Influence of cadmium on Ru/xCd/Al2O3 catalyst for benzene partial hydrogenation

Article abstract of DOI:10.1016/j.mcat.2020.111288

The thermodynamic of the partial hydrogenation of benzene with ruthenium catalyst drives the reaction to the cyclohexane formation. To stop the reaction in cyclohexene, promoters such as zinc are added. In this work, we carried out a preliminary DFT study showing that the addition of cadmium to a structure containing Ru and Al could be as effective or better than zinc in promoting partial hydrogenation of benzene. Experimentally, Ru/Al₂O₃ catalysts with 1% Ru and different cadmium contents were prepared by successive impregnations of CdCl₂ and RuCl₃ and characterized by textural analysis, XRD, EDXRF, SEM-EDS, DRS, TPR and H₂-chemisorption. The catalysts were subjected to partial hydrogenation of benzene in batch reactor at 150 °C and 50 bar H₂. The catalysts presented RuO₂ and RuCl₃ in function of the remaining chlorine content after calcination. The Ru0.2CdAl catalyst with Cd:Ru ratio 0.2:1 presented 2-fold higher activity and cyclohexene yield than Ru/Al₂O₃ without cadmium. This catalyst presented highest reducibility with Ru^o, Ru-Cd interaction and higher H₂ chemisorption. Additionally, a reaction with benzene and water showed that the Ru0.2CdAl catalyst obtained twice the activity of the same catalyst with pure benzene.

Full text of DOI:10.1016/j.mcat.2020.111288

Molecular Catalysis 499 (2021) 111288
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
Influence of cadmium on Ru/xCd/Al2O3 catalyst for benzene
partial hydrogenation
,
Paulo Victor C. Azevedoa b, Mateus V. Diasc, Arthur Henrique A. Gonçalvesa,
,
Luiz Eduardo P. Borgesb, Alexandre B. Gaspara *
a National Institute of Technology, Avenida Venezuela, 82, Sl. 518, Centro, Rio Janeiro, RJ, 20081-321, Brazil
b Military Engineering Institute, Praça General Tibúrcio, 80, Urca, Rio de Janeiro, RJ, 22290-270, Brazil
c
´
˜
´
Fluminense Federal University – Engineering School, R. Passo da Patria, 156 – Sao Domingos, Niteroi, RJ, 24210-240, Brazil
A R T I C L E I N F O
A B S T R A C T
Keywords:
The thermodynamic of the partial hydrogenation of benzene with ruthenium catalyst drives the reaction to the
cyclohexane formation. To stop the reaction in cyclohexene, promoters such as zinc are added. In this work, we
carried out a preliminary DFT study showing that the addition of cadmium to a structure containing Ru and Al
could be as effective or better than zinc in promoting partial hydrogenation of benzene. Experimentally, Ru/
Al2O3 catalysts with 1% Ru and different cadmium contents were prepared by successive impregnations of CdCl2
and RuCl3 and characterized by textural analysis, XRD, EDXRF, SEM-EDS, DRS, TPR and H2-chemisorption. The
catalysts were subjected to partial hydrogenation of benzene in batch reactor at 150 ◦C and 50 bar H2. The
catalysts presented RuO2 and RuCl3 in function of the remaining chlorine content after calcination. The
Ru0.2CdAl catalyst with Cd:Ru ratio 0.2:1 presented 2-fold higher activity and cyclohexene yield than Ru/Al2O3
without cadmium. This catalyst presented highest reducibility with Ruo, Ru-Cd interaction and higher H2
chemisorption. Additionally, a reaction with benzene and water showed that the Ru0.2CdAl catalyst obtained
twice the activity of the same catalyst with pure benzene.
Benzene hydrogenation
Cyclohexene
Ruthenium
Cadmium
Water
DFT
Introduction
a zinc salt added to the aqueous phase. The main drawbacks of the
process are the use of the expensive bulk ruthenium catalyst and the
Due to the severe progressive restrictions regarding the presence of
aromatic compounds in fossil fuels, partial hydrogenation of benzene
rises as an interesting alternative to produce chemical intermediates,
such as cyclohexene and cyclohexane [1]. As an example, cyclohexene
can be used in adipic acid synthesis [2] and caprolactam as well as in the
production of cyclohexanone and cyclohexanol using lysine [3]. The
main challenge of the benzene → cyclohexene → cyclohexane series
reaction is to stop the process in the intermediate compound. Consid-
ering thermodynamics, hydrogenation of benzene generates cyclo-
hexane (ΔG = ꢀ 98 kJ molꢀ 1) instead of cyclohexene (ΔG = ꢀ 23 kJ
molꢀ 1) [4].
addition of aqueous zinc sulfate solution as a promoter. The low solu-
bility of cyclohexene in water increases its yield. In fact, cyclohexene is 6
times less soluble in water than benzene [4].
Thus, many efforts are verified in the literature with the aim of
interrupting the reaction in the formation of cyclohexene, avoiding over
hydrogenation to cyclohexane. The studies are focused on adding pro-
moters to the catalyst [3,8–13] or the liquid medium [14,15] or the use
of solvents [4,16]. In summary, NaOH is used in the liquid medium in
order to increase the hydrophilic characteristic of the catalyst surface.
Water, as mentioned before, is used because the solubility of cyclo-
hexene in this solvent is low, thus promoting the desorption of cyclo-
hexene formed from the surface of the hydrophilic catalyst and
inhibiting additional hydrogenation. Finally, zinc is added to the reac-
tion medium or incorporated into the catalyst because the Ru-Zn
interaction causes an increase in the desorption rates of cyclohexene
from catalyst surface and delays the re-adsorption of cyclohexene to the
Ruthenium-based catalysts are the most active for this reaction [5,6].
Asahi Chemical Industry Co. Ltd. has developed a process in which
cyclohexene can be obtained by inhibiting the complete hydrogenation
of benzene to cyclohexane [7]. This process uses a bulk ruthenium
catalyst in a four-phase system: gaseous, aqueous and organic, including
* Corresponding author.
E-mail address: alexandre.gaspar@int.gov.br (A.B. Gaspar).
https://doi.org/10.1016/j.mcat.2020.111288
Received 28 April 2020; Received in revised form 19 October 2020; Accepted 22 October 2020
Available online 4 November 2020
2468-8231/© 2020 Elsevier B.V. All rights reserved.

P.V.C. Azevedo et al.
Molecular Catalysis 499 (2021) 111288
active sites, improving the selectivity in relation to cyclohexene [3].
Thus, several authors have observed an improvement in activity and
selectivity for cyclohexene with Ru catalysts using salts added to the
catalyst or to the liquid medium, mainly sulphates. Hu and Chen [8]
prepared Ru-Zn/SiO2 catalysts with 5 wt.% Ru and a zinc content ranged
from 0.3–3.0 wt.%. The authors observed that the addition of small
amounts of Zn delayed the reduction of ruthenium oxide and increased
H2 consumption by 1 wt.% Zn, suggesting a partial reduction from ZnO
to metallic Zn. The hydrogenation of benzene was carried out at 150 ◦C,
34 atm, 2.5 g of catalyst, 0.62 mol Lꢀ 1 NaOH, 75 mL of benzene and 100
mL of water. With the addition of zinc, the activity was reduced, prob-
ably by covering the Ru sites with a layer of zinc during the reduction.
However, selectivity has been increased. For 5% Ru-1% Zn/SiO2, 55 %
benzene conversion and 31 % cyclohexene selectivity were obtained,
which suggests an inhibition of complete hydrogenation to cyclohexane
and/or a desorption of cyclohexene faster than benzene.
hydrogenation were considered in the study.
Considering that CdSO4 is toxic and harmful, the addition of Cd to
the catalyst is preferable. Wang et al. [21] studied Ru and Cd bimetallic
catalysts using bentonite as a support, a phyllosilicate clay whose
composition may contain Al, Si, Ca, Mg, as well as potassium or iron. The
Ru-Cd/bentonite catalyst with a 1:1 ratio in nominal content of the Ru
and Cd metals, presents the best results, compared to the same catalyst
with different metal ratios, being, 2Ru:1Cd, 1Ru:2Cd, among others. The
catalyst with a 1:1 ratio, showed 42.8 % selectivity and 54.6 % con-
version (yield = 23.4 %) for the partial hydrogenation reaction of ben-
zene to cyclohexene. The reduction of the catalyst occurs after its
preparation (before the reaction), at a temperature of 300 ◦C, for 1 h.
The reaction took place in a 6 mL batch reactor, with the addition of 1
mL of benzene and 1 mL of water, the reaction conditions were at a
temperature of 150 ◦C and 50 atm, with stirring.
Thus, this work aims to evaluate, for the first time, Ru/Al2O3 cata-
lysts prepared with different contents of cadmium in the partial hy-
drogenation of benzene. The effects of the promoter on the reducibility
and dispersion of ruthenium as well as on textural, structural and
morphological characteristics were considered. The effect of cadmium
has also been studied on partial hydrogenation of pure benzene or with
the addition of water and reuse. A preliminary DFT study was carried out
to identify which additive (Cd, Ag, Ga, In, Zn and Cu) for Ru/Al2O3
catalyst could be more effective in the reaction considering simplified
catalyst structures.
Yuan et al. [9] observed the same effect when studying Ru-Zn/ZrO2
catalysts using theoretical and experimental approaches. Ru-Zn/ZrO2
samples were prepared by co-precipitation of ZrOCl2 and RuCl3 with
KOH followed by ZnSO4 impregnation and reduction (Zn content:
2.4–3.0 wt. %). The catalysts were evaluated in the hydrogenation of
benzene under the following conditions: 2.0 g of catalyst, 80 mL of
benzene, 150 mL of water and ZnSO4 solution were loaded into the
reactor. The authors observed that the activity decreased, but the
selectivity for cyclohexene increased, attributed to the decrease in
benzene and cyclohexene adsorption on catalysts influenced by the
presence of Zn in the prepared catalyst. The best result was obtained
with a Zn of 2.72 % by weight with a yield at cyclohexene up to 44 %.
The work of Yuan et al. [9] also studied the benzene hydrogenation
using calculations being made in the VASP program. These theoretical
calculations proposed that the Ru-Zn catalyst adsorbs benzene and
cyclohexene more weakly than the Ru-only catalyst and that this effect is
more important for cyclohexene production. The authors also observed
that the proposal is valid in the presence or absence of water.
Experimental
Preparation of the catalysts
The γ-Al2O3 support (Degussa 221) was previously crushed and
sieved in the 270 mesh size. The support was taken to a muffle for
calcination up to 500 ◦C for two hours with a heating rate of 10 ◦C
minꢀ 1. The precursor salts used were RuCl3.xH2O (Sigma-Aldrich) and
CdCl2.xH2O (Sigma-Aldrich) added by dry impregnation to the calcined
support to obtain catalysts with 1% wt. Ru. CdCl2.xH2O was impreg-
nated followed by RuCl3.xH2O. After each impregnation, a muffle
calcination step was carried out up to 500 ◦C for two hours with a
heating rate of 10 ◦C minꢀ 1. Five catalysts were synthesized: 1%Ru-0.2
%Cd/Al2O3, 1%Ru-0.4 %Cd/Al2O3, 1%Ru-1%Cd/Al2O3, 1%Ru-6%Cd/
Al2O3, 1%Ru/Al2O3 with the resulting nomenclatures: Ru0.2CdAl,
Ru0.4CdAl, Ru1CdAl, Ru6CdAl and RuAl.
Gonçalves et al. [17] studied Ru/x-ZnO/Al2O3 catalysts with addi-
tion of zinc (× = 0, 10, 20 and 90 %) in the partial hydrogenation of
benzene in a batch reactor. The reaction was carried out at 150 ◦C and
50 atm of H2 with 100 mL of pure benzene and 0.5 g of catalyst. The
addition of zinc (10 wt. % ZnO) promoted the reaction being better than
the catalyst without zinc (RuAl). The authors found that the size of the
ruthenium particle and its reducibility were strongly influenced by the
composition of the support and by the presence of species of Al2O3, ZnO
and ZnAl2O4. The most active catalyst, Ru10ZnAl, showed greater
dispersion of ruthenium and partial reduction of ZnO to metallic zinc,
influencing the adsorption of benzene and cyclohexene, resulting in an
initial selectivity to cyclohexene of 92.5 % and yield of 37 %.
Density functional theory (DFT) calculations
The interaction between benzene, cyclohexene and cyclohexane
with the theoretical catalytic surface and the calculations of heat of
adsorption of these molecules were simulated using the B3LYP density
functional method in conjunction with the 6ꢀ 31+G* basis set. Distinct
simple combinations between Ru, Al and a promoter (Cd, Ag, Ga, In, Zn
and Cu) were made as approximations of the catalytic surface (active
phase and support), and have set a carbon-ruthenium atom distance of
2.3 Å to simulate the interaction of the compound and the active phase
of catalyst. The calculations were all performed with the software
Spartan Student®. Default convergence criteria were used for the ge-
ometry optimization.
The review by Foppa and Dupont [18] reported the use of catalysts
with Ru as the main component and the occurrence of a second
component, in addition to zinc as a promoter, for example, Cu, La, Fe,
Co, Ba, Ce, Mn, Cd, among others. In 1992, Struijk et al. [19] reported
that Cd2+, In3+, Ga3+ and Cr2+ are more strongly adsorbed on ruthe-
nium than Zn2+, Fe2+ and Co2+. Thus, Cd seems to be a good candidate
to promote the reaction. Liu et al. [20] compared ZnSO4 and CdSO4 as
promoters for RuLa/SBA-15 catalysts. Both salts presented distinct be-
haviors. While ZnSO4 acts accelerating the desorption of cyclohexene,
CdSO4 suppress the adsorption of cyclohexene more than that of ben-
zene. The maximum cyclohexene yield for RuLa/SBA-15 with the
addition of 1.56. 10ꢀ 3 mol Lꢀ 1 was 28 % after 6 min with the following
reaction conditions: 1.0 g of catalyst, 50 mL of benzene, 100 mL of H2O,
temperature 140 ◦C, H2 pressure of 40 atm and stirring rate of 1000 rpm.
The authors also developed an interesting theoretical study of the for-
mation energies (kcal mol-1) of the complexes formed between Cd2+ or
Zn2+ ions with benzene or cyclohexene. The authors concluded that
Zn2+ ions form more stable complexes with both molecules than Cd2+
ions. However, neither other components of the catalyst (for example:
Ru, La) nor the presence of cyclohexane as the final product of
Characterization of the catalysts
The textural analysis of the catalysts was measured in an ASAP 2020
equipment from Micromeritics, for specific area (BET) and pore volume
(BJH) by adsorption of N2 at ꢀ 196 ◦C. The catalysts were previously
dried under vacuum, at 350 ◦C, for 2 h.
The quantification of each metal was performed by X-ray dispersive
energy fluorescence spectrometry (EDXRF) in a Shimadzu / EDX-800HS
equipment, with Rh detector, 1 mm collimator, under vacuum, in the Fe-
2

P.V.C. Azevedo et al.
Molecular Catalysis 499 (2021) 111288
K
α
line and zirconium filter to eliminate interference from the detector
Nch = number of moles of cyclohexane
in Ru analysis. CdCl2.xH2O and metallic Ru were used as reference
standards.
Results and discussion
The X-ray Diffraction (XRD) analysis was performed on a Bruker
equipment, model ASX D8 of high resolution, with CuKα radiation (λ
=1.5406 Å; 40 kV, 40 mA). The samples were previously compacted in
the sample holder and analyzed in the region of 2θ between 5 and 100◦,
with a step of 0.01◦ and counting time of 0.75 s per step. The particle
diameter was calculated following the Scherrer equation, using the
highest intensity line for the RuO2 species.
The heats of adsorption of benzene, cyclohexane and cyclohexene in
ruthenium in different groups of atoms were determined by using DFT
approach. The catalysts were named as Ru-2Al-P where one atom of
ruthenium, two atoms of aluminum and one atom of promoter (P) were
arranged to simulate the surface. Based on the work of Yuan et al. [9],
the defined distance between the carbon and ruthenium atoms was
defined as equal to 2.3 Å.
The analyzes of diffuse reflectance spectroscopy (DRS) were per-
formed in the Varian Cary 500 spectrophotometer with scanning range
in the UV–vis in the region of 800 to 200 nm (UV–vis), adopting the
support as reference. The analysis made it possible to identify the elec-
tronic states of ruthenium and cadmium in the catalyst.
Table 1 shows the adsorption heats of benzene, cyclohexane and
cyclohexene for the different configurations tested. In this work, cal-
culations related to the adsorption of benzene, cyclohexene and cyclo-
hexane molecules on ruthenium catalyst surfaces were performed. The
heat of adsorption can be given by Eq. 4:
Scanning electron microscopy (SEM) analyzes with X-ray energy
dispersion spectroscopy (EDS) were obtained on a FEI equipment
(INSPECT model) with tungsten filament, under high vacuum, operating
under a maximum voltage of 20 kV, spot of 5.0, and ETD detector of
secondary electrons in the magnifications of 500x and 1000 × .
The programmed temperature reduction (TPR) analyzes were per-
formed on an AutoChem II 2920 equipment from Micromeritics with
thermal conductivity detector (TCD). The samples were dried at 100 ◦C
in a pure N2 flow of 50 mL minꢀ 1 and reduced to 500 ◦C for 1 h under a
ΔHads = Hads - (Hcat + Hm olecule
)
(4)
Where:
Hads represents the enthalpy of the reagent molecule (benzene,
cyclohexene or cyclohexane) adsorbed on the catalyst;
H
H
cat represents the enthalpy of the catalyst molecule;
molecule represents the enthalpy of the reagent molecule (benzene,
cyclohexene or cyclohexane) before adsorption.
10 % H2/N2 mixture flow at 50 mL minꢀ 1
.
Analyzing these data, it is noticed that practically all the adsorption
energies of benzene and cyclohexene with ruthenium are lesser than
zero. This indicates that the hydrogenation of these molecules occurs
spontaneously, which was already expected. Thus, more negative ΔHads
values for benzene and more positive ΔHads values for cyclohexene
would be more favorable to obtain higher cyclohexene yields. The
presence of only ruthenium and zinc in the catalyst provides the most
positive ΔHads of benzene when compared to the other Ru-Al structures.
However, this does not happen for cyclohexene, because the ΔHads of
this molecule decreases, moving away from zero and, thus, favoring the
formation of more cyclohexane. As can be seen in Table 1, the addition
of zinc to a Ru-2Al configuration changes the adsorption heat of cyclo-
hexene ꢀ 137.96 to ꢀ 3.27 kJ molꢀ 1, increasing its selectivity in partial
hydrogenation of benzene, which was observed experimentally in the
literature [8,9,17,19,20]. Considering these results, it can be seen that
the presence of zinc increases ΔHads of cyclohexene, which means that
the spontaneity of adsorption decreases. This is explained by the fact
that zinc repels electrons from carbon orbitals. However, when there is
only zinc and ruthenium in the catalyst that interacts with benzene,
ΔHads becomes positive, that is, adsorption is no longer spontaneous,
and consequently, hydrogenation becomes more difficult.
H2 chemisorption was carried out at a temperature of 100 ◦C, where
the sample is cooled after TPR, under a flow of pure N2 of 50 mL minꢀ 1
.
After cooling, 20 pulses of 10 % H2/N2 mixture were performed in a
fixed volume of 2 mL each in order to measure H2 adsorption.
Partial hydrogenation of benzene
Catalytic tests were performed in a 300 mL PARR Instruments
reactor. The catalyst (0.5 g) was reduced in situ at a temperature of 250
◦C, for 60 min, under a flow of pure H2 of 50 mL minꢀ 1. After reduction,
the reactor was cooled under a flow of high purity N2 and 100 mL of the
reagent was transferred. Then, the reaction was carried out at 150 ◦C, at
a pressure of 50 atm of ultrapure H2 and with 600 rpm of stirring. The
aliquots were obtained every 10 min in the first hour and every 30 min
in the following period until reaching 3 h. The samples were analyzed on
a HP 6890 gas chromatograph equipped with a KCl/Al2O3 column (60 m
× 0.53 mm) and a flame ionization detector (FID).
Bimetallic Ru-Cd and monometallic Ru catalysts were tested. The
reaction medium was composed of pure benzene or benzene with water.
The reaction with water was carried out with the most selective catalyst
to cyclohexene, using 50 mL of benzene and 50 mL of water and 0.25 g of
catalyst. The reuse of the most selective catalyst was also evaluated.
Thus, at the end of the reaction, the catalyst was separated from the
liquid phase of the reactor by filtration, dried in an oven at 100 ◦C for 24
h. The conditions of the reuse reaction, including reduction with H2 at
250 ◦C, were maintained in order to compare values such as selectivity
and conversion.
Performing the same exercise for other elements of the periodic table
around zinc, Ga, Cu, Ag, In and Cd, the only one that brought promising
results was cadmium that further reduced the adsorption heat of
cyclohexene (ꢀ 1.12 kJ molꢀ 1), suggesting that a Ru-Cd/Al2O3 catalyst
may be a good formulation for this reaction.
Fig. 1 shows the ionization energies and energy of the HOMO
The conversion (X (%)) of benzene, can be calculated by Eq. 1:
Table 1
Benzene, cyclohexene and cyclohexane adsorption energy for all studied catalyst
configurations.
X (%) = (NoA - NA). 100/NoA
(1)
(2)
(3)
Heat of adsorption (kJ molꢀ 1
)
For the calculation of selectivity (S (%)), Eq. 2 was used:
S (%) = N=ch. 100/(Nch + N=ch)
Configuration
Benzene
Cyclohexene
Cyclohexane
Ru
ꢀ 198.07
35.06
ꢀ 137.96
ꢀ 75.82
ꢀ 28.97
ꢀ 3.27
437.96
92.00
ꢀ 29.16
ꢀ 0.39
ꢀ 3.12
ꢀ 7.87
ꢀ 5.53
ꢀ 7.01
1.31
The yield (R(%)) was calculated according to Eq. 3:
R (%) = X(%). S(%)
Ru-Zn
Ru-2Al
ꢀ 6.68
Ru-Zn-2Al
Ru-Cu-2Al
Ru-Ga-2Al
Ru-Ag-2Al
Ru-In-2Al
Ru-Cd-2Al
ꢀ 7.52
ꢀ 24.13
ꢀ 34.66
ꢀ 30.79
ꢀ 27.96
ꢀ 8.26
ꢀ 18.12
ꢀ 394.24
ꢀ 18.55
ꢀ 20.04
ꢀ 1.12
Where,
NoA = initial number of moles of benzene
NA = number of moles of benzene at time i
N=ch = number of moles of cyclohexene
3

P.V.C. Azevedo et al.
Molecular Catalysis 499 (2021) 111288
Fig. 1. Homo orbitals and ionization energies for Ru-Zn-2Al, Ru-Ag-2A, Ru-Cd-2Al and Ru-Ga-2Al configurations interacting with cyclohexene molecule.
orbitals, also calculated with the aid of Spartan software, for the Ru-Zn-
2Al, Ru-Ga-2Al, Ru-Ag-2Al and Ru-Cd-2Al configurations. In general,
the closer to red, the easier it is to remove an electron by electrophilic
attack, while the closer to blue, the more difficult. Comparing the values
of ΔHads with the HOMO and Ionization Energy images, it is possible to
infer that zinc and cadmium allow the desorption of cyclohexene, which
is associated with selectivity increasing. In fact, in the Ru-Ag-2Al and
Ru-Ga-2Al configurations, the charge density is distributed between the
catalyst and the benzene molecule, resulting in a more negative heat of
adsorption. However, in the Ru-Zn-2Al and Ru-Cd-2Al configurations,
the charge density is concentrated only in the catalyst, suggesting a
weaker cyclohexene-catalyst interaction, which is reflected in Table 1.
Table 2 shows the values obtained from the textural properties and
chemical analysis of the synthesized catalysts previously calcined. It is
possible to observe that the specific area and pore volume of the com-
mercial support (204 m2 gꢀ 1 of surface area and 0.45 cm3 gꢀ 1 of pore
volume) are the largest of the whole series of samples and decrease as
impregnation of cadmium and ruthenium occurs, as expected.
the catalyst with the higher cadmium content (6 wt.% Cd) presented a
significant decrease in the specific area, possibly related to the deposi-
tion of cadmium in the pores of the catalyst. Just as the specific area
decays, as the levels of Ru and Cd increase, the pore volume also
decreases.
In Fig. 2, it is possible to observe the adsorption and desorption
profiles of N2 at ꢀ 196 ◦C, after drying the samples under vacuum, at a
temperature of 350 ◦C, for 2 h, for the RuAl and Ru6CdAl catalysts. The
same treatment was carried out for the other prepared catalysts. Ac-
cording to the IUPAC classification, both isotherms, characterized as
type IV, present a hysteresis loop caused by the capillary condensation
characteristic of mesoporous solids. Isotherms were presented for two
catalysts, RuAl, without cadmium, and Ru6CdAl, with the highest cad-
mium content, where it was possible to observe that the isotherms did
not change their profiles as the content of this component increased. All
The differences in areas are small for the low Ru and Cd contents. But
Table 2
Prepared catalysts, textural analysis, chemical analysis and RuO2 crystallite di-
ameters (numbers in parentheses: EDXRF analyses).
Catalyst
S (m2
P.V. (cm3
Ru (wt.
%)
Cd (wt.
%)
Cl (wt.
%)
DRu
gꢀ 1
)
gꢀ 1
)
(nm)
RuAl
196
185
191
180
174
0.45
0.43
0.41
0.42
0.38
0.90
0
1.70
1.73
2.37
2.99
4.81
116
180
147
162
120
(0.80)
0.80
Ru0.2CdAl
Ru0.4CdAl
Ru1CdAl
Ru6CdAl
0.25
(0.63)
0.88
(0.25)
0.55
(0.94)
0.91
(0.38)
1.06
(0.88)
0.88
(1.26)
3.66
Fig. 2. Adsorption and desorption isotherm profiles at ꢀ 196 ◦C of RuAl and
(0.84)
(6.87)
Ru6CdAl catalysts.
4

P.V.C. Azevedo et al.
Molecular Catalysis 499 (2021) 111288
the other catalysts showed the same type IV isotherm profile, as also
reported by Wang et al. for Ru–Cd/Bentonite catalysts [21].
The contents of Ru, Cd and Cl in the catalysts were determined by
EDS. The analyzes were carried out in two different regions of the
catalyst, with three points in each region. The values obtained are re-
ported in Table 2. The values obtained for ruthenium are within the
expected range. However, the semiquantitative analysis of the atomic
content of cadmium available in the catalysts was barely satisfactory,
mainly for Ru6CdAl.
Fig. 3 shows the SEM image for the Ru6CdAl catalyst. All catalysts
presented particles with irregular spheric shape. Fig. 3 also displays the
EDS analysis of this catalyst. The region of occurrence of the species
ruthenium, cadmium and chlorine, are overlapping, making the semi-
quantitative determination difficult. A fact that can be evidenced is
the lower cadmium content than expected in the Ru6CdAl catalyst,
indicating that chlorine or ruthenium directly interferes with cadmium
present in the catalyst. Another important point is the occurrence of
chlorine in the catalyst. Chlorine contents were verified in all catalysts.
Despite the problems reported above about the overlapping of the Ru,
Cd and Cl regions, analyzed under the same conditions, it appears that
the contribution of residual chlorine is equal from RuCl3 and CdCl2. In
fact, the chlorine content increases linearly with the sum of the Ru and
Cd contents. As reported by Gonçalves et al. [17], calcination is not
enough to remove chlorine from RuCl3 impregnated on alumina.
The results obtained by EDXRF are reported in Table 2 in paren-
theses. All analyzes were performed in triplicate, further confirming the
values of the intensities obtained. For the calculation, the means of the
three intensities were used, and it was identified that, even in triplicate,
the intensities did not have significant variations, that is, they were
practically identical. In this way, quantifications were performed, first
for ruthenium and later for cadmium. Following with the results, it was
possible to observe that the ruthenium levels were close to the theo-
retical amount. The ruthenium and cadmium levels obtained by EDXRF
and MEV-EDS were similar for all catalysts. However, there was
discrepancy for the Ru6CdAl catalysts with the highest Cd content,
probably hindered by the overlaps between Ru, Cd and Cl must have
made it difficult to obtain correct levels in the SEM-EDS.
Fig. 4. XRD patterns of γ-Al2O3 support and RuxCdAl catalysts (* RuO2,
# CdO).
It is also possible to observe the formation of crystalline CdO with
peaks at 2θ = 33◦, 38◦ and 55◦ (JCPDS-40ꢀ 1290), as evidenced by
Mandal et al. [22]. As the RuO2 and CdO diffraction peaks present close
values of 2θ, CdO was only observed for the Ru6CdAl catalyst. Thus, the
formation of crystalline RuO2 is more visible than the formation of
crystalline CdO, justified by the intensity of the peaks. However, there is
a decrease in the intensity of the peak at 28◦ with the increase in the Cd
content, suggesting a coating of ruthenium by cadmium. Even with the
increase in cadmium content, the intensity remained almost identical for
all catalysts. The diameter (nm) of the ruthenium oxide particles was
estimated by the Scherrer equation, in the region 2θ = 28◦, and the
values are reported in the Table 2. It is stated that the impregnation of
cadmium favored the decrease in the size of the RuO2 crystallite, which
may lead to the conclusion that ruthenium is more dispersed on the
surface of catalysts with higher cadmium contents or is covered by
cadmium.
Fig. 5 shows the DRS spectra of the prepared RuxCdAl catalysts. As
γ-Al2O3 support was used as reference, it is possible to verify the
contribution of ruthenium and cadmium. In all spectra, a band of greater
intensity is observed at 326 nm, and another, at 480 nm. The spectra are
very similar to that also reported by Lopez et al. [23] for a Ru/SiO2
catalyst containing 0.5 wt.% Ru. Thus, the band around 326 nm is
attributed to the electronic transfer between the chloride T orbitals and
The diffractograms of the support (γ-Al2O3) and of the RuxCdAl
catalyst, prepared and calcined, are shown in Fig. 4. It is possible to
verify characteristic peaks of γ-alumina at 2θ = 37◦, 46◦ and 67◦ (JCPDS
1ꢀ 088-2112), used as a support in the preparation of catalysts and the
appearance of peaks of RuO2 at 2θ = 28◦, 35◦ and 54◦ (JCPDS-
40ꢀ 1290).
the ruthenium orbitals [
π 1 t2u-2t2g]. The d-d band at 480 nm is
attributed to the 2T2g - 4TIg transition of ruthenium in complexes of the
type [RuCIx(H2O)y]. The increase in cadmium concentration did not
Fig. 3. SEM image and EDS analysis of the Ru6CdAl catalyst.
Fig. 5. DRS patterns of the catalysts.
5

P.V.C. Azevedo et al.
Molecular Catalysis 499 (2021) 111288
result in new bands, but led to an enlargement of the band by 326 nm. In
fact, samples of 0.1CdAl and 1CdAl without ruthenium were also
analyzed by DRS using γ-Al2O3 as a reference. In both spectra, a band
around 300 nm is observed. A similar spectrum has been reported by
Urbiola et al. [24] for a Cd(O2)0.88(OH)0.24 film. The formation of cad-
mium peroxide (CdO2) is not expected unless there is an oxidizing at-
mosphere during the preparation (eg preparation with H2O2). It is
possible to assume that calcination in atmospheric air has supplied the
necessary oxidizing environment. According to Selvam et al. [25], the
CdO2 → CdO + ½ O2 reaction should occur between 190ꢀ 400 ◦C.
Considering the spectra of the xCdAl samples of this work and the
spectra of Cd(O2)0.88(OH)0.24 and CdO calcined at 500 ◦C presented by
Urbiola et al. [24], this reaction did not happen to a large extent.
The TPR profiles of the catalysts prepared by successive impregna-
tions of CdCl2 and RuCl3 are shown in Fig. 6. The catalysts showed wide
peaks with a maximum reduction temperature around 230 ◦C, attributed
to the reduction of bulk RuO2 [17,26,27]. As the consumption of H2
starts around 180 ◦C, it is also possible to infer the presence of
well-dispersed ruthenium species. However, no reductions of RuCl3
species were found in these samples that would appear at 140 ◦C [26].
These results are in accordance with the lower chlorine content verified
by the SEM-EDS analysis in the catalysts prepared by successive im-
pregnations (Table 1). The Ru6CdAl catalyst showed a TPR profile with
a wide peak shifted to higher reduction temperatures, with a maximum
around 235 ◦C, attributed to the reduction of bulk RuO2. Considering the
higher Cd: Ru ratio in this catalyst, the displacement of the RuO2
reduction peak to higher temperatures suggests that the reduction pro-
cess of this oxide is delayed by the presence of cadmium oxide, as also
observed by Hu and Chen [8] for Ru-Zn/SiO2 catalysts.
Fig. 7. TPR profiles of the catalysts: calcined and previously reduced in situ at
250 ◦C with pure H2.
considering the contents of 0.88 and 0.84 wt.% of Ru, respectively.
Ruthenium is 0.5 eV more electronegative than cadmium. Thus, ruthe-
nium tends to attract cadmium electrons, creating an interaction be-
tween the two metals, which can promote the reduction of Cd2+
particles. This effect was notably greater in the Ru6CdAl catalyst.
Table 3 shows the consumption of H2 in the TPR of the catalysts
prepared by successive impregnation, using the Ru contents of the
EDXRF analysis. The experimental results can be discussed considering
the stoichiometric H2 consumption for the reduction of RuO2 and RuCl3
species as observed by XRD and DRS: RuO2 + 2 H2 → Ru◦ + 2 H2O 19.8
The catalysts were also subjected to a second TPR up to 650 ◦C in the
samples already reduced in situ with pure H2 up to 250 ◦C. In this case,
the objective was to verify if all ruthenium would be reduced to 250 ◦C
and if there would be a reduction of cadmium oxide in the range up to
650 ◦C. In Fig. 7, it is possible to highlight the TPR profiles made up to
650 ◦C. Only three profiles were presented, since, for catalysts with low
cadmium contents, there was no detection of reduction by the equip-
ment above 300 ◦C. The Ru6CdAl catalyst showed a TPR peak around
450 ◦C attributed to the reduction of cadmium oxide [28]. The Ru1CdAl
catalyst and the 1CdAl support showed very similar and less intense
profiles than Ru6CdAl, considering that it has 6 times more cadmium in
mass. Hydrogen consumption for the Ru1CdAl and Ru6CdAl catalysts
μ
mol H2 mgRuꢀ 1 and RuCl3 + 3/2 H2 → Ru◦ + 3 HCl 14.8
μmol H2
mgRuꢀ 1. Values around 18
μ
mol H2 mgRuꢀ 1 are observed. This value
suggests the predominance of RuO2 over RuCl3, according to the chlo-
rine content of these catalysts obtained. The Ru0.2CdAl catalyst
consumed 24.5
μ
mol H2 mgRuꢀ 1, much higher than the stoichiometric
for reducing RuO2. The reduction profile in the TPR of this catalyst
(Fig. 6) does not suggest the reduction of other species, and even CdO or
CdCl2. The Ru content of this catalyst was 0.63 wt.% Ru obtained by
EDXRF. However, considering an average content of 0.9 % Ru, con-
sumption would drop to 17.1
μ
mol H2 mgRuꢀ 1 within expectations for
the reduction of RuO2, and suggesting that the value of 0.63 wt.% Ru is
underestimated.
for CdO reduction was 1.1 and 3.4
μ
mol H2 mgꢀ 1 Cd, considering the
contents of 1.26 and 6.87 wt.% of Cd, respectively and 1.7
μ
mol H2 mgꢀ 1
The H2 chemisorption values are also shown in Table 3. The H2
chemisorption goes through a maximum of 646
μ
mol H2 gRuꢀ 1 for the
Cd for 1CdAl support. In Fig. 7, it is still possible to observe for the
Ru1CdAl and Ru6CdAl catalysts that there is a small consumption of
Ru0.2CdAl catalyst and falls with an increase in cadmium contents for
the catalysts. According to Narita et al. [29] and Lin et al. [30], the use of
alumina as a catalytic support promotes the retention of chlorine species
even after calcination. This favors the formation of well-dispersed
ruthenium particles [26]. However, even with an increase in the chlo-
rine content, observed by SEM-EDS, the chemisorption values decrease
with an increase in the cadmium content.
ꢀ 1
hydrogen in the RuO2 reduction range of 8.6 and 9.5
μmol H2 mg
,
Ru
Although there seems to be no reduction in CdO because of the
reduction profile, the higher consumption of H2 in TPR and the greater
Table 3
TPR, H2 chemisorption and reaction results.
Catalyst
TPR (
μ
mol
H2 chem.
SC=
Reaction rate
(mol minꢀ 1
TOF
mgRuꢀ 1
)
(μ
mol
(%)a
(minꢀ 1
)
gRuꢀ 1
)
gRuꢀ 1
0.7
)
RuAl
17.5
24.5
514
646
16.9
30.6
(26.9)
24.5
–
663
Ru0.2CdAl
1.4 (2.5)b
1045
(1967)
859
Ru0.4CdAl
Ru1CdAl
Ru6CdAl
18.6
17.8
18.8
223
84
0.4
0.08
446
82
–
0.01
60
a
Fig. 6. TPR profiles of the calcined catalysts.
Considering 10 % conversion. b result in parenthesis: reuse.
6

P.V.C. Azevedo et al.
Molecular Catalysis 499 (2021) 111288
chemisorption of H2 suggest a hydrogen spillover effect in this sample.
This effect can also be favored by the larger RuO2 particle size observed
by XRD for this Ru0.2CdAl catalyst (Table 2). Gonçalves et al. [17] also
reported this phenomenon studying a Ru/Zn/Al2O3 catalytic system.
The calcined catalysts were tested in the benzene partial hydroge-
nation. The samples were reduced in situ with H2 at 250 ◦C and sub-
mitted to pure benzene at 150 ◦C, 50 bar, benzene/catalyst mass ratio
175.3 during 3 h. The catalytic performances are shown in Fig. 8 in
terms of cyclohexene yield with the ascending order Ru6CdAl <<
Ru1CdAl < RuAl < Ru0.4CdAl < Ru0.2CdAl. This sequence can also be
ruthenium heterogeneous catalyst, synthesizing the Ru-Cd/BEN cata-
lyst. The authors followed the work reported by Liu et al. [20] that used
CdSO4 in the reaction medium, performing homogeneous catalysis and,
thus, obtained improvements in selectivity to cyclohexene. Wang et al.
[21] synthesized Ru-Cd catalysts in the Ru:Cd ratio of 0:1 to 1:5, varying
the contents of Ru in 0, 1 and 5, and 0, 1 and 5 for Cd, % by mass. They
used 10 mg of catalyst for the reaction with 1 mL of water and 1 mL of
benzene (benzene:catalyst mass ratio 87.7) at 150 ◦C, under a pressure
of 40 bar. The Ru/BEN catalyst presented 7.7 % cyclohexene yield, while
the catalyst containing Ru:Cd 5:1, equivalent to Ru0.2CdAl, presented
12.8 %, demonstrating the promoting effect of cadmium.
observed for the reaction rates in Table 3, calculated according the
ꢀ 1
literature [17,32]. Thus, the highest reaction rate is 1.4 mol minꢀ 1
g
The results of the present work demonstrated the promoting effect of
cadmium for Ru/Al2O3 catalysts in the benzene partial hydrogenation,
even in the absence of water. In fact, both Ru0.2CdAl and Ru0.4CdAl
catalysts presented better performance than RuAl. The increase of the Cd
content to 1 wt.% or 6 wt.% worsened the performance in relation to the
catalyst without cadmium. Thus, considering XRD, TPR and H2 chemi-
sorption results, the Ru0.2CdAl catalyst presents large particles of RuO2
by XRD. When subjected to a reduction in TPR, this catalyst has a higher
H2 consumption than the stoichiometric for RuO2 or RuCl3, suggesting a
partial reduction of the CdO present in the support, at a temperature of
up to 250 ◦C. This reduced catalyst also adsorbs more H2 than the others
in the chemisorption due to the spillover effect. This greater H2
adsorption capacity added to the presence of reduced cadmium species
are important factors for the greater activity in the partial hydrogena-
tion of benzene.
Ru
for the Ru0.2CdAl catalyst. This catalyst also presented the highest
cyclohexene selectivity considering 10 % conversion (30.6 %). Direct
comparison of these reaction rate data with the literature is difficult
because different conditions were employed. However, while in this
work, a reaction rate value for the RuAl catalyst of 0.7 mol minꢀ 1 gꢀ 1
was obtained, Gonçalves et al. [17] found 1.5, Supino et al. [31] ach-
ieved 1.7 and Milone et al. [32] obtained 1.7 mol minꢀ 1 gRuꢀ 1 for
Ru/Al2O3 catalysts. Table 3 also presents results of reaction frequency
(TOF) using the reaction rate and H2 chemisorption data. In this case,
both Ru0.2CdAl and Ru0.4CdAl catalysts showed higher TOF than RuAl,
while Ru1CdAl and Ru6CdAl showed less activity per site.
Then, in Fig. 8, it is also possible to observe that the cyclohexene
yield reduces for Ru0.2CdAl catalyst from 40 min of reaction while the
performance of the Ru0.4CdAl remains constant throughout the reac-
tion. Following in the Figure, the Ru1CdAl and Ru6CdAl catalysts pre-
sented very low conversions, leaving its cyclohexene yield lower than
RuAl.
In addition, as can be seen in Table 1, the cyclohexene Hads for the
Ru-Cd-2Al is -1.12 kJ.molꢀ 1, whilst Fig. 1 shows that for the same
structure the charge density is concentrated in the catalyst, suggesting a
weaker cyclohexene-catalyst interaction, thus suppressing the cyclo-
hexene further hydrogenation. Both results endorse the Ru0.2CdAl and
Ru0.4CdAl higher yields compared to RuAl catalyst. Thus, the promot-
ing effect of cadmium was also demonstrated in the yield of cyclohexene
as seen in the DFT study (Table 1) and in previous works for both cad-
mium [21] and zinc [8,9,17].
Based on the characterizations carried out, it is possible to compare
the reaction performance in Table 3 and Fig. 8. As the RuO2 particle size
of the Ru0.2CdAl catalyst was the largest among all samples and the
results of TPR and H2 chemisorption suggest the possibility of partial
reduction of CdO to Cdo or H2 spill over, both conditions should favor
activity in partial hydrogenation of benzene.
Comparing the results of the present work with the literature, Gon-
çalves et al. [17] obtained their best activity for the benzene hydroge-
nation based on Ru10ZnAl catalyst reaching a maximum cyclohexene
yield of 4.2 % after only 10 min of reaction at 50 bar, 150 ◦C, and
benzene/catalyst mass ratio 175.3. The complete conversion of benzene
was obtained after 40 min of reaction. The authors associated the
presence of Ru◦ and also Ruδ+ with the better activity of the Ru10ZnAl
catalyst. Their results also suggested stronger Ru-Zn interaction, pro-
moting partial reduction of zinc in the support, increasing the hydro-
philic character of the catalyst and increasing the yields to cyclohexene.
Wang et al. [21] used cadmium as a promoter for the production of a
The reuse reaction was carried out with pure benzene, following the
same reaction parameters used in the previous tests, as well as the
proportion of benzene to catalyst (benzene/mass ratio catalyst). In
Table 3, the reaction activity is presented in parenthesis, 2.5 mol minꢀ 1
gꢀ 1. It is possible to observe that the reused catalyst was almost twice
more active than the fresh catalyst for the reaction. This surprising in-
crease in activity is difficult to explain. However, the catalyst was
filtered and dried after the first reaction and again reduced to 250 ◦C
with pure H2. Fig. 7 shows the behavior of Ru1CdAl and Ru6CdAl cat-
alysts where, after reduction to 250 ◦C with pure H2, the catalysts still
show residual H2 reduction in the RuO2 reduction region.
A quantitative analysis of Cd2+ ions in the filtered solution after the
1st. cycle of Ru0.2CdAl catalyst was performed using ICP-OES analysis.
This analysis revealed a Cd2+ ions concentration of 1.66 × 10ꢀ 5 mol Lꢀ 1
.
This value corresponds to 14.8 % Cd2+ ions leached from the Ru0.2CdAl
catalyst in the 1st. cycle. This fact could impact both activity and
selectivity results comparing 1st. and 2nd. cycles according to Struijk
et al. [19], Liu et al. [20] and Wang et al. [21] works. These authors
compared the effects on the activity and selectivity of the addition of
Zn2+ and Cd2+ salts to ruthenium catalysts in the partial hydrogenation
of benzene. They observed that Cd2+ ions are more easily adsorbed on
the catalyst than Zn2+ ions. They also observed that both affect the ac-
tivity and selectivity to cyclohexene in different ways. While Cd2+ af-
fects the active ruthenium site, Zn2+ acts mainly by stabilizing
cyclohexene in solution, hindering its readsorption in the catalyst
leading to over hydrogenation to cyclohexane.
Thus, considering our results and these references, the remarkable
increase in hydrogenation activity and decrease of cyclohexene yield in
the results of reuse experiments could be resultant of both additional
reduced ruthenium active sites obtained in the second reduction at 250
◦C and the lower Cd amount in the Ru0.2CdAl catalyst affecting active
Fig. 8. Yield of cyclohexene for the catalysts.
(50 bar, 150 ◦C, 3 h, benzene/catalyst mass ratio 175.3).
7

P.V.C. Azevedo et al.
Molecular Catalysis 499 (2021) 111288
sites because of Cd2+ ions leaching.
In order to bring more light to this point, an additional in situ
reduction experiment at a higher temperature (275 ◦C, 1 h, pure H2) was
performed with the Ru0.2CdAl catalyst. The reduction procedure at 275
◦C resulted in a reaction rate of 1.2 mol minꢀ 1 gRuꢀ 1 and a TOF of 905
minꢀ 1. These values are quite similar to those found after reduction at
250 ◦C, that is, 1.4 mol minꢀ 1 gRuꢀ 1 and 1045 minꢀ 1, respectively
(Table 3). Likewise, selectivity to cyclohexene considering 10 % con-
version of benzene resulted in 26.3 % when the catalyst was reduced to
275 ◦C against 30.6 % for the catalyst reduced to 250 ◦C. Thus, the
surprisingly greater activity of the Ru0.2CdAl catalyst in reuse can be
better explained by the leaching of cadmium during the first reaction
than by the reduction of RuO2 between the 1 st. and the 2nd. cycle.
Fig. 9 displays the cyclohexene yield as a function of the reaction
time. As the benzene hydrogenation reaction is a series reaction of
benzene - cyclohexene - cyclohexane, the increase in activity also pro-
vided an increase in the formation of cyclohexane at the expense of
selectivity to cyclohexene. In fact, while the maximum yield of cyclo-
hexene in the first cycle was 3.4 %, in the second cycle it reached only
2.5 %.
Fig. 9. Yield of cyclohexene for the Ru0.2CdAl catalyst: 1st. reaction x reuse.
(50 bar, 150 ◦C, 3 h, benzene/catalyst mass ratio 175.3).
heat of adsorption of cyclohexene, increasing its selectivity.
The reaction with water was carried out with the most active catalyst
Ru0.2CdAl. The reaction conditions adopted are the same in all the
catalytic tests performed previously. The difference occurred in the
amount of pure benzene and water added to the reaction medium, 50 mL
of water and 50 mL of benzene were added to the reaction medium. This
relationship was chosen with the help of theoretical contribution of
Foppa and Dupont [18]. In order to maintain the benzene/catalyst mass
ratio, the catalyst mass was also reduced to 0.25 g. In this case, quan-
tification by gas chromatography was carried out only at the end of the
reaction after 3 h. Thus, the conversion obtained was 22.0 % and
selectivity to cyclohexene was 39.1 %. In conditions without water, the
Ru0.2CdAl catalyst showed 65 % conversion and selectivity to cyclo-
hexene less than 5% at the end of the reaction. With these results, yields
of 3.4 % were obtained for the reaction without water and 8.6 % for the
reaction with water, with a factor of 2.5 times higher.
CRediT author statement
P.V.C.A., M.V.D., A.H.A.G., L.E.P.B. and A.B.G. conceived the idea of
the project. P.V.C.A. synthesized the catalysts, performed the charac-
terization measurements (except DFT calculations), carried out the
catalytic tests, collected the characterization and reactions data and
performed the data analysis. M.V.D. and A.H.A.G. performed the DFT
calculations. L.E.P.B. and A.B.G. supervised the findings of this work. All
authors discussed the results and contributed to the final manuscript.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
As reported in the literature for the promotion of zinc in Ru catalysts
for benzene hydrogenation [7,16], the addition of the appropriate ad-
ditive and water promotes the formation of a stagnant film on the sur-
face of the catalyst and, during the reaction, both hydrogen and benzene
need to break through this barrier to reach the active sites. Thus, the
hydrophilic character of the catalyst plays a fundamental role. Through
this property in aqueous media, the adsorption of cyclohexene formed
decreases, preventing its subsequent hydrogenation to cyclohexane.
The results indicated the promotion of ruthenium by cadmium in Ru/
Al2O3 catalysts. As reported in the literature for zinc, addition of cad-
mium should alter the reducibility of the catalyst. Thus, ruthenium tends
to attract cadmium electrons, promoting the interaction between the
two metals and the reduction of Cd2+ particles. Therefore, cadmium
must act by reducing the adsorption force of cyclohexene by ruthenium
and promoting the repulsion of hydrogen atoms, reducing the reduction
of cyclohexene to cyclohexane. In this work, the beneficial effect of
cadmium in Ru/Al2O3 was demonstrated for the benzene partial hy-
drogenation with or without addition of water.
Acknowledgements
Paulo Victor C. Azevedo, Mateus V. Dias and Arthur Henrique A.
Gonçalves thanks CAPES/Ministry of Education and CNPq/Ministry of
Science, Technology and Innovations (PIBIC and PCI programs),
respectively, for the financial support, and SisNANO/MCTI for the SEM-
EDS analyzes carried out at CENANO/INT.
References
¨
[1] E. Dietzsch, P. Claus, D. Honicke, The partial gas-phase hydrogenation of benzene
to cyclohexene on supported and coated ruthenium catalysts, Topics Catal. 10
(2000) 99–106, https://doi.org/10.1023/A:1019164001135.
[2] J.C.S. Soares, A.H.A. Gonçalves, F.M.Z. Zotin, L.R.R. Araújo, A.B. Gaspar,
Cyclohexene to adipic acid synthesis using heterogeneous polyoxometalate
catalysts, Mol. Catal. 458 (2018) 223–229, https://doi.org/10.1016/j.
mcat.2018.02.020.
[3] P. Zhang, T. Wu, T. Jiang, W. Wang, H. Liu, H. Fan, Z. Zhang, B. Han, Ru–Zn
supported on hydroxyapatite as an effective catalyst for partial hydrogenation of
benzene, Green Chem. 15 (2013) 152–159, https://doi.org/10.1039/
C2GC36596K.
Conclusions
[4] H. Sun, W. Guo, X. Zhou, Z. Chen, Z. Liu, S. Liu, Chin, Progress in Ru-based
amorphous alloy catalysts for selective hydrogenation of benzene to cyclohexene,
J. Catal. 32 (2011) 1–16, https://doi.org/10.1016/S1872-2067(10)60154-4.
[5] M.C. Schoenmaker-Stolk, J.W. Verwijs, J.J.F. Scholten, The catalytic
hydrogenation of benzene over supported metal catalysts: I. Gas-phase
hydrogenation of benzene over ruthenium-on-silica, Appl. Catal. 27 (1987) 73–90,
https://doi.org/10.1016/S0166-9834(00)82608-1.
The addition of cadmium to the Ru/Al2O3 catalyst in 0.2:1, 0.4:1, 1:1
and 6:1 Cd:Ru ratios in the partial hydrogenation of benzene showed
that the promoter increases activity per site and also selectivity in
cyclohexene in relation to the Ru/Al2O3 catalyst. The 0.2:1 Cd:Ru ratio
provides greater reductibility of ruthenium and Ru-Cd interaction, in
addition to increasing the amount of H2 chemisorption. The promoting
effect of cadmium was verified in the reaction with or without the
addition of water. A preliminary DFT study showed that the Ru-Cd-2Al
configuration is as promising as the Ru-Zn-2Al configuration in the re-
action by reducing the heat of adsorption of cyclohexane and raising the
[6] H. Kubicka, The specific activity of technetium, rhenium, ruthenium, platinum,
and palladium in catalytic reactions of benzene with hydrogen, J. Catal. 12 (1968)
223–237, https://doi.org/10.1016/0021-9517(68)90102-4.
[7] H. Nagahara, M. Ono, M. Konishi, Y. Furuoka, Partial hydrogenation of benzene to
Cyclohexene, Appl. Surf. Sci. 121/122 (1997) 448, https://doi.org/10.1016/
S0169-4332(97)00325-5.
8

P.V.C. Azevedo et al.
Molecular Catalysis 499 (2021) 111288
[8] S.C. Hu, Y.W. Chen, Partial hydrogenation of benzene on Ruꢀ Zn/SiO2 catalysts,
Ind. Eng. Chem. Res. 40 (2001) 6099–6104, https://doi.org/10.1021/ie0007926.
[9] P.Q. Yuan, B.Q. Wang, Y.M. Ma, H.M. He, Z.M. Cheng, W.K. Yuan, Partial
hydrogenation of benzene over the metallic Zn modified Ru-based catalyst, J. Mol.
Catal. A Chem. 309 (2009) 124–130, https://doi.org/10.1016/j.
benzene to Cyclohexene, J. Catal. 268 (2009) 100–105, https://doi.org/10.1016/j.
jcat.2009.09.007.
[21] W. Wang, H. Liu, G. Ding, P. Zhang, T. Wu, T. Jiang, B. Han, Ru–Cd/Bentonite for
the partial hydrogenation of benzene: a catalyst without additives, ChemCatChem.
4 (2012) 1836–1843, https://doi.org/10.1002/cctc.201200272.
molcata.2009.05.006.
[22] R.K. Mandal, M.D. Purkayastha, T.P. Majumder, Silver modified cadmium oxide–A
novel material for enhanced photodegradation of malachite green, Optik. 180
(2019) 174–182, https://doi.org/10.1016/j.ijleo.2018.11.066.
[10] W. Xue, Y. Song, Y. Wang, D. Wang, F. Li, Effect of hydrazine hydrate on the
Ru–Zn/SiO2 catalysts performance for partial hydrogenation of benzene, Catal.
Comm. 11 (2009) 29–33, https://doi.org/10.1016/j.catcom.2009.08.009.
[11] H.J. Sun, Y.J. Pan, H.B. Jiang, S.H. Li, Y.X. Zhang, S.C. Liu, Z.Y. Liu, Effect of
transition metals (Cr, Mn, Fe, Co, Ni, Cu and Zn) on the hydrogenation properties
of benzene over Ru-based catalyst, Appl. Catal. A Gen. 464–465 (2013) 1–9,
https://doi.org/10.1016/j.apcata.2013.05.021.
[23] T. Lopez, M. Villa, R. Gomez, UV-visible diffuse reflectance spectroscopic study of
platinum, palladium, and ruthenium catalysts supported on silica, J. Phys. Chem.
95 (1991) 1690–1693, https://doi.org/10.1021/j100157a038.
[24] I.R.C. Urbiola, R.R. Bon, Y.V. Vorobiev, The transformation to cadmium oxide
through annealing of cadmium oxide hydroxide deposited by ammonia-free SILAR
method and the photocatalytic properties, Thin Solid Films 592 (2015) 110–117,
https://doi.org/10.1016/j.tsf.2015.08.052.
[12] X.L. Yu, Y. Li, S.M. Xin, P.Q. YUAN, W.K. Yuan, Ind. Eng. Partial Hydrogenation of
Benzene to Cyclohexene on Ru@ XO2 (X= Ti, Zr, or Si), Chem. Res. 57 (2018)
1961–1967, https://doi.org/10.1021/acs.iecr.7b04642.
[25] N.C.S. Selvam, R.T. Kumar, K. Yogeenth, L.J. Kennedy, G. Sekaran, J.J. Vijaya,
lament Sagaya, et al., Simple and rapid synthesis of cadmium oxide (CdO)
nanospheres by a microwave-assisted combustion method, Powder Technol. 211
(2011) 250–255.
[13] X. Xue, J. Liu, D. Rao, S. Xu, W. Bing, B. Wang, S. He, M. Wei, Double-active site
synergistic catalysis in Ru–TiO2 toward benzene hydrogenation to cyclohexene
with largely enhanced selectivity, Catal. Sci. Technol. 7 (2017) 650–657, https://
doi.org/10.1039/C6CY02471H.
[26] P. Betancourt, A. Rives, R. Hubaut, C.E. Scott, J. Goldwasser, A study of the
ruthenium–alumina system, Appl. Catal. A Gen. 170 (1998) 307–314, https://doi.
org/10.1016/S0926-860X(98)00061-1.
[14] H.J. Sun, Y.J. Pan, H.B. Wang, Y. Dong, Z.Y. Liu, S.C. Liu, Selective hydrogenation
of benzene to cyclohexene over a Ru-Zn catalyst with diethanolamine as an
additive, Chin. J. Catal. 33 (2012) 610–620, https://doi.org/10.1016/S1872-2067
(11)60351-3.
´
[27] A. Arcoya, X.L. Seoane, L.M. Gomez-Sainero, Activity of Pd/Al2O3 and Ru/Al2O3
catalysts in the hydrogenation of o-xylene: effect of thiophene, Appl. Surf. Sci. 211
(2003) 341–351, https://doi.org/10.1016/S0169-4332(03)00354-4.
[28] B.M. Abu-Zied, A.M. El-Awad, The synergism of cadmium on the catalytic activity
of Cd–Cr–O system: II. Ethanol decomposition, catalysts reducibility, and in situ
electrical conductivity measurements, J. Mol. Catal. A Chem. 176 (2001) 227–246,
https://doi.org/10.1016/S1381-1169(01)00263-1.
[15] E.V. Spinace, J.M. Vaz, Liquid-phase hydrogenation of benzene to cyclohexene
catalyzed by Ru/SiO2 in the presence of water–organic mixtures, Catal. Comm. 4
(2003) 91–96, https://doi.org/10.1016/S1566-7367(02)00270-4.
[16] J. Struijk, R. Moene, T. van der Kamp, J.J.F. Scholten, Partial liquid-phase
hydrogenation of benzene to cyclohexene over ruthenium catalysts in the presence
of an aqueous salt solution: II. Influence of various salts on the performance of the
catalyst, Appl. Catal. A Gen. 89 (1992) 77–102, https://doi.org/10.1016/0926-
860X(92)80079-R.
[29] T. Narita, H. Miura, M. Ohira, H. Hondou, K. Sugiyama, T. Matsuda, R.D. Gonzalez,
The effect of reduction temperature on the chemisorptive properties of Ru/Al2O3:
effect of chlorine, Appl. Catal. 32 (1987) 185–190, https://doi.org/10.1016/
S0166-9834(00)80624-7.
[17] A.H.A. Gonçalves, J.C.S. Soares, L.R.R. de Araújo, F.M.Z. Zotin, F.M.T. Mendes, A.
B. Gaspar, Surface investigation by X-ray photoelectron spectroscopy of Ru-Zn
catalysts for the partial hydrogenation of benzene, Mol. Catal. 483 (2020), 110710,
https://doi.org/10.1016/j.mcat.2019.110710.
[30] B. Lin, R. Wang, J. Lin, S. Du, X. Yu, K. Wei, Preparation of chlorine-free alumina-
supported ruthenium catalyst for ammonia synthesis base on RuCl3 by hydrazine
reduction, Catal. Comm. 8 (2007) 1838–1842, https://doi.org/10.1016/j.
catcom.2007.02.021.
[18] L. Foppa, J. Dupont, Benzene partial hydrogenation: advances and perspectives,
Chem. Soc. Rev. 44 (2015) 1886–1897, https://doi.org/10.1039/C4CS00324A.
[19] J. Struijk, R. Moene, T. Van der Kamp, J.J.F. Scholten, Partial liquid-phase
hydrogenation of benzene to cyclohexene over ruthenium catalysts in the presence
of an aqueous salt solution: II. Influence of various salts on the performance of the
catalyst, Appl. Catal. A Gen. 89 (1992) 77–102, https://doi.org/10.1016/0926-
860X(92)80079-R.
[31] R.S. Supino, R. Landers, A.J.G. Cobo, Partial hydrogenation of benzene on Ru
catalysts: effects of additives in the reaction medium, Appl. Catal. A Gen. 452
(2013) 9–16, https://doi.org/10.1016/j.apcata.2012.11.034.
[32] C. Milone, G. Neri, A. Donato, M.G. Musolino, L. Mercadante, Selective
hydrogenation of benzene to cyclohexene on Ru/γ-Al2O3, J. Catal. 159 (1996)
253–258, https://doi.org/10.1006/jcat.1996.0086.
[20] J.L. Liu, Y. Zhu, J. Liu, Y. Pei, Z.H. Li, H. Li, H.X. Li, M.H. Qiao, K.N. Fan,
Discrimination of the roles of CdSO4 and ZnSO4 in liquid phase hydrogenation of
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