The effect of supports on hydrogenation and water-tolerance of copper-based catalysts

Article abstract of DOI:10.1039/d0nj05302c

The effect of supports (SiO2, ZnO, ZrO2 and Al2O3) on the hydrogenation and water-tolerance of copper catalysts were studied at reaction condition containing water. The copper catalysts with different supports showed different hydrogenation and water-tolerance performances. The result of XRD, BET, Raman, TPR, XPS, N2O titration and TEM confirmed the interactions between copper and supports, and the formation of Cu-MxOy (M = Zn, Zr, and Al) interfaces had great effects on the catalytic activity and water-tolerance performance. In particular, too strong interactions suppressed the reduction of copper oxides, resulting in a low catalytic activity. Nevertheless, the formation of Cu-MxOy could provide more active sites, which provided more chances for the reactants to touch the active sites. By this method, the loss of active sites due to competitive adsorption between water and ethyl acetate could be made up, so that the water-tolerance of copper catalysts was improved.

Full text of DOI:10.1039/d0nj05302c

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The effect of supports on hydrogenation and
water-tolerance of copper-based catalysts†
Cite this: New J. Chem., 2021,
45, 9967
Zheng Chen, * Xueying Zhao, Shuwei Wei, Dengfeng Wang and Xuelan Zhang*
The effect of supports (SiO2, ZnO, ZrO2 and Al2O3) on the hydrogenation and water-tolerance of copper
catalysts were studied at reaction condition containing water. The copper catalysts with different
supports showed different hydrogenation and water-tolerance performances. The result of XRD, BET,
Raman, TPR, XPS, N2O titration and TEM confirmed the interactions between copper and supports, and
the formation of Cu–MxOy (M = Zn, Zr, and Al) interfaces had great effects on the catalytic activity and
water-tolerance performance. In particular, too strong interactions suppressed the reduction of copper
oxides, resulting in a low catalytic activity. Nevertheless, the formation of Cu–MxOy could provide more
active sites, which provided more chances for the reactants to touch the active sites. By this method,
the loss of active sites due to competitive adsorption between water and ethyl acetate could be made
up, so that the water-tolerance of copper catalysts was improved.
Received 29th October 2020,
Accepted 30th April 2021
DOI: 10.1039/d0nj05302c
rsc.li/njc
widespread use in chemical, energy, pharmaceutical and food
industries. Recently, the hydrogenation of ethyl acetate to
1. Introduction
With the development of clean utilization of coal and biomass, ethanol has attracted considerable attention.15,16 This method
the catalytic hydrogenation reaction of some specific compounds had the advantages of high selectivity towards ethanol, low
originating from coal or biomass have attracted considerable production cost and convenient operation, and could also solve
attention among researchers.1–3 In these catalytic hydrogenation the overcapacity problem of ethyl acetate. However, water
reactions, copper-based catalysts have been widely used due to impurities might exist in ethyl acetate during the hydrogenation
their good selective hydrogenation of CQO bonds and relatively reaction due to the non-ideal selectivity or raw materials derived
low activity for C–C bond hydrogenolysis.4,5 Nonetheless, the from industrial scrap. Therefore, it is worth investigating that
particle size of the copper species, the dispersion of metal copper how the supports affect the hydrogenation and water-tolerance
and the interaction effects between copper and the supports were of copper catalysts. Simultaneously, our previous study has
the key factors affecting the catalytic performance of Cu-based demonstrated that the doping of Zn into the Cu/SiO2 catalyst
catalysts. These factors depended greatly on the selection of could effectively improve the hydrogenation water-tolerance of
supports with different surface properties. Although the same Cu/SiO2.17 Moreover, a published study had also confirmed that
preparation method was used, copper-based catalysts possessed water easily promoted the growth of copper particles, which led
different physicochemical properties due to the use of different to the deactivation of the catalyst.18 The strong interactions
supports. At present, the effects of supports on the catalytic between copper and the support can hinder the particle growth
performance have been extensively researched by changing metal in the reaction. Therefore, a strong interaction is beneficial
dispersion, surface area, oxygen storage capacity, basicity, and for the enhancement of the water-tolerance of copper-based
interactions between active species and support of the catalysts. ZrO2 and Al2O3 have been widely used as support
catalysts.6–14 However, to the best of our knowledge, the effect materials of copper catalysts.19 The metal-support interaction
of supports on the water-tolerance of copper-based catalysts has can be enhanced due to the existence of surface coordinatively
not been systematically researched so far.
unsaturated Al3+ and Zr4+O2À 20
. ZrO2- and Al2O3-supported
In the process of clean utilization of coal and biomass, the copper catalysts have been used in CO and CO2 hydrogenation,
production of ethanol was extensively researched due to its which showed high activity and hydrothermal stability.21,22
Therefore, SiO2, ZnO, ZrO2 and Al2O3 were used as supports to
College of Chemistry, Chemical Engineering and Materials Science, Zaozhuang
study the effects of supports on the water-tolerance of copper-
based catalysts.
University, Zaozhuang 277160, Shandong, China. E-mail: chenzhengtt@163.com,
mhszxl@sxicc.ac.cn
In this study, the hydrogenation of ethyl acetate to ethanol
† Electronic supplementary information (ESI) available: Physicochemical prop-
was selected to study the influence of supports (SiO2, ZnO, ZrO2
and Al2O3) on the hydrogenation water-tolerance performance
erty characterization of Cu/SiO2 is given in Figures S1–S5. See DOI: 10.1039/
d0nj05302c
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of copper-based catalysts. The obtained hydrogenation results
were associated with characterizations to reveal the essence
of the affection of the supports on the hydrogenation water-
tolerance performance. We aimed at understanding the
advantages of different supports for copper catalysts at reaction
condition containing water, which is beneficial for researchers
to develop highly efficient water-tolerant catalysts.
2. Experimental
2.1. Material
Copper(II) nitrate hydrate, aluminum nitrate nonahydrate, zinc
nitrate hexahydrate, zirconium oxychloride octahydrate, tetraethyl
orthosilicate, ethyl alcohol absolute and ammonium bicarbonate
were bought from Sinopharm Chemical Reagent Co., Ltd.
All reagents used in this study were of analytically pure grade
and used without further purification.
Fig. 1 XRD profiles of the Cu/SiO2, Cu/Al2O3, Cu/ZnO and Cu/ZrO2
catalysts.
were assigned to ZrO2 (PDF#49-1746) and ZnO (PDF#36-1451),
respectively.
2.2. Catalyst preparation
The Brunauer–Emmett–Teller (BET) specific surface area
and BJH adsorption pore size distribution of all catalysts were
measured via N2 adsorption–desorption measurements, and
the relative results are shown in Fig. 2 and Table 1. Fig. 2a
shows the adsorption–desorption isotherms, and it was clear
that physisorption isotherms with hysteresis loops were different
from each other. According to the IUPAC Classification, the Cu/
SiO2 catalyst was of type III, and the hysteresis loops belonged to
the H3 type with aggregates of plate-like particles giving rise to
slit-shaped pores; Cu/ZrO2 and Cu/Al2O3 catalysts were of type II,
and the hysteresis loops belonged to the H4 type with narrow
slit-like pores; and Cu/ZnO catalyst was of type V, and the
hysteresis loops belonged to the H2 type with ‘‘ink bottle’’
pores.25,26 These results indicated that the copper catalysts with
different supports possessed different pore features. From
Fig. 2b, the most probable distributions of pore sizes were 18.1
and 6.5 nm for the Cu/ZnO and Cu/Al2O3 catalysts, suggesting
the existence of mesopores. The distribution of pore sizes was
concentrated on 10–200 nm or larger, indicating that mesopores
and macropores existed in the Cu/SiO2 catalyst. The BJH pore
size distribution of the Cu/ZrO2 catalyst was focused on 1.8 and
115 nm, suggesting that micropores and large-pores were
present. Although the peak intensity of the pore size distribution
for the Cu/ZnO catalyst was stronger than that of others, its pore
sizes were bigger than the particle size (Fig. 5). This result
indicated that the accumulation of particles was the formation
reason of the pores.
Copper catalysts were prepared by precipitation using ammonium
bicarbonate as the precipitant. The obtained catalysts were
marked as Cu/SiO2, Cu/ZrO2, Cu/ZnO and Cu/Al2O3 on the basis
of different supports. The nominal copper loading of the catalysts
was 25%.
For copper catalysts, 6.05 g of Cu(NO3)2Á3H2O was dissolved
in the mixture of 50 ml deionized water and 50 ml ethanol.
Then, C8H20O4Si (53.72 g), Al(OH)3Á9H2O (32.38 g), Zn(NO3)Á
6H2O (16.08 g) and ZrOCl2Á8H2O (11.51 g) were added in 6
portions to the mixed solution under agitation, respectively,
until they completely dissolved. Subsequently, 6 portions of a
200 ml NH4HCO3 aqueous solution with different concentrations
were added into the above-mentioned mixed solutions under
agitation. Finally, the mixed solutions were stirred for 3 h at
90 1C, and then aged for 12 h.
Thereafter, the mixtures were filtered and dried at 80 1C for
12 h. The obtained precursors were calcined at 400 1C in a
muffle furnace for 4 h, and then pelletized, shed and sieved to
40–60 meshes.
The detailed information of the characterization methods
and hydrogenation reaction conditions of all catalysts can be
found in the ESI.†
3. Result and discussion
3.1. Characterization of the catalysts
Fig. 1 shows the XRD profiles of the Cu/SiO2, Cu/Al2O3, Cu/ZnO
The Raman spectra of the as-prepared catalysts were
and Cu/ZrO2 catalysts to analyse their crystal structure and recorded to study the effects of the support on the molecular
phases. For the Cu/SiO2 and Cu/Al2O3 catalysts, there were no structure of CuO. As presented in Fig. 3a, the block CuO
obvious peaks corresponding to the copper species observed, showed three peaks in the range of 50–1000 cmÀ1, which were
indicating that the copper species was highly dispersed. Just assigned to the Ag mode (282.8 cmÀ1) and Bg mode (332.5 and
weak and broad characteristic peaks of SiO2 (2y = 221, PDF#29- 616.7 cmÀ1) of CuO.29 It was clear that the three peaks of CuO
0085)4,23 and Al2O3 (2y = 34.81 and 64.91, PDF#52-0803) were moved to lower wavenumbers when the supports were used,
found, respectively. For the Cu/ZrO2 and Cu/ZnO catalysts, the indicating that an interaction existed between the supports and
characteristic peaks of CuO, located at 35.21 and 38.51 (PDF#44- CuO.30 However, the Raman shift was also influenced by the
0706), could be clearly observed.4,24 Other characteristic peaks particle sizes. A published study confirmed that particle growth
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Fig. 2 (a), N2 adsorption–desorption isotherms and (b), BJH adsorption pore size distribution for all catalysts.
Table 1 The textural properties of copper catalysts from BET and N2O peak assigned to the reduction of Cu2+ (233 1C), the peak at
titration
303 1C corresponded to the reduction of the center of bulk
copper oxide, and an additional peak (476 1C) could also be
found, which was assigned to the reduction of Cu2+ with strong
interactions between Cu and Zr.32 Moreover, a broad reduction
peak was also observed at 423 1C for the Cu/Al2O3 catalyst,
which was attributed to the reduction of the bulk copper oxide
phase. From the result of TPR, it was referred that the particle
Catalysts SBET (m2 gÀca1t) VP (cm3 gcÀa1t) SCu (mC2u gÀca1t) DCu (%) dCa u (nm)
Cu/SiO2 40.7
Cu/ZnO 41.0
Cu/Al2O3 26.0
Cu/ZrO2 55.9
0.07
0.16
0.04
0.03
33.0
24.5
14.3
7.7
19.5
14.5
8.4
5.1
6.9
11.9
22.0
4.5
SCu = 1353 Y/X, DCu = 2 Y/X, daCu = 0.5 X/Y. X = the area of the TPR1 peak
and Y = the area of the TPR2 peak, which determined by N2O size of copper species in the Cu/Al2O3 and Cu/ZrO2 catalysts was
titration.27,28
larger than that of the Cu/SiO2 and Cu/ZnO catalysts, which was
confirmed by TEM (Fig. 5 and Fig. S1, ESI†).
would lead to Raman peaks move towards higher wavenumbers.31
The specific surface area (SCu), average volume-surface diameter
In our study, the particle size of the as-prepared catalysts had a big (dCu) and dispersion (DCu) of metal copper were measured by N2O
difference, which could be confirmed by TEM (Fig. 5). Therefore, titration. The typical N2O titration profiles are shown in Fig. 4,
the Raman shift could not totally show the size of interactions for and the results calculated by a relative formula27,28 were
the catalysts with different particle size. The following character- summarized in Table 1. From Fig. 4, it was clearly observed
ization explains it further.
that there were two peaks for TPR2 of the Cu/Al2O3 and Cu/ZrO2
Fig. 3b shows the profiles of H2-TPR, indicating that the catalysts, which distinguished that of the Cu/SiO2 and Cu/ZnO
reduction temperatures of different catalysts had a great catalysts. More specifically, Cu2+ could be reduced absolutely
distinction. For Cu/SiO2 and Cu/ZnO catalysts,
a main for the Cu/SiO2 and Cu/ZnO catalysts by TPR1, and just one
reduction peak along with a shoulder could be observed, which peak (180 1C), which was assigned to the reduction of Cu+,
were located at 240, 258 1C and 219, 248 1C, respectively. These could be found from TPR2. For the Cu/Al2O3 and Cu/ZrO2
reduction peaks could be assigned to the reduction of Cu2+ to catalysts, except for the peak assigned to the reduction of Cu+
Cu+, and then to Cu0. For the Cu/ZrO2 catalyst, except for the at 200 1C, another peak could be found from TPR2, which
Fig. 3 (a) Raman spectra and (b) TPR pattern of bulk CuO and the prepared copper catalysts.
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Fig. 4 TPR profiles of copper catalysts before and after N2O oxidation at 50 1C.
corresponded to the reduction of Cu2+. For the Cu/Al2O3 (7.7 m2 gcÀa1t). Clearly, the strong interaction and the large
catalyst, the big particle size was responsible for the peak at particle size hindered the reduction of partial Cu2+, so that
360 1C in TPR2 because its reaction temperature was basically Cu/Al2O3 and Cu/ZrO2 had a little SCu as compared to that of
in line with the reduction temperature of TPR1. Therefore, the Cu/SiO2 and Cu/ZnO after the reduction.
peak at 360 1C in TPR2 was assigned to the reduction of the
The XPS of the Cu/SiO2, Cu/Al2O3, Cu/ZnO and Cu/ZrO2
center of bulk copper oxide for the Cu/Al2O3 catalyst. However, catalysts were also performed, and the relative results are
the reduction temperature of another peak was 460 1C in TPR2 shown in Table 2 and Fig. S2 (ESI†). From Fig. S2 (ESI†), two
for the Cu/ZrO2 catalyst, which was much higher than the peaks could be observed from about 935.2 and 955.5 eV, along
reduction temperature in TPR1. Therefore, it was referred that with the satellite peaks of 2p - 3d at about 943.4 and 963.7 eV
the strong interaction between Cu and Zr was responsible for for all catalysts, indicating the existence of Cu2+3,33,34 Moreover,
this reduction peak. That is, the strong interaction suppressed it was clear that the binding energies of Cu 2p3/2 could be
the reduction of partial Cu2+ under the conditions of the N2O deconvoluted into two symmetric peaks (Fig. S2, ESI†), indicating
titration (350 1C, 40 min), and these Cu2+ ions could be reduced the existence of two Cu2+ species with different valence states.
at 460 1C in TPR2. Therefore, just the first peaks belonged to From Table 2, it was found that two types of Cu2+ centered
the reduction of Cu+ for the Cu/Al2O3 and Cu/ZrO2 catalysts. around 935.0 (Cu12+) and 933.5 eV (Cu22+). The binding energies
Based on the above-mentioned analysis, the SCu, dCu and DCu of at around 933.5 eV were assigned to Cu2+ in the CuO species.
reducible copper under the conditions of N2O titration were The binding energies at around 935.0 eV, which indicated
calculated, and the result is shown in Table 1. From Table 1, it charge transfer from Cu2+ toward the supports, were assigned
was found that SCu followed the order of Cu/SiO2 (33.0 m2 gcÀa1t) 4 to Cu2+ with strong interaction with the supports.35,36
Cu/ZnO (24.5 m2 gÀca1t) 4 Cu/Al2O3 (14.3 m2 gcÀa1t) 4 Cu/ZrO2 Moreover, Cu21+/(Cu12+ + Cu22+) was calculated to show the
relative content of two types of Cu2+. In particular, the values
of Cu12+/(Cu12+ + Cu22+) were 35.8, 18.6, 38.8 and 15.6% for Cu/
SiO2, Cu/Al2O3, Cu/ZnO and Cu/ZrO2 catalysts, respectively,
Table 2 Binding energies of Cu 2p3/2 on the studied samples
which indicated the relative content of the CuO species on
Sample
Cu/SiO2
Cu/Al2O3
Cu/ZnO
Cu/ZrO2
the catalyst surface. That is, the order of the relative content of
Cu2+ with a strong interaction with the support was Cu/ZrO2,
Cu/Al2O3, Cu/SiO2 and Cu/ZnO. Therefore, Cu/ZrO2 and Cu/
Al2O3 were more difficult to be reduced than the Cu/ZnO and
Cu/SiO2 catalysts, which was also in agreement with the results
Cu21+ (eV)
Cu22+ (eV)
933.5
935.8
35.8
933.6
935.2
18.6
933.0
934.0
38.8
933.4
934.5
15.6
Cu21+/(Cu21++ Cu22+ a
) (%)
a
Percent of relative amount of the metal species evaluated by XPS.
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of TPR (Fig. 3b). From the above, it was inferred that the For the Cu/Al2O3 and Cu/ZnO catalysts, the conversion of 95%
different supports had a different effect on the interactions was obtained, when reaction temperature was 280 1C, suggesting
between Cu and supports.
a good hydrogenation performance. Nonetheless, the selectivity
The TEM images of the spent catalysts before and after the of ethanol for the Cu/ZnO catalyst was higher than that of
ethyl acetate hydrogenation under reaction condition containing the Cu/Al2O3 catalyst, and the main by-product was ethane.
5 wt% water are shown in Fig. 5 and Fig. S1 (ESI†). From Fig. 5a, Moreover, the conversion of ethyl acetate for the Cu/ZnO catalyst
it was found that copper species were distributed evenly before was also higher than that of the Cu/Al2O3 catalyst at each
the reaction, and the particle sizes were mainly concentrated in temperature. In particular, above 90% conversion was obtained
3–5 nm. Furthermore, although the particle sizes increased after for the Cu/ZnO catalyst at 240 1C, while the conversion of the Cu/
the reaction (7–9 nm), the copper species were still highly Al2O3 catalyst was just 38%. Moreover, the turnover frequency
dispersed on the silica supports for Cu/SiO2, as shown in (TOF) was calculated to further show the effects of the supports
Fig. 5b. For the Cu/ZnO catalyst, the copper species were on the catalytic hydrogenation in the presence of water (Fig. 6b).
dispersed on the ZnO particles with sizes of 11–22 nm It was clear that the TOF increased with the of the reaction
(Fig. 5c). The particle sizes also increased after the reaction for temperature. Although the TOF of the Cu/Al2O3 catalyst
Cu/ZnO (18–36 nm), as shown in Fig. 5d. In addition, the sheet increased from 1.72 to 28.03 hÀ1 (above 90% conversion), a high
structure of the Cu/SiO2 catalyst was in accordance with the reaction temperature was necessary. For the Cu/ZnO catalyst, the
result of BET that slit-shaped pores were found due to the value of TOF was 14.38 hÀ1 at 220 1C, and further increased to
aggregates of plate-like particles. For the Cu/ZnO catalyst, 16.18 hÀ1 with above 90% conversion at 240 1C. These results
obvious channel structures could be observed from Fig. S3 showed that the Cu/ZnO catalyst had better catalytic perfor-
(ESI†). However, the particle sizes of the Cu/ZrO2 and Cu/Al2O3 mance and water-tolerance. Therefore, the Cu/ZnO catalyst
catalysts were larger than that of the Cu/SiO2 and Cu/ZnO was chosen to evaluate its stability at 240 1C at reaction
catalysts, which can be found in Fig. S1 (ESI†). This result was condition containing 5 wt% water, and the relative result is
in agreement with the result of TPR, and it was bad for the shown in Fig. S4 (ESI†). From Fig. S4 (ESI†), there was no
catalytic performance.
apparent inactivation observed within 200 h for the conversion
and ethanol selectivity of Cu/ZnO, suggesting that it had a good
stability.
3.2. Catalytic results
In order to confirm that the reason of low catalytic perfor-
mance was due to the existence of water rather than the catalyst
essential nature, the hydrogenation performances of all
catalysts were evaluated at reaction condition no-containing
water, as shown in Fig. 7 and Fig. S5 (ESI†). Except for the
reaction temperature, other reaction conditions were the same.
In particular, the reaction temperature of the Cu/ZnO catalyst
was 240 1C, and the reaction temperature for other catalysts was
280 1C. From Fig. 7, it was found that 90% conversion was
obtained for the Cu/ZnO catalyst at a reaction temperature of
240 1C. Although 87% conversion can be observed for Cu/SiO2,
and it required a reaction temperature of 280 1C. For the
Cu/Al2O3 and Cu/ZrO2 catalysts, their conversion increased
with the reaction time. In particular, the conversions of the
Cu/Al2O3 and Cu/ZrO2 catalysts increased from 70 to 87% and
from 25 to 79% within 50 h, respectively.
The hydrogenation performance of the copper catalysts was
evaluated to study the effects of supports on the gas-phase
hydrogenation of ethyl acetate with 5 wt% water (Fig. 6a), and
the calibration graphs of the products are shown in Fig. S3
(ESI†). It was found that the conversion of ethyl
acetate increased with the increase in the reaction temperature.
The above results from Fig. 6 and Fig. 7 indicated that 90%
conversion can be obtained for the Cu/ZnO catalyst under two
reaction conditions (240 1C), suggesting that the water had only
a little effect on the activity of the Cu/ZnO catalyst. However, an
obvious effect on the activity of the Cu/SiO2 catalyst is that the
conversion markedly decreased, as observed in Fig. 6 and Fig. 7.
3.3. Discussion
From Fig. 7, all catalysts showed a good hydrogenation perfor-
mance. However, the Cu/Al2O3 and Cu/ZrO2 catalysts had an
induction period within 50 h. This may be because of the
existence of strong interactions between Cu and Al2O3 or
ZrO2, and the big particle size, which had been confirmed by
XPS, N2O titration, TPR and TEM, led partial copper oxide
Fig. 5 The TEM images of Cu/SiO2 (a and b) and Cu/ZnO (c and d)
catalysts before (a and c) and after (b and d) the reaction.
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Fig. 6 The catalytic performance (a) and turnover frequencies (b) of the catalysts at reaction conditions (T = 220, 240, 260 and 280 1C): P = 2.5 MPa,
n(H2)/n(ethyl acetate) = 40 (molar ration), m(H2O)/m(ethyl acetate)/m(ethanol) = 5/91/4 (mass ration) and LHSV of ethyl acetate = 1 hÀ1
.
As mentioned above, an obvious effect on the activity of the
Cu/SiO2 catalyst could be found from Fig. 6 and Fig. 7. From
Fig. 6, it was found that the Cu/SiO2 catalyst had just 30%
conversion of ethyl acetate at reaction condition (280 1C)
containing water. However, 87% conversion was obtained for
the Cu/SiO2 catalyst at reaction condition no-containing water,
and no induction period was observed from Fig. 7. This
phenomenon showed that water had a great effect on the
hydrogenation performance of the Cu/SiO2 catalyst. A published
study has demonstrated that the doping of Fe, Co, Ni and Zn into
the Cu/SiO2 catalyst could form crystal defects at the border of
copper species, and the doped metal species were seen as active
sites of hydrogenation.17 Furthermore, the adsorption/reaction
sites with complementary chemical properties could be obtained
in a metal–oxide interface.37 Moreover, Ding et al. found that the
formation of Cu–MxOy (M = Zn, Zr, Al) interfaces was beneficial
to ethyl ester hydrogenations.32 Therefore, more active sites were
obtained for the Cu/ZnO, Cu/Al2O3 and Cu/ZrO2 catalysts than
Fig. 7 The catalytic performances of all catalysts at reaction conditions:
T = 240 1C (Cu/ZnO) or 280 1C (Cu/SiO2, Cu/Al2O3, Cu/ZrO2), P = 2.5 MPa,
n(H2)/n(ethyl acetate) = 40 (molar ration), pure ethyl acetate and LHSV of
ethyl acetate = 1 hÀ1
.
difficult to be reduced at the same reduction conditions, and that of the Cu/SiO2 catalyst due to the formation of Cu–MxOy
further resulting in a small number of active sites. Our previous (M = Zn, Zr, Al) interfaces. Also, competitive adsorption existed
studies have confirmed that the competitive adsorption between water and ethyl acetate on the active sites.18 Therefore,
between water and ethyl acetate existed in the ethyl acetate the great negative impact on the Cu/SiO2 catalyst was observed
hydrogenation reaction.18 The high number of active sites due to the existence of water. However, the strong interactions
played an important role in the activity of ethyl acetate and big particle sizes for the Cu/Al2O3 and Cu/ZrO2 catalysts were
hydrogenation at reaction condition containing water.17 bad for the catalytic performance, so that they showed worse
Therefore, the conversion of Cu/Al2O3 and Cu/ZrO2 catalysts hydrogenation performances than that of the Cu/ZnO catalyst.
were relatively low due to the incomplete reduction of CuO, That is, proper interaction, relatively small particle size and the
and the conversion increased with the reaction time because formation of Cu–ZnxOy interfaces were responsible for the Cu/
of the further reduction of copper oxides under the reaction ZnO catalyst to exhibit good water-tolerant hydrogenation
conditions.
performance.
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4. Conclusion
In this study, we chose SiO2, ZnO, ZrO2 and Al2O3 as supports to
study the effects of supports on the hydrogenation and water-
tolerance of copper catalysts. The results demonstrated that the
strong interactions and channel structures had great effects on
the catalytic activity of copper catalysts. The strong interactions
between copper and supports hindered the reduction of copper
oxides resulting in a low catalytic activity. Furthermore, the
formation of Cu–MxOy (M = Zn, Zr, Al) interfaces were also seen
as the active sites of hydrogenation, so that their water-tolerant
hydrogenation performance was better than that of the Cu/SiO2
catalyst. The Cu/ZnO catalyst, which possessed an appropriate
interaction between Cu and ZnO, showed a good hydrogenation
and water resistance performance. This work provides a factual
basis for the effects of supports on the hydrogenation and water
resistance performance of copper catalysts, which would have a
positive effect on industrial production.
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Conflicts of interest
The authors declare no competing financial interest.
Acknowledgements
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12 M. M. Najafpour, M. Abasi, M. Hołynska and B. Pashaei,
This work was supported by the Natural Science Foundation of
Shandong Province (ZR2020MB030 and ZR2020QB049), the
Science and Technology Research Program for Colleges and
Universities in Shandong Province (J18KA107) and the Key
Research and Development Program of Shandong Province
(2018GGX107010).
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