Stabilizing copper species using zeolite for ethanol catalytic dehydrogenation to acetaldehyde

Article abstract of DOI:10.1016/S1872-2067(19)63378-4

The selective dehydrogenation of ethanol to acetaldehyde is a promising route for acetaldehyde production. Although Cu-based catalysts exhibit high activity in ethanol dehydrogenation, a rapid deactivation due to Cu sintering always occurs. In this study, highly dispersed Cu species were sta-bilized using the silanol defects in Beta zeolite (denoted as Beta) resulting from dealumination, and applied as robust catalysts for ethanol-to-acetaldehyde conversion. Typically, a long catalyst life-time of 100 h with an acetaldehyde yield of ～70% could be achieved over 5% Cu/Beta. The pres-ence of Cu⁺ and Cu⁰ species and the agglomeration of Cu particles after a long-term reaction for 180 h were revealed by transmission electron microscopy, thermogravimetric analysis, and CO-diffuse-reflectance infrared Fourier transform spectroscopy, and were responsible for the deac-tivation of the Cu/Beta catalyst in the ethanol-to-acetaldehyde conversion.

Full text of DOI:10.1016/S1872-2067(19)63378-4

Chinese Journal of Catalysis 40 (2019) 1375–1384
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article (Special Issue on Celebrating the 100th Anniversary of Nankai University)
Stabilizing copper species using zeolite for ethanol catalytic
dehydrogenation to acetaldehyde
Dongni Yu a, Weili Dai a,*, Guangjun Wu a, Naijia Guan a,b, Landong Li a,b,#
a School of Materials Science and Engineering & National Institute for Advanced Materials, Nankai University, Tianjin 300350, China
b Key Laboratory of Advanced Energy Materials Chemistry of the Ministry of Education, Collaborative Innovation Center of Chemical Science and
Engineering, Nankai University, Tianjin 300071, China
A R T I C L E I N F O
A B S T R A C T
Article history:
The selective dehydrogenation of ethanol to acetaldehyde is a promising route for acetaldehyde
production. Although Cu-based catalysts exhibit high activity in ethanol dehydrogenation, a rapid
deactivation due to Cu sintering always occurs. In this study, highly dispersed Cu species were sta-
bilized using the silanol defects in Beta zeolite (denoted as Beta) resulting from dealumination, and
applied as robust catalysts for ethanol-to-acetaldehyde conversion. Typically, a long catalyst lifetime
of 100 h with an acetaldehyde yield of ~70% could be achieved over 5% Cu/Beta. The presence of
Cu+ and Cu0 species and the agglomeration of Cu particles after a long-term reaction for 180 h were
revealed by transmission electron microscopy, thermogravimetric analysis, and
CO-diffuse-reflectance infrared Fourier transform spectroscopy, and were responsible for the deac-
tivation of the Cu/Beta catalyst in the ethanol-to-acetaldehyde conversion.
Received 28 February 2019
Accepted 2 April 2019
Published 5 September 2019
Keywords:
Ethanol dehydrogenation
Acetaldehyde
Zeolite
Cu-based catalyst
Deactivation
© 2019, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
good selectivity toward acetaldehyde, in part, because Cu can
cleave C–C and C–O bonds at much lower rates than other tran-
Bio-derived ethanol is regarded as a green feedstock to
produce valuable chemicals, e.g., ethene, butadiene, 1-butanol,
and acetaldehyde [1,2]. Among these chemicals, acetaldehyde is
an important building block for the production of acetic acid,
acetate esters, and pentaerythritol [3]. In the past few decades,
the bioethanol-based dehydrogenation route to produce acet-
aldehyde and the important hydrogen co-product has attracted
significant attention.
Generally, the catalysts for alcohol dehydrogenation can be
mainly divided into two types, i.e., metal (Cu, Ag, Pd, Au, etc.)
and metal oxide (CuO, ZnO, MgO, Cr2O3, etc.) [4–15]. Among
these catalysts, Cu-based catalysts exhibit very high activity and
sition metals (e.g., Pd and Pt) [16]. However, a rapid deactiva-
tion due to Cu particle sintering often occurs on Cu-based cata-
lysts [12,17]. Hence, special attention has been paid to prevent
this Cu sintering and prolong the catalyst lifetime. Some inert
substances that inhibit Cu sintering have been widely used as
textural promoters, and amphoteric or basic metal oxides have
been proven to be beneficial for improving the dehydrogena-
tion activity [18–21]. In addition, the effects of the Cu content
and the chemical state of the active Cu species on the catalytic
activity have also been investigated [16,22–27]. Bueno et al.
[22] found that the selectivity for the products was related to
the metal dispersion defined by the Cu content. Cassinelli et al.
* Corresponding author. Tel/Fax: +86-22-85358536; E-mail: weilidai@nankai.edu.cn
# Corresponding author. Tel/Fax: +86-22-23500341; E-mail: lild@nankai.edu.cn
This work was supported by the National Natural Science Foundation of China (21872072, 21573113), Municipal Natural Science Foundation of
Tianjin (18JCZDJC37400) and Sinopec (417012).
DOI: S1872-2067(19)63378-4 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 40, No. 9, September 2019

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Dongni Yu et al. / Chinese Journal of Catalysis 40 (2019) 1375–1384
[23,24] reported that Cu+ cations were the most active species
in the ethanol dehydrogenation reaction, whereas other groups
found that the metallic Cu species were responsible for the
ethanol dehydrogenation activity [16,25–27]. Furthermore, the
catalytic activity and stability of the catalyst also depend on the
nature of the support. A good support should ensure a suitable
dispersion of the Cu active phase, possibly stabilizing it during
the high-temperature reaction [28]. Inert SiO2 [11,12], meso-
porous carbon [13], silicon carbide [29], and carbon lay-
er–coated SiO2 [15] as well as rice husk ash [17] have already
been tested as candidate supports.
In our previous work, zeolite-confined catalyst systems
were successfully constructed via a post-synthesis route; they
exhibited remarkable activity in ethanol-to-butadiene conver-
sion [30–32]. The silanol groups in the hydroxyl nests of
[Si]Beta zeolite acted as the exclusive anchoring sites, and
could strongly anchor the cations upon calcination decomposi-
tion of the metal precursors. Considering the rapid Cu sintering
of Cu-based catalysts in ethanol-to-acetaldehyde conversion,
we expected that the zeolite-confined Cu/Beta zeolite catalyst
will be more structurally stable against Cu sintering, and exhib-
it a good performance in the ethanol-to-acetaldehyde conver-
sion. In this study, a Cu/Beta zeolite catalyst was prepared via
indicates the weight loadings of Cu metal).
For reference, Cu/Beta was also prepared using different Cu
precursors via wet impregnation, and the obtained samples
were denoted as CuCl2-Beta, Cu(NO3)2-Beta, and Cu(Ac)2-Beta
(Cu loading: 5%).
2.2. Characterization of catalysts
The XRD patterns of the as-prepared catalysts were rec-
orded on a Rigaku SmartLab powder diffractometer using Cu Kα
radiation (λ = 1.5418 Å), with a scanning rate of 5°/min in the
2θ range of 5°–50°.
The surface areas and pore volumes of the calcined samples
were measured using nitrogen adsorption on a Quantachrome
iQ-MP gas adsorption analyzer at –196 °C. Before nitrogen ad-
sorption, the samples were dehydrated at 300 °C for 6 h. The
total surface area was calculated using the Brunau-
er-Emmett-Teller (BET) equation. The micropore size distribu-
tion was determined using the t-plot method.
TEM and high-resolution TEM (HRTEM) images were rec-
orded on a JEOL JEM-2100F electron microscope at an acceler-
ation voltage of 200 kV.
H2-TPR experiments on the samples were carried out on a
chemisorption analyzer (Chemisorb 2720, Micromeritics) with
5 vol% H2/Ar, at a heating rate of 10 °C/min from room tem-
perature to 500 °C. Prior to reduction, the samples (100 mg)
were pretreated under Ar gas at 350 °C for 1 h.
the
post-synthesis
route
and
applied
in
etha-
nol-to-acetaldehyde conversion. The effects of the Cu contents,
reaction temperatures, and Cu precursors on the catalytic per-
formance were investigated. After optimization, a long lifetime
of 100 h with an acetaldehyde yield of ~70% could be achieved
over the 5% Cu/Beta zeolite catalyst. The physicochemical
properties of the fresh catalysts were analyzed using X-ray
diffraction (XRD), nitrogen physisorption, transmission elec-
tron microscopy (TEM), ultraviolet-visible spectroscopy
The diffuse-reflectance UV-Vis spectra of the Cu/Beta zeo-
lites were recorded against BaSO4 in the region of 200–800 nm
on a PerkinElmer Lambda 750 UV-Vis-NIR spectrophotometer.
1H MAS NMR measurements were performed on a Bruker
Avance III spectrometer at a resonance frequency of 400.1
MHz. The 1H MAS NMR spectra were obtained using a sin-
gle-pulse excitation of π/2 with a pulse duration of 2.6 μs and a
(UV-Vis),
(H2-TPR). The existing states of Cu species and the organic de-
posits occluding the catalyst as result of the etha-
nol-to-acetaldehyde conversion were then characterized using
TEM, thermogravimetric analysis (TGA), and
and
H2-temperature-programmed
reduction
1
a
repetition time of 20 s. All the H MAS NMR studies were per-
formed with dehydrated samples, which were treated at 450 °C
in vacuum (below 10‒2 Pa) for 12 h.
CO-diffuse-reflectance infrared Fourier transform spectroscopy
(CO-DRIFTS). On the basis of the catalytic and spectroscopic
results, the deactivation behavior of the Cu/Beta zeolite cata-
lyst in the ethanol-to-acetaldehyde conversion is discussed
herein.
2.3. Catalyst evaluation
Ethanol-to-acetaldehyde conversion was performed in a
fixed-bed reactor at atmospheric pressure, as described in our
previous report. In a typical experiment, 0.4 g of the Cu/Beta
catalyst (sieve fraction: 20–40 mesh) was placed in the
fixed-bed reactor and treated with flowing nitrogen (20
mL/min) at 400 °C for 1 h. After cooling down to the desired
temperatures, ethanol was introduced into the reactor, using a
Shimadzu LC-2AT Dual Reciprocating Plunger HPLC pump, at a
weight hourly space velocity (WHSV) of 1 h−1. The reaction
products were analyzed using an on-line gas chromatograph
(Shimadzu GC-2010 plus) with a TM-PLOT-Q column and a
flame ionization detector (FID). The ethanol conversion and
2. Experimental
2.1. Catalyst preparation
Cu/Beta zeolite was prepared via a two-step metallation
procedure [30]. Firstly, the dealuminated zeolite Si-Beta zeolite
was synthesized by treating commercial H-Beta zeolite (nSi/nAl
= 13.5, Sinopec Co.) in 13 mol/L nitric acid aqueous solution.
Thereafter, the [Si]Beta host zeolite was impregnated with
Cu(Ac)2·6H2O. Typically, the Cu precursors were dissolved in
excess water to prepare a solution, to which [Si]Beta zeolite
was added under stirring for 8 h. Then, the homogenous slurry
was evaporated at 80 °C and calcined at 550 °C for 6 h under
flowing air. The final product was denoted as x% Cu/Beta (x
acetaldehyde selectivity were defined as follows:
[Ethanol]inlet − [Ethanol]outlet
ꢀ
ꢁ
=
Ethanol conversion mol%
× 100%
× 100%
[Ethanol]inlet
[Acetaldehyde]outlet
ꢀ
ꢁ
=
Acetaldehyde selectivity mol%
[Ethanol]inlet − [Ethanol]outlet

Dongni Yu et al. / Chinese Journal of Catalysis 40 (2019) 1375–1384
1377
2.4. TGA and CO-DRIFTS investigations of spent Cu/Beta
catalysts
Table 1
Chemical composition and surface area of H–Beta, [Si]Beta and Cu/Beta
samples under study.
b
c
Cu loading a
(wt%)
SBET
Vmicro
TGA data were acquired using a Q600 SDT simultaneous
Samples
Si/Al a
(m2/g)
590
620
601
583
571
562
501
458
(cm3/g)
0.204
0.210
0.205
0.198
0.192
0.186
0.163
0.141
thermal analyzer (TA Instruments). Typically, 0.1 g of spent
Cu/Beta catalyst was placed in an Al2O3 crucible and heated up
to 800 °C at a constant rate of 10 °C/min under flowing O2/Ar
(20 vol%/80 vol%, 30 mL/min).
13.5
H-Beta
[Si]Beta
—
—
>1800
>1800
>1800
>1800
>1800
>1800
>1800
1%Cu/Beta
5%Cu/Beta
7%Cu/Beta
9%Cu/Beta
20%Cu/Beta
0.9
4.9
6.8
8.8
19.7
49.4
DRIFTS measurements of the CO adsorption on the spent
Cu/Beta zeolites were conducted on a spectrometer (Bruker
‒
1
Tensor 27), with 128 scans at a resolution of 4 cm . The
self-supporting pellets of the catalyst samples were placed into
the reaction chamber, pretreated under helium flowing at 300
°C for 1 h, and subsequently cooled to 25 °C. Then, 2% CO (he-
lium balanced) was introduced into the DRIFTS cell at a gas
flow rate of 15 mL/min; subsequently, the cell temperature
was increased from 25 to 50 and 100 °C. The DRIFTS spectra
were obtained by subtracting the spectrum of the bare catalyst
(background).
50%Cu/Beta
a
Determined by ICP-AES. b Determined by N2-absorption. Micropore
c
volume was estimated by using the t-plot method.
indicating that the small crystallites of CuO gradually aggre-
gated due to the increase in Cu content, resulting in an increase
in the particle sizes. Simultaneously, the surface areas and mi-
cropore volumes of the Cu/Beta zeolites decreased obviously
(Table 1).
3. Results and discussion
On the basis of our previous work, the dealumination of
H-Beta and incorporation of Ti, Sn, Zr, or Ce species are associ-
ated with the evolution of silanols related to the vacant sites
during post-synthesis [33–36]. This phenomenon was also
observed when preparing the Cu/Beta zeolites using the wet
impregnation method under study. The DRIFTS spectrum (Fig.
2(a)) of the parent H-Beta exhibits two obvious characteristic
3.1. Physicochemical properties of catalyst materials
To verify the possible structural changes during the
post-synthesis procedures, the XRD patterns of the parent
H-Beta and post-treated samples were collected; they are
shown in Fig. 1. The typical diffraction line characteristics of the
BEA topology were observed for all the samples, indicating that
the primary BEA structure was well-preserved after dealumi-
nation and Cu introduction [33]. In addition, no obvious dif-
fraction lines attributable to Cu species were observed for the
Cu/Beta zeolites with low Cu contents (1%–9%), indicating
that the copper species, which were either amorphous or very
small, were highly dispersed within the matrix. These results
are consistent with those of the N2 adsorption-desorption
analysis; no significant loss in the surface areas and micropore
volumes was observed for the samples with low Cu contents
(Table 1). However, upon increasing the Cu content to >20 %,
characteristic peaks of CuO (35.5° and 38.9°) appeared [17],
bands in the hydroxyl stretching region: one band at 3740 cm‒1
,
ascribed to isolated external Si–OH groups, and another band at
3600 cm‒1, ascribed to the bridging hydroxyl Si–OH–Al [33].
The treatment of H–Beta with concentrated HNO3 solution re-
sulted in the complete disappearance of the band at 3600 cm‒1
associated with Si–OH–Al groups, evidencing the complete
elimination of Al from the framework, which was also sup-
ported by the higher Si/Al ratio (>1800) of [Si]Beta (Table 1).
Simultaneously, intensification of the band at 3735 cm‒1 corre-
sponding to isolated internal Si–OH groups and the band at
3520 cm‒1 related to hydrogen-bonded silanol groups was
clearly observed, indicating the formation of vacant T atom
sites, in accordance with earlier assignments [33]. After the
introduction of the Cu species, an obvious decrease in the in-
tensity of the OH bands at 3735 and 3520 cm‒1 occurred. Ac-
cording to our previous studies [30], this observation indicated
that catalyst systems of copper species confined in Beta zeolite
were successfully constructed via the post-synthesis route.
Upon calcination, the Cu precursors decomposed and reacted
with the silanols created by dealumination. Thus, Cu functional
sites could be introduced into zeolite cages, at former Al posi-
tions of the parent zeolite, to build confined catalytic reactors.
The hydroxyl groups in H-Beta, and the Si-Beta zeolites be-
fore and after the introduction of Cu species were analyzed
using 1H MAS NMR spectroscopy. As shown in Fig. 2(b), a
strong signal at δ1H = 1.5 ppm due to silanol groups at frame-
work defects, and two weak resonance signals at 2.5 and 3.9
ppm ascribable to extra-framework Al–OH groups and bridging
hydroxyl Si–OH–Al, respectively, appeared for the H-Beta sam-
H-Beta
[Si]Beta
1% Cu/Beta
5% Cu/Beta
7% Cu/Beta
9% Cu/Beta
20% Cu/Beta






50% Cu/Beta




CuO
10

20
30
40
50
2θ
Fig. 1. XRD patterns H–Beta, [Si]Beta, Cu/Beta and bulk CuO samples.

1378
Dongni Yu et al. / Chinese Journal of Catalysis 40 (2019) 1375–1384
1.5
(b)
(a)
H-Beta
2.5
3.9
H-Beta
[Si]Beta
2.7
1.7
[Si]Beta
1% Cu/Beta
1% Cu/Beta
5% Cu/Beta
7% Cu/Beta
5% Cu/Beta
7% Cu/Beta
9% Cu/Beta
9% Cu/Beta
-2.0 -4.0
4000
3800
3600
3400
3200
3000
10.0
8.0
6.0
4.0
2.0
0.0
Wavenumber / cm-1
δ1H / ppm
316
(c)
(d)
1% Cu/Beta
5% Cu/Beta
7% Cu/Beta
9% Cu/Beta
CuO * 0.1
251
9% Cu/Beta
7% Cu/Beta
237
218
2
2Eg T2g
O2- Cu2+
5% Cu/Beta
1% Cu/Beta
100
301
300
15000 20000 25000 30000 35000 40000 45000 50000
50
150
200
250
350
400
Wavenumber / cm-1
Temperature / oC
Fig. 2. (a) DRIFT spectra in the hydroxyl stretching vibration region and (b) 1H MAS NMR spectra of H-Beta, [Si]Beta and Cu/Beta samples; (c)
UV-Vis-NIR spectra of Cu/Beta samples with different Cu contents; (d) H2-TPR profiles of bulk CuO and Cu/Beta zeolite samples with different Cu
contents.
ple [34]. After dealumination, the band of the hydroxyl groups
associated with Al species responsible for the signals at 2.5 and
3.9 ppm disappeared, and a broad signal at about 2.7 ppm due
to the hydrogen bonds between some SiOH groups and neigh-
boring framework oxygen species inside the silanol nests ap-
peared for [Si]Beta [34]. The introduction of Cu species result-
ed in the disappearance of the band for silanol groups in the
hydroxyl nests, leaving the band for the remaining unreacted
silanols located at 1.7 ppm, in good agreement with the DRIFTS
results in Fig. 2(a). On the basis of the results of DRIFTS and 1H
MAS NMR spectroscopy, the Cu/Beta-confined catalyst systems
could be prepared in a way similar to that of Zn/Beta and
Y/Beta in our previous work [30].
The chemical environment of the Cu species within the
[Si]Beta matrix was characterized using UV-Vis-NIR spectros-
copy. As shown in Fig. 2(c), two absorption bands centered at
about 14000 and 48000 cm‒1 appeared for all the Cu/Beta zeo-
lites. The former band can be attributed to the 2Eg→2T2g transi-
tion of Cu2+ cations in an octahedral environment, whereas the
latter can be assigned to O2‒→Cu2+ charge transfer transitions,
which are characteristic of isolated Cu2+ [37,38]. With an in-
crease in the Cu loadings from 1% to 9%, the intensities of the-
se bands gradually increased, indicating an increase in the
amount of Cu2+ species in the [Si]Beta matrix. Additionally, a
broad band in the range of 20000–25000 cm‒1, attributable to
the d–d transition of Cu with an octahedral environment in
CuO, appeared when the Cu loading increased to >5% [38,39],
indicating an increase in the particle size of the Cu species; this
is in good agreement with the TPR results (ut infra).
TPR experiments were performed to evaluate the reducibil-
ity of the Cu/Beta zeolites. This technique is suitable for study-
ing low-load and highly dispersed systems whose characteris-
tics are beyond the limits of detection of other direct structural
analysis methods such as XRD [22]. Fig. 2(d) shows the TPR
profiles of the bulk CuO and Cu/Beta samples with different Cu
loadings. The maximum of the TPR peaks (Tmax) of the Cu/Beta
samples should have been related to the reduction of support-
ed Cu species in different interactions with the zeolite matrix as
well as to the particle size and Cu content. Bulk CuO exhibited a
single hydrogen consumption peak centered at 316 °C. For the
Cu/Beta samples, the reduction temperatures were signifi-
cantly lowered, in good agreement with earlier reports [17,22],
due to the presence of highly dispersed CuO with an octahedral
environment. The broadening of the TPR peak and its shift to
higher temperatures were related to an increase in the CuO
particle sizes with an increase in the Cu content from 5% to

Dongni Yu et al. / Chinese Journal of Catalysis 40 (2019) 1375–1384
1379
(301 °C) than those of the other samples. This indicated that
Cu/Beta with a very low Cu content probably contained highly
dispersed isolated Cu2+ species that strongly interacted with
the support or were possibly incorporated into the zeolite
framework, and were hard to reduce.
100
80
60
40
20
(a)
3.2. Optimization of catalysts and reaction conditions for
ethanol conversion
Ethanol conversion
Acetaldehyde selectivity
The effects of the Cu contents, reaction temperatures, and
Cu precursors on the ethanol conversion and acetaldehyde
selectivity are given in Figs. 3 and 4. The Cu content of the cat-
alyst clearly shows a strong impact on the ethanol conversion
(Fig. 3(a)). Upon increasing the Cu loading from 1% to 5%, the
ethanol conversion dramatically increased from 25% to 85%.
However, a further increase in the Cu loading from 5% to 50%
resulted in a gradual decrease in the ethanol conversion (from
85% to 48%). For comparison, the catalytic performance of
bulk CuO was also studied, and a poor activity with 7% ethanol
conversion and 97% selectivity toward acetaldehyde was
achieved. According to earlier studies [16,22], large metal spe-
cies are not conducive to ethanol conversion. This indicates
that when approaching saturation of highly dispersed Cu spe-
cies in 5% Cu/Beta, more Cu species will be forced to aggregate
into larger CuO particles, as revealed by the XRD and H2-TPR
results. Therefore, lower catalytic activities were observed for
the Cu/Beta catalysts with higher amounts of Cu loading. In
addition, an increase in the Cu loading had no significant influ-
ence on the acetaldehyde selectivity, suggesting that selective
dehydrogenation of ethanol to acetaldehyde was the dominant
reaction over the Cu/Beta catalysts.
0
0
2
4
6
8
20
40
60
80
100
Cu weight / %
100
(b)
80
60
40
20
0
Ethanol conversion
Acetaldehyde selectivity
200
225
250
275
300
325
350
Temperature / oC
Fig. 3. (a) Ethanol conversion and acetaldehyde selectivity over
Cu/Beta zeolite catalysts with different Cu contents at 300 °C with a
TOS of 2 h; (b) Ethanol conversion and acetaldehyde selectivity over
5% Cu/Beta zeolite catalysts at different temperatures with a TOS of 2
h.
9%. In addition, 1% Cu/Beta exhibited a relatively higher Tmax
(a)100
Ethanol conversion
Acetaldehyde selectivity
Acetaldehyde yield
(e)
(d)
(c)
80
60
40
20
0
(f)
(g)
HRTEM
Si-K
(b)
218
Cu(Ac) -Beta
2
260
230
Cu-K
O-K
Cu(NO ) -Beta
3 2
296
337
CuCl -Beta
2
50
100
150
200
250
300
350
400
Temperature / oC
Fig. 4. (a) Catalytic perfpormance of 5% Cu/Beta prepared with different precursors in the ethanol to acetaldehyde conversion at 300 °C with a TOS =
2.0 h; H2-TPR profiles (b) and TEM pictures (c and d) of 5% Cu/Beta zeolites prepared with different precursors; High-resolution TEM images (f) of
Cu(Ac)2-Beta sample with the corresponding element mapping (g).

1380
Dongni Yu et al. / Chinese Journal of Catalysis 40 (2019) 1375–1384
Fig. 3(b) shows the ethanol conversion and acetaldehyde
100
80
60
40
20
100
80
60
40
20
0
selectivity over 5% Cu/Beta at reaction temperatures of
200–350 °C. Clearly, the ethanol conversion gradually in-
creased from 30% to 95% with an increase in the reaction
temperature from 200 to 350 °C. Simultaneously, a stable acet-
aldehyde selectivity of 85%–90% was maintained up to 300 °C;
however, it started to decrease at 325 °C, as much more con-
densation products from acetaldehyde and some ethene from
ethanol dehydration were produced. Therefore, 300 °C was
used as the reaction temperature in subsequent studies.
(a)
Deactivation
5% Cu/Beta
CuO
Fig. 4(a) shows the effect of the Cu precursors, i.e., CuCl2,
Cu(NO3)2, and Cu(Ac)2, on the ethanol conversion and acetal-
dehyde selectivity over the 5% Cu/Beta catalyst at 300 °C. Ob-
viously, the Cu precursors had a strong impact on the catalytic
performance. The use of inorganic precursors, i.e., CuCl2 and
Cu(NO3)2, resulted in a lower catalytic activity, with ~50%
ethanol conversion and ~80% acetaldehyde selectivity, in
comparison with that achieved using the organic precursor
Cu(Ac)2. The H2-TPR profiles indicated that the use of inorganic
precursors would lead to the broadening of the TPR peaks and
their shift toward higher temperatures (Fig. 4(b)). This sug-
gested that the Cu species obtained from the inorganic precur-
sors interacted more strongly with the [Si]Beta matrix or pos-
sibly had larger particle sizes. The TEM images of these
Cu/Beta catalysts also indicated that the inorganic precursors
could produce larger particles of Cu species (Fig. 4(c) and (d)),
which were not easy to reduce and were not conducive to the
ethanol-to-acetaldehyde conversion. For Cu/Beta prepared
with the organic precursor, the HRTEM image showed the clear
lattice fringes of the zeolite support but no evidence of large
particles of Cu species on the support (Fig. 4(f)), indicating that
Cu species were homogeneously dispersed in the [Si]Beta ma-
trix. This was also supported by the corresponding element
mapping results, which showed that the distribution of the Cu
species was homogeneous and that no obvious Cu-containing
aggregates were formed (Fig. 4(g)). Hence, a good catalytic
activity could be expected and also achieved for the
Cu(Ac)2–Beta catalyst with a homogeneous distribution of Cu
species.
100
80
60
40
20
0
(b)
Deactivation
5% Cu/Beta
5% Cu/Beta
CuO
CuO
0
2
4
6
20
40
60
80
100
120
140
160
180
Time-on-stream / h
Fig. 5. Ethanol conversion (a) and acetaldehyde selectivity (b) as well
as acetaldehyde yields over 5 % Cu/Beta and bulk CuO samples during
the ethanol to acetaldehyde conversion at 300 °C with a TOS = 180 h.
be achieved, a rapid deactivation due to Cu sintering started to
occur even at a TOS of 2.0 h, and the ethanol conversion dra-
matically decreased from 49% to 2% over 6 h. These results
clearly indicated that Cu/Beta-confined catalyst systems were
successfully constructed via the post-synthesis route, and ex-
hibited good structural stability against Cu sintering. In addi-
tion, by increasing the WHSV from 1.0 to 4.0 h−1, a high acetal-
dehyde productivity of 1.99 gAcH/gcat/h with a stable acetalde-
hyde selectivity of 84% (see Table S1 for direct comparison
with the literature) could be achieved at 300 °C over the same
5% Cu/Beta catalyst.
The 5% Cu/Beta catalyst also exhibited a remarkable activ-
ity in propanol dehydrogenation to propylaldehyde; a stable
activity with ~92% propanol conversion and ~95% propylal-
dehyde selectivity could be achieved for a TOS of 15 h (Fig. 6).
This suggested that the Cu/Beta zeolites under study were
general dehydrogenation catalysts, and their use could be ex-
panded to the selective conversion of alcohols to the corre-
sponding aldehydes.
To verify the recyclability of 5% Cu/Beta, the spent catalyst
after ethanol-to-acetaldehyde conversion for 180 h was regen-
erated under synthetic air at 550 °C for 4 h, and its catalytic
activity was re-evaluated at 300 °C. In general, the catalytic
performance of the regenerated catalyst could be
well-recovered if carbon deposits were exclusively responsible
for the catalyst deactivation, as shown in Fig. 7; however, a
rapid deactivation occurs after a TOS of 10 h. These observa-
tions revealed that carbon deposits were associated with the
deactivation of the 5% Cu/Beta catalyst under study. Other
3.3. Stability of optimal catalyst in ethanol conversion
Under the optimized reaction conditions (vide supra), i.e.,
5% Cu/Beta catalyst prepared with Cu(Ac)2 precursor and a
reaction temperature of 300 °C, the long-term stability of the
catalyst was tested for 180 h, and the results are shown in Fig.
5. For comparison, the stability of bulk CuO was also studied.
Using the 5 % Cu/Beta catalyst, an initial ethanol conversion of
86% could be achieved, and it only dropped by ~15% after 100
h of reaction. Moreover, the acetaldehyde selectivity gradually
increased from 82% to 92%, and a stable acetaldehyde yield of
~70% could be obtained during the long-term reaction for 100
h. With further progress in the ethanol-to-acetaldehyde con-
version, the deactivation of the 5% Cu/Beta catalyst occurred,
and the ethanol conversion gradually decreased to ~50% at a
time-on-stream (TOS) of 180 h. For the bulk CuO catalyst, alt-
hough a higher initial acetaldehyde selectivity of >95% could

Dongni Yu et al. / Chinese Journal of Catalysis 40 (2019) 1375–1384
1381
Cu/Beta catalyst after treatment with 2% CO (He balanced) at
25, 50, and 100 °C, followed by purging with He for 30 min.
Two dominant bands at 2119 and 2170 cm‒1 appeared for all
the spent 5% Cu/Beta catalysts treated at different tempera-
tures. The band at 2119 cm‒1 with a shoulder at 2133 cm‒1
could be attributed to either the Cu+–CO carbonyl species or the
atop binding of CO on Cu surface atoms [14,22,37]. However,
upon increasing the treatment temperature, the intensities of
the bands at 2119 and 2133 cm‒1 significantly decreased,
whereas no obvious changes occurred for the band at 2170
cm‒1. According to the literature, the Cu0-CO species can be
easily removed during flushing or heating, whereas the
Cu+-(CO)x carbonyl species are highly resistant to He flushing or
heating [22]. Therefore, the band at 2119 with a shoulder at
2133 cm‒1 should be assigned to CO binding to Cu surface at-
oms. The band at 2170 cm‒1 could be attributed to Cu+-(CO)2
dicarbonyl species. These results indicated that the CuO species
confined in the [Si]Beta matrix could be gradually reduced to
Cu+ and Cu0 species during the ethanol dehydrogenation reac-
tion. With an increase in the production of Cu2+, the selectivity
to acetaldehyde gradually increased, as shown by the catalytic
results (Fig. 5). This indicates that the presence of Cu+ and Cu0
species is beneficial to the acetaldehyde selectivity; this is sup-
ported by the H2 pretreatment results. As shown in Fig. S1,
after H2 pretreatment at 215 and 300 °C, the acetaldehyde se-
lectivity increases to more than 90%, whereas the ethanol
conversion obviously decreases. According to the TPR results
(Fig. 2(d)), the 5% Cu/Beta sample treated at 215 °C should
contain Cu+ species, whereas that treated at 300 °C mainly
contains Cu0 species. This indicates that Cu2+, Cu+, and Cu0 spe-
cies are all active in the ethanol dehydrogenation to acetalde-
hyde; however, the Cu+ and Cu0 species are beneficial to the
acetaldehyde selectivity. After the emergence of Cu0 nanoparti-
cles due to over-reduction, metal sintering occurred, as re-
vealed by the TEM pictures (Fig. 8(a)), leading to catalyst deac-
tivation.
To quantify the carbon deposits formed on the 5% Cu/Beta
catalyst after the ethanol-to-acetaldehyde conversion for 180 h,
the weight contents of these deposits were determined using
TGA in the temperature range of 25–800 °C. In general, two
obvious weight losses were recognized: a low-temperature
weight loss at <300 °C and a high-temperature weight loss at
>400 °C. These were ascribed to volatile compounds and oc-
cluding coke compounds, respectively. According to Fig. 8(c), a
small weight loss of about 0.32% due to the weakly adsorbed
water and a weight loss of 3.07% attributable to the occluding
carbon compounds appeared in the TGA curves. With regard to
the catalytic performance of the regenerated catalyst (Fig. 7),
the low weight losses of 3.07% had a minor impact on the cat-
alyst deactivation under study. Additionally, a special wave
curve with two weight gains appeared in the temperature
range of 135–320 °C. According to the standard TGA curves of
blank Cu and Cu2O samples (Fig. 8(d)), the former weight gain
in the temperature range of 135–225 °C should have been due
to the oxidation of Cu0, whereas the latter in the temperature
range of 300–320 °C could be assigned to the oxidation of Cu+
species. These results indicate the presence of Cu and Cu+ spe-
100
80
60
40
20
0
Propanol conversion
Propylaldehyde selectivity
0
2
4
6
8
10
12
14
16
Time-on-stream / h
Fig. 6. Propanol conversion and propylaldehyde selectivity over 5%
Cu/Beta zeolite catalyst during the propanol to propylaldehyde conver-
sion at 300 °C with a TOS = 14 h.
issues related to catalyst deactivation did exist; these are dis-
cussed in the next section.
3.4. Deactivation of Cu/Beta catalyst during ethanol conversion
To get more detailed information on the deactivation be-
haviors of the 5% Cu/Beta catalyst in the etha-
nol-to-acetaldehyde reaction, the existing states of Cu species
and organic deposits after the reaction were investigated using
TEM, CO-DRIFTS, and TGA. As shown in Fig. 8(a), the agglom-
eration of Cu species occurred in the deactivated catalyst; large
aggregates of about 25 nm could be observed. According to
earlier reports [12,17], the main reason for the deactivation of
Cu catalysts is Cu sintering. Therefore, in comparison with the
state of a fresh catalyst, the agglomeration of Cu species in the
spent 5% Cu/Beta catalyst under study may have been a key
reason for the catalyst deactivation.
The existing states of Cu species in the spent 5% Cu/Beta
catalyst after ethanol-to-acetaldehyde conversion for 180 h
were investigated using DRIFTS, with CO as the probe mole-
cule. Fig. 8(b) shows the DRIFTS spectra of the spent 5%
100
1st cycle
Fresh
80
60
40
20
0
0
10
20
30
40
0
20
30
40
10
Time-on-stream / h
Fig. 7. Catalytic properties of regenerated 5% Cu/Beta zeolite used as
ethanol to acetaldehyde catalyst at 300 °C with a TOS of 40 h.

1382
Dongni Yu et al. / Chinese Journal of Catalysis 40 (2019) 1375–1384
100
c
(c)
(a)
a
b
320
225
135
99
98
97
d
30 nm
Cu-K
a = 0.32%
b = 0.31%
c = 0.08%
d = 3.07%
200
400
600
800
Si-K
O-K
Temperature / oC
130
120
110
100
(d)
(b)
Spent 5% Cu/Beta
2119
25 oC
50 oC
100 oC
25.6%
2170
11.3%
2250
2200
2150
2100
2050
2000
200
400
600
800
Wavenumber / cm-1
Temperature / oC
Fig. 8. TEM image and the corresponding element mapping (a), and CO-DRIFTS spectra (b) as well as TGA curves (c) of the spent 5% Cu/Beta zeolite
catalyst obtained after the ethanol to acetaldehyde conversion at 300 °C with a TOS of 40 h; (d) TGA curves of the bulk Cu and Cu2O samples.
cies in the spent 5% Cu/Beta catalyst after the reaction for 180
h, again, in good agreement with the CO-DRIFTS results (Fig.
8(b)).
4. Conclusions
To summarize the results of the TEM, CO-DRIFTS, and TGA
investigations, we came to the conclusion that the highly dis-
persed Cu2+ species confined in the [Si]Beta matrix could be
gradually reduced to Cu+ and Cu0 species during the etha-
nol-to-acetaldehyde conversion. With the emergence of Cu0
particles, Cu sintering would occur after the long-term reaction,
causing catalyst deactivation (Scheme 1). However, in compar-
ison with bulk CuO, the Cu/Beta-confined catalyst constructed
via the post-synthesis route under study exhibited much better
structural stability against Cu sintering; furthermore, a long
lifetime of 100 h with an acetaldehyde yield of ~70% could be
achieved.
In this work, copper species confined in [Si]Beta zeolite
were prepared via a post-synthesis route and applied as cata-
lysts for ethanol dehydrogenation to acetaldehyde. The homo-
geneous distribution of Cu species with tiny particles in the
[Si]Beta matrix resulted in good structural stability against Cu
sintering; a long lifetime of 100 h with an acetaldehyde yield of
~70% could be achieved over the 5% Cu/Beta catalyst. How-
ever, in the long-term reaction, the confined CuO species were
gradually reduced to Cu+ and Cu0 species, and the agglomera-
tion of metallic Cu particles occurred, eventually leading to
catalyst deactivation.
Acknowledgments
Cu2+
Cu+
Cu0
This work was supported by the National Natural Science
Foundation of China (21872072, 21573113), Municipal Natural
Science Foundation of Tianjin (18JCZDJC37400) and Sinopec
(417012).
Deactivation
CH3CHO
H2
CH3CHO
H2
C2H5OH
C2H5OH
Aggregation
Reduction
References
Scheme 1. Schematic illustration of the ethanol conversion on Cu/Beta
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Graphical Abstract
Chin. J. Catal., 2019, 40: 1375–1384
doi: S1872-2067(19)63378-4
Stabilizing copper species using zeolite for ethanol catalytic dehydrogenation to acetaldehyde
Dongni Yu, Weili Dai *, Guangjun Wu, Naijia Guan, Landong Li *
Nankai University
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分子筛稳定的Cu物种催化乙醇脱氢制乙醛
于东霓a, 戴卫理a,*, 武光军a, 关乃佳a,b, 李兰冬a,b,#
a南开大学材料科学与工程学院, 国家新材料研究院, 天津300350
b南开大学先进能源材料化学教育部重点实验室, 化学化工协同创新中心, 天津300071
摘要: 生物乙醇作为一种绿色原料, 常用于制备具有更高附加值的化学品, 如乙烯、丙烯、丁二烯、1-丁醇和乙醛等. 其中,
乙醛作为重要的化学原料又可以进一步转化生成乙酸, 乙酸酯和季戊四醇等。因此, 由生物乙醇脱氢制乙醛的可持续路线
受到了研究者的广泛关注. 在众多催化体系中, Cu基催化剂表现出较高的活性和乙醛选择性, 但Cu物种的烧结又会导致催
化剂的快速失活. 因此, 抑制Cu物种的烧结是提高Cu基催化剂稳定性的常用手段. 本文通过后合成路线构筑了具有限域
效应的Cu/Beta分子筛催化剂, 并成功应用于乙醇脱氢制乙醛反应. 我们首先考察了Cu的负载量、反应温度和Cu的前驱体
对反应的影响, 发现以醋酸铜为前驱体的5% Cu/Beta催化剂在乙醇脱氢制乙醛反应中表现出最佳催化性能：催化剂寿命达
到100 h, 乙醛收率稳定在70%左右(反应条件: 300 °C, WHSV=1.0 h–1). XRD, TEM, DRIFT, 1H MAS NMR, UV-Vis以及
H2-TPR等结果表明以醋酸铜为前驱体的Cu/Beta分子筛上的主要活性中心为CuO物种, 并且该物种以高度分散的形式限域
在分子筛脱铝产生的硅羟基缺陷位上. 因此, 在乙醇脱氢制乙醛反应中, 限域的Cu/Beta分子筛催化剂与孤立的CuO催化剂
相比, 具有优异的抗烧结能力. 然而, 当反应至180 h时, 催化剂会有明显的失活现象. 再生后, 尽管催化剂的初始活性能较
好的恢复, 但催化剂的寿命却大大下降. 我们利用TEM, TGA以及CO-DRIFTS对反应后的催化剂进行表征, 发现失活的
Cu/Beta催化剂上存在Cu+和Cu0物种, 并且伴有大块烧结现象. 这表明随着反应的进行, Cu/Beta催化剂上的CuO物种会逐渐
还原成Cu+和Cu0物种, 而Cu0物种的出现又会诱导烧结现象的发生, 进而导致催化剂的失活.
关键词: 乙醇脱氢; 乙醛; 沸石分子筛; Cu基催化剂; 失活
收稿日期: 2019-02-28. 接受日期: 2019-04-02. 出版日期: 2019-09-05.
*通讯联系人. 电话/传真: (022)85358536; 电子信箱: weilidai@nankai.edu.cn
#通讯联系人. 电话/传真: (022)23500341; 电子信箱: lild@nankai.edu.cn
基金来源: 国家自然科学基金(21872072, 21573113); 天津市自然科学基金(18JCZDJC37400); 中国石化(417012).
本文的电子版全文由Elsevier出版社在ScienceDirect上出版(http://www.sciencedirect.com/science/journal/18722067).