q600 TGA/DSC thermobalance, with a heating rate of
ꢀ
ꢁ1
ꢀ
ꢀ
10 C min from 30 to 600 C.
dried at 60 C for 12 h. The resulting catalyst precursors were
ꢀ
Acid sites of the Mo–V mixed oxides were measured by
heat treated in oxidant atmosphere at 500 C during 2 h. The
temperature programmed desorption of ammonia (TPD-NH
3
).
sample synthesized in the absence of surfactant was named
reference and samples synthesized in the presence of SDS and
CTAB were named SDS-0.10 and CTAB-0.10, respectively.
In these experiments, a mass of 200 mg of sample was placed
ꢁ
1
ꢀ
under a He ow (60 mL min ) at 300 C for 1 h, followed by
ꢀ
cooling to 100 C. At this temperature, the sample was exposed
to a 60 mL min ow of 1% NH in He for 1 h. The excess and
ꢁ
1
3
Characterization
ꢀ
physically adsorbed NH
3
was then purged at 100 C under a ow
The amounts of V and Mo present in the synthesis solutions of He during 60 min. Finally, ammonia was desorbed in a He
ꢁ
1
ꢀ
(
mother liquors) were determined by chemical analysis by using ow of 60 mL min , with heating from 100 to 500 C at a rate of
ꢀ
ꢁ1
an Optima 8000 ICP-OES spectrometer. The amounts of V and 10 C min . The amount of desorbed ammonia per gram of
Mo present in the solid catalysts were determined by mass catalyst was calculated from measurements made using a mass
balance between the initial quantities fed into the hydrothermal spectrometer (PrismaPlus QMG 220, Pfeiffer).
reactor and those obtained by chemical analysis.
XPS measurements of the catalysts were performed using
Crystalline phases of the catalysts, before and aer the a UNI-SPECS UHV spectrometer with Mg-Ka source (l ¼ 1253.6
catalytic reaction, were assessed by X-ray diffraction (XRD) eV), pass energy of 10 eV, and 0.2 eV energy step. The pelleted
ꢁ11
using a Rigaku Miniex 600 diffractometer with CuKa radiation samples were rst le overnight under vacuum (<3.7 ꢃ 10
0.15418 nm) selected with a curved graphite monochromator. Pa), aer which the analysis was performed at a residual pres-
(
ꢀ
ꢁ11
Data were collected in the 2q ( ) range from 5 to 60, with a scan sure below 3.7 ꢃ 10
Pa inside the chamber. The binding
step of 0.02 and counting time of 1 s. All the crystalline phases energies of the chemical elements were determined by tting
11976 | RSC Adv., 2018, 8, 11975–11982
This journal is © The Royal Society of Chemistry 2018

View Article Online
Paper
the measured spectra and referencing to the C1s (285.1 eV) where ngl and ngl
RSC Advances
input
output
are the molar ow rates of glycerol in the
ꢁ
1
peak, with accuracy of ꢄ0.1 eV. The tting of the curves was input and in the output (mol min ), n is the molar ow of
i
performed with CasaXPS soware, using the Shirley baseline product i, zgl is the number of carbon atoms in the glycerol
function and Voigt prole (70% Gaussian and 30% Lorentzian). molecule, and z is the number of carbon atoms in the
i
27
The critical packing parameters or shape factors (g) of CTAB products.
and SDS surfactants were calculated according to eqn (1):
g ¼ v/a
0
l
c
(1)
Results and discussion
where v is the volume of hydrophobic portion of the molecule,
The hydrothermal synthesis procedure allowed a very high solid
yield to be obtained, with consequently a high assimilation of
molybdenum of 98.8% and vanadium of 97.3% in the catalysts.
In fact, by visual observation, the mother liquor was very clear
and therefore qualitatively free from reactants.
a
0
is the optimum head group area and l is the critical length of
hydrophobic tail. The area of head groups, a , for micelles of
CTAB and SDS in an aqueous solution are 0.64 and 0.62 nm ,
c
6
0
2
24,25
respectively.
The volume and length of hydrophobic tails
were calculated by eqn (2) and (3), respectively:
The diffraction patterns (Fig. 1) show that despite the use of
the surfactants in the syntheses, the formed Mo V O crystalline
À
Á
0
ꢃ 10ꢁ3 nm
3
x
y z
v ¼ 27:4 þ 26:9n
c
(2)
(3)
0.35 2 8
phases are essentially the same, i.e. Mo4.65V O14, MoV O and
0
MoO3 (Table S1†). The results obtained by X-ray diffraction
suggests that vanadium in crystalline phase was incorporated in
the mixed oxide structures, since no peaks referent to crystalline
V O were identied. Table 1 shows the textural and surface
2 5
properties of the mixed oxides, BET area, pore volume, porosity
and surface acidity. The use of both surfactants increased the
l
c
¼ 0:15 þ 0:1265n
c
nm
0
where n c is one less than the total number of carbons in the
surfactant tail, n , since the rst carbon aer the head group is
c
26
highly solvated and is considered part of it.
Glycerol oxydehydration
ꢀ
The catalytic reaction was conducted in the gas phase at 320 C,
under atmospheric pressure and using a xed bed reactor.
Firstly, 200 mg of the calcined catalyst was supported on a glass
wool placed inside the reactor. The reactor was heated to the
ꢁ1
reaction temperature under a ow of 30 mL min of 100% of
and maintained at this temperature for 15 min before
O
2
initiating the reaction. An aqueous solution containing 10 wt%
of glycerol (Sigma-Aldrich, 99%) was fed to the reactor at a ow
ꢁ1
rate of 0.05 mL min . The gas composition was glycerol/
oxygen/water ¼ 2/28/70 (% mole). The products outowing the
ꢀ
reactor were condensed in a gas–liquid separator kept at 1 C
and aliquots were withdrawn at every 1 h. A previous test was
performed to conrm the efficiency of the condensation and the
full recovery of the condensable products outowing the
reactor, by injecting an aqueous solution containing 10 wt% of
acetaldehyde (i.e. the most volatile product detected, with
ꢀ
a boiling point of 20 C) into the reactor and determining the
mass balance in the gas–liquid separator. The products were
quantied using a gas chromatograph (Shimadzu GC-2014)
equipped with a capillary column (RTX-1, 30 m, 0.32 mm, 1
mm) and a ame ionization detector (FID). As part of the prod-
ucts was converted to CO and CO
able to detect them, the produced CO
2
and the FID detector is not
(CO + CO ) was deter-
X
2
mined by mass balance with the liqueed products. The
quantication of the products was done by adding a known
mass of n-butanol as an internal standard before each injection
and comparing with calibration curves of the products.
The glycerol conversion (Xgl) and selectivity of products (S )
i
were calculated according to eqn (4) and (5), respectively.
input
Xgl (%) ¼ [(ngl
output input
ꢁ ngl
)/ngl ] ꢃ 100
(4)
Fig. 1 XRD patterns and identification of phases for the Mo–V mixed
oxides. The net results of the Rietveld analyses are shown in red and
input
output
S
i
(%) ¼ [n
i
/(ngl
ꢁ ngl
)] ꢃ (z
i
/zgl
)
(5) the deviations from the measured diffractograms are depicted in blue.
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 11975–11982 | 11977

View Article Online
RSC Advances
Paper
Table 1 Textural and acidic properties of the Mo–V mixed oxides
a
b
Porosity
(%)
Pore volume
BET area
Chemisorbed
XRD
MoV
size (nm)
2
O
8
crystallite
Critical packing
parameter (g)
3
ꢁ1
2
ꢁ1
ꢁ1 c
d
e
f
Catalyst
(cm g
)
(m g
)
NH
3
(mmol g
)
intensity (I001/I111
)
Reference
CTAB-0.10
SDS-0.10
69.3
77.9
82.7
0.22
0.51
1.21
11.4
5.9
1.4
38.9
33.9
27.0
3.51
5.65
5.37
41.0 ꢄ 1.3
72.1 ꢄ 5.4
88.5 ꢄ 7.7
—
0.33
0.34
a
b
c
d
Determined by He-pycnometry. Determined by Hg-porosimetry. Determined by TPD-NH
3
.
Intensity ratio of (001) and (111) crystalline plans
f
e
from MoV
2 8
O phase. Determined by LVol-IB of full peak prole from Rietveld analysis performed on TOPAS V4.2 soware. Critical packing
parameter values are referent to surfactant micelles formed in the synthesis mixture.
28
porosity and the open pore volume as evidenced by He- present in the synthesis solution. We tried to perform the
pycnometry and Hg-porosimetry measurements, but no syntheses with lower concentration of surfactants, below the
2
changes in the BET area of catalysts were noted by N phys- critical micelle concentration (CMC), however, in this condi-
isorption measurements, indicating that the presence of tion, changes in the crystal shape were not observed, probably
surfactants during synthesis are considerable inuencing the explained by the non-effective micelle formation, and the
porosity in a macropore scale but not generating mesoporosity presence of surfactant only as isolated molecules, as already
28
in the materials. The catalyst SDS-0.10 had the best improve- reported. In this work, the surfactant concentration used in
ment in porosity from 69.3% in the reference sample to 82.7%. the synthesis mixture was, approximately, 44 mM, a value
In fact, the Hg-intrusion curves in Fig. 2A show that SDS-0.10 higher than the CMC of CTAB and SDS in water, of 0.9–1.0 and
29,30
presented a higher pore volume and a narrower pore size 8.2 mM, respectively.
The formation of rod-like crystals in
distribution when compared with CTAB-0.10 and the reference the presence of CTAB and SDS ionic surfactants was already
31
samples, with a maximum point at, approximately, 1 mm.
reported in the synthesis of other materials. The shape factor
Additional differences have arisen from scanning electron or critical packing parameter (g) values for these surfactants
microscopy images in Fig. 2B, in which nanorod crystals could (Table 1), calculated from head group area, tail length and
be clearly observed from very dispersed fragments in CTAB-0.10 volume of hydrophobic portion (Scheme 2A and eqn (1)), are in
and SDS-0.10 samples. The use of both surfactants drastically the range expected for the structure aggregation to form cylin-
26
changed the shape of crystals during synthesis when compared drical or rod-shaped micelles, and the result ts well with that
to the reference sample, leading to the formation of randomly observed experimentally. The presence of the rod-shaped
stacked nanorods. Considering surfactant-assisted synthesis of micelles limits the crystal nucleation on lateral crystalline
materials, the kind and shape of the crystals obtained are plans, which favours a preferential growth in the direction
32
directly dependent from type and concentration of surfactant normal to the surfactant free surface (Scheme 2A). Carefully
Fig. 2 (A) Representation of ionic surfactant preferentially adsorbed on the side planes of Mo
B) organization of CTAB and SDS micelles around the crystallites.
x y z
V O crystal leading the nanorod morphology and
(
11978 | RSC Adv., 2018, 8, 11975–11982
This journal is © The Royal Society of Chemistry 2018

View Article Online
Paper
RSC Advances
Scheme 2 (A) Representation of ionic surfactant preferentially adsorbed on the side planes of Mo
V
x y
O
z
crystal leading the nanorod morphology
and (B) organization of CTAB and SDS micelles around the crystallites.
analyzing the contributions of each phase in the diffraction molybdenum in the samples (Fig. S2† and Table 2). The
patterns, a consistent relation between the increasing in the chemical speciation showed that molybdenum oxidation state
crystallite size and the peak intensity ratio referent to (001) and remained invariant in the samples produced by surfactant-
4
+
(
2 8
111) crystalline plans of MoV O phase with the formation of assisted synthesis compared to reference, and that V was
nanorods was observed (see Fig. S1† and Table 1). These present in low quantity in all samples. According to the stoi-
observations lead us to believe that the presence of surfactants chiometry of the detected Mo and V crystalline phases described
are basically affecting the crystal morphology of MoV O phase, in Table S1,† in MoV O , vanadium bears the valence 5+.
2
8
2 8
and not considerably inuencing in the formation of MoO and However, in Mo4.65V0.35O14 vanadium has a mixed valence, that
3
4
+
Mo4.65V0.35O14 phases, when compared to the reference sample. can be the rst reason for detection of V in these samples.
The nanorod morphology were already detected and correlated Even presenting a mixed valence of 4+ and 5+, the low molar
3
3,34
to MoV
2
O
8
phase in previous studies.
0.35
Differences in the ratio concentration of vanadium atoms in Mo4.65V O14 phase are
4
+
of intensity between (001) and (111) crystalline plans are very not sufficient to justify the presence of 13–15% of V in the
coherent with the thickness of nanorods observed in the Fig. 2B. samples, so some V can also be present in the framework of
The catalyst CTAB-0.10, that presented the higher I(001)/I(111) MoV mixed oxide because, despite the larger atomic radius
4
+
2 8
O
4
+
ratio have the crystallites with smaller thickness (approxi- of molybdenum, the lower valence of vanadium (V ) makes the
mately, 90 nm), suggesting a fast growth in the direction of (111) metal–oxygen bond distances closely similar for both elements
8
facets and a limited growth in the direction normal to the side in the Mo V O structures, leading to its partial reduction.
x
y z
plans, like (001). The ratio I(001)/I(111) slightly decreased for the Finally, these reduced forms of vanadium could be present in
catalyst SDS-0.10 leading to the formation nanorods with, small quantities of amorphous phases, not detectable by XRD
approximately, 150 nm of thickness. The differences in crystal but, previously reported in the synthesis of vanadium-
6,8,36
thickness in the presence of CTAB and SDS surfactants are molybdenum based mixed oxides.
Another important fact
probably associated with the limitations in the transfer of Mo–V noted is that the catalysts synthetized with surfactants showed
active phase to lateral plans due to the different aggregation of higher quantities of O–Mo bonds compared to the reference
surfactant monomers to form the micelles (Scheme 2B). The sample, what can be associated with the higher quantities of
aggregation number increases linearly with the increase of MoO crystalline phase formed in these samples. The quantity
3
surfactant concentration for CTAB and SDS surfactants in of surface O–H bonds decreased in CTAB-0.10 and SDS-0.10
35
water. The concentration of CTAB used in the synthesis was samples, inuencing the total acidity of these catalysts (Table
about 44 times the CMC, and an aggregation number above 80 1). This observation are probably related to the decrease of
35
is expected for this compound. On the other hand, the surface defects on crystal surfaces formed in the presence of the
concentration of SDS was about 6 times the CMC and an surfactants. The decrease in low temperature peaks (100–200
35
ꢀ
aggregation number of, approximately, 54 is expected. The
C) of NH -TPD proles (Fig. S3†) observed in the CTAB-0.10
3
presence of a high number of monomers in CTAB micelles are and SDS-0.10 in relation to the reference catalyst, are very
more effective in preventing the penetration of Mo–V atoms in coherent with the amount of surface hydroxyl bonds provided
the sides of micelle and, consequently, the growth perpendic-
ular to the crystal side plans, decreasing the thickness of
nanorods, when compared to SDS micelles. Apparently, the
presence of SDS promoted the formation of a larger amount of
Table 2 XPS analyses of the fresh catalysts
2 8
the MoV O rod-like particles when compared with catalyst
Speciation of V and Mo
synthesized in the presence of CTAB, as observed by SEM
images (Fig. 2B) and by increase in macroporosity, attributed to
the intercrystallite spaces generated between the nanorods
conrmed by Hg-porosimetry (Fig. 2A).
The XPS analyses provided information concerning the
surface composition and the oxidation states of vanadium and SDS-0.10
(
%)
Surface oxygen atoms (%)
4+
5+
6+
Catalyst
V
V
Mo
O–H
O–V
O–Mo
Reference
CTAB-0.10
15
13
13
85
87
87
100
100
100
10.0
5.0
5.6
32.4
30.8
21.6
57.6
64.2
72.8
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 11975–11982 | 11979

View Article Online
RSC Advances
Paper
5
+
4+
by XPS, suggesting that these peaks are related to weakly associated with the V /V equilibrium established during
bonded ammonia adsorbed on hydroxyl groups of surface. reaction by reduction/oxidation dynamics where acrolein is
The results by applying the catalysts in the glycerol oxy- oxidized into acrylic acid, generating oxygen vacancy in the
dehydration reaction are depicted in Fig. 3. The optimal reac- catalyst structure, and subsequent lling of the vacancy with O
tion conditions for these materials are determined using the as predicted by Mars-van-Krevelen mechanism.
2
37
6
reference sample from our previous study. For comparison, the
catalytic activities of pure vanadium and molybdenum oxides aer reaction suggesting the occurrence of solid-state reactions
were determined (Fig. S4†). The MoO and V showed very between the single and bicomponent oxide phases. These reac-
low selective for acrylic acid formation, 13 and 11%, with tions were already observed for MoVTe-based oxides in high
Signicant changes were observed in the phase composition
3
2 5
O
20
conversions of 95 and 98%, and high CO production, 72 and temperatures and oxidant atmospheres. Considering the refer-
X
7
0%, respectively. The presence of mixed oxide phases consid- ence sample, MoV O phase disappeared and molybdenum oxide
2 8
erably enhanced selectivity when compared to the pure oxide phase, MoO , considerably increased (from 15 to 30%) in the
3
6
samples. The reference sample showed a gradual increase in spent catalyst. Analysing the mole percentage of vanadium,
the conversion and selectivity to acrylic acid until the fourth molybdenum and oxygen in the crystalline phases of fresh and
hour of reaction, when the conversion reached 100%. Aer the spent catalysts (Fig. 4B) for reference sample, a signicant

h hour, the acrylic acid selectivity started to decrease in this decrease in the mole percentage of vanadium (from 12.6 to 2.9%),
sample. The catalysts prepared with surfactants presented followed by an increase in the amount of molybdenum (from 14.2
a considerable increase in the acid acrylic formation compared to 22.8%) was observed. These observations strongly suggest that
to the reference sample, with selectivities of 36–45% for CTAB- MoV O is very unstable in this sample, and the most part of
2
8
0.10, and 45–57% for SDS-0.10 catalyst. The catalysts showed vanadium originally present in this mixed oxide structure is
a conversion of 100% with no deactivation aer 6 h on stream transferred to amorphous phase as shown in eqn (6). Most part of
and no coke was detected by elemental analysis of CNHS (not the low amount of vanadium that remained in the crystalline
shown) or by thermogravimetry (Fig. S5†), only a weight loss in phase is present in the reduced phase MoVO
5
.
ꢀ
1
00–200 C range due to elimination of adsorbed water and an
ꢀ
MoV O / MoO + V O
(6)
increase above 400 C corresponding to the capture of molec-
ular oxygen to site reoxidation from V to V were observed.
Both CTAB-0.10 and SDS-0.10 showed 100% of glycerol
conversion and presented lower CO formation when compared
to the reference catalyst. The CO
decreased in the presence of CTAB-0.10 and SDS-0.10 catalysts
with selectivities in the range of 49–53% and 36–45%, respec-
tively. It is important to note that there was a signicant
X
decrease in the amount of CO compared to the reference
sample (60–66%). The catalyst SDS-0.10 had the best activity in
2
8
3
2
5(amorphous)
4
+
5+
CTAB-0.10 and SDS-0.10 catalysts showed decreases of 2.3%
from 11.7 to 9.4%) and 0.2% (from 10.5 to 10.3%) in the mole
percentage of vanadium present in crystalline phases, respec-
tively. The lower migration of vanadium to amorphous phase in
the catalyst SDS-0.10, compared to CTAB-0.10, are probably
associated with the presence of higher MoV O phase in
(
X
X
production considerable
2
8
nanorod morphology in this sample, as observed by SEM
images. In these catalysts MoV
2
O
8
phase are still present aer
reaction, and the reduced MoVO
5
phase appears as a result of
terms of higher acrylic acid selectivity and low CO formation,
X
3
the vanadium redox cycle. The percentage of MoO phase is
considerably low in CTAB-0.10 and SDS-0.10 catalysts, suggest-
ing a restructuration of phases according to reaction in eqn (7),
with 57% and 36%, respectively, in the sixth hour of reaction.
The crystalline phases present in the catalysts identied
before and aer reaction (Fig. 4A) were carefully analyzed and
quantied by Rietveld renement (Fig. 1 and S6†). Before the
reaction, all catalysts showed the phases Mo4.65
MoV and MoO . Aer reaction, the formation of an addi-
tional phase, MoVO , was observed. The presence of this phase,
2 8 3
where vanadium is being transferred from MoV O to MoO to
form MoVO phase and an oxygen remaining from the structure
5
0.35 14
V O ,
that are reacting with acrolein to produce acrylic acid. Feeding
molecular oxygen to the reaction media and considering the
restructuration of phases a reversible process, the MoV O
2 8
2
O
8
3
5
4
+
that contains basically reduced forms of vanadium (V ), is
ꢀ
Fig. 3 Catalytic results of glycerol oxydehydration performed during 6 h at 320 C under a flow of 100% of O
2
.
11980 | RSC Adv., 2018, 8, 11975–11982
This journal is © The Royal Society of Chemistry 2018

View Article Online
Paper
RSC Advances
Fig. 4 (A) Quantification of the crystalline phases determined by Rietveld refinement of fresh and spent catalysts (a – reference, b – CTAB-0.10
and c – SDS-0.10), (B) % in mole of vanadium, molybdenum and oxygen present in fresh and spent catalysts calculated through Rietveld
quantification (only crystalline phases identified in XRD patterns were considered in the calculation) and (C) schematic representation of
modifications in the catalyst structure during catalytic reaction.
phase are being re-established as shown in eqn (8), and this
Conclusions
cycle generates the equilibrium between MoV O /MoVO pha-
2 8 5
ses observed in CTAB-0.10 and SDS-0.10 spent catalysts. The In summary, Mo–V mixed oxides were successfully synthetized
decrease in acrylic acid selectivity observed for reference in the absence and presence of ionic surfactants by a facile
sample, between the h and sixth hour of reaction could be hydrothermal method and applied as catalysts in the glycerol
associated with the migration of vanadium from MoV O crys- oxydehydration to acrylic acid. Addition of surfactants in the
2
8
talline phase to amorphous phase, but the presence of synthesis mixture not signicantly changed the nature of pha-
14 phase in the catalysts is contributing to the ses present in the catalyst but strongly inuenced the crystal
oxidation of acrolein during reaction, since it was already re- morphology of MoV O phase, orienting the nucleation across
0.35
Mo4.65V O
2
8
38
ported as active in the glycerol oxydehydration, and is, prob- specic plans during synthesis and leading to the formation of
ably, maintaining some activity in the reference catalyst. Fig. 4C nanorod-type crystals. Rod-like morphology stabilized the
shows a schematic representation of key structural points MoV O crystalline phase, favouring the redox process, through
2
8
observed for fresh and spent catalysts prepared in the presence the MoV O /MoVO equilibrium, during the reaction, avoiding
2
8
5
of CTAB, SDS and in the absence of surfactant, and a proposed the migration of vanadium atoms to amorphous phase, as
mechanism of phase restructuration during reaction.
MoV + MoO / 2MoVO
MoVO + 1/2O / MoV O + MoO
observed for the reference sample, resulting in a signicant
improvement in the performance of the catalysts. The SDS-0.10
sample achieved the best catalytic result with 100% of conver-
sion, a maximum of 57% of selectivity to acrylic acid and
2
O
8
3
5
+ [O]structure
(7)
(8)
2
5
2(g)
2
8
3
a considerable decrease in CO production from 66 to 36%.
X
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 11975–11982 | 11981

View Article Online
RSC Advances
Paper
1
6 C. Doornkamp and V. Ponec, J. Mol. Catal. A: Chem., 2000,
Conflicts of interest
162, 19.
There are no conicts to declare.
17 S. Ishikawa, M. Tashiro, T. Murayama and W. Ueda, Cryst.
Growth Des., 2014, 14(9), 4553.
1
8 S. Ishikawa, X. D. Yi, T. Murayama and W. Ueda, Catal.
Today, 2014, 238, 35.
Acknowledgements
This work was supported by the Brazilian agencies CNPq (grant 19 S. Ishikawa and W. Ueda, Catal. Sci. Technol., 2016, 6, 617.
#
2
304698/2014-8) and FAPESP (grants #2013/10204-2, #2014/ 20 L. Chen, J. Liang, W. Weng, Y. Wang, H. Wan and
0116-6 and #2016/10597-2). Authors also thank Dr Peter
J. C. V ´e drine, Catal. Commun., 2004, 5(11), 697.
Hammer (LEFE/IQ-UNESP) for XPS measurements and data 21 A. Taguchi and F. Sch u¨ th, Microporous Mesoporous Mater.,
analysis.
2005, 77, 1.
2
2
2
2 G. Leofanti, M. Padovan, G. Tozzola and B. Venturelli, Catal.
Today, 1998, 41, 207.
3 J. H. de Boer, A. van den Heuvel and B. G. Linsen, J. Catal.,
References
1
2
3
M. Anitha, S. K. Kamarudin and N. T. Koi, Chem. Eng. J.,
016, 295, 119.
B. Katryniok, S. Paul and F. Dumeignil, ACS Catal., 2013, 3,
819.
L. H. Vieira, K. T. G. Carvalho, E. A. Urquieta-Gonz ´a lez,
S. H. Pulcinelli, C. V. Santilli and L. Martins, J. Mol. Catal.
A: Chem., 2016, 422, 148.
1
964, 3, 268.
4 F. Zhao, Y.-K. Du, J.-K. Xu and S.-F. Liu, Colloid J., 2006,
8(6), 784.
25 H.-J. Butt, K. Graf and M. Kappl, Physics and Chemistry of
2
6
1
Interfaces, Wiley-VHC Verlag GmbH
&
Co. KGaA,
Weinheim, Germany, 2003.
26 D. Myers, Surfactant Science and Technology, John Wiley &
4
5
6
7
8
M. V. Rodrigues, C. Vignatti, T. Garetto, S. H. Pulcinelli,
C. V. Santilli and L. Martins, Appl. Catal., A, 2015, 495, 84.
L. G. Possato, R. N. Diniz, T. Garetto, S. H. Pulcinelli, C. V
Santilli and L. Martins, J. Catal., 2013, 300, 102.
L. F. Rasteiro, L. H. Vieira, L. G. Possato, S. H. Pulcinelli,
C. V. Santilli and L. Martins, Catal. Today, 2017, 296, 10.
L. G. Possato, W. H. Cassinelli, T. Garetto, S. H. Pulcinelli,
Sons, Hoboken, United States, 2006.
7 W. Suprun, M. Lutecki, T. Haber and H. Papp, J. Mol. Catal.
A: Chem., 2009, 309, 71.
8 B. Naik and N. N. Ghosh, Recent Pat. Nanotechnol., 2009, 3,
2
2
213.
29 J. M. Goronja, A. M. J. Le ˇz ai ´c , B. M. Dimitrijevi ´c and
A. M. Malenovi ´c , Hem. Ind., 2016, 70, 485.
C. V. Santilli and L. Martins, Appl. Catal., A, 2015, 492, 243. 30 R. F. Ali, A. H. Nazemi and B. D. Gates, Cryst. Growth Des.,
L. G. Possato, W. H. Cassinelli, C. I. Meyer, T. Garetto,
S. H. Pulcinelli, C. V. Santilli and L. Martins, Appl. Catal.,
A, 2017, 532, 1.
2017, 17, 4637.
3
1 C. Ni, P. A. Hassan and E. W. Kaler, Langmuir, 2005, 21(8),
3334.
9
L. H. Vieira, L. G. Possato, T. F. Chaves, S. H. Pulcinelli,
32 M. S. Bakshi, Cryst. Growth Des., 2016, 16, 1104.
C. V. Santilli and L. Martins, Mol. Catal., 2017, DOI: 33 M. Shahid, J. Liu, Z. Ali, I. Shakir and M. F. Warsi, J. Power
0.1016/j.mcat.2017.11.027, in press.
Sources, 2013, 230, 277.
0 P. Okoye, A. Abdullah and B. Hameed, Renewable Sustainable 34 Z. Yin, Y. Xiao, X. Wang, W. Wang, D. Zhao and M. Cao,
Energy Rev., 2017, 74, 387.
Nanoscale, 2016, 8, 508.
1 L. Zheng, X. Li, W. Du, D. Shi, W. Ning, X. Lu and Z. Hou, 35 M. Pis ´a r ˇc ik, F. Dev ´ı nsky and M. Pup ´a k, Open Chem., 2015,
1
1
1
1
Appl. Catal., B, 2017, 203, 146.
13, 922.
2 D. Sun, Y. Yamada, S. Sato and W. Ueda, Green Chem., 2017,
´
3
6 A. Chieregato, J. M. Lopez Nieto and F. Cavani, Coord. Chem.
19, 3186.
Rev., 2015, 301–302, 3.
1
1
3 R. K. Grasselli and F. Trir `o , Rendiconti Lincei, 2017, 28, 59. 37 P. Mars and D. W. Van Krevelen, Chem. Eng. Sci., 1954, 3, 41.
4 L. G. Possato, T. F. Chaves, W. H. Cassinelli, S. H. Pulcinelli, 38 L. Liu, B. Wang, Y. Du, Z. Zhong and A. Borgna, Appl. Catal.,
C. V Santilli and L. Martins, Catal. Today, 2016, 289, 20.
5 J. Deleplanque, J. L. Dubois, J. F. Devaux and W. Ueda, Catal.
Today, 2010, 157, 351.
B, 2015, 174–175, 1.
1
11982 | RSC Adv., 2018, 8, 11975–11982
This journal is © The Royal Society of Chemistry 2018