Cu-decorated Ru catalysts supported on layered double hydroxides for selective benzene hydrogenation to cyclohexene

Article abstract of DOI:10.1002/cctc.201402895

The selective hydrogenation of benzene to cyclohexene is of high value for the chemical industry owing to its inexpensive feedstock, atom economy, and operational simplicity. A tunable catalytic behavior towards the selective hydrogenation of benzene was obtained over Cu-decorated Ru catalysts supported on a layered double hydroxide (denoted as Ru_xCu_y/MgAl-LDH), reaching a maximum cyclohexene yield of 44.0-% over Ru_1.0Cu_0.5/MgAl-LDH at 150-°C and 5.0 MPa without employment of any additives. CO-TPD (TPD=temperature-programmed desorption) and in situ CO-FTIR techniques demonstrated that Cu atoms preferentially deposit on the surface of low-coordinated Ru atoms in Ru_xCu_y/MgAl-LDH catalysts, resulting in a low adsorption energy of cyclohexene on the modified sites as revealed by DFT calculations. This work not only gives an understanding of the correlation between the surface exposure of Ru active sites and the resulting selectivity, but also provides a green and additive-free catalytic process for the selective hydrogenation of benzene. Selective hydrogenation: Cu-decorated Ru catalysts supported on hierarchical MgAl-LDH (LDH=layered double hydroxide) were fabricated by a facile two-step procedure and exhibit a superior selectivity for hydrogenation of benzene to cyclohexene without employment of any additives. CO-TPD (TPD=temperature-programmed desorption) and in situ CO-FTIR results demonstrate that Cu atoms occupy the low-coordinated Ru active sites, which possess a high adsorption energy for cyclohexene as revealed by DFT calculations.

Full text of DOI:10.1002/cctc.201402895

DOI: 10.1002/cctc.201402895
Full Papers
Cu-Decorated Ru Catalysts Supported on Layered Double
Hydroxides for Selective Benzene Hydrogenation to
Cyclohexene
Jie Liu, Simin Xu, Weihan Bing, Fei Wang, Changming Li, Min Wei,* David G. Evans, and
[a]
Xue Duan
The selective hydrogenation of benzene to cyclohexene is of
high value for the chemical industry owing to its inexpensive
feedstock, atom economy, and operational simplicity. A tunable
catalytic behavior towards the selective hydrogenation of ben-
zene was obtained over Cu-decorated Ru catalysts supported
on a layered double hydroxide (denoted as Ru Cu /MgAl-LDH),
grammed desorption) and in situ CO-FTIR techniques demon-
strated that Cu atoms preferentially deposit on the surface of
low-coordinated Ru atoms in Ru Cu /MgAl-LDH catalysts, re-
x
y
sulting in a low adsorption energy of cyclohexene on the
modified sites as revealed by DFT calculations. This work not
only gives an understanding of the correlation between the
surface exposure of Ru active sites and the resulting selectivity,
but also provides a green and additive-free catalytic process
for the selective hydrogenation of benzene.
x
y
reaching
a maximum cyclohexene yield of 44.0% over
Ru Cu /MgAl-LDH at 1508C and 5.0 MPa without employ-
1.0
0.5
ment of any additives. CO-TPD (TPD=temperature-pro-
Introduction
Cyclohexene is one of the most important commodity petro-
chemicals, and is a raw material for the production of value-
added adipic acid, nylons, caprolactam, and other fine chemi-
needs to be addressed to obtain a high yield of cyclohex-
[4,9,12–14]
ene.
Fundamental studies have been conducted based
on the employment of hydrophilic Ru-based catalysts in an
aqueous-phase-assisted catalytic system, resulting in the kineti-
[
1]
cals. Recently, growing interest has been focused on the gen-
eration of cyclohexene by selective hydrogenation of benzene
owing to the advantages over traditional production routes
[1a,15,16]
cally enhanced desorption of cyclohexene.
In addition,
the structure of active sites and reaction conditions (e.g., tem-
perature, pressure, and medium) also play important roles in
(e.g., dehydration of cyclohexanol, dehydrogenation of cyclo-
hexane, and Birch reduction), in terms of inexpensive feed-
the adsorption behavior of cyclohexene on the catalyst sur-
[
2]
[4,17,18]
stock, atomic economy, and operational simplicity. However,
obtaining high yields of cyclohexene from the selective hydro-
genation of benzene is challenging because of further hydro-
face.
The reported tailoring, by additives, of Ru-based cat-
alysts with a favorable electronic and geometric structure pro-
vides important approaches for the enhancement of selectivity
[
3]
[6,13,19,20]
genation from cyclohexene to cyclohexane. Ruthenium-
based catalysts are commonly used in this reaction and gener-
ally exhibit a relatively high yield of cyclohexene with the assis-
towards cyclohexene.
However, very few investigations
were performed to reveal the influences of surface defects on
the hydrogenation selectivity on an atomic basis. This investi-
gation would be particularly interesting because metal cata-
lysts with controlled exposure of active sites have been proven
to exhibit excellent catalytic behavior in the field of hydroge-
[4–12]
tance of large amounts of additives (typically, ZnSO4).
Al-
though the cyclohexene yield can be increased effectively, the
use of zinc salts results in corrosion and separation problems,
which limit the long-term development of such a system.
Therefore, the exploration of highly efficient, economical, and
green catalysts for this reaction remains a challenging goal.
Given the well-known characteristics of the consecutive hy-
drogenation of benzene, the knowledge of how to facilitate
the desorption of cyclohexene from the catalyst surface, as
well as to inhibit its readsorption remains the key issue that
[21,22]
nation reactions.
One of the most promising approaches to tune metal cata-
lysts with controlled exposure of active sites is surface modifi-
[23,24]
cation by a second metal.
However, it is rather difficult to
incorporate a suitable second metal and a facile synthesis
strategy to obtain a core-shell structure similar to bimetallic
particles, in which the main metal particles exhibit a controlled
atom exposure owing to the surface coverage or segregation
of the second metal. Layered double hydroxides (LDHs), with
[a] J. Liu, S. Xu, W. Bing, F. Wang, Dr. C. Li, Prof. M. Wei, Prof. D. G. Evans,
2
þ
3þ
nꢀ
Prof. X. Duan
the formula of [M1 M (OH) ](A ) ·mH O, have been exten-
2
x/n
2
ꢀx
x
State Key Laboratory of Chemical Resource Engineering
Beijing University of Chemical Technology (P.R. China)
Fax: (+86)10-64425385
sively explored as sorbents and heterogeneous catalysts and/
or precursors because of their versatility in composition, mor-
[25–29]
phology, and architecture.
Considerable interest has been
E-mail: weimin@mail.buct.edu.cn
focused on the design and preparation of mono-/bimetallic
nanocatalysts owing to the unique topotactic transformation
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201402895.
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of LDH materials into supported metal nanocatalysts on
powdered XRD patterns of these precursors are shown in Fig-
ure S1A (see the Supporting Information), in which the typical
(003) and (006) reflections at 2qꢁ12 and 248 are observed,
respectively, verifying the formation of hydrotalcite phase. No
other crystalline phase was detected, indicating the high purity
[
30–34]
a metal oxide matrix under reducing conditions.
Inspired
by the unique merits of LDH materials, we explored the idea
that bimetallic catalysts can be fabricated by a co-reduction
approach. Treatment of RuCl -impregnated CuMgAl-LDH by
3
hydrogen resulted in the formation of a supported bimetallic
Ru–Cu catalyst on a MgO–Al O mixed metal oxide (denoted
of these LDH precursors. After impregnation of RuCl , followed
3
by subsequent reduction process, two peaks at 2qꢁ43 and
638 were observed (Figure S1B), which can be indexed to the
typical (200) and (220) reflections of a face-centered MgO
phase. This finding confirms the topotactic transformation of
the LDH precursors to mixed metal oxides accompanied by
the co-reduction of Ru and Cu (Ru Cu /MgAl-MMO). However,
2
3
as MgAl-MMO) matrix. This strategy possesses the following
desirable features: 1) The atomic-scale distribution of Cu in the
LDH precursors favors uniform decoration of Ru nanoparticles
(NPs) so as to achieve a tunable catalytic performance; and
2) the co-reduction process based on the topotactic transfor-
x
y
mation of LDH guarantees the high stability of the resulting
supported Ru Cu NPs.
no reflections corresponding to metallic Ru and Cu were de-
tected, possibly because of a high dispersion with tiny particle
size or low concentration of metal species in these samples,
which was outside of the XRD detection limit.
x
y
Herein, we report the fabrication of Cu-decorated Ru cata-
lysts supported on MgAl-LDH by a facile two-step procedure
involving the reduction of RuCl -impregnated CuMgAl-LDH by
Taking into account the fact that the selective hydrogena-
tion of benzene is actually performed in an aqueous microen-
3
hydrogen, followed by the rehydration of the resulting Ru Cu /
x
y
[1a,15,16]
MgAl-MMO to Ru Cu /MgAl-LDH (Scheme 1). SEM and TEM
vironment system,
the choice of a good hydrophilic sup-
x
y
port for the immobilization of the metal catalyst is important.
Herein, the Ru Cu /MgAl-MMO samples underwent further
x
y
treatment by a rehydration process, so as to achieve the trans-
formation of the catalyst support from MMO to LDH based on
[36]
the so-called “memory effect” of LDH materials. It was found
that the typical (003) and (006) reflections for the hydrotalcite
phase reappear in the XRD patterns of the regenerated sam-
ples (Figure 1), indicative of successful fabrication of Ru Cu /
x
y
MgAl-LDH samples from Ru Cu /MgAl-MMO. Compared with
Scheme 1. The synthesis of Cu-decorated Ru nanoparticles supported on
x
y
MgAl-LDH.
the original CuMgAl-LDH precursors (Figure S1A), the full
width at half maximum (FWHM) of the regenerated Ru Cu /
x
y
measurements reveal that Cu-decorated Ru particles with
a mean size of ꢁ2.1 nm were well-dispersed on the hierarchi-
cal LDH matrix. CO-TPD (TPD=temperature-programmed de-
sorption) and CO-FTIR measurements demonstrated that the
metallic Cu tends to deposit on the surface of Ru particles at
the low-coordinated sites. The resulting Ru Cu /MgAl-LDH cata-
MgAl-LDH is much wider, suggesting a decrease in the crystal-
line degree of the LDH phase. FTIR spectra show a strong peak
ꢀ
1
at 3470 cm , attributed to the stretching mode of surface hy-
[37]
droxyl groups (Figure S2), which implies good hydrophilicity
ꢀ
1
for these LDH materials. The peaks at 1360 and 3070 cm are
2ꢀ
assigned to the stretching vibration of interlayer CO and the
x
y
3
2
ꢀ
lysts exhibit a tunable catalytic performance toward the selec-
tive hydrogenation of benzene, and the best catalytic behavior
was obtained over Ru Cu /MgAl-LDH with a maximum cyclo-
bridging mode of H OꢀCO , respectively, which confirms the
2
3
formation of carbonate-containing LDH materials.
1.0
0.5
hexene yield of 44.0% (1508C, 5.0 MPa). A correlation between
the exposure of Ru active sites and the selectivity was estab-
lished based on kinetic investigations and DFT calculations.
The low-coordinated Ru atoms possess a high surface free
energy, which is unfavorable for the desorption of cyclohex-
ene; an appropriate coverage of the low-coordinated Ru by Cu
gives rise to a largely enhanced selectivity toward cyclohexene.
This work provides an understanding of the discriminatory hy-
drogenation behavior of Ru active sites, and demonstrates
a promising approach for the fabrication of green catalysts for
structure-sensitive hydrogenation reactions.
Results and Discussion
Structural and morphological characteristics
Figure 1. XRD patterns of a) Ru/MgAl-LDH, b) Ru1.0Cu0.2/MgAl-LDH,
c) Ru1.0Cu0.5/MgAl-LDH, d) Ru1.0Cu1.0/MgAl-LDH, and e) Cu/MgAl-LDH-2
derived from CuMgAl-LDH-2 precursor.
The CuMgAl-LDH precursors with various Cu contents were
[35]
prepared by the urea hydrolysis–precipitation method. The
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The SEM images of three typical samples: CuMgAl-LDH-2
precursor, Ru Cu /MgAl-MMO, and Ru Cu /MgAl-LDH are
1.0
0.5
1.0
0.5
shown in Figure 2. The LDH precursor displays smooth and
uniform hexagonal nanocrystals with a diameter of ꢁ2 mm
(
Figure 2a and b). Hydrogen reduction of the RuCl -impregnat-
3
ed CuMgAl-LDH-2 resulted in its transformation to Ru Cu /
1.0
0.5
MgAl-MMO, which still maintains the original morphology of
the LDH precursor (Figure 2c). After the further rehydration
treatment of Ru Cu /MgAl-MMO, an interesting morphologi-
1.0
0.5
cal change occurs. A large number of nanosheets are observed
on the surface of an individual hexagonal nanocrystal (Fig-
ure 2d). A similar phenomenon was also observed for other
Ru Cu /MgAl-LDH samples (Figure S3). This structural transfor-
x
y
mation from Ru Cu /MgAl-MMO to Ru Cu /MgAl-LDH is due to
x
y
x
y
the so-called “memory effect” of calcined hydrotalcite, that is,
it can recover the original LDH structure with a positively
2ꢀ
charged host layer and an interlayer CO as a compensation
3
[
36]
anion.
HRTEM measurements were employed to investigate the
structure and distribution of metal species (i.e., Ru and Cu) on
MgAl-MMO and the regenerated MgAl-LDH. For the Ru Cu /
Figure 2. SEM images of a,b) CuMgAl-LDH-2 precursor, c) Ru1.0Cu0.5/MgAl-
MMO, and d) regenerated Ru1.0Cu0.5/MgAl-LDH.
x
y
MgAl-MMO samples, metal nanoparticles with high density dis-
perse uniformly in a platelet-like matrix, as shown in Fig-
ure 3a –d . The histograms (Figure 3a –d , inset) show that all
metallic Ru and Cu in the Ru Cu /MgAl-MMO samples. The
x
y
peak at 280.2 eV can be attributed to the binding energy of
1
1
1
1
[40]
four samples possess a narrow size distribution from 1.5 to
.7 nm, with a mean size of 2.1 nm based on 300 particles
Ru3d5/2 (Figure S6A); whereas the two peaks at 952. 5 and
932.8 eV can be attributed to Cu2p and Cu2p1/2, respectively
2
3/2
[41]
counted in different regions. The corresponding HRTEM
images with high magnification (Figure 3a –d ) reveal nanopar-
(Figure S6B). Both of the binding energies of Ru and Cu do
not show any shift for these samples with various Ru/Cu ratios,
indicating the absence of obvious electron interaction. Regard-
2
2
ticles with characteristic lattice spacing of 0.206 nm, which can
be ascribed to the (101) plane
of hexagonal close-packed (hcp)
Ru. This finding is further con-
firmed by the fast Fourier trans-
form (FFT) patterns (Figure 3a2–
d2, inset). A metallic Cu phase is
not observed for the four sam-
ples, suggesting that Cu is de-
posited onto the surface of Ru
particles because the two metals
are immiscible in bulk. To study
the evolution of Cu, we in-
creased the Cu content gradual-
ly, and found an individual met-
allic Cu atom cannot be identi-
fied until the Cu/Ru molar ratio
increases to 3:1 (Figure S5). Simi-
lar phenomena were also report-
ed for Ru–Cu bimetal NPs sup-
ported on other carriers (e.g.,
[
38,39]
KL-zeolite and SiO2),
in
which the tiny metallic Cu atoms
deposit on the surface of Ru at
a low concentration, and tend to
form segregated Cu particles at
a high concentration. The XPS
spectra of Ru3d and Cu2p (Fig-
Figure 3. HRTEM images of the Ru
MgAl-MMO, b ,b ) Ru1.0Cu0.2/MgAl-MMO, c
) Ru1.0Cu0.2/MgAl-LDH, c ) Ru1.0Cu0.5/MgAl-LDH, d
x
Cu
y
/MgAl-MMO and the corresponding Ru
,c ) Ru1.0Cu0.5/MgAl-MMO, d ,d ) Ru1.0Cu1.0/MgAl-MMO, a
) Ru1.0Cu1.0/MgAl-LDH. Insets in (a ), (b ), (c ), and (d
1
x
Cu
y
/MgAl-LDH samples: a
) Ru/MgAl-LDH,
) are the
1 2
,a ) Ru/
1
2
1
2
1
2
3
b
3
3
3
1
1
1
ure S6) reveal the presence of corresponding particle-size frequency distribution histograms (300 particles analyzed).
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ing the low intensity of the XPS spectra of Cu, Cu K-edge X-
ray-absorption near-edge structure (XANES) spectra are also
given in Figure S7. It was found that the Cu K-edge XANES of
the three Cu-containing samples was rather close to that of Cu
foil, further verifying the presence of metallic Cu in these Ru–
Cu samples. Figure 3a –d3 further shows that the metal NPs
3
were still well-dispersed on the basal hexagonal matrix after
the regeneration process, with epitaxially-formed LDH nano-
sheets. The inset in Figure 3 a shows a hexagonal arrange-
3
ment of spots from the fresh LDH nanosheets in the corre-
sponding selected-area electron diffraction (SAED) pattern. This
arrangement demonstrates
a
well-crystallized hydrotalcite
[
42]
phase
owing to the epitaxial growth of nanosheets. This
finding is in accordance with the XRD results (Figure 1). More-
over, HRTEM images of the Ru Cu /MgAl-LDH samples with
x
y
high magnification (Figure S8) further shows that the morphol-
ogy of the bimetal NPs remains unchanged after the rehydra-
tion process.
Hydrogen temperature-programmed reduction (H -TPR)
2
measurements were performed to provide information on the
redox properties of RuCl -impregnated CuMgAl-LDH samples
3
(Figure 4A). Two peaks at around 125 and 2708C can be ob-
served in the H -TPR profiles of these samples. The first peak is
2
[
43]
attributed to the reduction of Ru species,
whereas the
2
+
second peak is attributed to the reduction of Cu species in
[
44]
the lamellar structure, verifying the strong reducibility of Ru
species, which undergoes preferential reduction. It can be seen
Figure 4. A) H -TPR profiles of: a) Ru/CuMgAl-LDH-0, b) Ru/CuMgAl-LDH-1,
2
2
+
that the hydrogen consumption for the reduction of Cu spe-
cies increases gradually along with the increase of Cu content
c) Ru/CuMgAl-LDH-2, and d) Ru/CuMgAl-LDH-3. B) CO-TPD profiles of: a) Ru/
MgAl-LDH, b) Ru1.0Cu0.2/MgAl-LDH, c) Ru1.0Cu0.5/MgAl-LDH, d) Ru1.0Cu1.0/MgAl-
LDH, and e) Cu/MgAl-LDH-2.
(Figure 4A, a–d), whereas no obvious change was observed for
the reduction peak of Ru. Therefore, it is deduced that the
preferentially reduced Ru particles may act as the seeds for the
subsequent reduction of Cu oxide, resulting in the Cu-decorat-
ed Ru particles in the above samples. Carbon monoxide tem-
perature-programmed desorption (CO-TPD) experiments were
further employed to investigate the influence of Cu on the sur-
face-atom exposure of metallic Ru particles in the final Ru Cu /
tion capacity of CO in the Ru Cu /MgAl-LDH samples. Based on
x
y
the CO chemisorption capacity, the apparent dispersion of Ru
in these Ru-based samples was calculated and is listed in
Table 1. Because the four samples possess a similar Ru particle
size (Figure 3), the apparent decline of Ru dispersion upon in-
creasing Cu content further confirms the deposition of Cu
atoms on the surface of the Ru particle.
x
y
MgAl-LDH samples (Figure 4B). A strong desorption peak of
CO at 1708C can be observed in the CO-TPD profiles of the
four Ru-based samples (Figure 4B, a–d), whereas no obvious
CO chemisorption can be found for the Cu/MgAl-LDH-2
sample derived from the CuMgAl-LDH-2 precursor (Figure 4B,
e). This result indicates that for the Ru–Cu bimetallic system,
CO chemisorption occurs on Ru, but not on Cu, at room tem-
In heterogeneous catalysis, metal particles involve various
active sites (e.g., corners, steps, and facets). The deposition of
Cu on the surface of Ru particles leads to a decreased expo-
sure of Ru active sites, but detailed information on the prefer-
entially covered active sites is still not available. In situ FTIR
spectra of CO adsorption is a useful method to probe the sur-
face structure of Ru particles. The IR spectra of CO absorbed
on the five Ru Cu /MgAl-LDH samples are shown in Figure 5.
[
38]
perature. In addition, for the four Ru-based samples, the in-
tensity of the desorption peak of CO decreases gradually with
the increase of Cu content, indicating a decreased chemisorp-
x
y
For the Cu/MgAl-LDH sample, no obvious CO vibration was ob-
Table 1. Structural parameters of various samples.
Catalyst
S
BET
2
Ru
[wt.%]
Molar ratio
of Ru/Cu
Apparent dispersion
of Ru [%]
Coverage degree (q)
of Cu [%]
Mean particle size
by TEM [nm]
ꢀ1
[a]
[a]
[b]
[b]
[m g
]
Ru/MgAl-LDH
38
38
40
40
2.39ꢂ0.05
2.48ꢂ0.05
2.51ꢂ0.07
2.45ꢂ0.04
1:0
26.5
21.1
17.9
13.8
0
2.1ꢂ0.6
2.0ꢂ0.6
2.0ꢂ0.6
2.1ꢂ0.6
Ru1.0Cu0.2/MgAl-LDH
Ru1.0Cu0.5/MgAl-LDH
Ru1.0Cu1.0/MgAl-LDH
1:0.18
1:0.45
1:0.82
17.2
29.1
46.6
[a] Values determined by ICP. [b] Values calculated based on CO-TPD.
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hydrogenation of benzene to cyclohexene. The corresponding
plots of the selectivity for cyclohexene versus the conversion
of benzene are shown in Figure 6 (reaction temperature:
1
508C; H pressure: 5.0 MPa). The benzene content decreased
2
gradually and the completely saturated product, cyclohexane,
increased linearly with reaction time. Regarding the cyclohex-
ene content, there was a maximum at a certain reaction time,
dependent on the catalyst, exhibiting the well-known behavior
of consecutive reactions. Among the four catalysts, the Ru/
MgAl-LDH catalyst shows the highest activity of benzene hy-
drogenation, but the lowest selectivity to cyclohexene. With
the enhancement of incorporated Cu, the activity of benzene
hydrogenation over the resulting Ru Cu /MgAl-LDH decreased
x
y
gradually, whereas the selectivity to cyclohexene increased and
reached a maximum value at a Ru/Cu molar ratio of 1:0.5. As
a result, a maximum cyclohexene yield of 44.0% at a benzene
Figure 5. In situ FTIR spectra of CO adsorption on a) Cu/MgAl-LDH-2, b) Ru/
MgAl-LDH, c) Ru1.0Cu0.2/MgAl-LDH, d) Ru1.0Cu0.5/MgAl-LDH, and e) Ru1.0Cu1.0
MgAl-LDH.
/
conversion of 71.8% was obtained over the Ru Cu /MgAl-
1.0
0.5
LDH catalyst. Although the obtained catalytic yield is not as
high as some reported values, the Cu-decorated Ru catalyst in
this work does not involve the use of any additive (e.g.,
served (Figure 5a), in accordance with the CO-TPD result (Fig-
0
ure 4e), further confirming that Cu hardly adsorbs CO species
ZnSO ), which has been commonly applied previously (see
4
[
38]
at room temperature.
In the case of the Ru/MgAl-LDH
Table S1). Therefore, this additive-free catalyst can serve as
a green and eco-friendly candidate for the selective hydroge-
nation of benzene.
sample, three bands can be identified (Figure 5b). The bands
ꢀ
1
at 1993 and 2062 cm can be attributed to the bridge-
bonded and linearly adsorbed CO on Ru atoms, respectively;
It can be seen from Figure 6 that all three Ru Cu /MgAl-LDH
x
y
ꢀ
1
whereas the band at 2120 cm can be assigned to the stretch-
ing mode of multicarbonyl species, which often adsorb on
low-coordinated sites (such as corners, steps, or defective sites)
catalysts display inhibited activity of benzene hydrogenation
with the addition of Cu, in comparison with the Ru/MgAl-LDH
catalyst. As summarized in Table 2, the weight-specific activity
[
23,45]
of finely dispersed noble metals.
Generally, low-coordinat-
(R ) of these catalysts also decreased with an increase of Cu
0
ed metal atoms possess relatively low electron density and
lead to the occurrence of the stretching vibration of multicar-
bonyls at a high wavenumber, owing to the weak electron
feedback to the adsorbed CO. Of the four Ru-based samples,
Ru/MgAl-LDH exhibits the strongest multi-CO adsorption (Fig-
ure 5b), indicating a high exposure of unsaturated Ru. With
the introduction of Cu, the adsorption intensity of multi-CO
decreases gradually and reaches a negligible value at a Ru/Cu
molar ratio of 1:0.5 and 1:1 (Figure 5d and e), implying almost
complete coverage of Cu on the low-coordinated Ru. The cov-
content. This inhibiting effect, to some extent, can be attribut-
ed to the coverage of Cu on the surface of Ru, resulting in a de-
creased exposure of Ru active sites. To give a deep insight into
the inhibition effect, the turnover frequency (TOF) of benzene
over these catalysts was further calculated on the basis of Ru
dispersion and R data. A plot of the TOF value as a function of
0
the coverage degree of Cu (q ) is shown in Figure 7a. It is ob-
Cu
served that initially the TOF value exhibits a linear decrease
with an increase of q , and is balanced at a q value of
Cu
Cu
29.1%. It has been proven that metal catalysts with high unsa-
turation/low coordination number generally show excellent hy-
erage degree of Cu (q ) for these two samples is ꢁ29.1 and
Cu
[47]
46.6%, respectively, according to the CO-TPD result (Table 1).
drogenation activity. For benzene hydrogenation, a high effi-
ciency of hydrogenation is also dependent on the exposure of
The results indicate a preferential coverage of Cu on the low-
coordinated Ru, in good agreement with previous investiga-
[20]
low-coordinated Ru atoms.
Compared with the Ru/MgAl-
[
46]
tions.
For the bridge-bonded CO adsorption peak at
LDH catalyst, the low-coordinated Ru atoms in the Ru Cu /
x
y
ꢀ
1
1
993 cm , a continuous decrease is observed with the en-
MgAl-LDH catalysts are continuously covered by Cu and under-
go a complete coverage at qCu value of 29.1%, according to
the CO-TPD result, which accounts for the gradually decrease
in the TOF of benzene.
hancement of Cu content to the Cu/Ru molar ratio of 1:1 (Fig-
ure 5e), implying a further coverage of Cu on the high-coordi-
nated Ru. Therefore, a tunable exposure of the active sites of
Ru NPs can be successfully achieved by changing the surface
coverage of Cu originating from a LDH support. This finding
imposes great influence on the hydrogenation selectivity,
which is discussed below.
Although the reaction rate of benzene hydrogenation over
the Cu-decorated Ru catalysts was inhibited to some extent,
the hydrogenation selectivity to cyclohexene was largely en-
hanced. In terms of the effect of Cu on the cyclohexene selec-
tivity for these catalysts, the value of S50,CHE was also obtained
(
defined as the cyclohexene selectivity at 50% benzene con-
Catalytic evaluation for selective hydrogenation of benzene
version). Unlike the negative correlation between the TOF
value and the coverage of Cu on the Ru surface, a positive re-
lationship was established for S50,CHE (Figure 7b), indicative of
The catalytic performances of the as-synthesized Ru/MgAl-LDH
and Ru Cu /MgAl-LDH samples were evaluated by the selective
x
y
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Figure 6. The time courses of benzene hydrogenation over a) Ru/MgAl-LDH, and the Ru
d) 1:1. Reaction conditions: catalyst (0.1 g), benzene (10 mL), H
x
Cu
y
/MgAl-LDH catalysts with various Ru/Cu ratios: b) 1:0.2, c) 1:0.5,
2
2
O (20 mL), temperature (1508C), H pressure (5.0 MPa), stirring rate (1000 rpm).
[
a]
Table 2. Catalytic performance of various catalysts.
Catalyst
t
Conv.
[%]
S
[%]
CHE
[b]
Y
[%]
CHE
[b]
S
[%]
50,CHE
STY50,CHE
R
0
TOF
[
b]
[b]
ꢀ1 ꢀ1Ru
ꢀ1 ꢀ1 [c]
ꢀ1
[min]
[mmolmin
g
]
[mmolmin
g
]
[s
]
Ru/MgAl-LDH
30.0
45.0
55.0
55.0
71.5ꢂ2.6
74.4ꢂ1.2
71.8ꢂ2.2
68.9ꢂ2.5
38.6ꢂ3.8
50.1ꢂ2.2
61.3ꢂ2.6
61.2ꢂ3.6
27.5ꢂ1.7
37.2ꢂ1.7
44.0ꢂ2.1
42.1ꢂ2.5
35.4ꢂ2.7
60.6ꢂ3.1
70.8ꢂ2.4
69.8ꢂ2.8
488ꢂ18
597ꢂ11
566ꢂ12
467ꢂ10
67.7ꢂ1.5
43.2ꢂ1.9
29.1ꢂ1.1
21.7ꢂ1.3
18.0ꢂ0.4
13.9ꢂ0.3
10.9ꢂ0.4
10.8ꢂ0.3
Ru1.0Cu0.2/MgAl-LDH
Ru1.0Cu0.5/MgAl-LDH
Ru1.0Cu1.0/MgAl-LDH
2 2
[a] Reaction conditions: Catalyst (0.1 g), benzene (10 mL), H O (20 mL), H pressure (5.0 MPa), 1508C. [b] The results are provided at the maximum yield of
cyclohexene. Conv.=conversion of benzene; SCHE =selectivity to cyclohexene; YCHE =yield of cyclohexene; S50,CHE refers to the selectivity to cyclohexene at
which a benzene conversion of 50% is obtained. STY50,CHE refers to the space-time-yield to cyclohexene and was calculated based on the amount of cyclo-
hexene produced per minute per gram of metal Ru at a benzene conversion of 50%. [c] Values were calculated based on the amount of converted
benzene per minute per gram of catalyst.
the TPD result, which accounts for the gradually decrease in
the TOF of benzene. Because the selectivity to cyclohexene is
determined by the relative hydrogenation rate of benzene to
cyclohexene (step 1) and of cyclohexene to cyclohexane
the Ru surface (Table 3); however, the decrease in the k and k
1 2
values is not to the same extent. As listed in Table 3, the evolu-
tion of the k /k ratio almost mimics that of S50,CHE, implying
1
2
that the coverage of Cu on the low-coordinated Ru improves
the selectivity to cyclohexene by modifying the hydrogenation
rates of both benzene and cyclohexene. As a result, both en-
hanced selectivity and space-time-yield (STY50,CHE) can be ob-
tained over the Cu-decorated Ru catalysts.
(step 2), further understanding of the reaction kinetics is neces-
sary. By fitting the reaction data presented in Figure 6, a first-
order reaction for step 1 and a zero-order reaction for step 2
[48]
were identified, and are consistent with previous reports.
The relative kinetic plots are shown in Figure 8, from which
Generally, Cu is considered to be inactive in the catalytic hy-
drogenation of benzene. The “hydrogen-spillover effect” be-
the rate constants k and k for the two hydrogenation steps
1
2
[49]
were calculated and are listed in Table 3. The results show that
both step 1 and step 2 are slowed by the coverage of Cu on
tween Ru and Cu, to some extent favors the hydrogenation
[50]
ability of Cu. However, in the case of the selective hydroge-
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Figure 7. a) TOF and b) S50,CHE value as a function of the surface coverage of
Ru over: 1) Ru/MgAl-LDH, 2) Ru1.0Cu0.2/MgAl-LDH, 3) Ru1.0Cu0.5/MgAl-LDH,
and 4) Ru1.0Cu1.0/MgAl-LDH catalyst.
nation of benzene to cyclohexene, the enhanced hydrogena-
tion capability of Cu could facilitate further hydrogenation of
cyclohexene to cyclohexane, which is unfavorable to improve
[
6]
the reaction selectivity. It is well-known that the low-coordi-
nated atoms are generally associated with defects, edges, cor-
ners, or high-index facets, which normally exhibit a high sur-
face-adsorption energy. The presence of the high-energy ad-
sorption sites does not favor desorption of the formed cyclo-
hexene and thus reduces the hydrogenation selectivity. To pro-
vide further evidence, DFT calculations were performed to
investigate the adsorption energy of cyclohexene on the sur-
face of Ru with different coordination numbers. As depicted in
Figure S9, the adsorption energy of cyclohexene on high-coor-
dinated metallic Ru(0001) facet is lower than that on
Figure 8. A) Curves of ln(c(benzene)) versus reaction time, B) curves of
c(cyclohexane) versus reaction time at 1508C and 5.0 MPa of hydrogen:
a) Ru/MgAl-LDH, b) Ru1.0Cu0.2/MgAl-LDH, c) Ru1.0Cu0.5/MgAl-LDH, and d)
Ru1.0Cu1.0/MgAl-LDH.
the low-coordinated Ru(1122) facet (Eads on
Ru(0001) and Ru (1122): 4.30 and 4.62 eV, respec-
tively). As far as the Ru Cu /MgAl-LDH catalytic
Table 3. The rate constants for the hydrogenation of benzene to cyclohexene (k
1
) and
cyclohexene
to
cyclohexane
(k
2
)
over
Ru
x
Cu
y
/MgAl-LDH
catalysts.
x
y
system is concerned, the occupancy of Cu on the
low-coordinated Ru can be regarded as a restrained
exposure of a high-index facet of Ru. Thus, desorp-
tion of cyclohexene on the Cu-decorated Ru particles
becomes easy, and its further hydrogenation to cyclo-
hexane is prevented. Furthermore, the adsorption
energy of cyclohexene on the typical Ru(0001) facet
with the modification of Cu cluster was calculated,
from which a decreased Eads (3.78 eV) was obtained
Catalyst
k
1
k
2
1 2
k /k
[10 Lmol ]
ꢀ
2
ꢀ1 [a]
ꢀ2
ꢀ1
ꢀ1 [a]
ꢀ2
ꢀ1
[
10 min
]
[10 molL min
]
Ru/MgAl-LDH
4.13
2.96
2.37
1.93
140
2.95
3.65
5.02
4.74
Ru1.0Cu0.2/MgAl-LDH
Ru1.0Cu0.5/MgAl-LDH
Ru1.0Cu1.0/MgAl-LDH
81.1
47.2
40.7
(
Figure S9c). Therefore, in view of the decreased hy-
[
a] Values obtained by fitting the reaction data in Figure 6.
drogenation activity with the addition of Cu, as well
as the low adsorption energy of cyclohexene on Cu-
decorated Ru catalyst, it is concluded that the “shielding
effect” of Cu gives a dominant contribution to the enhanced
selectivity towards cyclohexene.
a much lower cyclohexene yield, with the highest value of
33.4% over Ru Cu /MgAl-MMO (Figure S10). This lower yield
1.0
0.5
can be attributed to the relatively poor hydrophilicity of the
MgAl-MMO support. For the selective hydrogenation of ben-
zene, a good hydrophilic support makes Ru catalysts well-dis-
persed in the aqueous phase, which is favorable for desorption
of low-soluble cyclohexene from the catalyst surface. During
the rehydration process, the MgAl-MMO support converts into
a hydroxyl-enriched MgAl-LDH support, which endows an ex-
cellent hydrophilicity for the resulting Ru Cu /MgAl-LDH cata-
In addition, it has been reported that the hydrophilicity of
the catalyst support imposes an important effect on its selec-
[
4,18]
tivity.
Therefore, we performed a study to investigate this
effect. The catalytic behavior of Ru Cu /MgAl-MMO samples
x
y
with the same active center, but a less-hydrophilic support was
also studied. Compared with the Ru Cu /MgAl-LDH catalysts,
x
y
the corresponding Ru Cu /MgAl-MMO samples exhibited
x
y
x
y
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lysts. As shown in the XPS spectra of O1s (Figure S11), a largely
enhanced oxygen-in-water (OH2O) and surface-hydroxyl-oxygen
to the inhibited exposure of the undesirable low-coordinated
Ru species owing to the coverage of Cu, thus facilitating de-
sorption of cyclohexene from the surface of Ru, as revealed by
the DFT calculations. Understanding the “shielding effect” of
the undesirable Ru active sites by an inactive ingredient could
contribute to the design of more efficient catalysts. In addition,
the Cu-decorated Ru catalyst does not involve the use of any
corrosive additives, which have been commonly applied previ-
ously, and can serve as a green and eco-friendly candidate for
the selective hydrogenation of benzene.
(
O ) concentration is present in the Ru/MgAl-LDH sample
OH
compared with Ru/MgAl-MMO, verifying the enhanced hydro-
philicity of the LDH material. Finally, the recyclability of the op-
timal catalyst, Ru Cu /MgAl-LDH, was tested. After each cata-
1.0
0.5
lytic run, the organic phase was removed under vacuum and
the residual catalyst suspension was reused in a successive run
without further catalyst reactivation. The catalyst exhibited
good reusability, with a stable activity and selectivity after five
recycles (Figure 9a). The TEM image of the used catalyst fur-
ther reveals that the particle size and morphology was main-
tained (Figure 9b), demonstrating a stable, effective, and
recyclable catalyst.
Experimental Section
Materials
RuCl ·3H O, Cu(NO ) ·3H O, Mg(NO ) ·6H O, and Al(NO ) ·9H O
3
2
3 2
2
3 2
2
3 3
2
were purchased from Sigma–Aldrich. Other chemicals, including
urea, benzene, cyclohexene, and cyclohexane were purchased
from the Beijing chemical Co., Ltd., and used without further pu-
rification. Deionized water was used in all experimental processes.
Synthesis of CuMgAl-LDH precursors with various Cu
content
CuMgAl-LDH precursors with various Cu content were synthesized
[35]
by
precipitation
through
urea
hydrolysis.
Typically,
Mg(NO ) ·6H O (10 mmol), Al(NO ) ·9H O (5 mmol), and urea
3
2
2
3 3
2
ꢀ2
(
1
50 mmol) with various contents of Cu(NO ) ·3H O (0, 4.8ꢁ10 ,
3 2 2
ꢀ
2
ꢀ2
2ꢁ10 , and 24ꢁ10 mmol) were dissolved in deionized water
(
50 mL). The mixed solution was added into a Teflon-lined stainless
steel autoclave (100 mL), sealed, and hydrothermally treated at
10 8C for 24 h. After cooling to RT, the resulting product was fil-
1
tered, washed thoroughly with deionized water and ethanol, and
dried in an air-circulating oven at 608C for 12 h. For convenience,
the resulting CuMgAl-LDH samples with Cu content in ascending
order are denoted as CuMgAl-LDH-0, CuMgAl-LDH-1, CuMgAl-LDH-
2
and CuMgAl-LDH-3.
Synthesis of Ru Cu /MgAl-MMO
x
y
Figure 9. a) The catalytic performances of Ru1.0Cu0.5/MgAl-LDH in five consec-
utive recycles. b) The TEM image of Ru1.0Cu0.5/MgAl-LDH sample after five
cycles.
Introduction of Ru was achieved through the impregnation
method. The obtained CuMgAl-LDH precursors were added to
a RuCl ·3H O solution (40 mL, 6 mm), resulting in the Ru/Cu molar
3
2
ratios 1:0, 1:0.2, 1:0.5, and 1:1. After vigorous stirring for 2 h, the
ꢀ
precipitate was washed with deionized water until no Cl was de-
tected by AgNO , and then dried in an air-circulating oven at
3
Conclusions
6
08C for 12 h, yielding Ru/CuMgAl-LDH-0, Ru/CuMgAl-LDH-1,
Ru/CuMgAl-LDH-2 and Ru/CuMgAl-LDH-3. Subsequently, the ob-
tained samples underwent a reduction process in 10% H (H /N
:9,
In summary, the Ru Cu /MgAl-LDH catalysts with a good hydro-
x
y
philicity were fabricated by the co-reduction of Ru-impregnat-
ed CuMgAl-LDH, followed by LDH regeneration, and exhibit
a tunable catalytic behavior towards the selective hydrogena-
tion of benzene to cyclohexene. CO-TPD analysis demonstrates
the deposition of Cu atoms on the surface of the Ru NPs, with
a preferential occupancy on low-coordinated Ru as revealed by
CO-FTIR analysis. The resulting Cu-decorated Ru catalysts dem-
onstrate significantly improved catalytic selectivity for cyclo-
hexene, and the best catalytic behavior can be obtained over
2
2
=
2
1
ꢀ1
v/v) at 4008C for 3 h with a heating rate of 58Cmin , yielding Ru–
Cu bimetal NPs supported on MgO–Al O3 mixed metal oxides
2
(
MgAl-MMO), which are denoted as Ru Cu /MgAl-MMO with vari-
x y
ous x(Ru):y(Cu) ratios of 1:0, 1:0.2, 1:0.5 and 1:1, according to the
Ru/Cu ratio in the feed.
Synthesis of Ru Cu /MgAl-LDH
x
y
the Ru Cu /MgAl-LDH catalyst (cyclohexene yield of 44.0% at
1.0
0.5
The Ru Cu /MgAl-MMO samples underwent a rehydration process.
x y
1508C and 5.0 MPa) without the use of any additives. The re-
In detail, Ru Cu /MgAl-MMO (0.5 g) was added into water (50 mL)
x y
markable increase in cyclohexene selectivity can be attributed
and aged in a round-bottomed flask (100 mL) at room temperature
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ꢀ
1
ꢀ1
for 1 h in CO atmosphere with a flow rate of 20 mLmin . The ob-
sample was subjected to a stream of Ar with a rate of 40 mLmin
2
ꢀ1
tained product was filtered and dried under vacuum at 508C for
h to yield the corresponding Ru/MgAl-LDH, Ru Cu /MgAl-LDH,
and a temperature ramp of 108Cmin to perform the TPD. Fur-
thermore, based on the difference of the CO chemisorption capaci-
ty over Ru/MgAl-LDH and Ru-Cu/MgAl-LDH (owing to the deposi-
5
1.0 0.2
Ru Cu /MgAl-LDH and Ru Cu /MgAl-LDH catalysts.
1
.0
0.5
1.0
1.0
tion of Cu on the surface of Ru particle), the Cu coverage (q ) was
Cu
calculated by applying the following equation:
Catalytic evaluation
CRu ꢀ C
RuꢀCu
qCu
¼
ꢃ 100
ð1Þ
The selective hydrogenation of benzene was performed in
a Teflon-lined stainless-steel autoclave (100 mL) equipped with
a magnetic stirrer (see Figure S12). In a typical experiment, ben-
zene (10 mL), catalyst (0.1 g), and water (20 mL) were introduced
into the reactor. After being purged by low-pressure hydrogen
four times to remove the air, the reactor was heated to the reac-
tion temperature. Hydrogen was then charged into the reactor
until the desired pressure was reached. The products were collect-
ed from the sample valve of the reactor and analyzed by using
a gas chromatograph (GC-2014C) equipped with a flame ionization
detector (FID) and a PEG-20m capillary column (0.25 mm in diame-
ter, 30 m in length).
C
Ru
in which CRu denotes the chemisorption capacity of CO per gram
of the Ru/MgAl-LDH catalyst and C refers to the chemisorption
capacity per gram of the Cu-decorated Ru catalyst.
RuꢀCu
In situ FTIR absorption spectroscopy of CO was performed in
a quartz cell equipped with KBr windows allowing sample activa-
tion and successive measurements in the range 25–6008C, at
a pressures as low as 10 Pa. The samples (ꢁ50 mg) were pressed
into a disk and activated in the same cell for the measurement.
FTIR spectra were collected with a Nicolet 380 instrument spectro-
photometer with a resolution of 4 cm and accumulation of
4 sans. After hydrogen treatment at 4008C, the sample was
cooled to 1508C and scanned to get a background record below
ꢀ
4
ꢀ1
6
Characterization
ꢀ
4
a pressure of 2ꢁ10 Pa. Then, the sample was exposed to a CO
flow at 308C for another 120 min. Sample scanning for adsorbed
CO was conducted after the pressure was again reduced below 2ꢁ
XRD patterns of samples were obtained on a Rigaku XRD-6000 dif-
fractometer using CuKa radiation (l=0.154 nm) at 40 kV, 40 mA,
ꢀ1
ꢀ1
a scanning rate of 108 min , a step size of 0.028 s , and a 2q
angle ranging from 3 to 708. The morphology of the samples was
investigated by using a Zeiss Supra 55 scanning electron micro-
scope with an accelerating voltage of 20 kV. The actual loadings of
Ru and Cu were analyzed by using a Shimadzu ICPS-7500 induc-
tively coupled plasma emission spectrometer (ICP-ES). Prior to anal-
ysis, the sample (0.1 g) was added into 20 mL Teflon-lined stainless-
steel autoclave containing concentrated nitric acid (2 mL) and con-
centrated hydrochloric acid (6 mL), sealed and hydrothermally
treated at 1508C for 4 h. The obtained solution was diluted with
water to 100 mL. Low-temperature N2 adsorption–desorption ex-
periments were carried out by using the Quantachrome Autosorb-
ꢀ4
1
0
Pa.
Computational methods
The Ru (0001), Ru (1122), and Cu cluster-decorated Ru (0001)
facet were presented as a p(4ꢁ4) slab with a thickness of four Ru
layers, only the bottom atoms in the slab were constrained to their
crystal lattice positions. The neighboring slabs were separated in
the direction perpendicular to the surface by a vacuum region of
1
5 ꢂ.
1
1
C-VP instrument. The sample was outgassed prior to analysis at
008C for 12 h under 10 Pa vacuum. The total specific surface
First-principle calculations within the DFT framework were per-
formed within the CASTEP code in the Materials Studio 6.1 soft-
ꢀ4
[51]
area was evaluated from the multipoint BET method. The vacuum
FTIR spectra were recorded by using a Vector22 (Bruker) spectro-
photometer in the range 4000–400 cm with 2 cm of resolution
under 10 Pa vacuum. The standard KBr disk method (1 mg of
sample in 100 mg of KBr) was used. HRTEM observations were car-
ried out on a JEM-2100 transmission electron microscope. X-ray
photoelectron spectra (XPS) were recorded on a Thermo VG Esca-
lab 250 X-ray photoelectron spectrometer at a pressure of ~2ꢁ
ware package. The exchange-correlation potential was described
by the Perdew–Burke–Ernzerhof (PBE) generalized gradient ap-
ꢀ
1
ꢀ1
[52]
proach (GGA). The structure optimizations are based on the fol-
ꢀ
8
ꢀ5
lowing points: 1) an energy tolerance of 5.0ꢁ10 eV per atom;
2) a maximum force tolerance of 0.1 eV; 3) a maximum displace-
ꢀ3
ment tolerance of 5’10 ꢂ. To study the interactions between Ru
surface and cyclohexene, the adsorption energy is defined as Eads
and calculated based on the following equation:
ꢀ9
1
0
Pa with AlKa X-rays as the excitation source. The Cu K-edge
XANES measurements were performed at beamline 1W1B of Bei-
jing Synchrotron Radiation Facility (BSRF), Institute of High Energy
Physics (IHEP), Chinese Academy of Sciences (CAS).
E
ads ¼ EðFacetÞ þ EðCyclohexeneÞꢀEðFacet with CyclohexeneÞ
ð2Þ
H -TPR and CO-TPD analyses were conducted in a quartz tube re-
2
actor on a Micromeritics ChemiSorb 2720 with a thermal conduc-
tivity detector (TCD). In a typical H -TPR process, 100 mg of Acknowledgements
2
a sample placed in a quartz tube reactor was first degassed under
flowing argon at 4008C for 2 h and cooled to RT. Then, a gaseous
This work was supported by the 973 Program (Grant No.
011CBA00504), the National Natural Science Foundation of
mixture of H and Ar (1:9, v/v) was fed into the reactor at
2
2
ꢀ
1
4
0 mLmin . The temperature was raised to 7008C at a heating
1
China (NSFC), the Scientific Fund from Beijing Municipal Commis-
sion of Education (20111001002) and the Fundamental Research
Funds for the Central Universities (ZD 1303). We acknowledge the
support of BSRF (Beijing Synchrotron Radiation Facility) during
the XAFS measurements. M. W. particularly appreciates the finan-
ꢀ
rate of 108Cmin . For the CO-TPD process, 100 mg of the reduced
sample was first sealed and calcined in the reactor in the gaseous
mixture of CO and Ar (1:9, v/v) at 4008C for 1 h. Subsequently, the
catalyst was purged in Ar at 5008C for 30 min to remove excess
CO, then cooled down to 258C for readsorption of CO; finally, the
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cial aid from the China National Funds for Distinguished Young
Scientists of the NSFC.
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Received: November 4, 2014
Revised: December 23, 2014
Published online on && &&, 0000
&
ChemCatChem 0000, 00, 0 – 0
www.chemcatchem.org
10
ꢀ 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

FULL PAPERS
J. Liu, S. Xu, W. Bing, F. Wang, C. Li,
M. Wei,* D. G. Evans, X. Duan
&& – &&
Cu-Decorated Ru Catalysts Supported
on Layered Double Hydroxides for
Selective Benzene Hydrogenation to
Cyclohexene
Selective hydrogenation: Cu-decorated
Ru catalysts supported on hierarchical
MgAl-LDH (LDH=layered double hy-
droxide) were fabricated by a facile two-
step procedure and exhibit a superior
selectivity for hydrogenation of benzene
to cyclohexene without employment of
any additives. CO-TPD (TPD=tempera-
ture-programmed desorption) and in
situ CO-FTIR results demonstrate that
Cu atoms occupy the low-coordinated
Ru active sites, which possess a high
adsorption energy for cyclohexene as
revealed by DFT calculations.
ChemCatChem 0000, 00, 0 – 0
www.chemcatchem.org
11
ꢀ 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ