Aluminum-doped zirconia-supported copper nanocatalysts: Surface synergistic catalytic effects in the gas-phase hydrogenation of esters

Article abstract of DOI:10.1002/cctc.201402674

The efficient gas-phase selective hydrogenation of a series of esters to the corresponding alcohols was achieved over well-dispersed aluminum-doped zirconia-supported copper nanocatalysts (Cu/Al-ZrO₂), which were prepared through a homogeneous coprecipitation route in the presence of cetyl trimethyl ammonium bromide. The characterization revealed that the structure and catalytic performance of Cu/Al-ZrO₂ nanocatalysts were profoundly affected by the addition of Al. Compared with the Al-free catalyst, Al-doped materials had higher specific surface areas and smaller copper nanoparticles. In particular, the results confirmed that the incorporation of Al into the ZrO₂ framework could form tetrahedrally coordinated Al³⁺ species, leading to the improvement of metal dispersion and the formation of more surface Lewis acid sites. In the gas-phase selective hydrogenation of dimethyl oxalate (DMO) to ethylene glycol (EG), 100% DMO conversion, 97.1% EG selectivity, and a high turnover frequency of 16.9'h^-1 were achieved over Cu/Al-ZrO₂ catalyst with a Al/(Cu+Zr+Al) mass ratio of 0.1. The high efficiency of Cu/Al-ZrO₂ catalysts in DMO hydrogenation was attributed mainly to the surface synergistic catalytic effect between highly dispersed metallic copper species and strong Lewis acid sites, which promoted the hydrogenation reaction related to the ester groups, unlike the single case of Cu⁺Cu⁰ synergy reported previously that was found to control the extent of hydrogenation. The obtained catalysts displayed excellent catalytic performance in the gas-phase hydrogenation of other esters including dimethyl succinate, dimethyl maleate, dimethyl adipate, and 1,4-cycolhexane dicarboxylate. The effects of doping: The gas-phase hydrogenation of dimethyl oxalate (DMO) to ethylene glycol (EG) is conducted on well-dispersed Al-doped Zr-supported Cu nanocatalysts, achieving 100% conversion with 97.1% selectivity. The unprecedented catalytic performance is ascribed to the surface synergistic catalytic effect between active metallic Cu sites and strong Lewis acid sites. MG=Methyl glycolate, 1,2-BDO=1,2-butanediol.

Full text of DOI:10.1002/cctc.201402674

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DOI: 10.1002/cctc.201402674
Aluminum-Doped Zirconia-Supported Copper
Nanocatalysts: Surface Synergistic Catalytic Effects in the
Gas-Phase Hydrogenation of Esters
Qi Hu, Guoli Fan, Lan Yang, and Feng Li*[a]
The efficient gas-phase selective hydrogenation of a series of
esters to the corresponding alcohols was achieved over well-
dispersed aluminum-doped zirconia-supported copper nano-
catalysts (Cu/Al-ZrO2), which were prepared through a homo-
geneous coprecipitation route in the presence of cetyl trimeth-
yl ammonium bromide. The characterization revealed that the
structure and catalytic performance of Cu/Al-ZrO2 nanocata-
lysts were profoundly affected by the addition of Al. Compared
with the Al-free catalyst, Al-doped materials had higher specific
surface areas and smaller copper nanoparticles. In particular,
the results confirmed that the incorporation of Al into the ZrO2
framework could form tetrahedrally coordinated Al3+ species,
leading to the improvement of metal dispersion and the for-
mation of more surface Lewis acid sites. In the gas-phase selec-
tive hydrogenation of dimethyl oxalate (DMO) to ethylene
glycol (EG), 100% DMO conversion, 97.1% EG selectivity, and
a high turnover frequency of 16.9 hÀ1 were achieved over Cu/
Al-ZrO2 catalyst with a Al/(Cu+Zr+Al) mass ratio of 0.1. The
high efficiency of Cu/Al-ZrO2 catalysts in DMO hydrogenation
was attributed mainly to the surface synergistic catalytic effect
between highly dispersed metallic copper species and strong
Lewis acid sites, which promoted the hydrogenation reaction
related to the ester groups, unlike the single case of Cu+ÀCu0
synergy reported previously that was found to control the
extent of hydrogenation. The obtained catalysts displayed ex-
cellent catalytic performance in the gas-phase hydrogenation
of other esters including dimethyl succinate, dimethyl maleate,
dimethyl adipate, and 1,4-cycolhexane dicarboxylate.
Introduction
Over the past two decades, owing to its academic and indus-
trial value, the selective hydrogenation of esters to the corre-
sponding alcohols has attracted increasing attention.[1] For in-
stance, a series of hydrogenation processes including those of
dimethyl oxalate (DMO) to ethylene glycol (EG) and dimethyl
maleate to 1,4-butanediol were developed on an industrial
scale.[2] In particular, significant attention has been focused on
the synthesis of EG from synthesis gas, on account of the di-
minishing fossil-fuel resources.[3] For this, the production of EG
has been performed by the catalytic coupling of carbon mon-
oxide with nitrite esters for the formation of dialkyl oxalates,
followed by hydrogenation.[4] However, the yield of EG still
needs improvement and the interaction of catalytically active
sites is not yet fully clear. In industry, esters are normally pro-
duced by catalytic hydrogenation over traditional Cr2O3-pro-
moted Cu-based heterogeneous catalysts.[2b,5] Despite the high
yield of desired products, the toxicity of such types of Cr-con-
taining catalysts can cause serious environmental pollution.
Therefore, base metals, especially nontoxic Cu catalysts, are
under extensive investigation as alternative hydrogenation cat-
alysts.
Recently, the design of efficient and “green” supported Cr-
free Cu-based catalysts for the hydrogenation of esters to alco-
hols has become an important issue, indeed a great challenge
from the viewpoint of sustainable development of resources
and economy.[6] For example, as one of the candidates for Cr-
free catalysts, the industrial Cu/ZnO catalyst has received much
attention[7] because ZnO can function as a physical spacer be-
tween Cu nanoparticles (NPs) to effect high Cu surface area.[8]
It is well documented that the surface Cu+ÀCu0 synergy can
remarkably control the extent of hydrogenation.[9] On the
other hand, the way that the ester molecules interact with the
catalyst surface may be greatly influenced by the acid–base
nature of the catalysts. Recently, our groups found that surface
Lewis base sites in intimate contact with metallic Cu0 sites
could easily interact with the p* acceptor orbital of the carbon-
yl group in ester molecules, thus significantly improving the re-
activity of the ester group.[1c] In addition, the high electron
density of oxygen in the carbonyl group can induce polarity to
ester molecules. Correspondingly, Lewis acid (LA) sites on the
catalyst surface acting as an electron acceptor may fix the car-
bonyl oxygen, thereby activating the carbonyl group.[10]
As for supported catalysts, a synthesis optimization, howev-
er, is far more advanced than a fundamental understanding of
their catalytic activity. A large number of approaches for pre-
paring supported Cu-based catalysts (e.g., deposition–precipi-
tation, incipient wetness impregnation, and colloidal deposi-
[a] Dr. Q. Hu, Dr. G. Fan, Prof. L. Yang, Prof. F. Li
State Key Laboratory of Chemical Resource Engineering
Beijing University of Chemical Technology
P.O. BOX 98, Beijing, 100029 (P.R. China)
Fax: (+86)10-64425385
E-mail: lifeng@mail.buct.edu.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201402674.
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tion methods) have been intensively explored to make full use
of their unique catalytic performance.[11] In most cases, espe-
cially at high metal loadings, it is difficult to obtain well-dis-
persed metal NPs with uniform particle size distribution and
good thermal stability because of the inhomogeneous distribu-
tion of the active species on supports and weak interactions
between them. Hence, an alternative method that involves the
uniform coprecipitation of active metal ions with other metal
ions to construct oxide supports that form highly homogene-
ous precursor colloid catalysts, followed by high-temperature
calcination and reduction, can be used to obtain well-dis-
persed supported metal nanocatalysts and contributes consid-
erably to the fundamental understanding of catalytic phenom-
ena.
Results and Discussion
Structural analysis
In the synthesis of Cu/Al-ZrO2 catalyst precursors, the added
cationic CTAB surfactant can form a large number of vesicles in
solution under vigorous stirring. In this way, the CTAB molecule
may come into contact with Cu2+, Zr4+, and Al3+ cations at
the end of its long carbon chain, leading to homogeneous co-
precipitation of various metal ions. As a result, metal ions can
precipitate more uniformly and integrate better with each
other in the presence of CTAB.
The XRD patterns of calcined catalyst precursors are shown
in Figure 1a. In each case, the XRD patterns exhibit the charac-
teristic diffractions of CuO (JCPDS no. 05-0661) and tetragonal
ZrO2 (JCPDS no. 50-1809) phases. However, no diffraction
As an interesting functional catalytic material, ZrO2 with
both Zr4+ÀO2À acid–base centers and high ion exchange capa-
bility has been used widely in
the field of catalysis.[12] ZrO2 not
only stabilizes the metal particles
but also improves the catalytic
performance through the metal–
support interaction. In addition,
surface LA sites may be generat-
ed by tetrahedrally coordinated
Al3+ species, which can be de-
veloped by Al atoms entering
into the oxide frameworks of an-
other quadrivalent element (e.g.,
SiO2).[13]
Inspired by the above results,
in this contribution, new envi-
ronmentally friendly well-dis-
persed Al-doped Zr-supported
Figure 1. XRD patterns of a) calcined and b) reduced samples.
Cu nanocatalysts (Cu/Al-ZrO2) for
the gas-phase selective hydroge-
nation of a series of esters were prepared through a homoge-
neous coprecipitation approach in the presence of cetyl tri-
methyl ammonium bromide (CTAB). In combination with sys-
tematic characterizations, the effects of Al addition on the
metal dispersion, surface acidity and catalytic performance of
Cu/Al-ZrO2 nanocatalysts were elucidated. It was found that
the incorporation of Al into the ZrO2 framework generated
more surface LA sites. The obtained Cu/Al-ZrO2 catalyst with
a Al/(Cu+Zr+Al) mass ratio of 0.1 showed the highest catalyt-
ic performance in the gas-phase hydrogenation of DMO with
complete DMO conversion and 97.1% EG selectivity. This could
be attributed to the surface synergistic catalytic effect between
highly dispersed metallic Cu species and strong LA (SLA) sites,
which was the main promoter of the hydrogenation reaction
related to ester groups, unlike the Cu+ÀCu0 synergy reported
once previously. Furthermore, the prepared Cu/Al-ZrO2 nanoca-
talysts exhibited excellent catalytic capacity in the hydrogena-
tion of a series of esters including dimethyl succinate, dimethyl
maleate, dimethyl adipate, and 1,4-cycolhexane dicarboxylate.
To the best of our knowledge, such LA-promoted supported
Cu-based catalysts for the hydrogenation of esters have not
been reported to date.
peaks related to crystalline Al2O3 can be found in the XRD pat-
terns. Interestingly, with the gradual introduction of Al, the cal-
cined sample presents a weakened diffraction of the ZrO2
phase at 2qꢀ30.68. This implies that Al species may be incor-
porated into the ZrO2 framework or exist in the amorphous
form, thus inhibiting the crystallization of ZrO2. The above re-
sults demonstrate that Al-doped ZrO2 is formed, and crystalline
CuO is loaded onto the AlÀZrO2 matrix.
27Al solid-state NMR spectroscopy, as an extremely sensitive
probe of the coordination environment of Al, can provide an
excellent means to investigate the site occupancy of Al3+ cat-
ions. In the 27Al NMR spectra of calcined precursors (Figure 2),
an intensive and symmetrical resonance centered at approxi-
mately 55 ppm corresponds to 4-coordinate Al3+ ions[14] and
a very small resonance with the chemical shift of approximate-
ly 0.8 ppm is assigned to 6-coordinate Al3+ ions.[15] The results
suggest the incorporation of Al into the ZrO2 framework in
a 4-coordinate position with good symmetry and the forma-
tion of a small amount of amorphous Al2O3 outside the frame-
work. This is consistent with the above XRD results.
The XRD patterns of reduced Cu/Al-ZrO2 catalysts are pre-
sented in Figure 1b. All samples present a strong (111) diffrac-
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type H2 hysteresis loop at P/P0 values of 0.4–0.8, owing to the
presence of an interconnected pore system.[16] The pore distri-
bution curves show that the most frequent pore diameter cen-
ters at approximately 3.5 nm. In the presence of Al, larger
mesopores in the range of 6–20 nm are developed. As listed in
Table 1, BET specific surface area and total pore volume of Cu/
Al-ZrO2 catalysts increase progressively with Al content, which
is associated mainly with the heightened disordering of the
crystalline ZrO2 structure in samples.
Usually, the high specific surface area can result in the good
dispersion of metal particles.[17] Typical TEM images of Cu/Al-
ZrO2 catalysts are shown in Figure S2. Black Cu clusters in R-
CAZ-0 and R-CAZ-5 (see the Experimental Section for a descrip-
tion of the notation) are poorly dispersed on the support and
undergo minor agglomeration. With the increase of Al doping
amount, Cu clusters without agglomeration of particles are
well-dispersed in the cases of R-CAZ-10 and R-CAZ-15. This is
in good agreement with the above N2O titration results. The
higher specific surface area seems to provide a favorable dis-
persing effect for the Cu species, thus preventing the growth
and agglomeration of Cu particles.
Figure 2. 27Al Magic angle spinning NMR of calcined samples.
tion peak at 2q=43.48 along with a weak one (200) at 2q=
50.38, which is characteristic of the face-centered cubic metallic
Cu phase (JCPDS no. 01-1241). Noticeably, the diffractions from
the ZrO2 phase are more difficult to distinguish with the intro-
duction of Al, which is indicative of the lowered crystallinity.
The average sizes of metallic Cu particles calculated by using
the Scherrer formula based on the (111) diffraction are listed
in Table 1. The introduction of a suitable amount of Al doping
Furthermore, high-resolution (HR)TEM images of representa-
tive R-CAZ-10 depict that a large number of approximately
spherical Cu NPs are well-dispersed on the support (Figure 3a).
However, the clear interface between the metallic Cu phase
and the support matrix is difficult to identify, implying the
presence of a strong metal–sup-
port interaction.
A
typical
HRTEM image depicts the lattice
fringes with an interplanar spac-
ing of approximately 2.09 ꢁ
within a single nanoparticle (Fig-
ure 3b) corresponding to the
(111) plane of the face-centered
cubic metallic Cu phase.
Table 1. Structural and textural data for Cu/Al-ZrO2 catalysts.
[b]
[c]
[d]
[e]
[f]
Catalyst
Content[a] [wt%]
SBET
Vp
Dp
D111
SCu
Di[g] Acid sites[h] SLA sites[i]
Cu
Zr
Al
[m2 gÀ1
]
[cm3 gÀ1
]
[nm] [nm] [m2 gÀ1
]
[%]
[mmolgÀ1
]
[mmolgÀ1
]
R-CAZ-0
R-CAZ-5
17.0 30.4
17.2 28.8 2.7
0
76.3
83.0
0.125
0.126
0.138
0.162
5.48 19.8
5.99 11.3
5.50
6.00
9.8
11.7
14.7
15.1
12.7 0.27
15.2 0.32
18.8 0.42
20.5 0.49
0.15 (0)[j]
0.23 (0)
0.34 (0)
R-CAZ-10 16.8 26.7 4.5 101.9
R-CAZ-15 17.4 26.4 7.1 120.5
8.8
8.5
0.19 (0.21)
The high-angle annular dark-
field–scanning TEM–energy-dis-
[a] Determined by using inductively coupled plasma atomic emission spectroscopy; [b] Specific surface area
calculated by the BET method; [c] Total pore volume; [d] Mean pore diameter; [e] Average crystallite size of
metallic Cu particles based on XRD patterns; [f] Metallic Cu surface area determined by using N2O titration;
[g] Cu dispersion degree; [h] Density of acid sites determined by using NH3-TPD at T=250–9008C; [i] Density of
SLA sites determined by using NH3-TPD at T=500–7508C; [j] Value in parentheses is the density of SSLA sites.
persive
X-ray
spectroscopy
(HAADF–STEM–EDS) image of
a representative R-CAZ-10 cata-
lyst is shown in Figure 3c. The
tiny bright spots in the image
is helpful for retarding the growth of metallic Cu crystallites.
The average Cu particle size, however, decreases from approxi-
mately 8.8 to 8.5 nm on an increase in Al content from 4.5 to
7.1 wt%. As shown in Table 1, both the Cu surface areas of cat-
alysts and the Cu dispersion measured by H2-N2O titration in-
crease monotonically with Al content from 9.8 to 15.1 m2 gÀ1
and from 0.127 to 0.205, respectively, which is consistent with
the change in the average size of metallic Cu particles based
on the XRD patterns.
correspond to Cu NPs supported on the matrix. Moreover, the
histograms of the size distributions of Cu NPs in Cu/Al-ZrO2
catalysts estimated from STEM images (Figure S3) reveal that
the average sizes of Cu NPs are consistent with those based
on XRD line broadening. Elemental mapping analysis based on
HAADF–STEM–EDS (Figure 3d) displays a uniform distribution
of Cu, Zr, Al, and O, respectively. The presence and distribution
of the elements Cu, Zr, and Al within one particle were verified
further by means of the STEM–EDS line-scan spectrum. As
shown in Figure 3e and f, two Cu and Zr lines show a similar
Gaussian distribution along each individual particle, indicating
their similarly homogeneous distributions over the sample. In-
terestingly, the distribution of elemental Al in the EDS line
spectrum appears to be mainly at the edges of the particles,
rather than in the middle, indicative of the formation of a large
As shown in Figure S1, in each case, the Cu/Al-ZrO2 catalyst
displays a type IV N2 adsorption–desorption isotherm with
a typical type H1 hysteresis loop at high relative pressures
(P/P0) greater than 0.7, which is characteristic of a mesoporous
material with 2-dimensional cylindrical channels. Interestingly,
a close observation of the isotherm reveals another small
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Figure 4. H2-TPR profiles of calcined samples.
introduction of Al can improve the metal dispersion and thus
the reducibility of Cu-containing species.
Usually, the dissociation of H2 for reduced samples can be
determined by using the H2 temperature-programmed desorp-
tion (H2-TPD) technique. As shown in Figure S4, H2-TPD profiles
of Cu/Al-ZrO2 catalysts present a broad peak at approximately
3838C, which is assigned to the H2 chemisorption on Cu0 spe-
cies. The total amount of desorbed H2 for catalysts increases in
the order R-CAZ-0<R-CAZ-5<R-CAZ-10<R-CAZ-15, indicating
the heightened dissociation of H2 as a result of higher Cu sur-
face area.
Surface acidity and chemical composition
Figure 3. a,b) HRTEM images of R-CAZ-10; c,d) high-magnification STEM
images with EDS mapping; e) high-magnification STEM image; f) EDS line
spectra of Cu, Zr, and Al along the red line shown in part e.
As for Cu/Al-ZrO2 catalysts, LA and Brønsted acid (BA) sites are
related to Zr4+ or Al3+ cations and OHÀ groups on the surface,
respectively. Additionally, it has been reported that the defects
produced by tetrahedrally coordinated Al3+ species would
generate LA sites.[13]
amount of AlÀZrO2 in strong interfacial contact with Cu NPs.
As a result, HAADF–STEM–EDS analysis verifies the formation
of a highly dispersed structure of the Cu/AlÀZrO2 catalysts,
which agrees with the HRTEM observations.
IR spectroscopy of adsorbed pyridine (Py-IR) spectra of Cu/
Al-ZrO2 catalysts at 258C are shown in Figure 5. All samples
present several characteristic bands from pyridine adsorption
on different acid sites, including LA (1613, 1456, and
1449 cmÀ1) and BA sites (1512 cmÀ1).[20] Among them, the band
at 1613 cmÀ1 can be assigned to pyridine adsorbed on SLA
sites.[21] In addition, the bands in the ranges of 1440–1460 and
1590–1600 cmÀ1 are associated with adsorption of pyridine by
H-bonding, whereas the bands at 1439 and 1584 cmÀ1 are as-
signed to the physical adsorption of pyridine.[20,22] Notably, Cu/
Al-ZrO2 catalysts show an increase with Al content in the inten-
sity of the band of LA sites at 1613 cmÀ1. This is related to the
generation of more defects originating from tetrahedrally coor-
dinated Al species in the ZrO2 framework, thus leading to the
formation of more LA sites on the surface.[13] In the case of R-
CAZ-15, only a weak adsorption of pyridine to BA sites is ob-
served. As for R-CAZ-0, R-CAZ-5, and R-CAZ-10 catalysts, how-
ever, the band of pyridine bonded to BA sites cannot be ob-
served distinctly. Moreover, the temperature-dependent Py-IR
spectra of catalysts (Figure S5) reveal that the bands of physi-
Redox behaviors and dissociation of H2
H2 temperature-programmed reduction (H2-TPR) characteriza-
tion was performed to investigate the redox properties of cal-
cined samples (Figure 4). In each case, the reduction peak
covers a wide range from approximately 125 to 2758C, which
can be attributed to the reduction of Cu2+ species. The fitted
low-temperature reduction peak (I) is ascribed to the reduction
of small isolated highly dispersed CuO on the surface,[18]
whereas the high-temperature peak (II) is associated with the
reduction of larger bulk CuO particles.[19] With an increase in
Al/(Cu+Zr+Al) mass ratio from 0 to 0.15, the amount of
highly dispersed CuO species increases gradually based on the
proportions of the two chemical states of the Cu2+ ions. Ac-
cordingly, as for C-CAZ-15, the proportion of highly dispersed
CuO reaches the maximum. The above results confirm that the
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Figure 5. Py-IR spectra of a) R-CAZ-0, b) R-CAZ-5, c) R-CAZ-10, and d) R-CAZ-
15 at 258C, with types of pyridine binding site indicated (HB=H-bonded,
PA =physically adsorbed).
Figure 6. NH3-TPD profiles of a) R-CAZ-0, b) R-CAZ-5, c) R-CAZ-10, and d) R-
CAZ-15. TCD=Thermal conductivity detector.
cally adsorbed, H-bonded, and BA site-bonded pyridine for R-
CAZ-15 decrease dramatically with increasing desorption tem-
perature and almost vanish above 2508C. Despite the reduced
intensities of adsorption bands, the pyridine adsorbed on LA
sites can survive high-temperature evacuation (at 3008C),
which is suggestive of the high strength of LA sites. The above
results demonstrate that the LA sites bind more tightly with
pyridine than the BA sites, and BA sites of relatively weak and
moderate strength are present on Cu/Al-ZrO2 catalysts. At
3008C, the remaining broad bands for R-CAZ-15 at 1613 and
1456 cmÀ1 show characteristics of LA sites. However, R-CAZ-10
possesses LA sites at 1449 cmÀ1 but lacks LA sites at 1613 and
1456 cmÀ1. This indicates that LA sites of different strengths
distribute on Cu/Al-ZrO2 catalysts.
ZrO2 framework improve the amount and/or strength of sur-
face LA sites. As a result, Cu/Al-ZrO2 catalysts differ in the
amount and strength of LA sites, which may play a prominent
part in the DMO hydrogenation.
It is well known that catalytic performance is greatly related
to the surface chemical composition. The surface composition-
al information on Cu/Al-ZrO2 catalysts was obtained by using
X-ray photoelectron spectroscopy (XPS; see Figure S6). In each
case, the Cu2p3/2 signal at 932.3 eV is associated with metallic
Cu and/or CuI species.[4,9] Further evidence of the Cu reduction
stems from the absence of XPS signals of Cu2+ at 933–935 eV
and the shake-up satellite of Cu2+ at 942–944 eV.[24] X-ray in-
duced Auger electron spectroscopy (XAES) was performed to
examine the chemical states of surface Cu after reduction (see
inset in Figure S6). Clearly, the asymmetry and broad Cu LMM
Auger kinetic energy peaks of all samples can be deconvoluted
into two symmetry peaks centered at 915.3 and 918.1 eV. The
kinetic energy at 915.3 eV is assigned to Cu+ species and that
at 918.3 eV to Cu0 species.[25] Notably, in all cases, the Cu0/Cu+
intensity ratio is approximately 0.34, which remains unchanged
with increasing Al content, probably as a result of the strong
interaction between Cu species and the ZrO2 matrix, which
leads to the formation of a Cu+ÀOÀZr4+-like structure in Cu/
Al-ZrO2 catalysts.
Furthermore, NH3-TPD measurements were taken to evalu-
ate the character of acid sites on reduced Cu/AlÀZrO2 catalysts
(Figure 6). Under identical experimental conditions, the posi-
tions of desorption peaks should give information about the
types and relative strengths of acid sites, and the peak areas
give information about the amounts of acid sites. With all sam-
ples, there are mainly three temperature ranges of NH3 desorp-
tion at approximately 50–2508C (I), 250–5008C (II) and 500–
7508C (III). According to the above Py-IR results and the litera-
ture,[23] desorption of NH3 from different weak acid sites
should be assigned to the first range (I), that from surface
moderate strength acid sites including LA sites and/or BA sites
to the second range (II), and that from surface SLA sites to the
third range (III). Specially, R-CAZ-15 also shows a large desorp-
tion peak in the range 750–9008C, owing to desorption of NH3
from super strong LA (SSLA) sites. According to a quantitative
analysis on the densities of acid sites on Cu/Al-ZrO2 catalysts
(Table 1), the density of acid sites in the temperature range
250–9008C increases with Al content, whereas that of SLA sites
increases from 0.15 to 0.34 mmolgÀ1 with the Al/(Cu+Zr+Al)
Gas-phase hydrogenation of esters
The catalytic performance of Cu/Al-ZrO2 catalysts was investi-
gated in the gas-phase DMO hydrogenation. The DMO hydro-
genation is known to comprise several continuous reactions,
including DMO hydrogenation to methyl glycolate (MG), MG
hydrogenation to EG, and further deep hydrogenation to etha-
nol. In addition, the dehydration reaction between EG and eth-
anol to form 1,2-butanediol (1,2-BDO) cannot be neglected.
mass ratio from
0 to 0.1, then drops dramatically to
0.19 mmolgÀ1 with continued increase in Al content. The
above results confirm that the Al species introduced to the
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The catalytic activity and selectivity for DMO hydrogenation
versus different Al/(Cu+Zr+Al) mass ratios in Cu/Al-ZrO2 cata-
lyst are given in Figure 7. Under the given reaction conditions,
the DMO conversion increases steadily from 89.8% over
R-CAZ-0 to 100% over R-CAZ-10 with the Al/(Cu+Zr+Al)
mass ratio from 0 to 0.1 and the selectivity to EG also increases
Figure 7. Catalytic performance of Cu/Al-ZrO2 catalysts in gas-phase DMO
hydrogenation. Reaction conditions: P=2.5 MPa, T=2208C, H2/DMO molar
ratio=120, WHSV=0.8 hÀ1
.
steadily from 92.3 to 97.1%. Although DMO can be completely
converted over R-CAZ-15, the EG selectivity (93.1%) is much
lower than that over R-CAZ-10 because more EG can be con-
verted further into ethanol and 1,2-BDO through deep hydro-
genation and dehydration. The R-CAZ-10 catalyst shows the
best catalytic performance for the DMO hydrogenation.
Figure 8. a) DMO conversion and b) EG selectivity versus WHSV of DMO over
different catalysts. Reaction conditions: P=2.5 MPa, T=2208C, H2/DMO
molar ratio=120.
In the gas-phase DMO hydrogenation, a variety of reaction
factors (e.g., H2 pressure, space velocity, temperature, and ratio
of substrates) can significantly affect catalytic performance. To
further understand the effect of Al introduction on the catalyt-
ic performance of Cu/Al-ZrO2 catalysts, the reactions under dif-
ferent weight hourly space velocity (WHSV) values were con-
ducted (Figure 8). The DMO conversion over R-CAZ-10 and R-
CAZ-15 catalysts remains above 99% between WHSVs of DMO
from 0.3 to 0.8 hÀ1, then decreases slowly with WHSV increas-
ing from 0.8 to 1.6 hÀ1. Subsequently, conversion over R-CAZ-0
and R-CAZ-5 declines continuously at a WHSV of 0.3 hÀ1 and
the conversion over Al-free R-CAZ-0 catalyst drops dramatically
at WHSVs of DMO above 1.1 hÀ1. With the WHSV increasing
from 0.3 to 0.8 hÀ1, the selectivity to EG remains approximately
unchanged over all of the Cu/Al-ZrO2 catalysts. Despite the
gradual decrease in selectivity to EG with the further increase
in WHSV, the R-CAZ-10 catalyst continues to shows higher EG
selectivity than the other catalysts. The best catalytic per-
formance, as well as high tolerance to the WHSV of DMO, is
achieved over R-CAZ-10. Apparently, the appropriate amount
of Al doping significantly improves the catalytic performance
for the DMO hydrogenation, which follows the order: R-CAZ-
10>R-CAZ-15>R-CAZ-5>R-CAZ-0. Moreover, compared with
other supported Cu-based catalysts,[2c,6b,17b] the R-CAZ-10 de-
scribed here shows better or comparable catalytic performance
in DMO hydrogenation based on catalytic activity and selectivi-
ty to EG.
As for hydrogenation catalyzed by metallic Cu, Cu dispersion
and surface area are always considered vital parameters that
greatly influence the catalytic performance. As shown in
Table 1, the dispersion and surface area of metallic Cu clearly
improve with increasing Al content in the catalysts. According
to the XAES analysis results, metallic Cu species exist as
a major component on the surface. It is deduced that Cu0 spe-
cies are the catalytically active centers for the hydrogenation.
The quantity of dissociative H2 on the active sites directly de-
termines the rate of the reaction. Correspondingly, a larger
metallic Cu surface area results in more dissociated H2, as evi-
denced by H2-TPD results. In addition, it is well documented
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that the Cu+ÀCu0 synergy can further control the extent of hy-
drogenation,[4,6,9] because electrophilic Cu+ sites may polarize
the C=O bond via the lone pair electron on the O atom. Ac-
cordingly, a balanced distribution of Cu+ and Cu0 is one of the
key issues for obtaining outstanding hydrogenation per-
formance. In the catalyst system presented here, although all
Cu/Al-ZrO2 catalysts possess the same Cu+/Cu0 ratio, the syner-
gy between Cu+ and Cu0 inevitably influences the catalytic
performance. Moreover, R-CAZ-10 catalyst with a Cu surface
area slightly lower than that of R-CAZ-15 and the highest den-
sity of SLA sites exhibits superior catalytic performance. If R-
CAZ-15 with the highest Cu surface area and the smallest den-
sity of SLA sites is used to catalyze the hydrogenation, the
yield of EG, however, decreases slightly. The results indicate
that Cu surface area alone cannot be used to explain the dif-
ference in the catalytic performances.
(16.2 hÀ1). The results demonstrate that the surface Cu species
on Cu/Al-ZrO2 catalysts are not the only important catalytically
active sites and the hydrogenation of DMO is a structure-sensi-
tive reaction. Therefore, some complicated interactions on cat-
alytically active sites with high efficiency in the DMO hydroge-
nation probably occur. It is suggested that the surface SLA
sites on Cu/Al-ZrO2 catalysts strongly activate ester molecules.
Thus, the surface acidic nature probably plays an important
role in enhancing the efficiency of catalysts during the DMO
hydrogenation.
Based on the above experimental results, a possible mecha-
nism for the DMO hydrogenation over Cu/AlÀZrO2 catalysts is
proposed. Firstly, the highly dispersed Cu0 species are the cata-
lytically active centers for DMO hydrogenation. The quantity of
H2 that dissociates on Cu0 species determines the reaction rate.
Secondly, Cu+ species as electrophilic sites can improve the re-
activity of the ester group in the substrate. In addition, the sur-
face LA sites make close contact with metallic Cu0 sites, espe-
cially SLA sites, and can easily interact with one of the O atom
lone pairs of the carbonyl groups in the nodal plane of the
C=O p bond, thus further improving the reactivity of the ester
group. Moreover, the formation of the C=O p bond can facili-
tate the nucleophilic attack of H· to the positively charged
carbon.[27] Therefore, except for Cu0 species, surface SLA sites
are also very important to the DMO hydrogenation. As a result
of the hydrogenation of C=O, DMO transforms gradually to
MG by eliminating CH3OH, which is evidenced by the forma-
tion of incomplete hydrogenation product. As a result, the su-
perior catalytic performance of R-CAZ-10 catalyst is attributed
to the presence of more highly dispersed Cu species and sur-
face SLA sites. In addition, owing to a small amount of H-
bonded sites present on Cu/Al-ZrO2 catalysts under the gas-
phase DMO hydrogenation conditions, as evidenced by the
temperature-dependent Py-IR spectra, H-bonded sites have
marginal effect on the gas-phase DMO hydrogenation. A possi-
ble catalytic mechanism for DMO hydrogenation over the pre-
pared Cu/Al-ZrO2 nanocatalyst is proposed to rationalize the
experimentally observed structure–performance relationship
(Scheme 1). In the proposed mechanism, the hydrogenation of
only one ester group is shown; the other can be assumed to
undergo the same hydrogenation path.
On the other hand, to rigorously compare the intrinsic
nature of catalytically active Cu sites on different Cu/Al-ZrO2
catalysts, DMO conversion data limited to less than 30% by ad-
justing the WHSV of DMO were used to calculate the initial
turnover frequency (TOF), based on the moles of DMO convert-
ed by each mole of surface Cu on the catalyst surface in the
first 1 h of reaction.[26] Variations in the initial TOF as a function
of the Al content are shown in Figure 9. Notably, the TOF value
As a result, the incorporation of Al into the ZrO2 frameworks
promotes the dispersion of Cu species and generates more
SLA sites on the surface of Cu/Al-ZrO2 catalysts. Correspond-
ingly, the DMO conversion rate increases significantly owing to
Figure 9. Initial TOF of DMO over Cu/Al-ZrO2 catalysts with different
Al/(Cu+Zr+Al) mass ratios.
increases progressively with the
Al/(Cu+Zr+Al) mass ratio and
reaches a maximum (16.9 hÀ1) at
Al/(Cu+Zr+Al)=0.1, which is
indicative of the significant pro-
motional effect of the addition
of Al on the catalytic activity. As
for R-CAZ-15 with the highest
Cu surface area, however, the
excess amount of Al leads to
a slight decrease in the TOF Scheme 1. Proposed hydrogenation mechanism over the Cu/Al-ZrO2 nanocatalyst.
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the facile dissociation of H2 molecules on highly dispersed cat-
alytically active Cu species and activation of ester molecules
on the surface SLA sites, which result in the enhanced catalytic
activity; however, a high amount of SSLA sites on R-CAZ-15
may be overactive towards the C=O group, leading to the pro-
duction of a detectable amount of deep hydrogenation and
dehydration byproducts (ethanol and 1,2-BDO). The excellent
catalytic performance of Cu/Al-ZrO2 nanocatalysts for the DMO
hydrogenation can be attributed to the surface synergistic cat-
alytic effect between highly dispersed metallic Cu species and
SLA sites, in addition to the contribution by Cu+ species. In
general, adding the appropriate amount of Al can be an effec-
tive way to improve the dispersion of active metal species and
the proportion of surface SLA sites on Cu/Al-ZrO2 catalysts to
achieve remarkable catalytic performance.
The resultant Al-containing Cu/Al-ZrO2 catalysts exhibited
much higher activity in the dimethyl oxalate hydrogenation
than the Al-free catalyst with larger Cu nanoparticles and
fewer acidic sites. In particular, Cu/Al-ZrO2 with an optimized
Al/(Cu+Zr+Al) mass ratio of 0.1 and highest density of strong
Lewis acid sites showed an exceptional catalytic hydrogenation
performance with 100% conversion and 97.1% selectivity to
ethylene glycol. A surface synergistic catalytic effect between
highly dispersed catalytically active Cu0 sites and strong Lewis
acid sites was proposed to explain the observation. The Cu0
sites contributed to the activation of H2, whereas surface
strong Lewis acid sites favored the activation of substrate ester
molecules. Furthermore, the prepared Cu/Al-ZrO2 nanocatalyst
was also active in the hydrogenation of other esters. From
both academic and industrial viewpoints, this work provides
a guide to the preparation of cost-effective and highly efficient
supported Cu-based catalysts for the selective hydrogenation
of esters to the corresponding alcohols.
Gas-phase hydrogenation of other esters
To further investigate the catalytic capacity of Cu/Al-ZrO2 nano-
catalysts, various saturated and unsaturated fatty acid methyl
esters including dimethyl succinate, dimethyl maleate, dimeth-
yl adipate, and 1,4-cycolhexane dicarboxylate were selected to
undergo the gas-phase hydrogenation to generate the corre-
sponding alcohols. The R-CAZ-10 catalyst is also very effective
in the hydrogenation of these esters (Table 2), exhibiting
a much higher catalytic performance with more than 98.0%
conversion and 91% selectivity to the target alcohols than the
Al-free R-CAZ-0 catalyst. Notably, dimethyl maleate can be con-
verted to 1,4-butanediol over R-CAZ-10 at a relatively low reac-
tion temperature of 1708C. The above finding indicates the ex-
cellent catalytic performance of Cu/Al-ZrO2 nanocatalysts in
the gas-phase hydrogenation of a series of esters.
Experimental Section
Synthesis of catalysts
Cu/Al-ZrO2 catalyst precursors were synthesized by a co-precipita-
tion method. Firstly, CTAB (3 g) was dissolved in deionized water
(200 mL) under vigorous stirring to form a surfactant solution, then
Cu(NO3)2·6H2O, Al(NO3)3·9H2O, and Zr(NO3)4·5H2O were dissolved
in the surfactant solution, followed by a dropwise addition of
aqueous Na2CO3 solution (1.4m, 100 mL). The resulting suspension
was aged for 12 h at 608C, recovered by dispersion and centrifuga-
tion in ethanol–water solutions (1:1 v/v), and dried at 708C over-
night. The obtained samples were calcined in air at 4508C for 6 h,
pelletized, crushed, and sieved to 40–60 meshes. The calcined sam-
ples were denoted as C-CAZ-x, in which x is the Al/(Cu+Zr+Al)
mass ratio. Subsequently, the C-CAZ-x samples were reduced in
a 5% H2/Ar atmosphere at 2508C for 2 h at a ramping rate of
28CminÀ1 and the obtained catalysts denoted as R-CAZ-x.
Table 2. Catalytic performance of Cu/Al-ZrO2 catalysts in the gas-phase
hydrogenation of a series of esters.[a]
Ester
Sample
T
P
Conversion Selectivity
Characterization
[8C] [MPa] [%]
[%]
85.2[b]
91.5[b]
90.5[c]
98.5[c]
87.5[d]
96.2[d]
90.2[e]
95.8[e]
XRD data were collected at RT on a Shimadzu XRD-6000 diffrac-
tometer with a graphite-filtered CuKa source (l=0.15418 nm) at
40 kV and 30 mA.
dimethyl succinate R-CAZ-0
220 3.0 90.0
R-CAZ-10 220 3.0
R-CAZ-0 170 6.0
R-CAZ-10 170 6.0
R-CAZ-0 220 6.0
R-CAZ-10 220 6.0
R-CAZ-0 220 8.0
R-CAZ-10 220 8.0
100.0
88.5
98.9
90.3
98.0
88.7
98.8
dimethyl maleate
dimethyl adipate
Elemental analysis was performed through inductively coupled
plasma atomic emission spectroscopy on a Shimadzu ICPS-7500
spectrometer after the samples were dissolved in dilute hydrochlo-
ric acid (6.0 m).
1,4-cycolhexane
dicarboxylate
[a] WHSV=0.8 hÀ1; [b] g-Butyrolactone; [c] 1,4-Butanediol; [d] 1,6-Hexane-
diol; [e] 1,4-Cyclohexanedimethanol.
TEM and HRTEM were performed on a JEOL 2100 microscope oper-
ated at an accelerating voltage of 200 kV. HAADF–STEM–EDS
images were recorded on a JEOL 2010F instrument combined with
an EDS system by using an electron probe (diameter 0.5 nm) at
a diffraction camera length of 10 cm.
Conclusions
Low-temperature N2 adsorption–desorption isotherms of the sam-
ples were obtained on a Micromeritics ASAP 2020 sorptometer.
The total specific surface areas were evaluated with the multipoint
BET method. The mesopore size distributions were determined by
the BJH method.
Well-dispersed aluminum-doped zirconia supported copper
nanocatalysts were prepared by uniform coprecipitation in the
presence of cetyl trimethyl ammonium bromide. The incorpo-
ration of Al into the ZrO2 framework, which could generate
tetrahedrally coordinated Al species, helped to disperse the Cu
species and enhance the amount of surface Lewis acid sites.
XPS was recorded on a Thermo VG ESCALAB250 X-ray photoelec-
tron spectrometer with AlKa X-ray radiation (hn=1486.6 eV). XAES
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was performed on a PHI Quantera SXM at a base pressure of
6.7ꢂ10À8 Pa with an AlKa X-ray as the excitation source.
methanol and H2 were fed into the reactor at a H2/DMO molar
ratio of 120 at 493 K. During hydrogenation, the total pressure was
retained at 2.5 MPa and the WHSV of DMO changed from 0.3 to
1.6 hÀ1. Finally, the liquid products were analyzed on an Agilent
GC7890 A gas chromatograph equipped with a flame ionization
detector and HP-5 capillary column. To investigate the effect of in-
ternal diffusion, catalysts with different grain diameters were used.
It was found that the conversion of DMO did not vary if the grain
diameter of the catalyst was less than 40 mesh. Also, there was no
difference in the DMO conversion under two different catalyst
loadings of 2.0 and 3.0 g if the applied values of WHSV and H2/
DMO molar ratio were constant, which indicated that the external
mass transfer limitation could be ignored in the present reaction
system. Other esters were evaluated by using a similar procedure,
H2-TPR was performed by using Micromeritics ChemiSorb 2720 in-
strument. The sample (0.1 g) was placed in a quartz U-tube reactor
and degassed at 2008C for 2 h under a flow of Ar (40 mLminÀ1).
Then, TPR was conducted in
a stream of 10% v/v H2/Ar
(40 mLminÀ1) with a heating rate of 58CminÀ1 up to 8008C. The ef-
fluent gas was analyzed by using a thermal conductivity detector.
With Ag2O as the external standard, the thermal conductivity de-
tector signal was calibrated after trapping H2O at 184 K.
The surface areas, dispersions, and crystal sizes of Cu0 particles
were determined through H2-N2O titration by using a Micromeritics
ChemiSorb 2720 instrument.[28] Firstly, the calcined sample under-
went H2-TPR in 10% H2/Ar from 50 to 3008C and the temperature
was held until H2 consumption ceased. After cooling down, the
gas was switched to 10% N2O/N2 and the sample was oxidized at
a flow rate of 40 mLminÀ1 at 608C for 1 h, followed by Ar purging
and cooling of the sample bed to RT. Finally, H2-TPR was performed
again with a gas mixture of 10% H2/Ar to 3008C. Cu dispersion
was calculated by dividing the amount of surface copper sites by
the total number of supported Cu atoms; Cu surface area was cal-
culated by assuming a spherical shape of the Cu metal particles
in which the WHSV of the esters was set at 0.8 hÀ1
.
Acknowledgements
We gratefully acknowledge the financial support from 973 Pro-
gram (2011CBA00506), the National Natural Science Foundation
of China and Program from Beijing Engineering Center for Hier-
archical Catalysts.
and a surface concentration of 1.47ꢂ1019 Cu atomsmÀ2
27Al solid-state magic-angle spinning NMR spectra were performed
.
on Bruker Avance III 400 NMR spectrometer operating at
104.261 MHz for Al with a spinning rate of 6 KHz.
a
Keywords: copper · heterogeneous catalysis · hydrogenation ·
Lewis acids · nanoparticles
The nature of the acid sites on reduced samples was characterized
by Py-IR analysis. The spectra were recorded with a 4 cmÀ1 spectral
resolution on a Thermo Nicolet 380 spectrometer equipped with
a temperature-controlling accessory. The sample powder (50 mg)
was pressed into a self-supporting wafer. The wafers were placed
in an evacuable IR cell with CaF2 windows, evacuated at 2008C for
2 h under Ar flow, and then cooling of the sample wafer to RT.
After the background spectrum was recorded, pyridine was intro-
duced and balanced at this temperature for 1 h. Physisorbed pyri-
dine was removed by evacuation and the samples were heated
stepwise under vacuum. The spectra were recorded at 25, 100,
150, 200, 250, and 3008C.
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