Keggin-type molybdovanadophosphoric acids loaded on ZSM-5 zeolite as a bifunctional catalyst for oxidehydration of glycerol

Article abstract of DOI:10.1016/j.mcat.2018.02.015

Glycerol is a promising renewable feedstock for the manufacture of C3 derivatives. We investigated the one-pass oxidehydrarion of glycerol through the dehydration of glycerol into acrolein, followed by the oxidation of acrolein. A novel bifunctional catalyst for this reaction was prepared by loading the Keggin-type molybdovanadophosphoric acid H_3+xPV_xMo_12-xO₄₀ (x = 0–3) on ZSM-5 (MFI) zeolite (Si/Al = 45) exhibiting both dehydration and oxidation activity. H₅PV₂Mo₁₀O₄₀ and H₆PV₃Mo₉O₄₀ were stable and dispersed on ZSM-5 zeolite, and the acidic property of the ZSM-5 zeolite was retained. The oxidehydration of glycerol was catalyzed by H₅PV₂Mo₁₀O₄₀ loaded on the ZSM-5 zeolite with high selectivity of acrylic acid. In-situ IR analysis suggests that acrolein molecules adsorbed on H₅PV₂Mo₁₀O₄₀/ZSM-5 were converted into acrylic acid due to the inhibition of side-reactions such as polymerization and auto-condensation, which induced coke formation, compared with the other Mo and V-based oxides loaded on ZSM-5 zeolite.

Full text of DOI:10.1016/j.mcat.2018.02.015

MolecularCatalysis449(2018)85–92
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Keggin-type molybdovanadophosphoric acids loaded on ZSM-5 zeolite as a
bifunctional catalyst for oxidehydration of glycerol
Satoshi Suganumaa,⁎, Takuya Hisazumib, Kohtaro Taruyab, Etsushi Tsujib, Naonobu Katadab
a
Center for Research on Green Sustainable Chemistry, Graduate School of Engineering, Tottori University, 4-101 Koyama-cho Minami, Tottori 680-8552, Japan
Department of Chemistry and Biotechnology, Graduate School of Engineering, Tottori University, 4-101 Koyama-cho Minami, Tottori 680-8552, Japan
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
Glycerol
Oxidehydration
Acrylic acid
ZSM-5 zeolite
Keggin-type heteropoly acid
Glycerol is a promising renewable feedstock for the manufacture of C3 derivatives. We investigated the one-pass
oxidehydrarion of glycerol through the dehydration of glycerol into acrolein, followed by the oxidation of ac-
rolein. A novel bifunctional catalyst for this reaction was prepared by loading the Keggin-type molybdovana-
dophosphoric acid H3+xPVxMo12-xO40 (x = 0–3) on ZSM-5 (MFI) zeolite (Si/Al = 45) exhibiting both dehy-
dration and oxidation activity. H5PV2Mo10O40 and H6PV3Mo9O40 were stable and dispersed on ZSM-5 zeolite,
and the acidic property of the ZSM-5 zeolite was retained. The oxidehydration of glycerol was catalyzed by
H5PV2Mo10O40 loaded on the ZSM-5 zeolite with high selectivity of acrylic acid. In-situ IR analysis suggests that
acrolein molecules adsorbed on H5PV2Mo10O40/ZSM-5 were converted into acrylic acid due to the inhibition of
side-reactions such as polymerization and auto-condensation, which induced coke formation, compared with the
other Mo and V-based oxides loaded on ZSM-5 zeolite.
1. Introduction
typically produced through the two-step oxidation of propylene, a
petroleum feedstock. Propylene is oxidized to acrolein, then further
Petroleum is a limited resource that has been a vital feedstock for
the manufacture of liquid fuels and petrochemicals, but there is in-
creasing interest in the world centering on countries with agriculture-
based economics to use biomass as a renewable resource. Biodiesel as
an alternative fuel comprises fatty acid esters and is typically produced
through the transesterification of vegetable fats or oils with alcohol [1].
The production of biodiesel is increasing, and simultaneously the pro-
cess induced the fact that ca. 10 wt% of glycerol is being generated as a
byproduct during transesterification [2]. Residual glycerol has been
burned for thermal power generation, but it is demanded to use it as a
feedstock for the manufacture of several C3 derivatives [3]. There are
approximately 1500 products currently marketed that contain glycerol,
such as cosmetics, pharmaceuticals, and food products [4]. Glycerol
contains three hydroxyl groups, which offer diverse opportunities for
catalytic conversion into high-value chemicals. The conversion of gly-
cerol into value-added products would improve the economics of bio-
diesel production and thus approaches for the transformation of gly-
cerol would be a key development towards alternative bio-based energy
resources and chemicals.
oxidized into acrylic acid. Over 3 million tons per year of acrylic acid
are manufactured using this approach and is increasing annually. Using
glycerol as a bio-based feedstock in the manufacture of acrylic acid is
therefore desirable to meet this growing demand.
Glycerol is typically converted into acrylic acid through multiple
steps of dehydration and subsequent selective oxidation, but acrolein
generated in the first reaction is difficult to store in chemical plants due
to its high reactivity, toxicity, and combustibility. The direct oxidehy-
dration of glycerol generates acrylic acid, eliminating the need to store
acrolein. However, realization of this challenging reaction requires a
bifunctional catalyst with acid sites (probably Brønsted type) for the
dehydration of glycerol and selective oxidation properties for the con-
version of acrolein into acrylic acid. The oxidehydration of glycerol can
be catalyzed by bifunctional catalysts [8–13] which contain vanadium
as a redox center to stabilize acrolein in the form of acrylate [14].
Furthermore, the addition of molybdenum and/or tungsten likely in-
duces the generation of acidic sites. In particular, the catalytic activity
of W-V mixed oxides with a hexagonal tungsten bronze structure can be
improved by the addition of niobium, which generates acidic properties
[11]. Vanadium oxide loaded on ZSM-5 zeolite, with negligible deac-
tivation during the dehydration of glycerol [15], has been investigated
for the oxidehydration of glycerol [16]. However, this approach
Glycerol dehydration followed by oxidation generates acrylic acid,
which is a widely used intermediate in the manufacture of water-ab-
sorbing polymers, paints, plastics, and rubber [5–7]. Acrylic acid is
⁎
Corresponding author.
E-mail address: suganuma@chem.tottori-u.ac.jp (S. Suganuma).
https://doi.org/10.1016/j.mcat.2018.02.015
Received 12 November 2017; Received in revised form 4 February 2018; Accepted 14 February 2018
2468-8231/©2018ElsevierB.V.Allrightsreserved.

S. Suganuma et al.
MolecularCatalysis449(2018)85–92
provided only moderate yields of acrylic acid. Conversely, vanadium-
substituted cesium salts (Keggin-type heteropoly acids) have been re-
ported to exhibit high selectivity towards acrylic acid in the oxidehy-
dration of glycerol, although catalytic activity gradually decreases [17].
In this work, we synthesized a novel catalyst for oxidehydration by
loading the Keggin-type molybdovanadophosphoric acid H3+xPVxMo12-
xO40 (x = 0–3) onto ZSM-5 zeolite. This catalyst shows high dispersion
and stability of the heteropoly acid. A single-bed reactor loaded with
this catalyst exhibits higher reaction rate to the desired product than
mixture and dual-bed reactors comprising ZSM-5 zeolite and
H5PV2Mo10O40/SiO2. To our knowledge, this study is the first example
using a heteropoly acid combined with ZSM-5 zeolite for the direct
oxidehydration of glycerol. In addition, in-situ IR analysis of the ad-
sorption of acrolein or acrylic acid on the catalyst and the oxidation of
acrolein by the catalyst will give new insights into the catalytic prop-
erties required for the oxidehydration of glycerol.
(MicrotracBEL). Powders of the catalysts were compressed at 20 MPa
into self-supporting disks 1 cm in diameter and pre-treated in a stream
of oxygen (37 μmol s−1, 100 kPa) at 623 K for 1 h in an IR cell. The
sample was heated at a ramp rate of 2 K min−1 from 343 to 623 K under
a helium stream (89 μmol s−1, 6.0 kPa) and IR spectra were collected at
1 K intervals. Next, ammonia was adsorbed at 343 K, and heating and IR
spectrum collection under a helium stream were conducted as the
temperature was raised from 343 to 803 K. The concentration of am-
monia in the gas phase was monitored by a mass spectrometer (MS)
operating at m/e = 16. The amount of acidic sites was calculated from
the intensity of desorbed ammonia in the TPD spectrum.
2.3. Gas-phase conversion of glycerol into acrylic acid
The activity of each catalyst for the oxidehydration of glycerol was
assessed by packing 0.45 g of the catalyst with 0.10 g of glass beads into
a Pyrex tube (i.d. 10 mm). The temperature of the catalyst bed was
monitored by a thermocouple located inside the catalyst bed, and the
temperature of this thermocouple was controlled at 623 K. The gas flow
was kept at 1.8 L h−1 and its composition was O2 (21 mol%) and N2
2. Experimental
2.1. Catalyst preparation
(79 mol%). Glycerol aqueous solution (30 wt%) were fed at 1.5 g h−1
;
H3+xPVxMo12-xO40 (x = 0-3, Japan New Metal), vanadium (IV)
oxide sulfate n-hydrate (99.9%, Wako) and hexaammonium heptamo-
lybdate tetrahydrate (99.0%, Wako) were used without further pur-
ification. ZSM-5 and SiO2 were provided by the Catalysis Society of
Japan (JRC-Z5-90NA(1) (Na-form, Si/Al = 45) and JRC-SIO-13, re-
spectively). The Na-form zeolite was ion-exchanged into the NH4-form
by stirring in a 5 wt% ammonium nitrate solution (NH4/Na = 10 in the
system) at 353 K for 4 h, filtered then washed with water 3 times. These
procedures (stirring, filtering and washing) were repeated 3 times. The
zeolite was dried at 383 K overnight to provide NH4-form zeolite. H-
form zeolite was prepared by calcination of it at 823 K in air.
glycerol and water were vaporized in the gas flow before the catalyst
bed. The molar ratio of glycerol/H2O/O2/N2 was 4.9/58/16/58. The
outlet effluent was trapped by water at 273 K after the system had
stabilized for 0.5 h at 623 K. The products were analyzed by a gas
chromatograph (GC) (Shimadzu GC-2014) with a capillary column (TC-
WAX) using a flame ionization detector (FID) and a packed column
(WG-100) using a thermal conductivity detector (TCD). The untrapped
gas products were analyzed through a six-way valve in the online FID-
GC system, and the collected COx (CO and CO2) with gas-tight syringe
was analyzed by the TCD-GC system.
In
a
typical procedure, 90 μmol of H3+x[PVxMo12-xO40]·nH2O
2.4. In-situ IR analysis of acrolein adsorbed on the catalysts
(x = 0-3) was dissolved in 100 mL of distilled water with stirring. After
adding 1.0 g of ZSM-5 zeolite, the suspension was evaporated to dry-
ness, and the powder was dried at 383 K in air for 12 h. The catalysts
were calcined at 573 K for 3 h in air. The product contained 17–18 wt%
In-situ IR analysis of molecules adsorbed on the catalyst samples was
carried out using an automatic IRMS-TPD analyzer (MicrotracBEL).
Catalyst powders were compressed at 20 MPa into self-supporting disks
1 cm in diameter and pre-treated at 373 K under vacuum for 1 h in an IR
cell to remove adsorbed water. Acrolein or acrylic acid was adsorbed at
H3+x[PVxMo12-xO40
]
and was designated “PVxMo12-x/ZSM-5”.
PV2Mo10/SiO2 was prepared from SiO2 and H5[PV2Mo10O40]·nH2O
using the same procedure. Mo-V/ZSM-5 without Keggin structure for
comparison was prepared through the impregnation of Mo-V mixed
oxides. Hexaammonium heptamolybdate tetrahydrate (120 μmol) and
240 μmol of vanadium (IV) oxide sulfate n-hydrate were dissolved in
100 mL of distilled water with stirring. After adding 1.0 g of ZSM-5
zeolite, the suspension was evaporated to dryness and the powder was
dried at 383 K in air for 12 h. The catalysts were calcined at 573 K for
3 h in air. Mo-V/ZSM-5 contained the same amount of ([Mo] + [V]) as
PVxMo12-x/ZSM-5 and PV2Mo10/SiO2.
303 K, and IR spectra were collected as 21 mol% of O2/N2 (0.24 L h−1
)
at 70 Pa was streamed over the sample and the temperature was in-
creased at a ramp rate of 2 K min−1 from 303 to 673 K.
3. Results and discussion
3.1. Characterization
Fig. 1 shows the XRD patterns of ZSM-5 zeolites before and after
loading PVxMo12-x or Mo-V mixed oxide, and of SiO2 before and after
loading PV2Mo10. The diffraction peaks of the ZSM-5 zeolite support
indicated an MFI zeolitic framework structure (Fig. 1(a)). Loading
PVxMo12-x on the support did not change the zeolitic structure
(Fig. 1(b)–(d)). The introduction of PVxMo12-x, x = 0–2, resulted in a
small peak at about 6° and likely indicates a periodic three-dimensional
structure in small aggregates of the heteropoly acids. This signal was
not observed in the PV3Mo9/ZSM-5 pattern (Fig. 1(e)). The zeolitic
structures in Mo-V/ZSM-5 was not collapsed, and periodic crystal
structures of the Mo and/or V oxides were not observed (Fig. 1(f)). The
SiO2 pattern before and after loading PV2Mo10 showed an amorphous
structure (Fig. 1(g)–(h)). However, Keggin structure and the other
crystalline structure of Mo and V-based oxides were not appeared in
XRD patterns, indicating the lack of large crystallites of these materials.
Fig. 2 shows representative nitrogen adsorption-desorption isotherms.
The steep increases at very low relative pressure (p/p0 < 0.1) and high
relative pressure (p/p0 > 0.4) in the bare ZSM-5 zeolite were asso-
ciated with microporosity and mesoporosity (Fig. 2(a)). These features
2.2. Characterization of the catalysts
The crystalline phases of the catalysts were analyzed by X-ray dif-
fraction using a Rigaku Ultima IV diffractometer, with Cu Kα radiation.
Data were collected in the 2θ range from 5 to 50°. The N2 adsorption-
desorption isotherms were determined on a BELSORP-max apparatus
(MicrotracBEL). The samples were pretreated at 573 K under vacuum
for 1 h before measurement. Raman spectra were recorded using a
JASCO NRS-7100 at a wavenumber of 785 nm with a CCD detector in
air. Thermogravimetry/differential thermal analyses (TG-DTA) were
determined on a Rigaku Thermos Plus instrument. Samples were heated
from 313 to 1073 K at a rate of 10 K min−1. SEM (scanning electron
microscope) images were collected using a Hitachi S-4800 SEM with
EDX (Energy Dispersive X-ray) spectrometry.
Ammonia infrared-mass spectroscopy/temperature programmed
desorption (IRMS-TPD) analysis for the measurement of acidic prop-
erties [18] was conducted on an automatic IRMS-TPD analyzer
86

S. Suganuma et al.
MolecularCatalysis449(2018)85–92
observed in the Raman spectra of PMo12 and PVMo11 loaded on ZSM-5
zeolite and arose from asymmetric OMoO, symmetric OMoO and
symmetric MoeOeMo stretching modes, and the OMoO wagging mode
of MoO3, respectively [24,25] (Fig. 4(b)–(c)). Therefore, some hetero-
poly acids on these catalysts had decomposed into molybdenum oxides.
However, no band assignable to vibration modes of V2O5 was observed.
In contrast, PV2Mo10 and PV3Mo9 were stable, and the structure of the
Keggin-type heteropoly acids was retained on ZSM-5 zeolite and SiO2
(Fig. 4(d)–(f)). In the Raman spectrum of Mo-V/ZSM-5 (Fig. 4(g)), the
bands at 980, 670, 290 and 240 cm−1 were ascribed to MoO3, and the
bands at 894, 885 and 836 cm−1 were likely due to a MoOx (x < 3)-
type structure [24,25]. Fig. S1 shows TG-DTA curves. Mass loss below
523 K was presumably due to removal of adsorbed water.
The acidic properties of the catalysts influence catalytic perfor-
mance during glycerol dehydration, as reported [15]. The ammonia
IRMS-TPD was used to quantify the Brønsted or Lewis acid sites [26]. IR
spectra of the 1050–1700 cm−1 region of NH3 adsorbed on the catalysts
are shown in Fig. S2. After pretreatment under a stream of helium at
623 K and cooling to 343 K, the reference spectrum was measured
under a stream of helium while increasing the temperature. This re-
ference spectrum was subtracted from the sample spectrum after ad-
sorption of NH3, recorded in the same atmosphere at the same tem-
perature. The bands corresponding to the adsorbed NH3 bending modes
were assigned as reported [26]. The bands around 1450 and 1625 cm−1
were assigned according to the literature [18] to the ν4 bending mode of
symmetric NH4 adsorbed on Brønsted acid sites and the δs of asym-
metric NH3 bending modes coordinatively adsorbed on Lewis acid sites,
respectively. The band assignable to ammonia bound to Brønsted acid
sites was observed for all the catalysts, while the band assignable to
ammonia coordinatively adsorbed on Lewis acid sites was rarely ob-
served. The amounts of Brønsted acid sites were determined by the
IRMS-TPD profile [26] and are shown in Table 1, on the assumption in
which MS-TPD showed the sum of TPD profiles of the observed
Brønsted acid sites. The amount of Brønsted acid sites on PMo12 and
PVMo11 loaded on ZSM-5 was less than that on the parent ZSM-5 alone,
while PV2Mo10, PV3Mo9 and Mo-V mixed oxide loaded on ZSM-5 pro-
vided the similar amount of Brønsted acid sites to the parent ZSM-5.
PV2Mo10/SiO2 had no or very weak Brønsted acid sites, therefore de-
Fig. 1. XRD patterns of (a) ZSM-5, (b) PMo12/ZSM-5, (c) PVMo11/ZSM-5, (d) PV2Mo10
ZSM-5, (e) PV3Mo9/ZSM-5, (f) Mo-V/ZSM-5, (g) SiO2 and (h) PV2Mo10/SiO2.
/
hydration of glycerol cannot proceed over PV2Mo10
.
3.2. Oxidehydration of glycerol
The oxidehydration of glycerol was performed under gas-phase
conditions with PVxMo12-x/ZSM-5, containing total amount of
a
Fig. 2. N2 adsorption-desorption isotherms at 77 K of (a) ZSM-5, (b) PMo12/ZSM-5, (c)
0.90 mol kg−1 of [Mo] and [V]. The catalytic behaviors were studied as
a function of time on stream (Fig. 5). Complete conversion of glycerol
was obtained with all catalysts (Fig. 5(a)). The yield of acrolein for
PVMo11/ZSM-5 was lower than that for PMo12/ZSM-5 (Fig. 5(b)). An
increase in the vanadium content in the heteropoly acids on the cata-
lysts resulted in a decrease in acrolein yield. The yield of acrylic acid
increased with introduction of V as the acrolein yield decreased
(Fig. 5(c)). An increase in the vanadium content in the catalysts en-
hanced the catalytic activity for the sequential oxidation of acrolein,
with an exception of PV3Mo9/ZSM-5. Acetol formed via dehydration of
the primary hydroxyl group in glycerol likely continuously decomposed
into acetaldehyde, which was also oxidized into acetic acid [27,28]. As
shown in Fig. 5(d) and (e), the yields of acetaldehyde and acetic acid
were much lower than those of acrolein and acrylic acid. An increase in
the vanadium content of the catalysts resulted in a decrease in the ac-
rolein and acetaldehyde yields and promoted the formation of acrylic
acid and acetic acid. Vanadium can induce the oxidation of C3 and C2
aldehydes into carboxylic acids. Fig. S3 shows TG-DTA curves of col-
lected PVxMo12-x/ZSM-5 after the reaction. The weight loss in the range
300–700 K and 700–900 K indicated elimination of water and carbon
deposit. There is not the difference of the amounts of water and carbon
deposit between the catalysts, and the amounts was estimated at about
PVMo11/ZSM-5, (d) PV2Mo10/ZSM-5, (e) PV3Mo9/ZSM-5 and (f) Mo-V/ZSM-5.
did not change after loading PVxMo12-x or Mo-V mixed oxide
(Fig. 2(b)–(f)). The heteropoly acids and the Mo-V mixed oxide were
thus dispersed on the ZSM-5 zeolite and SiO2. Fig. 3 shows SEM images
and the corresponding EDX mapping images of PV2Mo10/ZSM-5 and
ZSM-5 alone; each have a particle size of about 20 μm. The EDX map-
ping images of PV2Mo10/ZSM-5 show silicon, molybdenum, aluminum,
and a small amount of vanadium, suggesting that PV2Mo10 is probably
uniformly supported on the outer surface of ZSM-5 zeolite particles
because the primary structure of the heteropolyacid (ca. 1 nm) cannot
enter the micropores of the MFI zeolite structures.
Raman spectra of the catalysts are presented in Fig. 4. The Raman
spectra of ZSM-5 zeolite and supported zeolites show a band around
364 cm−1, assigned to νs (SieOeSi) modes due to framework vibration
of five-membered rings in the MFI crystals [19,20]. The band at
1003 cm−1, with a shoulder band at 986 cm−1, and the bands at 608
and 250 cm−1, were assigned to the terminal MoO stretching vibration
mode, MoeOeMo stretching vibration mode, and MoeOeMo bending
vibration mode of the intact Keggin-type heteropoly acid, respectively
[21–23]. Additional bands at 980, 815, 670 and 290 cm−1 were
87

S. Suganuma et al.
MolecularCatalysis449(2018)85–92
Fig. 3. SEM images and the corresponding EDX mapping images of (a) PV2Mo10/ZSM-5 and (b) ZSM-5.
Table 1
Amount of Brønsted acid sites on the catalysts.
Sample
Amount of Brønsted acid site/mol kg−1
ZSM−5a
0.30
0.19
0.23
0.32
0.32
0.31
0.00
PMo12/ZSM−5a,b
PVMo11/ZSM−5a,b
PV2Mo10/ZSM−5a,b
PV3Mo9/ZSM−5a,b
Mo-V/ZSM−5a,b
PV2Mo10/SiO2
a
H-type, Si/Al = 45.
([Mo] + [V])/[Al] = 3.0.
b
Brønsted acid sites and stable heteropoly acids on PV2Mo10/ZSM-5, and
therefore the catalytic performance of PV2Mo10/ZSM-5 was in-
vestigated in detail.
The effect of the loading amount of PV2Mo10 on the conversion and
yield of products in the oxidehydration of glycerol at 6 h of the time on
stream is shown in Fig. 6. Complete conversion of glycerol was achieved
on all the catalysts. PV2Mo10 (5.8 wt%; 0.36 mol kg−1 of ([Mo] + [V])
content) loaded on ZSM-5 zeolite provided a high yield of the aldehydes
such as acrolein (ca. 60%) and acetaldehyde (ca. 16%), but low yield of
the carboxylic acids such as acrylic acid and acetic acid. An increase in
the amount of PV2Mo10 resulted in decrease of the yields of the alde-
hydes but increased the yields of the carboxylic acids. The highest yield
of acrylic acid (31%) was obtained by using 17.3 wt% of PV2Mo10
(1.08 mol kg−1 of ([Mo] + [V]) content) loaded on ZSM-5 zeolite.
However, the excess amount of PV2Mo10 resulted in a slightly decreased
yield of acrylic acid and decreased the yield of acrolein down to ca.
15%. PV2Mo10 (17.3 wt%) loaded on ZSM-5 zeolite provided the
highest total yield of acrolein and acrylic acid.
Fig. 4. Raman spectra of (a) ZSM-5, (b) PMo12/ZSM-5, (c) PVMo11/ZSM-5, (d) PV2Mo10
ZSM-5, (e) PV3Mo9/ZSM-5, (f) PV2Mo10/SiO2 and (g) Mo-V/ZSM-5.
/
1 and 2%, respectively. The carbon deposit at 6 h was slight amount,
therefore there is not effect on catalytic activity. PV2Mo10 and PV3Mo9
loaded on ZSM-5 zeolite were more stable than PMo12 and PVMo11, as
shown by the Raman spectra in the previous section, and thus PV2Mo10
and PV3Mo9 on ZSM-5 zeolite likely contain larger amount of active
sites for the oxidation reaction. The high yield of acrylic acid was thus
obtained in the oxidehydration of glycerol by a more balance of the
The combination of oxidation catalyst (heteropoly acids) and acid
88

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MolecularCatalysis449(2018)85–92
Fig. 5. Time course of catalytic activities of PVxMo12-x/ZSM-5 (17–18 wt%) in the oxidehydration of glycerol. (a) Conversion and yields of (b) acrolein, (c) acrylic acid, (d) acetaldehyde
and (e) acetic acid.
catalyst (zeolite) to form a bifunctional catalyst is the aim of this study.
To clarify the effect of PV2Mo10 loading on catalytic performance for
the oxidehydrarion of glycerol, we here compared PV2Mo10/SiO2, Mo-
V/ZSM-5, a physically mixed catalyst (0.39 g of ZSM-5 zeolite + 0.45 g
of PV2Mo10/SiO2 (17.3 wt%)), a separate sequential bed (1st bed:
0.39 g of ZSM-5 zeolite, 2nd bed: 0.45 g of PV2Mo10/SiO2 (17.3 wt%)),
and PV2Mo10/ZSM-5. PV2Mo10/SiO2 and Mo-V/ZSM-5 contained the
same amount of [Mo] + [V] on PV2Mo10/ZSM-5. Fig. 7 shows time
courses for the oxidehydration of glycerol by the various catalytic
systems, as described above. Full conversion was obtained with all the
systems (Fig. 7(a)). PV2Mo10/SiO2 provided a much low yield of acro-
lein and acrylic acid compared to the other catalysts. The negligible
glycerol was dehydrated on PV2Mo10/SiO2 with very weak Brønsted
acid sites, but the other products were formed through oxidation. The
physically mixed catalyst resulted in the formation of more acrolein and
acrylic acid than PV2Mo10/SiO2. Glycerol was dehydrated into acrolein
over ZSM-5 zeolite, and acrolein was then oxidized into acrylic acid
over PV2Mo10/SiO2. Locating ZSM-5 zeolite and PV2Mo10/SiO2 into the
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MolecularCatalysis449(2018)85–92
metal atom of a heteropoly acid, C]C stretching, and CH2 bending
vibration, respectively [29]. In the IR spectra measured at elevated
temperatures up to 673 K (Fig. S6(a)), the band at 1660 cm−1 im-
mediately decreased as the temperature increased, and the band at
1710 cm−1 gradually disappeared. A broad band at 1480–1580 cm−1
was observed at 673 K and probably assigned to the C]C stretching
vibration of aromatic compounds, since the sample disk changed from
yellow to gray during IR analysis. The spectrum of ZSM-5 without
heteropoly acid (Fig. 8(b)) shows bands at 1675, 1605, 1460 and
1365 cm−1, attributed to ν (C]O) of a compound coordinated with
Brønsted acid sites, C]C stretching, CeH rocking of the COH of alde-
hyde, and CH2 bending vibration, respectively [30]. These vibration
modes were observed at 323–373 K (Fig. S6(b)). The bands at 1730 and
1555 cm−1 were assigned to ν (C]O) of aldehyde in polymers formed
through 1, 2-addition of acrolein and to cyclic structures formed by the
auto-condensation of aldehyde. As the temperature increased, these
structures gradually transformed into aromatics. Acrolein molecules
interacted with the Brønsted acid sites on ZSM-5 zeolite were activated,
and this interaction was likely stronger than that of acrolein with het-
eropoly acids.
Fig. 6. Influence of loading amount of PV2Mo10/ZSM-5 on the conversion and yield of
products in the oxidehydration of glycerol at 6 h of time on stream.
Bands associated with acrolein absorption onto ZSM-5 were present
in the spectra of ZSM-5 zeolites modified with heteropoly acids and Mo-
V mixed oxide. The vanadium content of heteropoly acids affected the
intensity ratio of several bands. PMo12/ZSM-5, which contained a
smaller amount of Brønsted acid sites than ZSM-5 zeolite alone, had a
weaker band at 1675 cm−1, assignable to ν (C]O) of a compound co-
ordinated with the Brønsted acid sites (Fig. 8(c)). The band at
1695 cm−1 was ascribable to ν (C]O) of a coordinated species on
molybdenum oxides derived from the decomposition of heteropoly
acids. An increase in the vanadium content increased the band intensity
at 1675 cm−1; this intensity was maximum for PV2Mo10 (Fig. 8(e)) due
to adsorption of acrolein on the acid sites of ZSM-5 and the stable
heteropoly acids, as shown in the Raman spectra. Fig. S6(c)–(f) shows
the IR spectra of ZSM-5 zeolite modified with heteropoly acids at ele-
vated temperatures up to 673 K, and Fig. S6(h) shows the difference IR
spectra of PV2Mo10/ZSM-5 without adsorbed acrolein. The bands as-
signable to acrolein adsorbed on PV2Mo10/ZSM-5 gradually weakened
as the temperature increased and a broad band at 1480–1580 cm−1
appeared, probably attributable to the C]C stretching vibration of
aromatic compounds. The spectra of other heteropoly acids loaded on
ZSM-5 zeolite also showed immediate decreases in the intensities of
acrolein bands and the appearance of the 1480–1580 cm−1 band
arising from aromatic compounds. In addition, in-situ IR analysis was
used to characterize acrylic acid adsorbed on the surface of ZSM-5 and
PV2Mo10/ZSM-5 (Fig. S7). The bands attributed to acrylic acid de-
creased immediately on spectra for the catalysts. Acrylic acid on the
catalyst surface may be desorbed at lower temperature compared to
acrolein. Probably due to this, the formation of acrylic acid was not
observed on the catalysts during in-situ IR analysis of acrolein adsorp-
tion (Fig. 8).
separate bed increased the yield of acrolein and acrylic acid compared
to the mixed bed. PV2Mo10/ZSM-5 was observed to be most active for
the production of acrylic acid, resulting in the high yield of acrolein and
acrylic acid. Mo-V/ZSM-5 provided a low total yield of acrolein and
acrylic acid, suggesting that molybdenum and/or vanadium oxides on
Mo-V/ZSM-5 catalyzed complete oxidation. Therefore, heteropoly acids
loaded on ZSM-5 zeolite can catalyze the efficient selective oxidation of
acrolein into acrylic acid.
Fig. S4 shows time course of the catalytic activity of PV2Mo10/ZSM-
5 for a long time. The conversion did not change. Acrylic acid was
observed at a selectivity of 30% after 6 h, but the selectivity decreased
to 10% at 72 h. The selectivity of acrolein increased. The behaviors of
acetic acid and acetaldehyde on time course were similar to those of
acrylic acid and acetaldehyde, respectively. Total selectivity of CO and
CO2, shown as COx, slightly increased from 23% at 1 h to 27% at 72 h.
The undetected product with FID- and TCD-GC systems were described
as others, and decreased from 33% at 1 h to 2% at 72 h. The behavior of
others on time course was similar to that of acrylic acid. It was
speculated that others was carbon deposit on heteropoly acids, there-
fore the deactivation of oxidation activity were related to the formation
of carbon deposit on PV2Mo10. Fig. S5 shows temperature dependence
of glycerol oxidehydration over PV2Mo10/ZSM-5 at 6 h of time on
stream. In the above, the reaction was carried out at 623 K. Selectivity
of acrylic acid in the reaction at 603 K was lower than that of acrolein.
In contrast with the values at 623 K, selectivity of acrylic acid at 643 K
decreased but that of acrolein increased, therefore the oxidation ac-
tivity decreased against ramping temperature. It was speculated that
formation rate of carbon deposit in the reaction at 643 K was faster than
623 K, therefore the oxidation activity declined.
It is suggested that inhibition of polymerization and auto-con-
densation of acrolein molecules was needed to increase selectivity of
acrylic acid in the oxidehydration of glycerol. The IR analysis thus in-
dicated that these side reactions were inhibited on PV2Mo10/ZSM-5
compared to the other catalysts, and therefore PV2Mo10/ZSM-5 showed
high catalytic activity and high selectivity of acrylic acid.
3.3. In-situ IR analysis of adsorption and reaction by the catalysts
In-situ IR analysis was employed to observe acrolein adsorbed on the
surface of the catalysts and its desorption and/or conversion into pro-
ducts. The samples were pretreated at 373 K for 1 h in vacuum. After
introduction of acrolein into an IR cell at 303 K, the cell was evacuated
and gradually ramped from 303 to 673 K as IR spectra were recorded at
each temperature. Fig. 8 shows difference IR spectra in which the
spectrum before acrolein adsorption at 303 K is subtracted from the
corresponding spectrum at elevated temperature to 323 K after acrolein
adsorption at 303 K. In the spectrum for PV2Mo10/SiO2 (Fig. 8(a)),
weak bands observed at 1710, 1660, 1625 and 1365 cm−1 were as-
signed to ν (C]O) of a carbonyl-bonded compound linked with an O
atom of a heteropoly acid, ν (C]O) of a compound coordinated with a
4. Conclusion
The oxidehydration of glycerol into acrylic acid was studied on a
bifunctional catalyst formed from Keggin-type molybdovanadopho-
sphoric acid and ZSM-5 zeolite. The heteropoly acid was dispersed on
the stable ZSM-5 zeolite. Based on Raman analysis, PV2Mo10 and
PV3Mo9 on the zeolite retained the original structure of the Keggin-type
heteropoly acids, but the structures of PMo12 and PVMo11 partially
decomposed. The amount of Brønsted acid sites on PV2Mo10/ZSM-5 and
90

S. Suganuma et al.
MolecularCatalysis449(2018)85–92
Fig. 7. Comparative yields of catalytic reaction systems for the oxidehydration of glycerol. (a) Conversion and yields of (b) acrolein, (c) acrylic acid, (d) acetaldehyde and (e) acetic acid.
[The systems: PV2Mo10/ZSM-5, Mo-V/ZSM-5, PV2Mo10/SiO2 (13.6 wt%), mixed bed (ZSM-5 zeolite + PV2Mo10/SiO2 (13.6 wt%)) and separate bed (1st bed: ZSM-5 zeolite, 2nd bed:
PV2Mo10/SiO2 (13.6 wt%))]
PV3Mo9/ZSM-5 was higher than those on PMo12/ZSM-5 and PVMo11
/
on PV2Mo10/ZSM-5 were converted into acrylic acid due to inhibition
of side-reactions such as polymerization and auto-condensation, which
induced coke formation, compared to other heteropoly acids loaded on
ZSM-5 zeolite investigated in this study.
ZSM-5. The total yields of acrylic acid and acrolein in the catalytic
oxidehydration of glycerol on PV2Mo10/ZSM-5 were about 60%, and
the catalytic activity of PV2Mo10/ZSM-5 was higher than that of either a
mixed bed or separate beds of ZSM-5 and PV2Mo10/SiO2, or of Mo-V/
ZSM-5. In-situ IR analysis suggested that acrolein molecules adsorbed
91

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MolecularCatalysis449(2018)85–92
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