MoO3-TiO2 synergy in oxidative dehydrogenation of lactic acid to pyruvic acid

Article abstract of DOI:10.1039/c7gc00807d

An efficient catalytic process for the oxidative dehydrogenation of biomass-derived lactic acid by earth-abundant MoO₃/TiO₂ mixed oxide catalysts is presented. A series of MoO₃/TiO₂ materials with varied MoO₃ loadings were prepared and their performance in the aerobic and anaerobic conversion of lactic acid was evaluated. A strong synergistic effect between MoO₃ and TiO₂ components of the mixed oxide catalyst was observed. Optimum catalysts in terms of activity and pyruvic acid selectivity can be obtained by ensuring a high dispersion of MoO_x species on the titania surface. Mo-oxide aggregates catalyze undesired side-reactions. XPS measurements indicate that the redox processes involving supported Mo ions are crucial for the catalytic cycle. A mechanism is proposed, in which lactic acid adsorbs onto basic sites of the titania surface and is dehydrogenated over the Mo=O acid-base pair of a vicinal tetrahedral Mo site. The catalytic cycle closes by hydrolysis of surface pyruvate and water desorption accompanied by the reduction of the Mo center, which is finally oxidized by O₂ to regenerate the initial active site. Under anaerobic conditions, a less efficient catalytic cycle is established involving a bimolecular hydrogen transfer mechanism, selectively yielding propionic and pyruvic acids as the major products. The optimum catalyst is 2 wt% MoO₃/TiO₂ predominantly containing tetrahedral Mo species. With this catalyst the oxidative conversion of lactic acid at 200 °C proceeds with a selectivity of ca. 80% to pyruvic acid. The pyruvic acid productivity is 0.56 g g^-1 h^-1.

Full text of DOI:10.1039/c7gc00807d

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MoO3-TiO2 synergy in oxidative dehydrogenation of lactic acid to
pyruvic acid
Kaituo Liu,a Xiaoming Huang,a Evgeny. A. Pidko,*a,b and Emiel J.M. Hensen*a
Received 00th January 20xx,
Accepted 00th January 20xx
An efficient catalytic process for the oxidative dehydrogenation of biomass-derived lactic acid by earth-abundant MoO3/TiO2
mixed oxide catalysts is presented. A series of MoO3/TiO2 materials with varied MoO3 loading were prepared and their
performance in the aerobic and anaerobic conversion of lactic acid was evaluated. A strong synergistic effect between MoO3
and TiO2 components of the mixed oxide catalyst was observed. Optimum catalysts in terms of activity and pyruvic acid
selectivity can be obtained by ensuring a high dispersion of MoOx species on titania surface. Mo-oxide aggregates catalyze
undesired side-reactions. XPS measurements indicate that the redox processes involving supported Mo ions are crucial for
the catalytic cycle. A mechanism is proposed, in which lactic acid adsorbs to basic sites of the titania surface and is
dehydrogenated over the Mo=O acid-base pair of a vicinal tetrahedral Mo site. The catalytic cycle closes by hydrolysis of
surface pyruvate and water desorption accompanied by the reduction of the Mo center, which is finally oxidized by O2 to
regenerate the initial active site. Under anaerobic conditions, a less efficient catalytic cycle is established involving a
bimolecular hydrogen transfer mechanism, selectively yielding propyonic and pyruvic acids as the major products. The
optimum catalyst is a 2 wt% MoO3/TiO2 predominantly containing tetrahedral Mo species. With this catalyst the oxidative
conversion of lactic acid at 200 °C proceeds with a selectivity of ca. 80 % to pyruvic acid. The pyruvic acid productivity is
0.56 g g-1 h-1.
DOI: 10.1039/x0xx00000x
www.rsc.org/
in developing direct oxidative dehydrogenation of lactic acid is
the high reactivity of PyA. Decarboxylation as well as over-
oxidation of PyA lower product selectivity, which is especially a
1. Introduction
The development of novel catalytic technologies for the
efficient production of chemicals and fuels from renewable
resources is pivotal for the transition to a greener chemical
industry. Especially, lignocellulosic biomass is an attractive
source of renewable carbon. Novel processes will hinge on the
availability of platform molecules, which can be efficiently
problem at the elevated temperatures necessary to achieve
appreciable reaction rate.11 The use of lower reaction
temperatures usually results in rapid catalyst deactivation due
to the blocking of the catalytic surface by oligomers of LA.3
12
Phosphate-based catalysts,11,
usually containing transition
metal promotors,13 are currently the benchmark systems for
obtained from biomass and converted into a range of chemical
intermediates and end-products.1,
2
Lactic acid (LA) is a
prominent platform molecule, which is typically obtained by
enzymatic conversion of sugars.3, 4 Scheme 1 depicts the large
number of chemical products that can be obtained from LA.
Catalytic dehydrogenation of LA to pyruvic acid (PyA) is of
considerable importance for the production of L-amino acids,5
cyanoacrylate adhesives,6 perfumes, food additives, dietary and
weight-control supplements, nutraceuticals, antioxidants7 and
photo-resistant solvents for electronic processing.8
The current industrial process for the production of PyA
involves the stoichiometric dehydrative decarboxylation of
tartaric acid. This approach was originally described by
Erlenmeyer in 1881.9-10 A more sustainable alternative is
desirable in order to avoid the formation of large amounts of
inorganic waste and to improve the atom efficiency. A challenge
a. Inorganic Materials Chemistry group, Schuit Institute of Catalysis, , Eindhoven
University of Technology, P.O. Box 513, 5600 MB Eindhoven, the Netherlands..
b. ITMO University, Lomonosova str. 9, St. Petersburg 191002, Russia
Scheme 1. Potential valorization pahts for biomass-derived lactic acid
* Corresponding authors: e.a.pidko@tue.nl (E.A.P.); e.j.m.hensen@tue.nl (E.J.M.H.)
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oxidative dehydrogenation of LA and its alkylated derivatives. Ai
et al. described an iron phosphate catalyst for the gas-phase
oxidation of LA, which gave a maximum PyA yield of 50% (66%
PyA selectivity at 76% LA conversion) at 260 °C.11 Doping this
catalyst with Mo strongly enhanced the catalytic performance
and a nearly similar yield of 52% could be achieved already at
200 °C.12
Reducible metal oxides represent another important class of
promising catalysts for oxidative dehydrogenation of LA. Their
acid-based properties stron
2. Experimental
2.1 Materials
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DOI: 10.1039/C7GC00807D
TiO2 (anatase, powder, −325 mesh, ≥ 99% trace metals basis),
(NH4)6Mo7O24·4H2O (≥ 99%) were purchased from Sigma–
Aldrich. Lactic acid (90%, Sigma-Aldrich), pyruvic acid (98%,
Aldrich), acetaldehyde (≥ 99.5%, Aldrich), methylsuccinic
anhydride (98%, Aldrich), propionic acid (≥ 99.5%, Aldrich),
acetic acid (≥ 99.85%, Aldrich) and citraconic anhydride (98%,
Aldrich) were used as received without further purification.
gly affect PyA selectivity. TiO2, ZrO2, and SnO2 typically result
in higher selectivity to pyruvates compared to conventional
2.2 Catalysts preparation
17
14, 18
oxidation catalysts such as MoO3 14-17, TeO2, and V2O5
.
Two methods were used to synthesize the catalysts, namely
physical mixing of the two oxides and wet impregnation of
ammonium heptamolybdate on titania. A TiO2-MoO3 catalyst
was prepared by thoroughly mixing MoO3 and TiO2 in a 1:2
molar ratio TiO2:MoO3 with a small amount of water. The
resulting paste was spread over a glass plate, dried overnight at
80 °C, crushed, and calcined at 500 °C in air for 6 h. To prepare
MoO3/TiO2, with MoO3 loadings in the 1-10 wt% range, an
appropriate amount of (NH4)6Mo7O24·4H2O was completely
dissolved in deionized water under vigorous stirring for 2 h at
room temperature. Then, an appropriate amount of TiO2 was
added to the solution and stirred for 4 h at room temperature.
Afterwards, the water was removed under reduced pressure
and the resulting solid was dried at 110 °C for 16 h before being
calcined at 500 °C for 6 h in static air. A typical procedure for
obtaining 5 g of 10 wt% MoO3/TiO2 catalyst was as follows:
0.6132 g (NH4)6Mo7O24·4H2O was dissolved in 15 mL deionized
water under vigorous stirring followed by addition of 4.5 g TiO2.
Catalysts prepared by wet impregnation are denoted as MTx,
where x represents the weight loading of MoO3.
Rothenberg and co-workers evaluated the activity of a wide
range of metal oxides including Fe2O3, V2O5/MgO-Al2O3, ZrO2,
TiO2, CeO2, and ZnO for the oxidative dehydrogenation of ethyl
lactate.19 TiO2 showed superior catalytic performance, reaching
up to 75% ethyl pyruvate selectivity at 80% conversion of ethyl
lactate. The unique reactivity of TiO2 is attributed to specific
surface sites that selectively activate the hydroxyl group in the
reactant.20 The catalytic properties of these oxides can be
further improved by addition of other metal oxides. The
properties of binary oxides such as surface area, acidity, and
redox behaviour are often quite different from their single-
oxide constituents. A previous study demonstrated the utility of
Ni–Nb–O mixed oxides to catalyze oxidative dehydrogenation of
LA.21 PyA yields of up to 18% were obtained at 280 °C. Binary
oxides containing Mo, e.g., Fe2O3-MoO3, and TeO2-MoO3, have
also been reported as active catalysts for LA or ethyl lactate
dehydrogenation. For these catalysts, lactate conversions of 75
% with pyruvate selectivities higher than 90% were reported.22
In this study we investigated the catalytic performance of
heterogeneous MoO3-TiO2 (anatase) catalysts for the oxidative
dehydrogenation of lactic acid to pyruvic acid. Titania-
supported MoO3 constitutes an important class of catalytic
2.3 Characterization
Powder X-ray diffraction (XRD) patterns were measured on a
Bruker D4 Endeavor powder diffraction system using Cu Kα
radiation (40 kV and 30 mA) with a scanning speed of 0.01 °/min
in the range of 5° ≤ 2θ ≤ 80°. Scanning electron microscopy
(SEM) measurements were taken using a Philips environmental
scanning electron microscope FEIXL-30 ESEM FEG in high
vacuum mode at low voltage. Textural analysis was done by
nitrogen physisorption on a Tristar 3000 automated gas
adsorption system. The samples were degassed at 300 °C for 6
h prior to analysis. Raman spectra were recorded using a Jobin-
Yvon T64000 triple-stage instrument at a spectral resolution of
2 cm-1. The excitation laser line at 600 nm was produced by a
Kimmon He-Cd laser. The power of the laser was 4 mW. Diffuse
reflectance UV-Vis spectra were recorded on a Shimadzu UV-
2401 PC spectrometer in a diffuse-reflectance mode using an
integrating sphere (internal diameter 60mm) and BaSO4 was
used as the reference. Fourier Transform Infrared (FT-IR)
materials showing promising performance in the oxidation of
23, 24
methanol,
propane,
the oxidative dehydrogenation of ethane,25
26, 27
28
1-butene, and o-xylene
as well as the
hydrodeoxygenation of m-cresol.29 The high catalytic
performance of these catalysts in selective oxidation of organic
molecules has been linked to highly reactive sites at the
interface between the two oxides.30, 31 Accordingly, it has been
proposed that the catalytic performance of such materials
crucially depends on the dispersion of the MoO3 phase on TiO2
.
32, 33 The formation of the binary system has been shown to be
crucial for the activity in alcohol dehydrogenation.34 It has also
been reported that higher MoO3 dispersion can be achieved
using anatase as a catalyst support instead of the rutile TiO2
phase.35
The present work introduces MoO3, TiO2 and their mixtures
as promising catalysts for obtaining pyruvic acid from lactic acid.
The surface properties of the metal oxides was extensively
investigated to understand the relation between the structure
and acidic properties and catalytic performance. A mechanism
is proposed for the strong synergy observed between isolated
tetrahedral Mo-oxide species and the titania support.
spectra were recorded on
a Bruker Vertex V70v FTIR
spectrometer in transmission mode. Typically, a powdered
sample was pressed into a self-supporting wafer with a density
of about 10 mg/cm2 and placed inside a controlled environment
infrared transmission cell, capable of heating and cooling, gas
dosing and evacuation. After calcining the catalyst wafer at 500
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°C, the catalyst was evacuated to a pressure better than 2x10-5 Stabilwax-DA capillary column, 30 m x 0.53 mm x V1ie.0w AμrmticleaOnndlinae
DOI: 10.1039/C7GC00807D
mbar and temperature was lowered to 150 °C. A background flame ionization detector (FID). The oven was set at 40°C, using
was recorded. Pyridine was introduced into the cell from a glass a hold time of 5 min, before increasing with a rate of 10°C/min
ampoule. The sample was then exposed to pyridine at 150 °C to reach 240°C, followed by another hold time of 20 min.
for 30 min. After removing physorbed pyridine by evacuation at Injector and detector temperatures were 250 °C. For HPLC and
150 °C for 1h, a spectrum was recorded. Additional spectra were GC-FID analysis, the products were quantified using external
recorded at 150 °C after outgassing at 200 °C and 400 °C for 1h. calibration curves generated with authentic compound
X-ray photoelectron spectroscopy (XPS) measurements solutions (see supporting information).
were performed using
a
Thermo Scientific K-Alpha
The conversion of LA (X) was calculated by
spectrometer, equipped with a monochromatic small-spot X-
ray source and a 180° double focusing hemispherical analyzer
with a 128-channel detector. Spectra were obtained using an
aluminum anode (Al Kα=1486.6 eV) operating at 72 W and a
spot size of 400 µm. Survey scans were measured at constant
pass energy of 200 eV and regions scans at 50 eV. The
ꢁꢂꢃ,0 − ꢁꢂꢃ
(
)
% =
ꢀ
× 100%
ꢁꢂꢃ,0
The (carbon-based) selectivity of product ꢄ was calculated as
ꢇ
ꢈꢉꢊꢋꢌꢍꢎ ꢏ
( )
% =
ꢅꢆ
×number of carbon atoms in product ꢆ × 100%
ꢇ ×ꢒ×3
ꢐꢑ,0
background pressure was
2 ×
10-9 mbar and during
where CLA,0, CLA and Cproduct i are the concentrations of LA in the
feed and after the reactor and the concentration of product i
after the reactor, respectively.
measurement 3 × 10-7 mbar Ar because of the charge
compensation dual beam source. Data analysis was performed
using Casa XPS software. The binding energy was corrected for
surface charging by taking the C 1s peak of adventitious carbon
as a reference at 284.5 eV.
2.4.2 Oxidative dehydrogenation of ethanol
Oxidative dehydrogenation of ethanol was carried out in a fixed-
bed plug flow reactor system. In a typical experiment, 100 mg
of catalyst (sieve fraction 125-250 µm) diluted with 300 mg of
α-Al2O3 was loaded into a stainless-steel reactor with an
internal diameter of 6 mm. Prior to reaction, the catalyst was
stabilized in a mixture 20 vol% O2 in N2 at 300 °C for 4 h. The
volumetric composition of these feed mixtures were
ethanol:O2:He = 1.5:4.5:94. The reactant feed mixture was
obtained by evaporating ethanol in a He flow in a controlled
evaporator mixer. All tubings were kept above 120 °C to avoid
condensation of the reactants and products. The total gas flow
rate was 167 mL/min with ethanol flow rate-2.5 mL/min (1.5
vol%) and a GHSV of 100,000 mL gcat-1h-1 for all experiments. The
reactor effluent was analyzed by online gas chromatography
(Interscience GC-8000 Top, permanent gases on Schincarbon
ST80/100 packed column connected to a TCD and hydrocarbons
on a Rtx-Wax column connected to a FID). The ethanol
conversion (X) and acetaldehyde selectivity (S) were calculated
by:
Temperature programmed reduction (TPR) experiments
were carried out in a flow apparatus equipped with a fixed-bed
reactor, a computer-controlled oven and a thermal conductivity
detector. Typically, 50 mg catalyst was loaded in a tubular
quartz reactor. The sample was reduced in 4 vol% H2 in N2 at a
flow rate of 8 ml min-1 whilst heating from room temperature
up to 800 °C at a ramp rate of 10 °C min-1. The H2 signal was
calibrated using a CuO/SiO2 reference catalyst.
2.4 Catalysts activity measurements
2.4.1 Oxidative dehydrogenation of lactic acid
Catalytic tests were carried out under atmospheric pressure in
a gas phase down-flow fixed-bed reactor (stainless steel, 6 mm
inner diameter, 150 mm length). In a typical experiment, 0.3 g
of catalyst powder (125 –250 µm) were loaded in the reactor
and the bed was fixed between quartz wool. The aqueous LA
solution (20 wt%) was fed using an HPLC pump at a flow-rate of
0.05 ml/min to the entry point of the reactor system, where it
was vaporized at 190°C and diluted with air (30 ml/min STP, STP
= standard pressure and temperature) . The feed molar LA: H2O:
air composition was 0.6:13.3:1. The liquid solution fed by the
HPLC was prepared by dissolving an appropriate amount of
lactic acid (90%, Sigma Aldrich) in deionized water. The reaction
temperature was 200°C. The products were collected in a cold
trap installed after the reactor and analyzed by HPLC and GC-
FID. The feed concentration and the reaction conditions were
selected such to ensure a complete vaporization of the LA
substrate. HPLC analysis of concentrations of LA, pyruvic acid
and acetic acid was carried out on a Shimadzu setup equipped
with UV detector and a Prevail Organic Acid 3u column (Grace,
2.1 x 100 mm column). HPLC analysis was done at room
temperature with 25 mM phosphate buffer of pH = 2 as a
mobile phase at a flow rate of 0.125 ml/min and the products
were detected by a UV detector. GC-FID analysis was used to
quantify acetaldehyde and other hydrocarbon products. The
analysis was carried out using a Thermo GC equipped with a
ꢁꢓꢔꢕꢖ,0 − ꢁꢓꢔꢕꢖ
( )
%
ꢀ
=
× 100%
ꢁꢓꢔꢕꢖ,0
ꢁꢗꢘꢙꢔꢗꢚꢛꢙℎꢜꢛꢙ
( )
% =
ꢅ
× 100%
ꢁꢓꢔꢕꢖ,0 × ꢀ
3. Results and discussion
3.1. Oxidative dehydrogenation of LA using TiO2, MoO3 and their
physical mixture
As a starting point of this study, we evaluated the catalytic
performance of TiO2 and MoO3 catalysts for the oxidative
dehydrogenation of LA in continuous flow at an air pressure of
1 bar. LA was fed to the reactor as a 20 wt% aqueous solution.
The reaction temperature was 200 °C. The results are shown in
Figure 1. For MoO3 and TiO2, stable but low conversion of LA
was observed with a relatively high PyA selectivity (~80 %). The
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Figure 1. The evolution of LA conversion (X(LA), %, black squares) and selectivities (S, %)
to PyA (SPyA(%),red circles) and other minor products (open symbols) in the presence of
1 bar air over (a) TiO2, (b) MoO3, (c) physical mixture of TiO2 and MoO3 with a molar ratio
of 1:2 and (d) MT2 binary catalyst (Conditions: T = 200 °C, p = 1 atm, 4.0 vol% LA, 6.7
vol% air, WHSV = 10 h-1).
Figure 2. An overview of LA conversion and PyA yields (respectively, X(LA), ◊ and Y(PyA),
○, %, right axes), and selectivities (bars, left axes) at 4 h reaction time over TiO2, MoO3,
and binary molybdena-titania catalysts with varying MoO3 loading (Conditions: T = 200
°C, p = 1 atm, 4.0 vol% LA, 6.7 vol% air, WHSV = 10 h-1).
product distribution is slightly different between these two bulk
oxide catalysts. Whereas methylsuccinic anhydride and
propionic acid are the major by-products with TiO2, products of
decarboxylation such as acetaldehyde and CO2 were observed
when MoO3 was the catalyst. The decarboxylation by-products
decreased with time on stream, suggesting that some highly
reactive sites on the MoO3 surface slowly deactivated. Other by-
products included acetic acid, propionic acid and citraconic
anhydride formed with selectivities below 1%. The reaction
outcome changed drastically when a physical mixture of the two
oxides was employed as the catalyst (Figure 1c). The LA
conversion was about three times higher than for the single
component oxides. A steady state was obtained after 3 hours
on stream. During the initial transient, the PyA selectivity
increased from 61% to 82%, the final selectivity level being
similar to that of the much less active single-component
catalysts. The decrease in LA conversion was relatively minor.
The increase in PyA selectivity was accompanied by lowered
formation rates of decarboxylation productsCO2, acetaldehyde
and the overoxidation product acetic acid. When 2 wt% Mo on
TiO2 (MT2) prepared by wet impregnation was employed as the
catalyst, higher activity and selectivity with similar stability as
the physical mixed catalyst was observed (Figure 1d). Notably,
the selectivity to the decarboxylation products acetaldehyde
and carbon dioxide and the overoxidation product acetic acid
became lower with time on stream. It should be remarked that
the amount of carbon dioxide was nearly equal to the amount
of acetaldehyde, indicating that carbon dioxide was the product
of decarboxylation of lactic acid and not from full combustion
reactions. These results clearly demonstrate the synergy
between TiO2 and MoO3 for the oxidative dehydrogenation of
LA.
3.2. Supported MoO3/TiO2 catalysts
We investigated further the synergy between MoO3 and TiO2 in
the oxidative dehydrogenation of LA by evaluating the
performance of a series of MoO3/TiO2 catalysts with varying
MoO3 loading (1, 2, 5, 8 and 10 wt%). The catalytic performance
of these catalysts after a reaction time of 4 h is shown in Figure
2. These data further emphasize the synergy between the two
oxides. Within this series, the highest LA conversion occurred
with the catalyst with the highest MoO3 loading (MT10).
However, its high activity was accompanied by a significant
contribution of products of decarboxylation and further
oxidation of acetaldehyde to acetic acid. For MT10, the acetic
acid selectivity was higher than the PyA selectivity. Lowering the
MoO3 loading resulted in a decrease of the LA conversion and
an increase of the PyA selectivity. For MT8, the overall
selectivity of C2 products was the same as for MT10, although
also acetaldehyde was observed next to acetic acid. MT1 with
the lowest MoO3 loading was the most selective catalyst,
reaching a PyA selectivity of about 90% at a LA conversion of
50%. No acetic acid was observed for this catalyst. The by-
products methylsuccinic anhydride and propionic acid are likely
due to LA conversion on titania, as the data show that the yield
of these by-products increased with increasing TiO2 content. On
the contrary, the selectivities towards oxidation products trend
well with the MoO3 content. These findings suggest that these
side-reactions were catalyzed by bulk MoO3 species present at
higher Mo loading. The highest PyA yield of 60% was obtained
using MT2 as the catalyst, which remains unchanged during 24
h on stream indicating a good stability of the MT2 catalyst (see
O
O
O
O
O
O
O
+1/2O2
-H
+1/2O2
OH
2O
-CO2
-H
O
-H
2O
O
2O
PyA
OH
O
OH
LA
+1/2O2
O
OH
-CO2
Scheme 2. The proposed reaction scheme for oxidative conversion of LA over
MoO3/TiO2.
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Figure 3. (a) XRD patterns and (b) UV-Raman spectra for MoO3, TiO2, and MT catalysts.
supporting information). The offline cumulative analysis of the
liquid products reveals a carbon balance of 96%. The PyA yield
of 60% is the highest reported for oxidative dehydrogenation of
LA under mild reaction conditions.21
Based on the results presented above, we propose that the
oxidative conversion of LA over the MoO3/TiO2 catalysts follows
the reaction paths shown in in Scheme 2. The dominant reaction
is dehydrogenation of LA to PyA. The side-reactions include the
further condensation of PyA product with concomitant
decarboxylation and dehydration to citraconic anhydride and
methylsuccinic anhydride as well as the decarboxylation to CO2,
acetaldehyde and the further oxidation of acetaldehyde to
acetic acid.
3.3. Catalyst characterization
To gain further insight into the MoO3/TiO2 synergy, we
characterized the various materials. Figure 3 shows powder XRD
patterns and UV-Raman spectra. The dominant diffraction
features in the XRD patterns relate to the anatase form of
titania. Characteristic diffraction features of crystalline MoO3
were absent in MT1 and MT2. The other catalysts displayed
clear (020) and (021) reflections of MoO3, which became more
intense and sharper at higher MoO3 content. No diffraction
peaks related to mixed oxide phases were observed.35 All of the
Raman spectra contained a broad band centered at around 790
cm–1, which is attributed to the first overtone of the O-Ti-O
vibration.36 For the Mo-containing samples, additional bands in
the 940-990 cm–1 regime are due to different MoOx species. The
band at 942 cm-1 in MT1 is due to isolated tetrahedral [MoO4]2–
species.36 With increasing MoO3 content, this band shifted to
higher wavenumbers. These changes point to the
agglomeration of Mo-oxides and an increasing amount of
octahedral Mo species.37-39 Oligomeric Mo-oxides are
characterized by the Raman band at ca. 980 cm-1, which was
also observed for Mo7O246- and Mo8O264- clusters in solutions.40
The band at 992 cm-1, which appeared in the Raman spectrum
of MT5 and which became more intense at higher MoO3
Figure 4. (a) TPR traces, (b) UV-Vis absorption spectra and (c) corresponding Tauc plots
for TiO2, MoO3 and MT catalysts.
content, is due to the Mo=O stretching vibration in Mo-oxide
aggregates. Based on these results, it appears that the
monolayer Mo-oxide coverage is reached between 2 wt% and 5
wt%. In a previous study, it was determined that the monolayer
capacity of anatase is about 0.16 wt% MoO3 per square meter
of surface area.41 Based on the surface area of the anatase
titania support used in this study (11.7 m2 g–1, Supporting
information Table S1), a MoO3 monolayer coverage of 2 wt% is
estimated, which explains the formation of bulk segregated
molybdena phases above a MoO3 loading of 2 wt%.
Further evidence of the intimate contact between the
components in the binary oxide catalysts is provided by TPR and
UV-Vis spectroscopy. The corresponding data are shown in
Figure 4. Pure TiO2 is hardly reducible at temperature below
900°C. Pure MoO3 shows three broad peaks with hydrogen
uptake in the temperature range between 600 and 850 °C.
According to Thomas et al.,42 the first two main reduction
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Figure 6. Mo 3d binding energy region of XPS spectra of (a) fresh and spent MT2 following
reaction with LA in (b) air and (c) nitrogen. The displayed ratios of Mo oxidation states
were obtained by deconvolution of the spectra.
Figure 5. Selectivities to PrA (propionic acid, right triangles, left Y axis), PyA (red circles,
left Y axis), and to other minor products (open symbols, right axis) and LA conversion
(black squares, right axis) under anaerobic conditions over (a) TiO2, (b) MoO3 and(c) MT2
(Conditions: T = 200 °C, p = 1 atm, 4.0 vol% LA, 6.7 vol% N2, WHSV = 10 h-1).
acid (~60 %), the remainder being PyA. We propose that the
reaction in the absence of the external oxidant proceeds via a
transfer hydrogenation mechanism, in which LA acts both as the
hydrogen donor and the hydrogen acceptor. Without an
oxidant that can remove the proton and hydride from LA,
hydrogenolysis is the dominant reaction, explaining why the
propionic acid yield is higher than that of PyA. As propionic acid
yield was higher with TiO2 than with MoO3, we expect that the
dehydrogenation and the associated hydrogenolysis reaction
are catalyzed by sites on the titania surface. The promoting role
of MoO3 is to provide mobile oxygen species, necessary for the
removal of adsorbed H obtained by dehydrogenation of LA. The
rapid deactivation of bulk MoO3 in the absence of oxygen is
associated with the reduction of surface Mo sites in the course
of the reaction.
This hypothesis about the role of Mo-oxide is supported by
Mo 3d XPS spectra, which are compared in Figure 6 for fresh
and spent MT2 recovered after reaction with LA in air and
nitrogen. The surface of the fresh MT2 catalyst contains Mo6+
and Mo5+ species in a 75:25 ratio. Spent catalysts contain a
higher amount of reduced Mo species. The sample contained
these species in a 65:35 ratio, showing that reduction of Mo-
oxide species is part of the catalytic cycle. Without oxygen
present, the extent of Mo reduction is significantly higher. The
Ti 3d XPS spectra show that Ti is not reduced under the reaction
conditions (Supporting Information, Figure S1). Scanning
electron micrographs show that the occurrence of line defects
in spent bulk MoO3 (Figure S2 a,b), which is due to the removal
of lattice oxygen on MoO3 surface. Such changes are expectedly
absent for the spent TiO2 sample (Figure S2c-e). The result that
Mo-oxides are reduced is consistent with the XPS findings.
Previous studies found that the redox and acid-base
properties of metal oxide catalysts correlated well with their
catalytic performance in selective oxidation reactions.44-46It has
also been demonstrated that oxidative dehydrogenation of
ethanol could be used as a model reaction to investigate the
features at 704°C and 818°C correspond to reduction of MoO3
to MoO2 and further reduction of MoO2 to Mo0 state,
respectively. The small peak at 740°C is attributed to reduction
of MoO3 to Mo4O11. For the MT catalysts, the first reduction
step occurred at much lower temperature than for bulk MoO3.
Among them, MT2 shows the highest reducibility with the first
reduction peak occurring already at 472°C. The second
reduction feature to metallic Mo took place at the same
temperature as for MoO3. The higher reducibility of isolated
surface Mo species implies a higher availability of oxygen
species, relevant to removal of hydrogen atoms from lactic acid
during its oxidative dehydrogenation.
According to Wachs and co-workers43 the band-gap energy
for the supported MoO3 catalysts correlates with the Mo
dispersion: isolated Mo-oxo species are characterized by a
wider band gap. We recorded UV-Vis spectra and derived Tauc
plots, depicted in Figure 4b and 4c, respectively. Pure TiO2 and
MoO3 have band gaps of 3.4 eV and 3.2 eV, respectively. For all
MT catalysts, the band gap is shifted to higher energies. The
highest band gap is observed for MT2 (Figure 4c), further
confirming the higher Mo dispersion.
3.4 Catalytic mechanism
In order to understand the mechanism better, we also carried
out the conversion of LA without a gaseous oxidant. For this
purpose, the reaction experiments were performed in a flow of
nitrogen. The results are given in Figure 5. TiO2 provided low
and stable LA conversion with a high selectivity (ca. 40%) to
propionic acid (Figure 5a). Pure MoO3 initially showed a higher
activity, but the conversion rapidly decreased with time on
stream (Figure 5b). The decrease in conversion was
accompanied by a pronounced increase in the selectivities of
PyA and propionic acid. Combined, these products made up
about 80% of the products after 4 h on stream. The conversion
was stable for MT2 (~10%) with a high selectivity to propionic
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HO
O
DOI: 10.1039/C7GC00807D
(a)
O
Ti
O
Ti
Mo
O
O
O
Ti
H2O
HO
O
O
HO
O
H
HO
VI O
Mo
VI
Mo
O
O
Ti
H
O
Ti
O
O
O
Ti
Ti
Ti
Ti
O
O
O
O
O
O
O
O
HOH
O2
O
IV
O
H
O
Ti
O
O
Ti
MoIV
Mo
O
Ti
O
Ti
Ti
Ti
Ti
O
O
O
O
O
O
O
O
Figure 7. Acetaldehyde yield (open circles) and product selectivities (bars) in the
oxidation of ethanol over MoO3, TiO2 and MT catalysts after 4 h time on stream.
(Conditions: T = 300⁰C, p = 1 atm, 1.5 vol% C2H5OH, 4.5 vol% O2, balance He, GHSV =
100,000 mL g-1h-1).
O
H2O
HO
O
O
H
(b)
redox properties of the V2O5/TiO2 catalysts.47 Inspired by these
studies, we also applied ethanol oxidation as a model reaction
to investigate these properties of the catalysts used for LA
conversion. The catalytic results are summarized in Figure 7.
The acetaldehyde yield with TiO2 is ca. 30% with a very high
acetaldehyde selectivity (ca. 99%). This confirms that TiO2 is
highly selective for oxidative dehydrogenation reaction. For
MT2, the acetaldehyde yield reached the highest value of 93%.
At higher loading, the yield decreased again. The best
performance for MT is in keeping with the high reducibility of
MT2. For the bulk MoO3 catalyst, a high acetaldehyde yield
(~79%) was obtained, although the acetaldehyde selectivity was
much lower than TiO2. This is caused by the formation of diethyl
ether by-product, which is catalyzed by acidic sites of the
catalysts.48, 49 The formation of diethyl ether is only noticeable
when MoO3 loading is higher than 1 wt% and with bulk MoO3.
The diethyl ether yield increased along with the MoO3 loading,
indicating an increase of the acidity with increasing MoO3
loading content.
OH
Ti
O
Ti
O
H
O
Ti
H2O
HO
HO
O
O
O
O
O
H
O
H
O
Ti
OH
Ti
O
Ti
OH
Ti
H
Ti
Ti
O
O
O
O
O
O
O
O
H2O
O
+
H2O
OH
O
OH
O
H
H2O
HO
O
O
O
Ti
OH
Ti
H
OH
Ti
H
H
O
H
O
Ti
Ti
Ti
O
O
O
O
O
O
HO
HO
O
Scheme 3. Proposed catalytic reaction mechanisms for the (a) aerobic and (b) anaerobic
conversion of LA over MoO3/TiO2 catalysts.
Pyridine FT-IR was further applied to characterize the Lewis
and Brønsted acidity of these catalysts (Figure S3). As expected,
Mo sites located on the titania support. The first step in the
oxidative dehydrogenation of LA (Scheme 3a) involves the
coordination of LA to the catalyst surface by reaction of basic
OH groups of titania, forming lactate species (CH3CH(OH)COO),
which have earlier been identified by IR spectroscopy.57 The
next step is deprotonation of the secondary alcohol group in
adsorbed lactate by the molybdate species, followed by hydride
abstract of the resulting activated lactate intermediate by the
Mo center, which results in the reduction of Mo and the
formation of pyruvate. Hydrolysis of the pyruvate adsorbate on
the surface yields PyA and regenerates the basic OH group on
the titania surface. After desorption of water, the Mo center is
reduced to the 4+ state. Molecular oxygen can reduce the
surface back to the 6+ state. In the absence of oxygen, the
removal of surface H species has to proceed via another
pathway. Bimolecular transfer hydrogenation resulting in
propionic acid as the major by-product in addition to PyA
TiO2 mainly contains Lewis acid sites at 1450 cm-1 and 1480 cm-
1 52, 53
.
The loading of MoO3 on TiO2 introduced new Brønsted
acid sites characterized by the peaks at 1540 cm-1 and 1638 cm-
1. Consistently, these two peaks are only noticeable for the bulk
MoO3 and MT catalysts with more than 1 wt% MoO3 loading.
The general trend also correlates well with the Raman results
where oligomeric Mo species start to form when MoO3 loading
is higher than 1 wt%. Therefore, it is reasonable to conclude that
the oligomeric Mo species on the titania surface introduce more
Brønsted acid sites that catalyze dehydration of ethanol and
result in the formation of diethyl ether side-product.50, 51 A high
amount of Brønsted acid sites is also known to catalyze
54-56
decarboxylation reactions
which explains the lower
selectivity at higher MoO3 content.
Scheme 3 shows tentative catalytic cycles for the aerobic
and anaerobic conversion of LA by highly dispersed tetrahedral
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product (Scheme 3b) requires synergy between Lewis acid and
base sites.58 The initial LA activation would involve dissociation
of the carboxyl and hydroxyl group to form a dual-site bound
activated lactate species. Oxidation of the surface lactate
species takes place either via hydride abstraction by the Lewis
acid sites of titania (shown in Scheme 3b) or via a concerted
electron-coupled proton transfer.59 Further in situ spectroscopy
and computational modeling would be required to understand
this mechanism. Hydrolysis of the resulting pyruvate adsorbates
regenerates the basic OH group. This OH group activates
another LA molecule to form a surface lactate, which then
undergoes hydrogenolysis by reacting with the surface hydride
and the proton with concomitant release of water and
formation of a surface propionate. The latter can desorb after
hydrolysis of its bond with the surface OH group.
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Acknowledgements
This work was performed in the framework of the European
Union FP7 NMP project NOVACAM ("Novel Cheap and
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Lactic acid oxidative dehydrogenation to pyruvic acid can be
efficiently carried out using mixed-oxide MoOx/TiO2
heterogeneous catalyst.
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