Effect of Co incorporation and support selection on deoxygenation selectivity and stability of (Co)Mo catalysts in anisole HDO

Article abstract of DOI:10.1016/j.apcata.2018.12.004

A series of supported Co modified Mo catalysts was prepared by varying the Co/Mo ratio in the range from 0 to 1 while maintaining the Mo loading at ca. 10 wt%. A sequential incipient wetness impregnation method, with Mo being introduced first, using aqueous solutions of the corresponding precursor salts was employed during the synthesis procedure. Three supports, i.e., Al₂O₃, ZrO₂, and TiO₂ differing in textural and acidic properties were investigated. Material physicochemical characteristics were evaluated through ICP-OES, N₂-sorption, XRD, H₂-TPR, NH₃-TPD, O₂-TPO, STEM-EDX and XPS techniques. The anisole HDO performance of these CoMo catalysts was evaluated at gas phase conditions in a fixed bed tubular reactor in plug flow regime. The catalysts performance is correlated with properties such as reducibility, acidity, and metal-support interactions. Cobalt addition enhanced the total HDO selectivity by 45% as compared to Mo catalysts. Alumina catalysts displayed higher initial activity (X_anisole≈97%) relative to titania and zirconia supported variants (X_anisole <40%) at identical operating conditions. Titania supported catalysts exhibited rather higher stability compared to zirconia and alumina catalysts over 50 h time on stream (TOS), while zirconia catalysts displayed the highest HDO selectivity (up to 86%). Characterization studies of pre and post-reaction catalysts indicate Mo⁵⁺ to be the main active phase while over-reduction to lower Mo states (Mo⁴⁺ and Mo³⁺) as well as carbon deposition are identified as the cause for catalyst activity decrease with TOS.

Full text of DOI:10.1016/j.apcata.2018.12.004

Applied Catalysis A, General 571 (2019) 61–70
Contents lists available at ScienceDirect
Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
Effect of Co incorporation and support selection on deoxygenation
selectivity and stability of (Co)Mo catalysts in anisole HDO
a
a
b
c
a,⁎
Chanakya Ranga , Vaios I. Alexiadis , Jeroen Lauwaert , Rune Lødeng , Joris W. Thybaut
a
b
Laboratory for Chemical Technology, Ghent University, Technologiepark 914, B-9052, Ghent, Belgium
Industrial Catalysis and Adsorption Technology, Ghent University, Valentin Vaerwyckweg 1, B-9000, Ghent, Belgium
c
SINTEF Materials and Chemistry, Kinetics and Catalysis research team, N-7465, Trondheim, Norway
A R T I C L E I N F O
A B S T R A C T
Keywords:
Bio-oil
A series of supported Co modified Mo catalysts was prepared by varying the Co/Mo ratio in the range from 0 to 1
while maintaining the Mo loading at ca. 10 wt%. A sequential incipient wetness impregnation method, with Mo
being introduced first, using aqueous solutions of the corresponding precursor salts was employed during the
Hydrodeoxygenation
Anisole
synthesis procedure. Three supports, i.e., Al
2
O
3
, ZrO
2
, and TiO
2
differing in textural and acidic properties were
-sorption, XRD, H
-TPO, STEM-EDX and XPS techniques. The anisole HDO performance of these CoMo catalysts
CoMo
investigated. Material physicochemical characteristics were evaluated through ICP-OES, N
2
2
-
Titania
TPR, NH -TPD, O
3
2
Alumina
Zirconia
was evaluated at gas phase conditions in a fixed bed tubular reactor in plug flow regime. The catalysts per-
formance is correlated with properties such as reducibility, acidity, and metal-support interactions. Cobalt ad-
dition enhanced the total HDO selectivity by 45% as compared to Mo catalysts. Alumina catalysts displayed
higher initial activity (Xanisole≈97%) relative to titania and zirconia supported variants (Xanisole < 40%) at
identical operating conditions. Titania supported catalysts exhibited rather higher stability compared to zirconia
and alumina catalysts over 50 h time on stream (TOS), while zirconia catalysts displayed the highest HDO
5
+
selectivity (up to 86%). Characterization studies of pre and post-reaction catalysts indicate Mo to be the main
4
+
3+
active phase while over-reduction to lower Mo states (Mo
identified as the cause for catalyst activity decrease with TOS.
and Mo ) as well as carbon deposition are
1
. Introduction
group, has already been widely investigated as a model compound for
lignin derived biooil [8–13]. The direct deoxygenation of anisole, i.e.,
breaking the Caromatic eO bond rather than the Caliphatic eO bond poses
a specific challenge as the former bond energy exceeds that of the latter
The high consumption of fossil fuels and corresponding environ-
mental impact continue to spark interest in finding cleaner-energy
sources. Lignocellulosic biomass processing through various thermo-
chemical techniques, e.g., fast pyrolysis, leading to biooils, is a very
promising alternative [1–3]. One of the most important research areas
in biooil valorization is catalytic hydrodeoxygenation (HDO) during
which fuels or suitable blending agents including aromatic and ali-
phatic hydrocarbons such as benzene, toluene, and cycloalkanes are
produced [4].
−1
by 84 kJ mol
[6]. Moreover, deoxygenation catalyst development
aims at selective oxygen removal rather than aromatic ring hydro-
genation for minimizing the hydrogen consumption as well as to
maintain an appropriate aromatic content [7,14,15].
The low sulfur content in biooil renders the use of traditional, sul-
fided catalysts such as NiMoS and CoMoS less interesting. Such catalysts
require a tailored amount of sulfur in the processed feeds to maintain
their activity and selectivity and, hence, end products are inevitably
contaminated by sulfur [16–19]. To mitigate this issue, non-sulfided
transition metal (Ni, Co, Mo) catalysts on various supports have been
investigated for hydrotreatment of biooil model compounds [20–22].
Yet, significant challenges remain to improve the catalyst activity,
stability, and HDO selectivity.
The complexity of lignin and, hence, of the correspondingly ob-
tained fast pyrolysis oil, has prompted the use of model compounds
such as phenolics, furans, ethers, acids etc. to study the intricacies of
the hydrodeoxygenation reactions. Among the oxygenates in biooil,
phenolics constitute about one fourth and are the most refractory to
HDO [5,6]. Alkoxy groups are among the majorly occurring moieties
within ligninderived phenolics [6,7]. Anisole, because of its methoxy
Another type of catalysts which are interesting for HDO reaction are
⁎
Corresponding author.
E-mail address: Joris.Thybaut@UGent.be (J.W. Thybaut).
https://doi.org/10.1016/j.apcata.2018.12.004
Received 25 June 2018; Received in revised form 4 December 2018; Accepted 7 December 2018
Available online 07 December 2018
0
926-860X/ © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license
(
http://creativecommons.org/licenses/BY-NC-ND/4.0/).

C. Ranga et al.
Applied Catalysis A, General 571 (2019) 61–70
those based on noble metals [23–26]. Noble metals generally exhibit
good HDO activity but are mostly selective towards aromatic ring hy-
drogenation rather than deoxygenation. This leads to higher hydrogen
consumptions at pressures ranging from atmospheric pressure to 4 MPa
were mineralized by alkaline fusion with sodium peroxide.
An AutoChem 2920 instrument with a thermal conductivity de-
tector (TCD) was applied for the temperature programmed reduction
using hydrogen, i.e., H -TPR. Sample amounts of ca. 100 mg were
2
[
25–28]. Even though exhibiting promising results, noble metals have
loaded in a U-shaped tubular quartz reactor, with an internal thermo-
their excessive price and limited availability as major disadvantages.
This renders processes employing them economically less feasible. To
remediate this issue, various non-noble transition metals have recently
been developed for bio oil HDO, among which Mo-based catalysts, that
have exhibited excellent activity and selectivity towards targeted
deoxygenation reactions [12,13,29].
couple positioned at the level of the sample bed. Prior to H -TPR, the
−1
2
sample was purged with high purity (99.999%) Argon (60 ml min ) at
200 °C for 2 h. To obtain the TPR profiles the temperature was pro-
−
1
gressively increased from ambient to 900 °C at a rate of 10 °C min in
a mixture of 10 vol.% H
Acidity measurements were performed by temperature programmed
desorption with NH (NH -TPD), also on the AutoChem 2920 instru-
2
/Ar.
Our previous work on zirconia supported MoO
3
catalysts has led to
3
3
adequate activities and stabilities, while the HDO selectivity did not
exceed 50% [30]. Several supported metal-Mo catalyst compositions
have already been tested under HDO conditions and among the pro-
moter metals (including noble metals such as Pd, Pt, Re), Co has been
found to induce one of the highest hydrodeoxygenation to aromatic ring
hydrogenation ratios [4,19,31]. On supported Mo and CoMo catalysts,
interactions between the metal (oxide) and the support have been re-
ported to determine the exact structure of this metal oxide, which can
affect reducibility, catalytic activity as well as total acidity of the cat-
alyst materials [21,29,32–37].
ment coupled with a TCD. Prior to NH -TPD, the sample was purged
3
−1
with high purity (99.999%) helium (60 ml min ) at 200 °C for 2 h.
After pretreatment, the sample was saturated with high purity anhy-
−1
drous ammonia employing 4 vol.% NH /He (75 ml min ) at 80 °C for
3
2 h and subsequently flushed at 110 °C for 1 h to remove physisorbed
ammonia. The TPD analysis was carried out from ambient temperature
−
1
to 700 °C at a heating rate of 10 °C min . A calibration factor was
determined by calibrating the detector with known volumes of NH
3
[38]. The amount of ammonia desorbed was correlated to the area
under the TPD curve.
In the present work, the effect of the Co/Mo ratio and the type of
support of Co-Mo materials on their HDO performance has been in-
Temperature programmed oxidation experiments using oxygen (O -
2
TPO) were also conducted on the AutoChem 2920 instrument. The
outlet gas stream was monitored online using a calibrated OmniStar
Pfeiffer mass spectrometer (MS). In a typical TPO experiment, the spent
catalyst sample was purged with high purity (99.999%) helium (60 ml
vestigated. In particular, three supports, i.e., ZrO
2
, Al
2
O
3
, and TiO ,
2
exhibiting different textural and acidic properties are probed. The im-
pact of metal and support properties, i.e., reducibility and acidity, as
well as of metal-support interactions on the catalysts activity, stability
and selectivity under HDO experimental conditions is presented.
−
1
min ) at 200 °C for 2 h followed by heating from ambient temperature
−
1
to 700 °C under 10 vol% O /He (45 ml min ). The heating rate used
−1
2
for O
different amu signals, the selection of which was based on the analysis
of the mass spectra of the individual components. The CO signal was
monitored at m/z = 44, that of CO at 28, that of He at 2, and that of O
at 16. The amount of carbonaceous species was quantified by calcu-
lating the evolved CO during a typical TPO experiment. The CO signal
observed to be negligible during the present experiments and any CO
produced probably converted to CO
2
−TPO was 10 °C min . For quantification, the MS is focused to
2
. Experimental methods
2
2.1. Catalyst preparation
2
A series of supported Co modified Mo catalysts was prepared by
2
varying the Co/Mo ratio in the range from 0 to 1 while maintaining the
Mo loading at ca. 10 wt%. A sequential incipient wetness impregnation
method using aqueous solutions of the corresponding precursor salts,
2
.
X-Ray Diffraction (XRD) patterns of the powdered catalyst samples
were recorded at room temperature on a Siemens Diffractometer
Kristalloflex D5000, using Cu Kα radiation (λ = 1.54 Å). The X-ray tube
voltage was set to 40 kV and the current to 50 mA. XRD patterns were
collected in the range of 2θ from 10° to 90° with a step size of 0.02°.
Xray Photoelectron Spectroscopy (XPS) analysis was performed
under ultrahigh vacuum conditions using an Axis Ultra DLD XP spec-
trometer from Kratos Analytical and monochromatic Al Kα radiation
(hν = 1486.6 eV). A pass energy of 160 eV was used for survey scans
and 20/40 eV was used for the individual core levels. Charge com-
pensation using low energy electrons was applied during acquisition.
The binding energy scales were calibrated to the adventitious carbon of
C1s component at 284.6 eV. The background was subtracted using a
Shirley function and the spectra were fitted using a convolution of
Gaussian and Lorentzian functions. The composition of Mo oxidation
states was estimated by the deconvolution of Mo 3d doublet. The fol-
lowing constraints were used for deconvolution: (1) Splitting energy of
i.e., ammonium heptamolybdate ((NH
4
)
6
2
Mo
7
O
24.4H O, Alfa Aesar) and
2
cobalt nitrate hexahydrate (CoN
2
O
6
.6H O, Alfa Aesar), was employed,
with the former being introduced first. Support pellets were first cru-
shed and sieved to obtain the 100–300 μm particle size fraction, and
were subsequently calcined at 500 °C for 5 h prior to impregnation.
After the impregnation of Mo salt on to the support, the samples were
dried at room temperature for 12 h and then at 120 °C for 24 h, followed
−
1
by calcination under flowing air (ca. 150 ml min ) at 550 °C for 6 h.
Subsequently, Co salt was impregnated with same post impregnation
steps as that of Mo impregnation. Three different supports, i.e., Al
2 3
O ,
ZrO
2
and TiO (Alfa Aesar), were used. The resulting calcined materials
2
were designated as (xCo)MoA, (xCo)MoZ, and (xCo)MoT respectively,
where “x” refers to the corresponding Co/Mo ratio present in the ma-
terial.
2.2. Catalyst characterization
3
.2 eV for Mo 3d5/2–Mo 3d3/2, (2) Area intensity ratio of 3:2 for Mo
N
2
adsorption–desorption isotherms of the powdered catalyst sam-
3d5/2–Mo 3d3/2, and (3) Equal full width at half maximum (FWHM) of
Mo 3d5/2 and Mo 3d3/2 [30]. Mo 3d5/2 and 3d3/2 components are lo-
cated at 232.55 and 235.7 eV respectively [39–41].
ples were measured at −196 °C using a Micromeritics TriStar II 3020
instrument. The specific surface area (SBET) was calculated by the
Brunauer–Emmett–Teller (BET) method. The average pore volume was
obtained using the Barrett–Joyner–Halenda (BJH) method. Prior to
these measurements, the samples were outgassed at 200 °C for 2 h to
remove any volatile adsorbates from the surface.
Scanning Transmission Electron Microscopy (STEM) was used for
structural analysis, while EDX yielded local elemental mapping. These
techniques were performed using a JEOL JEM-2200FS, Cs-corrected
microscope operated at 200 kV, which was equipped with a Schottky-
type field-emission gun, FEG, and EDX JEOL JED-2300D. All samples
were deposited by immersion onto a lacey carbon film on a copper
support grid.
The bulk elemental composition of as-prepared catalysts was de-
termined by means of inductively coupled plasma optical emission
spectroscopy (ICP-OES, ICAP 6500, Thermo Scientific). The samples
62

C. Ranga et al.
Applied Catalysis A, General 571 (2019) 61–70
Table 1
component k. The selectivity, Sel
i
, for product i coming from the feed
Overview of the range of performance test conditions.
component k is calculated using Eq. (2) on an elemental carbon basis,
where cn
i
is the number of carbon atoms in molecule i.
Operating condition
Experimental range
cni Fi
Catalyst pellet diameter (μm)
Temperature (°C)
100–200
340
Seli =
cnk (Fk0
Fk)
(2)
Total pressure (MPa)
0.5
−
1
The selectivity towards hydrodeoxygenation products, SHDO, is
Space time (kgcat s molanisole
)
5–230
50
H
2
/anisole (molH2 molanisole−1)
given by Eq. (3).
nHDO
cni Fi,HDO
i=1
SHDO =
0
2
.3. Catalyst activity measurements
cnk (F
Fk)
(3)
k
F
i,HDO represents the outlet molar flow rate of hydrodeoxygenated
2
.3.1. Experimental setup and operating conditions
product i and nHDO represents the total number of deoxygenated pro-
Kinetic experiments for gas phase anisole HDO were carried out in a
ducts, created from feed component k.
state of the art high throughput kinetic screening setup, HTKS [42]. A
schematic representation of the setup is provided in SI Fig. 1 and a more
elaborate description can be found in our previous work [30]. It com-
prises 16 parallel reactors (L = 85 cm, i.d. = 0.21 cm), in which cata-
lyst particles with size 100–200 μm are loaded, diluted with α-alumina
inert of the same size, between two inert α-alumina layers. The inert
layer on top of the catalyst bed ensures complete liquid feed vapor-
ization and adequate mixing, as well as the development of the plug
flow regime before the reacting fluid reaches the catalyst bed. Hy-
drogen diluted with helium was used as gas feed and anisole dissolved
in n-hexane with n-octane as internal standard, as the liquid feed. The
absence of heat and mass transport limitations and the establishment of
the ideal plug flow regime were verified via adequate correlations, see
SI Table 1 [43]. The investigated range of the operating conditions is
presented in Table 1. The product stream was analyzed with an on-line
gas chromatograph, i.e., a DHA (Detailed Hydrocarbon Analyzer) Tra-
ceGC1310 equipped with 2 flame ionization detectors (FIDs, front and
back). For the present analysis, the Front FID with a Rtx-PONA column
3
. Results and discussion
3
.1. Material characterization
3
.1.1. Textural properties and elemental composition
The metal loading and specific surface area (SBET) of the materials
obtained through ICP analysis and BET measurements respectively are
reported in Table 2 and SI Table 2. As can be seen, SBET decreases with
increasing metal loading, i.e., initially with Mo and subsequently with
Co introduction in the catalyst, which is attributed to the gradual filling
of the pore volume [30,44,45]. All metal loaded samples exhibit lower
surface area and pore volume compared to the pure support (see SI
Fig. 2).
3.1.2. Structural composition
X-ray diffraction analyses of pure and metal loaded supports are
given in Fig. 1. As can be seen, the most prominent crystalline phases
(
L = 100 m, i.d. = 0.25 mm) was used.
Prior to the kinetics measurements, the catalyst was activated in situ
identified in the metal loaded samples are pure MoO
3
, and Co
3
O as
4
by drying first at 200 °C for 2 h under helium and subsequently by re-
well as a mixed CoMoO . The monometallic Mo-supported material
4
−
1
duction under H
2
/He (70% v/v) till 500 °C at 5 °C min . The catalyst
MoA presents sharper peaks, characteristic of MoO phase, than MoZ,
3
was maintained at that temperature for 3 h at a total pressure of
while MoT does not present such peaks. This can be attributed to the
0
.5 MPa.
different Mo-support interactions, which, particularly in the case of
MoT, appear to result in more amorphous or highly dispersed MoO
3
onto the support. Interestingly, the incorporation of Co in the MoT
sample results in the aggregation of the MoO phase, i.e., the MoO
peak in the XRD pattern is becoming more pronounced. In the case of
the other two monometallic Mo-supported samples, MoO dispersion
2
.3.2. Data treatment
3
3
The conversion of feed component k, X
k
, is defined on a molar basis
(1)
as shown in Eq. (1),
3
0
seems to be promoted by incorporation of Co, as derived from the re-
F
Fk
k
Xk =
0
duction of the MoO peak height. With increasing Co loading, peaks
F
3
O
k
characteristic of Co
3
4
and CoMoO phases become sharper in the case
4
0
F
and Fk represent the inlet and outlet molar flow rates of
of MoZ and MoA, yet these phases are highly dispersed and/or rather
k
Table 2
Catalyst and support properties.
2
−1)
uptake (μmol g−1
metal/acid site ratioa
(-)
Material
SBET (m
g
Co (Wt.%)
H
2
)
3
NH uptake
Region I
Region II
Total
(μmol g−1)
(μmol m−2
)
ZrO
2
99 ±
80 ±
80 ±
67 ±
66 ±
1
2
4
4
4
–
–
–
–
432
356
306
237
207
1112
980
955
895
817
517
476
367
4.4
4.5
3.8
3.5
3.2
4.9
4.5
4.6
4.6
4.5
3.4
4.0
3.9
–
MoZ
–
67
40
73
50
–
120
131
189
111
–
187
172
262
161
–
0.5
0.6
1.1
0.8
–
0
0
1
.25CoMoZ
2.2 ± 0.1
5.8 ± 0.1
8.9 ± 0.2
–
.6CoMoZ
CoMoZ
2
Al O
3
225 ± 10
MoA
220 ±
206 ±
1
4
–
65
43
42
46
–
141
189
204
169
–
205
231
246
215
–
0.2
0.2
0.3
0.3
–
0
0
1
.25CoMoA
.6CoMoA
CoMoA
2.8 ± 0.1
6.1 ± 0.1
9.4 ± 0.2
–
193 ± 10
181 ±
152 ±
119 ±
94 ±
7
1
2
TiO
2
MoT
–
128
199
110
84
238
283
0.5
0.8
0
.6CoMoT
1
5.9 ± 0.1
1
1
a
H2uptake (µmol g
NH3uptake (µmol g
)
)
.
63

C. Ranga et al.
Applied Catalysis A, General 571 (2019) 61–70
XRD amorphous in the MoT sample, see Fig. 1(c).
3
.1.3. Reduction behavior
Fig. 2 depicts the H
2
TPR profiles of the supported CoMo oxide
samples along with the pure supports. These profiles are interpreted in
terms of two stages: the first within a temperature range 300–600 °C
(
(
region I), followed by the second one at temperatures exceeding 600 °C
region II). No significant reduction has been observed during pure
supports’ TPR, see Fig. 2.
Within region I, the first peak, i.e., between 300 and 400 °C, is at-
tributed to the reduction of Co
3
O to CoO [46,47]. No such peak is
4
observed on 0.6CoMoT. This, however, may correspond to the high
dispersion (as also evident from the XRD measurements, see Fig. 1) and
the low reducibility of the Co
3
O phase and/or peak overlap with the
4
one corresponding to the reduction of octahedral molybdena (MoO
3
) to
4
+
Mo
corresponds to the reduction of octahedrally co-ordinated Mo
[48]. The second peak, i.e., the one between 430 and 550 °C
6
+
pre-
[32,49]. It is reported that during
4
4+
+
sent in MoO
3
and CoMoO to Mo
4
6
+
TPR, Mo
transforms into Mo
through an intermediate state
5
+
(
(
Mo ) [50], see also Section 3.3 where the surface characterization
6
+
XPS) of the reduced material is discussed in more detail. This Mo
reduction peak shifts to higher reduction temperatures with increasing
Co/Mo ratio (see particularly Fig. 2(b) for the alumina support), po-
tentially because the CoMoO crystalline phase, which is emerging with
4
6+
the Co content, limits the reducibility of Mo
[46,51].
Region II reflects the further reduction of the hardly reducible Co
and Mo species. The broad peaks around 720–850 °C are ascribed to the
reduction of CoO to Co and of tetrahedrally co-ordinated molybdena
4
+
6+
(
MoO ) to Mo [30,47]. The tetrahedral phase of Mo is difficult to
3
reduce due to strong metalsupport interactions [32]. The further re-
4
+
3+
duction of Mo
to Mo
species could also be associated with this
peak [29]. The corresponding hydrogen uptake values are given in
Table 2. Interestingly, alumina and zirconia supported CoMo catalysts
exhibited higher hydrogen consumption in region II compared to their
Mo variants and to titania supported ones, see Table 2.
3
.1.4. Acidic properties
Fig. 3 depicts the NH -TPD profiles of the supported CoMo oxide
3
samples. The corresponding ammonia uptake values are given in
Table 2, showing that the catalysts, in terms of total acidity, are ranked
as follows: CoMoA > CoMoT > CoMoZ. The impregnation of the ac-
tive phase decreased the total acidity of the catalysts compared to their
respective bare supports, see Table 2 [52]. The decline in total acid
strength could indicate that Co and Mo oxide phase partially covered
the acid sites on the supports [52]. Acid site densities expressed as NH
3
−2
uptake per SSA of the catalyst, μmol m , were found to be rather si-
milar, yet the metal to acid site ratios vary somewhat more across
differently supported catalysts, see Table 2. Through the acidity mea-
surements performed in this work, no distinction could be made be-
tween the Lewis (LAS) and Brønsted (BAS) acid sites present on the
catalyst materials. However, typically Al
2
O
3
and TiO contain both LAS
2
and BAS while ZrO contains predominantly LAS [27,53,54].
2
As can be observed in Fig. 3, all the catalysts exhibit broad deso-
rption peaks, indicating a wide distribution of acid strength on the
surface of the catalysts. Depending on the desorption temperature (T
d
),
< 250 °C), medium,
). Sites of medium acid
acid sites were designated as: weak, (T
250 < T < 450 °C) and strong, (450 °C < T
d
(
d
d
strength seem to dominate in the structure of all investigated materials,
followed by weakly and strongly acidic sites. In particular, the TPD
profiles of the alumina catalysts exhibit a clear tailing behavior at
temperatures exceeding 450 °C indicating the presence of some strong
acidic sites. Zirconia and titania supported catalysts mainly have weak
and medium acidic sites, while strong acidic sites on these materials are
negligible.
Fig. 1. X-Ray Diffractograms of (a) zirconia (b) alumina and (c) titania sup-
ported as prepared calcined materials.
64

C. Ranga et al.
Applied Catalysis A, General 571 (2019) 61–70
Fig. 2. H
2
-Temperature Programmed Reduction (H -TPR) of (a) zirconia (b) alumina and (c) titania supported as prepared calcined materials.
2
3
.2. Catalytic performance evaluation
.2.1. Activity and stability
(instead of 128 kgcats molanisole−1 used for zirconia and titania catalysts,
see Fig. 4). In terms of activity, titania supported catalysts displayed a
comparable level of initial conversions to zirconia supported ones at
identical space-time, see Fig. 4(c). In contrast to their zirconia and
alumina supported counterparts, Co incorporation into the MoT cata-
lytic system did not significantly impact on the deactivation behavior of
the material, yet it decreased the initial activity compared to that of the
monometallic MoT, see Fig. 4(c). In the present work, even though
catalysts with different supports exhibited similar acid site densities,
see Table 2, Section 3.1.4, the differences in strength of these acid sites
results in different levels of initial activity per gram of the catalysts
[37,53,55]. Alumina supported catalysts with relatively more strong
acid sites exhibited higher activity (up to 97% conversion, see SI Fig. 3)
in comparison to zirconia and titania supported catalysts, where more
weak and medium acid sites are present.
3
As relevant conditions for HDO activity testing a reaction tem-
perature and total pressure of 340 °C and 0.5 MPa, respectively were
selected [30], employing a H
2
to anisole inlet molar ratio of 50 mol
−
1
−1
mol
within a space time between 5–230 kgcat s molanisole. In case of
zirconia-supported catalysts, tested at a space-time of 128 kgcats mo-
−
1
l
anisole , the activity of the monometallic MoZ decreases during the
first 10 h TOS and then remains rather stable up to 50 h TOS, see
Fig. 4(a). The bimetallic CoMoZ catalysts with the highest Co/Mo ratio,
i.e., 0.6 and 1, exhibited a higher initial activity than MoZ and
0
.25CoMoZ. However, the activity of bimetallic CoMoZ materials seems
to decrease more significantly during the first 10 h TOS, as compared to
MoZ, while their deactivation continued for higher TOS, albeit less
steeply.
Our previous work on zirconia supported Mo catalysts [30] in-
dicated that the decreasing catalyst activity with time on stream could
be, in part, attributed to carbon deposition during the course of the
reaction, which decreases the exposure of the catalyst’s active phase to
the reactants and results in a reduced overall activity [4,29]. The cor-
relation between time on stream performance and catalyst properties is
discussed in more detail further in Section 4.
The alumina supported catalyst series displayed a similar perfor-
mance in terms of catalyst stability as the zirconia supported ones. As
evident from Fig. 4(b), the monometallic MoA material exhibited a less
pronounced deactivation than the bimetallic CoMo variants. Overall,
however, alumina catalysts exhibited a significantly higher initial ani-
sole conversion, i.e., up to 97%, see SI Fig. 3, in the same space-time
range as employed for the zirconia based catalysts. Thus, to generate
the intrinsic kinetics character of the data and achieve comparable
conversions among the investigated materials, the alumina catalysts
were evaluated at a much lower space-time, i.e., 12 kgcat s molanisole−1
3.2.2. Product selectivity and reaction pathways
The typical product spectrum for all the investigated catalysts
contained benzene, phenol, cresol, toluene, xylenes, methyl anisole,
65

C. Ranga et al.
Applied Catalysis A, General 571 (2019) 61–70
3
Fig. 3. NH -Temperature Programmed Desorption (NH
3
-TPD) of (a) zirconia (b) alumina and (c) titania supported as prepared calcined materials.
and dimethyl phenol. Trace amounts of heavier aromatic products such
as trimethylbenzenes were also detected. Methane was the only light
hydrocarbon by-product observed. The typical mass and carbon bal-
ances were closed within 5%. Main product selectivities are displayed
in Fig. 5(a) and SI Fig. 4 for bimetallic catalysts, with a Co/Mo ratio of
catalysts [30]. It was also found that, irrespective of the support, CoMo
catalysts exhibit higher deoxygenation selectivity, as compared to the
monometallic Mo catalysts at iso-conversion, i.e., 30%, see Fig. 5 (b).
Incorporation of Co into Mo catalytic system selectively affected the
kinetics of the already considered steps and resulted in an enhanced
total HDO product (BTX - benzene, toluene, and xylenes) selectivity
rather than opening up new reaction pathways. The reaction network
that is consistent with our results is presented in Fig. 6.
0
.6. For all catalysts, the deoxygenated product selectivity, i.e., towards
benzene and toluene, exhibited a decreasing trend with TOS, similar to
that observed for the anisole conversion (see Fig. 4), in contrast to the
phenolic product selectivity, i.e., phenol, cresol and dimethyl phenol,
which increases with TOS. Given the decrease in conversion with TOS,
the above-described behavior is in line with what can be expected from
a mere conversion effect in a consecutive reaction mechanism [30]. In
terms of deoxygenated product selectivity, the investigated materials
are ranked as follows: CoMoZ > CoMoT > CoMoA. All materials ex-
hibited a negligible selectivity towards aromatic ring hydrogenation
products, under the investigated range of operating conditions.
Anisole conversion on the investigated CoMo catalysts mainly
started via the cleavage of the Caliphatic– O bond with formation of
phenol and methane, i.e., demethylation. Methanol was not observed in
the product stream, suggesting that the catalysts preferably cleave
Caliphatic – O rather than Caromatic – O bond in anisole HDO [56–58]. The
former is, indeed, weaker than the latter as already pointed out in
Section 1 [6,19]. Phenol hydrodeoxygenation leads to benzene pro-
duction. Further, aromatic hydrogenation of benzene to cyclohexene
was found to be very limited i.e., selectivity < 1%. The formation of
methyl anisole, cresol, and dimethyl phenol also confirms the occur-
rence of intermolecular and intramolecular methyl group transfer.
Cresol and dimethyl phenol were found to further transform into
The acquired data suggest that anisole transformation under the
aforementioned HDO conditions over the mono- and bi metallic mate-
rials occurs through a complex reaction network, similar to that re-
ported in our previous work over monometallic zirconia supported Mo
66

C. Ranga et al.
Applied Catalysis A, General 571 (2019) 61–70
Fig. 4. (a, b, c) Total activity of CoMo catalysts with TOS for zirconia, alumina, and titania supported catalysts respectively. Operating conditions: T = 340 °C,
= 0.5 MPa, H /anisole = 50 mol mol , W/Fº = 128 kgcat s molanisole (except for alumina supported catalysts the space-time tested was 12 kgcat s molanisole ).
−
1
−1
−1
P
T
2
toluene and xylenes, respectively, through direct hydrogenolysis
59,60]. Furthermore, aromatic ring methylation leading to tri-methy-
anisole resulting in methyl anisole. Titania supported CoMo materials
presented medium strength acidity and their selectivities towards
transalkylation and deoxygenation products were found to be com-
parable, i.e., 60 and 40% respectively with benzene and phenol as
major part of the product spectrum followed by their methylated de-
rivatives and methyl anisole, see Fig. 5(a).
[
lated benzenes was found to take place only to a negligible extent. The
introduction of Co into the Mo catalysts greatly enhanced their per-
formance in terms of HDO selectivity through hydrogenolysis of phe-
nolic oxygenates (phenol, cresol, and dimethyl phenol) into alkylated
aromatics (benzene, toluene and xylenes). The presence of Co around
the active Mo oxide species apparently promotes the breakage of phe-
nolic Caromatic eO bond, e.g., by lowering the activation energy in
comparison to an un-promoted Mo catalyst [19,31,61].
In comparison to our previous work on zirconia supported Mo cat-
alysts [30], CoMoZ catalysts in this work exhibited 45% higher HDO
selectivity. In addition, the present CoMoZ catalysts exhibited higher
deoxygenation selectivity (86%) with negligible aromatic ring hydro-
genation compared to noble metals such as Ru (50% HDO selectivity at
similar hydrogen pressures, 0.5 MPa) [23] and Pt, Pd (up to 60% HDO
selectivity at higher hydrogen pressures, 3–4 MPa) [25,26].
Optimal HDO performance has been reported to not only depend on
the metal (oxide) but also on the acid strength of the catalyst [4,62]. It
is reported that, the acidic site strength of the affects the adsorption of
reactants and intermediates, as well as dictates the course of the reac-
tion [4,53,63]. In our work, alumina supported CoMo catalysts, ex-
hibiting the highest acidity among the investigated materials, see
Table 2 and Fig. 3, were found to yield more transalkylation rather than
deoxygenation products (SHDO = 8%), see Fig. 5. Due to relatively
higher amount of strong acid sites on alumina supported (Co)Mo cat-
alysts, the phenol intermediate can dissociate on the support and does
not necessarily strongly interact with the active metal phase [17,63]. As
a result, phenol and its methylated variants constituted the major
product fraction in case of alumina catalysts. In contrast, zirconia
supported bimetallic materials with more weak acid sites and lower
total acidity, mainly led to deoxygenation reactions, resulting in a SHDO
as high as 86%. Benzene constituted the major share (66%) of deox-
ygenated products with mono-, di-, and traces of tri-methylated ben-
3.3. Spent catalyst study
As evident from the results presented above, the catalysts HDO
performance in terms of activity, stability and selectivity varies with the
incorporation of Co as well as with the selected support. The active sites
attributed to Mo are still believed to be the main contributors to the
reaction. Yet the catalyst performance is significantly affected by the
incorporation of Co and the differences in support via changes in re-
ducibility and acidity, mainly.
To gain more insight into the changes that the catalyst structure and
the active surface undergo during anisole HDO, both reduced, i.e, prior
to testing, and spent, after 50 h TOS testing, catalyst samples were
characterized via XPS and O -TPO. With respect to XPS, emphasis was
2
zenes being the rest, on ZrO
2
supported catalysts, see Fig. 5(a). Along
placed on the high-resolution window of Mo 3d transitions. After de-
with deoxygenated products, zirconia supported catalysts displayed a
convolution of the XPS spectra, the contribution of the various Mo
species, i.e., Mo6+, Mo , Mo and Mo was determined, see Fig. 7.
5+
4+
3+
good amount (selectivity = 10%) of aromatic ring methylation onto
67

C. Ranga et al.
Applied Catalysis A, General 571 (2019) 61–70
species and the electronic interaction between Co and Mo leads to a
deeper reduction of Mo species [48]. The differences in deeper re-
ducibility between zirconia, alumina supported catalysts and titania
supported one, are attributed to differences in metal support interac-
tions [37]. This finding can also account for the relatively high hy-
drogen uptake, observed for CoMoZ and CoMoA, during H -TPR ana-
2
lysis, in the region (II), compared to their monometallic counterparts
and titania supported catalysts, see Table 2 and Fig. 2.
5
+
In our previous work, it was found that the presence of Mo
spe-
cies was crucial to achieve high catalyst stability and activity, whereas
reduction to lower oxidation states induced a negative effect [30]. As
can be seen in Fig. 7, the reduced alumina supported monometallic
5
+
material presents the highest concentration of Mo , which is only
moderately reduced during HDO testing. This provides an explanation
for the relatively high initial activity of MoA and its subsequent stabi-
lity, see Fig. 4. In addition, the high activity even at lower space-times
(
see Fig. 4(b)), could be partially attributed to alumina’s high acidic
strength. However, as mentioned before the incorporation of Co in the
supported Mo oxide catalyst leads to steeper deactivation with TOS for
MoA and MoZ, whereas it presents no stability effect for MoT, see
Fig. 4. This can be ascribed to the over-reduction of the Mo oxides and
3
+
the formation of Mo phase during the anisole HDO testing over MoA
3
+
and MoZ, see Fig. 7. In contrast, there is no Mo phase detected on the
surface of the spent MoT sample, but only constrained reduction of
6
+
5+
4+
Mo to Mo and Mo phases, see Fig. 7. This is probably due to the
strong metalsupport interactions in case of titania supported catalysts
leading only to a limited reduction under reaction conditions such as
anisole HDO [68].
Carbon deposition on the catalyst surface may be another phe-
nomenon responsible for catalyst deactivation with TOS under HDO
conditions [29,30]. O -TPO was performed on the spent bimetallic
2
Fig. 5. (a) A comparison of main product selectivity and (b) deoxygenation
materials, with Co/Mo = 0.6, to follow the carbon dioxide evolution as
a measure of the nature and quantity of carbonaceous species on the
catalyst surface. As can be seen in the corresponding patterns in Fig. 8,
product selectivity of Mo and CoMo catalysts during anisole HDO (T = 340 °C,
−
1
−1
P
T
= 0.5 MPa, H
2
/anisole = 50 mol mol , space-time = 5–180 kgcat s mol
)
at iso-conversion (≈30%).
MS responses corresponding to CO evolution were identified for all the
2
The binding energies of Mo6+, Mo , Mo , Mo species were found
5+
4+
3+
materials in the temperature range 350–380 °C, indicating the presence
of similar carbonaceous deposits, irrespective of the nature of the
support. In terms of amount of deposited carbon, see SI Table 4, the
investigated materials can be ranked as follows: CoMoA > CoMoT >
CoMoZ, identically to the ranking with respect to total acidity, see
Table 2 and Fig. 3. The presence of a relatively higher number of strong
acid sites in CoMoA induces the formation of a higher amount of coke,
which leads to a more pronounced reduction of specific surface area
to be at 232.0–232.4, 231.0–231.6, 229.1–229.7, and 228.5–228.9 eV
respectively, which is consistent with previously reported values
[
29,41,64,65]. It was found that the reduced catalyst samples still
6+
contain the original Mo species as well as lower Mo oxidation states,
5
5
+
+
4+
3+
Mo
Mo
and Mo , but no Mo or metallic Mo phase. The existence of
4+
and Mo
species in reduced catalysts indicates the creation of
3+
active Mo oxide defects as reported elsewhere [11,12,29,30]. Mo was
identified only in the spent bimetallic samples CoMoZ and CoMoA. As
can be seen from Fig. 7, the incorporation of Co in the zirconia and
(
SSA) and porosity, see SI Table 4, and contributes to the deactivation
of the catalytic material, see Fig. 4(b), by covering active sites. Even
though CoMoT was found to develop slightly higher amount of coke
than CoMoZ, attributed to the relatively higher acidity of the former, it
presented a more stable behavior [69]. CoMoZ exhibited a more
4
+
alumina supported Mo oxide catalysts, facilitates the reduction of Mo
3
+
to Mo
species during reaction, while it has a negligible effect in the
case of titania supported material [66,67]. Easier reducibility of Co
Fig. 6. Proposed anisole HDO reaction pathways.
68

C. Ranga et al.
Applied Catalysis A, General 571 (2019) 61–70
Fig. 7. High-resolution XPS of Mo 3d doublet transitions of catalysts with (a, b) zirconia, (c, d) alumina, and (e, f) titania supports in their reduced and spent form.
6
+
5+
4+
3+
Red: Mo , Orange: Mo , Green: Mo , Blue: Mo (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of
this article).
pronounced deactivation trend under 50 h TOS HDO conditions, see
Fig. 4(a, c). This could be assigned mainly, as mentioned above, to the
3
+
formation of the less active Mo
phase.
STEM analysis along with EDX elemental mapping has been per-
formed to determine the structural features of CoMo catalysts with Co/
Mo ratio of 0.6, prior to and after reaction. The particle size is difficult
to estimate as molybdenum is practically homogeneously spread over
the support. The structural morphology of the catalyst materials is in-
tact even after 50 h on stream. No evidence of sintering is identified, see
SI Fig. 5.
4
. Conclusions
The roles of Co incorporation into Mo based catalysts and support
properties on catalyst stability and HDO selectivity are correlated to key
properties such as reducibility, acidity and metal-support interaction
through a wide range of physicochemical analyses of the as prepared,
reduced as well as spent catalysts. Titania supported catalysts remain
relatively stable over 50 h TOS during anisole HDO as compared to
zirconia and alumina supported variants. Bimetallic CoMo catalysts
exhibit higher (up to 45%) HDO selectivity than their Mo counterparts
within the investigated range of operating conditions. While alumina
Fig. 8. Evolution of CO
catalysts measured by O
Conditions: in Helium (10% v/v), flow rate = 45 ml min
rate = 10 °C min . Catalyst mass ≈ 100 mg. Reaction conditions: T = 340 °C,
= 0.5 MPa, H /anisole = 50 mol mol , W/Fº = 12–125 kgcat s molanisole
TOS = 50 h.
2
2
(m/z = 44) as a function of temperature for spent
-Temperature Programmed Oxidation (TPO) using MS.
Ramp
−1
O
2
−
1
−
1
−1
P
T
2
,
69

C. Ranga et al.
Applied Catalysis A, General 571 (2019) 61–70
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