Synthesis of Cu-Zn-Zr-Al-O catalysts via a citrate complex route modified by different solvents and their dehydrogenation/hydrogenation performance

Article abstract of DOI:10.1039/c5ra13778k

The quaternary Cu-Zn-Zr-Al-O catalysts have been prepared by a citrate-complex method using deionized water, ethanol, and ethyl acetate as solvents. The solvents with different polarity and solubility have a prominent influence on the reaction pathway(s) of the synthesis. When using ethanol as solvent to prepare the catalysts, the selectivity and yield to ethyl acetate of 89.5 wt% and 70.6 wt%, respectively, corresponding to 78.9 wt% conversion of ethanol, are achieved. This catalyst also shows good catalytic activity and stability (>120 h at 220 °C) in the conversion of ethanol containing 7 wt% of water. Its remarkable performance is due to the existence of smaller CuO particles and their more uniform dispersion, which is beneficial to form Cu-M_xO_y (M = Zn, Zr, Al) interfaces. The catalyst showing more strong basic sites associated with the high density of O^2- that migrated from the interaction of Cu-M_xO_y, which is favorable for the dehydrogenative dimerization of ethanol to ethyl acetate; furthermore, the moderate chemisorption and desorption of H₂ makes this catalyst more favorable for ethanol dehydrogenation. This catalyst also proves to be effective for a series of ethyl ester hydrogenations (e.g., ethyl acetate and diethyl oxalate), indicating a general promotion of these reactions using ethanol as the solvent to prepare the catalysts.

Full text of DOI:10.1039/c5ra13778k

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Synthesis of Cu–Zn–Zr–Al–O catalysts via a citrate
complex route modified by different solvents and
Cite this: RSC Adv., 2015, 5, 82822
their dehydrogenation/hydrogenation
performance†
a
Jian Dingab and Jiangang Chen
*
The quaternary Cu–Zn–Zr–Al–O catalysts have been prepared by a citrate-complex method using
deionized water, ethanol, and ethyl acetate as solvents. The solvents with different polarity and solubility
have a prominent influence on the reaction pathway(s) of the synthesis. When using ethanol as solvent
to prepare the catalysts, the selectivity and yield to ethyl acetate of 89.5 wt% and 70.6 wt%, respectively,
corresponding to 78.9 wt% conversion of ethanol, are achieved. This catalyst also shows good catalytic
activity and stability (>120 h at 220 ꢀC) in the conversion of ethanol containing 7 wt% of water. Its
remarkable performance is due to the existence of smaller CuO particles and their more uniform
dispersion, which is beneficial to form Cu–MxOy (M ¼ Zn, Zr, Al) interfaces. The catalyst showing more
strong basic sites associated with the high density of O2ꢁ that migrated from the interaction of Cu–
MxOy, which is favorable for the dehydrogenative dimerization of ethanol to ethyl acetate; furthermore,
the moderate chemisorption and desorption of H2 makes this catalyst more favorable for ethanol
dehydrogenation. This catalyst also proves to be effective for a series of ethyl ester hydrogenations (e.g.,
ethyl acetate and diethyl oxalate), indicating a general promotion of these reactions using ethanol as the
solvent to prepare the catalysts.
Received 14th July 2015
Accepted 15th September 2015
DOI: 10.1039/c5ra13778k
www.rsc.org/advances
deactivation mainly due to metal sintering.6–8 To overcome
these problems, methods such as the Pd–Ag membrane
reactor9 and acceptorless dehydrogenation reactions1,10 have
1. Introduction
The dehydrogenation/hydrogenation reactions are important
facets of synthetic chemistry.1 Taking the future large avail-
ability of bioethanol into account and for reducing greenhouse
gas emissions that contribute to the global warming, the use of
ethanol (EtOH) based on green chemistry as feedstock can be
foreseen. Direct conversion of EtOH to ethyl acetate (AcOEt)
through dehydrogenative dimerization with the liberation of
H2 is an attractive alternative and represents a simple, non-
corrosive, non-toxic, and economical process.2–5 The reaction
is a combination of dehydrogenation, which is preferred at low
pressures, and dimerization, which is preferred at high pres-
sures. In the case of ethanol (EtOH) dehydrogenation, the
copper-based catalysts have been successfully employed
because of their ability to maintain the C–C bond intact while
dehydrogenating the CO–H bond. Nevertheless, there are two
limitations of the Cu catalysts, namely, their inherent activity
is lower than those of noble metals and irreversible
recently been developed, but these systems require the
extensive use of noble metals and hazardous substances such
as phosphorus. From environmental and economic view-
points, it is important to synthesize and develop new catalysts
with a higher activity for the conversion of ethanol and better
selectivity to ethyl acetate under mild conditions. Herein, we
highlight a catalyst design for improving the activity and
stability of Cu-based catalysts and a signicant improvement
in ethyl acetate selectivity based on the understanding of the
reaction mechanism.
In a metal–oxide interface, one can obtain adsorption/
reaction sites with complementary chemical properties,
whereas truly bi-functional sites would be very difficult to
create on the surface of a pure metal or alloy system.11 More-
over, the metal–oxide interface determines the formation and
stability of the intermediates present in the ethanol trans-
formation process. Based on the study by Bueno,12,13 a combi-
nation of the Cu+/Cu0 pair or Cu0 interfaced to ZrO2, which may
provide synergism, is needed to efficiently transform ethanol to
ethyl acetate. In the case of the Cu–Zn–Zr–Al–O (CZZA) cata-
lyst,7,14 the active site for the coupling of ethanol and aldehyde
is at the mixed metal–oxide surface, not at the Cu metal
aState Key Laboratory of Coal Conversion, Shanxi Institute of Coal Chemistry, Chinese
Academy of Sciences, Taiyuan 030001, Shanxi, PR China. E-mail: chenjg@sxicc.ac.cn;
Tel: +86 0351 4040290
bUniversity of Chinese Academy of Sciences, Beijing 100049, PR China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra13778k
82822 | RSC Adv., 2015, 5, 82822–82833
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
surface. According to many reports,15–17 by applying a citrate
complex method as an advanced wet chemical route, multi-
component catalysts can be prepared which have higher
uniformity in particle size distribution and combination, and
which can exhibit higher synergistic interaction and coopera-
tive catalysis than those prepared by coprecipitation and
impregnation. This technique is based mainly on the forma-
tion of metal chelate complexes in solution, followed by elim-
ination of the solvent via drying, resulting in a gel that contains
the starting cations. The organic fraction of this gel is then
removed by calcination, resulting in a very ne and reactive
oxide powder. More encouragingly, chelating of solvated metal
ions, as an exception, is governed by a relatively high contri-
bution of entropy, due to the replacement of many solvent
molecules around a metal ion by one chelating ligand.18
Thanks to this behavior, most metal chelate complexes exhibit
relatively high stability. However, a few systematic studies have
been made, taking the parameters of the rst step, namely,
citrate synthesis, into account. For example, the contribution
of different solvents used in the citrate complex method is
seldom investigated. In the complexation process specically,
not only does the solvent play an important role in the hydro-
lysis and condensation of metal ions, but it also facilitates the
nanoparticle growth of the synthesized materials.19 Along this
line, adsorption characteristics of the citrate–metal complex
with the aid of solvents including the interaction between
hydrogen-bonded citrate chains, surface coverage, and poten-
tial intermolecular interactions between citrate anions during
nanoparticle growth are signicantly important.20 Further-
more, the stability of the citrate complex in solution is inu-
enced by the presence of short-range repulsive forces, i.e., steric
or electrostatic repulsion, which further affect nanoparticle
growth.20,21 Therefore, the solvent used may affect not only the
surface features but also the morphological characteristics of
the synthesized materials.
2. Experimental
2.1 Catalyst preparation
All the reagent grade chemicals were supplied by Sinopharm
Chemical Reagent Co. Ltd, China, and used without further
purication. Each CZZA quaternary catalyst sample was prepared
using Cu(NO3)2$3H2O, Zn(NO3)2$6H2O, Zr(NO3)4$5H2O,
Al(NO3)3$9H2O, and citric acid monohydrate (CA). The method
was essentially a similar procedure as for an amorphous citrate
process introduced by Sato et al.,16 where deionized water,15
ethanol, and ethyl acetate22 were used as solvents. The catalysts
prepared were referred to as CZZA–x, where x ¼ am, wa, et, and
eta represent the amorphous citrate process, and those using
water, ethanol, and ethyl acetate as solvents, respectively. Taking
CZZA–et as an illustration, the mixed nitrates of Cu, Zn, Zr and Al
with a desired molar ratio (12 : 1 : 4 : 2) were dissolved in
ethanol. An alcoholic solution of CA (calculated as 1/3 mol CA
per g eq. of each metal plus an extra 5%) was dropped rapidly
into the above mentioned mixed nitrates solution with vigorous
stirring. The mixture was then evacuated under a pressure of
1.0 kPa at 60 ꢀC. Aer the resulting solid had been heated in air
at 170 ꢀC for 2 h, it was calcined in air at 550 ꢀC for 2 h to obtain a
CZZA sample. Similarly, the CZZA–wa catalyst was prepared
through mixing an aqueous solution of CA with nitrates of Cu,
Zn, Zr and Al, which were dissolved in deionized water; the
CZZA–eta catalyst was prepared by adding an ethyl acetate
solution of CA to nitrates of Cu, Zn, Zr and Al, which were dis-
solved in ethyl acetate. The preparation of CZZA–am catalyst was
essentially the same procedure as for an amorphous citrate
process, which involved physical mixing of CA with nitrates of
Cu, Zn, Zr and Al without the aid of solvent. Equally, the hybrid
catalysts (CZZA–m) were prepared by mechanically mixing ZnO,
ZrO2, and Al2O3 with CuO as a base for comparison. To investi-
gate the effects of the addition of ZnO and Al2O3 on the structural
evolution of CZZA–et, a sample of Cu/ZrO2 (denoted as CZr–et)
with the same Cu loading (ca. 54.2 wt%) as the CZZA catalysts
was prepared by the citrate complex method employing ethanol
as solvent.
To gain an insight into the effect of solvents on the struc-
tural properties of Cu–Zn–Zr–Al–O catalysts in relation to their
performance, we prepared CZZA catalysts by citrate-complex
method employing deionized water, ethanol, and ethyl
acetate as solvents. The products formed during ethanol
conversion on CZZA were ethyl acetate (AcOEt), acetaldehyde
(AcH), diethyl ether (DEE), n-butanol (BOL), methyl ethyl
ketone (MEK), crotonaldehyde (CROT), propanone (PrO), CO,
CO2, ethylene (ETE), and 1-butene (BTE). On the basis of our
Before the following utilization, the raw CZZA catalyst was
tabulated, crushed and sieved to obtain particles with a size
range of 0.18–0.25 mm.
experiments, and with respect to maximum catalytic activities 2.2 Characterization of catalysts
of the CZZA catalysts, ethanol still appears to be the solvent of
The powder X-ray diffraction (XRD) patterns in the 2q range of
5–85ꢀ at a scanning speed of 2ꢀ minꢁ1 were collected on a DX-
2700 with monochromatic Cu-Ka radiation operating at 40 kV
and 30 mA. Average particle sizes were calculated by the
Scherrer equation using the (111) peak position of CuO from the
XRD data [eqn (1)].
choice for the precursor synthesis. This strategy raises the
prospect of using the citrate-complex method employing
ethanol as solvent to prepare catalysts for the efficient
dehydrogenation of alcohols to functionalized carbonyls, and
consequently contributes to the sustainable development of
green chemistry. Based on the understanding of selective
activation of C–O and O–H bonds, the catalysts can also be
applied to the hydrogenolysis reactions of ethyl esters, which
Kl
d ¼
(1)
FW cos q
are the simple esters and good model compounds for studying where K is a constant generally taken as unity (0.89), l is the
the selective hydrogenation of C]O and C–O bonds to produce wavelength of the incident radiation (0.15405 nm), FW is the
various chemicals.
full width at half maximum and q is the peak position.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 82822–82833 | 82823

View Article Online
RSC Advances
Paper
Nitrogen physisorption isotherms were recorded using a room temperature by a He gas ow (50 mL minꢁ1). Several CO2
Micromeritics TriStar II 3020 instrument. The samples were pulses (1.2 mL CO2 of one pulse) were then introduced to the
outgassed at room temperature for 24 h before the measure- sample to obtain a saturated adsorption of CO2. The sample
ments. Specic surface area (SBET) calculations were carried out was purged by He for 1 h at 100 ꢀC to eliminate the physically
using conventional BET calculations. Pore volume (Vpore) and adsorbed CO2; then, the sample was heated to 650 ꢀC at 15 ꢀC
average pore diameter (dp) were deduced using BJH pore anal- minꢁ1 in He and the effluent was continuously monitored by
ysis based on the desorption branch of the nitrogen adsorption/ mass spectrometer. The mass number used was 44 for the CO2
desorption isotherm. The surface morphology and the particle species.
size were observed by transmission electron microscopy (TEM).
The purity of the N2, Ar, He, N2O, CO2, and H2 gases were
The TEM measurements were performed with a JEM-2100F greater than 99.99%, and all of them were pretreated by silica
˚
microscope operating at 200 kV. Samples for TEM were gel desiccant, 5 A molecular sieve, and some of them were
prepared by depositing a drop of an ultrasonically dispersed deoxygenated by a silver molecular sieve before use.
solution onto a standard amorphous carbon-coated copper
grid.
2.3 Catalytic activity tests
The reducibility of the catalysts was studied by H2
Evaluations were conducted on a stainless steel xed-bed
temperature-programmed reduction (H2-TPR) on a homemade
apparatus.23 A ow of 5% H2/N2 (50 mL minꢁ1) was passed
reactor (internal diameter ¼ 9.8 mm). The catalyst (2 g) was
placed between two layers of quartz sand and reduced in a
stream of diluted hydrogen (10% H2 in N2) at 250 ꢀC under
through 20 mg of sample with a ramping rate of 10 ꢀC minꢁ1 to
ꢀ
a nal temperature of 600 C. The effluent gas formed during
the TPR experiment was passed through a molecular sieve trap
to remove the water. In the meantime, the hydrogen consumed
atmospheric pressure for 16 h with a ow rate of 30 mL minꢁ1
.
The catalyst was then heated to the desired reaction temper-
ature at steps of 2 ꢀC minꢁ1. The liquid reactant was injected by
double-plunger pump, enabling a tunable liquid hourly space
velocity (LHSV). Aer vaporizing by a pre-heater, the vapor was
mixed with N2 or H2. Details of the reaction conditions are
presented below each result. The condensable obtained
products were collected in a trap and analyzed by gas chro-
matography (Shimadzu GC-14C) using a capillary column
(19091n-213, HP-INNOWAX, 0.25 mm ꢂ 30 m) with FID and a
packed column (TDX-101) with TCD as the detectors. The gas
products were periodically analyzed by online gas chromato-
graphs (Shanghai Haixin GC-950) with a packed column lled
with carbon molecular sieve. The products were determined
quantitatively by calibrated area normalization. Moreover,
conversion and selectivity were based on the mass basis
products (wt%) and calculated by the equations given in the
ESI.†
was monitored continuously by
a thermal conductivity
detector (TCD). Calibration of the instrument by the hydrogen
consumed was carried out with 20 mg standard samples of
CuO (Aldrich). The dispersion and specic surface area of
metallic copper (DCu and SCu) were measured by one-pulse N2O
ꢀ
oxidation at 50 C using the procedure described by van der
Gri et al.24,25 The calculation was based on the total amount of
N2O consumption with 1.46 ꢂ 1019 copper atoms per m2. For
more details, see the ESI.† The activation and desorption
properties of hydrogen were tested by the temperature-
programmed desorption of adsorbed H2 (H2-TPD) using the
same apparatus employed by the TPR experiment. Prior to H2
adsorption, the sample (120 mg) was reduced in a H2 (50
mL minꢁ1) gas ow at 250 ꢀC for 4 h. The sample was then
pretreated under an Ar ow (50 mL minꢁ1) at 350 ꢀC for 1 h to
sweep the surface of the reduced catalyst. Aer cooling to room
temperature by Ar ushing, the sample was exposed to H2 (50
mL minꢁ1) for 1 h and heated to 600 ꢀC at a rate of 15 ꢀC minꢁ1
The effluent formed during the experiment was puried by
.
3. Results and discussion
3.1 Textural and structural properties
˚
silica gel desiccant and a 5 A molecular sieve to eliminate water
and other substances produced in the test, and subsequently The X-ray patterns of all the prepared catalysts are shown in
monitored by TCD. H2O-TPD measurement was carried out to Fig. 1. All the samples exhibited main sharp peaks at 2q ¼ 32.6ꢀ,
investigate the adsorption/desorption of water on the catalysts. 35.6ꢀ, 38.6ꢀ, 48.8ꢀ, 53.6ꢀ, 58.3ꢀ, 61.6ꢀ, 66.4ꢀ, 68.1ꢀ, 72.3ꢀ and
Before any measurements, all the samples were in situ satu- 75.1ꢀ corresponding to a crystalline CuO phase with tenorite
rated with water aer being reduced in H2 (50 mL minꢁ1) for 4 structure (JCPDS 481548). There were no peaks obtained cor-
h. The temperature was subsequently ramped from 50 ꢀC to responding to ZnO and Al2O3, indicating that the dispersion of
700 ꢀC with a linear rate of 15 ꢀC min under He ow (30 these was more uniform and they were present in highly
mL minꢁ1), and the tail gas was continuously monitored on disordered or amorphous states.26,27 A decrease of the peak
stream by a TCD. Temperature-programmed desorption of intensity of copper species (Fig. 1) was apparent. The average
adsorbed CO2 (CO2-TPD) on the reduced catalysts was CuO crystallite sizes, estimated by Scherrer's equation, were
analyzed using a quadrupole mass spectrometer (HIDEN, Hpr- 19.8, 14.1, 10.9, and 8.6 nm for CZZA–eta, CZZA–am, CZZA–wa,
20; Pfeiffer Vacuum Technology AG) equipped with the same and CZZA–et, respectively. Table 1 shows the dispersion and
apparatus employing the TPR experiment. Prior to CO2 surface area of Cu0, surface area, pore volume and average pore
adsorption, the sample (200 mg) was reduced in a H2 (30 diameter values for the CZZA catalysts. Although the samples
mL minꢁ1) gas ow at 250 ꢀC for 4 h. Aer reduction, the did not show much difference in Vp and dp, SBET and SCu
sample was swept at 350 ꢀC for 1 h and subsequently cooled to determined by N2O titration method (Fig. S1†) were different,
82824 | RSC Adv., 2015, 5, 82822–82833
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
which increased by employing ethanol as solvent, can be
directly observed from the TEM image of the CZZA–et sample
(Fig. 2c). Moreover, the image for the CZZA–et sample showed
little obvious evidence of bulk copper species condensed on the
substrate MxOy (M ¼ Zn, Zr, and Al), which further conrms
that the copper species were uniformly dispersed in the CZZA–
et aer the synthesis process and the following high-
temperature thermal treatment. As can be seen in Fig. S2,†
contrary to the agglomerated particles in CZZA–wa, the
magnied TEM images of CZZA–et show small particles of
narrow dispersity embedded in an amorphous matrix, which
are, most probably, formed from the substrate MxOy. Further-
more, the higher agglomeration and thus higher density of
particles enables additional conrmation of the crystallinity of
the CZZA–wa using selected area electron diffraction (SAED)
(Fig. S2e†). Moreover, in the amorphous structure of MxOy, the
small and even structure of CuO can be further conrmed by a
successive diffraction halo in an attached SAED image for
CZZA–et (Fig. S2c†). Generally speaking, this was because small
CuO particles adhered to the surface of MxOy, and hence the
sintering of copper particles was obstructed, as seen in the
schematic illustration (Fig. 2d). This is in agreement with the
results from XRD and metallic copper surface area measure-
ment (Table 1 and Fig. S1†). However, the metallic Cu particle
size of the CZZA–wa catalyst observed by TEM was larger than
the crystallite size derived from XRD and N2O titration, indi-
cating the polycrystalline nature of the metallic Cu particles in
CZZA–wa as compared to the particles of other samples. This
could probably be connected to the contribution of the water,
which facilitated the intimate contacting of Cu species in the
precursors by building strong hydrogen-bonded networks, and
then affected the sintering behavior in the following high-
temperature thermal treatment, for example, forming the
agglomerated or polycrystalline of CuO. Thus, the nano-
structure was not distinctive in the images of TEM. In a word,
the big difference of the active copper surface area among
CZZA–x may originate from the differences in copper particle
size and dispersion. Moreover, the increase in Cu0 surface area
and Cu–MxOy interfaces is assumed to be partially responsible
for the improvement of dehydrogenation and dimerization
activity.
Fig. 1 XRD patterns of calcined CZZA–x samples prepared by citrate-
complex method.
and the dispersion of copper species followed the sequence
CZZA–et > CZZA–wa > CZZA–eta > CZZA–am. Among the
samples, CZZA–et displayed the highest surface area (84.7 m2
gcatꢁ1) and copper surface area (49.2 m2 gcatꢁ1). Probably the
increase in Cu surface areas was caused by the decrease in the
particle size of CuO species, as shown in the XRD patterns of
CZZA–et (Fig. 1). Based on the data mentioned above, we
concluded that the CuO particle sizes in the CZZA–et became
the smallest and the dispersion of Cu species was more
uniform, indicating that the solvents used in the preparation
process had a great inuence on the dispersion of the copper
species.
Fig. 2a–c compares the TEM images of the calcined CZZA–x
catalysts. These show that the catalysts consisted of clusters of
interconnected particles with sizes of 5–50 nm (Fig. S2e†). For
CZZA–am and CZZA–wa (see Fig. 2a and b), CuO particles were
agglomerated into clusters larger than 15 nm in size, which was
also veried by XRD. The morphology of the two calcined
catalysts clearly demonstrated that the CZZA–am and CZZA–wa
catalysts oen appeared in larger agglomerates than the CZZA–
et, which resulted in a lower BET surface area and dispersion of
Cu0 (Table 1). The corresponding dispersion of copper species,
The TEM images of the Cu/ZrO2 catalysts are shown in
Fig. 2e. The morphology of the CZr–et and CZZA–et catalysts
Table 1 Physicochemical properties and catalytic performance of the CZZA–x catalysts
a
a
b
b
d
SCu
DCua (%) (m2 gcat
dp
SBET
Vpore
Conversiond Sel. of AcOEtd,e Yield of AcOEtd Yield of H2
(nm) (m2 gcat
)
(m3 gcat
)
(SI/SII)c (wt%)
(wt%)
(wt%)
(wt%)
ꢁ1
ꢁ1
ꢁ1
Catalyst
)
CZZA–am
CZZA–wa 11.9
CZZA–et
CZZA–eta
6.5
24.6
41.5
49.2
29.3
15.4
8.4
7.1
71.2
68.6
84.7
65.1
0.095
0.079
0.080
0.084
0.28
0.45
0.65
0.31
46.5
58.6
63.8
44.9
58.2
70.4
84.9
52.9
27.1
41.2
54.2
23.8
1.9
2.6
3.2
1.9
14.1
8.4
11.9
a
Cu dispersion and surface area of Cu0 were determined by N2O titration. b Measured by BET method for the calcined sample. c H2O-TPD peak area
ratios of the lower temperature desorption peak I to the higher temperature desorption peak II of CZZA–x. d Reaction conditions: P ¼ 0.1 MPa, LHSV
¼ 1.0 mL gcatꢁ1 hꢁ1, and T ¼ 220 ꢀC. e Other condensable by-products (e.g., AcH, MEK, CROT, PrO, and H2O) and gaseous products (e.g., CH4, C2H6,
CO, and CO2) were also formed.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 82822–82833 | 82825

View Article Online
RSC Advances
Paper
Fig. 2 TEM images of the calcined quaternary catalysts: (a) CZZA–am; (b) CZZA–wa; (c) CZZA–et and (d) the calcined binary catalyst Cu/ZrO2
and (e) schematic illustration of the structural evolution of the nanostructured CZZA–x catalysts as a function of solvent.
clearly demonstrate that the binary catalyst consists of small
particles of similar shape, while they oen appear in larger
agglomerates when compared to the quaternary catalyst. The
aggregation of CuO nanoparticles in CZZA–et was not signif-
icant as in CZr–et. The combination of Al and Zn oxides with
Zr oxide had been described as preventive elements for sin-
tering of Cu crystallites and therefore was considered as a
structural promoter and as an alternative support.27 More-
over, the introduction of ZnO and Al2O3 effectively stabilized
copper and prevented crystallite growth. As shown in Fig. 2c,
it is clear that CZZA–et catalysts exhibited better resistance to
Cu/CuO particle growth upon calcination than the binary CZ–
et catalyst. The results show that the addition of ZnO and
Al2O3 had a signicant inuence on the particle size distri-
bution and the structure of the catalyst with the aid of ethanol
as solvent. In addition, based on “X-ray amorphous” features
for CZZA–et, zinc, zirconium, and aluminum phases were
present in an amorphous-like state in the quaternary
catalysts.
3.2 H2-TPR analysis
Fig. 3a compares the H2-TPR proles for CuO with those of the
four prepared CZZA–x catalysts. All the samples displayed
reduction proles characterized by a main peak with a
maximum between 160 and 270 ꢀC, well below that of the
standard bulk CuO (ca. 285 ꢀC); this suggested the presence of a
copper–oxides interaction in the present samples, which facil-
itated the reduction of copper oxides. In fact, the peak was
sharper and much more symmetrical for CZZA–am, CZZA–wa,
and CZZA–eta. The prole of CZZA–et displayed a weak reduc-
ꢀ
tion peak centered at ca. 162 C, which was lower than that of
the sharp one. Fig. 3b shows typical TPR proles before and
aer N2O oxidation for CZZA–et. The peak area of the rst TPR
prole (TPR 1) corresponded to all CuO in the sample, and that
of the second TPR (TPR 2) corresponded to Cu2O produced by
N2O oxidation. In fact, the TPR peak was around 130 ꢀC for
surface Cu+ to Cu0 compounds and 170 ꢀC for small CuO
particles to Cu0 in Fig. 3b. As stated, the peak occurring at
82826 | RSC Adv., 2015, 5, 82822–82833
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
obtained by H2-TPD and elsewhere33 regarding the generation
of sites for medium absorption of H2.34 More details about this
will be given below. On the other hand, the product of CuO
reduction strongly interacted with the support; although it
existed in the other three samples, can be partially covered up
by the reduction product of large CuO crystallites. As a conse-
quence, the reduction temperature of CZZA–x catalysts
increased and followed the order CZZA–et < CZZA–wa < CZZA–
am < CZZA–eta. Thus, the increasing reduction temperature in
the proles of TPR indicated the increasing size of CuO particles
and/or possibly a strong interaction between copper and the
support, which is in accordance with XRD and TEM results.
Furthermore, it should be noted that the TPR measurement can
be explained partially in terms of copper dispersion, especially
for the sample containing zirconia because of the introduction
of ZrO2 in the sample facilitating the reduction of CuO.27,29 The
CZZA–et catalyst displayed the lowest reduction temperature, in
which both the weak reduction peak centered at ca. 162 ꢀC and
the main peak appeared at approximately 220 ꢀC. This indicated
the presence of smaller CuO particles, and hence a higher
dispersion than that of other samples. These results correlate
well with the metallic copper surface area measurements and
XRD results.
Fig. 3 (a): TPR profiles of the CZZA–x catalysts prepared by citrate-
complex method and bulk CuO; (b) reduction profiles of CZZA–et
catalyst before and after N2O oxidation at 50 ꢀC.
192 ꢀC for CZZA–et was unlikely to be caused by the reduction of
Cu+, but rather by CuO particles with small sizes.28 As is known,
the more facile reduction–oxidation of Cu species in the small
CuO particles was presumably due to a higher degree of surface
defective dominant features, which meant a poor crystallinity
with plenty of weakly bonded surface oxygen ions and higher
surface area exposed to H2, whereas large crystallites would
appear in the TPR as species reducible at a higher temperature
because of the diffusion hindrance on the reduction process
and/or their relatively lower surface area exposed to H2.27,29 In
this case, the reducing temperature in the proles of TPR can be
used to reect the size of the Cu particles, e.g., CuO phase with a
low reduction temperature denoted a small particle size.30
Second, according to Behrens et al.,31 the crystalline and pure
CuO species are more easily reduced than the highly dispersed
CuO in amorphous materials as a result of diffusion effects and
the strong interactions between Cu species and MxOy. The
higher reduction temperature of second peak as opposed to the
former peak for CZZA–et indicated the presence of highly
dispersed CuO species embedded into the supports of MxOy.
Moreover, interactions with supports can decrease the rate of
CuO reduction.32 The consumption of H2 for CZZA–et was lower
than that of CZZA–am and CZZA–wa, which further conrmed
that highly dispersed CuO particles strongly interacted with the
MxOy thus decreasing the rate of CuO reduction. In a word, we
proposed that there were two reducible copper species in the
CZZA–et catalyst. One was represented by small crystalline CuO
particles, which were nely dispersed and reduced at lower
temperature (peak I); another was represented by highly
dispersed CuO particles, which strongly interacted with the
MxOy supports, and reduced at higher temperature (peak II).
Deconvolution of the two TPR peaks indicated that the amounts
of small CuO and Cu–MxOy were ca. 10% and 90%, respectively.
Interestingly, the two-peak H2-TPR prole of CZZA–et was
consistent with a phenomenon discussed in detail for data
3.3 H2-TPD analysis
The H2-TPD proles of the reduced CZZA–x catalysts are shown
in Fig. 4. Two obvious H2 desorption peaks (centered at 152 and
ꢀ
ꢀ
460 C) and a slight peak (centered at 580 C) appeared in the
whole T range over all the samples, which were at similar
temperatures, but differed greatly in the size. According to
published studies in this area,35–37 the resolved peak at low
temperatures (120–240 ꢀC) could be ascribed to the hydrogen
moderately adsorbed on Cu (hydrogen on surface Cu sites),
while a considerably broader signal in the range of 350–530 ꢀC
monitored the desorption of hydrogen from the split H–H on
the surface of Zn–Al–Zr–O oxides or CuHx, which denotes the
Fig. 4 H2-TPD profiles of the reduced CZZA–x catalysts.
RSC Adv., 2015, 5, 82822–82833 | 82827
This journal is © The Royal Society of Chemistry 2015

View Article Online
RSC Advances
Paper
hydrogen strongly adsorbed on Cu, and the weak peak aer
ꢀ
540 C would be due to the oxidation of the metal by support
protons. Thus, the H2-TPD patterns of CZZA–x catalysts span-
ꢀ
ning a wide range of temperature (50–670 C) is diagnostic of
different adsorption states of hydrogen species across the
catalyst structure (Fig. 4). At the rst stage (corresponding to
peak I), sites were probably being formed that had a high effi-
ciency for the adsorption of H2 and thus produced moderately
adsorbed hydrogen on surface Cu sites.36,38 Apparently, these
peaks were enhanced in CZZA–et and CZZA–wa, while lowered
in CZZA–eta and CZZA–am because of the decrease of small
CuO particles, which were liable to be transformed to Cu0. The
high-temperature peak (corresponding to peak II, denoted as
hydrogen strongly adsorbed on catalysts) was attributed to the
generation of sites for facile H2 dissociation, which facilitated
the formation of CuHx. The catalytic activities test for CZZA–am
indicated that it had a strengthening of peak II and weakening
of peak I in the TPD prole, showing a relatively low catalytic
activity for ethanol dehydrogenation (46.5 wt% conversion of
ethanol and 58.2 wt% selectivity to AcOEt). The same
phenomenon was observed over CZZA–m (16.5 wt% conversion
of ethanol and 26.5 wt% selectivity to AcOEt, data not shown
here), which only displayed surface sites for H2 dissociation.
CZZA–et, which showed the best catalytic activities (ca. 64 wt%
conversion of ethanol), had the highest TPD peak area of the
lower temperature peak I. Therefore, the dehydrogenation
activity of the catalyst decreased with increasing amounts of
strong desorption of H2. These results suggested that hydrogen
Fig. 5 H2O-TPD profiles of the reduced CZZA–x catalysts.
more active sites for the adsorption of ethanol. As discussed
above, one can suppose that CZZA–et would show better catalytic
activities in the conversion of hydrous ethanol because the
substantial desorption of H2O avoided the competitive adsorp-
tion of H2O and ethanol or acetaldehyde on metallic active sites
at the reaction temperatures.
3.5 CO2-TPD analysis
activation driven by the adsorption–desorption of H2 for CZZA–x As is known, zirconia possesses surface Lewis basic sites, which
markedly contributed to the H2 activation ability, and these were able to adsorb CO2, while ZnO enhanced the affinity of the
enhanced H2 activation abilities (by the way, the desorption of system for CO2.36 Moreover, the combination of Al oxide with Zn
H atom would be very difficult) may impair its activity for the and Zr oxides probably reduced the acidity of alumina.27 CO2-
dehydrogenation of ethanol. Qualitatively, the key for TPD proles of CZZA–x samples are shown in Fig. 6. As it can be
increasing catalytic activity derives from the balance between veried, these samples showed different proles regarding their
the number of active sites and the ease of product desorption. interaction with CO2. Several obvious CO2 desorption peaks (at
In other words, the moderate chemisorptions and desorption of 150–180 and 320–500 ꢀC) appeared over the entire temperature
H2 avoided the competitive adsorption of H and ethanol or range. The weak basic sites were related to a curve, which
acetaldehyde on metallic active sites. Moreover, the recombi- showed a maximum at a temperature of ca. 180 ꢀC; the ones
nation of H and ethoxide yields ethanol again.12,13
between 350 and 500 ꢀC were strong basic sites and, nally, the
peaks above 550 ꢀC were due to the release of CO2 from the
support.39 Thus, the CO2-TPD prole of CZZA–x discloses a clear
concentration of varied basic sites on the surface (Fig. 6). As it
3.4 H2O-TPD analysis
The H2O-TPD proles of the CZZA–x catalysts are shown in Fig. 5. can be observed, CZZA–et showed the largest peak for strong
There were two obvious H2O desorption peaks (centered at 152 basic sites followed by CZZA–wa and CZZA–am. There were no
and 382 ꢀC), which appeared for all CZZA–x catalysts. Both were distinct peaks of desorption of CO2 for CZZA–eta, while there
similar in terms of temperature, but differed greatly in the size. was a broad and lower peak with maximum between 350 and
Clearly, the TPD peak area of the lower temperature peak I 520 ꢀC, which was similar to that of CZZA–et (curves in Fig. 6); a
enhanced in CZZA–et and lowered in CZZA–eta, indicating that broad peak for the desorption of CO2 with maximum between
ꢀ
there were more active sites for the adsorption/desorption of H2O 400 and 520 C appeared, which was similar to that of CZZA–
at the reaction temperature. The considerably broader peak II is am. Therefore, the CO2-TPD proles indicated increasing
attributed to the H2O strongly adsorbed on Cu. This strong contribution of stronger basic sites associated with the inter-
adsorption of H2O competed with ethanol or acetaldehyde on the action between Cu and MxOy. In other words, the high density of
surface of CZZA–x catalysts and led to the hindering of ethanol basic sites associated with the oxygen sites originated from the
conversion. Table 1 compares TPD peak area ratios of the peak I oxygen-rich interface at Cu–MxOy by oxygen diffusion. This
to the peak II for the CZZA–x catalysts. Analyses indicated that implied that the location of O preferred a high coordination
the catalyst having a high SI/SII ratio showed a relatively high environment on a neutral Cu system, where the surface electron
desorption activity for water on the catalyst; moreover, it afforded density was high, which is in line with the strong ionic bonding
82828 | RSC Adv., 2015, 5, 82822–82833
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
Solution chemistry of citrate ligands and metal ions is
complicated because citrates form complexes containing metal
ions of different identity (heteronuclear or mixed-metal
complexes). Generally, the polarity of the solvent made a great
contribution to the citric complexes of Cu–Zn–Zr–Al–O cata-
lysts. In this case, the reduction in hydration/solvation was
related to preorganization or intrinsic basicity of the ligands for
the complexation of metal ions.44 Apart from that, we observed
that solvents having low dielectric constants (ethanol, 24.3)
accelerated the homogeneous complexing process, while
solvents having high dielectric constants (water, 80.4) did not
have an equivalent effect on the process. Comparing CZZA–et
with CZZA–wa, small copper oxide particles (8–10 nm) and
highly dispersed copper species in a strong interaction with the
MxOy exist in CZZA–et. This could probably be connected with
the capability of solvent for stabilizing the polymeric structure
of the precursors by building weak/strong hydrogen-bonded
networks, which then affected the sintering behavior in the
following high-temperature thermal treatment. In addition, the
nanostructure was enhanced by solvents of lower polarity.45
Despite the low polarity of ethyl acetate, it contributed little to
the catalytic activities of CZZA–eta because of the low solubility
of metal nitrates and CA in ethyl acetate (z15 wt% here). For
example, ethyl acetate exhibited a negative inuence on the
catalytic activities of CZZA–eta. The low solubility of CA in ethyl
acetate, the dissolution and repolymerization might be inhibi-
ted by the non-uniform contact in the heterogeneous suspen-
sion, thereby lowering the formation of mixed-metal complexes,
as was experimentally observed for CZZA–am without solvent.
When water or ethanol was used as the solvent, they signi-
cantly increased the formation of mixed-metal complexes and
increased the dispersion of Cu species in the MxOy matrix by
dissolving the components and forming a uniform solution
during the process of catalyst preparation. Regarding the
abovementioned facts, it was obvious that the formation of
agglomerated structure of CZZA particles prepared by citrate
complex method was induced by the morphology of the
precursor gel by using different solvents.46,47
Fig. 6 CO2-TPD profiles of the reduced CZZA–x catalysts.
character of O and Cu.32 As was veried, acetaldehyde produced
on the dehydrogenation catalyst migrated towards the oxides
and reacted with the ethoxide species, which were generated by
the oxide basic sites. The resulting hemiacetal was dehydro-
genated and the ethyl acetate obtained was desorbed. Clearly,
oxides with strong basic sites generated more effective systems
than weak ones for the ethyl acetate synthesis.14,40 Of all the
samples used in these experiments, CZZA–et showed the best
catalytic activities (Table 1). Actually, when CZZA–eta was
employed, a lower selectivity to ethyl acetate was observed than
that of CZZA–et (52.9 wt% vs. 84.9 wt%). The low selectivity to
ethyl acetate observed from the others might be associated with
the low density of the strong basic sites. Above all, we can
conclude that the basic sites on the CZZA–et interacted strongly
with Cu nanoparticles and formed Cu–MxOy, generating a
catalytically active nanoenvironment for reaction coupling
between alcohol dehydrogenation and dimerization.41 This
suggested the existence of a synergism of metal Cu dehydro-
genation and oxide basic sites dimerization, which provided the
metal/oxide interface on the functionality of Cu–Zn–Zr–Al–O
catalysts and enhanced the dual-site nature.27,42,43
3.7 Performance of CZZA–et catalyst in the reaction of
ethanol conversion
3.6 Effect of solvent on the structure evolution of Cu–Zn–Zr–
Al–O catalyst
As shown in Table 1, the selectivity of ethyl acetate over CZZA–x
catalysts was in the order CZZA–et > CZZA–wa > CZZA–am >
For amorphous citrate process, the mixture of metal nitrates CZZA–eta, i.e., selectivity decreased from 84.9 wt% to 70.4 wt%,
reacted with CA and formed hexanuclear complexes. In this 58.2 wt%, and 52.9 wt%, respectively. This sharp decrease in
process, CA molecules exist as a polymeric skeleton structure selectivity can be attributed to the decreased amount of strong
containing free carboxyl and carboxylate groups attached to the basic sites on the CZZA–x catalysts that catalyzed the bimolec-
metal cation, which act as a chelating agents and help in the ular condensation or dimerization of EtOH to AcOEt.40 Owing to
dispersion of the metal component. For example, the coordi- the decreased dispersion of Cu species and increased particle
nation of bidentate carboxylate anion on metal ion is observed size of CuO for CZZA–x catalysts, the observed conversion of
in the composites of copper, alumina, titania, and zirconia with ethanol decreased in the order of CZZA–et > CZZA–wa > CZZA–
carboxylic acids.17 The presence of Cu–MxOy interaction in the am > CZZA–eta along with the selectivity to AcOEt (Table 1).
samples but at different levels was due to this. However, the
Among all the catalysts, it was noted that CZZA–et exhibited
CuO in CZZA–am was liable to accumulate partially due to the the best catalytic activities for the synthesis of AcOEt (Table 1).
absence of solvent and thus formed the heterogeneous distri- Fig. 7a shows the changes in EtOH conversion, AcOEt selec-
ꢀ
bution and agglomerated structure.
tivity, and yield of AcOEt and H2 with LHSV at 220 C and 0.1
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 82822–82833 | 82829

View Article Online
RSC Advances
Paper
MPa over the CZZA–et catalyst. At LHSV ¼ 0.4 mL gcatꢁ1 hꢁ1, the greatly increased with increasing reaction pressure, and
EtOH conversion was 78.9 wt%. The ethanol conversion reached a plateau at around 0.8 MPa (90.1 wt%), which then
decreased signicantly with increasing LHSV. The ethyl acetate increased slightly regardless of the sharp increase in the pres-
selectivity showed a maximum (89.5 wt%) at LHSV ¼ 0.4 mL sure. In contrast, the observed conversion of ethanol decreased
gcatꢁ1 hꢁ1, which then decreased slightly with increasing LHSV. with increasing reaction pressure. Combining the proles from
This indicated that in the formation of ethyl acetate from Fig. 7b, it was inferred that an optimum pressure was about 0.8
ethanol, the stepwise reaction requires a prolonged contact MPa in a practical operation because of the relatively high yield
time. In the analysis of gaseous products, hydrogen and carbon of ethyl acetate (56.3 wt%) without any higher pressure.
dioxide were observed; more than 98 mol% of the gaseous
The reactant ethanol containing 7 wt% of water was fed over
product was hydrogen, with a small amount of CO2. The CZZA–et catalysts under varied pressure (Fig. 8a). A selectivity to
amount of hydrogen produced corresponded to the sum of the AcOEt of 80.1 wt%, which is based on 64.4 wt% conversion of
ꢀ
derivatives from the dehydrogenation, and the data mentioned ethanol, was achieved at 220 C, 0.1 MPa, and LHSV ¼ 1.0 mL
above was checked by the yield of H2 and was close to 95 mol% gcat hꢁ1. Compared with the reaction of ethanol conversion,
ꢁ1
for each experiment. Fig. 7b shows the changes in the selectivity the conversion of hydrous ethanol and formation of the diethyl
to ethyl acetate and the conversion of the ethanol with reaction ether and C4-species were decreased slightly because the aldol
pressure. According to Inui6 and Santacesaria,8 it was advanta- addition or hydration reaction was retarded by the presence of a
geous that the rate of formation of ethyl acetate was accelerated little water. However, acetic acid was formed together with the
at high pressure by the increase in intermolecular collision products of the dehydrogenation of ethanol (data not shown).
frequency. Under an ambient pressure of 0.2 MPa, the selec- As the reaction pressure was increased from 0.2 MPa to 0.8 MPa,
tivity to ethyl acetate was 85.1 wt% at the temperature of 220 ꢀC the selectivity of ethyl acetate increased from 80.1 wt% to 87.5
and the LHSV of 1.0 mL gcat hꢁ1. The selectivity to AcOEt
ꢁ1
Fig. 8 (a) Changes in the conversion of ethanol and selectiꢁvi1ty to ethyl
acetate with pressure at 220 ꢀC and LHSV ¼ 1.0 mL gcat hꢁ1 over
Fig. 7 Changes in conversion of ethanol and selectivity to ethyl CZZA–et. (b) Stability evaluation of CZZA–et for the dehydrogenative
ꢀ
acetate over CZZA–et catalyst with (a): LHSV at 220 C and 0.1 MPa; dimerization of ethanol to ethyl acetate at 220 ꢀC and LHSV ¼ 1.0
and with (b): pressure at 220 ꢀC and LHSV ¼ 1.0 mL gcatꢁ1 hꢁ1
.
mL gcatꢁ1 hꢁ1
.
82830 | RSC Adv., 2015, 5, 82822–82833
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
wt%, while the observed conversion of ethanol decreased from
64.4 wt% to 55.1 wt%, respectively. It was also found that there
was no noticeable improvement in selectivity when the pressure
was increased from 0.8 to 1.6 MPa, while the observed conver-
sion of ethanol decreased to 51.5 wt%. Therefore, the higher
pressure would promote the competitive adsorption of H2O and
ethanol or acetaldehyde on metallic active sites, which resulted
in the decrease of the conversion of ethanol.
Scheme 1 Reaction pathway for the hydrogenation of DEO to EGT,
EG, and ethanol.
As is well known, water vapor increases the sintering rate of
supported metals and leaching of the metal in the reaction
system, while carbon deposition is another important reason
for catalyst deactivation.48–50 The stability of the CZZA–et cata-
lyst was evaluated by a test run of 120 h under the following
reaction conditions: 220 ꢀC, atmospheric pressure, and LHSV ¼
activity of all the samples remained unchanged aer 120 h on
stream.
It is known that the hydrogenation of DEO proceeds via ethyl
glycolate (EGT) to ethylene glycol (EG), while EG can dehydrate
further to ethanol (Scheme 1).53,54 For pressures above 1.0 MPa,
with the ratio of H2/ester more than 30/1, the formation of EG
was favored.55 The activity and durability of the catalyst under
severe conditions, including low H2/ester and high space
velocity, was tentatively employed in this study. Under the harsh
reaction conditions specied in Table 2, CZZA–x (x ¼ am, wa,
and eta, respectively) showed low activities for hydrogenation of
DEO, while the efficiencies were greatly improved when ethanol
was used as the solvent to prepare the CZZA–et catalyst. The
CZZA–et catalyst, which had the highly dispersed Cu, also
showed better catalytic performances for the hydrogenation of
AcOEt (Table 2). The results here indicated that the prominent
role of ethanol upon preparing Cu–Zn–Zr–Al–O catalyst could be
generalized into hydrogenation of esters, facilitating future
studies on the CZZA synergy for the hydrogenation of carbonyl
groups.
1.0 mL gcat hꢁ1. As shown in Fig. 8b, at initial reaction, the
ꢁ1
ethanol conversion was nearly 51.5 wt% and the selectivity to
ethyl acetate was 79.3 wt%. However, the catalytic activity
evidently increased with the reaction time, which gradually
increased during the rst 48 h of testing. The ethanol conver-
sion reached 62.3 wt%, and the selectivity of ethyl acetate
reached 80.1 wt% at 48 h. For the rst 48 h of testing, the
catalytic activity increased because of the instability of the Cu
particle aer the gradual reduction and formation of active sites
for ethanol conversion.48–50 Aer 48 h, the catalytic activity was
stabilized at the ethanol conversion of ca. 63 wt% and the
selectivity to ethyl acetate was stable at ca. 80 wt%. However,
both the conversion of ethanol and selectivity to AcOEt
remained unchanged, and no deactivation phenomenon was
observed for CZZA–et during the 120 h stability test. In addition,
mobile oxygen from the MxOy support may participate in the
oxidation of carbonaceous species at higher temperatures,
preventing carbon deposition.51
3.9 Structure and performance correlation
As can be seen in Table 1, all the samples showed dehydroge-
nation activity to some extent in the conversion of ethanol, even
CZZA–eta which was prepared by employing ethyl acetate as
solvent. Nevertheless, the catalyst employing ethanol as solvent
was distinguished from the other samples because of the exis-
3.8 Applications for the hydrogenation of carbonyl
compounds
The hydrogenation of esters is one of the most fundamental and tence of quantitatively small CuO particles and abundant Cu–
widely employed reactions. As representative reactions, we have MxOy interfaces, as observed from XRD, TEM, TPR, H2-TPD, and
chosen the hydrogenation of diethyl oxalate (DEO) and ethyl CO2-TPD analysis. However, catalysis was controlled not only by
acetate, which is known to be critically affected by the amount the chemical composition and size of the catalyst used but also
of metal-active sites on copper-based catalysts.52 The catalytic by the type of surface sites available at the catalyst surface. As a
performances of the CZZA–x catalysts prepared using different result, a nonlinear relationship between the dispersion of CuO
solvents are listed in Table 2. Steady-state product compositions and catalytic activity may be present in the dehydrogenative
were obtained aer about 36 h on stream and the catalytic dimerization of ethanol to ethyl acetate. Moreover, limited by
Table 2 Hydrogenation of diethyl oxalate and ethyl acetate over the CZZA–x catalysts
Selectivitya/%
EG
Selectivityb/%
EtOH
Catalyst
Conversiona of DEO/%
EtOH
EGT
Conversionb of AcOEt/%
ETA
CZZA–am
CZZA–wa
CZZA–et
51.5
63.4
77.1
34.0
25.6
22.1
55.1
15.4
48.6
41.0
31.6
68.5
19.6
26.7
6.1
81.2
81.6
95.8
79.3
92.6
91.3
98.3
95.9
1.6
2.3
0.4
1.1
CZZA–eta
10.7
a
Reaction conditions for hydrogenation of DEO: P ¼ 2.4 MPa, T ¼ 240 ꢀC, H2/DEO ¼ 25 mol molꢁ1, LHSV ¼ 1.2 mL gcatꢁ1 hꢁ1. Other by-products
included ethoxyethanol (2-EE), AcOEt, ETA, 1,2-butanediol (1,2-BDO), CO, and CO2. b Reaction conditions for hydrogenation of AcOEt: P ¼ 2.4 MPa, T
¼ 240 ꢀC, H2/AcOEt ¼ 25 mol molꢁ1, LHSV ¼ 1 mL gcatꢁ1 hꢁ1. Other byproducts included AcH, MEK, CROT, PrO, CO, CO2, ETE, BTE, DEE, and BOL.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 82822–82833 | 82831

View Article Online
RSC Advances
Paper
the equilibrium conversion of ethanol under the reaction
Acknowledgements
conditions employed in this study (ca. 74 wt% conversion of
ethanol at 0.1 MPa and 220 C),6 the improvement of ethanol
ꢀ
We would like to thank Mr Wenbin Zhang and Dr Yanting Liu
for the facilitation in the equipment maintenance, Dr Shenke
Zheng and Zhikai Li for the helpful discussions. This study was
supported by the Natural Science Foundation of China (Grant
No. 21373254) and PetroChina (Project No. 14-08-05-02).
conversion over the as-prepared Cu–Zn–Zr–Al–O catalysts was
not obvious, irrespective of the solvent used during the prepa-
ration. However, the equilibrium of the dehydrogenation of
ethanol shied toward the synthesis of ethyl acetate, which was
achieved by a synergism of metal Cu for the dehydrogenation of
ethanol and strong basic sites for the coupling of ethanol and
aldehyde in the CZZA–et. Thus, a high yield of ethyl acetate with
increasing conversion of ethanol was obtained. This method
was also applied to other systems where high dispersion and
small size of Cu species were important, e.g. hydrogenation of a
series of ethyl esters (e.g., ethyl acetate and diethyl oxalate),
indicating a general promotion of these reactions due to the
highly dispersed Cu species in the CZZA–et catalyst.
References
1 C. Gunanathan and D. Milstein, Science, 2013, 341, 249.
2 C. Angelici, B. M. Weckhuysen and P. C. Bruijnincx,
ChemSusChem, 2013, 6, 1595–1614.
3 A. L. Wang, H. Xu, J. X. Feng, L. X. Ding, Y. X. Tong and
G. R. Li, J. Am. Chem. Soc., 2013, 29, 10703–10709.
4 J. Llorca, N. S. Homs, J. Sales and P. R. R. de la Piscina, J.
Catal., 2002, 209, 306–317.
5 T. G. Kelly and J. G. Chen, Green Chem., 2014, 16, 777–784.
6 K. Inui, T. Kurabayashi and S. Sato, J. Catal., 2002, 212, 207–
215.
4. Conclusions
The morphologies and catalytic activities of quaternary Cu–Zn–
Zr–Al–O catalysts prepared by citrate-complex method were
signicantly altered by the solvent utilized. When a solvent with
lower polarity and excellent solubility such as ethanol was
employed, the catalyst showed excellent performance in dehy-
drogenative dimerization of ethanol to ethyl acetate. When a
7 K. Inui, T. Kurabayashi and S. Sato, Appl. Catal., A, 2002, 237,
53–61.
8 E. Santacesaria, G. Carotenuto, R. Tesser and M. Di Serio,
Chem. Eng. J., 2012, 179, 209–220.
9 G. Zeng, T. Chen, L. He, I. Pinnau, Z. Lai and K. W. Huang,
Chem.–Eur. J., 2012, 18, 15940–15943.
solvent having higher polarity such as water or low dissolving 10 M. Nielsen, H. Junge, A. Kammer and M. Beller, Angew.
capacity such as ethyl acetate was used, the catalyst performed Chem., Int. Ed., 2012, 51, 5711–5713.
poorly in the reaction. Actually, the sample prepared by amor- 11 J. Graciani, K. Mudiyanselage, F. Xu, A. E. Baber, J. Evans,
phous citrate process using no solvent also performed poorly in
S. D. Senanayake, D. J. Stacchiola, P. Liu, J. Hrbek,
the reaction. Thus, the solute–solvent interaction between
J. F. Sanz and J. A. Rodriguez, Science, 2014, 345, 546–550.
ethanol and precursors played an important role in the 12 A. G. Sato, D. P. Volanti, I. C. de Freitas, E. Longo and
synthesis process and the nanostructure was enhanced by the J. M. C. Bueno, Catal. Commun., 2012, 26, 122–126.
lower polarity and prominent solubility of ethanol. Further- 13 I. Freitas, S. Damyanova, D. Oliveira, C. Marques and
more, the decreased hydration within the alcoholic solution led J. Bueno, J. Mol. Catal. A: Chem., 2014, 381, 26–37.
to a decrease in the size of the metal ions and thus an increased 14 A. B. Gaspar, F. G. Barbosa, S. Letichevsky and L. G. Appel,
ability to form complex compounds with CA. In fact, the catalyst Appl. Catal., A, 2010, 380, 113–117.
employing ethanol as solvent was distinguished from the other 15 J. H. Ryu, S. M. Koo, J. W. Yoon, C. S. Lim and K. B. Shim,
samples because of the existence of quantitatively small CuO Mater. Lett., 2006, 60, 1702–1705.
particles and abundant Cu–MxOy interfaces, as observed from 16 S. Sato, K. Koizumi and F. Nozaki, J. Catal., 1998, 178, 264–
XRD, TEM, and TPR testing. The change in catalytic activities on 274.
both samples used was paralleled by a dramatic change in the 17 S. Samantaray, D. K. Pradhan, G. Hota and B. G. Mishra,
hydrogen adsorption–desorption properties, as seen from the Chem. Eng. J., 2012, 193–194, 1–9.
H2-TPD proles. Based on CO2-TPD, it was veried that the 18 D. W. Kim and S. G. Oh, Mater. Lett., 2005, 59, 976–980.
selectivity to ethyl acetate increased with increase in strong 19 R. A. Lucky, R. Sui, J. M. H. Lo and P. A. Charpentier, Cryst.
basic sites. The promotional roles of solvents can be generalized
into hydrogenation of esters (e.g., ethyl acetate and diethyl 20 J. W. Park and J. S. Shumaker-Parry, J. Am. Chem. Soc., 2014,
oxalate), where high dispersion and small size of Cu species 136, 1907–1921.
were important, indicating a general promotion of these reac- 21 Z. S. Pillai and P. V. Kamat, J. Phys. Chem. B, 2003, 108, 945–
tions due to the highly dispersed Cu species. Our study high- 951.
lighted the design of highly active, selective, and stable Cu 22 J. Słoczynski, R. Grabowski, P. Olszewski, A. Kozłowska,
Growth Des., 2010, 10, 1598–1604.
´
catalysts via a simple route, simultaneously solving the low
reactivity and deactivation caused by the sintering of supported
J. Stoch, M. Lachowska and J. Skrzypek, Appl. Catal., A,
2006, 310, 127–137.
metal catalysts. The results will be useful in the development of 23 H. Boer, W. Boersma and N. Wagstaff, Rev. Sci. Instrum.,
supported metal catalysts for a range of dehydrogenation or 1982, 53, 349–361.
hydrogenation reactions and have great implications for prac- 24 J. Evans, M. Wainwright, A. Bridgewater and D. Young, Appl.
tical applications.
Catal., 1983, 7, 75–83.
82832 | RSC Adv., 2015, 5, 82822–82833
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
25 C. van der Gri, A. Wielers, B. Jogh, J. van Beunum, M. de 39 Z. Y. Ma, C. Yang, W. Wei, W. H. Li and Y. H. Sun, J. Mol.
Boer, M. Versluijs-Helder and J. Geus, J. Catal., 1991, 131,
178–189.
26 L. R. W. Dai, in Handbook of Heterogeneous Catalysis, ed. G.
Catal. A: Chem., 2005, 227, 119–124.
40 P. C. Zonetti, J. Celnik, S. Letichevsky, A. B. Gaspar and
L. G. Appel, J. Mol. Catal. A: Chem., 2011, 334, 29–34.
Ertl, H. Knozinger, F. Schuth and J. Weitkamp, Wiley-VCH 41 F. Wang, R. Shi, Z. Q. Liu, P. J. Shang, X. Pang, S. Shen,
Verlag GmbH, 1st edn, 2008, vol. 3, pp. 1064–1077. Z. Feng, C. Li and W. Shen, ACS Catal., 2013, 3, 890–894.
27 J. Agrell, H. Birgersson, M. Boutonnet, I. Melian-Cabrera, 42 S. H. Kang, J. W. Bae, H. S. Kim, G. M. Dhar and K. W. Jun,
´
R. M. Navarro and J. L. G. Fierro, J. Catal., 2003, 219, 389–
403.
28 R. X. Zhou, T. M. Yu, X. Y. Jiang, F. Chen and X. M. Zheng,
Appl. Surf. Sci., 1999, 148, 263–270.
29 W. P. Dow, Y. P. Wang and T. J. Huang, J. Catal., 1996, 160,
155–170.
30 M. F. Luo, Y. J. Zhong, X. X. Yuan and X. M. Zheng, Appl.
Catal., A, 1997, 162, 121–131.
Energy Fuels, 2010, 24, 804–810.
43 H. W. Lim, M. J. Park, S. H. Kang, H. J. Chae, J. W. Bae and
K. W. Jun, Ind. Eng. Chem. Res., 2009, 48, 10448–10455.
44 A. Martell, R. Hancock and R. Motekaitis, Coord. Chem. Rev.,
1994, 133, 39–65.
45 E. Krupicka, A. Reller and A. Weidenkaff, Cryst. Eng., 2002, 5,
195–202.
46 R. Takahashi, S. Sato, T. Sodesawa, M. Kawakita and
K. Ogura, J. Phys. Chem. B, 2000, 104, 12184–12191.
47 R. T. Wang, L. B. Kong, J. W. Lang, X. W. Wang, S. Q. Fan,
Y. C. Luo and L. Kang, J. Power Sources, 2012, 217, 358–363.
48 C. H. Bartholomew, Appl. Catal., A, 2001, 212, 17–60.
¨
31 M. Behrens, I. Kasatkin, S. Kuhl and G. Weinberg, Chem.
Mater., 2010, 22, 386–397.
32 A. Sato, D. Volanti, D. Meira, S. Damyanova, E. Longo and
J. Bueno, J. Catal., 2013, 307, 1–17.
33 C. S. Polster, H. Nair and C. D. Baertsch, J. Catal., 2009, 266, 49 J. T. Sun, I. S. Metcalfe and M. Sahibzada, Ind. Eng. Chem.
308–319. Res., 1999, 38, 3868–3872.
34 F. Arena, K. Barbera, G. Italiano, G. Bonura, L. Spadaro and 50 R. Hughes, Chem. Eng. Sci., 1989, 44, 1747–1748.
F. Frusteri, J. Catal., 2007, 249, 185–194. 51 H. Song and U. S. Ozkan, J. Catal., 2009, 261, 66–74.
35 S. Xia, Z. Yuan, L. Wang, P. Chen and Z. Hou, Appl. Catal., A, 52 Y. N. Wang, X. Duan, J. Zheng, H. Lin, Y. Yuan, H. Ariga,
2011, 403, 173–182.
36 F. Arena, G. Italiano, K. Barbera, S. Bordiga, G. Bonura,
S. Takakusagi and K. Asakura, Catal. Sci. Technol., 2012, 2,
1637–1639.
L. Spadaro and F. Frusteri, Appl. Catal., A, 2008, 350, 16–23. 53 D. J. Thomas, J. T. Wehrli, M. S. Wainwright, D. L. Trimm
37 H. Lin, X. Duan, J. Zheng, X. Zheng, P. He, Y. Yuan and
Y. Yang, RSC Adv., 2013, 3, 11782–11789.
38 H. Wilmer, T. Genger and O. Hinrichsen, J. Catal., 2003, 215,
188–198.
and N. W. Cant, Appl. Catal., A, 1992, 86, 101–114.
54 Y. Cui, C. Wen, X. Chen and W. L. Dai, RSC Adv., 2014, 4,
31162–31165.
55 G. H. Xu, Y. C. Li, Z. H. Li and H. J. Wang, Ind. Eng. Chem.
Res., 1995, 34, 2371–2378.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 82822–82833 | 82833