Insights in supported rhenium carbide catalysts for hydroconversion of lignin-derived compounds

Article abstract of DOI:10.1016/j.apcata.2020.117600

Rhenium supported over activated carbon (AC) was carburized under different temperatures (from 500 to 700 °C) under a mixture of H₂/C₂H₄ (75/25). Resulting catalysts were characterized by N_2-sorption, CO chemisorption, temperature programed reduction (TPR), ammonia temperature programed desorption (TPD-NH₃), STEM, and XPS and catalytic properties were measured in a batch reactor at 350 °C, 5.0 MPa for the hydrodeoxygenation (HDO) of guaiacol. Results have shown that Rhenium was gradually reduced and carburized upon increasing the temperature. From 650 °C, a Re_xC phase was obtained and transformed into ReC at 700 °C. Re_xC was the most active phase obtained and maximum yield in benzene of 51% was reached at complete conversion. ReC phase showed to inhibit the hydrogenolysis of phenol into benzene.

Full text of DOI:10.1016/j.apcata.2020.117600

Applied Catalysis A, General 599 (2020) 117600
Contents lists available at ScienceDirect
Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
Insights in supported rhenium carbide catalysts for hydroconversion of
lignin-derived compounds
Elodie Blanco *, Ana Belén Dongil , José Luis García-Fierro , Néstor Escalonaa,b,d,e,*
b
c
a,b,
c
c
a
Departamento De Ingeniería Química y Bioprocesos, Pontificia Universidad Católica De Chile, Santiago, Chile
Millenium Nuclei on Catalytic Processes Towards Sustainable Chemistry (CSC), Chile
Instituto De Catálisis y Petroleoquímica, CSIC, Cantoblanco, 28049 Madrid, Spain
d
e
Departamento De Química Física, Facultad De Química y De Farmacia, Pontificia Universidad Católica De Chile, Santiago Chile
Unidad De Desarrollo Tecnológico, Universidad De Concepción, Coronel, Chile
A R T I C L E I N F O
A B S T R A C T
Keywords:
Rhenium supported over activated carbon (AC) was carburized under different temperatures (from 500 to
700 °C) under a mixture of H /C H (75/25). Resulting catalysts were characterized by N sorption, CO che-
Rhenium carbide
Guaiacol
2
2
4
2-
misorption, temperature programed reduction (TPR), ammonia temperature programed desorption (TPD-NH ),
3
Hydrodeoxygenation
Carburization
Biomass conversion
STEM, and XPS and catalytic properties were measured in a batch reactor at 350 °C, 5.0 MPa for the hydro-
deoxygenation (HDO) of guaiacol. Results have shown that Rhenium was gradually reduced and carburized upon
increasing the temperature. From 650 °C, a Re
x
C phase was obtained and transformed into ReC at 700 °C. Re C
x
was the most active phase obtained and maximum yield in benzene of 51% was reached at complete conversion.
ReC phase showed to inhibit the hydrogenolysis of phenol into benzene.
1
. Introduction
cyclohexanol. Inspired by traditional HDS catalysts, supported Co(Ni)
Mo ulfided catalysts resulted in highly useful for the synthesis of aro-
Biomass is a promising renewable feedstock for producing sustain-
matics [20]. Nevertheless, the addition of sulfur is required for pre-
venting the deactivation. An alternative to sulfide catalysts is to use
nitride, phosphide, or carbides of transition metals [21]. Among the
able fine chemicals such as benzene, toluene, and xylenes, also known
as BTX, essential for polymer synthesis, currently generated from the
petrochemical industry [1,2]. Lignin, cellulose and hemicellulose are
the three fractions that composed biomass. Lignin is a polyphenolic
macromolecule that can be depolymerized and upgraded through
thermochemical, catalytic or biological treatment [3–5].
phosphide family, Ni P has shown to be better HDO catalysts than other
2
phosphides (Fe, Co, Mo and W) [22], although Ni P led to saturated
2
compounds. Nitride catalysts have been barely explored, and have
shown improvement of the catalytic activity compared to sulfided
homolog material and led mainly to the formation of phenol and ca-
techol [23–25]. Over carbide catalysts, in the case of guaiacol conver-
sion, phenol was the primary product obtained [26–34], and benzene as
a major product could also be achieved [35,36]. BTX could be obtained
in the case of anisole, phenol and cresol conversion [37–44]. A com-
parison between Mo-phosphides, nitride and carbide supported over
The resulting bio-oil holds high content in oxygen that needs to be
removed in order to improve the thermal stability, viscosity, etc. and for
obtaining the products of interest [6]. Hydrodeoxygenation (HDO) is
one of the alternatives for bio-oil upgrading [7–10]. As lignin is a
complex molecule, its study needs the implementation of several ana-
lytical tools [11]. Thus, the study of model compounds is more often
employed instead of real feedstock. Furthermore, the study of model
compounds provides a better understanding of the conversion pathway
TiO , Bollousa et al. have observed that carbides were the most active
2
in phenol conversion toward benzene [45].
[
4].
Transition metal carbides have attracted attention lately as they
have shown versatile properties in different catalytic reactions such as
hydrogenolysis, hydrogenation, isomerization, and HDO of other bio-
mass model compounds [46–66]. Such properties can be tuned during
the synthesis or post-synthesis by controlling the oxygen content at the
surface [38,44,67]. Carbides can be generally prepared by temperature
Several catalytic systems have been extensively studied for the HDO
of model compounds. Noble metal-based catalysts show to be highly
active in obtaining saturated products [12–14]. Non-noble metals such
as Ni [15,16], Fe [17], Co [18], and Re [19] showed to be efficient in
producing mono-oxygenated compounds such as phenol and
⁎
Corresponding authors at: Departamento De Ingeniería Química y Bioprocesos, Pontificia Universidad Católica De Chile, Santiago, Chile.
E-mail addresses: elblanco@uc.cl (E. Blanco), neescalona@ing.puc.cl (N. Escalona).
https://doi.org/10.1016/j.apcata.2020.117600
Received 11 February 2020; Received in revised form 21 April 2020; Accepted 24 April 2020
Available online 29 April 2020
0926-860X/ © 2020 Elsevier B.V. All rights reserved.

E. Blanco, et al.
Applied Catalysis A, General 599 (2020) 117600
programed reduction (TPR) or the carbothermal hydrogen reduction
15, 17, 18, 28, 44 correspondings to the main masses of methane,
ammonia, water, carbon monoxide and carbon dioxide, respectively.
Scanning transmission electron microscopy (STEM) images were
obtained in a FESEM instrument Quanta FEG 250 (from FEI), the
sample was previously dispersed over a copper grid using ethanol
(analytical grade from Aldrich).
(
CHR). In the case of TPR method, the metal gets carburized by heating
with a mixture of hydrogen and a light hydrocarbon (as the carbon
source) while in the case of the CHR, the heat is carried out in pure
hydrogen and the carbon source is generally obtained from the support
or by using a carbon-based metal precursor such as methane, ethane,
etc. [63,68]. Several studies have reported the effect of different
synthesis parameters on Mo carbide based-catalysts and it has been
observed that the carburization degree could be controlled by adjusting
synthesis parameters such as carburization temperature [31,69,70],
time [69–71], heating rate [31,69,70] and the gas mixture composition
X-ray photoelectron spectra of catalysts were recorded on a VG
Escalab 200R electron spectrometer using an Mg Kα (1253.6 eV)
photon source. Passivated catalysts were reduced in situ for 1 h at 350 °C
with H in order to remove the passivation layer. The binding energies
2
(BE) were adjusted according to the C 1s level of the carbon support at
284.8 eV. Fitting of experimental data was obtained from a sum of
Gaussian and Lorentzian lines (90G-10L) using a least-squares mini-
mization procedure. For the obtention of semi-quantitative data (Re/C
atomic ratio), the peak area of each element (Re4f for Re and C1s for C)
was divided by the tabulated sensitivity.
[
70].
In previous studies, rhenium sulfide or oxide has presented better
activity toward HDO products than Molybdenum based catalysts
[
19,72]. Recently, Re C has shown to be a promising material for hy-
2
drogen production from biomass [73]. In previous work, it has been
found that metallic rhenium was carburized by using a mixture
H
2
+C
2
H at 700 °C [26]. Qi et al. observed that graphite was first
4
formed and then decomposed/dissolved to form rhenium carbide spe-
cies [74]. Hence, with the aim of studying the effect of the carburiza-
tion degree on HDO properties, the present work investigates the im-
pact of the carburization temperature, from 500 to 700 °C, on rhenium
carbide.
2.3. Catalytic properties
Catalytic properties were measured in a stirred-batch reactor (Parr
Model 4590) of 100 mL for four 4 h. Typically, 200 mg of catalyst, 1.7 g
of guaiacol, 32 g of dodecane, and 700 μL of hexadecane (all reagent
grade purchased from Merk) were mixed. Prior heating at 350 °C with
stirring (645 rpm), the reactor was purged with nitrogen. Once the
temperature was reached, the reactor was charged with hydrogen to
5 MPa. Under these conditions, the reaction rates are not limited by
internal or external mass transfer [75]. The beginning of the reaction
2
. Experimental
2.1. Synthesis
Metal carbides were prepared by incipient wetness impregnation of
was assumed to be the time when the H was introduced into the re-
2
a commercial activated carbon (Type CO-850, Petrochil S.A) followed
by carburization treatment. Support was first ground, sieved
actor. During the reaction, hydrogen was added to keep the pressure
constant. Small aliquots of less than 0.1 mL were periodically collected
during the reaction and quantified by gas chromatograph Nexis GC-
2030 (Shimadzu) equipped with an Elite-1 column (Perkin Elmer,
30 m × 0.32 mm, film thickness of 0.25 μm). The initial temperature
was held at 37 °C for 6 min, then heated up to 80 °C at 8 °C/min and
finally up to 270 °C at 15 °C/min for 1 min. All the compounds detected
were quantified by using a calibration curve of external standards and
using hexadecane as an internal standard. Conversion and product se-
lectivity were defined according to the equations as reported elsewhere
[26]:
(
< 150 μm), and dried for 2 h at 110 °C. An aqueous solution of
NH
8%wt), left for maturation for 6 h, and dried overnight at 80 °C. The
carburization was conducted under different temperatures (500, 550,
00, 650, and 700 °C) for 90 min under the following atmosphere
composition: 75/25 of H /C (%vol). Finally, the catalyst was pas-
sivated at ambient temperature for 1 h with 5% O /N mixture
30 mL.min ). Resulting catalysts will be labeled according to the
carburization temperature.
4
ReO
4
(99% from Aldrich) was then impregnated on the support
(
6
2
H
2 4
2
2
−
1
(
products
2
.2. Characterizations
Conversion(%) =
Selectivity (%) =
100
reagent remaining+ products
Textural, chemical properties, and temperature programed experi-
ments were performed using the 3Flex instrument from Micromeritics.
products i
Textural properties were measured from the sorption isotherm of N
2
100
products
at −196 °C. Sample (10 mg) were previously degassed at 4 h at 300 °C
under vacuum using a SmartVacPrep instrument from Micromeritics.
The surface area was calculated from the adsorption branch in the
−1
−1
The initial reaction rate r
0
(mol g
s
) was calculated from the
−1
initial slope (b) of the conversion (≤30%) vs. time plot (s ) according
range 0.02 ≤ p/p ≤ 0.25 using the Brunauer–Emmett–Teller (BET)
0
to the formula:
theory. Total pore volume was defined as the single-point pore volume
at p/p = 0.99. Micropore volume was calculated from the t-plot
0
0
b × n
GUA
equation, and pore size distribution was obtained by applying the
Horvath–Kawazoe equation. CO uptakes were measured at 35 °C. The
catalyst was first degassed at 110 °C (10 °C/min) for 30 min and then
reduced at 350 °C at 60 min. All the temperature programed experi-
ments were carried out using the 3Flex (Micromeritics) combined with
a mass spectrometer (Cirrus 2, MKS Spectra Product). For temperature
programed reduction (TPR), the sample was heated at a rate of 10 °C/
r =
0
m
0
where
n
(mol) is the initial guaiacol mol in the reactor; m (g) is the
GUA
mass of the catalyst.
r0
TOF
CO uptake
min until 1050 °C with a mixture of 5% H
2
-Ar (100 mL/min). For
−1
temperature programed desorption of NH (TPD-NH
3
3
), the sample was
TOF (s ) was deduced by considering that active sites chemisorb
−1
−1
first pretreated 30 min at 350 °C (10 °C/min) under He (50 mL min ),
CO (μmol g ) according to the following formula:
then adsorption of NH
3
was carried out at 100 °C for 15 min (30 mL
Mass balance was about 90% in all the reactions and was de-
termined by comparison of the conversion calculated from guaiacol
disappearance to that from product formation.
−
1
−1
min ) prior desorption under He (100 mL min ) at 10 °C/min until
5
00 °C. Products formed during the TPR and TPD-NH
3
treatments were
identified by a mass spectrometer by following specific fragments, m/z
2

E. Blanco, et al.
Applied Catalysis A, General 599 (2020) 117600
Table 1
Physico-chemical properties obtained for Re/AC carburized at different temperature.
Carburization temperature
500 °C
550 °C
600 °C
650 °C
700 °C
2
S
BET (m /g)
926
876
864
771
668
Micropore volume (mL/g)
CO uptake (μmol/g)
0.27
103
0.27
0.26
0.26
95
0.23
104
107
56
H
2
consumption in TPR experiment (mmol/g)
1.8
1.6
1.5
1.5
1.2
CO MS signal in TPR experiment (a.u/g)
Re 4f7/2 BE (eV) [%]
52
54
46
48
14
40.3 [83]
40.3 [88]
44.5 [12]
0.0092
40.4 [86]
44.4 [14]
0.0089
40.4 [100]
40.8 [85]
43.4 [15]
0.0520
4
4.4 [17]
Re/C Atomic surface ratio (from XPS)
0.0090
0.0090
3
. Results and discussion
displayed similar values with carburization temperature from 500 to
00 °C, then a slight decrease was observed at 650 °C, which is much
6
3.1. Characterization results
more pronounced after carburization at 700 °C. The CO uptake depends
on the particle size, the nature of metal surface species (metallic,
oxides, oxycarbides, or carbides) and the presence of some polymeric
carbon on the metal surface that can inhibit CO chemisorption [77–79].
For the sample carburized at 700 °C, a higher CO uptake could be
expected merely considering its smaller particle size. Thus, the higher
and similar CO uptake of the samples carburized in the range
500−650 °C compared to that of the sample carburized at the highest
temperature points to other reasons. A higher ratio of carbide in the
sample treated at 700 °C could explain this behavior since it is well
known that carbides are less sensitive to CO chemisorption, as already
observed in tungsten carbides [77–79]. For that reason, particle size
from CO uptake was not calculated as it might not be representative of
the actual particle size as observed by microscopy.
The evolution of the physicochemical properties as a function of
carburization temperature was measured, and results are summarized
in Table 1. The table shows that the surface areas decrease gradually
with the increasing of carburization temperature. The similar SBET of
the sample carburized at 500 °C compared to that of the bare support
2
−1
(
ca. 950 m g ) suggests that after impregnation/carburization at
5
00 °C, no pore blocking occurs. This is also confirmed by the micro-
pore volume that is quite similar to the bare support (ca. 0.32 mL/g).
On the contrary, the diminishing in the surface area upon increasing the
carburization temperature can be ascribed to a partial pore-blocking,
which could be generated by carbon growth during the carburization
process and/or the metal nanoparticles [74] or to partial gasification of
the support [76].
In order to get more insight into the surface species, XPS analyses
were performed. The Re 4f region displays the characteristic doublet of
Re 4f 7/2 and Re 4f 5/2. Re 4f 7/2 was used to estimate the relative
proportion of each species which is included in parenthesis in Table 1,
and the deconvolution curve is provided in Fig. S2. Table 1 shows that
two contributions were identified at the surface of all the catalysts,
except when the catalyst was carburized at 650 °C that displayed only
one. The main contribution obtained in all the cases presented a BE
between 40.3 and 40.8 eV. The BE at 40.8 eV was ascribed previously to
To investigate if the particle size was affected by the carburization
temperature, STEM was carried out. Average particle size distributions
were calculated from the images and are presented in Fig. 1; an ex-
ample of the images obtained is also provided in Fig. S1. Fig. 1 shows
that carburization of the catalyst at 500 °C resulted in a very hetero-
geneous particle size distribution with an average centered at around
1
3 nm. The histogram obtained by counting at least 200 particles in-
dicates that particle sizes were mainly below 50 nm of diameter. When
carburization temperature was increased to 550 °C, particle size dis-
tribution seems more homogenous, but the particle size was bigger
+
Reδ in Rhenium carbide [74], while BE at 40.3 eV can be assigned to
metallic Re [74] or carbide [80]. Other species were observed at a BE of
44.5 eV (carburized at 500, 550 and 600 °C) and 43.4 eV (carburized at
700 °C) and could be associated with oxide species [19,81,82]. Ac-
cording to the observed B.E at 40.3 eV, when the catalyst was carbur-
ized at 500 °C and 550 °C, Re° is the main specie present at the surface
of the catalyst. When the temperature was raised to 600 and 650 °C, a
shift to higher BE, i.e., 40.4 eV, was observed that further shifted to
40.8 eV after carburization at 700 °C. According to the work of Qi et al.
[74], graphene started to get formed between 630−680 °C over Re
(
average centered around 33 nm) than when the catalyst was carbur-
ized at 500 °C. A similar trend was observed when carburization was
carried out at 600 °C, and particle size increased compared to the
sample carburized at 550 °C. Interestingly, when the catalyst was car-
burized at 650 °C, particle size became much smaller (average around
1
3 nm) than when the catalyst was carburized at 600 °C. Finally, after
carburization at 700 °C, the average size distribution of the particle
slightly decreases to 10 nm compared to the size of the sample car-
burized at 650 °C. The CO chemisorption uptake, shown in Table 1,
(0001) when submitting under vacuum to
2
C H4. Therefore, the
Fig. 1. Average particle size distribution obtained from STEM images for each carburization temperature.
3

E. Blanco, et al.
Applied Catalysis A, General 599 (2020) 117600
observed shift of 0.1 eV could indicate that rhenium was getting par-
tially carburized. A similar shift (ca. 0.14 eV) was already reported
during graphene growth in UHV conditions and was ascribed to surface
carbide formation and carbon dissolution into the bulk [83]. The B.E of
the maximum for the sample treated at 700 °C, at 40.8 eV, can be as-
cribed to fully carburized rhenium ReC [74,83]. On the other hand, in
consumed), at least half of the ReO initially present was reduced and/
x
or carburized. These species could be either reduced into metallic
Rhenium and/or to oxycarbide/carbide. However, it is also possible
that ReO results from the passivation step. To evaluate the carbur-
x
ization extent, the formation of CO during the TPR of the passivated
samples was studied. Table 1 shows that while for the samples car-
burized at 500−650 °C the CO formed is in the range 54−46 a.u/g, the
sample carburized at 700 °C displayed a much lower value of 14 a.u/g.
Furthermore, TPR of the catalyst that has not been subjected to car-
burization does not show any CO formation. This indicates that con-
tribution from the support on m/z 28 is negligible and that the CO
observed post-carburization could be associated with the formation of
oxycarbides and/or carbides from the Rhenium oxides of the surface
and solid carbon. The higher CO production in the samples treated at
temperatures in the range 500−650 °C can be explained by the higher
concentration of oxygen at the surface, in agreement with XPS results.
Fig. 2a. shows that by increasing the carburization temperature
from 500 to 600 °C, there is a slight shift to a higher temperature of the
reduction profile. This shift cannot be attributed to the larger particle
size displayed by the samples carburized at 600 and 550 °C, as observed
by microscopy since bigger particles should be easier to get reduced.
However, as previous characterization suggested, there might be some
carbon formed over the particles which would make it more difficult to
reduce, or some bulk composition changes to a phase harder to reduce
despite the bigger particle size. When catalyst was carburized at 650 °C,
the TPR band shifted to lower temperature while according to the mi-
croscopy, particle size started to decrease. Furthermore, as observed by
CO chemisorption and XPS, some changes in surface composition were
observed and, textural analysis evidenced that partial gasification of the
support occurs from this temperature. Thus, there could be generation
of defects that would avoid sintering of the particles. Hence, from
650 °C it seems that the carbon formed has partially get dissolved over
the particle to form rhenium carbide as proposed by Qi et al. [74].
Finally, when catalyst was carburized at 700 °C, the band shifted to
higher temperature, which seems to indicate that the average particle
size decreases. This is in agreement with the results obtained by STEM
that showed an average particle size of 10 nm.
the case of tungsten carbide, several phases were reported like W C,
2
W C and WC and the corresponding W 3d BE shifted from lower to
x
higher BE respectively [28]. Thus, XPS results indicate that after for-
mation of metallic Rhenium at lower temperatures, different carbides
species could be formed at 600, 650 and 700 °C. At this point, only the
formation of ReC can be assessed from the data available in the lit-
erature and we cannot distinguish between the other rhenium species
obtained.
Moreover, the Re/C ratio also in Table 1 is very similar for all the
catalysts and in the range 0.0089−0.0092, except for the sample car-
burized at 700 °C that displays a significantly higher value of 0.052.
While the larger Re/C ratio of the sample carburized at 700 °C is in
quite good agreement with the smaller particle size of this sample, as
estimated from CO chemisorption and microscopy, the similar Re/C
ratio of the other samples is likely due to several opposed effects such as
their different dispersion and the Rhenium being encapsulated inside
the carbon.
In order to get information on the nature of chemical species of the
catalyst, we performed temperature programed reduction followed by
mass spectroscopy (TPR-MS) of the passivated catalysts. Several MS
fragments were recorded, but for more clarity, only m/z = 28 corre-
sponding to CO formation, which is the most relevant fragment, will be
presented. This fragment is formed at a higher temperature, i.e.
6
00−850 °C when the metal starts to get carburized by the carbon of
the support structure [26,31,68,69], but it could also be associated to
the remaining oxygen at the surface and/or incomplete carburization
[
84]. A comparison of the TCD signal representative of the reducibility
is presented in Fig. 2a., while the evolution of CO formation is given in
Fig. 2b. The respective integrations are also shown in Table 1.
The TPR profiles of Re/AC catalysts present a well-resolved peak at
around 300 °C and a broad feature under the well-resolved peak that
can be associated with the reduction of different ReO
x
species [72]. The
TPD of NH was carried out to probe the acidity of the catalysts and
3
amount of H
2
consumed during the TPR, in Table 1, shows that it de-
the desorption signal measured by MS spectroscopy is presented in
creases by increasing the carburization temperature that could be in-
dicative of incomplete carburization during the preparation, or to the
reduction of the passivation layer. Compared to the catalyst that was
Fig. 3.
As it can be observed, all the catalysts displayed one single con-
tribution with maxima in the range 156−176 °C. Moreover, the max-
imum shifted to lower temperatures as the carburization temperature
−1
not subjected to the carburization process (4 mmol
g
of
H
2
Fig. 2. Evolution of a. TCD signal and b. CO MS signal during TPR with the carburization temperature.
4

E. Blanco, et al.
Applied Catalysis A, General 599 (2020) 117600
Table 2
Effect of the carburization temperature on the initial rate at 350 °C, 50 bar in a
batch reactor, and the respective TOF calculated.
Carburization temperature
500 °C
550 °C
600 °C
650 °C
700 °C
0
−6
mol g−1.s−1
)
r
(10
)
15.3
0.15
14.9
0.14
12.8
0.12
18.1
0.19
5.7
−
1
TOF (s
0.10
temperature, some carbon started to be generated over the particle,
affecting the reduction temperature. From 600 °C, some Re began to get
fully carburized and particle continued to grow. At 650 °C, it was pro-
posed that carbon started to decompose, leading to a decrease of the
reduction temperature and and increased dispersion of the particles.
Finally, at 700 °C, the surface composition significantly changed and
another carbide phase was proposed to be formed resulting in a change
in the CO sensitivity during CO-chemisorption. The carbides obtained
exhibited weak acid sites that were suggested to be located on the metal
center.
3.2. Catalytic properties
Table 2 summarizes the initial rate and turnover frequency (TOF) as
a function of carburization temperature. The TOF was calculated con-
sidering that the active sites in the reaction were probed by CO che-
misorption.
Fig. 3. Effect of the carburization temperature over the catalyst acidity by TPD-
Table 2 shows that the catalyst carburized at 650 °C displayed the
highest initial rate and TOF, followed by those at 500, 550, and 600 °C
and finally, the lowest was obtained for the one carburized at 700 °C. It
appears that the initial rate and TOF first decreases from 500 to 600 °C;
such behavior could be explained by the changes observed in the
average particle size distribution by STEM. On the other hand, as al-
ready exposed above, during the carburization process, carbon forma-
tion was observed. Hence the formation of carbon could also explain
the decrease of the initial rate from 500 to 600 °C, since it may block
some of the active sites of the catalysts. After carburization at 650 °C
activity increased and, as the average particle size distribution obtained
at 650 °C was similar to the one at 500 °C, the enhancement of the
catalytic activity can be assigned to the formation of another active
phase as suggested in the previous part (Fig. 4). Finally, after carbur-
ization at 700 °C, despite having a smaller average particle size com-
pared to the catalyst carburized at 650 °C, 10 nm vs. 13 nm, it displayed
significantly lower initial rate and TOF that could be attributed to the
formation of a new carbide phase less active than the one obtained at
650 °C.
NH .
3
increases. This contribution corresponds to weak acid sites [85] which
are the only ones detected and, by increasing the carburization tem-
perature, the density of acid sites increases. Acid sites can be located on
the metal center (Lewis acid site), some defective carbon and/or on
some OH groups (Brønsted acid site) that may remain on the support
[
86]. By increasing the synthesis temperature, it was observed that the
metal was getting carburized and the CO formation during the TPR
decreased, this suggesting that there was less oxygen content on the
metal surface. Hence, since the density of acid sites increases with the
carburization temperature, this suggests that acid sites are more likely
located on the metal center.
To summarize, Fig. 4 depicts the different phases proposed upon
changing the carburization temperature. After carburization at 500 °C,
rhenium was present as metal, oxide, and oxycarbide phase. By in-
creasing the carburization temperature to 550 °C, a similar surface
composition was observed by XPS, while the oxide phase decreased and
particle size increased. Furthermore, it was proposed that from this
Guaiacol can be converted through several pathways according to
the nature of the active sites. An overview of the different main pro-
ducts observed under our reaction conditions and a possible path is
presented in Fig. 5.
Comparison of the product selectivities at similar conversion
(
around 30%) was carried out (Fig. 6) in order to study the effect of the
carburization temperature on the active site.
Fig. 6 shows that the main product obtained, whatever the carbur-
ization temperature, was phenol in agreement with all other works on
carbides [27,29–31,34,71,87–89]. The highest phenol selectivity was
obtained after carburization at 500 °C followed by catalyst carburized at
7
00 °C, while the selectivity to phenol obtained over catalysts carbur-
ized at 550, 600, and 650 °C was similar. The second main product
obtained depended on the carburization temperature. For the catalyst
carburized at 500 °C, benzene and cresols were obtained. In the case of
the catalyst carburized at 550, 600, and 650 °C, the second main pro-
duct obtained was labeled as others and assigned to methylated com-
pound (see Fig. 5) followed by benzene, cresol, anisole, and methya-
nisole. Finally, in the case of the catalyst carburized at 700 °C, the
second main product observed was cresol, followed by methylanisole
and catechol. Small amounts of anisole and benzene were also found.
Fig. 4. Proposed transformation of Re state upon changing the carburization
temperature.
5

E. Blanco, et al.
Applied Catalysis A, General 599 (2020) 117600
Fig. 5. Different possible pathways for guaiacol conversion.
The product distributions obtained in the case of the catalysts after
product observed followed by cresol and/or BTX, depending on the
support [19,90]. Consequently, it appears that when the catalyst was
carburized at 500 °C, the oxide phase reactivity predominates over the
metallic one leading to the formation of benzene, while no catechol was
carburization at 550, 600, and 650 °C were very similar, which could
indicate that the same active phase was formed under such conditions.
On the other hand, the product distribution differs when the catalyst
was carburized at 500 and 700 °C, suggesting that different active phase
was formed.
identified. On the other hand, Ghampson et al. reported over ReO /
x
CNF, after analysis of the spent catalyst, the formation of an oxycarbide
From the characterization results, after carburization at 500 °C,
metallic Re, ReOx, and/or oxycarbide phase were identified. According
to the literature review, metallic Re is poorly active for HDO of guaiacol
phase during guaiacol conversion [72]. Hence, it is possible that active
site on ReO
x
is similar to the one on ReO
x
C and could lead to similar
y
product distribution. The same observation was made by Murugappan
et al., where the active phase identified on MoO and Mo C was in both
cases an oxycarbide, although with different electronic properties [92].
compared to ReO
x
[19,90,91], and leads mainly to phenol, cresol and
3
2
catechol, while in the case of ReO , phenol was also the primary
x
Fig. 6. Comparison of the product selectivities obtained at different carburization temperature near iso-conversion and the corresponding initial rate of guaiacol
conversion.
6

E. Blanco, et al.
Applied Catalysis A, General 599 (2020) 117600
When the catalyst was carburized at 550, 600, and 650 °C, the
showed to change in the reactivity on anisole conversion [38,93]. Based
characterization results have shown that Re/AC catalyst started to get
carburized, and new products were formed such as anisole methylani-
sole and others methylated compounds. Then, it can be proposed that
carbide and/or oxycarbide can activate the hydrogenolysis (HYL) and
transalkylation (TRA) reactions.
on these results and making parallelism between W and Re, we can
postulate that Re C carbide could be first formed from 500 to 650 °C
2
together with some oxycarbide, and would favor phenol and benzene
formation. By increasing the carburization temperature up to 700 °C,
the carbide phase transition occurred from Re C to ReC passing through
2
Finally, when the catalyst was carburized at 700 °C, another carbide
was postulated to be formed, i.e. ReC and lower selectivity to “others,”
anisole and benzene were observed at the benefit of phenol, cresol and
catechol.
Re
using DFT calculation, that ReC phase was the most probable phase
obtained after heating at 710 °C under C and consistent with our
x
C. This would be in agreement with Qi et al. that have concluded,
2 4
H
XPS results, where BE at 40.8 eV was assigned to ReC [74]. Finally, it is
also coherent with the activity measured where ReC showed to be less
active than the other catalysts, as observed in the case of WC [28].
To further investigate, the evolution of the product yields was
plotted versus the reaction time, and the results are presented in Fig. 7.
The figure shows that at low conversion/reaction time, phenol is the
main product obtained, suggesting that guaiacol is first converted into
phenol through the direct demethoxylation pathway (see DMO in
Fig. 5). At higher conversion, the yield of phenol reaches a maximum
and then started to decreases at the benefit of benzene indicating that
phenol hydrogenolysis (see HYL in Fig. 5) is favored at high conversion.
This can be attributed to competitive adsorption between phenol and
guaiacol on the active sites. Similar observations can be made in the
case of the transformation of cresol to toluene. When the catalyst was
carburized at 650 °C, the higher conversion was reached after 4 h and it
According to the work of Iglesias et al., over tungsten carbide cat-
alysts in the case of alkane conversion, two kinds of active sites were
identified [77–79]. WC sites that were able to catalyze HYL reaction
x
and WO
x
sites generated by chemisorbed oxygen on WC sites that lead
x
to the formation of Brønsted acid sites that catalyze isomerization re-
actions. Furthermore, more recently, Fang et al. were able to control the
surface composition of tungsten carbide during the preparation and
showed that different reaction pathways for guaiacol conversion could
be achieved [28]. Over metal W, catechol was the main product ob-
tained, while WC showed to favor DMO leading to phenol, and W
enhanced benzene formation. WC presented a lower activity than W
2
2
C
C
and W
x
C. Additionally, a correlation between C/W surface atomic ratio
and phenol formation was established. This was also confirmed by Bhan
et al. over Mo C catalysts, where the presence of oxygen on the surface
2
Fig. 7. Evolution of the guaiacol conversion and product yield during the reaction at 350 °C, 50 bar for catalyst carburized at different temperatures.
7

E. Blanco, et al.
Applied Catalysis A, General 599 (2020) 117600
was observed that benzene could get converted into other products.
Interestingly, in opposition with the work on rhenium-based catalyst
hydrogenation reactions that would lead to cyclohexane were inhibited
under our reaction condition (ca. 300 °C vs. 350 °C in the present work)
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
[
19,90], probably because the hydrogenation reaction became ther-
modynamically unfavorable [36,94].
The main difference between those catalysts was the final conver-
sion reached (associated with the different initial reaction rates) and
the evolution of the secondary products. For instance, the yield of
Acknowledgments
The authors gratefully acknowledge funding from ANID Chile for
the following projects: Fondecyt N° 3170072, 1180982, PIA CCTE
AFB170007 and Fondequip N° EQM160070 and EQM150101.
Additional funding was provided under the Millennium Science
Initiative of the Chilean Ministry of Economy, Development and
Tourism, for the grant “Nuclei on Catalytic Processes towards
Sustainable Chemistry (CSC)”.
“
others” was not changing with time in the case of the catalyst car-
burized at 500 and 550 °C. In the case of the catalyst carburized at
6
00 °C, it decreases with time, while it increases gradually with time
when the catalyst was carburized at 650 °C. Anisole formation was
observed in all the cases but only the catalyst carburized at 650 °C was
able to convert it further. Finally, in the case of methylanisole forma-
tion, the yield obtained was higher when catalyst carburized at 650 °C,
consistent with the TPD-NH results. It was proposed that methylani-
3
Appendix A. Supplementary data
sole was achieved according to a bimolecular transalkylation reaction
[
95,96], and recently over 2-D Mxene nanosheet, acid-base pair was
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.apcata.2020.117600.
intended to catalyze such reaction [97]. Thus, the slights changes ob-
served can be ascribed to the differences in the different ratios between
uncarburized and carburized Rhenium (Re + ReO
x
/Re
x
C + ReO
x
y
C ).
References
As exposed above, from 550 °C Re started to get carburized, the density
of ReO sites decreases at the benefit of oxycarbide sites that appears to
x
[
1] R.A. Sheldon, Green Chem. 16 (2014) 950–963.
[2] C.O. Tuck, E. Pérez, I.T. Horváth, R.A. Sheldon, M. Poliakoff, Science 337 (2012)
95.
3] J. Zakzeski, P.C.A. Bruijnincx, A.L. Jongerius, B.M. Weckhuysen, Chem. Rev. 110
2010) 3552–3599.
be less active but enhanced HYD and TRA reactions (see Fig. 5).
In the case of the catalyst carburized at 700 °C, the evolution of the
products with time differs from the other catalyst, this confirming the
proposal exposed above that a different active phase was formed and on
such phase HYL reaction was inhibited.
6
[
(
[4] M.P. Pandey, C.S. Kim, Chem. Eng. Technol. 34 (2011) 29–41.
[
5] A.J. Ragauskas, G.T. Beckham, M.J. Biddy, R. Chandra, F. Chen, M.F. Davis,
B.H. Davison, R.A. Dixon, P. Gilna, M. Keller, P. Langan, A.K. Naskar, J.N. Saddler,
T.J. Tschaplinski, G.A. Tuskan, C.E. Wyman, Science 344 (2014) 1246843.
Finally, Table S1 provides a small sample of reaction conditions
employed for guaiacol conversion over carbides. It shows that our
catalytic results could only be compared with the work of Jonguerius
et al. and Cai et al. [29,98]. Results indicate that high yield in benzene
could be reached over Re-650 °C (e.g., 51%) while over Mo-based car-
[6] G.W. Huber, S. Iborra, A. Corma, Chem. Rev. 106 (2006) 4044–4098.
[7] E. Furimsky, Appl. Catal. A 199 (2000) 147–190.
[
8] M. Saidi, F. Samimi, D. Karimipourfard, T. Nimmanwudipong, B.C. Gates,
M.R. Rahimpour, Energy Environ. Sci. 7 (2014) 103–129.
[9] S. De, B. Saha, R. Luque, Bioresour. Technol. 178 (2015) 108–118.
[10] W. Jin, L. Pastor-Pérez, D. Shen, A. Sepúlveda-Escribano, S. Gu, T. Ramirez Reina,
ChemCatChem 11 (2019) 924–960.
bide and W C/CNF, phenol was the main product obtained at similar
2
conversion. Therefore, the Re
the obtention of BTX.
x
C phase could be promising catalysts for
[
11] B. Joffres, C. Lorentz, M. Vidalie, D. Laurenti, A.A. Quoineaud, N. Charon,
A. Daudin, A. Quignard, C. Geantet, Appl. Catal. B 145 (2014) 167–176.
12] D.C. Elliott, T.R. Hart, Energy Fuels 23 (2009) 631–637.
[
[
13] A. Gutierrez, R.K. Kaila, M.L. Honkela, R. Slioor, A.O.I. Krause, Catal. Today 147
(
2009) 239–246.
4
. Conclusions
The present work gives some insight into the carburization process
[
14] L. Bomont, M. Alda-Onggar, V. Fedorov, A. Aho, J. Peltonen, K. Eränen, M. Peurla,
N. Kumar, J. Wärnå, V. Russo, P. Mäki-Arvela, H. Grénman, M. Lindblad,
D.Y. Murzin, Eur. J. Inorg. Chem. 2018 (2018) 2841–2854.
[
15] M.V. Bykova, O.A. Bulavchenko, D.Y. Ermakov, M.Y. Lebedev, V.A. Yakovlev,
V.N. Parmon, Catal. Ind. 3 (2011) 15–22.
of rhenium during the TPR method using a mixture of H
2
+ C
2
4
H .
Rhenium metal was first obtained, which lead to carbon growth upon
increasing the temperature. Then carbon started to dissolve over
Rhenium particle leading to partially carburized Rhenium. From
[16] M. Alda-Onggar, P. Mäki-Arvela, A. Aho, I.L. Simakova, D.Y. Murzin, React. Kinet.
Mech. Catal. 126 (2019) 737–759.
[
17] R. Olcese, M.M. Bettahar, B. Malaman, J. Ghanbaja, L. Tibavizco, D. Petitjean,
A. Dufour, Appl. Catal. B 129 (2013) 528–538.
7
00 °C, ReC was formed, and between 500 and 600 °C a mixture of
[18] N.T.T. Tran, Y. Uemura, S. Chowdhury, A. Ramli, Appl. Catal. A 512 (2016)
9
3–100.
metal, oxide, oxycarbide and carbide was obtained and at 650 °C only
one phase of rhenium carbide, Re C, was prepared. Catalytic guaiacol
C presented
[
19] K. Leiva, N. Martinez, C. Sepulveda, R. García, C.A. Jiménez, D. Laurenti, M. Vrinat,
C. Geantet, J.L.G. Fierro, I.T. Ghampson, N. Escalona, Appl. Catal. A 490 (2015)
71–79.
x
conversion was investigated and it was observed that Re
x
the highest rate of conversion, while over ReC, the lowest rate was
obtained. The main product obtained for all the catalysts was phenol,
however, at high conversion (ca from 80%), phenol started to further
convert into benzene, suggesting that the same site undergo the DMO
and the HYL. The highest yield of benzene (51%) was reached for the
[20] V.N. Bui, D. Laurenti, P. Afanasiev, C. Geantet, Appl. Catal. B 101 (2011) 239–245.
[
[
[
21] S. Ramanathan, S.T. Oyama, J. Phys. Chem. 99 (1995) 16365–16372.
22] H.Y. Zhao, D. Li, P. Bui, S.T. Oyama, Appl. Catal. A 391 (2011) 305–310.
23] I.T. Ghampson, C. Sepúlveda, R. Garcia, B.G. Frederick, M.C. Wheeler, N. Escalona,
W.J. DeSisto, Appl. Catal. A 413–414 (2012) 78–84.
[
[
[
[
[
[
24] I.T. Ghampson, C. Sepúlveda, R. Garcia, L.R. Radovic, J.L.G. Fierro, W.J. DeSisto,
N. Escalona, Appl. Catal. A 439–440 (2012) 111–124.
catalyst presenting only Re C at the surface, while benzene formation
x
25] C. Sepúlveda, K. Leiva, R. García, L.R. Radovic, I.T. Ghampson, W.J. DeSisto,
J.L.G. Fierro, N. Escalona, Catal. Today 172 (2011) 232–239.
26] E. Blanco, C. Sepulveda, K. Cruces, J.L. García-Fierro, I.T. Ghampson, N. Escalona,
Catal. Today (2019), https://doi.org/10.1016/j.cattod.2019.08.029.
27] Z. Cai, F. Wang, X. Zhang, R. Ahishakiye, Y. Xie, Y. Shen, Mol. Catal. 441 (2017)
28–34.
seems to be inhibited over ReC.
CRediT authorship contribution statement
28] H. Fang, A. Roldan, C. Tian, Y. Zheng, X. Duan, K. Chen, L. Ye, S. Leoni, Y. Yuan, J.
Catal. 369 (2019) 283–295.
Elodie Blanco: Conceptualization, Investigation, Methodology,
Visualization, Writing - original draft, Funding acquisition. Ana Belén
29] A.L. Jongerius, R.W. Gosselink, J. Dijkstra, J.H. Bitter, P.C.A. Bruijnincx,
B.M. Weckhuysen, ChemCatChem 5 (2013) 2964–2972.
Dongil: Writing
-
review
&
editing. José Luis García-Fierro:
[30] R. Ma, K. Cui, L. Yang, X. Ma, Y. Li, Chem. Commun. (Camb.) 51 (2015)
1
0299–10301.
Resources. Néstor Escalona: Conceptualization, Supervision, Project
[
31] E. Ochoa, D. Torres, R. Moreira, J.L. Pinilla, I. Suelves, Appl. Catal. B 239 (2018)
administration, Writing - review & editing, Funding acquisition.
8

E. Blanco, et al.
Applied Catalysis A, General 599 (2020) 117600
4
63–474.
32] E. Santillan-Jimenez, M. Perdu, R. Pace, T. Morgan, M. Crocker, Catalysts 2073-
344 (5) (2015) 424-S424.
33] C.-C. Tran, Y. Han, M. Garcia-Perez, S. Kaliaguine, Catal. Sci. Technol. 9 (2019)
387–1397.
34] R. Li, A. Shahbazi, L. Wang, B. Zhang, A.M. Hung, D.C. Dayton, Appl. Catal. A 528
[66] U. Akhmetzyanova, Z. Tišler, N. Sharkov, L. Skuhrovcová, L. Pelíšková,
[
[
[
[
[
O. Kikhtyanin, P. Mäki-Arvela, M. Opanasenko, M. Peurla, D.Y. Murzin,
ChemistrySelect 4 (2019) 8453–8459.
4
[67] Y. Deng, Y. Ge, M. Xu, Q. Yu, D. Xiao, S. Yao, D. Ma, Acc. Chem. Res. 52 (2019)
3372–3383.
1
[68] D. Mordenti, D. Brodzki, G. Djéga-Mariadassou, J. Solid State Chem. 141 (1998)
114–120.
(
2016) 123–130.
35] F.G. Baddour, V.A. Witte, C.P. Nash, M.B. Griffin, D.A. Ruddy, J.A. Schaidle, ACS
Sustain. Chem. Eng. 5 (2017) 11433–11439.
[69] R. Guil-López, E. Nieto, J.A. Botas, J.L.G. Fierro, J. Solid State Chem. 190 (2012)
285–295.
36] A. Sulman, P. Mäki-Arvela, L. Bomont, M. Alda-Onggar, V. Fedorov, V. Russo,
K. Eränen, M. Peurla, U. Akhmetzyanova, L. Skuhrovcová, Z. Tišler, H. Grénman,
J. Wärnå, D.Y. Murzin, Catal. Letters 149 (2019) 2453–2467.
[70] T. Mo, J. Xu, Y. Yang, Y. Li, Catal. Today 261 (2016) 101–115.
[71] E. Ochoa, D. Torres, J.L. Pinilla, I. Suelves, Catal. Today (2019).
[72] I.T. Ghampson, C. Sepulveda, R. Garcia, J.L.G. Fierro, N. Escalona, Catal. Sci.
Technol. 6 (2016) 4356–4369.
[
37] T. Iida, M. Shetty, K. Murugappan, Z. Wang, K. Ohara, T. Wakihara, Y. Román-
Leshkov, ACS Catal. 7 (2017) 8147–8151.
[73] M.G. Granados-Fitch, J.M. Quintana-Melgoza, E.A. Juarez-Arellano, M. Avalos-
Borja, Int. J. Hydrogen Energy 44 (2019) 2784–2796.
[74] Y. Qi, C. Meng, X. Xu, B. Deng, N. Han, M. Liu, M. Hong, Y. Ning, K. Liu, J. Zhao,
Q. Fu, Y. Li, Y. Zhang, Z. Liu, J. Am. Chem. Soc. 139 (2017) 17574–17581.
[75] I.T. Ghampson, G. Pecchi, J.L.G. Fierro, A. Videla, N. Escalona, Appl. Catal. B 208
(2017) 60–74.
[
[
[
38] A. Kumar, A. Bhan, Chem. Eng. Sci. 197 (2019) 371–378.
39] W.-S. Lee, Z. Wang, R.J. Wu, A. Bhan, J. Catal. 319 (2014) 44–53.
40] Q. Lu, C.J. Chen, W. Luc, J.G.G. Chen, A. Bhan, F. Jiao, ACS Catal. 6 (2016)
3
506–3514.
[
41] A.A. Smirnov, Z. Geng, S.A. Khromova, S.G. Zavarukhin, O.A. Bulavchenko,
A.A. Saraev, V.V. Kaichev, D.Y. Ermakov, V.A. Yakovlev, J. Catal. 354 (2017)
[76] H. Wang, S. Liu, K.J. Smith, Energy Fuels 30 (2016) 6039–6049.
[77] E. Iglesia, F.H. Ribeiro, M. Boudart, J.E. Baumgartner, Catal. Today 15 (1992)
307–337.
6
1–77.
[
[
42] S. Boullosa-Eiras, R. Lødeng, H. Bergem, M. Stöcker, L. Hannevold, E.A. Blekkan,
Catal. Today 223 (2014) 44–53.
[78] E. Iglesia, J.E. Baumgartner, F.H. Ribeiro, M. Boudart, J. Catal. 131 (1991)
523–544.
43] P.M. Mortensen, H.W.P. de Carvalho, J.-D. Grunwaldt, P.A. Jensen, A.D. Jensen, J.
Catal. 328 (2015) 208–215.
[79] E. Iglesia, F.H. Ribeiro, M. Boudart, J.E. Baumgartner, Catal. Today 15 (1992)
455–458.
[
[
44] C.J. Chen, A. Bhan, ACS Catal. 7 (2017) 1113–1122.
45] S. Boullosa-Eiras, R. Lødeng, H. Bergem, M. Stocker, L. Hannevold, E.A. Blekkan,
Catalysis 26 (2014) 29–71.
[80] G. Soto, H. Tiznado, J.A. Díaz, E.C. Samano, A. Reyes-Serrato, Thin Solid Films 519
(2011) 3236–3241.
[
[
[
46] Y. Aoki, H. Tominaga, M. Nagai, Catal. Today 215 (2013) 169–175.
47] A.J. Brungs, A.P.E. York, M.L.H. Green, Catal. Letters 57 (1999) 65–69.
48] L. He, Y. Qin, H. Lou, P. Chen, Journal of Zhejiang University, Science Edition 42
[81] J. Okal, W. Tylus, L. Kȩpiński, J. Catal. 225 (2004) 498–509.
[82] E.S. Shpiro, V.I. Avaev, G.V. Antoshin, M.A. Ryashentseva, K.M. Minachev, J. Catal.
55 (1978) 402–406.
(
2015) 631–636 and 643.
[83] E. Miniussi, M. Pozzo, T.O. Menteş, M.A. Niño, A. Locatelli, E. Vesselli, G. Comelli,
S. Lizzit, D. Alfè, A. Baraldi, Carbon 73 (2014) 389–402.
[84] J.-S. Choi, G. Bugli, G. Djéga-Mariadassou, J. Catal. 193 (2000) 238–247.
[85] E. Blanco, P. Delichere, J.M.M. Millet, S. Loridant, Catal. Today 226 (2014)
185–191.
[
[
49] J.S. Lee, S. Locatelli, S.T. Oyama, M. Boudart, J. Catal. 125 (1990) 157–170.
50] W.-S. Lee, Z. Wang, W. Zheng, D.G. Vlachos, A. Bhan, Catal. Sci. Technol. 4 (2014)
2
340–2352.
[
[
[
51] C. Liu, M. Lin, K. Fang, Y. Meng, Y. Sun, RSC Adv. 4 (2014) 20948–20954.
52] P. Liu, J.A. Rodriguez, J. Phys. Chem. B 110 (2006) 19418–19425.
[86] C. Moreno-Castilla, A.n.F. Pérez-Cadenas, F.J. Maldonado-Hódar, F. Carrasco-
53] M. Lu, F. Lu, J. Zhu, M. Li, J. Zhu, Y. Shan, React. Kinet. Mech. Catal. 115 (2015)
Marın, J.L.G. Fierro, Carbon 41 (2003) 1157–1167.
́
2
51–262.
[87] R. Moreira, E. Ochoa, J.L. Pinilla, A. Portugal, I. Suelves, Catalysts 8 (2018) 127.
[88] E. Santillan-Jimenez, M. Perdu, R. Pace, T. Morgan, M. Crocker, Catalysts 5 (2015)
424.
[
54] Y.F. Ma, G.Q. Guan, X.G. Hao, J. Cao, A. Abudula, Renewable Sustainable Energy
Rev. 75 (2017) 1101–1129.
[
[
[
[
[
[
[
55] J.R. McManus, J.M. Vohs, Surf. Sci. 630 (2014) 16–21.
[89] C.-J. Chen, W.-S. Lee, A. Bhan, Appl. Catal. A 510 (2016) 42–48.
[90] C. Alvarez, K. Cruces, R. Garcia, C. Sepulveda, J.L.G. Fierro, I.T. Ghampson,
N. Escalona, Appl. Catal. A 547 (2017) 256–264.
56] R. Michalsky, Y.-J. Zhang, A.A. Peterson, ACS Catal. 4 (2014) 1274–1278.
57] M.M. Sullivan, A. Bhan, ACS Catal. 6 (2016) 1145–1152.
58] A. Villa, S. Campisi, C. Giordano, K. Otte, L. Prati, ACS Catal. 2 (2012) 1377–1380.
59] K. Xiong, W. Yu, J.G. Chen, Appl. Surf. Sci. 323 (2014) 88–95.
60] W. Zhang, Y. Zhang, L. Zhao, W. Wei, Energy Fuels 24 (2010) 2052–2059.
61] P.A. Alaba, A. Abbas, J. Huang, W.M.A.W. Daud, Renewable Sustainable Energy
Rev. 91 (2018) 287–300.
[91] K.B. Jung, J. Lee, J.-M. Ha, H. Lee, D.J. Suh, C.-H. Jun, J. Jae, Catal. Today 303
(2018) 191–199.
[92] K. Murugappan, E.M. Anderson, D. Teschner, T.E. Jones, K. Skorupska, Y. Román-
Leshkov, Nat. Catal. 1 (2018) 960–967.
[93] M.M. Sullivan, J.T. Held, A. Bhan, J. Catal. 326 (2015) 82–91.
[94] X. Liu, W. An, C.H. Turner, D.E. Resasco, J. Catal. 359 (2018) 272–286.
[95] X. Zhu, L.L. Lobban, R.G. Mallinson, D.E. Resasco, J. Catal. 281 (2011) 21–29.
[96] X. Zhu, R.G. Mallinson, D.E. Resasco, Appl. Catal. A 379 (2010) 172–181.
[97] E. Blanco, A. Rosenkranz, R. Espinoza-González, V.M. Fuenzalida, Z. Zhang,
S. Suárez, N. Escalona, Catal. Commun. 133 (2020) 105833.
[98] Z. Cai, F. Wang, X. Zhang, R. Ahishakiye, Y. Xie, Y. Shen, Mol. Catal. 441 (2017)
28–34.
[
[
[
[
62] E.F. Mai, M.A. Machado, T.E. Davies, J.A. Lopez-Sanchez, V. Teixeira da Silva,
Green Chem. 16 (2014) 4092–4097.
63] M. Abou Hamdan, S. Loridant, M. Jahjah, C. Pinel, N. Perret, Appl. Catal. A 571
(
2019) 71–81.
64] J. Pang, J. Sun, M. Zheng, H. Li, Y. Wang, T. Zhang, Appl. Catal. B 254 (2019)
10–522.
65] K.J. Smith, Curr. Opin. Green Sustain. Chem. 22 (2020) 47–53.
5
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