Effect of activation temperature on the surface copper particles and catalytic properties of Cu-Ni-Mg-Al oxides from hydrotalcite-like precursors

Article abstract of DOI:10.1016/j.catcom.2011.03.011

The Cu-Ni-Mg-Al oxides catalysts for furfural hydrogenation were prepared from the hydrotalcite-like precursors, and the effect of activation temperature on the Cu⁰ particles and catalytic properties of the catalyst was thoroughly investigated. The catalyst activated by H₂ at 300 °C was found to exhibit the best catalytic activity, due to the presence of the smallest Cu⁰ particles with a high dispersion. Moreover, the bigger Cu⁰ particles were active for furfuralcohol hydrogenolysis to 2-methylfuran in the liquid-phase (ethanolic solution), and the hydrogenation of the furan ring of furfuralcohol and 2-methylfuran on Cu⁰ particles was easily achieved in the vapour-phase.

Full text of DOI:10.1016/j.catcom.2011.03.011

Catalysis Communications 12 (2011) 996–999
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Effect of activation temperature on the surface copper particles and catalytic
properties of Cu–Ni–Mg–Al oxides from hydrotalcite-like precursors
⁎
Chenghua Xu , Liangke Zheng, Danfeng Deng, Jianying Liu, Shengyu Liu
College of Resources and Environment, Chengdu University of Information Technology, Chengdu 610225, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The Cu–Ni–Mg–Al oxides catalysts for furfural hydrogenation were prepared from the hydrotalcite-like
precursors, and the effect of activation temperature on the Cu0 particles and catalytic properties of the catalyst
was thoroughly investigated. The catalyst activated by H2 at 300 °C was found to exhibit the best catalytic
activity, due to the presence of the smallest Cu0 particles with a high dispersion. Moreover, the bigger Cu0
particles were active for furfuralcohol hydrogenolysis to 2-methylfuran in the liquid-phase (ethanolic
solution), and the hydrogenation of the furan ring of furfuralcohol and 2-methylfuran on Cu0 particles was
easily achieved in the vapour-phase.
Received 15 October 2010
Received in revised form 5 March 2011
Accepted 8 March 2011
Available online 15 March 2011
Keywords:
Hydrotalcite-like precursors
Cu0 metal particles
Furfural hydrogenation
Furfuralcohol
© 2011 Elsevier B.V. All rights reserved.
2-Methylfuran
1. Introduction
particle size; and iii) to investigate the FFR catalytic hydrogenation
over this catalyst activated by H2 at different temperatures both in the
Furfural (FFR) can be hydrogenated to produce an important
chemical, furfuralcohol (FFA), which is mainly used in polymer
industry, synthetic fibers, rubber-resins, farm chemical and in other
products from fine chemical industry. The commercial catalyst used in
the FFR hydrogenation is a Cr2O3 promoted Cu-containing catalyst,
wherein MgO generally acts as support. Nagaraja et al. [1,2] have
studied Cu–MgO coprecipitated catalysts for FFR hydrogenation to
FFA in the gas phase and concluded that the presence of both Cu+ and
Cu0 species was responsible for the high selectivities obtained. The
same group has reported the use of a Cu–MgO–Cr2O3 catalyst for FFR
gas-phase hydrogenation to FFA using cyclohexanone dehydrogena-
tion as hydrogen resource [2]. Their conclusion is that Cr2O3 can
stabilize the Cu0/Cu+ species required for both the hydrogenation/
dehydrogenation reactions. However, the toxicity of Cr2O3 in Cr2O3
promoted Cu-containing catalysts will cause severe environmental
pollution. Recently, a Cu–Fe/SiO2 catalyst has been reported to show a
high selectivity towards the FFR hydrogenation to 2-methyfuran via
the selective hydrogenolysis of the intermediate FFA [3].
liquid and vapour-phase.
2. Experimental
The Cu–Ni–Mg–Al mixed oxides catalysts (CuNiMgAlOy) were
prepared by the calcination of the corresponding hydrotalcite-like
precursors (CuNiMgAl(OH)x) at 500 °C in air for 4 h. The precursors
were synthesized according to the procedure reported in the
literature [4], wherein the molar composition of metal ions was
11.2Cu2+:4.7Ni2+:50Mg2+:50Al3+. The obtained catalysts were cha-
racterized by XRD, N2 adsorption–desorption, XPS, H2-TPR and H2–
N2O titration. The Cu surface area (SCu), dispersions (DCu) and average
particle size (rav) of the reduced catalysts were calculated according to
H2–N2O titration as the previous reports [5,6]. The FFR liquid-phase
and vapour-phase hydrogenation were carried out at 200 and 220 °C,
respectively. The catalysts were activated by H2 in the temperature
range of 220–400 °C. The detailed procedures are described in the
Supplementary material.
The present work aimed: i) to prepare a new environmentally
friendly FFR hydrogenation catalyst (CuNiMgAlOy) with a high Cu
dispersion from the hydrotalcite-like precursors according to our
previous work [4]; ii) to study thoroughly the effect of activation
temperature on the characteristics of Cu0 species at the catalyst
surface such as their dispersion, the metal specific surface area and the
3. Results and discussion
3.1. Textural and structural properties
The XRD patterns of the calcined catalyst (CuNiMgAlOy) and its
precursor (CuNiMgAl(OH)x) (Fig. 1) clearly indicate that CuNiMgAl
(OH)x exhibits a typical hydrotalcite-like phase, which will be destroyed
to form spinel species such as MgAlO2, CuAlO2 or NiAlO2 [7,8], and even
a small number of oxides [9] during the calcination. Meanwhile, the
⁎
Corresponding author. Tel.: +86 28 85966011; fax: +86 28 85966089.
E-mail address: xch@cuit.edu.cn (C. Xu).
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.03.011

C. Xu et al. / Catalysis Communications 12 (2011) 996–999
997
(c)
(b)
hydrtalcite
CuO
MgAlO2, NiAlO2 or CuAlO2
MgO
(b)
(a)
(a)
10
20
30
40
2
50
60
70
80
100
200
300
400
500
600
700
800
Temperature /oC
/(o)
Fig. 2. H2-TPR profiles of the CuNiMgAlOy catalyst (a), Cu11.2/MgAlOx (b) and Ni4.7
MgAlOx (c).
/
Fig. 1. XRD patterns of the calcined catalyst CuNiMgAlOy (a) and its precursor (b).
characteristic peaks for CuO in CuNiMgAlOy are very weak and no NiO
species are detected, indicating that Cu2+ and Ni2+ species in
CuNiMgAlOy are mainly located in the spinel phase or highly dispersed
onto MgAlO2 or MgO.
is increased with the rise of activation temperature, however the peak at
933.8 eV ascribed to Cu2+ species almost disappears at the activation
temperature of 300 °C. Therefore, the Cu2+ species in the catalyst are
deduced to be completely reduced by H2 to Cu0 when the activation
temperature is beyond 300 °C.
The N2 adsorption–desorption analysis (Fig. S1 in the Supplementary
material) shows that the isotherm for catalyst precursor is typical of Type
II in nature and exhibits a clear open H3 hysteresis loop according to the
IUPAC classification, which is often recorded in layered materials with
long and narrow cleft [10,11]. However, the calcined catalyst gives a
close H3 hysteresis loop, indicating that the layered structure of
hydrotalcite-like precursor has been destroyed to form a new phase,
and its surface area, pore diameter and pore volume (Table 1) are
increased after calcination due to the gradual disappearance of the
interlay OH− and CO23− anions.
Meanwhile the H2–N2O titration studies are also introduced to
investigate the state of the surface Cu metal particles. From the results
(Fig. 2S in Supplementary material), all H2 consumption peaks are
clearly observed to appear at about 220 °C, which is lower than that for
the Cu2+ reduction in CuNiMgAlOy (287 °C, in Fig. 2), indicating that all
copper species titrated by N2O are active particles. And the intensity of
H2 consumption peak decreases with the increasing activation
temperature. According to the surface stoichiometry of the titration
Cu0 with N2O and using the calculation methods reported [5,6], the Cu
dispersion (DCu), the Cu metal specific area and the average Cu0 particle
size (rav) of the catalyts activated by H2 at above 300 °C have been
calculated and the values obtained are shown in Table 2. The results
clearly show that the Cu dispersion and metal surface area decrease,
however the Cu average particle sizes increase with the increasing
activation temperature, indicating that the Cu particles are obviously
agglomerated at a high temperature. Moreover the average Cu particle
3.2. Surface species and reducibility properties
In the previous studies [1,2,12], the Cu0 species were considered to
be the catalytic active centers for FFR hydrogenation over the Cu-
containing catalysts. The H2-TPR results of catalysts (Fig. 2) show that
CuNiMgAlOy gives an intensive H2 consumption peak at 287 °C and a
weak peak at about 675 °C. The former can be ascribed to the reduction
of the highly dispersed Cu2+ species including the isolated Cu2+ ions in
spinel and CuO [8], the latter one is assigned to the reduction of Ni2+
species. Moreover, it can also be observed that the Cu2+ species in
catalyst from hydrotalcite-like precursors are reduced more easily than
those in Cu-supported catalyst, however the Ni2+ species are difficultly
reduced in comparison to those in the supported Ni catalyst. It is
possibly due to the bigger ionic radius of Cu2+ (0.073 nm) than that of
Ni2+ (0.069 nm), resulting in the migration of Cu2+ cations to the
surface spinel phase to form the highly dispersed CuO phase and the
easy penetration of Ni2+ species into the bulk phase, which has been
proved by the above XRD results.
sizes in the reduced catalysts (3–4.5 nm) are much smaller and the DCu
%
values are larger than those of the supported catalysts as reported [9,14].
300oC
260oC
220oC
The XPS analysis of catalysts activated by H2 at different tempera-
tures (Fig. 3) shows that the intensity of peaks at 932.4 eV, assigned to
the characteristic Cu2p3/2 binding energy values of metallic copper [13],
Table 1
Textural characteristics for the calcined catalyst and its precursor.
Catalysts
Most probable pore
diameter (nm)
BET surface area
BJH pore volume
925 930 935 940 945 950 955 960 965 970
Binding energy/eV
(m2 g−1
)
(cm3 g−1
)
CuNiMgAl
(OH)x
CuNiMgAlOy 5.9
1.9
98.8
0.97
Fig. 3. Cu2p XPS patterns of CuNiMgAlOy catalyst activated by H2 at 220, 260 and
300 °C, respectively.
148.9
1.07

998
C. Xu et al. / Catalysis Communications 12 (2011) 996–999
92
Table 2
90
88
86
84
82
80
Cu dispersion, metal area, and average particle size of the catalysts activated by H2 at
different temperature.a
Activation temperature/°C
DCu
%
Cu metal area (m2 g−1-cat)
rav/nm
300
360
400
35.84
29.89
23.39
30.67
25.58
20.01
2.9
3.5
4.5
FFR conversion
FFA selectivity
2-MF selectivity
2-dEMF selectivity
78
14
a
According to the H2–N2O titration.
12
10
8
It shows that the catalysts with highly dispersed catalytic active centers
can be obtained from the hydrotalcite-like materials as precursors.
6
4
2
3.3. Catalytic performance
0
200 220 240 260 280 300 320 340 360 380 400 420
Activation temperature/oC
From the catalytic performance of different catalysts in the FFR
liquid-phase (ethanolic solution) hydrogenation (Table 3), it is found
that MgAlOy has almost no catalytic activity for the FFR hydrogena-
tion, and NiMgAlOy or CuMgAlOy gives above 52% of FFR conversion.
However, NiMgAlOy gives a higher proportion of 2-diethoxylmethxyl-
furan (2-DEMF) than CuMgAlOy. According to the H2-TPR results, it
can be deduced that the FFR acetalization [15] easily occurs due to the
presence of nickel cations as Lewis acids on the surface. The copper
cations will be less acidic and, thus, less efficient to catalyze the
acetalization of FFR. Meanwhile, CuNiMgAlOy gives the highest FFR
conversion (90.5%) and the lowest 2-DEMF selectivity, indicating that
the Ni2+ species have a synergistic effect on the FFR hydrogenation on
the Cu0 active centers in the catalysts.
Fig. 4 shows the results on the FFR liquid-phase hydrogenation
over CuNiMgAlOy catalysts activated by H2 at different temperatures.
There are three main products formed, FFA, 2-methylfuran (2-MF)
and 2-diethoxylmethxyl-furan (2-DEMF). FFA arises from FFR
hydrogenation and 2-DEMF results from FFA hydrogenolysis. More-
over, both FFR conversion and FFA selectivity clearly increase first and
then decrease upon increasing the activation temperature. However,
the 2-MF selectivity follows an inverse trend. The highest FFR
conversion (90.5%) and FFA selectivity (85.4%) are obtained along
with the lowest 2-MF selectivity (4.3%) at 300 °C. It shows that the
activation temperature is an important factor affecting the catalytic
properties of CuNiMgAlOy and 300 °C seems to be an optimum
activation temperature. According to the XPS and H2-TPR results, it is
also deduced that Cu0 species are the effective catalytic centers for the
FFR hydrogenation, which is in agreement with the previous reports
[1,2,12]. From the H2–N2O titration results, the decrease on both FFR
conversion and FFA selectivity upon increasing the temperature are
concluded to be due to the agglomeration of active Cu0 particles at a
high activation temperature, the presence of larger Cu0 particles
favours the FFA hydrogenolysis to 2-MF.
Fig. 4. Effectof thecatalyst activationtemperature onFFR liquid-phase hydrogenation over
CuNiMgAlOy. Conditions: 90 mL ethanol, 30 mL FFR, 1 MPa H2, 1 g catalyst activated for
2 h, stirring at 500 rmin−1, reaction at 200 °C for 2 h.
FFR conversion in the vapour-phase hydrogenation shows a trend
similar to that in the liquid-phase hydrogenation, it decreases. This
suggests that FFR hydrogenation is inhibited to some extent by the
agglomeration of the Cu particles. On the other hand, the selectivities
of FFA and 2-MF formation show an inverse trend. Clearly 300 °C is an
important break point at which there is change of the trends for FFA
and 2-MF selectivities: the FFA selectivity increases from 32% to 80%
and the 2-MF selectivity decreases from 48% to 10% between 300 °C
and 400 °C. The increase of the FFA selectivity and the decrease of the
2-MF selectivity with the increase of the activation temperature are
most probably due to the short contact time of FFR and FFA with the
catalytic active centers.
From the catalytic cycle results (Fig. 3S and 4S in the Supplemen-
tary material) of CuNiMgAlOy activated by H2 at 300 °C, it is shown
that the catalytic activity of CuNiMgAlOy has no obvious decrease after
3 cycles, and only about 10% decrease on the FFR conversion after 6
cycles in the FFR liquid-phase hydrogenation. And the catalytic
properties of CuNiMgAlOy in vapour-phase hydrogenation stay nearly
unchanged after 36 h. This indicates that CuNiMgAlOy from the
hydrotalcite-like precursors exhibits a stable catalytic performance in
FFR hydrogenation.
90
FFR conversion
80
From the FFR vapour-phase hydrogenation over CuNiMgAlOy
(Fig. 5), two other by-products including tetrahydrofurfuralcohol
(THFFA) and 2-methyltetrahydrofuran (2-MHF) are found besides the
two main products FFA and 2-MF. The two by-products result from
the hydrogenation of furan ring of FFA and 2-MF, respectively. When
the catalyst activation temperature is raised from 220 to 400 °C, the
FFA selectivity
2-MF selectivity
2-MHF selectivity
70
THFFA selectivity
60
50
40
30
20
10
Table 3
Catalytic performance of the different catalysts.a
Catalysts
FFR
Products distribution/mol%
conversion/
mol%
FFA
2-MF
2-dEMF
0
200 220 240 260 280 300 320 340 360 380 400 420
MgAlOy
9.8
55.6
52.7
90.5
59.8
53.2
78.1
85.4
–
40.2
46.8
13.9
10.1
Activation temperature/oC
NiMgAlOy
CuMgAlOy
CuNiMgAlOy
–
8.0
4.7
Fig. 5. Effect of the catalyst activation temperature on FFA vapour-phase hydrogenation
over CuNiMgAlOy. Conditions: catalysts activated for 2 h, GHSV=4000 h−1, H2/FFR
molar ratio=10.1, reaction at 220 °C.
a
1 g catalyst activated by H2 at 300 °C for 2 h, 90 mL ethanol, 30 mL FFR, 1 MPa H2,
stirring at 500 rmin−1, reaction at 200 °C for 2 h.

C. Xu et al. / Catalysis Communications 12 (2011) 996–999
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4. Conclusions
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In FFR hydrogenation, the catalyst activation temperature is found
to be an important factor affecting the dispersion of the active centers
of catalysts (CuNiMgAlOy) prepared from the hydrotalcite-like
precursors. 300 °C is an optimum activation temperature to obtain
the best FFR hydrogenation catalysts with a high Cu dispersion, a large
metal specific area and small particle sizes. FFR hydrogenation is
inhibited by the agglomeration of active Cu particles. FFA hydro-
genolysis to 2-MF easily occurs over the larger Cu particles in FFR
liquid-phase hydrogenation (ethanolic solution) and hydrogenation
of the furan ring of FFA and 2-MF occurs in the vapour-phase
hydrogenation in the absence of ethanol.
Appendix A. Supplementary data
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Supplementary data to this article can be found online at
doi:10.1016/j.catcom.2011.03.011.