Highly active Ce, Y, La-modified Cu/SiO2 catalysts for hydrogenation of methyl acetate to ethanol

Article abstract of DOI:10.1039/c9ra08780j

Rare earth element (Ce, Y, and La) modified Cu/SiO₂ catalysts via hydrolysis precipitation and impregnation method were fabricated for the vapor-phase hydrogenation of methyl acetate to ethanol. LaO_x showed the most pronounced promotion in the catalytic tests. After detailed characterizations, via N₂ adsorption-desorption, XRD, N₂O chemisorption, FTIR, H₂-TPR, H₂-TPD, TEM, XPS, and TG/DTA, we found that the addition of promoter LaO_x can decrease the particle size while in turn, it can increase the dispersion of copper species. The strong interactions between copper and lanthanum atoms alter the surface chemical states of the copper species. This results in the generation of more Cu⁺ species and high S_Cu⁺ values, which are responsible for the excellent activity and stability during hydrogenation. In addition, the content of additive LaO_x and reaction conditions (reaction temperature and LHSV) were optimized. Then, the long-term stability performance was evaluated over the selected catalyst in contrast with Cu/SiO2.

Full text of DOI:10.1039/c9ra08780j

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Highly active Ce, Y, La-modified Cu/SiO2 catalysts
for hydrogenation of methyl acetate to ethanol†
Cite this: RSC Adv., 2020, 10, 5590
c
cd
Zhiheng Ren,abc Muhammad Naeem Younis,
Chunshan Li,
Zengxi Li,e
*
ab
Xiangui Yang
and Gongying Wangab
*
Rare earth element (Ce, Y, and La) modified Cu/SiO2 catalysts via hydrolysis precipitation and impregnation
method were fabricated for the vapor-phase hydrogenation of methyl acetate to ethanol. LaOx showed the
most pronounced promotion in the catalytic tests. After detailed characterizations, via N2 adsorption–
desorption, XRD, N2O chemisorption, FTIR, H2-TPR, H2-TPD, TEM, XPS, and TG/DTA, we found that the
addition of promoter LaOx can decrease the particle size while in turn, it can increase the dispersion of
copper species. The strong interactions between copper and lanthanum atoms alter the surface
Received 25th October 2019
Accepted 19th December 2019
chemical states of the copper species. This results in the generation of more Cu+ species and high SCu
+
values, which are responsible for the excellent activity and stability during hydrogenation. In addition, the
content of additive LaOx and reaction conditions (reaction temperature and LHSV) were optimized. Then,
the long-term stability performance was evaluated over the selected catalyst in contrast with Cu/SiO2.
DOI: 10.1039/c9ra08780j
rsc.li/rsc-advances
behavior and low cost of the catalyst, making it suitable for
industrial development.11–13 Moreover, relatively high copper
1. Introduction
loading (>30%) is used to synthesize the catalyst to ensure high
efficiency of the reaction.14 In the main studies on Cu/SiO2
catalysts, the proper distribution of Cu species and their strong
interactions with the support or promoter are vital in deter-
mining the catalytic performance. In the past, different prepa-
ration methods have been successfully developed to generate
high-efficiency catalysts, including impregnation,15 urea-
assisted precipitation,16,17 ion exchange,18 and ammonia evap-
oration (AE method).19,20 Recently, the development of a novel
hydrolysis-precipitation method (HP method) by using TEOS as
the silicon source exhibited better performance in the hydro-
genation of dimethyl oxalate (DMO) to ethylene glycol than that
prepared by the AE method.21 This could be due to stronger
interactions between the copper particles and the silica support,
which enhanced the metal dispersion.
The chemical structure, composition, and surface and bulk
components greatly inuence the properties of the catalysts. In
particular, the synergy between Cu0 and Cu+ species is widely
accepted in the hydrogenation of esters. The Cu0 active sites are
benecial for activation of H2 molecules, while Cu+ species
could adsorb and activate the C]O groups in MA. The latter is
believed to be the rate controlling step.22,23 In previous studies,
it has been shown that adding a second metal such as Zn, Ni, B,
In, Ag, Mn, or Mg to the Cu/SiO2 catalyst could improve the
catalytic activity.17,24–29 The promotion is attributed to the
enhancement in copper species dispersion and the appropriate
Cu+/(Cu+ + Cu0) ratio on the surface of the catalyst. However,
there are still many challenges in Cu-based catalysts that need
to be overcome, especially their short lifespan.
As a renewable and sustainable energy source, ethanol has been
widely used as a solvent and disinfectant; it also acts as an
alternative fuel for vehicles. At present, a promising approach to
produce ethanol from the hydrogenation of dimethyl ether
(DME) has been proposed, which has attracted worldwide
attention.1–3 This process consists of two stages: DME carbon-
ylation to methyl acetate (MA) over zeolites and MA to ethanol
over copper based catalysts.4–8 The rst step acquired higher
activity and stability over mordenite, while the second step
employed cheap copper catalysts since they possess selective
hydrogenation ability.9,10
The Cu/SiO2 catalyst is typically used for MA hydrogenation
mainly because of the non-reducible and relatively inert
aChengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu,
610041, China. E-mail: yangxg@cioc.ac.cn
bNational Engineering Laboratory & Technology, University of Chinese Academy of
Science, Beijing 101408, China
cCAS Key Laboratory of Green Process and Engineering, State Key Laboratory of
Multiphase Complex Systems, The National Key Laboratory of Clean and Efficient
Coking Technology, Beijing Key Laboratory of Ionic Liquids Clean Process, Institute
of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China
dZhengzhou Institute of Emerging Technology Industries, Zhengzhou 450000, PR
China. E-mail: csli@ipe.ac.cn
eSchool of Chemical Science, University of Chinese Academy of Sciences, Beijing
100049, People's Republic of China
† Electronic supplementary information (ESI) available: Product analysis by
GC-MS; N2 adsorption–desorption isotherm of SiO2 and LaOx/SiO2; grain size of
different catalysts; FTIR spectra; N2O titration method; XPS spectra of Ce 3d
and Y 3d; relation between SC0u and STYEtOH; TG and DTA analysis; TEM image
of the spent catalyst; summary of Cu-based catalysts. See DOI: 10.1039/c9ra08780j
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Rare-earth metal oxides have been used in many catalytic
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reactions due to their superb properties, such as enhanced anti-
agglomeration and sinterability. Huang et al. showed that the
rare earth additives (Y, La, Ce, Pr, and Sm) could strengthen the
structure of the Cu/SiO2 catalyst to improve stability and also
prevent the leaching of active metals, especially Y and La.30
Zheng et al. found that the introduction of La promoter could
enhance the interaction between Cu species and silica support,
and restrain the sintering of the catalyst in DMO hydrogena-
tion.31 Additionally, cerium as a catalyst promoter has exhibited
an excellent effect in promoting the hydrogenation of DMO
reaction in recent years. Ye et al. showed that adding an
appropriate amount of cerium could decrease the size of the
copper crystallite, improve its dispersion, and enrich the
surface Cu+ species.32 Ai et al. found that strong interaction
between the Ce promoter and Cu species substantially changed
the redox properties of the catalysts. Moreover, the addition of
Ce could remarkably increase the dispersion of Cu and retard
the sintering of Cu species.33 Generally, the introduction of rare-
earth metal oxides to Cu/SiO2 could increase the copper
dispersion and maintain a thermally stable catalyst structure in
the hydrogenation reaction. However, to the best of our
knowledge, in the hydrogenation reaction of MA, no detailed
study on Ce, Y, and La modied Cu/SiO2 catalyst system
prepared by HP method has been reported.
Fig. 1 Preparation process of Cu/SiO2–xM catalysts.
represents the weight percentage of the rare earth metal M.
Here, x was calculated according to the equation:
mM
x ¼
ꢂ 100
m
CuꢁSiO2
Before testing the reduced catalyst, a certain amount of the
Cu-based catalyst was placed into the tube furnace and heated
at 350 ꢀC for 4 h under a H2 atmosphere (100 mL minꢁ1). Aer
cooling to room temperature, the reduced catalyst was sealed in
a centrifuge tube to avoid oxidation. The preparation ow chart
is shown in Fig. 1. For comparison, a LaOx/SiO2 catalyst was
prepared by impregnation using a selected SiO2 and the preset
weight percentage of La was 5%.
In this work, a series of Ce-, Y-, and La-promoted Cu/SiO2
(Cu-HP) catalysts for the hydrogenation of MA to ethanol were
synthesized using the impregnation method. Several charac-
terization techniques were used to evaluate the interaction
between copper and the promoters. In addition, the structure–
activity relationship of La-doped Cu/SiO2 catalyst and the
deactivation analysis of Cu/SiO2 catalyst were also investigated.
2.2 Catalyst characterization
The actual metal loading was determined by ICP-OES on an IRIS
Intrepid II XSP (Thermosher, USA) instrument. X-ray diffrac-
tion (XRD) analysis was done on a Rigaku Smart Lab X-ray
powder diffractometer using Cu Ka (l ¼ 0.15406 nm) as the
radiation source. The data was recorded from 2q ¼ 5ꢀ to 90ꢀ
with a scanning speed at 10ꢀ minꢁ1. N2 adsorption–desorption
was performed using a Micromeritics ASAP 2460 analyzer at
liquid nitrogen temperature. Prior to testing, all the samples
were degassed at 350 ꢀC for 4 h. Fourier Transform infrared
spectra (FTIR) was obtained on a Nicolet 6700 spectrometer
with a spectral resolution of 4 cmꢁ1 and was recorded from 400
to 4000 cmꢁ1. KBr was mixed with the sample and then pressed
into a wafer. The in situ DRIFTs of MA was carried out on the
same apparatus as above to identify the nature of the adsorbed
species on the catalyst. Prior to testing, the catalyst was rstly
reduced at 350 ꢀC in the hydrogenation atmosphere for 4 h. MA
was introduced into the cell through the Ar stream. Aer the
catalyst was saturated with MA, the cell was purged with Ar ow
and spectra were recorded at different purging times until the
spectrum was unchanged. H2-TPR curves were recorded to
investigate the reduction behavior of the catalyst, which was
conducted on an Autochem II 2920 Chemisorption Apparatus
(Micromeritics). About 50 mg of the sample was loaded in the U-
2. Experimental
2.1 Catalyst preparation
The Cu/SiO2 catalyst (30 wt% Cu) was prepared using the
hydrolysis precipitation method (HP), as reported previ-
ously.21,24 Briey, an appropriate amount of TEOS mixed ethanol
solution was rst added in the Cu(NO3)3$3H2O aqueous solu-
tion and then stirred for 1 h. Subsequently, 0.25 M ammonium
carbonate was used as a precipitant and added dropwise to the
above solution under vigorous stirring. The pH of the mixed
solution was maintained at about 7 and the resultant mixture
was aged at 80 ꢀC for 18 h. Aer ltering, washing, and drying,
the dried Cu/SiO2 was obtained. Finally, the dried Cu/SiO2 was
calcined at 500 ꢀC for 4 h in static air and Cu/SiO2 powder was
obtained. The La, Ce, and Y-modied Cu/SiO2 catalysts were
prepared by impregnation method. Typically, a certain amount
of the above synthesized Cu/SiO2 was added to the La(NO3)3-
$6H2O, Ce(NO3)3$6H2O, and Y(NO3)3$6H2O solution. Aer stir-
ring for 12 h, excess water was evaporated from the suspension
at 80 ꢀC using a vacuum rotary system. The catalyst Cu/SiO2–xM
was nally obtained aer calcining at 450 ꢀC for 4 h in air,
where M represents three additive elements and x (x ¼ 1, 5, 10)
ꢀ
tube and dried at 120 C for 1 h under a He atmosphere and
ꢀ
then cooled down to 50 C. 10% H2–Ar was switched into the
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ꢀ
tube and the sample was heated to 500 C with a ramp rate of
10ꢀ minꢁ1. H2-TPD was measured on the same device above.
moles of ethanolðoutÞ
moles of MAðinÞ
Yieldð%Þ ¼
ꢂ 100
(3)
ꢀ
The sample was rst reduced at 350 C for 4 h in 10% H2–Ar
atmosphere before cooling down to 50 ꢀC, then the adsorption
of H2 was performed at 50 ꢀC for 1 h. Aer the removal of
unabsorbed H2 by the Ar purge, the desorption data of H2 was
collected from 50 ꢀC to 800 ꢀC at a heating rate 10 ꢀC minꢁ1. The
copper surface of the catalyst was calculated in combination
with N2O-titration and CO-TPD, which was also tested in the
above apparatus. As reported previously, it was assumed that
Cu+ ions and Cu0 atoms occupy the same area (1.47 ꢂ 1019
copper atoms per m2).24,33 The detailed process is shown in the
ESI.† X-ray photoelectron spectroscopy (XPS) and Auger electron
spectroscopy (XAES) measurements were performed on an ESCA
Lab220i-XL electron spectrometer (VG Scientic) with an Al Ka
X-ray radiation source (hy ¼ 1486.6 eV). Before testing, the
samples were ex situ reduced at 350 ꢀC for 4 h in pure hydrogen
and sealed in a centrifuge tube. The C 1s peak (284.6 eV) was
used to calibrate the binding energies (BE). TEM micrographs
were obtained using the JEM-2100 system. The analysis of
elemental dispersion was performed by EDS mapping
measurement in the STEM mode. TG and DTA measurements
of the spent catalyst were conducted on a simultaneous Shi-
madzu thermal analyzer. The samples were heated from room
temperaꢁtu1re to 700 ꢀC in air ow with a heating rate of
The indicator of space time yield of ethanol (STY) was used to
evaluate the catalytic performance of the catalyst. The value of
ꢁ1
STYEtOH (gEtOH gcat hꢁ1) is the mass of ethanol produced per
gram of the catalyst and per hour.
3. Results and discussion
3.1 Catalytic performance and stability
To investigate the effect of modication by rare earth elements
(Ce, Y, and La), the catalytic activities were tested under iden-
tical conditions. As shown in Table 1, the MA conversion and
ethanol selectivity could be improved by introducing rare earth
elements. In addition, under the same amount of additive (5%),
the order of activity is Cu/SiO2–5La > Cu/SiO2–5Y > Cu/SiO2–5Ce.
For comparison, LaOx/SiO2 was also tested but is not active,
indicating a synergistic effect between copper and lanthanum
species. Furthermore, the yields of La-doped catalysts exhibit
volcano-type trend with increasing La content. The 5% La-
doped Cu/SiO2 catalyst shows the maximum value of STY
(1.04 gEtOH gcatꢁ1 hꢁ1), further increasing the La content to 10%,
which decreases to 0.96 gEtOH gcat hꢁ1 be due to partial
ꢁ1
10ꢀ min
.
covering of the active sites.30
The different catalytic properties between optimized Cu/
SiO2–5La and Cu/SiO2 were further evaluated as a function of
temperature in the range of 190 ꢀC to 280 ꢀC. As shown in
Fig. 2a, it is apparent that both conversion and selectivity are
improved aer La modication. In particular, the La-doping
catalyst has a better low-temperature activity. Furthermore,
2.3 Catalyst tests
The catalytic performance of hydrogenation of MA was studied
on a xed bed reactor. A certain catalyst with 40–60 mesh was
loaded at the center of the reactor and some silica sand was
lled on both sides, to ensure that the feed had a plug ow
prole. Prior to the evaluation, all catalysts were pretreated in
pure H2 ow (100 mL minꢁ1) at 350 ꢀC for 4 h, with a ramp rate
of 5 ꢀC min. Aer that, MA was injected into the vaporizer using
a high pressure metering pump, heated, and then carried into
the reactor by a H2 ow. The reaction was conducted at 2.5 MPa,
the liquid hourly space velocity (LHSV) was in the range of 1–4
hꢁ1, and the gas hourly space velocity (GHSV) was xed at 3000
hꢁ1. The liquid products were separated and analyzed using
a Ruihong SP-7890 gas chromatograph equipped with a ame
ionization detector (FID). The main by-products were identied
by GC-MS (QP2010, Shimadzu, Japan). The internal standard
method was used to calculate the conversion of MA and selec-
tivity of ethanol. Iso-butanol was selected as the internal stan-
dard material. The conversion of MA, selectivity, and yield of
ethanol were based on the following eqn (1)–(3):
ꢀ
when the reaction temperature is lower than 220 C, both the
conversion of MA (X) and the selectivity of ethanol (S) sharply
rise. On further increasing the temperature, the conversion
curves of MA change slightly. However, the selectivity of ethanol
for the two catalysts rst reaches the maximum (94.6% and
84.4%) at 250 ꢀC and then decreases to 85.6% and 78.9%,
respectively. This is because more side reactions occur at higher
temperature (280 ꢀC) and the by-product analysis is listed in
Table S1.†
Table 1 Catalytic performance for the hydrogenation of MA over Cu/
SiO2–xM catalystsa
MA
Ethanol
STY/gEtOH
ꢁ1
Catalyst
conversion/% selectivity/% Yield/% gcat hꢁ1
Cu/SiO2
90.4
92.2
95.1
94.7
95.4
85.1
86.2
90.2
91.6
94.8
91.1
0
76.9
79.5
85.7
86.7
90.4
83.4
0
0.89
0.92
0.99
1.00
1.04
0.96
0
moles of MAðinÞ ꢁ moles of MAðoutÞ
Conversionð%Þ ¼
ꢂ 100
Cu/SiO2–5Ce
Cu/SiO2–5Y
Cu/SiO2–1La
Cu/SiO2–5La
Cu/SiO2–10La 91.5
LaOx/SiO2
moles of MAðinÞ
(1)
moles of ethanolðoutÞ
Selectivityð%Þ ¼
ꢂ 100
0
moles of MAðinÞ ꢁ moles of MA ðoutÞ
a
(2)
Reaction conditions: T ¼ 250 ꢀC, P ¼ 2.5 MPa, GHSV ¼ 3000 hꢁ1, LHSV
¼ 1 hꢁ1
.
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Fig. 2 Effect of reaction conditions on the reaction activity. (a) Effect of reaction temperature, P ¼ 2.5 MPa, LHSV ¼ 1 hꢁ1, GHSV ¼ 3000 hꢁ1. (b)
Effect of liquid hourly space velocity (LHSV), T ¼ 250 ꢀC, P ¼ 2.5 MPa, GHSV ¼ 3000 hꢁ1
.
high activity under the optimal conditions (LHSV ¼ 1 hꢁ1). X ¼
96.1%, S ¼ 97.8% were obtained for the Cu/SiO2–5La catalyst
and X ¼ 92.1%, S ¼ 89.5% were obtained for the Cu/SiO2
catalyst. Excellent performance was obtained at lower LHSV
since the catalyst surface possesses enough active sites to
adsorb and activate the reactant molecule. As the LHSV
increases, the catalytic activity of the two catalysts gradually
decline. Because the value of GHSV (dwell time) remain
unchanged, the ratio of H2/MA will be decreased on increasing
the LHSV, which may result in a lower hydrogenation rate.
However, for the Cu/SiO2–5La catalyst, the conversion and
selectivity were maintained above than 90% with an LHSV as
high as 2 hꢁ1. This suggests the 5% doping La could provide
more active species than bare Cu/SiO2 catalyst. Moreover, La
decorated Cu/SiO2 catalyst has a better tolerance capacity for
higher LHSV. When the LHSV increases to 4 hꢁ1, the conversion
and selectivity of Cu/SiO2 decrease to 55.5% and 57.8%,
respectively. On the other hand, MA conversion and ethanol
selectivity of the Cu/SiO2–5La catalyst could maintain a better
level (75.1% and 70.2%).
Fig. 3 Long-term catalytic performance of Cu/SiO2 and Cu/SiO2–5La
catalysts as a function of reaction time.
The inuence of LHSV on the conversion of MA and selec-
tivity of ethanol over Cu/SiO2–5La and Cu/SiO2 catalysts was
tested. As exhibited in Fig. 2b, two catalysts show a relatively
In order to investigate the long-term stability of Cu/SiO2 and
optimized Cu/SiO2–5La catalysts, the comparison of catalytic
Table 2 Structural properties of the Cu/SiO2–xM catalyst
Contenta
(wt%)
c
Catalyst
Cu
M
SBETb (m2 gꢁ1
)
Vpb (cm3 gꢁ1
)
Dpb (nm)
S0Cu (m2 gꢁ1
)
SCu+c (m2 gꢁ1
)
SCu (m2 gꢁ1
)
Cu/SiO2
28.2
26.6
27.0
25.8
27.0
24.2
—
—
613
575
596
610
569
522
419
452
1.16
1.10
1.13
1.14
0.92
0.98
0.99
1.06
6.8
7.2
7.3
7.4
6.9
7.3
9.4
9.2
28.7
30.1
31.5
26.6
25.9
29.4
—
35.0
39.9
43.4
44.8
46.9
43.4
—
63.7
70.0
74.9
71.4
72.8
72.8
—
Cu/SiO2–5Ce
Cu/SiO2–5Y
Cu/SiO2–1La
Cu/SiO2–5La
Cu/SiO2–10La
LaOx/SiO2
SiO2
4.4
4.3
0.99
4.5
9.1
4.3
—
—
—
—
—
a
c
Obtained by ICP-OES. b Obtained from N2 isotherm adsorption. Caculated SCu by N2O–CO titration combined with LMM XAES spectra.
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Fig. 4 Calcined Cu/SiO2–xM catalysts (a) N2 adsorption–desorption isotherm and (b) BJH pore size distribution curves.
performance was evaluated under the same conditions, viz., T ¼ 250 h, indicating that the copper supported SiO2 catalysts
250 ꢀC, P ¼ 2.5 MPa, LHSV ¼ 1 hꢁ1, and GHSV ¼ 3000 hꢁ1. As prepared by the HP method have excellent stability. However,
displayed in Fig. 3, the two catalysts exhibit good stability within compared with the 5La-doped Cu/SiO2 catalyst, MA conversion
Fig. 5 XRD patterns of (a) calcined, (b) reduced Cu–SiO2, and Cu/SiO2–xM catalysts, and (c) the dried Cu/SiO2.
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Fig. 6 TEM images of reduced (a) Cu/SiO2 and (b) Cu/SiO2–5La catalyst; particle size distribution of Cu/SiO2 (a-1) and Cu/SiO2–5La (b-1); (c) and
(d) HRTEM images of reduced Cu/SiO2–5La catalyst (the zoom-in area of green region); (e) STEM image elemental map of the reduced Cu/SiO2–
5La catalyst and the corresponding EDS elemental mappings of Cu, Si, O, and La.
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and ethanol selectivity of the unmodied Cu/SiO2 catalyst
decreased dramatically aer 250 h. The Cu/SiO2–5La catalyst
could maintain its high activity ever aer 280 h, suggesting that
modication with a suitable La content could improve the
stability of the Cu/SiO2 catalyst.
3.3 Crystalline phase and morphology
As shown in Fig. 5a, the broad diffraction peak that appears at
2q ¼ 22ꢀ in the calcined catalyst is assigned to amorphous SiO2.
For the Cu/SiO2 precursor, very faint peaks are ascribable to the
Cu2SiO5(OH)2 structure (Fig. 5c).34 The structure was also
conrmed using FTIR (Fig. S2†), where the band at 670 cmꢁ1 is
ascribed to the dOH band of Cu2SiO5(OH)2. As could be found, all
the calcined Cu/SiO2–xM catalysts have shown the Cu2SiO5(-
OH)2 structure. Aer calcination, the crystallinity weakens,
demonstrating that this phase gradually vanishes instead of the
emergence of diffraction peaks belonging to the CuO center at
2q ¼ 35.4ꢀ and 38.6ꢀ (JCPDS05-0661). Aer doping with the
additive of 5% Ce, 1% La, and 5% La, the CuO diffraction peaks
are weakly visible. However, for 5% Y and excess La (10%) doped
catalysts, the CuO diffraction peaks become more obvious,
possibly due to partial aggregation of metallic copper particles.
In addition, except for the insignicant diffraction peaks
ascribed to CeO2 at 28.5ꢀ and 56.3ꢀ (JCPDS 34-0394), no
diffraction peaks assignable to YOx or LaOx were observed for Y
and La modied Cu/SiO2 catalysts, respectively. This demon-
strates that MOx (CeO2, YOx, and LaOx) are highly dispersed in
these samples.
As displayed in Fig. 5b, besides amorphous SiO2, other
diffraction peaks of the catalysts are observed aer reduction in
pure H2 at 350 ꢀC for 4 h. The characteristic peaks at 2q ¼ 43.3ꢀ
and 50.5ꢀ belong to the Cu0 species (JCPDS 04-0836), and the
peak center at 2q ¼ 36.4 is ascribable to the Cu2O phase (JCPDS
05-0667).17,33 On the whole, aer doping Ce and La, the
diffraction peaks of Cu0 species become relatively broad, sug-
gesting that the addition of these promoters could decrease the
crystallite size of Cu0 and increase the dispersion of Cu0 species.
The results of grain size of the copper crystallite of different
catalysts are shown in Table S2.† It is interesting to note that the
sharp diffraction at about 43ꢀ in Cu/SiO2–5Y and Cu/SiO2–10La
catalysts might be caused by a small degree of agglomeration of
the copper grains. Furthermore, when 5% Y, 1% La, and 5% La
are added, the intensity of the Cu2O characteristic peak
3.2 Physicochemical properties of the catalysts
The actual copper loading was measured by ICP-OES. As can be
seen from Table 2, this value is slightly lower than the theo-
retical value due to elution of copper ion during washing. As
exhibited in Fig. 4a, all the calcined catalysts show typical IV
type isotherms, indicating the existence of a mesoporous
structure.24 Aer introducing the rare earth element into the Cu/
SiO2 system, the shape of the hysteresis loop changes to some
extent but the distributions of pore size of all the samples are
concentrated at about 3 nm (in Fig. 4b), suggesting that the
addition of Ce, Y, and La has no signicant change on the pore
structure.
The BET surface area (SBET) and pore volume (Vp) of the
calcined catalyst are summarized in Table 2. Obviously, the
unmodied Cu/SiO2 catalyst has a relatively high SBET of 613 m2
gꢁ1 and Vp of 1.16 cm3 gꢁ1, which would decrease aer intro-
ducing the additive may be due to their blockage in the meso-
porous structure. Additionally, the distribution of copper
species is critical for determining the catalytic activity of the
copper based catalyst in the hydrogenation of esters. Therefore,
the copper surface areas of the reduced catalysts were estimated
by the results of N2O titration and combined with CO-TPD and
XAES. As shown in Table 2, the Cu+ surface area of the reduced
sample increases aer introducing the promoter, whereas that
of the Cu0 surface area has no signicant difference compared
with Cu/SiO2. It is worth noting that the SCu+ values of La-doped
Cu/SiO2 are higher than the other catalysts, in particular, Cu/
SiO2–5La has the highest SCu+ (46.9 m2 gꢁ1). Nevertheless, as the
+
lanthanum content further increased, the value of SCu is
somewhat reduced, which may be due to the coverage of LaOx
on the catalyst's surface.
Fig. 7 H2-TPR profiles (a) of Cu–SiO2 and Cu/SiO2–xM catalysts, (b) H2-TPD profiles of reduced Cu–SiO2 and Cu/SiO2–xM catalysts.
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becomes stronger, suggesting that the amount of Cu2O is SiO2–5La catalyst, which is consistent with the XRD results.
simultaneously increased. However, adding 5% Ce or Furthermore, the HAADF-STEM and STEM-EDS mappings in
increasing the La loading to 10% will decrease that strength. Fig. 6e show that the Cu and La species are co-existent and
These results demonstrate that the Cu/SiO2–5La catalyst may uniformly dispersed on the surface of texture of the support,
have a higher content and a higher dispersion of the Cu2O which overlap with each other, suggesting that they are in
species.
intimate contact. The strong interaction between Cu and La
HRTEM was used to further determine the morphology and might be derived from their close proximity, which would retard
distribution of elements of the optimized Cu/SiO2–5La catalyst. the transformation of Cu2+ to Cu0 to some extent.36
As can be seen from Fig. 6a and b, traces of the typical whisker-
shaped copper phyllosilicates remain in the reduced Cu/SiO2
Cu/SiO2–5La catalyst, which are inherited from the structural
characteristics of the dried catalyst.21 Aer reduction and acti-
vation in pure H2 at 350 ꢀC, black metallic Cu particles are
formed and are uniformly dispersed on the surface of SiO2
texture. The mean particle size is 4.6 nm for the Cu/SiO2 catalyst
and 3.8 nm for the Cu/SiO2–5La catalyst. This means that the
addition of La could decrease the particle size of the copper
species. In addition, it can be seen from the results of the
HRTEM images in Fig. 6c and d that Cu, Cu2O, and La2O3
nanoparticles are present simultaneously in the reduced Cu/
3.4 H2-TPR and H2-TPD
To clarify the reducibility of the rare earth element modied Cu/
SiO2 catalyst, the H2-TPR proles of the calcined catalysts were
recorded. As shown in Fig. 7a, all the samples exhibit a center of
H2 consumption peak at about 194 ꢀC, which could be attributed
to the collective effect from highly dispersed CuO to Cu0 and
copper phyllosilicate to Cu2O. The further reduction of Cu2O to
Cu0 requires a high temperature above 600 ꢀC.37–39 However,
a small H2 consumption peak located at 212 ꢀC for 5% Y and 10%
La doped Cu/SiO2 catalysts may be caused by conversion of large
CuO particles to Cu, in accordance with the XRD diffraction
Fig. 8 Cu 2p spectra (a), La 3d spectra (b), and (c) Cu LMM Auger spectra of reduced Cu/SiO2 and Cu/SiO2ꢁxM catalysts.
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Table 3 Deconvolution results of XPS and Cu LMM XAES of Cu/SiO2–
xM catalysts
peaks of the calcined catalysts in Fig. 5a. Aer introducing the
promoter, the reduction temperature of the catalyst shis to
a lower temperature than that for Cu/SiO2, suggesting that
a suitable amount of the promoter could prove the dispersion of
copper species and be easily reduced.31 Moreover, it has been
found that the introduction of different contents of La would
have a great effect on the reducibility of the catalyst. The reduc-
tion peak shis to lower temperature as the La content is no more
than 5%. However, on increasing the La content to 10%, the
reduction temperature would shi to a higher temperature due
to the signicant interaction between the Cu and La species,
which is consistent with Huang's results.27
KEa (eV)
Cu+ Cu0
APb (eV)
Cu+
+
Catalyst
Cu/SiO2
Cu0
Cu 2p3/2 BEc (eV) XCu
0.55
913.6 918.1 1846.1 1850.6 932.5
Cu/SiO2–5Ce 913.6 918.1 1846.2 1850.7 932.6
Cu/SiO2–5Y 913.6 918.1 1846.2 1850.7 932.6
Cu/SiO2–1La 913.6 918.1 1846.2 1850.7 932.6
Cu/SiO2–5La 913.6 918.1 1846.3 1850.8 932.7
Cu/SiO2–10La 913.6 918.1 1846.2 1850.7 932.6
0.57
0.58
0.63
0.65
0.60
a
Kinetic energy (KE). b Auger parameter (AP). c Binding energy (BE).
H2-TPD was used to analyze the adsorption behavior of H2 on
the catalyst surface aer activation in pure H2. As presented in
Fig. 7b, the H2-TPD proles for all the copper based catalysts
show two types of desorption peaks, a lower temperature range
(lower than 100 ꢀC), and a broad higher temperature range
(150–550 ꢀC). The lower temperature desorption peak is
ascribed to the chemisorption of H2 at the Cu active sites, while
the higher one corresponds to the chemisorbed splitting H
species and the broad width of which may be related to the
formation of a small size of particles.40,41
The strength of a broad high temperature peak can be
increased by the introduction of a promoter, suggesting that the
addition of a promoter to the Cu/SiO2 catalyst can increase the
adsorption concentration of the active H species on the catalyst
surface. Zheng and Ai believed that the high dispersion of
copper species and large SCu was benecial for the activation
and adsorption of H2.31,33 However, in combination with our
results from Table 2, the SCu values of the modied catalysts are
almost the same; even the 1% La and 5% La doped catalysts
have lower SCu values than other catalysts.
Based on previous studies, there was an electronic effect
between copper and the rare earth element, in which the d-
electrons of Cu0 easily ow to the d orbital of the rare earth
element, thereby forming an unoccupied d-orbital and possibly
bonding with the H2 species.42,43 The electrons of Cu–M (M ¼
Ce, Y, and La) catalysts can be transferred to the H2 molecule,
which promotes dissociation by weakening the H–H bond when
H2 is adsorbed on the catalyst surface.44,45 Therefore, we spec-
ulate that the presence of an electronic effect between Cu0 and
these additive species is the main cause for promoting the
adsorption ability of active H species. Among these catalysts,
5% La-doped Cu/SiO2 has the maximum adsorption capacity of
H species, further increasing the La content; thus, the capacity
would be decreased. Therefore, the introduction of an appro-
priate amount of La species could greatly increase the amount
of splitting H species through electronic effects.
between 942–944 eV were observed.35 In addition, the binding
energy of Cu 2p3/2 shis to higher values aer doping with the
additive. This conrms the signicant electronic effect in the
reduced Cu/SiO2 catalysts containing Ce, Y, and La, which is
consistent with the H2-TPD results. The Ce 3d and Y 3d spectra
are shown in the ESI† (Fig. S3†). Overall, it could be found that
the Ce3+ species may be partially present in the reduced Cu/
SiO2–5Ce catalyst aer reduction in our test. It is also worthy of
noting that for the La-doped reduced catalysts, the binding
energies of La 3d5/2 and 3d3/2 are centered at 835.5 and 852.1 eV,
respectively (Fig. 8b). With the increase in La content, the La
3d5/2 binding energy decreases from 842.8 eV to 841.5 eV due to
the increase in electron density of the LaOx species on the
surface of the catalyst.36
Cu LMM Auger electron spectra (XAES) was used to
discriminate Cu+ and Cu0 species because these two species are
almost located at the same position of binding energy. As shown
in Fig. 8c, the Cu+ and Cu0 species co-exist since the XAES peaks
of the reduced catalysts are asymmetric and broad. These peaks
could be deconvoluted into two overlapped peaks at ca. 913.6 eV
and 918.1 eV, which represent Cu+ and Cu0, respectively.46 As
summarized in Table 3, it can be seen that the Cu+/(Cu+ + Cu0)
molar ratio increases in the Cu/SiO2 catalyst, when Ce, Y, and La
are doped. As reported previously, the production of more Cu+
species may be due to electronic interaction between the copper
species and the promoter.36 This effect would retard the degree
of reduction of Cu2+ species and generate more Cu+ on the
catalyst surface. Compared to Ce and Y, the La-doped Cu/SiO2
catalyst exhibits a higher ratio of Cu+/(Cu+ + Cu0), indicating
that La is more favorable for increasing the surface concentra-
tion of Cu+ species. Moreover, the ratio of reduced La-doped
catalyst is initially increased to a maximum value of 0.65
when introducing 5% La and then gradually decreased with
a further increase in the La loading, which is similar to the
results of XRD of the reduced catalysts. The Auger parameters
(AP) of Cu+ and Cu0 are close to the reported values of 1847.0 eV
and 1851.0 eV, respectively.46
3.5 Chemical states of the surface species
Fig. 8a illustrates the valence state of the copper species on the
surface of the reduced catalysts. Apparently, the spectra of the
reduced catalyst in our test only have two peaks, which center at
3.6 In situ DRIFTs of MA adsorption
932.6 eV and 952.5 eV corresponding to Cu 2p3/2 and Cu 2p1/2
,
respectively. The results demonstrate that the Cu2+ species are In order to affirm the surface species on the catalyst aer
mostly reduced to Cu0 or Cu+ since no 2p to 3d satellite peaks adsorption of MA and to understand the mechanism of MA
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Fig. 9 In situ DRIFTs of MA adsorption and desorption (D-time) of reduced Cu/SiO2–5La and LaOx/SiO2: (a) and (b) the spectra during MA
desorption process; (c) the final spectra of reduced LaOx/SiO2, Cu/SiO2, and Cu/SiO2–5La.
hydrogenation, the in situ DRIFTs of MA was performed with assigned to the C]O stretching vibration in gaseous MA, and
reduced LaOx/SiO2, Cu/SiO2, and Cu/SiO2–5La. As shown in the bands at 1375 cmꢁ1 and 1442 cmꢁ1 are linked to the
Fig. 9a and b, the bands at 1778 cmꢁ1 and 1760 cmꢁ1 are symmetrical and asymmetric C–H bending stretching
Scheme 1 Schematic of MA hydrogenation on the Cu/SiO2–La catalyst.
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vibrations of acyl species, respectively. This suggests that MA and Cu+ surface area. However, the relationship between Cu0
could be adsorbed on the surface of the reduced catalyst or surface area and STYEtOH is irregular (Fig. S4†). Some
LaOx/SiO2 catalyst and may be partially decomposed into researchers have found that more Cu+ content on the surface
methoxy and acyl species.47 The same trend was observed for the would improve the catalytic activity in hydrogenation esters,
intensities of the four bands decrease on prolonging the such as dimethyl oxalate and MA.21,24,31 They pointed out that
purging time. However, there is a discrepancy between the Cu+ species could polarize C]O bond in the esters since they
adsorption ability of the two catalysts. Compared with the stable act as electrophilic or Lewis acidic sites, which was the key
adsorption of the reduced Cu/SiO2–5La catalyst, the bands at factor and the rate-controlling step. Moreover, it is noteworthy
1375 cmꢁ1 and 1442 cmꢁ1 of the reduced LaOx/SiO2 catalyst that Cu/SiO2–5La has a large Cu+ surface area but the lowest
almost disappear aer being purged by Ar. This demonstrates surface area of Cu0 (Fig. 10b). Wang et al. had reported that the
that the adsorption of methoxy and acyl species on the reduced catalytic activity of hydrogenation of MA is linearly increased
LaOx/SiO2 is not stable. In addition, the stretching of n(C]O), with increasing Cu0 surface area when the available metallic Cu
shown as the bands at 1725 cmꢁ1 and 1560 cmꢁ1, is retained in surface was insufficient, otherwise it would be inuenced by the
the reduced Cu/SiO2–5La catalyst probably due to the strong Cu+ surface area.22 Combined with our results, we speculate that
chemical adsorption of MA.48
the Cu0 sites in our prepared catalysts are sufficient to adsorb
The nal spectra of the three catalysts are shown in Fig. 9c. and activate molecular H2; thus, an increase in Cu+ concentra-
Aer normalization, the results show that the addition of La tion leads to heightened catalytic activity.
could indeed increase the adsorption and dissociation of MA,
which is consistent with the XPS results. It could be inferred
that the dissociated species (methoxy and acyl species) are
mainly adsorbed on the active sites of copper, while the La
species themselves may not adsorb these organic species. The
adsorption and stabilization of these organics has been
3.8 Comparison of the catalytic activity with various
catalysts
As summarized in Table S3,† the MA conversion of Cu-based
catalysts prepared using the HP method was superior to the
proposed to be important in the hydrogenation of esters. In our
traditional AE method. The AE method used colloidal silica as
catalytic system, the addition of a certain La species can create
enough Cu0 active sites and more Cu+ sites, which facilitates the
the silica source and ammonia as the precipitant. In contrast,
the HP method used ammonium carbonate as the precipitation
enrichment of surface concentrations of active H-species and
agent, which could apparently decrease environmental pollu-
organics decomposed from MA. These promotional effects are
tion by reducing ammonia evaporation. Besides, HP method
presumably responsible for the improvement of catalytic
used TEOS as the silica source, which can hydrolyze to generate
performance and stability. Thus, combined with the above
silanol intermediate in the aqueous solution. This procedure
analysis, the possible path for the hydrogenation of MA over the
could promote the formation of copper phyllosilicate structure.
Thus, the interaction between copper species and support
Cu/SiO2–La catalyst is proposed, as shown in Scheme 1.
would be enhanced compared with the AE method. Zhao et al.
have found that the HP method showed higher dispersion and
3.7 Structure–activity relationship
The balanced effect between Cu0 and Cu+ species is widely larger surface areas of Cu0 and Cu+, which was the main reasons
accepted for copper-based catalysts in the hydrogenation of for the excellent performance in DMO hydrogenation.21 As seen
esters. Herein, the structure–performance relationship was from Table S3,† the Cu/SiO2 catalyst prepared by HP method
investigated by correlating the value of STY and Cu0 or Cu+ exhibited a better performance (X ¼ 86.8%) than that of the
species surface area, as shown in Fig. 10a and Fig. S4.† Our catalyst prepared by AE method (X ¼ 83.7%) even at a lower
results show that there is a direct correlation between STYEtOH reaction pressure.
Fig. 10 (a) Correlation of STYEtOH and Cu+ surface area; (b) effect of La content on STYEtOH
.
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illustrated in Fig. 11, the XRD analysis of spent Cu/SiO2 and Cu/
SiO2–5La catalysts was also carried out. Compared to the 5%
doped Cu/SiO2 catalyst, it can be observed that the intensity of
the diffraction peak ascribable to metallic Cu0 of spent Cu/SiO2
become sharper. Compared with the fresh samples, it could be
seen that the addition of La could restrain the aggregation of
copper. As can be demonstrated from Table S2,† the copper
crystallite size of spent Cu/SiO2 increased from 4.5 nm to
8.6 nm. From the TEM results of the spent catalysts (Fig. S9†), it
could be found that the grain size of the spent Cu/SiO2 catalyst
aer the stability test increased sharply from 4.6 nm to 9.9 nm,
which is much larger than that of the spent Cu/SiO2–5La cata-
lyst (5.4 nm). Besides, as exhibited in Fig. 12, the Cu+/(Cu+ +
Cu0) molar ratio of spent Cu/SiO2 declines greatly from 0.55 to
0.28 aer long-term evaluation but the 5% La-doped Cu/SiO2
catalyst remains relatively stable. Therefore, we speculate that
the activity of the Cu/SiO2 catalyst aer 250 h is greatly reduced
due to the concurrent effect of sintering and valence transition
of copper species. It can be seen from the above analysis that
introducing an appropriate amount of La can not only improve
the stability of the original Cu/SiO2 catalyst but also modulate
the distribution of copper species on the surface of the catalyst,
thereby improving the activity and stability under the reaction
conditions.
Fig. 11 XRD spectra of spent Cu/SiO2 after the long-term stability test.
In comparison to previous literature, the Cu/SiO2–5La cata-
lyst shows competitive activity at relatively low ratio of H2/MA
and low pressure. Therefore, the Cu/SiO2–5La catalyst prepared
with HP method is a promising new catalyst for the hydroge-
nation of MA to ethanol.
3.9 Deactivation analysis of the Cu/SiO2 catalyst
4. Conclusion
To investigate the deactivation of Cu/SiO2 catalyst during the
stability test, the carbon depositions of the spent Cu/SiO2 Three selected rare earth element (Ce, Y, and La) modied Cu/
catalyst were measured by TG analysis.49,50 As shown in Fig. S5,† SiO2 catalysts exhibited great improvement in the catalytic
the carbon deposition of the Cu/SiO2 catalyst is not the main activity for hydrogenation of methyl acetate to ethanol.
cause of deactivation.51,52 Moreover, the copper mass loading of Compared to the unmodied Cu/SiO2 catalyst (lower MA
the Cu/SiO2 and Cu/SiO2–5La catalysts aer stability testing was conversion of 90.4% and ethanol selectivity of 85.1%), the 5%
measured by ICP. The copper mass loading was slightly La-doped Cu/SiO2 catalyst showed the highest catalytic perfor-
decreased from 28.2% to 27.5% for Cu/SiO2 and 27% to 26.1% mance with the MA conversion of 95.4% and ethanol selectivity
for the Cu/SiO2–5La catalyst, respectively. This suggests that the of 95.8% under the same reaction conditions. There was
loss of copper species is also not the main factor causing evidence that strong interactions between copper and
deactivation.
lanthanum species can modulate the state of surface copper
In general, agglomeration of copper species is believed to be species. The higher ratio of Cu+/(Cu+ + Cu0) and the corre-
a major cause of deactivation of copper based catalysts.53,54 As sponding higher Cu+ surface area were responsible for the
Fig. 12 (a) Cu LMM Auger spectra of spent Cu/SiO2 and (b) Cu/SiO2–5La catalysts.
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higher value of STYEtOH. The Cu+ surface area was determined 18 A. Yin, X. Guo, K. Fan and W. Dai, ChemCatChem, 2010, 2,
by the content of lanthanum loading. Furthermore, the optimal 206–213.
Cu/SiO2–5La catalyst showed excellent long-term stability, 19 L. Chen, P. Guo, M. Qiao, Y. Run, H. Li, W. Shen, H. Xu and
which could maintain its initial activity for more than 280 h K. Fan, J. Catal., 2008, 257, 172–180.
without any deactivation. The stability may originate from the 20 Z. Wang, Z. Xu, S. Peng, M. Zhang, G. Lu, Q. Chen, Y. Chen
ability of resistance to sintering and transition of surface copper
species by introduction of the lanthanum species.
and G. Guo, ACS Catal., 2015, 5, 4255–4259.
21 Y. Zhao, S. Li, Y. Wang, B. Shan, J. Zhang, S. Wang and X. Ma,
Chem. Eng. J., 2017, 313, 759–768.
22 Y. Wang, Y. Shen, Y. Zhao, J. Lv, S. Wang and X. Ma, ACS
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Conflicts of interest
The authors declare no competing nancial interest.
23 S. Li, Y. Wang, J. Zhang, S. Wang, Y. Xu, Y. Zhao and X. Ma,
Ind. Eng. Chem. Res., 2015, 54, 1243–1250.
24 Y. Zhao, B. Shan, Y. Wang, J. Zhou, S. Wang and X. Ma, Ind.
Eng. Chem. Res., 2018, 57, 4526–4534.
Acknowledgements
We acknowledge Key Research Program of Frontier Sciences, 25 H. Liu, Z. Huang, H. Kang, X. Li, C. Xia, J. Chen and H. Liu,
CAS, (Grant No. QYZDB-SSW-SLH022); The National Natural Appl. Catal., B, 2018, 220, 251–263.
Science Funds (NSFC-NRCT, 51661145012); K. C. Wong 26 C. Ye, C. Guo, C. Sun and Y. Zhang, RSC Adv., 2016, 6,
Education Foundation (GJTD-258 2018-04).
113796–113802.
27 Y. Huang, H. Ariga, X. Zheng, X. Duan, S. Takakusagi,
K. Asakura and Y. Yuan, J. Catal., 2013, 307, 74–83.
28 S. Zhao, H. Yue, Y. Zhao, B. Wang, Y. Geng, J. Lv, S. Wang,
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29 Y. Liu, J. Ding, J. Sun, J. Zhang, J. Bi, K. Liu, F. Kong, H. Xiao,
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