Surface Lewis acid-promoted copper-based nanocatalysts for highly efficient and chemoselective hydrogenation of citral to unsaturated allylic alcohols

Article abstract of DOI:10.1039/c5cy01084e

Chemoselective hydrogenation of α,β-unsaturated aldehydes or ketones to unsaturated alcohols (UAs) is one of the key processes for the production of various important intermediate chemicals. In the present work, well-dispersed ZnO-promoted supported copper nanocatalysts were generated from Cu-Zn-Al layered double hydroxide (CuZnAl-LDH) precursors for liquid-phase chemoselective hydrogenation of citral to allylic alcohols (geraniol and nerol isomers). A series of characterizations including XRD, TEM, STEM, XPS, H₂-TPR, and Py-IR demonstrated that the microstructure and catalytic performance of as-formed Cu-based nanocatalysts were significantly affected by the incorporation of Zn into catalyst precursors. It was found that the addition of more ZnO to catalysts could result in better metal dispersion and an increase in the surface Cu⁺/(Cu⁺ + Cu⁰) ratio and surface Lewis acid sites. In liquid-phase chemoselective hydrogenation of citral, a high selectivity toward allylic alcohols (>75%) at complete citral conversion was achieved successfully on as-formed non-noble-metal Cu-based nanocatalysts with a Cu/Zn molar ratio of 2:1 under mild reaction conditions (e.g. 80°C, 1.0 MPa). The high efficiency of the catalysts was attributed mainly to both the synergism between Cu⁰ and Cu⁺ species and the promotion of surface Lewis acid sites, thereby improving the dissociation of hydrogen and facilitating the adsorption of the citral molecule and the following activation of the carbonyl group during the citral hydrogenation.

Full text of DOI:10.1039/c5cy01084e

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Surface Lewis acid-promoted copper-based
nanocatalysts for highly efficient and
chemoselective hydrogenation of citral to
unsaturated allylic alcohols†
Cite this: DOI: 10.1039/c5cy01084e
*
Wei Li, Guoli Fan, Lan Yang and Feng Li
Chemoselective hydrogenation of α,β-unsaturated aldehydes or ketones to unsaturated alcohols (UAs) is
one of the key processes for the production of various important intermediate chemicals. In the present
work, well-dispersed ZnO-promoted supported copper nanocatalysts were generated from Cu–Zn–Al lay-
ered double hydroxide (CuZnAl-LDH) precursors for liquid-phase chemoselective hydrogenation of citral
to allylic alcohols (geraniol and nerol isomers). A series of characterizations including XRD, TEM, STEM,
XPS, H2-TPR, and Py-IR demonstrated that the microstructure and catalytic performance of as-formed
Cu-based nanocatalysts were significantly affected by the incorporation of Zn into catalyst precursors. It
was found that the addition of more ZnO to catalysts could result in better metal dispersion and an
increase in the surface Cu+/(Cu+ + Cu0) ratio and surface Lewis acid sites. In liquid-phase chemoselective
hydrogenation of citral, a high selectivity toward allylic alcohols (>75%) at complete citral conversion was
achieved successfully on as-formed non-noble-metal Cu-based nanocatalysts with a Cu/Zn molar ratio of
2 : 1 under mild reaction conditions (e.g. 80 °C, 1.0 MPa). The high efficiency of the catalysts was attributed
mainly to both the synergism between Cu0 and Cu+ species and the promotion of surface Lewis acid sites,
thereby improving the dissociation of hydrogen and facilitating the adsorption of the citral molecule and
the following activation of the carbonyl group during the citral hydrogenation.
Received 15th July 2015,
Accepted 9th November 2015
DOI: 10.1039/c5cy01084e
www.rsc.org/catalysis
conjugated CC bond and isolated CC bond). However,
selectively hydrogenating the carbonyl group in citral at a
1. Introduction
Nowadays, chemoselective hydrogenation of α,β-unsaturated
aldehydes or ketones to unsaturated alcohols (UAs) is one of
the key processes for the production of various important
intermediate chemicals.1–3 From the point of view of thermo-
dynamics and kinetics, the CO bond is hydrogenated more
difficultly than the conjugated CC bond in α,β-unsaturated
aldehydes or ketones due to greatly higher activation barriers
to hydrogenation of the CO functional group.4–8 Therefore,
the design and development of high-performance catalysts
that are chemoselective to UAs still constitutes a challenging
task. Typically, industrially important allylic alcohols (geraniol
and nerol isomers) are widely used for the production of
flavors, cosmetics, pharmaceuticals, and insect repellants in
several fine chemical industries and can be produced by
selectively hydrogenating the CO functional group in citral
molecule with three unsaturated sites (i.e. carbonyl group,
high conversion rate still remains quite difficult. Therefore,
the realization of improvement in selectivity toward the CO
band in the hydrogenation of citral bears a very important
realistic and research significance.
At present, as to liquid-phase hydrogenation of α,β-unsat-
urated aldehydes, many efforts have been devoted to develop-
ing a variety of supported group VIII noble metal catalysts
(e.g. Pt, Pd, Ru, Rh, Os and Ir).4,9 Usually, Pt, Ru and Os pre-
fer the hydrogenation of the CO bond, leading to the for-
mation of UAs, whereas Pd and Rh are more selective toward
the formation of saturated aldehydes.9–15 In order to further
improve the selectivity toward UAs, a second metal like Sn
and Fe, which can act as an electronic or structural promoter
in the catalytic systems, has always been incorporated into
catalysts.16–21 In spite of the high yields of the desired UA
products, unfortunately, both the high cost of noble metals
and the complicated modification of catalysts greatly limit
their large-scale industrial applications.22
State Key Laboratory of Chemical Resource Engineering, Beijing University of
Chemical Technology, P. O. BOX 98, Beijing, 100029, PR China.
E-mail: lifeng@mail.buct.edu.cn; Fax: +86 10 64425385; Tel: +86 10 64451226
† Electronic supplementary information (ESI) available: Additional experimental
results. See DOI: 10.1039/c5cy01084e
In this regard, some mono- and bimetallic heterogeneous
catalysts based on non-noble metals (e.g. Co and Ni) have
been explored to improve the activity and selectivity for UAs
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in the selective hydrogenation of α,β-unsaturated aldehydes
in the literature.9,23–25 The catalytic performance of these
catalysts mainly depends on the character of the active sites,
the metal–support interactions, and the nature of the sec-
ond metals. In most cases, Ni-based catalysts are less selec-
tive toward the hydrogenation of the CO bond26,27 com-
pared with noble metal catalysts. For example, it was
reported that supported Ni catalysts could attain a high
selectivity of about 83% toward citronellol by simultaneously
hydrogenating CO and conjugated CC bonds in citral.28
In contrast, Co-based catalysts, such as Co/SiO2 catalysts9,29,30
and CoB alloy catalysts,26,31 showed much higher selectivity
toward allylic alcohols in citral hydrogenation. For instance,
a high selectivity of 90% toward allylic alcohols was attained
over the Co/SiO2 catalyst prepared from a water-in-oil micro-
emulsion at about 30% citral conversion.30 Nonetheless, Co-
based catalysts have some drawbacks such as complex prepa-
ration procedure and environmentally unfriendly preparation
processes. In addition, supported or unsupported Cu-based
catalysts are found to be active for selective hydrogenation of
α,β-unsaturated aldehydes (e.g. cinnamaldehyde and croton-
aldehyde) to UAs.32–35 However, reports about Cu-based cata-
lysts for the selective hydrogenation of citral to allylic alcohols
are still scarce. Therefore, regarding the chemoselective hydro-
genation of citral to produce allylic alcohols, the design of
new catalytic technology based on low-cost and highly efficient
non-noble-metal catalysts has become an important issue.
In this study, with the aim of developing new highly effi-
cient and cheap supported copper catalysts for the
chemoselective hydrogenation of citral to geraniol and nerol,
a series of ZnO-modified copper-based nanocatalysts were
generated from Cu–Zn–Al layered double hydroxide (CuZnAl-
by our group.41 Typically, CuIJNO3)2·6H2O, ZnIJNO3)2·6H2O and
AlIJNO3)3·9H2O with an [M2+]/[M3+] molar ratio of 3 : 1 and a
[Zn2+]/[Al3+] molar ratio of x (x = 0, 0.5, 1 and 1.5) (solution A)
were dissolved in 100 mL of deionized water. NaOH and
Na2CO3 ([OH−] = 1.6[M2+ + M3+] and [CO32−] = 2[M3+]) (solu-
tion B) were dissolved in another 100 mL of deionized
water. Subsequently, the prepared A and B solutions were
simultaneously added to a colloid mill with the rotor speed
set at 3000 rpm and mixed for 3 min. The resulting suspen-
sion was aged at 40 °C for 12 h and centrifuged and washed
with deionized water until pH = 7.0. The resulting LDH
solid was dried at 70 °C overnight. The LDH precursor was
calcined in static air at 600 °C for 6 h and denoted as
C-LDH-x. The obtained C-LDH-x samples were placed in a
tube furnace and reduced in 10% v/v H2/Ar atmosphere at
300 °C for 1 h at a ramping rate of 2 °C min−1. The
obtained reduced catalysts were denoted as CuZn-x. Finally,
after cooling to room temperature in 10% H2/Ar atmo-
sphere, the reduced catalysts were removed rapidly and
immediately sealed in a sample tube filled with N2 before
characterization and reactions.
2.2. Characterization
Powder X-ray diffraction (XRD) patterns of samples were
obtained using a Shimadzu XRD-6000 diffractometer with
graphite-filtered Cu Kα radiation (λ = 0.15418 nm).
Transmission electron microscopy (TEM) and high-
resolution TEM (HRTEM) images were recorded on a JEOL
2100 instrument. High-angle annular dark-field scanning
TEM-energy-dispersive X-ray spectroscopy (HAADF-STEM)
photographs were recorded on a JEOL 2010F instrument
combined with an X-ray energy dispersive spectroscopy (EDS)
system.
LDH) precursors. LDHs with the general formula
3+
[M2+
M
xIJOH)2]IJAn−)x/n·mH2O are known as a class of two-
1−x
dimensional multimetal clays.36,37 Therefore, a variety of
supported metal catalysts with tunable particle size may be
formed through structural transformation of LDHs with dif-
ferent compositions.38–40 The effect of the incorporation of
zinc into catalyst precursors on the catalytic hydrogenation
performance was investigated comprehensively. The results
demonstrated that a high selectivity of about 75% toward
allylic alcohols at almost complete conversion was success-
fully achieved over as-formed copper-based nanocatalyst with
a Cu/Zn molar ratio of 2 : 1 under mild reaction conditions
(e.g. 80 °C, 1.0 MPa), mainly due to both the synergism
between Cu0 and Cu+ species and the promotion of surface
Lewis acid sites. To the best of our knowledge, such a highly
efficient surface Lewis acid-promoted non-noble metal
copper-based catalyst for the chemoselective hydrogenation
of citral to allylic alcohols has never been reported to date.
Low-temperature N2 adsorption–desorption isotherms of
the samples were obtained using a Micromeritics ASAP 2020
sorptometer apparatus. The total specific surface area was
evaluated with the multipoint Brunauer–Emmett–Teller (BET)
method.
X-ray photoelectron spectra (XPS) were recorded on a
Thermo VG ESCALAB 250 X-ray photoelectron spectrometer
using monochromatic Al Kα X-ray as the excitation source
(1486.6 eV). X-ray induced Auger spectra (XAES) were carried
out on a PHI Quantera SXM using Al Kα X-ray as the excita-
tion source.
Hydrogen temperature-programmed reduction (H2-TPR)
was performed using a Thermo Fisher TPDRO-1100 chemi-
cal adsorption instrument with a thermal conductivity detec-
tor (TCD). The sample (30 mg) was placed in a quartz tube
reactor and degassed at 200 °C for 2 h under argon flow
(30 mL min−1). Then TPR was conducted in a stream of
10% v/v H2/Ar (30 mL min−1) with a heating rate of 5 °C min−1
up to 800 °C.
2. Experimental
2.1. Synthesis of supported copper nanocatalysts
The active copper surface area and dispersion of Cu0 parti-
cles were determined according to H2–N2O titration using
a Micromeritics ChemiSorb 2720 instrument. Firstly, the
Catalyst precursors, CuZnAl-LDHs, were prepared by a sepa-
rate nucleation and aging step method previously developed
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calcined sample underwent a H2-TPR process in 10% H2/Ar 3. Results and discussion
mixture from 50 to 300 °C and the temperature was held
until there was no more H2 consumption. After cooling down,
3.1. Characterization of catalysts
Fig. S1† shows the XRD patterns of CuZnAl-LDH precursors
with different Zn/Al molar ratios, which can be indexed to a
rhombohedral structure with a series of characteristic diffrac-
tions of hydrotalcite-like materials corresponding to the
(003), (006), (012), (015), (018), (110) and (113) planes. As
shown in Fig. 1A, the XRD patterns of all calcined CuZnAl-
LDH samples present the characteristic diffractions of CuO
(JCPDS no. 05-0661), and no diffractions associated with
crystalline Al2O3 are detected due to the highly dispersed
nature of Al-containing phases as well as the presence of an
amorphous phase. With the gradual introduction of Zn, the
diffractions of CuO are weakened and broadened progres-
sively, indicative of the decreased crystallite size of CuO parti-
cles. Meanwhile, the ZnO phase (JCPDS no. 36-1451) also
begins to appear. It is noted from Fig. 1B that the XRD pat-
terns of reduced CuZn-x samples present a relatively sharp
(111) diffraction at 2θ of 43.3°, along with two weak (200)
and (220) peaks at 50.4° and 74.1°, which is characteristic of
face-centered cubic (fcc) metallic copper phase (JCPDS no.
04-0836). With increasing Zn/Al molar ratio, the average crys-
tallite size of metallic copper particles estimated by the
Scherrer formula according to the (111) plane decreases grad-
ually from about 14.3 to 7.6 nm (Table 1), which can be
attributed to the dispersing effect of ZnO formed as a physi-
cal spacer between copper species in samples. Moreover, the
change in the crystallite size of copper particles is in good
agreement with the change in the degree of copper disper-
sion. It demonstrates that a larger amount of ZnO phase can
prevent the growth and agglomeration of copper particles,
thus leading to a higher copper dispersion than that of other
samples.
the gas was switched to 10% N2O/N2, and the sample was oxi-
dized at a flow rate of 40 ml min−1 at 60 °C for 1 h, followed
by Ar purging and cooling the sample bed down to room tem-
perature. Finally, H2-TPR was carried out again with a gas
mixture of 10% H2/Ar to 300 °C. Cu dispersion was calculated
by dividing the amount of surface copper sites by the total
number of supported Cu atoms, and copper surface area was
calculated by assuming a spherical shape of the copper metal
particles and a surface concentration of 1.47 × 1019 copper
atoms per m2.
In situ infrared spectroscopy of adsorbed pyridine (Py-IR)
was carried out on a Thermo Nicolet 380 spectrometer. The
calcined sample (30 mg) was pressed into a self-supporting
wafer. The wafer was placed in an evacuable IR cell with CaF2
windows, reduced with 10% H2/Ar mixture from 25 to 300 °C
and held at this temperature for 1 h. After cooling to room
temperature, the wafers were evacuated at 200 °C for 2 h
under argon flow and then the sample wafer was cooled
down to room temperature again. Finally, pyridine was intro-
duced and balanced at this temperature for 1 h, and the sam-
ple was heated stepwise under vacuum to 300 °C. The Py-IR
spectra were recorded at 25, 100, 150, 200, 250, and 300 °C.
Ammonia temperature-programmed desorption (NH3-
TPD) experiments were carried out using a Thermo Fisher
TPDRO-1100 chemical adsorption instrument. The calcined
sample (200 mg) was placed in a quartz tube reactor and
reduced under the same conditions as that for preparing the
catalyst. After cooling to room temperature, the sample was
degassed by heating under a He environment at 450 °C for
1 h followed by treatment with flowing ammonia gas mixture
(5% NH3 in He) for 1.5 h at 100 °C. Chemisorbed NH3 was
desorbed by heating from the saturation temperature up to
900 °C at a rate of 10 °C min−1.
HRTEM analyses were conducted to investigate the micro-
structure of copper nanoparticles in the CuZn-x samples
(Fig. 2). In the Zn-free sample, several black aggregated
2.3. Selective hydrogenation of citral
The liquid phase hydrogenation of citral was carried out in a
200 mL stainless steel autoclave. At the beginning of each
experiment, the catalyst (0.1 g) was first added to 100 mL of
ethanol, and subsequently the citral (58.5 mM) was injected
into the ethanol. Afterward, the air was flushed out of the
reactor with nitrogen, and the reactor was sealed and flushed
with 2.0 MPa H2 ten times. The autoclave was placed in a
heating jacket and preheated to a reaction temperature of
80 °C. Then H2 was fed into the reactor to a total pressure of
1.0 MPa and the reaction was initiated by starting stirring at
a speed of 1000 rpm. The liquid products were analyzed
quantitatively by gas chromatography using an Agilent
GC-7890B instrument equipped with a DB-WAX capillary
column (30.0 m × 250 μm × 0.25 μm) using a flame ioniza-
tion detector. The injector temperature was 250 °C, and the
detector column temperature was increased from 100 to
Fig. 1 XRD patterns of C-LDH-x (A) and reduced CuZn-x (B) samples
with different Zn/Al molar ratios. (a) x = 0, (b) x = 0.5, (c) x = 1, (d) x = 1.5.
150 °C with a ramp rate of 5 °C min−1
.
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Table 1 The textual and structural data of different samples
Contenta (wt%)
b
c
d
D111
SBET
SCu
Dise
(%)
Cu+/(Cu+ +
Cu0) ratiof
Acid sitesg
SLA sitesh
(mmol g−1
)
Sample
Cu
Zn
(nm)
(m2 g−1
)
(m2 g−1
)
(mmol g−1
)
CuZn-0
CuZn-0.5
CuZn-1
67.6
48.2
44.2
33.2
0
14.3
10.9
7.8
30
46
50
58
21.9
21.7
19.2
17.2
7.3
9.4
11.1
12.7
0.29
0.59
0.70
0.74
0.136
0.409
0.446
0.514
0.033
0.079
0.083
0.101
10.8
22.5
32.9
CuZn-1.5
7.6
a
b
c
Determined by ICP-AES. Crystallite size of metallic Cu based on XRD line broadening of the (111) plane. BET specific surface area.
d
e
f
Metallic copper surface area determined by N2O titration. Copper dispersion degree. Intensity ratio between Cu+ and (Cu+ + Cu0) by
g
h
deconvolution of Cu LMM XAES spectra. Density of acid sites determined by NH3-TPD at T = 350–900 °C. Density of strong LA sites
determined by NH3-TPD at T = 700–900 °C.
irregular copper clusters are poorly dispersed on the support,
although copper particles cannot be observed clearly due to
the low particle/support contrast. With the introduction of
Zn, small copper nanoparticles without agglomeration can be
observed in CuZn-1 and CuZn-1.5 samples. Further, the
HRTEM image of a CuZn-1 sample indicates the close inter-
face between the copper particle and the ZnO phase as well
as the adjacent amorphous Al2O3 zone. The above result dem-
onstrates that the nanometric Cu particles may be strongly
anchored on the support matrix composed of the ZnO phase
and the amorphous Al2O3 component, accounting for the
presence of the interactions between them.
In order to gain further insight into the microstructure of
CuZn-x samples, HAADF-STEM observation of a represen-
tative CuZn-1 sample was performed (Fig. 3). The bright
spots in Fig. 3a correspond to small copper particles. Ele-
mental mapping analysis shows the homogeneous distribu-
tion of Cu, Zn, Al and O species in the sample. The pres-
ence and spatial positions of the Cu, Zn, Al and O elements
within one individual particle were further verified by line-
scan spectra (Fig. 3g). It is noted that the Cu and Zn lines
show a similar Gaussian distribution along the particle,
demonstrating the close contact of the Cu particle with the
ZnO phase over the Al-containing matrix. In addition, the
slightly higher intensity of zinc than that of copper within
the particle implies that Cu nanoparticles may be surrounded
or partly covered by the ZnO component, thereby leading to
the interactions between them.
3.2. Redox behaviors
H2-TPR characterization was carried out to investigate the
redox properties of C-LDH-x samples. As shown in Fig. 4, cop-
per species in different samples are reduced at different
Fig. 3 HAADF-STEM (a) and high-magnification STEM (b) images of
CuZn-1 sample with the EDS mapping of Cu (c), Zn (d), Al (e) and O (f);
EDS line spectra (g) of Cu, Zn, Al and O along the red line shown in (b).
Fig. 2 HRTEM images of CuZn-0 (a), CuZn-0.5 (b), CuZn-1 (c and e)
and CuZn-1.5 (d).
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temperatures. The C-LDH-0 sample exhibits a broad reduc-
tion peak in the temperature range of 175–350 °C, which is
mainly assigned to the reduction of bulk CuO particles.42,43
With increasing Zn/Al molar ratio from 0 to 1.0, the reduc-
tion peak shifts to lower temperatures. Meanwhile, with the
Zn/Al molar ratio from 0 to 1.0, a shoulder peak at the lower
temperature, which accounts for about 16.3%, 31.5% and
9.9% of the total reduction peak area, is mainly associated
with the reduction of isolated highly dispersed CuO particles
on the surface. Such a decrease in reduction temperature
demonstrates the enhanced reducibility of catalysts due to
the formation of smaller CuO particles with the incorporation
of ZnO. As the Zn content further increases, in turn, the
reduction peak shifts to the higher temperature, which
should be related to the presence of strong interactions
between Cu nanoparticles and the support, thus leading to
reduced reducibility of Cu2+ species and depressed sintering
of Cu nanoparticles.
Fig. 5 Cu 2p XPS (A) and Cu LMM XAES (B) spectra of reduced CuZn-x
samples.
the broad and asymmetrical Cu LMM XAES spectra clearly
demonstrate the presence of one more component. The
kinetic energy at about 916.3 eV corresponds to Cu+ species
and about 918.6 eV to Cu0 species.46 It is noted from Table 1
that the surface Cu+/(Cu+ + Cu0) intensity ratio increases pro-
gressively with increasing Zn content in samples, suggestive
of the formation of more Cu+–O–Zn-like structures at the
interface between Cu species and the adjacent ZnO phase
due to the enhanced interactions between Cu–Zn species.47,48
Similarly, Cu+ species are also found to exist in other
previously reported copper-based catalysts supported on
oxide supports49–53 due to the interaction between Cu species
and supports.
3.3. Surface chemical states and compositions
It is known that the catalytic performance of catalysts is
greatly related to their surface chemical states and composi-
tions. The surface/near-surface chemical states of reduced
CuZn-x samples were analyzed by XPS measurements
(Fig. 5A). In all cases, the Cu 2p 3/2 peak at ca. 932.6 eV is
related to metallic copper and/or CuIJI) species in the low
valence states,44,45 which are formed during the reduction
process. The absence of Cu 2p 3/2 XPS signals of Cu2+ at
933–935 eV and the shake-up satellite peak of Cu2+ between
940 and 945 eV further confirms the reduction of copper
species.44 Given that the Cu 2p 3/2 and Cu 2p 1/2 binding
energies for Cu+ and Cu0 species are nearly the same, XAES
analysis was conducted to discriminate the chemical states of
surface copper species after reduction. As shown in Fig. 5B,
3.4. Surface acidity
As for CuZn-x samples, surface acid sites, such as Lewis acid
(LA) and Brønsted acid (BA) sites, could be associated with
the ZnO component and surface hydroxyl groups. Py-IR spec-
tra were obtained to probe the type and strength of the
surface acid sites of samples. As shown in Fig. 6, reduced
CuZn-x samples present several adsorption bands for pyri-
dine adsorbed on different acid sites, including LA site-
bonded pyridine (LA-Py, 1613, 1505 and 1457 cm−1) and BA
site-bonded pyridine (BA-Py, 1641 and 1542 cm−1).54,55
Among them, the band at 1613 cm−1 is assigned to the
adsorption of pyridine on strong LA sites.56,57 Additionally,
the bands in the ranges of 1440–1447 and 1590–1600 cm−1
are related to the adsorption of pyridine by H-bonding
(HA-Py), while the band at about 1584 cm−1 is associated with
physically adsorbed pyridine (PA-Py).58 Noticeably, the inten-
sities of the bands for LA-Py at 1613 and 1457 cm−1 are
increased gradually with increasing Zn content due to the
increasing amount of Zn–O pairs on the surface, thereby
generating more LA sites. In the case of CuZn-1.5, strong
adsorption of pyridine at BA sites can be clearly observed. In
addition, the stronger adsorption of pyridine by H-bonding is
Fig. 4 H2-TPR profiles of C-LDH-x samples: (a) x = 0, (b) x = 0.5, (c) x
= 1, (d) x = 1.5.
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weak acid sites, surface moderate strength LA sites, and sur-
face strong LA (SLA) sites, respectively, according to the above
Py-IR results and the literature.59–61 Based on a quantitative
analysis on the areas of desorption on CuZn-x samples
(Table 1), the densities of total LA sites and SLA sites in the
temperature ranges of 350–900 °C and 700–900 °C increase
with increasing Zn content. The aforementioned results dem-
onstrate that the ZnO component introduced to catalysts can
improve the strength and/or amount of surface LA sites,
which may play an important role in citral hydrogenation.
3.5. Selective hydrogenation of citral
Scheme 1 shows the main reaction pathways that occur in
the course of citral hydrogenation. Generally, the first step is
the reduction of either the CO functional group or the con-
jugated CC functional group in citral to produce geraniol/
nerol isomers or citronellal, respectively. Occasionally, the
isolated CC bond can be solely reduced to produce 3,7-
dimethyl-2-octenal. Consecutive hydrogenation of the above
citronellal and geraniol–nerol leads to citronellol and finally
to 3,7-dimethyloctanol. It is known that highly chemoselective
hydrogenation toward geraniol and nerol usually is difficult
because the hydrogenation of the CC group is thermody-
namically favorable over that of the CO group by ∼35 kJ
mol−1 and the reactivity of the CC group is higher than that
of the CO group in the hydrogenation.62
Fig. 8 displays the dependence of the compositions of the
main products (allylic alcohols, citronellal, and citronellol)
on time on stream over different CuZn-x catalysts for the
hydrogenation of citral performed in ethanol solvent at a
hydrogen partial pressure of 1.0 MPa and 80 °C. Under the
present reaction conditions, the obtained products are
mainly allylic alcohols, citronellal and citronellol. The
amount of complete hydrogenation product is negligible. In
addition, the reduction of the isolated CC bond does not
take place over all catalysts, indicating that the hydrogena-
tion of citral is exclusively performed on the CO and conju-
gated CC double bonds. As shown in Fig. 8, in the initial
reaction period, the concentration of citral decreases linearly
with the reaction time over each catalyst. In the case of the
CuZn-0 catalyst, citral is rapidly converted with a complete
conversion being reached at the reaction time of 150 min.
With increasing Zn content, CuZn-x catalysts present reduced
activity for the reaction. Based on the initial reaction rates of
citral hydrogenation (Table 2), the catalytic activity decreases
in the order CuZn-0 > CuZn-0.5 > CuZn-1 > CuZn-1.5. Such
activity evolutions may be related to the number of accessible
copper atoms within the series.
Fig. 6 Py-IR spectra of reduced CuZn-x samples. (a) x = 0, (b) x = 0.5,
(c) x = 1, (d) x = 1.5.
observed in CuZn-0.5 and CuZn-1 samples. The temperature-
dependent Py-IR spectra of samples demonstrate that the
bands of HA-Py, PA-Py, and/or BA-Py are remarkably reduced
with increasing desorption temperature and almost vanish
above 250 °C (Fig. S2†). As for the CuZn-1 and CuZn-1.5 sam-
ples, the remaining reduced bands at 1613 and 1457 cm−1 at
the desorption temperature of 300 °C are unambiguously
attributed to the LA sites, indicative of the presence of LA
sites with high strength.
Further, NH3-TPD was performed to evaluate the surface
acidity of CuZn-x samples (Fig. 7). Notably, there are three
main temperature ranges for NH3 desorption at approxi-
mately 50–350 °C (I), 350–700 °C (II), and 700–900 °C (III),
which are associated with desorption of NH3 from different
Note that in each case, the concentrations of citronellal
and allylic alcohols increase gradually with the reaction time
and remain relatively high during the initial reaction period,
demonstrating the competitive hydrogenation of the conju-
gated CC bond and CO group in the citral molecule. In
the case of the CuZn-0 catalyst, the citral is selectively
converted to citronellal at the initial reaction period. The
Fig. 7 NH3-TPD profiles of reduced CuZn-x samples. (a) x = 0, (b) x =
0.5, (c) x = 1, (d) x = 1.5.
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Scheme 1 Main reaction pathways for citral hydrogenation.
hydrogenation of the CO group in citral to produce gera-
niol and nerol is ongoing in the whole reaction time, but the
selectivity for allylic alcohols remains low (less than 35%). It
signifies that the activation of the conjugated CC bond in
citral is favored over that of the CO one. Correspondingly,
the concentration of citronellal is increased much faster than
that of allylic alcohols and reaches the maximum of about
40% at 70% conversion. At the same time, the consecutive
conversion of citronellal and allylic alcohols into citronellol
is also observed with the reaction time. Especially, the reduc-
tion of the CO bond in citronellal is much more rapid than
that of the conjugated CC bond in allylic alcohols with
prolonged reaction time. Finally, the yield of citronellol
reaches about 64% at 100% citral conversion. The above
results demonstrate that the CC/CO adsorption competi-
tion of citral mainly favors the CC bond on the Zn-free
CuZn-0 catalyst.
Notably, in the cases of the ZnO-modified supported cop-
per catalysts, the hydrogenation of the conjugated CO bond
in citral to produce allylic alcohols is always dominant in the
whole reaction process, in spite of the gradually reduced
activity with increasing Zn content. Correspondingly, the
selectivity for allylic alcohols is continuously increased with
the reaction time, whereas the selectivity for citronellal
reaches a maximum value at a certain reaction time and then
begins to drop slowly. It demonstrates that the introduction
of the ZnO component into catalysts facilitates the formation
of geraniol and nerol, suggesting that the adsorption of the
carbonyl bond is favored over that of the olefinic bond in cit-
ral, thereby leading to the improved activation of the CO
bond. Especially, a much higher selectivity toward allylic alco-
hols (>75.0%) over the CuZn-1 catalyst is attained at almost
100% conversion in comparison with other catalysts (Table 2).
In this sample, the allylic alcohol products become the larg-
est quantity of products formed and become constant in our
investigated time scale. It suggests that the adsorption of
allylic alcohols on the catalyst surface is weaker than that of
citral and citronellal during the reaction. Further, we carried
out the hydrogenation of citronellal with a single CO bond
and geraniol with a single CC bond over CuZn-1 under the
same conditions (Fig. S3†). It is found that the conversion of
citral and citronellal increases rapidly with the reaction
time. However, the conversion of geraniol increases slowly
and only reached 9.0% within 180 min. It demonstrates that
compared with geraniol, citral and citronellal can be hydro-
genated preferentially.2,9,63,64 Due to the adsorption and
electrostatic interaction between the reactants and the cata-
lysts, the conjugated CC–CO group in citral can be
delocalized as the Cδ+–Cδ−–Cδ+–Oδ− group. Correspondingly,
the adsorption strength for reactants on CuZn-x catalysts
should decrease in the order citral > citronellal ≫ geraniol/
nerol, thus leading to the greatly reduced hydrogenation
rates for geraniol and nerol. Traditionally, supported copper
on inert oxides are indicated to be unselective for the reduc-
tion of α,β-unsaturated compounds.33,65 In this context,
such a notably high selectivity for allylic alcohols even at
high conversion levels can be associated with the modifica-
tion of the electronic structure of the active metal species
and the surface acid–base nature of the catalysts. It is
assumed that with increasing Zn content, the more favor-
able adsorption of citral molecule on the catalyst surface is
probably correlated with large amounts of ZnO component
and Cu+ species, which should be the origin of enhanced
selectivity for geraniol and nerol.
In order to compare the intrinsic nature of metallic copper
sites on CuZn-x catalysts, the initial turnover frequency (TOF)
based on the moles of citral converted or allylic alcohols pro-
duced per mole of metallic copper in the initial 15 min was
calculated. As shown in Table 2, the TOF values for the
conversion of citral decrease gradually with increasing Zn
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Fig. 8 Catalytic performance in citral hydrogenation as a function of the reaction time over CuZn-0 (A), CuZn-0.5 (B), CuZn-1 (C), and CuZn-1.5 (D).
Table 2 Catalytic results for citral hydrogenationa
c
d
Rateb (−μ1mol
Conv.
(%)
TOFcitral
TOFg+n
(min−1
Selectivity (%)
Allylic alcohols
Catalysts
s−1 gCu
)
(min−1
)
)
Citronellal
Citronellol
Otherse
CuZn-0
CuZn-0.5
CuZn-1
22.0
16.2
15.8
15.6
1.38
0.92
0.90
0.76
0.35
0.42
0.53
0.36
100
100
99.8
79.6
34.4
61.5
75.1
63.2
0.4
2.3
5.8
64.1
35.4
18.0
15.5
1.1
0.8
1.1
0.5
CuZn-1.5
20.8
a
b
c
Reaction temperature: 80 °C; H2 pressure: 1.0 MPa; reaction time: 3.5 h. Initial reaction rate of citral hydrogenation. Calculated based on
d
the moles of citral converted per mole surface metallic copper in the initial 15 min. Calculated based on the moles of geraniol and nerol
produced per mole surface metallic copper in the initial 15 min. e Cyclization by-products.
content, indicative of the decline in the catalytic activity. In
addition, compared with other CuZn-x catalysts, CuZn-1
exhibits a higher TOF value for the formation of geraniol and
nerol, demonstrating that the appropriate incorporation of
ZnO into catalysts can greatly promote the selective hydroge-
nation towards the CO bond in citral. To the best of our
knowledge, the selective hydrogenation of citral using copper
as the single catalytically active component has not been
reported to date. The present TOF values over CuZn-x cata-
lysts are comparable to those over other Cu-based catalysts
employed in the hydrogenation of α,β-unsaturated aldehydes
or even to those over some noble metal catalysts.65–67
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In order to study the hydrogenation behaviors of E-citral
and Z-citral isomers, the compositions of citral and allylic
alcohol isomers in the reaction mixture were further analyzed
(Fig. S4†). Notably, the E-/Z- ratio of citral concentrations
exhibits a sustained decrease with reaction time, and the con-
version rate of E-citral is slightly faster than that of Z-citral.
Similarly, the formation rate of geraniol is slightly faster than
that of nerol over the reaction time. Furthermore, the initial
percentage of E-citral in citral isomers (about 60.1%) is quite
similar to the percentage of geraniol (58.7%) at the end of
the reaction, indicating that the conversion rates of E-citral
and Z-citral are equal to the respective formation rates of
geranial and nerol. As a result, the trans to cis isomerization
between E- and Z-citral or geraniol and nerol do not occur in
the present reaction system, in good agreement with the pre-
viously reported results by Vannice et al.68 On the other
hand, the TOF dependence for citral isomers and allylic alco-
hol isomers on their concentration reveals that the E-citral
and Z-citral exhibit near first- and near zero-order kinetics,
respectively, which are similar to those for the E- and
Z-isomers of allylic alcohols (Fig. S5†). The above results
demonstrate that E-citral is more active than Z-citral due to
the steric hindrance resulting from the stereochemistry of
the double bond.
As for hydrogenation catalyzed by metallic copper, accessi-
ble catalytically active metallic copper sites always play a vital
role in governing the activity of catalysts because Cu0 species
are mainly responsible for the dissociation of H2 in hydroge-
nation. In the present catalyst system, the copper surface area
of CuZn-x catalysts decreases gradually with increasing Zn
content, accounting for the reduced hydrogenation activity.
However, the difference in the copper surface area alone
cannot explain the different reaction pathways observed over
catalysts.
results, the incorporation of ZnO brings out a quite differ-
ent surface acid nature of CuZn-x catalysts. According to the
literature, theoretical predictions supported by experimental
results show that the selectivity for the CO group versus
CC bond hydrogenation in citral is determined by the
activation of the CO group to a large extent. It was
reported that the activation of the carbonyl group could be
achieved by improving the metallic catalyst with an ionic
compound that has a positively charged cationic site, which
is able to accept electrons and act as an LA site.20,69 Com-
pared with the Zn-free CuZn-0 sample, more LA sites includ-
ing SLA sites appear on the surface of Zn-containing cata-
lysts, which originates from surface Zn–O pairs as well as
enhanced ZnO–metal interactions. These LA sites further
facilitate the anchoring of the carbonyl double bond in cit-
ral, which makes a greater contribution to the improvement
of the selectivity toward allylic alcohols.10,70 Therefore, the
surface acid nature of as-formed CuZn-x catalysts should
play a crucial role in affecting the selectivity for allylic alco-
hols in citral hydrogenation.
Based on the above experimental results, a plausible
mechanism for the citral hydrogenation over CuZn-x catalysts
is proposed (Scheme 2). Firstly, well-dispersed Cu0 species
act as the catalytically active centers for H2 dissociation in cit-
ral hydrogenation, which are directly responsible for the
activity of catalysts. Secondly, the Cu+ species, which mainly
locate at the interface between Cu nanoparticles and ZnO,
function as electrophilic sites to polarize the CO bond by
the lone electron pair in oxygen. Thirdly, surface LA sites are
formed with the introduction of Zn into catalysts. Such LA
sites in close contact with metallic Cu0 sites can act as
electron acceptors and easily form a complex with the CO
group through the interaction with one of the oxygen atom
lone pairs in the nodal plane of the CO π bond, which
facilitates the nucleophilic attack of H to the carbon bearing
a positive charge.71 Therefore, an enhanced adsorption capac-
ity of CO bond in citral can be attained on Zn-containing
catalysts, whereas the CC adsorption can be effectively
inhibited at the same time. After the selective hydrogenation
of the CO group, citral transforms gradually to allylic alco-
hols as the main products with a high yield. Excess amounts
of ZnO species, however, can block the surface of active Cu
nanoparticles, thus inducing a more negative effect on the
activity. As a result, the incorporation of ZnO into catalysts
enhances the dispersion of Cu species and creates more sur-
face LA sites. The superior selectivity toward allylic alcohols
over the CuZn-1 catalyst should be attributable to both the
reasonable balance of Cu0–Cu+ species and the promoting
effect of surface LA sites.
On the one hand, it has been reported that the Cu+ sites
on the catalyst surface can function as electrophilic centers
to polarize the CO bond via the lone electron pair in
oxygen. Such an electronic configuration may facilitate the
activation of the electronic doublet of oxygen in the carbonyl
group to some extent,46,53,65 thus improving the hydrogena-
tion to the carbonyl group. The above Cu LMM XAES results
of CuZn-x catalysts reveal an increase in the surface
Cu+/(Cu+ + Cu0) ratio with increasing Zn content in spite of
the decreasing copper surface area. Therefore, the increas-
ing Cu+/(Cu+ + Cu0) ratio may be partially responsible for the
significant promotion to selectivity for allylic alcohols
through the polarization of the CO bond in citral by Cu+
species. Simultaneously, a higher Cu+/(Cu+ + Cu0) ratio,
however, can also facilitate the activation of the CO bond
in citronellal and thus the formation of more citronellol. It
demonstrates that the balance between Cu0 and Cu+ species
is one of the key factors affecting the selectivity for allylic
alcohols. On the other hand, the different reaction pathways
over CuZn-x catalysts can also be related to the interphase
interactions between copper species and the ZnO compo-
nent. Based on the above Py-IR spectra and NH3-TPD
In addition, for heterogeneous catalysts, catalytic reusabil-
ity in the catalytic reaction is an important feature for practi-
cal applications. We investigated the reusability of CuZn-1
catalyst. After each cycling test, the reactant mixture was
centrifuged, and then the collected catalyst was washed with
ethanol and deionized water and finally dried overnight. As
shown in Fig. 9, the CuZn-1 catalyst still maintains its
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Scheme 2 Proposed possible mechanism of citral hydrogenation to allylic alcohols over as-formed CuZn-x catalysts.
4. Conclusions
In summary, the catalytic behaviors of liquid-phase selective
hydrogenation of citral to UAs over ZnO-modified supported
copper nanocatalysts were investigated. A series of characteri-
zation techniques including XRD, STEM, XAES, Py-IR spectra
and NH3-TPD revealed that the incorporation of ZnO helped
to disperse the Cu nanoparticles and enhance the metal–sup-
port interactions, resulting in the increase in the surface Cu+/
(Cu0 + Cu+) ratio and the amount of surface LA sites. A dra-
matic improvement in catalytic selectivity toward allylic alco-
hols was observed after the introduction of ZnO. In particu-
lar, Cu-based catalysts with an optimized Cu/Zn molar ratio
of 2 : 1 showed an exceptional catalytic hydrogenation perfor-
mance with 100% conversion and 75% selectivity toward
allylic alcohols under mild reaction conditions (80 °C, 1.0
MPa). The excellent efficiency of the CuZn-1 catalyst could be
associated with both the synergism between Cu0–Cu+ species
and the promotion of surface LA sites. The Cu0 species as cat-
alytically active centers provided the sites for H2 dissociation,
while the Cu+ species and surface LA sites were able to
adsorb the citral molecule and thus activate the CO bond,
greatly improving the selectivity toward allylic alcohols. Com-
pared with noble metal catalysts, as-synthesized highly effi-
cient, cheap and stable copper-based nanocatalysts exhibited
a comparative catalytic performance in selective citral hydro-
genation to allylic alcohols and would be regarded as an
alternative for further applications in terms of green and sus-
tainable chemistry.
Fig. 9 The reusability of the CuZn-1 catalyst at a reaction time of
3.5 h.
excellent catalytic activity and selectivity for allylic alcohols
after four consecutive cycles (attaining a 69% selectivity to
allylic alcohols at 93% conversion), as a result of the strong
interactions between metal and support in catalysts. The
result demonstrates that the as-formed Cu-based nano-
catalyst is chemically and structurally stable.
In general, the present strategy of utilizing the LDH pre-
cursor route can be an effective way to reasonably manipulate
the surface structures of CuZn-x catalysts and thus create
new catalytic centers for facilitating the adsorption and fol-
lowing activation of reactants, besides catalytically active
metallic copper species. Correspondingly, over LDH-derived
copper-based nanocatalysts, three different types of catalytic
centers (e.g. Cu0, Cu+, LA) synergistically control the reactions
to achieve a high catalytic efficiency in terms of both good
activity and high chemoselectivity in citral hydrogenation.
Acknowledgements
We are gratefully thankful for the financial support from
National Natural Science Foundation of China (21325624),
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973 Program (2011CBA00506), Program for Beijing Engineer-
ing Center for Hierarchical Catalysts and Fundamental
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