Facile Fabrication of Ultrasmall Copper Species Confined in Mesoporous Silica for Chemo-Selective and Stable Hydrogenation Ethylene Carbonate Derived from CO2

Article abstract of DOI:10.1002/cctc.201800828

The hydrogenation of ethylene carbonate to co-produce commodity methanol and ethylene glycol has attracted growing interests due to the potential chemical utilization of CO₂ in large scale. In this work, we report a novel and facile protocol for the preparation of mesoporous Cu@SiO₂ catalysts and successfully applied the as-synthesized catalysts in the hydrogenation of ethylene carbonate. The catalysts were characterized in detail by means of N₂ physisorption, N₂O titration, XRD, FT-IR, H₂-TPR, TEM and XPS (XAES). This strategy is an effective method for fabricating the unique flower-like mesoporous Cu@SiO₂ with sub-2.0 nm ultrasmall Cu particles. The results revealed that the involvement of β-cyclodextrin improved the Cu dispersion and facilitated exposing more copper active sites, which also indicated that the confined catalyst inhibiting the sintering of copper particles. Meanwhile, the stability of the attained catalyst was superior with the modification of carbon. Importantly, among the catalysts tested in the hydrogenation of ethylene carbonate, 25Cu@SiO₂-β-P with appropriate copper loading as well as moderate Cu⁺ ratio exhibited superior catalytic performance. Accordingly, the synergistic effect between the Cu⁺ and metallic Cu⁰ species was crucial for obtaining better catalytic activity.

Full text of DOI:10.1002/cctc.201800828

Accepted Article
Title: Facile fabrication of ultrasmall copper species confined in
mesoporous silica for chemo-selective and stable hydrogenation
ethylene carbonate derived from CO2
Authors: Liguo Wang
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10.1002/cctc.201800828
ChemCatChem
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Facile fabrication of ultrasmall copper species confined in
mesoporous silica for chemo-selective and stable hydrogenation
ethylene carbonate derived from CO2
Chanjuan Zhang,[a,b,c,d] , Liguo Wang*[a,c,d], Jiaju Liu,[a] Yanmi Yang,[a] Peng He,[a] Yan Cao,[a] Jiaqiang
Chen,[a] Huiquan Li*[a,c,d]
Abstract: The hydrogenation of ethylene carbonate to co-produce
commodity methanol and ethylene glycol has attracted growing
interests due to the potential chemical utilization of CO2 in large scale.
In this work, we report a novel and facile protocol for the preparation
the mesoporous Cu@SiO2 catalysts and successfully applied the as-
synthesized catalysts in the hydrogenation of ethylene carbonate. The
catalysts were characterized by means of N2 physisorption, N2O
titration, XRD, FT-IR, H2-TPR, TEM and XPS (XAES) in details. In
particular, this strategy was demonstrated an effective method in
fabricating the unique flower-like mesoporous Cu@SiO2 with sub-2.0
nm ultrasmall Cu particles. The results revealed that the involvement
of β-cyclodextrin improved the Cu dispersion and facilitated exposing
more copper active sites, which also indicated that the confined
catalyst inhibiting the sintering of copper particles. Meanwhile, the
stability of the attained catalyst was superior with the modification of
carbon. Importantly, among the catalysts tested in the hydrogenation
of ethylene carbonate, 25Cu@SiO2-β-P with appropriate copper
loading as well as moderate Cu+ ratio exhibited superior catalytic
performance. Accordingly, the synergistic effect between the Cu+ and
metallic Cu0 species was considered to be the crucial for obtaining
better catalytic activity.
recent years, the chemical utilization of carbon dioxide as an
important feedstock for the syntheses of fuels, materials and
chemicals has drawn great attentions.[1-2] In this regard, the
strategy for converting CO2 into value-added oxygenates are
being paid intensive study.[3-6] Recently, the catalytic
hydrogenation of CO2 derivations, e.g. carbonates, carbamates
and urea, provides an alternative approach for the efficient
transformation of CO2 indirectly.[7-9] Particularly, methanol is the
important raw material for the production of oxygen-containing
chemicals and fuels, e.g. formaldehyde, acetic acid, olefins, which
is also expected the excellent hydrogen carrier.[10-14] Nevertheless,
the direct utilization of CO2 to methanol requires harsh reaction
conditions due to its kinetic inertness and the high activation
energy for cleavage of C=O bonds in CO2 molecular. Meanwhile,
the direct process usually suffers from low product yield owing to
the thermodynamic limitations. In order to enhance the utilization
efficiency, indirect route has provided alternative pathway and
demonstrated promising application.
In recent years, the hydrogenation reaction of ethylene carbonate
(EC) to co-produce important commodities of methanol and
ethylene glycol (EG) has attracted special concerns due to the
merits of environmentally friendliness, atomic economy and low
energy consumption.[15-17] Therefore, many researchers have
devoted great efforts for developing effective catalysts. The
homogeneous princer-type RuII complex catalyst with high
efficiency was applied for the hydrogenation of EC.[18-19] On
account of the convenient catalyst recovery, much work was
devoted to develop effective and easily recyclable heterogeneous
catalysts for the chemoselective hydrogenation of EC. Because
the unique property demonstrated in the selective hydrogenation
of carbonyl bonds, copper-based catalysts were studied
extensively in the catalytic hydrogenation of ester for the
synthesis of alcohols, thus ensuring the good selectivity of target
products.[20-24] As reported in previous literatures for the
Introduction
Huge emission of carbon dioxide into the atmosphere by human
activity has imposed significant influence on global warming. In
[a]
C. Zhang, Dr. L. Wang, J. Liu, Y. Yang, Dr. P. He, Dr. Y. Cao, J.
Chen, Prof. Dr. H. Li,
CAS Key Laboratory of Green Process and Engineering, Institute of
Process Engineering,
Chinese Academy of Sciences
1 North 2nd Street, Zhongguancun, Haidian District, Beijing, PR
China
E-mail: lgwang@ipe.ac.cn (L. Wang), hqli@ipe.ac.cn (H. Li)
C. Zhang
Chemical and Biochemical Engineering
Technical University of Denmark
Building 229, 2800 Kgs. Lyngby, Denmark
analogous reaction, Cu/SiO2 catalysts as
a
versatile
hydrogenation catalyst showed an excellent application in the
hydrogenation of dimethyl oxalate to EG or ethanol.[25-26] In
particular, heterogeneous copper-based catalysts, e.g.,
CuCr2O4,[27] Cu-SiO2-PG,[28] Cu/HMS,[29] Cu/SBA-15[20] and
Cu/SiO2-AE[16] were investigated for EC hydrogenation to co-
produce methanol and EG, and displayed relatively good catalytic
performance. Among these catalysts tested, a maximum turnover
frequency (TOF) of 776 mgEC·g cat-1·h -1 was obtained over
Cu/SBA-15. However, more effective, low-cost heterogeneous
catalysts with high efficiency are highly desired. As also proposed,
[b]
[c]
[d]
C. Zhang, Dr. L. Wang, Prof. Dr. H. Li
Sino-Danish College
University of Chinese Academy of Sciences
No.19(A) Yuquan Road, Shijingshan District, Beijing, P.R.China
C. Zhang, Dr. L. Wang, Prof. Dr. H. Li
Sino-Danish center for Education and Research
University of Chinese Academy of Sciences
[25-28]
the size of the copper particals are considered to be the
crucial factors in hydrogenation reaction. Therefore, the protocol
for the preparation of the copper-based catalysts with finely tuned
particles is of great significance.[20,26] Recently, Yao et al reported
No.19(A) Yuquan Road, Shijingshan District, Beijing, P.R.China
Supporting information for this article is given via a link at the end of the
document.((Please delete this text if not appropriate))
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10.1002/cctc.201800828
ChemCatChem
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a robust copper-based catalysts with the introduction of low-cost
dextrin and employed it in the system of dimethyl oxalate
hydrogenation sucessfully.[30] The catalytic activity and stability of
the as-prepared catalysts were enhanced greatly with the
formation of dextrin-coating. Meanwhile, the deactivation reason
responsible for the stability was also highly concerned for giving
insights into stabilizing mechanism. [16, 20, 28]
In this work, we provide a facile method shown in Scheme 1 of
one-step hydrolysis precipitation with the aid of β-cyclodextrin and
templating agent of P123 to prepare uniformly dispersed copper-
based catalysts for the first time and apply the as-prepared
catalysts in hydrogenation of ethylene carbonate derived from
CO2. Furthermore, the structures and the physicochemical
properties of catalysts were systematically characterized by
different techniques, e.g., N2 physisorption, ICP-AES, N2O
titration, FT-IR, XRD, H2-TPR, TEM, XPS and XAES. Moreover,
the catalytic performance, the effects of reaction parameters and
the reusability of catalyst were also investigated. In addition, the
correlation between the catalytic performance and the unique
roles of Cu0 and Cu+ sites was also studied to elucidate the effect
of different copper species on the catalytic performance.
(x≤25%) and 25Cu@SiO2-P catalysts distributed between 10-30
nm and the peak of pore distribution shifted right with copper
loadings (x<25%), while the amounts of micropores in xCu@SiO2-
β-P(x≥ 25%) showed a dramatic decrease after the nominal
copper-loading over 25%, which was also observed in the
literature.[32] The declined pore sizes of xCu@SiO2-β-P catalysts
(x>35%) implied that the micropores were blocked by the larger
copper nanoparticles or the formation process of micropores was
influenced by the excess copper loading, which was consistent
with the N2 physisorption isothermal curves of catalysts.The
relatively larger surface area of 105.6 m2/g was obtained for
25Cu@SiO2-β-P with appropriate copper loading. As expected,
the enlarged surface area for 25Cu@SiO2-P revealed that P123
as templating agent facilitates obtaining a larger surface area. In
addition, the distribution of average pore sizes with the copper
loading showed unregular tendency. Furthermore, compared with
the xCu@SiO2-β-P catalysts with different copper loadings and
25Cu@SiO2-P catalyst, the larger pore size of 0.43 cm3/g was
obtained over 25Cu@SiO2-β-P due to the combination of β-
cyclodextrin and P123. ICP-AES results demonstrated that the
actual copper contents of catalysts were close to the nominal
value, indicating the copper is almost completely deposited on the
catalyst. There was another interesting phenomenon that the
actual copper contents increased sharply while the nominal value
over 25%, the copper content difference between actual and
nominal contents indicates that the excess copper inhibits the
formation of the spherical structure. Copper dispersion of
xCu@SiO2-β-P and 25Cu@SiO2-P catalysts were also
determined by N2O titration, which were summarized in Table 1.
Obviously, the Cu dispersion of 15Cu@SiO2-β-P is 26.8% which
decreased with the increment of copper loading, and it can be
speculated that the excess copper content influences the
formation of catalyst which is consistent with the assumption from
ICP-CAS results. On the contrast, compared to that of
25Cu@SiO2-β-P catalyst (16.1%), the copper dispersion of
25Cu@SiO2-P without the introduction of β-cyclodextrin
decreased to 10.0%. The atomic ratios of Cu/Si on the surface of
catalysts were also measured by XPS and summarized in Tab. 1.
The copper ratio initially increased with the copper loading of
xCu@SiO2-β-P catalysts (x≤ 25%) and subsequently decreased
to 0.033 of 35Cu@SiO2-β-P from the highest value 0.047 of
25Cu@SiO2-β-P. Furthermore, the introduction of β-cyclodextrin
promoted the copper ratio on the surface of catalysts. The
tendency of the atomic ratios Cu/Si matched well with that of BET
surface area. Combining with the actual copper loading from ICP-
AES and the copper dispersity from N2O titration, copper
dispersity decreased with copper loadings, while the atomic ratio
of Cu/Si on the surface of catalysts increased with copper loading
(x≤25%), indicating the catalysts with moderate copper contents
were beneficial for exposing more copper active sites. Meanwhile,
the relative lower copper contents led to a higher dispersity.
Moreover, both the results of copper dispersity and the atomic
ratio of Cu/Si for 25Cu@SiO2-β-P and 25Cu@SiO2-P catalysts
suggested that β-cyclodextrin palyed a significant role in
promoting the copper dispersity.
Scheme 1. Schematic illustration for the fabrication of carbon promoted
catalyst xCu@SiO2-β-P
Results and Discussion
Catalyst characterization
N2 physisorption, ICP-AES and N2O titration
N2 physisorption isotherms and the pore size distribution curves
of xCu@SiO2-β-P and 25Cu@SiO2-P catalysts were depicted in
Fig.1, and the physico-chemical properties of these samples were
summarized in Table 1. According to the N2 physisorption
isotherms with the IUPAC classification, type IV with H1-type
hysteresis loops was observed for all these samples, suggesting
the mesoporous structures were successfully fabricated.[31] For
xCu@SiO2-β-P catalysts, the surface area of xCu@SiO2-β-P
catalysts increased firstly when Cu loadings lower than 25%,
while it declined when Cu loadings further elevated to 35%.
Moreover, the pore shape of the xCu@SiO2-β-P catalysts
changed from “spherical” to “slit-like” with increasing the copper
loading.[16] As shown in Fig. 1b, the pore sizes of xCu@SiO2-β-P
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10.1002/cctc.201800828
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TGA analysis of beta-cyclodextrin was carried out to confirm the
existence of residual carbon from beta-cyclodextrin, which was
shown in Fig. S1. Obviously, there was two main weight losses
from ambient temperature to 400 oC in pure nitrogen atmosphere.
The first loss of 13.4 % could be ascribed to the removal of
(a)
o
absorbed water when the temperature was lower than 120 C.
o
Another drastic weight loss started at around 300 C and ended
at around 400 oC with obvious heat adsorption of 320 oC, and the
majority weight loss of 74.8% indicated the thermal
decomposition of beta-cyclodextrin which was consitent with the
previous report,[33] indicating beta-cyclodextrin was primarily
eliminated before 450oC. Therefore, we deduced that the residual
carbon formed in the as-prepared catalysts after the thermal
decomposition in pure nitrogen atmosphere at 450 oC.
Accordingly, it can be speculated that beta-cylcodextrin was
elimitinated primarily after the calcination process at 450 oC for 4h
and the residual carbon was introduced in the as-prepared
catalysts sucessfully. This phenominon was also observed in the
reference.[30]
(b)
FT-IR
The FT-IR spectra of the dried precursors and the reduced
catalysts of xCu@SiO2-β-P and 25Cu@SiO2-P were illustrated in
Fig.2. For the dried precursors, two obvious peaks of 1080 cm-1
and 1250 cm-1 were observed in the all dried precursors of
xCu@SiO2-β-P, which can be attributed to the vibration modes of
Si-O bands. Besides, both the symmetric stretching vibration
peak of 800 cm-1 and the bending vibration bands of 457 cm-1
reflected the existence of Si-O-Si bands. The bands at 620 cm-1
and 580 cm-1 ascribed to the stretching vibration of Cu-O bond in
monoclinic CuO. The obvious presence of 1385 cm-1 and 1502
cm-1 when the copper loading above 15% can be assigned to the
NO3 and CO3 groups from Cu2(OH)3NO3 and Cu2(OH)2CO3,[34]
respectively, implying the formation of copper complexes. The
presence of 1502 cm-1 peak in dried precursor was observed,
suggesting that the existence of mode ν3 of interlayer carbonate
species.[35] The peak around 1027 cm-1 can be assigned to the
formation of copper phyllosilicate, which was considered to be
originated from the strong interaction between copper species
and silica support.[16,36] It can be concluded that the dried
Figure 1. (a) N2 physisorption isotherms (b) pore size distributions of
xCu@SiO2-β-P catalysts, x=5, 15, 25, 35 and 45 and 25Cu@SiO2-P.
Table 1. Physico-chemical properties of xCu@SiO2-β-P and 25Cu@SiO2-P
catalysts.
[b]
[c]
[b]
Cu
loading[a]
(%)
SBET
DCu
Vpore
Cu/Si[d]
(mol
ratio)
[b]
Catalyst
dpore
(m2g-
(%)
(cm3g-
(nm)
1
1
)
)
5Cu@SiO2-
β-P
15Cu@SiO2-
β-P
25Cu@SiO2-
β-P
35Cu@SiO2-
β-P
45Cu@SiO2-
β-P
25Cu@SiO2-
P
4.7
54.7
79.2
48.0
26.8
16.1
14.8
17.5
30.2
28.8
43.8
23.1
22.4
0.17
0.34
0.43
0.24
0.18
0.36
0.025
0.045
0.047
0.033
-
precursors
xCu@SiO2-β-P
contained
Cu2(OH)3NO3,
Cu2(OH)2CO3 and copper phyllosilicate, while that of
25Cu@SiO2-P catalyst contained copper phyllosilicate as well as
Cu2(OH)3NO3. The peaks of 1502 cm-1 and 1385 cm-1
disappeared for the reduced catalysts, as shown in Fig. 2b,
indicating that the NO3 and CO3 groups were destroyed in the
subsequent calcination and reduction processes, while there still
was an obvious peak on the curve of 45Cu@SiO2-β-P which
implied that the excess copper loading disturbed the completed
reduction process. The diverse copper species in the precursor
may impose a great effect on the variation of copper species in
the reduced catalyst.
14.9
25.9
40.9
58.8
25.5
105.6
57.6
31.7
-
10.0
128.6
0.032
[a] Measured by ICP-AES. [b] Obtained by N2 physisorption. [c] Determined
by N2O titration. [d] Attained by XPS
TGA
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10.1002/cctc.201800828
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increasing the copper loading, indicating relatively larger particle
size were formed with the increment of copper content. Besides,
there was no detectable diffraction peak of CuO, suggesting that
all of the Cu2+ was primarily reduced to copper species with low
valences, i.e. Cu0 or Cu+ after reduction. [27,38-39] Moreover, the
increased intensity of diffraction peaks with increasing copper
loading suggested the size of copper particles also increased with
copper loadings. The evolution of the copper species during the
preparation process is in agreement with the results of FT-IR.
These results proved the Cu2O and metallic Cu species were well
dispersed on silica support, especially for the samples with lower
copper loadings, which is in agreement of N2O titration result.
(a)
(a)
(b)
(b)
Figure 2. FT-IR of (a) the dried precursors, (b) reduced catalysts of
xCu@SiO2-β-P and 25Cu@SiO2-P.
XRD
Fig. 3 showed the XRD patterns of the dried percursors and the
reduced catalysts of xCu@SiO2-β-P and 25Cu@SiO2-P. As
shown in Fig. 3a, only one characteristic diffraction peak at 2θ of
22.80 assignable to amorphous SiO2 was observed for
5Cu@SiO2-β-P. For the catalysts with copper loadings in the
region of 15% and 35%, there are distinguished diffraction peaks
at 2θ of 14.70, 17.60, 24.20, 17.60, 31.30 and 36.60 assigned to
cupric subcarbonate Cu2(OH)2CO3 could be observed. It can be
clearly found that the corresponding peak intensities increased
with the copper loadings, indicating the enhanced crystallinity.
Furthermore, the new crystal phase appeared when copper
loading greater than 25%, and the diffraction peaks at 2θ of 13.00-
63.00 ascribable to the monoclinic Cu2(OH)3NO3.[37]
For the reduced catalyst of xCu@SiO2-β-P and 25Cu@SiO2-P, as
presented in Fig. 3b, the weak diffraction peaks of Cu2(OH)3NO3
and Cu2(OH)2CO3 existed in the dried precursors disappeared
completely after reduction. Specifically, for the xCu@SiO2-β-P
samples with x above 5%, the presence of diffraction peaks at 2θ
of 36.50, 43.40 and 61.60 could be indexed to the (111), (200) and
(220) crystal planes of Cu2O, respectively. Meanwhile, the evident
diffraction peaks at 2θ of 50.60 and 74.00 could be assigned to the
(220) and (220) crystal planes of metallic fcc Cu, respectively. The
diffraction peaks for Cu2O and Cu0 become more intensive when
Figure 3. XRD of (a) dried precursors, (b) reduced catalysts of xCu@SiO2-β-
P.
H2-TPR
Fig. 4 showed the H2-TPR profile of the xCu@SiO2-β-P and
25Cu@SiO2-P catalysts. All of the catalysts exhibited
a
symmetrical reduction peak. The onset of the reduction peaks of
these catalysts centered at about 529 K-546 K could be primarily
ascribable to the collective reduction of copper phyllosilicate and
well dispersed CuO particles.[20,40] The TPR reduction peak
gradually shifted to higher temperature regions with the augment
of Cu content, presumably ascribed to the larger CuOx particle
size with deteriorated dispersion or strongly interactions between
copper species and silica support at higher copper loading.
Moreover, compared to 25Cu@SiO2-P, the reduction peak of
25Cu@SiO2-β-P with the same copper loading shifted towards
the lower temperature regions, suggesting that the introduction of
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β-cyclodextrin is beneficial for enhancing the reducibility of
oxidized copper species. It is generally accepted that the entire
reduction of Cu+ species to metallic Cu0 at the premise of high
reduction temperature above 873 K.[16] In this case, after
reduction, copper phyllosilicate or Cu-O-Si was also transformed
into Cu+, while CuO was reduced to metallic Cu0 species,[16,20,39]
which is consistent with the findings from FT-IR and XRD.
35Cu@SiO2-β-P catalyst. Although the mean particle sizes lower
than 2 nm, the large particles with diameter larger than 2 nm were
also observed and collected in the distribution of copper particles
by TEM. Meanwhile, the existence of minority particles with larger
sizes could not be completely excluded, which possibly also
contributed to the peaks of copper species in XRD spectra. For
45Cu@SiO2-β-P, the mean copper particle size was 9.6 nm,
which can be obviously detected by XRD. Therefore, it can be
speculated that the copper particle sizes of xCu@SiO2-β-P
increased with copper-loadings. Moreover, the peak intensities of
Cu and Cu2O increased with the copper loadings, implying the
increase of copper particles sizes in xCu@SiO2-β-P catalysts,
which was consistent with the results obtained from TEM. Notably,
the statistical results revealed that average particle sizes mainly
distributed in the regions of sub-2 nm if the copper loading less
than 35%, indicating the preparation method used in this study
has provided an effective protocol for controllable syntheses of
the catalysts with ultrasmall Cu nanoparticles in uniform
distribution. In combination of the results demonstrated in N2
physisorption, N2O titration and H2 TPR, the introduction of β-
cyclodextrin is favorable for the copper dispersion and improved
the uniformity of particle size distribution.
(a)
(d)
(g)
(b)
(e)
(h)
(c)
(f)
(i)
Figure 4. H2-TPR of the catalysts of xCu@SiO2-β-P, x=5, 15, 25, 35 and
25Cu@SiO2-P.
TEM
The catalyst morphology and particle size distributions of
xCu@SiO2-β-P and 25Cu@SiO2-P were shown in Fig. 5 and Fig
S2. The morphology was controlled to be nanoflower-like
structure. The distinct mesoporous channel structure can be
observed in Fig 5a-b and Fig S2, which is exclusively favorable
for improving copper dispersion and promoting the catalytic
performance in EC hydrogenation. The histograms of particle size
distributions for the different copper loading were also obtained
by measuring particles from TEM images. As a result, the average
particle diameters of xCu@SiO2-β-P catalysts increased with the
copper loading, and the average particle diameters were 0.9 nm,
1.0 nm, 1.6 nm, 1.8 nm and 9.6nm with x= 5, 15, 25, 35 and 45,
respectively. The particle size increased from 1.8 nm to 9.6 nm
rapidly, implying the excess copper loading impacted the
generation of the particles and lead to the aggregation of particles.
In combination with the results from XRD shown in Fig. 3b, there
was no detectable peak of metallic Cu and Cu2O in 5Cu@ SiO2-
β-P with 5% copper content, possibly because that all of the
copper particles sizes were lower than 2 nm. Actually, as also
reported by Tomishige et al,[37] the peaks of copper crystallites
were difficult to be detected by XRD when the copper particles
were lower than 2 nm. Whereas, the peaks related to Cu0 and
Cu2O appeared when copper loading was 15% and the peak
intensities further enhanced with the increase of copper loadings.
As shown in Fig. 3b and Fig. 5, the copper particle sizes increased
with copper loadings and there was a dramatic increase when
copper loadings over 35%. The appearance of peaks ascribed to
Cu2O were observed in 15-35Cu@SiO2-β-P catalysts, indicating
that the larger copper particle sizes (d≥2nm) presented in 15-
Figure 5. TEM images for the catalyst morphology: (a) 15Cu@SiO2-β-P, (b)
25Cu@SiO2-β-P, the particle size distribution of (c) the dry precursor of
25Cu@ SiO2-β-P, the reduced xCu@SiO2-β-P catalysts: (d) x=5, (e) x=15, (f)
x=25, (g) x=35 (h) x=45, (i) 25Cu@SiO2-P.
Element mapping by energy-dispersive X-ray (EDX)
EDX spectroscopy showed that elemental copper and carbon
were uniformly distributed on mesoporous silica, as presented in
Fig 6. Furthermore, the co-existence copper and carbon
illustrated these two species were synchronously distributed, in
which the residual carbon was deemed to be originated from β-
cyclodextrin during the carbonization process in inert atmosphere.
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In combination of TEM images with the result from EDX mapping,
it can be concluded that the one-step method by hydrolysis
precipitation is an excellent protocol for preparing the catalyst with
high-dispersed Cu species and petal morphology, which was
considered to facilitate inhibiting the agglomeration of copper
species and stabilize the copper species in some extent. From the
results aforementioned, It can be deduced that the Cu species are
also decorated by residual carbon and confined in mesoporous
silica channels.
resulting in a balanced copper species distribution. The results
from XPS and XAES demonstrated that Cu0 and Cu+ co-existed
on the surfaces of these copper-based catalysts with varied
proportions.
(a)
(a)
(b)
(c)
(b)
(d)
(e)
Figure 6. EDX mapping of the reduced catalyst 25Cu@SiO2-β-P: (a) Overall
state, (b) C-mapping, (c) Cu-mapping, (d) O-mapping, (e) Si-mapping.
XPS
The surface properties of chemical states and compositions of the
reduced xCu@SiO2-β-P and 25Cu@SiO2-P catalysts were
achieved by XPS and XAES illustrated in Fig.7, and the relevant
quantitative data were collected in Table 2. The slim binding
energy range of Cu 2p3/2 without obvious satellite peaks of Cu2+
was determined from 933.1 eV to 933.9 eV except the binding
energy of 45Cu@SiO2-β-P which indicated the uncompleted
reduction of higher copper loading catalyst and was consistent
with the result of FT-IR and TEM. The variation of the Cu 2p3/2
binding energy values of xCu@SiO2-β-P suggested the different
interactions between the copper species and the supports. Owing
to the limitation of XPS measurement, XAES was performed to
distinguish Cu0 and Cu+ species. The asymmetric and broad
peaks of xCu@SiO2-β-P catalysts were deconvoluted into two
overlapping Cu LMM Auger kinetic energy peaks at ca. 912 eV
and 915 eV, which represented Cu+ and Cu0, respectively.[41-42]
According to the previous reports,[16,39] Cu+ could be ascribed to
the reduction of copper phyllosilicate, and Cu0 was derived from
the reduction of CuO. As summarized in Table 2, the proportion
of Cu+/(Cu0+Cu+) increased with the copper loading less than
25%, while the proportion of Cu+ decreased from 0.54 to 0.43 at
copper loading of 35%. Among the catalysts tested, the maximum
Cu+ proportion of 0.54 was attained over 25Cu@SiO2-β-P with the
medium copper loading of 25%. Compared to 25Cu@SiO2-P,
25Cu@SiO2-β-P with the introduction β-cyclodextrin greatly
influenced the ratio of Cu+ to the total amount of (Cu0+Cu+),
Figure 7. (a) Cu 2p XPS and (b) Cu LMM XAES spectra of the reduced
xCu@SiO2-β-P catalysts.
Table 2. Assignments of copper species in copper-based catalysts
attained from Cu LMM XAES spectra.
Binding Energy
(eV)
Kinetic Energy
(eV)
Cu+/(Cu++Cu0)
(mol ratio)
Catalyst
Cu
Si 2p
2p3/2
Cu+
Cu0
5Cu@SiO2-β-
933.7
932.6
933.7
933.1
933.1
933.9
103.3
103.1
103.3
103.1
103.2
103.2
912.2
911.8
912.1
912.0
912.1
912.1
914.8
914.8
915.3
915.0
914.9
915.1
0.29
0.29
0.54
0.43
0.44
0.36
P
15Cu@SiO2-
β-P
25Cu@SiO2-
β-P
35Cu@SiO2-
β-P
45Cu@SiO2-
β-P
25Cu@SiO2-
P
Catalyst performance
The catalytic performance tests of xCu@SiO2-β-P and
25Cu@SiO2-P catalysts for the hydrogenation of ethylene
carbonate was carried out under 453 K for 4 h, and the results
were summarized in Table 3. Merely 9.4% EC conversion with
95.2% EG and 11.0% methanol selectivities was obtained over
5Cu@SiO2-β-P, entry 1. The catalytic activity increased gradually
with further increasing the copper loadings to maximum 45% at
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10.1002/cctc.201800828
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the identical interval of 10%, entries 2-5. Among them, the
relatively moderate catalytic activity, i.e. 41.7% EC conversion
with 98.1% EG and 50.2% methanol selectivities could be
obtained over 25Cu@SiO2-β-P, entry 3. These results suggest
that surface areas, pore volumes of these catalysts play a weak
role on the hydrogenation efficiency. One possible reason can be
deduced that the unique mesoporous catalysts with flower-like
nanostructures facilitated the mass transfer of the reactants,
thereby leading to overcome the limit of mass transfer diffusion in
the catalyst channel during reaction. As a result, the intrinsic
catalytic activity was considered to play a dominant role in this
case. It should be noted that the gas composition after reaction is
qualitatively analyzed by Mass Spectrometer LC-D200M, and the
result of mass spectrometry test is shown in the Fig S2. Besides
H2, a small amount of water, carbon monoxide, methanol, and
carbon dioxide were also observed. When the copper loading was
finally increased to utmost value of 45%, entry 5, 67.7% EC
conversion with 97.7% EG and 52.5% methanol selectivities
could be obtained over 45Cu@SiO2-β-P. Compared to
25Cu@SiO2-β-P, the counterpart of 25Cu@SiO2-P without
introduction of β-cyclodextrin exhibited lower catalytic activity, i.e.
30.6% EC conversion with 98.2% EG and 33.3% much lower
methanol selectivity, entry 6. These results indicated that the
involvement of β-cyclodextrin could effectively promoted the
catalytic activity, simultaneously improved the methanol
selectivity. According to the previous reports, [16,20] it also can be
deduced that Cu+ could stabilize the methoxy and acyl species of
the reaction intermediate, and Cu+ could act as Lewis acidic sites
to polarize the carbonyl group of EC via the interaction with the
electron lone pair in oxygen, thus improving the reactivity of the
cyclic ester group.
The introduction of appropriate amount of copper and additive is
deemed to be crucial for obtaining higher catalyst stability. In
order to evaluate the possible stability of catalyst prior to choosing
the appropriate catalyst, the recycling experiments for selective
hydrogenation of EC were preliminarily tested over 25Cu@SiO2-
β-P, 25Cu@SiO2-P and 35Cu@SiO2-β-P, and the results were
shown in Table 3. Importantly, EC conversion increased to 47.4%
during 2nd run over 25Cu@SiO2-β-P, and methanol selectivity
increased to 58.1% with the similar EG selectivity, entry 7. It can
be speculated that induction time at the initial stage probably
existed, thus active sites of 25Cu@SiO2-β-P could be enhanced
after the first run. In comparison with 25Cu@SiO2-P catalyst,
entry 8, β-cyclodextrin modified 25Cu@SiO2-β-P exhibited higher
catalytic activity during the recycling experiments, implying that
the involvement of β-cyclodextrin promoted the catalyst stability.
For 35Cu@SiO2-β-P with relatively higher copper loading, the
selectivities of methanol and ethylene glycol obviously decreased
to 51.4% and 95.3% in the second run, respectively. According to
the viewpoints by Ma et al, [39] the catalyst powder is difficult to be
shaped when the copper loading exceeded over 25% in Cu@SiO2,
which is necessary for meeting the requirement of continuous
reaction mode at industrial scale. Therefore, the preliminary
reusability tests showed that 25Cu@SiO2-β-P catalyst with
superior comprehensive catalytic performance was the choice for
further evaluation.
Table 3. Catalytic performance of various copper-based catalysts in the
hydrogenation of EC
EC
Yield (%)
Sel. (%)
MeOH
Entry
Catalysts
Conv.
(%)
MeOH
EG
10.8
34.1
EG
95.2
96.0
1
2
5Cu@SiO2-β-P
9.4
1.2
11.0
50.2
15Cu@SiO2-β-
36.1
18.3
P
25Cu@SiO2-β-
3
4
41.7
53.3
21.0
31.9
39.9
51.8
50.2
60.5
98.1
99.9
P
35Cu@SiO2-β-
P
45Cu@SiO2-β-
P
5
6
7
67.7
30.6
47.4
35.5
10.2
27.1
66.1
29.0
47.6
52.5
33.3
58.1
97.7
98.2
99.5
25Cu@SiO2-P
25Cu@SiO2-β-
P[a]
25Cu@SiO2-
P[a]
8
9
30.6
50.9
15.6
26.4
33.8
48.0
30.9
51.4
95.0
95.3
35Cu@SiO2-β-
P[a]
Reaction condition: 10 mmol EC, 5 MPa H2, 20 mL THF, 20wt% catalyst
dosage (based on the weight of EC),453 K and 4 h.
[a] reused for the second run.
The effect of reaction temperature on the catalytic activity was
carried out in the range of 443-483 K at 4 h, as shown in Fig. 8a.
It can be seen that the EC conversion increased monotonically
with the increasing reaction temperature. The EC conversion was
as low as 34.1 % when the temperature was 443 K. In contrast,
methanol selectivity gradually increased accompanied with
elevating reaction temperature. The relatively higher EC
conversion of 97.4 % was obtained when the temperature was
473 K. The EC conversion further increased to when the
temperature increased to 483 K, corresponding to excellent TOF
of 1195 mgEC•gcat-1•h-1. On the contrast, EG selectivity decreased
to 95.5 %. This result indicated that the reaction temperature has
significant influence on the catalyst behavior, and relatively higher
reaction temperature was more favorable for the hydrogenation
reaction. Nevertheless, too high temperature is unfavorable due
to the risk of deteriorated EG selectivity. Comprehensively
speaking, 473 K was chosen as the preferential reaction
temperature. The influence of reaction time on the catalytic
performance was also investigated, as shown in Fig 8b., 51.5 %
higher EC conversion with 96.4 % EG selectivity could be
obtained. The EC conversion further increased to 75.9 % at 5 h.
The EC conversion increased to 92.2 % when the reaction time
further prolonged to 6 h in subsequent investigation. Therefore,
25Cu@SiO2-β-P exhibited excellent catalytic activity.
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10.1002/cctc.201800828
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(a)
(b)
Figure 9. The correlation of between the surface Cu0 site density and the
formation rate of MeOH.
The mechanism of the hydrogenation of ethylene carbonate
Based on the analysis of the characterization of catalysts and the
catalytic performance, the mechanism of EC hydrogenation to
methanol and ethylene glycol over xCu@SiO2-β-P catalysts was
proposed, as shown in Scheme 2. Accordingly, it could be
speculated that Cu0 species was responsible for the dissociation
[43]
of H2, while Cu+ species polarized the C=O groups in EC.
Moreover, the reaction processes of EC hydrogenation could be
[18]
divided to three main steps.
Ethylene carbonate was firstly
Figure 8. The effect of reaction temperature at 4 h and reaction time at 473 K
on the hydrogenation of EC based on the dosage of 10mmol EC, 20 mL THF
and 0.176 g 25Cu@SiO2-β-P.
reduced to 2-hydroxyethylformate and 1,3-dioxolan-2-ol.
Subsequently, 2-hydroxyethylformate was reduced to ethylene
glycol and formaldehyde. The final reduction step was the
formation of methanol via the hydrogenation of formaldehyde
intermediate. The existed two kinds of copper species on the
catalyst surface could facilitate the hydrogenation of EC to co-
produce methanol and ethylene glycol.
The correlation of catalytic activity and structure
In order to give insights into the intrinsic active sites in the
hydrogenation of ethylene carbonate, the structure-activity
relationship was primarily studied. The results showed that the
catalytic activity, which was expressed in terms of methanol
formation rate, increased proportionly with the increment of Cu0
active site density and attained the peak value, as shown in Fig.9.
However, the methanol formation rate decreased with further
increase in Cu0 active site density, indicating the cooperative
effect of Cu0 and Cu+, and it could be speculated that Cu0 played
different roles in the different regions with valcano-like trend.
Therefore, the correlation between the methanol formation rate
and surface area of Cu0 probably indicated that the H2
dissociation is the rate-determining step during the reaction when
Cu0 active site density was lower than the peak value. In contrast,
Cu+ possibly played an important role in the catalytic performance
for the EC hydrogenation when Cu0 active site desity exceeded,
which is responsible for the polarization of C=O groups in EC.
Scheme 2. Schematic illustration of the reaction mechanism over xCu@SiO2-
β-P catalysts for the hydrogenation of ethylene carbonate.
Reusability of catalysts
The stability of the copper-based catalyst is very important for the
practical application in industrial scale. Unfortunately, copper-
based catalyst is prone to deactivate in previous literatures. [16,44-
46]
The reusability of 25Cu@SiO2-β-P was investigated at a
temperature of 483K, and the result was shown in Fig. 10. In the
second run of the 25Cu@SiO2-β-P, the conversion of EC slightly
decreased to 95.8% which was 1.6% less than the first run, while
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the selectivity of MeOH increased to 71.8% with an increment of
7.2% which can be ascribed to the excitation of the catalytic and
active site. In the subsequent runs, the activity of this catalyst
showed the stable performance with the selectivity of EG
maintained at a high level, 97.8%, 97.7% and 97.8% hereby the
conversion of EC and the selectivity of MeOH also maintained a
stable level. Therefore, it can be speculated that β-cyclodextrin
stabilized catalysts during the hydrogenation of EC as well as
improved the catalytic activity.
(b)
(a)
(c)
(d)
Figure 11. (a) FT-IR, (b) XRD, (c) TEM, (d) XPS of the reused 25Cu@SiO2-β-
P catalysts
Figure 10. The reusability of 25Cu@SiO2-β-P catalyst.
Conclusions
In order to explore the crucial influencing effect on catalyst
stability, FT-IR, XRD, TEM and XPS characterizations of the
reused catalyst 25Cu@SiO2-β-P were performed. As identified by
FT-IR (Fig. 11a), compared to the fresh catalyst, there is no
noticeable change in the catalyst structure for the used catalyst.
Accordingly, as revealed in Fig. 11b, the average diameter of
25Cu@SiO2-β-P-Reused was around 1.6 nm thereby there was
no change of the particle size almost, thus the β-cyclodextrin
promoted catalyst exhibited marginal change in particle size
during the recycling experiments. It can be deduced that the
introduction of cyclodextrin could effectively inhibit the
aggregation of copper particles. Connecting with the low melting
point of copper with a lowing THutting around 451 K, [47] the catalytic
activity of the traditional copper-based catalyst decreased with the
reaction cycles under the high temperature. Whereas, the
introduction of cyclodextrin inhibited the aggregation of catalysts
which can be presumably attributed to that the residual carbon
promoted the sintering temperature THutting of catalysts and
inhibited the volume diffusion of catalysts. On the contrast, the
intensity of the characteristic diffraction peaks ascribable to Cu0
crystal plane became narrower and stronger after use (Fig. 11c),
and a weaker and broader peak at around 36.2o ascribable to the
Cu2O (111) plane could be observed. Meanwhile, the distribution
of Cu0 and Cu+ was further distinguished by XAES, and the
Cu+/(Cu++Cu0) also maintained a stable level where the reused
one was 0.55, suggesting the balanced copper species was not
influenced under the reaction temperature. These results
indicated that the introduction of cyclodextrin promoted the
stability of the catalyst effectively. The results further proved the
important role of balanced Cu+ and Cu0, and the synergistic effect
of these species is responsible for the superior catalytic activity.
In this study, β-cyclodextrin modified Cu@SiO2 catalysts were
prepared by a facile hydrolysis precipitation method with the
assistance of P123 and was successfully applied in the
hydrogenation of ethylene carbonate to co-produce methanol and
ethylene glycol. In particular, mesoporous xCu@SiO2-β-P
catalyst in flower-like nanostructure were successfully prepared
with sub-2 nm ultrasmall Cu particles. In addition, the Cu species
are also decorated by residual carbon and confined in
mesoporous silica channels. The results demonstrated that the
distribution of Cu+ and Cu0 could be modulated by varying the
initial copper loadings. Among the catalysts tested, the
25Cu@SiO2-β-P catalyst exhibited relatively higher catalytic
activity, and 97.4% EC conversion with 95.5% EG and 64.6%
methanol selectivities could be achieved, corresponding to
excellent TOF of 1195 mgEC•gcat-1•h-1. Accordingly, the
relationship between the structure and activity disclosed that the
maximum content of Cu+ species on the catalyst surface, which is
beneficial for achieving superior catalytic performance. The
cooperative effect between Cu0 and Cu+ species was considered
to be responsible for the enhanced catalytic activity. This work has
provided a new preparation method for preparing the uniformly
dispersed Cu catalyst with sub-2 nm size and gives insights into
the modulation method for obtaining the inexpensive copper-
based catalyst with high efficiency and excellent stability, which is
beneficial for the indirectly upgrading CO2 to co-produce
commodity methanol and ethylene glycol.
Experimental Section
Materials
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10.1002/cctc.201800828
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Cu(NO3)2•3H2O (>99%), β-cyclodextrin (>99%), p-xylene (>98.85%) and
ammonium carbonate (>99%) were provided by Sinopharm Chemical
Reagent Co., Ltd., China. Poly (ethylene glycol)-block-poly (propylene
glycol)-block-poly (ethylene glycol) (P123) was purchased from Aldrich.
Ethylene carbonate (EC, 99wt%) was purchased from Alfa Aesar.
Tetrahydrofuran (THF, > 99%) and tetraethyl silicate (TEOS) were
commercially available from Xilong Chemical Co., Ltd., China. Hydrogen
was provided by Beijing Haikeyuanchang PracticalGas Co., Ltd., China.
Other reagents were all of analytical grade and used as received in the
experiments.
of 10 K min-1 and maintained for 2 h. After cooling to the room temperature
again, argon was introduced for purging the sample for 1 h. Then, the
sample was heated to 363 K, 10 vol.% N2O/Ar was introduced under the
titration model and the reaction volume of N2O was counted until there is
no change of peaks. The reactions were described by the following
equations:
CuO + 퐻2 → 퐶푢 + 퐻2푂
(1)
(2)
2Cu + N2푂 → 퐶푢2푂 + 푁2
The copper dispersities were determined based on the actual copper
loadings determined from ICP.
X-ray diffraction patterns of samples were obtained through a PANalytical
Empyrean diffractometer with Cu Kα radiation (λ=0.15406 nm) over 2θ
range of 10 o – 90 o.
Catalyst preparation
The xCu@SiO2-β-P catalysts (the loadings of Cu were based on the total
weight of the catalyst, x=5, 15, 25, 35, 45) were synthesized by one-step
hydrolysis precipitation (HP) method primarily in the presence of β-
cyclodextrin and P123 and the details were described as follow.
According to copper loadings, the desired weights of Cu(NO3)2•3H2O, 1.5
g P123 and 0.5 g β-cyclodextrin were dissolved in a mixture liquid
composed of 100 mL water and 50 mL ethanol under stirring.
Subsequently, the mixed solution was heated to 313 K, and 100 mL
prepared ammonium carbonate aqueous liquid (0.5 mol/L) was injected
into the prepared copper aqueous dropwise around 1 h. Afterwards, the
corresponding weight of TEOS was dropped with the same drop velocity,
hereby the temperature of liquid was maintained at 313 K simultaneously.
The attained suspension was stirred for 4 h under ambient temperature
when the drop process finished, thereby copper species were deposited
with TEOS. After the aging stage, the suspension was heated to 363 K to
evaporate ammonia until pH reached 6~7. Thereafter, the resulting
suspension was filtered, washed by deionized water and ethanol in
sequence for at less five times. The attained solid was dried at 363 K
overnight and calcined under nitrogen atmosphere at 723 K for 4 h. Finally,
the catalyst reduction was performed with 10 vol%H2/N2 at 623 K for 4 h,
and the resulting catalysts were denoted as xCu@SiO2-β-P. In addition,
the catalyst without the introduction of β-cyclodextrin (25Cu@SiO2-P) was
also prepared for references.
Fourier-transform infrared spectra (FT-IR) was obtained on a Bruker
Tensor 27 spectrometer. The catalyst was ground and dispersed in KBr to
attain pellets utilized for FT-IR characterizations.
H2 temperature-programmed reduction (H2-TPR) studies of catalysts were
performed on a Quantachrome Chembet pulsar TPR/TPD instrument.
Around 30 mg calcined catalysts were pretreated with He for 1 h under
473 K, and the aforementioned sample was cooled to room temperature
before reduction process. Subsequently, the TPR was implemented from
323 K to 803 K with a heating rate of 10 K•min-1 by introducing 10 vol%
H2/N2.
Transmission electron microscopy (TEM) and high-resolution transmission
electron microscopy (HRTEM) images were attained by field-emission
transmission electron microscopy (JEOL, JEM-2100F), which was
operated at an acceleration voltage of 200 kV to characterize the
morphologies and the crystal structures of the xCu@SiO2. Prior to the
investigation, the samples were ultrasonically dispersed in ethanol at room
temperature for around 30 min. The scanning mode was employed
together with EDX spectroscopy. Afterwards, the suspension was
deposited on copper grids supported by holey carbon films, and the
measurement was tested after the sample drying.
X-ray photoelectron spectroscopy (XPS) and X-ray Auger electron
spectroscopy (XAES) were employed to probe the surface chemical states
of samples. The equipment was conducted under an ultrahigh vacuum
using an ESCALAB 259Xi spectrometer with Al Kα radiation (1486.6 eV)
and a multichannel detector. The obtained binding energies were
calibrated by using C1s peak at 284.6 eV as the reference with an
uncertainty of ±0.2 eV.
Catalyst characterization
N2 physisorption measurements were executed on Quantachrome
Autosorb-1 at liquid nitrogen temperature (77 K) after the samples were
outgassed at 573 K in vacuum for 3 h. The special surface area (SBET) was
determined by the Brunauer-Emmett-Teller (BET) method. The total pore
volume (Vpore) was obtained from the absorbed volume of nitrogen with a
relative pressure of 0.99. Based on the desorption branch of the isotherm,
pore size distribution was preliminarily estimated by the Barrett, Joyner
and Halenda (BJH) method.
Catalyst performance tests
The catalytic activities of the xCu@SiO2 catalysts were implemented for
the hydrogenation of EC in a 50 mL stainless steel autoclave under
magnetic stirring. Typically, 10 mmol EC, 20 mL THF and 0.176 g catalyst
were weighted and introduced into the autoclave. After the autoclave
sealed and flushed repeatedly with H2 five times, the autoclave was
charged with 5 MPa H2. The reactor was heated to the desired temperature
for a desired time with a magnetic stirring rate of 550 rpm. After the reaction
finished, the autoclave was cooled to the ambient temperature by ice-water.
The residual hydrogen was released to atmosphere carefully. The
quantitative analysis of the reaction mixture was measured on Shimadzu
GC-2014 with a GsBP-1 column (30 m×0.32 mm×1.0 μm) and a flame
The loadings of catalysts were identified by inductively coupled plasma-
atomic emission spectroscopy (ICP-AES) on PerkinElmer optima 5300DV.
Copper dispersity was determined by N2O titration on a Quantachrome
Chembet pulsar TPR/TPD instrument. Weight around 30 mg into quartz
tube and use argon gas to purge the sample under 473 K for 1h. After
cooling to the ambient temperature, the mixture gas of 10 vol.%H2/N2 was
introduced with the sample temperature increasing to 723 k by heating rate
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10.1002/cctc.201800828
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FULL PAPER
ionization detector (FID) using p-xylene as the internal standard. As for the
recycling experiments, the catalyst was separated by centrifugation,
washed by THF for five times. The obtained catalysts were designated as
25Cu@SiO2-β-P-Reused and used for the next run.
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Acknowledgements
We are grateful to the National Natural Science Foundation of
China (21576272), "Transformational Technologies for Clean
Energy and Demonstration", Strategic Priority Research Program
of the Chinese Academy of Sciences, Grant No. XDA 21030600,
and the project from Jiangsu Collaborative Innocation Center for
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Keywords: Carbon dioxide • Ethylene Carbonate • Mesoporous
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10.1002/cctc.201800828
ChemCatChem
FULL PAPER
β-cyclodextrin modified Cu@SiO2
catalysts were prepared by a facile
hydrolysis precipitation method and
was successfully applied in the co-
production of methanol and ethylene
glycol via hydrogenation of ethylene
carbonate derived from CO2.
Chanjuan Zhanga,b,c,d , Liguo Wanga,c,d*,
Jiaju Liua, Yanmi Yanga, Peng Hea, Yan
Caoa, Jiaqiang Chena, Huiquan Lia,c,d
*
Page No. – Page No.
Facile fabrication of ultrasmall copper
species confined in mesoporous
silica for chemo-selective and stable
hydrogenation ethylene carbonate
derived from CO2
This article is protected by copyright. All rights reserved.