Sonochemical synthesis of Zn-promoted porous MgO-supported lamellar Cu catalysts for selective hydrogenation of dimethyl oxalate to ethanol and their long-term stability

Article abstract of DOI:10.1039/C8NJ03766C

The environmentally benign hydrogenation reaction of dimethyl oxalate (DMO) to produce the important platform chemicals methyl glycolate (MG), ethylene glycol (EG) and ethanol (EtOH) has gained significant importance in recent years. In this work, novel and highly efficient catalysts composed of Zn-doped lamellar Cu nanocrystals supported on porous MgO nanoparticles were synthesized using an innovative and facile sonochemical approach, and applied in the DMO hydrogenation reaction. The obtained catalytic activity revealed that the addition of Zn was found to significantly enhance the catalytic performance. The product selectivity dramatically changed from MG selectivity (88%) for the un-doped catalyst to a high EtOH selectivity of 98% for the Zn-doped Cu/MgO catalyst. To understand the relationship between the catalyst structure and the catalytic performance, the prepared, reduced and spent Cu/MgO and Zn-doped Cu/MgO catalysts were thoroughly characterized using XRD, TEM, HR-TEM, EDS mapping analysis, N₂ physical adsorption, XPS, Cu-LMM, FTIR and H₂-TPR techniques. The enhancement in the deep hydrogenation reaction for the Zn-doped catalyst to produce high EtOH selectivity is attributed to the improved dispersion, crystal defects, surface segregation and the synergistic ratio effect between Cu⁰/(Cu⁰ + Cu⁺) in the catalyst. Moreover, the produced catalysts maintained high efficiency for DMO conversion and EtOH selectivity with long-term stability for at least 200 h. The developed sonochemical approach in this study seems to be promising for the green and surfactant-free aqueous synthesis of highly efficient heterogeneous catalysts.

Full text of DOI:10.1039/C8NJ03766C

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Sonochemical synthesis of Zn-promoted porous
MgO-supported lamellar Cu catalysts for selective
hydrogenation of dimethyl oxalate to ethanol and
their long-term stability
Cite this: New J. Chem., 2018,
42, 17553
ab
a
ac
a
Mohamed Abbas,
*
Juan Zhang, Zheng Chen and Jiangang Chen*
The environmentally benign hydrogenation reaction of dimethyl oxalate (DMO) to produce the
important platform chemicals methyl glycolate (MG), ethylene glycol (EG) and ethanol (EtOH) has gained
significant importance in recent years. In this work, novel and highly efficient catalysts composed of
Zn-doped lamellar Cu nanocrystals supported on porous MgO nanoparticles were synthesized using an
innovative and facile sonochemical approach, and applied in the DMO hydrogenation reaction. The
obtained catalytic activity revealed that the addition of Zn was found to significantly enhance the catalytic
performance. The product selectivity dramatically changed from MG selectivity (88%) for the un-doped
catalyst to a high EtOH selectivity of 98% for the Zn-doped Cu/MgO catalyst. To understand the
relationship between the catalyst structure and the catalytic performance, the prepared, reduced and spent
Cu/MgO and Zn-doped Cu/MgO catalysts were thoroughly characterized using XRD, TEM, HR-TEM, EDS
2 2
mapping analysis, N physical adsorption, XPS, Cu-LMM, FTIR and H -TPR techniques. The enhancement
in the deep hydrogenation reaction for the Zn-doped catalyst to produce high EtOH selectivity is
attributed to the improved dispersion, crystal defects, surface segregation and the synergistic ratio effect
Received 28th July 2018,
Accepted 15th September 2018
0
0
+
between Cu /(Cu + Cu ) in the catalyst. Moreover, the produced catalysts maintained high efficiency
for DMO conversion and EtOH selectivity with long-term stability for at least 200 h. The developed
sonochemical approach in this study seems to be promising for the green and surfactant-free aqueous
synthesis of highly efficient heterogeneous catalysts.
DOI: 10.1039/c8nj03766c
rsc.li/njc
glycolate (MG), ethylene glycol (EG), and ethanol (EtOH) has
gained significant and great interest in recent few years as an
1
. Introduction
EtOH is an interesting and versatile candidate for the synthesis environmentally benign and clean reaction for the utilization of
4
of various important products such as polymers, synthetic fuel biomass and coal.
to be used in automobiles and other important chemicals.
1
Numerous research groups have focused their effort towards
Currently, EtOH is produced using mainly two approaches, the the development of several approaches for the synthesis of
first is the fermentation process of sugars, while the second highly efficient catalysts to be used in the DMO hydrogenation
approach is through the catalytic conversion of syngas reaction and obtain high selectivity ratios for MG, EG and EtOH
(
CO + H ). However, several disadvantages such as poor selectivity, under mild reaction conditions. Particularly, Cu-based catalysts
2
fast chain growth of C2 intermediates, low yield, and the high exhibited superb catalytic activity in the production of high EG
5
,6
cost required for carrying out massive research and develop- and EtOH selectivity. Nonetheless, the deactivation of the
2
,3
ment effort have restricted their wide practical application.
catalyst, which results from Cu sintering and leaching of silica,
Alternatively, the hydrogenation reaction of dimethyl oxalate makes the longevity as well as stability unsatisfactory and con-
7
(DMO) to produce the important platform chemicals methyl sequently limits the catalyst application in the industrial scale.
Therefore, the development of approaches for enhancing the
a
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry,
Chinese Academy of Sciences, Taiyuan, 030001, China.
E-mail: mohamed_abbas83@yahoo.com, mohamed@dgist.ac.kr,
chenjg@sxicc.ac.cn; Tel: +86-15334635354
catalyst dispersion and making the sintering resistance highly
efficient in the DMO hydrogenation reaction is of great interest.
Incorporation of second metals as promoters to the catalysts or
modification of the catalyst support via another oxide material
could be an effective approach to modify the electronic char-
acteristics and crystal structures of the active phase and thus
b
c
Ceramics Department, National Research Centre, El-Behouth Str., 12622 Cairo,
Egypt
University of Chinese Academy of Sciences, Beijing, China
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control the selectivity to the target products. For instance, Kong heating the sample in a boat crucible at a temperature of
et al. obtained high EG selectivity through the hydrogenation 400 1C in open air.
2
+
reaction of DMO using Mg doped nanoscale Cu–Mg/ZnO cata-
50 wt% Cu with a lamellar structure wrapped on the MgO NPs
8
lysts prepared by the co-precipitation method. Jung Li et al. was synthesized using a one-pot in situ sonochemical method.
3+
reported that doping of Ga promoted Cu catalysts with Zn can Typically, the above prepared MgO NPs and Cu(NO ) Á3H O were
3
2
2
dramatically improve their stability against metal sintering and dispersed in a suitable amount of di-H
2
O using a magnetic
enable high EG selectivity. Later, Qi et al. synthesized CuO–ZnO/ stirrer. The mixture was transferred to an ultrasonication reactor
SiO catalysts with various Zn/Cu molar ratios using a precipita- chamber for 30 min. Within a few minutes of initiating the
tion method and observed high EG selectivity.
9
2
7
ultrasonication, a solution of NaOH was injected rapidly. The
Herein, unlike the previously employed conventional synthesis reaction was continued, and after that, the ultrasonication was
approaches (the hydrolysis precipitation, urea-assisted gelation, turned off. The resultant Cu/MgO catalyst was washed several
hydrothermal and ammonia evaporation methods) for the syn- times using water and ethanol and the precipitate was collected
10–12
thesis of Cu-based catalysts
with a long synthesis reaction by means of centrifugation, and then dried at 80 1C. The pre-
time and additional rigorous protection conditions, we have pared Cu/MgO catalyst was calcined at 400 1C in an oven.
successfully developed a novel, convenient and eco-friendly sono-
2
.3. Synthesis of the Zn-doped Cu/MgO catalyst
chemical approach for the synthesis of Zn-doped lamellar Cu
nanocrystals supported on MgO NPs and then both un-doped and A one-pot sonochemical route was designed to synthesize the
Zn-doped Cu/MgO catalysts were employed in the DMO hydro- Zn doped-Cu/MgO NP catalyst. Typically, 1 mol of Zn(NO O,
Á6H
3 2
)
2
genation reaction. Porous MgO NPs are used as the catalyst 3 mol of Cu(NO ) Á6H O and the above prepared MgO NPs were
3
2
2
support due to the irreducible and relatively inactive oxide, and dispersed in an aqueous solution. After that, the mixture was
they are also widely used as a catalyst support in several cata- transferred to an ultrasonication vessel. 3 M NaOH was injected
lysis reactions such as CO
p-nitrophenol and oxidative dehydrogenation of propane.
2
Unlike the most inorganic materials such as SiO , ZrO , Al O
2
reforming of methane, reduction of rapidly within 7 min of starting the ultrasonication process and
1
3
the reaction was continued for 30 min and then turned off. The
produced Zn-doped Cu/MgO NPs were washed several times
2
2
3
and active carbon, which have been extensively reported in the using mixture of water and ethanol and the precipitated was
last few years as supports to Cu NPs in the DMO hydrogenation collected through the centrifugation, and then dried at 80 1C
reaction, in this study, MgO NPs were employed as robust for 6 h. The precipitate was collected using centrifugation, and
support materials. The synthesized Zn-doped Cu/MgO catalysts then dried at 80 1C for 6 h. Finally, the catalyst was calcined at
were subjected to comprehensive TEM, HR-TEM, EDS mapping, 400 1C for 3 h.
2 2
XRD, N physical adsorption, XPS, FTIR, H -TPR, and Cu-LMM
analyses to well understand the relationships between the
crystal structure and the catalytic activity.
2.4. Characterization methods
Powder X-ray diffraction (XRD) patterns were used to monitor the
crystal structure and were collected on a Shimadzu XRD-6000
diffractometer using Cu Ka radiation (l = 1.5418 Å) at 40 kV and
150 mA and in the 2y scanning range between 51 and 851. A Hitachi
H-7650 instrument, operated at an acceleration voltage of 100 kV
and (HRTEM, JEM 2100F) was employed to investigate the shape
and size of the catalysts, meanwhile, the chemical compositions of
the catalysts were determined through EDS point and mapping
analysis coupled with TEM. The pore size distribution and specific
2
. Experimental section
2.1. Chemicals and materials
Magnesium nitrate (Mg(NO ) Á6H O), cupric nitrate (Cu(NO ) Á
3
2
2
3 2
3H
2
O), zinc nitrate (Zn(NO
3
)
2
Á6H
2
O), sodium hydroxide (NaOH),
OH), were purchased from
and ammonium hydroxide (NH
Sinopharm Chemical Reagent Co., Ltd, China.
4
2
surface area were characterized using a N adsorption analyzer.
The BET surface area and the pore size distribution curves were
estimated based on the Brunauer–Emmett–Teller model and
2.2. Synthesis of the Cu/MgO catalyst
Firstly, the synthesis of MgO NPs as a support material was the Barrett–Joyner–Halenda method. An inductively coupled
carried out using a one-pot sonochemical method. Typically, plasma optical emission spectrometer (ICP-OES, Optima2100DV,
4
g of Mg(NO
di-H O and the solution was transferred immediately to an prepared catalysts. X-ray photoelectron spectroscopy was utilized to
ultrasonication vessel with a pulsed system of 3 s (ON):1 s identify the elemental valence of the catalysts. H -TPR experiments
OFF). After that, NaOH was rapidly added into the reaction were carried out to measure the reducibility of the catalysis in the
3
)
2
Á6H
2
O was dispersed in a suitable amount of PerkinElmer) was used to determine the actual Cu loading in the
2
2
(
À1
mixture within a few minutes of the ultrasonication. The temperature range of 0 to 800 1C. In the range of 4000–500 cm
,
reaction was continued for 60 min and the total reaction time Fourier transform infrared spectroscopy (FTIR) was performed to
is 20 min, and the ultrasonication was then turned off. The analyse the functional groups on the catalyst surfaces.
obtained MgO NPs were washed several times using water and
2.5. DMO hydrogenation reaction test
ethanol. The precipitate was collected by means of centrifuga-
tion and then dried in a vacuum evaporator. Crystallization The catalytic performance experiment was performed using 0.3 g
of the as-synthesized porous MgO NPs was carried out by of the calcined catalysts and using a stainless steel fixed-bed
1
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reactor at different reaction temperatures (200–240 1C). However,
before starting the reaction, the catalyst was reduced in a stream
of mixed 10% H in N at 280 1C for a period of 4 h. For the
2
2
reaction, 12.5 wt% DMO (purity 4 99.5%) in methanol and pure
H were fed into the reactor at a molar ratio of 200 for H /DMO,
2
2
meanwhile, the pressure system is set at 2.5 MPa. A gas chromato-
graph (Shimadzu Trace GC ultra) fitted with a 30 m HP-5 capillary
column and a flame ionization detector (FID) was used to analyze
the catalyst performance.
3. Results and discussion
The morphology and size of the synthesized un-doped and
Zn-doped Cu/MgO catalysts were analyzed using transmission
electron microscopy. Fig. 1a and b show the TEM images of the
porous MgO NPs synthesized using a sonochemical method.
Fig. 1c and d show the TEM images for the Cu/MgO NPs, and
from the figures, it can be seen that the lamellar Cu nanocrystals
were impregnated into the MgO NPs with a homogenous and
uniform narrow particle size distribution. Fig. 1e and f show the
HRTEM images of the Cu/MgO NP catalyst, and from the figures,
the lattice fringes of the lamellar CuO nanocrystal could be
Fig. 2 EDS mapping analysis of the Cu/MgO catalyst.
observed clearly, which confirms the highly crystalline structure
of the CuO nanocrystals. The interplanar distance value was
calculated from the adjacent lattice fringes to be 0.247 nm for
1
3
the (002) plane of the CuO phase. Fig. 2 shows the EDS
mapping analysis images of the MgO NPs impregnated with
the lamellar Cu NP catalyst. From the figure, a spatial distribu-
tion of the elements Mg, O, and Cu is observed. Fig. 2a shows
the TEM image of the Cu/MgO NP catalyst, which is to be
characterized by EDS mapping analysis. Fig. 2b displays the EDS
mapping analysis of the three elements Mg, O and Cu together
in the Cu/MgO catalyst and the images in Fig. 2c–e reveal the
elemental maps of the three elements separately.
Fig. 3 shows the TEM and HRTEM images for the Zn-doped
Cu/MgO catalyst. From Fig. 3a and b, it can be observed that
rod-like shapes are obtained within particle grain growth and
are highly crystalline after doping with Zn metal. Besides, the
calculated interplanar distance values from the HRTEM images
in Fig. 3c and d were found to be 0.241 and 0.5 nm for the (002)
and (001) planes of the CuO and ZnO phases, respectively, in
the Zn-doped Cu/MgO catalyst, confirming the co-existence of
both Cu and Zn NPs in the catalyst. It is noteworthy that the
addition of Zn metal as a dopant has created the so-called
crystal defects in the Zn-doped Cu/MgO catalyst, as shown in
the HRTEM images in Fig. 3e and f. The presence of Zn, Cu, Mg
and O elements together in the Zn-doped Cu/MgO catalyst is
confirmed through EDS elemental analysis as shown in Fig. 4b,
and the elemental composition is 56, 21, 16 and 7 wt% for Cu,
O, Mg and Zn, respectively. Also the good homogenous distri-
bution of the elements in the Zn-doped Cu/MgO catalyst is
confirmed through EDS mapping analysis, as shown in Fig. 4.
In addition, the obtained results from ICP-OES analysis demon-
strated that the real Cu loading values in the un-doped and
Zn-doped catalysts are 48.1 and 48.5, respectively, which are
close to each other and a pre-set value of 50 wt%.
To study the crystal structure and Cu dispersion of the two
prepared catalysts, XRD patterns for the calcined and reduced
Cu/MgO and Zn doped Cu/MgO catalysts are recorded and shown
in Fig. 5. From the XRD pattern of the calcined Cu/MgO catalyst,
both un-doped and Zn-doped catalysts are found to show char-
acteristic peaks for the crystalline phase of MgO (PDF no.: 45-0946),
Fig. 1 (a and b) TEM images of the MgO NPs, (c and d) TEM images of the
Cu/MgO NPs and (e and f) HRTEM images for the Cu/MgO NP catalyst.
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Fig. 4 EDS mapping and elemental analysis of the Zn-doped Cu/MgO NP
catalyst.
Fig. 3 (a–d) TEM and HRTEM images of the Zn-doped Cu/MgO catalyst
and (e and f) TEM images showing the crystal defects in the Zn-doped
Cu/MgO catalyst.
Zn-doped Cu/MgO catalysts and the calculated sizes are 31.4 and
3
8 nm, respectively.
The BET N adsorption/desorption and BJH methods were
employed to measure the specific surface area (SBET) and pore
2
and the CuO phase (PDF no.: 45-0937), while additional peaks size distribution of the prepared catalysts. Fig. 6a shows the N2
for ZnO are only detected for the Zn-doped catalyst (PDF no.: adsorption/desorption isotherms for the Cu/MgO NP catalysts,
001-1136). It is noteworthy that significantly high intensities of and from the figure, the un-doped and Zn doped Cu/MgO
the CuO peaks are observed for the Zn-doped catalyst, reflecting catalysts were found to exhibit Langmuir type IV isotherms.
the role of Zn doping on the dramatically improved crystalline The obtained IV isotherm for the prepared catalysts is reflecting
structure of the catalyst. Fig. 5b shows the XRD patterns for the typical shape of a mesoporous material based on the IUPAC
1
6
the reduced catalysts. From the figure, for both catalysts, classification. Besides, a H2 hysteresis loop type is observed for
characteristic peaks for the reduced Cu metal are detected the Zn-doped catalyst, which mostly comes from the presence of
at 2y = 43.31, 50.51 and 74.11 (card no.: 04-0836), meanwhile, uniform channel-like pores and pores within narrow mouths
1
7
a similar behaviour with large differences in the peak intensi- (ink-bottle pores). The estimated specific surface area values
2
À1
ties is observed again for the reduced catalysts, which confirms through the BET method are 32 m g for the Cu/MgO catalyst
2
À1
the importance of Zn in enhancing the reduced catalyst crystal- and 39 m g for the Zn doped Cu/MgO catalyst. Fig. 6b shows
linity. Besides, an additional small peak at 371 corresponding to the pore diameter curves for the prepared materials, and the
+
Cu
2
O (JCPDS 05-0667) is observed, which confirms that Cu
estimated average pore diameters based on the BJH methods are
0
3
existed in both catalysts along with Cu after reduction. It is 50 nm for the Cu/MgO NPs and 48 nm for the Zn doped catalyst.
noteworthy that the observed shift to a higher 2y value for the In addition, the estimated copper surface areas for the un-doped
2
À1
reduced Zn-doped Cu/MgO catalysts in the high magnification and Zn-doped Cu/MgO catalysts are 18 and 26 m gcat
,
image in the inset of Fig. 5b is mostly attributed to the crystal respectively, according to the calculation from the dissociative
1
4,15
disorders and defects,
which supports the obtained above
N
2
O chemisorption method.
TEM result. On the other hand, as can be seen from the XRD
The surface properties of the prepared catalysts were inves-
pattern, the Scherrer formula (D = Kl/b cos y) was employed tigated through FTIR absorption analysis, which was performed
À1
to measure the crystallite sizes of the reduced un-doped and in the spectral range of 4000 to 400 cm . Fig. 7 shows the FTIR
1
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Fig. 5 XRD pattern of the (a) calcined and (b) reduced Cu/MgO and
ZnCu/MgO catalysts synthesized by a sonochemical method.
Fig. 6
N
2
physisorption results. (a) N
2
adsorption–desorption isotherms
spectra for the un-doped Cu/MgO and Zn doped catalysts
synthesized by a sonochemical method. In the spectra of both
and (b) BJH pore size distribution curves for the Cu/MgO and Zn-doped
Cu/MgO catalysts.
À1
catalysts, the peak at 400 cm is assigned to the stretching
À1
vibration of Cu–O, while the peak at 490 cm for the Zn-doped
catalyst corresponds to the stretching vibration of Zn–O. the observed high intensity and widening in the reduction peak
À1
In addition, a broad absorption band at around 3500 cm is for the Zn-doped Cu/MgO catalyst reflected the strong inter-
observed, which corresponds to the OH-stretching vibrations of action between the Zn and CuO NPs, which reduced the strong
the hydroxyl group in the MgO NPs. Besides, the peak at around interaction between the catalyst support MgO and Cu species,
À1
4
00 cm is attributed to the Mg–O stretching vibration, while and thereby improved the catalyst dispersion, which leads to
À1
the bands in the range of 1500 cm are assigned to the –OH the consumption of more hydrogen, and consequently many
asymmetric stretching mode of water molecules.
1
8
more CuO NPs were reduced into the active metallic Cu species.
Reducibility measurement is an important tool to measure Therefore, Zn incorporation into the catalyst increased the
the reduction behaviour of a catalyst and consequently dispersion and the reducibility. Similar results were reported
7
the active phase in the catalysis reaction. Therefore, H
analysis was utilized to study the degree of reduction as well as
the metal–support interaction for the prepared catalysts. Fig. 8 position and valence state of the calcined Cu/MgO catalysts.
shows the H -TPR profile for the un-doped and Zn-doped Fig. 9a shows peaks corresponding to Mg, Cu and O in the XPS
2
-TPR previously by Qi et al.
XPS analysis was carried out to determine the surface com-
2
Cu/MgO catalysts. From the H -TPR profile, only a single survey spectra for the un-doped Cu/MgO catalyst and Mg, Cu, Zn
2
distinguished peak is observed for both catalysts, meanwhile, and O in the XPS survey spectra for the Zn-doped catalyst. Fig. 9b
the Zn-doped Cu/MgO catalyst displayed a reduction peak at a shows high resolution spectra of Cu, and from the figure, two
lower temperature of 287 1C compared with a reduction tem- major peaks positioned at 933 eV and 953 eV for Cu(2p3/2) and
2
+
0
perature of 296 1C for the un-doped Cu/MgO catalyst, which Cu(2p1/2) with a splitting of 20 eV corresponding to Cu of Cu
reflected the easy reduction of the Zn-doped catalyst. Moreover, on the surface of both catalysts were observed, while two other
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0
+
concentration of the active sites Cu and Cu in the catalysts.
From the figure, two peaks at 911.3 eV and 916.2 eV for the
sonochemicaly synthesized catalysts are found to be overlapping
+
0
and they correspond to the kinetic energy of Cu and Cu ,
respectively. The calculated deconvolution results revealed that the
0
0
+
intensity of Cu /(Cu + Cu ) is 59% and 68.5% for the un-doped
and Zn-doped Cu/MgO catalysts, respectively.
The catalytic activities of the calcined catalysts were inves-
tigated for the hydrogenation reaction of DMO at different
temperatures (200–240 1C). Fig. 11 shows the effect of the
reaction temperature on the DMO conversion ratio. From the
figure, it can be seen that the DMO conversion rate increased
progressively and reached the maximum of 100% at 230 1C
and then remained stable for the un-doped Cu/MgO catalyst,
while the DMO conversion ratio is much improved for the
Zn-doped Cu/MgO catalyst and reached 100% at 220 1C and
then remained stable at 100% ratio with a further increase in
the reaction temperature up to 240 1C.
Fig. 7 FTIR spectra for the Cu/MgO and Zn-doped Cu/MgO catalysts.
Fig. 12 shows the EtOH and MG selectivity at different
reaction temperatures for the un-doped and Zn-doped Cu/MgO
catalysts. As can be seen from Fig. 12a, the Cu/MgO catalyst shows
a relatively low EtOH selectivity at all reaction temperatures.
However, a remarkable enhancement in the EtOH selectivity is
observed for the Zn-doped Cu/MgO catalyst and the selectivity
increased dramatically with reaction temperature to reach the
maximum value of 98% at 240 1C. Fig. 12b shows the MG
selectivity of the two catalysts, and from the figure, it can be
observed that the MG selectivity of the un-doped Cu/MgO
catalyst reached the maximum value of 88% at 210 1C and then
decreased along with the increase in the reaction temperatures.
Generally, the DMO hydrogenation reaction involves the
following successive reactions: MG formation, followed by EG
formation and finally the deep hydrogenation reaction leading
to the EtOH product. It is worth mentioning that, in most of the
reported studies on the DMO hydrogenation reaction, it has
0
Fig. 8
H
2
-TPR profile for the Cu/MgO and Zn-doped Cu/MgO catalysts. been confirmed that Cu is the main active site and primarily
responsible for the activity of the catalyst, and also a proper
+
0
amount of Cu /Cu plays a crucial role in controlling both DMO
shake-up peaks at 942 eV and 962 eV further confirmed the conversion and product selectivity ratios in the DMO hydro-
2
+
19
1
existence of the Cu chemical state.
genation reaction. Thus, in this study, the obtained high DMO
Fig. 9c shows Mg high resolution spectra, and the spectra conversion ratio for the Zn-doped Cu/MgO catalyst at all reaction
0
show a single peak at 1303.2 for Cu/MgO and 1304 eV for the temperatures below 230 1C is mostly attributed to the high Cu
Zn doped-Cu/MgO catalysts, which are typically due to the ratio in the catalyst. A similar result is also recently reported by
2
+
Mg species in the two catalysts. High resolution O spectra our group for the obtained high DMO conversion ratio at a high
0
23
are shown in Fig. 9d, and from the figure, two similar peaks concentration of Cu in the Cu/SiO catalytic system.
2
were found for the two catalysts at 529.4 and 531.4 eV. The first
According to the series of characterization experiments
peak corresponds to the lattice oxygen in MgO and Cu, while performed for our Zn-doped Cu/MgO catalyst, the obtained
the second peak at 531.4 eV may have originated from the remarkable high EtOH selectivity will be explained in the
adsorbed oxygen and water molecules on the surface of the following points. Incorporation of Zn into the Cu/MgO catalyst
2
0,21
catalyst.
It is worth mentioning that the shift in the binding resulted in an improvement in the copper dispersion, and
energy peak for the Zn-doped catalyst (Fig. 9) is mostly attributed subsequently increased the active sites i.e., Cu species and
to the resulting crystal defects of the catalyst after doping with the catalytic activity. Furthermore, the generated crystal defects
Zn NPs, which is in good agreement with the obtained above in the Zn-doped catalyst, as confirmed by the obtained above
2
2
TEM and XRD results.
TEM images, also contributed to promote the dispersion of the
Fig. 10 shows Cu-LMM Auger electron spectra for the reduced Cu species, and consequently provided more active sites and
un-doped and Zn-doped Cu/MgO catalysts to distinguish the then boosted the deep hydrogenation reaction to produce EtOH.
1
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Fig. 9 (a) XPS survey spectra of the Cu/MgO and Zn-doped Cu/MgO catalysts and (b–d) are high resolution Cu, Mg and O spectra, respectively.
0
0
+
+
Moreover, the ratio of Cu /(Cu + Cu ) increased from 59% to be stabilize the Cu concentration in the Ag–CuO
x
catalyst to
+
6
8.5% for the Zn-doped Cu/MgO catalyst, and based on the obtain high MG selectivity since the high concentration of Cu
work reported by Gong et al., Cu is the sole active site for the on the catalyst surface would enhance the activation of the acyl
0
chemoselective synthesis of ethanol and is responsible for and methoxy groups of MG to compensate for the weakening of
0
the activity of the catalyst, therefore, the obtained high EtOH
H
2
activation because of the fewer Cu species in the catalyst
2
5
selectivity in the Zn-doped catalyst compared with the un-doped surface. Thus, this agrees well with the obtained results in our
one is mostly ascribed to the pivotal role of the Cu species. study and the obtained relatively high MG selectivity of the
In addition, from the obtained H -TPR results, incorporation of un-doped Cu/MgO catalyst is ascribed to a high Cu /(Cu + Cu )
2
0
+
+
0
Zn into the Cu/MgO catalysts caused a reduction in the strong ratio of 41%. Moreover, based on the above obtained FTIR result,
interaction between the catalyst supports MgO and Cu species, the un-doped catalyst possesses a large amount of OH groups on
and thereby improved the Cu dispersion as well as reducibility, the surface and it is worth noting that Wen et al. and Yin et al. have
and thus the catalytic activity.
found that the presence of hydroxyl groups on the surface of the
On the other hand, to understand the obtained relatively catalysts profoundly promotes the hydrogenation process of DMO
24,26
high MG selectivity of 88% for the un-doped Cu/MgO catalyst, to MG,
because the increase in the OH amount leads to
and based on the recent research on the DMO hydrogenation an increase in the amount of weak acids for the catalysts and
+
reaction, Cu is more disposed to stabilize the intermediate consequently makes the desorption process of MG over EG and
2
7,28
product of DMO hydrogenation such as MG compared with EtOH easier.
Cu . Therefore, MG is formed when the concentration of Cu
0
24
+
The long term stability of catalysts is an important indicator
is more or in a proper amount. Chen et al. employed Ag NPs to of the potential of their catalytic industrial application. Thus,
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Fig. 10 Cu-LMM Auger spectra for the (a) Cu/MgO and (b) Zn-doped
Cu/MgO catalysts.
Fig. 12 EtOH and MG selectivity for the un-doped and Zn-doped Cu/MgO
catalysts at different reaction temperatures.
Fig. 11 DMO conversion ratios for the un-doped and Zn-doped Cu/MgO
catalysts at different reaction temperatures.
the Zn-doped Cu/MgO catalyst was subjected to the DMO
hydrogenation reaction for a period of 200 h and the catalyst
performance versus time on stream is shown in Fig. 13. As
Fig. 13 Catalytic performance of the Zn-doped Cu/MgO catalyst for 200 h
shown in the figure, the catalyst maintained high ratios of both of time on stream.
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while the Zn-doped Cu/MgO catalyst displayed a high EtOH
selectivity of 98% at 240 1C. Moreover, the synthesized Zn-doped
Cu/MgO catalyst maintained high efficiency for DMO conversion
and EtOH selectivity with long-term stability for at least 200 h. In
light of the full characterization of the catalyst, the obtained high
EtOH selectivity for the Zn-doped Cu/MgO catalyst is attributed to
the improved dispersion, crystal defects, surface segregation with
0
0
+
Zn doping and also a high ratio of 68.5 of Cu /(Cu + Cu ) in the
catalyst. However, the relatively high MG selectivity of the
un-doped Cu/MgO catalyst is mainly due to the high ratio of
+
+
0
Cu /(Cu + Cu ) in the catalyst. The developed facile sonochemical
route in this study is expected to extend to the synthesis of several
highly efficient heterogeneous catalysts.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the NSFC (National Natural Science
Foundation of China project 21673272 and 21373254) and the
Postdoctoral fellowship project of President’s International Fellow-
ship Initiative (PIFI) by the Chinese Academy of Sciences, China.
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