Effect of Calcination Temperature on the Textural Properties and Catalytic Behavior of the Al2O3 Doped Mesoporous Monometallic Cu Catalysts in Dimethyl Oxalate Hydrogenation

Article abstract of DOI:10.1007/s10562-020-03453-z

Abstract: Al₂O₃ doped mesoporous monometallic Cu catalysts were successfully synthesized though the self-assembly Cu species derived from the oxalate precursor undergoing thermal decomposing. The evolutions of microstructures, physicochemical and surface properties of the CuAl catalysts have been systematically characterized focusing on the effect of the calcination temperature during catalyst preparation. It is found that the textural and surface properties of the CuAl catalysts were profoundly affected by the calcination temperature, further determining the resultant catalytic behavior in dimethyl oxalate (DMO) hydrogenation. Particularly, the CuAl-500 possessing the maximum surface Cu⁺ sites and proper surface acid features exhibits?100.0% DMO conversion and 98.0% ethylene glycol (EG) selectivity in presence of the adequate active Cu⁰ sites, which is superior to that of the other catalysts under the identical reaction conditions. And no activity loss occurred for more than 200?h demonstrated of the outstanding stability of the CuAl-500 catalyst. Moreover, the synergistic effect between surface Cu⁺ and Cu⁰ sites should be responsible for DMO selective hydrogenation. Additionally, the strengthened chemical interaction between Cu and Al species endows the catalysts outstanding stability by suppressing the dispersive Cu NPs agglomeration during DMO hydrogenation. Graphic Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-020-03453-z

Catalysis Letters
https://doi.org/10.1007/s10562-020-03453-z
Efect of Calcination Temperature on the Textural Properties
and Catalytic Behavior of the Al2O3 Doped Mesoporous Monometallic
Cu Catalysts in Dimethyl Oxalate Hydrogenation
Xiangpeng Kong1,2 · Yuehuan Wu1 · Peihong Yuan3 · Man Wang1 · Peng Wu2,4 · Lifeng Ding1 · Ruihong Wang1 ·
Jiangang Chen2
Received: 9 September 2020 / Accepted: 30 October 2020
© Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
Al2O3 doped mesoporous monometallic Cu catalysts were successfully synthesized though the self-assembly Cu species
derived from the oxalate precursor undergoing thermal decomposing. The evolutions of microstructures, physicochemical
and surface properties of the CuAl catalysts have been systematically characterized focusing on the efect of the calcina-
tion temperature during catalyst preparation. It is found that the textural and surface properties of the CuAl catalysts were
profoundly afected by the calcination temperature, further determining the resultant catalytic behavior in dimethyl oxalate
(DMO) hydrogenation. Particularly, the CuAl-500 possessing the maximum surface Cu+ sites and proper surface acid fea-
tures exhibits 100.0% DMO conversion and 98.0% ethylene glycol (EG) selectivity in presence of the adequate active Cu0
sites, which is superior to that of the other catalysts under the identical reaction conditions. And no activity loss occurred
for more than 200 h demonstrated of the outstanding stability of the CuAl-500 catalyst. Moreover, the synergistic efect
between surface Cu+ and Cu0 sites should be responsible for DMO selective hydrogenation. Additionally, the strengthened
chemical interaction between Cu and Al species endows the catalysts outstanding stability by suppressing the dispersive Cu
NPs agglomeration during DMO hydrogenation.
Graphic Abstract
Keywords Mesoporous monometallic Cu catalysts · Al2O3 dopant · Dimethyl oxalate · Hydrogenation · Ethylene glycol
2
* Xiangpeng Kong
State Key Laboratory of Coal Conversion, Institute of Coal
Chemistry, Chinese Academy of Sciences, Taiyuan 030001,
People’s Republic of China
kongxiangpeng@sxicc.ac.cn
* Jiangang Chen
3
4
chenjg19@163.com; chenjg@sxicc.ac.cn
Taiyuan Institute of Mine Design and Research,
Taiyuan 030012, People’s Republic of China
1
Department of Chemistry and Chemical Engineering,
University of Chinese Academy of Sciences, Beijing 100049,
People’s Republic of China
Taiyuan Institute of Technology, Taiyuan 030008,
People’s Republic of China
Vol.:(0123456789)
1 3

X. Kong et al.
reactions [22]. Also, the grain-boundary strengthening and
Orowan strengthening of the Cu nanoparticles could be
efectively improved by forming chemical bonding phase
at the Cu-Oxide interface [21]. Moreover, uniform distribu-
tion of Al2O3 in the Cu matrix was identifed to be essential
for the order of alumina dispersion strengthened Cu system
[13, 23, 24].
1 Introduction
Ethylene glycol (EG), as a versatile fne chemicals and
industrial intermediate, has been popular in the various
chemical felds [1–3]. Currently, the feedstock for EG was
mainly derived from the dwindling oil resource, hinder-
ing the desired production capacity [4–6]. Alternatively,
syngas has been deemed as a promising stock for valuable
chemical product, owing to the benign virtues of abundant
resources, high atom economy and environmental benign
[7–9]. Particularly, converting syngas to dimethyl oxalate
(DMO) technology has been commercialized successfully,
presenting a potential routine for indirect producing EG from
syngas [10]. Wherein, a talented catalyst for DMO selective
hydrogenation to EG has become crucial for implementing
this technology.
Previous reports identifed that self-assembly Cu species
derived from the Cu contained precursors though thermal
decomposing has been proposed to be an efective approach
to synthesize Cu NPs, constructing the porous Cu nanomate-
rials [25, 26]. Particularly, perfect porous Cu nanomaterials
with high specifc surface area, low density and intercon-
nected pore can be gained though the copper oxalate pyroly-
sis [27]. And the co-precipitation method has been chosen to
synthesize the copper oxalate precursor using the Cu(NO3)2
and oxalate as the start reagent [28, 29]. What is more, the
thermal process has demonstrated to show drastically efect
on the resultant physicochemical features of the catalysts
[30, 31]. Thus, it is deduced that the calcination temperature
should have a great infuence on the resultant physicochemi-
cal properties and catalytic behavior of the catalysts.
In this work, porous Cu nanomaterials reinforced by the
Al2O3 dopants were successfully synthesized by self-assem-
bly of Cu species, originating from thermal decomposition
of the precursor derived from diferent calcination tempera-
tures. The aim of this study is to have a better understanding
of the efect of the thermal process on the physicochemical
properties and catalytic behavior in DMO hydrogenation of
the as-synthesized catalysts.
With regarding to DMO hydrogenation, metallic Cu spe-
cies have been deemed as perfect candidate, due to the vir-
tues of less expensive, abundantly available and superior
selectivity in C–O/C=O bonds [11]. Nonetheless, metal-par-
ticle aggregation resulting from the essential low Hüttig and
Tamman temperatures challenges the Cu catalysts designing
[12, 13]. In general, desired efciency and thermal stability
has been proposed to be essential for the Cu based catalysts.
For this case, support and promoter has been adopted to
achieve improved Cu dispersion, reasonable active sites dis-
tribution and strengthened metal-support interaction (MSI),
favoring the perfect performance [14, 15]. Nonetheless, the
local overheating due to the poor thermal conductivity of
the oxide support aggravates the Cu NPs agglomerating
during the DMO hydrogenation [13]. On the other hand,
the needless surface active sites aforded by the superfuous
oxide used for support or promoter can promote the side
reaction (e.g. dehydration and etherifcation), corroding the
stability and EG yield [12, 13]. Recently, nanoporous metal
materials have attracted considerable interests in catalysis,
flter, sensor, surface-enhanced Raman spectroscopy and so
forth, owing to its high specifc surface area, low density
and interconnected pore [16, 17]. And the high surface area
to volume ratio properties of the mesoporous monometallic
Cu crystallites endow them excellent thermal conductiv-
ity features [18, 19]. Furthermore, the active corner sites
and structural fexibility of the metallic NPs can efectively
reduce the barriers of the hydrogenation reaction, facilitating
improved hydrogenation activity [20]. Nonetheless, the mon-
ometallic Cu NPs cannot hold adequate thermal endurance
and mechanical properties [21]. Particularly, embedding or
anchoring dopants on the Cu NPs has been demonstrated to
be an efective strategy to optimize the reactivity and stabil-
ity [21]. Most of all, alumina (Al2O3) ceramic has been used
as reinforcement for Cu NPs to guarantee their thermody-
namic stability, gaining particular interests in hydrogenation
2 Experiment
2.1 Catalyst Preparation
The CuAl catalysts were prepared by though the self-assem-
bly Cu species derived from the oxalate precursor undergo-
ing thermal decomposing as follows. The starting solution
were 15.0 g Cu(NO3)2·3H2O and 10.0 g Al(NO3)3·9H2O in
20 ml deionized water, and 20.0 g oxalic acid in 20 ml deion-
ized water. A mixed aqueous solution of the starting materi-
als with given atomic ratios was used as a precursor solution,
and a oxalic acid solution was used as precipitating agent.
Co-precipitation was performed at 30 °C in a water bath.
The resultant suspension was aged for 12 h at 30 °C. The
precipitate was fltrated and dried in air at 120 °C for 12 h.
After that, the precursors were calcined at the diferent tem-
perature for 6 h. The calcined catalysts were denoted as CAl-
x, and the corresponding reduced catalysts were denoted as
CuAl-x, wherein the C, Al and x stands for the CuO, Al2O3
and calcination temperature, respectively.
1 3

Efect of Calcination Temperature on the Textural Properties and Catalytic Behavior of the…
2.2 Characterization
a
The powder X-ray difraction (XRD) patterns were collected
on a Shimadzu XRD-6000 difractometer with graphite-fl-
tered Cu Ka source. The average crystallite size (Dhkl) was
calculated by Scherrer equation.
Textual properties of the samples were determined by a
nitrogen adsorption method using a Micromeritics Tristar II
3000 Analyzer at 77 K, and the specifc surface areas were
calculated from the isotherms using the Brunauer-Emmett-
Teller (BET) method.
The number of surface metallic copper sites was deter-
mined by N2O oxidation and followed H2 titration using the
procedure described by the previous report [32].
Temperature programmed reduction by hydrogen
(H2-TPR) was carried out using a dynamic analyzer
(Micromeritics, Model 2920). About 30 mg of catalyst was
treated in 5% H2/95% Ar (v/v) (fow rate 50 ml/min), and the
reduction temperature was increased from room temperature
to 600 °C at a heating rate of 10 °C·min−1.
b
The morphology of the catalysts was investigated by
scanning electron microscopy (SEM) (JSM-6701F, Japan).
NH3-TPD experiments were performed in an atmospheric
quartz tube fow reactor (5 mm i.d.). In each NH3-TPD
experiment, 0.3 g of catalyst was activated at 300 °C for 1 h,
then evacuated using the attached vacuum system for 0.5 h.
The sample was cooled to 80 °C and equilibrated with NH3
gas for 0.5 h, and the unabsorbed NH3 was evacuated using
as attached vacuum system for 0.5 h. Afterward, the sample
switched to He atmosphere for 1 h. Finally, the temperature
was increased to 600 °C at ramp of 10 °C min−1 under He
fow (40 mL· min−1), and the desorbed NH3 was detected by
an AMETEK mass spectrometer.
Fig. 1 XRD patterns of as-prepared catalysts: (a) after calcinations;
(b) after reduction
X-ray photoelectron spectroscopy (XPS) was carried out
on a Quantum 2000 Scanning ESCA Microprob instrument
(Physical Electronics) equipped with an Al Ka X-ray radia-
tion source (hm=1486.6 eV).
analyzed by a gas chromatograph ftted with a 30 m HP-5
capillary column and a fame ionization detector (FID).
Amount of feed after reaction (mol)
Conversion(%) =
× 100
Total amount of feed (mol)
2.3 Catalytic Activity Test
Amount of the product (mol)
The hydrogenation of DMO was conducted on the fxed-
bed tubular reactor with an inner diameter of 10 mm. The
calcined samples (3.0 g) was loaded into the tubular reac-
tor with quartz powders (20–40 meshes) packed in both
sides of the catalyst bed with a height of approximately
40 mm, and then were reduced in a 5% H2/Ar atmosphere
at 300 °C for 4 h at a heating rate of 2 °C min−1. After acti-
vation, 12.5 wt% DMO (purity (99%) in methanol and H2
were fed into the reactor at a H2/DMO molar ratio of 100 at
210 °C. During hydrogenation, the total pressure was kept
at 2.5 MPa, and the room-temperature liquid space veloc-
ity of DMO was 2.0 h−1. The products were condensed and
Selectivity(%) =
× 100
Total amount of feed converted (mol)
3 Results and Discussions
3.1 Textural Properties and Structure Evolution
of the Catalysts
The XRD patterns of the as-synthesized catalysts after
calcination and reduction are shown in Fig. 1a, b. From
the Fig. 1a, it is found that a series of difractions peaks
1 3

X. Kong et al.
Fig. 2 N2 sorption isotherms of
(a) and BJH pore size distri-
bution (b) of the as-prepared
catalysts
b
a
3.5 nm
2.8 nm
4.3 nm
CuAl-600
CuAl-500
CuAl-400
CuAl-600
CuAl-500
35.7 nm
30.5 nm
CuAl-400
28.1 nm
CuAl-300
CuAl-300
10
100
0.0
0.2
0.4
0.6
0.8
1.0
Pore Diameter (nm)
P / P0
characteristic of CuO (JCPDS 48–1548) can be detected for
the CAl-300. Notable, the intensity of the difractions peak
became weakened gradually via the calcination temperature
increasing from 300 °C to 500 °C, implying that the CuO
NPs size grew reduced. Whereas, excessive calcination
temperature of 600 °C caused the CuO difractions peaks
strengthen adversely, suggesting of larger CuO NPs in the
CAl-600. On the other hand, the 38.97° (200) peak became
more intensive with the calcination temperatures increasing
referred to the 35.47° (002) peak, indicating that elevated
calcination temperatures promote CuO preferentially grew
along the (200) plane for the sufcient nucleation energy
[33–35]. As for the reduced CuAl catalysts, the XRD pat-
terns (Fig. 2b) show only three difractions peaks at the 2θ
of 43.2°, 51.5° and 75.8° assigned to Cu (JCPDS 04-0836),
indicating that CuO species were completely reduced at the
set conditions. Additionally, the change of the metallic Cu
NPs size with the calcination temperature exhibited the same
trend with the CuO NPs in the calcined catalysts, suggesting
that the calcination temperature shows signifcant efect on
the dispersion of the Cu species. Additionally, no charac-
teristic difractions related to crystalline Al2O3 phase can
be detected in the calcined and reduced catalysts, imply-
ing of amorphous Al2O3 formation. Thus, it is inferred that
the thermal treatment has great efect on the crystallization
direction, further determining the Cu crystallinity.
The textural properties of the as-synthesized samples
were characterized by the N2 sorption isotherms. The N2
sorption isotherms in the Fig. 2a present Langmuir type
IV isotherms with a H3-type hysteresis loop, correspond-
ing to the type of mesoporous capillary condensation [36].
The BJH pore size distribution curves (Fig. 2b) of the cata-
lysts suggest that all the samples exhibit main mesoporous
structure together with partial micropore. And the pore
Table 1 Physicochemical properties of the as-prepared CuAl samples
0
Catalyst
Al Content (%)a
SBET (m2·g−1
)
Vp (cm3·g−1
)
D p (nm)
DCu (nm)b
SCu
Cu Dispersion
(%)c
(m2·g−1 c
)
CuAl-300
CuAl-400
CuAl-500
CuAl-600
3.41
3.57
3.49
3.61
45.1
48.3
54.8
26.4
0.034
0.032
0.026
0.010
17.39
17.20
15.80
13.07
19.6
17.9
15.4
29.0
7.15
9.80
14.3
3.59
4.09
5.60
8.14
1.97
aAl element contents based on the XPS surface element peak intensity
bDCu were calculated from the refections of Cu (111) planes in the XRD using the Scherrer equation
cSCu and copper dispersion (DCu) were measured by N2O adsorption
1 3

Efect of Calcination Temperature on the Textural Properties and Catalytic Behavior of the…
315
358
CuAl-600
CuAl-500
CAl-600
333
78
207
295
CAl-500
CAl-400
CAl-300
305
CuAl-400
CuAl-300
282
50
100 150 200 250 300 350 400 450 500
100
200
300
Temperature (oC)
400
500
o
Temperature ( C)
Fig. 4 NH3-TPD profles of the as-prepared CuAl catalysts
Fig. 3 H2-TPR profles of the calcined CAl catalysts prepared at the
diferent calcination temperatures
the reducing temperature adversely shifted to 315 °C, which
should be closely related to reduced CuO dispersion for Cu
species excessive agglomeration, as shown in the XRD
results (Fig. 1) [37]. On the other hand, the main reduction
peaks became broader and more symmetrical, suggesting
that the Cu particle size distribution becomes more uniform
at the higher calcination. Therefore, it is confrmed that the
calcination temperature can modulated the distribution and
chemical interaction mode of the Cu species.
size became increased from 28.1 nm to 35.7 nm with the
calcination temperature increasing from 300 to 500 °C.
Nonetheless, the pore size of the CuAl-600 catalyst mainly
concentrates at 3.5 and 4.3 nm, originating from micropore
structure collapsing at the excessive calcination temperature.
These results in the Table 1 show that the physicochemi-
cal properties of the as-prepared catalysts depend critically
on the calcination temperatures, and elevated calcination
temperature intends to decrease the SBET. Most of all, the
calcination temperature of 500 °C was found to possess
the maximal SBET of 54.8 m2·g−1. On the other hand, there
is the same change trend for the metallic Cu specifc area
Figure 4 gives the NH3-TPD profles of the CuAl cata-
lysts calcined at diferent temperatures. It is found that the
CuAl-300 features three distinct desorption peaks: the peak
I at 78 °C, the peak II at 207 °C and peak III at 333 °C. The
peak I should be assigned to physically adsorbed ammonia
on the catalyst surface, the peak II should be attributed to
the weakly adsorbed NH3 on the weak Lewis acid sites, and
the peak III should be originating from the coordinated NH3
bound to Brønsted acid sites [40–42]. Obviously, increasing
the calcination temperature to 400 °C promotes all the peaks
intensity strengthened, indicating of increased surface acid
sites. Nonetheless, further increasing the calcination tem-
perature to 500 °C lead to the reduced desorption intensity,
suggesting of the decreased Lewis and Brønsted acid sites.
Furthermore, excessive calcination temperature of 600 °C
induced the peak III shifted to the higher desorption tem-
perature of 358 °C, declaring of stronger surface acid sites
formed. Moreover, the acidity was found to be relevant to
the SBET and SCu0 of the catalysts, except for the CuAl-500
catalyst, arguing that the surface acid features are independ-
ent on metallic Cu species concentration exposed [43]. On
the other hand, previous report identifed that the metallic
Cu species could suppress the acidic sites of the oxide sup-
port or dopant by forming the Cu-oxide interface, facilitating
ionic Cu species [44, 45]. Based on the above results, it is
deduced that the Cu dispersion shows great infuence on the
surface acid properties of the CuAl catalysts, and improved
0
(SCu0) via the calcination temperature, and the highest SCu
of 14.3 m2·g−1 was gained at 500 °C.
H2-TPR profles of the as-prepared calcined CuAl cata-
lysts are shown in Fig. 3, wherein the total hydrogen con-
sumption corresponds to the reduction of CuO species.
Notable, the CAl-300 catalyst displays a main unsymmet-
rical broad reduction peak locating at 282 °C with a tail
peak at the lower temperature. These hydrogen consump-
tion peaks should be corresponding to the Al2O3 dopants
anchored and isolated highly-dispersed CuO NPs reducing
[37, 38]. Although the elevated calcination temperature
facilitates improved Cu dispersion, the reduction tempera-
ture still shifted to 305 °C, due to the strengthened interac-
tion between the Cu species and Al2O3 [39]. Nonetheless,
increasing the calcination temperature to 500 °C caused the
main reduction peaks dropped to 295 °C, which is higher
than that of the CAl-300. Considering the increased Cu dis-
persion at the elevated calcination temperature, the chemi-
cal interaction between the Cu species and oxide should be
another vital factor determining the Cu dispersion. Thus,
it is deduced that the high calcination temperature intend
to favor strengthened Cu-Oxide chemical interaction force.
While excessive calcination temperature of 600 °C causes
1 3

X. Kong et al.
The SEM images of the CuAl sample prepared from dif-
ferent calcination temperatures were presented in the Fig. 5,
identifying that all samples are in the nanoscale between
15 and 30 nm. As shown in the Fig. 5, the morphology of
the CuAl catalysts was found to be changed from spherical
shape grains prepared at 300 °C to fower-like CuO architec-
ture at 500 °C, further changed to grain shaped at the calci-
nation temperature of 600 °C. Moreover, the fower-shaped
grains in the CuAl-500 intend to promote the enhanced Cu
dispersion, favoring the maximum SBET
.
Figure 6 shows the TEM images of the pre-reduced CuAl
catalysts prepared at diferent calcination temperatures.
From the Fig. 6a–d, it is found that the Cu metal was rela-
tively well dispersed in the as-synthesized samples with a
particle size range of 10–20 nm, which consists well with
the XRD calculation results. And the CuAl catalysts are con-
structed by the aggregations, which mainly composed of
the Cu crystals surrounded by an amorphous layer. Particu-
larly, the more Cu crystal exposed in the CuAl-500 intend
to endow the porous catalyst enhanced SBET and SCu0. Addi-
tionally, amorphous Al2O3 phase was approximates to the
dark Cu crystalline boundary, facilitating chemical interac-
tion by decorating the Cu NPs. In contrast, the CuAl-600
sample showed large metal particles, indicating the agglom-
eration of Cu metal at high temperatures, as shown in the
Fig. 6d. These results further confrmed that the morphology
evolution of the catalysts depends critically on the calcina-
tion temperature by controlling the crystal growth, as shown
in the Fig. 1. Accordingly, the morphology results prove
that the Cu nanoparticles comprise small nanocrystalline
structures with diferent orientation of the single crystal
difraction pattern, having a good agreement with the cor-
responding XRD results.
Fig. 5 SEM images of the pre-reduced CuAl catalysts prepared from
diferent calcination temperature. (a) CuAl-300, (b) CuAl-400, (c)
CuAl-500, (d) CuAl-600
Figure 7 shows the Al2p (a), Cu2p (b) and Cu LMM (c)
spectrums of the as-synthesized CuAl catalysts. From the
Fig. 7a, it is found that the CuAl-300 features a main peak
at 77.4 eV with a low binding energy peak at 76.0 eV, cor-
responding to two diferent types of surface Al species [46,
47]. On the other hand, the high binding energy peak of the
catalysts prepared at the elevated calcination temperature
shifted to the low binding energy, which should be asso-
ciated with a larger electron density at the emitting atom
migrating from the interacted Cu species [48]. Particularly,
the higher Cu binding energies of other catalyst than that
of the CuAl-300 confrms that more electron transfer from
Cu to the oxide dopant, facilitating strengthened Cu-oxide
interaction force [49]. Figure 7b shows the presence of only
the metallic Cu binding energy peak, presenting that Cu2+
species were completely reduced [50].
Fig. 6 TEM images of the pre-reduced samples: (a) CuAl-300, (b)
CuAl-400, (c) CuAl-500 and (d) CuAl-600
Cu dispersion intend to facilitate enhanced Cu-Oxide inter-
face area, depriving the Al3+ acid species amount exposed.
However, the excessive strengthened Cu-Oxide chemical
interaction would promote the polarity of the Cu species,
contributing to stronger acidic sites.
In order to distinguish the surface Cu+ and Cu0 species,
the kinetic energy of the Cu LMM and the binding energy
of the Cu2p3/2 together make up an Auger parameter to dis-
tinguish the Cu0 and Cu+ sites. As shown in Fig. 7c, the
1 3

Efect of Calcination Temperature on the Textural Properties and Catalytic Behavior of the…
75.2
b
a
77.4
932.8
Al 2p
Cu 2p
952.7
CuAl-600
CuAl-500
CuAl-400
CuAl-600
CuAl-500
CuAl-400
CuAl-300
75.4
75.4
76.0
CuA-300
84
82
80
78
76
74
72
70
68
66
965
960
955
950
945
940
935
930
925
Binding Energy (eV)
Binding Energy (eV)
916.3
c
918.5
CuAl-600
CuAl-500
CuAl-400
CuAl-300
925
920
915
910
905
Kinetic Energy (eV)
Fig. 7 Al 2p (a) and Cu 2p (b) and XPS spectra and Cu LMM spectra (c) of the activated catalysts
The catalytic behavior of the as-synthesized catalysts
was evaluated in gas-phase hydrogenation of DMO, and the
results were collected in the Fig. 8. From the Fig. 8a, it is
found that all the catalysts exhibited excellent activity with
100.0% DMO conversion. Specially, CuAl-500 exhibited
EG selectivity of 98.0%. Then, the CuAl-500 catalyst was
selected to explore the long-term stability for EG synthesis.
As shown in the Fig. 8b, almost constant DMO conversion
(100%) and EG selectivity (98.0%) for 200 h tests indicated
of the outstanding stability of the CuA-500 catalyst.
Table 2 Surface Cu component of the reduced samples based on Cu
LMM deconvolution
+ a
(m2·g−1
Catalysts
KE (eV)
Cu0
X Cu+ a
(%)
S Cu
)
Cu+
CuAl-300
CuAl-400
CuAl-500
CuAl-600
916.0
916.0
916.0
917.1
917.5
917.5
917.5
913.2
57.1
70.0
61.9
70.6
8.55
11.3
11.5
3.96
aIntensity ratio between Cu+ and (Cu+ + Cu0) by deconvolution of Cu
LMM spectra
The above results reveal that the calcination tempera-
ture shows great infuence on the microstructures, physico-
chemical and surface properties of the as-prepared catalysts.
Theoretically, elevating calcination temperature intends to
produce the catalysts with decreased Cu dispersion and
SCu0, due to Cu particles agglomeration. Nonetheless, the
facts show that increasing the calcination temperature from
300 °C to 500 °C promote the crystals morphology change
from spherical shape grains to flower-like architecture,
presence of Auger kinetic energy peaks ranging from 905.0
to 925.0 eV suggests the co-existence of the surface Cu+
and Cu0 sites [51]. The deconvolution of Cu LMM (Table 2)
shows that the CuAl-400 and CuAl-500 catalyst possesses
the higher Cu+ density of 11.3 and 11.5 m2·g−1, respectively,
which is higher than that of the others.
1 3

X. Kong et al.
reports identifed that the Cu0 sites can aford H atoms by
splitting H–H bond adsorbed on the threefold sites via a
homolytic mechanism, but immune to the ester groups [53].
And the surface Cu+ species was discerned to dissociate
C–O/C=O bonds [54]. Thus, it is deduced that the pres-
ence of both Cu0 and Cu+ sites should be vital for DMO
hydrogenation. Particularly, carbonyl group dissociated
on the Cu+ sites generated on the Cu-oxide interface was
reported to be the rate-controlling step over the Cu based
catalysts [55]. As shown in the Fig. 8, the activity of the
CuAl catalysts became increased with the surface Cu+ sites
concentration increasing, confrming that increased sur-
face Cu+ sites can actually promote DMO hydrogenation.
Whereas, the adversely lower activity of the CuAl-400 was
found to be not proportionally with the SCu0, arguing that
sufcient surface Cu0 sites should be indispensable. Because
that bare Cu cluster ions (Cu+) isolated in the gas phase
do not react with H2, theoretical calculations predict that
H2 is dissociated at a corner Cu atom and forms hydrides
on the Cu NPs surface [56]. This result further confrmed
that the synergistic cooperation of Cu0 and Cu+ sites should
be responsible for the achieved activity [57]. On the other
hand, the proper surface chemical features can efectively
suppress the side reaction, ensuring desired EG yield [58].
Moreover, the preserved mesoporous structures and stronger
metal-support interaction may suppress the copper aggrega-
tion and facilitate stability [59].
a
DMO Conver.
EG Selec.
MG Selec.
Ethanol Selec.
100
80
60
40
20
0
CuAl-300
CuAl-400
CuAl-500
CuAl-600
Catalysts
b
100
80
60
40
20
0
DMO Conver.
EG Selec.
0
50
100
Time on stream ( h )
150
200
4 Conclusions
Fig. 8 Catalytic behavior of the CuAl samples in DMO hydrogena-
tion: Activity of samples (a), Stability test of the CuAl-500 (b).
Reaction conditions: Reaction pressure=2.5 MPa, Reaction tempera-
ture= 210 °C, LHSV= 2.0 h−1, and H2/DMO molar ratio= 100
The present work demonstrated that the Al2O3 doped
mesoporous monometallic Cu catalyst synthesized from the
oxalate precursor undergoing thermal decomposing exhibit
excellent catalytic behavior in DMO hydrogenation to EG.
And the calcination temperatures were found to have great
infuence on the resultant textural and surface features of
the catalysts, further determining the catalytic behavior. The
CuAl-500 shows 100% DMO conversion, 98% EG selectiv-
ity and excellent stability in DMO hydrogenation, due to
the synergistic efect between surface Cu+ and Cu0 sites.
Additionally, the strengthened chemical interaction between
Cu and Al2O3 species endows the catalysts excellent stability
by suppressing the dispersive Cu NPs agglomeration during
DMO hydrogenation.
facilitating increased SBET and SCu0. Excessive calcina-
tion temperature of 600 °C caused the crystals morphology
degenerate to the spherical shape grains again, leading to
reduced SBET and SCu0, as shown in the Table 1. On the other
hand, Fig. 7 and Table 2 confrmed that higher calcination
temperature endows the catalysts enlarged Cu-Oxide inter-
face area by enhancing Al2O3 dispersion, facilitating more
surface Cu+ sites existing in form of Cu-O-Al bonds. Also,
more strengthened Cu-Oxide chemical interaction force
and appropriate surface chemical properties was achieved,
as shown in the Figs. 3 and 4. Accordingly, the resultant
catalytic behavior in DMO hydrogenation was found to be
dependent critically on the calcination temperature.
Acknowledgements The authors gratefully acknowledge the fnancial
support from the Scientifc and Technological Innovation Programs of
Higher Education Institutions in Shanxi (Grant STIP No. 2019 L0928),
National Natural Science Foundation of China (Grant No. 21503256
and 51604180), Applied Basic Research Programs of Science and
Technology Department of Shanxi Province (201701D221036).
It is well established that adsorbed hydrogen and unsatu-
rated compounds adsorption dissociation was proposed to be
essential for the heterogeneous hydrogenation [52]. Previous
1 3

Efect of Calcination Temperature on the Textural Properties and Catalytic Behavior of the…
29. Zhang L, Liu R, Yang H (2012) Phys E 44:1592–1597
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