Dynamic redox cycle of Cu0 and Cu+ over Cu/SiO2 catalyst in ester hydrogenation

Article abstract of DOI:10.1039/c5ra04389a

Esters (methyl acetate, ethyl acetate, dimethyl oxalate) can oxidize Cu⁰ of the reduced Cu/SiO₂ catalyst to Cu⁺ and the obtained Cu⁺ can be reduced back to Cu⁰ by H₂ under reaction conditions. Cu⁰ and Cu⁺ are in a dynamic cycle during ester hydrogenation. The ratio of Cu⁺/Cu⁰ in the presence of esters under reaction conditions is probably different from that in the absence of esters under characterization conditions. The effect of the dynamic cycle on the Cu⁺/Cu⁰ ratio should be taken into consideration when establishing the relationship between the Cu⁺/Cu⁰ ratio and reaction performance.

Full text of DOI:10.1039/c5ra04389a

RSC Advances
View Article Online
View Journal | View Issue
COMMUNICATION
Dynamic redox cycle of Cu0 and Cu+ over Cu/SiO2
catalyst in ester hydrogenation†
Cite this: RSC Adv., 2015, 5, 37581
a
b
b
b
a
*
Xiangang Ma, Zhiqiang Yang, Xuebin Liu, Xingzhi Tan and Qingjie Ge
Received 12th March 2015
Accepted 20th April 2015
DOI: 10.1039/c5ra04389a
www.rsc.org/advances
Esters (methyl acetate, ethyl acetate, dimethyl oxalate) can oxidize Cu0 reported that a Cu/SiO2 catalyst prepared by ammonia evap-
of the reduced Cu/SiO2 catalyst to Cu+ and the obtained Cu+ can be oration method at 363 K with the highest Cu+/Cu0 ratio of
reduced back to Cu0 by H2 under reaction conditions. Cu0 and Cu+ are 1.22 exhibited the best catalytic activity for the hydrogenation
in a dynamic cycle during ester hydrogenation. The ratio of Cu+/Cu0 in of dimethyl oxalate to ethylene glycol.5 Yin et al. also prepared
the presence of esters under reaction conditions is probably different Cu/SiO2 catalysts by the same method and investigated the
from that in the absence of esters under characterization conditions. effect of the preparation temperature on the Cu+/Cu0 ratio.6 In
The effect of the dynamic cycle on the Cu+/Cu0 ratio should be taken contrast, they gained the best performance at their minimum
into consideration when establishing the relationship between the Cu+/Cu0 ratio of 0.16 for the same reaction. In the published
Cu+/Cu0 ratio and reaction performance.
papers, the ratio of Cu+/Cu0 detected by XPS/XAES (X-ray
Photoelectron Spectroscopy and X-ray Auger Electron Spec-
troscopy) before the reaction was adopted to establish the
relationship with the reaction performance. However, the
XPS/XAES analysis was exclusively carried out in high vacuum
and without ester present. He et al. observed the Cu2O phase
in the spent Cu/SiO2 catalyst, which did not appear in the
fresh reduced catalyst.7 This hinted at the variation of the
Cu+/Cu0 ratio during ester hydrogenation. Here we present
proof that the esters (methyl acetate, ethyl acetate, dimethyl
oxalate) can oxidize Cu0 to Cu+ and the obtained Cu+ can be
reduced back to Cu0, implying a dynamic cycle of Cu0 and Cu+
during ester hydrogenation. The Cu+/Cu0 ratio in the presence
of esters under reaction conditions is likely to be different
from that in the absence of esters under characterization
conditions. This may be the reason why different researchers
observed diverse results about the effect of the Cu+/Cu0
ratio.5,6
Fig. 1 shows TPR proles of Cu/SiO2 catalyst prepared by
ammonia evaporation method (ESI†). With a heating rate of
10 K minꢀ1, the fresh calcined catalyst exhibited a broad
reduction peak centered at 565 K. Ethyl acetate (EA) was intro-
duced onto the catalyst aer the reduction by a carrier gas of
helium at 493 K, the temperature at which ester hydrogenation
is usually carried out. Aer the treatment with EA, a second
reduction demonstrated a smaller H2-consumption peak at a
relatively low temperature. This implies that EA oxidized Cu0 to
its oxidation state, which showed similar reduction temperature
to Cu2O obtained from the oxidation of Cu0 in the reduced
Cu/SiO2 catalyst by N2O (454 K vs. 458 K).
Hydrogenation of esters to the corresponding alcohols is a
fundamental reaction in organic chemistry and is employed in
a large number of chemical processes. Of particular interest
are the hydrogenation of diesters to diols, which are used in
the production of polyesters, and the hydrogenation of acetic
esters to the promising fuel ethanol. Cu/SiO2 catalysts have
been identied as active and selective catalysts for these
reactions.1 In order to gain better activity and stability, much
work has been done to explore the nature of the active species
for ester hydrogenation. The activity was frequently found to
be proportional to the surface area of metallic copper and thus
Cu0 was deemed as the sole active site.2 Based on this, Cu0
activates both H2 and the ester. However, it is reported that
Cu+ can polarize the C]O bond of ester molecules and
stabilize the intermediates formed during the reaction,
implying that Cu+ could play a role in ester hydrogenation.3
Therefore, a synergetic effect of Cu0 and Cu+ for ester hydro-
genation was advanced and proper ratio of Cu+/Cu0 is believed
to benet the catalytic performance of Cu/SiO2 catalysts in
addition to improved dispersion of copper species.4 Chen et al.
aDalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics,
Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, P. R. China.
E-mail: geqj@dicp.ac.cn
bEnergy Innovation Laboratory, BP (China) Holdings Limited, 457 Zhongshan Road,
Dalian 116023, P. R. China
† Electronic supplementary information (ESI) available: Experimental details and
characterization data. See DOI: 10.1039/c5ra04389a
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 37581–37584 | 37581

View Article Online
RSC Advances
Communication
decreased signicantly aer H2-treatment while the peak of Cu0
at 2q ¼ 43.4ꢁ became obvious again (Fig. 2d). This demonstrated
that Cu2O derived from the oxidation of Cu0 by EA can be
reduced back to Cu0 at 493 K. The highly dispersed metallic
copper might be oxidized to CuO to some extent when exposure
to air. The weak peak at about 2q ¼ 36.5ꢁ existing in the reduced
catalyst in Fig. 2b should be ascribed to this type of CuO
because it did not appear during the in situ XRD (Fig. S1, ESI†).
This type of CuO may also make some contribution to the
intensity of the peak at 2q ¼ 36.5ꢁ in Fig. 2c and d. However, we
think its contribution was small. All in all, it can be drawn from
the XRD and TPR results that the oxidation of Cu0 to Cu2O by EA
and the reduction of Cu2O to Cu0 by H2 occur simultaneously
during EA hydrogenation. The existing formation of Cu+ during
the reaction was determined by the catalyst system together
with reactants and products. Cu+ in the copper chromite cata-
lyst responsible for CO chemisorption in methanol synthesis
from syngas exists as a crystalline CuCrO2 phase.8 The active site
Cu+ in ester hydrogenation seems to be in the form of Cu2O.6,7,9
The Cu2O obtained from oxidation of Cu0 by EA in this work and
the Cu+ species in the published papers should be same in
essence. Therefore, the active sites Cu0 and Cu+ are in a
dynamic redox cycle during EA hydrogenation. This dynamic
cycle also exists during the hydrogenation of other esters, such
as methyl acetate and dimethyl oxalate (Fig. S2, ESI†).
Fig. 3 shows the XPS/XAES spectra of the Cu/SiO2 catalyst.
The copper in the calcined catalyst existed in the oxidation state
of Cu2+, as evidenced by the broad Cu 2p3/2 peak at about 933 eV
and the 2p to 3d satellite at 942–945 eV characteristic of Cu2+
with electron conguration of 3d9. For the catalyst reduced at
623 K, the binding energy of Cu 2p3/2 shied to 932.2 eV and the
satellite disappeared, suggesting that the copper species
became reduced (Cu0 or Cu+). Because the BE values of Cu0 and
Cu+ are almost identical, the distinction between these two
species has to be made on the basis of the XAES spectra. Two
overlapping Cu LMM Auger kinetic energy peaks centered at
Fig. 1 TPR profiles of Cu/SiO2 catalyst. (a) First reduction for the
calcined catalyst, (b) second reduction after treatment with ethyl
acetate.
XRD patterns of Cu/SiO2 catalyst with different treatments
are illustrated in Fig. 2. The broad diffuse peak at 2q ¼ 21.7ꢁ can
be attributed to amorphous silica. No diffraction peak of copper
species was detected in the calcined catalyst, indicating that the
cupric compounds were in amorphous state. Fig. 2b implies
that the characteristic diffraction peak of Cu0 at 2q ¼ 43.4ꢁ
showed up aer the reduction at 623 K (JCPDS 003-1015). Fig. 2c
was for the catalyst suffering the reduction at 623 K and EA
treatment at 493 K in series. Compared to Fig. 2b, the peak at
2q ¼ 43.4ꢁ abated drastically or even disappeared aer the
treatment with EA. Instead, two new peaks emerged at 2q ¼
36.5ꢁ and 42.4ꢁ, respectively, characteristic of Cu2O (JCPDS 065-
3288). This suggests that EA oxidized Cu0 to Cu2O, which is
consistent with the TPR results. Aer EA oxidation, H2 was
introduced onto the catalyst to check the reducibility of the
newly generated Cu2O at 493 K. The peak of Cu2O at 2q ¼ 36.5ꢁ
Fig. 2 XRD patterns of Cu/SiO2 catalyst. Ethyl acetate (EA) was fed
onto the catalyst by helium.
Fig. 3 XPS/XAES spectra of Cu/SiO2 catalyst.
37582 | RSC Adv., 2015, 5, 37581–37584
This journal is © The Royal Society of Chemistry 2015

View Article Online
Communication
RSC Advances
about 918.8 eV and 914.2 eV can be ascribed to Cu0 and Cu+,
respectively, impling the coexistence of Cu0 and Cu+ on the
surface (2–10 nm) of the reduced catalyst. Cu0 and Cu+ of
Cu/SiO2 catalyst prepared by ammonia evaporation method
were attributed to the reduction of highly dispersed CuO and
copper phyllosilicate respectively in some papers and further
reduction of this kind of Cu+ to Cu0 was believed to require a
high temperature of 873 K.5–7,9,10 However, the appointed
reduction peak for the Cu+ was not observed in these papers
although XPS/XAES gave a high Cu+ content of more than 50%
sometimes. Copper phyllosilicate used as
a
reference
compound in Van Der Gri's work did not show hydrogen
consumption above 600 K in TPR process with a heating rate of
3.4 K minꢀ1
.
Lin et al.12 and Zheng et al.13 reported that the
11
reduction of copper phyllosilicate yielded Cu nanocrystallites
and increased amount of copper phyllosilicate boosted the
dispersion of Cu0. It seems that the source of Cu+ is not clear.
Deconvolution results of the XAES peak show that the propor-
tion of surface Cu+ species was 21% for our 623 K reduced
catalyst. If there was so much Cu+ in the bulk of the catalyst,
there should be a reduction peak above 623 K for further
reduction of Cu+. However, no other peak was obtained until
1200 K except the peak centered at about 565 K (Fig. S3, ESI†). In
addition, the H2 consumption implied that almost all the Cu2+
was reduced to Cu0 at 623 K. Therefore, it is reasonable to
deduce that the freshly reduced catalyst contained only trace
amount of Cu+, which may mainly exist on the surface of the
catalyst. Its formation may be related to the special surface
topographies such as terrace, kink or edge. Yet, the possibility
of Cu+ coming from the reduction of copper phyllosilicate could
not be excluded.
In general, the reaction rate is mainly determined by the
surface active sites of the catalyst. Our results showed that Cu2O
derived from the oxidation of Cu0 by the esters yielded distinct
XRD diffraction peaks and an obvious TPR H2-consumption
peak, suggesting that the Cu0 in the bulk of the catalyst can be
reached and oxidized to Cu+ by the esters. Therefore, we
tentatively proposed that the copper species in the bulk should
be involved in ester hydrogenation in addition to the species on
the surface. Herein, we attempted to combine the Cu+/Cu0 ratio
of the entire catalyst to the reaction performance. Two methods
were established by applying the TPO and TPR techniques to
determine the Cu+/Cu0 ratio of the entire catalyst aer the
reaction (ESI†). The two methods gave identical results, which
proved them feasible and reliable.
Fig. 4 Catalytic activity and Cu+/Cu0 ratio of Cu/SiO2 catalyst in the
hydrogenation of ethyl acetate (EA). Reaction conditions: (a) 493 K,
1.0 MPa, H2/EA ratio ¼ 80; (b) 493 K, 0.1 MPa, H2/EA ratio ¼ 80. The
ꢀ1
ꢀ1
.
feed rate of EA was 3.3 mmol h
g
cat
and 9 h, respectively, with a Cu+/Cu0 ratio of 49/51 and 60/40.
Fig. 4 displays that EA conversion initially increased and then
reached a stable level. Repeat experiments conrmed the rising
period at the beginning but its duration varied with reaction
conditions. The initial increase indicated an evolution of the
active sites. At the start of the reaction, the catalyst possessed
high Cu0 content, but exhibited a low EA conversion. Aer a few
hours, EA conversion stabilized at a relatively high level,
however the catalyst was in a low Cu0 content. We think that Cu+
and Cu0 attained a dynamic equilibrium at the moment. These
results implied that EA conversion was not proportional to the
content of Cu0 and a synergetic effect existed between Cu+ and
Cu0 for EA hydrogenation. Therefore, the Cu+/Cu0 ratio is
important for the catalytic performance of the catalyst.
Promoters and preparation methods were reported to inuence
the reaction performance via regulating the Cu+/Cu0 ratio.14 In
these papers, the Cu+/Cu0 ratio was detected by XPS/XAES
without the presence of the reactants and it was just for the
surface of the catalyst. Ester and H2 are actors in the inter-
conversion of Cu0 and Cu+. As excepted, high reaction pres-
sure promoted the reduction of Cu+ to Cu0 and resulted in a
lower ratio of Cu+/Cu0 of the entire catalyst (Fig. 4). It seems that
the Cu+/Cu0 ratio of the entire catalyst was affected by the
reaction conditions, which certainly can alter the surface
Cu+/Cu0 ratio of the catalyst. We think that the relationship
between the Cu+/Cu0 ratio and reaction performance should be
estimated under operando conditions, no matter whether the
active species in the bulk of the catalyst make some contribu-
tion to the reaction or not.
Fig. 4 shows the activity of Cu/SiO2 catalyst for EA hydroge-
nation and the Cu+/Cu0 ratio of the spent catalyst. The Cu/SiO2
catalyst gave more than 98% selectivity to ethanol (not shown
here). The by-products were ethane, diethyl ether and acetal-
dehyde. Two experiments were carried out under same reaction
conditions of 493 K, 0.1 MPa and H2/EA ratio of 80, but termi-
nated at different reaction times of 30 min and 9 h. The catalysts
aer 30 min and 9 h reaction possessed a Cu+/Cu0 ratio of 67/33
and 80/20 and exhibited 13% and 28% EA conversion, respec-
tively. High pressure was benecial for the reaction. Under
reaction conditions of 493 K, 1.0 MPa and H2/EA ratio of 80, the
catalysts demonstrated 22% and 96% EA conversion at 20 min
One ester molecule turns into two alcohol molecules
according to the equation of ester hydrogenation. The two
oxygen atoms in the ester molecule should be present in the
alcohol molecules aer the reaction. In this work, the trans-
formation of Cu0 to Cu2O should take some of the oxygen from
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 37581–37584 | 37583

View Article Online
RSC Advances
Communication
the esters. If Cu2O changed into water and Cu0 with the
reduction of H2, then signicant amounts of water along with
alkanes should be detected in the products. However, the
amount of alkanes was very small. In addition, the stoichio-
metric yield of alcohol molecules per ester molecule was close to
two. Therefore, the inter-conversion of Cu0 and Cu+ did not
transform the oxygen of the esters into water. Although how
esters and H2 participate in the dynamic cycle of Cu0 and Cu+ is
not clear, we think that esters and H2 may be activated during
the inter-conversion of Cu0 and Cu+ and then transformed into
the target products.
Y. Zhao, S. Zhao, L. Zhao, J. Lv, S. Wang and X. Ma, J. Am.
Chem. Soc., 2012, 134, 13922–13925.
3 (a) E. K. Poels and D. S. Brands, Appl. Catal., A, 2000, 191, 83–
96; (b) M. A. N. Santiago, M. A. Sanchez-Castillo,
R. D. Cortright and J. A. Dumesic, J. Catal., 2000, 193, 16–28.
4 (a) A. Yin, C. Wen, X. Guo, W. Dai and K. Fan, J. Catal., 2011,
280, 77–88; (b) X. Ma, H. Chi, H. Yue, Y. Zhao, Y. Xu, J. Lv,
S. Wang and J. Gong, AIChE J., 2013, 59, 2530–2539; (c)
A. Yin, X. Guo, W. Dai, H. Li and K. Fan, Appl. Catal., A,
2008, 349, 91–99.
5 L. Chen, P. Guo, M. Qiao, S. Yan, H. Li, W. Shen, H. Xu and
K. Fan, J. Catal., 2008, 257, 172–180.
6 A. Yin, X. Guo, W. Dai and K. Fan, Catal. Commun., 2011, 12,
412–416.
7 Z. He, H. Lin, P. He and Y. Yuan, J. Catal., 2011, 277, 54–63.
8 (a) G. Apai, J. R. Monnier and M. J. Hanrahan, Appl. Surf. Sci.,
1984, 19, 307–314; (b) J. R. Monnier, M. J. Hanrahan and
G. Apai, J. Catal., 1985, 92, 119–126.
9 A. Yin, X. Guo, W. Dai and K. Fan, J. Phys. Chem. C, 2009, 113,
11003–11013.
In summary, Cu0 and Cu+ are in a dynamic redox cycle
during the hydrogenation of esters such as methyl acetate, ethyl
acetate and dimethyl oxalate. As the participators in inter-
conversion of Cu0 and Cu+, ester and H2 can shi the equilib-
rium of Cu0 and Cu+ and affect the Cu+/Cu0 ratio. Our results
implied that the Cu+/Cu0 ratio is closely related to the reaction
performance. Operando study using X-ray Absorption Fine
Structure will be carried out to further explore the relationship
between the Cu+/Cu0 ratio and reaction performance.
10 A. Yin, X. Guo, W. Dai and K. Fan, J. Phys. Chem. C, 2010, 114,
8523–8532.
11 C. J. G. Van Der Gri, A. Mulder and J. W. Geus, Appl. Catal.,
1990, 60, 181–192.
12 H. Lin, X. Zheng, Z. He, J. Zheng, X. Duan and Y. Yuan, Appl.
Catal., A, 2012, 445, 287–296.
Acknowledgements
This work was nancially supported by Energy Innovation
Laboratory of BP company and National Natural Science
Foundation of China (no. 21403215).
13 X. Zheng, H. Lin, J. Zheng, X. Duan and Y. Yuan, ACS Catal.,
2013, 3, 2738–2749.
Notes and references
14 (a) Z. Huang, F. Cui, H. Kang, J. Chen, X. Zhang and C. Xia,
Chem. Mater., 2008, 20, 5090–5099; (b) L. Lin, P. Pan,
Z. Zhou, Z. Li, J. Yang, M. Sun and Y. Yao, Chin. J. Catal.,
2011, 32, 957–969; (c) C. Wen, Y. Cui, A. Yin, K. Fan and
W. Dai, ChemCatChem, 2013, 5, 138–141.
1 (a) J. Lin, X. Zhao, Y. Cui, H. Zhang and D. Liao, Chem.
Commun., 2012, 48, 1177–1179; (b) C. Wen, Y. Cui, W. Dai,
S. Xie and K. Fan, Chem. Commun., 2013, 49, 5195–5197; (c)
D. S. Brands, E. K. Poels and A. Bliek, Appl. Catal., A, 1999,
184, 279–289.
2 (a) M. Mokhtar, C. Ohlinger, J. H. Schlander and T. Turek,
Chem. Eng. Technol., 2001, 24, 423–426; (b) J. Gong, H. Yue,
37584 | RSC Adv., 2015, 5, 37581–37584
This journal is © The Royal Society of Chemistry 2015