Paving the way towards green catalytic materials for green fuels: Impact of chemical species on Mo-based catalysts for hydrodeoxygenation

Article abstract of DOI:10.1039/c9ra03208h

A series of Mo-based catalysts were synthesized by tuning the sulfidation temperature to produce mixtures of MoO₃ and MoS₂ as active phases for the hydrodeoxygenation (HDO) of palmitic acid. Differences in the oxidation states of Mo, and the chemical species present in the catalytic materials were determined by spectroscopic techniques. Palmitic acid was used as a fatty-acid model compound to test the performance of these catalysts. The catalytic performance was related to different chemical species formed within the materials. Sulfidation of these otherwise inactive catalysts significantly increased their performance. The catalytic activity remains optimal between the sulfidation temperatures of 100 °C and 200 °C, whereas the most active catalyst was obtained at 200 °C. The catalytic performance decreased significantly at 400 °C due to a higher proportion of sulfides formed in the materials. Furthermore, the relative proportion of MoO₃ to MoS₂ is essential to form highly active materials to produce O-free hydrocarbons from biomass feedstock. The transition from MoS₂ to MoO₃ reveals the importance of Mo-S and Mo-O catalytically active species needed for the HDO process and hence for biomass transformation. We conclude that transitioning from MoS₂ to MoO₃ catalysts is a step in the right direction to produce green fuels.

Full text of DOI:10.1039/c9ra03208h

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Paving the way towards green catalytic materials
for green fuels: impact of chemical species on Mo-
based catalysts for hydrodeoxygenation†
Cite this: RSC Adv., 2019, 9, 18292
a
Leonardo Dıaz-Garcıa,a Luis Felipe Ramırez-Verduzco,a
*
Diego Valencia,
´
´
´
Amir Qamar,b Alexander Moewes
and Jorge Aburtoa
b
A series of Mo-based catalysts were synthesized by tuning the sulfidation temperature to produce mixtures
of MoO3 and MoS2 as active phases for the hydrodeoxygenation (HDO) of palmitic acid. Differences in the
oxidation states of Mo, and the chemical species present in the catalytic materials were determined by
spectroscopic techniques. Palmitic acid was used as a fatty-acid model compound to test the
performance of these catalysts. The catalytic performance was related to different chemical species
formed within the materials. Sulfidation of these otherwise inactive catalysts significantly increased their
performance. The catalytic activity remains optimal between the sulfidation temperatures of 100 ꢀC and
200 ꢀC, whereas the most active catalyst was obtained at 200 ꢀC. The catalytic performance decreased
significantly at 400 ꢀC due to a higher proportion of sulfides formed in the materials. Furthermore, the
relative proportion of MoO3 to MoS2 is essential to form highly active materials to produce O-free
hydrocarbons from biomass feedstock. The transition from MoS2 to MoO3 reveals the importance of
Mo–S and Mo–O catalytically active species needed for the HDO process and hence for biomass
transformation. We conclude that transitioning from MoS2 to MoO3 catalysts is a step in the right
direction to produce green fuels.
Received 29th April 2019
Accepted 5th June 2019
DOI: 10.1039/c9ra03208h
rsc.li/rsc-advances
oxygen content, which leads to a low heating value, immiscibility
with fossil fuels, a tendency for polymerization, thermal instability
and high viscosity.8 Catalytic hydrotreating (HDT) is an available
1 Introduction
Currently about 82% of the world's energy consumption is based
on products derived from fossil fuels. These hydrocarbons con-
taining natural resources are rst rened to intermediates, addi-
tives, or polymers before they become suitable energy sources for
different purposes. However, there are currently several concerns
regarding their use.1,2 These are non-renewable energy resources,
which take millions of years to form and their extraction has faced
continuous challenges due to rapid depletion of known reserves.
Furthermore, fossil fuels have been a major contributor to global
warming, and we must take steps to investigate new sustainable
and more environmental friendly energy sources for the future.3,4
Generation of bio-fuels has become a major research and
industrial topic during this decade.5,6 Vegetable oils, especially
palm oil, are considered good candidates to develop environ-
mental friendly and high quality fuels.7 However, these bio-based
oils cannot be used immediately as fuels due to their high
technology for the conversion of biomass to liquid biofuels. HDT
has been used in past decades to eliminate targeted atoms within
hydrocarbon molecules.9 Hydrodeoxygenation (HDO) is the main
catalytic process to eliminate oxygen from the organic molecules
by hydrogenation and hydrogenolysis reactions.10 This catalytic
process takes place at higher H2 pressures and temperatures in the
presence of a heterogeneous catalyst, typically MoS2-based active
phases supported on metal oxides.11–13
MoS2-based catalysts were developed for the hydro-
desulfurization (HDS) process and this metal sulde is highly
active in the elimination of sulfur atom in crude oils. In general,
this active phase provides catalytically active sites for hydrogena-
tion and hydrogenolysis reactions, where numerous chemical
processes can take place. Its application in biomass conversion has
shown a tremendous potential as a catalytic system. Heteroge-
neous catalysts consisting of MoS2 active species have been tested
successfully in the HDO of fatty acids and triacylglycerides. The
operational conditions are almost identical to the ones reported
for the hydrotreating processes (high temperature and H2 pres-
sure). However, it faced several disadvantages due to the rapid
deactivation from coke formation on the catalytic surface or the
presence of water that leaches metal species in uid phase. MoO3
as an active phase is a promising alternative because it does not
a
´
´
´
Direccion de Investigacion en Transformacion de Hidrocarburos, Instituto Mexicano
´
´
´
Del Petroleo, Eje Central Lazaro Cardenas 152, Col. San Bartolo Atepehuacan, CP
07730, Mexico City, Mexico. E-mail: dvalencia@imp.mx
bDepartment of Physics and Engineering Physics, University of Saskatchewan, 116
Science Place, Saskatoon, SK, S7N 5E2, Canada
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra03208h
18292 | RSC Adv., 2019, 9, 18292–18301
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
need any prior suldation stage,14,15 but further investigation is still
The second step involved transformation of the oxide to the
necessary to improve its catalytic performance in HDO reactions. sulde forms. We deliberately chose the suldation conditions
This catalytic active phase has been tested in the HDO of aromatics to produce mixtures of MoO3/MoS2 within the catalytic mate-
at low H2 pressure in different oxide supports. It is remarkable that rials. The catalyst prepared in the rst step was sulded under
the oxidation states of Mo atoms vary depending on the support a mixture of 15 vol% of H2S and H2 at atmospheric pressure.
used, and these changes might play a crucial role in the elimina- The suldation reaction took place with a heating rate of
tion of O atoms contained in biomass-derived products.16
5
ꢀC minꢁ1 under a continuous ux of 50 cm3 minꢁ1. We
The supported MoS2 active phase is prepared from the prepared four different catalysts by varying the suldation
conversion of MoO3 under suldation conditions. Depending temperature. The ꢀnal temperature was kept constant at 100,
on the activation temperature, reaction time, pressure, and the 200, 300 and 400 C, respectively, for 2 hours under the same
support used, different mixtures of MoO3 and MoS2 are formed gas mixture and ow rate as above. The temperature was then
within the catalytic materials.17,18 The inuence of the sulda- lowered to room temperature by simply turning off the oven and
tion process on the HDO of aromatics is very important for this letting the samples cool down. Different temperatures were
type of catalysts.19 The NiMo sulded catalysts supported on chosen to understand the effect of different Mo species present
Al2O3 have been tested in the HDO of triacylglycerides and were in the HDO. Each catalyst in this study is identied by its sul-
the best option among different supports to achieve complete dation temperature.
deoxygenation mixture of hydrocarbons.20 The use of different
supports, metals or combination with zeolites could help to
optimize the performance of the newer generation of HDO
Characterization of catalysts
catalyst.21 It is also desirable to change the MoS2 active phase to The catalysts were characterized by powder X-ray diffraction
an oxidic form with a proper support or zeolite, which is (XRD), temperature-programmed reduction (TPR), Raman
a challenging task, but new generation of biomass-oriented spectroscopy, X-ray photoelectron spectroscopy (XPS), and X-ray
catalysts might help to overcome this obstacle.22–24 Since the absorption (XAS) spectroscopy. XRD patterns were recorded in
oxidation states and oxide/sulde species play a crucial role in the 4–80ꢀ range on a Siemens 5005 diffractometer, using Cu Ka
ꢀ
˚
this process, we utilized a catalytic system that allows us to radiation (l ¼ 1.5405 A) and a goniometer speed of 0.02
study the importance of the oxidation states of Mo, sulde or (2q) minꢁ1. The TPR experiment was performed using a ZETON
oxide characteristics of these materials, and their impact on the Altamira model AMI-200 with a reduction mixture of hydrogen/
HDO of palmitic acid. In this study, we prepared a series of Mo- argon (10 vol% of H2) with a standard thermal conductivity
based catalysts supported on Al2O3–TiO2 and sulded at detector (TCD). The samples were studied in a temperature
ꢀ
different temperatures to produce mixtures of different Mo range of 50–850 C. Micro-Raman measurements were carried
catalytically active species. The selection of this support is based out with an excitation wavelength of 532 nm using the objective
on the high activity of the corresponding MoS2-based catalysts with magnication 50. A Horiba Jovin Yvo laser instrument was
that were used in HDS industrial plants in the past. The HDS used with incident radiation power of about 9 mW. XPS analysis
catalysts supported on Al2O3–TiO2 mixed oxide were proved to was performed using a Thermo VG Escalab 250 equipment
be much more active than those supported on pure Al2O3. Our where Al Ka (1486.6 eV) was used as a source of incident X-rays.
main aim is to understand the importance of oxide vs. sulde The chamber pressure was kept at 3 ꢂ 10ꢁ8 mbar. The spectra
characteristics of these materials, oxidation states of Mo-based were calibrated using the carbon 1 s excitation at the binding
catalysts and their effect on the HDO of palmitic acid.
energy of 285 eV. BL 8.0.1 at Advanced light source (ALS)25 was
used for O K-edge absorption spectra. Resolving power (E/DE)
values for the monochromator at this beamline is 7000, which
corresponds to an energy resolution value (DE) of 0.07 eV at
530 eV. All spectra were collected at room temperature with
2 Experimental
Preparation of catalysts
First, commercial supports with a composition of g-Al2O3/TiO2 dwell time of 1 second at each energy step in TFY mode. Energy
¼ 94.3/5.7 wt% were used to prepare oxidic form of the catalysts calibration was performed by measuring O K edge XAS for
used as the base material. The catalysts were prepared by co- bismuth germanium oxide (BGO), and comparing its features to
impregnation of the metal salts dissolved in aqueous solution the known energy values (532.7 eV and 538.6 eV for XAS).
to the support (g-Al2O3/TiO2). We used MoO3 (Productos
´
Quımicos de Monterrey SA de CV) and nickel(II) carbonate
Catalytic activity
hydroxide tetrahydrate (2NiCO3$3Ni(OH)2$4H2O, Sigma-
Aldrich). These metal precursors were added to ammonia The catalysts were tested in the HDO of palmitic acid (PA);
solution, 29 wt% (Fermont). This solution was impregnated to Sigma Aldrich >99%; as a model compound of fatty acids. For
the dried support to achieve a homogeneous green pistachio each test, 0.1 g of the catalyst was used with 40 mL of a solution
catalyst. The catalyst was then calcined under static air atmo- containing PA (39 mM) in n-dodecane (Sigma Aldrich, anhy-
sphere at 500 ꢀC for 4 hours. The composition of the nal drous > 99%). The catalysts were tested in a Parr reactor (100
catalyst obtained was 11.9 and 2.9 wt% base oxides (MoO3 and mL) at 280 ꢀC and 5.2 MPa of H2 pressure for 6 hours. The time
NiO, respectively), while the rest consisted of the support evolution of the reaction was observed by withdrawing aliquots
material.
each hour and analyzing them on a gas chromatograph. Product
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 18292–18301 | 18293

View Article Online
RSC Advances
Paper
identication was analyzed on a Hewlett Packard GC-MS is too low to be detectable. The Raman peaks for the MoS2 rst
instrument with a FID.
appear for the catalyst sulded at 200 ꢀC conrming the
formation of sulded species at the catalyst's surface. These
Raman features become strongest for the catalyst sulded at
400 ꢀC (S-400), which is expected due to the more favorable
conditions to form these Mo species in this catalyst.
3 Results and discussion
Characterization
Mo 3d XPS spectra for all the materials sulded at different
temperatures are shown in Fig. 2. The binding energies for the
spin–orbit splitting for the Mo 3d5/2 and 3d3/2 are ascribed to
Mo6+, Mo5+ and Mo4+. As expected, the relative proportions of
Mo species with different oxidation states depend on the sul-
dation temperature. For example, the catalyst sulded at 100 ꢀC
(S-100) showed a larger concentration of higher valent molyb-
denum species (Mo6+), while the Mo4+ was scarce, but never-
theless it was measurable even at this suldation temperature
(Fig. 2a). A suldation temperature of 200 ꢀC resulted in
formation of more Mo species with several different oxidation
states (Fig. 2b). This spectrum showed several overlapping
absorption bands due to contributions from Mo6+, Mo5+ and
Mo4+. The catalysts sulded at 300 and 400 ꢀC also exhibit
contributions from all these three different Mo species, but for
these two catalysts the predominant contribution comes from
Mo4+, which is due to the presence of MoS2 in the catalysts
(Fig. 2c and d). Thus, the concentration of Mo species with
lower oxidation states increases as the suldation temperature
was raised.
XRD measurements (Fig. S1†) show the support had a crystal-
line structure, identied as g-Al2O3 according to the JCPDS 10-
0425. The impregnation of the metal species into the support
did not change the crystalline structure of the support, evident
from the identical diffraction pattern aer the impregnation
was done. This parent material, supported with metal species,
was then sulded at different temperatures. No detectable
crystalline phase of the catalytically active metals in oxide form
was found. Thus, we conclude that the parent material exhibi-
ted high dispersion of metal species, which would then be
transformed into the sulde species at higher temperatures.
The reduction prole of the oxide supported catalyst is
shown in Fig. S2.† The reduction of the Mo species shows
hydrogen consumption in a wide temperature range (400–800
ꢀC). The main reduction peak of the catalysts is located at
464 ꢀC, which can be attributed to the partial reduction of Mo in
octahedral polymeric species (Mo6+ + 2eꢁ / Mo4+) weakly
bound to the Al2O3–TiO2 support. The broad shoulder at higher
temperature is associated with the complete reduction of Mo
species (Mo4+ + 4eꢁ / Mo0) in polymeric octahedral, tetrahe-
dral and bulk crystalline MoO3.26 The Mo species interacting
with the alumina support needs to be reduced and sulded for
the production of MoS2 active species.
Fig. 1 shows the Raman spectra for the catalysts sulded in
this study. MoS2 characteristic peaks, A1g and E2g1 located at 410
and 387 cmꢁ1, respectively,27,28 become stronger as the sul-
dation temperature is increased. The in-plane E2g1 mode is due
to antisymmetric vibration of two S atoms with respect to the
Mo atom. The A1g mode is ascribed to the out-of-plane vibration
of the S atoms in opposite directions.29 The catalyst sulded at
100 ꢀC (S-100) does not show any peaks related to MoS2 species
on the surface of the catalyst, which suggests that Mo in the
catalyst is mainly in oxide form, and the amount of suldation
Binding energies and the relative concentrations of different
Mo species present in these catalysts are summarized in Table
1. A change in suldation temperature causes a shi in the
binding energy for the Mo species with same oxidation state,
because binding energies are sensitive to not only the oxidation
state, but also the local environment around the element pro-
bed. For instance, Mo6+ species exhibit a peak at 231.9 eV for S-
100 and S-200, while this feature is shied to 230.8 eV for S-300
and S-400. The catalyst S-100 showed the lowest concentration
for Mo4+ (27.3), while highest for Mo6+ (65.9). On the other
hand, the catalyst S-400 exhibited the dominant concentration
for Mo4+ (55.2), while lowest for Mo6+ (40.3). The relative
concentrations of Mo6+ for both S-300 and S-400 are identical.
The increment in the Mo4+ species from S-300 to S-400 is related
to the reduction of Mo5+ species during the suldation process.
It is important to mention that the concentration of Mo5+
,
which is an intermediate species, increased when suldation
5+
ꢀ
temperature changed from 100 to 200 C. Mo concentration
then decreased when the suldation temperature was changed
ꢀ
to 400 C. This is in agreement with the consecutive reduction
reactions Mo6+ + eꢁ ¼ Mo5+ + eꢁ ¼ Mo4+ during suldation. The
largest concentration of Mo5+ was obtained for S-200. We found
that the predominant oxidation states in these catalysts were
Mo4+ and Mo6+, corresponding to MoS2 and MoO3, respectively.
However, the presence of Mo5+ was found in all the catalysts,
which might play an important role in the HDO process. In
Table 1, we also report calculated ratios of Mo6+/Mo5+ and Mo6+/
Mo4+ concentrations to get an estimation of the production of
Mo6+ or Mo4+ due to the suldation temperature. The Mo6+/
Mo5+ ratio is the largest for the catalysts S-100 and S-400 due to
the small amount of Mo5+ in these catalysts. Mo6+/Mo4+ ratio
Fig. 1 Raman spectra for the catalysts (a) S-100, (b) S-200, (c) S-300
and (d) S-400.
18294 | RSC Adv., 2019, 9, 18292–18301
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
Fig. 2 XP spectra for Mo 3d obtained from the catalysts (a) S-100, (b) S-200, (c) S-300 and (d) S-400.
reects the conversion of Mo6+ to Mo4+ during suldation. This illustrate that different suldation conditions result in different
ratio is almost identical for S-300 and S-400 catalysts. S-400 proportions of the two sulfur-based species which can have
showed largest difference between Mo6+/Mo5+, and Mo6+/Mo4+ a signicant effect on the catalytic performance. The concentration
ratios due to an overall decrement in the Mo5+ species. These of S6+, which was largest for S-100, decreased by raising the sul-
results are in agreement with the effect of the temperature on dation temperature due to increasing formation of MoS2. The
the formation of MoS2 under different suldation conditions.
catalyst S-200 exhibited strong presence of both of these sulfur
To better understand the incorporation of sulfur-based species species, reected by two convoluted peaks present at binding
into the catalytic materials, S 2p XPS spectra were also collected energies of 158 to 173 eV. The catalyst sulded at 400 ꢀC exhibited
(Fig. 3). Two different types of sulfur species were detected with the largest proportion of S2ꢁ species identied by their binding
binding energies in the range of 160–165 eV and 168–172 eV that energies in the range of 160–164 eV related to the formation of
belong to S2ꢁ and S6+ species, respectively.30–32 These two S species MoS2 in agreement with our Raman results.
are present due to formation of suldes and oxysuldes/sulfates in
Synchrotron-based measurements were carried out to get more
all the catalysts where H2S acted as a source of S. This involves information about the atomic species in the catalysts under study.
a series of complex chemical reactions dependent on various These measurements allowed us to selectively probe the local
experimental factors.33,34 Our aim here is not to uncover detailed electronic environment of oxygen sites in the catalytic materials.
mechanism for the formation of these S species, but we simply Oxygen K-edge X-ray absorption spectra (XAS) were obtained in the
Table 1 Relative concentration of the Mon+ species sulfided at different temperatures
Binding energyb (eV)
Relative concentrationc (%)
Ratio of Mo species
Mo6þ
Mo5þ
Mo6þ
Mo4þ
Catalyst
Ta (ꢀC)
Mo4+
Mo5+
Mo6+
Mo4+
Mo5+
Mo6+
S-100
S-200
S-300
S-400
100
200
300
400
227.4
229.6
227.4
227.1
228.9
230.9
228.5
228.5
231.9
232.4
230.1
230.8
27.3
36.8
49.0
55.2
6.8
13.2
10.2
4.5
65.9
50.0
40.8
40.3
9.7
3.8
4.0
9.0
2.4
1.4
0.8
0.7
a
c
Temperature of suldation of MoO3 supported catalysts. b The value of binding energy corresponds to Mo 3d5/2
.
Concentration of the Mo species
was estimated by the integration of the peak area.
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 18292–18301 | 18295

View Article Online
RSC Advances
Paper
Fig. 3 XP spectra for S 2p obtained from the catalysts (a) S-100, (b) S-200, (c) S-300 and (d) S-400.
total uorescence yield (TFY) mode for the materials under study. (Fig. 4). The catalyst sulded at 100 ꢀC showed the highest inten-
XAS spectra collected for the two reference materials, MoO2 and sity for band located at 531.1 eV near pre-edge, but it decreased for
MoO3 pure compounds, look identical (Fig. S3†) where the main S-200 catalyst. This feature, due to MoO2 and MoO3, is present for
absorption energy is located around 531 eV. The positions of the both the pure Mo-oxide samples and for samples on Al2O3–TiO2
pre-edge and edge spectral regions does not show any signicant support. The decrement in intensity of this feature indicates that
differences between Mo6+ and Mo4+. To study the effect of reduc- Mo–O species are transformed into suldes (Mo–S) during sul-
tion on the electronic structure of O, we reduced some of the dation. Importantly, the O K-edge displayed an energy shi in this
catalysts under H2 atmosphere without performing any suldation peak when the suldation temperature changed from 200 to
process. O K-edge spectra for these materials are shown in Fig. S4.† 300 ꢀC. The intensity of this feature remained almost the same, but
Two main common aspects were found between the pure MoO2/ it belongs to a different species as evident from the shied posi-
MoO3 and the ones supported on Al2O3–TiO2. Reduction of these tion of this band at 531.5 eV. For S-400, this peak remains at
catalysts shows no effect on the electronic structure of O atoms. 531.5 eV, but its intensity decreases compared to S-300. This
The spectral features and the intensity of the absorption bands are implies increasing suldation temperature not only reduces the
very similar for pure MoO2/MoO3 and for the samples reduced. concentration of oxygen based species, but also causes formation
However, we found important differences in the O K-edge XAS for of species with different local environment around the O atoms. S-
the catalysts which were sulded in the range of 100–400 ꢀC 100 and S-200 have one type of O sites, while S-300 and S-400 have
a different type of O sites.
Fig. 4 O K-edge X-ray absorption spectra (TFY mode) for the catalysts Fig. 5 Conversion of palmitic acid over the catalysts: (-) S-100, (C)
sulfided at different temperatures.
S-200, (,) S-300, (B) S-400 and (>) non-sulfide reference.
18296 | RSC Adv., 2019, 9, 18292–18301
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
All the different experimental techniques discussed above impact on the catalytic performance. We chose two tempera-
ꢀ
indicate one common theme that a signicant structural tures, 200 and 400 C, for the reduction under H2 atmosphere
ꢀ
ꢀ
change occurs around 200 C to 300 C of suldation temper- with the same conditions as the sulded counterparts, but
ature. Raman spectra indicated the formation of sulfur species using only H2 gas in the reaction chamber. These catalysts can
in the catalyst's surface starts about 200 ꢀC of suldation convert PA about 38% and 48%, respectively. The conversion of
temperature, in agreement with the S 2p XPS results. The O K- PA over the catalyst reduced at 200 ꢀC is very similar to the one
edge XAS measurements also show that Mo–O compounds obtained with the oxide catalyst. The catalytic activity slightly
presꢀent in the catalytic materials changed in the range of 200– increased at higher reducing temperature, but was still lower
300 C.
compared with the sulded counterparts. We found very similar
Mo–O species for these catalytic materials by XAS measure-
ments which explains their similar behavior in HDO. Therefore,
we stress that the presence of Mo–O and Mo–S in the catalysts is
crucial for their performance to more effectively convert PA.
We enlist the kinetic parameters in Table 2 for the catalysts
under study. The highest conversion of PA was achieved over
the S-200 catalyst, which reached 50% aer only 1.2 h, while for
S-400 it took 2.7 hours. The kinetic parameters of the catalysts
follow a similar trend as the conversion. The pseudo-rate
constants for the S-100 and S-200 have the highest values of
11.13 and 11.73 hꢁ1 gcatꢁ1, respectively. Pseudo-rate constants
normalized per mmol of Mo atoms within the catalytic mate-
rials are also higher for the lower temperature suldation
materials. S-100, S-200, and S-300 catalysts have almost one
order of magnitude higher pseudo-rates compared to the non-
sulded oxide supported catalyst (Ox). The normalized
constants are given per mmol of Mo atoms, which shows only
a tiny amount is needed to convert the PA over these catalysts,
especially S-200 (14.10 hꢁ1 mmolMoꢁ1). The fact that these
higher conversions and pseudo-rates are achieved at relatively
low suldation temperatures is an important step toward
transitioning from sulde-based catalysts (originally developed
for petroleum industry) to MoO3 based catalysts for biomass
feedstock. We have shown that it is not necessary to have only
MoS2 present in these materials to achieve high conversion of
PA, mixture of Mo–O and Mo–S active species work equally well
to convert PA. The change of MoS2 for MoO3 as active species in
these catalysts is a greener alternative since no H2S is needed for
suldation. We consider that tuning properly the chemical or
redox properties can provide high active materials based on
MoO3 for biomass conversion in the future. On the other hand,
MoO3 can be handled and stored easier than MoS2.
Hydrodeoxygenation of palmitic acid
We used palmitic acid (PA) as a model compound of fatty acids
contained in vegetable oils or animal fats to test the perfor-
mance of these catalysts. The conversion of PA is shown in
Fig. 5. The catalyst without prior suldation, in its oxide form,
showed very little activity. All the catalysts sulded were active
under these HDO conditions. However, the catalysts sulded at
lower temperatures were more active. The catalysts sulded
ꢀ
between 100–300 C reached a conversion of 100% aer 6 h of
HDO reaction. The catalyst sulded at 200 ꢀC (S-200) showed the
highest conversion of PA aer 1 h of reaction. The suldation
temperature of these catalysts played a major role in their
performance. We discovered differences in the oxidation states
of Mo, and presence of Mo–S and Mo–O compounds in the
catalytic materials discussed above. We also found changes in
the sulfur species due to different reaction temperatures, where
a mixture of sulfates/suldes was formed. The suldation
process depends on two different variable, temperature and the
type of gas present. H2 in the mixture of the atmosphere can
reduce MoO3 to lower oxidation states. On the other hand, the
presence of H2S can also produce MoS2 species that are active in
the HDS petroleum-based process, but not necessarily in the
HDO reactions. We also detected several intermediate species
from sulfur and oxygen that are also present in the catalytic
materials.
In order to clarify the questions about the effects of the
suldation conditions on the catalytic performance, we tested
some catalysts without any source of sulfur present during their
thermal treatment. PA was converted up to only 41% over the
oxide catalyst without any thermal or chemical pretreatment,
which is too low for a good candidate for the HDO of biomass
feedstocks. We tested the catalysts characterized by XAS
(Fig. S4†) reduced under H2 atmosphere without any sulfur
source, to elucidate the effect of Mo–S compounds and their
The HDO network of this PA consists of consecutive
hydrogenation reactions of the carboxylic acid moiety to the
aldehyde, then to its alcohol version, and nally to n-
Table 2 Kinetic parameters for the catalysts tested in HDO of palmitic acid
Conversionb (%)
kc (hꢁ1 gcat
)
k
d (hꢁ1 mmolMo
)
roe, 102 (mol Lꢁ1 hꢁ1 gcat
)
ꢁ1
ꢁ1
ꢁ1
Catalyst
t50a (h)
S-100
S-200
S-300
S-400
Ox
1.8
1.2
2.1
2.7
nd
100
100
100
93
11.13
11.76
7.95
4.06
0.83
13.14
14.10
9.53
4.87
0.99
8.22
8.70
7.76
5.53
1.97
41
a
Time to reach 50% conversion of PA. b Conversion of HDO for 6 h of reaction. c Pseudo rst-order rate constant per gram of catalyst. d Pseudo rst-
e
order rate constant per mmol of Mo in the catalyst. Initial rate of reaction.
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 18292–18301 | 18297

View Article Online
RSC Advances
Paper
In Fig. 7, we have plotted the distribution of HDO products
as a function of conversion of PA. The relative proportions of the
products strongly depend on the suldation temperature of the
catalyst. Table 3 lists the distribution of products obtained at
the same conversion for all the catalysts, where we can easily
compare the HDO routes over this series of catalysts. The main
products are 1-hexadecanol and n-hexadecane for the materials
which were sulded, while the oxide catalyst produces large
amounts of 1-hexadecanol, hexadecanal, and cetyl palmitate.
The oxide form of the catalyst is less active in the HDO, and the
mixture of products contains large amounts of oxygenated
molecules. This specic catalyst is not able to convert PA and
the main products are poor in O-free molecule yields. Therefore
the effect of suldation of these catalysts is positive on the HDO
of fatty acids for its catalytic activity and selectivity. The highly
active catalyst (S-200) produces larger quantities of n-hex-
adecane (0.51) and 1-hexadecanol (0.32) at 40% of PA conver-
sion. Importantly, changes in suldation temperature change
the selectivity because n-pentadecane starts appearing while 1-
hexadecanol decreases as temperature is increased. We
analyzed the composition of products for 6 h of reaction,
revealing that the products contained mostly deoxygenated
molecules. It is important to mention that PA showed three
main products over the S-200 catalyst at 6 h (100% conversion)
of reaction, n-hexadecane (0.87), n-pentadecane (0.05) and 1-
hexadecanol (0.08). The distribution of products in the HDO of
PA over the S-400 catalyst at 6 h is different, where the presence
of O-containing compounds were still present. We found
Fig. 6 Reaction network for the Mo-based catalysts in the HDO of
palmitic acid (R ¼ C15H31). (1) Palmitic acid, (2) hexadecanal, (3) 1-
hexadecanol, (4) n-hexadecane, (5) n-pentadecane and (6) cetyl
palmitate.
hexadecane.35 Parallel reactions take place such as decar-
boxylation or decarbonylation. The reaction network of the
PA obtained over these catalysts is shown in Fig. 6. The
hydrogenation reactions in the HDO of fatty acids over MoS2-
based catalysts are favored due to the easy detection of the
products. In this case, we found a very small amount of the
decarbonylation or decarboxylation product (n-pentadecane).
We also found an unexpected product from condensation of
PA with its alcohol product, which is the ester product
formed from these catalysts. This product has been scarcely
observed, but recently reported.36–38
a
composition of products of n-hexadecane (0.61), 1-
Fig. 7 Distribution of HDO products over the catalysts (a) S-100, (b) S-200, (c) S-300 and (d) S-400. The products identified are: (-) 1-hex-
adecanol, (C) n-hexadecane, (,) hexadecanal, (B) n-pentadecane and (:) cetyl palmitate.
18298 | RSC Adv., 2019, 9, 18292–18301
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
Table 3 Composition of reaction products at 40% conversion of palmitic acid (relative concentration)
Catalyst
1-Hexadecanol
Hexadecanal
n-Hexadecane
n-Pentadecane
Cetyl palmitate
S-100
S-200
S-300
S-400
Ox
0.34
0.32
0.32
0.28
0.34
0.07
0.05
0.02
0.03
0.39
0.48
0.51
0.51
0.50
0.06
0.04
0.05
0.08
0.13
0.00
0.07
0.07
0.06
0.06
0.21
hexadecanol (0.19), n-pentadecane (0.13) and cetyl palmitate a catalytic material with Mo6+ (50%) and increasing the Mo5+
(0.06) over this catalyst aer 6 h of reaction. The products of the species with a relative concentration of Mo4+ between 27–35
HDO over the catalyst sulded at 400 ꢀC contained larger would be a highly active catalytic material in the HDO of fatty
amounts of O-containing molecules that are undesirable for the acids.
purpose of green fuels. Compared to the products at 40% of
We have established differences in the characteristics of the
conversion, the composition at the end of the HDO test showed Mo species in the supported catalysts. The oxidation states of
that the highly active catalyst is also the most selective (S-200) Mo atoms played a major role in their catalytic performance in
toward the complete HDO product (n-hexadecane). The the HDO of fatty acids, depending on the suldation tempera-
production of n-pentadecane due to decarboxylation or decar- ture. Sulfur is necessary to produce highly active materials by
bonylation in both S-200 and S-400 occurs only at the beginning introducing suldes or sulfate species in the catalysts. We
of the HDO process for these catalysts.
illustrate the catalytic cycles of Mo species for the reduction
The catalytic performance of the Mo-based catalysts in this processes (palmitic acid and derived products in the HDO) and
study showed the importance of intermediate oxide/sulde the oxidation reaction (hydrogen) in Fig. 8. It is important to
active sites being present. The pure oxide form of these cata- note that lower oxidation states for Mo atoms would be involved
lysts is not very active in the HDO of palmitic acid and more in the reduction reactions from the fatty acids and their deriv-
importantly, neither is the selectivity directed toward O-free atives, while higher Mo oxidation states would activate
molecules. The suldation of the catalysts signicantly hydrogen to be added into the fatty acids and related products.
improved their performance, with the most active catalyst The number of electrons are also important for the complete
ꢀ
achieved at suldation temperature at 200 C. The amount of catalytic cycle. We expect that a new Mo-based catalyst needs to
sulde species increased at higher suldation temperatures, possess a combination of those chemical species to become
but the catalytic performance degraded. We also found highly active and selective towards HDO reactions. The Mo-
a decrement in the Mo–O species with increasing temperature, based catalysts do not need to be fully sulded to be highly
and a formation of O species in different local environment for active in the HDO process. This nding can play an important
S-300 and S-400 than S-100 and S-200 catalysts. Spectroscopic role to developing cleaner catalytic materials for the production
methods provide strong evidence for the participation of both of clean fuels, and provides a way for improving the catalytic
Mo–O and Mo–S species to achieve higher conversion of PA and performance of Mo-based heterogeneous catalysts by tuning
selectivity towards O-free products, more specically for the their oxidation states and chemical species.
catalysts S-100 and S-200. Therefore, a proper balance of Mo
species (6+, 5+ and 4+) with different oxide/sulde characteris-
tics is of paramount importance to synthesize highly active
catalysts for HDO of fatty acids and related compounds. This
mixture of species in the catalyst for HDO also results in higher
yields of O-free products for these catalysts. This S-200 catalyst
was the best in terms of conversion of palmitic acid as well as
the production of O-free hydrocarbons. Catalyst S-200 exhibited
a relative concentration of 50%, 13.2% and 36.8% for Mo6+
,
Mo5+ and Mo4+, respectively. This was the best catalytic material
in the HDO of PA, and it contained half of the Mo in the high
valent species, while the rest was in the low valent species. On
the other hand, Mo5+ showed the largest relative concentration
in this catalyst compared to all the series of materials, which
might indicate that the performance strongly depends on
a proper balance of all the Mo species. This study illustrates that
the catalytic performance depends on the Mo species, but
increasing Mo4+ species is not benecial for the activity and
selectivity of these Mo-based materials. We hypothesize that
Fig. 8 Catalytic cycles for the active Mo species. (A) Activation of H2
and hydrodeoxygenation of fatty alcohol by Mo5+/Mo6+ and (B) acti-
vation of H2 and reduction of fatty acid by the Mo4+/Mo5+ active
species.
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 18292–18301 | 18299

View Article Online
RSC Advances
Paper
8 M. Mohammad, T. K. Hari, Z. Yaakob, Y. C. Sharma and
K. Sopian, Renewable Sustainable Energy Rev., 2013, 22,
121–132.
9 P. M. Mortensen, J.-D. Grunwaldt, P. A. Jensen,
K. G. Knudsen and A. D. Jensen, Appl. Catal., A, 2011, 407,
1–19.
10 E. Furimsky, Appl. Catal., A, 2000, 199, 147–190.
11 M. J. Girgis and B. C. Gates, Ind. Eng. Chem. Res., 1991, 30,
2021–2058.
12 T. V. Choudhary and C. B. Phillips, Appl. Catal., A, 2011, 397,
1–12.
4 Conclusions
We have successfully synthesized a series of Mo-based catalysts
with different chemical characteristics by using a range of sul-
dation temperatures. Different conditions during the sulda-
tion process result in formation of different chemical species
and performance of the catalysts. The suldation of these
catalysts, supported on Al2O3–TiO2, signicantly improved their
performance for the HDO of palmitic acid. However, catalysts
with high formation of suldes resulted in lower catalytic
performance. Thus, a proper combination of Mo oxidation
states as well as chemical characteristic (such as oxide, sulde
or intermediate) of the materials is needed to produce highly
active and selective heterogeneous catalysts for oxygen elimi-
nation in fatty acids. The presence of different Mo species is
necessary to promote different reactions under HDO condi-
tions. We suggest that a new class of Mo-based catalysts for the
production of green fuels should be considered, which incor-
porate mixed active species ranging from oxides to suldes to
achieve higher yields of O-free compounds.
13 M. Toba, Y. Abe, H. Kuramochi, M. Osako, T. Mochizuki and
Y. Yoshimura, Catal. Today, 2011, 164, 533–537.
´
14 T. Prasomsri, M. Shetty, K. Murugappan and Y. Roman-
Leshkov, Energy Environ. Sci., 2014, 7, 2660–2669.
´
´
15 D. Valencia, I. Garcıa-Cruz, L. F. Ramırez-Verduzco and
J. Aburto, ACS Omega, 2018, 3, 14165–14172.
16 M. Shetty, K. Murugappan, T. Prasomsri, W. H. Green and
´
Y. Roman-Leshkov, J. Catal., 2015, 331, 86–97.
17 N. Chen, S. Gong and E. W. Qian, Appl. Catal., B, 2015, 174–
175, 253–263.
18 L. van Haandel, G. M. Bremmer, E. J. M. Hensen and
Th. Weber, J. Catal., 2016, 342, 27–39.
19 M. Ferrari, S. Bosmans, R. Maggi, B. Delmon and P. Grange,
Catal. Today, 2001, 65, 257–264.
Conflicts of interest
There are no conicts of interest to declare.
´
ˇ
´ˇ
20 J. Horacek, Z. Tisler, V. Rubas and D. Kubicka, Fuel, 2014,
121, 57–64.
Acknowledgements
21 T. L. Chew and S. Bhatia, Bioresour. Technol., 2009, 100,
2540–2545.
22 M. Shahinuzzaman, Z. Yaakob and Y. Ahmed, Renewable
Sustainable Energy Rev., 2017, 77, 1375–1384.
23 N. Arun, R. V. Sharma and A. K. Dalai, Renewable Sustainable
Energy Rev., 2015, 48, 240–255.
24 Y. Shi, Y. Cao, Y. Duan, H. Chen, Y. Chen, M. Yang and
Y. Wu, Green Chem., 2016, 18, 4633–4648.
25 J. J. Jia, T. A. Callcott, J. Yurkas, A. W. Ellis, F. J. Himpsel,
M. G. Samant, J. Stohr, D. L. Ederer, J. A. Carlisle,
E. A. Hudson, L. J. Terminello, D. K. Shuh and
R. C. C. Perera, Rev. Sci. Instrum., 1995, 66, 1394–1397.
´
The authors wish to thank Project Y.61023 CEMIE-BIO Cluster
Bioturbosina (Instituto Mexicano del Petroleo) and Fondos
´
SENER-CONACYT (248090) for nancial support. R. I. Conde is
gratefully acknowledged for her help with batch reactors. J.
Navarrete, D. Zaragoza and P. Morales are also acknowledged
for suldation of samples, GC-MS, XRD, Raman spectroscopic
and XPS measurements, respectively. The synchrotron-based
measurements were supported by the Natural Sciences and
Engineering Research Council of Canada (NSERC) and the
Canada Research Chair program.
´
´
26 R. Lopez Cordero and A. Lopez Agudo, Appl. Catal., A, 2000,
202, 23–35.
References
27 Y. Wang, C. Cong, C. Qiu and T. Yu, Small, 2013, 17, 2857–
1 E. Vine, Energy Efficiency, 2008, 1, 49–63.
2861.
´
˜
2 R. Banos, F. Manzano-Agugliaro, F. G. Montoya, C. Gil, 28 J. Jadwiszczak, C. OCallaghan, Y. Zhou, D. S. Fox, E. Weitz,
´
´
A. Alcayde and J. Gomez, Renewable Sustainable Energy Rev.,
D. Keane, C. P. Cullen, I. O'Reilly, C. Downing,
2011, 15, 1753–1766.
3 S. Chu and A. Majumdar, Nature, 2012, 488, 294–303.
A. Shmeliov, P. Maguire, J. J. Gough, C. McGuinness,
M. S. Ferreira, A. L. Bradley, J. J. Boland, G. S. Duesberg,
V. Nicolosi and H. Zhang, Sci. Adv., 2018, 4, 5031.
´
´
4 G. S. Aleman-Nava, V. H. Casiano-Flores, D. L. Cardenas-
´
´
Chavez, R. Dıaz-Chavez, N. Scarlat, J. Mahlknecht, 29 P. A. Bertrand, Phys. Rev. B: Condens. Matter Mater. Phys.,
J.-F. Dallemand and R. Parra, Renewable Sustainable Energy
1991, 44, 5745–5749.
Rev., 2014, 32, 140–153.
5 A. Corma, G. W. Huber, L. Sauvanaud and P. OConnor, J.
Catal., 2007, 247, 307–327.
6 L. Wu, T. Moteki, A. A. Gokhale, D. W. Flaherty and
F. D. Toste, Chem, 2016, 1, 32–58.
30 A. Galtayries, S. Wisniewski and J. Grimblot, J. Electron
Spectrosc. Relat. Phenom., 1997, 87, 31–44.
31 D. Nilolova, R. Edreva-Kardjieva, G. Gouliev, T. Grozeva and
P. Tzvetkov, Appl. Catal., A, 2006, 297, 135–144.
32 E. J. Romano and K. H. Schulz, Appl. Surf. Sci., 2005, 246,
262–270.
´
7 P. Tamunaidu and S. Bhatia, Bioresour. Technol., 2007, 98,
3593–3601.
18300 | RSC Adv., 2019, 9, 18292–18301
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
33 A. G. Vandeputte, M.-F. Reyniers and G. B. Marin, J. Phys. 36 E. Kordouli, B. Pawelec, K. Bourikas, C. Kordulis,
Chem. A, 2010, 114, 10531–10549.
J. L. G. Fierro and A. Lycourghiotis, Appl. Catal., B, 2018,
229, 139–154.
´
´
´
´
34 L. Dıaz-Garcıa, V. Santes, T. Viveros-Garcıa, A. Sanchez-
´
´
Trujillo, J. Ramırez-Salgado, C. Ornelas and E. Rodrıguez- 37 G. Zafeiropoulos, N. Nikolopoulos, E. Kordouli, L. Sygellou,
´
Castellon, Catal. Today, 2017, 282, 230–239.
K. Bourikas, C. Kordulis and A. Lycourghiotis, Catalysts,
2019, 9, 210.
´
´
35 D. Valencia, I. Garcıa-Cruz, V. H. Uc, L. F. Ramırez-Verduzco,
M. A. Amezcua-Allieri and J. Aburto, Biomass Bioenergy, 2018, 38 E. Kordouli, L. Sygellou, C. Kordulis, K. Bourikas and
112, 37–44.
A. Lycourghiotis, Appl. Catal., B, 2017, 209, 12–22.
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 18292–18301 | 18301